Encyclopedia of Astrobiology
Muriel Gargaud (Editor-in-Chief) Ricardo Amils, Jose´ Cernicharo Quintanilla, Henderson James (Jim) Cleaves II, William M. Irvine, Daniele L. Pinti and Michel Viso (Eds.)
Encyclopedia of Astrobiology With 547 Figures and 68 Tables
Editor Muriel Gargaud
Astrophysicist Laboratoire d’Astrophysique de Bordeaux BP 89 33270 Floirac France
ISBN 978-3-642-11271-3
e-ISBN 978-3-642-11274-4
Print and electronic bundle under ISBN 978-3-642-11279-9 DOI 10.1007/978-3-642-11274-4 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2011927757 © Springer-Verlag Berlin Heidelberg 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Printed on acid-free paper Springer is part of Springer ScienceþBusiness Media (www.springer.com)
Foreword Are we alone? Long an object of speculation or fiction, if not heresy, this question entered the field of science on November 1, 1961, at the National Radio Astronomy Observatory in Green Bank, Virginia, where a number of scientists, including Melvin Calvin, who had just been awarded the Nobel prize in chemistry for his work on photosynthesis, and the charismatic Carl Sagan, gathered at the invitation of a young astronomer, Frank Drake, to launch the Search for ExtraTerrestrial Intelligence (SETI) project. Since then, batteries of increasingly powerful radiotelescopes have been scanning space for messages sent out by some extraterrestrial civilization. So far in vain. At the same time, in the wake of widening space exploration, a new discipline was born that has the distinctive peculiarity of having three names – exobiology, astrobiology, bioastronomy – and no as-yet-known object. The purpose of this new discipline is more modest than that of the SETI project: to detect signs of extraterrestrial life, not necessarily intelligent. To guide this quest, we have available vast knowledge that has been gained in the last few decades concerning the basic mechanisms of life. This knowledge, in turn, has illuminated our concept of the origin of life. Even though we do not know how or under what conditions this phenomenon took place, we may safely affirm that if life arose naturally, which is the only scientifically acceptable assumption, its origin must have depended on “chemistry.” By its very nature, chemistry deals with highly deterministic, reproducible events that are bound to take place under prevailing physicalchemical conditions. If even a very slight element of chance affected chemical reactions, there would be no chemical laboratories, no chemical factories. We could not afford the risk. A conclusion that emerges from this consideration is that life, as a product of environmentally enforced chemistry, was bound to arise under the physical-chemical conditions that prevailed at the site of its birth. This statement, at least, holds true for the early steps in the origin of life, until the appearance of the first replicable substance, most likely RNA. Once this happened, “selection” became added to chemistry, introducing an element of chance in the development of life. Contrary to what has often been claimed in the past, this fact does not necessarily imply that the process was ruled by contingency. There are reasons to believe that, in many instances, chance provided enough opportunities for selection to be optimizing, and, therefore, likewise obligatory under prevailing conditions. Thus, in so far as chemistry and optimizing selection played a dominant role in the process, the development of life appears as the obligatory outcome of prevailing conditions. Hence the assumption that the probability of the appearance elsewhere in the universe of forms of life resembling Earth life in their basic properties is approximately equal to the probability of the occurrence elsewhere in the universe of the physical conditions that obtained at the site where Earth life arose. In the eyes of many astronomers, this probability is very high. It is estimated that some 30 billion sunlike stars exist in our galaxy alone and that the total number of galaxies in the universe is on the order of 100 billion. This means, to the extent that our galaxy may be taken as a representative sample of galaxies in general, there may be some 3,000 billion sunlike stars in the universe. Unless our solar system should be the product of extremely unlikely events, the probability of there being planets similar to Earth (or to whatever celestial object served as the cradle of Earth life) seems very strong. Recent findings are most encouraging in this respect, by revealing that planet formation is not a rare event, with more than 400 planets already identified around a number of nearby stars. Although no habitable Earthlike extrasolar planet has yet been found, this may be partly due to technical limitations. The prospects that, with improved technologies, such a planet may be discovered some time in the future are far from negligible. Signs of life on such a planet, although more difficult to detect, may likewise yield to technological progress. As by now, the enormous research effort expended within the framework of the new discipline of exobiologycum-bioastronomy-cum-astrobiology has already produced a wealth of new findings, in fields ranging from physics and
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cosmology to chemistry, biochemistry, and molecular biology. These findings have provided rich material for this Encyclopedia. Its editors and authors are to be commended for making this material widely available in easily accessible form. Christian de Duve 14 January 2011
Reference 1. de Duve C (2005) Singularities landmarks on the pathways of life. Cambridge University Press, New York
Acknowledgments A brief note is warranted about how we constructed the Encyclopedia. A glossary of terms was first compiled by a team of experts in each field. It was then cross-referenced between fields to check for conceptual overlap and was then both expanded and pared down to produce a consensus entry list. Authors with peer-recognized contributions to their fields of study were then invited to contribute entries appropriate to their expertise. After a final draft was submitted, entries were proofread and vetted for scientific accuracy and readability by a team of field editors, then edited and modified to be accessible by a reader with general knowledge of college-level science. Finally, the entries were cross-referenced, and edited for stylistic consistency and ease of reading. The editors would like to sincerely thank all the authors of the content of the Encyclopedia for their efforts and understanding throughout the long and at times difficult triple review process. We are particularly grateful to those who also accepted to act as non-specialist reviewers for fields other than their own. We would also like to thank several people who, although not authors, served as external reviewers for a significant number of entries: Maxence Claeys (Ecole Centrale Paris, France), Carlos Garcia-Ferris (Universitat de Vale`ncia, Spain), David Hochberg (CAB, Madrid, Spain), Pierre Le´na (Acade´mie des Sciences, Paris, France), Susan Leschine (University of Massachusetts Amherst, USA), Jean Vandenhaute (University of Namur, Belgium). We express our gratitude to our respective institutions, especially those who facilitated and aided in the organization and funding of editorial meetings: Centre National de la Recherche Scientifique (CNRS, France), Centre National d’E´tudes Spatiales (CNES, France), Laboratoire d’Astrophysique de Bordeaux (France), Universite´ Bordeaux 1 (France), GEOTOP research center for geochemistry and geodynamics (Universite´ de Que´bec a` Montre´al, Canada), Natural Sciences and Engineering Research Council of Canada, European Science Foundation (Archean Environment Research Networking Program), Centro de Astrobiologı´a (CAB, INTA-CSIC, Madrid, Spain), Universidad Auto´noma de Madrid, NASA Goddard Space Flight Center Cooperative Agreement NNX09AH33A with the University of Massachusetts Last, but certainly not least, we express our sincere appreciation to the editorial staff of Springer, in particular Saskia Ellis, Marion Kraemer and Jana Simniok, who lent technical and administrative support throughout the entire process. The Editors
Preface Where do we come from? Are we alone in the Universe? Where are we going? These are the questions addressed by astrobiology – the study of the origin, evolution, distribution, and the future of life in the Universe. Encyclopedias are unusual works. A quote from the prologue of one of the more famous early encyclopedias is instructive: "
“. . .the purpose of an encyclopedia is to collect knowledge disseminated around the globe; to set forth its general system to the men with whom we live, and transmit it to those who will come after us, so that the work of preceding centuries will not become useless to the centuries to come; and so that our offspring, becoming better instructed, will at the same time become more virtuous and happy, and that we should not die without having rendered a service to the human race in the future years to come”. Diderot and d’Alembert, Encyclope´die (1751).1
Diderot and d’Alembert’s eighteenth century Encyclope´die was indeed ground-breaking, but perhaps more remarkable is the degree to which their description resembles the modern concept of genetic inheritance and natural selection: a civilization’s accumulated knowledge being analogous to the traits encoded in an organism’s time-tested DNA genome. In many ways, the Encyclope´die addressed the goals of astrobiology; between the lines, we find aspects of what makes biology biology. Encyclopedias have now existed for approximately 2,000 years, the first being Pliny the Elder’s Naturalis Historia, which was a compendium of the knowledge available to a citizen of the Roman Empire as documented by the first century AD.2 It contained 20,000 facts from 2,000 sources written by 200 authors. The present volume contains an unknown number of “facts” (indeed, some of the content will likely be proven false, as science is a living, breathing accumulation of presently accepted knowledge, all subject to future revision), but it does include more than 1,700 contributions, references uncounted thousands of prior publications, and is written by 385 authors. Modern encyclopedias are derived from the dictionaries of the eighteenth century. The two are similar in that both are arranged alphabetically and generally are the work of a team of expert contributors. They differ in that encyclopedias contain a deeper level of analysis of the included terms and attempt to cross-reference and place the assembled contents in a useful context. The first encyclopedias attempted to cover all human knowledge. This is now impossible for a printed work because the body of human knowledge is presently growing exponentially, with no end in sight. Encyclopedias now exist for almost every definable field of study. A field requires a certain degree of maturity to have an encyclopedia, and conversely, the publication of an encyclopedia commonly records the birth of a definable field of study. Astrobiology is an interdisciplinary field, spanning geology, chemistry, physics, astronomy, biology, engineering, and computer science, to name only the core fields of study. While some of these fields of research are fairly well mapped, many others are in rapid flux, and still others remain perennially enigmatic, awaiting future breakthroughs by the scientists of tomorrow. To this end, the Encyclopedia of Astrobiology is primarily aimed at younger scientists or scientists new to the field who wish to understand how their expertise coincides with current knowledge in other areas of study. It is hoped that the encyclopedia will serve to orient researchers to the current state of the art. A more in-depth discussion of many of the topic areas can be obtained by referring to college or graduate level texts or to the articles cited at the end of many of the entries. Encyclopedias are snapshots of the state of knowledge at a particular time. In 1844, the book Vestiges of the Natural History of Creation was published anonymously (it was later found to have been written by Scottish publisher William Chambers) and created a public sensation.3 It offered a sweeping and very secular view of the development of the Solar System, stretching from the nebular hypothesis to the development of man. While primitive by modern standards (it was, after all, based on state-of-the-art early nineteenth century science), it was in many ways remarkably similar to modern cosmology. In broad brushstrokes, it is the precursor to the worldview developed in Carl Sagan’s Cosmos4 and the grand view of myriads of habitable planets implicit in the Drake equation. The implication of Vestiges was simply this: the
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Universe operates everywhere and at all times according to physical principles, and the evolution of matter is largely predictable and often progressive, proceeding from the simple to the complex. Science has advanced dramatically since Chambers’ book was published. It is truly a long way from Sir William Hershel’s 40-ft telescope to the Herschel Space Telescope,5 and from a Universe with seven known planets orbiting the Sun to one with more than 500 planets orbiting other stars. It is also a long way from the work of Black, Priestly, and Lavoisier6 to SELEX technology and high-throughput automated chemical screening and analysis, and from Lyell’s Principles of Geology7 to plate tectonics and isotope geochemistry. Nonetheless, certain questions permeate the sciences across time and discipline. Woese’s three domains of life8 are direct descendents of Linnaeus’ early classification scheme, and both are attempts to unify and classify terrestrial organisms. Darwinism has offered an underlying mechanism for doing so that has allowed for unification of the assorted observations of the living world. However, the question of whether terrestrial life is unique in the universe has fascinated mankind for millennia. It was not until 1959, when NASA began funding the search for life in the Universe in its Exobiology program, that we at last achieved the technological prowess to try to answer this question.9 The paleontologist George Gaylord Simpson famously noted shortly thereafter that Exobiology was a science “that has yet to demonstrate that its subject matter exists.” NASA’s first exobiology grant was awarded to Wolf Vishniac for the construction of the Wolf Trap, a device for detecting bacteria on Mars. Due to size limitations, the device never flew, but various descendants have made the trip to Mars and returned various negative or tantalizingly ambiguous results. These results are, amusingly, either disappointingly or encouragingly ambiguous, depending on one’s point of view. Despite remarkable progress in the sciences, humanity still has no answer to the question, “Are we alone?,” though the question is in principle answerable. The search continues enthusiastically. Why should we think there might be life elsewhere in the Universe? In 1960, the radio-astronomer Frank Drake developed his now-famous equation for estimating the number of communicating civilizations in the Galaxy: N ¼ R f p ne f e f i f c L; where N is the number of civilizations in our galaxy for which communication might be possible, R* is the average rate of star formation per year, fp is the fraction of stars that have planets, ne is the average number of planets that can support life per star with planets, fℓ is the fraction of the planets that can support life on which life actually develops, fi is the fraction of those on which intelligent life develops, fc is the fraction of those on which civilizations communicate using detectable signals, and L is the length of time these civilizations communicate. When Drake unveiled his equation in 1960 and estimated that there were maybe ten communicating civilizations in the Galaxy, few of the parameters were known with any certainty; the rate of star formation was perhaps the only solid measurable value. Fifty years later, the flourishing search for exoplanets has placed the focus on the second value (notably, it now appears to be close to what Drake estimated, 50%). Hundreds of exoplanets have been found around other stars, and current technology allows the observation of even small planets. Theory suggests that the fraction of stars with Earthlike planets is somewhere near 10% (again, surprisingly, and a tribute to back-of-the-envelope calculations, not far from Drake’s initial estimate). The least well-known value is the question of how difficult is it for life to begin (one of the “perennially enigmatic” facts mentioned above). Based on present knowledge, the fraction of planets on which life actually emerges (fl) could be anywhere from very, very close to 0 or far closer to 1. We simply do not know. On the ends of the spectrum, the scientific community is divided into two equally “hunch-”based camps: first, life is inevitable and is a cosmic imperative (where conditions are appropriate) and, second, the origin of life requires such a concatenation of improbable events that it is the scientific equivalent of a miracle. On the one planet we know of with life, our own, putative evidence in the form of isotopically light carbon appears in the earliest known sedimentary rocks, suggesting life emerged relatively early in the history of the planet, although we do not know whether this took place 100 years or 700 million years after the planet formed. This implies that either something extraordinary happened on Earth, or that the origin of life is a mundane phenomenon on young planets, given appropriate chemistry, environmental conditions, and enough time. Radioastronomy has provided a glimpse of the chemical inventory of the cosmos which does appear to be universal. Spectral signatures of a veritable zoo of organic compounds suggest that the Universe is strewn with the potential precursors of life. Organic carbon (in the form of carbon monoxide) has now been observed as far back as 13 billion years ago, only some 700 million years after the birth of
Preface
the Universe in the Big Bang. The picture emerging, reminiscent of Chambers’ universe, is that physics and chemistry are the same everywhere in the Universe, and that the Earth, although remarkable in many respects, may not be unique. As in any factorial equation, the most important values are the ones with the largest uncertainty. Two approaches could shed light on the “fl problem”: the duplication of the process in the laboratory or the discovery of life on another planet. It is difficult to say whether the first approach will ever succeed to anyone’s complete satisfaction, given that the origin of life on Earth was a historical event that happened when no one was around to witness it. The second approach, while fraught with technological difficulties, is perhaps more promising. To that end, numerous instruments and space missions have been designed and launched to explore the Solar System and beyond. The spectral signatures of planets around nearby stars are being monitored for the characteristic signs of life such as the signature of disequilibrium chemistry in the form of the presence in their atmospheres of both oxidized and reduced gases. While the answers to the vast questions that define astrobiology as a field of study are unclear, it is evident that answering them will require an interdisciplinary effort, stretching across international borders. One is hesitant to speculate what the answer to the question, “Are we alone?” will ultimately be. As good scientists, we should probably withhold judgment until the data are in. As better scientists, we must join hands and find the data. The editors of the Encyclopedia of Astrobiology hope that this volume will contribute to this effort. The Editors
Notes 1. A complete English and French version of the Encyclope´die can be found at http://quod.lib.umich.edu/d/did/ 2. For a complete English translation of Pliny the Elder’s The Natural History by John Bostock see http://www.perseus. tufts.edu/hopper/text?doc=Plin.+Nat.+toc&redirect=true. A complete Latin version can be found at http://www. perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0138:toc&redirect=true 3. Chambers R (1994) Vestiges of the natural history of creation and other evolutionary writings. University of Chicago Press 4. Cosmos was a remarkable 13-part popular science series narrated by Carl Sagan which aired in 1980. Most if not all of the episodes can be viewed on line, and a book was spun off: Sagan C (1985) Cosmos. Ballantine Books 5. For a survey of the early developments in astronomy, see Lankford J (ed) (1996) History of astronomy: an encyclopedia, first edition. Routledge 6. For an excellent discussion of the early history of chemistry (including the work of Black, Priestly and Lavoisier) see Partington JR (1989) A short history of chemistry, 3rd revised edition. Dover Publications 7. Lyell C (2010) Principles of Geology: Being an Inquiry How Far the Former Changes of the Earth’s Surface Are Referable to Causes Now in Operation. Nabu Press (March 1, 2010). Originally published in three volumes between 1830–1833 8. Woese C, Kandler O, Wheelis M (1990) Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proc Nat Acad Sci USA 87(12): 4576–4579 9. For an insightful recounting of the early history of NASA’s early efforts in exo- and astrobiology (including discussion of the roles of Wolf Vishniac and Frank Drake) see Dick SJ, Strick JE (2005) The living universe: NASA and the development of astrobiology. Rutgers University Press
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Editor-in-Chief Muriel Gargaud Laboratoire d’Astrophysique de Bordeaux CNRS and Universite´ Bordeaux 1 2 Rue de l’Observatoire 33270 Floirac France
[email protected] Field: Astrophysics
Book Editors Ricardo Amils Laboratory of Extremophiles Departament of Planetology and Habitability Centro de Astrobiologı´a (INTA-CSIC) Ctra de Ajalvir km 4 28850 Torrejo´n de Ardoz Madrid Spain
[email protected] Field: Geomicrobiology Jose´ Cernicharo Quintanilla Department of Astrophysics Centro de Astrobiologı´a (INTA-CSIC) Ctra de Ajalvir km 4 28850 Torrejo´n de Ardoz Madrid Spain
[email protected] Field: Astrochemistry and Astrophysics Henderson James (Jim) Cleaves II Geophysical Laboratory Carnegie Institution of Washington Washington, DC 20015 USA
[email protected] Field: Chemistry
William M. Irvine Goddard Center for Astrobiology and Astronomy Department University of Massachusetts 619 Lederle Graduate Research Center Amherst, MA 01003 USA
[email protected] Field: Astrochemistry and Planetary Science Daniele L. Pinti GEOTOP and De´partement des sciences de la Terre et de l’atmosphe`re Universite´ du Que´bec a` Montre´al CP 8888, succ. Centre-Ville Montre´al, Que´bec, H3C 3P8 Canada
[email protected] Field: Earth Sciences Michel Viso Astrobiology CNES/DSP/EU Paris France
[email protected] Field: Space Missions and Planetary Protection
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Field Editors
Field Editors Francis Albarede Ecole Normale Supe´rieure de Lyon 46, Allee d’Italie 69364 Lyon Cedex 7 France
[email protected] Field: Early Earth Geochemistry Nicholas Arndt Maison des Ge´osciences LGCA Universite de Grenoble 1381 rue de la Piscine 38400 St Martin d’He`res France
[email protected] Field: Archean Geology Hugues Bersini IRIDIA Universite´ Libre de Bruxelles Avenue Franklin Roosevelt 50 B-1050 Brussels Belgium
[email protected] Field: Artificial Life Carlos Briones Laboratory of Molecular Evolution Centro de Astrobiologia (INTA-CSIC) Instituto Nacional de Te´cnica Aeroespacial Carretera de Ajalvir, Km. 4 28850 Torrejon de Ardoz Madrid Spain
[email protected] Field: Genetics and Evolution Steven Charnley Astrochemistry Laboratory Solar System Exploration Division, Code 691 Science and Exploration Directorate NASA Goddard Space Flight Center Greenbelt, MD 20771 USA
[email protected] Field: Interstellar Medium (gas phase)
Therese Encrenaz LESIA Observatoire de Paris 92190 Meudon France
[email protected] Field: Outer Solar System and Comets
Felipe Go´mez Laboratory of Extremophiles Department of Planetology and Habitability Centro de Astrobiologia (INTA-CSIC) Ctra de Ajalvir km 4 28850 Torrejon de Ardoz Madrid Spain
[email protected] Field: General Biology
Gerda Horneck DLR German Aerospace Center Institute of Aerospace Medicine, Radiation Biology 51170 Koeln Germany
[email protected] Field: Microbiology in Space
Emmanuelle Javaux Geology Department University of Lie`ge 17 alle´e du 6 Aouˆt B18 Sart-Tilman Lie`ge 4000 Belgium
[email protected] Field: Traces of Life
Lisa Kaltenegger Harvard University MS-20 60 Garden Street Cambridge, MA 02138 USA
[email protected] Field: Planetary Atmospheres
Field Editors
Kensei Kobayashi Grad. School of Engineering Yokohama National University 79-5 Tokiwadai Hodogaya-ku, Yokohama 240-8501 Japan
[email protected] Field: Chemistry David W. Latham Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge, MA 02138 USA
[email protected] Field: Extra Solar Planets Juli Pereto Institut Cavanilles de Biodiversitat i Biologia Evolutiva and Departament de Bioquı´mica i Biologia Molecular University of Valencia Evolutionary genetics group APDO. 22085 46071 Vale`ncia Spain
[email protected] Field: Biochemistry Nikos Prantzos Institut d’Astrophysique de Paris 98bis Bd Arago 75014 Paris France
[email protected] Field: Nucleosynthesis Sean Raymond Laboratoire d’Astrophysique de Bordeaux CNRS and Universite´ Bordeaux1 2 rue de l’Observatoire 33270 Floirac France
[email protected] Field: Planetary Formation and Dynamics
Daniel Rouan LESIA Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot 5 place Jules Janssen 92195 Meudon France
[email protected] Field: General Astrophysics
Tilman Spohn DLR - German Aerospace Center Institute of Planetary Research Rutherfordstraße 2 12489 Berlin Germany
[email protected] Field: Inner Solar System and Asteroids
Steven Stahler Department of Astronomy University of California Berkeley, CA 94720 USA
[email protected] Field: Stars Formation
Stephane Tirard Centre Franc¸ois Vie`te d’Histoire des Sciences et des Techniques EA 1161 Faculte´ des Sciences et des Techniques de Nantes 2 rue de la Houssinie`re, BP 92 208 44322 Nantes CEDEX 3 France
[email protected] Field: History of Sciences
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List of Contributors JOSE´ PASCUAL ABAD Facultad de Ciencias Departamento de Biologı´a Molecular Universidad Auto´noma de Madrid Cantoblanco, Madrid Spain
[email protected] ANGELES AGUILERA Laboratorio de Extremo´filos Centro de Astrobiologı´a (INTA-CSIC) Torrejo´n de Ardoz, Madrid Spain
[email protected] FRANCIS ALBARE`DE Ecole Normale Supe´rieure de Lyon Lyon Cedex 7 France
[email protected] WLADYSLAW ALTERMANN Department of Geology University of Pretoria Pretoria South Africa
[email protected] LINDA AMARAL-ZETTLER Marine Biological Laboratory Josephine Bay Paul Center for Comparative Molecular Biology and Evolution Woods Hole, MA USA
[email protected] RICARDO AMILS Departamento de Planetologı´a y Habitabilidad Centro de Astrobiologı´a (CSIC-INTA) Universidad Auto´noma de Madrid Campus Cantoblanco Torrejo´n de Ardoz, Madrid Spain
[email protected] CONEL MICHAEL O’DONEL ALEXANDER Department of Terrestrial Magnetism Carnegie Institution of Washington NW Washington, DC USA
[email protected] ARIEL D. ANBAR School of Earth & Space Exploration and Department of Chemistry & Biochemistry Arizona State University Tempe, AZ USA
[email protected] ABIGAIL ALLWOOD Jet Propulsion Laboratory Pasadena, CA USA
[email protected] LUC ANDRE´ Department of Earth Sciences Royal Museum of Central Africa Tervuren Belgium
[email protected] CONCEPCIO´N ALONSO Universidad Autonoma de Madrid Madrid Spain
[email protected] PHILIPPE ANDRE´ Laboratoire AIM, IRFU/Service d’Astrophysique CEA Saclay Gif-sur-Yvette France
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RALF H. ANKEN German Aerospace Center (DLR) Institute of Aerospace Medicine Cologne Germany
[email protected] JOSEFA ANTO´N Departamento de Fisiologı´a, Gene´tica y Microbiologı´a Universidad de Alicante Alicante Spain
[email protected] NICHOLAS ARNDT Maison des Ge´osciences LGCA Universite´ Joseph Fourier, Grenoble St-Martin d’He`res France
[email protected] ANDREW AUBREY NASA Jet Propulsion Laboratory Pasadena, CA USA
[email protected] JEFFREY BADA Scripps Institution of Oceanography La Jolla, CA USA
[email protected] JUAN P. G. BALLESTA Genome Dynamics and Function Centro de Biologia Molecular Severo Ochoa Cantoblanco, Madrid Spain
[email protected] NADIA BALUCANI Dipartimento di Chimica Universita` degli Studi di Perugia Perugia Italy
[email protected] RORY BARNES Astronomy Department University of Washington Seattle, WA USA
[email protected] MARIA ANTONIETTA BARUCCI LESIA Observatoire de Paris Meudon Principal Cedex France
[email protected] GIBOR BASRI Astronomy Department, MC 3411 University of California Berkeley, CA USA
[email protected] [email protected] UGO BASTOLLA Unidad de Bioinforma´tica Centro de Biologı´a Molecular “Severo Ochoa,” CSIC-UAM Cantoblanco, Madrid Spain
[email protected] CHRISTA BAUMSTARK-KHAN German Aerospace Center (DLR) Institute of Aerospace Medicine Cologne Germany
[email protected] ANDREY BEKKER Department of Geological Sciences University of Manitoba Winnipeg, MB Canada
[email protected] G. FRITZ BENEDICT McDonald Observatory The University of Texas Austin, TX USA
[email protected] List of Contributors
STEFAN BENGTSON Department of Palaeozoology The Swedish Museum of Natural History Stockholm Sweden
[email protected] JOHN H. BLACK Chalmers University of Technology Onsala Space Observatory Onsala Sweden
[email protected] KARIM BENZERARA Institut de Mine´ralogie et de Physique des Milieux Condense´s, UMR 7590 CNRS, Universite´ Pierre et Marie Curie & Institut de Physique du Globe de Paris Paris France
[email protected] LAURENT BOITEAU Institut des Biomole´cules Max Mousseron – UMR5247 CNRS University Montpellier-2 Montepellier Cedex France
[email protected] JOSE BERENGUER Centro de Biologı´a Molecular Severo Ochoa, UAM-CSIC Madrid Spain
[email protected] [email protected] SYLVAIN BERNARD Section 4.3, Organic Geochemistry GFZ German Research Centre for Geosciences Potsdam Germany
[email protected] HUGUES BERSINI IRIDIA Universite´ Libre de Bruxelles Brussels Belgium
[email protected] BRUNO BE´ZARD LESIA Observatoire de Paris Meudon France
[email protected] JEAN-PIERRE BIBRING Institut d’Astrophysique Spatiale Universite´ Paris-Sud Orsay Cedex France
[email protected] JESSICA C. BOWMAN School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA USA
[email protected] SAMUEL A. BOWRING Department of Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology Cambridge, MA USA
[email protected] ANDRE´ BRACK Directeur de Recherche Centre de Biophysique Mole´culaire CNRS Orle´ans cedex 2 France
[email protected] DORIS BREUER German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] CARLOS BRIONES Laboratory of Molecular Evolution Centro de Astrobiologı´a (INTACSIC) Instituto Nacional de Te´cnica Aeroespacial Torrejon de Ardoz, Madrid Spain
[email protected] xxi
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GILLES BRUYLANTS Laboratory of Molecular and Biomolecular Engineering Universite´ Libre de Bruxelles C.P. 165/64 Brussels Belgium
[email protected] IAN CAMPBELL Research School of Earth Sciences The Australian National University Canberra, ACT Australia
[email protected] SERGEY BULAT Russian Academy of Sciences Petersburg Nuclear Physics Institute Gatchina Russia
[email protected] [email protected] DONALD E. CANFIELD Institute of Biology University of Southern Denmark Odense Denmark
[email protected] MICHEL CABANE LATMOS/IPSL B102/T45-46 Universite´ Pierre et Marie Curie UPMC-Paris 6 Paris Cedex 05 France
[email protected] JEAN CADET Institut Nanosciences et Cryoge´nie CEA/Grenoble Grenoble cedex 9 France
[email protected] MICHAEL P. CALLAHAN NASA Goddard Space Flight Center Astrochemistry Laboratory, Code 691 Greenbelt, MD USA
[email protected] JAN CAMI Department of Physics and Astronomy The University of Western Ontario London, ON Canada and SETI Institute Mountain View, CA USA
[email protected] MARI´A LUZ CA´RDENAS Centre National de la Recherche Scientifique Unite´ de Bioe´nerge´tique et Inge´nierie des Prote´ines Marseille Cedex 20 France
[email protected] DAMIEN CARDINAL Department of Earth Sciences Royal Museum of Central Africa Tervuren Belgium and LOCEAN Universite´ Pierre & Marie Curie Paris cedex 5 France
[email protected] LETICIA CARIGI Instituto de Astronomı´a, Universidad Nacional Auto´noma de Me´xico Me´xico, D.F. Mexico
[email protected] PIERGIORGIO CASAVECCHIA Dipartimento di Chimica Universita` degli Studi di Perugia Perugia Italy
[email protected] List of Contributors
CLAUDE CATALA LESIA Observatoire de Paris Meudon Cedex France
[email protected] STEVEN B. CHARNLEY NASA Goddard Space Flight Center Solar System Exploration Division, Code 691 Astrochemistry Laboratory Greenbelt, MD USA
[email protected] FRANCO CATALDO Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania Catania Italy and Actinium Chemical Research Rome Italy
[email protected] MARC CHAUSSIDON CRPG-Nancy Universite´-CNRS Vandoeuvre les Nancy France
[email protected] DAVID C. CATLING Department of Earth and Space Sciences University of Washington Seattle, WA USA
[email protected] CECILIA CECCARELLI Laboratoire d’Astrophysique de Grenoble (LAOG/IPAG) Universite´ J.Fourier de Grenoble, CNRS Grenoble cedex 9 France
[email protected] JOSE CERNICHARO Observatorio Astronomico Nacional Centro Astronomico de Yebes Guadalajara Spain
[email protected] JOHN H. CHALMERS Scripps Institute of Oceanography Geosciences Research Division University of California, San Diego La Jolla, CA USA
[email protected] GLENN E. CIOLEK New York Center for Astrobiology, Rensselaer Polytechnic Institute Troy, NY USA
[email protected] PHILIPPE CLAEYS Earth System Science Vrije Universiteit Brussel Brussel Belgium
[email protected] HENDERSON JAMES (JIM) CLEAVES II Geophysical Laboratory Carnegie Institution of Washington Washington, DC USA
[email protected] ALAIN COC Centre de Spectrome´trie Nucle´aire et de Spectrome´trie de Masse (CSNSM) CNRS/IN2P3 Universite´ Paris Sud 11, UMR 8609 Orsay France
[email protected] CHARLES S. COCKELL Geomicrobiology Research Group, PSSRI Open University Milton Keynes UK
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MEGAN COLE School of Biology Georgia Institute of Technology Atlanta, GA USA
[email protected] ALFONSO F. DAVILA SETI Institute – NASA Ames Research Center MS 245-3 Mountain View Moffett Field, CA USA
[email protected] CATHARINE A. CONLEY NASA Headquarters Washington, DC USA
[email protected] DAVID DEAMER Department of Biomolecular Engineering University of California Santa Cruz, CA USA
[email protected] MARTIN A. CORDINER The Goddard Center for Astrobiology NASA Goddard Space Flight Center Greenbelt, MD USA
[email protected] ATHEL CORNISH-BOWDEN Centre National de la Recherche Scientifique Unite´ de Bioe´nerge´tique et Inge´nierie des Prote´ines Marseille Cedex 20 France
[email protected] HERVE´ COTTIN Laboratoire Interuniversitaire des Syste`mes Atmosphe´riques (LISA) Universite´ Paris Est-Cre´teil Cre´teil Cedex France
[email protected] ATHENA COUSTENIS Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA) (Baˆt. 18) Observatoire de Paris-Meudon Meudon Cedex France
[email protected] JACQUES CROVISIER LESIA - Baˆtiment ISO (n 17) Observatoire de Paris Meudon France
[email protected] LUIS DELAYE Departamento de Ingenierı´a Gene´tica CINVESTAV-Irapuato Irapuato, Guanajuato Mexico
[email protected] [email protected] RENE´ DEMETS ESTEC (HSF-USL) Noordwijk The Netherlands
[email protected] DIDIER DESPOIS Observatoire de Bordeaux Floirac France
[email protected] LOUIS D’HENDECOURT Institut d’Astrophysique Spatiale Universite´ Paris-Sud 11 Orsay France
[email protected] [email protected] ERNESTO DI MAURO Department of Biology and Biotechnologies “Charles Darwin” University of Rome “Sapienza” Rome Italy
[email protected] List of Contributors
MARK DO¨RR Institute for Physics and Chemistry University of Southern Denmark Odense, Fyn Denmark
[email protected] J. CYNAN ELLIS-EVANS UK Arctic Office Strategic Coordination Group, British Antarctic Survey Cambridge UK
[email protected] THIERRY DOUKI Laboratoire Le´sions des Acides Nucle´iques, SCIB-UMR-E n_3, CEA/Grenoble (CEA/UJF), FRE CNRS 3200, Institut Nanosciences et Cryoge´nie Grenoble cedex 9 France
[email protected] JOSEF ELSTER Faculty of Science Institute of Botany Academy of Sciences of the Czech Republic Trˇebonˇ Czech Republic and University of South Bohemia Cˇeske´ Budeˇjovice Czech Republic
[email protected] CLAUDE D’USTON Centre d’Etude Spatiale des Rayonnements Toulouse France
[email protected] JASON P. DWORKIN NASA Goddard Space Flight Center Astrochemistry Laboratory, Code 691 Greenbelt, MD USA
[email protected] PATRICK EGGENBERGER Geneva Observatory University of Geneva Sauverny Switzerland
[email protected] JENNIFER EIGENBRODE NASA Goddard Space Flight Center Greenbelt, MD USA
[email protected] SYLVIA EKSTRO¨M Faculte´ des Sciences Observatoire astronomique de l’Universite´ de Gene`ve Universite´ de Gene`ve Sauverny, Versoix Switzerland
[email protected] THERESE ENCRENAZ LESIA - Baˆtiment ISO (n 17) Observatoire de Paris Meudon France
[email protected] GO¨ZEN ERTEM National Institutes of Health Bethesda, MD USA
[email protected] ALBERTO G. FAIRE´N NASA Ames Research Center Moffett Field, CA USA
[email protected] JAMES FARQUHAR Department of Geology University of Maryland College Park, MD USA
[email protected] VICTOR M. FERNA´NDEZ Institute of Catalysis, CSIC Madrid Spain
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DAVID C. FERNA´NDEZ-REMOLAR Centro de Astrobiologı´a (INTA-CSIC) Torrejo´n de Ardoz Spain ferna´
[email protected] RICARDO FLORES Instituto de Biologı´a Molecular y Celular de Plantas (UPV-CSIC) Universidad Polite´cnica de Valencia - Consejo Superior de Investigaciones Cientificas Valencia Spain
[email protected] FRANC¸OIS FORGET Institut Pierre Simon Laplace, Laboratoire de Me´te´orologie Dynamique, UMR 8539 Universite´ Paris 6 Paris Cedex 05 France
[email protected] YVES FOUQUET Institut franc¸ais de recherche pour l’exploitation de la mer Issy-les-Moulineaux Cedex France
[email protected] DIONYSIOS FOUSTOUKOS Geophysical Lab Carnegie Institution of Washington Washington, DC USA
[email protected] JOSE´ CARLOS GASPAR Institute of Geosciences University of Brası´lia Brası´lia, DF Brazil
[email protected] STEPHAN VAN GASSELT Planetary Sciences and Remote Sensing Institute of Geological Sciences, Free University of Berlin Berlin Germany
[email protected] MARI´A GASSET Instituto Quı´mica-Fı´sica Rocasolano, Consejo Superior de Investigaciones Cientı´ficas Madrid Spain
[email protected] ERIC GAUCHER School of Biology Georgia Institute of Technology Atlanta, GA USA
[email protected] B. SCOTT GAUDI Department of Astronomy Ohio State University Columbus, OH USA
[email protected] STEPHEN FREELAND University of Hawaii NASA Astrobiology Institute Honolulu, HI USA
[email protected] CARLOS GERSHENSON Instituto de Investigaciones en Matema´ticas Aplicadas y en Sistemas Universidad Nacional Auto´noma de Me´xico DF Mexico
[email protected] STE´PHANE LE GARS Centre Franc¸ois Vie`te Universite´ de Nantes Nantes, BP France
[email protected] ROSARIO GIL Institut Cavanilles de Biodiversitat i Biologia Evolutiva Universitat de Vale`ncia Paterna (Vale`ncia) Spain
[email protected] List of Contributors
STANLEY I. GOLDBERG Department of Chemistry University of New Orleans New Orleans, LA USA
[email protected] FELIPE GO´MEZ Centro de Astrobiologı´a (CSIC-INTA) Instituto Nacional de Te´cnica Aeroespacial Torrejo´n de Ardoz, Madrid Spain
[email protected] ALDO GONZA´LEZ Centro de Biologı´a Molecular, lab 104 CBMSO Consejo Superior de Investigaciones Cientificas Universidad Auto´noma de Madrid Cantoblanco, Madrid Spain
[email protected] ELENA GONZA´LEZ-TORIL Laboratorio de Extremo´filos Centro de Astrobiologı´a (INTA-CSIC) Torrejo´n de Ardoz, Madrid Spain
[email protected] ANDREW GOULD Department of Astronomy Ohio State University Columbus, OH USA
[email protected] MATTHIEU GOUNELLE Laboratoire de Mine´ralogie et Cosmochimie du Muse´um (LMCM) MNHN USM 0205 – CNRS UMR 7202 Muse´um National d’Histoire Naturelle Paris France
[email protected] PIERRE-HENRI GOUYON Muse´um National d’Histoire Naturelle Paris cedex 05 France
[email protected] THIJS DE GRAAUW ALMA Vitacura, Santiago Chile
[email protected] FELIX M. GRADSTEIN University of Oslo Blindem, Oslo Norway
[email protected] BRADLEY DE GREGORIO Materials Science and Technology Division U.S. Naval Research Laboratory, Code 6360 Washington, DC USA
[email protected] RODERICH GROß Department of Automatic Control & Systems Engineering The University of Sheffield Sheffield UK
[email protected] MANUEL GU¨DEL Department of Astronomy University of Vienna Vienna Austria
[email protected] STEPHANE GUILLOT LGCA, Universite de Grenoble St Martin d’He`res France
[email protected] TRISTAN GUILLOT Observatoire de la Coˆte d’Azur Universite´ de Nice-Sophia Antipolis, CNRS Nice France
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WEIFU GUO Carnegie Institution of Washington Washington, DC USA
[email protected] RAFAEL R. DE LA HABA Department of Microbiology and Parasitology Faculty of Pharmacy University of Sevilla Sevilla Spain
[email protected] ALAN W. HARRIS German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] EMMA HART School of Computing Edinburgh Napier University Edinburgh UK
[email protected] THOMAS H. P. HARVEY Department of Earth Sciences University of Cambridge Cambridge UK
[email protected] KO HASHIZUME Department of Earth and Space Sciences Osaka University Toyonaka, Osaka Japan
[email protected] ERNST HAUBER German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] ROBERT HAZEN Carnegie Institution of Washington Geophysical Laboratory Washington, DC USA
[email protected] JO¨RN HELBERT German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] RUTH HEMMERSBACH German Aerospace Center (DLR) Institute of Aerospace Medicine Cologne, Germany Berlin Germany
[email protected] JUDITH HERZFELD Brandeis University Waltham, MA USA
[email protected] TORI M. HOEHLER Exobiology Branch NASA Ames Research Center Moffett Field Mountain View, CA USA
[email protected] PAUL HOFFMAN Department of Earth & Planetary Sciences Harvard University Cambridge, MA USA
[email protected] HARALD HOFFMANN German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] List of Contributors
AXEL HOFMANN Department of Geology University of Johannesburg Auckland Park, Johannesburg South Africa
[email protected] SUSANA IGLESIAS-GROTH Instituto de Astrofisica de Canarias La Laguna, Tenerife Spain
[email protected] MICHIEL R. HOGERHEIJDE Leiden Observatory Leiden University Leiden The Netherlands
[email protected] HESHAN “GRASSHOPPER” ILLANGKOON Department of Chemistry University of Florida Gainesville, FL USA
[email protected] PENTTI HO¨LTTA¨ Department of Geosciences and Geography University of Helsinki Finland and Geological Survey of Finland Espoo Finland
[email protected] [email protected] GERDA HORNECK German Aerospace Center (DLR) Institute of Aerospace Medicine cologne Germany
[email protected] DAVID P. HORNING Department of Molecular Biology The Scripps Research Institute La Jolla, CA USA
[email protected] NICHOLAS V. HUD School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA USA
[email protected] ELIZABETH HUMPHREYS ESO European Southern Observatory Garching Germany
[email protected] EIICHI IMAI Nagaoka University of Technology Nagaoka Japan
[email protected] WILLIAM M. IRVINE Department of Astronomy University of Massachusetts Lederle Graduate Research Amherst, MA USA
[email protected] AKIZUMI ISHIDA Graduate School of Science Tohoku University Sendai Japan
[email protected] RALF JAUMANN German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany and Department of Earth Sciences Institute of Geosciences Remote Sensing of the Earth and Planets Freie Universita¨t, Berlin Germany
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EMMANUELLE J. JAVAUX Department of Geology Paleobotany-Paleopalynology-Micropaleontology Research Unit University of Lie`ge Lie`ge Belgium
[email protected] NATASHA M. JOHNSON NASA Goddard Space Flight Center Greenbelt MD USA
[email protected] MICHAEL J. KAUFMAN Department of Physics and Astronomy San Jose´ State University San Jose, CA USA
[email protected] KUNIO KAWAMURA Department of Applied Chemistry Osaka Prefecture University Naka-ku, Sakai Japan
[email protected] TAKESHI KAKEGAWA Graduate School of Science Tohoku University Sendai Japan
[email protected] YOKO KEBUKAWA Geophysical Laboratory Carnegie Institution of Washington Washington, DC USA
[email protected] PAUL KALAS Astronomy Department University of California Berkeley, CA USA
[email protected] LAURA KELLY Geomicrobiology Research Group, Planetary and Space Sciences Research Institute Open University Milton Keynes UK
[email protected] LISA KALTENEGGER Harvard University Cambridge, MA USA and MPIA Heidelberg Germany
[email protected] BALZ SAMUEL KAMBER Department of Earth Sciences Laurentian University Sudbury, ON Canada
[email protected] INGE TEN KATE NASA Goddard Space Flight Center Greenbelt, MD USA
[email protected] PIERRE KERVELLA LESIA Observatoire de Paris Meudon France
[email protected] MARTIN F. KESSLER European Space Agency (ESA), European Space Astronomy Centre (ESAC) Madrid Spain
[email protected] GU¨NTER VON KIEDROWSKI Lehrstuhl fu¨r Organische Chemie I Ruhr-Universita¨t Bochum Bochum, NRW Germany
[email protected] List of Contributors
ADRIENNE KISH Institut de Ge´ne´tique et Microbiologie Universite Paris-Sud 11 Orsay Cedex France
[email protected] MARTIN JULIAN VAN KRANENDONK Department of Mines and Petroleum Geological Survey of Western Australia East Perth, WA Australia
[email protected] DAVID M. KLAUS Aerospace Engineering Sciences Department University of Colorado/429 UCB Boulder, CO USA
[email protected] RAMANARAYANAN KRISHNAMURTHY Department of Chemistry The Scripps Research Institute La Jolla, CA USA
[email protected] THORSTEN KLEINE Institut fu¨r Planetologie Westfa¨lische Wilhelms-Universita¨t Mu¨nster Mu¨nster Germany
[email protected] MARC KUCHNER NASA Goddard Space Flight Center Exoplanets and Stellar Astrophysics Laboratory Greenbelt, MD USA
[email protected] KATERYNA KLOCHKO Carnegie Institution of Washington Washington, DC USA
[email protected] JANA KVI´DEROVA´ Institute of Botany Academy of Sciences of the Czech Republic Trˇebonˇ Czech Republic
[email protected] KENSEI KOBAYASHI Yokohama National University Tokiwadai Hodogaya-ku, Yokohama Japan
[email protected] JEAN-FRANC¸OIS LAMBERT Laboratoire de Re´activite´ de Surface Universite´ Pierre et Marie Curie Paris France
[email protected] KURT O. KONHAUSER Department of Earth and Atmospheric Sciences University of Alberta Edmonton, AB Canada
[email protected] DAVID W. LATHAM Harvard-Smithsonian Center for Astrophysics Cambridge, MA USA
[email protected] AKIRA KOUCHI Institute of Low Temperature Science Hokkaido University Kita-ku, Sapporo, Hokkaido Japan
[email protected] AMPARO LATORRE Institute Cavanilles for Biodiversity and Evolutionary Biology Universitat de Valencia Valencia Spain
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ESTER LA´ZARO Laboratory of Molecular Evolution Centro de Astrobiologı´a (CSIC-INTA) Torrejo´n de Ardoz, Madrid Spain
[email protected] ANTONIO LAZCANO Facultad de Ciencias Universidad Nacional Auto´noma de Me´xico (UNAM) Cd. Universitaria Mexico D.F Mexico
[email protected] MICHAEL LEBERT Biology Department, Plant Ecophysiology Friedrich-Alexander-University Erlangen/Nuremberg Erlangen Germany
[email protected] LAURA M. LECHUGA Nanobiosensors and Bioanalytical Applications Group Research Center on Nanoscience and Nanotechnology (CIN2) CSIC Barcelona Spain
[email protected] GUILLAUME LECOINTRE Directeur du De´partement Syste´matique et Evolution UMR 7138 CNRS-UPMC-MNHN-IRD CP39, Muse´um National d’Histoire Naturelle Paris Cedex 05 France
[email protected] EMMANUEL LELLOUCH Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA) Observatoire de Paris Section de Meudon Meudon France
[email protected] TOM LENAERTS De´partement d’Informatique Universite´ Libre de Bruxelles Brussels Belgium
[email protected] KEVIN LEPOT De´partement de Ge´ologie, UR Pale´obotanique, Pale´opalynologie et Micropale´ontologie Universite´ de Lie`ge Lie`ge Belgium
[email protected] HUGUES LEROUX Unite´ Mate´riaux et Transformations (UMET) University Lille 1 Villeneuve d’Ascq Ronchin, Nord-Pas-de-Calais France
[email protected] ANNY-CHANTAL LEVASSEUR-REGOURD UPMC Univ. Paris 6/LATMOS-IPSL Paris France
[email protected] RICHARD LE´VEILLE´ Space Science and Technology Canadian Space Agency Saint-Hubert, QC Canada
[email protected] MATTHEW LEVY Michael F. Price Center Albert Einstein College of Medicine Bronx, NY USA
[email protected] PURIFICACIO´N LO´PEZ-GARCI´A Unite´ d’Ecologie, Syste´matique et Evolution CNRS UMR8079 Universite´ Paris-Sud 11 Paris, Orsay cedex France
[email protected] List of Contributors
CHRISTOPHE MALATERRE Institut d’Histoire et Philosophie des Sciences et Techniques (IHPST) Universite´ Paris 1-Panthe´on Sorbonne Paris France
[email protected] IRENA MAMAJANOV School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA USA
[email protected] ROCCO MANCINELLI Bay Area Environmental Research Institute, NASA Ames Research Institute USA
[email protected] AVI M. MANDELL NASA Goddard Space Flight Center Greenbelt, MD USA
[email protected] SUSANNA C. MANRUBIA Laboratory of Molecular Evolution Centro de Astrobiologı´a (INTA-CSIC) Torrejon de Ardoz, Madrid Spain
[email protected] IRMA MARIN Departamento de Biologı´a Molecular Universidad Auto´noma de Madrid Madrid Spain
[email protected] OLIVIER LA MARLE Centre National d’Etudes Spatiales DSP/EU Paris Cedex 01 France
[email protected] MARK S. MARLEY NASA Ames Research Center Moffett Field, CA USA
[email protected] JEAN-EMMANUEL MARTELAT LST UMR5570, Universite´ Claude Bernard Lyon 1 St Martin d’He`res, Grenoble France
[email protected] HERVE´ MARTIN Laboratoire Magmas et Volcans Universite´ Blaise Pascal, OPGC, CNRS, IRD Clermont-Ferrand France
[email protected] BERNARD MARTY Institut Universitaire de France Ecole Nationale Supe´rieure de Ge´ologie Centre de Recherches Pe´trographiques et Ge´ochimiques (CRPG), CNRS Vandoeuvre les Nancy Cedex France
[email protected] KOICHIRO MATSUNO Nagaoka University of Technology Nagaoka Japan
[email protected] THOMAS MCCOLLOM Laboratory for Atmospheric and Space Physics University of Colorado Boulder, CO USA
[email protected] FRANCIS MCCUBBIN Geophysical Laboratory Carnegie Institution for Science Carnegie Institution of Washington Washington, DC USA
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CHRISTOPHER P. MCKAY NASA Ames Research Center Moffett Field, CA USA
[email protected] NICOLA MCLOUGHLIN Department for Earth Science and Centre for Geobiology University of Bergen Bergen Norway
[email protected] UWE J. MEIERHENRICH Laboratoire de Chimie des Mole´cules Bioactives et des Aroˆmes Institut de Chimie de Nice Faculte´ des Sciences University of Nice-Sophia Antipolis Nice France
[email protected] H. JAY MELOSH Departments of Earth and Atmospheric Sciences, Physics and Aerospace Engineering Purdue University West Lafayette, IN USA
[email protected] FRANCESCA MERLIN University of Paris-Sorbonne Paris France
[email protected] FRANC¸OIS MIGNARD CNRS, Observatoire de la Coˆte d’Azur University of the Nice Sophia-Antipolis Nice France
[email protected] STEFANIE N. MILAM Astrochemistry Laboratory NASA Goddard Space Flight Center Code 691 Greenbelt, MD USA
[email protected] THOMAS J. MILLAR Astrophysics Research Centre, School of Mathematics and Physics Queen’s University Belfast Belfast UK
[email protected] SHIN MIYAKAWA Ribomic Inc Shirokanedai Usuibiru 6F, Tokyo Japan
[email protected] A. M. MLOSZEWSKA Department of Earth and Atmospheric Sciences University of Alberta Edmonton, AB Canada
[email protected] ROBERT MOCHKOVITCH Institut d’Astrophysique de Paris Paris France
[email protected] RALF MOELLER German Aerospace Center (DLR) Institute of Aerospace Medicine Cologne Germany
[email protected] STEPHEN J. MOJZSIS Department of Geological Sciences University of Colorado Boulder, CO USA
[email protected] PIERRE-ALAIN MONNARD FLinT center Institute for Physics and Chemistry University of Southern Denmark Odense M Denmark
[email protected] List of Contributors
FRANCISCO MONTERO Department of Biochemistry and Molecular Biology I Facultad de Ciencias Quı´micas Universidad Complutense de Madrid Madrid Spain
[email protected] STEFANO MOTTOLA German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] THIERRY MONTMERLE Institut d’Astrophysique de Paris CNRS/Universite´ Paris 6 Paris France
[email protected] DENIS J. P. MOURA Centre National d’Etudes Spatiales Paris France
[email protected] MICHEL MORANGE Centre Cavaille`s USR 3308 CIRPHLES Ecole normale supe´rieure Paris Cedex 05 France
[email protected] ALESSANDRO MORBIDELLI Observatoire de la Cote d’Azur Nice France
[email protected] DAVID MOREIRA Unite´ d’Ecologie, Syste´matique et Evolution CNRS UMR8079 Universite´ Paris-Sud 11 Paris, Orsay cedex France
[email protected] ALVARO MORENO Department of Logic and Philosophy of Science University of the Basque Country (UPV/EHU) Donostia, San Sebastia´n Basque Country, Spain
[email protected] HAROLD MOROWITZ George Mason University Fairfax, VA USA
[email protected] ARMEN Y. MULKIDJANIAN School of Physics University of Osnabrueck Osnabrueck Germany and Moscow State University Moscow Russia
[email protected] HOLGER S. P. MU¨LLER I. Physikalisches Institut Universita¨t zu Ko¨ln Ko¨ln Germany
[email protected] SAGI VASUDEVA NAIDU Department of Chemistry The Scripps Research Institute La Jolla, CA USA
[email protected] KAZUMICHI NAKAGAWA Graduate School of Human Development and Environment Kobe University Nada, Kobe Japan
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HIROSHI NARAOKA Department of Earth and Planetary Sciences Kyushu University Fukuoka Japan
[email protected] ANN NOWE´ Vrije Universiteit Brussel Brussels Belgium
[email protected] GOPAL NARAYANAN Five College Radio Astronomy Observatory University of Massachusetts Amherst, MA USA
[email protected] JOSEPH A. NUTH, III Solar System Exploration Division NASA Goddard Space Flight Center Greenbelt, MD USA
[email protected] ALICIA NEGRO´N-MENDOZA Instituto de Ciencias Nucleares Universidad Nacional Auto´noma de Me´xico Coyoaca´n, DF Mexico
[email protected] KARIN I. O¨BERG Harvard-Smithsonian Center for Astrophysics Cambridge, MA USA
[email protected] GERHARD NEUKUM Planetary Sciences and Remote Sensing Institute of Geological Sciences Free University of Berlin Berlin Germany
[email protected] SHOHEI OHARA Geophysical Laboratory Carnegie Institution of Washington Washington, DC USA
[email protected] WAYNE L. NICHOLSON Space Life Sciences Laboratory Kennedy Space Center University of Florida FL USA
[email protected] HIROSHI OHMOTO NASA Astrobiology Institute and Department of Geosciences The Pennsylvania State University University Park, PA USA
[email protected] PETER E. NIELSEN The Panum Institute University of Copenhagen Copenhagen N Denmark
[email protected] JOSE´ OLIVARES Esztacio´n Experimental del Zaidı´n. CSIC Granada Spain
[email protected] NORA NOFFKE Department of Ocean, Earth & Atmospheric Sciences Old Dominion University Norfolk, VA USA
[email protected] MARC OLLIVIER Institut d’Astrophysique Spatiale, CNRS Universite´ de Paris-Sud Orsay France
[email protected] List of Contributors
JONATHAN O’NEIL Carnegie Institution of Washington Washington, DC USA
[email protected] SILVANO ONOFRI Department of Ecology and Sustainable Economic Development University of Tuscia Viterbo Italy
[email protected] TULLIS C. ONSTOTT Department of Geosciences Princeton University Princeton, NJ USA
[email protected] CORINNA PANITZ German Aerospace Center (DLR) Institute of Aerospace Medicine Cologne Germany
[email protected] DOMINIC PAPINEAU Geophysical Laboratory Carnegie Institution of Washington Washington, DC USA
[email protected] V´ICTOR PARRO Molecular Evolution Department Centro de Astrobiologı´a (INTA-CSIC) Torrejo´n de Ardoz, Madrid Spain
[email protected] CAMILLE PARTIN Department of Geological Sciences University of Manitoba Winnipeg, MB Canada
[email protected] ROBERT PASCAL Institut des Biomole´cules Max Mousseron CC1706 Universite´ de Montpellier II Montpellier France
[email protected] MATTHEW PASEK University of Southern Florida Tampa, FL USA
[email protected] ERNESTO PECOITS Department of Earth and Atmospheric Sciences University of Alberta Edmonton, AB Canada
[email protected] ELS PEETERS Department of Physics and Astronomy The University of Western Ontario London, ON Canada and SETI Institute Mountain View, CA USA
[email protected] JULI PERETO´ Cavanilles Institute for Biodiversity and Evolutionary Biology and Department of Biochemistry and Molecular Biology University of Vale`ncia Vale`ncia Spain
[email protected] JE´ROˆME PEREZ Applied Mathematics Laboratory ENSTA ParisTech Paris Cedex 15 France
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JEAN-ROBERT PETIT CNRS-UJF Laboratoire de Glaciologie et Ge´ophysique de l’Environnement (LGGE) St Martin D’Heres Cedex France
[email protected] PASCAL PHILIPPOT Equipe Ge´obiosphe`re Actuelle et Primitive Institut de Physique du Globe de Paris (IPGP) Paris France
[email protected] RAY PIERREHUMBERT Department of the Geophysical Sciences University of Chicago Chicago, IL USA
[email protected] GO¨RAN L. PILBRATT ESA Astrophysics and Fundamental Physics Missions Division Research and Science Support Department European Space Agency (ESA) AZ, Noordwijk The Netherlands
[email protected] SAMANTA PINO Department of Biology and Biotechnologies “Charles Darwin” University of Rome “Sapienza” Rome Italy
[email protected] DANIELE L. PINTI GEOTOP & De´partment des Sciences de la Terre et de l’Atmosphe`re Universite´ du Que´bec a` Montre´al Montre´al, QC Canada
[email protected] SANDRA PIZZARELLO Department of Chemistry & Biochemistry Arizona State University Tempe, AZ USA
[email protected] NOAH PLANAVSKY Department of Earth Sciences University of California Riverside, CA USA
[email protected] RAPHAE¨L PLASSON Nordita Stockholm Sweden
[email protected] FRANCK POITRASSON Geosciences Environnement Toulouse, CNRS Toulouse France
[email protected] NIKOS PRANTZOS Institut d’Astrophysique de Paris Paris France
[email protected] DANIEL PRIEUR Universite´ de Bretagne Occidentale (University of Western Britanny) Brest France and Institut Universitaire Europe´en de la Mer (IUEM) Technopoˆle Brest–Iroise Plouzane´ France
[email protected] VENKATESHWARLU PUNNA The Scripps Research Institute La Jolla, CA USA
[email protected] List of Contributors
FRANC¸OIS RAULIN LISA – UMR CNRS/IPSL 7583 Universite´s Paris Est-Cre´teil & Denis Diderot Cre´teil France
[email protected] WAYNE G. ROBERGE New York Center for Astrobiology Rensselaer Polytechnic Institute Troy, NY USA
[email protected] FLORENCE RAULIN-CERCEAU Maıˆtre de Confe´rences Centre Alexandre Koyre´ (UMR 8560-CNRS/EHESS/ MNHN/CSI) Muse´um National d’Histoire Naturelle Brunoy France
[email protected] FRANC¸OIS ROBERT Laboratoire de Mine´ralogie et Cosmochimie du Muse´um (LMCM) Muse´um National d’Histoire Naturelle UMR 7202 CNRS Paris Cedex 05 France
[email protected] SEAN RAYMOND Laboratoire d’Astrophysique de Bordeaix CNRS; Universite de Bordeaux Floirac France
[email protected] JACQUES REISSE Universite´ Libre de Bruxelles Brussels Belgium
[email protected] ANTHONY J. REMIJAN NRAO Charlottesville, VA USA
[email protected] PETRA RETTBERG German Aerospace Center (DLR) Institute of Aerospace Medicine Cologne Germany
[email protected] ALONSO RICARDO Harvard University Boston, MA USA
[email protected] MICHAEL P. ROBERTSON Department of Molecular Biology MB42, Bldg. MB110 The Scripps Research Institute La Jolla, CA USA
[email protected] BERND MICHAEL RODE Universita¨t Innsbruck Institute for General, Inorganic and Theoretical Chemistry Leopold-Franzens University Innsbruck Austria
[email protected] FRANCISCO RODRIGUEZ-VALERA Microbiologia Universidad Miguel Hernandez San Juan, Alicante Spain
[email protected] FRANC¸OISE ROQUES Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA) Observatoire de Paris Meudon France
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MINIK T. ROSING Nordic Center for Earth’s Evolution Natural History Museum of Denmark University of Copenhagen Copenhagen Denmark
[email protected] RAMON ROSSELLO´-MO´RA Grup de Microbiologia Marina IMEDEA (CSIC-UIB) C/Miquel Marque´s 21 Esporles, Illes Balears Spain
[email protected] DANIEL ROUAN LESIA Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot Meudon France
[email protected] KEPA RUIZ-MIRAZO Department of Logic and Philosophy of Science and Biophysics Research Unit (CSIC-UPV/EHU) University of the Basque Country Donostia, San Sebastia´n Basque Country, Spain
[email protected] NITA SAHAI University of Wisconsin Madison, WI USA
[email protected] CRISTINA SANCHEZ-PORRO Faculty of Pharmacy Department of Microbiology and Parasitology University of Sevilla Sevilla Spain
[email protected] LEOPOLDO G. SANCHO Universidad Complutense de Madrid Facultad de Farmacia Departamento de Biologia Vegetal II Madrid Spain
[email protected] JOSE´ LUIS SANZ Departamento de Biologı´a Molecular Universidad Auto´noma de Madrid Madrid Spain
[email protected] PIERRE SAVATON Universite´ de Caen Basse-Normandie Caen France
[email protected] KAREL SCHULMANN Ecole et Observatoire de Science de la Terre Institute de Physique de Globe Universite´ de Strasbourg Strasbourg France
[email protected] PETER SCHUSTER Institut fu¨r Theoretische Chemie der Universita¨t Wien Wien Austria
[email protected] ALAN W. SCHWARTZ Radboud University Nijmegen Nijmegen The Netherlands
[email protected] WILLIAM G. SCOTT Department of Chemistry and Biochemistry The Center for the Molecular Biology of RNA University of California at Santa Cruz Santa Cruz, CA USA
[email protected] List of Contributors
ANTIGONA SEGURA Instituto de Ciencias Nucleares Universidad Nacional Auto´noma de Me´xico. Circuito Exterior C.U.A Me´xico D.F Mexico
[email protected] FRANCK SELSIS Universite´ de Bordeaux-CNRS Bordeaux France
[email protected] DMITRY SEMENOV Max Planck Institute of Astronomy Heidelberg Germany
[email protected] SILKE SEVERMANN Institute of Marine & Coastal Sciences and Department of Earth & Planetary Sciences Rutgers University New Brunswick, NJ USA
[email protected] IAN W. M. SMITH Chemistry Laboratory University of Cambridge Cambridge UK
[email protected] RONALD L. SNELL Department of Astronomy 517 K Lederle Graduate Research Center University of Massachusetts Amherst, MA USA
[email protected] FRANK SOHL German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] ALESSANDRO SOZZETTI Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Torino Pino Torinese Italy
[email protected] M. A. SHEA Air Force Research Laboratory (Emeritus) Bedford, MA USA
[email protected] PIETRO SPERONI DI FENIZIO CISUC, Department of Informatics Engineering University of Coimbra Coimbra Portugal
[email protected] DON F. SMART Air Force Research Laboratory (Emeritus) Bedford, MA USA
[email protected] TILMAN SPOHN German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] ALEXANDER SMIRNOV Department of Geosciences Stony Brook University Stony Brook, NY USA
[email protected] GREG SPRINGSTEEN Furman University Greenville, SC USA
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STEVEN W. STAHLER Department of Astronomy University of California Berkeley, CA USA
[email protected] LUCAS J. STAL Department of Marine Microbiology Netherlands Institute of Ecology NIOO-KNAW Yerseke The Netherlands
[email protected] VLADA STAMENKOVIC German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] KENNETH MARK STEDMAN Biology Department Portland State University Center for Life in Extreme Environments Portland, Oregon, OR USA
[email protected] JENNIFER C. STERN Planetary Environments Laboratory NASA Goddard Space Flight Center Greenbelt, MD USA
[email protected] JUN-ICHI TAKAHASHI NTT Microsystem Integration Laboratories Atsugi, Kanagawa Japan
[email protected] OLGA TARAN Lehrstuhl fu¨r Organische Chemie I Ruhr-Universita¨t Bochum Bochum, NRW Germany
[email protected] CHRISTOPHE THOMAZO UMR CNRS 5561 Bioge´osciences Universite´ de bourgogne Dijon France
[email protected] PHIL THURSTON Laurentian University Sudbury, ON Canada
[email protected] SIMON TILLIER De´partement Syste´matique et Evolution, UMR 7138 CNRS-MNHN-UPMC-IRD, CP 41 Muse´um National d’Histoire Naturelle Paris Cedex 05 France
[email protected] BARBARA STRACKE DLR Institut fu¨r Planetenforschung Berlin Germany
[email protected] STE´PHANE TIRARD Faculte´ des Sciences et des Techniques de Nantes Centre Franc¸ois Vie`te d’Histoire des Sciences et des Techniques EA 1161 Nantes France
[email protected] HARALD STRAUSS Institut fu¨r Geologie und Pala¨ontologie Westfa¨lische Wilhelms-Universita¨t Mu¨nster Mu¨nster Germany
[email protected] DANIELA TIRSCH German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] List of Contributors
MARCO TOMASSINI Information Systems Department University of Lausanne Switzerland
[email protected] MELISSA G. TRAINER Space Scientist NASA Goddard Space Flight Center Code 699 Greenbelt, MD USA
[email protected] JORGE L. VAGO European Space Agency – ESA/ESTEC (SRE-SM) Noordwijk The Netherlands
[email protected] ANTONIO VENTOSA Department of Microbiology and Parasitology Faculty of Pharmacy University of Sevilla Sevilla Spain
[email protected] JEAN-PIERRE DE VERA German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] ENRIQUE VIGUERA Genetics Department, Sciences Faculty University of Malaga Malaga Spain
[email protected] MICHEL VISO Astrobiology CNES/DSP/EU Paris France
[email protected] ROLAND WAGNER German Aerospace Center (DLR) Institute of Planetary Research Berlin Germany
[email protected] SARA IMARI WALKER School of Chemistry and Biochemistry Georgia Institute of Technology Center for Chemical Evolution NAI Center for Ribosomal Origins and Evolution Atlanta, GA USA
[email protected] FRANCES WESTALL Centre de Biophysique Mole´culaire CNRS Orle´ans cedex 2 France
[email protected] HUBERT WHITECHURCH Ecole et Observatoire de Science de la Terre Institute de Physique de Globe Universite´ de Strasbourg Strasbourg France
[email protected] DOUGLAS WHITTET Department of Physics, Applied Physics & Astronomy New York Center for Astrobiology Rensselaer Polytechnic Institute Troy, NY USA
[email protected] SIMON A. WILDE Department of Applied Geology Curtin University of Technology Perth, WA Australia
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IAN S. WILLIAMS Research School of Earth Sciences ANU College of Physical and Mathematical Sciences The Australian National University Canberra, ACT Australia
[email protected] LOREN DEAN WILLIAMS School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA USA
[email protected] CHARLES T. WOLFE Unit for History and Philosophy of Science University of Sydney Sydney, NSW Australia
[email protected] MARK G. WOLFIRE Astronomy Department University of Maryland College Park, MD USA
[email protected] ALEXANDER WOLSZCZAN Department of Astronomy & Astrophysics and Center for Exoplanets & Habitable Worlds The Pennsylvania State University University Park, PA USA
[email protected] KOSEI E. YAMAGUCHI Geochemical Laboratory Department of Chemistry Toho University Funabashi, Chiba Japan
[email protected] MASAMICHI YAMASHITA Institute of Space and Astronautical Science/JAXA Sagamihata, Kanagawa Japan
[email protected] BRUCE YARDLEY School of Earth and Environment University of Leeds Leeds UK
[email protected] REIKA YOKOCHI Department of Geophysical Sciences University of Illinois Chicago, IL USA and Department of Earth and Environmental Sciences University of Illinois at Chicago Chicago, IL USA
[email protected] PHILIPPE ZARKA LESIA Observatoire de Paris, CNRS, UPMC Universite´ Paris Diderot Meudon France
[email protected] TANJA ELSA ZEGERS Institute of Earth Sciences Paleomagnetic Laboratory “Fort Hoofddijk” Utrecht University Utrecht The Netherlands
[email protected] A AAN ▶ Aminoacetonitrile
Abiogenesis Definition Thomas Huxley used the term abiogenesis in an important text published in 1870. He strictly made the difference between spontaneous generation, which he did not accept, and the possibility of the evolution of matter from inert to living, without any influence of life. Since the end of the nineteenth century, evolutive abiogenesis means increasing complexity and evolution of matter from inert to living state in the abiotic context of evolution of primitive Earth.
See also ▶ Darwin’s Conception of Origins of Life ▶ Huxley’s Conception on Origins of Life ▶ Origin of Life
Abiogenic Photosynthesis ▶ Abiotic Photosynthesis
can hardly escape the activity of the biosphere, some processes do not depend on biological activities. For example, this is the case of the formation of hydrothermal deposits that are based on redox, volatile fugacity, and high thermal conditions. Some abiotic processes are involved in the production of surface oxidants through photochemical reactions in planet atmospheres as has been proposed to explain the presence of perchlorates on Mars. Paradoxically, different abiotic pathways (thermal, radiolytic, or photochemical) create the chemical disequilibrium which is strictly necessary to fuel physical and chemical cycles on planets depleted of life. Some abiotic pathways have likely been essential to originate the primary biochemical machinery that drove the emergence of life on Earth. In this sense, most abiotic processes support the production of the compounds and chemical disequilibrium essential for a region of the Universe to become habitable.
See also ▶ Hydrothermal Environments ▶ Origin of Life ▶ Photochemistry ▶ Prebiotic Chemistry
Abiotic Photosynthesis ARMEN Y. MULKIDJANIAN School of Physics, University of Osnabrueck, Osnabrueck, Germany Moscow State University, Moscow, Russia
Abiotic Synonyms Definition Abiotic refers to the physical and chemical processes that take place in natural environments but are driven by mechanisms that do not involve any biological activity. Although major physical and chemical cycles on Earth
Abiogenic photosynthesis; Prebiotic photosynthesis
Keywords Bacterial photosynthesis, carbon dioxide, photochemistry, semiconductors
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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Definition Abiotic or abiogenic photosynthesis is the synthesis of organic compounds with the aid of radiant energy and various inorganic or organic catalysts.
History In September 1912, Benjamin Moore suggested at a discussion on the origin of life held by the joint sections of Zoology and Physiology of the British Association for the Advancement of Science, that “the first step towards the origin of life must have been the synthesis of organic matter from inorganic by the agency of inorganic colloids acting as transformers or catalysts for radiant solar energy” (Moore and Webster 1913).
Overview In spite of Haldane’s well-known idea that UV light may have served as a driving force for formation of the first virus-like organisms (Haldane 1929), the idea of directly driving abiogenesis by solar energy had not won much support at that time, despite the fact that the Sun is by far the most powerful energy source on Earth. The limited acceptance of the idea was partly due to the low quantum yield of abiotic photosynthetic reactions and the poor
reproducibility of experimental results. Specifically, low-yield abiotic photoreduction of CO2 was generally observed, e.g., in the presence of ions of divalent iron (Mauzerall 1992). Only in the 1980s, were robust procedures of producing colloidal nanoparticles of photoactive semiconductors, such as zinc sulfide (ZnS) or cadmium sulfide (CdS), developed (Henglein 1984). These particles (see Fig. 1), due to their high surface-tovolume ratio, provided experimental systems where the photoreduction of CO2 to diverse organic compounds could be studied. The photoreduction proceeded with high and reproducible quantum yield (up to 80% for CO2 reduction to formate at the surface of colloidal ZnS particles (Henglein 1984)). Recent studies have demonstrated high-yielding ZnS- and MnS-mediated photosynthesis under simulated primeval conditions (Zhang et al. 2004, 2007; Guzman and Martin 2009). In the modern oceans, ZnS and MnS are found at the sites of the geothermal activity, where minute particles of these minerals continuously precipitate around hot, deep-sea hydrothermal vents (Tivey 2007). It is noteworthy that under the possibly high pressure primordial CO2 atmosphere, very hot metal-enriched hydrothermal fluids may have discharged at the surface of the first continents, so that
E, V (redox potential) Surface electron trap
-2 Conduction band 0
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S2-
CO2 + 2H+ HCOOH
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Abiotic Photosynthesis. Figure 1 Abiogenic photosynthesis under a primordial high CO2 atmosphere. Left panel: light-induced reactions in a ZnS particle combined with an energy diagram. The absorption of a UV quantum by a minute crystal of ZnS, an n-type semiconductor, leads to the separation of electric charges and to the transition of the excited electrons into the conducting zone. The electrons can migrate inside the crystal until they are trapped at the surface, where they can be picked up by appropriate acceptors, e.g., molecules of CO2. The residual electron vacancies (holes) are initially reduced by the S2 ions of the crystal, which then eventually can be replenished by external electron donors, e.g., H2S (cf. with the mechanism of anoxygenic photosynthesis). Right panel: the precipitation of ZnS particles (gray dots) around a Hadean continental hot spring (Figure is taken from Mulkidjanian 2009)
Absorption Cross Section
particles of ZnS and MnS could have precipitated within regions exposed to solar radiation (Mulkidjanian 2009). These sulfide minerals could have been present in shallow waters (Guzman and Martin 2009) or may have formed photosynthesizing and habitable rings around terrestrial hot springs (Mulkidjanian 2009). The development of the first life forms within photosynthesizing, ZnS-containing precipitates might explain cellular enrichments in Zn2+, the equilibrium concentration of which in the primordial ocean should have been extremely low, but which could be steadily released as byproducts of the ZnS-mediated photosynthesis (Mulkidjanian and Galperin 2009). Several proteins shared by all extant organisms believed to form the core of the Last Universal Common Ancestor (LUCA) are particularly enriched in Zn and Mn; this may also support the role of abiogenic photosynthesis in the earliest stages of evolution (Mulkidjanian and Galperin 2009). Since these ubiquitous proteins are depleted in iron, it remains to be established whether and to what extent divalent iron, the predominant transition metal in the primeval waters, was involved in the abiogenic photosynthesis.
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Henglein A (1984) Catalysis of photochemical reactions by colloidal semiconductors. Pure Appl Chem 56(9):1215–1224 Mauzerall D (1992) Light, iron, Sam Granik and the origin of life. Photosynth Res 33(2):163–170 Moore B, Webster TA (1913) Synthesis by sunlight in relationship to the origin of life. Synthesis of formaldehyde from carbon dioxide and water by inorganic colloids acting as transformers of light energy. Proc R Soc Lond B Biol Sci 87:163–176 Mulkidjanian AY (2009) On the origin of life in the Zinc World: 1. Photosynthetic, porous edifices built of hydrothermally precipitated zinc sulfide (ZnS) as cradles of life on Earth. Biol Direct 4:26 Mulkidjanian AY, Galperin MY (2009) On the origin of life in the Zinc World. 2. Validation of the hypothesis on the photosynthesizing zinc sulfide edifices as cradles of life on Earth. Biol Direct 4:27 Tivey MK (2007) Generation of seafloor hydrothermal vent fluids and associated mineral deposits. Oceanography 20(1):50–65 Zhang XV, Martin ST, Friend CM, Schoonen MAA, Holland HD (2004) Mineral-assisted pathways in prebiotic synthesis: photoelectrochemical reduction of carbon(+IV) by manganese sulfide. J Am Chem Soc 126(36):11247–11253 Zhang XV, Ellery SP, Friend CM, Holland HD, Michel FM, Schoonen MAA, Martin ST (2007) Photodriven reduction and oxidation reactions on colloidal semiconductor particles: Implications for prebiotic synthesis. J Photochem Photobiol A Chem 185(2–3):301–311
See also ▶ Anoxygenic Photosynthesis ▶ Black Smoker ▶ Carbon Dioxide ▶ Charge Transfer ▶ Earth’s Atmosphere, Origin and Evolution of ▶ Electron Acceptor ▶ Electron Donor ▶ Energy Sources ▶ Extreme Ultraviolet Light ▶ Formic Acid ▶ Haldane’s Conception of Origins of Life ▶ Hot Spring Microbiology ▶ Hydrothermal Vent Origin of Life Models ▶ Iron ▶ LUCA ▶ Transition Metals and Their Isotopes ▶ Origin of Life ▶ Photochemistry ▶ Photosynthesis ▶ UV Radiation ▶ White Smoker
References and Further Reading Guzman MI, Martin ST (2009) Prebiotic metabolism: production by mineral photoelectrochemistry of alpha-ketocarboxylic acids in the reductive tricarboxylic acid cycle. Astrobiology 9(9):833–842 Haldane JBS (1929) The origin of life. Rationalist Annual. Watts & Co, London, pp 3–10
Ablation Definition Ablation is the erosion of the surface of a solid object in a flow (e.g., during the entrance of an object into the atmosphere) through a process, such as fusion, vaporization, or friction.
Absorption ▶ Absorption Cross Section ▶ Absorption Spectroscopy
Absorption Cross Section Synonyms Absorption
Definition When a parallel, monochromatic beam of light traveling in some specific direction encounters a medium of finite
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Absorption Spectroscopy
extent, a certain amount of the flux will be absorbed and a certain amount will be scattered into other angles. The rate at which energy is taken out of the beam by absorption and scattering can be characterized in terms of coefficients with dimensions of area, which are known as cross sections. The term absorption cross section is often used to include both the portion due to scattering and that due to true absorption (loss of the photon into another form of energy, such as heat). For atmospheric gases, this total absorption cross section is defined by the Beer’s law expression: I ¼ I0 expðsnlÞ where I0 and I are the incident and transmitted light intensities, respectively, s is the absorption cross section (cm2 molecule1), n is the molecular density, and l is the pathlength in cm.
See also ▶ Spectroscopy
Abundances of Elements NIKOS PRANTZOS Institut d’Astrophysique de Paris, Paris, France
Keywords Chemical composition, nucleosynthesis, nuclides
Definition
Absorption
The relative amount (or fraction) of a given nuclide in a sample of matter is called the abundance of that nuclide. It can be expressed either in absolute terms (i.e., with respect to the total amount of matter in the sample) or in relative terms (with respect to the amount of some key element, e.g., the most abundant one, in the sample). Similarities and differences in the elemental and isotopic composition of ▶ stars and galaxies are key ingredients for understanding their origin and evolution.
Definition
Overview
Absorption Spectroscopy Synonyms
In absorption ▶ spectroscopy the spectral features of interest appear in absorption with respect to a background continuous spectrum. In the interstellar medium the background continuum may be supplied by a radiation source, such as a star, located behind the region of interest. The absorbing material may be either in the gas or the solid phase (e.g., interstellar dust or ices). Solid state features are much broader than atomic or molecular absorptions, and are consequently more difficult to assign to a specific carrier. Much of the solar (Fraunhofer) spectrum is seen in absorption, as the outer cooler layers of the solar atmosphere absorb radiation from the deeper photosphere. Spectral lines in planetary atmospheres are typically seen in absorption, against the continuous thermal spectrum from the planetary or satellite surface.
History The first person to notice a number of dark features in the solar spectrum was the English chemist Willam Wollaston in 1802. This absorption spectrum was first systematically investigated by Joseph von Fraunhofer, starting in 1814, and the spectral features are now known as Fraunhofer lines.
The composition of remote objects (the Sun, ▶ stars, interstellar gas, and galaxies) is determined through spectroscopy, which usually allows determination of elemental abundances; in rare cases, particularly for interstellar clouds, some isotopic abundances may be determined in those objects. For Earth, lunar, and meteoritic samples, nuclear mass spectroscopy allows precise determination of most isotopic abundances; this is also the case for cosmic rays, albeit only for the most abundant nuclides at present. Hydrogen (H) being the most abundant element in the Universe, spectroscopists express the abundance of element i as the number ratio of its nuclei with respect to those of H: ni = Ni/NH , and they use a scale where log(NH) = 12. In the meteoritics community, the silicon scale of log(NSi) = 6 is used. Theoreticians use the mass fraction Xi = NiAi/ ∑NjAj, where Aj is the mass number of nuclide j ; obviously, ∑Xi = 1. Conversion of mass fractions to abundances by number requires use of the quantity Yi = Xi/Ai called the mole fraction (notice that ∑Yi 6¼ 1). According to our current understanding, the material of the proto-solar nebula had a remarkably homogeneous composition, as a result of high temperatures (which
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Abundances of Elements
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Abundances of Elements. Figure 1 Solar system abundances (by number) of the 92 chemical elements, in a logarithmic scale where log(N) = 6 for Silicon (from a compilation in Lodders 2003)
caused the melting of nearly all the dust grains) and thorough mixing. This composition characterizes the present-day surface layers of the Sun, which remain unaffected by nuclear reactions occurring in the solar interior (with a few exceptions, e.g., the fragile D and Li). Furthermore, after various physicochemical effects are taken into account, it appears that the elemental composition of the Earth and meteorites matches extremely well with the solar photospheric composition. The composition of stars in the Milky Way presents both striking similarities and considerable differences with the solar composition. The universal predominance of H (90% by number, but 70% by mass) and He (9% by number, but 25% by mass) and the relative abundances of “metals” (to astronomers, elements heavier than He) is the most important similarity. On the other hand, the fraction of metals (metallicity, about 1.5% in the Sun) appears to vary
considerably within the solar vicinity (where the oldest stars have a metallicity of 0.1 solar), across the Milky Way disk (with young stars in the inner Galaxy having three times more metals than the Sun), or in the galactic halo (with stellar metallicities ranging from 0.1 to 0.00001 solar). These variations in composition reflect the history of “chemical evolution” of the Milky Way.
See also ▶ Stars
References and Further Reading Asplund M, Grevesse N, Sauval AJ, Scott P (2009) The chemical composition of the sun. Ann Rev Astron Astrophys 47:481–522 Lodders K (2003) Solar system abundances and condensation temperatures of the elements. Astrophys J 591:1220–1247
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Acasta Gneiss
Acasta Gneiss SAMUEL A. BOWRING Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
Synonyms Gneiss
Keywords Geochronology, oldest rocks, zircon
Definition The Acasta gneisses are the oldest known rocks on the surface of the Earth. They are exposed in northern Canada, north of Great Slave Lake, east of Great Bear Lake with the approximate position of 65 100 N and 115 300 W. They have a composition close to granitic and are interpreted to have formed, at least in part, from even older rocks that may be as old as 4.2 Ga.
Overview The Acasta gneisses are the ▶ oldest dated rocks on Earth. They are exposed in northwestern Canada (65 100 N and 115 300 W) along the western margin of the Archean Slave craton (>2.5 Ga), in the core of a north-trending fold in the foreland of Wopmay orogen, a 2.02–1.84 Ga orogenic belt. The Acasta gneisses range in ▶ age from 4.03 Ga to ca. 3.6 Ga with distinct groupings at 4.03–3.94 Ga, 3.74–3.72 Ga, and 3.66–3.58 Ga. Rocks from these three distinct groups are compositionally diverse and range from ▶ granite to quartz diorite to tonalite. Rocks have been deformed several times resulting in welldeveloped foliations (planar fabric present in metamorphic rocks and produced by reorientation of minerals). Lens-shaped boudins (cylinder-like structures making up a layer in a deformed rock) of serpentinized ultramafic rocks, up to several hundred meters long, occur throughout the gneisses. No ca. 4.0–3.6 Ga metasedimentary rocks have been discovered although sparse outcrops of locally tightly folded quartzite, iron-formation, and pelite are found in the older gneisses. Weakly deformed ca. 3.6 Ga granitic dikes cut many outcrops. During the 1.88 Ga Calderian orogeny to the west, sheets of 1.9–2.5 Ga rocks were thrust over western edge of Slave craton resulting in a set of north-trending folds and metamorphism of
underlying Archean rocks. Ar–Ar biotite and U–Pb apatite dates record complex reheating during this event at ca. 1.77 Ga. The protoliths of the Acasta gneiss range from granite to tonalite/diorite in composition. Their U–Pb zircon dates indicate that the oldest igneous crystallization ages are 4.03–3.96 Ga. Many zircons from all rock types contain older cores with the oldest at 4.06 and 4.2 Ga, which is consistent with the involvement of even older crust in their generation by partial melting or assimilation. In general, the geochemistry of the Acasta gneisses is not different from other Archean and younger rocks: they are on average enriched in light rareearth elements with variable depletion in heavy rare earth elements, features that are thought to reflect the presence of garnet in the source area. ▶ Radiogenic isotope systematics in whole rocks (Sm–Nd) and zircon (Lu–Hf) are also consistent with the involvement of older ▶ continental crust. Many of the rocks have zircons with thin overgrowths likely related to metamorphism at ca 3.65 Ga, 3.6 Ga, and 3.4 Ga. The formation and preservation of ▶ continental crust early in earth history is of broad interest to earth scientists because the oldest continental crust provides a record of magma formation and the role of water in generating granitic magmas over 4 billion years ago. The ca. 4 Ga granitoids are very similar to those formed much later in earth history by plate tectonic processes. No evidence of the late heavy bombardment is preserved in the Acasta gneisses.
See also ▶ Canadian Precambrian Shield ▶ Continental Crust ▶ Earth, Formation and Early Evolution ▶ Geochronology ▶ Granite
References and Further Reading Bowring SA, Housh TB (1995) The Earth’s early evolution. Science 269:1535–1540 Bowring SA, Housh TB, Isachsen CE (1990) The Acasta gneisses: remnant of Earth’s early crust. Origin of the Earth. Oxford University Press, New York Bowring SA, Williams IS (1999) Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada. Contrib Mineralog Petrol 134:3–16 Iizuka T, Horie K, Komiya T, Maruyama S, Hirata T, Hidaka T, Windley BF (2006) 4.2 Ga zircon xenocryst in an Acasta gniess from northwestern Canada: evidence for early continental crust. Geology 34:245–248
Acetaldehyde Iizuka T, Komiya T, Ueno Y, Katayama I, Uehara Y, Matuyama S, Hirata T, Johnson SP, Dunkley DJ (2007) Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: new constraints on its tectonothermal history. Precambrian Res 153:179–208
Accretion Disks ▶ Planet Formation ▶ Solar System Formation (Chronology)
Accretion Shock Definition Generally, an accretion shock is a shock wave occurring at the surface of a compact object or dense region that is accreting matter supersonically from its environment. In the context of astrobiology, an accretion shock is normally understood to mean the shock wave present at the surface of the ▶ protosolar nebula, or the corresponding nebula surrounding a ▶ protostar, as it accretes interstellar matter from the surrounding molecular cloud.
See also ▶ Protoplanetary Disk ▶ Protosolar Nebula (Minimum Mass) ▶ Protostars ▶ Shocks, Interstellar
own gravity. The object being built up in this manner is a protostar, and represents the first phase of stellar evolution. Some infalling gas impacts the protostar directly. Much of the gas, however, has sufficient angular momentum that it goes into orbit around the young star. The accreting gas thus creates a circumstellar disk. Matter spirals in through the disk onto the surface of the protostar. The remaining part of the disk eventually gives rise to planets.
See also ▶ Collapse, Gravitational ▶ Dense Core ▶ Free-Fall Time ▶ Molecular Cloud ▶ Protoplanetary Disk ▶ Protostars ▶ Protostellar Envelope ▶ Star Formation
Acetaldehyde Synonyms Acetic aldehyde; Ethanal
Definition Acetaldehyde is an organic compound with the chemical H
▶ Planet Formation
Accretion, Stellar Definition Stellar accretion refers to the inflow of ambient gas onto the surface of a star. During the process of star formation, accretion builds up the object to its final mass. The infalling gas is the interior portion of a ▶ dense core, a small ▶ molecular cloud that collapses under the influence of its
O
formula CH3CHO H C C . It is the second smaller H
Accretion, Planetary
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H
aldehyde after formaldehyde. It is a colorless liquid at room temperature with an irritating odor. It can be obtained by the oxidation of ▶ ethanol and by the reduction of ▶ acetic acid. When we drink alcohol, it is oxidized to acetaldehyde by alcohol dehydrogenase, and then oxidized to acetic acid by aldehyde dehydrogenase in the liver. It can be formed easily from gas mixtures containing methane by ultraviolet light and electric discharges, among others. It reacts with hydrogen cyanide and ammonia to give 2-aminopropionitrile, which gives ▶ alanine (amino acid) after hydrolysis. It has been detected in extracts from carbonaceous ▶ chondrites. Melting point: 123.5 C, boiling point: 20.2 C, density: 0.788 g cm3.
See also ▶ Acetic Acid ▶ Alanine
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Acetic Acid
▶ Aldehyde ▶ Chondrite ▶ Formaldehyde
Definition The organic compound acetone (CH3COCH3) is the simplest example of a ketone. Under standard conditions it is a colorless, flammable liquid. Acetone is naturally produced by normal metabolic processes in the human body. Since it is miscible with water, it serves as an important laboratory solvent. Rotational transitions in both the ground vibrational state and in the first excited torsional state have been detected by radio astronomers in ▶ molecular clouds.
Acetic Acid Synonyms Ethanoic acid
Definition Acetic acid is a ▶ carboxylic acid with the chemical H
O
formula CH3COOH H C C H
. It is a colorless liquid O
H
at room temperature with an irritating odor. Pure anhydrous acetic acid is sometimes called glacial acetic acid. It can be obtained by oxidation of ▶ acetaldehyde, which occurs in human liver by enzyme (aldehyde dehydrogenase), or by the hydrolysis of acetonitrile. It is easily formed in chemical evolution experiments, e.g., it was found among the products of spark discharge experiment in a gas mixture of methane, ammonia, hydrogen and water by S. L. Miller in 1953. It has been found in extracts from carbonaceous ▶ chondrites and has also been identified in ▶ molecular clouds. Melting point: 16.6 C, boiling point: 117.8 C, density: 1.0492 g cm3, acidity constant (pKa): 4.76.
See also ▶ Acetaldehyde ▶ Aldehyde ▶ Carboxylic Acid ▶ Chondrite ▶ Miller, Stanley ▶ Molecular Cloud
History Although detection of acetone in a molecular cloud toward the center of our ▶ Milky Way galaxy was reported by radio astronomers in 1987, secure confirmation of its presence in interstellar clouds was not achieved until some 15 years later.
See also ▶ Molecular Cloud ▶ Milky Way
References and Further Reading Friedel DN, Snyder LE, Remijan AJ, Turner BE (2005) Detection of acetone toward the orion-KL hot core. Astrophys J 632:L95–L98
Acetonitrile Synonyms CH3CN; Cyanomethane; Methylcyanide
Definition Acetonitrile is the simplest organic ▶ nitrile with the H
chemical formula CH3CN
C
C
N. It is a colorless
H H
Acetic Aldehyde ▶ Acetaldehyde
Acetone Synonyms Propanone
liquid at room temperature with an ether-like odor. It can be obtained by dehydration of acetamide, or by hydrogenation of a mixture of carbon monoxide and ammonia. It gives ▶ acetic acid and ammonia after hydrolysis and gives ethylamine after reduction. Acetonitrile itself is only slightly toxic, but gives extremely toxic ▶ hydrogen cyanide by metabolism in the body. It is detected in ▶ molecular clouds as an interstellar molecule and also found in cometary comas. When aminated on the methyl group, aminoacetonitrile is produced, which is an important precursor of ▶ glycine. It is completely miscible with water and often used as an eluant in high performance
Achondrite
liquid chromatography (HPLC). Melting point: 45.7 C, boiling point: 82 C, density: 0.786 g cm3.
See also ▶ Acetic Acid ▶ Comet ▶ Glycine ▶ Hydrogen Cyanide ▶ Molecular Cloud ▶ Nitrile
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References and Further Reading Brooke TY, Tokunaga AT, Weaver HA, Crovisier J, Bockele´e-Morvan D, Crisp D (1996) Detection of acetylene in the infrared spectrum of comet Hyakutake. Nature 383:606–608 Hartquist TW, Williams DA (1995) The chemically controlled cosmos. Cambridge University Press, Cambridge Lacy JH, Evans NJ II, Achtermann JM, Bruce DE, Arens JF, Carr JS (1989) Discovery of interstellar acetylene. Astrophys J 342:L43–L46
Achiral Acetylene
Synonyms Mirror symmetric
Synonyms
Definition
Ethyne
The term “achiral” is applied to any object – in astrobiology most commonly a molecule, a two-dimensional crystal surface, or a three-dimensional crystal structure – that is invariant (i.e., superimposable) with its mirror image. Achiral objects possess a plane of symmetry, either a mirror or a glide plane symmetry operator. Common achiral objects include a soccer ball, a pencil, and the letter “X,” in contrast with chiral objects such as a snail shell, your left hand, and the letter “R.” Common achiral molecules include H2O, CH4, and NH3, in contrast with such chiral biomolecular species as alanine and ribose. In chemistry, achiral should not be confused with racemic, although in neither case is the optical rotation of polarized light affected.
Definition Acetylene (C2H2) is the simplest alkyne (hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n2). Under standard conditions in the laboratory, it is a colorless but unstable gas. Because of its symmetry, linear of the form HCCH, it lacks a permanent electric dipole moment and hence has no allowed pure rotational transitions, making it undetectable at millimeter wavelengths. Astronomers have observed it in the infrared, in both ▶ molecular clouds and in the envelopes of evolved stars. It is an important link in the chemistry of heavier carbon chain molecules and related species in these regions. Acetylene is also found as a minor component in the atmospheres of gas giants like the planet Jupiter, in the atmosphere of Saturn’s satellite Titan, and in ▶ comets.
History Acetylene was discovered in 1836 by Edmund Davy and then rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name “acetylene.” It was first observed in the interstellar medium by Lacy et al. (1989) and in ▶ Comet Hyakutake and ▶ Comet Hale-Bopp (Brooke et al. 1996).
See also ▶ Chirality ▶ Enantiomeric Excess ▶ Homochirality ▶ Racemic (Mixture) ▶ Stereoisomers
Achondrite
See also
Definition
▶ Comet ▶ Comet Hale–Bopp ▶ Comet Hyakutake ▶ Molecular Cloud ▶ Stellar Evolution
Achondrites are differentiated stony ▶ meteorites and constitute a minority among the stony meteorites. The term literally means “without ▶ chondrules” and therefore underlines the main difference with ▶ chondrites. Achondrites are igneous ▶ rocks or ▶ breccias of igneous rock
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fragments and thus their parent body has experienced partial melting and recrystallization. The class of Achondrites includes the primitive Achondrites (e.g., Ureilites) and Achondrites in general (including, e.g., Aubrites, Eucrites, Howardites, Diogenites, Martian meteorites, and Lunar meteorites).
of boiling the macerate in hot HCl removes fluorides formed during the previous acid step. This protocol may vary according to the nature of the rock, of the fossils, and their degree of preservation. After neutralization of the final macerate with distilled water, the residue is filtered on sieves of desired size fractions, then mounted on microscopic slides or kept in vials for other analyses.
See also ▶ Breccia ▶ Chondrite ▶ Chondrule ▶ Meteorites ▶ Rock
See also ▶ Acritarch ▶ Biomarkers, Morphological ▶ Fossil ▶ Kerogen
Acidophile Achondrites ▶ Meteorites
Acid Hydrolysis Definition Hydrolysis (greek: udor [hydor] = “water” and lύsiς [ly´sis] = “solution”) is a chemical reaction in which a compound is cleaved by water. If a proton-donating compound (Brønsted acid) catalyzes the reaction, it is called “acid hydrolysis.” Formally one part of the cleaved reaction product receives a proton (H+), the other a hydroxyl (OH) moiety of a water molecule. Hydrolysis can also be catalyzed by a base. The reverse reaction is called a “condensation reaction.”
FELIPE GO´MEZ Centro de Astrobiologı´a (CSIC-INTA), Instituto Nacional de Te´cnica Aeroespacial, Torrejo´n de Ardoz, Madrid, Spain
Keywords Archaea, chemolithoautotroph, eukaryote, iron cycle, prokaryote, sulfur cycle
Definition Acidophiles are ▶ microorganisms that thrive under acidic conditions, usually at very low pH ( 1 keV) coronal emission.
where proximity allows detailed, localized studies; (2) considering “stars as suns,” allowing statistical approaches on magnetic activity properties as a function of spectral type (mass, age, evolutionary status) for thousands of stars, in particular for young stars at the stage of planet formation.
See also ▶ Faint Young Sun Paradox ▶ Magnetic Field ▶ Sun (and Young Sun) ▶ X-Rays (Stellar)
References and Further Reading Charles P, Seward FD (2010) Exploring the x-ray universe. Cambridge University Press, Cambridge Feigelson ED, Montmerle T (1999) High-energy processes in young stellar objects. Annu Rev Astron Astrophys 37:363 Friedmann H, Lichtman S, Byram E (1951) Photon counter measurements of solar x-rays and extreme ultraviolet light. Phys Rev 83:1025 Gu¨del M (2007) The sun in time: activity and environment. Living Rev Sol Phys 4(3), http://solarphysics.livingreviews.org/Articles/lrsp-2007-3/ Gu¨del M, Naze´ Y (2009) X-ray spectroscopy of stars. Astron Astrophys Rev 17:309 Lanza AF (2009) Stellar coronal magnetic fields and star-planet interaction. Astron Astrophys 505:339 Montmerle T et al (2006) From suns to life: a chronological approach to the history of life on earth 3 solar system formation and early evolution: the first 100 million years. Earth Moon Planet 98:39–95
Applications For present-day human environment, the solar magnetic activity is strongly related to the notion of “space weather,” i.e., the interaction of the solar wind with the Earth’s magnetic field. For astrobiology, X-rays from the young Sun (and from young stars in general) may have had an important influence, via ionization of atoms and molecules, on early planetary atmospheres. More generally, the study of solar and stellar magnetic activity is related to fundamental plasma physics: low-density plasmas in the corona and (in the case of young stars) interaction with a circumstellar disk; and high-density plasmas in motion, leading to the generation of magnetic fields in outer convective zones (dynamo effect). Theoretical developments are now based on sophisticated numerical 3D simulations.
Future Directions From an astronomical point of view, the similarities between solar and stellar magnetic activity allow two different approaches to the same problem, i.e., the origin of stellar magnetic fields: (1) considering the “Sun as a star,”
Adaptation SUSANNA C. MANRUBIA Laboratory of Molecular Evolution, Centro de Astrobiologı´a (INTA-CSIC), Torrejon de Ardoz, Madrid, Spain
Keywords Ecosystem, environmental change, genetic change, mutation, phenotype, selection
Definition Adaptation is a dynamical process whereby populations become better suited to their habitat. It is promoted by ▶ environmental changes, be they abiotic (climatic change, e.g.,) or biotic (the appearance of a new trait in a predator or the extinction of a competitor ▶ species, e.g.,). Adaptation is the outcome of ▶ natural selection
Adaptive Optics
acting on heritable variation and leading to a change in the genetic make-up of a population. It may involve changes in any ▶ phenotypic trait, among others, in morphology, physiology, dispersal, defense and attack mechanisms, development and growth, reproduction, behavioral patterns, and ecological interactions.
Overview Adaptation has been an on-going process ever since the emergence of the first self-replicating molecules. Populations of replicating entities generate continuous variability chiefly due to ▶ mutations and (in the case of organisms) to the migration of ▶ genes and other mobile genomic sequences. Adaptation is a gradual process that occurs over many generations as the offspring (i.e., the genes) of individuals best suited to the current habitat become increasingly abundant. Under environmental changes, species can react in three different ways. In case of slow change, they (1) perform habitat tracking if the change is exogenous (i.e., the population shifts with the ▶ environment to maintain the characteristics of its habitat) or (2) undergo genetic change. Only in the latter case are they truly adapting. If changes are too sudden, species cannot adapt and (3) become extinct. Even if the abiotic environment remains constant, the steady generation of mutants within a population compels related species to change. The appearance of teeth and claws in predators forces a simultaneous improvement in defense organs (as skeletons) to escape extinction. This sustained competition is called the Red Queen effect (Van Valen 1973; Stenseth and Maynard Smith 1984). Many adaptive changes in the history of life are gradual and involve a significant number of sequential modifications, as the evolution of tetrapod limbs from the fins of precursor fish. Some changes (the invention of lungs, of vascular systems in terrestrial plants or of organs for flight) permitted organisms to colonize new ecological niches (Gould 1993). Certain major transitions in evolution (Maynard Smith and Szathma´ry 1995) are probably not the result of adaptation, but of contingency. Such might be the case for the appearance of eukaryotic cells or of multicellular organisms. Since natural selection acts on the phenotype as a whole, it is impossible to simultaneously improve all its traits in the same degree (Mayr 1982). Also, traits that were once the result of adaptation may turn to be disadvantageous when the habitat changes. Hereditary hemochromatosis causes an excessive absorption of iron that accumulates in human body tissues. This disease is more common among Europeans: it protects against bacterial
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infections, and probably became frequent at the time of the Black Death (Moalem and Prince 2006). This is an example of the many currently non-adaptive traits that were permitted to thrive at some point of time in the history of a species.
See also ▶ Colonization (Biological) ▶ Environment ▶ Evolution (Biological) ▶ Evolution, Molecular ▶ Gene ▶ Mutation ▶ Natural Selection ▶ Phenotype ▶ Species
References and Further Reading Gould SJ (ed) (1993) The book of life: An illustrated history of the evolution of life on Earth. WW Norton, New York Maynard Smith J, Szathma´ry E (1995) The major transitions in evolution. Oxford University Press, New York Mayr E (1982) The growth of biological thought: diversity, evolution and inheritance. Belknap Press of Harvard University Press, Cambridge Moalem S, Prince J (2006) Survival of the sickest. Harper Collins, New York Rose MR, Lauder GV (eds) (1996) Adaptation. Academic, San Diego Stenseth NC, Maynard Smith J (1984) Coevolution in ecosystems: Red Queen evolution or stasis? Evolution 38:870–880 Van Valen L (1973) A new evolutionary law. Evol Ther 1:1–30
Adaptive Optics Definition Adaptive optics is a technique, used while performing visible or infrared imaging from a ground-based telescope that improves the image quality that is otherwise degraded by the atmospheric turbulence. The wave-front distortions, measured on the studied object or a nearby star, are compensated by deforming a small, thin mirror at a speed higher than turbulence (1 kHz typically) (Fig. 1). The deformable mirror is conjugated to the pupil of the telescope. The use of this technique is mandatory in any method, such as coronagraphy, that aims at a direct imaging of exoplanets. Adaptive optics should not be confused with active optics that corrects the large primary telescope mirror deformed under gravity, and which is applied at a much lower frequency (0.1 Hz).
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See also
Light from telescope Adaptive mirror
Distorted wavefront
Beamsplitter Control system
Corrected wavefront
Wavefront sensor
High-resolution camera
Adaptive Optics. Figure 1 Cartoon describing the principle of adaptive optics: the wave front deformed by atmospheric turbulence is corrected thanks to a deformable mirror that produces an inverse deformation several hundred times per second, after a device called a wave-front sensor has measured the residual distortion. The final improved image is obtained on a camera. A computer is used to analyze the residuals and to compute the proper surface to give to the mirror
See also ▶ Coronagraphy ▶ Imaging ▶ Telescope
Adenine Definition Adenine (C5H5N5), with molecular weight 135.13, is one of the four nucleic acid bases found in ▶ DNA and ▶ RNA. In double-stranded ▶ DNA and ▶ RNA, adenine base-pairs with ▶ thymine (T) and ▶ uracil (U), respectively. It is hydrolyzed to give ▶ hypoxanthine. The halflife to hydrolysis in aqueous solution at pH 7 is 1 year at 100 C and 6 105 years at 0 C. It has a UV absorption maximum at 260 nm. It has been found in the ▶ Murchison meteorite and can be synthesized in HCN polymerizations, ▶ Fischer–Tropsch type reaction, and electric discharges acting on gas mixtures such as NH3–CH4– C2H6–H2O.
▶ DNA ▶ Fischer-Tropsch-Type Reaction ▶ HCN Polymer ▶ Hydrogen Cyanide ▶ Hypoxanthine ▶ Meteorite (Murchison) ▶ Nucleic Acid Base ▶ RNA ▶ Thymine (T) ▶ Uracil (Ura)
Adenosine 5’-triphosphatase ▶ ATPase
Adenosine Triphosphatase ▶ ATPase
Adenosine Triphosphate ▶ ATP
Adiabatic Processes Definition A process affecting a parcel of matter is said to be adiabatic if it occurs without addition or loss of heat from the parcel. In a planetary atmosphere, the adiabatic lapse rate is the change in air temperature with changing height, resulting from pressure change. The so-called dry adiabatic lapse rate has the slope d (ln T)/d (ln p) = R/cp, where T is temperature, p is pressure, R is the specific gas constant (which depends on the mean molecular weight of the mixture), and cp is the specific heat at constant pressure. R/cp = 2/7 for Earth’s air. Adiabatic processes leave entropy unchanged, provided that the changes in state of the system are slow enough that the system remains close to thermodynamic equilibrium at all times.
Aerobic Mesophilic Bacterial Spores
See also ▶ Atmosphere, Model 1D ▶ Atmosphere, Structure ▶ Atmosphere, Temperature Inversion ▶ Exoplanets, Modeling Giant Planets ▶ Grey Gas Model ▶ Non-Grey Gas Model: Real Gas Atmospheres
Adsorption Definition Adsorption is the accumulation of molecules from the gas phase, or more generally from any fluid (adsorption operates in liquids as well as gases) onto a solid surface, e.g., of an ▶ interstellar dust grain. The molecules may be bound by physical (van der Waals), electrostatic, or chemical (e.g., hydrogen bonding) forces.
See also ▶ Interstellar Dust
AEB Synonyms Ageˆncia Espacial Brasileira; Brazilian space agency
Definition Numerous committees since 1961 have been charged with space activities in Brazil. In February 1994, the Brazilian Space Agency was established with the Department of Science and Technology. The agency is partnering with four institutes in charge of specific missions. The National Institute for Space Research (INPE) in charge of developing satellites and products for civilian use, the Institute of Aeronautics and Space in charge of developing planes and the launch vehicles, and two launching bases in Alcantara and Barriera do Inferno. The space agency is developing numerous cooperation with Europe, Japan, China and the United States. For further information: http://www.aeb.gov.br/
Aerobe Definition Aerobes are organisms that can tolerate or require the presence of (strict aerobe in this case) oxygen. Oxygen is
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an extremely strong oxidant and produces very reactive radicals, which react with amino acids or nucleic acids inactivating the functional sites of enzymes or producing lethal mutations. Practically, all animals are aerobes; most fungi and many prokaryotes can survive in the presence of oxygen. To do so aerobic organisms require the presence of detoxification activities, like catalases and peroxidases. Among aerobes there are different kinds of organisms: obligate aerobes, which require oxygen for growth and use oxygen as final ▶ electron acceptor in the ▶ respiration process; facultative aerobes, which can use oxygen or not, to obtain energy; microaerophiles, which require low levels of oxygen; and aerotolerants, which are not affected by the presence of oxygen.
See also ▶ Aerobic Respiration ▶ DNA Damage ▶ Electron Acceptor ▶ Respiration
Aerobic Mesophilic Bacterial Spores Definition “Spores” or more precisely “bacterial endospores” are resistant dormant bodies produced by some ▶ microorganisms (Gram positive bacteria) upon exposure to stressful environmental conditions. Aerobic microorganisms grow while exposed to oxygen, while mesophilic microorganisms grow on nutrient-rich media at temperatures comfortable for humans (roughly between 15 C and 40 C). Spores are capable of surviving extreme environmental conditions in a dormant state and proliferating when reintroduced into a hospitable environment. This particular type of spores is commonly used in ▶ planetary protection as reference microorganisms for the qualification of ▶ bioburden reduction processes. Numbering the spores per unit of surface is also used as a proxy when measuring the relative cleanliness of spacecraft components and systems.
See also ▶ Aerobe ▶ Bacteria ▶ Bioburden ▶ Spore ▶ Survival
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Aerobic Respiration
Aerobiology (from Greek άήr, ae¯r, “air”; bίος, bios, “life”; and -lοgίa, -logia) is the study of the occurrence, movement, and dispersal of living or once-living material through the atmosphere.
fact, one of the critical questions that has yet to be answered unequivocally is, “do microbes metabolize and divide while airborne?” If they do, then the atmosphere may be considered a true habitat rather than just a place where they are transient interlopers. Given the hostility of the environment, Earth’s atmosphere just above the surface contains a variety of airborne microorganisms that are thought to originate from the soil, lakes, oceans, animals, plants, as well as any process causing ▶ aerosols or dust. The numbers of viable airborne microbes recovered from the atmosphere vary seasonally with the highest numbers obtained during the summer and fall and the lowest in the winter. The distances that airborne organisms may travel range from a few kilometers to thousands of kilometers. There is a statistically significant positive correlation between the total number of viable ▶ bacteria isolated from urban air and the concentration of suspended particulate matter in the air. The organisms may be protected from drying by adsorbed water on the surfaces of these suspended particles. Studies of the biology of the upper troposphere and lower stratosphere (5–20 km) date back to the late 1800s using balloons. But these studies are few in number, and not well controlled. The organisms collected included fungi and ▶ spore forming bacteria. Later studies reported a larger variety of nonspore forming microbes, especially a variety of pigmented bacteria. Using meteorological rockets, fungi and pigmented bacteria have been isolated from as high as 77 km, the highest altitude from which microbes have been isolated. These studies, however, all used culturing methods to determine microbial counts. It has been estimated that those methods allow studying only between 0.1% and 10% of the total microbial flora in any given environment. Therefore, it is speculated that a number of microbes may exist in the upper atmosphere that we do not have the ability to culture and go unnoticed and uncounted.
Overview
See also
Aerobic Respiration Synonyms Oxygen respiration
Definition Aerobic ▶ respiration is a respiration in which dioxygen (O2) serves as the terminal ▶ electron acceptor of an electron transport chain.
See also ▶ Anaerobic Respiration ▶ Electron Acceptor ▶ Respiration
Aerobiology ROCCO MANCINELLI Bay Area Environmental Research Institute, NASA Ames Research Institute, USA
Synonyms Bioaerosol
Keywords Biology of the atmosphere, microorganisms in the atmosphere, pollen, spores
Definition
The atmosphere presents a series of challenges for life from radiation to ▶ desiccation. The absolute amount of solar radiation and the proportional contribution of UVB and UVC increase with altitude, both of which are particularly hazardous to bio-molecules. The low temperature and pressure at 29 km above the surface of the Earth are similar to those on ▶ Mars and create problems due to freezing and desiccation. Finally, the lack of nutrient availability in the atmosphere creates an additional challenge for life. The ▶ survival of airborne microbes should not be confused with growth and division while airborne. In
▶ Aerosols ▶ Bacteria ▶ Desiccation ▶ Extreme Environment ▶ Mars ▶ Solar UV Radiation (Biological Effects) ▶ Spore ▶ Survival
References and Further Reading Horneck G, Klaus D, Mancinelli RL (2010) Space microbiology. Mol Microbiol Rev 74:121–156
Affinity Constant Lacey ME, West JS (2007) The air spora – a manual for catching and identifying airborne biological particles. XV, Springer, Dordrecht, The Netherlands Mandrioli P, Caneva G, Sabbioni C (eds) (2003) Cultural heritage and aerobiology – methods and measurement techniques for biodeterioration monitoring. Kluwer
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▶ Saturn ▶ Tholins ▶ Titan
Affinity Chromatography Aeronautics and Space Agency of FFG ▶ ASA
Aerosols Synonyms Atmosphere, dust; Atmosphere, particle; Haze particle
Definition Aerosols are small liquid or solid particles in suspension in a gas. Solid smoke particles from the burning of vegetation, dust formed by wind erosion of soil, or liquid droplets produced by the ocean waves are examples of aerosols. Aerosols are present in many planetary environments, for example, in the atmospheres of Mars, the giant planets and ▶ Titan, and they were probably present in the primitive atmosphere of the Earth, much as they are presently. Atmospheric aerosols can play an important role in climate, producing an anti-green house effect, like the haze in Titan’s atmosphere. Atmospheric photochemistry in several extraterrestrial environments, like Titan, produces organic aerosols which are similar to laboratorysynthesized ▶ tholins, which can produce a complex set of prebiotic chemicals when reacted with water.
Definition Affinity ▶ chromatography is a (bio-) chemical separation method based on highly specific molecular interactions (affinity) such as between antigens and antibodies, ▶ enzymes and ▶ substrates, or receptors and ligands. The stationary phase is commonly composed of beads of a gel (e.g., agarose gel) with a covalently bound ligand (e.g., an ▶ antibody). Affinity chromatography is currently one of the most powerful separation methods, as it combines the size fractionation capability of gel permeation chromatography with specific, reversible interactions of molecules.
History The method was introduced by P. Cuatrecasas, M. Wilchek and C.B. Anfinsen in 1968 (Cuatrecasas et al. 1968). Pedro Cuatrecasas and Meir Wilchek were jointly rewarded the Wolf Prize in Medicine 1987 for this discovery.
See also ▶ Antibody ▶ Chromatography ▶ Enzyme ▶ HPLC ▶ Substrate
References and Further Reading Cuatrecasas P, Wilchek M, Anfinsen CB (1968) Selective enzyme purification by affinity chromatography. Proc Natl Acad Sci USA 61:636–643. doi:10.1073/pnas.61.2.636
See also ▶ Atmosphere, Structure ▶ Carbon ▶ Cassini ▶ Cassini–Huygens Space Mission ▶ Clouds ▶ Flux, Radiative ▶ Hydrocarbons ▶ Jupiter ▶ Methane ▶ Refractory Molecules
Affinity Constant Synonyms Association constant; Binding constant
Definition In chemistry and biochemistry, the affinity constant is the reciprocal of the dissociation constant, where both are equilibrium constants describing the strength of binding
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Age Measurement
between a catalyst such as an ▶ enzyme or ▶ ribozyme and its ▶ substrate. A Km value is a specific example of an affinity constant in enzymatic reactions. For example, the equilibrium for the formation of an enzyme-substrate (ES) complex between an enzyme (E) and a substrate (S), Ex þ Sy $ Ex Sy ; can be represented as
E x Sy Km ¼ x y ½E ½S
Agenzia Spaziale Italiana ▶ ASI
ð1Þ
AIB ð2Þ
where [E], [S], and [ES] are the concentrations of enzyme, substrate, and the enzyme-substrate complex, respectively, and x and y represent their stoichiometric coefficients. The affinity constant, also known as the Michaelis Constant, has units of per molar (M1), or l/mole. Affinity constants can vary significantly with solution conditions (e.g., temperature, pH, and ionic strength). This equilibrium is also the ratio of the rate of association (kass) and rate of dissociation (kdiss). Two different enzyme-substrate complexes may have the same affinity constants, but one could have a high kass and kdiss, while the other may have a low kass and kdiss.
See also ▶ Enzyme ▶ Ribozyme ▶ Substrate
Age Measurement ▶ Geochronology ▶ Solar System Formation (Chronology)
▶ Aminoisobutyric Acid
Akilia Definition Akilia is the name of a small island near the town of Nuuk on the SW coast of ▶ Greenland (63.933 N 51.667 W). The Akilia sequence of rocks has its name from this island, but they occur throughout the Eoarchean of West Greenland. The Akilia sequence consists mainly of granitic gneiss, and includes some of the oldest rocks on Earth. The largest member of this group of rocks is the ca. 3.8 Ga old ▶ Isua Supracrustal Belt composed of metamorphosed pillow basalts, clastic, and chemical sediments that constitute the oldest known supracrustal sequence. Fractionated C, N, and S isotopic compositions of materials of probable sedimentary origin are thought to be biogenic and to provide the oldest record of life on Earth.
See also ▶ Earth, Formation and Early Evolution ▶ Greenland ▶ Isua Supracrustal Belt
Alanine Ageˆncia Espacial Brasileira ▶ AEB
Definition One of the twenty a-amino acids found in coded proteins with the formula C3H7NO2 and the structure. H
O
Agentur fu¨r Luft- und Raumfahrt der FFG ▶ ASA
C HO
C CH3
H N H
The a-carbon of alanine is chiral so there are two optical isomers (▶ enantiomers), L- and D-alanine. Alanine was
Albedo
the first ▶ amino acid synthesized in the laboratory by Adolph Strecker in 1850, who reacted acetaldehyde with hydrogen cyanide and ammonia in aqueous solution. It is also readily produced in ▶ spark-discharge experiments from a reduced gas mixture of methane, ammonia, and hydrogen, and has been found in carbonaceous chondrites.
See also ▶ Amino Acid ▶ Enantiomers ▶ Spark Discharge ▶ Strecker Synthesis
Albedo MARK S. MARLEY NASA Ames Research Center, Moffett Field, CA, USA
Synonyms Reflectivity
Keywords Clouds, equilibrium temperature, planet, spectroscopy
Definition Albedo is a unitless measure of the reflectivity of an object. Albedo can range between zero and one. Several different types of albedos have been defined and it is important to appreciate their unique characteristics.
History Albedo is derived from the Latin “albus” or “white.” The term was first applied to optics by Johann Heinrich Lambert (of the Lambert disk) in his text Photometria in 1760.
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which equates the energy thermally radiated by the planet (left side) to the stellar (solar) energy absorbed. The Bond albedo is frequently (as in eq. 1) computed by integrating the total reflectivity of a planet over the incident flux from the parent star and thus depends both on the reflectivity spectrum and the spectral type of the star. A planet that efficiently scatters in the blue and absorbs in the red portion of the spectrum will thus have a higher integrated Bond albedo when illuminated by a blue star than by a red star, since a greater proportion of the incident light is scattered away in the former case. However, the Bond albedo can be defined as a function of wavelength, A(l); e.g., Irvine et al. (1968). The geometric albedo is defined as the ratio of a planet’s reflectivity measured at zero phase angle (opposition) to that of a Lambert disk (which has the same apparent brightness at all viewing angles) of the same radius. The geometric albedo is a function of wavelength and, because it is measured at opposition (when the phase angle ’ = 0), does not require information on the dependence of scattering with phase to measure. For a perfectly reflecting Lambert sphere the geometric albedo is 2/3 and for a semi-infinite purely ▶ Raman scattering atmosphere it is 3/4. Both such idealized, perfectly scattering objects would have a Bond albedo of 1, but the Rayleigh atmosphere sends more light directly back to the observer at zero phase angle and thus has a higher geometric albedo. The geometric albedo is a fixed quantity for a given planet, so the computation of the geometric albedo does not depend on the type of incident flux. Other types of albedos have been defined, including the spherical albedo and the single scattering albedo (reflectivity of a single particle). Great care thus must always be exercised to be certain that the correct albedo is being discussed.
See also ▶ Atmosphere, Structure ▶ Clouds ▶ Rayleigh Scattering
Overview From a planet-wide perspective the albedo of most importance is the Bond albedo, A, the ratio of incident energy reflected into all angles by a planet to the total incident energy received from its star. The Bond albedo appears in the equation for the equilibrium temperature of a rapidly rotating planet Teq, with radius R receiving an incident flux F, 4 ¼ ð1 AÞpR2 F; 4pR 2 sTeq
ð1Þ
References and Further Reading Cahoy K, Marley M, Fortney J (2010) Exoplanet albedo spectra and colors as a function of planet phase, separation, and metallicity. Astrophys J 724:189–214 de Pater I, Lissauer J (2010) Planetary Sciences, 2nd edn. Cambridge University Press Irvine WM, Simon T, Menzel DH, Pikoos C, Young AT (1968) Multicolor photometric photometry of the brighter planets, III. Astron J 73:807–828 Seager S (2010) Exoplanet Atmospheres. Princeton University Press
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Albedo Feature
Aldehyde
Synonyms
Definition
Regio
Aldehydes are organic compounds containing the RCHO functional group where R can be hydrogen or another carbon containing moiety, i.e., a part or functional group of a molecule. Aldehydes are named after the corresponding carboxylic acids by dropping the -ic or -oic suffix and adding -aldehyde or -al (for example, acetic acid ! ▶ acetaldehyde, ethanoic acid ! ethanal), or if derived from acyclic aliphatic hydrocarbons, by dropping the final e and adding -al (for example, propane ! ▶ propanal). Formyl derivatives of ring compounds may be called carbaldehydes as in cyclopentanecarbaldehyde. See the IUPAC rules for more complex cases. Aldehydes can be reduced to alcohols or oxidized to carboxylic acids, and undergo a variety of addition and condensation reactions, for example, addition of cyanide in the cyanohydrin synthesis, or condensation with another aldehyde in aldol reactions.
Definition A geographic area on the surface of a ▶ planet that is distinguished from adjacent terrains by a difference in brightness or ▶ albedo. Classical albedo features have been identified with Earth-based telescopes while no detailed morphology could be resolved. Time-varying albedo features can be due to seasonal changes, for example, due to frost covers or due to the redistribution of dark sand sheets by Aeolian activity. Albedo feature nomenclatures are increasingly replaced with detailed feature descriptions thanks to high-resolution imagery on board of space probes.
See also ▶ Albedo ▶ Planet ▶ Regio
History The name may be derived from the phrase “▶ alcohol dehydrogenatum.”
See also
Alcohol Definition An Alcohol is a general term for an organic compound containing a hydroxyl (-OH) group. Compounds whose hydroxyl group is connected to benzene ring are referred to as phenols. In the IUPAC nomenclature system, the suffix “-ol” is used in the name of each alcohol. The simplest alcohol is ▶ methanol (methyl alcohol, CH3OH), and the most commonly encountered alcohol is ▶ ethanol (ethyl alcohol, C2H5OH) which is contained in alcoholic beverages. Alcohols with three or more carbons have isomers: Propyl alcohol has two isomers, which are propan-1-ol (CH3CH2CH2OH) and propan-2-ol (CH3CH(OH)CH2). Many low-molecular-weight alcohols, such as ethanol and propan-2-ol, are used as disinfectants, since they diffuse easily through cell membranes and denaturize proteins.
See also ▶ Ethanol ▶ Methanol
▶ Acetaldehyde ▶ Alcohol ▶ Aldose ▶ Amino Acid Precursors ▶ Carboxylic Acid ▶ Formaldehyde ▶ Formose Reaction ▶ Glyceraldehyde ▶ Propanal ▶ Propionaldehyde
Aldose Definition An aldose is a ▶ monosaccharide where one end of the molecule is sp2 hybridized and thus present as a CHO or ▶ aldehyde group. These are typically reactive ends of the molecules which may be involved in redox or addition reactions. Examples of biologically important aldoses include ▶ ribose and glucose. ▶ Glycolaldehyde is the simplest aldose.
Algae
See also ▶ Aldehyde ▶ Glycolaldehyde ▶ Monosaccharide ▶ Ribose
Algae LINDA AMARAL-ZETTLER Marine Biological Laboratory, Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Woods Hole, MA, USA
Synonyms Photosynthetic eukaryotes; Protists with chloroplasts
Keywords Brown algae, coccolithophorids, diatoms, dinoflagellates, euglenoids, golden algae, green algae, phytoplankton, red algae, seaweeds
Definition The algae are an eclectic grouping of photosynthetic eukaryotes that ranges from microscopic picoeukaryotes (1 mm) (Courties et al. 1994) to macroscopic multicellular seaweeds (50 m) (Sze 2003). Algae are phylogenetically and morphologically diverse, occur in benthic and planktonic forms, and can be free-living, symbiotic, predatory, or parasitic. They inhabit diverse environments including several extreme environments of astrobiological interest, such as desert varnish, permafrost, and highly acidic Mars analog environments (Seckbach 2007). Green algae have unicellular members that share common ancestry with land plants (Charales). Eukaryotic algae along with cyanobacteria dominate global oceanic primary production.
History The term “blue-green algae” is a colloquial term and refers to cyanobacteria that are members of the domain Bacteria. This term is seldom used in current literature, but is still frequently encountered in popular articles and other media.
Overview Algae are differentiated by various means, including phylogenetic affiliation, photosynthetic pigment type, and
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morphological features including: the number of flagella, external ornamentation (e.g., scales, frustules), sub-cellular ultrastructure and colony-forming abilities. They are polyphyletic and possess representatives in several major lineages of the Eukarya including Alveolates (e.g., dinoflagellates), Chlorarachniophytes (e.g., Chlorarachnion), Cryptomonads, Euglenids, Glaucophytes (e.g., Cyanophora), Haptophytes (e.g., Emiliana huxleyi), Red Algae (Rhodophyta), Stramenopiles (e.g., diatoms, brown algae) and Viridaeplantae (e.g., Chlorophyta). Newer taxonomic classifications based on molecular phylogenetic methods employing multigene approaches place representatives from these lineages into higher taxonomic level groupings called supergroups (Keeling et al. 2005). These six major supergroups are the “Plantae,” “Chromalveolata,” “Rhizaria,” “Excavata,” “Amoebozoa,” and “Opisthokonta” – the first four of these contain algal representatives. Plantae (also referred to as the Archaeplastida (Adl et al. 2005)) include the Red Algae, Green Algae and Streptophytes; Chromalveolates include Alveolates and Stramenopiles; Excavates include Euglenids; Rhizaria include Chlorarachniophytes that group within the Cercozoa. A note of caution is that the resilience of these supergroups has been called into question (Parfrey et al. 2006; Yoon et al. 2008), so it is important to take this into consideration when using these terms. Of these six major supergroups, only the Opisthokonta appear to be strongly supported in robust phylogenetic analyses. In addition to chlorophyll a, algae are further distinguished on the basis of other types of photosynthetic pigments they possess. Alveolates, Cryptomonads, Haptophytes, and Stramenopiles also contain chlorophyll c, while Chlorarachniophytes, Euglenids, Viridaeplantae, and Cryptomonads contain chlorophyll b. Other assessory pigments, such as phycobilins, further distinguish Cryptomonads, Glaucophytes, and Red Algae. A feature that all algae share is the ability to photosynthesize. There is strong evidence that this characteristic was the result of a single endosymbiotic event that occurred between a cyanobacterium and an ancestor of the glaucophytes, red algae, and green algae (including plants) (Keeling 2010). The uptake of both green and red algae by other eukaryotes has occurred multiple times in what is referred to as “secondary symbioses.” In some dinoflagellates, tertiary symbioses can occur via plastid replacement. Plastids are also sometimes stolen and used by their host for brief periods of time by sea slugs, dinoflagellates, as well as other protists, such as ciliates and foraminifera – this process is called kleptoplasty.
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See also ▶ Eukarya
▶ rock has been launched from Mars by an impact event 15 million years ago. 13,000 years ago the meteorite landed on Earth.
References and Further Reading Adl SM et al (2005) The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. J Eukaryot Microbiol 52(5):399–451 Courties C et al (1994) Smallest eukaryotic organism. Nature 370(6487):255–255 Keeling PJ (2010) The endosymbiont origin, diversification and fate of plastids. Philos Trans R Soc B 365:729–748 Keeling PJ et al (2005) The tree of eukaryotes. Trends Ecol Evol 20(12):670–676 Parfrey LW et al (2006) Evaluating support for the current classification of eukaryotic diversity. PLoS Genet 2(12):e220 Seckbach J (ed) (2007) Algae and cyanobacteria in extreme environments. Springer, Dordrecht, p 811 Sze P (2003) A biology of the algae, 4th edn. McGraw-Hill Companies, Boston Yoon HS et al (2008) Broadly sampled multigene trees of eukaryotes. BMC Evol Biol 8(1):14 Internet Resources The Tree of Life Project: http://tolweb.org/ AlgaeBase: http://www.algaebase.org/
Algorithmic Chemistry ▶ Automaton, Chemical
ALH 84001 Synonyms Allan Hills 84001
Definition ALH 84001 (abbreviation of Allan Hills 84001) is a 1.93 kg ▶ meteorite found in the Allan Hills ice field, Antarctica (Victoria Land) 1984 by US meteorite searchers. ALH 84001 has been classified as ▶ achondrite and is thought to be from ▶ Mars. It mainly consists of coarse-grained cataclastic orthpyroxenes and among the ▶ SNC meteorites it defines the class of SNC-orthopyroxenites. In 1996 NASA scientists announced that the meteorite might contain microscopic ▶ fossils of Martian ▶ bacteria, a view that has been widely criticized. Radiometric dating suggests that ALH 84001 is 4.1 billion years old. The piece of
See also ▶ Achondrite ▶ Bacteria ▶ Fossil ▶ Mars ▶ Meteorites ▶ Rock ▶ SNC Meteorites
Alignment of Dust Grains Definition ▶ Interstellar Dust produces not only extinction of transmitted starlight, but also introduces polarization of that light, with a positive correlation between the amount of reddening and the linear polarization. This effect is normally ascribed to the alignment of asymmetric grains in the galactic magnetic field. When the direction of alignment changes along the line of sight, a circularly polarized component is produced. Consequently, observations of this polarization provide (model dependent) information on both dust grain properties and on the galactic magnetic field. Various mechanisms have been proposed to produce the grain alignment. Since circularly polarized light could conceivably affect the chiral symmetry of irradiated molecules such as amino acids, it could possibly play a role in producing the observed ▶ enatiomeric excess in some meteoritic organics, although this is far from being demonstrated.
See also ▶ Chirality ▶ Enantiomeric Excess ▶ Interstellar Dust ▶ Reddening, Interstellar
Aliphatic Carboxylic Acids ▶ Fatty Acids, Geological Record of
Alkaliphile
Aliphatic Hydrocarbon Definition An aliphatic hydrocarbon is an organic compound composed of carbon and hydrogen which does not contain aromatic rings. It may be linear or cyclic and may contain unsaturated double or triple bonds, thus alkanes, alkenes, and alkynes are all aliphatic compounds. Some illustrative examples are ▶ methane, ethylene, ▶ acetylene and cyclopentane.
See also ▶ Acetylene ▶ Aromatic Hydrocarbon ▶ Methane
Alkaline Lakes ▶ Soda Lakes
Alkaliphile ANTONIO VENTOSA, RAFAEL R. DE LA HABA Department of Microbiology and Parasitology, Faculty of Pharmacy, University of Sevilla, Sevilla, Spain
Keywords Alkaline, extreme habitat, extremophile, pH, soda lake
Definition Alkaliphiles are microorganisms that grow optimally or very well at pH values above 9, often between 10 and 12, but cannot grow or grow slowly at the near-neutral pH value of 6.5 (Horikoshi 1999).
Overview There is no precise definition of what characterizes an alkaliphilic organism. Several microorganisms exhibit more than one optimum pH for growth depending on growth conditions, particularly nutrients, metal ions, and temperature. However, the definition given above is the most extended one. Many different taxa are represented among the alkaliphiles, including ▶ prokaryotes (aerobic ▶ bacteria
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belonging to the genera Bacillus, Micrococcus, Pseudomonas, and Streptomyces; ▶ anaerobic bacteria from the genera Amphibacillus, Anaerobranca, and Clostridium; halophilic ▶ archaea belonging to the genera Halorubrum, Natrialba, Natronomonas, and Natronorubrum; methanogenic archaea from the genus Methanohalophilus; anaerobic archaea from the genus Thermococcus; cyanobacteria; spirochetes; actinomycetes; sulfuroxidizing and sulfate-reducing bacteria), ▶ eukaryotes (▶ yeasts and filamentous ▶ fungi); and even phages (Horikoshi 1998, 1999). Alkaliphiles require alkaline environments and, in most cases, sodium ions for their growth, germination, and sporulation (Kudo and Horikoshi 1983). Isolation of alkaliphilic microorganisms in laboratory conditions must be carried out in alkaline media containing sodium carbonate, sodium bicarbonate, or sodium hydroxide, following conventional means. Alkaliphiles are widely distributed in different habitats and isolated from soils, feces, and alkaline and/or saline lakes. The frequency of alkaliphilic microorganisms in neutral “ordinary” soil samples is 102 to 105/g of soil, which corresponds to 1/10 to 1/100 of the population of the neutrophilic microorganisms (Horikoshi 1991). Some studies show that alkaliphilic bacteria have also been found in deep-sea sediments collected from depths of up to the 10,898 m in the Mariana Trench (Takami et al. 1997). Most alkaliphiles have an optimal growth at around pH 10, which is the most significant difference from well-investigated neutrophilic microorganisms. These alkaliphilic microorganisms can grow in such extreme environments because their internal pH is maintained at 7.5–8.5, despite a high external pH of 8–13 (Aono et al. 1997). Therefore, one of the key features in alkaliphily is associated with the cell surface, which discriminates and maintains the intracellular neutral environment separate from the extracellular alkaline environment. Alkaliphiles have two mechanisms of cytoplasmic pH regulation. The first one involves the cell wall structure, which contains acidic polymers that function as a negatively charged matrix and may reduce the pH value at the cell surface (Aono and Horikoshi 1983). The surface of the cytoplasmic membrane must presumably be kept below pH 9, because the cytoplasmic membrane is very unstable at alkaline pH values (pH 8.5–9.0) much below the pH optimum for growth (Aono et al. 1992). The second strategy to maintain pH ▶ homeostasis consists of the use of the Naþ/Hþ membrane antiporter system (Dc dependent and DpH dependent), the Kþ/Hþ antiporter, and ATPase-driven Hþ expulsion (Krulwich et al. 1998).
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The flagella motility of alkaliphiles is considered to be driven by a sodium-motive force instead of a proton-motive force, as shown by neutrophiles. These alkaliphiles are most motile at pH 9.0–10.5, whereas no motiliy is observed at pH 8; in addition, they require Naþ for motility (Horikoshi 1998). Studies of alkaliphiles have led to the discovery of many types of enzymes that exhibit interesting properties. Alkaliphilic microorganisms produce some enzymes such as proteases, amylases, cyclomaltodextrin glucanotransferases, pullulanases, cellulases, lipases, xylanases, pectinases, chitinases, and alginate lyases that are of great interest (Horikoshi 1999; Kobayashi et al. 2009).
See also ▶ Anaerobe ▶ Archea ▶ Bacteria ▶ Cyanobacteria ▶ Eukaryote ▶ Fungi ▶ Homeostasis ▶ Methanogens ▶ Prokaryote ▶ Soda Lakes ▶ Yeast
References and Further Reading Aono R, Horikoshi K (1983) Chemical composition of cell walls of alkalophilic strains of Bacillus. J Gen Microbiol 129:1083–1087 Aono R, Ito M, Horikoshi K (1992) Instability of the protoplast membrane of facultative alkaliphilic Bacillus sp. C-125 at alkaline pH values below the pH optimum for growth. Biochem J 285:99–103 Aono R, Ito M, Horikoshi K (1997) Measurement of cytoplasmic pH of the alkaliphile Bacillus lentus C-125 with a fluorescent pH probe. Microbiology 143:2531–2536 Horikoshi K (1991) Microorganisms in alkaline environments. Kodansha-VCH, Tokyo Horikoshi K (1998) Alkaliphiles. In: Horikoshi K, Grant WD (eds) Extremophiles: microbial life in extreme environments. Wiley-Liss, New York, pp 155–179 Horikoshi K (1999) Alkaliphiles: some applications of their products for biotechnology. Microbiol Mol Biol Rev 63:735–750 Kobayashi T, Uchimura K, Miyazaki M, Nogi Y, Horikoshi K (2009) A new high-alkaline alginate lyase from a deep-sea bacterium Agarivorans sp. Extremophiles 13:121–129 Krulwich TA, Ito M, Hicks DB, Gilmour R, Guffanti AA (1998) pH Homeostasis and ATP synthesis: studies of two processes that necessitate inward proton translocation in extremely alkaliphilic Bacillus species. Extremophiles 2:217–222 Kudo T, Horikoshi K (1983) Effect of pH and sodium ion on germination of alkalophilic Bacillus species. Agric Biol Chem 47:665–669 Takami H, Inoue A, Fuji F, Horikoshi K (1997) Microbial flora in the deepest sea mud of the Mariana Trench. FEMS Microbiol Lett 152:279–285
Alkanoic Acids ▶ Fatty Acids, Geological Record of
Allan Hills 84001 ▶ ALH 84001
ALMA THIJS DE GRAAUW ALMA, Vitacura, Santiago, Chile
Synonyms Atacama Large Millimeter/submillimeter Array
Definition The Atacama Large Millimeter/submillimeter Array (ALMA) is an international radio telescope under construction on a dry site at 5,000 m elevation in the Atacama Desert of northern Chile. ALMA is a partnership of Europe, Japan, and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere, in Japan by the National Institutes of Natural Sciences (NINS) in cooperation with the Academia Sinica in Taiwan, and in North America by the National Science Foundation in cooperation with the National Research Council of Canada and the National Science Council of Taiwan. Construction and operation of the facility are led on behalf of Europe by ESO, on behalf of Japan by the National Astronomical Observatory of Japan (NAOJ), and on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by the Associated Universities, Inc. (AUI). The telescope will consist of 66 high-precision antennas operating over the instrument wavelength range of 0.3–10 mm.
Alpha Helix Definition The alpha (or a-) helix is one of the two most common polypeptide secondary structural motifs and consists
Alunite
of a right-handed helix with 3.6 amino acid residues per turn. The helix has a pitch of 0.54 nm, a width of 1.2 nm, and is stabilized by hydrogen bonds between the peptide –C=O– and –NH– moieties of every fourth bond. Proline and glycine tend to kink or break alpha helices while alanine, leucine, methionine, lysine and glutamate stabilize them. Protein alpha-helical regions often fold into supercoiled configurations that can span membranes, bind DNA, or serve structural roles.
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radioactive alpha decay have low penetration depth. Helium nuclei, which form 10–12% of cosmic rays, are usually of much higher energy than those produced by radioactive decay.
See also ▶ Beta Rays ▶ Gamma Rays ▶ Radiochemistry
History The term alpha helix was coined by William Astbury in the 1930s. Linus Pauling worked out the structure accurately in 1948.
Alteration
See also
Definition
▶ Amino Acid ▶ Oligopeptide ▶ Peptide ▶ Polypeptide ▶ Protein ▶ Secondary Structure (Protein)
Alteration refers to processes by which the mineralogy, composition and texture of a rock is changed as a result of re-equilibration under conditions of lower temperature and pressure or through interaction with aqueous or CO2-rich fluids. The minerals of the original rock, which may be magmatic, sedimentary, or metamorphic, are transformed into an assemblage of low-temperature, usually finer grained minerals. A typical example is the replacement of magmatic minerals such as olivine, pyroxene, and feldspar by chlorite, clay minerals, or carbonates. ▶ Weathering is a type of alteration that takes place close to the surface through interaction of rock with the atmosphere and with ground- or surface-waters. Alteration is also used in chemistry and biology (e.g., DNA alterations).
Alpha Particles ▶ Alpha Rays
See also
Alpha Rays
▶ DNA Damage ▶ Weathering
Synonyms Alpha particles; Helium nuclei
Definition An alpha ray is a stream of alpha particles. An alpha particle consists of two protons and two neutrons bound together into a particle identical to a helium nucleus. It is radioactive and is produced in the process of alpha decay. Alpha particles, like helium nuclei, have a net spin of zero. The energy of alpha particles varies, with higher energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3 and 7 MeV, corresponding to extremely long to extremely short halflives of alpha-emitting nuclides. They are a highly ionizing form of particle radiation, that when resulting from
Aluminilite ▶ Alunite
Alunite Synonyms Aluminilite
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Definition Alunite is a secondary mineral of chemical formula KAl3(SO4)2(OH)6 (trigonal crystal system). It forms solid solutions with ▶ jarosite KFe(III)3(SO4)2(OH)6 and results from low-medium temperature (80–150 C) hydrothermal alteration of feldspar-rich volcanic rocks. Acid fluids formed during the oxidation and leaching of metal sulfide deposits commonly control the alteration. Detection of alunite at Terra Sirenum on the surface of ▶ Mars could be an indicator of basalt alteration in contact with H2SO4-rich water, possibly derived from past acid saline lakes.
See also ▶ Hydrothermal Environments ▶ Jarosite ▶ Mars ▶ Weathering
This slippage occurs when the ionization fraction is so low that collisions between neutral species and ions become relatively rare. At this point, the neutral atoms can move relative to the ions, which are effectively tied to the magnetic field. Ambipolar diffusion is thought to occur in ▶ molecular clouds, which are dense enough to shield much of the external, ionizing radiation. The cloud’s self-gravity can then cause the gas to condense, in spite of its internal magnetic field. This condensation ultimately leads to star formation.
See also ▶ Collapse, Gravitational ▶ Dense Core ▶ Fragmentation (Interstellar Clouds) ▶ Molecular Cloud ▶ Star Formation
Amide Amazonian Definition The youngest of three systems (of time-stratigraphic units) or periods (the chronologic equivalents to systems) in the Martian stratigraphic scheme, named after the region of Amazonis Planitia (Amazonis: From the classical land of the Amazons on the island Hesperia; see U.S. Geological Survey Gazetteer of Planetary Nomenclature). Depending on the different models to determine absolute ages on planetary surfaces by crater statistics, the Amazonian began at some point in time between 3.55 and 1.8 billion years ago and lasts until the present.
See also ▶ Chronology, Cratering and Stratography ▶ Hesperian ▶ Mars ▶ Mars Stratigraphy ▶ Noachian
Definition In chemistry an amide is an organic compound or functional group containing a ▶ carboxyl group modified by the replacement of the hydroxyl (–OH) group by an ▶ amine or ammonia. Some important amides include peptides, urea, and formamide. Amides are also important intermediates in the Strecker amino acid synthesis. The amide functional group exists as a hybrid between two states as shown in Fig. 1 below, and hydrogen bonding between amide functional groups in polypeptides allows the formation of secondary structural motifs such as a-helices and b-sheets. Amides are weak bases, and can be hydrolyzed back to the constituent amine and carboxylic acid. Cyclic amides are known as lactams.
See also ▶ Amine ▶ Carboxylic Acid ▶ Polypeptide ▶ Strecker Synthesis
Ambipolar Diffusion Definition Ambipolar diffusion is the slippage of neutral matter in a plasma with respect to an internal magnetic field.
O–
O R2 R1
N H
R1
Amide. Figure 1 Two states of amide
N H+
R2
Amino Acid
Amidocyanogen
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Amino Acid
▶ Cyanamide
JEFFREY BADA Scripps Institution of Oceanography, La Jolla, CA, USA
Synonyms Amino alkanoic acid
Amidogen
Keywords
▶ Amino Radical
Amino group, carboxyl group
Definition Amino acids are organic molecules that contain at least one primary amino group (NH2) and one carboxyl group (COOH). The general formula for amino acids with alkyl side-chains that have one amino and one carboxyl group, known as amino alkanoic acids, is CnH2nNH2COOH.
Amine Definition An amine is an organic compound containing an amino group (-NH2). Since the nitrogen atom in an amino group has a lone electron pair, it can associate with a proton; thus, amines are generally basic. Amines are classified as primary amines, secondary amines, tertiary amines, or quaternary amines depending on the number of alkyl substituents. Primary amines have a single alkyl substituent, secondary amines have two and so on. The simplest amine is methylamine (CH3NH2). Methylamine was found as an interstellar molecule in 1974. Amines have also been detected among organic compounds extracted from carbonaceous ▶ chondrites. Amines having a carboxylic group (-COOH) are referred as to ▶ amino acids, which are important bioorganic compounds.
History Most of the biologically important amino acids were isolated and characterized in Europe in the early nineteenth century (Vickery and Schmidt 1931). For example, asparagine, the first amino acid discovered, was isolated from asparagus by Vauquelin and Robiquet in 1806. Glycine was isolated by Braconnot in 1820. Laboratory syntheses were developed shortly thereafter.
Overview The structural isomers with the amino group on the sequential carbon atoms adjacent to the carboxyl group are called a, b, g, etc. -amino acids. Thus a-amino acids are 2-amino alkanoic acids, b-amino acids are 3-amino alkanoic acids, etc. General structural formulae for a, b, and g amino acids are shown in Fig. 1.
See also ▶ Amino Acid ▶ Chondrite ▶ Molecular Cloud
Amino Acid. Table 1 The numbers of possible structural isomers for amino alkanoic acids (with the formula CnH2nNH2COOH) are (Henze and Blair 1934) Number of carbon atoms
N
H
R1
N R2
H Primary amine
H
R1
Secondary amine
Amine. Figure 1 Amine
N
R3
R1 R2
Tertiary amine
Number of possible isomers
2
1
3
2
4
5
5
12
6 10
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31 1,479
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Amino Acid
NH2
NH2
COOH H2N
COOH α-Amino-n-butyric acid
β-Amino-n-butyric acid
COOH γ-Amino-n-butyric acid
Amino Acid. Figure 1 Generalized structural formulas for a-, b-, and g-amino acids
The common names of the amino acid isomers with up to five carbon atoms are given in Table 2. Some amino acids have aromatic side-chains: examples are phenylglycine (a-aminophenylacetic acid), phenylalanine (a-amino-b-phenylpropanoic acid), and tyrosine (a-amino-b-(4-hydroxyphenyl)propanoic acid). Amino acids can also have side-chains consisting of an indole (a benzene ring linked to a five-membered nitrogen-containing pyrrole ring) or an imidazole (fivemembered diunsaturated ring composed of three carbon atoms and two nitrogen atoms at non-adjacent positions): two examples of this type of amino acid found in biochemistry are tryptophan and histidine, respectively. There are also amino acids with hydroxyl and sulfur containing side-chains. The common names of some examples of these amino acids are serine (a-amino-b-hydroxypropionic acid), threonine (a-amino-b-hydroxybutanoic acid), cysteine (a-amino-bmercaptopropanoic acid), and methionine (a-aminog-(methylthio)butyric acid). Selenium can also substitute for sulfur in the sulfur containing amino acids in some organisms. Some amino acids have more than one amino group (for example, lysine or a, ε-diaminohexanoic acid) and/ or more than one carboxyl group: for example, a-aminomalonic acid, aspartic acid (a-aminobutanedioic acid), and glutamic acid (a-aminopentanedioic acid). Asparagine (a-amino-b-carbamoylpropanoic acid) and glutamine (a-amino-d-carbamoylbutyric acid) are the side group carboxamides of aspartic and glutamic acids, respectively. Interestingly, asparagine was the first amino acid discovered in 1806 when it was crystallized from the “juice” squeezed from asparagus shoots. The amino acid arginine (a-amino-ε-guanidinopentanoic acid) has a guanidinium group attached to the end of its alkyl side chain. Some amino acids have cyclic secondary amine rather than a primary amino group. Examples include proline (pyrrolidine-2-carboxylic acid, C5H9NO2) where the primary amino group is replaced with a five-membered pyrrolidine ring or tetrahydropyrrole. Another example
Amino Acid. Table 2 The names of the structural isomers for 2, 3, 4, and 5 carbon amino alkanoic acids Number of carbon atoms
Common names
2
Glycine
3
Alanine, b-alanine
4
a-amino-n-butyric acid, b-amino-n-butyric acid, a-aminoisobutyric acid, baminoisobutyric acid, g-amino-n-butyric acid
5
Valine, isovaline, b-aminopentanoic acid, g-aminopentanoic acid, d-aminopentanoic acid, a-methyl-b-aminobutyric acid, allo-a-methyl-b-aminobutyric acid, a-methyl-g-aminobutyric acid, b-methyl-b-aminobutyric acid, b-methyl-g-aminobutyric acid, a-ethyl-b-aminoproponic acid, a-dimethyl-b-aminoproponic acid
is pipecolic acid (piperidine-2-carboxylic acid, C6H11NO2) where the amino group is replaced by sixmembered piperidine ring (Fig. 2). The ionization constants (pKa) at 25 C of the amino and carboxyl groups of amino alkanoic acids are in the range 8–10 and 2–4, respectively. Thus, at neutral pH, the amino group is protonated while the carboxyl group is deprotonated, producing a doubly-charged zwitterion with no net charge. Amino acids with other amino or carboxyl groups have additional ionization constants characteristic of the particular group. The pKa of the b-carboxyl group of aspartic acid is 3.9 at 25 C, thus at neutral pH aspartic acid has a net negative charge. The pKa of the guanidinium group of arginine is 12.5 and arginine is thus positively charged at neutral pH. When a carbon atom in an amino acid has four different groups attached to it, referred to as an asymmetric or chiral carbon, it is optically active. For those amino acids with one chiral carbon there are two possible
Amino Acid
O
H N
HOOC
H N
+H
HOOC
R
Pipecolic acid
Amino Acid. Figure 2 Some cyclic amino acids, proline, and pipecolic acid
H2N
COOH
H2N
COOH
Enantiomers
L-Isoleucine
D-Isoleucine Diastereomers
H2 N
COOH
H2N
COOH
Enantiomers
L-Alloisoleucine
D-Alloisoleucine
Amino Acid. Figure 3 Enantiomers and diastereomers of iso- and alloisoleucine
optically active isomers designated the L- and D-enantiomers. Some amino acids have more than one chiral carbon so several stereoisomers are possible. An amino acid with two chiral carbons is said to be diastereomeric and there are thus two diastereomers, each which has two enantiomers, for a total of four possible optical isomers. For the diastereomeric pair L-isoleucine/D-alloisoleucine (aamino-b-methylpentanoic acid) the two sets of enantiomers are L- and D-isoleucine, and L- and D-alloisoleucine, respectively (Fig. 3). Amino acids can be linked together by the formation of a ▶ peptide bond that involves the amino group of one
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N H Proline
A
COO−
Amino Acid. Figure 4 A generic dipeptide
amino acid and the carboxyl group of another. Two amino acids connected in this fashion are called a dipeptide which has the structure given below (in this case one amino acid is glycine and the other has a generic R-group side chain) (Fig. 4). Polypeptides and proteins consist of a large number of amino acids connected in peptide linkages. A total of 20 different amino acids (for this discussion the amino acids selenocysteine and pyrrolysine are not included because they are relatively rare coded amino acids) are decoded from DNA sequences and encoded into RNA for incorporation into proteins. A list of the “canonical” 20 protein amino acids and their abbreviations are given in Table 3. There are peptides that contain additional amino acids other than the standard protein amino acids, but these are incorporated by post-translational modifications or the peptides themselves are synthesized by nonribosomal peptide synthetases (NRPSs). For example a-aminoisobutyric acid and isovaline are found in some fungal peptides synthesized by NRPSs. With the exception of achiral glycine, only the L-enantiomers of the proteinogenic amino acids are incorporated into proteins. The discrimination against the incorporation of D-amino acids during the protein synthesis process is estimated to be greater than 104. However, there are D-amino acids present in some peptides but these are introduced either by the conversion of L-amino acids by post-translational isomerization enzymes or are introduced by NRPSs. Some D-amino acid-containing peptides have potent antimicrobial activity. The total number of amino acids theoretically possible is huge and several hundred different amino acids have been isolated from organisms and an even larger number have been made in the laboratory by a variety of synthetic methods. Moreover, the synthesis of amino acids is not confined to terrestrial biology or laboratory synthesis: amino acids have been detected in meteorites and there are hints that at least the simplest amino acid glycine is present in interstellar clouds and comets (see, however, ▶ Molecules in Space). One of the meteorites most extensively studied is the Murchison carbonaceous chondrite that fell in southeastern Australia in 1969. Over 75 different amino acids have been detected in
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Amino Acid. Table 3 The 20 amino acids commonly found in proteins and their commonly used abbreviations Amino acid common name
Three letter abbreviation
One letter abbreviation
Aspartic acid
Asp
D
Glutamic acid
Glu
E
Asparagine
Asn
N
Glutamine
Gln
Q
Glycine
Gly
G
Alanine
Ala
A
Valine
Val
V
Isoleucine
Iso
I
Leucine
Leu
L
Phenylalanine
Phe
F
Tyrosine
Tyr
Y
Serine
Ser
S
Threonine
Thr
T
Cysteine
Cys
C
Methionine
Met
M
Lysine
Lys
K
Histidine
His
H
Arginine
Arg
R
Tryptophan
Trp
W
Proline
Pro
P
Murchison (Sephton 2002), with only 8 of these also being found in biological proteins. These amino acids are clearly of extraterrestrial origin: many are unique to the meteorite and do not occur naturally on Earth and those with a chiral carbon are racemic (or close to racemic). The Murchison amino acids are thought to have been synthesized by natural reactions, such the ▶ Strecker synthesis, directly on the juvenile meteorite parent body or in the early solar nebula before incorporation into planetesimals. Amino acids may also have been synthesized by natural processes on the early Earth as demonstrated by the classic Miller spark discharge experiment carried out in 1953 (Miller 1953; Johnson et al. 2008). These amino acids could have accumulated on the Earth and been available for incorporation into the first living entities. To date, 12 of the amino acids found in the proteins of terrestrial organisms have been synthesized in spark discharge experiments with various reduced gas mixtures.
See also ▶ Diastereomers ▶ Enantiomers ▶ L-Amino Acids ▶ Molecules in Space ▶ Peptide ▶ Protein ▶ Strecker Synthesis
References and Further Reading Henze HR, Blair CM (1934) The number of structural isomers of the more important types of aliphatic compounds. J Am Chem Soc 56:157 Johnson AP, Cleaves HJ, Dworkin JP, Glavin DP, Lazcano A, Bada JL (2008) The Miller volcanic spark discharge experiment. Science 322:404 Miller SL (1953) Production of amino acids under possible primitive Earth conditions. Science 117:528 Sephton MA (2002) Organic compounds in carbonaceous meteorites. Nat Prod Rep 19:292–311 Vickery HB, Schmidt CLA (1931) The history of the discovery of the amino acids. Chem Rev 9(2):169–318
Amino Acid N-Carboxy Anhydride LAURENT BOITEAU Institut des Biomole´cules Max Mousseron – UMR5247 CNRS, University Montpellier-2, Montepellier Cedex, France
Synonyms 1,3-Oxazolidine-2,5-dione; Leuchs’ anhydride; NCA
Definition An amino acid N-carboxy anhydride (or NCA) is a cyclic organic compound structurally related to an ▶ amino acid, which is an intramolecular mixed anhydride of a carboxylic and carbamic acid (structure -CO–O–CO– NH-), making it both an N-protected and a CO-activated amino acid. NCAs are structurally related to hydantoins, but have very different chemical reactivity. They are rather unstable in water and physiological media. The term NCA is usually used to refer to the NCAs of a-amino acids (although NCAs of b-amino acids etc. are also possible). NCAs can condense to give oligo- or polypeptides, with release of CO2. NCAs are postulated or observed intermediates in many prebiotically relevant reactions leading to
Amino Acid N-Carboxy Anhydride
▶ peptides from AA derivatives, especially in aqueous media in the presence of carbonate. NCAs are also considered to be potentially prebiotic reagents, as they are versatile free energy carriers which can potentially activate other biologically relevant chemical species, such as nucleotides.
History Although speculations that NCAs might have played a role in prebiotic chemical evolution arose in the mid 1970s, notwithstanding their use since the late 1970s in “model” prebiotic reactions, NCAs gained significant status as prebiotically relevant compounds in the early 2000s after conclusive experimental evidence.
Overview Discovered by Hermann Leuchs in 1906, NCAs are wellknown reactants in both organic and polymer synthesis (Kricheldorf 2006). Since their most popular preparative method, involving the reaction of free amino acids with phosgene is not prebiotically relevant, NCAs themselves were long considered as prebiotically irrelevant (Pascal et al. 2005). Nevertheless, NCAs have been continuously used from the 1970s in model reactions of prebiotic peptide formation, especially to assess stereoselection hypotheses in relation with the emergence of homochirality of the natural amino acid pool, e.g., enantiomeric excess amplification processes (Kricheldorf 2006; Pascal et al. 2005; Illos et al. 2008). NCAs have long been postulated as likely intermediates in the reaction of activated amino acid esters (e.g., adenylates, thioesters) based on the observation that the formation of peptides is accelerated by the presence of CO2 or bicarbonate. Since the late 1990s, several prebiotically relevant pathways for NCAs formation have been identified, thus confirming the prebiotic status of NCAs (Kricheldorf 2006; Pascal et al. 2005): ● The nitrosation of N-carbamoyl amino acids (CAA) promoted by nitrogen oxides (Kricheldorf 2006; Pascal et al. 2005) ● The decomposition of diacyldisulfides ● The reaction of amino acids with carbon oxysulfide in the presence of oxidizing or alkylating agents (Leman et al. 2004) ● The spontaneous decomposition of N-carbamoyl amino acids (CAA) in water (Danger et al. 2006) NCAs represent both: (1) the structurally simplest activated amino acid (formally resulting from condensation
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with CO2), (2) an unavoidable intermediate from any form of CO-activated amino acid in a bicarbonate/ CO2-rich environment, and (3) the most activated amino acid species achievable in water in a prebiotic environment. Thermodynamic calculations show NCAs to be quite stable (because of the cyclic structure) compared to other anhydrides, although kinetically they are as reactive as the latter. Furthermore, NCAs may be kinetically competent intermediates from almost any inactivated amino acid derivatives, provided their spontaneous hydrolysis is slower than NCA formation (Pascal et al. 2005). Such thermodynamic and kinetic features make NCAs potential energy carriers in an amino acid–based protometabolism, as exemplified by their ability to activate inorganic phosphate (Pascal et al. 2005) or nucleotides (Biron et al. 2005; Leman et al. 2006), which could be coupled to a peptide/nucleic acid coevolution scenario supporting speculations on the emergence of the translation apparatus (Pascal et al. 2005).
See also ▶ Amino Acid ▶ Chirality ▶ Metabolism (Prebiotic) ▶ N-Carbamoyl-Amino Acid ▶ Peptide ▶ Prebiotic Chemistry
References and Further Reading Biron JP, Parkes AL, Pascal R, Sutherland JD (2005) Expeditious prebiotic aminoacylation of nucleotides. Angew Chem Int Ed 44:6731–6734 Danger G, Cottet H, Boiteau L, Pascal R (2006) The peptide formation mediated by cyanate revisited. N-Carboxyanhydrides as accessible intermediates in the decomposition of N-carbamoylamino acids. J Am Chem Soc 128:7412–7413 Illos RA, Bisogno FR, Clodic G, Bolbach G, Weissbuch I, Lahav M (2008) Oligopeptides and copeptides of homochiral sequence, via b-sheets, from mixtures of racemic a-amino acids, in a one-pot reaction in water; relevance to biochirogenesis. J Am Chem Soc 130(27): 8651–8659 Kricheldorf HR (2006) Polypeptides and 100 years of chemistry of a-amino acid N-carboxyanhydrides. Angew Chem Int Ed 45:5752–5784 (and references cited therein) Leman L, Orgel LE, Ghadiri MR (2004) Carbonyl sulfide-mediated prebiotic formation of peptides. Science 306:283–286 Leman LJ, Orgel LE, Ghadiri MR (2006) Amino acid dependent formation of phosphate anhydrides in water mediated by carbonyl sulfide. J Am Chem Soc 128(1):20–21 Pascal R, Boiteau L, Commeyras A (2005) From the prebiotic synthesis of a-amino acids towards a primitive translation apparatus for the synthesis of peptides. Top Curr Chem 259:69–122 (and references cited therein)
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Amino Acid Precursors
Amino Acid Precursors Definition ▶ Amino acids precursors are compounds that give amino acids after some reactions (usually hydrolysis). One of the typical amino acid precursors is ▶ aminoacetonitrile, which is converted to glycine by hydrolysis via glycine ▶ amide: NH2CH2CN + 2H2O ! NH2CH2CONH2 + H2O ! NH2CH2COOH + NH3. Complex organic polymers with large molecular weights are also possible amino acid precursors. ▶ Tholins, which are formed by reactions of mixtures of nitrogen and methane, are large complex molecules and give amino acids after hydrolysis. Thus, tholins are also regarded as amino acid precursors. Amino acids are frequently detected in carbonaceous ▶ chondrites (meteorites), but the amount of amino acids recovered usually increases after hydrolysis, suggesting that some of amino acids detected in meteorites are present in the form of amino acid precursors.
butyric acid are found: (1) a-Amino butyric acid (aABA) a key intermediate in the biosynthesis of ophthalmic acid, (2) b-amino butyric acid (bABA), and (3) g-amino butyric acid (GABA) modulates the excitability of neurons of vertebrates and the muscle tone, (4) a-Amino isobutyric acid (aAIB), which is found in some fungal peptides. a, b, and g denote the position of the amino group relative to the carboxyl group in the ▶ amino acid molecule: a refers to the first, b the second, and g the third position. O NH2 OH β
OH
NH2 α-amino butyric acid
β-amino butyric acid O
See also ▶ Amide ▶ Aminoacetonitrile ▶ Amino Acid ▶ Complex Organic Molecules ▶ Chondrite ▶ Hydrolysis ▶ Tholins
O
α
H2N
γ OH γ-amino butyric acid
See also ▶ Amino Acid
Amino Alkanoic Acid ▶ Amino Acid
Amino Radical Synonyms
Amino Butyric Acid
Amidogen; Aminyl radical; NH2
Definition Synonyms Butyrine; Ethyl-glycine
Definition Amino butyric acid is the term for a variety of structural isomers of amino acids derived from n- or isobutyric acid with the chemical formula C4H9NO2. They belong to the substance class of amino acids, since they contain an amino functional group and a carboxylic acid functional group. In nature, several different isomers of amino
This triatomic radical is an important intermediary in the interstellar chemistry of ▶ ammonia, NH3. Like many light hydrides, its pure rotational transitions occur at far infrared/submillimeter wavelengths, making its observation difficult because of the opacity of the terrestrial atmosphere.
History The NH2 radical was first detected in the ▶ interstellar medium in 1993, at submillimeter wavelengths.
Aminonitrile
See also ▶ Ammonia ▶ Interstellar Medium
Aminoisobutyric Acid Synonyms
References and Further Reading
AIB
van Dishoeck EF, Jansen DJ, Schilke P, Phillips TG (1993) Detection of the Interstellar NH2 Radical. Astrophys J Lett 416:L83–L86
Definition
Aminoacetic Acid ▶ Glycine
Aminoacetonitrile
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Amino isobutyric acid (AIB) is an amino acid derived from isobutyric acid. There are two structural isomers of amino isobutyric acid (Fig. 1), a-aminoisobutyric acid (aAIB), which is achiral, and b-aminoisobutyric acid (bAIB), which has two ▶ stereoisomers, a D and L form. Both isomers have been found in carbonaceous chondrites, with aAIB often being one of the most abundant amino acids. This is thought to be significant as aAIB is not found in proteins, suggesting an extraterrestrial origin of this compound. However, several fungi are now known to synthesize this compound for incorporation in nonribosomally encoded peptide antibiotics. H2N
Synonyms
COOH
COOH
AAN; Cyanomethylamine; Glycinonitrile; NH2CH2CN H2N
Definition Aminoacetonitrile (IUPAC name 2-Aminoacetonitrile) is a (toxic) liquid at room temperature and standard pressure. It is a precursor of the simplest amino acid, ▶ glycine, which it forms by reaction with liquid water. It is also an intermediary in the ▶ Strecker synthesis of glycine. It was identified in the interstellar medium in 2008.
α AIB
β AIB
Aminoisobutyric Acid. Figure 1
See also ▶ Carbonaceous Chondrites (Organic Chemistry of ) ▶ Stereoisomers
History Although its rotational spectrum has been studied since the 1970s, and modeled explicitly for a search in the interstellar medium in 1990, aminoacetonitrile has only been detected recently in space in a large molecular cloud Sagittarius B2 (Sgr B2), at the center of the Galaxy (Belloche et al. 2008).
See also ▶ Glycine ▶ Molecular Cloud ▶ Molecules in Space ▶ Strecker Synthesis
Aminonitrile Definition An amino nitrile is a compound containing both an amino and a nitrile functional group. The simplest amino nitrile is ▶ cyanamide. a-amino nitriles, such as a-amino acetonitrile, are important intermediates in the ▶ Strecker synthesis of amino acids, as they are hydrolyzed consecutively to a-amino amides and finally to a-amino acids (Fig. 1).
See also References and Further Reading Belloche A, Menten KM, Comito C, Mu¨ller HSP, Schilke P, Ott J, Thorwirth S, Hieret C (2008) Detection of amino acetonitrile in Sgr B2(N). Astron Astrophys 482:179–196
▶ Amino Acid ▶ Amino Acid Precursors ▶ Cyanamide ▶ Strecker Synthesis
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Aminyl Radical
a-amino nitrile
a-amino amide
a-amino acid
Aminonitrile. Figure 1 Strecker amino acid synthesis via aminonitrile
Aminyl Radical ▶ Amino Radical
Ammonia ALEXANDER SMIRNOV Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
Synonyms Azane; Nitro-sil; Trihydrogen nitride
Keywords Prebiotic synthesis, nitrogen, abiotic reduction
Definition Ammonia (NH3) is a chemical compound composed of ▶ nitrogen and hydrogen which exists as a gas at standard conditions of temperature and pressure. In the trigonal pyramidal ammonia molecule, the lone electron pair of the nitrogen atom is responsible for its dipole moment (polarity) and its behavior as a base (proton acceptor). It dissolves readily in water and its protonation results in the formation of the conjugate acid ammonium ion (NH4+) with both species co-existing in a pH-dependent equilibrium (pKa NH4+ = 9.25 at 25 C). Liquid ammonia (boiling point 33.35 C at atmospheric pressure) is an ionizing solvent with physical properties and behavior similar to water (Lagowski 2007).
History Ammonia has been known since ancient times, although it was first isolated by Priestly in 1774. In 1785, Berthollet determined its composition. The Haber-Bosch process to synthesize ammonia from nitrogen and hydrogen was developed by Fritz Haber and Carl Bosch in 1909. It was found in space by Cheung et al. (1968)
and in comets by Altenhoff et al. (1983) using radioastronomical techniques.
Overview Ammonia has been detected throughout our solar system as well as in interstellar space. It is found as a gas in planetary atmospheres and in the solid form (ice) in cometary nuclei and planetary surfaces. Ammonia is hypothesized to be present in liquid form in a subsurface ocean on some outer planet satellites (e.g., ▶ Titan) where it effectively lowers the freezing point of water and offers exciting opportunities for our continued search for life in the Universe (Raulin 2008). On the early Earth, ammonia was likely a necessary precursor for prebiotic organic synthesis, such as the ▶ Strecker synthesis of amino acids. It was used as the nitrogen source in the Miller–Urey experiment which produced a suite of organic compounds such as amino acids from a mixture of reduced gases simulating the primordial atmosphere (Miller 1953). However, most current models suggest the early atmosphere was only mildly reducing, with the redox state linked to the evolution and oxidation state of the Hadean and early Archaean mantle, with ▶ dinitrogen (N2) as the dominant nitrogen species (Kasting and Catling 2003). It has been experimentally shown that ammoniacontaining environments are more efficient in organic synthesis than those dominated by dinitrogen in both aqueous and gaseous environments. This notion is not unexpected, considering that the strong triple bond (948 kJ.mol1) of the N2 molecule results in large reaction activation energy barriers even if the overall reaction is thermodynamically favored. The process of conversion (e.g., reduction) of unreactive dinitrogen to reactive and prebiologically useful ammonia is referred to as ▶ nitrogen fixation. Mechanisms suggested for abiotic ammonia production on the early Earth include reduction of atmospherically derived nitrite (NO2) by ferrous iron or iron bearing minerals (Summers and Chang 1993); hydrolysis of atmospherically produced HCN (Zahnle 1986), reduction of dinitrogen on mineral surfaces (sulfides, metals, alloys) in hydrothermal systems (Brandes et al. 2008;
Amorphous Carbon
Smirnov et al. 2008) or delivery of reduced nitrogen (nitride, N3) in iron meteorites followed by dissolution and reaction with H+(Smirnov et al. 2008). The concentrations of ammonia and/or ammonium ion in the prebiotic atmosphere and hydrosphere were likely controlled by mechanisms such as photolytic destruction, sequestration in clay minerals by substitution for K+ and formation of N-bearing organic molecules.
See also ▶ Amino Acid ▶ Dinitrogen ▶ Hydrogen Cyanide ▶ Mildly Reducing Atmosphere ▶ Nitrogen ▶ Nitrogen Fixation ▶ Prebiotic Chemistry ▶ Strecker Synthesis ▶ Titan
References and Further Reading Altenhoff WJ, Batrla W, Huchtmeirs WK (1983) “Radio observations of Comet 1983 D”, A.&A., 187:502 Brandes JA, Hazen RM, Yoder HS (2008) Inorganic nitrogen reduction and stability under simulated hydrothermal conditions. Astrobiology 8:1113–1126 Cheung AC, Rank DM, Townes CH, Thornton DD, Welch WJ (1968) “Detection of NH3 Molecules in the Interstellar Medium by Their Microwave Emission”, Physics Review Letters 21:1701 Kasting JF, Catling D (2003) Evolution of a habitable planet. Annu Rev Astron Astrophys 41:429–463 Lagowski JJ (2007) Liquid Ammonia. Synth React Inorg Me 37:115–153 Miller SL (1953) A production of amino acids under possible primitive Earth conditions. Science 117:528–529 Raulin F (2008) Astrobiology and habitability of Titan. Space Sci Rev 135:37–48 Smirnov A, Hausner D, Laffers R, Strongin D, Schoonen MA (2008) Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle. Geochem Trans 9:5 Summers DP, Chang S (1993) Prebiotic ammonia from reduction of nitrite by iron(II) on the early Earth. Nature 365:630–632 Zahnle K (1986) Photochemistry of methane and the formation of Hydrocyanic acid (HCN) in Earth’s early atmosphere. J Geophys Res 91:2819–2834
Amoebae
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(amoeboid or crawling-like movement). They represent a large diversity of unrelated groups of eukaryotes. Some are surrounded by a cell coat (glycocalyx); others are naked. Some are pathogens. Others produce a mineral test made of siliceous plates, an organic test, or an agglutinated test made of external organic or mineral particles (thecamoebae or testate amoebae). Some amoebae demonstrate social behavior when several individuals join to form complex multicellular structures such as slugs or fruiting bodies. The oldest fossil amoeba reported so far is 750 Ma old.
See also ▶ Eukaryote ▶ Protists
Amorphous Carbon AKIRA KOUCHI Institute of Low Temperature Science, Hokkaido University, Kita-ku, Sapporo, Hokkaido, Japan
Synonyms Glassy carbon; Vitreous carbon
Keywords Amorphous carbon, carbon star, carbonaceous chondrites, cometary particles, hydrogenated amorphous carbon, interplanetary dust particles
Definition Amorphous carbon is a noncrystalline solid allotropic form of carbon. There is no long-range order in the positions of the carbon atoms, but some short-range order is observed. Chemical bonds among atoms are a mixture of sp2 and sp3 hybridized bonds with a high concentration of dangling bonds. Because amorphous carbon is thermodynamically in a metastable state and the ratio of sp2 and sp3 hybridized bonds is variable, the properties of amorphous carbon vary greatly depending on the formation methods and conditions (Silva and Ravi 2003). Amorphous carbon is often abbreviated as “a-C”.
Overview Definition Amoebae are microscopic unicellular ▶ eukaryotes (▶ protists) able to deform their cytoplasm to move
In the laboratory, amorphous carbon can be produced by physical vapor deposition, chemical vapor deposition, sputtering, and ion irradiation of diamond or graphite.
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Amorphous Solid
The structure of amorphous carbon has been analyzed by X-ray and electron diffraction methods. The ratio of sp2 and sp3 hybridized bonds can be determined by electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Amorphous carbon whose dangling bonds are terminated with hydrogen is called hydrogenated amorphous carbon (a-C:H). Depending on the sp2 and sp3 ratios, the properties of amorphous carbon differ greatly. When a significant fraction of sp3 bonds is present in amorphous carbon, this is called tetrahedral amorphous carbon (ta-C) or diamondlike carbon. Tetrahedral amorphous carbon is hard, transparent, electrically insulating, and has higher density than a-C and a-C:H. In space, the occurrence of amorphous carbon is observed in circumstellar envelopes around carbon stars. When carbon stars lose mass to stellar winds, carbonaceous materials, such as polycyclic aromatic hydrocarbons (PAH), SiC, and amorphous carbon (a-C/a-C:H), that condense in their extended atmospheres are released to the interstellar medium. The conditions for the formation of amorphous carbon (a-C) grain have been investigated theoretically (Gail and Sedlmayr 1984), and the occurrence of amorphous carbon (a-C) and SiC has been deduced by observing the spectra of carbon stars (Blanco et al. 1990). Very recently, amorphous carbon has been found in various extraterrestrial materials. Cometary particles from comet 81P/Wild 2, captured by NASA’s Stardust mission, were analyzed by transmission electron microscopy, and a small amount of amorphous carbon grains less than 200 nm in size was found (Matrajt et al. 2008). In interplanetary dust particles (IDPs), investigated with Raman and infrared spectroscopy, the dominant type of carbon is found to be either a form of amorphous carbon (a-C) or of hydrogenated amorphous carbon (a-C:H), depending on the type of IDP (Mun˜oz Caro et al. 2006). It has been proposed that amorphous carbon in cometary particles and IDPs was formed by energetic processing (UV photons and cosmic rays) of icy grains in interstellar molecular clouds (Greenberg 1998; Kouchi et al. 2005). Amorphous carbon grains have also been found in the matrix of carbonaceous chondrites (Brearley 2008). These grains are essentially made of pure carbon embedded in an amorphous silicate matrix. It has been proposed that these grains were originally primitive macromolecular organic material that has undergone mild thermal metamorphism in the parent bodies of carbonaceous chondrites.
See also ▶ Insoluble Organic Matter ▶ Kerogen
▶ Molecular Cloud ▶ Organic Refractory Matter ▶ Polycyclic Aromatic Hydrocarbons
References and Further Reading Blanco A et al (1990) Amorphous carbon and carbonaceous materials in space II.—Astrophysical implications. Nuovo Cimento C 13: 241–247 Brearley AJ (2008) Amorphous carbon-rich grains in the matrices of the primitive carbonaceous chondrites, ALH77307 and Acfer 094. Lunar Planet Sci XXXIX:1494 Gail H-P, Sedlmayr E (1984) Formation of crystalline and amorphous carbon grains. Astron Astrophys 132:163–167 Greenberg JM (1998) Making a comet nucleus. Astron Astrophys 330:375–380 Kouchi A et al (2005) Novel routes for diamond formation in interstellar ices and meteoritic parent bodies. Astrophys J 626:L129–L132 Matrajt G et al (2008) Carbon investigation of two Stardust particles: A TEM, NanoSIMS, and XANES study. Meteor Planet Sci 43:315–334 Mun˜oz Caro GM et al (2006) Nature and evolution of the dominant carbonaceous matter in interplanetary dust particles: effects of irradiation and identification with a type of amorphous carbon. Astron Astrophys 459:147–159 Silva S, Ravi P (eds) (2003) Properties of amorphous carbon, institution of engineering and technology. INSPEC, London
Amorphous Solid Definition An amorphous solid lacks long-range order in the positioning of its constituent atoms; glass is an example. This contrasts with a crystalline solid, where such order is present, e.g., quartz. Both the ices and the silicates in interstellar grains are typically amorphous, although crystalline silicates are present in some circumstellar and cometary dust. The conversion of amorphous to crystalline water ice has often been invoked as an energy source in cometary outbursts at large heliocentric distances. The presence of crystalline silicates (presumably formed in the hot and dense inner solar system, possibly under the action of energetic particles from the young Sun) in ▶ comets, which are formed in the cold, outer part of the solar system, suggests that mixing of material was important in the ▶ solar nebula.
See also ▶ Comet ▶ Interstellar Dust ▶ Interstellar Ices ▶ Solar Nebula
Anabolism
Amphibolite Facies Definition Amphibolite facies refers to rocks formed under ▶ metamorphic conditions of moderate to high temperatures (400–600 C) and pressures (200–900 MPa). Rocks in most Archean gneiss belts are metamorphosed at the amphibolite facies. An amphibolite is a dense, and dark green to black rock generated by metamorphism under moderate temperature (ca. 500 C) and pressure (1 GPa) from a ▶ mafic (basaltic) protolith or more rarely from impure dolostone (carbonate rock). It consists mainly of hornblende, a type of amphibole, with lesser amounts of plagioclase, and in some cases biotite, epidote, titanite, and iron oxides. Amphibolite is a common constituent of metamorphosed oceanic crust or mafic intrusions in orogenic belts.
See also ▶ Metamorphic Rock ▶ Oceanic Crust
Amphiphile Synonyms Detergent; Lipid; Surfactant
Definition An amphiphile is a molecule having both a hydrophobic nonpolar group and a hydrophilic polar group. The nonpolar hydrophobic portion of the molecule is typically a hydrocarbon chain ranging from 10 to 20 or more carbons in length, and the polar moiety can be a carboxylic acid, phosphate, sulfate, amine, or alcohol group, among other possibilities. Examples of amphiphiles are fatty acids, detergents, and all lipids including phospholipids and sterols. All amphiphiles are surface active and form monolayers at air–water interfaces. Some amphiphiles, particularly those with a single hydrocarbon chain, assemble into micelles in aqueous solutions. Other amphiphiles with two hydrocarbon chains, for instance, phospholipids, typically self-assemble into bilayer membranes that are the permeability barriers defining most forms of cellular life. Amphiphilic molecules resembling fatty acids are present in carbonaceous meteorites, and are plausible membrane-forming components of the first living cells.
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See also ▶ Lipid Bilayer ▶ Self Assembly
Amplification (Genetics) Definition In molecular biology, amplification is a process by which a ▶ nucleic acid molecule is enzymatically copied to generate a progeny population with the same sequence as the parental one. The most widely used amplification method is ▶ Polymerase Chain Reaction (PCR). The result of a PCR amplification of a segment of ▶ DNA is called an “amplicon.” Nucleic acids can also be amplified in an isothermal reaction involving a reverse transcriptase, which copies ▶ RNA!DNA, and a DNA-dependent RNA polymerase, which transcribes DNA!RNA. Isothermal amplification does not generate double-stranded DNA, and it is mainly used for copying RNA. Ligasebased methods, including the so-called Ligase Chain Reaction (LCR), can be also used for specific DNA or RNA amplification. A fourth general method for nucleic acid amplification involves ▶ cloning the selected DNA molecule into bacterial or eukaryotic cells, allowing them to reproduce, and collecting the amplified DNA.
See also ▶ Cloning ▶ DNA ▶ Nucleic Acids ▶ Plasmid ▶ Polymerase Chain Reaction ▶ Replication (Genetics) ▶ RNA
Anabolism Synonyms Biosynthesis
Definition Anabolism is the subset of metabolic networks by which cell components are derived from organic or inorganic precursors. Anabolism requires a source of energy – usually in the form of ATP– and reducing power –usually as NADPH.
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Anaerobe
See also ▶ Assimilative Metabolism ▶ Catabolism ▶ Metabolism (Prebiotic)
Anaerobe JOSE´ LUIS SANZ Departamento de Biologı´a Molecular, Universidad Auto´noma de Madrid, Madrid, Spain
Synonyms Non-aerobic
Definition Anaerobes are organisms that do not require oxygen to obtain energy or to grow. Anaerobic metabolism is restricted to microorganisms, both prokaryotic (▶ Bacteria and ▶ Archaea) and eukaryotic (yeast, microsporidia), although an anaerobic multicellular organism (phylum Loricifera) has been recently discovered in marine sediments.
Overview There are two main categories of anaerobic microorganisms: (1) facultative anaerobes that can use oxygen for ▶ respiration if it is present, but in its absence obtain energy from ▶ fermentation (such as enterobacteria or yeasts), ▶ anaerobic respiration (some Pseudomonas, Thiobacillus, Bacillus, and many others): and anoxygenic photosynthesis (some proteobacteria) (2) obligate anaerobes, which never use oxygen. These can, in turn, be divided into two subcategories: (a) strict or obligate anaerobes, for whom oxygen is poisonous (i.e., oxygen is extremely toxic to ▶ methanogens); and (b) aeroduric or aerotolerant anaerobes that can grow in the presence of oxygen, although they never use it (i.e., bacteria involved in lactic acid fermentations). Cultivating strict anaerobes in the laboratory is an arduous task due to their extreme sensitivity to oxygen. Anaerobic jars or chambers are necessary for the isolation and growth of methanogenic archaea, sulfur reducing bacteria, or bacteroids. Some anaerobes are etiological agents of important diseases, such as tetanus (Clostridium tetanii), botulism (Clostridium botulinum), cholera (Vibrio cholera), salmonellosis and typhoid fever (Salmonella enterii), or peptic ulcers (Helicobacter pilori). Others, such as the lactic acid fermenters Lactobacillus and Lactococcus, are involved
in the production of food from dairy (yogurt, cheese, kefir, sourcream), vegetables (sauerkraut, olives, pickles) or meat (sausages). Some yeast (Saccharomyces) are responsible for bread, beer, and wine production. Finally, the methanogenic archaea carry out the last step of anaerobic degradation of organic matter in the absence of oxygen and, therefore, play a key role in anaerobic wastewater treatment and biomethanization of municipal solid waste processes. It is important to underline that Earth’s atmospheric O2 is of biological origin, and for an extended period of biological evolution, including the period in which the ▶ origin of life is suggested, the Earth remained strictly anaerobic. Anaerobes are of astrobiological interest because anaerobic conditions prevail on many planets, for instance, ▶ Mars.
See also ▶ Anaerobic Respiration ▶ Anoxygenic Photosynthesis ▶ Archea ▶ Bacteria ▶ Fermentation ▶ Mars ▶ Methanogens ▶ Origin of life ▶ Respiration
References and Further Reading Madigan M, Martinko J, Dunlap P, Clark D (2009) Brock Biology of microorganisms, 12th edn. Person Education, Inc, Benjamin Cummings, Chapters 18, 21 Sowers KR, Noll KM (1995) Techniques for anaerobic growth. In: Robb FT, Place AR, Sowers KR, Schreier HJ, Dassarma S, Flischmann EM (eds) Archaea. A laboratory manual: Methonogens. Cold Spring Harbor Laboratory Press, New York, pp 15–47 Willey JM, Sherwood LM, Woolverton CJ (2008) Prescott, Harley, and Kleins. Microbiology, 7th edn. McGraw-Hill Inc, Boston, Chap. 9
Anaerobic Photosynthesis ▶ Anoxygenic Photosynthesis
Anaerobic Respiration Definition Anaerobic ▶ respiration is a metabolic process in which oxidized organic compounds, such as fumarate, or an
Angular Momentum
inorganic molecules, such as nitrate, sulfate or ferric ion, serve as the terminal ▶ electron acceptor of an electron transport chain.
See also ▶ Aerobic Respiration ▶ Electron Acceptor ▶ Respiration
Analogue Sites ▶ Mars Analogue Sites ▶ Terrestrial Analogues
Angular Diameter Synonyms Angular size
Definition The angular diameter of a celestial object, seen from Earth, is the apparent diameter measured in angular units. Planets in the solar system have typical angular diameters between a few arcsec up to 50 arcsec (0.25 m-radians).
Angular Momentum JE´ROˆME PEREZ Applied Mathematics Laboratory, ENSTA ParisTech, Paris cedex 15, France
Keywords Conserved quantity, rotation
Definition In mechanics, angular momentum is the vector cross product between the position vector and the momentum vector of a point mass system. This definition can be extended to a solid by summation.
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boldfaced) and then they are relative to some reference system. This movement splits into two parts: a movement of translation and a movement of rotation. The amount of movement is measured by the linear momentum (impulsion) p = mv (for simple cases), which is a conserved quantity for a translation invariant system. The amount of rotation is measured by the angular momentum L = r p, which is a conserved quantity for a rotation invariant system. Note that the vector cross product a b = ab sin y n, where y is the smaller angle between a and b (0 y 180 ), a and b are the magnitudes of vectors a and b, and n is a unit vector perpendicular to the plane containing a and b in the direction given by the right-hand rule. Variations of velocity are produced by forces, in accordance with Newton’s second law for dynamics. If forces that apply on the point are aligned with the position vector r, they are called central, the system is invariant under rotation and its angular momentum is conserved. As ▶ gravitation is a central force, this conservation occurs frequently in astronomy. When the system is extended, such as a solid planet described by a distribution of points whose relative distances are fixed, the total angular momentum is the sum of the contributions of all these points. In this case, it is distributed between the spin of the planet itself and the angular momentum of its orbit. The conservation of angular momentum of celestial bodies is a fundamental tool for analyzing their properties. For example: ● In a two-body problem (see gravitation), if one of the two bodies is much heavier than the other, the conservation of angular momentum implies Kepler’s third law (see ▶ orbital resonance) and allows us to obtain the value of the large mass from observations of the period and of the semi major axis of the small mass. ● If a planet is found to rotate slower than expected, one can suspect that this planet is accompanied by a satellite, because the total angular momentum is shared between the planet and its satellite in order to be conserved. ● The tidal torque the Moon exerts on the Earth implies a slowing down of the rotation rate of Earth (at about 42 nsec/day). As a consequence, and because the total angular momentum of the whole system is conserved, the distance between Earth and Moon gradually increases by 4.5 cm/year.
Overview
See also
The movement of a point mass m is defined by its position r and its velocity v. These quantities are vectors (italic
▶ Gravitation ▶ Orbital Resonance
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Angular Size
Angular Size ▶ Angular Diameter
Anoxic Definition Anoxic is a term used to describe a condition, environment, or habitat depleted of oxygen.
See also
Anions
▶ Anaerobic
Definition Anions are atoms or molecules that have gained an electron (e.g., CN, OH, C4H). Neutral molecules with large electron affinities (EA’s) can attach an electron in a variety of chemical reactions, such as photo attachment. Long carbon-chain molecules have large EA values and C6H was the first anion discovered in the interstellar medium in 2006.
Anoxic Ocean ▶ Sulfidic Oceans
See also ▶ Photochemistry
Anoxygenic Photosynthesis Synonyms Anaerobic photosynthesis
Anorthosite Definition Anorthosite is a magmatic intrusive rock. It is light colored (or leucocratic) and has a medium to coarse grain size (or phaneritic). It is mainly composed of plagioclase (andesine, labradorite, bytownite) and minor pyroxene, olivine, and iron-titanium oxides (ilmenite, magnetite). Proterozoic anorthosite forms large massifs associated with granitoids (North America, Scandinavia). Archean coarse grained (megacrystic) anorthosite occurs in intrusions (dikes and sills) and flows of basaltic composition. Anorthosite is a common constituent of the lighter surfaces of the Moon called the lunar highlands or terrae. Formation of anorthosite requires concentration of plagioclase from mafic magma by flotation in a magma ocean (as is proposed to have occurred on the Moon), ascent of plagioclase-rich mushes, or low-pressure crystallization in magma chambers.
See also ▶ KREEP ▶ Mafic and Felsic ▶ Moon, The
Definition Anoxygenic ▶ photosynthesis is a bacterial photosynthesis that occurs under anaerobic conditions, using the photosynthetic electron transport chain in a non–cyclic mode and reduced inorganic electron donors, such as hydrogen sulfide, hydrogen, or ferrous ion, as ▶ electron donors. There are also cases of anaerobic photosynthetic electron transport chains acting cyclically, in this case, the generation of reducing power is not needed or it is decoupled from the photosynthetic reaction. The prototypical non– cyclic anoxygenic photosynthesis is present in green bacteria.
See also ▶ Electron Donor ▶ Oxygenic Photosynthesis ▶ Photosynthesis
Antarctic Continent ▶ Antarctica
Anticodon
Antarctica
other chemotherapeutic agents varies significantly and is the base of its pharmacological use.
Synonyms
See also
Antarctic continent
▶ Cell Membrane ▶ Cell Wall ▶ Gram Negative Bacteria ▶ Gram-Positive Bacteria ▶ Replication (Genetics) ▶ Ribosome ▶ Sporulation ▶ Transcription ▶ Translation
Definition Antarctica is the ice-covered continent located in the southern hemisphere of the Earth. Ninety-eight percent of its 14 Mkm2 surface area is covered by a 1.6 km thick ice sheet, corresponding to 90% of the world’s ice. Antarctica reached its present position 25 Ma ago after the breakup of the supercontinent ▶ Gondwana. It has been covered by ice for the past 15 Ma. About 400 ▶ subglacial lakes lie at the base of the continental ice sheet, the best known being the Lake Vostok, which is ca. 250 km long, 50 wide, and 200–800 m deep. Lake Vostok has remained isolated for 14 million years, making it a valuable analog for exploring deep biosphere niches. The surface of the ice sheets shares similarities with those of Jupiter’s moon ▶ Europa, and the ice-free Dry Valleys are an analog of the martian surface.
See also ▶ Europa ▶ Gondwana ▶ Mars Analogue sites ▶ Vostok, Subglacial Lake
Antibiotic Synonyms
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Antibody Definition Antibody is a complex ▶ protein (immunoglobulin) produced as a response to a chemical agent (antigen) as a part of a defensive system (immune system) in multicellular animals. The combination of antibody-antigen is specific (albeit not necessarily absolute), non-covalent, and reversible. There are many methodological applications of antibodies, either as a heterogeneous population of immunoglobulins (polyclonal antibodies) or as a homogeneous preparation (monoclonal antibodies). Antibodies show a broad applicability in biotechnology, including the development of affinity ▶ biosensors.
See also ▶ Biosensor ▶ Protein
Antimicrobial agents; Functional inhibitors
Definition Antibiotics are chemical substances produced by a wide range of microorganisms, among them fungi and bacteria, that kill or inhibit the growth of other organisms. A large number of antibiotics have been identified in nature, most of them as products of secondary metabolism. Antibiotic producers must be resistant to the active form of the antibiotic. Important targets of antibiotics are the synthesis of ▶ cell membrane and ▶ cell wall, ▶ replication, ▶ transcription and ▶ translation. Antibiotics are considered regulators of microbial populations rather than part of microbial warfare. The susceptibility of organisms to individual antibiotics or
Anticodon Definition Anticodon is a triplet of nucleotides in a tRNA, complementary to a codon in the mRNA.
See also ▶ Codon ▶ Genetic Code ▶ RNA ▶ Translation ▶ Wobble Hypothesis (Genetics)
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Antimicrobial Agents
Antimicrobial Agents ▶ Antibiotic
abiotic processes under those temperatures. If biogenic, microfossils could represent remains of thermophile chemothrophs living close to hydrothermal vents.
See also
Apex Basalt, Australia Definition The Apex Basalt is a ca. 3.46 Ga-old formation comprising tholeiitic pillow basalts, komatitic basalts, and komatiites intercalated with thin chert layers. It is located near Marble Bar in the ▶ Pilbara Craton of Western Australia. ▶ Microfossils, morphological biomarkers, and filamentous carbon structures in the lower chert beds have been interpreted as fossil prokaryotes (mainly cyanobacteria but also thermophiles) and are claimed to represent the oldest fossil record of life on Earth. For this reason, outcrops of this formation are considered one of the most important astrobiological sites on Earth.
See also ▶ Analogue Sites ▶ Apex Chert ▶ Apex Chert, Microfossils ▶ Microfossils ▶ Pilbara Craton
▶ Apex Basalt, Australia ▶ Apex Chert, Microfossils ▶ Archean Traces of Life ▶ Biomarkers, Isotopic ▶ Biomarkers, Morphological ▶ Carbon Isotopes as a Geochemical Tracer ▶ Cyanobacteria ▶ Pilbara Craton ▶ Rubisco ▶ Stromatolites
Apex Chert, Microfossils DANIELE L. PINTI1, WLADYSLAW ALTERMANN2 1 GEOTOP & Department of Earth and Atmospheric sciences, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada 2 Department of Geology, University of Pretoria, Pretoria, South Africa
Keywords
Apex Chert
▶ Apex chert, ▶ apex basalt, biomarkers, cyanobacteria, ▶ microfossils
Definition
Definition
The 3.465 Ga Apex Chert is a chert unit within the ▶ Apex Basalt in the Warrawoona Group, which is part of the oldest greenstone sequence in the Pilbara granite greenstone terrain. The Apex Basalt is stratigraphically below the Strelley Pool Chert, a unit known for hosting the oldest ▶ stromatolite on Earth. In Apex chert, small carbonaceous filaments with d13C as low as 22.5 to 25‰ were reported to represent evidence for ▶ cyanobacteria able to recycle inorganic carbon through ▶ RubisCO. The Apex Chert microfossils occur in rounded grains of microcrystalline silica, which have been interpreted as clasts in a conglomerate deposited in a wave-washed beach or a stream mouth, an ideal environment for cyanobacteria. But later work suggested that the chert was deposited from hydrothermal fluids with a temperature higher than 250 C, and that the micro-textures may result from
The Apex Chert is a bedded, microcrystalline silica (SiO2) deposit interlayered with pillow lavas and massive flows of the Apex Basalt Formation, ▶ Pilbara Craton, Western Australia. The basalts were dated at 3,465–3,458 Ma. The origin of the chert is disputed and interpretations of primary silica deposition on the ocean floor or alternatively secondary, hydrothermal silicification (chertification) of clastic or carbonate sedimentary and volcano-sedimentary rocks rival one another. The putative microfossils of Apex Chert are carbonaceous filaments found in ca. 3,465 Ma old chert lenses at the so-called “Schopf locality,” Chinaman Creek near Marble Bar.
Overview The name of the locality derives from that of the American paleontologist and paleobiologist, J. William (Bill)
Apex Chert, Microfossils
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suggested that these filaments were cyanobacteria. The hypothesis of cyanobacterial life 3.5 billion years ago implied, for uniformitarianism, that oxygenic photosynthesis might have acted very early in the Earth’s history and that life was already well advanced one billion years after the Earth formation. Though the evidence of microfossils in Apex Chert, as well as the paleoenvironmental and depositional conditions, have been later highly debated, the Apex chert and its putative microfossils have greatly contributed to a general interest in the origin and evolution of life and in many ways towards astrobiology, encouraging the search for extraterrestrial life, the improvement of our understanding of abiotic and biotic, evolutionary and taphonomic (processes by which organisms become fossilized) processes, and to the development
Schopf, who, at this site, reported 11 morphological taxa of prokaryotic, filamentous, and coccoidal microfossils embedded in chert clasts. At that time, the host rock was thought to constitute a sedimentary layer and later reinterpreted as sedimentary fill of a hydrothermal vein. The kerogenous (carbonaceous) filaments, up to several tens of micrometers long and 1–20 mm wide, show in most cases a typical cyanobacteria-like septation and terminal cells of varying morphology (Fig. 1). They form single cell chains and single coccoids that were interpreted as the Earth’s oldest microfossils (Schopf 1993). The morphology of the filaments and their organic carbon isotopic composition (d13C) ranging from 22‰ to 26‰ (whole rock measurements), and the sedimentary environment interpreted as shallow marine, strongly
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Apex Chert, Microfossils. Figure 1 Microfossils from the early Archean Apex Chert of Australia (From Schopf 1993). Stromatolite-like clasts (A, B, C) and microfossils (D-O, holotypes) with interpretative drawings. Except as indicated, magnification is as shown in (N). (a) shows clast, with boxed area enlarged in; (b) arrows point to minute filamentous microfossils. (c) is another clast showing stromatolite-like laminae. (d,e) Archaeotrichion septatum; (f) Eoleptonema apex; (g,h) Primaevifilum minutum; (i,j,k) Primaevifilum delicatum; (l,m,n,o) Archaeoscillatoriopsis disciformis
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of careful investigation methods and definition of unambiguous biosignatures.
Geology of the Schopf Locality The Apex Chert is a unit within the 4 km thick Apex Basalt Formation of the Salgash Subgroup of the Warrawoona Group of Pilbara Craton, Western Australia. The Apex Basalt consists of greenschist facies metamorphosed basalts, komatiitic basalts, and serpentinized peridotites and minor felsic volcaniclastic rocks with locally intruded dolerite sills which forms part of the Marble Bar greenstone belt. The “Schopf locality” (Schopf and Packer 1987; Schopf 1993), north of Marble Bar, is well known for its chert formations. The Apex Chert, with an assigned age of 3,465 5 Ma, is a bedded unit consisting of up to 10 m of white, gray, and black-layered chert, interbedded with felsic tuff, which contains sills of massive black silica. The bedded deposits overlie a swarm of weakly radiating black silica veins that extend up to 750 m stratigraphically down into metabasalts, but which do not penetrate above the bedded chert horizon, cut by an unconformity. The veins themselves are composed of several phases of intrusive silica that vary in color from very dark blue-black, through shades of blue-gray, to white. They migrate sidewards into the sedimentary layers replacing them with silica. The veins comprise dominantly massive dark blueblack silica, but can include multiple generations of dark gray to black silica to white quartz, core zones of felsic tuff breccia, and phreatomagmatic breccias with a jigsaw puzzle fit with exploded fragments at their tops (Van Kranendonk and Pirajno 2004). The bedded chert of the Schopf microfossil locality is the stratigraphically lowest of five bedded chert units within the pillowed Apex Basalt.
Microfossils at the Schopf Locality The “Schopf locality,” where the microfossils were discovered, is controversially within a black chert vein radiating from the bedded chert unit and is not from the bedded Apex Chert Unit itself. However, the fossiliferous specimen deposited by Schopf at the Natural History Museum, London, shows bedded structure and brownish color, implying that they come from the bedded part of the section. Microfossils were discovered in rounded grains of microcrystalline silica, apparently within one of the blue-black veins beneath the lowermost of the bedded chert units of the Apex Basalt Formation. Schopf (1993, 1999) interpreted the grains as clasts of a conglomerate deposited in a wave-washed beach or a stream mouth, an ideal environment for cyanobacteria. Fragments of stromatolites provide evidence that at least part of the clasts is of sedimentary origin. Schopf (1993)
observed hundreds of filaments, tens of micrometers long and 1–20 mm wide, some of them showing a septate division that morphologically resembled cyanobacteria (Fig. 1). Next to the filaments, hundreds of solitary unicell-like spheroidal structures resembling coccoidal microfossils were identified. Based on morphology, Schopf and Packer (1987) and Schopf (1992, 1993, 2006) recognized 11 morphotypes of putative microfossils in the Apex Chert, as listed below: 1. Narrow unbranched septate prokaryotic filaments Incertae Sedis cf. bacteria? (Archaaeotrichion septatum) 2. Narrow unbranched septate prokaryotic filaments Incertae Sedis cf. bacteria? (Eoleptonema apex) 3. Narrow unbranched septate prokaryotic filaments Incertae Sedis cf. bacteria? or cyanobacteria? (Primaevifilum minutum) 4. Narrow unbranched septate prokaryotic filaments Incertae Sedis cf. bacteria? or cyanobacteria? (Primaevifilum delicatulum) 5. Intermediate-diameter unbranched septate prokaryotic filaments Incertae Sedis cf. cyanobacteria? (Primaevifilum amoenum) 6. Intermediate-diameter unbranched septate prokaryotic filaments having disk-shaped medial cells Incertae Sedis cf. cyanobacteria? (Archaeoscillatorioposis disciformis) 7. Broad unbranched septate prokaryotic filaments having conical end cells Incertae Sedis cf. cyanobacteria? (Primaevifilum conicoterminatum) 8. Broad unbranched septate prokaryotic filaments having equant medial cells Incertae Sedis cf. cyanobacteria? (Primaevifilum laticellulosum) 9. Broad unbranched septate prokaryotic filaments Incertae Sedis cf. cyanobacteria? (Archaeoscillatorioposis grandis) 10. Broad unbranched markedly tapering septate prokaryotic filaments Incertae Sedis cf. cyanobacteria? (Primaevifilum attenuatum) 11. Broad unbranched septate prokaryotic filaments having hemispheroidal end cells Incertae Sedis cf. cyanobacteria? (Archaeoscillatorioposis maxima)
The Debate Remapping of the Marble Bar area including the Schopf locality, and detailed petrology and mineralogy of the Apex Basalt Formation and Apex Chert Unit (Brasier et al. 2002, 2005; Van Kranendonk and Pirajno 2004) revealed that the Apex Chert is largely a breccia infilling
Apex Chert, Microfossils
one of multiple generations of metalliferous hydrothermal veins. These veins, crosscut pillow basalts and feed into, and are continuous with the overlying stratiform-bedded chert unit of the Apex Basalt Formation. The discussion on the reality of Schopf ’s (1993) findings was triggered by claims that the filaments are branching, unlike prokaryotic filaments, and do not contain carbon. Simultaneously, it was claimed that life did not existed on Earth prior to ca. 2,500 Ma (Brasier et al. 2002, 2004). At reexamination, the Apex Chert, however, was found to contain cellular-preserved kerogenous microfossil remains, revealing advanced biostratonomic to metamorphic, taphonomic changes. It was suggested that thermal alteration is the cause of taphonomic changes in cyanobacterial microfossils, resulting in the present form of microfossil preservation in the Apex Chert (Kazmierczak and Kremer 2002). The preserved morphological variation indicates biological behavior and fulfills the requirements for microfossil recognition (Buick 1990). Claims of branching of the filaments or of incomplete, selective photomontages of the microstructures, mimicking a biological appearance (Brasier et al. 2002, 2004), result from misinterpretation of auto-montages of photographs taken at different depth of focus and superimposed on each other (Fig. 2). However, the lack of assessment of the geological context of the Apex microfossil assemblage together with the generally poor preservation due to possible biological, diagenetic, and metamorphic degradation, cast some doubts on the applied taxonomy in some cases (Altermann 2005). Scanning electron microscopy (SEM) showed the presence of metals (Ni, Cu, Zn, Sn), sulfides, barite, jarosite, alunite, phyllosilicates, and Fe oxides, suggesting a high-temperature hydrothermal environment where microbial life could hardly have survived (Brasier et al. 2002), except for chemoautolithotroph thermophiles (Brasier et al. 2006). Alternatively, an acid-sulfate epithermal environment of alteration, syn- or post-genetic with the precipitation of the chert, has also been suggested (van Kranendonk and Pirajno 2004). The abundance of sulfate and lack of argilitic alteration indicated depositional temperatures up to 350 C. Recently, Pinti et al. (2009) showed that medium-low temperature weathering processes could explain the mineralogy of the Apex chert so that high-temperature hydrothermal fluid–rock interactions are not required. These observations invigorated the debate on the ▶ biogenicity of the carbonaceous filaments and their putative inclusion in the Phylum Cyanobacteria. LaserRaman imagery of carbonaceous filaments (Schopf et al. 2002, 2007; Brasier et al. 2002) and disseminated
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carbonaceous (kerogeneous) matter in the Apex Chert (De Gregorio and Sharp 2006) gave controversial results. Schopf et al. (2002) interpreted the carbon as of biological origin. Brasier et al. (2002) proposed that it rather could be amorphous carbon reorganized in the form of filamentous strains after devitrification processes of the chert veins. De Gregorio and Sharp (2006) suggested that the carbonaceous material is similar in structure to microfossil kerogen, but may also be produced abiotically via Fischer-Tropsch-type (FTT) synthesis reactions, in an ancient hydrothermal vent. However, it has never been confirmed that the FTT process can produce particulate carbon. Three dimensional Confocal laser microscopy and Raman imagery demonstrated that the structures are indeed cellular-made filaments and coccoids (Schopf and Kudryavtsev 2005). The carbon isotopic composition is also controversial. The in situ, on single microfossils, measured d13C values from 27‰ to 34‰ could be related to photosynthesis (d13C = 25‰ 10‰; Schopf 2006), methanogenesis (Brasier et al. 2002), or abiotic FTT reactions (e.g., McCollom and Seewald 2006). Buick (1984) suggested that carbonaceous filaments in the silica swarm dykes of the North Pole and Marble Bar, including Apex Chert, were contaminants introduced in the microfracturing of the silica veins during the tectonic uplift of the region, 2.75 Ga ago. Pinti et al. (2009) observed branched microstructures suggesting postdepositional colonization of microcracks and fissures by microbes. However, Schopf ’s kerogenous microfossils are embedded in primary chert, in clasts deposited within the Apex Chert dyke or beds. Some clasts contain stromatolitic laminae and relict carbonate minerals and therefore must have been silicified during early diagenesis at their source of origin (Fig. 3). The hydrothermal chert distinctly differs from these clasts. However, in some places, hydrothermal recrystallization strongly affects the clasts and they become almost non-discernible from the hydrothermal chert matrix. The clasts are thus clearly older than the hydrothermal dike (Altermann and Kazmierczak 2003; Altermann 2007). The Apex Chert seems to be have been affected by several hydrothermal and supergene episodes of weathering, suggesting that it is unlikely to have preserved any early forms of life. Nevertheless, it contains stromatolitic clasts, and stromatolites are known within this stratigraphic succession. Moreover, even older microfossils and stromatolites were described from equally metamorphosed and altered shallow marine and hydrothermal environments of the underlying Dresser Formation (3,490 Ma) (Awramik et al. 1983; Ueno et al. 2001; Allwood et al. 2006).
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Apex Chert, Microfossils. Figure 2 The same specimen of Archaeoscillatorioposis disciformis filament photographed from the original material deposited by Schopf at the Natural History Museum, London. Upper left, as depicted by Schopf (1993), and upper right, as shown by Brasier et al. 2002: The difference created the impression that Schopf (1993) has manipulated the microphotographs in reality showing a branching and therefore impossibly cyanobacterial filament. The following lower micrographs show the same filament at different depth of focus within the several tens of mm thick petrographic thin section (from left to right and downwards). It becomes clear that Schopf (1993) has shown only one filament located closer to the surface of the section, while Brasier et al. (2002) have shown a sandwich photograph, including all depth of focus and exhibiting two filaments coincidentally superimposed one above the other within the thickness of the section (Photograph by W. Altermann in M. in Brasier’s lab, 2003)
Whether the carbonaceous filaments of J. William Schopf are genuine ancient fossilized prokaryotes (Schopf 1993; Altermann 2005), later biological contamination (Pinti et al. 2009), or abiotic products (Brasier et al. 2005), this rock is still the most fascinating challenge in Archean paleobiology and astrobiology.
Sharp 2006) and several among them were specifically developed to resolve the dilemma of the Schopf microfossils (e.g., Schopf et al. 2002, 2005). This rock represents thus the best challenge for determining the reality of very ancient traces of life and developing successful methodologies and strategies of search for extraterrestrial life.
Applications
Future Directions
Most of all the techniques developed for determining the biogenicity and singenicity of Archean traces of life have been tested on Apex Chert (e.g., De Gregorio and
The uniqueness of these microfossils constrains the use of destructive methods for determining the environmental context of deposition of this chert unit and the reality of
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Apex Chert, Microfossils. Figure 3 Stromatolitic clast within the microfossiliferus samples deposited by Schopf at the Natural History Museum, London, exhibiting microbial lamination, pyrite grains, and dark organic matter. The clast is cut by two parallel silica veinlets with pyrite enrichment (Photograph by W. Altermann in M. Brasier’ s lab, 2003)
these microfossils. New nondestructive techniques such as NanoSIMS imagery of microfossils (Oehler et al. 2009) or a combination of analytical techniques could be useful for determining whether the chemical structure of such putative microfossils is consistent with a biological origin (Derenne et al. 2008).
See also ▶ Apex Basalt, Australia ▶ Apex Chert ▶ Archean Traces of Life ▶ Biogenicity ▶ Biomarkers ▶ Biomarkers, Isotopic ▶ Biomarkers, Morphological ▶ Cyanobacteria, Diversity and Evolution of ▶ Dubiofossil ▶ Earth, Formation and Early Evolution ▶ Microfossils ▶ Microfossils, Analytical Techniques ▶ Pilbara Craton ▶ Pseudofossil ▶ Syngenicity
References and Further Reading Allwood AC, Walter MR, Kamber BS, Marshall CP, Burch IW (2006) Stromatolite reef from the Early Archaean era of Australia. Nature, 441, doi:10.1038 Altermann W (2005) The 3.5 Ga Apex fossil assemblage – consequences of an enduring discussion. 14th Internat. Conference on the Origin of Life, ISSOL’05, Beijing, China, 136–137 Altermann W (2007) The early Earth’s record of enigmatic cyanobacteria and supposed extremophilic bacteria at 3.8 to 2.5 Ga. In: Seckbach J (Ed) Algae and cyanobacteria in extreme environments. cellular
origin, life in extreme habitats and astrobiology (COLE) 11, pp 759–778, Springer, Berlin Altermann W, Kazmierczak J (2003) Archean microfossils: a reappraisal of early life on Earth. Res Microbiol 154:611–617 Awramik SM, Schopf JW, Walter MR (1983) Filamentous fossil bacteria from the Archean of Western Australia. Precambrian Res 20:357–374 Brasier MD, Green OR, Jephcoat AP, Kleppe AK, Van Kranendonk MJ, Lindsay JF, Steele A, Grassineau NV (2002) Questioning the evidence for Earth’s oldest fossils. Nature 416:76–81 Brasier M, Green O, Lindsay J, Steele A (2004) Earth’s oldest (similar to 3.5 Ga) fossils and the “Early Eden hypothesis”: questioning the evidence. Orig Life Evol Biosph 34:257–269 Brasier M, Green O, Lindsay J, Mcloughlin N, Steele A, Stoakes C (2005) Critical testing of Earth’s oldest putative fossil assemblage from the 3.5Ga Apex chert, Chinaman Creek, Western Australia. Precambrian Res 140:55–102 Brasier M, Mcloughlin N, Green O, Wacey D (2006) A fresh look at the fossil evidence for early Archaean cellular life. Phil T Roy Soc B 361:887–902 Buick R (1984) Carbonaceous filaments from North Pole Western Australia: are they fossil bacteria in Archaean stromatiolites? Precambrian Res 24:157–172 Buick R (1990) Microfossil recognition in archean rocks: an appraisal of spheroids and filaments from a 3500 M.Y. Old Chert-Barite Unit at North Pole, Western Australia. Palaios 5:441–459 De Gregorio BT, Sharp TG (2006) The structure and distribution of carbon in 3.5 Ga Apex chert: implications for the biogenicity of Earth’s oldest putative microfossils. Am Mineral 91:784–789 Derenne S, Robert F, Skrzypczak-Bonduelle A, Gourier D, Binet L, Rouzaud J-N (2008) Molecular evidence for life in the 3.5 billion year old Warrawoona chert. Earth Planet Sci Lett 272:476–480 Kazmierczak J, Kremer B (2002) Thermal alteration of the Earth’s oldest fossils. Nature 420:447–478 McCollom T, Seewald J (2006) Carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions, Earth Planet. Sci Lett 243:74–84 Oehler DZ, Robert F, Walter MR, Sugitani K, Allwood A, Meibom A, Mostefaoui S, Selo M, Thomen A, Gibson EK (2009) NanoSIMS: insights to biogenicity and syngeneity of Archaean carbonaceous structures. Precambrian Res 173:70–78
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Pinti DL, Mineau R, Clement V (2009) Hydrothermal alteration and microfossil artefacts of the 3, 465-million-year-old Apex chert. Nat Geosci 2:640–643 Schopf JW (1992) Paleobiology of the Archean. In: Schopf JW, Klein C (eds) The Proterozoic biosphere. Cambridge University Press, New York, pp 25–39 Schopf JW (1993) Microfossils of the early Archean apex chert: new evidence of the antiquity of life. Science 260:640–646 Schopf JW (1999) The cradle of life. Princeton University Press, New York Schopf WJ (2006) Fossil evidence of Archaean life. Phil T Roy Soc B 361:869–885 Schopf JW, Kudryavtsev AB (2005) Three-dimensional Raman imagery of Precambrian microscopic organisms. Geobiology 3:1–12 Schopf JW, Packer BM (1987) Early Archean (3.3- billion to 3.5-billionyear-old) microfossils from Warrawoona Group, Australia. Science 237:70–73 Schopf JW, Kudryavtsev AB, Agresti DG, Wdowiak TJ, Czaja AD (2002) Laser-Raman imagery of Earth’s earliest fossils. Nature 416:73–76 Schopf JW, Kudryavtsev AB, Agresti DG, Czaja AD, Wdowiak TJ (2005) Raman imagery: a new approach to assess the geochemical maturity and biogenicity of permineralized Precambrian fossils. Astrobiology 5:333–371 Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB (2007) Evidence of Archean life: stromatolites and microfossils. Precambrian Res 158:141–155 Ueno Y, Maruyama S, Isozaki Y, Yurimoto H (2001) Early Archean (ca. 3.5 Ga) microfossils and 13C-depleted carbonaceous matter in the North Pole area, Western Australia. In: Nakashima S, Maruyama S, Brack A, Windley BF (eds) Field occurrence and geochemistry, in geochemistry and the origin of life. Universal Academic Press, Tokyo, pp 203–236 Van Kranendonk MJ, Pirajno F (2004) Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits. Implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia. Geochem Explor Environ Anal 4:253–278
Aphelion
Definition In interstellar chemistry, apolar molecules are molecules lacking a permanent electric dipole moment. The lack of a dipole moment results from the symmetry of the charge density distribution in the molecule.
See also ▶ Polar Molecule
Apollo (Asteroid) Definition An Apollo ▶ asteroid is a near-Earth asteroid with a semimajor axis of more than 1 astronomical unit (AU, the mean Sun–Earth distance) and a perihelion distance of less than 1.017 AU (the Earth’s aphelion distance). The ▶ orbit of such an asteroid may intersect that of the Earth, giving rise to an impact hazard. Apollo asteroids are named after the asteroid 1862 Apollo, which is the first to be discovered having these dynamical characteristics.
See also ▶ Asteroid ▶ Near-Earth Objects ▶ Orbit
Apollo Mission GERDA HORNECK German Aerospace Center (DLR), Institute of Aerospace Medicine, Cologne, Germany
Definition The aphelion is the point on a body’s orbit around the Sun (planets, comets, asteroids) where the body is farthest from the Sun.
Synonyms
See also
Keywords
▶ Periastron
Biological effects of space, exposure experiments, human space flight, lunar missions
Apolar Molecule Synonyms Nonpolar molecule
NASA lunar landing mission
Definition The Apollo missions were the heart of NASA’s manned Lunar Landing Program that took place between 1969 and 1972 with 6 successful landings of 12 astronauts on the Moon.
Apollo Mission
History On July 20, 1969, the astronauts N.A. Armstrong and E.E. Aldrin were the first humans to set foot on the Moon. Herewith NASA had reached the ambitious goal of its manned Lunar Landing Program. It was made possible by the strong commitment of the United States to manned lunar exploration with President J.F. Kennedy’s announcement in 1961 of sending an American safely to the Moon before the end of the decade, and at same time the progress in the technical capabilities of space transportation. The Apollo program ultimately placed 12 men on the lunar surface. In 1972, with Apollo 16 and 17, the era of human exploration beyond Earth orbit was terminated and so far it has not been resumed.
Overview The Apollo missions to the Moon were performed between 1968 and 1972 (Table 1). The Apollo missions were the first and so far only human space missions beyond Earth’s orbit. They provided in depth knowledge of the geology of the Moon (Schaber 2005) and biomedical data on human health issues during space flight (Johnston et al. 1975). Biological responses to the parameters of outer space were studied in the following experiments: ● ▶ Biostack experiments on board of the Apollo 16 and 17 Command Module on the responses of a
variety of biological systems in resting state to the heavy ions component of cosmic rays (Bu¨cker and Horneck 1975) ● ALFMED experiment during the Apollo 16 and 17 mission that demonstrated that the light flash phenomenon observed by the crew members after dark adaptation was attributed to the passage of cosmic ray ions through the retina of the eye (Johnston et al. 1975; Benton et al. 1977) ● BIOCORE experiment during the Apollo 17 mission that studied brain effects in pocket mice caused by the passage of single heavy ions (▶ HZE particles) of cosmic radiation (Klein 1981) ● MEED during the Apollo 16 mission that studied the effects of space vacuum and ▶ solar UV radiation on different functions of ▶ microorganisms (Taylor 1974) The radiobiological experiments performed during the Apollo missions are the only ones that studied the biological effects of the complete interplanetary radiation field, not attenuated by the Earth’s magnetic field.
See also ▶ Biostack ▶ Cosmic Rays in the Heliosphere ▶ HZE Particle ▶ MEED
Apollo Mission. Table 1 Summary of human flights in the Apollo program to the Moon Apollo mission Mission description
Launch date Stay lunar day/month/year surface (h)
Astronauts
7
Earth orbit test
11/9/68
–
Schirra, Cunningham, Eisele
8
Circumlunar flight
21/12/68
–
Borman, Lovell, Anders
9
Earth orbit test of LM
3/3/69
–
McDivitt, Scott, Schweickert
10
Circumlunar flight, LM separation
18/5/69
–
Stafford, Cernan, Young
11
Lunar landing, sample return
16/7/69
22.2
Armstrong, Collins, Aldrin
12
Lunar landing, surface experiment package
14/11/69
31.5
Conrad, Gordon, Bean
13
Lunar landing aborted
11/4/70
–
Lovell, Swigert, Haise
14
Lunar landing, highland exploration
31/1/71
33.5
Shepard, Roosa, Mitchell
15
Lunar landing and rover, geological sampling
26/7/71
67
Scott, Worden, Irwin
16
Lunar landing and rover, geological sampling, Biostack and MEED experiments
16/4/72
71
Young, Mattingly, Duke
17
Lunar landing and exploration of the Moon’s geology and history, Biostack experiments
7/12/72
75
Cernan, Evans, Schmitt
LM = lunar module.
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▶ Microorganism ▶ Moon, The ▶ Radiation Biology ▶ Solar UV Radiation (Biological Effects) ▶ Space Vacuum Effects
References and Further Reading Benton EV, Henke RP, Peterson DD (1977) Plastic nuclear track detector measurements of high-LET particle radiation on Apollo, Sskylab, and ASTP space missions. Nucl Track Detect 1:27–32 Bu¨cker H, Horneck G (1975) The biological effectiveness of HZE-particles of cosmic radiation studied in the Apollo 16 and 17 Biostack experiments. Acta Astronaut 2:247–264 Golombeka MP, McSween Jr HY (2007) Mars: landing site geology, mineralogy and geochemistry. In: McFadden L-A, Weissman PR, Johnson TV (eds) Encyclopedia of the solar system, 2nd edn. Elsevier, Amsterdam, The Netherlands, pp 331–348 Johnston RS, Dietlein F, Berry CA (eds) (1975) Biomedical results of Apollo. NASA SP-368. NASA, Washington, DC Klein HP (1981) U.S. biological experiments in space. Acta Astronaut 8:927–938 Schaber GG (2005) The U.S. geological survey, branch of astrogeology – a chronology of activities from conception through the end of project Apollo (1960–1973). U.S. Department of the Interior U.S. Geological Survey, Open-File Report 2005-1190. http://www.legislative.nasa. gov/alsj/Schaber.html Taylor G (1974) Space microbiology. Ann Rev Microbiol 28:121–137
Aptamer Definition An aptamer is a target-binding ▶ nucleic acid molecule obtained by ▶ in vitro evolution. Targets specifically recognized by DNA or RNA aptamers cover a wide range of size and complexity, including simple ions, small molecules – such as amino acids, nucleotides, antibiotics, or metabolites – ▶ peptides, proteins, nucleic acids, macromolecular assemblies, viruses, organelles, or even whole cells. The sensitivity and specificity of the molecular recognition between an aptamer and its target rival those of the antibody-antigen pairs. The preparation of a desired aptamer is currently easy and quick, since only 6–15 rounds of in vitro selection or evolution are usually required. These reasons, together with their cost-effectiveness, make aptamers very useful tools in biotechnology with increasing applications in ▶ biosensing, diagnostics, and therapy. Although aptamers are artificial molecules, riboswitches have been considered as “natural aptamers” embedded in messenger RNAs since they act as regulatory elements for gene expression by directly sensing small effector molecules. In addition to nucleic acid aptamers, peptide aptamers have been obtained from randomized peptide libraries using different modifications of in vitro selection procedures.
See also
Apparent Motion ▶ Proper Motion
▶ Biosensor ▶ Evolution (Biological) ▶ Evolution, In Vitro ▶ Nucleic Acids ▶ Peptide ▶ Ribozyme ▶ RNA World
Apsidal Angle Definition In planetary dynamics, the apsidal angle is the angle between the directions of closest approach (the apse) of two planets, as measured from the origin of the coordinate system (usually the center of the star). This angle may change with time and is coupled to the eccentricity of the orbits. The apsidal angle may oscillate about a fixed value (called apsidal libration) or circulate.
See also ▶ Secular Dynamics ▶ Secular Resonance
Arachnoid Definition Presumably volcanic landform only seen on the surface of ▶ Venus. Arachnoids get their name from their resemblance to spider webs. They appear as concentric ovals surrounded by a complex network of fractures, and can span 200 km. Over 30 arachnoids have been identified on Venus, so far.
See also ▶ Venus
Archea
Archaeobacteria ▶ Archea
Archea ANTONIO VENTOSA, RAFAEL R. DE LA HABA Department of Microbiology and Parasitology, Faculty of Pharmacy, University of Sevilla, Sevilla, Spain
Synonyms Archaeobacteria
Keywords Domain, evolution, extremophiles, molecular adaptation, phylogeny, 16S rRNA sequencing
Definition The Archaea are a phylogenetically coherent group of ▶ prokaryotes that have a different organization than the ▶ Bacteria.
History Woese and Fox (1977) proposed that prokaryotes were not a monophyletic group. Based on the comparison of their small subunit ribosomal RNA sequences, the prokaryotes comprise two distinct evolutionary lineages that are represented by the ▶ Bacteria and the Archaea (that formerly were designated as Archaebacteria [Woese et al. 1978]). The concept of a third ▶ domain of life, which explained several structural, metabolic, and molecular differences with respect to other prokaryotes, was initially poorly accepted by the scientific community. However, the concept of the Archaea was advanced through studies and meetings carried out by O. Kandler, W. Zillig, and K.O. Stetter, among others. The Archaea have similarities and are considered phylogenetically more closely related with the ▶ Eukarya (Woese et al. 1990).
Overview Archaea have distinct molecular characteristics that clearly distinguish them from the Bacteria and the Eukarya, and evolutionary studies have highlighted their role on the development of life on our planet. Archaea have been associated with ▶ extreme environments and many of
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them are extremophilic microorganisms, showing interesting characteristics and applications for industrial and other purposes. Many of them are considered to be microorganisms that are able to grow on the limits of life. Their ability to thrive in extreme environments has expanded the horizons for Astrobiology as they are considered counterparts for extraterrestrial life. The Archaea are characterized by a cellular morphology similar to those of most Bacteria (rods, cocci, irregular cells, etc.). However, myceliar or multicellular stages with cellular differentiation have not been described. On the contrary, unique morphologies have been described for some Archaea, such as the square flat cells of some haloarchaea (Haloquadratum walsbyi) or amoeba-like cells (Thermoplasma and other microorganisms). Other characteristics of the Archaea that define their differential status with respect to the other living organisms are (1) the presence of phytanyl ether instead of fatty acid ester lipids in their membranes; (2) the absence of peptidoglycan (murein) in their cell walls and a frequent presence of proteinaceous S-layers (only a few have a polysaccharide cell wall), as well as the absence of a periplasmic space; (3) their complex DNA-dependent RNA polymerases (early in vitro studies using several inhibitors showed that the transcription machinery in Archaea is more closely related to that of Eukarya than to Bacteria); the sequences of the archaeal RNA polymerases resemble some eukaryotic RNA polymerases and consist of up to 13 different units; (4) although the translation machinery of Archaea is similar to that of bacteria (70S ribosomes with 50S and 30S subunits, similar length ribosomal RNAs, transcriptional and translational coupling, etc.), there are an important number of specific features not present in Bacteria, some of which are specific to Archaea while others are similar to Eukarya. For example, almost all antibiotics that inhibit bacterial translation are ineffective in the Archaea; Bacteria use N-formyl-methionyltRNA for translational start codons, while Archaea use unmodified initiator methionine in translation, similar to Eukarya. Besides, Archaea and Eukarya share a common characteristic, elongation factor 2 (EF-2), which is ADP-ribosylated by diphtherial toxin. The Archaea are subdivided into five phyla, of which two, the Crenarchaeota and the Euryarchaeota, are most extensively studied. The classification of Archaea has been widely discussed and several proposals have been published. The most widely accepted include the Archaea as a higher taxon with the range of “Domain,” which includes the following five phyla: Crenarchaeota, Euryarchaeota, Korarchaeota, Nanoarchaeota, and
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Thaumarchaeota (Euzeby 2010). The phylum Crenarchaeota includes a single class, Thermoprotei, with 4 orders: Acidilobales, Desulfurococcales, Sulfolobales, and Thermoproteales. The phylum Euryarchaeota includes eight classes: Archaeoglobi, Halobacteria, Methanobacteria, Methanococci, Methanomicrobia, Methanopyri, Thermococci, and Thermoplasmata. The phylum Korarchaeota includes non-cultivated Archaea designated as “Candidatus Korarchaeum,” the phylum Nanoarchaeota includes the genus “Nanoarchaeum,” and finally, the phylum Thaumarchaeota includes the genus “Cenarchaeum.” The phylum Euryarchaeota includes two of the most typical groups that were identified in the early studies by Woese and coworkers as members of the Archaea: the ▶ methanogens and the haloarchaea (also designated as halobacteria). The methanogenic Archaea are anaerobic organisms that produce methane as the major end product of their metabolism. Phylogenetically methanogens are very diverse and are represented by a large number of species belonging to many genera, grouped in 12 families within 6 orders. They are found on a variety of anoxic environments such as ocean and lake sediments, hydrothermal vents, animal digestive tracts, anaerobic sludge digesters, etc. The typical growth compounds of methanogens are H2 and CO2, or short-chain (C1–C5) organic compounds (formate, acetate, ethanol, trimethylamine, etc.). H2 is used as electron donor for CO2 reduction, and electrons can also be derived from formate, CO, or specific alcohols. Among the microorganisms that have been used as models for studying methanogenesis are species of the genera Methanobacterium, Methanothermobacter, Methanobrevibacter, Methanosarcina, Methanococcus, among others (Dworkin et al. 2002; Madigan et al. 2008). Haloarchaea are represented by a group of extremely halophilic aerobic Archaea, which taxonomically are placed within a single class, Halobacteria, order Halobacteriales, family Halobacteriaceae (Grant et al. 2001). Currently they are represented by more than 20 genera and a large number of species that are characterized by their Na+ requirements. They are considered to be organisms that are able to grow under higher salt concentrations, in saturated NaCl habitats. Their optimal NaCl requirements are in the range 3.5–4.5 M NaCl and they are not able to grow in media without NaCl, thus, they have a specific requirement for NaCl, which has led to detailed studies of their mechanisms of haloadaptation. In contrast to most other prokaryotes, which accumulate intracellular organic compounds designated as ▶ compatible solutes,
haloarchaea compensate for the high salt concentration in the environment by accumulating ions, mainly up to 5 M KCl. They are normal inhabitants of hypersaline environments, being the predominant microbiota of saturated ponds of salterns and salt lakes (they may reach high cell densities, > 107 cell ml-1); they are also found in salt or salted products (salted fish or meats, salted fermented foods), salt deposits (mines), salted hides and saline soils. Most haloarchaea grow at neutral pH values but some species are haloalkaliphilic, being able to grow optimally at alkaline pH and inhabiting soda lakes. Other typical features of haloarchaea are their production of red- to pink-pigmented colonies due to the presence of bacterioruberins (C50 carotenoids), although there are a few exceptions, the presence, in some of them, of retinal-based pigments (bacteriorhodopsin), that act as a proton pump driven by light energy, or the presence of typical archaeal polar lipids, with ether-linked phosphoglycerides that can be easily detected by thinlayer chromatography (a feature that is widely used for the taxonomic differentiation of most genera of haloarchaea) (Grant et al. 2001). Haloarchaea are excellent models for the study of the molecular biology and other structural features of Archaea, as well as their mechanisms of adaptation to extreme conditions of salinity, alkaline pH, and moderate temperature, and several species have been used for such purposes due to their ease of manipulation under laboratory conditions: they grow in complex media (with the appropriate salt content) under aerobic conditions using the standard procedures utilized for most non-fastidious prokaryotes. Some species used for such studies include Halobacterium salinarum, Haloarcula marismortui, Haloferax volcanii, and more recently, the square haloarchaeon Haloquadratum walsbyi (recently isolated and referred to as “Walsby’s square bacterium”). In addition, several biotechnological applications have been suggested, such as the commercial production of bacteriorhodopsin, the production of extracellular hydrolytic enzymes or exopolysaccharides, the use of polyhydroxyalkanoates (PHAs) as bioplastics, or the production of halocins (archaeocins, proteinaceous archaeal antimicrobials). With a few bacterial exceptions, most ▶ hyperthermophiles (defined as organisms showing optimal growth at 80 C or higher) are species of Archaea. They are inhabitants of hot springs, solfataric and volcanic areas, deep-subsurface aquifers, submarine vents (“black smokers”), etc. Hyperthermophiles include several methanogens, as well as members of a variety of genera
Archean Biosignatures
of the Archaeoglobales, Thermococcales, Desulfurococcales, Thermoproteales, or Sulfolabales. They are excellent models for the study of the metabolisms of sulfur and inorganic sulfur compounds; many species use inorganic sulfur compounds as electron acceptors or donors. Some of the most hyperthermophilic organisms known are Pyrolobus fumarii (optimal growth at 106 C, range 90–113 C), Pyrodictium occultum (optimal growth at 105 C, range 85–110 C), Pyrococcus furiosus and Pyrococcus woesei (optimal growth at 100–103 C, range 70–105 C), Pyrobaculum aerophylum (optimal growth at 100 C, range 75–104 C), and Pyrobaculum islandicum (optimal growth at 100 C, range 74–102 C). The phylum Nanoarchaeota is known for a single species, Nanoarchaeum equitans, to date a hyperthermophilic archaeon that lives in a symbiotic association with the Crenarchaeote Ignicoccus, a sulfur-dependent anaerobic hyperthermophile. The cells are spherical and only about 400 nm in diameter; they grow attached to the surface of a specific archaeal host (Hubber et al. 2002). This archaeon was isolated from a submarine hot vent, but recent studies have shown that ▶ nanoarchaea may be widely dispersed in hyperthermophilic and mesophilic halophilic environments (Casanueva et al. 2008).
See also ▶ Bacteria ▶ Compatible Solute ▶ Crenarchaeota ▶ Domain (Taxonomy) ▶ Eukarya ▶ Euryarchaeota ▶ Extreme Environment ▶ Halophile ▶ Hyperthermophile ▶ Korarchaeota ▶ Membrane ▶ Methanogens ▶ Nanoarchaeota ▶ Phylogenetic Tree ▶ Prokaryote
Brochier-Armanet C, Boussau B, Gribaldo S, Forterre P (2008) Mesophilic crenarchaeota: proposal for a third archaeal phylum, the Thaumarchaeota. Nat Rev Microbiol 6:245–252 Casanueva A, Galada N, Baker GC, Grant WD, Heaphy S, Jones B, Yanhe M, Ventosa A, Blamey J, Cowan DA (2008) Nanoarchaeal 16S rRNA gene sequences are widely dispersed in hyperthermophilic and mesophilic halophilic environments. Extremophiles 12: 651–656 Cavicchioli R (ed) (2007) Archaea: molecular and cellular biology. ASM Press, Washington, DC Dworkin M, Falkow S, Rosenberg E, Schleifer K-H, Stackebrandt E (eds) (2002) The prokaryotes: an evolving electronic resource for the microbiological community, 3rd edn., release 3.19 ed. SpringerVerlag, New York. http://link.springer-ny.com/link/service/books/ 10125/ Euzeby JP (2010) List of prokaryotic names with standing in nomenclature. http://www.bacterio.cict.fr/ Garret RA, Klenk H-P (eds) (2007) Archaea: evolution, physiology and molecular biology. Blackwell, Oxford Grant WD, Kamekura M, McGenity TJ, Ventosa A (2001) Class III. Halobacteria class. nov. In: Boone DR, Castenholz RW, Garrity GM (eds) Bergey’s manual of systematic bacteriology, vol 1, 2nd edn, The Archaea and the deeply branching and phototrophic Bacteria. Springer, New York Hubber H, Hohn MJ, Rachel R, Fuchs T, Wimmer VC, Stetter KO (2002) A new phylum of archaea represented by a nanosized hyperthermophilic symbiont. Nature 417:63–67 Kates M, Kushner DJ, Matheson AT (1993) The biochemistry of Archaea (Archaebacteria). Elsevier, Amsterdam Madigan MT, Martinko JM, Dunlap PV, Clark DP (2008) Brock biology of microorganisms, 12th edn. Benjamin Cummings, San Francisco Pfeifer F, Palm P, Schleifer K-H (1994) Molecular biology of Archaea. Gustav Fischer Verlag, Stuttgart Robb FT, Place AR, Sowers KR, Schreier HJ, DasSarma S, Fleischmann EM (eds) (1995) Archaea: a laboratory manual. Cold Spring Harbor, New York Ventosa A (2006) Unusual micro-organisms from unusual habitats: hypersaline environments. In: Logan NA, Lappin-Scott HM, Oyston PCF (eds) Prokaryotic diversity: mechanisms and significance. Cambridge University Press, Cambridge Woese CR, Fox GE (1977) The phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proc Natl Acad Sci USA 74:5088–5090 Woese CR, Kandler O, Wheelis ML (1990) Towards a natural system of organisms: proposal for the domains Archaea, Bacteria and Eukarya. Proc Natl Acad Sci USA 87:4576–4579 Woese CR, Magrum LJ, Fox GE (1978) Archaebacteria. J Mol Evol 11:245–251 Woese CR, Wolfe RS (eds) (1985) The bacteria: a treatise on structure and function, vol VIII, Archaeabacteria. Academic Press, New York
References and Further Reading Blum P (ed) (2001) Archaea: ancient microbes, extreme environments, and the origin of life. Academic Press, San Diego Blum P (ed) (2008) Archaea: new models for prokaryotic biology. Caister Academic Press, Norfolk Boone DR, Castenholz RW, Garrity GM (2001) Bergey’s manual of systematic bacteriology, vol 1, 2nd edn, The Archaea and the deeply branching and phototrophic Bacteria. Springer, New York
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Archean Biosignatures ▶ Archean Traces of Life
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Archean Drilling Projects
Archean Drilling Projects NICHOLAS ARNDT Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France
Definition Several scientific drilling programs that have been carried out in ▶ Archean terrains in the past decade and others that are planned in the near future. The aim of most of these drilling projects is to recover relatively well-preserved rock samples from below the present weathering profile and to obtain continuous rock cores that retain soft or friable units that outcrop poorly at the surface. Astrobiologyrelated studies such as search of pristine morphological or chemical traces of early life form an important part of these projects.
Overview Both volcanic and sedimentary sequences have been targeted and the recovered cores have been analyzed to investigate conditions at the surface of the Archean Earth: the composition, temperature, and redox state of the Archean ocean and atmosphere; the volcanic and sedimentary processes that operated early in Earth history; and above all, to search for evidence of primitive life. The focus has been the ▶ Pilbara craton in Western Australia, where four separate programs have been carried out, each involving collaboration between geologists from Australian universities, the Western Australian Geological Survey, and foreign agencies. The four programs are (1) the Archean Biosphere Drilling Project (ABDP) cosponsored by several Japanese Universities, (2) the Deep Time Drilling Project (DTDP) of the NASA Astrobiology Institute, (3) the Pilbara Drilling Project (PDP) of IPG Paris, and (4) Dixon Island–Cleaverville Drilling Project (DXCLDP) supported by the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT). Two programs have focused on the transition from the Archean to the ▶ Proterozoic. A series of short holes have been drilled in Russian Fennoscandia to samples the 500 million-year interval defining the Archean– Palaeoproterozoic transition and the Agouron Griqualand Paleoproterozoic Drilling Project straddled a similar interval in Northern Cape of South Africa.
See also ▶ Archea ▶ Archean Traces of Life
▶ Barberton Greenstone Belt ▶ Microfossils ▶ Pilbara Craton ▶ Proterozoic (Aeon)
Archean Environmental Conditions NICHOLAS ARNDT Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France
Keywords Chert, komatiite, oceans, sediment, traces of life
Definition The general term “Archean environmental conditions” refer to the geological, physical, and chemical conditions of the surface of the Earth during the ▶ Archean eon. The surface of the Archean Earth was in many ways similar to that of today. Oceans likely covered most of the globe, but there were also regions of dry land. Oceanic crust was almost as thick as ▶ continental crust, mountain ranges were not very high, parts of oceanic ridges and plateaus (thick, piles f flat-lying lava flows) were emergent. Geological processes such as volcanism, erosion, and sediment deposition operated as now, but were influenced by a lack of vegetation, higher ocean temperatures, and a hotter, more aggressive, acidic atmosphere.
Overview The surface of the Archean Earth was in many ways similar to that of today. Oceans covered most of the globe, but there were also regions of dry land. The total area covered by oceans was greater than now, for two reasons. First, the volume of continental crust may have been less, if this crust had grown progressively through time (Benn et al. 2006). Second, the oceans might have been more voluminous because high temperatures in the mantle (Nisbet et al. 1993) destabilized hydrous minerals and drove water to the surface. Mountain ranges existed but were not as high as those of today because the continental crust was heated internally and rendered more ductile by more abundant radioactive elements. Continental crust was relatively thin while oceanic crust, produced by high-degree melting of the hotter mantle, was far thicker (Sleep and Windley 1982). The subdued topography, the limited contrast between the thicknesses of oceanic and continental crust, combined
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with bigger oceans, meant that much of the continental crust was flooded (Arndt 1998). Just as during more recent geological history, global temperatures waxed and waned. Periods of global glaciation, the most pronounced being during the Proterozoic “▶ snowball Earth” episodes (Hoffman et al. 1998), alternated with periods when temperatures were relatively high. The O and Si isotopic compositions of Archean ▶ cherts suggest that ocean temperatures were commonly above 40 C and possibly as high as 80 C (Knauth and Lowe 2003). The atmosphere contained a little to no free oxygen but was rich in CO2; rainwater was acid. The normal cycle of erosion, transport, and deposition of sediment operated, but the rivers flowed through a landscape that was very different from that of today. The feature that most starkly distinguished the Archean and modern land surface was the lack of vegetation. Microbes no doubt colonized the subsurface and constructed biofilms (slime mats) that covered moist areas, but most of the landscape was a Martian vista of bare rocks and soil. The rate of erosion was enhanced by the lack of vegetation, high temperatures, and aggressive atmosphere but restrained by modest heights of mountain belts. Active volcanism covered much of the surface with lava flows or pyroclastic deposits. The oceanic crust was composed of basaltic lavas like that of modern crust, but more magnesian (picritic) in places (Sleep and Windley 1982). Parts of mid-ocean ridges and the summits of oceanic plateaus may have been emergent forming what might be called “melano- (dark colored) continents.” The pelagic sediment that covered this crust was different from that of today. An absence of shellforming organisms precluded the formation of biogenic calcareous or siliceous oozes; in their place were Si- or Ferich sediments that precipitated directly from the hightemperature seawater that contained high concentrations of these elements. Hydrothermal circulation of Si-charged seawater resulted in massive silicification of all nearsurface rocks with formation of chert horizons. Expulsion of fluids at hydrothermal vents led to the deposition of exhalative sediments variably composed of sulfides, sulfates, carbonates or silica minerals (Russell et al. 2005). The earliest Archean coincided with the end of the ▶ Late Heavy Bombardment, a time of massive meteorite impacts. The largest of these would have vaporized large expanses of the oceans and pulverized large parts of the continents, but their overall impact was local, not global.
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▶ Barberton Greenstone Belt ▶ Chert ▶ Continental Crust ▶ Craton ▶ Earth, Formation and Early Evolution ▶ Hydrothermal Environments ▶ Isua Supracrustal Belt ▶ Komatiite ▶ Late Heavy Bombardment ▶ Ocean, Chemical Evolution of ▶ Oxygen Isotopes ▶ Pilbara Craton ▶ Silicon Isotopes ▶ Snowball Earth ▶ Weathering
References and Further Reading Arndt NT (1998) Why was flood volcanism on submerged continental platforms so common in the Precambrian? Precambrian Res 97:155–164 Benn K, Mareschal J-C, Condie KC (2006). Archean geodynamics and environments. geophysical monograph series, American geophysical union 164, p 320 Hoffman PF, Kaufman AJ, Halverson GP, Schrag DP (1998) A Neoproterozoic snowball Earth. Science 281:1342–1346 Knauth LP, Lowe DR (2003) High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa. Geol Soc Am Bull 115:566–580 Nisbet EG, Cheadle MJ, Arndt NT, Bickle MJ (1993) Constraining the potential temperature of the Archaean mantle: a review of the evidence from komatiites. Lithos 30:291–307 Russell MJ, Hall AJ, Boyce AJ, Fallick AE (2005) On hydrothermal convection systems and the emergence of life. Econ Geol 100:419–438 Sleep NH, Windley BF (1982) Archaean plate tectonics: constraints and inferences. J Geol 90:363–379
Archean Eon HERVE´ MARTIN1, DANIELE L. PINTI2 1 Laboratoire Magmas et Volcans, Universite´ Blaise Pascal, OPGC, CNRS, IRD, Clermont-Ferrand, France 2 GEOTOP & De´partement des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada
Synonyms Precambrian
See also
Keywords
▶ Archean Eon ▶ Archean Tectonics
Continental crust, greenstone belts, komatiite, plate tectonics, TTG
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Definition
Overview
The Archean (Archaean in British English) is the second major period in the geological history. Preceded by the Hadean and followed by the Proterozoic, its start is usually taken as the age of the oldest preserved rocks, either the 4.0 Ga-old (Ga = 109 years = billion years) ▶ Acasta gneisses (Canada) or the 3.85 to 3.80 Ga-old Amitsoˆq gneisses (Greenland). The transition to the Proterozoic is typically taken at 2.5 Ga, which was thought to mark a major change in the Earth’s geodynamic style and corresponds roughly to the ▶ Great Oxygenation Event. It encompasses an approximately 1.5-Ga period during which the oldest well-preserved rocks formed and life likely originated. The ▶ tectonic style was different from today, with more abundant mantle plumes, greatly fragmented tectonic plates, and longer mid-oceanic ridges. It is commonly believed that plate tectonics started in this period.
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Geographical and Temporal Distribution of Archean Terranes The Archean eon is characterized by the extraction from the mantle and the subsequent differentiation of huge amounts of ▶ continental crust. Indeed, at the end of the Archean eon, about 75% of the juvenile continental crust is formed. Large portions of this Archean crust, named cratons or shields, have been preserved on all continents (Condie 1994; Fig. 1), including: ● In Europe, the 3.1 to 2.5 Ga Baltic (sometimes referred to as Fennoscandian) and 3.8 to 3.2 Ga Ukranian shields as well as a few outcrops north of Scotland (Lewisian gneisses in the Hebrides with a debated age of 3520 160 Ma)
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Archean Eon. Figure 1 Geographical distribution of the Archean provinces (After Condie 1994; redrawn). (1) Baltic Shield, (2) Ukranian Shield, (3) Scotland Shield, (4) Siberian Shield (5) Indian Craton, (6) Sino-Korean Craton, (7) Pilbara craton, (8) Yilgarn craton, (9) Northern Australia craton, (10) Napier complex, (11) Kaapvaal craton, (12) Zimbabwe Craton, (13) Madagascar Craton, (14) Central Africa Craton, (15) West Africa Craton, (16) Sa˜o Francisco and Amazonian Cratons, (17) Guyana Shield, (18) Wyoming Province, (19) Superior Province, (20) Slave Province, (21) Labrador Shield, and (22) Greenland Shield. Dotted areas represent exposed Archean terranes, while striped areas represent regions underlain by Archean rocks
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● In Asia, the Siberian Aldan ▶ Shield (3.5 to 3.0 Ga), the Indian (3.6 to 2.5 Ga), and the Sino-Korean cratons (3.8 to 3.0 Ga) ● In Australia, the Pilbara (3.6 to 2.5 Ga), Yilgarn (2.94 to 2.63 Ga), Gawler (2.5 Ga) and Northern Australia cratons ● In Antarctica, the Napier complex (orthogneisses dated at 3.95 to 2.46 Ga) ● In Africa, the Kaapvaal (3.6 to 2.5 Ga), Zimbabwe (3.5 to 2.5 Ga) and Madagascar cratons, as well as the Central and West Africa cratons ● In South America, the Sa˜o Francisco and Amazonian cratons (3.5 to 2.4 Ga), in Brazil and the 3.4 Ga Guyana Shield ● In North America, the Wyoming Province, USA (3.5 to 2.5 Ga); Superior Province (3.7 to 2.7 Ga); Slave Province (dominated by 2.73 to 2.63 Ga greenstone sequences but with ▶ Acasta gneiss dated back to 4.03 Ga); and Labrador Shield (Canada); and the Greenland Shield (3.8 to 2.6 Ga with older units at ▶ Akilia and ▶ Isua up to 3.88 Ga)
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The granitic gneisses are the most abundant, composing up to 80% of the Archean continental crust. Better known under the acronym ▶ TTG for Tonalite– Trondhjemite–Granodiorite association (Jahn et al. 1981), these rocks are coarse-grained, gray orthogneisses (which means derived from magmatic rocks, in this case granitoids) with well-developed banding consisting in the alternation of whitish quartz-plagioclase layers with biotite- and amphibole-rich gray layers (Fig. 2). Contrarily to typical modern granites, the TTG contain very low amounts of potassic feldspars (KAlSi3O8). The parent magma from which TTG derived results from the melting at high pressure of a hydrated mafic rock of basaltic composition (Fig. 3a). Indeed, when the pressure increases, basalt is transformed into amphibolite (amphibole garnet plagioclase feldspar-rich rock) and then into eclogite (pyroxene + garnet rock). These products are melted to give the parental magmas of TTG (e.g., Martin and Moyen 2002). Although all geologists agree on the basaltic source of TTG, the geodynamic environment where melting took place is
The largest continuously exposed outcrop of Archean rocks is the Amitsoˆq gneiss in Greenland with an area of 3,000 km2. The protolith of these rocks consists in older granitoids, metamorphosed into gneisses with emplacement ages of 3.822 0.005 Ga. The oldest supracrustal rocks (volcaniclastic and sedimentary) are in Akilia and the Isua Supracrustal belt with older ages at 3.872 0.010 Ga, together with banded iron formation at Nuvvuagittuq greenstone belt (3.817 0.016 Ga or older). The recognized oldest rocks on the Earth (covering a surface of about 20 km2) are the Acasta gneisses in Canada (Slave Province) with an age of 4.030 0.003 Ga (Bowring and Williams 1999) while the oldest known minerals are the now famous, ▶ Jack Hills detrital zircons (Western Australia) with recorded ages as old as 4.404 Ga (Wilde et al. 2001). Similarly, zircon inherited cores from Acasta provided an age of 4.20 0.06 Ga (Isuka et al. 2006). These zircon crystals are thus the only records of Hadean crust existing on Earth. Recently, a model age of 4.28 Ga has been proposed for an amphibolitic rock (Faux amphibolite), outcropping in the ▶ Nuvvuagittuq greenstone belt (O’Neil et al. 2008), though this age is strongly debated.
Geology of the Archean Terranes The Archean terranes all show the same lithological association, independent of their age: (1) granite gneiss, (2) ▶ greenstone belts, and (3) late granitoids.
Archean Eon. Figure 2 Photo of typical gray gneisses (TTG) from Sand River, Limpopo, South Africa. These 3.283 0.008 Ga-old rocks consist in the alternation of whitish quartzplagioclase layers with biotite and amphibole-richer gray layers (Photo H. Martin)
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Archean Eon. Figure 3 (a) Pressure–temperature (P-T) diagram and schematic cross section of both Archean (b) and modern (c) subduction zones. (a) The P-T diagram shows the dry and 5% hydrous solidus of tholeiite as well as main dehydration reactions of oceanic lithosphere. H is hornblende out, A is anthophyllite out, C is chlorite out, Ta is talc out, Tr is tremolite out, Z is zoisite out. G outlines stability field of garnet. The grey field is the P-T domain where slab melts can coexist with hornblendeand garnet-bearing residue. In the Archean (inset b), geothermal gradient along Benioff plane was very high; subducted slab melts at shallow depth before dehydration could take place. After 2.5 Ga, Earth was cooler and the geothermal gradient along the Benioff plane was lower (inset c) such that slab dehydration generally occurs before its melting can begin. The liberated volatiles (mainly water) ascend through the mantle wedge, thus lowering its solidus temperature, which induces its melting. OC is oceanic crust; CC is continental crust; MS is solidus of hydrated mantle; black tears indicate magma
still debated: (1) ▶ basalts from a subducted oceanic crust (Fig. 3; Martin 1995; Martin and Moyen 2002); or (2) underplated basalts melted during the passage of a mantle plume (Smithies 2000). The first hypothesis can be explained if we assume a hotter Archean mantle, as inferred by the occurrence in Archean times of Mg-rich magmas called ▶ komatiites. It can also be explained by the subduction of young and consequently hotter, oceanic plates. In modern ▶ tectonic regimes, the subducted plate is old and cold, such that it dehydrates during its descent into the mantle; indeed, there, the oceanic plate undergoes both higher pressure and temperature, such that it dehydrates. Volatiles, mainly water are liberated and ascend through the mantle wedge, thus lowering its melting temperature (the mantle wedge lies between the descending or subducting oceanic plate and the continental (or oceanic) plate). Mantle wedge melting generates magmas with andesitic to granitic composition that are accreted to form new continental crust (Fig. 3b). Modern subduction systems (both mantle and oceanic crust) are normally too cold to allow subducted basalt melting. If we assume a hotter Archean mantle associated or not to the subduction of a younger oceanic crust, then the latter cannot
rapidly dehydrate and consequently, direct melting of hydrated basalts is possible, resulting in TTG magma genesis (Fig. 3c). ▶ Greenstone belts represent only 5–10% of the Archean terrains. These elongated structures (typically >100 km 20 km) contain variable amounts of metamorphosed mafic to ultramafic volcanic sequences associated with sedimentary rocks. The name “greenstone” comes from the green hue imparted by the color of the metamorphic minerals within the mafic rocks. Chlorite, actinolite, and other green amphiboles are the usual green minerals. In some cases, greenstone belts show a specific stratigraphic polarity, with ultramafic lavas (komatiites) at the base of the sequence, followed by basalts (often erupted subaqueously with typical pillowed structures). Variably metamorphosed sedimentary rocks (▶ metasediments) are emplaced at the top of the sequence. Komatiites are ultramafic volcanic rocks (Arndt et al. 2008), almost exclusively restricted to the Archean eon, which distinguished from more common basalts by a higher content of MgO (>18%) and correlated low contents of most other elements. The high Mg content is explained by a higher degree of melting of the mantle;
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their emplacement temperatures ranges between 1400 C and 1650 C (Arndt et al. 2008) against the 1100–1300 C for modern basalts. They demonstrate that the internal Earth heat production in the Archean was higher than today. After emplacement, these magmas cooled very rapidly resulting in acicular and dendritic textures, referred to as “spinifex” textures, which are typical of komatiites. The Earth almost totally ceased to produce komatiites after 2.5 Ga. Mafic volcanics are mainly tholeiitic basalts (Fig. 4), while calc-alkaline lavas are rare. By contrast with the modern Earth, Archean andesites are scarce. At the top of the sequence, more felsic rocks (dacites to rhyolites) can be intercalated within the sedimentary successions. Sedimentary cycles classically start with coarsegrained clastic rocks containing volcanic material (conglomerates and greywacke, Fig. 5) and often turbidites (well-preserved 3.5 Ga old sequences can be observed in
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the Komati River valley, South Africa). They are overlain by shales, chert, and banded iron formations (BIF; Fig. 6). Cherts and BIF are common lithologies in Archean greenstone belts and they are likely the result of an intense hydrothermal activity on the ocean floor (Westall 2005; Van Kranendonk et al. 2006). Both the TTG basement and the greenstone belts were later intruded by high-magnesium granitoids or sanukitoids (Fig. 7). These calc-alkaline granites are rich in potassic feldspars and magnesium and they might derive by melting of a mantle peridotite, whose initial composition was modified by assimilation of TTG (Martin et al. 2009).
Archean Eon. Figure 4 Pillow lavas of 2.65 Ga-old tholeiitic basalt from Kuhmo (Finland) (Photo H. Martin)
Archean Eon. Figure 6 Banded Iron Formation (BIF) from Copping Gap (Australia). These rocks consist in alternation of silica and iron-rich layers (Photo H. Martin)
Archean Eon. Figure 5 Clastic sediments (conglomerate) at the base of the 3.1 Ga Moodies group, Barberton, South Africa (Photo H. Martin)
Archean Eon. Figure 7 Dyke of high-Mg granodiorite (sanukitoid) intrusive into the 2.65 Ga-old Kuhmo greenstone belt (Finland) (Photo H. Martin)
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Archean Geodynamics Modern plate tectonics induces horizontal forces that cause thrusting during orogenesis. These structures are known in most Archean terranes, indicating that tectonics similar to modern plate tectonics was operating since at least 4.0 Ga ago. However, Archean terranes also show large evidence of major vertical deformation that produce dome-and-basin structures (Fig. 8) which are exclusive to Archean times. This type of tectonics is driven by gravity (as opposed to plate tectonics which is driven by mantle convection) and this process has been known as sagduction since the 1970s (Gorman et al. 1978). Sagduction structures result from the down motion of high density greenstones (such as komatiites; density = 3.3 g/cm3) into the TTG basement (density = 2.7 g/cm3) and the concomitant upward motion of low-density TTG into the greenstones creating inverse diapirs. At the top of the inverted diapirs, a basin is created allowing deposition of sedimentary rocks. Several authorities have suggested that horizontal forces acted mainly at the plate boundaries (as today) while sagduction processes were concentrated within plates. Another difference compared with today is the supposed length of the mid-ocean ridges, i.e., the divergent boundaries between two plates. Indeed, the large amounts of internal heat produced during Archean times has necessarily been released, otherwise, the accumulated heat should have resulted in melting the external part of our
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planet, which is not attested by geological record. As conduction is not efficient at all to evacuate internal heat, Archean convection should have played this role and, as today, heat must have been released by oceanridge systems. As the amount of heat to evacuate was greater, it can be concluded that the excess of heat has been released through convective processes. Convection rate could have been slightly greater, but mainly the ridge length was significantly greater than today. The amount of heat dissipated is correlated with cubic square of the ridge length (Hargraves 1986). Because the Earth volume and surface did not significantly change since 4.5 Ga, a greater ridge length should result in smaller plates (Fig. 9). The greater ridge length can also account for the abundance of cherts and BIFs in Archean greenstone belts.
Hydrosphere, Atmosphere, and Climate There is good evidence that oceans were present on the Earth already in the Early Archean ( 3.8 Ga ago) after the ▶ Late Heavy Bombardment (Abe 1993; Sleep et al. 2001). The convincing evidence comes from one of the oldest areas of volcanic and sedimentary rocks – the ▶ Isua Supracrustal Belt, West Greenland. The ages of the rocks have been established at about 3.7–3.8 Ga (for Nuvvuagittuq a date of 4.28 Ga has been proposed; O’Neil et al. 2008). In the Isua Supracrustal Belt, ▶ pillow basalts provide evidence of underwater eruption and metasedimentary rocks (banded iron formations,
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Archean Eon. Figure 8 (Left) Sketch depicting the three main steps of sagduction: (1) In a greenstone belt, high-density komatiites (d = 3.3 g/cm3) emplace over lower density (d = 2.7 g/cm3) TTG basement rocks, thus generating an inverse density gradient; (2) komatiites sink downward into the TTG basement which favor a relative upward motion of the TTG; (3) the movement is amplified creating a sedimentation basin at the center of the greenstone belt. Dark gray is komatiites, light gray is TTG basement, and white are sediments. (Right) Satellite photo of the sagduction structures at the Pilbara craton, Western Australia. The greenstone belts (in dark gray) are localized between TTG domes (white). The width of the photo is 300 km
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Archean Eon. Figure 9 Sketch representing the size of the tectonic plates that presently cover the surface of the Earth (left) and that supposed for the Archean plates (right)
metapelite, and ferruginous quartzite) are the products of erosion, fluvial transport, and subaqueous deposition (Rosing et al. 1996). Primary fluid inclusions were found in quartz crystals in iron oxide structures from the 3.5 to 3.2 Ga ▶ Barberton greenstone belt, South Africa (Channer et al. 1997), intra-pillow quartz from the Dresser Formation (3.49 Ga), ▶ Pilbara craton, Western Australia (Foriel et al. 2004), and in the 2.7 Ga Abitibi Greenstone Belt, Ontario, Canada (Weiershauser and Spooner 2005). The analysis of major cations and anions indicates that the chemistry of the seawater was similar to today, with some noticeable differences in iron, iodine, and bromine abundances indicating a larger influence of hydrothermal fluids (today, the chemistry of seawater is mainly controlled by weathering of continents with a minor role for hydrothermal fluids). Salinity was basically NaCl-dominated, though salinities up to 10 times the present values have been measured, possibly related to seawater evaporation in closed basins (Foriel et al. 2004). The atmosphere was possibly mildly reducing. The amount of N2 was likely close to the present level (Kasting 1993); CO2 might have been present in larger amounts (up to 1% in volume or higher; Kasting 1987), while oxygen was likely 1 ppmv against the 21% by volume today. Oxygen concentrations rose only at the end of Archean to values close to 1% of their present-day level, probably because of shifts in the competition between the production of oxygen derived from cyanobacteria photosynthesis and the rate of consumption of oxygen by different geological processes. The amount of CO2 in the atmosphere is still a matter of debate, but the occurrence of larger amounts of greenhouse gases (CO2, CH4) may have been needed in the Archean atmosphere to counterbalance the
lower radiation from the faint young Sun, which was 30% less than the present-day value. Archean terranes do not contain evidence for major glaciations during the first 2 billion years of the Earth’s history indicating that a warmer climate (as suggested by ▶ high ocean temperatures; Robert and Chaussidon 2006) dominated during the eon.
Life Though the timing of the origin of life is unknown, the Archean world likely saw the emergence of the first organisms. Several morphological, molecular, and chemical traces of life punctuate the Archean sedimentary record. Currently it is difficult to declare with certainty what the oldest trace of life is, and importantly what its nature and habitat were. Life can be traced unambiguously to approximately 2.7–3.0 Ga ago (Lopez-Garcia et al. 2006). Beyond this point many claims for biological processes have been made, and all of them have to some degree been questioned. Some of the intriguing but controversial early Archean traces include (1) isotopically light graphite inclusions in rocks older than 3.8 Ga from Akilia island and the Isua Supracrustal Belt in southwest Greenland (Mojzsis et al. 1996; Van Zuilen et al. 2002); (2) kerogenous microstructures, stromatolites, and diverse stable isotope ratio anomalies in 3.5 Ga cherts from the Pilbara Granitoid–Greenstone Belt in Western Australia (Schopf 1993; Brasier et al. 2002; Ueno et al. 2006; Pinti et al. 2009) (Fig. 10); (3) kerogenous microstructures, stromatolites, and diverse stable isotope ratio anomalies in cherts, as well as microscopic tubes in altered pillow basalts from the 3.4 to 3.2 Ga Barberton Greenstone Belt in South Africa (Staudigel et al. 2008).
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Archean Eon. Figure 10 3.5 Ga-old stromatolites from North Pole (Pilbara, Australia)
See also ▶ Akilia ▶ Amphibolite Facies ▶ Archean Traces of Life ▶ Barberton Greenstone Belt ▶ Basalt ▶ Canadian Precambrian Shield ▶ Chronological History of Life on Earth ▶ Craton ▶ Earth, Formation and Early Evolution ▶ Gneiss ▶ Granite ▶ Greenstone Belts ▶ Igneous Rock ▶ Isua Supracrustal Belt ▶ Metamorphic Rock ▶ Metamorphism ▶ Metasediments ▶ Pilbara Craton ▶ Pillow Lava ▶ Shield ▶ Volcaniclastic Sediment
References and Further Reading Abe Y (1993) Physical state of the very early Earth. Lithos 30:223–235 Arndt N, Lesher MC, Barnes SJ (2008) Komatiite. Cambridge University Press, New York, 488 pp Bowring SA, Williams IS (1999) Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada. Contrib Mineral Petrol 134:3–16 Brasier M, Green O, Lindsay J, Mcloughlin N, Steele A, Stoakes C (2005) Critical testing of Earth’s oldest putative fossil assemblage from the 3.5 Ga Apex chert, Chinaman Creek, Western Australia. Precambrian Res 140(1–2):55–102
Channer DMDR, de Ronde CEJ, Spooner ETC (1997) The Cl-Br-I composition of 3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry. Earth Planet Sci Lett 150:325–335 Condie KC (1994) The Archean crustal evolution. Developments in Precambrian Geology, Elsevier, Amsterdam, 528 pp Foriel J, Philippot P, Rey P, Somogyi A, Banks D, Menez B (2004) Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia. Earth Planet Sci Lett 228:451–463 Gorman BE, Pearce TH, Birkett TC (1978) On the structure of Archean greenstone belts. Precambrian Res 6:23–41 Hargraves RB (1986) Faster spreading or greater ridge length in the Archean. Geology 14:750–752 Jahn B-M, Glikson AY, Peucat JJ, Hickman AH (1981) REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution. Geochim Cosmochim Acta 45:1633–1652 Kasting JF (1993) Earth’s early atmosphere. Science 259:920–926 Lopez-Garcia P, Moreira D, Douzery E, Forterre P, van Zuilen MA, Claeys P, Prieur D (2006) Ancient fossil record and early evolution (ca. 3.8 to 0.5 Ga). Earth Moon Planet 98:248–268 Martin H (1995) The Archean grey gneisses and the genesis of the continental crust. In: Condie KC (ed) The Archean crustal evolution. Elsevier, Amsterdam, pp 205–259 Martin H, Moyen J-F (2002) Secular changes in TTG composition as markers of the progressive cooling of the Earth. Geology 30:319–322 Martin H, Moyen J-F, Rapp R (2009) The sanukitoid series: magmatism at the Archean-Proterozoic transition. Earth Environ Sci Trans R Soc Edinb 100:15–33 Mojzsis SL, Arrhenius G, Friend CRL (1996) Evidence for life on Earth before 3,800 million years ago. Nature 384:55–57 O’Neil J, Carlson RW, Francis D, Stevenson RK (2008) Neodymium-142 Evidence for Hadean Mafic Crust. Science 321:1828–1831 Pinti DL, Hashizume K, Sugihara Y, Massault M, Philippot P (2009) Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara Carton, Western Australia: origin of 15N-depleted nitrogen. Geochim Cosmochim Acta 73(13):3819–3848 Robert F, Chaussidon M (2006) A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443:969–972 Rosing MT, Rose NM, Bridgwater D, Thomsen HS (1996) Earliest part of Earth’s stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence. Geology 24:43–46 Schopf JW (1993) Microfossils of the early Archean apex chert: New evidence of the antiquity of life. Science 260(5108):640–646 Sleep NH, Zahnle K, Neuhoff PS (2001) Initiation of clement surface conditions on the earliest Earth. Proc Natl Acad Sci USA 98(7):3666–3672 Smithies RH (2000) The Archean tonalite-trondhjemite-granodiorite (TTG) series is not an analogue of Cenozoic adakite. Earth Planet Sci Lett 182:115–125 Staudigel H, Furnes H, Mcloughlin N, Banerjee N, Connell L, Templeton A (2008) 3.5 billion years of glass bioalteration: Volcanic rocks as a basis for microbial life? Earth Sci Rev 89:156–176 Ueno Y, Yamada K, Yoshida N, Maruyama S, Isozaki Y (2006) Evidence from fluid inclusions for microbial methanogenesis in the early Archean era. Nature 440:516–519 Van Kranendonk MJ (2006) Volcanic degassing, hydrothermal circulation and the flourishing of early life on Earth: A review of the evidence from c. 3490–3240 Ma rocks of the Pilbara Supergroup, Pilbara Craton, Western Australia. Earth Sci Rev 74(3–4):197–240
Archean Tectonics van Zuilen MA, Lepland A, Arrhenius G (2002) Reassessing the evidence for the earliest traces of life. Nature 418(6898):627–630 Weiershauser L, Spooner E (2005) Seafloor hydrothermal fluids, Ben Nevis area, Abitibi Greenstone Belt: Implications for Archean (2.7Ga) seawater properties. Precambrian Res 138(1–2):89–123 Westall F (2005) The geological context for the origin of life and the mineral signatures of fossil life. In: Gargaud M, Barbier B, Martin H, Reisse J (eds) Lectures in Astrobiology. Springer-Verlag, Berlin, pp 195–226 Wilde SA, Valley JW, Peck WH, Graham CM (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Ga ago. Nature 409:175–178
Archean Mantle NICHOLAS ARNDT1, DANIELE L. PINTI2 1 Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France 2 GEOTOP & Department of Earth and Atmospheric sciences, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada
Definition Archean mantle refers to the terrestrial mantle during the Archean eon, which differed both physically and chemically from the modern day mantle.
Overview The mantle is that part of Earth or other planets between the crust and the core. The upper mantle, from the base of the crust at about 9 km (oceanic) or 30 km (continental) to the transition zone at 660 km, is composed mainly of ▶ peridotite, an ultramafic rock mainly composed of olivine, pyroxene, and minor garnet. In the lower mantle, which extends to the core at 2,990 km, the minerals are mainly Mg- and Ca-perovskite ((Mg, Ca)TiO3) and magnesiowu¨stite ((Mg, Fe)O). The mantle is solid except for localized zones of partial melting, but it convects with velocities of a few tens of centimeters per year. The Archean mantle differed from the modern mantle in several important ways. Because the main sources of heat – radioactivity, residual heat from accretion and core crystallization – were more active than today, the mantle was hotter and it convected more vigorously. Higher temperatures in mantle upwelling beneath mid-ocean ridges produced larger melt volumes and a thicker oceanic crust. Higher temperatures at depth may have resulted in larger and hotter mantle plumes. The abundance of ▶ komatiite only in the Archean provides evidence of higher mantle temperatures.
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The composition may also have been different, if, as many authors believe, the continental crust was less voluminous through the Archean. Continental crust contains far higher concentrations of elements such as Si, Al, K, and the “incompatible” trace elements, and the segregation of this crust has left the upper part of the modern mantle depleted in these elements. If crustal growth were incomplete in the Archean, either the volume of depleted mantle, or the degree of depletion, would have been less. The isotopic composition of rocks from the Archean mantle should, in theory, cast some light on the problem but at present the message is ambiguous. The Hf isotope compositions of zircons show evidence of extraction, before 4 Ga ago, of enriched material, perhaps continental crust; the Nd isotopic compositions of rocks from the oldest areas of West Greenland show evidence of derivation from strongly depleted mantle. Either these rocks were extracted from a small and localized volume of mantle, or a large volume of continental crust had formed at this time. If the mantle were significantly hotter, it would have been drier because the reactions that liberate water in upwelling mantle, where degassing takes place, or in subduction zones, where the mantle is rehydrated, are temperature dependent. The proportion of water on the surface was larger and therefore the volume of the oceans may have been greater. The oxidation state of the Archean mantle has been investigated using redox sensitive elements like vanadium; no significant difference from that of the modern mantle has been established.
See also ▶ Archean Environmental Conditions ▶ Archean Tectonics ▶ Isua Supracrustal Belt ▶ Jack Hills (Yilgarn, Western Australia) ▶ Komatiite ▶ Peridotite
Archean Tectonics MARTIN JULIAN VAN KRANENDONK Department of Mines and Petroleum, Geological Survey of Western Australia, East Perth, WA, Australia
Synonyms Crustal deformation; Early earth
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Archean Tectonics
Keywords ▶ Archean, ▶ continents, crust, lithosphere, nonuniformitarian, plate tectonics, tectonics, thermal evolution of the Earth, TTG
Definition Archean tectonics is the study of the formation, interaction, and deformation of the Earth’s continental and oceanic crust during early Earth history (the Archean Eon; ca. 4.0–2.5 Ga) and the driving forces behind these processes, including mantle plumes, subduction, and accretion/collision. This topic remains highly controversial due, in part, to a fragmentary rock record, but also to nonunique interpretations of complex geological datasets in the absence of actualistic plate configurations. Historically, Archean tectonics has been polarized into uniformitarian (i.e., analogous with modern, or Phanerozoic Earth) and non-uniformitarian views, but recent studies have favored modern Earth processes in the Archean, complicated by problems arising mainly from greater heat production and higher mantle temperatures (Condie 1994; Benn et al. 2006; Brown and Rushmer 2006). The discussion revolves around the basic question if and how tectonics in the Archean was different from modern-style plate tectonics.
History Although the relative antiquity of some parts of the continental crust was recognized more than 150 years ago (Logan 1857), it was not until the advent of radiometric dating, 100 years later, that the antiquity of much of the continental crust was fully appreciated (Stockwell 1961). Only in the past decade it has been discovered that the preserved crustal record on Earth extends back to within 150 Ma of the age of formation of the Solar System (Wilde et al. 2001). Early geological studies found that continental crust older than about 2.5 Ga was different from younger crust. Archean crust showed distinct regional patterns defined by: overlapping, elliptical areas of granitic rocks (gregarious batholiths; Macgregor 1951); steeply dipping, generally synclinal, greenstone keels (granite-greenstone crust: Hickman 1984; Chardon et al. 1996); a unique type of ultramafic lava known as ▶ komatiite derived from high-temperature mantle melts (Viljoen and Viljoen 1969; Arndt 2003). Many authors also noted that Archean granite-greenstone crust lacked the diagnostic features of modern subduction/collision zones, including accretionary tectonic me´lange, ophiolites, and high-pressure/ low-temperature metamorphism, leading some to suggest – even recently – that plate tectonics either did not operate in the Archean (Hamilton 1998; McCall 2003; Stern 2005),
or operated in conjunction with other processes (e.g., Rey et al. 2003; Sandiford et al. 2004).
Overview The Archean mantle was probably 100–300 K hotter than today, which significantly affected the dominant tectonic style. Geochemical, geophysical, and modeling evidence suggests some form of plate tectonics in the Archean, although the absence of key characteristics such as ophiolites and blueschists implies that it probably differed from modern ▶ plate tectonics. Evidence for Archean plate tectonics was recognized quite early on in the type of Archean crust known as high-grade gneiss terranes. This evidence included the presence of large-scale recumbent isoclinal folds associated with crustal thickening (Bridgwater et al. 1974; Myers 1976; Wilks 1988; Hanmer and Greene 2002), voluminous sodic granitoids derived from high-pressure melting of basalt (Martin et al. 2005; Rapp et al. 1991), high-pressure metamorphism (Riciputi et al. 1990; Harley 2003), and structures consistent with terrane accretion (Nutman et al. 2002; Windley and Garde 2009). Over the past three decades, abundant evidence for Archean plate tectonics has also been found in some Archean granite-greenstone terrains in the form of thrusts and recumbent isoclinal folds, coupled high-pressure– low-temperature metamorphism, fossil subduction zones, accreted terranes, rift sequences, and subductionzone magmatism (Card 1990; Calvert et al. 1995; Smithies et al. 2005; Moyen et al. 2006; Wyman et al. 2006; Van Kranendonk 2007; Van Kranendonk et al. 2010). However, an absence of hallmark characteristics of modern subduction-accretion zones in many granitegreenstone terrains (Hamilton 1998; McCall 2003; Stern 2005), the autochthonous nature of some major greenstone successions, and suggestions that mantle roots form through in situ melting events rather than subduction stacking indicates local crustal development as volcanic plateaus developed on older continental basement (e.g., Blenkinsop et al. 1993; Bleeker et al. 1999; Van Kranendonk et al. 2007). Indeed, many studies suggest that some pieces of Archean crust contain features that cannot be ascribed to uniformitarian, Phanerozoictype, plate tectonics, but rather formed as a result of largescale infra-crustal differentiation accompanying periods of mantle plume–related magmatism (Stein and Hofmann 1994; Whalen et al. 2002; Rey et al. 2003; Smithies et al. 2009; Van Kranendonk et al. 2009). The fact that different processes have been recognized from studies of different pieces of Archean crust indicates that there was no single Archean tectonic process, but rather that – as with modern Earth – Archean continental crust formed through
Archean Tectonics
a variety of processes, including plate tectonics and mantle-derived upwellings, and the probable interaction between these two end-member processes.
Basic Methodology Direct field evidence from Archean continental lithosphere provides a record of Archean tectonic processes. Geophysical methods include paleomagnetism and seismic evidence. Solidifying magma registers the paleolatitude, and thus can record (relative) continental motion. Seismic profiles through Archean crust indicate the presence of dipping seismic reflectors, which could be interpreted as the remnants of a fossil subduction zone. Amongst the geological evidence, ophiolites (slivers of oceanic lithosphere that escaped subduction), and blueschists and ultrahigh metamorphic rocks (partly subducted rocks that emerged again at the Earth’s surface) are all well-understood features associated with modern plate tectonics. Their occurrence throughout Earth’s history is thought to provide clear indicators of platetectonic activity. Large-scale tectonic structures can be indicative as well: linear structures are often interpreted as remnants of subduction trenches, whereas large oval-shape structures are thought to be diapir- or domerelated that might not need any plate-tectonic activity. Geochemical and petrologic techniques provide the “fingerprints” of the chemical processes associated with Archean tectonics. Mantle melting will deplete the mantle of “incompatible” elements (Rollinson 2007). Another form of element separation occurs due to differences in fluid mobility, so that some elements will preferentially move with any pore fluids, while others will stay in the residue. These processes will leave geochemical fingerprints that might be used to recognize ancient subduction processes (e.g., Shirey et al. 2008). In addition, geochemical dating of crustal rocks and mantle material shows how mantle material was depleted through time by continent formation. Since continents today are formed primarily at subduction zones, this has been interpreted as another indication for the presence of ancient subduction and therefore plate tectonics. Various modeling techniques are used to further constrain the range of dynamically viable tectonic processes during the Archean. Tectonic vigor is related to mantle convection, and intimately couples to the cooling rate of the Earth: tectonic activity leads to increased heat loss from the mantle, and mantle temperature influences the vigor of tectonic activity. Mantle temperature through time therefore provides an important constraint, and parameterized and numerical modeling techniques provide a means to link mantle temperature and tectonic
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activity. Today’s plate tectonics is primarily driven by the subduction process, and subduction dynamics is, to a large extent, influenced by mantle temperature due to melting events and temperature-dependent strength of the lithosphere. The viability and style of subduction in an early, hotter Earth is investigated using parameterized and numerical modeling techniques.
Key Research Findings Field evidence has been used to argue for or against modern-style plate tectonics in the Archean. Ophiolites are preserved pieces of old oceanic lithosphere that escaped subduction, and the occurrence of old ophiolites is therefore a clear indicator of plate-tectonic activity. They are widespread since 1 Ga, but are much rarer before that. Recently, Furnes et al. (2007) reported a 3.8 Ga-old ophiolite in Isua, West Greenland, although their interpretation has been disputed. Other direct types of evidence for plate tectonics are blueschists and ultrahigh metamorphic rocks, which are both generally believed to form inside subduction zones, where they are brought down to large pressures and temperatures, and subsequently make it back to the surface. The oldest blueschists are ca. 850–700 Ma old, while the oldest UHP localities are 600 Ma. These data could indicate that modern-style plate tectonics did not start until the Neoproterozoic (Stern 2005), or that the appearance of plate tectonics evolved over time (van Hunen and van den Berg 2008). Earth’s thermal evolution provides further constraints. Today, plate tectonics forms the dominant cooling mechanism for the Earth, and is therefore closely linked to the thermal evolution of the Earth. The Archean Earth had an amount of radiogenic heat production, 2–3-times larger than today due to gradual decay of the dominant heatproducing elements uranium, thorium, and potassium in the mantle. Today, the surface heat flux (heat escaping the Earth’s interior) is 30% provided by internal radiogenic heating, and the remaining 70% results comes from cooling of the Earth (Turcotte and Schubert 2002; Korenaga 2006). This shows that surface heat flow from radiogenic heating was a lot more important in the Archean, and implies that either surface tectonics were such that the total surface heat flux was higher, or that the Earth was not cooling (significantly), or perhaps even heating up. Inferred liquidus temperatures from ophiolites and greenstone belts suggest a gradual mantle temperature drop of 200 K since the Archean. Komatiitic melt data suggests a mantle potential temperature (i.e., mantle T extrapolated to surface P, T conditions) reduction of 300 K for dry melting, to 100 K if melting took
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place under much wetter conditions. Jaupart et al. (2007) provide a recent overview of the thermal evolution of the Earth. The dynamical viability of subduction in the Archean has been questioned. Today, plate tectonics is primarily driven by dense slabs sinking into the mantle, and thereby pulling the trailing lithosphere across the surface. Archean plate tectonics would require a similar driving mechanism. A hotter mantle provides more melt, and therefore a thicker, low-density mafic crust (up to 20 km instead of today’s 6–7 km) that doesn’t easily subduct. Although dehydration during melting will make the plate compositionally stronger and could allow for similar plate-tectonic rates in the Archean as today (Korenaga 2006), thermal weakening would probably dominate if the mantle was substantially hotter, and would result in weaker plates. The combined buoyancy and plate strength effects make subduction inefficient for mantle temperatures more than 150 K hotter than today (van Hunen and van den Berg 2008). Today’s continental crustal rocks (loosely termed andesites) differ significantly in trace-element composition from their Archean counterparts (trondhjemitetonalite-granodiorite, or TTGs). Whereas andesites are thought to ultimately derive from melting in the hydrated supra-subduction mantle wedge, TTGs seem to form from wet melting of oceanic basalts at sometimes >50 km depth, and the most popular formation model is melting of subducting oceanic crust (Foley et al. 2002). So the differences between modern and Archean continental crust suggest a secular evolution of the subduction process. But at the same time, it indicates the need for a process to bring fluids to 50–100 km depth throughout the Earth’s history. At present, no other mechanism than subduction seems capable of doing that, which is regarded as one of the strong arguments in favor of Archean plate tectonics. However, although most studies support Archean plate tectonics, perhaps in some modified form, the possibility of other dominant tectonic processes should not be excluded. If indeed in the Archean plate tectonics was absent, such alternative tectonic models were probably essential to provide a mechanism for the observed steady mantle cooling of 50–100 K/Gyr. One popular model is the crustal delamination model (e.g., Zegers and van Keken 2001), in which mantle melting events would thicken the continental crust until its base becomes gravitationally unstable and would cause lithospheric overturn events. Such models would explain early Archean observations such as the ovoid-shaped intrusions in the eastern Pilbara.
Applications The style and vigor of tectonics in the Archean has important consequences for many aspects of the evolution of the Earth. Tectonic style directly influences (a) how and when continents formed and why cratons remained stable and preserved over much of the Earth’s history (Lenardic et al. 2003); (b) how tectonics-related events such as melting, remelting, and fluid-related alteration has changed the composition of the mantle from its primitive composition shortly after core formation to its modern composition (Shirey et al. 2008); (c) the composition of the atmosphere and oceans through ▶ degassing (during volcanism) and regassing (at subduction zones) of volatiles and surface weathering (Lowe and Tice 2007; Rollinson 2007), and through that the emergence and evolution of life on Earth. Furthermore, if plate tectonics has been operative throughout the changing conditions of the Earth during its history, how does that relate to the viability of plate tectonics on the other terrestrial planets of our solar system, such as on Mars (which has no plate tectonics today, but might have had some during its earliest history) or Venus (which doesn’t have plate tectonics, probably because of the lack of liquid water, but perhaps experiences episodic large-scale mantle overturns).
Future Directions Integrated, four-dimensional lithospheric studies of Archean lithosphere are the key future research directions, particularly in poorly studied regions. Detailed geochronology within a well-established map framework continues to be key to understanding formation processes of ancient crust, particularly when coupled with ongoing reevaluation of uniformitarian assumptions given known aspects of secular change. Hf isotope determination of zircon can help discriminate juvenile crustal growth through subduction from volcanism and crustal recycling during episodes of plume magmatism. Further understanding of crustal growth processes will be aided by more complete knowledge on the origin of subcontinental lithospheric mantle: detailed studies of primary duniteharzburgite xenoliths are required to determine the age, composition, and history of these more depleted rocks, and their properties tied into detailed regional seismic studies, including physical modeling of their geophysical response. Additional work is also required on the metamorphism of granite-greenstone terranes; specifically, precise dating of mineral assemblages related to magmatic-deformational events, and P–T studies of granitic rocks as a counterpart to greenstones, to test models of cold greenstone diapirs in hot rising granites (partial
Archean Tectonics
convective overturn; Smithies et al. 2009). Additional studies are required on the origin of Archean calc-alkaline felsic volcanic rocks and mafic granites to establish whether they are really the products of volcanic arc magmatism over an active subduction zone, as widely assumed, or the products of fractionation and crustal contamination of large tholeiitic magma chambers derived from mantle plumes. Additional research is required across the interval 3.3–2.9 Ga, in order to assess the tantalizing clues that there may have been a global change in crust-formation processes at this time, perhaps due to an increase in plate size and concomitant cooling of oceanic lithosphere and steepening of the angle of subducting oceanic lithosphere. Key open questions include the following. How did subduction first initiate, and did plate tectonics maintain operative from its first onset until today, or was it once more episodic? Is the onset of plate tectonics related to any atmospheric and ocean changes in the Archean, to the habitat that provided early life and its evolution, and to the presence of the Earth’s magnetic field? How does Archean tectonics relate to the observed peaks in continental crust formation at 3.3, 2.7, 1.9, and 1.2 Ga (Condie 1998; Parman 2007)?
See also ▶ Archea ▶ Continental Crust ▶ Degassing ▶ Komatiite ▶ Ophiolite ▶ Plate Tectonics
References and Further Reading Arndt N (2003) Komatiites, kimberlites, and boninites. J Geophys Res 108(B6):ECV 5-1–5-11 Benn K, Mareschal J-C, Condie KC (2006) Archean geodynamics and environments, vol 164, Geophysical monograph series. American Geophysical Union, Washington, DC, p 320 Bleeker W, Ketchum J, Jackson V, Villeneuve M (1999) The central slave basement complex, part I: its structural topology and autochthonous cover. Can J Earth Sci 36:1083–1109 Blenkinsop TG, Fedo CM, Bickle MJ, Eriksson KA, Martin A, Nisbet EG, Wilson JF (1993) Ensialic origin for the Ngezi Group, Belingwe greenstone belt, Zimbabwe. Geology 21:1135–1138 Bridgwater D, McGregor VR, Myers JS (1974) A horizontal tectonic regime in the Archean of Greenland and its implications for early crustal thickening. Precambrian Res 1:179–197 Brown M, Rushmer T (2006) Evolution and differentiation of the continental crust. Cambridge University Press, Cambridge Calvert AJ, Sawyer EW, Davis WJ, Ludden JN (1995) Archean subduction inferred from seismic images of a mantle suture in the Superior Province. Nature 375:670–674 Card KD (1990) A review of the Superior Province of the Canadian Shield, a product of Archean accretion. Precambrian Res 48:99–156
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Chardon D, Choukroune P, Jayananda M (1996) Strain patterns, decollement and incipient sagducted greenstone terrains in the Archean Dharwar craton (southern India). J Struct Geol 18:991–1004 Collins WJ, Van Kranendonk MJ, Teyssier C (1998) Partial convective overturn of Archean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications. J Struct Geol 20:1405–1424 Condie KC (1994) Archean crustal evolution. Elsevier, Amsterdam Condie KC (1998) Episodic continental growth and supercontinents: a mantle avalanche connection? Earth Planet Sci Lett 163:97–108 Foley S, Tiepolo M, Vannucci R (2002) Growth of early continental crust controlled by melting of amphibolite in subduction zones. Nature 417:837–840 Furnes H, de Wit M, Staudigel H, Rosing M, Muehlenbachs K (2007) A vestige of Earth’s oldest ophiolite. Science 315:1704–1707 Hamilton W (1998) Archean magmatism and deformation were not the products of plate tectonics. Precambrian Res 91:143–179 Hanmer S, Greene DC (2002) A modern structural regime in the Paleoarchean (3.64 Ga); Isua Greenstone Belt, southern West Greenland. Tectonophysics 346:201–222 Harley SL (2003) Archean to Pan-African crustal development and assembly of East Antarctica: metamorphic characteristics and tectonic implications. In: Yoshida M, Windley BF (eds) Proterozoic East Gondwana: supercontinent assembly and breakup. London, Geological Society, pp 203–230, Special Publication 206 Hickman AH (1984) Archean diapirism in the Pilbara Block, Western Australia. In: Kro¨ner A, Greiling R (eds) Precambrian tectonics illustrated. E. Schweizerbarts’che Verlagsbuchhandlung, Stuttgart, pp 113–127 Jaupart C, Labrosse S, Mareschal J-C (2007) Temperatures, heat and energy in the mantle of the Earth. In: Bercovici D (ed) Treatise on geophysics, vol 7, Mantle convection. Elsevier, Amsterdam, pp 253–303 Korenaga J (2006) Archean geodynamics and the thermal evolution of Earth. In: Benn K, Mareschal J-C, Condie K (eds) Archean Geodynamics and Environments. American Geophysical Union, Washington, DC, pp 7–32 Lenardic A, Moresi L-N, Mu¨hlhaus H (2003) The longevity and stability of cratonic lithosphere: Insights from numerical simulations of coupled mantle convection and continental tectonics. J Geophys Res 108:2303, doi:10.1029/2002JB001859 Logan WE (1857) On the division of the Azoic rocks of Canada into Huronian and Lawrentian. Proc Am Assoc Adv Sci 1857:44–47 Lowe DR, Tice MM (2007) Tectonic controls on atmospheric, climatic, and biological evolution 3.5–3.4 Ga. Precambrian Res 158:177–197 Macgregor AM (1951) Some milestones in the Precambrian of Southern Rhodesia. Proc Geol Soc SA 54:27–71 Martin H, Smithies RH, Rapp R, Moyen J-F, Champion D (2005) An overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79:1–24 McCall JGH (2003) A critique of the analogy between Archean and Phanerozoic tectonics based on regional mapping of the MesozoicCenozoic plate convergent zone in the Makran, Iran. Precambrian Res 127:5–18 Moyen J-F, Stevens G, Kisters AFM (2006) Record of mid-Archean subduction from metamorphism in the Barberton terrain, South Africa. Nature 442:559–562 Myers JS (1976) Granitoid sheets, thrusting and Archean crustal thickening in West Greenland. Geology 4:265–268
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Nutman AP, Friend CRL, Bennett VC (2002) Evidence for 3650–3600 Ma assembly of the northern end of the Itsaq Gneiss complex, Greenland: implications for early Archean tectonics. Tectonics 21:10.1029/ 2000TC001203 Parman SW (2007) Helium isotopic evidence for episodic mantle melting and crustal growth. Nature 446:900–903 Rapp RP, Watson EB, Miller CF (1991) Partial melting of amphibolite/ eclogite and the origin of Archean trondhjemites and tonalites. Precambrian Res 51:1–25 Rey PF, Philippot P, Thebaud N (2003) Contribution of mantle plumes, crustal thickening and greenstone blanketing to the 2.75-2.65 Ga global crisis. Precambrian Res 127:43–60 Riciputi LR, Valley JW, McGregor VR (1990) Conditions of Archean granulite metamorphism in the Godthab-Fiskenaesset region, southern West Greenland. J Metmaorphic Geol 8:171–190 Rollinson H (2007) Early Earth Systems: A geochemical Approach. Blackwell, Maldon, USA Sandiford M, Van Kranendonk MJ, Bodorkos S (2004) Conductive incubation and the origin of dome-and-keel structure in Archean granitegreenstone terrains: a model based on the eastern Pilbara Craton, Western Australia. Tectonics 23, TC1009, DOI: 10.1029/ 2002TC001452 Shirey SB, Kamber BS, Whitehouse MJ, Mueller PA, Basu AR (2008) A review of the isotopic and trace element evidence for mantle and crustal processes in the Hadean and Archean: Implications for the onset of plate tectonic subduction. In: Condie KC, Pease V (eds) When did plate tectoincs begin on Earth? Geol Soc America, Spec Paper 440, pp 1–29 Smithies RH, Champion DC, Van Kranendonk MJ, Howard HM, Hickman AH (2005) Modern-style subduction processes in the MesoArchean: geochemical evidence from the 3.12 Ga Whundo intraoceanic arc. Earth Planet Sci Lett 231:221–237 Smithies RH, Champion DC, Van Kranendonk MJ (2009) Formation of Paleoarchean continental crust through infracrustal melting of enriched basalt. Earth Planet Sci Lett 281:298–306 Stein M, Hofmann AW (1994) Mantle plumes and episodic crustal growth. Nature 372:63–68 Stern RJ (2005) Evidence from ophiolites, blueschists, and ultrahighpressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time. Geology 33: 557–560 Stockwell CH (1961) Structural provinces, orogenies and time classification of rocks of the Canadian Shield. Geol Surv Can Pap 61–17:108–118 Turcotte DL, Schubert G (2002) Geodynamics, 2nd Edition. Cambridge University Press, New York, 456 p van Hunen J, van den Berg AP (2008) Plate tectonics on the early Earth: limitations imposed by strength and buoyancy of subducted lithosphere. Lithos 103:217–235 Van Kranendonk MJ (2007) Tectonics of early Earth. In: Van Kranendonk MJ, Smithies RH, Bennet V (eds) Earth’s oldest rocks. Developments in precambrian geology, vol 15. Elsevier, Amsterdam, pp 1105–1116 Van Kranendonk MJ, Smithies RH, Hickman AH, Champion DC (2007) Secular tectonic evolution of Archaean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia. Terra Nova 19:1–38 Van Kranendonk MJ, Kro¨ner A, Hegner E, Connelly J (2009) Age, lithology and structural evolution of the c. 3.53 Ga Theespruit formation in the Tjakastad area, southwestern Barberton Greenstone Belt, South Africa, with implications for Archean tectonics. Chem Geol 261:114–138
Van Kranendonk MJ, Smithies RH, Hickman AH, Wingate MTD, Bodorkos S (2010) Evidence for Mesoarchean (3.2 Ga) rifting of the Pilbara Craton: the missing link in an early Precambrian Wilson cycle. Precambrian Res 177:145–161 Viljoen MJ, Viljoen RP (1969) The geology and geochemistry of the lower ultramafic unit of the Onverwacht Group and a proposed new class of igneous rocks. Geol Soc SA Spec Publ 2:55–86 Whalen JB, Percival JA, McNicoll VJ, Longstaffe FJ (2002) A mainly crustal origin for tonalitic granitoid rocks, Superior Province, Canada: implications for late Archean tectonomagmatic processes. J Petrol 43:1551–1570 Wilde SA, Valley JW, Peck WH, Graham CM (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409:175–178 Wilks ME (1988) The Himalayas – a modern analogue for Archean crustal evolution. Earth Planet Sci Lett 87:127–136 Windley BF, Garde AA (2009) Arc-generated blocks with crustal sections in the North Atlantic craton of West Greenland: crustal growth in the Archean with modern analogues. Earth Sci Rev 93:1–30 Wyman DA, Ayer JA, Conceic¸a˜o RV, Sage RP (2006) Mantle processes in an Archean orogen: evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths. Lithos 89:300–332 Zegers TE, van Keken PE (2001) Middle Archean continent formation by crustal delamination. Geology 29:1083–1086
Archean Traces of Life NICOLA MCLOUGHLIN Department of Earth Science and Centre for Geobiology, University of Bergen, Bergen, Norway
Synonyms Archean biosignatures; Trace of life
Keywords Earliest evidence of life on earth, emergence of life, oldest fossils
Definition The Archean is the period of geological time between 3.8 and 2.5 billion years ago when life is thought to have emerged on Earth. Traces of Archean life are preserved in rare, fragmentary and often highly altered rock sequences. Morphological evidence for Archean life is provided by ▶ microfossils, microborings, ▶ stromatolites, and wrinkle mats. Chemical evidence for life is recorded by stable isotope ratios of C and S especially. These different biosignatures are yet to provide a consistent and complete picture of early Archean ecosystems and there is currently little scientific consensus about when and where life first emerged on Earth. Refining our understanding of
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microbial biosignatures in the Archean rock record is essential to designing strategies for seeking life elsewhere in our universe and for ratifying this evidence.
Overview This entry first explains where to look for Archean traces of life, what evidence astrobiologists seek, and how these rocks are investigated, especially the techniques and approaches involved. Then a review of current research findings is given focusing on selected case studies of microfossils, stromatolites, and microborings along with stable isotope evidence from the early Archean. Lastly, new frontiers in early earth research are described with the aim of better understanding the nature of Archean life and environments and how this translates to recognizing signatures of life beyond earth.
Basic Methodology Locating Well-Preserved Archean Rocks from Habitable Environments The search for Archean traces on life relies upon geological mapping and radiometric dating to locate rocks of Archean age. Worldwide, there are two ▶ cratons that preserve intact sequences of early Archean age where now metamorphosed volcanic and sedimentary rocks are preserved in ▶ greenstone belts – so called because of the green color conferred by their typical metamorphic minerals. The first of these is the Pilbara of Western Australia and the second is the Kapvaal Craton of South Africa and the ▶ Barberton Greenstone Belt. In recent years, there have been several ▶ Archean scientific drilling project that have targeted these sequences to seek the earliest evidence for life. Scientific drilling yields continuous sequences of rock unaffected by alteration at the earth’s surface allowing more complete investigation of well-preserved biosignatures within their geological context. Older Archean rocks of between 3.8 and 3.7 Ga from the ▶ Isua supracrustal belt of Western Greenland and also Labrador are of much higher metamorphic grade and more intensely deformed. Thus, any morphological traces of life have been almost completely destroyed in these earliest rocks and discussions regarding the evidence for life center on chemical evidence alone. The search for Archean life has traditionally centered on meta-sedimentary rocks in particular cherts and carbonates. Geological environments where microbial remains are most likely to be preserved are those where rapid, contemporaneous mineralization entombs and permineralizes living organisms. Precipitation of microcrystalline silica is
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a good example and can preserve cellular remains with exceptionally high fidelity. Cherts are formed in the vicinity of hydrothermal vents, hot springs, and as chemical sediments on the Archean seafloor. The retention of fossilized biosignatures over geological time frames is increased if the host rock comprises phases that are resistant to postdepositional alteration processes such as diagenetic recrystallization, dissolution, or replacement. In recent years, a new approach to seeking traces of Archean life has come to prominence and this involves seeking “footprints” of life or tunnels created by microbes that etch rocks rather than the organic remains of the microorganisms themselves. Meta-volcanic glass from Archean seafloor ▶ pillow lavas have been found to contain such “microbial footprints” and Archean carbonate sequences are now also being reexamined for evidence of such rock-tunneling microorganisms. An important criterion for establishing the ▶ biogenicity of a candidate Archean traces of life is the demonstration that the geological environment was viable for life. This translates to the assessment of habitability or, in other words, mapping out the environmental limits to life. There are a number of first-order differences between the Archean world and the recent earth that should be borne in mind. Firstly, Archean surface environments were largely anoxic, the atmosphere was probably rich in carbon dioxide and methane and there was no ozone layer. Secondly, seawater was supersaturated with silica, its pH, temperature, and salinity are widely debated and certainly differed from today, with the ▶ ocean temperatures likely being warmer. Thirdly, exactly when Archean plate tectonics began and the nature of early tectonic processes is unclear with profound implications for the cycling of nutrients on the earth and the oxidation of key geological reservoirs. A comprenhensive review of the latest research investigating early Archean rocks can be found in Van Kranendonk et al. (2007).
Textural Evidence of Life: Investigating Morphological Complexity Morphological ▶ complexity is often regarded as a diagnostic criterion for life. But it must be remembered that complex shapes do not require complex causes and can arise naturally in physiochemical systems as shown, for example, by snowflakes growth. There are three key morphological traces of life: microfossils, stromatolites, and microborings, and interpretation of this morphological evidence proceeds hand in hand with studies of modern analogues and exploration of potential abiological mimics as explained below.
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Chemical Evidence of Life: Elemental and Isotopic Signatures ▶ Isotopic ratios preserved in ancient rocks may record past biological activity and can be measured by mass spectrometry to test for the presence of life and sometimes identify the metabolisms involved. Carbon and sulfur are the main isotopic tools used in the search for Archean life and these are introduced below. Elemental mapping in the vicinity of microbial remains can also be highly informative as the life activity and/or the subsequent decay of a microorganism can modify the composition of the surrounding minerals. Examples are given below along with the techniques and instruments capable of undertaking such isotopic and elemental mapping.
Carbon Isotopes ▶ Carbon isotopes are probably the most studied isotopic tracer of life on Earth. Carbon isotope systematics is described elsewhere in this volume and rather here the focus is on their application to seeking traces of Archean life. The d13C of the biosphere through time has been measured directly from carbonaceous material found in ancient sediments and if this can be shown to be both syngenetic and endogenetic then it records the microbial metabolisms employed at that time. Typical Archean organic matter is found to have a d 13C values of 20‰ relative to inorganic carbonate leading many researchers to claim that biological activity began 3.8 billion years ago (Schidlowski 2001). Specific microbial metabolisms have also been inferred on the basis of the magnitude of carbon ▶ isotopic fractionations measured from Archean rocks and several examples will be discussed below.
Sulfur Isotopes Microorganisms that metabolize sulfur compounds are one of the most deeply rooted groups in the Tree of Life. ▶ Sulfur isotopes preserved in ancient sulfides especially pyrite along with sulfate minerals like barite are used to trace ancient microbial metabolisms. A review of sulfur isotopes in early Archean rocks can be found in Van Kranendonk et al. 2007. The baseline for interpreting such data is provided by studies of modern microbes that employ sulfur-based metabolisms including sulfur oxidation, sulfate reduction, and sulfur disproportionation (Canfield 2001). Sulfur cycling and isotope systematics are explained in detail elsewhere in this volume. Of particular interest to Archean studies is the development of mass spectrometry techniques that can measure mass independent sulfur isotope fractionations (MIF) as will be explained below. Sulfur isotopes have also provided a hotly debated tracer for the rise of
atmospheric oxygen (e.g., Farquhar et al. 2000) and this will not be discussed further here. The S isotope record from 2 Ga onward shows d34S fractionations of 50–60‰ between sulfides that are depleted relative to coexisting sulfates and this has been attributed to microbial sulfate reduction (Canfield and Raiswell 1999). In older rocks such fractionations are much smaller with most sedimentary sulfides older than 2.7 Ga showing a narrow d34S range and this has several possible explanations. Firstly, a nonbiological origin from H2S derived from hydrothermal or volcanogenic processes always needs to be tested. Secondly, sulfate reducers only discriminate sulfur isotopes when seawater sulfate concentrations are above 1 millimolar. Thus, the absence of a large d34S signal before 2.7 Ga could mean that either seawater sulfate levels were low or that sulfate reduction had not yet evolved. Evidence in support of the former explanation is discussed below.
Additional Stable Isotope Systems Additional isotopic systems that have been utilized to investigate Archean environments and traces of life are explained in a rich literature that includes silicon, oxygen, and deuterium isotopes measured on cherts to investigate seawater temperatures (e.g., Hren et al. 2009); ▶ nitrogen isotopes measured especially on ▶ kerogen (e.g., Godfrey and Falkowski 2009) and iron isotopes in a range of rock types (e.g., Dauphas et al. 2004).
Instrumental Techniques for Seeking Traces of Archean Life Having outlined the morphological and chemical basis for seeking Archean traces of life, the techniques used for deciphering these traces are reviewed. All such investigations begin with geological mapping to identify the nature of the host rocks, to determine if the context was plausible for life, and to establish age relationships with other rock units. The key techniques are: Optical microscopy: Examines the morphology of the putative biosignature in two dimensions and if z-plane stacking is available in three dimensions, also the mineralogy of the enclosing rock and relative age of the candidate biosignature with respect to other fabrics in the rock. Scanning electron microscopy – with energy dispersive X-rays (SEM-EDX): Examines the shape and surface morphology of a putative biosignature. Accompanying element distribution maps can be created using EDX. Focussed ion beam milling – transmission electron microscopy (FIB-TEM): FIB is used to mill a very thin 100 nm wafer from a chosen site within a sample targeting, for example, fossilized cell walls. This can then
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be imaged by TEM at the nanometer scale to reveal cellular and crystalline structures. Electron diffraction patterns can also be generated to identify crystalline phases. Electron microprobe: Is used for nondestructive analysis of the chemical composition of a biosignature including the quantification of elements present at levels as low as 100 ppm. Confocal laser Raman micro-spectroscopy: Generates spectra that are diagnostic of different mineral and organic polymorphs and can be used for rapid mineral identification. Also the spectra can be used for nondestructive 2-D and 3-D morphological mapping of, for instance, microfossils. Atomic force microscopy (AFM): Can be used to image and measure the atomic surface structure of a sample at the nanoscale by “feeling” the surface with a cantilever tip (can be coupled to a Raman microscope). Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of stable Isotopes C and S: Can measure isotopic ratios for a target spot several microns across to detect potential biological processing of these elements. Secondary ion mass spectrometry (▶ SIMS and nanoSIMS): A surface analytical technique that enables in situ elemental mapping of major and trace elements and measurement of isotopic ratios at the micron scale or submicron scale in the case of nanoSIMS. Can detect elements present in the parts per billion range. Gas chromatography – mass spectrometry (▶ GC/MS): Used to identify organic molecules that are ▶ biomarkers for specific groups of organisms, for example, ▶ cyanobacteria. Radiogenic isotopes: The abundances of naturally occurring radioactive elements is measured to calculate absolute ages of rocks. Synchrotron X-ray tomography: Nondestructive 3-D morphological images created from a series of 2-D X-ray images taken around a single axis of rotation. Yields spectacular images of paleontological samples. Synchrotron X-ray Spectroscopy and Microscopy: Uses the absorption of X-rays to image samples at the micron to nanometer scale and to investigate, for example, the redox state or coordination chemistry of the sample. There are many astrobiological applications, for instance, to investigate microbe–mineral interfaces.
Key Research Findings Microfossils Microfossils are the permineralized remains of carbonaceous microbial cells and display a range of shapes that in
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the Archean include coccoids or simple spheres and filaments that may be septate and/or branched. An instructive example of how the biogenicity of candidate Archean microfossils is assessed comes from the 3.45 Ga ▶ Apex Chert of Western Australia that is now famous for engendering a vigorous debate regarding the oldest microfossillike objects (Fig. 1c). In the 20 years since their discovery these “microfossils” have become the cornerstone of textbook descriptions of an early Archean biosphere. This changed however, when a reexamination of the “microfossils” called into question their biogenicity (Brasier et al. 2002). These authors argued that the geological context, morphology, and distribution of the “microfossils” are more consistent with an origin as ▶ abiotic graphite artifacts, produced by the recrystallization of amorphous silica to spherulitic chert. The principal lines of evidence from Brasier et al. (2005) are summarized in Table 1 and contrasted with the original interpretation of Schopf and Packer (1987) and Schopf (2002). An origin for these “microfossils” as oxygen-producing cyanobacteria-like organisms now seems highly unlikely. There have been several subsequent studies that have investigated the morphology of the Apex microfossil-like structures and compared them to younger less metamorphosed samples, also studies that have looked at the carbon ultrastructure and likened this to younger biogenic microfossils. But no studies have yet investigated the correlation between seafloor-hydrothermal depositional gradients within the Apex Chert and changes in the candidate chemical and/or morphological biosignatures to conclusively reconstruct a microbial ecosystem. Moreover, SEM investigations of the Apex Chert have revealed multiple episodes of hydrothermal alteration at temperatures 250 C, also recent groundwater alteration that have generated micrometer-sized silica structures resembling microbial exopolymers and textures formed by the partial dissolution of tubular minerals that mimic some fossilized microbial mat textures (Pinti et al. 2009). In addition, there is evidence for postdepositional colonization of micro-cracks in the Apex Chert and taken together these processes point to several sources of nonindigenous carbonaceous material within this unit. In summary, the Apex Chert has proven to be a highly controversial but also instructive example of how the biogenicity and antiquity of microfossil-like structures can be tested. Further examples of putative Archean microfossils including spheroids, ellipsoids, and filaments have been reported from the 2.6 Ga Ghaap Subgroup of South Africa, the 2.7 Ga ▶ Tumbiana formation of West Australia, the 3 Ga Farrell Quartzite of West Australia,
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a
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Archean Traces of Life. Figure 1 Precambrian morphological traces of life. (a) Photomicrograph of microbial biotextures in an inter-pillow breccia from the Hooggenoeg formation of the Barberton Greenstone Belt, South Africa, the meta-volcanic glass comprises a greenschist facies assemblage of chlorite and quartz with titanite-filled tubular textures, these are curvilinear and unbranched, radiate from a central “root zone,” and are segmented by crosscutting chlorite; (b) scanning electron micrograph of a twisted filamentous ▶ pseudofossil made experimentally by precipitating barium-carbonate crystals in sodium silicate gel; (c) branched, septate “microfossil” composed of carbonaceous material (orange) in a silica matrix (yellow) from the 3.45 Ga Apex Chert of West Australia; (d) transmitted light image of coccoid microfossils from the 1 Ga Boorthana Chert of South Australia; (e) putative microbial mat layer from the 3.5 Ga Buck Reef Chert of South Africa; a lower layer of rounded, composite, carbonaceous grains is overlain by putative microbial mat rip-up fragments that show plastic deformation features; (f) automontaged, transmitted light image of intertwined filamentous microfossils from the 3.2 Ga Sulphur Springs Group of West Australia. Scale bars: (a) 50, (b) 20, (c) 10, (d) 50, (e) and (f) 50 mm
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Archean Traces of Life. Table 1 Contrasting lines of evidence and their interpretation collected from the 3.45 Ga Apex Chert of Western Australia and contained “microfossil” structures Lines of evidence
Brasier et al.
Schopf et al.
Environment Deep marine seafloor cherts with intrusive of deposition hydrothermal dyke cherts
Shallow marine silicified sediments
“Microfossil” morphology
Sheets and wisps of carbonaceous material Eleven taxa of filamentous “microfossils” concentrated around the rims of silica spherulites and rhombic crystal inclusions
Laser Raman analysis
The carbonaceous material has a graphitic Raman signature and the “microfossil” signature is indistinguishable from the matrix carbonaceous material
The “microfossils” have a Raman signature that is argued to be comparable to disordered kerogenous carbon from younger biogenic assemblages
Carbon isotopes
d13Corg of 30‰ to 26‰ which cannot exclude abiotic Fischer–Tropsch synthesis
d13Corg of 30‰ to 23‰ lies within the range of biological fractionation
Interpretation Abiotic artifacts created by the recrystallization of amorphous silica that displaced graphitic margins forming a spectrum of arcuate to dendritic artifacts
the 3.2 Ga Dixon Island Fm of W Australia, 3.41 Ga Kromberg Fm of South Africa, and the 3.5 Ga Dresser Formation of the Pilbara; these are illustrated and discussed in Wacey (2009). The perennial difficulty with interpreting all such structures is that they comprise shapes that can be difficult to distinguish from natural abiotic, crystal habits that could grow under similar conditions and develop complex self-organized morphologies (Brasier et al. 2006). This has been illustrated by “crystalgarden” type experiments that precipitate microfossil-like biomorphs in sodium silicate gels (Garcı´a-Ruiz et al. 2003; Fig. 1b). Moreover, organic compounds produced by the abiogenic breakdown of iron-carbonate can condense onto these biomorphs during mild heating thereby mimicking both the morphological and chemical signatures of 3.5 Ga “microfossils” (Garcı´a-Ruiz et al. 2003). Perhaps more robust microfossil evidence may come from the younger Sulphur Springs Group at 3.24 Ga in the form of pyritic filaments from a deep sea, volcanogenic, massive sulfide deposit, interpreted as the fossilized remains of thermophilic, chemotrophic prokaryotes (Rasmussen 2000; Fig. 1f). These straight, curved, or sinuous filaments exhibit putative biological behavior including preferred orientations, clustering, and intertwining. They are found in an early chert fabric in a subsurface drill core that is crosscut by later fractures. Thus, these filaments appear to satisfy criteria for the ▶ syngenicity and biogenicity of candidate microfossils and await supporting lines of geochemical evidence.
Silica permineralization of filamentous “microfossil” cells that could have included oxygen-producing cyanobacteria and possibly larger, beggiatoacean microfossils
Siliclastic sediments such as sandstones, siltstones, and mudstones have also been somewhat overlooked in the search for Archean traces of life. A recent study by Javaux et al. (2010) reports large, hollow spherical organic-walled microfossils known as acritarchs from the 3.2 Ga Moodies Group of S Africa. These structures pass syngenicity and endogenicity tests and appear to be the oldest and largest organic-walled spheroidal microfossils reported to date. They may record a planktonic ecosystem contemporaneous with benthic microbial mat textures described by Noffke et al. (2006) and will reinvigorate the search for traces of Archean life in siliclastic sediments with low-organic carbon contents.
Microborings Microborings are micron-sized cavities created by the activities of rock-dwelling microorganisms termed endoliths. Microborings have long been known from Precambrian silicified carbonates and have more recently been reported from the glassy margins of pillow lavas from modern to Archean volcanic rocks (Staudigel et al. 2008). A rock-dwelling mode of life in the Archean subseafloor may have offered many attractions including proximity to geothermal heat; a source of reductants, principally Fe and Mn which are abundant in basalts; and access to both oxidants and carbon sources carried by circulating fluids. Such habitats would also have offered protection from the elevated UV radiation and meteoritic and cometary impacts on the early earth. First, a brief
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overview of what is known about these organisms in the modern subseafloor is given (for a more complete review, see Thorseth 2011), then these are compared to mineralized, tubular structures from the Archean. Given that pillow lavas constitute an estimated 99% of Archean greenstone belts they represent perhaps the largest potential habitat for seeking traces of early Archean life. Trace of ▶ endolithic microbes has been reported from both the modern oceanic crust and older seafloor fragments, for a comprehensive review, see Furnes et al. (2008). These are microtubular and granular cavities found at the interface of fresh and altered glass, along fractures in the rims of pillow basalts and around the margins of volcanic glass fragments. These are both texturally and chemically distinct from abiotic alteration textures found in basalts and include diverse tubular shapes such as spiralled, annulated, and branched forms (McLoughlin et al. 2009). Studies of recent material have found nucleic acids and bacterial and archeal RNA concentrated within these microborings. These textures may later be mineralized by clays and iron-oxyhydroxides that can preserve localized enrichments in C, N, and P along the margins that are interpreted as decayed cellular remains. Quantitative studies of the distribution and abundance of these alteration textures with depth in the modern oceanic crust have found that in the upper 350 m of the crust the granular type is dominant. Meanwhile, the tubular alteration textures constitute only a small fraction of the total zone of alteration and show a clear maximum at 120–130 m depth corresponding to lower temperatures of 70 C and thermophilic metabolisms. Comparisons of seafloor and drill core samples of different ages suggest that bioalteration commences early soon after crystallization of the basalt flows. In the Archean tubular bioalteration, textures have been reported from the formerly glassy rims of pillow basalts and inter-pillow breccias from both South Africa (Fig. 1a) and West Australia (Furnes et al. 2007). Some of the best examples come from the 3.46 Ga Hooggenoeg Complex of South Africa and are mineralized by titanite (CaTiO3) that ensured preservation of the textures when the host glass was transformed to a greenschist facies metamorphic mineral assemblage. These mineralized tubular structures are 1–10 mm wide, up to 200 mm long, and extend away from “root zones” of fine-grained titanite that is associated with fractures in the basaltic glass (Fig. 1a). These microtubes can have a segmented appearance caused by overgrowths of metamorphic chlorite. Morphologically comparable microtubular structures have also been reported from inter-pillow breccias within
the 3.35 Ga Eurobasalt Fm of W Australia (Furnes et al. 2007). These are also infilled with titanite that has been dated directly using U-Pb systematics, confirm that the microborings formed prior to an Archean 2.7 Ga phase of metamorphism (Banerjee et al. 2007). Late Archean microborings have now also been described from 2.5 Ga pillow lavas of Wutai China. In summary, microborings provide an important tool for mapping the deep subseafloor biosphere that may represent one of the earliest habitats for life on earth and perhaps other planetary surfaces.
Stromatolites and Wrinkle Mats Stromatolites are the most abundant macrofossil in the Precambrian rock record and are a volumetrically significant component of Precambrian carbonate platforms. Stromatolites comprise laminated, centimeterto-decimeter-scale domes, cones, columns, and planiform surfaces that are built through a complex interplay of physical, chemical, and biological processes producing an array of micro-fabrics and laminar geometries. The processes that lead to the growth of stromatolites and how these can be identified in the ▶ fossil record are reviewed in detail elsewhere in this volume. Here a nongenetic definition of a stromatolite is adopted because it can be difficult to demonstrate active biological participation in stromatolite growth. There have been many attempts to develop stromatolite biogenicity criteria in an effort to distinguish laminated seafloor precipitates formed by purely chemical processes from microbially mediated deposits. Most of these biogenicity criteria are so exacting, however, that the majority of Precambrian stromatolites of widely regarded biogenic origin fail to qualify. The task of distinguishing biogenic from abiogenic stromatolites is unfortunately, especially difficult in the Archean where diagenetic recrystallization and low-grade metamorphism can destroy any organic micro-textures that were once present. The oldest putative stromatolites are reported from the 3.49 Ga Dresser Formation of the West Australia, including wrinkled planiform surfaces, broad domes, and columnar forms. These occur at several localities in the ▶ North Pole Dome both in syn-depositional barite mounds and hydrothermal dykes and in silicified and hydrothermally altered ferruginous-carbonates (Wacey 2009). They are of disputed biological origin and are discussed elsewhere in this volume. Some of the next oldest putative stromatolites are described from the 3.4 Ga Strelley Pool Chert of West Australia. The discovery of large coniform stromatolites with rare
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flank structures and domal and laterally linked pseudocolumnar morphologies lead to a biological origin for these structures being advanced (Hofmann et al. 1999). Subsequently, detailed mapping of the stromatolites and investigation of rare outcrops with good micro-textural preservation has found evidence for a spatiotemporal correlation between stromatolite morphology, micro-fabric, and depositional environment (Allwood et al. 2009). Regionally however, the more typical, small, unbranched coniform stromatolites of the Strelley Pool Chert do not show unambiguous biological characteristics or depth-controlled distribution and/or changes in morphology with depth (Wacey 2010). In short, the Strelley Pool Chert includes a spectrum of stromatolitic structures some of which are biogenic, but we are still a way from confidently distinguishing those that are undoubtedly biogenic from those that are not microbially mediated. A morphological biosignature related to stromatolites is that of wrinkle-mat-textures or microbially induced sedimentary structures (MISS). These are formed by the interaction of benthic microbiota with physical sediment dynamics and some of the oldest come from the 3.2 Ga Moodies Group of South Africa (Noffke et al. 2006). These types of structures have been described as orange-peel textures on bedding surfaces with microscopic reticulated filaments of carbonaceous material among the sediment grain that have carbon isotopic signatures that are consistent with a biological origin. Older putative wrinkle-mat-horizons are described from cherts of the Barberton (e.g., Walsh and Lowe 1999). These comprise carbonaceous laminae and wisps with examples of plastically deformed carbonaceous fragments interpreted as microbial mat rip-up clasts (Fig. 1e) and purported examples of filamentous microfossils. These wrinklemat-type textures and morphologies are certainly very suggestive of microbial processing and arguably better preserved than anything hitherto reported from Western Australia. Recent studies documenting the facies and depth-dependent distribution of this carbonaceous material using detailed petrography and elemental analysis have strengthened the case for biogenicity and argued for the presence of anoxygenic photosynthesizers 3.4 Gyr (Tice and Lowe 2004).
Biomarker Compounds In rocks that do not preserve cellular microbial remains biomarker compounds found in soluble hydrocarbon fractions have been used as markers of specific biological pathways (Brocks and Summons 2003). Such compounds
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are derived from lipids in cell membranes and represent an important source of information about the diversity and evolution of life. For example, a suite of lipid biomarkers extracted from 2.7 Gyr organic-rich shales from Western Australia included hopane and sterane compounds that were interpreted, respectively, as the membrane remnants of cyanobacteria, a group of organisms characterized by oxygen-producing ▶ photosynthesis; and of eukaryotes organisms that have a membrane-bound nucleus and a complex cytoskeleton (Brocks et al. 2003). These findings in rocks of 2.7 Ga greatly extended the age of first appearance of cyanobacteria previously estimated at 2.15 Gyr old from fossil evidence and eukaryotes previously estimated at between 1.78 and 1.68 Gyr. Moreover, these findings also seemed to suggest an early accumulation of atmospheric oxygen. Such biomarker studies, however, have long been surrounded by concerns of contamination from nonindigenous hydrocarbons, especially since the carbon isotope ratios of the extracted biomarkers was significantly enriched relative to the bulk sedimentary organic matter. A recent nanoSIMS study has shown that the carbon isotope values of pyrobitumen (thermally altered petroleum) and kerogen contained within these rocks are strongly depleted in 13C, confirming that the indigenous petroleum is 10–20‰ lighter than the extracted hydrocarbon biomarkers (Rasmussen et al. 2008). These findings are inconsistent with an indigenous origin for the biomarkers that, moreover, have carbon isotopic values that are atypical of late Archean organic matter. Thus, it appears that the biomarkers derived from these 2.7 Ga-rich shales are not indigenous to the rock and are not robust evidence for cyanobacteria and eukaryotes at 2.7 Ga (Rasmussen et al. 2008). This in situ nanoSIMS approach to measuring carbon isotopes on different carbon bearing phases will be used to test the antiquity and endogenicity of other late Archean and younger reports of biomarkers.
Carbon Isotopes Various microbial metabolisms have been argued for on the basis of carbon isotopic ratios measured on organic matter contained within Archean rocks. These include anoxygenic photosynthesis from C isotopes in the range of 20‰ to 30‰ measured on kerogen (e.g., Tice and Lowe 2004), and methanogenesis from very low C ratios of 56‰ measured on methane bearing fluid inclusions (Ueno et al. 2006). These d13C values are certainly consistent with life but, unfortunately, carbon isotope fractionation patterns when taken alone are not a uniquely biological signal. This is because there are alternative
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nonbiological explanations for such light carbon isotopic values that need to be excluded and these are the source of much debate in Archean rocks. For example, ▶ Fischer– Tropsch type (FTT) reactions between CO and metals (Sherwood Lollar et al. 2002) or the metamorphic reduction of siderite (van Zuilen et al. 2002) can generate carbon isotope fractionations that lie within the “biological domain.” Thus, in the early rock record, carbon isotopes need to be integrated with other isotope systems along with the geological context and any candidate morphological traces of life. One of the most ancient claims for life comes from isotopically light carbon found in 3.8 Ga highly metamorphosed rocks from the island of ▶ Akilia off the West coast of Greenland. The material analyzed was graphitic carbon found as inclusions within grains of apatite, with a d 13C signature of –20‰ to –50‰ (Mojzsis et al. 1996). The vigorous debate that has surrounded these observations provides an illustrative case study of the need to understand the complete geological history of a rock argued to contain chemical traces of life. Different workers have subsequently challenged the evidence for life in the Akilia rocks on the basis of the age of the outcrop; the apatite petrography, and the fact that the protolith is not sedimentary in origin. A parallel debate has played out regarding the original biogenic interpretation of isotopically light graphite in apatite crystals from another site in the ▶ Isua Greenstone Belt of Greenland (Mojzsis et al. 1996). Here, petrographic and geochemical studies have also rejected the original biogenic interpretation and proposed that the metamorphic decomposition of ferrous carbonate (siderite) is the more likely source of the depleted carbonaceous material (van Zuilen et al. 2002). There does remain, however, one locality in the Isua region where the association of graphite with metasedimentary rocks may still be suggestive of life (Rosing and Frei 2004) and this occurrence awaits further verification.
Sulfur Isotopes The earliest sulfur isotope evidence suggestive of life comes from microscopic sulfides contained within barite crystals in the 3.49 Gyr Dresser Formation of North Pole, Western Australia (Shen et al. 2001). Fractionations of up to 21.1‰ between the sulfides and coexisting sulfates together with the co-occurrence of organic carbon were used to argue that sulfate-reducing bacteria had evolved by 3.49 Ga. More recent investigations of material from ▶ North Pole have measured mass-independently fractionated sulfur isotopic anomalies (MIF) in these sulfides
that differs from their host barite (Philippot et al. 2007). These authors interpret this combined negative d34S and positive MIF signature of the sulfides as the product of microorganisms that disproportionate elemental sulfur and not sulfate-reducing bacteria. In contrast, Ueno et al. (2008) and Shen et al. (2009) found that these same microscopic sulfides possessed – D33S values. They used these data together with D33S and D36S relationships to argue that their sulfides formed dominantly by microbial sulfate reduction. These conflicting conclusions may in part be due to methodological differences between the studies. An alternative approach has been taken by Wacey et al. (2010) who investigated mm-sized, diagenetic pyrite grains from a 3.4 Gyr, regionally extensive shallow marine sandstone unit. They reported high-resolution multiple S isotope analysis (32S, 33S, 34S) by secondary ion mass spectrometry, both nanoSIMS and traditional large-radius ion microprobe to reveal d34S values between –12‰ and +6‰, and D33S values between –1.65‰ and +1.43‰, from pyrite grains within a single thin section. A large spread of d34S values over only 5–10 mm, together with the spatial association of pyrite with C and N indicates biological processing of sulfur. The presence of both +D33S and –D33S signals overprinted by significant mass-dependent d34S fractionation in this pyrite population indicates that both microbial sulfate reduction of aqueous sulfate (–D 33S), and microbial disproportionation of elemental sulfur (+D33S) were co-occurring in an open-marine, sedimentary-hosted ecosystem in the early Archean. A parallel sulfur isotope story is starting to emerge from rocks of the ▶ Barberton Greenstone Belt of South Africa. Shales and black cherts from the 3.3 Ga Mendon Formation yield d34S values with 12‰ variation that was argued to be greater than that expected from purely magmatic or hydrothermal H2S, and due to bacterial sulfate reduction (Ohmoto et al. 1993). But as yet, no corresponding MIF signature suggestive of sulfur disproportionating bacteria has been reported from the Barberton rocks. In late Archean rocks, sulfur isotope evidence in conjunction with carbon isotopes and rare earth element studies give more definitive evidence for the emergence of sulfur metabolisms. Investigations of the 2.7–2.6 Ga Belingwe Belt of Zimbabwe have found pyrites in sulfidic shales with a wide range of d34S values from 21.1‰ to +16.7‰. This range together with the pyrite morphology and isotopic heterogeneity provides good evidence for sulfate-reducing and possible sulfur-oxidizing bacteria at this time (Grassineau et al. 2001). Carbon isotopic investigation accompanied by rare earth element analysis of
Archean Traces of Life
associated stromatolitic and non-stromatolitic sediments across an onshore–offshore gradient have also been used to argue for the presence of a diverse microbial ecosystem including anoxygenic photosynthesizer, methanogens, and methanotrophs at this time (Grassineau et al. 2001). This type of integrated approach is the best way of deciphering Archean traces of life.
Future Directions The controversies that currently surround the earliest claims for life on earth help astrobiologists to develop criteria for testing new and existing claims for extraterrestrial life. Some new techniques and approaches to seeking and verifying Archean traces of life are now highlighted. Firstly, emerging nanoscale techniques that allow highresolution elemental and isotopic analysis using NanoSIMS and synchrotron-based techniques give the opportunity to investigate putative microbial fabrics at a scale never previously obtainable and will help assess the biogenicity of stromatolites and microfossils in particular (e.g., Wacey 2009 and references therein). Secondly, renewed interest is being paid to a wider range of rock types in the search for Archean traces of life including microborings in not just volcanic glass but also silicate minerals and carbonates, and also fine-grained siliclastics to locate organic-walled microfossils and stromatolitic structures in a range of lithologies in addition to classical carbonates. In conclusion, our current understanding of Archean ecosystems is like an unfinished and jumbled up jigsaw – as new techniques, preservational windows and rock types are found new pieces in this jigsaw of early life and environments will come together refining our global picture.
See also ▶ Abiotic ▶ Akilia ▶ Apex Chert ▶ Archean Drilling Projects ▶ Archean Environmental Conditions ▶ Archean Eon ▶ Archean Tectonics ▶ Barberton Greenstone Belt ▶ Biogenicity ▶ Biomarkers ▶ Biomarkers, Isotopic ▶ Biomarkers, Morphological ▶ Carbon Isotopes as a Geochemical Tracer ▶ Chemolithoautotroph
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▶ Chronological History of Life on Earth ▶ Complexity ▶ Cyanobacteria ▶ Endogenicity ▶ Endolithic ▶ Fischer–Tropsch Effects on Isotopic Fractionation ▶ GC/MS ▶ Geochronology ▶ Greenstone Belts ▶ Isotopic Ratio ▶ Isua Supracrustal Belt ▶ Kerogen ▶ Metasediments ▶ Microbial Mats ▶ Microfossils ▶ Nitrogen Isotopes ▶ North Pole Dome (Pilbara, Western Australia) ▶ Ocean, Temperature of ▶ Oxygen Isotopes ▶ Photosynthesis ▶ Pilbara Craton ▶ Pillow Lava ▶ Pseudofossil ▶ Raman Spectroscopy ▶ SIM ▶ Steranes, Rock Record ▶ Stromatolites ▶ Sulfur Isotopes ▶ Synchrotron Radiation ▶ Syngenicity ▶ Tumbiana Formation (Pilbara, Western Australia)
References and Further Reading Allwood AC, Grotzinger JP, Knoll AH, Burch IW, Anderson MS, Coleman ML, Kanik I (2009) Controls on development and diversity of Early Archean stromatolites. Proc Natl Acad Sci 106:9548–9555 Banerjee NR, Simonetti A, Furnes H, Staudigel H, Muehlenbachs K, Heaman L, Van Kranendonk MJ (2007) Direct dating of Archean microbial ichnofossils. Geology 35:487–490 Brasier MD, Green OR, Jephcoat AP, Kleppe AK, van Kranendonk MJ, Lindsay JF, Steele A, Grassineau NV (2002) Questioning the evidence for earth’s oldest fossils. Nature 416:76–81 Brasier MD, Green OR, Lindsay JF, McLoughlin N, Jephcoat AP, Kleppe AK, Steele A, Stoakes CP (2005) Critical testing of Earth’s oldest putative fossil assemblage from the 3.5 Ga Apex chert, Chinaman Creek, Western Australia. Precamb Res 140:55–102 Brasier MD, McLoughlin N, Wacey D (2006) A fresh look at the fossil evidence for early Archaean cellular life. Phil Trans R Soc B 361:887–902 Brocks JJ, Summons RE (2003) Sedimentary hydrocarbons. Biomarkers for early life. In: Holland HD, Turekian KK (eds) Treatise on geochemistry, vol 8. Elsevier, Amsterdam, p 63
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Brocks JJ, Buick R, Summons RE, Logan GA (2003) A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia. Geochim Cosmochim Acta 67:4321–4335 Canfield DE (2001) Biogeochemistry of sulphur isotopes. Reviews in Mineralogy and Geochemistry 43:607–636 Canfield DE, Raiswell R (1999) The evolution of the sulphur cycle. Am J Sci 299:697–723 Canfield DE, Kristensen E, Thamdrup B (2005) The sulphur cycle. In Aquatic geomicrobiology. Advances in Marine Biology 48:313–381 Dauphas N, van Zuilen M, Wadhwa M, Davis AM, Marty B, Janney PE (2004) Clues from Fe isotope variations on the origins of Early Archean BIFs from Greenland. Science 306:2077–2080 Farquhar J, Bao H, Thiemens M (2000) Atmospheric influence of earth’s earliest sulfur cycle. Science 289:756–758 Furnes H, Banerjee NR, Staudigel H, Muehlenbachs K, McLoughlin N, de Wit M, Van Kranendonk M (2007) Comparing petrographic signatures of bioalteration in recent to Mesoarchean pillow lavas: tracing subsurface life in oceanic igneous rocks. Precamb Res 158:156–176 Furnes H, McLoughlin N, Muehlenbachs K, Banerjee NR, Staudigel H, Dilek Y, de Wit M, Van Kranendonk M, Schiffmann P (2008) Oceanic pillow lavas and hyaloclastites as habitats for microbial life through time – a review. In: Dilek Y, Furnes H, Muehlenbachs K (eds) Links between geological processes, microbial activities, and evolution of life (Springer Book Series). Springer, Heidelberg, pp 1–68 Garcı´a-Ruiz JM, Hyde ST, Carnerup AM, Christy AG, Van Kranendonk MJ, Welham NJ (2003) Self-assembled silica carbonate structures and detection of ancient microfossils. Science 302:1194–1197 Godfrey LV, Falkowski PG (2009) The cycling and redox state of nitrogen in the Archean Ocean. Nat Geosci 2:725–729 Grassineau NV, Nisbet EG, Bickle MJ, Fowler CMR, Lowry D, Mattey DP, Abell P, Martin A (2001) Antiquity of the biological sulphur cycle: evidence from sulphur and carbon isotopes in 2700 million-year old rock of the Belingwe Belt, Zimbabwe. Proc R Soc Lond B 268:113–119 Hofmann HJ, Grey K, Hickman AH, Thorpe RI (1999) Origin of 3.45 Ga coniform stromatolites in the Warrawoona Group, Western Australia. Bull Geol Soc Am 111:1256–1262 Hren MT, Tice MM, Chamberlain CP (2009) Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago. Nature 462:205–208 Javaux EJ, Marshall CP, Bekker A (2010) Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliclastic deposits. Nature 463:934–938 McLoughlin N, Furnes H, Banerjee NR, Muehlenbachs K, Staudigel H (2009) Ichnotaxonomy of microbial trace fossils in volcanic glass. J Geol Soc Lond 166:159–170 Mojzsis SJ, Arrenhius G, McKeegan KD, Harrison TM, Nutman AP, Friend CRL (1996) Evidence for life on earth 3,800 million years ago. Nature 384:55–59 Noffke N, Eriksson KA, Hazen RM, Simpson EL (2006) A new window into Early Archean life: microbial mats in Earth’s oldest siliclastic tidal deposits (3.2 Ga Moodies Group, South Africa). Geology 34:253–256 Ohmoto H, Kakegawa T, Lowe DR (1993) 3.4-billion-year-old pyrites from Barberton, South Africa: sulfur isotope evidence. Science 262:555–557 Philippot P, van Zuilen MA, Lepot K, Thomazo C, Farquhar J, Van Kranendonk MJ (2007) Early Archean microorganisms preferred elemental sulfur, not sulfate. Science 317:1534–1537
Pinti DL, Mineau R, Clement V (2009) Hydrothermal alteration and microfossil artefacts of the 3, 465-million-year-old Apex Chert. Nat Geosci 2:640–643 Rasmussen B (2000) Filamentous microfossils in a 3, 250-million-year-old volcanogenic massive sulphide deposit. Nature 405:676–679 Rasmussen B, Fletcher IR, Brocks JJ, Kilburn MR (2008) Reassessing the first appearance of eukaryotes and cyanobacteria. Nature 455:1101–1104 Rosing MT, Frei R (2004) U-rich Archean sea-floor sediments from Greenland – indications of >3700 Ma oxygenic photosynthesis. E P S L 217:237–244 Schidlowski M (2001) Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of Earth history: evolution of a concept. Precamb Res 106:117–134 Schopf JW (2002) When did life begin? In: Schopf JW (ed) Life’s origin: beginnings of biological evolution. University of California Press, Berkeley, pp 158–180 Schopf JW, Packer BM (1987) Early Archean (3.3 billion to 3.5 billionyear-old) microfossils from Warrawoona Group, Australia. Science 237:70–73 Shen Y, Buick R, Canfield DE (2001) Isotopic evidence for microbial sulphate reduction in the early Archean era. Nature 410:77–81 Shen Y, Farquhar J, Masterson A, Kaufman AJ, Buick R (2009) Evaluating the role of microbial sulfate reduction in the early Archean using quadruple isotope systematics. Earth Planet Sci Lett 279:383–391 Sherwood Lollar B, Westagate TD, Ward JA, Slater GF, LacrampeCouloume G (2002) Abiogenic formation of alkanes in the Earth’s crust as a minor source for global hydrocarbon reservoirs. Nature 416:522–524 Staudigel H, Furnes H, McLoughlin N, Banerjee NR, Connell LB, Templeton A (2008) 3.5 Billion years of glass bioalteration: Volcanic rocks as a basis for microbial life? Earth Sci Rev 89:156–176 Thorseth IH (2011) Basalt (glass, endoliths). In Reitner T (ed) Encyclopedia of geobiology. Springer-Verlag, Berlin/Heidelberg Tice MM, Lowe DR (2004) Photosynthetic microbial mats in the 3, 416-Myr-old ocean. Nature 431:549–552 Ueno Y, Yamada K, Yoshida N, Maruyama S, Isozaki Y (2006) Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era. Nature 440:516–519 Ueno Y, Ono S, Rumble D, Maruyama S (2008) Quadruple sulfur isotope analysis of ca. 3.5 Ga Dresser formation: new evidence for microbial sulfate reduction in the early Archean. Geochim Cosmochim Acta 72:5675–5691 Van Kranendonk MJ, Smithies RH, Bennett VC (eds) (2007) Earth’s oldest rocks: developments in Precambrian geology, vol 15. Elsevier, London van Zuilen MA, Lepland A, Arhenius G (2002) Reassessing the evidence for the earliest traces of life. Nature 418:627–630 Wacey D (2009) Early life on earth: a practical guide. Topics in Geobiology, vol 31. Springer-Verlag, Berlin, 274 p Wacey D (2010) Stromatolites in the c.3400 Ma Strelley Pool formation, Western Australia: examining biogenicity from the macro- to the nano-scale. Astrobiology 10:381–395 Wacey D, McLoughlin N, Whitehouse MJ, Kilburn MR (2010) Two coexisting sulfur metabolisms in a ca. 3,400 Ma sandstone. Geology 38:1115–1118 Walsh MM, Lowe DL (1999) Modes of accumulation of carbonaceous matter in the early Archean: a petrographic and geochemical study of the carbonaceous cherts of the Swaziland Supergroup. In Lowe DR, Byerley GR (eds) Geologic evolution of the Barberton Greenstone Belt, South Africa. Geological Society of America special paper 329, Boulder, pp 167–188
Aromatic Hydrocarbon
Areology Definition Areology (Greek, Ares “Mars,” logos “speech, science”) is the science of the planet ▶ Mars, excluding its atmosphere. It comprises the study of the structure, composition, physical properties, dynamics, origin, and evolution of Mars as well as the processes that formed and shaped Mars. The term areology is not frequently used because scientifically and methodologically it is synonymous to the term “Geology of Mars.” Thus it is common to use the latter term instead of “areology.”
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positive charge in guanidinium ion (protonated guanidine) is delocalized to the three nitrogen atoms in it. The isoelectric point (pI) of arginine is 10.76, which is the highest among the protein amino acids. Adult humans are able to biosynthesize arginine, but infants cannot. Thus, it is an essential amino acid only for infants. To date, arginine has not been found in extraterrestrial bodies like carbonaceous chondrites.
See also ▶ Amino Acid ▶ Protein
See also ▶ Chronology, Cratering and Stratography ▶ Geological Time Scale, History of ▶ Mars ▶ Selenology ▶ Mars Stratigraphy
Argillaceous Earth ▶ Clay
Arginine Definition Arginine is one of 20 ▶ protein ▶ amino acids, whose structure is shown in Fig. 1. Its molecular weight is 174.21. Its three-letter symbol is Arg, and its one-letter symbol is R. It has a guanidino group (H2NC(=NH)NH2) in its side chain. Guanidine shows strong basicity since
H2N
Ariel Definition Ariel is one of the five big satellites of ▶ Uranus, and the closest to the planet. It was discovered by William Lassen in 1851. Its diameter is 1,160 km and its distance to Uranus is 191,000 km or 7.5 planetary radii. Its density is 1.66 g/cm3. Ariel has been explored by the Voyager 2 spacecraft which flew by the Uranian system in January 1986. Ariel is assumed to consist in about 30% silicates and 70% ices. It has a bright surface that shows a network of canyons and faults, such that, after Miranda, Ariel is the most geologically active among Uranus’ satellites. The longest canyon (622 km) is Kachina Chasmata. The activity may result from tidal heating due to the proximity of Uranus at the time of the satellite’s formation.
See also ▶ Giant Planets ▶ Uranus ▶ Voyager (Spacecraft)
Aromatic Hydrocarbon
NH
HENDERSON JAMES (JIM) CLEAVES II Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
NH
H O H2N OH
Arginine. Figure 1 Structural formula of arginine
Definition An aromatic hydrocarbon is a cyclic hydrocarbon where the series of saturated and unsaturated carbon–carbon bonds satisfies Hu¨ckel’s rules, i.e., where the number of
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electrons in double and triple bonds in the ring is 4n+2. The name derives from the fact that the first such molecules discovered tended to have an aromatic odor. They are typically somewhat more stable than their hydrogensaturated analogues. Aromatic hydrocarbons may be monocyclic or polycyclic.
See also ▶ Benzene ▶ PAHs
radiation pressure from the Sun and thereby seeding life from one planet to another, or even between planets of different stellar systems. Arrhenius based his considerations on the fact that the space between the planets of our Solar System is teeming with micron-sized cosmic dust particles, which at a critical size below 1.5 mm would be blown away from the Sun with high speed pushed by radiation pressure of the Sun. Herewith Arrhenius provided a scientific rationale for the theory of ▶ Panspermia, now called Radiopanspermia.
See also
Arrhenius Plot Definition In 1889, the Swedish chemist Svante Arrrhenius showed that the rate of a chemical reaction as a function of temperature could be described by the equation ln k ¼ ln A Ea =RT where k is the reaction rate, R is the universal gas constant, Ea is the ▶ activation energy (the energy required in order for the reactants to react), T the absolute temperature (K), and A the so-called pre-exponential factor (associated with collision and transition state theory). A plot of ln k versus 1/T often yields a straight line, the slope of which is equal to the activation energy of the reaction divided by the universal gas constant (Ea/R) and the y-intercept of which is equal to ln A.
▶ Arrhenius Plot ▶ Lithopanspermia ▶ Panspermia ▶ Spore
References and Further Reading Arrhenius S (1903) Die Verbreitung des Lebens im Weltenraum. Umschau 7:481–485 Arrhenius S (1908) Worlds in the making: the evolution of the universe. Harper & Row, New York
Artificial Cells ▶ Cell Models ▶ Protocell
See also ▶ Activation Energy
Arrhenius Svante
Artificial Chemistries PIETRO SPERONI DI FENIZIO CISUC, Department of Informatics Engineering, University of Coimbra, Coimbra, Portugal
History Svante August Arrhenius (1859–1927), Swedish scientist, received the Nobel prize in Chemistry in 1903 “in recognition of the extraordinary services he has rendered to the advancement of chemistry by his electrolytic theory of dissociation.” Among other achievements, Arrhenius is famous for the Arrhenius equation, which gives the dependence of the rate constant k of a chemical reaction on the temperature T (in K) and the activation energy of the reaction. For astrobiologists, Arrhenius is famous for his thoughts that microscopic forms of life, for example ▶ spores, can be propagated in space, driven by the
Synonyms Reaction network
Keywords Artificial chemistries, artificial life, evolution, reaction network
Definition Artificial Chemistry (AC) is a field of research that studies systems that are similar to, and commonly a generalization of, chemical networks. Those studies are usually done
Artificial Chemistries
through computer simulations which are also called artificial chemistries. An AC can be defined as triplet {M, R, A}, with M being the set of possible molecules (sometimes infinite), R the set of possible reactions, and A the algorithm (Dittrich et al. 2001).
Overview Artificial Chemistry grew as a field from the early 1990s. Different aims converged in producing this field. One of the aims of Artificial Chemistry was to investigate evolution, and the appearance of life. Some of the main questions that have been investigated deal with biology, proto-biology, and evolution and take a bottom-up approach to ▶ Artificial Life: ● Can we generate an artificial chemistry that can generate an artificial life? ● What kind of chemistry can sustain life? ● How does the ▶ Darwinian evolutionary process depend upon the chemistry on which it is based? Of course, as we extrapolate out of chemistry the basic network of relations that can produce life, this would eventually help to recognize life in different contexts. Another, more general, aim was to study Reaction Networks. Reaction Networks appear in multiple fields, but our ability to study them has generally been limited by a difficulty in solving differential equations where the number of different interacting elements (diversity, i.e., number of equations) were too high, and the quantity of each participating element in the reaction network (population size) was too low (which could lead to an element disappearing, and the set of reactions changing). Some Artificial Chemistries investigated exactly this problem. As such the aim of Artificial Chemistries have often been a qualitative study, looking for what sets of molecules would be present in the reactor, more than how many elements would be present for each type of element in the system (Fontana and Buss 1996). Due to the computational difficulty in simulating a chemical system that is both unbounded in the complexity of the molecules involved and that permits a faithful representation of space, the field tends to be divided into two main subfields: Complex Artificial Chemistries and Spatial Artificial Chemistries. Complex Artificial Chemistries are the most simple type of system and assume a well mixed reactor. Each molecule is either present or absent, with no information about the position. Pairs of molecules are then randomly selected and reacted. Such a simple algorithm frees the computational resources permitting the presence of complex molecules, with different behaviors. It is not
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uncommon for artificial chemistries to study the behavior of little program strings with computational capabilities (traditionally lambda terms). Spatial Artificial Chemistries traditionally study systems made up of many simple elements, floating in a 2D or in a 3D spatial environment. The basic elements (in some systems called atoms) can often link together creating long chains (usually called molecules). In the first pioneering works of Spatial Artificial Chemistries, the positions of the molecules were imprecise, as each molecule was generally assigned to a position on a lattice. Recent computational progress permitted a more faithful representation of the movement using differential equations (Fellermann 2009).
Basic Methodology The methodology is slightly different for Spatial Artificial Chemistries and for Complex Artificial Chemistries. Complex Artificial Chemistries, as mentioned before, are essentially reaction networks with a high diversity and a low population size. Such an AC can be defined as a triplet {Molecules, Reactions, Algorithm}. Reactions are usually binary (i.e., two molecules reacting and generating a third). Yet not all pair of molecules can react. When two molecules cannot react, they are said to be elastic. The basic algorithm is generally very simple: 1. Define a Soup S as a multiset of m molecules out of M. 2. Two molecules are randomly selected. 3. If the two molecules can react, a reaction takes place: (a) The result of the reaction is inserted in the Soup. (b) A random molecule is extracted from the soup and eliminated. 4. Go to 2. In many Artificial Chemistries, the reactions are catalytic; so when two molecules react, they are not taken away from the Soup while their product is added to the Soup (3a). Instead, the molecules catalytically induce the formation of the new molecule out of a substrate of basic material (too vast and ubiquitous to be explicitly modeled), while the disappearance (3b) would model the outflux due to molecules being washed away or breaking apart (Dittrich et al. 2001). This traditional structure has been strongly criticized (for not considering conservation of mass, for example) and many alternatives have been offered, but generally with only partial differences in the observed behavior. Traditionally the standard way to investigate such a system is to run it until no new molecule would be generated, at which point the resulting multiset is studied. Lately, a more advanced method has been to calculate all the possible sets where the system could stop (called organizations) and study the structure of the lattice
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of organizations, and use this to map the movement of the artificial chemistry as time progresses (Kaleta 2009). Spatial Artificial Chemistries start in general with a set of atoms, to which a position in space is randomly assigned. At each time step all the atoms are randomly moved, and when two atoms end up near each other, they have the possibility of colliding. Unless the system is superimposed on a lattice, the movement of the molecules would follow a Brownian motion style of movement or a dissipative particle dynamic style, where the forces interacting on a molecule are taken into account in greater detail (Fellermann 2009).
Key Research Findings The first result in AC is that there are sets of molecules, called organizations, that are qualitatively stable, in the sense that each molecule present in the set can be generated by the reactions inside the set, and that all reactions inside the cell can only generate molecules that are already inside the cell. Such organizations form an algebraic lattice, and in the absence of external inputs, each experiment eventually leads to the system reaching one of those sets. As artificial chemistries can be represented using ordinary differential equations (ODE), it has been shown that fixed points in the ODE of the system exist only inside organizations. In other words, if we take a fixed point of the system, find the molecules that are present with a quantity higher than 0, then this set of molecules forms an organization. Those results permit a preliminary study of an artificial chemistry by finding (through algebraic studies) the lattice of organizations, and produce a map of the system on which it is possible to track the changes in the system (Dittrich and Speroni di Fenizio 2007). Spatial Artificial Chemistries have been successfully used to model the lipid bilayer of cell membranes and to investigate the spontaneous emergence of artificial life protocells. More recently, experiments have been carried out generating protocells that are able to reproduce (Rasmussen et al. 2007).
Applications While the long-term aim of artificial chemistry, to investigate the appearance of life, has not been reached, a number of partial findings were recognized as being useful in different fields. As Complex Artificial Chemistry can investigate reaction network with a high diversity, they are often the right tool to study ▶ biological networks. In particular, studies have been done on gene regulatory networks and the internal metabolism of a cell. In this regard, AC has started merging with the other tools inside systems biology and bioinformatics. Other studies have
tried to consider social systems, language, and economical systems (Dittrich et al. 2001).
Future Directions Spatial Artificial Chemistries and Complex Artificial Chemistries are going in separate ways, mostly answering different questions about nature. Spatial Artificial Chemistries try to produce a minimal cell, but have not yet succeeded in generating a spontaneous emergence of a full Darwinian evolutionary system. So we can expect more research to go in this direction. Results in Complex Artificial Chemistries have been more connected with Systems Biology, and Artificial Chemistries have been used to explicitly study system biology models, and recent studies show that it is possible to use AC to build programmable systems made up of many interacting components. In future, we can expect those two trends to merge, as scientists investigate chemistries that can evolve and that can be programmed to evolve.
See also ▶ Artificial Life ▶ Autopoiesis ▶ Biological Networks ▶ Chemical Reaction Network ▶ Darwin’s Conception of Origins of Life ▶ Evolution (Biological)
References and Further Reading Dittrich P, Speroni di Fenizio P (2007) Chemical organisation theory. Bull Math Biol 69(4):1199–1231 Dittrich P et al (2001) Artificial chemistries – a review. Artif Life 7:225–275 Fellermann H (2009) Spatially resolved artificial chemistry. In: Adamatzky A, Komosinski M (eds) Artificial life models in software, 2nd edn, Springer, p 343 Fontana W (1992) Algorithmic chemistry. In: Proceedings of artificial life II conference, vol 5, pp 159–210 Fontana W, Buss LW (1996) The barrier of objects: from dynamical systems to bounded organizations. In: Casti J, Karlqvist A (eds) Boundaries and barriers. Addison-Wesley, pp 56–116 Kaleta C (2009) From artificial chemistries to systems biology. In: Adamatzky A, Komosinski M (eds) Artificial life models in software, Vol 2. Springer, London, pp 319–342 Rasmussen et al (2007) Life cycle of a minimal protocell – a dissipative particle dynamics study. Artif Life 13(4):319–345
Artificial Evolution ▶ Evolution, In Vitro
Artificial Life
Artificial Life HUGUES BERSINI IRIDIA, Universite´ Libre de Bruxelles, Brussels, Belgium
Synonyms Life, artificial; Theoretical biology
Keywords Bioinformatics, genetic algorithms, networks, selfreplication, software simulations, Von Neumann
Definition Artificial ▶ life uses software simulation and, to a lesser degree, robotics, in order to abstract and elucidate the fundamental mechanisms common to living organisms. It focuses on the rule-based mechanisms making life possible, supposedly neutral with respect to their underlying material embodiment, and to replicate them in a non-biochemical substrate. In artificial life, the importance of the substrate is purposefully understated for the benefit of the function. Minimal life begins at the intersection of a series of processes that need to be isolated, differentiated, and duplicated as such in computers. Only software development and running make it possible to understand the way these processes are intimately interconnected in order for life to appear at the crossroads.
Overview Artificial life obviously relates to astrobiology, this other recent interdisciplinary field of scientific research equally centered on life and the study of its origins, not only on the obvious environment of Earth, but also throughout the universe. Astrobiology cannot restrict itself to a mere materialistic view of life, in order to detect it elsewhere, as the material substrate could be something totally different. This substrate could be as much singular on a distant planet as it could be in the RAM memory somewhere in a university computer laboratory. The presence of life might be suspected through its functions, much before scientists are able to dissect it. Artificial life does not attempt to provide an extra thousandth attempt at the definition of life, any more than do most biologists. As a matter of fact, the concept of “life,” as opposed to “gravity” or “electromagnetism” or “quantum reduction of a wave packet,” has already been in widespread existence prior to any scientific
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reading or reification. The rejection of an authoritative definition of “life” is often compensated for by a list of functional properties that never finds unanimity among its authors. Some demand more properties, others require fewer of those properties that are often expressed in terms of a vague expression such as “self-maintenance,” “self-organization,” “metabolism,” “autonomy,” “▶ self-replication,” “open-ended evolution.” A first determining role of artificial life consists in the writing and implementing of software versions of these properties and of the way they do connect, so as to render them unambiguous, making them algorithmically precise enough that, at the end, the only reason for disagreement on the definition of life would lie in the length or the composition of this list and on none of its items. The biologist obviously remains the most important partner; but what may he expect from this “artificial life”? These computer platforms could be useful in several ways, presented in the following in terms of their increasing importance or by force of impact. First of all, they can open the door to a new style of teaching and advocating of the major biological ideas; that is, computer software as pedagogical help, as, for example, Richard Dawkins (1986) who, bearing the Darwinian good news, did so with the help of a computer simulation where sophisticated creatures known as “biomorphs” evolve on a computer screen by means of a genetic algorithm. These same platforms and simulations can, insofar as they are sufficiently flexible, quantifiable, and universal, be used more precisely by the biologist, who will find in them a simplified means of simulating and validating a given biological system under study. Cellular automata, Boolean networks, ▶ genetic algorithms, and algorithmic chemistry are excellent examples of software to download, parameterize, and use to produce the natural phenomena required. Their predictive power varies from very qualitative (their results apparently reproduce very general trends of the real world) to very quantitative (the numbers produced by the computer may be precise enough to be compared with those measured in the real world). Although being at first very qualitative, a precise and clear coding is already the guarantee of an advanced understanding accepted by all. Algorithmic writing is an essential stage in formalizing the elements of the model and making them objective. The more the model allows us to integrate what we know about the reality being reproduced, that is, the detailed structures of objects and relationships between them, the more the predictions will move from qualitative to precise and the easier the model will be to validate according to the Karl Popper ideal falsifying process.
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Finally, through systematic software experiments, these platforms can lead to the discovery of new natural laws, whose impact will be greater if the simulated abstractions will be present in many biological realms. In the 1950s, when Alan Turing (Turing 1952) discovered that a simple diffusion phenomenon propagating itself at different speeds, depending on whether it is subject to a negative or positive influence, produces zebra or alternating motifs, it had a considerable effect on a whole section of biology studying the genesis of forms (animal skins, shells of sea creatures (Meinhardt 1998)). When some scientists discovered that the number of attractors in a Boolean network or a neural network exhibits a linear dependency on the number of units in these networks (Kauffman 1993, 1995), these results equally well applied to the number of cells expressed as dynamic attractors in a genetic network or the quantity of information capable of being memorized in a neural network. Entire chapters of biology dedicated to networks (neural, genetic, protein, immune, hormonal) had to be rewritten in the light of these discoveries. When some scientists recently observed a nonuniform connectivity in many networks, whether social, technological, or biological, showing a small number of key nodes with a large number of connections and a greater number of nodes with far fewer, and when, in addition, they explained the way in which these networks are built in time (Barabasi 2002) by preferential attachment, again biology was clearly affected. Artificial life is of course at its apogee when it reveals new biological facts, destabilizing the presuppositions of biologists or generating new knowledge, rather than simply illustrating or refining the old. In the next section I shall attempt to set out the history of life as the disciples of artificial life understand it, by placing the different landmark steps on a temporal and causal axis, showing which one is indispensable to the appearance of the next and how it connects to the next. This history will certainly be very incomplete and full of numerous unknowns, but most people involved in artificial life will be in agreement. They will mainly disagree on the number of these functions and on the causal sequence of their appearance, acknowledging, however, that the appearance of any would have been conditioned by the presence and the functioning of the previous ones. The task of artificial life is to set up experimental software platforms where these different lessons, whether taken in isolation or together, are tested, simulated, and, more systematically, analyzed. I shall sketch some of these existing software platforms whose running delivers interesting take-home messages to open-minded biologists.
The History of Life as Seen by Artificial Life Proponents Appearance of Chemical Reaction Cycles and Autocatalytic Networks In order for a system to emerge and maintain itself inside a soup of molecules that are potentially reactive and contain very varied constituents (which could correspond to the initial conditions required for life to appear, i.e., in the primordial soup), this reactive system must form an internally cycled network or a closed organization, in which every molecule is consumed and reproduced by the network. Above all, in order for life to begin, all of the constituent components must have been able to stabilize themselves in time. These closed networks of chemical reactions are thus perfect examples of systems, which, although heterogeneous, are capable of maintaining themselves indefinitely, despite the shocks and impacts that attempt to destabilize them. This comes about through a subtle self-regeneration mechanism, where the molecules end up producing those molecules that have produced them. It may be obtained on a basic level in a perfectly reversible chemical reaction but can be obtained more subtly in the presence of a lot of intermediary molecules and catalysts. By this reactions-based roundabout in which they all participate, all molecules contribute to maintaining themselves at a constant concentration, compensating and reestablishing any disruption in concentration undergone by any one of them. The bigger the network, the more stable it should be and the more molecules it will maintain in a concentration zone that will vary very little, despite external disruptions. A network of this kind will be materially closed but energetically open if none of the molecules appears in or disappears from the network as a result of material fluxes, whereas energy, originating in external sources, is necessary for the reactions to start and take place. The presence of such an energy flux, maintaining the network far from the thermodynamic equilibrium, is needed, since, without it, no reactive flow would be possible circulating through the entire network. A molecular end of the cycle must be reenergized in order to start again the whole circular reaction process. This cycle thus acts as a chemical machine, energetically driven from the outside. As soon as one of the molecules is being produced in the network without, in its turn, producing one of the molecules making up the network, it absorbs and thus destroys the network. In the presence of molecules of this kind, produced but nonproductive (a kind of waste), the only way of maintaining the network becomes to feed it materially and to make it open to material influx. The network acts
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on the flow of material and energy as an intermediate ongoing stabilization zone, made up of molecules that may be useful to other vital functions (such as the composition of enclosing membranes or catalyzing self-replication), to be described in the following sections. It transforms, as much as it “keeps on,” all the chemical agents that it recruits. Biologists generally agree that a reactive network must exist prior to the appearance of life, at least to catalyze and make possible the other life processes such as genetic reading and coding; it is open to external influences in terms of matter and energy, but necessarily contains a series of active cycles. They are most often designated as “▶ metabolism” or “proto-metabolism,” the most popular and active advocates of this “metabolism-first” hypothetical scenario of the origin of life being (De Duve 2002; Ganti 2003; Maynard Smith and Szathmary 1999; Kauffman 1993; Shapiro 2007; Dyson 1999). Lenaerts and Bersini (2009) give priority to the study of chemical reaction networks, viewing them as key protagonists in the appearance of life. These chemical reaction networks, where the nodes are the molecules participating in the reactions and the connections are the reactions linking the reacting molecules to the molecules produced, are generally characterized by fixed-point dynamics, the chemical balances during which the producers and the products mutually support each other. The attractors in which these networks fix themselves are as dynamic – the concentrations slowly stabilize – as they are structural – the molecules participating in the network are chosen and “trapped” by the network as a whole. These networks are perfect examples of systems that combine dynamics (the chemical kinetics in this case) and metadynamics (the network topological change), as new molecules may appear as the results of reactions while some of the molecules in the network may disappear if their concentration vanishes in time. Both the structure of the network and the concentration of its constituents tend to stabilize over time. Kauffman (1993, 1995) and Fontana (1992) were the forerunners in the study of the genesis and properties of these networks. Figure 1 illustrates the work of these two artificial life pioneers, dedicated to the study of prebiotic chemistry, limiting the reactions studied to polymerization, such as aa + bb ! aabb or inversely, depolymerization or hydrolysis, such as abaa ! ab + aa. Kauffman showed that provided the probability that a reaction takes place is affected by the presence of a catalyst, which is itself produced by the network (in such a case the whole network is said to be autocatalytic), a phenomenon of percolation or phase transition, characteristic of this type of simulation, is produced. For probabilities that are
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Artificial Life. Figure 1 Representation of a network of chemical reactions of polymerization (a + b ! ab) and depolymerization (ab ! a + b) taking place in a simulated chemical reactor. Molecules are represented by circles and reactions by square. Each reaction can be catalyzed, like the arrows pointing to the squares show, by a molecule of the network (giving rise to an autocatalytic network). Some molecules can appear (like the molecule “aab”) or simply disappear from the network. Reaction cycles can appear, like the one surrounded in the figure (aa ! baaaa ! baaaaaab ! baaa ! aa)
too low, the network does not pop up because the reactions are too improbable, but as soon as a threshold value is reached for this same probability, the network “percolates,” giving rise to multiple molecules produced by multiple reactions. Kauffman grants a privileged status to this threshold value and to the giant “explosive” network resulting from it (in his scenario of the origin of life), without really arguing the reason why such a status should exist, but passing the immense interest and enthusiasm that the phenomena of phase transitions arouse among physicists on to the world of biology. Fontana for his part is concerned with the inevitable appearance of reaction cycles (such as that illustrated in Fig. 1). All the molecules
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produced by these cycles in the network in turn produce molecules of the network. He is among those many biologists who see these closed networks or organizations as forming a key stage in the appearance of life, due both to their stability and to the fact that they form structural and dynamic attractors for the system. They cause a stabilization and internal regulation zone together with an energetic motor in a chemical soup, which is continually being crossed by a flow of matter and energy. Fontana goes on to show how these networks are also capable of selfregeneration and self-replication. Lenaerts and Bersini (2009) have programmed the genesis of these chemical reaction networks by adopting the Object-Oriented (OO) programming paradigm. The OO simulator aims to reproduce a chemical reactor and the reaction network that emerges from it (like that shown in Fig. 2). This coevolutionary (dynamics + metadynamics) model incorporates the logical structure of constitutional chemistry and its kinetics on the one hand and the topological evolution of the chemical reaction network on the other hand. The network topology influences the kinetics and the other way round, since only molecules with a sufficient concentration are allowed to participate in new reactions (to avoid a combinatorial explosion of molecules and reactions).
The model is expressed in a syntax that remains as close as possible to real chemistry. Starting with some initial molecular objects and some initial reaction objects, the simulator follows the appearance of new molecules and the reactions in which they participate, as well as the development of their concentration over a period of time. The molecules are coded as canonical graphs. They are made up of atoms and bonds that open, close, or break during the reactions. The result of the simulation consists in various reaction networks, unfolding in time, and whose properties can be further studied (for instance the presence and the properties of reaction cycles or the nature of the network-particular topology such as scalefree or random). One of these reaction schemes, in addition to just cycling, can also be ▶ autocatalytic, when a product of the reaction cycle has twice the concentration of one of the reactant: a + b ! a + a. This is, for instance, the case of the so-called formose reaction (that Ganti and Szatmary have discussed at large in Ganti 2003), during which a two-carbon molecule, reacting twice with a monomer composed of one carbon, leads to a fourcarbon molecule, which then splits in order to duplicate the original molecule. This is the chemical variant of genetic self-replication, since in both cases an original
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Artificial Life. Figure 2 The OO chemical simulator developed by Lenaerts and Bersini (2009). On the left, the molecules are represented as canonical graphs. On the right, the outcome of the simulator is an evolving reaction network, which can be studied in its own right (the presence of cycles, the type of topology)
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molecule is duplicated. As will be discussed later, Ganti has been the first to connect and synchronize these two replication processes: chemical and genetic, in order for the cell to simultaneously duplicate its boundary, its metabolism, and its informational support. In the presence of autocatalysis, the reaction kinetics amounts to an exponential increase and, more interestingly, when various autocatalytic cycles enter in antagonistic interaction, turns out to be responsible for symmetry breaking (one of the cycle, initially favored, wins and takes it all). The early origin of life should not be studied without taking account of the self-organization of chemical networks, the emergence and antagonism of autocatalytic cycles, and how energy flows drive the whole process. Such chemical networks are, for instance, appealing to the effort to understand the onset of biological homochirality as the destabilization of the racemic state resulting from the competition between enantiomers and from amplification processes concerning both autocatalytic competitors (one leftoriented and the other right, see Plasson et al. 2007). The chemical reaction network under study (shown in Fig. 3) is made up of the same type of polymerization and depolymerization reactions as the one studied by Fontana. In the additional presence of epimerization reactions allowing the transformation of a right-hand monomer into a left-hand one and vice versa, the concentration of one family of monomers (for instance the left one) vanishes in favor of the other. The flux of energy is transferred and efficiently distributed through the system, leading to cycle competitions and to the stabilization of asymmetric states.
Production by This Network of a Membrane Promoting Individualization and Catalyzing Constitutive Reactions The appearance of a reaction network of this kind undeniably creates the stability necessary for exploiting its constituents in many reactive systems such as the ones dedicated to the construction of ▶ membranes or the replication of molecules carrying the ▶ genetic code. This network also acts as a primary filter as it can accept new molecules within it, but can equally well reject other molecules seeking to be incorporated within it. They will be rejected, as they do not participate in any of the reactions making up the network. Can we see a primary form of individualization in this network? No, because by definition it can only be unique as no spatial frontier allows it to be distinguished from another network. Although it is roughly possible to conceive of an interpenetration of several chemical networks, establishing a clear separation between these networks would remain a problem.
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Artificial Life. Figure 3 The prebiotic chemical reactor system responsible for a homochiral steady state studied by Plasson et al. (2007). The complete set of reactions is indicated, containing activation (the necessary energy source), polymerization and hydrolysis (which together shape the cycles), and epimerization (which induces the competition between the enantiomers)
It would seem fundamental that a living organism of any kind can be differentiated from another. We know that the reproduction of a second organism from a first is a central mechanism of life and can only operate if the “clone” elaborates something to spatially distinguish itself from its “original.” The best way of successfully completing this individualization and to be able to distinguish between these networks is to revert to a spatial divide, which can only be produced by some form of container capable of circumscribing these networks in a given space. Biochemists are well acquainted with an ideal type of molecule, the raw material for these membranes in the form of lipid/amphiphilic molecules or fatty acids, the two extremities of which behave in an antagonistic fashion – the first hydrophilic, attracted to water, and the second hydrophobic, repulsed by it. Quite naturally, these molecules tend to assemble in a double layer (placing the two
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opposing extremities opposite to each other), formed by the molecules lining up and finally adopting the form of a sphere to protect the hydrophobic extremities from water. Like soap bubbles, these lipid spheres are semipermeable and imprison the many chemical components trapped during its formation. They do, however, actively channel in and out the most appropriate chemicals for maintaining themselves. In assimilating living organisms to autopoietic systems, Varela et al. (1974) were the first to insist that this membrane should be endogenously produced by the elements and the reactions making up the network (e.g., lipids would come from the reactions of the network itself) and would in return promote the emergence and self-maintenance of the network. The membrane can help with the appearance of the reactive and growing network by the frontiers that it sets up, the concentration of certain molecules trapped in it, or by acting as a catalyst to some of the reactions due to its geometry or its makeup. Basically, autopoiesis requires a cogeneration of the membrane and of the reactive network that it “walls up.” The network presents a double closure – one chemical, linked to the cycling chain of its reactions, and another physical, due to the frontiers produced by the membrane. In the cellular automata model of Varela (Varela et al. 1974; McMullin and Varela 1994) illustrated in Fig. 4, there are three types of particles capable of moving around a two-dimensional
Artificial Life. Figure 4 Simulation by means of a cellular automata of the autopoietic model originally proposed by Varela. The minimal cell can easily be seen, together with the catalysts and the substrates that it encapsulates
surface: “substrates,” “catalysts,” and “links.” The working and updating rules of this cellular automata go as follows: ● If two substrates are near a catalyst, they disappear to create one single link where one of the two was located. ● If two links are near each other, they link up and attach themselves to each other. Once attached these links become immobile. ● Each link is only allowed to attach itself to two other links at the most. This allows the links to form chains and to be able to make up a closed membrane. ● The substrates can diffuse through the links and their attachments, while the catalysts and the other links cannot. We can therefore understand how the process of the cogeneration comes about. The membranes shut in the catalysts and the links, which in turn support the membrane by being essential to its formation and regeneration. ● The reactions creating the links are reversible, as the links can recreate the two original substrates (and thus cause the membrane to deteriorate), but at a lower speed. When this happens, the attachment between the links also disappears. Continuous updating and execution of these rules produces minimal versions of reactive systems, physically closed and confined by means of a membrane, which is itself produced by the reactive system. For Varela and the others following him, this turns out to be an essential stage in the road to life. Running the software, many difficulties are encountered such as the simple attainment of a closed cell on account of the many more possibilities for the membranes to unfold in a straight way. Only software simulations can interconnect the physical compartment played by the membrane with the generating metabolism, and further show how far from obvious it is for these two systems to mutually sustain each other. The whole, interactive “metabolism and membrane” prefigures a minimal elementary ▶ cell, which already seems capable both of maintaining itself and detaching itself from its environment and from cells similar to it. It is at this stage on the way to establishing a better and more exact characterization of life that the definition given by Luigi Luisi (Luisi 2002) takes on its full meaning (restating the idea of autopoiesis in more biological terms). “Life is a system which can be self-maintaining by using external energy and nutritional sources to the production of its internal constituents. This system is spatially circumscribed by a semipermeable membrane of its composition.” In the footsteps of Varela, considering life impossible without a way for individualization and compartmentalization, the constitution of the membrane
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circumscribed by the membrane for that same membrane to close on itself. However, in contrast with this autopoietic model, once in place the membrane cannot deteriorate and thus no further internal chemistry is required to endogenously produce what would be needed to fix it. Ultimately, this membrane should exhibit some selective channeling in and channeling out (akin, for some authors (Luisi 2002), to a very primitive form of cognition) providing its internal metabolism with the right nutrients and the right evacuating way out so as to facilitate the cell’s self-maintenance. These two software models raise interesting questions for the biologists like: how are the molecular parts of the membrane generated (endogenously or exogenously) and is this cogeneration of the membrane and the internal metabolism the signature of minimal life? Artificial Life. Figure 5 Simulation of a minimal cell based on A-B (A is hydrophobic and B hydrophilic) and water molecules. All molecules move in reaction to repulsive forces of different intensity and thermal agitation. The pink dots are the A, the gray dots are the B that do connect to give A-B (represented in red and black) by a simple chemical reaction. The blue dots are the water molecules
by simple self-organization or self-assembly processes of bipolar molecules (hydrophilic and hydrophobic) has become a very popular field of artificial life. It is indeed rather simple to reproduce this phenomenon in software (as illustrated in Fig. 5). You need water molecules that just randomly move, in blue in the figure. You need two kinds of submolecules (call then A and B), which when meeting form, though the only authorized additive chemical reaction, an A-B molecule (A is hydrophobic and B hydrophilic) whose two poles are connected by a small string. You need also to adjust the degree of repulsion between A and water, between B and water, the strength of the string of the A-B molecule, and the random component (akin to the thermal noise) to add on each of the intermolecular forces. Nevertheless, the final outcome turns out to be rather robust. The bilayer of B-A/A-B molecules will very naturally and spontaneously form just as for real cells. Again as for the Varela’s minimal cell, the closure turns out to be quite delicate to obtain. One very simple way to obtain it is to locate the source of A submolecules (the pink dots in the figure) in a singular point, so that the closed membrane will simply surround that source, the circular shape being the local minimal of the mechanical energy connecting all A-B together. Like in Varela’s model, and somewhat paradoxically, the source needs to be
Self-replication of This Elementary Cell Self-replication, or the ability of a system to produce a copy of itself on its own, is one of the essential characteristics that has most intrigued and impassioned disciples of artificial life, beginning with John Von Neumann. Biology, and in particular this faculty of self-replication, fascinated Von Neumann. For if we want to compare a cell to a computer and a genome to a code, we need to explain how the computer itself was able to be created out of this code. Let us follow the reasoning of this genius step by step, as it is the perfect illustration of an “artificial life” type of approach: no material realization but just pure functions or rules. Through a sequence of purely functional questions and showing an almost complete ignorance of actual biology, his reasoning led to a logical solution, the content of which retraces astonishingly closely those lessons we have since learned about the way biology functions. Von Neumann begins from the principle that a universal constructor C must exist, which, based on the plan of some kind of machine PM (P the plan, M the machine), must be capable of constructing the machine MP. This idea may be simply translated by C(PM) = MP. The question of self-replication that is then raised is “Is this universal constructor capable of constructing itself?” In order to do so, it must, following the example of other construction products, have a plan of what it wants to construct; in this specific case, it is the constructor’s plan PC. The problem is then expressed as follows: can C(PC) give C(PC) in order for there to be a perfect replication of the original? Von Neumann therefore realized that the question at issue is that of the fate of the construction plan, because if the constructor constructs itself, it has to add the plan itself to the product of the construction. Von Neumann proposed then
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allotting two tasks to the universal constructor; first, that of constructing the machine according to the given plan and thus adding the original plan to this construction. The constructor’s new formula then becomes: C(PM) = MP(PM). If the constructor applies itself to its own plan, this time the replication will be perfect: CP(PC) = CP(PC). The fascinating aspect of Von Neumann’s solution is that it anticipated the two essential functions that, as we have since discovered, are the main attributions of the protein tools constituting the cell: constructing and maintaining this cell, and also duplicating the code in order for this construction to be able to prolong itself for further generations. Starting with ▶ DNA, the whole ▶ protein machinery first of all builds the cell then, by an additional procedure, duplicates this same DNA. Von Neumann did not stop at duplication, because, at the same time, he imagined how this same machinery could evolve and become gradually more complex as a result of random
Artificial Life. Figure 6 The Langton’s self-replicating cellular automata
mutations taking place while the plan recopies itself. Von Neumann gave also a cellular automata solution of the problem in which each cell of the automata possessed 5 neighbors and 29 states, and around 200,000 cells were necessary for the phenomenon of self-replication to take place. Many years later, Chris Langton (Langton 1984, 1989), the organizer of the first conference on artificial life in 1989, proposed an extremely simplified version of this (8 states, but 219 rules remain necessary), although it still follows the pattern mapped out by Von Neumann. This automaton, shown in Fig. 6 incessantly reproduces a little motif shaped as a loop. For many biologists, as opposed to Varela, Luisi, Ganti, Maynard-Smith, life is not simply indissociable from but also essentially reducible to this capacity for self-replication. Nevertheless, they still need to explain how life can actually reproduce without an entire preexistent metabolic chemical machinery. Departing from the elementary cell introduced in the preceding section, and in the interest of an unbroken narrative, let us imagine a simpler scenario leading to self-replication. The closed circuit of chemical reactions could be destabilized by some kind of disturbance, causing a growth in concentration of some of its constituents, including those involved in the formation of membranes. This would also be the case provided all the reactions of the metabolism turn out to be autocatalytic, entailing the exponential growth in concentration of all its molecular elements (including again the membrane constituents). The membrane and the elements that it captures begin to grow (as illustrated in Fig. 7) until they reach the fatal point where the balance is upset. This is followed by the production of a new cell produced by and from the old one. When the new one comes, it quickly grows fast enough to catch up with the “generator” and “nursing” cell, as a chemical network is capable of some
Artificial Life. Figure 7 The elementary minimal cell of Fig. 5 in a process of self-replication induced by the growing and the division of the chemical metabolic network together with the membrane enclosing it. A lot of random thermal noise is here indispensable to destabilize the initial cell
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degree of self-regeneration due to its intrinsic stability; each molecule looks around for another that it can couple up to. This reconstitutes the natural chain reaction of the whole. The new membrane and the new chemical network reconstitute on their own by helping each other. Again, obtaining such duplication is far from obvious since, any cell being intrinsically stable, only a thermal but quite unnatural agitation would do the job. Rather than this elementary form of chemical selfreplication coupled to the physical self-replication induced by the growth and division of the membrane, life has opted for a more sophisticated physico-chemical version of it, more promising for the evolution to come: self-replication by the interposing of an “information template.” Each element of the template can only couple itself with one complementary element. The new elements will as a whole naturally reconstitute the template they were attracted to, causing then the replication of the entire template. In biology, it is the extraordinarily emblematic double helix of DNA that acts as a template, shouldering the major role in the history of life – that of the first known replicator. Our elementary cell must now be internally equipped with this information template. Since the 1950s, Timor Ganti (Ganti 2003) proposed a first minimum mathematical system, named “chemoton,” represented in the Fig. 8. This is the first abstract computational proto-cell that we know, constructed by Ganti as the original ancestor of living organisms. It possesses three autocatalytic chemically linked subsystems: a metabolic network, a membrane, and an information template responsible for scheduling and regulating self-replication. All three grow exponentially until they are able to reproduce, and they depend on each other for their existence and stability. The metabolism feeds the membrane and the template, the membrane concentrates the metabolites, and the template mechanism dictates the reproduction of the whole. The triad ensemble is indeed capable of a whole synchronous self-replication and tries to computationally answer questions about the three subsystems and their interdependency, such as “how does the selfreplication of the template automatically accompany the self-replication of the whole.” This complex software object, the “chemoton,” has also become the topic of many software developments and experimentations and is emblematic of artificial life at its best.
Genetic Coding and Evolution by Mutation, Recombination, and Selection In the information template introduced in the last section, each letter constituting it contributes to the code of a functional component essential to the cell and designed
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pVn Vn
pVnV′
T
3 Tm+1
Tm
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Tm
Artificial Life. Figure 8 The schematic representation of Ganti’s chemoton. One can easily see the three autocatalytic subsystems: the metabolism, the membrane, and the information template, chemically coupled
on the basis of that code – a protein. As soon as he hears anyone talking about code, the software specialist has, quite legitimately, to put his head in thorough the window, because it is to him and him alone that we in fact owe the metaphor of the genetic code. Since ▶ Darwin and thereafter throughout all evolutionary science, we have a good idea of what the last chapter of the history of life is. Doubtless what has stimulated most developments in “artificial life” (primarily from the point of view of engineering) is the fact that the genetic code can evolve through ▶ mutation and sexual crossing between the old machines, evolving so as to produce new machines that are more and more efficient. Over the past 20 years, many of those developments into artificial life have been eager to show how beneficial this idea is for the research and the automated discovery of sophisticated solutions to complex problems. As illustrated in Fig. 9, this research can take place through a succession of mutations and recombinations operating at the level of the code, with the best solutions proposed being preserved in the next generation in order to be used for a new cycle of these same operations. The brute force of the computer is used to its full effect. These are the same genetic algorithms that Dawkins used in his Darwinian crusade, when he developed his
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GA Flowchart
Create initial design population
λ = 9000 λ = 5000 λ = 1000
Evaluate obj. function of designs
old Select and Reproduce (Create new designs) new
Next Generation Replace designs of the old population with new designs
Stop?
Artificial Life. Figure 9 Illustration of the genetic algorithms: the recurrent iterated sequence of selection, mutation, and recombination easily lead to an interesting solution of a complex optimization problem
biomorphs. It should also be stressed that another element in Dawkins’ program is that, when it is finally evaluated, the phenotype is not directly obtained from the genotype, as would be the case for classical optimization in a real or combinatory space. In his work, the biomorphs are the product of a recursive sophisticated program, which is carried out starting from a given ▶ genotype to give a ▶ phenotype. A great “semantic distance” is maintained between these genotypes and phenotypes, which reflect the long process of cell construction from the genetic code and the need for a sophisticated metabolism building the machine out of the code. In brief, this constant program, able to interpret the evolving genotype, is much more important for the complexity of the final outcome than the genotype itself. Similarly, a very prized derivative of these algorithms is genetic programming (Koza 1992), where the individuals now to be optimized are software codes.
Conclusions Parallelism, functional emergence, and adaptability are the conditions necessary to allow these new biologically inspired artifacts to emerge, to “face the world.” We are jumping straight into the robotics branch of artificial life (Brooks 1991). The interfacing with the real world
required by these robots needs a parallel information reception mechanism, because the environment subjects it to a constant bombardment of stimuli. They have to learn to organize and master this avalanche falling on their perceptions. They have to learn to build their own concepts, fed and stimulated by this environment, which, in turn, allows them to master it. The conceptual high level cognitive processes are born out of motor-sensory interactions and serve to support them. Cognitive systems extend at new levels what the minimal cell in the primitive soup does, with a flow of matter and energy crossing straight through, maintaining itself by selectively integrating this influx to form a closed reactor network and the membrane enclosing it. My conclusions are addressed to the three partners: the biologist, the engineer, and the philosopher. To the first, the outcomes of artificial life consist in bringing out what the computer and biology share intimately: an elementary way of working at the ultimate lowest level, but which by the brute force of parallelism and incessantly repeated iterations, can make unknown and sophisticated phenomena to emerge at higher levels. The qualitative aspect of these simulations can give them new roles in the vast scientific register: use it for education, illustrate biological principles that are already understood, open up possible
Artificial Meteorite
experiences of thought, play and replay multiple biological scenarios very quickly, titillate the imagination by onscreen representations, call into question some of the ambiguously interpreted but commonly accepted facts, and, when detailed at most, be able to predict experimental measurements. The second partner, the engineer, is vigorously encouraged to use the computer for what it is best at doing – this infinite possibility of trial and error. There is a perfect synergy, where both participants complement each other ideally: the engineer must bow to the computer in terms of calculating power, but this is compensated for by his judgment. Genetic algorithms, ant colonies, neural networks, and reinforced learning have enriched the engineer’s toolbox. Finally, for the philosopher, for each attempt at a definition of life, artificial life makes a real attempt to achieve a computerized version in conformity with this definition. For the skeptic, unhappy with this computerized “lining,” the question now becomes how to refine his definition, to complete it, or to renounce the possibility that there is no definition that cannot be computerized. The other possibility, doubtless more logical but more difficult for many philosophers to accept, would be that life poses no problem for a computer snapshot since it is computational at its roots.
See also ▶ Autocatalysis ▶ Bioinformatics ▶ Biological Networks ▶ Cellular Automata ▶ Code ▶ Complexity ▶ Emergence of Life ▶ Genetic Algorithms ▶ Life ▶ Membrane ▶ Self Replication
References and Further Reading Barabasi L-A (2002) Linked. The new science of networks. Perseus, Cambridge Bersini H (2004) Whatever emerges should be intrinsically useful. In: Proceedings of artificial life, Vol 9. MIT Press, Cambridge, pp 226–231 Billoud B (2010) Origins of life: computing and simulation approaches. In: Gargaud M, Lopez-Garcia P, Martin H (eds) Origin and evolution of life: an astrobiology perspective. Cambridge University Press (Chapter 5)
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Brooks R (1991) Elephants don’t play chess. In: Maes P (ed) Designing autonomous agents. MIT Press, Cambridge, MA Dawkins R (1986) The blind watchmaker. WW Norton, New York. ISBN 0-393-31570-3 De Duve C (2002) Life evolving: molecules, mind, and meaning. Oxford University Press, Oxford Dyson F (1999) Origins of life, 2nd edn. Cambridge University Press, Cambridge Fontana W (1992) Algorithmic chemistry. In: Langton CG, Farmer JD, Rasmussen S, Taylor C (eds) Artificial life II: a proceedings volume in the SFI studies in the sciences of complexity, vol 10. Addison-Wesley, Reading Ganti T (2003) The principles of life. Oxford University Press, Oxford Goldberg DE (11 January 1989) Genetic algorithms in search, optimization, and machine learning, 1st edn. Addison-Wesley Professional, Reading Kauffman S (1993) The origins of order: self-organization and selection in evolution. Oxford University Press, Oxford Kauffman S (1995) At home in the universe. The search for the laws of self-organisation and complexity. Oxford University Press, New York Koza J (1992) Genetic programming. MIT Press, Cambridge Langton CG (1984) Self-reproduction in cellular automata. Phys D 10:135–144 Langton CG (ed) (1989) Artificial life I. Addison-Wesley, Reading Lenaerts T, Bersini H (2009) A synthon approach to artificial chemistry. Artif Life 15(1):89–103 Lovelock J (2000) Gaia: a new look at life on earth. Oxford University Press, Oxford Luisi PL (2002) Some open questions about the origin of life. In: Fundamentals of life. Elsevier, Paris, pp 287–301. ISBN 2-84299-303-9 Maynard Smith J, Szathmary E (1999) The origins of life: from the birth of life to the origin of language. Oxford University Press, Oxford McMullin B, Varela FR (1994) Rediscovering computational autopoiesis. In: Husband P, Harvey I (eds) Proceedings of the fourth European conference on artificial life. MIT Press, Cambridge, pp 38 Meinhardt H (1998) The algorithmic beauty of sea shells, 2nd edn. Springer, Heidelberg/New York Nagel T (1974) What is it like to be a bat? Philos Rev 83:435–450, Repr. Mortal questions. Cambridge University Press, New York, pp 165–180 Plasson R, Kondepudi DK, Bersini H, Commeyras A, Asakura K (2007) Emergence of homochirality in far-from-equilibrium systems: mechanisms and role in prebiotic chemistry. Chirality 19:589–600 Shapiro R (2007) A simpler origin for life. Sci Am 296:46–53 Turing AM (1952) The chemical basis of morphogenesis. Philos Trans R Soc Lond B 237:37–72, Also in Saunders PT (ed) (1992) The collected works of A. M. Turing: morphogenesis. North-Holland, Amsterdam Varela FR, Maturana HR, Uribe R (1974) Autopoiesis: the organisation of living systems, its characterization and a model. BioSystems 5:187–196
Artificial Meteorite ▶ STONE
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ASA
ASA Synonyms Aeronautics and Space Agency of FFG; Agentur fu¨r Luft- und Raumfahrt der FFG; Austrian Space Agency, Austria
Definition The FFG’s Aeronautics and Space Agency (ASA) is the gateway to the international aerospace industry for Austria’s industry and science sectors and aims to strengthen their international standing in these key technologies. The agency supports the participation of Austrian researchers in international and bilateral aerospace collaborations and fosters the creation and development of international networks. It implements Austrian aeronautical space policy and represents Austria’s interests in international aeronautical and space organizations. The FFG’s main focus is on managing the contributions of the Republic of Austria to the programs of the ▶ European Space Agency (ESA) and FFG is responsible for the management of the Austrian Space Applications Programme (ASAP), a bottom-up program targeted to Space Science, Technology, Space Technology Transfer, direct applications of space technology, and international cooperation. Bilateral cooperative projects were undertaken in particular with the former Soviet Union, such as the development of Austrian instruments for space probes and missions. These projects include, for example, the two Venus probes, Venera 13 and 14 (1981–1982), the ▶ Vega 1 and 2 (1984–1986) missions to ▶ Halley’s Comet, and the PHOBOS Mars probes (1988–1989). The highlight of bilateral cooperation with the former Soviet Union was the AUSTROMIR-91 mission – the flight of the first Austrian cosmonaut, Franz Viehbo¨ck, to the MIR space station. Other bilateral projects were, and still are, run in partnership with Norway, Sweden, France, Switzerland, and Germany. Among other activities, FFG is organizing annually since 1975, the well-known Alpbach Summer School that has a long tradition in providing in-depth teaching on aspects of space science and space technology with the aim of advancing the training and working experience of European graduates, postgraduate students, young scientists, and engineers. Number of Employees of the Aeronautics and Space Agency of FFG is 10 in 2010.
History The Space Research Institute of the Austrian Academy of Sciences was founded in 1970, and the Austrian Space Agency in 1972 (which had been merged into FFG in 2004). Austria has been participating in ESA programs since 1975 and became a full member in 1987.
Aseptic Process Definition Any operation which is carried out under conditions that minimize the potential for contamination by ▶ microorganisms.
ASI Synonyms Agenzia Spaziale Italiana; Italian Space Agency
Definition The Italian Space Agency was established in 1988 to coordinate all of Italy’s efforts and investments in the space sector that had begun in the 1960s. Today, ASI has a key role at the European level where Italy is the third contributing country to the ▶ European Space Agency. Italy is directly involved in major European and international programs. It provides several elements for the International Space Station like the multipurpose logistic module (MPLM) used to transfer cargo with the US space shuttle, Nodes 2 and 3 and is participating within ESA in the European Automated Transfer Vehicle activities. ASI selected five astronauts flying either through bilateral cooperation or through ESA. Franco Malerba was the first Italian in Space in 1992 onboard the STS 46 flight. Italy is involved also in many missions dedicated to planetology, astronomy, and exploration. It is playing a major role in ▶ Cassini Huygens mission, the ▶ Exomars ESA mission, and many others. Italy is taking also a major share (65%) in the medium launcher program (Vega rocket) of ESA. Beyond the headquarters in Rome, ASI has three bases and one center. “Luigi Broglio” Space Centre of Malindi, Kenya, was used to launch US Scout rockets with Italian satellites from oceanic platforms (the marine segment) up
Assay
to 1988. Nowadays this base (the ground segment) is dedicated to receive data from satellites and launchers. A stratospheric balloon launch base is located on a former airport in Trapani (since 1975). This base is, in particular actively involved in trans-Mediterranean flights of research balloons. In Matera, in collaboration with several institutions, ASI opened in 1983 a Space Geodesy Center dedicated to this discipline. Now this base is diverting its activities welcoming some technical activities required for robotic exploration. Finally, an ASI Science Data Center (ASDC) was established in September 2000 for the management and analysis of scientific data collected by scientific satellites. This center is located in the ESA facility (European Space Research Institute-ESRIN) in Frascati, which is dedicated to the Earth observation.
Asparagine
A
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Aspartic Acid Definition Aspartic acid (HCOOCH2CH(NH2)COOH) is an ▶ amino acid with chemical structure shown in Fig. 1. It is among the 20 protein amino acids, and its three-letter symbol is Asp, and one letter symbol is D. It is monoaminodicarboxylic acid, and it is classified as acidic amino acid. Aspartic acid has the lowest isoelectric point (pI) 2.77 of the protein amino acids. It is among the five amino acids that were detected in Miller’s electric discharge experiment in 1953 and is found in extracts from carbonaceous chondrites. Since it has two carboxyl groups (a- and b-carboxyl group), it can make both a- and b-peptide bonds with other amino acids. In biosynthesis of ▶ proteins, only a-peptide bonds are formed. As the ▶ racemisation of aspartic acid is relatively rapid, the D/L ratio of aspartic acid can be used for dating biological materials such as bone.
See also
Definition Asparagine is one of the 20 ▶ protein ▶ amino acids, whose chemical structure is shown in Fig. 1. Its threeletter symbol is Asn and one-letter symbol is N. It has a molecular weight of 132.12. It has an amide group (-CONH2) in its side chain, and it is easily hydrolyzed to give ▶ aspartic acid (Asp) and ammonia. In the hydrolysis of proteins and prebiotic organic materials, both aspartic acid and asparagine are determined as aspartic acid. The original source of the aspartic acid (Asp or Asn) cannot be determined by this method, thus it is denoted as Asx. It is classified as a neutral amino acid with an isoelectric point (pI) of 5.41.
▶ Amino Acid ▶ Miller, Stanley ▶ Protein ▶ Racemization
Assay CATHARINE A. CONLEY NASA Headquarters, Washington, DC, USA
Definition
See also
In ▶ planetary protection, an assay is the suite of actions performed during the integration of a spacecraft or an instrument to collect and measure the biological contamination using a specified procedure, in order to estimate
▶ Amino Acid ▶ Aspartic Acid ▶ Protein
O
O
O
O OH NH2
NH2
Asparagine. Figure 1 Structural formula of asparagine
OH OH
101
NH2
Aspartic Acid. Figure 1 Structural formula of aspartic acid
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Assimilative Metabolism
the number or types of ▶ microorganisms associated with an item of interest (exposed surface, material, environment, etc.).
Keywords Asteroids: dynamical and physical properties, impact hazard, mineralogy, mitigation, taxonomy
Definition
Assimilative Metabolism Definition Assimilative ▶ metabolism is the process by which an inorganic compound (NO3, SO42, CO2) is reduced for use as a cell nutrient source. Assimilative metabolism is conceptually different from the ▶ reduction reactions that take place when the same inorganic compounds are used as electron acceptors to obtain energy by ▶ anaerobic respiration (▶ dissimilative metabolism). There are important differences between both types of ▶ metabolism. In the assimilative metabolism, only enough of the compound is reduced to satisfy the needs for cell growth, and the products are normally converted into cell material, while in the dissimilative metabolism, a large amount of ▶ electron acceptor must be reduced to guaranty the generation of sufficient energy and the product is excreted into the environment.
See also ▶ Anaerobic Respiration ▶ Dissimilative Metabolism ▶ Electron Acceptor ▶ Metabolism (Prebiotic) ▶ Reduction ▶ Sulfate Reducers
Association Constant ▶ Affinity Constant
Asteroid ALAN W. HARRIS German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Synonyms Minor planet; Planetoid; Small Solar System body
An asteroid is an irregularly shaped rocky body orbiting the ▶ Sun that does not qualify as a ▶ planet or a ▶ dwarf planet under the International Astronomical Union’s (IAU) definitions of those terms introduced in 2006. In contrast to planets and dwarf planets, asteroids do not have sufficient mass for their self-gravity to overcome rigid body forces and assume a hydrostatic equilibrium (nearly round) shape. In contrast to ▶ comets, asteroids are inert bodies that do not display a coma of gas and dust. Very small objects with a size of less than about 10 m are normally referred to as meteoroids.
Overview The first asteroid, ▶ 1 Ceres, was discovered in 1801 by the Italian astronomer Giuseppe Piazzi, quickly followed in succeeding years by the discovery of 2 Pallas, 3 Juno, and ▶ 4 Vesta. Ironically, Ceres is now classed as a dwarf planet under IAU Resolution B5 of 2006 and Pallas and Vesta are candidates for transfer to the new category of dwarf planets. Since about 1850, the discovery rate has increased dramatically, especially in recent years, leading to the current tally of over 220,000 numbered asteroids. An asteroid is assigned a permanent designation, i.e., a sequential number, once its orbit has become accurately established through a sufficient number of astrometric observations. Asteroids and comets are considered to be remnant bodies from the epoch of planet formation. Planet embryos formed in the ▶ protoplanetary disk about 4.5 billion years ago via the accretion of dust grains and collisions with smaller bodies (planetesimals). A number of planet embryos succeeded in developing into the planets we observe today; the growth of other planet embryos and ▶ planetesimals was terminated by catastrophic collisions or a lack of material in their orbital zones to accrete. Most asteroids are thought to be the fragments of bodies that formed in the ▶ inner Solar System and were subsequently broken up in collisions. Comets and related icy bodies are thought to have accreted in the cold outer regions of the protoplanetary disk where volatile materials, such as ▶ water and ▶ carbon dioxide, were abundant as ices. Most numbered asteroids are in the ▶ main asteroid belt between the orbits of ▶ Mars and ▶ Jupiter. The existence of the main belt is thought to be due to the collisional fragmentation of remnant planetesimals that
Asteroid
were prevented from accreting into planets by the gravitational perturbations of the nearby massive planet Jupiter. Main-belt asteroids consist largely of silicates and metals and come in all shapes and sizes up to about 1,000 km in diameter. Table 1 lists physical data for the first ten asteroids discovered. Large asteroids with diameters of several hundred kilometers tend to be roughly ellipsoidal but smaller objects generally have very irregular shapes. Asteroid surfaces appear to consist of loose dust mixed with gravel and boulders (regolith), whereby there is evidence that the regolith of kilometer-sized bodies is coarser and less dusty than that of large main-belt asteroids.
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asteroids 4 Vesta, 8 Flora, and 10 Hygiea (Table 1). In these cases, the families presumably arose as the result of cratering events that gave rise to ejecta from the surfaces of the large asteroids. In other cases, precursor asteroids were apparently completely broken up. In both scenarios, the result is a family of fragments with similar orbits (and dust particles that probably contribute to the cloud of dust associated with the ecliptic plane and give rise to the zodiacal light). In most cases, the family members have very similar compositions. Other major dynamical groups are described below and listed in Table 2. Jupiter Trojans are asteroids that are trapped in dynamically stable zones 60 ahead of and behind Jupiter in its orbit. The stable zones are associated with the L4 and L5 Lagrangian points of Jupiter’s orbit. There are some 3,800 known Jupiter Trojans orbiting between about 5.0 and 5.4 AU from the Sun. ▶ Trans-Neptunian objects (TNOs) are very distant, presumably icy, bodies with semimajor axes larger than 30 AU. Most TNOs are classed as asteroids according to the formal definition of an asteroid given above, but in terms of their physical characteristics they may have more in common with comets. The first TNO was discovered in August 1992; some 1,300 have been discovered since. The ▶ Centaurs have orbits between that of Jupiter and the TNOs. Centaurs are possibly objects that originated as TNOs but due to perturbations by ▶ Neptune are now in orbits that bring them closer to the Sun. The number of known Centaurs is currently approaching
Asteroid Dynamical Groupings Asteroids are classified dynamically according to their orbital elements (▶ semimajor axis, period, ▶ inclination, ▶ eccentricity, etc.). The most significant grouping of asteroids is the main belt between about 2.0 and 3.5 astronomical units (AU, the mean Sun–Earth distance) from the Sun. The main belt is populated by millions of asteroids, some 220,000 of which have been assigned sequential numbers to date. A number of asteroid families exist in the main belt. Family members have very similar dynamical characteristics and may be fragments from relatively recent (relative to the history of the Solar System) collisions. For example, there are large families associated with the main-belt
Asteroid. Table 1 The first ten numbered asteroids Number and name
Discovery: year, site, discoverer
1 Ceres
1801, Palermo, G. Piazzi
949 11c
G, C
2.12 0.04
9.074
2 Pallas
1802, Bremen, H. W. Olbers
533 6
B
2.71 0.11
7.813
3 Juno
1804, Lilienthal, K. Harding
234 11d
S
4 Vesta
1807, Bremen, H. W. Olbers
529 10c
V
5 Astraea
1845, Driesen, K. L. Hencke
d
119 7
S
–
6 Hebe
1847, Driesen, K. L. Hencke
185 3d
S
–
7.274
7 Iris
1847, London, J. R. Hind
200 10d
S
–
7.139
8 Flora
1847, London, J. R. Hind
136 3
S
–
9 Metis
1848, Markree, A. Graham
172 13e
S
–
1849, Naples, A. de Gasparis
407 7
C
10 Hygiea a
Bus and Binzel (2002). b Harris et al. (2008). c Britt et al. (2002). d Tedesco et al. (2002). e Mu¨ller and Barnes (2007).
Bulk density Diameter (km) Taxonomic classa (g cm3)c Rotation period (h)b c
d
d
– 3.44 0.12
2.76 1.2
7.210 5.342 16.80
12.80 5.079 27.62
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Asteroid. Table 2 Selected asteroid dynamical groups Dynamical category
Semimajor axis (AU)
Approx. no. knowna
Notes
Trans-Neptunian objects (TNOs)b
>30
1,300
Centaursb
5.2–30
100
Jupiter Trojans
5.05–5.4
3,800
2.0–3.5
4.5 10
>1
2,800
1.017 < Perihelion 1.3 AU
Main belt Amors
c
Possibly TNOs whose orbits have been perturbed by Neptune Associated with the Lagrangian points of Jupiter’s orbit 5
2.2 105 numbered asteroids
1
3,300
Perihelion 1.017 AU
Atensc
10
H2CO
>10
HOCH2CH2OH
1
HCOOH
3
HCOOCH3
1
CH3CHO
1
NH2CHO
1
NH3
3
HCN
>10
HNCO
4
HNC
10
CH3CN
8
HC3N
2
H2S
>10
OCS
2
SO2
1
CS2
>10
H2CS
1
NS
1
S2
5
10−3
10−1 1 101 10−2 Relative abundances (% relative to water)
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Comet. Figure 2 Relative production rates of cometary volatiles and their comet-to-comet variations. These rates are believed to trace the relative abundances in cometary ices. The red part of each bar indicates the range of variation from comet to comet. The number of comets in which the species was detected is indicated on the right (Adapted from Bockele´e-Morvan et al. 2004)
Composition of Comets: Dust and (Semi) Refractories Knowledge of the composition of cometary dust stems from infrared spectroscopy (with ground-based telescopes and the ISO and Spitzer space observatories) and from the samples collected in the coma of the Jupiter-family comet 81P/Wild 2 by the Stardust mission, providing groundtruth for the remote-sensing investigations (Hanner and
Zolensky 2010). Additional information comes from the analysis of interplanetary dust particles (IDPs), collected in the upper Earth’s atmosphere, which could be of cometary origin. The analysis of the material excavated in the Jupiter-family comet 9P/Tempel 1 by the Deep Impact experiment revealed an inner-nucleus composition similar to that observed in more active, Oort-cloud comets, such as 1P/Halley or C/1995 O1 (Hale-Bopp).
C
336
C
Comet
Comet. Table 2 The relative composition of volatiles observed in comet C/1995 O1 (Hale-Bopp), normalized to water (From Bockele´e-Morvan et al. 2005, with updates) Water
H2O
100
Carbon monoxide
CO
12–23
Carbon dioxide
CO2
6
Methane
CH4
1.5
Acetylene
C2H2
0.1–0.3
Ethane
C2H6
0.6
Methanol
CH3OH
2.4
Formaldehyde
H2CO
1.1
Formic acid
HCOOH
0.09
Methyl formate
HCOOCH3
0.08
Acetaldehyde
CH3CHO
0.02
Ethylene glycol
CH2OHCH2OH
0.25
Formamide
NH2CHO
0.015
Ammonia
NH3
0.7
Hydrogen cyanide
HCN
0.25
Isocyanic acid
HNCO
0.10
Hydrogen isocyanide
HNC
0.04
Methyl cyanide
CH3CN
0.02
Cyanoacetylene
HC3N
0.02
Hydrogen sulfide
H2S
1.5
Carbonyl sulfide
OCS
0.4
Sulfur dioxide
SO2
0.2
Carbon disulfide
CS2
0.2
Thioformaldehyde
H2CS
0.05
NS radical
NS
0.02
Hydrogen peroxide
H2O2
3.8 Ga life in SW Greenland. J Geol Soc 166:335–348 Wyche S, Nelson DR, Riganti A (2004) 4350–3130 Ma detrital zircons in the Southern Cross Granite-Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton. Aust J Earth Sci 51:31–45
Earth, Surface Evolution
Earth, Surface Evolution NICHOLAS ARNDT Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France
Keywords Chert, Greenstone belt, komatiite, sediment, South Africa, traces of life
Definition The surface of the Archean Earth was in many ways similar to that of today. Oceans covered most of the globe, but there were also regions of dry land. Oceanic crust was almost as thick as continental crust, mountain ranges were not very high, parts of oceanic ridges and plateaux were emergent. Geological processes such as volcanism, erosion, sediment deposition operated as now, but were influenced by a lack of vegetation, higher ocean temperatures, and a hotter more aggressive, acidic atmosphere.
Overview The total area covered by oceans was greater than now, for two reasons. First, the volume of continental crust may have been less, if this crust had grown progressively through time (Benn et al. 2006). Second, the oceans were more voluminous because high temperatures in the mantle (Nisbet et al. 1993) destabilized hydrous minerals and drove water to the surface. Mountain ranges existed but were not as high as those of today because the continental crust was heated internally and rendered more ductile by more abundant radioactive elements. Continental crust was relatively thin while oceanic crust, produced by high-degree melting of the hotter mantle, was far thicker (Sleep and Windley 1982). The subdued topography, the limited contrast between the thicknesses of oceanic and continental crust, combined with bigger oceans, meant that much of the continental crust was flooded (Arndt, 1998). Just as during more recent geological history, global temperatures waxed and waned. Periods of global glaciation, the most pronounced being during the Proterozoic “▶ snowball Earth” episodes (Hoffman et al. 1998), alternated with periods when temperatures were relatively high. The O and Si isotopic compositions of Archean cherts suggest that ocean temperatures were commonly above 40 C (Knauth and Lowe 2003). The atmosphere contained a little to no free oxygen but was rich in CO2; rainwater was acid.
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The normal cycle of erosion, transport, and deposition of sediment operated, but the rivers flowed through a landscape that was very different from that of today. The feature that most starkly distinguished the Archean and modern land surface was the lack of vegetation. Microbes no doubt colonized the subsurface and constructed biofilms (slime mats) that covered moist areas, but most of the landscape was a Martian vista of bare rocks and soil. The rate of erosion was enhanced by the lack of vegetation, high temperatures, and aggressive atmosphere but restrained by modest heights of mountain belts. Active volcanism covered much of the surface with lava flows or pyroclastic deposits. The ▶ oceanic crust was composed of basaltic lavas like that of modern crust, but more magnesian (picritic) in places (Sleep and Windley 1982). Parts of mid-ocean ridges and the summits of oceanic plateaux may have been emergent forming what might be called “melano(dark colored) continents.” The pelagic sediment that covered this crust was different from that of today. An absence of shell-forming organisms precluded the formation of biogenic calcareous or siliceous oozes; in their place were Si- or Fe-rich sediments that precipitated directly from the high-temperature seawater that contained high concentrations of these elements. Hydrothermal circulation of Si-charged seawater resulted in massive silicification of all near-surface rocks. Expulsion of fluids at hydrothermal vents led to the deposition of exhalative sediments variably composed of sulfides, sulfates, or carbonates or silica minerals (Russell et al. 2005). The earliest Archean coincided with the end of the ▶ Late Heavy Bombardment, a time of massive meteorite impacts. The largest of these would have vaporized large expanses of the oceans and pulverized large parts of the continents, but their overall impact was local, not global. Opinions differ concerning the surface environment in the Hadean. One school envisages a ▶ cool early Earth in which clement conditions reigned (Valley et al. 2002); another imagines a hellish environment in which periods of intense heating subsequent to meteorite impact alternated with global glaciation when the atmosphere budget of greenhouse gases was insufficient to counteract the weak luminescence of the young sun (Sleep and Zahnle 2001).
See also ▶ Archean Traces of Life ▶ Earth, Formation and Early Evolution ▶ Impact Melt Rock ▶ Komatiite ▶ Oceans, Origin of ▶ Pilbara Craton ▶ Plate Tectonics
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References and Further Reading Arndt NT (1998) Why was flood volcanism on submerged continental platforms so common in the precambrian? Precambrian Res 97: 155–164 Benn K, Mareschal J-C, Condie KC (2006) Archean geodynamics and environments. geophysical monograph series. Am Geophys Union 164:320 Hoffman PF, Kaufman AJ, Halverson GP, Schrag DP (1998) A neoproterozoic snowball earth. Science 281:1342–1346 Knauth LP, Lowe DR (2003) High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland supergroup, South Africa. Geol Soc Am Bull 115:566–580 Nisbet EG, Cheadle MJ, Arndt NT, Bickle MJ (1993) Constraining the potential temperature of the Archaean mantle: a review of the evidence from komatiites. Lithos 30:291–307 Russell MJ, Hall AJ, Boyce AJ, Fallick AE (2005) On hydrothermal convection systems and the emergence of life. Econ Geol 100:419–438 Sleep NH, Windley BF (1982) Archaean plate tectonics: constraints and inferences. J Geol 90:363–379 Sleep NH, Zahnle K (2001) Carbon dioxide cycling and implications for climate on ancient Earth. J Geophys Res 106:1373–1399 Valley JM, Peck WH, King EM, Wilde SA (2002) A cool early Earth. Geology 30:351–354
to another one. It quantifies the amount by which the orbit deviates from a circle: e = 0 for a circle, e = 1 for a parabola, e > 1 for a hyperbola, and 0 < e < 1 for an ellipse, where e = c/a, with c the distance from the center to a focus and a the semi-major axis. See Fig. 1 for a geometrical definition. The eccentricities of planets belonging to the solar system are rather small, while a significant fraction of the discovered exoplanets exhibit large eccentricities, in excess of 0.3.
See also ▶ Exoplanet, Detection and Characterization ▶ Orbit
Eclipse Synonyms Eclipsing binary; Primary eclipse; Secondary eclipse; Transit
Earth-Like Planet
Definition ▶ Terrestrial Planet
Eccentricity Definition The eccentricity is a parameter (denoted by e) characterizing the orbit’s shape for an object gravitationally bound
e=0 (circle)
e = 0.43
e = 0.23
e = 0.77
Eccentricity. Figure 1 Several examples of orbits with different eccentricities
When orbits are viewed edge-on, the components periodically pass in front of each other, thus reducing the amount of light that reaches the observer. If both components are stars, the system is called an eclipsing binary. Normally the deeper eclipse occurs when the cooler object passes in front of the hotter one. This is traditionally denoted the primary eclipse, while the secondary eclipse refers to the shallower event when the cooler component passes behind the hotter. However, there can be interesting exceptions to this general pattern. For example, with highly eccentric orbits and modest inclinations, one or the other of the eclipses may not occur. By convention, the eclipse of a star by a much smaller and cooler object such as a planet is called a ▶ transit, while the passage of the planet behind the star is termed either an occultation or a secondary eclipse. Another interesting case is a binary consisting of a normal star and a ▶ white dwarf. White dwarfs are small enough to produce transit-like events when they pass in front of the star. But, if they are young enough, they can be hotter than the star, thus producing a deeper eclipse when they pass behind the star. Eclipsing binaries for which the spectra of both stars can be detected are especially valuable, because an analysis of the light curve together with the spectroscopic orbit derived from the radial velocities of both stars can yield the absolute masses and radii of both stars. If the spectrum of only one of the stars is detected (presumably for the
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brighter primary), then only the ratios of the masses and radii can be derived directly. Interestingly, the density of the primary star and surface gravity of the unseen companion can also be derived from the light curve and orbital solution. If the mass of the primary can be estimated from other observations, such as spectroscopy and/or parallaxes combined with stellar models, then the actual mass and radius of the secondary can be derived. Transiting planets are simply an extreme example of the case where only the spectrum of the primary star yields an orbital solution.
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Ecopoesis ▶ Planetary Ecosynthesis
Ecosphere ▶ Biosphere
See also ▶ Activity (Magnetic) ▶ Transit ▶ Transiting Planets ▶ White Dwarf
Eclipsing Binary ▶ Eclipse
Ecosystem J. CYNAN ELLIS-EVANS UK Arctic Office, Strategic Coordination Group, British Antarctic Survey, Cambridge, UK
Keywords Abiotic, biogeochemical cycles, biological populations, biotic, environment, physico-chemical conditions
Definition
Ecliptic Definition The ecliptic is the geometric plane containing the Earth’s ▶ orbit. More precisely, it is the average plane of the orbit of the center of mass of the Earth-Moon system. It is also the line delineating the projection of the Earth’s orbital plane onto the celestial sphere. On the sky, the ecliptic is at the middle of the Zodiacal belt that extends about 9 on either side. The term ecliptic also designates the adjective that specifies quantities associated with the ecliptic: for example, ecliptic coordinates.
See also ▶ Coordinate, Systems ▶ Orbit ▶ Solar System Formation (Chronology)
Ecohydrology ▶ Deep-Subsurface Microbiology
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An ecosystem is part of the broad ▶ environment, comprising of both physical (▶ abiotic) and interdependent biological (biotic) components, in which the community of living organisms continually interacts with all abiotic components (e.g., rock, water, air, soil, sunlight) through various interconnected relationships. The flow of energy within an ecosystem, manifested through cycling of materials between the biotic and abiotic components, defines its inherent trophic structure and biological diversity.
Overview The ecosystem concept developed, as a more abstract replacement of the concept of community, out of theorizing in the 1930s on the organization and dynamics of natural systems. While a community is a group of populations of different organisms that interact with one another in a given ▶ habitat or area and are interdependent, an ecosystem is considered an area where living communities exchange materials “. . .with the whole complex of physical factors that form what we term an environment” (Tansley 1935). Ecosystems are therefore essentially self-contained energy and nutrient cycles. An ecosystem can be long lived or temporary, and balance is a critical element of any viable ecosystem because disturbances and instability in its physical factors (e.g., rapid climate change) can threaten the existence of
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its component organisms. Ecosystems can be small or large, and the physical boundaries of an ecosystem can be defined as in a pond, or relatively blurred, as in the case of a marsh draining into a stream or river. Ecosystems are everywhere and the diversity is enormous on a planet such as Earth, and indeed we are still discovering environments (e.g., the deep sub-surface, sub-glacial lakes) that harbour as yet barely researched ecosystems. Extreme environments on Earth provide analogues of possible extraterrestrial ecosystems. Potential locations include sub-surface permafrost lithosols on Mars, the ice-covered ocean of the Jovian moon, Europa, the hydrocarbon lakes on Titan, or the sub-surface water on Encaladus.
Ediacara Biota ▶ Ediacaran Biota
Ediacaran Biota THOMAS H. P. HARVEY Department of Earth Sciences, University of Cambridge, Cambridge, UK
See also ▶ Abiotic ▶ Biogeochemical Cycles ▶ Environment ▶ Extreme Environment ▶ Habitat
Synonyms
References and Further Reading
Definition
Costanza R, D’Arge R, de Groot R, Farber S, Grasso M, Hannon B et al (1997) The value of the world’s ecosystem services and natural capital. Nature 387:253–260. doi:10.1038/387253a0 Tansley AG (1935) The use and abuse of vegetational terms and concepts. Ecology 16:284–307 United Nations Environment Programme. Convention on biological diversity. June 1992. UNEP Document no. Na.92–78
A particular suite of fossil organisms of the Ediacaran Period of geologic time.
ECSS Synonyms European Cooperation for Space Standardization
Definition The European Cooperation for Space Standardization (ECSS) is, since 1993, an organization that works to improve standardization within the European space sector. This organisation is supported by the ▶ European Space Agency and the space agencies of some member states. The industry is also participating in the technical working groups which define the standards to be applied to the management, the organization, the quality assurance of the projects, as well as the engineering methods in various domains (electrical system, communication, mechanic, . . .).
See also ▶ ESA ▶ ISO
Ediacara biota; Ediacarian biota
Keywords Evolution, Paleobiology, Precambrian
Overview This somewhat loose term (sometimes Ediacara or Ediacarian biota) is widely used to refer to the macroscopic, often complex, soft-bodied and notably problematic marine organisms that are characteristic of the later part of the Ediacaran Period (ca. 575–542 Ma), although there is potential confusion with a broader usage referring to Ediacaran-age life in general. The biota is named after the Ediacara Hills region, South Australia, but almost 40 localities are now known worldwide, with further key assemblages described from England, Russia, Namibia, and Newfoundland, Canada. Evidence for the origins and fate of the biota is sparse, although a rather earlier (>635 Ma) assemblage of simple forms from northwestern Canada has been reported, as have some possible Cambrian “survivors.” The Ediacaran biota includes the earliest known fossils of conspicuously large and complex organisms. Because this occurrence is preceded by a long period of apparently slow-paced microbial evolution, and succeeded by the sudden appearance of diverse and unambiguous metazoan (animal) fossils of the Cambrian explosion, the phylogenetic identities and ecological capabilities of the Ediacaran organisms are of pivotal significance. At first, the tendency was to interpret them as members of various familiar metazoan groups, such as cnidarians,
Effective Temperature
echinoderms, and annelids. Subsequently, however, their often bizarre-looking morphologies prompted the proposal of alternative phylogenetic positions ranging across the tree of life. Those of modular construction were suggested to constitute a distinct (and extinct) group termed the Vendobionta. Increasingly, the Ediacaran biota is being viewed as a collection of probably rather diverse organisms, perhaps including some true members of extant metazoan lineages (such as the putative bilaterian Kimberella) alongside members of extinct stem-lineages to more inclusive groups (including possible stem-metazoans), with others potentially identifiable as protists, algae, fungi, and microbial colonies. The ecologies of the Ediacaran organisms are often challenging to interpret. Some, notably Kimberella, appear to be preserved alongside feeding traces. Most, however, have no clear capabilities for movement or ingestion, and some lived at too great a depth in the oceans to have been photosynthetic. The forms with modular construction show adaptations consistent with osmotrophy, the direct absorption of nutrients across body surfaces. Interpreting the phylogeny, ecology, and evolutionary dynamics of the Ediacaran biota remains a key challenge in paleobiology.
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the Ediacaran Period provide the earliest abundant evidence for complex, large-bodied organisms, notably in the distinctive Ediacaran biota.
See also ▶ Ediacaran Biota ▶ Proterozoic (Aeon)
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ee ▶ Enantiomeric Excess
Effective Temperature
See also ▶ Ediacaran Period ▶ Eukaryote ▶ Proterozoic (Aeon)
References and Further Reading Knoll AH (2003) Life on a young planet: the first three billion years of evolution on Earth. Princeton University Press, Princeton Narbonne GM (2005) The Ediacara biota: Neoproterozoic origin of animals and their ecosystems. Annu Rev Earth Planet Sci 33:421–442 Vickers-Rich P, Komarower P (eds) (2007) The rise and fall of the Ediacaran biota. Geological Society Special Publication 286. The Geological Society, London Xiao S, Laflamme M (2009) On the eve of animal radiation: phylogeny, ecology and evolution of the Ediacara biota. Trends Ecol Evol 24:31–40
Ediacaran Period Definition The period of geologic time between c. 635 and 542 Ma, and the youngest period of the Proterozoic Eon. The upper boundary marks the base of the Cambrian Period and the beginning of the Phanerozoic Eon. The lower boundary is defined by the upper limit of Marinoan glaciation rocks in Enorama Creek, Australia. Fossils from
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DANIEL ROUAN LESIA, Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot, Meudon, France
Definition The effective temperature of a star is the temperature of a ▶ Black Body of the same size as the star and that would radiate the same total amount of electromagnetic power as emitted by the star.
Overview Noted Teff, the effective temperature is one of the fundamental parameters that characterizes a star. If P is the power radiated by a star of radius R, then Teff is derived by applying the Stefan–Boltzmann law: P = 4 p R2 s Teff4. Indeed a star is not actually a black body, as its ▶ Emissivity varies with wavelength; however, the effective temperature generally provides a fair approximation of the actual temperature of the stellar photosphere. This is so because at any wavelength, the radiation comes from a more or less deep layer but which is always on the skin of the star and thus, at approximately a constant temperature. The effective temperature is directly related to the color of the star: the higher the temperature, the bluer the light emitted by the star. The effective temperature is one
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of the two parameters, with the stellar luminosity, used to build the classical ▶ HR diagram that permits astronomers to classify stars. The effective temperature of the Sun is 5780 K, while it is 40,000 K for an O star and 3000 K for an M star. The term has been extended to planets, with a similar definition, but there, the effective temperature can sometimes be fairly different from the surface temperature. This is, for instance, the case of Venus, where the greenhouse effect in the atmosphere that blocks outward radiation imposes a ground temperature of 735 K, much higher than the effective temperature of 225 K, in order to balance the absorbed solar flux. Note that the effective temperature depends on the reflectivity, or albedo, of the planet: the larger the ▶ albedo, the lower the effective temperature.
a ▶ star, either quiescently (e.g., in coronal mass ejection or through the wind of an evolved star, like a ▶ red giant or an AGB star) or in a stellar explosion like in a ▶ supernova or nova.
See also ▶ Breccia ▶ Crater, Impact ▶ Impactite ▶ Red Giant ▶ Stars ▶ Supernova ▶ Volcano
See also ▶ Blackbody ▶ Bolometric Magnitude ▶ Color Index ▶ Emissivity ▶ Grey Body ▶ Hertzsprung–Russell Diagram
EH ▶ Redox Potential
Ejecta Synonyms Pyroclastics (in volcanology)
Definition Ejecta is solid, liquid, or gaseous material ejected from a source region. Three different kinds of ejecta exist: impact ejecta, volcanic ejecta (or pyroclastics), and (stellar) ejecta. Impact ejecta is solid, liquid, or vaporized rock debris ejected ballistically from an impact crater during a meteorite or cometary impact. These ejecta can be dispersed around the crater and forming specific patterns or partially building the crater rim. The volcanic ejecta is rock debris ejected by an explosive ▶ volcano and normally grouped in the more general term pyroclastics. In astrophysics, ejecta is the material expelled from
Ejection (Hyperbolic) Definition Hyperbolic ejection is the removal of an object from a gravitational system (e.g., a planetary system) on a hyperbolic ▶ orbit, so that the object is permanently lost from the system. This is caused by an increase in a body’s orbital energy that exceeds the gravitational binding energy of the system. In planetary systems, the increase in orbital energy usually comes from a close encounter with a massive planet. Objects that are ejected from planetary systems are thought to include a large number of ▶ planetesimal-sized bodies (asteroids and comets), a small number of planetary embryo-sized bodies, and in some cases, fully-grown terrestrial, ice giant or gas giant planets. In fact, the observed distribution of exoplanet eccentricities is consistent with the majority of planetary, systems having undergone a dynamical instability and ejected a planet in the past.
See also ▶ Escape Velocity ▶ Exoplanets, Discovery ▶ Orbit ▶ Planet Formation ▶ Planetesimals
References and Further Reading Rasio FA, Ford EB (1996) Dynamical instabilities and the formation of extrasolar planetary systems. Science 274:954–956 Weidenschilling S, Marzari F (1996) Gravitational scattering as a possible origin for giant planets at small stellar distances. Nature 384:619–621
Electrochemical Potential
Electric Discharge JEFFREY BADA Scripps Institution of Oceanography, La Jolla, CA, USA
Synonyms Spark Discharge
Definition An electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas. Several types of electric discharges occur naturally on Earth (American Geophysical Union, 1986): 1. Atmospheric lightning, which is thought to be caused by the frictional generation, and separation, of positive and negative charges on ice and dust particles. As the charge on these particles builds up, the result is the often-spectacular discharge of electricity known as lightning. On average, there are 50–100 lightning strikes per second on Earth with most of the activity taking place in equatorial and northern latitudinal regions. 2. Corona discharges, which are caused by an electrical discharge produced by the ionization of the surrounding atmosphere, generating a luminous plasma (sometime referred to as St. Elmo’s fire, a term used by sailors to describe the glow observed at the top of a ship’s mast during a thunderstorm). Unlike lightning, which is instantaneous and transient, coronal discharges are less brilliant, can last for a significant period of time (minutes or more), and occur over a large surface area. Continuous corona discharges have been observed on the nose cones of airplanes during thunderstorms. 3. Volcanic lightning, which is ubiquitous in most volcanic eruptions, especially water-rich eruptions. The cause of the lightning is not completely understood, but is thought to be the charging of ash particles by frictional processes and the ionization of gases expelled in the eruption. Volcanic lightning has been observed to be nearly continuous during an eruption.
Jupiter and Saturn and the NASA/ESA Cassini/Huygens mission has tentatively detected lightning in the atmosphere of Titan. Electric discharges on Earth, and by implication elsewhere, play an important role in various aspects of atmospheric chemistry. On the present-day Earth, the major products of lightning acting on the gases in the atmosphere are NO, along with lesser amounts of ▶ ozone. On the primitive Earth, with reducing atmosphere containing nitrogen and methane, the major product would have been HCN. In contrast, with a neutral nitrogen–carbon dioxide rich primordial atmosphere, the major product would have been a mixture of NO and CO, as well as ammonia and HCN (Cleaves et al. 2008). In the hydrogen-rich Jovian atmosphere, the products produced by lightning are much more diverse and include, besides HCN, low molecular weight hydrocarbons and formaldehyde. Lightning in the solar nebula may have also been important in the synthesis of reduced species of importance in prebiotic chemistry.
See also ▶ Ozone
References and Further Reading American Geophysical Union, National Research Council (U.S.). Geophysics Study Committee (1986) The earth’s electrical environment. National Academy Press, Washington, DC, pp 263 Aplin KL (2006) Atmospheric electrification in the solar system. Surv Geophys 27:63–108 Cleaves HJ, Chalmers JH, Lazcano A, Miller SL, Bada JL (2008) A reassessment of prebiotic organic synthesis in neutral planetary atmospheres. Orig Life Evol Biosph 38:105–115
Electrochemical Potential Definition Electrochemical potential is the free energy change (ΔG) associated with the movement of chemical species through a gradient of electric potential and/or a gradient of concentration established between the two sides of a biomembrane. The electrochemical potentials of H+ and/or Na+ are a main fuel for the cell energy transductions.
Overview Electrical discharges have also been observed in the atmospheres of other bodies in the solar system (Aplin 2006). Lightning has been detected during observations of
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See also ▶ ATP Synthase ▶ ATPase
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Electromagnetic Radiation
▶ Bioenergetics ▶ Photosynthesis ▶ Proton Motive Force ▶ Respiration ▶ Transduction
Electron Acceptor Synonyms Oxidant; Oxidative agent
Definition
Electromagnetic Radiation Definition The expression electromagnetic radiation designates the energy that is transported in the form of a propagating wave made of two perpendicular components, one an electric field, the other a magnetic field, which oscillate at the same frequency. This description applies to light in all its forms: radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays. It is by far the most important vector of information reaching us from the Universe and the objects it contains. Different physical processes give rise to electromagnetic radiation, and are generally classified as thermal (resulting from the thermal motion in a medium) or nonthermal radiation.
See also ▶ Blackbody ▶ Bremsstrahlung Radiation ▶ Free-free Emission ▶ Radiative Processes
Electromagnetic Spectrum
Electron acceptor is any of the organic or inorganic oxidative agents participating in an enzymatic reaction. In a restrictive sense, it is the terminal oxidant of an electron transport chain.
See also ▶ Anaerobic Respiration ▶ Photosynthesis ▶ Respiration
Electron Attachment Definition Electron attachment is the chemical process whereby an electron is attached to a neutral molecule. Examples include electron photo-attachment to CN to produce CN and a photon, and dissociative electron attachment to HCN to produce CN and a hydrogen atom.
See also ▶ Interstellar Chemical Processes
Electron Carrier
Definition The electromagnetic spectrum is the domain of all possible frequencies or wavelengths over which electromagnetic waves can be observed. This is a huge domain spanning 1023 decades in frequencies or wavelengths. It includes radio waves, millimetric waves, infrared, visible light, ultraviolet, x-rays, and gamma rays. The distribution versus frequency of the radiation intensity emitted by a particular celestial object is called the ▶ electromagnetic spectrum of this object; it contains in general extremely rich information on the nature and physical properties of the object.
See also ▶ Electromagnetic Radiation ▶ Spectroscopy
Definition Electron carrier is any of the soluble (e.g., NADH) or protein-bound (e.g., ▶ cytochromes) ▶ cofactors participating as reversible ▶ electron donor and/or acceptor in an enzymatic reaction.
See also ▶ Coenzyme ▶ Cofactor ▶ Cytochromes ▶ Electron Acceptor ▶ Electron Donor ▶ NADH, NADPH ▶ Photosynthesis ▶ Respiration
Electron Transport
Electron Dissociative Recombination
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Hþ to produce H plus a photon; this process is responsible for the red color of many low-density nebulae.
See also Definition Electron dissociative recombination is the chemical process whereby an electron reacts with a positive molecular ion to produce two neutral species. Examples include electron recombination with H3O+ to produce OH and 2H. This is a very important process in gas phase interstellar chemistry.
See also ▶ Interstellar Chemical Processes
▶ Electron Dissociative Recombination
Electron Transport
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FRANCISCO MONTERO Department of Biochemistry and Molecular Biology I, Facultad de Ciencias Quı´micas Universidad Complutense de Madrid, Madrid, Spain
Synonyms Electron transport chain
Electron Donor
Keywords
Reducing agent; Reductant
Electron carriers, respiration
Definition
Definition
Synonyms
Electron donor is any of the organic or inorganic substances acting as reducing agents in an enzymatic reaction. In a restrictive sense, the reducing agent at the beginning of an ▶ electron transport chain or the substrate used to obtain energy by ▶ respiration.
See also ▶ Electron Transport ▶ NADH, NADPH ▶ Photosynthesis ▶ Respiration
Electron Radiative Recombination Synonyms Radiative recombination
Definition Electron radiative recombination is a chemical process whereby an electron reacts with a positive atomic ion to produce a neutral atom with the emission of a photon (i.e., radiation). Examples include electron reaction with
photosynthesis,
redox
reactions,
Electron transport refers to the transfer of electrons from an initial donor (reducing substance) to a final acceptor (oxidizing substance) across different intermediaries, which takes place in biological membranes. Electron transport processes are usually associated with respiratory and photosynthetic processes.
Overview The most common ways by which living organisms obtain energy from their surroundings is by the ▶ oxidation of external electron donors (chemotrophic organisms), or by the absorption of electromagnetic radiation (phototrophic organisms). In both cases, the energy translation mechanisms take place in membranes (the cytoplasmic membranes in the case of prokaryotes, or subcellular particles such as mitochondria and chloroplasts in the case of eukaryotes). In the translation process, an electron transport takes place from a substance, which is an initial ▶ electron donor to a final acceptor, and since the ▶ redox potential of the donor is more negative than that of the acceptor, the global process is exergonic. This oxidation-reduction process takes place across a series of intermediaries (▶ electron carriers), which are not free in the intracellular medium as the majority of them form highly structured complexes inserted in the membranes. Therefore, the transfer of electrons that takes place between the different
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components does not occur using the conventional mechanism that takes place in the oxidation-reduction reactions of a homogenous system in dissolution. Instead, a directional electron transport takes place along the membrane and, in some cases, a transverse one as well. This explains how energy coupling between an oxidationreduction reaction and a transport phenomenon is possible, as first, the free energy released by these processes is used to produce a proton transport against the gradient across the membrane, and then the proton ▶ electrochemical potential gradient is translated, by means of processes that also take place in the membrane, in ▶ ATP. For many years, the mechanism whereby electrons are transferred between the different electron carriers, which are frequently separated from each other when the structure of the complexes is greater than 10 A˙, has been the subject of much debate. However, there is growing experimental and theoretical evidence that suggests that on some occasions the transport may be brought about by the “tunnel effect.” The molecules that take part in the electron transport are quite ubiquitous. The most common molecules are cytochromes, quinones, iron-sulfur proteins, and flavoproteins. In addition, chlorophylls and pheophytins are involved in ▶ photosynthesis.
See also ▶ ATP ▶ Electrochemical Potential ▶ Electron Acceptor ▶ Electron Carrier ▶ Electron Donor ▶ Oxidation ▶ Photosynthesis ▶ Redox Potential ▶ Reduction ▶ Respiration
Electrophoresis Synonyms Capillary electrophoresis; Gel electrophoresis
Definition Electrophoresis is the migration of charged molecules (for example ▶ nucleic acids or ▶ proteins) through a medium according to charge and molecular size as a consequence of an applied electric current and interactions with the medium. Parameters including matrix composition and concentration, ionic strength, electrical current strength and field angle, and migration time can be varied. Electrophoretic methods commonly employed include capillary electrophoresis (CE), gel electrophoresis (GE), and pulsed-field gel electrophoresis (PFGE) for separation of chromosomal DNA; denaturing and/or temperature gradient gel electrophoresis (DGGE/TGGE) for separation according to sequence; polyacrylamide gel electrophoresis (PAGE) and SDS-PAGE for separation of proteins in their native and denatured conformations, respectively; and isoelectric focusing (IEF) and 2D electrophoresis for separation of proteins by isoelectric point with or without PAGE first.
See also ▶ DNA Sequencing ▶ Nucleic Acids ▶ Protein
Elemental Carbon ▶ Carbon
References and Further Reading Nelson DL, Cox MM (2005) Lehninger principles of biochemistry, 4th edn. W.H. Freeman, New York Nicholls DG, Ferguson SJ (2002) Bioenergetics, 3rd edn. Academic Press, London Voet D, Voet JG (2004) Biochemistry, 3rd edn. Wiley, New York White D (1999) The physiology and biochemistry of prokaryotes, 2nd edn. Oxford University Press, New York
Electron Transport Chain ▶ Electron Transport
Elemental Depletion Definition Elemental depletion denotes the difference in elemental composition of an astronomical source relative to some standard of reference, usually the Sun. Lines of sight through the local interstellar medium indicate much lower ▶ abundances of many chemical elements compared to the solar composition. This is largely due to the differential incorporation of these elements into
Embden-Meyerhof-Parnas Pathway
▶ interstellar dust grains. Different lines of sight through ▶ diffuse clouds show different depletions.
History The definition of “cosmic abundances” takes account of the relative abundances in ▶ carbonaceous chondrites, which generally correlate well with those in the solar photosphere (Anders and Grevesse 1989; Przybilla et al. 2008).
See also ▶ Abundances of Elements ▶ Carbonaceous Chondrite ▶ Diffuse Clouds ▶ Interstellar Dust
References and Further Reading Anders E, Grevesse N (1989) Abundances of the elements – Meteoritic and solar. Geochim Cosmochim Acta 53:197–214 Przybilla N, Nieva M-F, Butler K (2008) A cosmic abundance standard: chemical homogeneity of the solar neighborhood and the ISM dustphase composition. Astrophys J 688:L103–L106
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theory one atom or molecule is adsorbed on the surface, and another reacts directly from the gas phase. This may be contrasted with the ▶ Langmuir–Hinshelwood mechanism. In the interstellar medium, formation of molecular hydrogen is thought to occur primarily through reactions on the surfaces of dust grains.
See also ▶ Adsorption ▶ Interstellar Dust ▶ Langmuir-Hinshelwood Mechanism
Embden-Meyerhof-Parnas Pathway Synonyms Glycolysis
Definition
Elephant Trunks Definition Tendrils of dark interstellar gas and dust surrounding, and pointing toward, massive stars are called elephant trunks. These tendrils form when the star’s copious ultraviolet radiation ionizes and disperses the more tenuous gas around it. Dense clumps remain, at least temporarily, and may shadow columns of gas lying behind them. The tendrils appear dark because the gas contains dust grains, which block the background light. Young stars may be present in the densest clumps at the tips of the trunks. These tips, where ionization is ongoing, also glow. A wellstudied region containing elephant trunks is the Eagle Nebula (M16) in the Serpens Constellation.
See also ▶ HII Region ▶ OB Association
Eley–Rideal Mechanism Definition This is a theoretical model by which some bimolecular chemical reactions can take place on solid surfaces. In the
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The Embden-Meyerhof-Parnas (EMP) pathway allows the metabolic use of glucose to generate ATP, NADH, and several biosynthetic precursors such as 3-phosphoglycerate or pyruvate. The EMP pathway can occur both anaerobically (leading to one or several ▶ fermentation pathways) and aerobically through the conversion of pyruvate to acetyl CoA and the connection with the tricarboxylic acids (TCA) cycle. The classical version of the EMP pathway is present in bacteria and eukaryotes whereas several modified versions are present in anaerobic archaea. The second half of the pathway is almost universal, and thus, it could represent the oldest part of the pathway, related to a primordial origin of ▶ gluconeogenesis.
History By 1940, the canonical glycolytic pathway (i.e., the one responsible for alcoholic fermentation in yeasts and anaerobic ▶ glycolysis in muscle) was elucidated. Actually, it was the result of a collective task with the contributions of, among others, Gustav Embden (1874–1933), Arthur Harden (1865–1940), Karl Lohman (1898–1978), Otto Fritz Meyerhof (1841–1951), Jakob Karol Parnas (1884–1949, and Otto Heinrich Warburg (1883–1970).
See also ▶ Entner–Doudoroff Pathway ▶ Fermentation ▶ Gluconeogenesis ▶ Metabolism (Biological)
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Embedded Bioburden ▶ Encapsulated Bioburden
Emergence of Life ▶ Origin of Life
50:50 Enantiomer Mixture ▶ Racemic (Mixture)
Enantiomeric Excess Synonyms Chiral excess; ee
Emission Nebula Definition That portion of an interstellar cloud that is hot and is ionized by radiation from nearby stars shines as an emission nebula. The spectrum is characterized by emission lines from the constituent atoms and ions. Emission nebulae are typically ▶ H II regions, although ▶ planetary nebulae are sometimes also referred to as emission nebulae.
See also ▶ HII Region ▶ Planetary Nebula
Emissivity Definition Emissivity is a quantity in physics that depends on wavelength and that characterizes the efficiency with which a surface radiates, compared to a ▶ black body at the same temperature. A black surface has an emissivity close to 1 and a polished reflecting surface, an emissivity close to 0. Because of Kirchhoff ’s law, the same quantity characterizes also the ability of a surface to absorb or to reflect impinging radiation. Emissivity is a positive, dimensionless quantity, lower than or equal to 1.
See also ▶ Albedo ▶ Blackbody
Empire ▶ Domain (Taxonomy)
Definition Enantiomeric excess (ee) is a measure of the deviation of a mixture of chiral (i.e., handed) molecules from the equimolar (or “racemic”) state. Enantiomeric excess is usually expressed as a percent; if D and L are the amounts of two chiral molecules, the enantiomeric excess is given as: ee ¼ ½ðD LÞ=ðD þ LÞ 100 Thus, the ee of a 60:40 mixture of D:L is 20 – a value that can be thought of as a mixture with 20% pure D plus 80% racemic mixture; furthermore, ee equals 0 and 100 for racemic and chirally pure mixtures, respectively. An enantiomeric excess can be measured in a number of ways, including specific optical rotation or chromatography with a chiral column. The enantiomeric excess is important in characterizing the purity of chiral pharmaceuticals and other chiral products. Many models of the origins of life postulate the local and/or global development of enantiomeric excesses in mixtures of amino acids or other biomolecules, and their subsequent chiral amplification, as a prelude to life’s ▶ homochirality.
See also ▶ Amino Acid ▶ Chirality ▶ Enantiomers ▶ Homochirality
Enantiomeric Pairs of Molecules ▶ Stereoisomers
Enantiomeric Ratio ▶ D/L-Ratio
Enceladus
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Enantiomers
▶ Amino Acid ▶ Chirality ▶ Stereoisomers
Definition An enantiomer is one of a pair of optical isomers, the structures of which are not superimposable on their mirror images. In organic compounds, enantiomers contain a carbon atom that has four different groups attached to it (asymmetric or chiral carbon) (see Fig. 1). The bestknown examples are the L- and D-enantiomers of amino acids. Enantiomers have identical physical and chemical properties, with the exception that they rotate light in equal but opposite directions. Enantiomers based on other central atoms such as silicon, phosphorus, and germanium are also known.
Encapsulated Bioburden Synonyms Embedded bioburden
Definition In planetary protection, the term “encapsulated ▶ bioburden” is used to indicated the number of viable ▶ microorganisms that are trapped inside nonmetallic spacecraft materials. They can be trapped between the layers of multilayer insulator, in the glue for honeycombs, or in the matrix itself. A spaceflight project may choose to measure the number of microorganisms present in a specific material. In addition, ▶ NASA and ▶ ESA have established consensus specifications describing the number of microorganisms that are assumed to be present in a variety of materials manufactured or prepared under defined conditions.
See also ▶ Bioburden ▶ Microorganism ▶ Planetary Protection
Enceladus C N
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THERESE ENCRENAZ LESIA, Observatoire de Paris, Meudon, France
H H
Keywords Saturn’s satellites R
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Enantiomers. Figure 1 An amino acid (bottom) showing four different atoms (R is another carbon substituent, for example CH3) attached to a central carbon atom. This configuration results in the property of asymmetry. The resulting enantiomers are mirror images of each other [much like the mirror images of your hands (top)], but they are otherwise chemical equivalent
Definition Enceladus is one of the midsized icy satellites of ▶ Saturn. It was discovered by William Herschel in 1789. Its distance to Saturn is 238,100 km or about 4 Saturnian radii. Its diameter is 500 km and its density is 1.0 g/cm3, corresponding to water ice. With an albedo of 0.9, Enceladus is the brightest object known in the solar system; such a high albedo results in very low surface temperature (70 K).
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Overview The exploration of Enceladus started with the ▶ Voyager missions. The first images of Voyager 1, taken in December 1980, already indicated a bright and young surface with relatively few craters. Voyager 2, in August 1981, not only confirmed this result but also provided evidence for tectonic activity, which was totally unexpected on such a small object. These results raised the interest of scientists who considered Enceladus as a prime objective for the ▶ Cassini mission. The exploration of Enceladus by Cassini started in 2005 with a series of flybys; the closest ones took place in March 2005 and July 2005, with respective distances of 501 and 172 km. Most of Enceladus’ surface is covered with impact craters distorted by viscous relaxation and also by more recent tectonic activity. Cassini images showed several different tectonic features: linear faults, curved stripes, and rifts (called ▶ fossae) of over 200 km in length, 10–15 km in width, and 1 km in depth. In addition, the Cassini instruments discovered at the South Pole a new region, previously unobserved, different from the rest of the surface. This region, which extends northward up to a latitude of about 55 S, is crater-free with many tectonic fractures and faults. The Cassini camera discovered four large faults distant from each other by about 35 km; they are up to 130 km in length, 2 km in width, and 0.5 km in depth. They are known as the “tiger stripes,” and are the youngest structures of the satellite. The Visible and Infrared Mapping Spectrometer of Cassini (VIMS) detected crystalline ice in this region, which indicates a very young origin (less than 1,000 years): if older, the water ice would have been transformed into amorphous ice by the solar UV radiation. The frontier between the South Pole region and its surrounding is marked by parallel cliffs and valleys. After the Voyager encounters, it was already proposed that ▶ cryovolcanism could be active on Enceladus thus accounting for its recent resurfacing. In addition, such cryovolcanism would provide a source for the E-ring of ▶ Saturn, which is located exactly at the same orbital distance as Enceladus. These hypotheses were fully confirmed by the Cassini observations in July 2005. First, observations by the magnetometer of Cassini showed, in the vicinity of Enceladus, a deviation of Saturn’s magnetic field, which could be attributed to the interaction between this magnetic field and ionized particles surrounding the satellite. In addition, ultraviolet spectra recorded during stellar occultation by the UVIS instrument detected a localized atmosphere, mostly made of water vapor around the South Pole. Simultaneously, the Cassini Ion and Neutral Mass Spectrometer (INMS) detected H2O,
but also N2 and CO2 in the cloud surrounding the South Pole. Finally, the Cosmic Dust Analyzer (CDA) detected near the tiger stripes microcrystals of water, similar to those of the E-ring (Fig. 1). Because of the low gravity field of Enceladus, the atmosphere surrounding the South Pole cannot be stable and must be continuously replenished. The most likely mechanism for feeding this atmosphere is cryovolcanism through the tiger stripes. This also accounts for the temperature excess of about 15 K measured by the Composite Infrared Spectrometer (CIRS) around the tiger stripes. In November 2005, a visual confirmation was brought by the camera of the Cassini orbiter that observed jets of icy particles above the South Pole region. Since 2005, repeated flybys have allowed scientists to accumulate data about the surface structure of the South Pole, the geysers, and the composition of the plumes. In addition to H2O, N2, CO2, and CO, other simple and complex hydrocarbon chains have been identified. The presence of nitrogen has been interpreted as a dissociation product of NH3, which suggests the possible presence of liquid NH4OH associated with H2O under the surface of Enceladus. Enceladus’ cryovolcanism is believed to be the feeding mechanism for the E-ring of Saturn, located at the same orbital distance. In June 2009, the Cosmic Dust Analyzer identified sodium salt (NaCl) in the icy grains that constitute the
Enceladus. Figure 1 Enceladus as observed by the Voyager spacecraft (©NASA)
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E-ring. This constitutes a strong argument in favor of the presence of a salty water ocean below the satellite’s surface. The presence of salts and carbonates in the geyser products suggests that a liquid ocean of water could be in direct contact with a silicate core, which might have important implications for astrobiology. What could be the origin of the internal energy that drives cryovolcanism on Enceladus? Tidal effects linked to the orbital resonance of Enceladus with ▶ Dione (another icy satellite of Saturn), could generate some internal energy, however, by far not enough to account for tectonic activity. According to some scientists, Enceladus, as other icy satellites of Saturn, might have formed very quickly. As a result, the satellite would have acquired a differentiated structure with a silicate core and an ice mantle. Subsequent radioactive and tidal heating would have raised the inner temperature enough to melt the icy mantle and part of the core, possibly creating magma chambers that would feed cryovolcanism. The exploration of Enceladus is the prime objective of the extended Cassini mission, which is planned to operate until 2017. In parallel, a future and more ambitious mission, TSSM (Titan and Saturn System Mission) is under study at NASA and ESA. Its first objectives will be the exploration of ▶ Titan and Enceladus. The mission could be launched at the horizon 2025.
See also ▶ Cassini–Huygens Space Mission ▶ Cryovolcanism ▶ Dione ▶ Fossa, Fossae ▶ Saturn ▶ Titan ▶ Voyager (Spacecraft)
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Terrile RJ, Cook AF (1981) Enceladus: “Evolution And Possible Relationship with Saturn’s E-ring”, 12th annual lunar and planetary science conference, Abstract 428 Waite JH et al (2006) Cassini ion and mass spectrometer: Enceladus plume composition and structure. Science 311:1419–1422
Endergonic Definition An endergonic reaction is one that requires ▶ free energy to proceed. An example of an endergonic reaction of biological interest is ▶ photosynthesis. Photosynthetic organisms conduct this reaction by using solar photons to drive the reduction of carbon dioxide to glucose and the oxidation of water to oxygen. The free energy change during a chemical reaction or a physical transformation which takes place at constant pressure is described by the familiar ΔG symbol where G is the Gibbs free energy. By definition, ΔG is positive for an endergonic process (and negative for an ▶ exergonic process). When the reaction takes place at constant volume, the free energy change is described by the symbol ΔF, where F is the Helmoltz free energy.
See also ▶ Exergonic ▶ Free Energy ▶ Photosynthesis
Endogenicity NICOLA MCLOUGHLIN Department for Earth Science and Centre for Geobiology, University of Bergen, Bergen, Norway
References and Further Reading Brown RH et al (2006) Composition and physical properties of Enceladus’ surface. Science 311:1425–1428 Castillo JC et al (2006) “A new understanding of the internal evolution of Saturn’s icy satellites from Cassini observations”, 37th annual lunar and planetary science conference, Abstract 2200 Dougherty MK et al (2006) Identification of a dynamic atmosphere at Enceladus with the Cassini magnetometer. Science 311:1406–1409 Hansen JC et al (2006) Enceladus water vapor plume. Science 311:1422–1425 Porco CC et al (2006) Cassini observes the south pole of Enceladus. Science 311:1393–1401 Smith BA et al (1982) A new look at the saturn system: the voyager 2 images. Science 215:504–537 Spahn F et al (2006) Cassini dust measurements at Enceladus and implications for the origin of the E-ring. Science 311:1416–1418
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Synonyms Indigenous
Keywords Biosignatures, contamination
Definition Endogenicity refers to a component that maybe textural, chemical, mineral, or biological and that formed within the host material; for example, a mineral phase or organic structure found in a rock that is indigenous to that rock and not derived from external sources. Endogenicity
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concerns the source of a component with respect to space and this differs from ▶ syngenicity, which concerns the source of a component with respect to time.
History Traditionally, the term endogenetic has been used in geology to refer to processes originating below the Earth’s surface as opposed to exogenetic, which describes geological processes acting at or near the Earth’s surface. This definition is modified here for the investigation of astrobiological samples obtained from a range of planetary surfaces and to operate at all observational scales from the field outcrop, hand sample to microscope.
Overview A component formed within the host material that is indigenous to that sample and formed in a closed system. Endogenetic components are not sourced from interactions with external environments – such components are termed exogenous. Most often, endogenetic components are also syngenetic and formed at the same time as the host material, but not always. For example, diagenetic and metamorphic phases produced by changes in pressure and temperature after formation of the host rock are endogenetic to that system (provided it was closed to fluid flow) but are not syngenetic, because they formed later in time. Often, exogenous components can be readily recognized because they occur in younger crosscutting veins or the alteration rims of, for example, meteorites and are derived from interactions with the terrestrial atmosphere or surface. Demonstration of the endogenicity of a component requires its composition, structure, and context to indicate an origin from within that system. So for instance, organic material found in a rock should occur in primary mineral phases or sedimentary fabrics; it should have an ▶ isotopic fingerprint that is distinct from externally sourced, migrating organic material; it should exhibit a crystallinity and/or reflectance that is compatible with the thermal maturity of the host rock. One way to demonstrate this for carbonaceous material is to show that the Raman spectra of the candidate biosignature and the host rock match, indicating that they have experienced the same degree of thermal alteration (e.g., Javaux et al. 2010). In addition, carbon isotopic measurements made both in situ and on extractable phases can be used to characterize and distinguish endogenetic from exogenous pools of organic material (e.g., Rasmussen et al. 2008). Fabric investigations are also crucial to eliminate exogenous components that are hosted by younger, possibly metastable phases produced by, for example,
hydrothermal alteration, or weathering near the planetary surface and occur near crosscutting features or external surfaces (e.g., Pinti et al. 2009). In meteorites for example, it has been shown by biological staining and epifluorescence microscopy that terrestrial microbial hyphae can rapidly colonize the interior (Toporski and Steele 2007). Thus, storage and handling protocols are essential to protect endogenetic remains collected by sample-return missions or delivered by impactors. Lastly, emerging nanoscale techniques for observing the interface between a candidate biosignature and host material can help to understand their preservational history and provide a further tool for assessing endogenicity (Wacey et al. 2008; Thomas-Keptra et al. 2009; Bernard et al. 2007).
See also ▶ Archean Traces of Life ▶ Biogenicity ▶ Biomarkers ▶ Biosignatures, Effect of Metamorphism ▶ Biomarkers, Isotopic ▶ Biomarkers, Morphological ▶ Diagenesis ▶ Raman Spectroscopy ▶ Syngenicity
References and Further Reading Bernard S, Benzerara K, Beyssac O, Menguy N, Guyot F, Brown JE, Goffe´ B (2007) Exceptional preservation of fossil plant spores in highpressure metamorphic rocks, Earth planet. Sci Lett 262:257–272 Javaux EJ, Marshall CP, Bekker A (2010) Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliclastic deposits. Nature 463:934–938 Pinti DL, Mineau R, Clement V (2009) Hydrothermal alteration and microfossil artefacts of the 3, 465-million-year-old Apex chert. Nat Geosci 2:640–643 Rasmussen B, Fletcher IR, Brocks JJ, Kilburn MR (2008) Reassessing the first appearance of eukaryotes and cyanobacteria. Nature 455:1101–1104 Thomas-Keptra KL, Clemett SJ, McKay DS, Gibsin EK, Wentworth SJ (2009) Origins of magnetite nanocrystals in Martian Meteorite ALH84001. Geochim Cosmochim Acta 73:6631–6677 Toporski J, Steele A (2007) Observations from a 4-Year contamination study of a sample depth profile through the Martian Meteorite Nakhla. Astrobiology 7:389–401 Wacey D, Kilburn MR, McLoughlin N, Parnell J, Brasier MD (2008) Using NanoSIMS in the search for early life on Earth: ambient inclusion trails in a c. 3400 Ma sandstone. J Geol Soc of London 165:43–53
Endogenous Synonyms Endogeny
Endogenous Synthesis
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Overview
In geology, endogenous refers to all the processes that are produced in the interior of the Earth (and other planets). It is commonly referred to the process that takes place in the ▶ mantle or the core of the planets but that can have subsequent effects on the surface of the planet. A good example is the mantle convection that drives the movement of tectonic plates at the surface of the Earth. The heat transfer towards its surface drives endogenous processes. The heat derives from the radioactive decay of elements U, Th, K, and the residual heat from planetary accretion.
It is generally believed that life on Earth arose in liquid water about 4 billion years ago. It is likely that the ingredients of primitive life were organic molecules based on carbon and hydrogen atoms associated with oxygen, nitrogen, and sulfur. The simplest sources of carbon susceptible to lead to prebiotic organic molecules are gaseous, that is, carbon dioxide, carbon monoxide, and methane. A prebiotic environment must produce reduced organic molecules in which carbon is dominantly associated with hydrogen rather than oxygen.
Production of Organics in the Atmosphere
See also ▶ Exogenous ▶ Geothermal Gradient ▶ Mantle ▶ Plate Tectonics
Endogenous Synthesis ANDRE´ BRACK Centre de Biophysique Mole´culaire CNRS, Orle´ans cedex 2, France
Synonyms Building blocks of primitive life
Keywords Amino acids, formaldehyde, hydrogen cyanide, hydrothermal systems, lipids, organic molecules, prebiotic chemistry, primitive atmosphere
Definition Life, defined as an open chemical system capable of selfreproduction and evolution, is thought to have originated from reduced organic matter in water. Carbon was available as gaseous compounds in the primitive atmosphere, either oxidized as carbon monoxide and carbon dioxide or reduced as methane. When subjected to energy sources (UV, heat, electric discharges, cosmic rays, shock waves) and mixed with water and ammonia or nitrogen, these gases generate hydrogen cyanide and formaldehyde that lead to many of the building blocks of life, such as amino acids. Deep-sea hydrothermal systems may also represent an environment for the synthesis of prebiotic organic molecules.
Oparin (1924) suggested that the small reduced organic molecules needed for primitive life were formed in a primitive atmosphere dominated by methane. The idea was tested in the laboratory by Miller (1953) who exposed a mixture of methane, ammonia, hydrogen, and water to spark discharge and silent electric discharge. In his initial experiment, he obtained three amino acids (glycine, alanine, and b-alanine) via the intermediary formation of hydrogen cyanide and aldehydes. More generally, simple gaseous molecules, like CH4, H2, NH3, and H2O, require a supply of energy (UV, heat, electric discharges, cosmic rays, shock waves) to react with each other. They generate compounds like formaldehyde and hydrogen cyanide that store chemical energy in their double and triple chemical bonds, respectively. The possible sources of atmospheric synthesis, including electric effects, solar UV, and impact shocks, have been reviewed by Chang (1993). Miller’s laboratory synthesis of amino acids occurs efficiently when a reducing gas mixture containing significant amounts of hydrogen is used. However, the actual composition of primitive Earth’s atmosphere is not known. The dominant view in recent years is that the primitive atmosphere consisted mainly of CO2, N2, and H2O along with small amounts of CO and H2 (Kasting and Brown 1998; Catling and Kasting 2007). Only small yields of amino acids are formed in such a mixture (Schlesinger and Miller 1983; Miller 1998). Recent studies show that the low yields previously reported appear to be the outcome of oxidation of the organic compounds during hydrolytic workup by nitrite and nitrate produced in the reactions. The yield of amino acids is greatly increased when oxidation inhibitors, such as ferrous iron, are added prior to hydrolysis, suggesting that endogenous synthesis from neutral atmospheres may be more important than previously thought (Cleaves et al. 2008). Additionally, 22 amino acids and five amines were obtained when reanalyzing archived samples of experiment run by Miller and simulating production of organic molecules in
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volcanic gases by lightning. The volcanic apparatus experiment suggests that, even if the overall atmosphere was not reducing, localized prebiotic synthesis could have been effective in volcanic plumes (Johnson et al. 2008). The escape of hydrogen from the early Earth’s atmosphere has recently been reevaluated (Tian et al. 2005). It may have occurred at rates slower by two orders of magnitude than previously thought. The balance between slow hydrogen escape and volcanic outgassing could have maintained a hydrogen mixing ratio of more than 30%, thus making endogenous organic synthesis more robust than previously thought. Intense bombardment probably caused some chemical reprocessing of the Earth’s primitive atmosphere by impact shock chemistry. An indication of the number and timing of the impacts onto the early Earth can be obtained by comparison with the crater record of the Moon, which records impacts from the earliest history of the solar system (Ryder 2003). Because of the larger size of the Earth and its greater gravitational pull, about 20 times as many impacts would have occurred on the early Earth as on the Moon. Computer modeling of impact shock chemistry shows that the nature of the atmosphere strongly influences the shock products (Fegley et al. 1986). A neutral CO2-rich atmosphere produces CO, O2, H2, and NO while a reducing CO-rich atmosphere yields primarily CO2, H2, CH4, HCN, NH3, and H2CO. The last three compounds are particularly interesting for prebiotic chemistry since they can lead to amino acids via the Strecker synthesis. However, a CO-rich primitive atmosphere may be unlikely due to the instability of CO to photolysis. In laboratory experiments, a gas mixture of methane, ammonia, and water subjected to shock heating followed by a rapid thermal quenching yielded the amino acids glycine, alanine, valine, and leucine (Bar-Nun et al. 1970). Here again, the gas mixture used does not represent a realistic primitive atmosphere, which was probably dominated by CO2. Laboratory simulations of shocks were also run with a high-energy laser. CH4-containing mixtures generated hydrogen cyanide and acetylene but no organics could be obtained with CO2-rich mixtures (McKay and Borucki 1997).
Submarine Hydrothermal Systems The reducing conditions in hydrothermal systems may have been an important source of biomolecules on the primitive Earth (Baross and Hoffman 1985; Holm and Andersson 1998, 2005). The reducing environment results from the flow of substances dissolved in sea water passing inorganic compounds present in very hot crustal material that reduce compounds in the sea water. These reduced
compounds flow out of the hydrothermal system and the inorganic sulfides formed precipitate when they mix with the cold (4 C) ocean water. For example, hydrocarbons containing 16–29 carbon atoms have been detected in the Rainbow ultramafic hydrothermal system, Mid-Atlantic Ridge (Holm and Charlou 2001). Hydrothermal vents are often disqualified as efficient reactors for the synthesis of bioorganic molecules, because of the high temperature. However, the products that are synthesized in hot vents are rapidly quenched in the surrounding cold water, which may preserve those organics formed. In laboratory experiments, one amino acid synthesis started with a mixture of hydrogen cyanide, formaldehyde, and ammonia, so is not really a hydrothermal process (Hennet et al. 1992). The same holds for a reported hydrothermal synthesis of amino acids that used calcium metal, formaldehyde, ammonia hydrogen, and oxygen in different combinations to generate amino acids (Marshall 1994). Amino acids and polymers of amino acids are formed by the reaction of methane and nitrogen at 325 C in “modified sea water” in a glass-lined vessel (Yanagawa and Kobayashi 1992). “Modified sea water” contains the principal metal ions present in sea water but at much higher concentrations so as to amplify the chemical processes catalyzed by the ions so they can be more easily detected. The role of the silicate and metal ions in this complex reaction mixture is not clear and this system may not be a good model of a hydrothermal system.
The Role of Minerals Clay minerals are formed by water weathering of silicate minerals. As soon as liquid water was permanently present in the surface of the Earth, clay minerals accumulated. The importance of clay mineral in the origins of life was first suggested by Bernal (1949). The advantageous features of clays for Bernal were their ordered arrangement, their large adsorption capacity, their shielding against sunlight, their ability to concentrate organic chemicals, and their ability to serve as polymerization templates. Since the seminal hypothesis of Bernal, many prebiotic experiments have been run with clays (Ponnamperuma et al. 1982; Brack 2006; Negron-Mendoza et al. 2010). If the carbon source for life was carbon dioxide, the energy source required to reduce the carbon dioxide might have been provided by the oxidative formation of pyrite from iron sulfide and hydrogen sulfide. Pyrite has positive surface charges and bonds the products of carbon dioxide reduction, giving rise to a two-dimensional reaction system, a “surface metabolism” (Wa¨chtersha¨user 1994, 1998, 2007). Laboratory work has provided some support for this promising hypothesis. An early laboratory
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simulation of hydrothermal synthetic reactions is the reduction of carbon dioxide to organic sulfides at 75 C in the presence of FeS and H2S. Methyl- and ethyl-thiol were the principal thiols formed along with smaller amounts of others containing up to five carbon atoms. The CO2 was also converted to CS2 and COS (Heinen and Lauwers 1996). The direct reduction of CO2 to acetic acid, acetaldehyde, ethanol, and smaller amounts of carbon compounds containing up to six carbon atoms was observed to take place at 350 C and high pressure in the presence of magnetite (FeO: Fe2O3) and small amounts (2%) of water. The yields of organics decreased 10–100% when the proportion of water was increased to 90% (Chen and Bahnemann 2000). A similar inhibitory effect of large amounts of water was observed in the reduction of nitrogen to ammonia in the presence of magnetite and 10 bars of CO2 at 350 C. The yield of ammonia decreased 10–100fold when the water/iron ratio was increased from 0.5 to 6 (Brandes et al. 1998). There are also reports of the reaction of CO in simulated hydrothermal systems. When a mixture of CO and CH3SH was reacted with a combination of a NiS-FeS at 100 C, acetic acid and its corresponding thioester were formed (Huber and Wa¨chtersha¨user 1997). This system was extended to the formation of keto esters at higher temperatures and pressures where CO inserted into the thioester to form a keto thioester that in turn hydrolyzed to pyruvic acid (Cody et al. 2000). More recently, a-hydroxy and a-amino acids have been obtained under possible volcanic origin-of-life conditions by heating CO in the presence of nickel or nickel/iron precipitates with carbonyl, cyano, and methylthio ligands as carbon sources (Huber and Wa¨chtersha¨user 2006). However, the plausibility of the conditions used has been debated (Bada et al. 2007).
See also ▶ Chemical Evolution ▶ Extraterrestrial Delivery (Organic Compounds)
References and Further Reading Bada JL, Fegley B Jr, Miller SL, Lazcano A, Cleaves HJ, Hazen RM, Chalmers J (2007) Debating evidence for the origin of life on Earth. Science 315:937–938 Bar-Nun A, Bar-Nun N, Bauer SH, Sagan C (1970) Shock synthesis of amino acids in simulated primitive environments. Science 168:470–473 Baross JA, Hoffman SE (1985) Submarine hydrothermal vents and associated gradient environment as sites for the origin and evolution of life. Orig Life Evol Biosph 15:327–345 Bernal JD (1949) The physical basis of life. Proc R Soc Lond 357A:537–558 Brack A (2006) Clay minerals and the origin of life. In: Bergaya F, Theng BKG, Lagaly G (eds) Handbook of clay science. Elsevier Science, Amsterdam, pp 385–398
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Brandes JA, Boctor NZ, Cody GD, Cooper BA, Hazen RM, Yoder HS Jr (1998) Abiotic nitrogen reduction on the early Earth. Nature 395:365–367 Catling D, Kasting JF (2007) Planetary atmospheres and life. In: Sullivan WT III, Baross JA (eds) Planets and Life. Cambridge University Press, Cambridge, pp 91–116 Chang S (1993) Prebiotic synthesis in planetary environments. In: Greenberg JM, Mendoza-Gomez CX, Pirronello V (eds) The chemistry of life’s origin. Kluwer Academic Publ, Dordrecht, pp 259–300 Chen QW, Bahnemann DW (2000) Reduction of carbon dioxide by magnetite: Implications for the primordial synthesis of organic molecules. J Am Chem Soc 122:970–971 Cleaves HJ, Chalmers JH, Lazcano A, Miller SL, Bada JL (2008) A reassessment of prebiotic organic synthesis in neutral planetary atmospheres. Orig Life Evol Biosph 38:105–115 Cody GD, Boctor NZ, Filley TR, Hazen RM, Scott JH, Sharma A, Yoder HS Jr (2000) Primordial carbonylated iron-sulfur compounds and the synthesis of pyruvate. Science 289:1337–1340 Fegley B Jr, Prinn RG, Hartman H, Watkins GH (1986) Chemical effects of large impacts on the earth’s primitive atmosphere. Nature 319: 305–308 Heinen W, Lauwers AM (1996) Sulfur compounds resulting from the interaction of iron sulfide, hydrogen sulfide and carbon dioxide in an anaerobic aqueous environment. Orig Life Evol Biosph 26:131–150 Hennet RJ-C, Holm NG, Engel MH (1992) Abiotic synthesis of amino acids under hydrothermal conditions and the origin of life: a perpetual phenomenon? Naturwissenschaften 79:361–365 Holm NG, Andersson EM (1998) Organic molecules on the early Earth: hydrothermal systems. In: Brack A (ed) The molecular origins of life: assembling pieces of the puzzle. Cambridge University Press, Cambridge, pp 86–99 Holm NG, Andersson EM (2005) Hydrothermal simulation experiments as a tool for studies of the origin of life on earth and other terrestrial planets: a review. Astrobiology 5:444–460 Holm NG, Charlou J-L (2001) Initial indications of abiotic formation of hydrocarbons in the Rainbow ultramafic hydrothermal system, Mid-Atlantic ridge. Earth Planet Sci Lett 191:1–8 Huber C, Wa¨chtersha¨user G (1997) Activated acetic acid by carbon fixation on (Fe, Ni)S under primordial conditions. Science 276:245–247 Huber C, Wa¨chtersha¨user G (2006) a-hydroxy and a-amino acids under possible Hadean, volcanic origin-of-life conditions. Science 314:630–632 Johnson AP, Cleaves HJ, Dworkin JP, Glavin DP, Lazcano A, Bada JL (2008) The Miller volcanic spark discharge experiment. Science 322:404 Kasting JF, Brown LL (1998) The early atmosphere as a source of biogenic compounds. In: Brack A (ed) The molecular origins of life: assembling pieces of the puzzle. Cambridge University Press, Cambridge, pp 35–56 Marshall WL (1994) Hydrothermal synthesis of amino acids. Geochim Cosmochim Acta 58:2099–2106 McKay CP, Borucki WJ (1997) Organic synthesis in experimental impact shocks. Science 276:390–392 Miller SL (1953) The production of amino acids under possible primitive Earth conditions. Science 117:528–529 Miller SL (1998) The endogenous synthesis of organic compounds. In: Brack A (ed) The molecular origins of life: assembling pieces of the puzzle. Cambridge University Press, Cambridge, pp 59–85 Negron-Mendoza A, Ramos-Bernal S, Mosqueira FG (2010) The role of clay interactions in chemical evolution. In: Basiuk VA (ed)
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Astrobiology: emergence, search and detection of life. American Scientific Publishers, Stevenson Ranch, California, pp 214–233 Oparin AI (1924) Proikhozndenie Zhizni. Izd. Moskowski Rabochi, Moscow Ponnamperuma C, Shimoyama A, Friebele E (1982) Clay and the origin of life. Orig Life 12:9–40 Ryder G (2003) Bombardment of the Hadean Earth: wholesome or deleterious? Astrobiology 3:3–6 Schlesinger G, Miller SL (1983) Prebiotic syntheses in atmospheres containing CH4, CO, and CO2 1. Amino acids. J Mol Evol 19:376–382 Tian F, Toon OB, Pavlov AA, De Sterck H (2005) A hydrogen-rich early atmosphere. Science 308:1014–1017 Wa¨chtersha¨user G (1994) Life in a ligand sphere. Proc Natl Acad Sci USA 91:4283–4287 Wa¨chtersha¨user G (1998) Origin of life in an iron-sulfur world. In: Brack A (ed) The molecular origins of life: assembling pieces of the puzzle. Cambridge University Press, Cambridge, pp 206–218 Wa¨chtersha¨user G (2007) On the chemistry and evolution of the pioneer organism. Chem Biodivers 4:584–602 Yanagawa H, Kobayashi K (1992) An experimental approach to chemical evolution in submarine hydrothermal systems. Orig Life Evol Biosph 22:147–159
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Endospore Synonyms Bacterial spore; Spore
Definition An endospore is a tough, dormant structure produced by certain species of bacteria, most notably of the genera Bacillus and Clostridium.
See also ▶ Sporulation
Endosymbiosis AMPARO LATORRE Institute Cavanilles for Biodiversity and Evolutionary Biology, Universitat de Valencia, Valencia, Spain
▶ Endogenous
Keywords
Endolithic Definition Endolithic refers to any ▶ microorganisms which is able to colonize and survive inside a rock. Endoliths are classified into three different classes depending of the part of the interior of the rock that the organisms colonize: chasmoendolith (chasm means cleft) are the organisms that colonize open spaces in the rock such as fissures and cracks, cryptoendolith (crypto means hidden), organisms that colonize the interior of porous rocks, and euendolith (eu which means true endolith) which are organisms that actively colonize the interior of the rock by forming tunnels with the shape of their bodies. These organisms are of extreme astrobiological interest because the interior of a ▶ meteorite can give shelter to endolithic organisms under the extreme conditions of an interplanetary voyage (▶ lithopanspermia).
See also ▶ Cryptoendolithic ▶ Lithopanspermia ▶ Microorganism ▶ Panspermia
Bacteriocyte, chloroplast, endosymbiont, mitochondria, obligate mutualism, symbiogenesis
Definition Endosymbiosis is a symbiotic association in which one partner, generally a prokaryote symbiont, lives sequestered inside specialized eukaryotic host cells called bacteriocytes. The association is obligate for both partners.
Overview Symbiogenesis, the evolutionary process of establishing a symbiotic association, has been one of the dominant forces in the early evolution of life on Earth. In 1967 Lynn Margulis postulated the serial endosymbiotic theory of eukaryotic ▶ cell evolution, currently accepted with respect to the origin of ▶ mitochondria and ▶ chloroplasts (Margulis 1993). These two eukaryotic ▶ organelles are beyond doubt the product of symbiogenic events between prokaryotes and primitive eukaryotes, as supported by many different types of genetic, biochemical, and phylogenetic evidence. The origin of mitochondria dates back to 2 109 years ago, whereas chloroplasts originated more than 1.2 109 years ago, because fossil red algae of that age have been found. DNA sequence
Energy
comparisons and phylogenetic analyses have identified alpha-proteobacteria and cyanobacteria as the bacterial groups to which the ancestors of mitochondria and plastids, respectively, belonged. Nowadays, symbiotic associations have been documented in practically every major branch of the ▶ tree of life and this observation reinforces the role played by ▶ symbiosis in the emergence of evolutionary innovations in ▶ eukaryotes. Recently, the genomic era has opened the access to the characterization of the organisms involved in symbiosis, mainly those non-cultivable microbial endosymbionts, allowing the comparison among the different evolutionary innovations carried out by these bacteria on their way from a free-living lifestyle to varied stages of integration with their respective hosts (Dale and Moran 2006; Moya et al. 2008). Thus, endosymbiosis is an ongoing phenomenon in the evolution of life and played a key role in the origin and evolution of the eukaryotic cell. In general, it is well established that endosymbiotic integration is a process that commonly changes profoundly the gene repertoire of the free-living ancestor. Depending on the type of symbiotic relationship (i.e., mutualistic or parasitic, facultative or obligate, etc.), age of the association, and host necessities (i.e., nutritional, defensive, waste recycling, etc.), the genetic and metabolic changes will be more or less dramatic (Wernegreen 2005; Moran et al. 2008). We should not disregard the discovery of new instances of endosymbionts currently en route to become organelles or fully fledged eukaryotic compartments of bacterial origin (Nakabachi et al. 2006; Pe´rez-Brocal et al. 2006; Nowack et al. 2008). Current “omics” (such as genomics, proteomics, or metabolomics) methodologies allow us not only to further unravel the steps toward the origin of the eukaryotic cell, but also to assess the question of how much of the eukaryotic complexity originated through evolutionary innovations by symbiogenesis.
See also ▶ Cell ▶ Chloroplast ▶ Eukarya ▶ Evolution (Biological) ▶ Mitochondrion ▶ Organelle ▶ Phylogenetic Tree ▶ Phylogeny ▶ Symbiosis ▶ Taxonomy
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References and Further Reading Dale C, Moran NA (2006) Molecular interactions between bacterial symbionts and their host. Cell 126:453–465 Margulis L (1993) Symbiosis in cell evolution, 2nd edn. W.H. Freeman, NJ, p 452 Moran NA et al (2008) Genomics and evolution of heritable bacterial symbionts. Ann Rev Genetics 42:165–190 Moya A et al (2008) Learning how to live together: genomic insights into prokaryote-animal symbioses. Nat Rev Genetics 9:218–229 Nakabachi A et al (2006) The 160-kilobase genome of the bacterial endosymbiont Carsonella. Science 314:267 Nowack ECM et al (2008) Chromatophore genome sequence of Paulinella sheds light on acquisition of photosynthesis by eukaryotes. Curr Bio 18:410–418 Pe´rez-Brocal V et al (2006) A small microbial genome: the end of a long symbiotic relationship? Science 314:312–313 Wernegreen JJ (2005) For better or worse: genomic consequences of intracellular mutualism and parasitism. Curr Opin Genet Dev 15:572–583
Endothermic Definition An endothermic reaction is a physical transformation which requires an energy supply if it takes place at constant volume or which requires an enthalpy supply if it takes place at constant pressure. A familiar endothermic physical transformation is the evaporation of water. The enthalpy change (ΔH ) or the energy change (ΔU ) during an endothermic reaction or transformation is positive by definition.
See also ▶ Exothermic
Energy FRANCISCO MONTERO Department of Biochemistry and Molecular Biology I, Facultad de Ciencias Quı´micas Universidad Complutense de Madrid, Madrid, Spain
Keywords ATP, bioenergetics, chemical work, electrochemical potential gradient, electromagnetic energy, energy transduction, osmotic work
Definition In physics, energy is defined as the capacity of a system to produce work. There are many forms of energy
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(e.g., mechanical, electrical, electromagnetic, chemical, etc.), and it is possible to convert some energy forms into others, with the condition that the energy is conserved.
History A coherent theory on energy and its transformations was first advanced in the nineteenth century, chiefly as a consequence of the Industrial Revolution. The propounding of the fundamental principles (or laws) of thermodynamics established the basic rules for energy conversions. Although important observations had already been made on the effect of energy on biological systems by the nineteenth century, it was not until the development of biochemistry in the twentieth century that energy conversion mechanisms in biological systems were explained. In 1941, Lipmann established that ATP is an energy currency in the biological system, while in 1961, Mitchell proposed the chemiosmotic hypothesis, which constitutes the base of all modern ▶ bioenergetics. Nowadays, the majority of energy transduction processes at molecular level are well understood.
Overview Ever since the Industrial Revolution, one of mankind’s greatest challenges has been to find interconversion mechanisms for different energy forms (conversion of thermal, wind, electromagnetic, or atomic energy into mechanical or electrical energy, etc.). Although all energy forms are interconvertible, there are limitations with regard to the amount and quality of the energy released in a process that can be used in another one. For this reason, the total energy in a system is usually classified into different categories, such as internal energy, free energy, enthalpy, entropic contribution, etc. Of these, free energy is the only one that can be transferred from one process to another. Therefore, processes are classified as ▶ endergonic or ▶ exergonic depending, respectively, on whether they need or release ▶ free energy during their development. Biological systems, when viewed as thermodynamic systems, are considered to be open systems, which means they can exchange matter and energy with their surroundings in order to maintain the dissipative structures, which characterize them. Basically, energy can be exchanged with the surroundings in two ways: through matter, by taking molecules from the environment whose catabolic processes (of degradation) release free energy, or by absorbing electromagnetic radiation. Furthermore, biological systems have mechanisms which can convert some forms of energy into others, e.g., electromagnetic energy into
osmotic energy, osmotic energy into chemical and mechanical energy, etc. These general mechanisms for the translation of energy within a biological system, which are the result of evolution and natural selection, constitute the field of study of bioenergetics, and are truly astonishing. A general law governing energy coupling in biological systems states that they are not usually direct. Instead, they are produced through universal intermediaries or “energy currencies”. An exergonic process produces an energy currency which can then be used in an endergonic process. All of the energy currencies can be used to produce chemical work (production of molecules whose formation requires energy, in general anabolic processes), osmotic work (the transport of substances across membranes against their ▶ electrochemical potential gradients), or mechanical work (movement of flagella and cilia, contractile movements, etc.)
See also ▶ ATP ▶ ATP Synthase ▶ Bioenergetics ▶ Electrochemical Potential ▶ Endergonic ▶ Endothermic ▶ Energy Conservation ▶ Energy Sources ▶ Enthalpy ▶ Entropy ▶ Exergonic ▶ Exothermic ▶ Free Energy
References and Further Reading Skulachev VP (1992) The laws of cell energetics. Eur J Biochem 208:203–209 Skulachev VP (1994) Bioenergetics: the evolution of molecular mechanisms and the development of bioenergetics concepts. Antonie Van Leeuwenhoek 65:271–284
Energy Conservation Definition ▶ Energy conservation refers to the mechanisms by which organisms can transform different ▶ energy sources (radiation, reduced organic or inorganic compounds) in cellular useful energy: proton motive force, ▶ ATP, reducing power. It is common for energy to be converted from one form to another; however, the law of conservation of energy, a fundamental law of physics, states that although
Energy Sources
energy can be changed in form it can be neither created nor destroyed. The main energy conservation mechanisms used by biological systems are ▶ respiration (aerobic and anaerobic), fermentation, and photosynthesis (oxygenic and anoxygenic).
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only two: ▶ radiation and reduced chemical compounds. Living systems transform external energy sources into energy useful for cellular processes according to the thermodynamic law of energy conservation.
History See also ▶ Aerobic Respiration ▶ Anaerobic Respiration ▶ Anoxygenic Photosynthesis ▶ ATP ▶ ATPase ▶ ATP Synthase ▶ Bioenergetics ▶ Electron Acceptor ▶ Electron Carrier ▶ Electron Transport ▶ Energy ▶ Energy Sources ▶ Fermentation ▶ NADH, NADPH ▶ Oxidation ▶ Oxygenic Photosynthesis ▶ Photosynthesis ▶ Proton Motive Force ▶ Proton Pump ▶ Respiration
Energy Sources FELIPE GO´MEZ Centro de Astrobiologı´a (CSIC-INTA), Instituto Nacional de Te´cnica Aeroespacial, Torrejo´n de Ardoz, Madrid, Spain
Keywords Electron donors, energy conservation, fermentation, photosynthesis, radiation, redox reactions, reduced inorganic compounds, reduced organic compounds, respiration
Definition Life implies work, in other words, it requires ▶ energy. Metabolite transports across membranes, cellular ▶ motility, or the biosynthesis of macromolecules are examples of cellular processes that require energy. Any living system relies on an external source of energy. The external sources of energy useful for living systems are
Peter Mitchell was awarded the 1978 Nobel Prize in Chemistry for his work in the description of the Chemiosmosis. Paul D. Boyer and John E. Walker were awarded part of the 1997 Nobel Prize in Chemistry for their clarification of the mechanism of ATP synthase.
Overview Energetic processes are basically related to oxidoreduction reactions associated with the transformation of the energy sources. Energy sources are key elements for the classification of organisms. According to this, organisms can be classified as chemotrophs if the external source of energy is chemical, or phototrophs if it is radiation. Chemotrophs can be subdivided as chemoorganotrophs if the source of energy is a reduced organic compound (e.g., glucose, acetate), or chemolithotrophs if the source of energy is a reduced inorganic compound (e.g., ferrous iron, ammonia). The mechanisms by which organisms transform the external source of energy in cellular energy are three: ▶ respiration, ▶ fermentation, which is used by chemothrophs, and ▶ photosynthesis, which is used by phototrophs. Cells can store the transformed energy as ▶ proton motive force (proton gradient) or in the form of energetic compounds like ▶ ATP or ▶ NADH. Some reactions or functions are driven by the proton motive force (transport of metabolites through the membrane, the rotation of the bacterial flagella); others require the use of ATP, such as the synthesis of macromolecules. An important achievement was the establishment of the chemiosmotic hypothesis coupling the ▶ electron transport in the respiratory chain with the generation of a proton motive force and the synthesis of ATP proposed by Peter Mitchell in 1961, and for which he was awarded the Nobel Price.
See also ▶ ATP Synthase ▶ Chemotroph ▶ Electrochemical Potential ▶ Electron Donor ▶ Electron Transport ▶ Energy ▶ Fermentation ▶ Metabolism (Biological) ▶ Motility
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▶ Photosynthesis ▶ Proton Motive Force ▶ Radiation ▶ Respiration
whereas the non-phosphorylative ED pathway is found in some fungi and thermophilic and hyperthermophilic archaea. It has been suggested that the ED pathway is older than the EMP pathway.
References and Further Reading
History
Amend JP, Shock EL (2001) Energetics of overall metabolic reactions of thermophilic and hyperthermophilic archaea and bacteria. FEMS Microbiol Rev 25:175–243 Mitchell P (1967) Proton current flow in mitochondrial systems. Nature 25(5095):1327–1328 Mitchell P, Moyle J (1967) Chemiosmotic hypothesis of oxidative phosphorylation. Nature 213(5072):137–139 Smith E, Morowitz HJ (2004) Universality in intermediary metabolism. Proc Natl Acad Sci USA 101:13168–13173 Temple KL, Colmer AR (1951) The autotrophic oxidation of iron by a new bacterium: thiobacillus ferrooxidans. J Bacteriol 62(5):605–611 Wa¨chtersha¨user G (1992) Groundworks for an evolutionary biochemistry – the iron sulphur world. Prog Biophys Mol Biol 58:85–201
The ED pathway was first discovered in 1952 in Pseudomonas saccharophila and in 1967 was shown to be also present in Escherichia coli.
See also ▶ Embden-Meyerhof-Parnas Pathway ▶ Metabolism (Biological)
Entropy Definition
Enthalpy Definition The enthalpy change is the heat produced or absorbed during a chemical or physical transformation taking place at constant pressure. The thermodynamic definition of enthalpy is H = U + PV, where H stands for enthalpy, U for internal energy, P for pressure, and V for volume.
Entropy is a thermodynamic concept introduced in 1865 by Clausius. Entropy, represented by S, increases in any spontaneous processes (where ΔS is positive) taking place in an isolated system, i.e., a system which does not exchange matter or energy with its surroundings. If two gases are in a constant-temperature box, initially separated by a partition and that partition is removed, the gases will mix without a change in enthalpy. Entropy, however, increases as the gases mix in this spontaneous process and acts as the driving force.
See also ▶ Free Energy
See also ▶ Enthalpy ▶ Free Energy
Entner–Doudoroff Pathway Synonyms
Environment
Glycolysis
Definition The Entner–Doudoroff (ED) pathway is the glycolytic pathway that allows to catabolize sugar acids, such as gluconate. Several enzymatic steps allow the conversion of six carbon sugar acids in two trioses that can be further metabolized through the universal lower part of the ▶ Embden-Meyerhof-Parnas (EMP) pathway. This pathway is present in bacteria, but modifications have been described in all three domains of life. For example, the semi-phosphorylative ED pathway has been demonstrated in several anaerobic bacteria and in the halophilic archaea,
JOSE´ LUIS SANZ Departamento de Biologı´a Molecular, Universidad Auto´noma de Madrid, Madrid, Spain
Synonyms Ecosystem; Habitat
Keywords Acidity, atmosphere, ionic strength, life, molecular ecology, oceans, pressure, radiation, redox conditions, temperature
Enzyme
Definition Natural environment refers to the collection of objects both living and inanimate as well as the fluid, gaseous, or solid media that together make up a ▶ habitat. The term includes soil, the subsurface, the atmosphere, the oceans and smaller bodies of water, and the interiors of plants and animals. The description of an environment requires definitions of its physical and chemical characteristics – temperature and pressure, acidity, redox conditions, ▶ radiation, etc.
Overview The ensemble of biological communities and their surroundings constitute an “▶ ecosystem,” and the science that studies these ecosystems is known as Ecology. From a microbiological point of view, “environmental microbiology” studies the microbial processes that take place in the soil, water, food, depuration systems, etc. and “microbial ecology” describes qualitatively and quantitatively the ▶ microorganisms that occupy a specific ecological niche and the activities that take place within it. Cultureindependent molecular biology techniques, such as in situ hybridization with fluorescent probes (FISH) or those based on PCR- denaturing gradient gel electrophoresis or the creation of genetic libraries – have given rise to “molecular microbial ecology” allowing great steps to be made in the understanding of microbial communities that live in specific habitats that would have been impossible with traditional microbiological research techniques. Microorganisms occupy extraordinarily diverse habitats needing only liquid water to develop. They not only thrive in soils and water (fresh or marine) with “normal” physical/chemical conditions but have also adapted to ▶ extreme environments: low and high temperatures and pHs, high salt concentrations, high pressure, etc. Furthermore, given the diminutive size of microorganisms, microniches or microenvironments can exist in conditions, which may differ greatly from those of the general environment, where they are contained. This is especially important in sediments and subsurfaces where the semisolid structure of the substrate allows microniches, whose properties may vary dramatically to coexist in proximity. Environmental conditions are very important in astrobiology because they can determine the habitability of a given planet or planetary body.
See also ▶ Biogeochemical Cycles ▶ Biosphere ▶ Biotope
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▶ Ecosystem ▶ Extreme Environment ▶ pH ▶ Radiation
References and Further Reading Johnson DL, Ambrose SH, Bassett TJ, Bowen ML, Crummey DE, Isaacson JS, Johnson DN, Lamb P, Saul M, Winter-Nelson AE (1997) Meanings of environmental terms. J Environ Qual 26:581–589 Madigan M, Martinko J, Dunlap P, Clark D (2009) Brock biology of microorganisms, 12th edn. Person Education, Inc. – Benjamin Cummings, San Francisco, CA Schlesinger WH (2005) Biogeochemistry, vol. 8. In: Treatise on geochemistry. Elsevier Science, Amsterdam, the Netherlands
Environmental Genome ▶ Metagenome
Environmental Sequence ▶ Phylotype
Enzyme ATHEL CORNISH-BOWDEN, MARI´A LUZ CA´RDENAS Centre National de la Recherche Scientifique, Unite´ de Bioe´nerge´tique et Inge´nierie des Prote´ines, Marseille Cedex 20, France
Synonyms Ferment (obsolete)
Keywords Active site, allosteric site, catalysis, isoenzyme, kinetics, metabolism, ribozyme
Definition An enzyme is a biological ▶ catalyst necessary for metabolic reactions to occur at an adequate rate under mild conditions of pH and temperature, typically consisting largely or entirely of protein, though catalytic RNA molecules are also known.
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History Enzyme catalysis was discovered by Lazzaro Spallanzani in the eighteenth century as an observation that digestion of food in the animal stomach was chemical rather than a purely mechanical process of grinding. Subsequent advances in the nineteenth century came primarily from studies of digestion and of the action of yeast in fermentation. Until the early twentieth century, the usual word for enzyme was ferment, now entirely superseded by enzyme, a name that also referred originally to yeast, as it was coined by Wilhelm Ku¨hne from Greek words meaning “in yeast.” When the last printed version of Enzyme Nomenclature was published in 1992, about 3,200 distinct enzyme activities were recognized, but the current number is about 5,000, and the number of distinct proteins known to be enzymes is very much larger.
Overview All metabolic reactions are catalyzed by specific molecules called enzymes. Even reactions that proceed spontaneously and rapidly, such as the hydration of CO2, are normally catalyzed in physiological systems. Before the discovery of catalytic RNA molecules (sometimes called ▶ ribozymes), all enzymes were believed to consist mainly or entirely of protein molecules, typically with more than 100 ▶ amino acid residues in each subunit. It is now widely accepted that catalytic RNA satisfies the definition of an enzyme, and the ▶ ribosome is a fundamental example of catalytic RNA (Steitz and Moore 2003). Biological catalysts also include metabolites that participate in cycles of reactions, such as citrate, regenerated in each turn of the tricarboxylate (Krebs) cycle, but also including many other metabolites. These are certainly catalysts, but they are not conventionally regarded as enzymes; however, from the point of view of astrobiology, they probably should be regarded as enzymes because at the origin of life there must have been catalysts produced by the first organisms that were neither protein nor RNA, but much simpler molecules. Although the effectiveness of enzymes as efficient catalysts is normally emphasized, their fundamental property is specificity because without specificity, there could be no organization or regulation of metabolic processes. Most enzymes catalyze just one or a small number of reactions, and are largely or completely inactive in others that appear quite similar. Bacterial glucokinase, for example, catalyzes the phosphorylation of glucose by ATP, but does not catalyze the phosphorylation of a hexose as similar as mannose. Even relatively unspecific enzymes, mostly involved in catabolic processes such as digestion, are highly specific by the standards of inorganic catalysts.
Another property exhibited by all enzymes is saturation: although the rate of reaction may be proportional to the concentration of each reactant at low concentrations, it never increases indefinitely with increases in concentration, the rate v typically depending on the substrate concentration a according to the Michaelis–Menten equation, v=Va/(Km +a), in which V, the limiting rate, and Km, the Michaelis constant, are constants. In vivo, however, enzymes typically operate in or close to the proportional zone, in other words at substrate concentrations similar to or smaller than Km. Certain enzymes, known as regulatory enzymes, are inhibited or activated by molecules structurally different from their substrates or products, which are called effectors, or display special kinetic laws that enable them to respond with high sensitivity to changes in conditions. The part of an enzyme where catalysis occurs is called the active site, and if a different site exists for binding effectors it is called an allosteric site. Enzymes are grouped in Enzyme Nomenclature into six classes, of which three (oxidoreductases, transferases, and hydrolases) catalyze group-transfer reactions, and three (lyases, isomerases, and ligases) catalyze other types of reaction. This is a classification of reactions catalyzed, not a classification of proteins catalyzing them. The number of known proteins acting as enzymes is very much larger than the number of known activities (because the proteins catalyzing any given reaction in different organisms are nearly always different), and the amount of variation in structure among the known cases suggests that the total number is huge. In most of the known cases, the proteins are similar enough to be clearly homologous, but some exceptions, exemplified by superoxide dismutase, are known. Even within a single species it is common to find multiple proteins, known as isoenzymes, catalyzing the same reaction, often with distinct regulatory properties.
See also ▶ Amino Acid ▶ Metabolism (Biological) ▶ Prebiotic Chemistry ▶ Ribosome ▶ Ribozyme
References and Further Reading Cornish-Bowden A (2004) Fundamentals of enzyme kinetics. Portland Press, London Enzyme Nomenclature: http://www.chem.qmul.ac.uk/iubmb/enzyme/ Fersht AR (1998) Structure and mechanism in protein science: a guide to enzyme catalysis and protein folding. Freeman, San Francisco Steitz TA, Moore PB (2003) RNA, the first macromolecular catalyst: the ribosome is a ribozyme. Trends Biochem Sci 28:411–418
Equation of State
Ephemeris Definition The ephemeris is the set of tabulated ▶ coordinates of an astronomical object in the sky at successive dates. Plural: ephemerides.
See also ▶ Coordinate, Systems ▶ Declination ▶ Right Ascension
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103P/Hartley 2 using the same instruments used for the Deep Impact observations of comet 9P/Tempel. Closest approach to the comet occured November 4, 2010. The name EPOXI is a hybrid of EPOCH (Extrasolar Planet Observation and CHaracterization Investigation) and DIXI (Deep Impact eXtended Investigation).
See also ▶ Deep Impact ▶ Transiting Planets
EPS Epilithic
▶ Exopolymers
Definition Epiliths are organisms or microbial communities that live on the surface of rocks. Examples are ▶ lichens, mosses, biofilms, and desert varnish. Epiliths are generally well adapted to resist environmental extreme conditions, such as large variations in temperature, extreme dryness, and intense solar irradiation. They are sometimes the only colonizers in desert and high mountain regions.
Equation of State TRISTAN GUILLOT Observatoire de la Coˆte d’Azur, Universite´ de Nice-Sophia Antipolis, CNRS, Nice, France
See also
Keywords
▶ Biofilm ▶ Desiccation ▶ Endolithic ▶ Extreme Environment ▶ Extreme Ultraviolet Light ▶ Hypolithic ▶ Lichens ▶ Microbial Mats
Density, pressure, temperature, thermodynamics
Episome ▶ Plasmid
Definition An equation of state is a relation (or by extension a series of relations) between thermodynamic state variables characterizing matter in a given state, for instance in a celestial body. It describes how a macroscopic entity is affected by external changes (e.g., changes in temperature, pressure, volume, etc.). When modeling stars and planets, equations of state are used in particular to calculate density as a function of pressure and temperature, the phases (e.g., gaseous, liquid, solid, etc.) of the materials considered, and how temperature changes with pressure during macroscopic motions.
Overview
EPOXI (mission) Definition EPOXI is an extension of the NASA Deep Impact mission (▶ Deep Impact) with two goals: to search for extrasolar planets using the transit method, and to study comet
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The first and most widely known equation of state is the so-called ideal gas law. It links the pressure P, volume V, absolute temperature T, and number of particles N in a gas composed of non-relativistic non-interacting mass particles through the relation PV = NkT, where k = 1.38065034 1023 J K1 is Boltzmann’s constant. Equations of state of arbitrary complexity can be calculated for elements that change phase (i.e., become gaseous, liquid, or solid), for
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mixtures, for photons, for charged particles, in the presence of magnetic fields, etc. Among other things, equations of state are crucial to model atmospheric structures: when a parcel of air (or fluid) is transformed (e.g., by its motion) without losing heat (i.e., adiabatically), it conserves its entropy. Entropy is a thermodynamic state variable and, as such, it also obeys an equation of state relation that links it to the other thermodynamic state variables of the problem (e.g., temperature, pressure). Using its conservation, one can therefore calculate the rate of change of temperature with pressure in such a transformation. In atmospheres that are thick enough for convection to occur nearly adiabatically, the temperature profile is mostly set by the temperature of the photosphere (the level at which most of the photons can escape freely to space) and by the equation of state of the atmospheric mixture. Equations of state are also at the heart of our knowledge of planetary and stellar interiors. They describe how matter is affected by the extreme conditions that occur there. With a pressure of about 360 GPa (about 3.6 million times the atmospheric pressure at sea level on Earth) and a temperature of about 7,000 K, the iron which forms most of the core at the center of the Earth is believed to be in solid form. However, the equation of state of iron tells us that at conditions relevant slightly higher up in the Earth, it should be liquid, therefore forming the so-called liquid outer core. In giant planets such as Jupiter and Saturn, the equation of state of hydrogen indicates that this element ionizes from a molecular state, H2, to a conducting state called metallic hydrogen at pressures of about 150 GPa and temperatures of 10,000 K or less.
See also ▶ Atmosphere, Structure ▶ Interior Structure (Planetary) ▶ Stars (Low Mass) ▶ Stellar Evolution
Equinox Synonyms Vernal point
Definition The equinox is one or the other of the two points on the celestial sphere, where the equatorial plane intersects the ▶ ecliptic plane. When the Sun is at one of these
points, daytime and nighttime have the same duration everywhere on Earth. The term equinox is also used for the date and time corresponding to those two positions of the Sun, which mark the beginning of spring (vernal equinox) and the beginning of fall (autumnal equinox).
See also ▶ Coordinate, Systems ▶ Ecliptic ▶ Right Ascension
ERA GERDA HORNECK German Aerospace Center (DLR), Institute of Aerospace Medicine, Cologne, Germany
Synonyms Exobiology radiation assembly
Definition The Exobiology Radiation Assembly (ERA) is a facility to expose various chemical and biological objects to the conditions of outer space, such as space vacuum, solar extraterrestrial radiation, and cosmic rays.
Overview ERA was mounted on the sun-pointing cold plate of ESA’s European Retrievable Carrier (▶ EURECA), which was launched to Low Earth Orbit (LEO) on July 31, 1992, and remained there until August 7, 1993 (Innocenti and Mesland 1995). ERA consisted of an exposure tray (Fig. 1) and a lid accommodating the following experiments: ● Exobiological Unit that studied the action of ▶ solar UV radiation and/or space vacuum on the survival and genetic changes of invertebrates, ▶ microorganisms, viruses, and DNA, and the role of chemical and physical protection mechanisms (Horneck et al. 1995) ● Space Biochemistry that elucidated dehydration and photochemical reactions in cellular, subcellular, and molecular systems under outer space conditions (Dose et al. 1995) ● Effects of solar UV on yeast cells ● Photoprocessing of grain mantle analogues that studied photolytical processes during chemical evolution of interstellar grains (Greenberg 2000)
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Horneck G (2007) Space radiation biology. In: Brinckmann E (ed) Biology in space and life on Earth. Wiley-VCH, Weinheim, pp 243–273 Horneck G, Eschweiler U, Reitz G, Wehner J, Willimek R, Strauch K (1995) Biological responses to space: results of the experiment “Exobiological Unit” of ERA on EURECA I. Adv Space Res 16(8):105–118 Horneck G, Klaus DM, Mancinelli RL (2010) Space microbiology. Microbiol Mol Biol Rev 74:121–156 Innocenti L, Mesland DAM (eds) (1995) EURECA scientific results. Adv Space Res 16(8):1–140 Reitz G, Atwell W, Beaujean R, Kern JW (1995) Dosimetric results on EURECA. Adv Space Res 16(8):131–137
ERE ERA. Figure 1 Tray of the exposure facility ERA with sample carriers and optical filters during assembly (credit ESA, from Horneck et al. 2010)
Beneath the tray was another compartment accommodating the following experiment: ● Free Flyer ▶ Biostack that studied the biological responses of different systems to the structured components of cosmic radiation, i.e., ▶ HZE particles (cosmic rays consisting of energetic nuclei with atomic number 3 or greater) and nuclear disintegration events (Reitz et al. 1995; Horneck 2007; Horneck et al. 2010)
See also ▶ Biostack ▶ Cosmic Rays ▶ EURECA ▶ Exobiology Unit ▶ Expose ▶ Exposure Facilities ▶ HZE Particle ▶ Interstellar Chemical Processes ▶ Microorganism ▶ Radiation Biology ▶ Solar UV Radiation (Biological Effects) ▶ Space Environment ▶ Space Vacuum Effects ▶ Yeast
▶ Extended Red Emission
Error Rate Definition In biology the error rate is the average number of ▶ mutations per nucleotide and round of copy produced during genomic ▶ replication. For example, an error rate of 1010 in a certain species means that, in average, one in every 1010 nucleotides will be misincorporated each time its genome is replicated. The error rate varies by orders of magnitude among species (from 103 – 105 in RNA viruses to 109 – 1012 in cellular organisms). It also can present intraspecific variations, depending on the nature and frequency of change of the selective pressures. The optimal value of the error rate results from the necessity to optimize different conflicting features, such as the generation of genetic diversity, the preservation of genetic information, the minimization of the number of deleterious mutations, and the metabolic costs of systems that increase replication fidelity.
See also ▶ Mutation ▶ Natural Selection ▶ Replication (Genetics)
References and Further Reading Dose K, Bieger-Dose A, Dillmann R, Gill M, Kerz O, Klein A, Meinert H, Nawroth T, Risi S, Stridde C (1995) ERA-experiment “Space biochemistry”. Adv Space Res 16(8):119–129 Greenberg JM (2000) The secret of stardust. Sci Am 283:46–51
ESA ▶ European Space Agency
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Escape Velocity Definition Escape velocity is the critical velocity above which a body is no longer gravitationally bound to a larger mass. This is important for numerous processes: atmospheric molecules escaping from a planet; a planet escaping from its host star; and stars escaping from either mutual orbits or from stellar clusters. For a simple Keplerian potential (where the gravitational force is proportional to the central mass and inversely proportional to the distance squared), the escape speed can be calculated as rffiffiffiffiffiffiffiffiffiffiffiffi M vesc ¼ 2G R where G is the gravitational constant, M is the central mass, and R is the distance from the central mass.
See also ▶ Ejection (Hyperbolic)
ESEF
energetic barrier to rotation about the X(=O)–O–C bond. Esters thus tend to be more volatile (i.e., have a lower boiling point, particularly with low molecular weight esters) than the corresponding amides. Their carbonyl group can serve as a hydrogen-bond acceptor, and this ability to engage in hydrogen bonding renders them somewhat water soluble. The inability of the linking ester oxygen atom to engage in hydrogen bonding with another ester group, unlike the protonated nitrogen atom in an amide or a protonated carboxylic acid, also results in their greater structural flexibility and volatility, but renders them unable to form the hydrogen-bonded structural motifs such as a-helices and b-sheets that polyamides can. Esters can react at one of two locations of their molecular structure. The carbonyl group is weakly electrophilic and is attacked by strong nucleophiles (amines, alkoxides, etc). The C–H bonds of the carbon atoms adjacent to the carbonyl group are weakly acidic but can be deprotonated by strong bases and can thus serve as nucleophiles. Esters undergo both acid and base catalyzed hydrolysis. Under basic conditions, hydroxide acts as a nucleophile, while an alkoxide is the leaving group. This reaction is known as saponification. Esters may also be cleaved by amines such as ammonia and primary or secondary amines to give amides, in a process known an aminolysis or amidation.
▶ COMET (Experiment)
Ethanal Ester HENDERSON JAMES (JIM) CLEAVES II Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
Definition In chemistry, esters are compounds derived by condensation of an oxoacid (one containing an oxo group, X=O, e.g., phosphoric acid (O=P(OH)3) or a carboxylic acid (RCOOH)) with an alcohol. Many biological lipids are the fatty acid esters of glycerol derivatives. Phosphoesters form the backbone of DNA molecules. Polyesters such as polyhydroxy butyrate are important energy storage molecules in some microbes. Cyclic esters formed from carboxylic acid and alcohol functional groups are called lactones.
▶ Acetaldehyde
1,2-Ethanediol ▶ Ethylene Glycol
Ethanoic Acid ▶ Acetic Acid
Ethanol Overview Esters contain a carbonyl functionality, but unlike amides, esters are structurally flexible because there is a low
Synonyms Ethyl alcohol
Ethyl Cyanide
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points of ethers and their parent alcohols becomes less significant as the carbon chains become longer, as van der Waals interactions between the extended carbon chains come to dominate intermolecular interaction. Ethers in general are of low chemical reactivity, and indeed in many extremophilic organisms the cell membranes contain glycerol ethers rather than glycerol esters.
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Ethanol. Figure 1 Rotational isomers of ethanol
Definition
Ethyl-glycine
Ethanol is an organic compound belonging to the group of ▶ alcohols whose structural formula is C2H5OH. It is a constituent of alcoholic beverages. IUPAC recommends using the term ethanol rather than ethyl alcohol. Ethanol is a colorless, volatile, and flammable liquid. Its melting and boiling point are 114.3 C and 78.4 C, respectively. It is made from sugar by fermentation, and it is metabolized in vivo to carbon dioxide via acetaldehyde and acetic acid. Ethanol has rotational isomers, and they are referred to as trans-ethanol and gauche-ethanol (Fig. 1). In 1975, trans-ethanol was identified as an interstellar molecule, while gauche-ethanol was found in 1996. Ethanol is condensed with carboxylic acids to form esters, and it is condensed with itself to form diethyl ether.
▶ Amino Butyric Acid
See also
CH3CH2CN; Cyanoethane; Propanenitrile; Propionitrile
▶ Alcohol ▶ Methanol ▶ Molecular Cloud
Ether Definition Ethers are a class of organic compound containing an ether group (R–O–R) – an oxygen atom connecting two alkyl or aryl groups. A typical example is diethyl ether, commonly referred to simply as “ether” (CH3–CH2–O– CH2–CH3). Polyethers are compounds with more than one ether group. Low to medium range molecular weight polyethers with a hydroxyl end-group are termed glycols. The crown ethers are examples of low-molecular polyethers. Common cyclic ethers include the furans (five-membered rings) and epoxides (three-membered rings). Ethers cannot form hydrogen bonds between each other, resulting in relatively low boiling points compared to their parent alcohols. The difference in the boiling
Ethyl Alcohol ▶ Ethanol
Ethyl Cyanide Synonyms Definition Under standard laboratory conditions ethyl cyanide is a clear liquid with a sweet odor. It is commonly found in the gas phase in interstellar ▶ molecular clouds, specifically in “▶ hot cores” (the regions where massive stars are forming), and has more recently been located in regions of low mass star formation (▶ hot corinos). Ethyl cyanide has a rich rotational spectrum that is observed by radio astronomers in the millimeter wavelength region. In addition to transitions in the ground vibrational state, rotational lines in low-lying vibrational states have been detected. All three 13-carbon isotopic species have been detected in hot cores.
History The first astronomical detection was made by Johnson et al. (1977); subsequently it has been found in most spectral surveys of hot cores (e.g., Nummelin et al. 2000).
See also ▶ Hot Cores ▶ Hot Corinos
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Ethylene Glycol
▶ Isotope ▶ Molecular Cloud
Ethylene Oxide
References and Further Reading
Synonyms
Johnson DR, Lovas FJ, Gottlieb CA, Gottlieb EW, Litvak MM, Thaddeus P, Gue´lin M (1977) Detection of interstellar ethyl cyanide. Astrophys J 218:L370–L376 Nummelin A, Bergman P, Hjalmarson A˚, Friberg P, Irvine WM, Millar TJ, Ohishi M, Saito S (2000) A three-position spectral line survey of sagittarius B2 between 218 and 263 GHZ. II. Data analysis. Astrophys J Suppl 128:213–243
C2H4O; Oxirane
Ethylene Glycol Synonyms 1,2-Ethanediol; Glycol; HOCH2CH2OH
Definition Ethylene glycol [IUPAC name 1,2-Ethanediol] is the simplest dialcohol (or diol). A (toxic) liquid under standard pressure from 197 C down to 12 C, its mixture (70% to 30%) with water remains liquid down to 51 C, which explains its use as antifreeze. It is currently produced by reaction of ethylene oxide (oxirane, C2H4O) with water. Ethylene glycol has been found in the interstellar medium toward the galactic center and in comet C/1995 O1 (Hale-Bopp).
Definition Under laboratory conditions, ethylene oxide is a colorless, flammable gas. Ethylene oxide (IUPAC name oxirane) is one of only a handful of cyclic molecules that have been found in the interstellar medium, all of which except benzene involve 3-membered rings. It was detected by radio astronomers by observing pure rotational transitions at millimeter wavelengths, in the Galactic Center ▶ hot core SgrB2(N). Ethylene oxide is ▶ isomeric with ▶ acetaldehyde (CH3CHO) and vinyl alcohol (CH2CHOH), both of which are also observed in the interstellar medium. NASA has used ethylene oxide for sterilization of spacecraft as part of its ▶ planetary protection program.
History Ethylene oxide was isolated in 1859 by the French chemist C.-A. Wurtz. It subsequently became commercially important as a precursor to ethylene glycol, and it was used during World War I in the production of mustard gas. The astronomical detection was carried out after a failed search for its derivative, oxiranecarbonitrile (C3H3NO), which had been suggested as a possible precursor to glycolaldehyde phosphate and hence to sugar phosphates (Dickens et al. 1996).
History
See also
Detection of ethylene glycol in interstellar matter was rather difficult, and performed for the first time by Hollis et al. (2002) toward the Galactic Center. This contrasts with the relative strength of the lines in comet C/1995 O1 (Hale-Bopp), where this molecule was identified in archive spectra as soon as the line frequencies were available (Crovisier et al. 2004).
▶ Acetaldehyde ▶ Hot Cores ▶ Isomer ▶ Molecular Cloud ▶ Planetary Protection
See also ▶ Comet ▶ Ethylene Oxide ▶ Molecules in Space
References and Further Reading Dickens JE, Irvine WM, Ohishi M, Arrhenius G, Pitsch S, Bauder A, Mu¨ller F, Eschenmoser A (1996) A search for interstellar oxiranecarbonitrile (C3H3NO). Orig Life Evol Biosph 26:97–110 Dickens JE, Irvine WM, Ohishi M, Ikeda M, Ishikawa S, Nummelin A, Hjalmarson A˚ (1997) Detection of interstellar ethylene oxide (c-C2H4O). Astrophys J 489:753–757
References and Further Reading Crovisier J, Bockele´e-Morvan D, Biver N, Colom P, Despois D, Lis DC (2004) Ethylene glycol in comet C/1995 O1 (Hale-Bopp). Astron Astrophys 418:L35–L38 Hollis JM, Lovas FJ, Jewell PR, Coudert LH (2002) Interstellar antifreeze: ethylene glycol. Astrophys J Lett 571:L59–L62
Ethyne ▶ Acetylene
Eukarya
Ethynyl Radical
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Keywords Classification, domain, tree of life
Synonyms
Definition
C2H
Eukarya is one of the three domains of life distinguished from ▶ Bacteria and ▶ Archaea at the morphological and molecular levels. All members of the Eukarya have a nucleus and are further distinguished from Bacteria and Archaea by a complex cellular organization with ultrastructural features including but not limited to nuclear pores, endoplasmic reticulum, 9 + 2 flagellar apparatus, mitotic spindle formation, acidified vacuoles, Golgi apparatus, multiple linear chromosomes, eukaryotic telomeres, and cellular processes that typically include mitosis, meiotic sex, endocytosis, and mitochondrial respiration. They are also differentiated from the other two domains by the presence of specific genes and proteins (e.g., tubulins, actin, dyneins, centrin, myosin, calmodulin, and ubiquitin).
Definition This triatomic ▶ radical plays an important role in the gas phase chemistry of interstellar ▶ molecular clouds, since it provides a link in the chemistry of acetylene and higher order polyacetylenes, the presence of the latter being deduced from observations of the corresponding nitriles (cyanopolyynes). Both carbon-13 and deuterated isotopic variants of C2H are observed in ▶ molecular clouds.
History The fundamental pure rotational transition (N = 10) of C2H at a frequency of 87 GHz was observed astronomically before corresponding laboratory measurements were available, with the identification resting on the pattern of the four detected hyperfine components (Tucker et al. 1974).
See also ▶ Cyanopolyynes ▶ Deuterium ▶ Molecular Cloud ▶ Radical
References and Further Reading Tucker KD, Kutner ML, Thaddeus P (1974) The ethynyl radical C2H – A new interstellar molecule. Astrophys J 193:L115–L119
Eucarya ▶ Eukarya
Eukarya LINDA AMARAL-ZETTLER Marine Biological Laboratory, Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Woods Hole, MA, USA
Synonyms Eucarya; Eukaryote
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Overview Members of the domain Eukarya include both unicellular and multicellular representatives from the 1-mm oceandwelling “pico” planktonic alga Ostreococcus to the blue whale (34 m) – a difference in size of over 7 orders of magnitude. Animals, plants, and ▶ fungi all have microbial eukaryotic (protistan) sister groups. Traditional taxonomic classifications of the Eukarya are morphologybased and have described 1.25 million animal species (mostly insects), 297,326 plant species and 75,000 fungal species, and 200,000 protistan species (http://www.eol. org/). Single-cell and some multicellular protists contribute to the greatest diversity of life on Earth in the domain Eukarya. Beginning in the 1960s, a large portion of this diversity was unveiled with the assistance of technologies including high-powered electron microscopy. Unlike bacteria and archaea, many microbial eukaryotes have ultrastructural features that can help to differentiate the major groups. These ultrastructural features are summarized in below (Patterson 1999). Ultrastructural features that differentiate major groups of eukaryotes: ● Shape of mitochondrial cristae (i.e., tubular, elongated, branching, flat, or discoidal) ● Presence or absence of hairs, scales, or other extensions on the flagella ● Organization of the axoneme into the nine triplet structure of the basal bodies ● Basal body length and orientation association with other organelles
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● Microtubule and rootlet structures arising from the basal bodies ● Source and deployment of microtubular or other cytoskeletal arrays within the cell ● Presence and nature of microtubule organizing centers ● Number, nature, and heterogeneity of nuclei, structures within the nuclear envelope, intranuclear and paranuclear inclusions ● Behavior of the nuclear envelope during mitosis ● Behavior of the mitotic spindle ● For chloroplast-containing cells: number of bounding membranes, thylakoids per lamella, and the presence and nature of contained (e.g., stigma) or associated (e.g., nucleomorph) organelles ● Identity and nature of other membrane-bound organelles in the cell Based on these ultrastructural features, Patterson (1999) defined 71 groups of protists but identified an additional 200 with no clear ultrastructural identity. While ultrastructure has its virtues in the eukaryotic domain, molecular approaches quickly superseded morphological ones in accelerating our ability to delineate species and higherorder taxa of eukaryotes. After Woese and Fox’s (Woese and Fox 1977) seminal work on applying ribosomal RNAbased strategy for inferring the relationships between the major branches of the tree of life, this approach quickly became the gold standard in molecular systematics studies in the 1980s and 1990s across all domains of life. In 1990, Woese et al. (Woese et al. 1990) formally erected the three domains including the Bacteria, Archaea, and Eukarya. Many studies employing rRNA-based approaches ensued and made important contributions to the placement of eukaryotes into major groups (Sogin 1991). Despite insights gained in applying molecular metrics to redefining the organization of life, modern taxonomic revisions have been slow to reach textbooks. The animal– plant dichotomy was replaced with the Whittaker five kingdoms recognizing Monera, Animalia, Plantae, Fungi, and Protista but the transition to the three-domain classification has been a gradual one at best. Another holdfast has been the prokaryote–eukaryote dichotomy. Pace (2006) pointed out the need to recognize the value of the three-domain system in doing away with the artificial lumping of bacteria and archaea via the term “prokaryote.” While ribosomal RNA approaches failed to unveil higher-order relationships between the three domains, amino acid–based analyses using vacuolar H+-ATPases showed that Archaea are more closely related to Eukarya than they are to Bacteria (Gogarten et al. 1989), further obfuscating the prokaryote–eukaryote division.
While important in establishing lower-level taxonomic relationships, single-gene approaches such as rRNA, tubulin, and others often returned ambiguous relationships between major groups of eukaryotes. The adoption of multi-gene phylogenetic approaches to inferring relationships between eukaryotes, followed by the use of genomic and Expressed Sequence Tag (EST) data ushered in the “phylogenomic” era in eukaryotic phylogeny research. These approaches allowed for the testing of relationships between major lineages of eukaryotes and lead to the adoption of the ‘supergroup’ concept of eukaryotic phylogeny that encompasses a varying number of higher taxonomic groupings than phyla depending on the extent of the supergroup. These include the Opisthokonta, Amoebozoa, Archaeplastida (Plantae), Rhizaria, Chromalveolata (Chromalveolates: chromists plus alveolates), and the Excavata (Baldauf 2003; Keeling et al. 2005; Simpson and Roger 2004). More detailed information on these supergroups can be found in the references listed above but a brief description follows. The Opisthokonta encompass the animal and fungal kingdoms, as well as several groups of protists related to both of these lineages, choanoflagellates and Mesomycetozoea (a group of parasitic protists, many in fishes), branch with the animals, and Nucleariids and Microsporidia (a group of obligate endoparasites) branch with the Fungi. Their name reveals the nature of the synapomorphy or shared derived trait that its members share – a single posterior flagellum. This higher-order relationship between two kingdom-level eukaryotic groups has been known for some time and was first proposed on the basis of morphological characters (CavalierSmith 1987), confirmed via single-gene rRNA-based molecular analyses (Wainright et al. 1993) and more recently supported by various phylogenomic investigations (Burki et al. 2007; Hampl et al. 2009; Parfrey et al. 2006). The Amoebozoa form a second supergroup comprising lobose amoebae, slime molds, as well as some amitochondriate amoebae including pelobionts and entamoebae (sometimes referred to as Archamoebae). Like the terms “alga” and “worm,” “amoeba” joins the ranks of ecological concepts as opposed to phylogenetic ones because the term does not refer to a coherent group of organisms that share common ancestry. Amoeboid organisms, that are organisms that possess pseudopodia or “false feet” that enable the cell to engulf food via phagocytosis, are sprinkled over the tree of life and occur in the Opisthokonta (i.e., Nucleariids), Amoebozoa, Rhizaria (i.e., Foraminifera, Polycystines, Phaeodaria, Acatharea), and Excavates (i.e., Heterolobosea). Amoebozoa include
Eukarya
free-living, facultative, and obligate parasites including the genus Amoeba, Acanthamoeba, and Entamoeba. As with the Opisthokonta, Amoebazoa have multicellular representatives including members of the slime molds such as Dictyostelium. The Plantae supergroup, also called the Archaeplastida, includes unicellular and multicelluar members such as the land plants, Charophytes, Chlorophytes, Red ▶ Algae, and Glaucophytes. All the members of this group are photosynthetic or contain relict plastids resulting from a primary endosymbiosis with a cyanobacterium. Surprisingly, a recent phylogenomic assessment of this supergroup along with other supergroups including 143 proteins and 48 taxa failed to support the monophyly of the Archaeplastida (Hampl et al. 2009). Beyond systematic errors associated with the analysis, one possible explanation is related to the symptomatic problem of the host nuclear genome being a potential mosaic of genes derived from multiple nuclei (Lane and Archibald 2008). The Chromalveolata supergroup itself contains higherorder groupings of eukaryotes including the Alveolates (i.e., ciliates, apicomplexans, dinoflagellates) and Chromists (Stramenopiles – brown algae, diatoms, pelagophytes, labyrinthulids, oomycetes, bicosoecids, opalindids), haptophytes, and cryptomonads. Members of this supergroup are ecologically diverse ranging from freeliving to obligately parasitic and heterotrophic, autotrophic, and mixotrophic. The Chromalveolates remains among the most controversial of supergroups and may require more taxon sampling to justify (Hampl et al. 2009). The Rhizaria consists of unicellular and colonyforming (therefore multicellular) protists that include the geologically significant Radiolaria and Foraminifera that are well preserved in the fossil record, as well as a hodgepodge of other amoeboid and flagellated forms collectively referred to as the Cercozoa. Members of the Radiolaria have been detected in molecular studies targeting picoplankton sized ( Qij and sufficiently large (see the error-threshold shown in Fig. 2). Since every replication product is either correct or a mutant, a conservation relation Si Qij = 1 holds. After the replication is completed both polynucleotide molecules, template and copy are released from the enzyme, which is free for the next replication round
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pcr at which the genotype distribution changes abruptly from the quasispecies centered around the master sequence into the uniform distribution (Fig. 2). Neglecting mutational backflow from mutants to the master sequence, the critical mutation rate is obtained as:
Evolution, Molecular. Figure 2 The accuracy limit for replication. The two plots show solution curves of equation (1) as functions of the mutation rate p. In order to illustrate the phase transition-like change in the mutant distribution at the error threshold (2) the relative concentration of error classes yk(p) are plotted against p: y0(p) represents the concentration of the master sequence, y1(p) is the sum of the concentrations of all one-error mutants, y2(p) is the sum of the concentrations of all two-error mutants, y3(p) is the sum of the concentrations of all three-error mutants, etc, and in general yk(p) is the sum of the concentrations of all k-error mutants. At the error-threshold the quasispecies changes abruptly into the uniform distribution, and the concentrations of the mutant classes become equal to the binomial coefficients. The lower part of the figure represents an enlargement of the curves around the error-threshold. Parameters used in the calculations: chain length ‘ = 100, fitness values f0 = 10, fn=1 for all n 6¼ 0
pcr ¼ 1 s1=‘ m and fm ¼
fm sm ¼ f m Xn fx: i¼1;i6¼m i i
with
1 1 xm
ð2Þ
In this equation the chain lengths of the genotype is denoted by ‘, xm is the relative concentration of the master sequence, sm is the superiority of the master sequence, and fm is the mean fitness of the population except the master sequence. Eq. 2 has two important consequences: It defines (1) a maximal chain length for constant replication accuracy (1p), ‘max ln sm =p, and (2) a maximal error rate for constant chain length ‘, pmax ln sm =‘. Both limitations have immediate consequences in biology: (1) In classes of organisms, the spontaneous mutation rates determined by the accuracy of the replication mechanism are inversely proportional to genome lengths (Drake et al. 1998) and (2) for RNA virus replication, which occurs at mutation rates as close to the error threshold as possible in order to guarantee maximal variability in fighting the defense system of the host, a drug-induced increase of the mutation rate can drive the population into extinction (lethal ▶ mutagenesis; Domingo et al. 2008). Molecular evolution experiments starting with the seminal work of Sol Spiegelman (Spiegelman 1971) and continued by Christof Biebricher, Jack Szostak, Gerald Joyce, and others (Watts and Schwarz 1997; Joyce 2004 and 2007) represent experimental realizations of the theory. The molecular approach to evolution is completed by introducing a relation between genotypes and phenotypes in the form of a mapping or fitness landscape, which in the simplest system, in vitro evolution of RNA, deals with RNA sequences and structures (Schuster 2003 and 2006). Evolutionary phenomena, for example, the existence of error thresholds, are highly sensitive to the fitness landscape and commonly applied. Oversimplified models based on the unrealistic assumptions that mutations contribute to fitness additively or in a multiplicative way give wrong results (Phillipson and Schuster 2009). Neutrality is a fundamental property of realistic fitness landscape and neutral evolution (Kimura 1983) in parts of the sequence space is indispensable for the success of the Darwinian mechanism (Schuster 2006). Molecular kinetics of evolution and most of population genetics are based on modeling by means of differential equations. The limitations of this approach primarily come from small particle numbers – every mutant has to start from a single copy, uniform distributions are completely unrealistic since there are orders of magnitude less molecules in the population than possible genotypes. Proper stochastic treatments and computer simulations
Evolution, Molecular
were performed, but a universal model of finite population size effects in evolution is not at hand.
Basic Methodology The methodology of molecular evolution is shared with molecular genetics, genome research, population genetics, bioinformatics, and biochemical kinetics. Specific developments concern the construction of special flow reactors and other equipment for evolutionary biotechnology (Watts and Schwarz 1997).
Key Research Findings The phylogenetic tree of conventional biology has been approved independently by molecular phylogenetics. The molecular clock of evolution yields a genetic measure of time. In vitro evolution has provided evidence that Darwinian evolution does not require reproduction at the cellular level. Quasispecies theory and existence of error thresholds for replication provide a solid theoretical basis for virus evolution at the molecular level.
Applications In vitro evolution of molecules was applied to the design of molecules with predefined properties in biotechnology and became a popular tool for Systematic Evolution of Ligands by Exponential enrichment (SELEX®, which is a registered trade mark of the company Gilead Sciences, Foster City, CA).Examples are the evolutionary design of nucleic acid molecules (Joyce 2004; Klussmann 2006), proteins (Brakmann and Johnsson 2002; Ja¨ckel et al. 2008), and synthetic compounds (Wrenn and Harbury 2007). Quasispecies theory and the concept of error propagation in successive replication gave rise to new antiviral strategies known as lethal mutagenesis (Domingo et al. 2008).
Future Directions The promising and challenging future program for molecular evolution is the integration of data from genomics and systems biology into the currently used methods. Whereas phylogenetics on the whole genome level is more or less solved, upscaling of modeling evolution from RNA molecules and viruses to cells and organisms will require new experimental and computational tools. Learning how genotypes are mapped into phenotypes represents the key for understanding evolution at the molecular level.
See also ▶ Evolution (Biological) ▶ Evolution, In Vitro
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▶ Fitness ▶ Flow Reactor ▶ Genotype ▶ Hypercycle ▶ Mutagenesis ▶ Mutation ▶ Phenotype ▶ Phylogenetic Tree ▶ Phylogeny ▶ Quasispecies ▶ Replication (Genetics) ▶ Selection
References and Further Reading Brakmann S, Johnsson K (eds) (2002) Directed molecular evolution of proteins. How to improve enzymes for biocatalysis. Wiley-VCh, Weinheim Domingo E, Parrish CR, Holland JJ (2008) Origin and evolution of viruses, 2nd edn. Academic, Elsevier, Amsterdam, NL Drake JW, Charlesworth B, Charlesworth D, Crow JF (1998) Rates of spontaneous mutation. Genetics 148:1667–1686 Eigen M (1971) Self-organization of matter and the evolution of biological macromolecules. Naturwissenschaften 58:465–523 Eigen M, Schuster P (1979) The hypercycle. A principle of natural selforganization, Springer-Verlag, Berlin Eigen M, McCaskill J, Schuster P (1989) The molecular quasispecies. Adv Chem Phys 75:159–264 Graur D, Li WH (2000) Fundamentals of molecular evolution, and Li WH (2006) Molecular evolution. Sinauer Associates, Sunderland, MA Ja¨ckel C, Kast P, Hilvert D (2008) Protein design by directed evolution. Annu Rev Biophys 37:153–173 Joyce GF (2004) Directed evolution of nucleic acid enzymes. Annu Rev Biochem 73:791–863 Joyce GF (2007) Forty years of in vitro evolution. Angew Chem Int Ed 46:6420–6436 Kimura M (1983) The neutral theory of molecular evolution. Cambridge University Press, Cambridge, UK Klussmann S (ed) (2006) The aptamer handbook. Functional oligonucleotides and their applications. Wiley-VCh, Weinheim Kumar S (2005) Molecular clocks: 4 decades of evolution. Nat Rev Genet 6:654–662 Morgan GJ (1998) Emile Zuckerkandl, Linus Pauling, and the molecular evolutionary clock, 1959–1965. J hist biol 31:155–178 Mount DW (2004) Bioinformatics. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, Sequence and genome analysis. Second Ed Nei M (2005) Selectionism and neutralism in molecular evolution. Mol Biol Evol 22:2318–2342 Phillipson PE, Schuster P (2009) Modeling by nonlinear differential equations. Dissipative and conservative processes. World Scientific, Singapore, pp 9–60 Schuster P (2003) Molecular insights into evolution of phenotypes. In: Crutchfield JP, Schuster P (eds) Evolutionary dynamics. Exploring the interface of selection accident, neutrality, and function. Oxford University Press, Oxford, UK, pp 163–215
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Schuster P (2006) Prediction of RNA secondary structures: from theory to models and real molecules. Rep Prog Phys 69:1419–1477 Spiegelman S (1971) An approach to the experimental analysis of precellular evolution. Q Rev Biophys 4:213–253 Takahata N (2007) Molecular clock: an anti-neo-Darwinian legacy. Genetics 176:1–6 Watts A, Schwarz G (eds) (1997) Evolutionary biotechnology – from theory to experiment. Biophys Chem 66:67–290 Wrenn SJ, Harbury PB (2007) Chemical evolution as a tool for molecular discovery. Annu Rev Biochem 76:331–349
Evolution of Planets ▶ Planetary Evolution
Evolution of Stars ▶ Stellar Evolution
may also be ▶ exothermic, but does not need to be (see ▶ entropy). An example of exergonic reaction is gasoline ▶ oxidation in the Earth’s atmosphere; burning gasoline releases sufficient free energy (work) to propel a car. It is important to make a clear distinction between thermodynamics and kinetics. A spontaneous reaction can be characterized by a rate so small that no change can be observed, as is the case with gasoline in the presence of oxygen at normal temperature. A temperature increase by a flame or spark is sufficient to initiate a combustion reaction which, being not only exergonic but also exothermic, then proceeds without any additional supply of heat. Many biochemical reactions are exergonic but extremely slow in the absence of a catalyst (for example an enzyme). In presence of enzymes, however, the reactions proceed at an appreciable rate at low temperature.
See also ▶ Entropy ▶ Exothermic ▶ Oxidation
Evolutionary Algorithms ▶ Genetic Algorithms
Exobiologie Experiment Definition
Evolutionary Sequence of Young Stellar Objects ▶ Spectral Classification of Embedded Stars
Evolutionary Tree ▶ Phylogenetic Tree
Exergonic Definition The term exergonic refers to a chemical reaction that releases free energy as it proceeds. An exergonic reaction
During the ▶ CNES sponsored mission “Perseus,” the experiment “Exobiologie” have been flown in 1999 on board the Russian MIR space station and exposed for 97 days outside the station (April 16–July 9). Samples containing ▶ chiral ▶ amino acid (▶ leucine and alphamethyl leucine) and ▶ peptides (leucine-diketopiperazine and trileucine thioethylester), mixed or not with montmorillonite or meteoritic powder were deposited on windows of Magnesium fluoride and exposed directly to the ▶ solar UV flux. Spores of Bacillus subtilis mixed with meteoritic powder were also exposed in cooperation with the ▶ DLR. As references, duplicates of the samples were placed in the instrument but hidden from the solar light as ground-based experiment was conducted. Analysis of the material after the flight did not reveal any racemization or polymerization of the amino acids but did provide information regarding photochemical pathways for the degradation of leucine and of the tripeptide. Very thin layers of inorganic material did not protect spores against the deleterious effects of energy-rich
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Exogenous Synonyms Exogeny
Definition In geology, exogenous refers to all the processes that are produced at the surface of the Earth (and other planets). Weathering, erosion, transportation, and sedimentation are the main exogenous processes. The result of these processes is the formation of sediments and ▶ sedimentary rocks.
See also Exobiologie Experiment. Figure 1 Experiment “Exobiologie” as seen from the interior of the Kvant module of the MIR space station (Photo CNES)
▶ Endogenous ▶ Sedimentary Rock
Exogeny UV radiation in space to the expected amount. Surprisingly, self-formed layers of UV radiation inactivated spores serve as a UV-shield by themselves. The hypothetical interplanetary transfer of life by the transport of ▶ microorganisms inside rocks through the solar system cannot be excluded, but requires the shielding of a substantial mass of inorganic substances (Rettberg et al. 2002) (Fig. 1).
References and Further Reading Boillot F, Chabin A, Bure´ C, Venet M, Belsky A, Jacquet R, Bertrand M, Delmas A, Brack A, Barbier B (2002) The PERSEUS exobiology mission on MIR. Behaviour of amino acids and peptides in Earth orbit. Orig Life Evol The Biosph 32(4):359–385 Rettberg P, Eschweiler U, Strauch K, Reitz G, Horneck G, Wa¨nke H, Brack A, Barbier B (2002) Survival of microorganisms in space protected by meteorite material: results of the experiment ‘EXOBIOLOGIE’ of the PERSEUS mission. Adv Space Res 30(6):1539–1545
▶ Exogenous
ExoMars JORGE L. VAGO European Space Agency – ESA/ESTEC (SRE-SM), Noordwijk, The Netherlands
Keywords Astrobiology, early life, exobiology, ESA, habitability, life on Mars, Mars, Mars evolution, Mars express, Mars geology, Mars missions, Mars rovers, MAX-C, MER, methane on Mars, MRO, MSL, NASA, origin of life, pathfinder, search for life, viking, water on Mars
Definition
Exobiology ▶ Astrobiology, History of
Exobiology Radiation Assembly ▶ ERA
ExoMars is a program of the ▶ European Space Agency. Two missions are foreseen within the ExoMars program, for the 2016 and 2018 launch opportunities to ▶ Mars. These missions are part of a broad cooperation between ESA and ▶ NASA for the robotic exploration of the red planet. The 2016 ExoMars Trace Gas Orbiter mission will study Martian atmospheric trace gases of possible biological importance, such as methane, including sources, distribution, and temporal variation. This mission will also deliver a European Entry, Descent, and Landing (EDL)
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demonstrator to the surface of Mars. The 2018 mission will land two similar size rovers, one from NASA and one from ESA, on the same location. ESA’s ExoMars rover will be devoted to the search for signs of past and present life, using a subsurface drill and novel analytical instruments.
History ExoMars was originally conceived as a rover mission carrying a drill capable to collect small samples from the subsurface and analyze them. The initial work aiming to define the scope of the rover mission and its payload started in 1997, when a group of scientists was tasked by ESA to make recommendation for future search-for-life missions on Mars and the rest of the Solar System. Their findings were collected in the so-called ESA “Red Book” report of 1999. During 2001, the nascent ESA Aurora Programme for Solar System exploration decided to pursue the Mars exobiology rover concept. In 2003, ESA issued a call for instrument proposals to be accommodated on the 2009 ExoMars rover. The ExoMars mission was approved in 2005. Unfortunately, a last-minute request to add a lander station resulted in a more complicated mission concept, requiring a larger launcher, which could not be realized within the available budget. A number of studies were necessary to redefine the mission, and thus the launch date was postponed, first to 2011, then to 2013. At the same time, NASA was experiencing cost overrun problems with their ▶ Mars Science Laboratory (MSL) mission that affected their possibilities to independently realize a mission in 2016. In the course of 2009, ESA and NASA decided that they could accomplish more by joining forces. A scenario was defined for an ESA-NASA Joint Mars Exploration Programme (JMEP) that has as ultimate goal the realization of an international ▶ Mars Sample Return (MSR) mission in the mid to late 2020s. Within this program, ESA and NASA have defined the first two missions, in 2016 and 2018. The European contribution to this international initiative is financed and realized through the ExoMars program, which now includes three major mission elements: an orbiter, a lander, and the ExoMars rover intended from the very beginning. ESA and NASA expect to realize other cooperative missions between 2020 and MSR. At the time of writing, these have still to be defined.
Overview Establishing whether life ever existed, or is still active on Mars today, is one of the outstanding scientific questions of our time. The ExoMars program seeks to timely address this important scientific goal and to demonstrate key
flight and in situ enabling technologies underpinning European ambitions for future exploration missions. The ExoMars scientific objectives are: – To search for signs of past and present life on Mars – To investigate the water/geochemical environment as a function of depth in the shallow subsurface – To study Martian atmospheric trace gases and their sources In support of these objectives, the following technologies will be achieved, beyond remote sensing: – Entry, Descent, and Landing (EDL) of a payload on the surface of Mars – Surface mobility with a Rover – Access to the subsurface to acquire samples – Sample acquisition, preparation, distribution, and analysis The ExoMars program includes missions to the red planet for the 2016 and 2018 launch opportunities (note that the orbits of Earth and Mars are such that favorable opportunities occur about every 2 years). The 2016 mission is ESA-led and launched by NASA. ESA will provide an Orbiter and an Entry, Descent, and Landing (EDL) Demonstrator. The Orbiter will accommodate scientific instruments for the detection of atmospheric trace gases, the study of their temporal and spatial evolution, and the localization of their source regions. The EDL Demonstrator will contain engineering sensors to evaluate the lander’s descent performance and instruments to study the landing site. The 2018 mission is NASA-led and includes the contribution of the ExoMars Rover from ESA. The ESA rover will share the journey to Mars with a NASA rover. Both rovers will be integrated in the same aeroshell and will be delivered to the same site on Mars. ESA and NASA will select this landing site jointly. The ESA rover will carry a comprehensive suite of analytical instruments named after Louis Pasteur dedicated to exobiology and geochemistry research. The ExoMars Rover will travel several kilometers searching for signs of past and present life, collecting and analyzing samples from within rocky outcrops and from the subsurface, down to a depth of 2 m. The NASA rover will include a robotic arm suite of instruments and the means to cache samples in a manner suitable for collection by a future MSR (Mars Sample Return) mission. The ESA and NASA rover missions and instrumentation are expected to be complementary. The NASA rover
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will be a more mobile platform dedicated to highresolution imaging and mineral mapping of surface targets, having a 5-cm corer to collect samples for caching, whereas the ESA rover will emphasize subsurface access and perform detailed analytical investigations on the samples it will collect. An international science working team has been tasked with recommending opportunities for scientific collaboration using the two rovers on Mars. Preliminary findings point to interesting possibilities. For example, using the NASA rover as a scout, to quickly explore a region for interesting targets, then studying them with the ExoMars rover’s drill and suite of analytical instruments.
Basic Methodology If life ever arose on the red planet, it probably did when Mars was warmer and wetter, sometime within the first billion years following planetary formation. Conditions then were similar to those when microbes gained a foothold on the young Earth. Both planets were habitable in the sense of having the necessary environmental conditions and ingredients for life; namely, liquid water, carbon, and other essential elements, as well as a source of energy. Life could have arisen in suitable locations, such as in the vicinity of hydrothermal activity, where all requirements and ingredients could have existed, even if the standing bodies of water were ice-covered. Not even intensive bombardment and possible volcanic resurfacing could have eradicated simple cells completely from the entire planet’s surface. This marks Mars as a primary target for the search for signs of life in our Solar System.
The Martian Environment and the Need for Subsurface Exploration For organisms to have emerged and evolved, liquid water must have been present on Mars. Without it, most cellular metabolic processes would not be possible. In the absence of water, life either ceases or slips into a quiescent mode. Hence, the search for extinct or extant life automatically translates into a search for liquid water-rich environments, past or present. The strategy to find traces of past biological activity rests on the assumption that any surviving signatures of interest will be preserved in the geological record, in the form of buried/encased remains, organic materials, and fossil communities. Because current Martian surface conditions are hostile to most known organisms, when looking for signs of extant life, the search methodology should focus on investigations in protected niches: in the
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subsurface and within surface outcrops. Therefore, the same sampling device and instrumentation can adequately serve both types of studies. The rover’s surface mobility and the 2-m vertical reach of the drill are both crucial for the scientific success of the mission. The ExoMars rover will search for two types of liferelated signatures: morphological and chemical. This will be complemented by an accurate determination of the geological context. Morphological information related to biological processes may be preserved on the surface of rocks. Possible examples include the bio-mediated deposition of sediments, fossilized bacterial mats, stromatolitic mounds, etc. Such studies require mobility and an imaging system capable of covering from the meter scale down to submillimeter resolution (to discern micro-textural information in rocks). Effective chemical identification of biomarkers requires access to well-preserved organic molecules. Because the Martian atmosphere is more tenuous than Earth’s, three important physical agents reach the surface of Mars with adverse effects for the long-term preservation of biomarkers: (1) The ultraviolet (UV) radiation dose is higher than on our planet and will quickly damage potential exposed organisms or biomolecules. (2) UV-induced photochemistry is responsible for the production of eactive oxidant species that, when activated, can also destroy biomarkers; the diffusion of oxidants into the subsurface is not well characterized and constitutes an important measurement that the mission must perform. (3) Finally, ionizing radiation penetrates into the uppermost meters of the planet’s subsurface. This causes a slow degradation process that, over many millions of years, can alter organic molecules beyond the detection sensitivity of analytical instruments. Please note that the ionizing radiation effects are depth dependent: the material closer to the surface is exposed to a higher dose than that buried deeper. A major goal of ExoMars is to study ancient (older than 3 billion years) sedimentary rock formations and evaporitic deposits. However, the record of early Martian life, if it ever existed, is likely to have escaped radiation and chemical damage only if it is trapped in the subsurface for long periods. Studies show that a subsurface penetration in the range of 2 m is necessary to recover well-preserved organics from the very early history of Mars. Additionally, it is essential to avoid loose dust deposits distributed by aeolian transport. While driven by the wind, this material has been processed by UV
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radiation, ionizing radiation, and potential oxidants in the atmosphere and on the surface of Mars. Any organic biomarkers would be highly degraded in these samples. For all the above reasons, the ExoMars drill will be able to penetrate and obtain samples from wellconsolidated (hard) formations, at various depths, from 0 down to 2 m. The successful NASA Mars Exploration Rovers (MER) have demonstrated the past existence of wet environments on Mars using a geologically oriented instrument package. Their results have persuaded the scientific community that mobility is a must-have requirement for future missions. Recent discoveries from ESA’s ▶ Mars Express spacecraft have revealed multiple deposits containing salt and clay minerals that can only form in the presence of liquid water. This reinforces the hypothesis that ancient Mars may have been wetter, and possibly warmer, than it is today. NASA’s Mars Science Laboratory (MSL), planned for a 2011 launch, will study surface geology and organics with the goal of identifying habitable environments. The 2018 ExoMars Rover constitutes the next logical step in Mars exploration. It will have instruments to investigate whether life ever arose on the red planet. It will also be the first mission combining mobility and access to subsurface locations where organic molecules may be well-preserved, thus allowing for the first time to investigate Mars’ third dimension: depth. This alone is a guarantee that ExoMars will break new scientific ground.
The Study of Martian Atmospheric Trace Gases Recent observations from the Planetary Fourier Spectrometer (PFS) on ESA’s Mars Express and from very high spectral resolution spectrometers using Earth-based telescopes, have detected variable amounts of methane in the atmosphere of Mars. Based on photochemical models and on the current understanding of the composition of the Martian atmosphere, methane has a chemical lifetime of 300–600 years, which is very short on geological time scales. Thus, its presence indicates a subsurface source that has recently (geologically speaking) released methane into the atmosphere. There are both geochemical and biochemical processes that could produce methane in the subsurface – its presence is not sufficient to establish the nature of the source. Current photochemical models cannot explain the reported rapid space and time variations in atmospheric methane concentration. Whether geochemical or biochemical in origin, methane observations indicates a dynamically active subsurface on Mars today.
A scientifically exciting and credible experiment can be conducted within the proposed mission concept of a 2016 science/telecom Orbiter delivering an EDL Demonstrator on direct entry to Mars (see next section). The scientific promise of this mission is that it will reveal just how active the Mars subsurface is, with the hope that it will explain the nature of that activity – geochemical or biological.
Descent and Environment Science The 2016 mission scenario foresees an Entry, Descent, and Landing (EDL) Demonstrator to be released by the Orbiter from its hyperbolic arrival trajectory, approximately 3 days before touchdown. Because this mission element constitutes a technological development rather than a science platform, the nature of the sensors that it can accommodate is relatively simple. The EDL Demonstrator will rely on batteries to power instruments after landing for a nominal surface mission duration of 4 sols (Martian days). The science possibilities of such a mission are very limited, but nevertheless useful. It will be possible to retrieve the atmospheric profile along the descent trajectory and obtain fundamental constraints for updating and validating the Mars standard atmospheric model. It will also be possible to accommodate a few, simple environment sensors.
See also ▶ Mars Express ▶ Mars Sample Return Mission ▶ Mars Science Laboratory ▶ MER, Spirit and Opportunity (Mars)
Exon Definition In interrupted (split) ▶ genes, exon refers to each of the sequences in the primary transcript present in the mature ▶ RNA sequence after removal of the ▶ introns (intervening sequences separating the exons) by the splicing process. It is present in mature RNA sequence but absent in the primary transcript.
See also ▶ Gene ▶ Intron ▶ RNA
Exoplanet, Detection and Characterization
Exoplanet, Detection and Characterization THERESE ENCRENAZ LESIA, Observatoire de Paris, Meudon, France
Synonyms Extrasolar planets detection
Keywords Astrometry, coronagraphy, micro lensing, planets, radial, transit, velocity
Definition An exoplanet, or extrasolar planet, is a planet in orbit around another star. The first of such objects was discovered in 1992 and almost 500 exoplanets had been identified by October 2010. Because an exoplanet’s reflected visible light is (in the case of a system comparable to ours) 109 times weaker than the stellar flux, indirect methods were used, based on the stellar motion induced by the planet relative to the center of mass of the system. Astrometry was first tried without success, as the available instrumental means were not sensitive enough to detect such small motions. The velocimetry technique was later successfully used, and this has led to the discovery of a very large fraction of the exoplanets currently known. Other methods have also been successful, in particular the observation of transits, microlensing surveys, and direct imaging (see ▶ Microlensing Planets, ▶ Direct Imaging Planets, ▶ Transiting Planets).
History The question of the plurality of worlds is probably as old as humanity. In Antiquity already, some philosophers suggested the possible existence of inhabited worlds outside the solar system. In the Middle Ages and later, the question was very much under debate among philosophers and scientists. Astronomical programs for detecting exoplanets started at the beginning of the twentieth century. Following the work of Friedrich Bessel (1784–1846) who had discovered a low-mass companion around Sirius, the astrometry technique was favored. In 1944, Piet Van de Kamp announced the discovery of a planet around Barnard’s star. After many studies, it turned out that the apparent star motion was due to an artifact, caused by systematic instrumental errors. In the 1980s, astronomers came to the view that, in light of the performances of the
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available instruments, astrometric techniques were not sufficiently accurate for detecting exoplanets. The first true exoplanet was discovered in 1992 around a pulsar, using another method called pulsar timing. Pulsars, first detected in 1967 by J. Bell and A. Hewish, are neutron stars, at the very last stage of their lifetime, which rotate extremely rapidly and have a very strong magnetic field. As a result, they emit a periodic radio signal which can be detected from Earth. If a planet is present around the pulsar, it creates a perturbation in the radio signal. In the 1970s, several tentative detections were announced but none of them was confirmed until, in 1992, Alexander Wolszczan discovered the presence of two planets around the pulsar PSR1257+12; this object has an especially fast rotation period of 1.5 ms. A few other objects of the same kind were later detected. These first detected exoplanets have of course no resemblance at all with solar-system planets and bear little interest for exobiology. Still in the 1970s, astronomers started developing another indirect method, called velocimetry or radial velocity measurement. The idea is, like astrometry, to detect the motion of the star with respect to the center of mass of the system, but this time the velocity is measured using the Doppler technique. At the beginning of the 1980s, three teams were especially active in carrying out such long-term programs: M. Mayor for the Observatory of Geneva, the team led by G. Marcy and P. Butler in the US, and the team led by G. Walker and B. Campbell in Canada. After over a first decade with no result, the first discovery was announced in 1995 by M. Mayor and D. Queloz, using the 1.93 m telescope of Saint-Michel Observatory (Fig. 1). An exoplanet was detected around a solar-type star, 51 Peg. More surprisingly, the object was a giant exoplanet (with a mass at least half of Jupiter), orbiting at only 0.05 AU from its star, with a revolution period of only 4 days! This outstanding discovery was followed by many others. In 1996, G. Marcy announced the detection of two other exoplanets, around 47 UMa and 70 Vir. The following detections confirmed the strange nature of the exoplanets: most of them were ▶ giant planets orbiting in the close vicinity of their star. This situation was completely unexpected: the formation scenario of the solar system predicts that planets, formed by the accretion of solid particles within the ▶ protoplanetary disk, should be small and rocky objects near their star and giant planets at larger distances. The first conclusion was that indeed exoplanets did exist around solar-type stars, but their formation scenario – or at least their evolutive process – was very different from that of the solar system. As of October 2010, nearly 500 exoplanets had been detected. Most of them have been detected by velocimetry,
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Exoplanet, Detection and Characterization. Figure 1 The velocity curve of 51 Pegasi, measured by M. Mayor and D. Queloz (1995) at the Haute Provence Observatory. The sinusoidal shape of the curve is the signature of the presence of a planet which orbits the star at the same period (After Ollivier et al. 2009)
and about 100 of them were detected by the transit method (in which the stellar flux is monitored as the exoplanet crosses the stellar disk).
Overview The main problem for detecting exoplanets is the huge flux contrast between the planetary flux and the stellar flux, and the very small angular distance between the planet and its star. As mentioned above, for a Jupiter-like planet located at 5 AU from a solar-type star, the flux contrast is 109 in visible light. As the planet is a cold object, the situation improves in the infrared with a contrast of about 106 at 10 mm. For a star located at 10 pc for the Sun (a typical distance for a sample of nearby stars), the star-planet angular separation is 0.5 arcsec. With the present means, it is impossible to obtain a direct image of a Jupiter-like planet around a solar-type star, even in the infrared. Direct imaging can be considered for star-planet systems with a more favorable contrast and a larger separation. This implies looking for cooler and smaller stars, i.e., M-type dwarfs, and large exoplanets far from their host stars. The first direct image of an exoplanet was obtained by G. Chauvin and his team from the Observatoire de Grenoble (Fig. 2). The star, 2MASS1207,
Exoplanet, Detection and Characterization. Figure 2 The planetary system 2MASS1207 observed from the ground in the near-infrared range by the NACO instrument on the VLT (After Chauvin et al. 2005 ((c)ESO))
has a mass of 0.025 solar masses. Its planet, located at 55 AU, has a mass of five Jovian masses. The observation was performed in the near-infrared range at the ▶ VLT, using adaptive optics and a coronographic system; the detection was made easier by the fact that the host star is a ▶ brown dwarf, much cooler than typical stars. This discovery is most likely the first one of a promising research field.
The Indirect Detection Techniques Astrometry and Velocimetry As mentioned above, both astrometry and velocimetry have been used for measuring the stellar motion around the center of mass of the system. In the case of the solar system, the motion induced on the Sun by Jupiter corresponds to a velocity of 12.5 m/s; in the case of the Earth, it is only 9 cm/s. In the case of astrometry, as seen by the observer, the motion of the star is an ellipse, which becomes a line segment if the observer is located in the plane of the planetary system. In the case of velocimetry, the situation is reversed. The observer measures the projection of the stellar motion along the line of sight (called the radial velocity). As a result, the velocity curve is sinusoidal and maximal if the observer is in the plane of the system, but is zero if the observer is aligned with the rotation axis of the system. As the viewing angle of the
Exoplanet, Detection and Characterization
system is unknown, only a lower limit of the planet’s mass is retrieved, in addition to its period. The detection of exoplanets by the astrometry method from the ground has been so far beyond instrumental capabilities. In the near future, it could become feasible for giant exoplanets with the interferometric instrument PRIMA at the ▶ VLTI. In addition, the ▶ Gaia astrometry space mission, to be launched by the European Space Agency in 2013, should lead to the detection of a huge number of exoplanets, especially massive objects at large distances from the stars. As mentioned above, velocimetry has been so far the main tool of exoplanets’ detection. The success of this method is due to the unexpected fact that massive objects are found very close to their stars, which is especially favorable for this technique. In the case of 51 Peg b and other exoplanets of this kind (also called Pegasides or ▶ hot Jupiters), the first exoplanet found around a solartype star, the motion of the star was close to 50 m/s. Thanks to the improving performances of high-resolution spectrographs, it is now possible to measure velocities smaller than 1 m/s, which allows a mass limit not far from a terrestrial mass. The extension of the velocimetry data base over more than a decade also allows searches for planets located at a few AU to a few tens of AU from their host star, i.e., searches for planetary systems which might be more similar to the solar system.
Under favorable circumstances, the observer may be located close to the plane of a planetary system. In this case, the exoplanet periodically crosses the stellar disk. During this event, the flux of the star is reduced because of the presence the occulting planet, and this flux variation can be measured if the stellar flux is continuously monitored with a very high stability. In the case of the solar system, the occultation of the Sun by Jupiter, as observed from a nearby star, would lead to a flux decrease of 1% (this is simply the square of the ratio between the planetary and solar radii). In the case of the Earth, the flux decrease would be only 0.01%. The detection of hot Jupiters is very favorable to the transit method, as the occultation events repeat over a short timescale (a few days). Measuring flux variations of 0.01% is not achievable from the ground, but variations of the order of a percent can be monitored from the ground. The first observation of a planetary transit was achieved using a ground-based telescope by D. Charbonneau and his team in 2000 (Fig. 3). The hot Jupiter, HD209458b, was later observed with the ▶ Hubble Space Telescope (HST) with a much higher precision. Since this observation, about 100 exoplanets have been detected by transit. The great advantage of the transit method is that, combined with velocimetry, several planetary parameters can be derived: the radius (from the flux extinction), the
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mass (from the velocimetry), and thus the density. Physical characterization of the planet thus becomes possible, as will be discussed below. Several ground-based monitoring programs have been developed for the systematic search for transits using small, robotic, wide-field telescopes. These programs are well designed for the search for giant exoplanets close to their stars. The Earth-like exoplanets, or even the rocky exoplanets, with a mass less than ten terrestrial masses, cannot be detected from the ground because the photometric stability is limited by the turbulence of the Earth’s atmosphere. This is why space missions have been developed. Launched by the French space agency CNES in December 2006, the ▶ CoRoT space mission was the first satellite designed for this project (and a complementary program of asteroseismology) which actually detected exoplanets. The instrument consists of a 27-cm diameter telescope equipped with four CCDs (two for the exoplanet survey) which image a 7 square-degree field for a maximum period of 5 months. After 4 years of operation in Earth orbit, CoRoT had detected 17 exoplanets of all kinds, and had identified tens of candidates which remained to be confirmed by velocimetry. The next transit space mission was ▶ Kepler, launched in March 2009. The instrument consists of a 95-cm telescope equipped with 42 CCDs which will observe a field of about 100 square degrees over a period of 4 years. Kepler has the possibility of detecting terrestrial size planets. Several hundreds of unconfirmed candidates had been reported by the fall of 2010, among them an exoplanet with a mass close to the terrestrial one.
The Microlensing Technique The phenomenon of gravitational lensing is a peculiar aspect of Einstein’s theory of relativity. As a consequence of the energy/matter equivalence, a photon is subject to gravitation just as classical baryonic matter. As a result, a photon approaching a stellar object is deflected relative to its direction of propagation. If a massive object is located on the light of sight between the observer and the distant object being observed, the image of the latter is modified. In the case of a massive deflecting object (usually a galaxy), the result is the well known gravitational lensing effect observed on distant quasars. When the mass of the deflecting object is low, the effect, called microlensing, is observed as a magnification of the signal of the source being observed. This magnification effect can be used to detect remote stars which were not observable before. The time of transit of the nearer object (the “lens”) in front of the distant star lasts typically a few tens of days. Using this method, several groups
have undertaken ground-based surveys to search for a population of brown dwarfs and thus to try to detect dark matter in the Galaxy. This method is also of interest for exoplanets’ detection, as the presence of a planet around the lensing star can be inferred from a special signature in the amplification curve of the distant (source) star. This method is very sensitive and has led to the detection of about ten objects around distant low-mass stars. A peculiar object is the very small planet (OGLE-05-390L) of 5.5 terrestrial masses, located at 2.6 AU from its star (Fig. 4). However, the method is neither predictable, nor repeatable, so no physical characterization can be undertaken on these presumably planetary objects.
Comparison of the Different Indirect Methods The advantages and limitations of the indirect detection methods can be summarized as follows: ● Astrometry is well adapted for the detection of massive exoplanets far from their star. The method has been not so far usable from the ground, but will be tried soon at the VLTI with the PRIMA instrument. Much progress is expected from the ▶ Gaia mission, to be launched by ESA in 2013. ● Velocimetry is the previledged tool for exoplanets’ detection. Constant improvements in the instrumental performances have allowed the detection of exoplanets close in mass to terrestrial analogs. Mostly short-period exoplanets were detected in the first decade, but the time extension of the database should now allow the detection of more and more solarsystem giant-planet analogs with longer periods. ● The transit method is well adapted to the detection of massive objects close to their star; ground-based observations are possible for giant exoplanets and are performed in the frame of several ground-based systematic campaigns. The limitation lies in the peculiar geometry of the planetary system which must be observed edge-on. The probability for such a geometry is about 10% for hot Jupiters. The transit method is always coupled with velocimetry which provides the confirmation of the planetary nature of the candidate detected by transit. The combination of both methods allows for characterization of the exoplanet with the determination of its period, mass, radius, and density. The detection of rocky planets by transit requires space missions like CoRoT and Kepler, presently in operation. ● The microlensing technique is very sensitive, and well adapted to low-mass objects distant from their stars.
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Exoplanet, Detection and Characterization. Figure 4 The signature of the gravitational-amplification event OB-05-390L. The artifact indicates the presence of a planet around the deflecting star (After Beaulieu et al. 2006. The figure is taken from Ollivier et al. 2009)
Exoplanets are found around distant stars, located in the halo of our Galaxy, and a statistical analysis of this population can be obtained from the ground by this method. However, there is no possible physical characterization of the detected objects, as the observations are not repeatable. ● Finally, the pulsar timing which led to the first detection of exoplanets is extremely sensitive to low-mass objects. However, the method is limited to a very small number of cases (pulsars with planets). Less than ten objects have been found since the early discovery of 1992.
Direct Detection Methods The direct detection of an exoplanet implies the detection of photons, at any wavelength, emitted by the exoplanet itself. The problem is thus to separate these photons from the stellar ones.
Spectrophotometric Measurements: Secondary Transits We have mentioned above the transit observations as an indirect method of exoplanet detection. In addition, when the planet’s flux is strong enough, the same observations can be used for a direct measurement of the exoplanet’s flux during the secondary transit, when the planet passes behind the star. By measuring the total flux of the system, before, during and after the secondary transit, it is possible
to retrieve by difference the flux emitted by the planet, and even to measure its spectrum. The method has been successfully used in the infrared range on a few hot Jupiters. Spectroscopy has been achieved between 1 and 20 mm using ground-based observations, the HST and Spitzer (see below).
Imaging Techniques Optimizing the spectral range of the observation results from a compromise between the planet/star flux ratio (higher in the infrared) and the spatial resolution (higher in the visible range). Two spectral domains are favored: ● In the near-infrared range, the star-planet pair can be resolved, using ▶ adaptive optics and coronographic techniques; this is how the first image of an exoplanet, 2MASS1207b, was obtained (see above). ● In the thermal infrared range, between 5 and 20 mm, the planet/star contrast is optimal; however, interferometry must be used to obtain the required angular resolution. A method has been proposed, called “nulling interferometry,” which allows the elimination of the stellar flux, by combining destructive interferences along the line of sight; if a planet is present at some distance from the line of sight, its signal can be detected if it corresponds to a bright fringe. These observations will have to be performed from space and still require important technological developments.
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Radio Detection The radio range, and especially the decametric range, is especially favorable in terms of planet/star flux ratio. Indeed, planets like Jupiter which posess a strong magnetic field emit a non-thermal auroral emission, coming from the interaction between charged particles and the magnetic field in the polar regions. In the case of Jupiter, the intensity of this emission is comparable to that of the Sun. These emissions have special characteristics: in the case of Jupiter, they have durations shorter than 1 s, and their spectrum extends from about 20 to 40 MHz. There are, however, sources of noise at the same frequencies, coming both from the Galactic emission and from the Earth itself, including human activities. Still, radioastronomers are studying the possibility of detecting radio emissions from exoplanets using large arrays of radio antennae like LOFAR in Europe, UTR-2 in Ukrainia, and later SKA (the proposed Square Kilometer Array). Calculations predict that the radio emission of a Jupiter-like exoplanet at 0.2 pc could be detected with UTR-2; however, there is no nearby star closer than 1 pc. In the case of hot Jupiters, close to their stars, the radio signal would be accordingly enhanced and such objects could possibly be detected up to distances of 20–25 pc. This would open the possibility of actually detecting exoplanets with this technique.
An Inventory of Exoplanets, 15 Years After the First Discovery What is the situation 15 years after the discovery of 51 Peg b? Almost 500 exoplanets have been confirmed, dozens of others are awaiting confirmation. Most of the objects have been detected by velocimetry. About 100 have been detected or observed by transits. More than half of the objects could belong to multiple systems. Exoplanets are common in the Galaxy. Statistical studies indicate that about 7% of observed stars (of spectral types F, G, K) have at least a giant exoplanet. This proportion increases up to 25% for the stars having a ▶ metallicity (abundance of elements heavier than helium) equal to twice the solar one. About 30% of all stars seem to have a rocky or more massive exoplanet. These first estimates will have to be refined in the light of more complete samplings, including the M dwarfs, by far the most numerous stars in the Galaxy. Also, there are still observational biases toward short-period and massive exoplanets, which are easier to detect.
them, about a third are orbiting at 0.05 AU from their star, i.e., with a period of 4 days only. Among them, the giant planets are called Pegasides, after the name of the first detected exoplanet around a solar-type star, 51 Peg b. These objects are so close to their star that, most likely, they are tidally locked with their star, in synchroneous rotation, showing always the same hemisphere to it (as in the case of the Earth–Moon system). In view of the short distance between the star and the planet, strong temperature contrasts are expected between the day side and the night side of the planet. With the exception of these hot objects, the distribution of exoplanets with distance is rather uniform below 5 AU. The diversity of parent stars, with very different masses, may contribute to smooth out the distance distribution.
A Majority of Exojupiters The velocimetry method allows the detection of exoplanets, but also the detection of more massive stellar companions. If the object has a mass lower than 0.01 solar mass (or 13 Jupiter masses), its internal energy is not sufficient to generate the thermonuclear reactions leading to stellar nucleosynthetis; the object is called a planet. If its mass is between 0.01 and 0.08 solar masses (between about 13 and 80 Jupiter masses), the central temperature is sufficient for initiating the first cycle of thermonuclear reactions which destroys deuterium, but not the next cycle which would transform hydrogen into helium; the object is called a brown dwarf. Beyond a mass of 0.08 solar masses, the object is a star. Several stellar types are defined (O, B, A, F, G, K, M) as a decreasing function of their mass and temperature. The brightest and hottest stars have the shortest lifetime. The Sun, a G-type star, is a very ordinary star, in the middle of the range; its lifetime is about 10 Gy. If we consider the distribution of stellar companions detected by velocimetry, we observe a bimodal distribution (Fig. 5), with a first peak between 0.001 and 0.01 solar masses (the exoplanets) and the other beyond 0.1 solar mass (the stars). The absence of objects between the two peaks is known as the “brown dwarf desert.” It probably illustrates the two different formation scenarios of these two classes of objects. Stars are formed from the collapse of a cloud fragment of interstellar matter; planets are formed by accretion of solid particles within the protoplanetary disk formed after the collapse of this cloud.
A Wide Range of Eccentricities Giant Exoplanets Close to Their Star Statistics have confirmed the early discoveries: hot Jupiters are very numerous. About half of the observed objects are located at less than 0.4 AU from their stars and, among
Within the solar system, we are used to planets on almost concentric orbits, i.e., with low eccentricities. Exoplanets are very different. With the exception of hot Jupiters whose orbits are very circular (possibly as an effect of
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The Effect of Metallicity It is interesting to the study the probability for a star to host a planet as a function of its relative fraction of heavy elements (i.e., elements heavier than helium), also called its metallicity. According to the scenario most commonly accepted for planetary formation, planets formed within a protoplanetary disk from the accretion of solid particles. The higher the metallicity of the star, the more solid material should be available for planetary embryos; we should thus expect a positive correlation between the metallicity and the frequency of planets. This correlation is indeed observed in the case of giant exoplanets; it is not clear in the case of rocky exoplanets, and a larger sample will be needed before any firm conclusion can be reached. It should be also noted that the Sun, in spite of its low metallicity, hosts a system of eight planets, including four giant ones.
Scenarios for Planetary Formation Exoplanet, Detection and Characterization. Figure 5 The bimodal mass distribution of stellar companions. The quantity [M. sin i] is determined by velocimetry. The exoplanet population appears on the left side, and the stellar population is on the right side. The hole at the center is known as the brown dwarf desert (The figure is taken from Ollivier et al. 2009)
strong tidal forces), exoplanets exhibit a large variety of eccentricities, in some cases as high as 0.90. A possible explanation could come from various types of interaction, especially between the planet and the protoplanetary disk. There is no clear apparent correlation between the masses of exoplanets and their eccentricities.
A Large Number of Multiple Systems Many multiple systems have been detected, and more are to be found. So far about 12% of exoplanets belong to a multiple system, but, in view of the observational bias, multiple systems might be as common as single ones. Thus, in this view, the solar system is not an exception. Multiple systems are of special interest for dynamicists who can study their stability with numerical simulations. A typical example of a multiple system is HD 69830. The star hosts three planets, with masses ranging between 5 and 20 terrestrial masses, located at 0.08, 0.2, and 0.6 AU from their star. In addition, a dusty disk could be present within 1 AU from the star. What could be the nature of these three planets? The innermost one might be rocky, while the outermost one could host a gaseous envelope around a core of ice and rocks.
It is generally accepted that planets formed within a protoplanetary disk, resulting from the collapse of a rotating fragment of an interstellar cloud. These early phases of star formation have been observed on many young stellar objects, and this scenario is also believed to be responsible for the solar-system formation. Within the disk, planets form from the accretion of solid particles. In the vicinity of the Sun, the temperature is such that only metallic compounds and silicates are in solid form. Because these elements are not very abundant in the Universe, the planets formed near the star are relatively small and dense: they are the rocky planets. In contrast, at farther distances from the star, where the temperature drops below about 200 K, most of the simple molecules (H2O, NH3, CO2, CH4, H2S, . . .) are in the form of ices. These compounds can thus be incorporated in planetary nuclei which can reach a mass of ten terrestrial masses. Calculations show that beyond this limit, the gravity field of the nuclei is sufficient to capture by gravitational collapse the surrounding material, which is mostly composed of hydrogen and helium. The process leads to the formation of giant planets, with a large volume and a low density. The collapse of the nebula surrounding such a planet leads to the formation of a small disk, which explains the presence of satellites and ring systems in the equatorial plane of the giant planets. In the case of the solar system, this nucleation formation scenario is fully consistent with the presence of terrestrial planets within 2 AU from the Sun and giant planets beyond 5 AU. How can we then explain the presence of giant exoplanets in the immediate vicinity of their star? The most common explanation proposed by theoreticians is migration. As an effect of interactions between the planet
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and the disk (mostly the gaseous disk), a planet can migrate from the outside to the inside and come in the vicinity of the star. A question is still open: what is the mechanism which stops the migration at 0.05 AU from the star? According to some dynamical models, after a period of high ▶ eccentricity, the planet could come into an approximately circular orbit; the strong gravity field of the star would tidally lock the planet to it. Another question is to be raised: why was there no major migration in the solar system? Actually, dynamical simulations also favor the existence of migration for the giant planets, but this migration turned out to be moderate. According to recent numerical simulations, Jupiter moved slightly inward after its formation while the three other giant planets moved outward. When the Jupiter:Saturn system crossed the 2:1 resonance, the orbits of small bodies were destabilized, with large variations of their inclinations and eccentricities. According to these models, this event was responsible for the ▶ Late Heavy Bombardment which took place some 3.8 Gy ago, the traces of it can be witnessed on the old craterized surfaces of small bodies.
From Detection to Characterization After 15 years of an intense observing campaign, we have discovered a wide variety of exoplanets, with different masses and orbital properties. We are now entering a new research era, the one of exoplanet characterization. The aim is to understand the nature of their atmospheres and the main tool for it is infrared spectroscopy of transiting exoplanets.
What Kind of Exoplanets Can We Expect? We can have a first guess of the possible nature of an exoplanet’s atmosphere, knowing its mass and its distance to the star, just by analogy with solar-system planets and satellites. The most important parameter is the stellar distance, as compared with the line of ices of the system (the “▶ snow line”). This limit corresponds to water condensation and occurs at a temperature of about 180 K. Within this limit, we expect rocky exoplanets; beyond it, we expect either icy planets, if their mass is below ten terrestrial masses, or giant planets (i.e., with some fraction of protostellar gas) if they are bigger. In the first case, we would expect a (CO2, N2) atmosphere, with some amount of H2O and CO; in the latter case, we would expect a reduced atmosphere, dominated by hydrogen and helium, with a minor contribution of CH4, NH3, and other hydrogenated molecules. Of course this “first guess” classification is extremely simplified, as many parameters (albedo, rotation period, obliquity, possible greenhouse effect, magnetic field, etc.) might be involved.
Even more importantly, no migration is assumed in the above scenario, and we know that it is a common process encountered in planetary systems.
The Infrared Spectrum of an Exoplanet As in the case of any solar-system object, the infrared spectrum of an exoplanet shows two main components, the reflected stellar radiation which peaks in the visible range for a solar-type star, and a thermal component with peaks in the infrared range corresponding, to first order, to the blackbody radiation at its effective temperature. The balance between the two components depends upon the albedo (reflectivity) of the exoplanet. For solar-system objects, the albedo ranges typically between 0.05 (comets) and 0.3–0.6 (planets and icy satellites). In the reflected regime, atmospheric signatures are observed in absorption in front of the stellar fiux. In the thermal regime, the situation is more complex, as the molecular features may appear in absorption or emission, depending upon the thermal vertical gradient in the atmosphere. For solarsystem planets, the signatures are in absorption if they are formed in the troposphere (where the temperature decreases with increasing altitudes), and in emission in the stratosphere (where the temperature increases with altitude). Understanding the thermal emission of an exoplanet thus simultaneously requires the determination of its thermal vertical structure.
Infrared Spectroscopy of Transit Exoplanets There are two ways of measuring the spectrum of a transiting exoplanet. During the primary transit, when the planet passes in front of the stellar disk, its atmospheric constituents can be observed in absorption in front of the stellar flux. Before and after the secondary transit, when it passes behind the star, the planet can be observed in emission. In both cases, the planetary flux is retrieved by difference with the stellar flux without the planet. Over the past few years, spectroscopic observations of a few hot Jupiters (HD209458b, TrES-1, HD439733b) have been obtained in both modes. Using primary transits, transmission spectra have been obtained in the visible and near-infrared ranges, using the Hubble Space Telescope and ground-based observations. Atomic lines of H, O, C, Na have been observed, as well as emission bands of CH4 and H2O. Secondary transits have been used to measure the exoplanets’ emission spectra with the Spitzer satellite in the near- and mid-infrared range. Signatures of CH4, H2O, and possibly CO2 and CO have been identified. This field of research is rapidly exploding and more discoveries are expected in the coming decade.
Exoplanet, Detection and Characterization
Conclusions and Perspectives The number of known exoplanets – over 500 – now allows statistical studies to be performed, and the characterization of their atmospheres is becoming technically feasible. In the future, studies will use both indirect and direct detection techniques.
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ESA. The instrumental payload includes visible and nearinfrared imaging and spectroscopy and is also well suited for microlensing monitoring.
Direct Detection Methods Coronagraphic Techniques
Indirect Detection Methods Velocimetry studies, the main tool currently used for exoplanets’ detection, are also essential for definitely identifying transit candidates (i.e., distinguishing true transits form other effects which can modulate the stellar intensity, such as star spots). Instruments currently used include, in particular, the ▶ HARPS spectrograph of ESO at La Silla and the SOPHIE instrument at the Observatoire de Haute Provence. This research will continue with the development of high-resolution spectrometers with improved stability, associated with large telescopes (VLT, Keck, and later the proposed Extremely Large Telescope or ELT). The goal is to reach velocity limits below 10 cm/s, sufficient to detect Earth equivalents around solar-type stars. The astrometry technique will become fully operational with the Gaia space mission, to be launched by ESA in 2013. With a star catalog of about one billion stars and with an expected astrometric accuracy of 4 microarcsec for a visual stellar magnitude V = 10 (11 microarcsec at V = 15), Gaia will be perfectly suited for detecting giant planets, down to the mass of Uranus. Planetary transits observations will continue, both from ground and space. There are about twenty groundbased programs currently operating or under development for detecting giant exoplanets by transit. Following the CoRoT and Kepler space missions, presently under operation, ESA is studying the ▶ PLATO mission which focuses on the observation of a large number of stars of all masses and ages over a very wide field (1,000 square degrees). The objective is to detect exoplanets around stars which will be bright enough to allow a velocimetry follow-up, needed to definitely confirm the planetary nature of the candidates. Microlensing ground-based campaigns were first designed for searching for brown dwarfs in the Galactic halo. Two of them, OGLE in the US and MOA in New Zealand, are continuously operating and provide photometric alerts for exoplanet hunters. The PLANET/ MICROFUN collaboration is a network of telescopes spread around the world which, upon alert, continuously monitor a microlensing event. So far about ten objects have been detected. The microlensing program could be an important secondary objective of the EUCLID mission, first devoted to cosmology and presently under study at
As mentioned above, the direct detection of giant exoplanets is possible from the ground, provided the planet/star contrast is maximized (by the choice of a low-mass star), the star-planet distance is large enough, and high angular resolution methods are used. The detection of 2MASS1207b (see above) was performed with the NACO (NAOS-CONICA) system at the VLT. The NAOS adaptive optics system was used in association with the CONICA camera, which included a coranagraphic mode. As a follow-up of this technique, the SPHERE instrument, a second-generation instrument of the VLT, is under development at the European Southern Observatory (ESO) and should improve the performances of NACO by an order of magnitude. In the US, the Gemini Planet Imager (GPI) is the equivalent of SPHERE. In a later future, the EPICS instrument is under definition phase at ESO as one of the instruments of the E-ELT. Among future space missions, the JWST, under development at NASA with participation from ESA, will be well suited for exoplanets’ detection and characterization, especially with its mid-infrared instrument MIRI, developed by ESA. This instrument will be used both in a coronagraphic mode as an imager, and in a photometric mode during planetary transits. Interferometric Missions
Later in the future, the direct imaging of Earth-like exoplanets will probably require interferometric space missions. As mentioned above, the technical concept is based on nulling interferometry which allows the cancellation of the stellar flux along the line of sight and constructive interference in the direction of the planet. Studies have been performed both at NASA with TPF-I and ESA with Darwin, but these projects still face technical challenges. The objective is the direct detection of Earth-like and giant exoplanets around nearby stars, and to characterize their atmospheres by mid-infrared low resolution spectroscopy. In the case of Darwin, the concept included several recombined telescopes (typically 3 m diameter in size) and a dark-fringe interferometer recombiner. The main difficulties of this project lay in the complexity of the recombination, the performance of the central extinction (which has to be better than 106), and the capability to extract the planetary signal from its surroundings
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Exoplanet, Detection and Characterization. Figure 6 The boundaries of the Habitable Zone as a function of the mass and stellar type of the central star. The oblique dashed line indicates the distance at which an Earth-like planet would be phase-locked (i.e., trapped in synchroneous rotation). The insert shows the relative populations of stars in the Galaxy as a function of their mass (The figure is taken from Ollivier et al. 2009)
(i.e., the zodiacal light of the exosystem). Precursor missions will probably be needed before such ambitious missions can be flown.
Search for Life Signatures in Exoplanets’ Atmospheres The ultimate goal for mankind is to discover an inhabited exoplanet. Astronomers have defined the concept of ▶ habitable zone, the region in a planetary system where the temperature (typically between 0 C and 100 C) allows the presence of liquid water (Fig. 6). The distance of the habitable zone to the center depends upon the mass and spectral type of the star. It extends between 0.8 and 1.5 AU in the case of solar-type stars but comes as close as 0.1 AU to the star in the case of M dwarfs, and is as far as 5 AU in the case of A-type stars. Once exoplanets are identified in the habitable zone, spectral signatures of biological markers will be searched for. The 5–20 mm region (also selected in the Darwin project) will be privileged, because it corresponds to the most favorable planet/star flux ratio (about 106) and because many molecules have strong vibrational– rotational bands in this region. Astronomers have identified a set of tracers which, if identified simultaneously,
might be proof of biological activity: [CH4, O2], or [CH4, CO2]. Because oxygen is not easily detectable spectroscopically, a better dignostic would be ozone, which has a strong spectral signature at 9.6 mm. In addition, CO2, CH4, and H2O have strong spectral signatures at 15, 7.7, and 6.2 mm, respectively.
See also ▶ Brown Dwarfs ▶ Coronagraphy ▶ CoRoT Satellite ▶ Direct-Imaging, Planets ▶ Eccentricity ▶ Exoplanets, Discovery ▶ Gaia Hypothesis ▶ Giant Planets ▶ Habitable Zone ▶ Hot Jupiters ▶ Hubble Space Telescope ▶ Late Heavy Bombardment ▶ Metallicity ▶ Microlensing Planets ▶ Nulling Interferometry ▶ Planet Formation
Exoplanets, Discovery
▶ Planetary Migration ▶ PLATO ▶ Protoplanetary Disk ▶ Pulsar Planets ▶ Snow Line ▶ Spitzer Space Telescope ▶ TPF/Darwin ▶ Transiting Planets ▶ VLT ▶ Water in the Solar System
References and Further Reading Beaulieu J-P et al (2006) Discovery of a cool planet of 5.5 Earth masses through gravitational lensing. Nature 439:437–440 Brown M, Charbonneau D, Gilliland D et al (2001) Hubble space telescope time-series photometry of the transiting planet of HD209458. Astrophys J 552:699–709 Casoli F, Encrenaz T (2007) The new worlds – extrasolar planets. Springer/Praxis, New York Charbonneau D, Brown TM, Latham DW, Mayor M (2000) Detection of planetary transit across a sun-like star. Astrophys J 529:L45–L48 Charbonneau D, Brown T, Burrows A, Laughlin G (2006) When extrasolar planets transit their parent stars. Protostars and Planets V. University of Arizona Press. http://arxiv.org/abs/astro-ph/0603376 Chauvin GL, Lagrange AM, Dumas AM et al (2005) Giant planet companion to 2MASSW 1207334-393254. Astron Astrophys 438: L25–L28 Marcy G et al (2005) Observed properties of exoplanets: masses, orbits and metallicities. Prog Theor Phys Suppl 158:24–42 Mayor M, Queloz D (1995) A Jupiter-mass companion to a solar-type star. Nature 378:355–359 Ollivier M, Encrenaz T, Roques F, Selsis F, Casoli F (2009) Planetary systems: detection, formation, habitability of extrasolar planets. Springer, New York Schneider J (2010) Interactive extrasolar planets catalog. In: The encyclopaedia of extrasolar planets. http://exoplanets.eu Wolszczan A, Frail DA (2003) A planetary system around the millisecond pulsar PSR1257+12. Nature 355(6356):145–147
Exoplanets, Discovery DAVID W. LATHAM Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
Synonyms Extrasolar planets
Definition Exoplanets are planets beyond the Solar System, orbiting around stars other than the Sun. Their discovery happened only recently (1995).
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Overview The quest for planets beyond the solar system achieved spectacular progress as the twentieth century drew to a close. By the year 2010, astronomers were poised to discover and characterize planets enough like the Earth orbiting stars enough like the Sun to imagine that they could support life as we know it. The initial discoveries involved giant planets with enough mass to induce measureable reflex motions in the stars they orbited, thus implying the presence of unseen planets. It was the motions along the line of sight based on measurements of the changes in radial velocities using Doppler spectroscopy at optical wavelengths that opened the exoplanet floodgates, not the orbital motion in the plane of the sky and across the line of sight based on astrometry, as many people had anticipated. The conventional thinking was dominated by the architecture of the solar system, with the giant planets in wide circular orbits out past the ▶ snow line, where conditions were cold enough for them to form. The first radial-velocity orbit for an unseen companion that could be called a giant planet (if the orbit happened to be oriented nearly edge on) was published in 1989, but it defied conventional wisdom on three counts. The orbital period of only 84 days placed it far inside the snow line, where conditions would have been too hot for a gas giant to form; the orbit was eccentric, unlike any of the giant planets in the solar system; and the mass was at least ten times that of Jupiter, which was uncomfortably large. Thus, most astronomers dismissed the unseen companion of ▶ HD114762 as a star or brown dwarf in an orbit seen nearly face on (thus minimizing the observed Doppler effect, which only measures the velocity along the line of sight). It was assumed that the observed orbital amplitude of 0.5 km/s was small because of projection effects and not because the mass of the companion was small enough to be a planet. That is how the search for planets around stars like the Sun stood until 1995 when an orbit for ▶ 51 Pegasi was announced that implied an unseen companion with a mass similar to Jupiter and a remarkably short period of only 4.2 days. Initial resistance to accepting this discovery as a planet was soon overwhelmed by the announcement of several other low-Doppler-amplitude orbits implying giant planets, often in eccentric orbits close to their parent stars, and some of them quite massive. Too many were being found for them all to lie in orbits facing the Earth. A population of extrasolar planets was being revealed, although for any particular candidate the mass could not be pinpointed until the inclination of the orbit to the line of sight could be established.
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In the meantime, in 1992 the detection of a system of unseen planets orbiting the pulsar PSR B1257 + 12 was announced, based on periodic variations in the arrival times of the radio pulses from the rotating neutron host star. Follow-up observations soon demonstrated that the masses were indeed small and similar to the Earth, based on the detection of perturbations in the pulse arrival times consistent with gravitational interactions involving planetary-mass objects. Astronomers were puzzled how planets could survive the supernova event that created the neutron star, or alternatively how they might have formed from the leftover debris, but it seemed clear if planets could find a way to live around a pulsar, maybe they would prove to be common around normal stars like the Sun. Ambitious Doppler surveys of hundreds and even thousands of solar-type stars rapidly expanded the known population of radial-velocity planet candidates. As the surveys were sustained over longer durations, planets with longer periods were discovered. As the instrumental techniques improved and the velocity precisions improved, the discoveries were extended to smaller masses. As richer data sets were accumulated, systems with more than one planet were discovered. These accomplishments are described in more detail in the entry on ▶ Radial-Velocity Planets. To remove the ambiguity in the actual mass of an unseen companion revealed by a radial-velocity orbit (traditionally termed a ▶ spectroscopic orbit by astronomers), the inclination of the orbit to the line of sight must be established so that the observed orbital velocity can be corrected for projection effects. Astrometry of the orbital motion on the plane of the sky can in principle resolve this ambiguity because of the extra information provided by the motion in two dimensions across the line of sight. Indeed, in a few cases, astrometry has provided orbital inclinations for radial-velocity planets. However, the real breakthrough for deriving actual masses for planets came with the discovery of systems with orbits oriented close enough to edge on as seen from the Earth so that the planets transited across the face of their host star, the first being ▶ HD209458. Not only does this allow the actual mass of the transiting planet to be determined, but also the amount of light blocked by the shadow of the planet allows the area, and therefore the radius to be measured as well. This in turn allows bulk properties such as the density and surface gravity of the planet to be calculated. It turns out that transiting planets also provide a rich variety of opportunities to learn additional planetary astrophysics. This includes observations of planetary
atmospheres during both the transits and the secondary eclipses (sometimes called occultations), when the planet disappears behind its star, yielding the temperature and chemical composition of the atmosphere. In some cases, the thermal emission from the planet has been tracked as it moves around in its orbit, thus revealing the changes in temperature as more of the dayside comes into view and providing insights into the weather on the planet. Another experiment uses the Rossiter–McLaughlin effect to see whether the planet’s orbit is aligned with the equator of its star or is tilted at an angle, thus providing important clues about the history of the planet’s formation and evolution. Additional planets in the system may reveal their presence by perturbing the times at which transits are observed. Perhaps surprisingly, the detailed shape of a transit can pin down the density of the host star, and if the shape is determined with sufficient precision, details such as the oblateness of the planet due to rotation can be determined. These topics are treated in more detail in the entry on ▶ Transiting Planets. An entirely different approach to the detection of unseen planets orbiting distant stars takes advantage of an effect called gravitational microlensing. When the line of sight to a distant star (the source) is intersected very precisely by an intervening star (the lens), the light of the distant star is magnified by the lensing effect of the gravity of the intervening star. As the relative motion of the two stars changes the exact alignment, the changing magnification produces a light curve that first brightens and then dims in a way that can reveal information about the lensing star, such as its mass. If a planet is orbiting the lens in such a way that it passes close to the line of sight, it can affect the brightening and introduce a feature in the light curve. Although the precise alignment needed to produce a microlensing event is very rare, and the alignment to reveal an orbiting planet is even more rare, surveys to monitor hundreds of millions of stars have detected and characterized several planets. These results and some details of the technique are described in the entry ▶ Microlensing Planets. An attractive way to study the characteristics of an extrasolar planet would be to isolate its light from that of the host star to produce a direct image for further studies such as astrometry to reveal the planet’s orbit or spectroscopy to explore the structure and chemical composition of its atmosphere. Such experiments present daunting technical challenges because of the extreme brightness ratio and the tiny angular separation between the planet and its host star, even for the nearest systems (See ▶ Coronagraphy). Direct images of a few giant planet candidates in wide orbits around hot stars have now
Exoplanets, Modeling Giant Planets
been obtained both with special techniques from the ground and with the Hubble Space Telescope. Earth-sized planets will be much more difficult to image and will require specialized space missions or ground-based “extremely large telescopes.”
See also ▶ Astrometric Planets ▶ Beta Pictoris b ▶ Coronagraphy ▶ Exoplanet, Detection and Characterization ▶ HD 114762B ▶ HD 209458b ▶ Microlensing Planets ▶ Nulling Interferometry ▶ 51 Pegasi B ▶ Pulsar Planets ▶ Radial-Velocity Planets ▶ Snow Line ▶ Spectroscopic Orbit ▶ Transiting Planets
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Definition Models of extrasolar giant planets facilliate data interpretation and suggest new observations.
Overview Atmosphere models of extrasolar giant planets are crafted with the goal of understanding the vertical structure, composition, thermal profile, and dynamics of a given planet. Comparisons of model predictions of emitted and reflected flux to observed spectra provide insight into whether fundamental properties of the planet are understood – or not. While there are several possible approaches to constructing an atmosphere model, the most traditional is forward modeling. Starting with a given set of assumptions, all of the relevant equations governing the various physical properties affecting the atmosphere are represented in a computer code, which predicts the variation in atmospheric temperature and composition with pressure in the atmosphere. In this entry, we consider the most important processes that must be accounted for in such models, as well as the ultimate goals of the modeling process.
References and Further Reading
Basic Methodology
Charbonneau D et al (2000) Detection of planetary transits across a Sunlike star. Astrophys J Lett 529:45–48 Henry GW et al (2000) A transiting “51Peg-like” planet. Astrophys J Lett 529:41–44 Latham DW et al (1989) The unseen companion of HD114762: A probable brown dwarf. Nature 339:38–40 Mayor M, Queloz D (1995) A Jupiter-mass companion to a solar-type star. Nature 378:355–359 Wolszczan A, Frail DA (1992) A planetary system around the millisecond pulsar PSR1257 + 12. Nature 335:145–147
The most fundamental process to capture in a planetary atmosphere model is the transport of energy. In general, at depth in an atmosphere energy is transported by convection. For a giant planet, this convection transports energy outward from the deep interior, as the planet cools slowly over time. As heat is transported outward, the atmosphere, which at depth is opaque, slowly becomes more transparent with increasing height. Eventually, the atmosphere becomes transparent and radiant energy escapes to space and convection ceases. The pressure level in the atmosphere at which this radiative–convective boundary lies depends upon the composition of the atmosphere, the opacity of the major atmospheric constituents, gravity, and the temperature. For example, in the Earth’s atmosphere, the surface temperature is about 290 K. Only at a temperature of about 220 K near 100 mb (at the tropopause) is the optical depth at the peak of the Planck function low enough that the air can radiate efficiently to space (Fig. 1). The tropopause lies at a few hundred millibars for the giant planets as well (also Fig. 1). Meanwhile, incident flux from the primary star is either absorbed by the atmosphere or scattered back to space. Any atmosphere model must account for and conserve both the internal and this incident flux. To find a self-consistent solution, a typical model atmosphere calculation begins with a draft temperature–pressure profile, which is then iterated upon until the energy flux
Exoplanets, Modeling Giant Planets MARK S. MARLEY NASA Ames Research Center, Moffett Field, CA, USA
Synonyms Model atmospheres
Keywords Atmosphere, atmospheric structure, planetary atmosphere, reflection spectrum, thermal emission
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Exoplanets, Modeling Giant Planets. Figure 1 Atmospheric temperature–pressure profiles for Uranus, Saturn, Jupiter, and Earth (Neptune is similar to Uranus). In all four planets, the temperature increases with depth below a few hundred millibars. In each planet, this atmospheric region – the troposphere – transports heat by convection from the deep interior, in the case of the giants, or the surface in the case of Earth. In each atmosphere, the temperature also rises at low pressure – the stratosphere – owing to the absorption of a fraction of the incident ultraviolet light by photochemical products (ozone in the case of Earth, various hydrocarbon products in the giants’ atmospheres). The upper boundary of the troposphere is called the tropopause. Giant planet data are from the Voyager Radio Science occultation experiments and the Earth profile is from the 1976 Standard Atmosphere (All data are available on-line http://atmos.nmsu.edu/planetary\_datasets/indextemppres.html)
through the atmosphere – accounting for deposited incident flux – is conserved. During the iterative process, the temperature–pressure profile must be adjusted such that the slope of the model profile, dT/dP, does not exceed that allowed by an ▶ adiabatic lapse rate. Profiles steeper than adiabatic initiate convection, which efficiently limits the maximum slope of the thermal profile. A final, self-consistent, profile is known as a radiative–convective equilibrium profile. Samples of such model profiles for extrasolar giant planets, including the computed convection zones, are shown in Fig. 2. Further details on the constraints that a model profile must satisfy are presented in the entry ▶ Atmosphere, Structure. Since the radiative energy fluxes are the fundamental components of a model atmosphere calculation, a necessary ingredient in any recipe for atmosphere model preparation is knowledge of the atmospheric opacity. The opacity at optical wavelengths must be known in order
to compute the amount of absorbed incident flux and the opacity at thermal wavelengths must be known in order to calculate the emitted fluxes. In practice, this means the atmospheric opacity must be known from the UV to the far infrared. There are two required components: a calculation of atmospheric composition and a calculation of the opacity of each individual component. The total atmospheric opacity is then the sum of the contribution of each individual absorber, weighted by its relative abundance. A determination of composition begins with a calculation of which species would be present in chemical equilibrium, given an assumed set of initial atomic abundances. Typically, some multiple of solar composition is assumed. The fraction of elements heavier than H and He can be varied to determine the effects of subor super-solar abundance. For any given assumed composition, a unique abundance of molecules and atoms can be
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calculated at any temperature and pressure, assuming chemical equilibrium. For example, for a certain abundance of O and C, one would find that at high temperatures, these atoms are primarily found as CO and H2O while at lower temperatures the C would be found in CH4 and there would be somewhat more H2O present. At very cold temperatures, the gaseous water would condense, leaving relatively little O in the gas phase. A rigorous chemical equilibrium calculation is done for each atomic species over the entire temperature and pressure range under consideration. These calculations must account for the rainout of species that condense. This means that once a species condenses it does not stay in the atmosphere to react at lower temperature but rather falls out by precipitation in a gravity field. State-of-the-art chemical equilibrium calculations account for the interaction of thousands of different species through a complex chemical network of reactions. Once a gas composition has been computed for a given temperature and pressure, the opacity of that gas must be computed as well. This requires knowledge of the absorption spectrum of atoms and molecules, often at high temperature. In some cases, particularly for atoms and some refractory molecules common in cool stars, opacities are well known. In other cases, particularly for the molecules methane and ammonia, opacities are not well understood at high temperatures because of the difficulty in
conducting appropriate laboratory experiments. Furthermore, even for well-understood molecules, the effects of line broadening by the ubiquitous H2 molecules are not usually well known. In addition to the molecular opacity, the opacity arising from condensate clouds must also be accounted for (▶ Clouds). Clouds can efficiently scatter incident flux back to space, making a planet brighter in reflected light, and they can also trap infrared flux below. The former effect can cool the atmosphere while the latter results in warming. Any complete atmosphere model must thus attempt to model cloud formation. This is a difficult challenge, since cloud formation is a leading source of uncertainty even in highly sophisticated studies of Earth’s atmosphere. But the sizes and vertical distribution of cloud particles play leading roles in any model. Approaches to cloud modeling can range from the simple to the complex. Complex models, while attempting to include more physics, can sometimes obscure underlying trends, so employing a variety of approaches is often wise. An example of a simple cloud model would be to assume a fixed particle size with condensate distributed over some vertical thickness of atmosphere, typically one scale height. The total mass of cloud particles could be set equal to the total mass of condensible gas above the condensation level. Parameter space in such models can be easily explored by varying the cloud thickness and
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particle size to understand the range of possible atmospheric responses. A more complex model might attempt to compute particle sizes and vertical condensate distribution given the computed or assumed vigor of atmospheric mixing. Highly complex approaches could even attempt to model the entire condensation process. While the latter approaches might be appropriate for studying a known object, the simpler techniques can quickly identify the range of planet brightnesses that might be expected, for example. Once all of these ingredients (composition, opacity, cloud model) are in hand, an atmospheric thermal profile for a given set of assumed conditions (interior heat flow, planet mass and gravity, distance from primary star, composition) can be computed (Fig. 2). The overall thermal profile of an irradiated atmosphere depends both upon the depth at which incident energy is absorbed as well as internal sources of energy. Figure 2 demonstrates that for planets more distant than a few AU from their primary star most incident energy is absorbed fairly deep in the atmosphere, below the depth at which the atmosphere becomes optically thick in the thermal infrared. This is because most gasses are more transparent in the optical than in the infrared. As a result the absorbed incident energy simply adds to the internal energy being transported outward by convection and the temperature profile resembles that of Jupiter shown in Fig. 1. For those giant planets found closer to their primary stars, the radiative–convective boundary is deeper but absorption of incident flux still occurs at a similar altitude (to the extent that composition is unchanged). Thus, the large incident flux upon a hot Jupiter is absorbed above the radiative–convective boundary (Fig. 2). As a consequence, an isothermal layer appears between the top of the deep convective zone and the region of the atmosphere in which incident flux is absorbed. In this case, the deep internal heatflow is distinct from the thermalized incident radiation and the global temperature distribution is no longer relatively homogeneous and equator to pole temperature gradients can be large. In the lower pressure region lying above the radiative– convective boundary (and generally above thick clouds), the atmosphere is in radiative equilibrium and – lacking any source of turbulence – generally stable, hence the name stratosphere. In this region, an idealized ▶ gray gas atmosphere would become isothermal at the skin temperature T0 = (1/2)1/4Teff, where Teff is the effective temperature (the temperature of a black body radiating the same total energy as the actual planet). However, a real, nongray, atmosphere can be opaque to incident radiation over some spectral range at low pressures and simultaneously
be relatively transparent at infrared wavelengths. In this case, more incident energy may be absorbed than can be emitted by an isothermal atmosphere with temperature T0. As a result, the atmospheric layer with strong absorption must, in the absence of other energy transport mechanisms, heat up until the thermal emission from the layer equals the absorbed incident flux. An inverted temperature structure with a warm, radiative stratosphere overlying a cooler tropopause is seen for several planetary atmospheres in Fig. 1. In fact, almost all solar system planets with an atmosphere exhibit a stratosphere. In Earth’s atmosphere, ozone absorbs ultraviolet (UV) light, which warms the stratosphere to 270 K, about 50 K warmer than the temperature at the top of the troposphere. Solar system giant planet atmospheres are heated by UV absorption by a combination of methane and hydrocarbon photochemical products, including C2H2 and C2H6, and photochemically produced hazes. The atmosphere of Jupiter (Fig. 1) provides a specific example. Without an energy source the planet’s middle atmosphere would be close to 104 K (the skin temperature for Jupiter with Teff = 124 K), as seen above the tropopause in Fig. 1. In the region where most of the incident UV flux is absorbed (near 10 mbar) there is little overlap between a 100 K Planck function and the important thermal opacity sources, so little flux can be emitted. Since the absorbed energy cannot be radiated away by a 100 K atmosphere, the atmosphere warms and the Planck function moves to shorter wavelengths. Eventually, the blue side of the Planck function overlaps the strong n4 methane fundamental vibrational band and the n9 ethane bands at 7.7 and 12.2 mm, allowing the atmosphere to radiatively cool, balancing the absorbed incident flux. As in other solar system giant planet atmospheres, these strong midinfrared bands of ethane and methane act as a thermostat, regulating the stratospheric temperatures. Models of exoplanet atmospheres must likewise account for gasses which may be important absorbers or emitters in the upper atmosphere. Important species may be those expected in chemical equilibrium, for example, TiO in the hottest atmospheres, or photochemical products. To account for the latter requires modeling complex reaction chains that can be triggered by the absorption of incident UV flux and adds yet another challenge to constructing complete atmospheric models.
Key Research Findings Atmosphere models resulting from the techniques discussed in the previous section have a number of applications. Their predictions can assist efforts to detect and characterize planets and, when applied to specific known
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planets, can identify important processes actually acting on exoplanet atmospheres. A complete review of both types of applications is well beyond the scope of this entry. Instead, here we present a few indicative research findings. For a detailed discussion of specific model predictions or planet interpretations, the relevant literature should be consulted.
Planets in Reflected Light The influence of the various physical processes discussed above can be found in reflected and transmitted light and thermal emission observations of extrasolar planets. For example, Fig. 3 compares observed ▶ albedo spectra of Jupiter and Uranus with a computed spectrum of the hot Jupiter HD 209458 b. Jupiter’s relative brightness at longer wavelengths arises from its bright, thick ammonia clouds (which scatter incident light efficiently) and ten times lower abundance of methane (which absorbs efficiently in the red). At shorter wavelengths, photochemically produced hazes on Jupiter absorb blue light while Uranus’ relatively clear atmosphere efficiently scatters incident flux. HD 209458b, which lacks any bright clouds and hosts multiple strong absorbing molecules, is almost black beyond 0.5 mm. Figure 4 illustrates model spectra for giant planets at a range of separations from a sun-like star. Once water clouds form beyond about 2AU, the
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planet brightens notably at red and longer wavelengths as can be seen from comparison of the dotted and solid lines. In the future, space-based coronagraphs will directly image planets in reflected light at optical wavelengths. For such observations, an understanding of exoplanet spectra and color will be needed. An example illustrates the need: given an orbital separation from the primary star, a single photometric detection, combined with an assumed phase function (relative brightness as a function of the angle star–planet–observer) and bounds placed on the geometric albedo, would allow a crude estimate of the planet’s size. Assuming an upper limit geometric albedo less than 0.75 (the pure Rayleigh scattering limit) and a lower limit of 0.06 (typical of low albedo asteroids), for example, would result in an uncertainty in the radius inferred for a directly imaged planet of a factor of 3.5. A bright planet with a radius slightly larger than Earth’s could not be distinguished from a dark planet with Neptune’s radius on the basis of brightness alone. If the planet were also detected by other means, for example, radial velocity or astrometric methods, then the known mass would discriminate between these two extremes. Without such detection, however, the nature of the planet would have to be discerned by spectroscopic or photometric methods. Even low resolution spectroscopy likely will be beyond the
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reach of modest aperture space-based coronagraphic telescopes. This means that planets will have to be characterized, at least initially, by their broadband colors. Indeed based on our experience in the solar system, broadband colors of giant planets at first seem to be promising markers for discerning planet type (Fig. 5 for the giants). Uranus and Neptune are blue while Jupiter and Saturn are red. However, as also shown in Fig. 5, model calculations of planet color reveal that color, as does spectra, depends sensitively on the presence or absence of clouds and atmospheric composition as well as viewing geometry. The right-hand panel of the figure shows model planet colors for a variety of star–planet separations, planet masses, and compositions. Cloudless planets, regardless of mass, are much bluer (because of methane absorption in the red) than cloudy, cooler planets. Reflectivity in the blue is further influenced by stratospheric hazes produced by photochemical processes. Thus, discerning planet characteristics from color alone will be challenging. Spectra, even low resolution spectra, as shown in Figs. 3 and 4, will be much more informative, if obtainable.
Understanding Thermal Emission The spectrum of any planet is composed of two components: scattered radiation incident from the planet’s star and thermal emitted flux from the planet. The thermal
flux represents both energy arising from processes interior to the planet and reradiated absorbed incident radiation. For solar system planets, these two components of the spectrum are usually well separated in wavelength, but for the hottest exoplanets there can be substantial overlap between thermal radiation and scattered incident light. For a planet, such as a transiting planet, with a known radius, the thermal emission spectrum is often equated for convenience, wavelength by wavelength, to the thermal emission from a blackbody. For an isothermal solid sphere with emissivity unity, the observed spectrum would equal that of a blackbody with a fixed temperature. However, for a real planet the flux will differ from that of a blackbody with the same radius, and at each wavelength a “brightness” temperature TB (l) may be defined (equal to the temperature of a black body radiating the observed intensity at a specific wavelength l). Although thermal emission data for transiting planets are often reported in terms of TB (l), such data must be regarded with some care. Except in special cases (e.g., an isothermal atmosphere) brightness temperature is not a measure of physical temperature or effective temperature. Rather it gives a weighted measure of atmospheric temperatures over a range of pressures from which flux emerges from the planet. Crudely, the brightness temperature can indeed be equated to the temperature in the atmospheric region which most contributes to the emergent flux. However, since atmospheric opacity can vary dramatically with wavelength, the brightness temperature can vary substantially with wavelength. In regions of low opacity, flux emerges from deeper in the atmosphere which, for a monotonically increasing temperature profile with depth, means higher brightness temperature. High opacity spectral regions correspond to lower pressures and lower temperatures. However, if there is an inverted temperature profile, then a situation could emerge where TB (l1) > TB (l2) and TB (l2) < TB (l3) where t (l1) > t (l2) > t (l3). This is a commonplace occurrence in the atmospheres of solar system giants and the chromospheres of stars. Furthermore, because of limb (edge) effects, this range in pressures from which flux emerges also varies over the disk. Thus, even for a gray atmosphere, with constant optical depth as a function of wavelength, the brightness temperature is in general not equal to the effective temperature at all wavelengths. For these reasons, while brightness temperatures are useful shorthands to convey information about planetary spectra, they must be regarded with some caution. For example, 8 mm Spitzer observation of the hot Neptune GJ 436b yield a brightness
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temperature of 712 36 K which is modestly above the predicted effective temperature. Since we do not expect, in general, for TB = Teff, the information content of this single datapoint is limited. With atmosphere models and additional data points the value of each brightness temperature measurement increases.
Hot Stratospheres One of the more intriguing observational findings of the hot Jupiters has been the ubiquity of hot stratospheres. Observations primarily by the Spitzer Space Telescope have ascertained that as many as half of the transiting hot Jupiters sport hot stratospheres, emitting at temperatures well in excess of the skin temperature. As in the solar system, jovian atmospheres discussed in the previous section, in exoplanet atmospheres a balance must be struck between the absorption of incident radiation and thermal emission. For hot Jupiters which are so warm that even the most refractory Ti- and V-bearing compounds do not condense, TiO and VO gas may be exceptionally important absorbers. These gasses, while not abundant, have extraordinarily large absorption cross sections across the entire optical spectrum. When present, these molecules can absorb much of the remarkably high incident flux at altitudes above 1 mbar, where the atmosphere is optically
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thin in the thermal infrared. The atmosphere therefore becomes very hot, as hot as 2,000 K or more, hot enough for emission by the near-infrared and optical bands of water, CO, and even TiO to balance this influx of energy. Discerning which extrasolar planets possess hot stratospheres has become a major endeavor for exoplanet science, particularly when utilizing the IRAC instrument aboard the Spitzer Space Telescope. It seems clear that the TiO/VO mechanism is plausible for some of the hottest extrasolar giant planets, but likely is not the full explanation for cooler exoplanets with hot stratospheres. It is likely that an as yet unknown absorber, perhaps one produced by disequilibrium or photochemical processes, is responsible for stratospheric heating in some fraction of the hot Jupiters. The techniques of forward modeling discussed here will continue to be required in the quest to understand the processes responsible for the observed heating.
Disequilibrium Chemistry As noted above, the first approach to constructing an exoplanet atmosphere model is to assume chemical equilibrium. However, an important limitation to conventional one-dimensional models of mean atmospheric structure is the neglect of vertical mixing. Vertical
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transport plays an important role when the dynamical timescale is short compared to a particular chemical equilibrium timescale, as is the case for CO in the atmosphere of Jupiter. While methane is the most abundant C-bearing molecule in Jupiter’s visible atmosphere, in the deep atmosphere, where temperatures are higher, the abundance of CO is substantially larger. Since the C–O bond is very strong, the conversion time from CO to CH4 in a parcel of rising gas is correspondingly long. As a consequence, vertical mixing through the atmosphere transports CO from the deep atmosphere to the visible atmosphere where it can be detected. Evidence for CO mixing ratios enhanced over that expected in chemical equilibrium is widely noted in the brown dwarf literature and tentative evidence is apparent in the available exoplanet data as well. The study of disequilibrium processes is a classic success of the traditional forward modeling approach to understanding planetary atmospheres. Models that assume pure chemical equilibrium fail to match available data (spectra and photometry of exoplanets), which leads to a search for additional physical mechanisms to consider. When an additional important physical process is accounted for in the models, the predictions compare more favorably with data and knowledge has been gained.
Dynamics Atmosphere models can also be used to study atmospheric dynamics on a global scale. The same models that compute mean, global conditions can also be adapted to compute profiles at specific points on a planet. Coupled with three-dimensional fluid flow computations, the nature of the atmospheric circulation of giant planets, accounting for energy transport and rotation, can be computed. The atmospheric redistribution of energy by winds is unquestionably of paramount importance for the hot Jupiters, and the efficiency of redistribution controls the global temperature map and consequently the phase variation of thermal emission, which has been successfully measured for multiple planets. Since a planet’s thermal emission can arise from different depths in the atmosphere at different wavelengths, any variation in redistribution efficiency with altitude will manifest itself as differing thermal emission maps as a function of wavelength. Ultimately coupled models of radiative transfer and dynamics, similar to terrestrial global circulation models, will be required to understand all of the contributing factors.
Future Directions As more and better data become available for the transiting extrasolar giant planets, particularly from the
James Webb Space Telescope, the opportunities for more new and interesting science – and the requirements for ever more sophisticated models – will grow. Ultimately direct imaging of cool, mature planets in reflected light will join the nascent field of direct imaging of young, luminous giant planets at the forefront of exoplanet science. In the nearer term, a number of new directions can be expected from theoretical models of giant exoplanet atmospheres. First, atmospheric molecules can be dissociated by the absorption of ultraviolet light, a process that happens high in the atmosphere before most incident UV light is scattered back to space. Photochemical products can then participate in complex reaction chains, producing various molecular products. A familiar example is atmospheric ozone in Earth’s stratosphere, which ultimately results from the photodissociation of molecular oxygen. Photochemical products can themselves become important players in the atmospheric radiative transfer of giant planets. Photochemistry has long been expected to be important for hot Jupiter atmospheres and will likely be far more complex than in the solar system. This is because molecular species that condense below the jovian clouds (e.g., H2O, H2S, NH3) and thus are protected from photodissociation will be gaseous in such hot atmospheres. Some of these species, such as H2S, are easily photodissociated, and will likely produce new or unexpected species. Sulfur and nitrogen compounds, in particular, may be important players in hot Jupiter photochemistry and perhaps haze production. While the carbon photochemistry has been studied, recent work on photochemistry in water-bearing H2 He atmospheres suggests that compounds including CO2, HCN, and C2H6 will be present well in excess of the abundance predicted by equilibrium chemistry. As a consequence, CH4 may be underabundant, as has indeed been found for the atmosphere of one exoplanet. Photochemical products may also play a role in the formation of hot stratospheres. This area is certainly rich for further study. Second, ever more sophisticated global circulation modeling of atmospheric dynamics will become increasingly important. Current models are hampered by the need to compute radiation balance and dynamics through a three-dimensional model atmosphere over tens of thousands of time steps. Such calculations are a timeconsuming process even for current generation computers. Continuing advances in computational speed and improvements in the computation and handling of molecular opacities will permit more sophisticated and, hopefully, more realistic model cases to be considered. Finally, of course, a great variety of exoplanets will likely be found. The variations in planet atmospheric
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composition, gravity, temperature, dynamics, photochemistry, and as yet unrecognized processes will certainly be great enough to attract the full attention of the next generation of atmospheric modelers.
See also ▶ Adiabatic Processes ▶ Albedo ▶ Atmosphere, Model 1D ▶ Atmosphere, Structure ▶ Atmosphere, Temperature Inversion ▶ Biomarkers, Spectral ▶ Clouds ▶ Color Index ▶ GCM ▶ Grey Gas Model ▶ Habitable Planet (Characterization) ▶ Habitability of the Solar System ▶ Mini-Neptunes ▶ Non-Grey Gas Model: Real Gas Atmospheres ▶ Rayleigh Scattering
References and Further Reading Burrows A, Hubbard WB, Lunine JI, Liebert J (2001) The theory of brown dwarfs and extrasolar giant planets. Rev Mod Phys 73:719 Cahoy K, Marley M, Fortney J (2010) Exoplanet albedo spectra and colors as a function of planet phase, separation, and metallicity. Astrophys J 724:189 Chamberlain J, Hunten D (1987) Theory of planetary atmospheres. Academic, Orlando Deming D, Seager S, Richardson LJ, Harrington J (2005) Infrared radiation from an extrasolar planet. Nature 434:740 Fortney JJ, Marley MS, Barnes JW (2007) Planetary radii across five orders of magnitude in mass and stellar insolation: Application to transits. Astrophys J 659:1661–1672 Fortney JJ, Lodders K, Marley MS, Freedman RS (2008) A unified theory for the atmospheres of the hot and very hot Jupiters: Two classes of irradiated atmospheres. Astrophys J 678:1419 Freedman RS, Marley MS, Lodders K (2008) Line and mean opacities for ultracool dwarfs and extrasolar planets. Astrophys J Suppl 174:504 Hubeny I, Burrows A, Sudarsky D (2003) A possible bifurcation in atmospheres of strongly irradiated stars and planets. Astrophys J 594:1011 Knutson HA, Charbonneau D, Allen LE, Burrows A, Megeath ST (2008) The 3.6-8.0 mm broadband emission spectrum of HD 209458b: Evidence for an atmospheric temperature inversion. Astrophys J 673:526 Lodders K, Fegley B (2002) Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. I. Carbon, nitrogen, and oxygen. Icarus 155:393 Marley MS, Gelino C, Stephens D, Lunine JI, Freedman R (1999) Reflected spectra and albedos of extrasolar giant planets. I. Clear and cloudy atmospheres. Astrophys J 513:879 Marley MS, Fortney J, Seager S, Barman T (2007) Protostars and Planets V. In: Reipurth B, Jewitt D, Keil K (eds). University of Arizona Press, Tucson, 733 p
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Rowe JF et al (2008) The very low albedo of an extrasolar planet: MOST space-based photometry of HD 209458. Astrophys J 689:1345 Spiegel DS, Silverio K, Burrows A (2009) Can TiO explain thermal inversions in the upper atmospheres of irradiated giant planets? Astrophys J 699:1487 Stevenson KB et al (2010) Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b. Nature 464:1161
Exopolymers LUCAS J. STAL Department of Marine Microbiology, Netherlands Institute of Ecology NIOO-KNAW, Yerseke, The Netherlands
Synonyms EPS; Extracellular polymeric substances; Extracellular polymers; Extracellular polysaccharides
Keywords Biofilm, cells, cyanobacteria, polymers
Definition Exopolymers are polymers that are deposited outside the (microbial) cell. These polymers are predominantly composed of carbohydrates, but many contain various other components such as proteins, DNA, and glycolipids. Some exopolymers consist of neutral sugars such as glucose, while others are acidic in nature and contain a variety of charged groups such as the uronic acids, carboxy groups, sulfated sugars, or pyruvate groups. The molecular structure and composition of exopolymers is therefore highly diverse and complex. Exopolymers form the matrix of biofilms in which the microorganisms are embedded, but also serve a plethora of other functions.
Overview Exopolymers are by definition polymers that are deposited outside the cell wall. These include sheaths or investments in which the cell, but often also aggregates or filaments of cells, are wrapped and represent a more or less structural part of the organism. Such sheaths may be highly structured and may have a typical morphology, while other infestations are more diffuse. Other exopolymers do not have a close association with the cells that produce them. They are exuded into the environment and “dissolve” as colloidal exopolymers in the water or form in the benthic environment the biofilm matrix in which the microorganisms are embedded.
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Exopolymers that are exuded into the environment are usually the result of unbalanced growth. For instance, when photosynthetic organisms fix CO2 but are limited by nutrients. In that case, the fixed carbon cannot be accommodated by the synthesis of structural cell material and will be deposited as polysaccharide. Part of it will be stored intracellularly as a reserve compound but the bulk will find its way as colloidal exopolymers. The same may be the case with chemotrophic microorganisms that live at the expense of energy-rich but nutrient-poor resources. Exopolymers can also exude as a result of the gliding motility by some bacteria, notably by ▶ cyanobacteria. The exopolymers are exuded through specific pores in the cell wall and interact with the substrate on which the organism glides, leaving behind a trail of exopolymer. The mechanism of gliding motility is still poorly understood. Exopolymers are not just waste products. They may serve a variety of important functions for the organism that produces them as well as for the ecosystem as a whole. It has been shown that benthic diatoms metabolize the exuded exopolymers as a carbon and energy source in the dark. Exopolymers form the matrix of biofilms. They may be important for attachment of microorganisms to surfaces. This may be important for the success of the organism but represents also a problem known as fouling. Exopolymers form coherent networks and matrixes that may interact covalently with mineral particles, resulting in sediments with a high erosion threshold. Hence, exopolymers have found application in sediment stabilization. Exopolymers may bind calcium and magnesium ions, thereby controlling calcification. It may act as an anti-calcification agent, but it may also allow local calcium carbonate precipitation and hence the tertiary structure of the polymer may determine the morphology of the calcium carbonate. This is the case with the calcium carbonate platelets known as coccoliths, which are produced by certain marine microalgae. Exopolymers protect the microorganisms from being grazed and immobilize (heavy) metals and other toxic compounds and antibiotics. The latter is an important problem for curing infections by pathogenic bacteria, while the former has found application in bioremediation of polluted soils. Many exopolymers may form a gel through the absorption of large amounts of water. This gel protects the organism from desiccation. Exopolymers can also scavenge important nutrients from the environment, thereby providing the biofilm organisms with an important advantage compared to free-living pelagic microorganisms. Finally, exopolymers can act as flocculants. Some benthic
cyanobacteria have been shown to produce it in order to bind mineral particles, thereby clearing in the water column above them.
See also ▶ Archean Traces of Life ▶ Biomarkers, Morphological ▶ Cyanobacteria
References and Further Reading Braissant O, Decho AW, Przekop KM, Gallagher KL, Glunk C, Dupraz C, Visscher PT (2009) Characteristics and turnover of exopolymeric substances in a hypersaline microbial mat. FEMS Microbiol Ecol 67:293–307 Branda SS, Vik A˚, Friedman L, Kolter R (2005) Biofilms: the matrix revisited. Trends Microbiol 13:20–26 Decho AW (1990) Microbial exopolymer secretions in ocean environments: their role(s) in food webs and marine processes. Oceanogr Mar Biol 28:73–153 Hoagland KD, Rosowski JR, Gretz MR, Roemer SC (1993) Diatom extracellular polymeric substances – function, fine structure, chemistry, and physiology. J Phycol 29:537–566 Hoiczyk E, Hansel A (2000) Cyanobacterial cell walls: news from an unusual prokaryotic envelope. J Bacteriol 182:1191–1199 Pereira S, Zille A, Micheletti E, Moradas-Ferreira P, De Philippis R, Tamagnini P (2009) Complexity of cyanobacterial exopolysaccharides: composition, structures, inducing factors and putative genes involved in their biosynthesis and assembly. FEMS Microbiol Rev 33:917–941 Stal LJ (2003) Microphytobenthos, their extracellular polymeric substances, and the morphogenesis of intertidal sediments. Geomicrobiol J 20:463–478 Sutherland IW (2001) Biofilm exopolysaccharides: a strong and sticky framework. Microbiology 147:3–9
Exothermic Definition The term exothermic refers to a reaction or a physical transformation which releases energy if it takes place at constant volume or which releases ▶ enthalpy if it takes place at constant pressure. A familiar exothermic reaction is the ▶ combustion of coal. The enthalpy change (ΔH) during an exothermic reaction or transformation taking place at constant pressure is negative by definition. The enthalpy release takes the form of a heat release.
See also ▶ Combustion ▶ Endothermic ▶ Enthalpy
Exozodiacal Light
Exozodi ▶ Exozodiacal Light
Exozodiacal Light DANIEL ROUAN LESIA, Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot, Meudon, France
Synonyms Exozodi
Keywords Dust particles, ▶ exoplanet, interplanetary dust, scattering
Definition The exozodiacal light is the analog in extrasolar planetary systems of the ▶ zodiacal light seen in the solar system. It corresponds to the light from the central star,
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scattered – or reemitted in the infrared – by interplanetary dust particles concentrated in the mean orbital planetary plane, that is, the local ecliptic. It is often abbreviated as exozodi.
Overview In the solar system, the zodiacal light appears as a flameshaped glow just above the horizon, visible to the naked eye in good atmospheric conditions and when the moon has not risen. It is seen only toward the west after sunset or in the east before sunrise and is aligned along the Zodiacal belt, hence its name. It is due to sunlight reflected by dust particles of 1–100 mm in size, concentrated in the plane of the ecliptic, mainly in its central region (approximately up to the asteroid belt). Because the dust particles are heated by the sunlight to a temperature comparable to the Earth’s (300 K), they also emit in the infrared. It is generally admitted that extrasolar planetary systems must harbor a similar content of dust and thus exhibit an analog of the zodiacal light in the visible and infrared. The origin of this dust can be primitive material in the protoplanetary disk, but more often, in mature systems, it corresponds to debris left after planet formation when the remaining planetesimals suffered collisions and
Exozodiacal Light. Figure 1 HST coronagraphic image of the star Fomalhaut at 0.6 mm, showing together the dust belt responsible for the exozodiacal light emission and the planet Fomalhaut b (white square) just within the inner boundary of the dust belt, at two different dates
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evaporation. The question of the intensity of the exozodiacal light is extremely important when dealing with the direct detection and characterization of planets in the habitable zone, because exozodiacal light intensity may be so large that it would dominate by several orders of magnitude the signal from a planet. In the solar system for instance, the zodiacal light is 300 times stronger than the emission of the Earth at a wavelength of 10 mm. The name exozodi was proposed as an abbreviation when this importance was realized and discussed in its details. It is generally assumed that the detection of an earthlike planet should be possible provided that the exozodi does not exceed ten times the solar system value. A consequence is that before launching an expensive space mission that aims at direct detection of exoplanets, precursor missions or possibly measurements from the ground should assess the relative frequency of large and medium exozodi intensities in the visible and in the infrared. Figure 1 shows an image of the exosystem Fomalhaut where the exozodi is resolved as a ring and contributes to the background against which a planet has been directly detected (Kalas et al. 2008). A few stars have been identified as exhibiting an exozodiacal light, at least in the infrared domain: beta Pictoris and 51 Ophiuchus are two examples.
Experimental Evolution ▶ Evolution, In Vitro
Explosive Nucleosynthesis ▶ Nucleosynthesis, Explosive
Expose HERVE´ COTTIN Laboratoire Interuniversitaire des Syste`mes Atmosphe´riques (LISA), Universite´ Paris Est-Cre´teil, Cre´teil Cedex, France
Synonyms EXPOSE-E; EXPOSE-R
Keywords See also ▶ Debris Disk ▶ Exoplanets, Discovery ▶ Exoplanet, Detection and Characterization ▶ Zodiacal Light
References and Further Reading Cockell CS, Herbst T, Le´ger A, Absil O, Beichman C, Benz W, Brack A, Chazelas B et al (2009) Darwin an experimental astronomy mission to search for extrasolar planet. Experimental Astronomy 23:435 Kalas P, Graham JR, Chiang R et al (2008) Optical images of an Exosolar planet 25 light years from Earth, Science 322:1345 Reach WT (1997) The structured zodiacal light: IRAS, COBE, and ISO observations, diffuse infrared radiation and the IRTS. ASP conference series no. 124, pp 33–40 Smith BA, Terrile RJ (1984) A circumstellar disk around Beta Pictoris. Science 226:1421–1424 Stark CC, Kuchner MJ, Traub WA, Monnier JD, Serabyn Eugene, Colavita Mark, Koresko Chris, Mennesson Bertrand et al (2009) 51 Ophiuchus: a possible beta pictoris analog measured with the Keck interferometer Nuller. Astrophysical Journal 703(2): 1188–1197
Exposure facility, International Space Station, low Earth orbit, photobiology, photochemistry, space radiations
Definition EXPOSE-E and EXPOSE-R are two exposure facilities developed by ESA in the 2000s to investigate the effect of space environment (especially energetic radiations) on various chemical and biological samples. EXPOSE-E was installed on the ▶ International Space Station outside of the European Columbus module on the European Technology Exposure Facility (EuTEF), from February 2008 to August 2009, while EXPOSE-R was installed outside of the Russian module Zvezda in March 2009, and should stay there for several years and be reloaded with new experiments every 2 years.
Overview The EXPOSE facilities are presented in Fig. 1. Data from six temperature sensors connected to the three trays, four UV-B sensors, and one radiometer are recorded every 10 s. Experiments selected by ESA to be implemented on EXPOSE-E and -R are presented in Tables 1 and 2 (from Rabbow et al. 2009).
Expose
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Tray 1 (Windows not shown)
Shutter assembly
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Tray 2
Tray 3
One lid for both carriers
Expose. Figure 1 The EXPOSE facility (480 520 327.5 mm) is made of three experiment trays into which four square sample carriers (77 77 26 mm) are fitted (Credit Kayser-Threde – Germany). In most of the experiment, one layer of samples is exposed to space, while another layer is fitted just below the first one to be used as flight controls
Expose. Table 1 Astrobiology experiments selected after the first call of ESA in 1996 and finally accommodated on EXPOSE-R Experiment
Topic of research
Principal investigator
AMINO
Photochemical processing of organic compounds, including amino acids, RNA fragments, and samples relevant to cometary and Titan chemistry
H. Cottin – LISA-Cre´teil (France)
ORGANIC
Study of the evolution of organic matter in space (PAHs)
P. Ehrenfreund – Leiden Observatory (NL)
ROSE-1/ENDO
Study of the impact of extraterrestrial UV radiation on microbial primary producers (algae, cyanobacteria)
C. Cockell – Open University(UK)
ROSE-2/OSMO Study of the protective effects of osmophilic microorganisms enclosed within gypsum–halite crusts
R. Mancinelli Seti Institute, NASA Ames (USA)
ROSE-3/ SPORES with R3D
Study of the protection of spores by meteorite material against space conditions: UV, vacuum, and ionizing radiation/Radiation dosimetry
G. Horneck – DLR (Germany)
ROSE-4/ PHOTO
Study of the photoproducts resulting from exposure of dry DNA samples or J. Cadet – C.E.A. Grenoble bacterial spores to solar UV radiation (France)
ROSE-5/ SUBTIL
Study of the mutational spectra of Bacillus subtilis spores induced by space N. Munakata – University of vacuum and/or solar UV radiation Tokyo (Japan)
ROSE-8/PUR
Study of the biologically effective dose of solar extraterrestrial UV radiation G. Ronto´ – Research Lab. for by biological dosimetry Biophysics, Budapest (Hungary)
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Expose. Table 2 Astrobiology experiments selected after the second call of ESA in 2004 and finally accommodated on EXPOSE-E Experiment Topic of research
Principal investigator
ADAPT
Study of molecular adaptation strategies of microorganisms to different P. Rettberg – DLR (Germany) space and planetary UV climate conditions
DOSIS/ DOBIES
Passive radiation dosimetry at the sample sites
G. Reitz – DLR (Germany)/ F. Vanhavere – SCK CEN (Belgium)
LIFE
Study of the resistance of lichens and lithic fungi at space conditions
S. Onofri – Universita` degli studi della Tuscia di Viterbo (Italy)
PROCESS
Study of photochemical organic chemistry relevant to comets, meteorites, Mars, and Titan
H. Cottin LISA – Cre´teil (France)
PROTECT
Study on the resistance of spacecraft isolates to outer space for planetary G. Horneck – DLR Germany protection purposes
R3D-2
Active radiation dosimetry (VIS, UV-A, UV-B, UV-C; LET spectra of cosmic D-P. Ha¨der – University of Erlangen radiation) (Germany)
SEEDS
Study of plant seed as a terrestrial model for a Panspermia vehicle and as D. Tepfer – CNRS, Versailles (France) a source of universal UV screens
See also
See also
▶ BIOPAN ▶ International Space Station ▶ Ionizing Radiation (Biological Effects) ▶ Lyman Alpha ▶ Photolysis ▶ Radiation Biology ▶ Solar UV Radiation (Biological Effects) ▶ Space Environment ▶ UV Radiation
▶ Bioburden ▶ Microorganism ▶ Planetary Protection
EXPOSE-E ▶ Expose
References and Further Reading Cottin H et al (2008) Heterogeneous solid/gas chemistry of organic compounds related to comets, meteorites, Titan and Mars: in laboratory and in lower Earth orbit experiments. Adv Space Res 42:2019–2035 Rabbow E et al (2009) EXPOSE, an Astrobiological Exposure Facility on the International Space Station – from Proposal to Flight. Orig Life Evol Biosph 39:581–598
EXPOSE-R ▶ Expose
Exposure Facilities Exposed Surface Bioburden
GERDA HORNECK German Aerospace Center (DLR), Institute of Aerospace Medicine, Cologne, Germany
Definition The term “exposed surface ▶ bioburden” is used to indicate the number of viable ▶ microorganisms that are carried on internal and external spacecraft surfaces that are available for particulate and gas exchange, and from which microorganisms could reach a planetary environment following the nominal landing of a spacecraft.
Synonyms Exposure platforms; Exposure trays
Keywords Outer space parameters, photochemical reactions, space experiments, survival
Exposure Facilities
Definition Exposure facilities are technical devices attached to the outer shell of a spacecraft with the purpose to expose ▶ organic molecules, ▶ microorganisms, and other small biological systems to outer space or to selected parameters of this extreme environment.
History Soon after the advent of space flight, the accessibility of space has been used to study specific questions of
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astrobiology, such as the role of stellar ▶ UV radiation in the evolution of potential precursors of life, the role of the UV radiation climate in prebiotic and biological evolution on Earth or any other celestial body, and the likelihood of viable interplanetary transfer of life and the limiting factors of ▶ Panspermia. First attempts were already performed during the Gemini program (Hotchin et al. 1968); they were continued in a more sophisticated manner during the Apollo program and up to the present studies on
Exposure Facilities. Table 1 Exposure facilities used on space missions to study the stability of organic molecules and the survival of biological systems in outer space (Horneck et al. 2010) Year
Mission name
1966
Gemini IX and XII
Mission Exposure characteristics facility
Exposure duration
Earth orbit (300 km alt)
Collecting/ exposure device
16 h 47 min (GIX) Space, solar UV
Space parameter studied
6 h 24 min (GXII)
1972
Apollo 16
Lunar mission
MEED
Vacuum: 1 h 20 min, UV: 10 min
Space vacuum; solar UV: 254, 280 nm
1983
Spacelab 1
Earth orbit (240 km alt.)
ES029
Vacuum: 9 days, UV: 19 min–5 h 17.5 min
Space vacuum; solar UV: >170; 220; 240; 260; 280
1984–1990
LDEF
Earth orbit (500 km alt.)
Exostack
2107 days
Space vacuum; solar UV
1992–1993
EURECA
Earth orbit, sun pointing
ERA
327 days
Space vacuum; solar UV: >110; >170; >280; >295; 220; 230; 260; 290 nm
1993
Spacelab D2
Earth orbit
RD-UVRAD
Vacuum: 10 days; Space vacuum; solar UV: 190; 210; 220; 230; UV: 5-120 min 260; 280; >190; >304; >313; >314; >315; >316; >317 nm
1994
Foton 9
Earth orbit
Biopan-1
14.8 days
Space vacuum; solar UV
1997
Foton 11
Earth orbit
Biopan-2
10 days
Space vacuum. solar UV
1999
Foton 12
Earth orbit
Biopan-3
12.7 days
Space vacuum. solar UV
1999
MIR-Perseus
Earth orbit
Exobiologie 98 days
Space vacuum, solar UV
1999
Terrier Black Brant Ballistic flight Rocket 280; >320; >400 nm
Biopan-5 Stone-5
2007
Foton-M3
Earth orbit
2008–2009
ISS-EuTeF
Earth orbit
2009–2011
ISS
Earth orbit
Meteorite entry in Earth’s atmosphere 10 days
Space vacuum, solar UV: > 110; >200; > 290; > 400 nm
EXPOSE-E
1.5 years
Space vacuum; solar UV: > 110 nm; simulated Martian atmosphere and UV climate: > 200 nm
EXPOSE-R
2 year
Space vacuum, solar UV: > 110; > 200 nm
Biopan-6 Stone-6
Meteorite entry in Earth’s atmosphere
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Experiment package Deployed
Exposure Facilities. Figure 1 Exposure facility MEED, which was mounted on the camera beam of the lunar orbiter of the Apollo 16 mission (Credit: NASA)
Exposure Facilities. Figure 2 Exposure tray of the ES029 experiment, which was mounted on a cold plate in the cargo bay of SL 1
the ▶ International Space Station (ISS) (Horneck et al. 2010).
Overview To expose chemical or biological samples to outer space or selected parameters of this extreme environment, several
Exposure Facilities. Figure 3 Exposure tray of the Exobiology Radiation Assembly (ERA), which was mounted on the EURECA platform (credit: ESA, from Horneck et al. 2010)
exposure facilities were developed for attachment to the outer shell of space craft (Table 1). The first sophisticated exposure device was built in 1972 by ▶ NASA, the Microbial Ecology Evaluation Device (▶ MEED) for the Apollo 16 mission (Taylor
Exposure Facilities
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et al. 1974). MEED was mounted to the distal end of the TV boom of the Command Module during the extravehicular activity phase of the trans-Earth coast. It was composed of 798 sample cuvettes with quartz windows as optical filters with the optional provision of ventilation holes for access to space vacuum (Fig. 1). Using a solar positioning device MEED was oriented directly perpendicular to the sun. In 1983, Spacelab 1 (SL 1) carried the exposure facility ES029 in its cargo bay (Horneck et al. 1984). ES029 consisted of an exposure tray partitioned in four square quartz-covered compartments (Fig. 2). Two of the compartments were vented to the outside, allowing access to space vacuum. The other two compartments were hermetically sealed with a constant pressure of 105 Pa. Each compartment accommodated 79 dry samples in the upper layer, allowing UV exposure, and the same number was kept in the bottom layer as flight dark controls. UV-irradiated samples were placed beneath an optical filtering system composed of interference filters for narrow wavebands (220 nm, 240 nm, 260 nm, and 280 nm) and neutral density filters. A nontransparent shutter with optical windows was used to achieve precise irradiation intervals during the “hot phase” of the mission, when during several orbits the cargo bay of the shuttle was perpendicularly pointing towards the sun. The samples were exposed to space vacuum for 10 days and for predefined periods (from 19 min to 5 h 17.5 min) to ▶ solar UV radiation. The temperature ranged from 17 C to 35 C, the highest values occurring during the “hot phase” of the mission. A similar device was flown in 1993 with the experiment UVRAD during the German SL D2 mission that provided a “hot phase” during two orbits at the end of the mission.
Long-term exposures of organic chemical compounds and microorganisms to space started with the NASA ▶ Long Duration Exposure Facility (LDEF) and were continued with the European Retrievable Carrier (EURECA) mission (Fig. 3) (Innocenti and Mesland 1995), and the French PERSEUS mission on the Russian MIR station (Rettberg et al. 2002). The longest exposure of microorganisms to space, about 6 years (1984–1990), was achieved during the LDEF mission within the German experiment Exostack (Horneck et al. 1994). Frequent opportunities for short-duration exposure experiments (up to 15 days) of molecules and biological systems were provided by ESA’s Biopan facilities, cylindrical pan-shaped containers with a deployable lid mounted on the outer surface of the descent module of a Russian Foton satellite (Fig. 4) (Demets et al. 2005). The Foton satellite was also used to study the mineral decomposition and microbial survival during atmospheric reentry within the ▶ STONE experiments (Brandsta¨tter et al. 2008; Cockell et al. 2007; de la Torre et al. 2010). Advanced exposure facilities with up to four times the capacity of the ES029 experiment of SL 1 were developed by the ▶ European Space Agency (ESA) with the ▶ Exobiology Radiation Assembly (ERA) for the ▶ EURECA mission (Fig. 3) and the ▶ EXPOSE facilities attached to the ▶ ISS (Rabbow et al. 2009). One EXPOSE unit consists of three trays (Fig. 5), each housing four compartments similar to those of the SL exposure trays and of ERA. The EXPOSE-E facility was mounted during extravehicular activity to the European Columbus Module of the ISS as part of the European Technology Facility (EuTeF)
Exposure Facilities. Figure 4 Biopan facility with lid opened, which was mounted on the outside of Foton satellites (Credit ESA, from Horneck et al. 2010)
Exposure Facilities. Figure 5 EXPOSE-E facility (arrow), which was mounted on the EuTef platform of the ESA Columbus facility of the ISS (Credit ESA)
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platform in February 2008 and retrieved in September 2009. Experiments on prebiotic chemical evolution were located in one tray of EXPOSE-E, the other two trays accommodated different microbial systems, either exposed to outer space conditions (space vacuum and solar UV spectrum of l > 110 nm), or to simulated Mars surface climate (600 Pa pressure, 95% CO2, and solar UV of l > 200 nm). The second EXPOSE facility, EXPOSE-R, was launched in November 2008 and will remain attached to the URM-D platform, an external ISS facility at the Russian Svezda module, for about 2 year. EXPOSE-E and EXPOSE-R house a total of 13 different experiments that are performed in international cooperation (Baglioni et al. 2007). Before launching into space, all EXPOSE experiments were tested in carefully designed ground simulation experiments and in an experiment sequence test using the Planetary and Space Simulation Facilities (PSI) at the ▶ German Aerospace Center DLR. The EXPOSE experiments will be continued with EXPOSE-R2.
See also ▶ BIOPAN ▶ Chemical Evolution ▶ Exobiologie Experiment ▶ Expose ▶ Extreme Environment ▶ Foton Capsule (Spacecraft) ▶ Inactivation ▶ International Space Station ▶ Lichens ▶ Lithopanspermia ▶ Long Duration Exposure Facility ▶ Mars ▶ MEED ▶ Panspermia ▶ Spore ▶ STONE
References and Further Reading Baglioni P, Sabbatini M, Horneck G (2007) Astrobiology experiments in low Earth orbit: facilities, instrumentation, and results. In: Horneck G, Rettberg P (eds) Complete course in astrobiology. Wiley-VCH, Berlin, New York, pp 273–320 Brandsta¨tter F, Brack A, Baglioni P, Cockell CS, Demets R, ME EH, Kurat G, Osinski GR, Pillinger JM, Roten C-A, Sancisi-Frey S (2008) Mineraogical alteration of artificial meteorites during atmospheric entry. The STONE-5 experiment. Planet Space Sci 56:976–984 Cockell CS, Brack A, Wynn-Williams DD, Baglioni P, Brandsta¨tter F, Demets R, Edwards HME, Gronstal AL, Kurat G, Lee P, Osinski GR, Pearce DA, Pillinger JM, Roten C-A, Sancisi-Frey S (2007) Interplanetary transfer of photosynthesis: an experimental
demonstration of a selective dispersal filter in planetary island biogeography. Astrobiology 7:1–9 de la Torre R, Sancho LG, Horneck G, de los Rı´os A, Wierzchos J, OlssonFrancis K, Cockell CS, Rettberg P, Berger T, de Vera J-PP, Ott S, Martinez Frı´as J, Melendi PG, Lucas MM, Reina M, Pintado A, Demets R (2010) Survival of lichens and bacteria exposed to outer space conditions – results of the Lithopanspermia experiments. Icarus 208:735–748 Demets R, Schulte W, Baglioni P (2005) The past, present and future of Biopan. Adv Space Res 36:311–316 Horneck G, Bu¨cker H, Reitz G, Requardt H, Dose K, Martens KD, Mennigmann HD, Weber P (1984) Microorganisms in the space environment. Science 225:226–228 Horneck G, Bu¨cker H, Reitz G (1994) Long-term survival of bacterial spores in space. Adv Space Res 14(10):41–45 Horneck G, Klaus DM, Mancinelli RL (2010) Space microbiology. Microbiol Mol Biol Rev 74:121–156 Hotchin J, Lorenz P, Hemenway C (1968) The survival of terrestrial microorganisms in space at orbital altitudes during Gemini satellite experiments. Life Sci Space Res 6:108–114 Innocenti L, Mesland DAM (eds) (1995) EURECA scientific results. Adv Space Res 16(8):1–140 Rabbow E, Horneck G, Rettberg P, Schott JU, Panitz C, L’Afflitto A, von Heise-Rotenburg R, Willnecker R, Baglioni P, Hatton J, Dettmann J, Demets R, Reitz G (2009) EXPOSE, an astrobiological exposure facility on the International Space Station – from proposal to flight. Orig Life Evol Biosph 39:581–598 Rettberg P, Eschweiler U, Strauch K, Reitz G, Horneck G, Wa¨nke H, Brack A, Barbier B (2002) Survival of microorganisms in space protected by meteorite material: results of the experiment EXOBIOLOGIE of the PERSEUS mission. Adv Space Res 30:1539–1545 Taylor GR, Spizizen J, Foster BG, Volz PA, Bu¨cker H, Simmonds RC, Heimpel AM, Benton EV (1974) A descriptive analysis of the Apollo 16 microbial response to space environment experiment. Bioscience 24:505–511
Exposure Platforms ▶ Exposure Facilities
Exposure Trays ▶ Exposure Facilities
Extended Red Emission Synonyms ERE
Extracellular Polysaccharides
Definition The broad emission band observed in some interstellar environments (e.g., ▶ reflection nebulae, ▶ planetary nebulae, ▶ HII regions, interstellar “▶ cirrus clouds”) extending between about 600 and 800 nm is referred to as extended red emission (ERE). It is thought to be the result of photoluminescence, although it is not clear whether the carrier is carbon-rich or silicate-rich material. The spatial distribution of ERE is, in many cases, similar to that of ▶ Polycyclic Aromatic Hydrocarbons (PAHs).
See also ▶ Cirrus Cloud ▶ HII Region ▶ Planetary Nebula ▶ Polycyclic Aromatic Hydrocarbons ▶ Reflection Nebula
References and Further Reading Whittet DCB (2003) Dust in the galactic environment, 2nd edn. Institute of Physics, Philadelphia
Extensive Plain ▶ Vastitas, Vastitates
Definition The extinction is the decrease of the light intensity of a celestial object due to scattering and/or absorption by an intervening medium. The medium can be of telluric origin (molecules, aerosol in the atmosphere) or astrophysical (interstellar cloud); in the later case one speaks of interstellar extinction. Extinction is measured in ▶ magnitudes and the wavelength at which it applies should be indicated. The extinction is larger in the blue than in the red and becomes less and less important as the wavelength increases in the infrared domain. The radio domain is essentially not affected by extinction. Interstellar extinction is generally noted AF, where F is the symbol of the color considered (U, B, V, etc.), defined by a passband filter. When no indication is given it is assumed that the extinction is in the visible band (V). Typical extinction by the average ▶ interstellar medium in our Galaxy is AV = 2 mag/kpc (1 kpc is about 3,000 light-years).
See also ▶ Color Index ▶ Dust Grain ▶ Interstellar Dust ▶ Interstellar Medium ▶ Magnitude ▶ Reddening, Interstellar
Extracellular Polymeric Substances ▶ Exopolymers
Extinction Event ▶ Mass Extinctions
Extinction-Level Event ▶ Mass Extinctions
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Extinct Radioactivity ▶ CAI ▶ Geochronology ▶ Solar System Formation (Chronology)
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Extracellular Polymers ▶ Exopolymers
Extracellular Polysaccharides ▶ Exopolymers
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Extrachromosomal Genetic Element
Extrachromosomal Genetic Element ▶ Plasmid
Extrasolar Planets ▶ Exoplanet, Detection and Characterization ▶ Exoplanets, Discovery
Extrasolar Planets Detection ▶ Exoplanet, Detection and Characterization
Extraterrestrial Delivery (Organic Compounds) ANDRE´ BRACK Centre de Biophysique Mole´culaire CNRS, Orle´ans cedex 2, France
Keywords Amino acids, chirality, ▶ comets, impacts, ▶ meteorites, micrometeorites, prebiotic compounds, space experiments
Definition The primitive Earth experienced a large spectrum of impactors ranging from the huge Mars-sized impactor which created the Moon to cosmic dust less than 1 mm in size. A great number of organic molecules, including amino acids, have been found in comets and in carbonaceous chondrites. Micrometeorite collection and analysis from the Greenland and Antarctic ice sheets suggest that the Earth accreted large amounts of extraterrestrial complex organic molecules. Intense bombardment probably caused some chemical reprocessing of the Earth’s primitive atmosphere. Laboratory and space experiments support plausibility of the extraterrestrial delivery of organics to the primitive Earth.
Overview Delivery of Extraterrestrial Organic Matter Comets Comets are the richest planetary objects in organic compounds known so far. Ground-based observations have detected hydrogen cyanide and formaldehyde in the coma of comets. In 1986, on board analyses performed by the two Russian missions Vega 1 et 2, as well as observations obtained by the European mission Giotto and the two Japanese missions Suisei and Sakigake, demonstrated that Halley’s comet contains substantial amounts of organic material. On average, dust particles ejected from the Comet Halley nucleus contain 14% of organic carbon by mass. About 30% of cometary grains are dominated by light elements C, H, O, and N, and 35% are close in composition to the carbon rich meteorites. The presence of organic molecules, such as purines, pyrimidines, and formaldehyde polymers, has also been inferred from the fragments analyzed by the Giotto PICCA and Vega PUMA mass spectrometers. However, there was no direct identification of the complex organic molecules probably present in the cosmic dust grains and in the cometary nucleus. Many chemical species of interest for exobiology have been detected in Comet Hyakutake in 1996, including ammonia, methane, acetylene (ethyne), acetonitrile (methyl cyanide), and hydrogen isocyanide. In addition, the study of the Hale-Bopp comet in 1997 led to the detection of methane, acetylene, formic acid, acetonitrile, hydrogen isocyanide, isocyanic acid, cyanoacetylene, formamide, and thioformaldehyde. The Stardust mission collected samples of Comet Wild 2 and returned them to Earth in January 2006 for laboratory analysis. Unexpectedly, most of the comet’s rocky matter formed inside the solar system at extremely high temperature. The grains contain a variety of organic functional groups (alcohol, ketone, aldehyde, carboxylic acid, amide, nitrile). The protein-building amino acid glycine has also been discovered. Comets orbit on unstable trajectories and sometimes collide with planets. The collision of Comet Shoemaker-Levy 9 with Jupiter in July 1994 gave a recent example of such events. Such collisions were probably more frequent 4 billion years ago, the comets orbiting around the Sun being more numerous. Comets may therefore be an important source of organic molecules delivered to the primitive Earth (Ehrenfreund and Charnley 2000; Despois and Cottin 2005). However, it is unlikely that whole comets could
Extraterrestrial Delivery (Organic Compounds)
have safely delivered organics to the Earth. They exploded either while crossing the atmosphere or when impacting the Earth’s surface.
Meteorites Carbonaceous chondrites delivered organic materials to the early Earth. They contain from 1.5% to 4% of carbon, for the most part as organic materials. One hundred kilograms of the Murchison meteorite, a CM2 type carbonaceous chondrite that fell in Australia in 1969, have been extensively analyzed (Pizzarello 2007; Pizzarello and Shock 2010, and references therein). Murchison organic materials are generally classified according to their solubility in water and organic solvents. Insoluble and soluble components represent, respectively, 70% and 30% of total carbon components. The insoluble organic material is referred as kerogen-like, a poorly identified insoluble macromolecular material of complex composition with average elemental formula C100H46N10O15S4.5. NMR, IR and pyrolysis analyses suggest the presence of aromatic ring clusters bridged by aliphatic chains, with peripheral branching and functional groups. The insoluble organic material releases a variety of aromatic and heteroatomic hydrocarbons as well as a suite of alkyl dicarboxylic acids up to C18 chain length under conditions similar to those of hydrothermal vents (Yabuta et al. 2007). The soluble organic compounds of the Murchison meteorite represent a diverse and abundant group of organics that vary from small water-soluble compounds such as amino acids and polyols up to 30 carbon-long hydrocarbons. This diversity has been analyzed in detail for the amino acid. The total number of meteoritic amino acids is about 100. All the possible a-amino acids up to seven-carbon were identified as well as large abundances of N-substituted, cyclic, b-, g-, d-, and e-amino acids. Eight protein-building amino acids (glycine, alanine, proline, leucine, isoleucine, valine, aspartic acid and glutamic acid) have been found. Nucleic acid bases, purines and pyrimidines, have also been found in the Murchison meteorite (Stoks and Schwartz 1982). No ribose, the sugar moiety which links together the nucleic acid building blocks, was detected in meteorites. Vesicle-forming fatty acids have been extracted from different carbonaceous meteorites (Deamer 1985, 1998). A combination of high-resolution analytical methods, composed of organic structural spectroscopy FTICR/MS, UPLC-QTOF-MS and NMR, applied to the organic fraction of Murchison extracted under mild conditions allowed to extend its indigenous chemical diversity to
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tens of thousands of different molecular compositions and likely millions of diverse structures (Schmitt-Kopplin et al. 2010). Most of the amino acids detected in the carbonaceous chondrites are chiral but present as racemate, i.e., the Land D-enantiomers are present in equal proportions. However, Cronin and Pizzarello (1997) found L-enantiomer excesses in six a-methyl-a-amino acids from the Murchison (e.e. 2.8–9.2%) and Murray (e.e. 1.0–6.0%) carbonaceous chondrites. An enantiomeric excess up to 18%, has been measured for isovaline, 2-methyl-2-aminobutyric acid. These amino acids (isovaline, 2-amino-2,3-dimethylpentanoic, a-methyl norvaline, a-methyl valine and a-methyl norleucine) are either unknown or rare in the terrestrial biosphere and cannot therefore be attributed to terrestrial contamination (Pizzarello 2007). In addition, the indigeneity of D- and Lisovaline enantiomers is supported by carbon and hydrogen isotopic data (Pizzarello et al. 2003; Pizzarello and Huang 2005). Several organic and inorganic phases of the carbonaceous chondrite matrix were separated and subjected to the Soai reaction, i.e., the addition reaction of i-Pr2Zn to pyrimidine-5-carbaldehyde. Asymmetric autocatalysis with amplification of chirality gave pyrimidyl alkanol with enantiomeric excesses of detectable level. The asymmetry resides in powders after extraction with water and solvents as well as in the insoluble organic material obtained after demineralization. Asymmetry is not found any longer in the insoluble organic matter after hydrothermal treatment and in meteorite powders from which all organics had been removed (Kawasaki et al. 2006). The meteoritic enantiomeric excesses may help to explain the emergence of a homochiral (one-handed) life. Each amino acid, with the exception of glycine, exists in two enantiomeric forms, L and D, but proteins use only the L ones. Proteins adopt asymmetric rigid geometries, a-helices and b-sheets, which play a key role in the catalytic activity. Homochirality is now believed to be not just a consequence of life, but also a prerequisite for life, because stereo-regular structures such as protein b-sheets, for example, do not form with mixtures of monomers of both handednesses. The use of one-handed biomonomers also sharpens the sequence information of the biopolymers. For a polymer made of n units, the number of sequence combinations will be divided by 2n when the system uses only homochiral monomers. Taking into account the fact that enzyme chains are generally made of hundreds of monomers, the tremendous gain in simplicity offered by the use of monomers restricted to one
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handedness is self-evident. The excess of L-amino acids found in the Murchison meteorite may result from the processing of the organic mantles of interstellar grains by circularly polarized synchrotron radiation from a neutron star remnant of a supernova (Bonner 1991). Strong infrared circular polarization, resulting from dust scattering in reflection nebulae in the Orion OMC-1 star-formation region, has been observed (Bailey et al. 1998). Circular polarization at shorter wavelengths might have been important in inducing chiral asymmetry in interstellar organic molecules that could be subsequently delivered to the early Earth (Bailey 2001). The discovery of a large number of meteorites since 1969 has provided new opportunities to search for organic compounds in CM type carbonaceous chondrites (Pizzarello et al. 2001; Glavin et al. 2006; Pizzarello and Shock 2010).
Micrometeorites Dust collections in the Greenland and Antarctic ice sheets (Maurette 1998, 2006) show that the Earth captures interplanetary dust as micrometeorites at a rate of about 20,000 tonnes per year. About 99% of this mass is carried by micrometeorites in the 50–500-mm size range. This value is about 2,000 times higher than the most reliable estimate of the meteorite flux, i.e., about 10 tonnes per year. This amazing dominance of micrometeorites already suggests their possible role in delivering complex organics to the early Earth 4.2 to 3.9 Ga ago when the micrometeorite flux was probably enhanced by several orders of magnitude. Antarctic micrometeorite flux measurements suggest that a huge mass (5 1024 g) of micrometeorites was accreted by the Earth during the first 300 Ma of the post-lunar period. At least, 20 wt% of the micrometeorites survive unmelted by atmospheric entry. As their kerogen fraction represents about 2.5 wt% of carbon, this amounts to a total mass of kerogen of 2.5 1022 g on the early Earth’s surface equivalent to a 40-m-thick global layer (Maurette and Brack 2006). This delivery represents more carbon than that present in the biomass of the present day Earth (1018 g). One amino acid, a-amino isobutyric acid, has been identified in Antarctic micrometeorites (Brinton et al. 1998; Matrajt et al. 2004). These grains also contain a high proportion of metallic sulfides, oxides, and clay minerals, a rich variety of inorganic catalysts which could have promoted the reactions of the carbonaceous material which lead to the origin of life. Analysis of the dust grains collected by the Cosmic Dust mission supports a cometary origin for the micrometeorites collected in Antarctica.
Laboratory and Space Experiments Supporting Extraterrestrial Delivery Synthesis Ultraviolet irradiation of dust grains in the interstellar medium results in the formation of complex organic molecules. The interstellar dust particles are assumed to be composed of silicate grains surrounded by ices of different molecules, including carbon-containing molecules. Ices of H2O, CO2, CO, CH3OH, and NH3 were deposited at 12 K under a pressure of 107 mbar and irradiated with electromagnetic radiation representative of the interstellar medium. The solid layer that developed on the solid surface was analyzed by enantioselective gas chromatography and mass spectrometry GC-MS. After the analytical steps of extraction, hydrolysis, and derivatization, 16 amino acids, including 6 protein amino acids, were identified in the simulated ice mantle of interstellar dust particles (Mun˜oz Caro et al. 2002). These amino acid identifications confirmed the preliminary amino acid formation obtained by Mayo Greenberg (Briggs et al. 1992). The chiral amino acids were racemic. Parallel experiments performed with 13 C-containing substitutes definitely excluded contamination by biological amino acids. The results strongly suggest that amino acids are readily formed in interstellar space. Irradiation experiments run with H2O, NH3, CH3OH, and HCN produced only three amino acids, namely, glycine, alanine, and serine (Bernstein et al. 2002).
Space Travel To estimate whether different amino acids could survive a trip in space embedded in micrometeorites, a suite of amino acids like those detected in the Murchison meteorite has been exposed to space conditions in Earth orbit onboard the unmanned Russian satellites FOTON 8 and 11, free and associated with clay minerals. Free exposed aspartic acid and glutamic acid were partially destroyed during exposure to solar UV. However, decomposition was prevented when the amino acids were embedded in clays (Barbier et al. 1998). Amino acids have also been subjected to solar radiation outside the MIR station for 97 days. The samples were exposed in free form and associated with different ground mineral supports to mimic micrometeorites, i.e., montmorillonite clay, powdered basalt, and powdered Allende meteorite. In the absence of mineral protection, about half of the amino acids were destroyed by UV radiation. The main photochemical degradation process was decarboxylation.
Extraterrestrial Delivery (Organic Compounds)
Significant protection from solar radiation was observed when the thickness of the added minerals was 4–5 mm or greater (Boillot et al. 2002).
Impact Shock Chemistry Intense bombardment probably caused some chemical reprocessing of the Earth’s primitive atmosphere by impact shock chemistry. An indication of the number and timing of the impacts onto the early Earth can be obtained by comparison with the cratering record of the Moon, which records impacts from the earliest history of the solar system (Ryder 2003). Because of the larger size of the Earth and its greater gravitational pull, about 20 times as many impacts would have occurred on the early Earth as on the Moon. Computer modeling of the impact shock chemistry shows that the nature of the atmosphere strongly influences the shock products (Fegley et al. 1986). A neutral CO2-rich atmosphere produces CO, O2, H2, and NO while a reducing CO-rich atmosphere yields primarily CO2, H2, CH4, HCN, NH3, and H2CO. The last three compounds are particularly interesting for prebiotic chemistry since they can lead to amino acids via the Strecker synthesis. However, a CO-rich primitive atmosphere may be unlikely. In laboratory experiments, a gas mixture of methane, ammonia, and water subjected to shock heating followed by a rapid thermal quenching yielded the amino acids glycine, alanine, valine, and leucine (Bar-Nun et al. 1970). Here again, the gas mixture used does not represent a realistic primitive atmosphere, which was likely dominated by CO2. Laboratory simulations of shocks were also run with a high-energy laser. CH4-containing mixtures generated hydrogen cyanide and acetylene, but no organics could be obtained with CO2-rich mixtures (McKay and Borucki 1997). Large impacts could also have promoted some fine ejecta particle chemistry. For 10–15-km-sized impactors, modeling (Lyons and Vasavada 1999) has shown that 100-mm fine ejecta particles would have been heated to about 200 C during reentry in a 0.3 bar CO2 atmosphere. For impactors larger than 20 km, the heat experienced by the fines would destroy all organics by pyrolysis or combustion. Furukawa and colleagues (Furukawa et al. 2009) investigated whether, rather than just transporting amino acids, meteorite impacts could themselves synthesize organic compounds. The group subjected a mixture of solid carbon, iron, nickel, water, and nitrogen to high-velocity impacts in a propellant gun. This experiment simulated the chemistry experienced by ordinary chondrites when hitting the Earth’s early oceans. They recovered several
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organic molecules after the impact, including complex molecules such as fatty acids and amines. Glycine, the simplest protein-building amino acid, was formed when the starting material contained ammonia, which is believed to have been formed in prior impacts on the early Earth. To avoid potential contamination by biological materials, the authors used solid carbon composed only of 13C. Subsequent analyses of the recovered organic products showed that they were all formed with the 13C, ruling out the presence of any naturally formed biological contaminants, which would have been enriched in 12C. The effects of impact shock on amino acids and a peptide in artificial meteorites composed of saponite clay were investigated (Bertrand et al. 2009). The samples were subjected to pressures ranging from 12 to 28.9 GPa, which simulated impact velocities of 2.4–5.8 km/s. Volatilization was determined by weight loss measurement, and the amino acid and peptide response was analyzed by gas chromatography–mass spectrometry. At the highest shock pressures, amino acids with an alkyl side chain were more resistant than those with functional side chains. Impact shock may therefore act as a selective filter to the delivery of extraterrestrial amino acids via carbonaceous chondrites. The peptide cleaved into its two primary amino acids. Some chiral amino acids experienced partial racemization during the course of the experiment, suggesting that the enantiomeric excesses measured in carbonaceous chondrites are probably underevaluated.
Conclusion Meteorite and micrometeorite collection and analysis as well as laboratory and space experiments strongly support an extraterrestrial pathway for the delivery of organic molecules at the time that life originated. The building blocks of life or their precursors could have been brought in by small impactors, possibly supplemented by prebiotic molecules formed in the atmosphere and oceans by meteorite impacts.
See also ▶ Comet ▶ Meteorites
References and Further Reading Bailey J (2001) Astronomical sources of circularly polarized light and the origin of homochirality. Orig Life Evol Biosph 31:167–183 Bailey J, Chrysostomou A, Hough JH, Gledhill TM, McCall A, Clark S, Me´nard F, Tamura M (1998) Circular polarization in star formation regions: implications for biomolecular homochirality. Science 281:672–674 Barbier B, Chabin A, Chaput D, Brack A (1998) Photochemical processing of amino acids in Earth orbit. Planet Space Sci 46:391–398
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Bar-Nun A, Bar-Nun N, Bauer SH, Sagan C (1970) Shock synthesis of amino acids in simulated primitive environments. Science 168: 470–473 Bernstein MP, Dworkin JP, Standford SA, Cooper GW, Allamandola LJ (2002) Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues. Nature 416:401–403 Bertrand M, van der Gaast S, Vilas F, Ho¨rz F, Haynes G, Chabin A, Brack A, Westall F (2009) The fate of amino acids during simulated meteoritic impact. Astrobiology 9:943–951 Boillot F, Chabin A, Bure´ C, Venet M, Belsky A, Bertrand-Urbaniak M, Delmas A, Brack A, Barbier B (2002) The Perseus exobiology mission on MIR: behaviour of amino acids and peptides in Earth orbit. Orig Life Evol Biosph 32:359–385 Bonner WA (1991) The origin and amplification of biomolecular chirality. Orig Life Evol Biosph 21:59–111 Briggs R, Ertem G, Ferris JP, Greenberg JM, McCain PJ, Mendoza-Gomez CX, Schutte W (1992) Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium. Orig Life Evol Biosph 22: 287–307 Brinton KLF, Engrand C, Glavin DP, Bada JL, Maurette M (1998) A search for extraterrestrial amino acids in carbonaceous Antarctic micrometeorites. Orig Life Evol Biosph 28:413–424 Cronin JR, Pizzarello S (1997) Enantiomeric excesses in meteoritic amino acids. Science 275:951–955 Deamer DW (1985) Boundary structures are formed by organic components of the Murchison carbonaceous chondrite. Nature 317: 792–794 Deamer DW (1998) Membrane compartments in prebiotic evolution. In: Brack A (ed) The molecular origins of life: assembling pieces of the puzzle. Cambridge University Press, Cambridge, pp 189–205 Despois D, Cottin HH (2005) Comets: potential sources of prebiotic molecules. In: Gargaud M et al (eds) Lectures in astrobiology. Springer-Verlag, Berlin, Heidelberg, pp 289–352 Ehrenfreund P, Charnley SB (2000) Organic molecules in the interstellar medium, comets and meteorites. Annu Rev Astron Astrophys 38: 427–483 Fegley B Jr, Prinn RG, Hartman H, Watkins GH (1986) Chemical effects of large impacts on the earth’s primitive atmosphere. Nature 319: 305–308 Furukawa Y, Sekine T, Oba M, Kakegawa T, Nakazawa H (2009) Biomolecule formation by oceanic impacts on early Earth. Nat Geosci 2:62–66 Glavin DP, Dworkin JP, Aubrey A, Botta O, Doty JH III, Martins Z, Bada JL (2006) Amino acid analyses of Antarctic CM2 meteorites using liquid chromatography-time of flight-mass spectrometry. Meteorit Planet Sci 41:889–902 Kawasaki T, Hatase K, Fujii Y, Jo K, Soai K, Pizzarello S (2006) The distribution of chiral asymmetry in meteorites: an investigation using asymmetric autocatalytic chiral sensors. Geochim Cosmochim Acta 70:5395–5402 Lyons JR, Vasavada AR (1999) Flash heating on the early Earth. Orig Life Evol Biosph 29:123–138 Matrajt G, Pizzarello S, Taylor S, Brownlee D (2004) Concentration and variability of the AIB amino acid in polar micrometeorites: implications for the exogenous delivery of amino acids to the primitive Earth. Meteorit Planet Sci 39:1849–1858 Maurette M (1998) Carbonaceous micrometeorites and the origin of life. Orig Life Evol Biosph 28:385–412 Maurette M (2006) Micrometeorites and the mysteries of our origins. Springer, Berlin, Heidelberg
Maurette M, Brack A (2006) Cometary petroleum in Hadean time? Meteorit Planet Sci 41:5247 McKay CP, Borucki WJ (1997) Organic synthesis in experimental impact shocks. Science 276:390–392 Mun˜oz Caro GM, Meierhenrich UJ, Schutte WA, Barbier B, Arcones Segovia A, Rosenbauer H, Thiemann WH-P, Brack A, Greenberg JM (2002) Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 416:403–405 Pizzarello S (2007) The chemistry that preceded life’s origin: a study guide from meteorites. Chem Biodivers 4:680–693 Pizzarello S, Huang Y (2005) The deuterium enrichment of individual amino acids in carbonaceous meteorites: A case for the presolar distribution of biomolecules precursors. Geochim Cosmochim Acta 69:599–605 Pizzarello S, Huang Y, Becker L, Poreda RJ, Nieman RA, Cooper G, Williams M (2001) The organic content of the Tagish Lake meteorite. Science 293:2236–2239 Pizzarello S, Shock E (2010) The organic composition of carbonaceous meteorites: the evolutionary story ahead of biochemistry. Cold Spring Harb Perspect Biol 2:a002105 Pizzarello S, Zolensky M, Turk KA (2003) Non racemic isovaline in the Murchison meteorite: chiral distribution and mineral association. Geochim Cosmochim Acta 67:1589–1595 Ryder G (2003) Bombardment of the Hadean Earth: wholesome or deleterious? Astrobiology 3:3–6 Schmitt-Kopplin P, Gabelica Z, Gougeon RD, Fekete A, Kanawati B, Harir M, Gebefuegi I, Eckel G, Hertkorn N (2010) High molecular diversity of extraterrestrial organic matter in Murchison meteorite revealed 40 years after its fall. Proc Natl Acad Sci USA 107:2763–2768 Stoks PG, Schwartz AW (1982) Basic nitrogen-heterocyclic compounds in the Murchison meteorite. Geochim Cosmochim Acta 46:309–315 Yabuta H, William LB, Cody GD, Alexander CMO’D, Pizzarello S (2007) The insoluble carbonaceous material of CM chondrites: a possible source of discrete compounds under hydrothermal conditions. Meteorit Planet Sci 42:37–48
Extreme Environment FELIPE GO´MEZ Centro de Astrobiologı´a (CSIC-INTA), Instituto Nacional de Te´cnica Aeroespacial, Torrejo´n de Ardoz, Madrid, Spain
Synonyms Extreme field sites
Keywords Extremophiles
Definition An extreme ▶ environment is a habitat characterized by harsh environmental conditions, beyond the optimal range for the development of humans, for example, pH 2 or 11, 20 C or 113 C, saturating salt concentrations,
Extreme Environment
high radiation, 200 bars of pressure, among others. Basically, these are all inhospitable conditions for life. By definition, the organisms that are able to live in extreme environments are known as ▶ extremophiles. Not so long ago it was thought that life could not occur under extreme conditions., In the 1960s, Professor Thomas D. Brock, from Wisconsin-Madison University, isolated and described the first organisms from Yellowstone National Park, USA. This organism, Thermus aquaticus, is capable of growing at temperatures higher than 70 C. Its DNA polymerase has been widely applied in molecular biology as it is the base of the polymerase chain reaction (PCR) based on the thermophilic properties of the microorganism that produce it. Sulfolobus acidocaldarius, growing at temperatures higher than 85 C, was also isolated and characterized at the same time and was the first characterized hyperthermophilic archaea.
History In 1974, R. D. MacElroy published Some comments on the evolution of extremophiles (Biosystems 6: 74–75); this was the first time the term ▶ “Extremophile” was used.
Overview Depending on the extreme physicochemical conditions that characterize the extreme environments, they are classified as follows: Extreme temperature. Two types of extreme ecosystems can be described: cold and hot. Extremely cold environments are those with temperatures consistently below 5 C. They can be found in deep ocean niches, at the peaks of high mountains, or in the Polar Regions. Organisms living in extreme cold environments are known as psychrophiles. Some of them, such as the organisms found in Vostok Lake are able to live at 20 C. Extremely hot environments are characterized by regular temperatures higher than 45 C. These environments are typically influenced by geothermal activity as geysers and fumaroles of continental volcanic areas or deep-sea vents. Typical extreme hot environments can be found in the geothermal areas of Yellowstone, in some locations of Iceland or Kamchatka. The organisms to develop in these environments are thermophiles. Some microorganisms are able to develop at temperatures higher than 80 C, and they are called hyperthermophiles. These organisms are associated with hydrothermal activities. Extreme pH. Extreme environments can be classified as acidic or alkaline according to their pH. Extreme acidic environments are natural habitats in which the pH is below 5. Some examples of extreme acidic environments are Rı´o Tinto (Iberian Pyritic Belt, SW
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Spain), mean pH 2.3 or Iron Mountain in California (USA) where the pH in some areas is below 1. Organisms developing in these acidic environments are known as acidophiles. Extreme alkaline environments are those with a pH above 9. Examples of this type of environment are the soda lake of Magadi, Mono Lake or saltpans. Organisms found in these environments are called alkaliphiles. Extreme ionic strength. Hypersaline environments have an ionic concentration higher than of seawater, >3.5%. Typical hypersaline environments are the Dead Sea (Israel), the Great Salt Lake (USA), or the salterns of Santa Pola (Spain). Organisms able to grow at high ionic strength are known as halophiles. Most alkaliphilic organisms are also halophiles. Extreme pressure environments are those under extreme hydrostatic or litho pressure, such as aquatic habitats at depths of 2,000 m or more or deep-subsurface ecosystems. Organisms living under high pressure can be classified as barotolerant if they can tolerate high pressure or barophilic if they depend on pressure to grow. High-radiation environments are those habitats exposed to abnormally high radiation doses, including ultraviolet of infrared radiation, like deserts, the top of high mountains, or in the surface of ISS. Xeric environments are extreme dry habitats with seriously limited water, an extremely important element for life. Cold and hot deserts are some examples of these extreme environments. Oligotrophic environments are extreme ecosystems that offer low levels of nutrients to sustain life. Oligotrophic environments include deep oceanic sediments, polar ice, or the deep subsurface.
See also ▶ Acidophile ▶ Alkaliphile ▶ Compatible Solute ▶ Deep-Sea Microbiology ▶ Deep-Subsurface Microbiology ▶ Desiccation ▶ Endolithic ▶ Environment ▶ Extremophiles ▶ Halophile ▶ Hyperthermophile ▶ Piezophile ▶ Psychrophile ▶ Radiation Biology ▶ Solar UV Radiation (Biological Effects) ▶ Terrestrial Analog
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References and Further Reading
Keywords
Amaral-Zettler L, Go´mez F, Zettler E, Keenan BG, Amils R, Sogin M (2002) Eukaryotic diversity in Spain’s river of fire. Nature 417:137. doi:10.1038/ 417137ª Karl DM, Bird DF, Bjo¨rkman K, Houlihan T, Shackelford R, Tupas L (1999) Microorganisms in the Accreted Ice of Lake Vostok, Antarctica. Science 286(5447):2144–2147. doi:10.1126/science.286.5447.2144 Rothschild LJ, Mancinelli RL (2001) Life in extreme environments. Nature 409:1092–1101. doi:10.1038/35059215 Wharton DA (2002) Life at the limits: organisms in extreme environments. Cambridge University Press, Cambridge. ISBN 0521782120
Acidophile, alkaliphile, extremophile, halophile, hydrostatic pressure, pH, piezophile, psychrophile, salt concentration, temperature, thermophile
Extreme Field Sites ▶ Extreme Environment
Extreme Ultraviolet Light Synonyms EUV
Definition ▶ Ultraviolet light (UV) is electromagnetic radiation with ▶ wavelengths between 10 and 400 nm in the domain between X-rays and optical visible light. The UV spectrum is subdivided in different energy domains, from near UV (400–300 nm; 3.10–4.13 eV) to Extreme UV (121–10 nm; 10.2–124 eV).
See also ▶ Extreme Ultraviolet Light ▶ VUV ▶ Wavelength
Extremophiles
Definition Life is influenced by physical parameters such as temperature, pH, salinity, pressure, etc. Extremophiles are organisms that thrive in ▶ ecosystems where at least one physical parameter is close to the known limits of life with respect to this parameter. While some organisms may temporarily survive harsh conditions by forming resistant stages (spores) or through specific mechanisms (heavy metal resistance), true extremophiles require these conditions. For instance, ▶ hyperthermophiles successfully complete their life cycle at optimal temperatures above 80 C, and commonly do not grow at all below 60–70 C.
History Before 1965, the upper temperature limit known for life was about 73 C. When Thomas Brock discovered thermophilic bacterium, Thermus aquaticus in the Octopus Hot Spring in Yellowstone National Park (USA), the research on extremophiles truely began. Thermus aquaticus lives in temperatures ranging from 50 C to 80 C. Since then, many ▶ alkaliphiles, ▶ acidophiles, ▶ halophiles, and ▶ piezophiles have been discovered and studied for their taxonomy, ▶ phylogeny, physiology, biochemistry, molecular biology, genetics, and applications. According to Gerday and Glansdorff (2007), “For biologists, the realization that many forms of life are actually confined to environments that are severely hostile by human standards will remain one of the most significant achievements of the second half of the twentieth century.” But “the questions raised by the molecular basis and the emergence of different forms of extremophily are not only deep and varied, but, not surprisingly, many remain controversial.” At the beginning of the XXI century, research on extremophiles is still an open field.
Overview DANIEL PRIEUR Universite´ de Bretagne Occidentale (University of Western Britanny), Brest, France Institut Universitaire Europe´en de la Mer (IUEM), Technopoˆle Brest–Iroise, Plouzane´, France
Synonyms Extremophilic organisms
Living organisms exposed to severe environmental conditions are frequently named “extremophiles.” However, they belong to different categories. Some organisms resist hostile conditions (elevated temperatures, ▶ desiccation, etc.) by forming sophisticated resistant and dormant stages such as the spores produced by several Grampositive ▶ bacteria. Such forms can survive for very long periods (millions of years in the view of some authors) and then produce growing cells after environmental
Extremophiles
conditions become favorable. Other organisms can temporarily resist toxic compounds such as heavy metals, or ionizing radiation, through specific mechanisms. But these two categories do not require such hostile conditions for growth. A third category truly deserves the name extremophile: these organisms require environmental conditions hostile for most of the other organisms to complete their whole life cycle: very low temperatures for ▶ psychrophiles, very high temperature for ▶ thermophiles, very low pH for acidophiles, very high pH for alkaliphiles, elevated salt concentrations for halophiles, or elevated hydrostatic pressure for piezophiles.
Basic Methodology Since extremophiles thrive in ▶ environments hostile for many organisms and particularly human beings, the main difficulty encountered during the study of extremophiles is access to ▶ extreme environments. Whereas access to continental hot springs (acidic or alkaline), glaciers, salt mines, or evaporating ponds only requires specific clothing and caution, access to the ▶ deep-sea ▶ hydrothermal vents, where psychro-piezophiles or hyperthermophiles are found, is much more complex. In this case, sophisticated underwater vehicles (manned submersibles or remote operated vehicles-ROV) are needed. The next challenge after sampling is the preservation of samples for further cultivation in the laboratory. Many extremophiles are strictly anaerobic and require anaerobic reduced conditions during preservation at low temperature. Probably many obligate piezophiles do not survive sampling procedures since it is very difficult (and very expensive) to deploy pressure retaining samplers. Psychrophiles are very sensitive to moderate temperature (room temperature), and all the glassware and reagents used must be pre-cooled. Finally, cultivation of extremophiles requires mimicking the main environmental conditions of their environments in the laboratory. The difficulties of growing litho-autotrophic organisms even under non extreme conditions are great but the most difficult to grow are probably piezophiles which require high-pressure bioreactors.
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reservoirs or deep-sea hydrothermal vents. According to their natural environments, many of them combine thermophily with acidophily or piezophily. They have representatives in various metabolic types: they may be aerobes, microaerophiles or anaerobes, and chimoorganotrophs or chemo-litho-autotrophs. As a whole, they may use a variety of electron donors and acceptors. Many of them have very short generation times, about 30 min under optimal conditions. The most thermophilic organism is an Archaea, Pyrolobus fumarii, isolated from a deep-sea hydrothermal vent. P. fumarii, which grows in a temperature range of 90–113 C, with an optimum at 106 C. It can survive an exposure of 24 h in an autoclave. Growth at 121 C of an iron reducer organism, also isolated from a deep-sea hydrothermal vent, has been reported. However, the strain has not been published as a novel organism nor deposited in type culture collections, and this result is still to be confirmed. Thermophiles may also harbor viruses that live in the same environmental conditions, but their life cycle and relationships with their hosts are still unknown.
Psychrophiles Psychrophiles are cold-adapted organisms that grow optimally at temperatures around 15 C, and never grow beyond 20 C. Their minimum temperature for growth can be below 0 C. Organisms that can grow at low temperatures but also beyond 20 C are named psychrotolerant. The lower temperature limit for growth is not easy to determine, since very low temperatures (80 C in deep freezer, 196 C in liquid nitrogen) are used for long-term preservation of both prokaryotic and eukaryotic cells. However, growth of bacterial cells at temperatures as low as 10 C to 12 C, and metabolic activity at 20 C in concentrated brines have been reported. Psychrophiles inhabit the deep cold oceans, but also polar regions, high mountains, glaciers, etc. They almost certainly belong to many phylogenetic lineages, but most of the cultivated species belong to the Bacteria domain, and particularly the proteobacteria.
Halophiles
Key Research Findings Thermophiles Thermophiles are organisms that grow optimally above 60 C, and hyperthermophiles grow optimally above 80 C. The most thermophilic and hyperthermophilic organisms are Prokaryotes and belong to the Bacteria, but essentially ▶ Archaea domains. They have been isolated from diverse hot environments such as continental hot springs, oil
Halophiles are organisms living in salty environments such as the oceans. However, many environments are much more saline and salt concentrations can reach 340 g/l. Typical settings are salt lakes, solar salterns, underground deposits of rock salts, but also salted food products. Organisms living optimally in these highly salted environments are named extreme halophiles. Several salty environments also have a high pH, and a rather high temperature, and consequently their inhabitants are
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polyextremophilic. Extreme halophiles (growing in salt concentrations above 100 g/l) are found in the three domains of life, but most of them are Prokaryotes, and the most extreme belong to the Archaea domain. Two lineages of halobacteriales and certain methanogens are included. The halobacteriales are heterotrophic organisms that produce red pigments, some of which (bacteriorhodopsin, halorhodopsin) are involved in light-driven proton and chloride pumps.
Acidophiles Acidophilic organisms grow optimally at pH below 7, but there is a distinction between moderate acidophiles that grow at pH from 5 to 3, and extreme acidophiles that grow at pH below 3. Acidophiles are found in natural acidic springs in volcanic areas, but also in sulfide ore or coal deposits. Acidophiles exist in the three domains of life, and several yeasts, fungi, and protozoa grow at pH below 2. Among the Prokaryotes, acidophiles are very diversified from a metabolic point of view, and both heterotrophs and autotrophs are found. Many acidophiles oxidize sulfides and sulfur, using oxygen as an electron acceptor. Since ferrous iron is stable at low pH under aerobic conditions, many acidophiles are also iron oxidizers. The most acidophilic organism on Earth is the Archaea Picrophilus, which has an optimal pH of 0.7, and still grows at pH 0.
Alkaliphiles Alkaliphiles are commonly divided into two categories: the alkali-tolerant organisms can grow at alkaline pH (8–9), but their optimum is around neutral pH. True alkaliphiles can grow at pH above 10, and/or grow equally well or better (rate, yield) at pH above 9. Some are facultative ▶ alkaliphile; true alkaliphiles cannot grow at pH below 8. Alkaliphiles occur in natural high pH environments such as ▶ soda lakes, underground alkaline waters, and hidden small niches such as insect guts. They are also found in artificial environments such as waste of foodprocessing industries. They have representatives in the three domains of life, but are most abundant in several lineages of the Bacteria domain, particularly the low G + C Gram-positive bacteria of the genus Bacillus. Some alkalphiles also exhibit slight thermophilic properties.
Piezophiles Piezophiles were first named barophiles, from the Greek word for weight. Piezophile is more appropriate because it comes from the Greek word for pressure. Piezophiles thrive in environments exposed to elevated hydrostatic pressure. Piezotolerant, piezophilic, and
obligate piezophilic exist. The latter require pressures above 1 bar to grow. They are found in habitats such as deep oceans, deep aquifers, oil fields, and deep sediments. The first obligate piezophile was isolated from a dead crustacean amphipod collected at the Marianna Trench (10,470 m depth). It is an aerobic heterotrophic bacterium that grows optimally at 2 C (it is also a ▶ psychrophile), under 690 bars (or 69 Mpa) with a generation time of 25 h. When exposed to hydrostatic pressure, this organism loses its ability to form colonies and dies. Piezophilic characteristics are also known in some hyperthermophiles isolated from deep-sea hydrothermal vents. Pyrococcus yayanosii strain CH1 grows in a temperature range of 80–105 C, with an optimum at 98 C, under an optimal hydrostatic pressure of 52 Mpa. Its pressure range extends from 20 to at least 120 Mpa. At all temperatures, no growth was observed for pressures below 20 Mpa.
Applications Extremophiles are able to grow under extreme conditions of temperature, pH, salt concentration, and pressure. Consequently, the metabolic pathways and ▶ enzymes involved in these pathways are also extremophilic and are named extremozymes. Extremozymes have two general properties: they are active under extreme conditions, but are also very stable. For those reasons they represent a powerful tool for industrial biotransformations, carried out under harsh conditions. Most of them come from thermophilic and hyperthermophilic organisms. Many extremozymes are involved in starch processing: alpha- and beta-amylases, glucoamylases, alpha-glucosidase, pullulanase, Cgtases, branching enzymes, and amylomaltases. There are extremozymes that can be used for other transformations such as cellulose, xylan, chitin, pectin-degrading enzymes, proteolytic enzymes, lipases and esterases, alcohol dehydrogenases, glucose and arabinose isomerases, nitrile-degrading enzymes, etc. Modern DNA technology also utilizes extremozymes from thermophiles. The most famous example is the thermostable taq polymerase, from the thermophilic bacterium Thermus aquaticus. This polymerase permitted the revolution in DNS technology through the ▶ PCR (polymerase chain reaction) technology. Other enzymes such as ligases, nucleases, and topoisomerases are also commonly used. Extremophiles can also be used as whole-cells biocatalysts for biomining, decontamination or bioremediation, and hydrogen production. Finally, biomolecules extracted from extremophiles such as proteins and peptides, biopolymers, compatible solutes or lipids have a wide range of applications.
Extremophilic Organisms
Future Directions At the beginning of the twenty-first century, the world of extremophiles has revealed a variety of fascinating organisms, most of which are Prokaryotes. However, despite the thousands of papers that have been published on the subject, many questions remain. Extremophiles thrive at the limits of the life on Earth, but what those limits are precisely is not yet fully understood, particularly for elevated temperatures and pressures. Although it is not possible to imagine an organism more acidophilic than picrophilus (already at the low pH limit), it may be possible to find an organism that combines extreme acidophily with extreme thermophily and piezophily, or any other combination. To discover such pluri-extremophiles, exploration of planet Earth for new biotopes must be continued. It was recently reported that living microorganisms exist in very deep (1,600 m thickness) and old (110 million years) marine sediments. At this location, temperature was estimated to be only 80– 100 C. But the known temperature limit for life is 113 C (perhaps more) and because temperature increases by only 30 C per kilometer, life could occur at even greater depths. Such organisms may also represent novel lineages and may use novel metabolic pathways or novel enzymes. For each of them, the cellular and molecular mechanisms of adaptation remain to be understood and may open a variety of potential applications. By pushing the definition of the limits for life to ever greater extremes, biologists engaged in research on extremophiles will help develop novel hypotheses about the origin of life on Earth and its existence elsewhere.
See also ▶ Acidophile ▶ Alkaliphile ▶ Anaerobe ▶ Archea ▶ Bacteria ▶ Barophile ▶ Chemolithoautotroph ▶ Deep-Sea Microbiology ▶ Deep-Subsurface Microbiology ▶ Desiccation ▶ Ecosystem ▶ Enzyme
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▶ Eukarya ▶ Extreme Environment ▶ Halophile ▶ Hot Spring Microbiology ▶ Hot Vent Microbiology ▶ Hydrothermal Vent Origin of Life Models ▶ Hyperthermophile ▶ Lithotroph ▶ PCR ▶ Piezophile ▶ Prokaryote ▶ Psychrophile ▶ Soda Lakes ▶ Thermophile
References and Further Reading Corliss JB et al (1979) Submarine thermal springs on the galapagos rift. Science 203:1073–1083 Gerday C, Glansdorff N (2007) Physiology and biochemistry of extremophiles. ASM, Whashington, DC Kato C (1999) Barophiles (piezophiles). In: Horikoshi K, Tsujii K (eds) Extremophiles in deep-sea environments. Springer, Tokyo, pp 91–111 Lopez-Garcia P (2005) Extremophiles. In: Gargaud M, Barbier B, Martin H, Reisse J (eds) Lectures in astrobiology, vol II, Advances in astrobiology and biogeophysics. Springer, Berlin Heidelberg, pp 657–682 Prieur D, Marteinsson VT (1998) Prokaryotes living under elevated hydrostatic pressure. Adv Biochem Eng Biotechnol 61:23–35 Prieur D et al (2010) Piezophilic prokaryotes. In: Sebert Ph (ed) Comparative high pressure biology. Science Publishers, Enfield (NH), Jersey, Plymouth, pp 285–322 Roussel E et al (2008) Extending the sub sea-floor biosphere. Science 320:1046 Xiang Z et al (2009) Pyrococcus CH1, an obligate piezophilic hyperthermophile isolated from a deep-sea hydrothermal vent. The ISME Journal 1–4 Yayanos AA (1986) Evolutional and ecological implications of the properties of deep-sea barophilic bacteria. Proc Natl Acad USA 83:9542–9546 Zeng X, Birrien JL, Fouquet Y, Cherkashov G, Jebbar M, Querellou J, Oger P, Cambon-Bonavita MA, Xiao X, Prieur D (2009) Pyrococcus CH1, an obligate piezophilic hyperthermophile: extending the upper pressure-temperature limits for life. ISME J 3:873
Extremophilic Organisms ▶ Extremophiles
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Keywords Climate, greenhouse gases, solar mass loss, young Sun
Definition
Definition
A facula is a bright area on the icy ▶ satellites of ▶ Jupiter, ▶ Ganymede, ▶ Callisto, and Amalthea, and on ▶ Saturn’s satellite ▶ Titan. Faculae on Ganymede and Callisto are circular or elliptical and up to several hundred kilometers in diameter. Faculae are thought to have been created by impacts into the icy ▶ crusts of these two Jovian satellites, possibly with plastic or liquid material present in the subsurface. Titan shows two globally abundant surface units characterized by either bright or dark ▶ albedo. Faculae on this satellite are irregularly shaped, represent slivers or islands of bright terrain, are located within extensive areas of dark terrain, and are possibly of non-impact origin.
The apparent contradiction between model calculations that indicate a dimmer young Sun and consequently a colder climate on Earth on the one hand, and the geological evidence that the early climates on Earth and Mars were mild and liquid water was abundantly present on the other hand.
See also ▶ Albedo Feature ▶ Callisto ▶ Crater, Impact ▶ Crust ▶ Ganymede ▶ Impact Basin ▶ Jupiter ▶ Macula, Maculae ▶ Satellite or Moon ▶ Saturn ▶ Titan
Faint Young Sun Paradox MANUEL GU¨DEL Department of Astronomy, University of Vienna, Vienna, Austria
Overview There is clear evidence for a mild climate on the young Earth and Mars, allowing liquid water – a prerequisite for the formation of life as we know it – to exist on the surface of both planets at ages of a few 100 Myr to 1 Gyr. Liquid water seems to have subsequently disappeared from the surface of Mars. However, detailed stellar evolutionary theory applied to the Sun during its main-sequence life indicates that it was fainter by about 30% when it arrived on the main sequence 4.6 billion years ago, and the radiative output increased only slowly in the next billion years. Both planetary surfaces would thus have been completely frozen. To solve this apparent paradox, various hypotheses have been proposed although a conclusive answer is still outstanding. The most popular theory assumes higher admixtures of greenhouse gases in the atmospheres of Earth and Mars; such gases include carbon monoxide, ammonia, and methane (CO2, NH3, and CH4, respectively). A radically different hypothesis posits that the young Sun was not faint – it may rather have been brighter because it was more massive by a few percent. Direct evidence for the implied strong mass loss in the younger epochs of solar evolution is still outstanding. Further hypotheses assume a lower albedo due to less efficient cloud formation, for example, as a consequence of more efficient suppression of cosmic rays in the young solar system, or a smaller fractional area of land, or the lack of biologically induced cloud condensation nuclei.
Basic Methodology Synonyms Faint young sun problem; Weak sun paradox; Weak young sun paradox
The average, effective equilibrium temperature of a planetary surface in the absence of an atmosphere follows from the energy balance between optical/near-infrared emission
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
Faint Young Sun Paradox
irradiating the planet and mid-infrared thermal radiation that is lost to space. The incoming power from the Sun is: Pin ¼ pR S ð1 AÞ 2
where S = 1,366 W m2 is the solar constant (solar radiative energy flux at the distance of the Earth), A 0.29 is the Earth’s Bond albedo (fraction of radiation that is reflected), and R = 6,378 km is the Earth’s radius. The power leaving from the Earth’s atmosphere can be approximated as radiation from a blackbody surface with temperature T, Pout ¼ 4pR2 esT 4 where s = 5.67 108 W m2 K4 is the Stefan– Boltzmann constant, and e is the surface mid-infrared emissivity. Equating Pin = Pout leads to, sT 4 ¼ S ð1 AÞ=ð4eÞ With the above numbers and e 0.9 for solid rock (Sagan and Mullen 1972), T 260 K. For a planet surrounded by an atmosphere, the effective temperature can be raised owing to the presence of greenhouse gases. The latter are nearly transparent to incident optical and near-infrared light but strongly absorb reemitted thermal radiation in the mid-infrared. Heat flow from the interior of the Earth is, in contrast, negligibly small (although it matters for maintaining plate tectonics). Considering the greenhouse effect for a present-day atmosphere leads to a somewhat elevated average temperature of 288 K (15 C), in agreement with measurements (e.g., Sagan and Mullen 1972; Kasting and Catling 2003). The total radiative output of the young Sun can be assessed from two sources: (1) The theory of stellar evolution indicates that the young Sun at a time when it started core hydrogen burning on the main sequence emitted 30% less electromagnetic radiation than at present (e.g., Sackmann and Boothroyd 2003). (2) The total luminosity of stars with known masses and ages can in principle be derived from observations; stellar masses and ages are, however, difficult to infer accurately. Our knowledge of the early climate history of the Earth and other planets, in particular Mars, rely on geological evidence: (1) oxygen isotopes measured in Jack Hills zircons dated at 4.4 to 4.3 Ga, which suggested that parent rocks interacted with liquid water; (2) 3.8 Ga old sedimentary rocks from Isua (West Greenland) clearly deposited in aquatic environments; and (3) extensive outflow channels and valley networks in the Martian highlands. All of these pieces of evidence indicate mild climates and the presence of liquid water on both young planets (Valley et al. 2002).
Key Research Findings Assuming the same atmospheric composition for the Earth as today but a solar constant reduced by about 30%, calculations yield average atmospheric temperatures of about 260 K for the young Earth around 4 billion years ago. The temperature would have remained below the freezing point until 2 billion years ago (Sagan and Mullen 1972; Kasting and Catling 2003; Fig. 1). This clearly contradicts geological evidence for a significantly warmer early climate on Earth (e.g., Kasting and Toon 1989), and especially also on Mars. This contradiction is the essence of the Faint Young Sun Paradox (FYSP). Various hypotheses have been put forward addressing the FYSP but the solution remains inconclusive. We summarize the key points of the proposed solutions below.
Different Atmospheric Composition: Greenhouses The composition of the young atmospheres may have been different, allowing for much stronger greenhouses on Earth or Mars. Many authors have favored this hypothesis, although it faces its own problems. Larger amounts of CO2 would strongly support the greenhouse together with water vapor (Kasting 1993). The required CO2 level to keep liquid water oceans would be self-regulated by the carbonate-silicate cycle (e.g., Walker et al. 1981; 300
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Faint Young Sun Paradox. Fig. 1 Illustration of the Faint Young Sun Paradox for the Earth, in the context of the atmospheric greenhouse. The solid line depicts the solar luminosity relative to the present value (right y-axis); the lower dashed curve is the effective temperature of the Earth without atmosphere, the upper dashed curve shows the calculated, mean global surface temperature affected by the greenhouse (with fixed CO2 mixing ratio and relative humidity; from Kasting and Catling 2003, reprinted, with permission, from the Annual Review of Astronomy and Astrophysics, Volume 41 2003 by Annual Reviews, www.annualreviews.org). (Courtesy of J. Kasting; credit: Princeton University Press.)
Faint Young Sun Paradox
Kasting and Catling 2003). Increased CO2 levels would at the same time also prevent massive losses of N2 under the influence of an enhanced early solar wind and enhanced extreme-ultraviolet radiation (Lichtenegger et al. 2010). There is, however, various geochemical and geological evidence against the required very massive CO2 atmospheres, such as the absence of siderite in paleosols (Rye et al. 1995; Rosing et al. 2010, and references therein); a dense Martian CO2 atmosphere would significantly enhance the Martian albedo, thus not raising the temperature sufficiently (Kasting 1991). Although these same clouds may also backscatter thermal radiation and therefore support the greenhouse mechanism (Forget and Pierrehumbert 1997), experiments suggest that this effect is too small to raise the temperatures above the freezing point (Glandorf et al. 2002). However, for a period around 2–2.5 Gyr ago, recent calculations indicate that a lower CO2 content of 2.9 Mb partial pressure would have been sufficient to keep liquid water on the Earth’s surface, and this pressure is in agreement with geological findings (von Paris et al. 2008). Another leading candidate among greenhouse gases is ammonia (NH3) which would be required in only relatively small amounts. However, NH3 dissociates rapidly if subject to solar UV radiation (Kuhn and Atreya 1979), but additional methane (CH4) may produce a high-altitude haze of organic solids through photolysis, which shields ammonia sufficiently from UV dissociation (Sagan and Chyba 1997). Methane itself is an efficient greenhouse gas (Kasting 1997; Pavlov et al. 2000). But the mixing ratio of CH4:CO2 should stay below unity to avoid increased albedo due to haze formation (Haqq-Misra et al. 2008), in turn requiring too high levels of CO2 for a significant greenhouse. Ethane (C2H6) and carbonyl sulfide (OCS) have been proposed as alternative efficient greenhouse gases in the young terrestrial atmosphere (Haqq-Misra et al. 2008; Ueno et al. 2009).
Cloud Formation, Land Coverage, and Albedo A much lower albedo, A, would allow T to rise. This explanation was initially deemed unlikely because the probably larger surface of the young Earth covered by ice would rather increase A (Sagan and Mullen 1972). The albedo could, however, have been lower due to a lower cloud coverage (Rossow et al. 1982; Charlson et al. 1987; Rosing et al. 2010). There is some evidence that elevated cosmic ray fluxes have a cooling effect on the Earth’s atmosphere because they ionize tropospheric layers, and charged ion clusters lead to condensation nuclei that form reflective clouds (Shaviv 2003).
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But because the ionized wind of the young Sun is thought to have been much stronger than today, the cosmic-ray flux reaching the inner solar system was more strongly suppressed, therefore suppressing cloud formation in the young Earth’s atmosphere, and in turn leading to a warmer climate (Shaviv 2003). A lower surface albedo may also be the result of the suggested smaller continental area in early epochs; furthermore, the absence of cloud condensation nuclei induced biologically (Charlson et al. 1987) decreases the albedo further. Considering these effects, a clement climate was inferred for the young Earth without the need for high amounts of greenhouse gases (Rosing et al. 2010).
A Brighter Young Sun A radical remedy of the FYSP would be a young Sun that was significantly more massive than at present, losing the excess mass during its main-sequence life in an enhanced ionized wind (Whitmire et al. 1995; Sackmann and Boothroyd 2003). The bolometric luminosity (i.e., the luminosity integrated over the electromagnetic spectrum) of main-sequence stars scales approximately with stellar mass to the third power; further, the radius of the Earth’s orbit would have been smaller for a more massive Sun (the orbital radius scales inversely with the solar mass). Both effects combine to a scaling of the received flux with the fifth power of the solar mass. Constraints on this model come from the requirement that the young Martian atmosphere was warm enough to maintain water in liquid form, but the Earth’s atmospheric temperature was moderate enough to prevent loss of the water oceans in a runaway greenhouse. The appropriate mass range for the young Sun was thus 1.03 M to 1.07 M. Corresponding solar models are in acceptable agreement with helioseismology results (Sackmann and Boothroyd 2003). The observational evidence is presently unclear. Upper limits to the ionized-wind mass-loss rates deduced from radio observations of young solar analogs are marginally compatible with the “Bright Young Sun” requirement, indicating a maximum Zero-Age Main-Sequence (ZAMS) solar mass of 1.06 M (Gaidos et al. 2000). Indirect observations suggest a smaller mass loss. If the solar wind had been constant throughout the Sun’s mainsequence life, then, including the losses from conversion of matter to energy in the hydrogen-burning core, a total mass loss of only 0.0004 M would have been achieved, which alters the solar radiative output insignificantly. However, observations of “hydrogen walls” in the astrospheres of neighboring solar-like stars support an increased wind-mass loss in younger stars (Wood et al. 2005).
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A power-law trend extrapolated back to the ZAMS indicates a total mass loss of order 0.01 M, but some evidence shows that the mass-loss rate was suppressed during the youngest epochs; including this latter effect leads to a total mass loss of only 0.003 M (Minton and Malhotra 2007), again too little for a significant luminosity effect.
See also ▶ Earth, Formation and Early Evolution ▶ Earth’s Atmosphere, Origin and Evolution of ▶ Precambrian Oceans, Temperature of ▶ Sun (and Young Sun)
References and Further Reading Charlson RJ, Lovelock JE, Andreae MO, Warren SG (1987) Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326:655–661 Forget F, Pierrehumbert RT (1997) Warming early Mars with carbon dioxide clouds that scatter infrared radiation. Science 278:1273–1276 Gaidos EJ, Gu¨del M, Blake GA (2000) The faint young Sun paradox: an observational test of an alternative solar model. Geophys Res Lett 27:501–503 Glandorf DL, Colaprete A, Tolbert MA, Toon OB (2002) CO2 snow on Mars and early Earth: experimental constraints. Icarus 160:66–72 Haqq-Misra JD, Domagal-Goldman SD, Kasting PJ, Kasting JF (2008) A revised, hazy methane greenhouse for the Archean Earth. Astrobiol 8:1127–1137 Kasting JF (1991) CO2 condensation and the climate of early Mars. Icarus 94:1–13 Kasting JF (1993) Earth’s early atmosphere. Science 259:920–926 Kasting JF (1997) Warming early Earth and Mars. Science 276:1213–1215 Kasting JF, Catling D (2003) Evolution of a habitable planet. Annu Rev Astron Astrophys 41:429–463 Kasting JF, Toon OB (1989) Climate evolution on the terrestrial planets. In: Atreya SK, Pollack JB, Matthews MS (eds) Origin and evolution of planetary and satellite atmospheres. University of Arizona Press, Tucson, pp 423–449 Kuhn WR, Atreya SK (1979) Ammonia photolysis and the greenhouse effect in the primordial atmosphere of the earth. Icarus 37:207–213 Lichtenegger HIM, Lammer H, Grießmeier J-M, Kulikov YN, von Paris P, Hausleitner W, Krauss S, Rauer H (2010) Aeronomical evidence for higher CO2 levels during Earth’s Hadean epoch. Icarus 210:1–7 Minton DA, Malhotra R (2007) Assessing the massive young Sun hypothesis to solve the warm young Earth puzzle. Astrophys J 660:1700–1706 Pavlov AA, Kasting JF, Brown LL, Rages KA, Freedman R (2000) Greenhouse warming by CH4 in the atmosphere of early Earth. J Geophys Res 105:11981–11990 Rosing MT, Bird DK, Sleep NH, Bjerrum CJ (2010) No climate paradox under the faint early Sun. Nature 464:744–747 Rossow WB, Henderson-Sellers A, Weinreich SK (1982) Cloud feedback: a stabilizing effect for the early Earth? Science 217:1245–1247 Rye R, Kuo PH, Holland HD (1995) Atmospheric carbon dioxide concentrations before 2.2 billion years ago. Nature 378:603–605
Sackmann I-J, Boothroyd AI (2003) Our Sun. V. A bright young Sun consistent with helioseismology and warm temperatures on ancient Earth and Mars. Astrophys J 583:1024–1039 Sagan C, Chyba C (1997) The early faint sun paradox: organic shielding of ultraviolet-labile greenhouse gases. Science 276:1217–1221 Sagan C, Mullen G (1972) Earth and Mars: evolution of atmospheres and surface temperatures. Science 177:52–56 Shaviv NJ (2003) Toward a solution to the early faint Sun paradox: a lower cosmic ray flux from a stronger solar wind. J Geophys Res 108(3):1–8 Ueno Y, Johnson MS, Danielache SO, Eskebjerg C, Pandey A, Yoshida N (2009) Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox. Proc Natl Acad Sci USA 106:14784–14789 Valley JW, Peck WH, King EM, Wilde SA (2002) A cool early Earth. Geology 30:351–354 Von Paris P, Rauer H, Greenfell JL, Patzer B, Hedelt P, Stracke B, Trautmann T, Schreier F (2008) Warming the early earth – CO2 reconsidered. Planet Space Sci 45:1254–1259 Walker JCG, Hays PB, Kasting JF (1981) A negative feedback mechanism for the long-term stabilization of the Earth’s surface temperature. J Geophys Res 86:9776–9782 Whitmire DP, Doyle LR, Reynolds RT, Matese JJ (1995) A slightly more massive young Sun as an explanation for warm temperatures on early Mars. J Geophys Res 100:5457–5464 Wood BE, Mu¨ller H-R, Zank GP, Linsky JL, Redfield S (2005) New massloss measurements from Astrospheric Lya absorption. Astrophys J Lett 628:L143–L146
Faint Young Sun Problem ▶ Faint Young Sun Paradox
Farbstreifen Sandwatt ▶ Microbial Mats
Far-Infrared (FAR IR) ▶ Infrared Astronomy
Fatty Acids ▶ Carboxylic Acids, Geological Record of ▶ Fatty Acids, Geological Record of
Fatty Acids, Geological Record of
Fatty Acids, Geological Record of JENNIFER EIGENBRODE NASA Goddard Space Flight Center, Greenbelt, MD, USA
Synonyms Aliphatic carboxylic acids; Alkanoic acids; Fatty acids
Keywords Carboxylic acids, cellular membranes, lipids, molecular fossils, organic acids
Definition Fatty acids are ▶ carboxylic acids with an aliphatic tail (chain) that may be branched, saturated, or unsaturated (i.e., contain double bonds). Fatty acids typically have C12-C36 chain lengths, though chains of only four carbons are considered “fatty.” Fatty acids are constituents in a variety of biomolecules and are released by the hydrolysis of the ester linkages. Fatty acids are important components of cellular ▶ membranes of ▶ bacteria and eukaryotes, regulating both fluidity and permeability, and are key energy stores. Fatty acids have also been observed in carbonaceous meteorites.
Overview In sediments and rocks, fatty acids may exist as free fatty acids (i.e., hydrolyzed form) or bound into larger biomolecules or geomolecules (as in macromolecular ▶ kerogen or bound to minerals). Fatty acids have varying specificity for particular phylogenetic groups of bacteria and eukaryotes but also environmental conditions. Most natural fatty acids have an even number of carbon atoms because they are biosynthesized by the multiple additions of C2-acetyl units via the acetyl-CoA coenzyme. Organisms also biosynthesize a limited range of chain lengths and have varying degrees of unsaturation (particularly plants, algae, and bacteria). Biologically-produced stereoisomeric configurations of unsaturated fatty acids are all cis (The latin term “cis” describes the orientation of functional groups, on the same side, within a molecule; the opposite term is “trans,” meaning “on the other side” or “across”). All of these features are regarded as general biosignatures and are not observed in abiological fatty acids, such as those observed in meteorites or in synthetics. Fatty acid abundance diminishes during early diagenesis primarily due to in situ microbial reworking.
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Preferential loss of unsaturated and short chain moieties is commonly observed. However, in the presence of other refractory materials, fatty acids will be incorporated into geomolecules that enhance preservation. Saturated, o-hydroxyl, and a,o-diacids are more resistant to diagenesis and have been observed in the ancient geological record. A variety of reactions can lead to the loss of carbon from the alkyl chain, which during later diagenesis leads to molecular patterns having greater relative abundance of odd-carbon chain lengths. For example, decarboxylation of even-carbon chain length fatty acids leads to odd-carbon alkane chains. Further, thermal maturation and carbon loss of alkanes leads to a diminishing preference for odd or even chain lengths. Thus, in the rock record, molecular distributions of fatty acids and alkanes can indicate biological sources and relative degree of diagenesis. In some cases, fatty acids from microbes living in rocks or petroleum reservoirs can complicate molecular interpretations and are important records of recent processes.
See also ▶ Acid Hydrolysis ▶ Bacteria ▶ Biomarkers ▶ Carbonaceous Chondrites (Organic Chemistry of ) ▶ Carbonyl ▶ Carboxylic Acid ▶ Carboxylic Acids, Geological Record of ▶ Cell wall ▶ Complex Organic Molecules ▶ Dicarboxylic Acid ▶ Eukarya ▶ Hydroxy Acid ▶ Kerogen ▶ Membrane ▶ Molecular Fossils
References and Further Reading Eigenbrode JL (2007) Fossil lipids for life-detection: a case study from the early Earth record. Space Sci Rev 135:161–185 Killops S, Killops V (2005) Introduction to organic geochemistry, 2nd edn. Blackwell Publishing, Oxford, p 393 Meyers PA, Leenheer MJ, Bourbonniere RA (1995) Diagenesis of vascular plant organic matter components during burial in lake sediments. Aquat Geochem 1:35–52 Petsch ST, Edwards KJ, Eglinton TI (2003) Abundance, distribution and d13C analysis of microbial phospholipid-derived fatty acids in a black shale weathering profile. Org Geochem 34:731–743 Petsch ST, Edwards KJ, Eglinton TI (2005) Microbial degradation of sedimentary organic matter. Palaeogeogr Palaeoclimatol Palaeoecol 219:157–170
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Petsch ST, Eglinton TI, Edwards KJ (2001) 14C-dead living biomass: evidence for microbial assimilation of ancient organic carbon during shale weathering. Science 292:1127–1131 Sephton MA (2005) Organic matter in carbonaceous meteorites: past, present and future research. Philos Trans R Soc A 363: 2729–2742 Summons RE, Albrecht P, McDonald G, Moldowan JM (2007) Molecular biosignatures: generic qualities of organic compounds that betray biological origins. Space Sci Rev 135:133–157
Definition Fennoscandia is the geographic term that covers the Scandinavian countries, Finland, and the northwesternmost part of Russia. The geologic term Fennoscandian shield is for the Precambrian formations of this area. The Precambrian of Fennoscandia consists of Archean and Proterozoic rocks, this description dealing with the former.
Overview
Feeding Zone Definition A planet’s feeding zone is the original radial distribution of material (dust, planetesimals and planetary embryos) from which the planet was formed in a ▶ protoplanetary disk. Given that radial temperature gradients are thought to translate into compositional variations in the disk, a planet’s feeding zone has consequences for its composition. A narrow feeding zone implies that the planet’s composition should represent material condensed within a small range in temperatures, and a wide feeding zone implies significant radial mixing during accretion, as is required for water/volatile delivery to the terrestrial planets.
See also ▶ Condensation Sequence ▶ Planetesimals ▶ Protoplanetary Disk ▶ Water, Delivery to Earth
Fennoscandia PENTTI HO¨LTTA¨ Department of Geosciences and Geography, University of Helsinki, Finland Geological Survey of Finland, Espoo, Finland
Synonyms Fennoscandian shield
Keywords Archean, fennoscandia, greenstone belt, TTG
Archean rocks comprise much of the eastern and northern parts of the Fennoscandian shield. They have been divided into three major provinces: the Karelian Province in the west, the Belomorian Province in the east, and the Kola province in the north. Neoarchean and Mesoarchean (3.2–2.7 Ga) lithologies prevail, but a high proportion fall in a relatively restricted range, ca. 2.84–2.67 Ga. Older gneisses of Paleoarchean age (ca. 3.5 Ga) have been found in a small area in the western part of the Karelian Province, and Mesoarchean 3.2–2.9 Ga rocks occur in restricted areas in the western and eastern parts of the Karelian Province. A ▶ tonalite–trondhjemite–granodiorite (TTG) association with subordinate ▶ greenstone belts, paragneisses, granulite complexes, and migmatitic amphibolites dominates the Archean terranes. The TTG magmatism mainly occurred between 2.83 and 2.74 Ga and was followed by a brief period of sanukitoid magmatism between 2.73 and 2.70 Ga. Many TTG have adakitic geochemical signatures. Sanukitoids originate from melting of a metasomatized mantle source, probably as a result of a slab break-off following a continental collision (Halla et al. 2009). The youngest 2.71–2.69 Ga granites are thought to result from melting of the crust during collisional processes. The youngest Archean igneous rocks are 2.67–2.61 Ga anorogenic alkaline granitoids that occur in the Keivy terrane in Kola (Zozulya et al. 2005), the 2.61 Ga Siilinja¨rvi carbonatite close to the western border of the Karelian Province, and some 2.61 Ga mafic dykes in the eastern part of the Karelian Province (Slabunov et al. 2006). The oldest, 3.10–2.90 Ga, volcanic rocks are found in greenstone belts in the eastern and northwestern parts of the Karelian Province. Most greenstone complexes are 2.88–2.78 Ga old, often showing an assembly of plume-related komatiites and tholeiites, island arc-type calc-alkaline volcanic rocks, as well as ▶ metasediments and ▶ banded iron formations. Neoarchean 2.72 Ga eclogites metamorphosed at 14–17 kbars are known in a few areas in the Belomorian Province (Volodichev et al. 2004). The Seriak and Iringora greenstone belts in the Belomorian Province have ophiolite-like features
Fermentation
(Shchipansky et al. 2004). Greenstone belts in the central parts of the Karelian Province are younger (2.75–2.73 Ga) than in its western and eastern parts.
See also ▶ Amphibolite Facies ▶ Archean Eon ▶ Banded Iron Formation ▶ Craton ▶ Earth, Formation and Early Evolution ▶ Greenstone Belts ▶ Igneous Rock ▶ Metamorphic Rock ▶ Metasediments ▶ Ophiolite ▶ Shield ▶ Tonalite–Trondhjemite–Granodiorite
References and Further Reading Halla J, van Hunen J, Heilimo E, Ho¨ltta¨ P (2009) Geochemical and numerical constraints on Neoarchean plate tectonics. Precambrian Res 174:155–162 Shchipansky AA, Samsonov AV, Bibikova EV, Babarina II, Konilov AN, Krylov KA, Slabunov AI, Bogina MM (2004) 2.8 Ga boninite-hosting partial suprasubduction ophiolite sequences from the North Karelian greenstone belt, NE Baltic Shield, Russia. In: Kusky T (ed) Precambrian ophiolites and related rocks. Elsevier, Amsterdam, pp 425–487 Slabunov AI, Lobach-Zhuchenko SB, Bibikova EV, Sorjonen-Ward P, Balagansky VV, Volodichev OI, Shchipansky AA, Svetov SA, Chekulaev VP, Arestova NA, Stepanov VS (2006) The archaean nucleus of the Baltic/Fennoscandian Shield. In: Gee DG, Stephenson RA (eds) European lithosphere dynamics. Geological Society of London, Memoir 32, pp 627–644 Volodichev OI, Slabunov AI, Bibikova EV, Konilov AN, Kuzenko T (2004) Archaean eclogites in the Belomorian mobile belt, Baltic shield. Petrology 2:540–560 Zozulya DR, Bayanova TB, Eby GN (2005) Geology and age of the late archean keivy alkaline province, Northeastern Baltic Shield. J Geol 113:601–608
Fennoscandian Shield ▶ Fennoscandia
Ferment (obsolete) ▶ Enzyme
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Fermentation JULI PERETO´ Cavanilles Institute for Biodiversity and Evolutionary Biology and Department of Biochemistry and Molecular Biology, University of Vale`ncia, Vale`ncia, Spain
Definition Fermentation is an ▶ anaerobic ▶ catabolism of a reduced carbon source (e.g., glucose) to generate ▶ ATP within a strict internal ▶ oxidation-▶ reduction balance. In many cases, the same substrate is used both as reductant and oxidant. The hallmark of fermentation is the accumulation of partially oxidized end products. Only a part of the chemical energy stored in the initial substrate is conserved during the synthesis of ATP, usually by a mechanism of substrate level phosphorylation. Nevertheless, there are some examples of the involvement of an electrochemical ion gradient in the synthesis of ATP (e.g., citrate fermentation).
History Antoine Laurent Lavoisier (1743–1794) collected the first quantitative data on alcoholic fermentation. Louis Pasteur (1822–1895) studied in depth the fermentation by intact living cells and defined this physiological process as “life without oxygen”. In 1897, Eduard Buchner (1860–1917) showed that fermentation could occur in cell-free extracts, inaugurating the genuine biochemical in vitro approach to living phenomena. Parallel studies on yeast alcoholic fermentation and anaerobic muscle glycolysis during the first half of the twentieth century contributed to the development of biochemistry as a science (Barnett 2003).
Overview Many microorganisms, obligate or facultative anaerobic, are able to degrade extracellular polymers (e.g., polysaccharides, proteins, nucleic acids) and use the monomers (e.g., hexoses, pentoses, amino acids, purines, pyrimidines) as fermentable substrates. There are other substrates of fermentations such as organic acids (e.g., citrate, succinate, or malonate) or even aromatic hydrocarbons. In this latter case, fermentation appears as a metabolic task of several microbial species working together. Regarding amino acid fermentations, many anaerobic bacteria can catabolize single amino acids but grow better with amino acid mixtures (the Stickland reaction). In this case, some amino acids of the mixture act as reductants whereas others are the oxidants.
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The stoichiometric yield of ATP in a fermentation depends on the particular pathway used and can range from less than one up to 4 mol of ATP per mol of fermentable substrate. Microorganisms are capable of generating a wide array of end products during fermentation, including carbon dioxide, ethanol, lactate, butyrate, acetate, and propionate. In comparison to respiration (i.e., electron transport chain–dependent processes), fermentations are less energetically efficient because a lot of potential chemical energy is still retained in most of the end products. Thus, to compensate this relatively low-energy yield, large amounts of fermentable substrate are used and most carbon from this can be recovered in the form of end products. Other anaerobic bacteria use the excreted end products of fermentations, building an anaerobic food chain with methanogens at the bottom. From a biotechnological point of view, however, since very ancient times, fermentations are widely used for the processing of food products, such as yogurt, cheese, beer, wine, or bread. In a few cases, some ATP synthesis during fermentation is associated with the dissipation of an electrochemical potential gradient either of protons or sodium ions. These ion gradients can be generated by electron transport (e.g., fumarate reduction in Propionibacterium), membrane decarboxylases (e.g., sodium-pumping succinate decarboxylase of Propionigenium modestum), or electrogenic substrate translocation through membranes (e.g., lactic acid bacteria like Lactococcus cremoris). When compared to the other energy-generating systems (such as respiration and photosynthesis), fermentation seems simpler at the structural and enzymatic levels. For this reason, in 1924, Oparin (Oparin 1924) postulated that fermentation was the earliest metabolic mode.
See also ▶ Anaerobe ▶ ATP ▶ Catabolism ▶ Chemoorganotroph ▶ Embden-Meyerhof-Parnas Pathway ▶ Glycolysis ▶ Origin of Life ▶ Oxidation ▶ Reduction
References and Further Reading Barnett JA (2003) A history of research on yeasts 5: the fermentation pathway. Yeast 20:509–543 Kim BH, Gadd GM (2008) Bacterial physiology and metabolism. Cambridge University Press, Cambridge, Chap. 8
Oparin AI (1924) Proiskhozhedenie Zhizni. Moscow: Mosckovskii Rabochii (Reprinted and translated in Bernal JD (1967) The origin of life. London: Weidenfeld and Nicolson) White D (1995) The physiology and biochemistry of prokaryotes. Oxford University Press, New York, Chap 13
Fermi Paradox NIKOS PRANTZOS Institut d’Astrophysique de Paris, Paris, France
Keywords Extraterrestrial civilizations
Definition The Fermi paradox, attributed to Italian physicist Enrico Fermi, concerns the apparent contradiction between the lack of any evidence for the presence of extraterrestrials on Earth and the view that extraterrestrial civilizations should be rather common in the Galaxy.
History Fermi formulated his paradox in 1950, during a casual conversation in Los Alamos Laboratory with E. Teller and colleagues, with the famous phrase “where are they?” (or “where is everybody?”). His point was that, if there are many extraterrestrial civilizations, Earth should have been visited by one or more of them, long ago and many times over. The discussion went completely unnoticed for many years. The phrase “where are they?”, attributed to Fermi but without comments, is first encountered in a paper published in 1963 by American astronomer Carl Sagan. Sagan referred to this problem as “Fermi’s paradox” after American astronomer Michael Hart independently rediscovered Fermi’s arguments in 1975.
Overview As all paradoxes, the Fermi paradox is better understood if its premises are explicitly stated: (a) There are many extraterrestrial civilizations in the Milky Way. (b) Our civilization is a typical one (not the first to have appeared, neither the most technologically advanced, nor the only one seeking to explore the cosmos and communicate with others). (c) Interstellar travel is not too difficult for civilizations slightly more advanced than ours.
Fischer–Tropsch Effects on Isotopic Fractionation
(d) Galactic colonization (either by some of those civilizations or their self-replicating robots) can be achieved in less than 108 years, i.e. less than 1% of the age of the Galaxy. If hypotheses (a) to (d) are valid, the conclusion “they should be here” is logically deduced and the Fermi paradox applies. It ceases to apply if one or more of its premises are refuted. Supporters of ETI (Extra-terrestrial Intelligence) reject one (or more) of the assumptions (b) to (d), in order to save the key hypothesis (a). In contrast, opponents of ETI uphold the plausibility of (c) and (d), while rejecting (b) and even (a). For instance, British astronomer Fred Hoyle thought that interstellar distances make interstellar travel impossible. However, arguments most often discussed refer to the sociological rather than the physical aspects of the problem. Fermi believed that a technological civilization would be too short-lived, destroying itself before mastering interstellar travel. Long before him, the Russian savant and father of astronautics Konstantin Tsiolkovsky preferred the so-called “zoo hypothesis” (reformulated independently in 1973 by American astronomer John Ball), according to which advanced civilizations observe us from afar without interfering, for various reasons (e.g. waiting for us to gain “maturity” before joining the “cosmic club”). All sociological arguments share a common weak point: It is hard to believe that any one of them applies to every single civilization in the Galaxy. Moreover, in such a case, assumption (b) would be implicitly violated since we would be the only civilization seeking communication with others. The most “economic” solution to Fermi’s paradox consists in straightforwardly rejecting hypothesis (a): Technologically advanced civilizations are rare in the Galaxy (and, perhaps, we are alone).
See also ▶ Drake Equation ▶ Galactic Habitable Zone
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devised by Emil Fischer in 1891. Fischer projections depict the stereochemistry of molecules, and are thus useful for depicting enantiomers of ▶ chiral molecules, especially monosaccharides. The carbon backbone of the molecule is depicted as a vertical line and the bonds of side chains are represented as horizontal lines, with carbon atoms represented by the center of crossing lines. The orientation of the carbon chain is such that the C1 carbon is at the top of the molecule diagrammed. Vertical lines lie either in or behind the plane of the paper, while horizontal lines project out from the surface. In a Fischer projection, the plane of symmetry (3D) between two enantiomers becomes a line of symmetry between their 2D representations.
See also ▶ Carbohydrate ▶ Chirality
Fischer–Tropsch Effects on Isotopic Fractionation DANIELE L. PINTI GEOTOP & De´partement des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada
Synonyms Isotopic fractionation, fischer–tropsch effect
Keywords ▶ Carbon isotopes, ▶ hydrothermal environments, isotopic biomarkers, metal-catalyzed reactions
References and Further Reading
Definition
Prantzos N (2000) Our cosmic future – humanity’s fate in the universe. Cambridge University Press, Cambridge Webb S (2002) Where is everybody? Fifty solutions to the Fermi paradox. Copernicus Books – Praxis, Chichester)
The Fischer–Tropsch-type reaction is a method for the synthesis of hydrocarbons and some aliphatic compounds in the presence of a metal catalyst. The carbon isotopic ratios of produced hydrocarbons (methane and short-chain hydrocarbons) and byproducts such as carbon dioxide show large fractionation relative to the reactants. Abiotically created organic products are depleted in 13C to a degree typically ascribed to biological processes, indicating that carbon isotopic composition may not be a particularly effective discriminant between biologic and non-biologic sources.
Fischer Projection Definition In chemistry, a Fischer projection is a two-dimensional representation of a three-dimensional organic molecule
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Fischer-Tropsch-Type Reaction
Overview ▶ Fischer–Tropsch-type reactions (FTT hereafter) can produce various light and heavy hydrocarbons, oxygencontaining compounds (alcohol, carboxylic acids), and basic N-compounds, including amino acids (Hayatsu and Anders 1981). It is now widely accepted that at mantle and crustal temperatures and pressures and in presence of natural metal catalysts such as iron, FTT can produce carbonaceous matter. Of particular interest for astrobiologists are FTT reactions in the seafloor hydrothermal environments, this being one of the possible environments where Hadean or Archean life could have originated and evolved. The carbon isotopic compositions of the carbonaceous products of FTT reactions display various extents of isotopic fractionation relative to the reactant. The majority of products are depleted in 13C, with d13C values (where d13C = [(13C/12C)sample/(13C/12C)standard 1] and standard is Pee Dee Belemnite) that show from 2 to 60‰ depletion compared to the reactant. Several investigators (e.g., Lancet and Anders 1970; Kerridge et al. 1989) synthesized hydrocarbons from CO and H2 at 102–500 C in the presence of metal catalysts (Fe, Co, Ni, magnetite, etc.) and proposed that such a process could explain the origin of carbonaceous and graphitic matter in meteorites. Their results indicated that short-chain hydrocarbons (C1–C4) were usually more depleted in 13C than the reactant CO. However, the carbon isotopic signature of experimentally produced methane (CH4) can vary significantly, from very depleted values of 60‰ to values even heavier than the reactant CO (Lancet and Anders 1970). Horita and Berndt (1999) conducted a series of experiments to investigate isotopic fractionation during hydrothermal production of CH4 from CO2 and H2 in the presence of Ni–Fe alloy. These experiments simulate the production of abiotic methane and other organic compounds during ▶ serpentinization. The d13C values of abiogenic CH4 produced from dissolved CO2 with a mantle-like 13C signature varied from 4 to 50‰, overlapping typical d13C values of microbial CH4. McCollom and Seewald (2006) produced H2, CO2, CH4, and light hydrocarbons (C2–C5) by heating an aqueous solution of formic acid (HCOOH) in the presence of Fe powder at 250 C and 325 bar in a hydrothermal cell. The d13C of the dissolved CO2 was 14‰ and values for methane and light hydrocarbons were as low as 49‰. Again these results show that organic products are depleted in 13C to a degree typically ascribed to biological processes, indicating that the carbon isotopic composition may not be an effective diagnostic means to differentiate
between biologic and non-biologic sources (McCollom and Seewald 2006).
See also ▶ Biomarkers ▶ Biomarkers, Isotopic ▶ Carbon Isotopes as a Geochemical Tracer ▶ Fischer-Tropsch-Type Reaction ▶ Hydrothermal Environments ▶ Serpentinization
References and Further Reading Hayatsu R, Anders E (1981) Organic compounds in meteorites and their origins. Top Curr Chem 99:1–37 Horita J (2005) Some perspectives on isotope biosignatures for early life. Chem Geol 218:171–186 Horita J, Berndt ME (1999) Abiogenic methane formation and isotopic fractionation under hydrothermal conditions. Science 285:1055–1057 Kerridge JF, Mariner R, Flores J, Chang S (1989) Isotopic characteristics of simulated meteoritic organic matter: 1. Kerogen-like material. Orig Life Evol Biosph 19:561–572 Lancet MS, Anders EA (1970) Carbon isotope fractionation in the Fischer–Tropsch synthesis and in meteorites. Science 170:980–982 McCollom TM, Seewald JS (2006) Carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions. Earth Planet Sci Lett 243:74–84
Fischer-Tropsch-Type Reaction NATASHA M. JOHNSON, JOSEPH A. NUTH, III NASA Goddard Space Flight Center, Greenbelt, MD, USA
Keywords Catalyst, hydrocarbons, hydrogenation, macromolecular organics, methanation, organics
Definition Fischer-Tropsch-type (FTT) synthesis produces hydrocarbons by hydrogenating carbon monoxide via surfacemediated reactions as shown in reaction (1). nCO þ ð2n þ 1ÞH2 þ catalyst ! Cn H2nþ2 þ nH2 O þ catalyst
ð1Þ
History Fischer-Tropsch-type synthesis is a common industrial process developed by coal researchers Franz Fischer (1877–1947) and Hans Tropsch (1889–1935) for obtaining
Fischer-Tropsch-Type Reaction
gasoline and other liquid fuels from coal by indirect catalytic hydrogenation of carbon oxides (Hindermann et al. 1993). FTT chemistry was a natural extension of the pioneering work of Sabatier and Senderens (1902), who synthesized ▶ methane by passing a mixture of ▶ hydrogen and CO/CO2 over a reduced nickel catalyst. It is important to remember that formation of methane (CH4) alone is not the ▶ Fischer–Tropsch reaction but simply methanation. FTT synthesis was widely used in Germany during World War II to manufacture organic fuels in the form of gasoline, diesel oil, liquefiable gases, and paraffin wax. Early FTT experimentation used nickel and then iron catalysts. However, iron suffered from excessive carbon deposition leading to blockage of the ▶ catalyst pores and deactivation (Pearce et al. 1989). As a result, modern FTT plants typically use precious metals, cobalt, or nickel dispersed on substrates (alumina, silica, etc.). In actuality, many of these “fresh” catalytic surfaces are oxides, and it is these metal oxides that have industrial catalytic properties.
Overview The origins of organics in the early Solar System are complex and still somewhat poorly understood (Ehrenfreund and Charnley 2000). However, FischerTropsch-type (FTT) catalytic reduction of CO by hydrogen to produce methane and other ▶ hydrocarbons has long been recognized as an important potential source of organic material in the ▶ Solar Nebula, since the “basic ingredients” of this reaction (H2, CO, and plausible catalysts) were ubiquitous in this environment (Kress and Tielens 2001). Hayatsu and Anders (1981) compared the results of a wide range of experimental studies of FTT reactions to detailed analyses of the organic residue extracted from ▶ carbonaceous chondrites. They reported that a significant fraction of the longer-chain hydrocarbons and more complex aromatic compounds in meteorites could be explained by FTT chemistry. However, there are some notable exceptions. In particular, ▶ amino acids are not produced efficiently enough or in the proportions and compositional distributions observed in meteorites via FTT reactions. This is in contrast to the ▶ Miller– ▶ Urey synthesis, which has been successful in generating many of the amino acids associated with living systems and carbonaceous chondrites. Experimental studies of FTT reactions have used pulverized iron–nickel meteorites, powdered carbonaceous chondritic meteorites, and synthetic substratesupported nickel–iron particles as the active catalyst (Hayatsu and Anders 1981; Llorca and Casanova 2000).
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Astrochemical experiments designed to measure the efficiency of FTT reactions (Hong and Fegley 1998; Fegley 1999) also used metallic iron (or iron oxide) as the catalyst. These materials are relevant in that they were surely present in the primitive nebula. However, the bulk form of iron meteorites is not conducive to efficient catalysis, nor is there evidence that iron–nickel particles that might have been found in the nebula would have been analogous to powdered iron meteorites. In addition to the abovementioned materials, preliminary experiments (Ferrante et al. 2000) demonstrated that iron silicate grain analogues—even those containing 10% total iron—are effective FTT catalysts. Amorphous iron silicates provide reasonably good surfaces to promote the reaction of H2, N2, and CO to organic materials resembling those found in meteorites (Hill and Nuth 2003; Gilmour et al. 2002). Although not as efficient, amorphous magnesium silicates, bronzite, and a range of other natural materials also promote the formation of organic materials from H2 and CO. Fortunately, the amorphous metal-silicate grains or “smokes” are straightforward to produce in the lab and serve as good early Solar Nebula catalyst analogues. In addition to producing volatile organics via the reaction of H2 and CO on these surfaces, a major reaction product appears to be a macromolecular organic coating on the grain surfaces. However, there is no significant decrease in the reaction rate of volatiles even when this coating comprises up to 10% by mass of the starting material (Gilmour et al. 2002). This implies that the organic coating formed as one of the initial products on any grain surface may also act as an additional effective catalytic surface for the conversion of H2, N2, and CO into organic materials. These coatings are composed of macromolecular organic phases (Johnson et al. 2004; Johnson et al. 2007). These experiments also showed that as the grains became coated, Haber–Bosch type reactions (catalytic reduction of N2 by hydrogen to make ammonia) took place resulting in nitrogen-bearing organics (Hill and Nuth 2003). These types of FTT reactions are an effective means to produce complex hydrocarbons. Organics generated by this technique could represent the carbonaceous material incorporated in comets and meteorites.
Basic Methodology The FTT catalytic sequence, as with other forms of catalysis, is rather complex and involves several related reactions on the catalyst surface. A typical FTT experiment involves circulating gases (typically CO and H2) through and/or over a catalyst at a specific temperature and then
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measuring the resulting gases (such as water, methane, and carbon dioxide) and residue on the grains as reaction time progresses. As the grains became coated with residue, organics are produced faster and in greater quantities and CO is quickly depleted. The reaction eventually reaches a plateau. Once this point is reached, the gases are removed and the system refilled to be run again.
Key Research Findings Dust grains falling into a protostellar system can provide surfaces that promote the reaction of H2, N2, and CO into both volatile organics and a macromolecular coating that continues to promote the formation of organic materials. Although the reaction is most efficient in the innermost regions of the nebula, this does not pose significant problems as the reaction products as well as the coated grains can migrate back out to the far reaches of the nebula, thus seeding the entire nebula with the organic building blocks of life (Nuth et al. 2008).
Applications In respect to Astrobiology, the application of the FTT process to the early Solar System is as described in the overview. The FTT catalytic process also has numerous applications in industry but an in-depth review of such applications is beyond the scope of this article.
Further Directions This is an active area of study and current research is probing the implications of these types of reactions for Astrobiology and the early Solar System.
See also ▶ Fischer–Tropsch Effects on Isotopic Fractionation ▶ Miller, Stanley ▶ Protoplanetary Disk, Chemistry ▶ Solar Nebula ▶ Urey’s Conception of Origins of Life
References and Further Reading Ehrenfreund P, Charnley SB (2000) Organic molecules in the interstellar medium, comets, and meteorites: a voyage from dark clouds to the early Earth. Annu Rev Astron Astr 38:427–483 Fegley B (1999) Chemical and physical processing of presolar materials in the Solar Nebula and the implications for preservation of presolar materials in comets. Space Sci Rev 90:239–252 Ferrante RF, Moore MH, Nuth JA, Smith T (2000) Laboratory studies of catalysis of CO to organics on grain analogs. Icarus 145:297–300 Gilmour I, Hill HGM, Pearson VK, Sephton MA, Nuth JA (2002) Production of high molecular weight organic compounds on the surfaces of amorphous iron silicate catalysts: implications for organic synthesis in the Solar Nebula. LPSC 33:1613
Hayatsu R, Anders E (1981) Organic-compounds in meteorites and their origins. Top Curr Chem 99:1–37 Hill HGM, Nuth JA (2003) The catalytic potential of cosmic dust: Implications for prebiotic chemistry in the solar nebula and other protoplanetary systems. Astrobiology 3(2):291–304 Hindermann JP, Hutchings GJ, Kienneman A (1993) Mechanistic aspects of the formation of hydro- carbons and alcohols from CO hydrogenation. Catal Rev Sci Eng 35:1–127 Hong Y, Fegley B (1998) Experimental studies of magnetite formation in the Solar Nebula. Meteorit Planet Sci 33:1101–1112 Johnson NM, Cody GD, Nuth JA (2004) Organics on Fe–silicate grains: potential mimicry of meteoritic processes? LPSC 35:1876 Johnson NM, Steiner ME, Nuth JA (2007) Fischer–Tropsch reactions and implications for protostellar systems. LPSC 38:2183 Kress ME, Tielens AGGM (2001) The role of Fischer–Tropsch catalysis in Solar Nebula chemistry. Meteorit Planet Sci 36:75–91 Llorca J, Casanova I (2000) Reaction between H2, CO, and H2S over Fe, Ni metal in the Solar Nebula: experimental evidence for the formation of sulfur-bearing organic molecules and sulfides. Meteorit Planet Sci 35:841–848 Nuth JA, Johnson NM, Manning S (2008) A self-perpetuating catalyst for the production of complex organic molecules in protostellar nebulae. Astrophys J Lett 673(2):L225–L228 Pearce BB, Twigg MV, Woodward C (1989) In: Twigg MW (ed) Catalyst handbook, 2nd edn. Wolfe, London, pp 340–378 Sabatier P, Senderens JB (1902) Direct hydrogenation of oxides of carbon in presence of various finely divided metals (in French). CR Acad Sci 134:689–691
Fitness SUSANNA C. MANRUBIA Laboratory of Molecular Evolution, Centro de Astrobiologı´a (INTA-CSIC), Torrejon de Ardoz, Madrid, Spain
Keywords Environment, natural selection, phenotype, reproduction, survival
Definition Fitness is a central concept in evolutionary biology. It usually refers to the average capacity of an organism (as described by its ▶ genotype) to produce viable progeny. The genotypes of fitter organisms become more abundant through the action of ▶ natural selection along subsequent generations. Fitness is however, a relative concept since the same genotype, which is the unique heritable material, might express different ▶ phenotypes in different environments. In turn, the ability of a phenotype (upon which natural selection acts) to reproduce successfully depends on the ▶ environment. The quantification
Flow Reactor
of the relationship between genotype and fitness remains an open problem.
Overview Fitness is commonly defined as the ability of an organism to pass its genes to the next generation. However, many different organismal and environmental traits are involved in the successful completion of this process, and the mathematical form of fitness is therefore difficult to cast. Often, the fitness of a single trait is instead addressed, that is, the advantages conferred by that trait to the individuals carrying it. Actually, the first quantification of fitness measured the relative advantage of an organism with a mutant trait (or genotype) with respect to the non-mutated original type (Haldane 1924). Still, the fitness value of a trait does not depend only on how it performs in comparison to other genotypes, as the case of the mutant allele causing sickle-cell anemia illustrates. In a normal environment, homozygous individuals may experience severe blood disorders. However, the disease confers certain resistance to malaria, enough for heterozygous individuals to be at an advantage in environments where malaria is common, thus enhancing the prevalence of sickle-cell anemia in those regions. The definition of fitness as the number of offspring of an organism has been challenged since the beginning of evolutionary theory. Already Fisher discussed the case of two individuals having the same number of offspring, the first one breeding only female offspring and the second one breeding half female and half male offspring. Though they are equally fit in terms of the definition above, the progeny of one of them cannot reproduce, and thus that lineage would go extinct if left on its own (Fisher 1930). Hamilton extended the idea of fitness as number of offspring (or number of genes that are passed to the next generation) to include the number of genes an individual can contribute to the next generation by helping kin relatives to reproduce. This is called inclusive fitness (Hamilton 1964). Several issues have prevented the reaching of a consensus definition for fitness. They include whether fitness is better represented as a short-term or as a longterm quantity, the role played by chance and causality in the fate of populations, whether it should be applied to species, organisms, or genotypes, and how to consider its dependence on the environment where organisms reproduce and compete with each other (Byerly and Michod 1991; Abrams 2009). As a result, fitness is used with different meanings in different situations, some of them being individual fitness, absolute fitness, and relative fitness (Orr 2009).
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See also ▶ Adaptation ▶ Environment ▶ Evolution (Biological) ▶ Genotype ▶ Natural Selection ▶ Phenotype ▶ Selection ▶ Survival
References and Further Reading Abrams M (2009) What determines biological fitness? The problem of the reference environment. Synthese 166:21–40 Byerly HC, Michod RE (1991) Fitness and evolutionary explanation. Biol phylosophy 6:1–22 Fisher RA (1930) The genetical theory of natural selection. Clarendon, Oxford Haldane JBS (1924) A mathematical theory of natural and artificial selection. Trans Cambridge Philos Soc 23:19–41 Hamilton WD (1964) The Genetical Evolution of Social Behaviour I and II. J Theor Biol 7:1–16, and 17–52 Orr EA (2009) Fitness and its role in evolutionary genetics. Nat Rev Genet 10:531–539
Flint ▶ Chert
Flood Basalt ▶ Trapps
Flow Reactor Synonyms Plug-flow reactor
Definition A flow reactor is a temperature and pressure-controlled reaction vessel with opposite inlet and outlet ends for isothermal exposure of aqueous or gaseous samples under laminar flow conditions. These systems consist of a narrow-bore reactor with an in-line pump and backpressure regulator and provide an alternative to batchtype reactors. Residence time is equivalent to the ratio of
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Fluid Inclusions Heating coil Effluent
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Pump
Flow reactor (>100⬚C)
Cooling stage
P-reg
Flow Reactor. Figure 1 Temperature-controlled flow reactor system schematic
V ) total reactor volume to the volumetric flow rate (tr ¼ ! V and variable exposures are achieved by adjusting flow rate within the laminar flow regime. Flow reactors are commonly used to investigate aqueous chemical reactivity in high-temperature, high-pressure systems and are commonly utilized in Astrobiology for investigation into prebiotic organic synthetic pathways that might be possible at submarine hydrothermal systems (SHSs). High temperature systems ( 100 C) include a rapid cooling stage before ambient sample collection (Fig. 1) to minimize hysteresis effects. Materials for high-temperature flow reactors include corrosion-resistant metals such as highnickel steel alloys or gold-plated surfaces.
See also ▶ Fischer-Tropsch-Type Reaction ▶ Strecker Synthesis
Fluid Inclusions PASCAL PHILIPPOT Equipe Ge´obiosphe`re Actuelle et Primitive, Institut de Physique du Globe de Paris (IPGP), Paris, France
Synonyms Fluid-bearing micro cavities; Trapped fluids
Keywords Composition, fluid inclusion, fluid–rock interaction, pressure, temperature
Definition A fluid phase present in the rock porosity or filling fractures that is trapped in microscopic cavities during mineral growth or fracture healing. Inclusion fluids can be of sedimentary, metamorphic, or magmatic origin. They can be thought of as time capsules storing information about ancient temperatures, pressures, and fluid compositions
relevant at the time of trapping. The composition of most fluids falls in the C-O-H-S-N (H2O, CO2, CH4, H2S, N2)+ NaCl system, with NaCl representing the salt content of the fluid (expressed in weight% NaCl equivalent).
Overview When crystals grow or recrystallize in a fluid medium of any kind, growth irregularities of many sorts trap microscopic portions of the ambient fluid in the solid crystal. These irregularities are called fluid inclusions. The sealing off of such irregularities may occur during the growth of the surrounding crystal, yielding primary fluid inclusions, or by recrystallization along fractures at some later time, yielding secondary inclusions. Fluid inclusions lining mineral growth zones are the best criteria for a primary origin. Secondary inclusions occur generally along planes cutting across several mineral grains. Fluid inclusions in rock samples are seldom of a unique origin. The presence of fluid inclusions in a rock is the rule rather than the exception. Indeed, the rock sample that contains no inclusions that are visible at high magnification (>600) is rare, and many minerals such as milky quartz may contain up to 109 fluid inclusions per cubic centimeter. Fluid inclusions are present in all types of rocks, being sedimentary, metamorphic, or igneous in origin. They have been traced down to mantle depths and were found in ultrahigh-pressure minerals such as diamond. They occur in rocks as old as 4,000 million years and have been identified in lunar and meteoric samples. Because fluid inclusions are almost ubiquitous in geological samples, their study is applicable to a variety of geological problems and areas. Fluid inclusions have played a central role in oil and ore exploration research and in evaluating the safety of sites for both nuclear reactors and atomic waste repositories. Gas inclusions in polar iced sheets have permitted reconstruction of CO2 concentrations of the recent atmosphere, fluid inclusions in speleothems (cave deposits) have provided data on paleotemperatures during the last 350,000 years, and inclusions in samples 3,500 million years old may provide geochemical constraints on the composition of the Earth’s primitive atmosphere. In mantle and high-pressure rocks, fluid inclusions have helped clarify the nature and role of volatiles in a variety of geodynamic processes including element recycling into the deep Earth, explosive volcanism at convergent margins, and the seismic behavior of the subducting plate. In metamorphic and sedimentary terranes, fluid inclusions have proven useful tools for containing the burial and exhumation history of crustal rocks and the pressure and temperature changes attending uplift and erosion of continental masses.
Fluid Inclusions
Fluid inclusions can be thought of as time capsules storing information about ancient temperatures, pressures, and fluid compositions relevant at the time of trapping. The basic assumption in all fluid inclusion studies is that the volume (density) and composition of the trapped fluid has not changed since formation of the inclusion or that, if an inclusion did leak, evidence for fluid loss is observable in the sample (decrepitation texture, microfracture, ...). With the exception of magmatic rocks that can contain glass inclusions, that is, solidified melt inclusions, most rocks contain liquid and/or gas inclusions. At room temperature, a general distinction can be made between liquid and gaseous inclusions. Both types are often found in the same rock. This might indicate immiscibility between two phases at the time of trapping. Miscibility is a function of both the salt content of the aqueous phase and the gas composition. One or more solid phases can also be present. Solid phases can be either accidentally trapped particles that were present in suspension when the mineral grew or represent newly formed mineral that crystallized out of the fluids trapped in inclusions after sealing. In this case, the solid phase is called a daughter mineral. Under the optical microscope, most fluid inclusions have a rather sharp outer boundary marking the edge of the inclusion cavity. This is because of a significant difference in refractive index between inclusion fluids and their mineral hosts: most aqueous fluids have refractive indices between 1.33 and 1.45 whereas the minerals in which they are included have refractive indices from 1.43 to as high as 3.22. Hydrocarbon liquids have refractive indices that may be similar to their mineral hosts and thus are not all easily visible. Given appropriate fluid inclusions, many aspects of fluid composition can be determined. The composition of most fluids falls in the C-O-H-S (H2O, CO2, CH4, H2S) +NaCl system, with NaCl representing the salt content of the fluid (expressed in weight% NaCl content) and accounting for KCl, CaCl2, and other chlorides. Some fluid inclusions can also contain significant amounts of nitrogen (N2). Bromine and fluorine can be important volatiles in some systems but their abundance (as HBr and HF) are generally very low. In sedimentary rocks, oil inclusions can be present. The nature and concentrations of a large variety of major and trace elements can be obtained by crushing and leaching the inclusion fluids out of small samples containing a large number of inclusions or in situ, within a single inclusion. Information on the isotopic composition of light elements (H, C, N, O, S, and some noble gases) of the fluid can also be obtained. If the composition of the inclusion can be determined and the liquid–vapor curve for that fluid is known, then
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the density can be determined from the temperature of homogenization of the vapor and liquid phases to one fluid phase. If the P-V-T properties of that fluid are known, then a line of constant density or molar volume is defined in a P-T space. This line, called an isochore, represents the range of P-T conditions over which a fluid of that density was trapped. If one can make an independent estimate of pressure or temperature at the time of trapping of the fluid in an inclusion, then both variables (P and T) can be uniquely determined using the isochore of the fluid. If both aqueous and gaseous inclusions are coexisting in the same healed fracture or fluid inclusion cluster, and the specific criteria for immiscibility between these two phases at the time of trapping can be ascertained precisely, then the intersection point between the two isochors in a P-T space corresponds to the temperature and pressure of trapping of the inclusion fluids.
History and Basic Methodology The presence of fluids in rocks has been noticed since at least the eleventh century, but the first fluid inclusion constituents (H2O, CO2, petroleum) were identified not before the eighteenth and nineteenth centuries (Dolomieu 1792; Sorby 1858). Sorby (1858) was the first to highlight a relation between metamorphism and filling of fluid inclusions. He suggested that the gas–liquid phases in fluid inclusions reflected thermally contracted fluids. He proposed that reheating inclusions would lead to disappearance of the bubble and that the temperature at which this occurred could serve as an estimate of the temperature of mineral formation. During the twentieth century, fluid inclusions were the subject of intense studies and debates by geologists from Russia, the USA, and Europe. The development and improvement of the equipments for quantitative analysis of fluid inclusions during the late 1960s and early 1970s, paved the way to a vast number of geological applications using fluid inclusions as a petrological tool. Especially, a cooling–heating stage was developed (Poty et al. 1976), which expanded to low temperatures the temperature range for easily and accurately measuring phase transition in fluid inclusions. This technique known as “microthermometry” is indispensable for the analysis of single fluid inclusions and opened the possibility for the determination of fluid salinities and densities, and the consequent interpretation on the rock-forming conditions. Numerous analytical methods were developed since then for measuring fluid chemical and isotopic compositions. Among these, the introduction of laser Raman spectroscopy around 1980 (Dubessy et al. 1982; Pasteris et al. 1988), and synchrotron radiation micro-x-ray fluorescence (Frantz et al. 1988;
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Vanko et al. 1993; Philippot et al. 1998), Laser AblationInductively Coupled Plasma-Mass Spectrometry (Heinrich et al. 2003) and Laser Microprobe Noble Gas Mass Spectrometry (Bohlke and Irwin 1992) around 1990 allowed quantitative analysis of anhydrous gas species (CO2, N2, CH4, H2S, and from then on H2O), major and trace elements, as well as noble gases in individual fluid inclusions. Much of this work has been reviewed in the treatise on fluid inclusions published by Roedder (1984). Some of the important concepts and technical achievements performed during this period are synthesized in Hollister and Crawford (1981) and Goldstein and Reynolds (1994).
Fluid Inclusions in Meteorites and Early Archean Sediments The discovery of water-bearing fluid inclusions in 4.5 billion years old halite (NaCl) crystals preserved in the matrix of the Monahans and Zag H chondrite regolith breccias (Zolensky et al. 1999) provided important constraints on the fluid composition and P-T conditions in the near-surface environment of the parent bodies. Minimum formation temperature of the inclusions has been estimated to be less than 100 C and could have been as low as about 0.01 C. The minimum pressure needed to stabilize a NaCl-saturated brine is 0.0754 MPa at 100 C and 3104 MPa at 0.01 C. These conditions are consistent with the alteration having occurred in the shallow subsurface of the parent bodies. Moreover, the P-T conditions inferred for the formation of these inclusions is within the range of temperatures in which life has been observed to exist and thrive on Earth. Tiny fluid inclusions have also been found in two Martian meteorites (one in Nakhla, NSNM 5891-3, and one in ALH 84001, 146) (Bodnar 2000) and carbonaceous chondrites (Saylor et al. 2001). Although the fluid inclusions in the Nakhla sample occur as healed fracture and therefore are of secondary origin, the inclusions in the ALH 84001, 146 sample do not occur along fractures and therefore could be of primary origin, that is, trapped during growth of the enclosing pyroxene. The fluid in the inclusions may represent the magmatic fluid that exsolved from the crystallizing melt as the igneous rock formed on Mars. As such, the inclusions can provide valuable information about degassing early in Mars history. Optical behavior of these inclusions suggests that they contain liquid and vapor carbon dioxide. Insights into the composition of the primitive terrestrial ocean and hydrothermal fluids (Cl/Br ratio, sulfur, and metal components) have been obtained by direct analysis of fluid inclusions preserved in poorly metamorphosed Archean sediments of the Kaapvaal Craton (South Africa) and Pilbara Craton (Western Australia).
Early studies performed in 3.2 Gyr-old (Channer et al. 1997; De Ronde et al. 1997) (but see Lowe and Byerly (2003) for a controversy on the origin of the samples considered) and paleoproterozoic 2.2 Gyr-old (Gutzmer et al. 2003) indicated that the Cl/Br ratio of the fluid trapped in the inclusions was below present-day seawater and resulted from mantle buffering, that is, hydrothermal in origin. However, these studies were based on bulk fluid analyses (i.e., crush-leach), which can result in fluid mixing if several fluid generations occur in a single sample. More recently, chemical analysis of individual fluid inclusions have been performed in 3.5 Gyr-old samples from the Pilbara Craton, Western Australia, by Synchrotron Radiation X-ray micro-Fluorescence (SR-XRF), thus allowing independent analysis of different fluids trapped in the same sample (Foriel et al. 2004). Individual fluid inclusion analysis using SR-XRF revealed the presence of three main fluid populations: a metal-depleted fluid, a Barich and S-depleted fluid, and a Fe–S-rich end-member. The Cl/Br ratio of metal-depleted fluid inclusions (630) is similar to the modern seawater value (649) and was thought to represent a relic testimony of the primitive ocean. By contrast, Ba- and Fe-rich brines have Cl/Br ratios (350 and 390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. Model composition of “seawater component” has been evaluated to be 1,100 mM Na, 2,250 mM Cl, and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt% salt equivalent. This high Cl concentration (ca. four times the present-day value) together with the occurrence of Cl/Br ratio similar to the modern seawater value (649) indicates that the Archean ocean formed on a typical modern-day seawater evaporation trend.
References and Further Reading Bodnar RJ (1983) A method of calculating fluid inclusion volumes based on vapor bubble diameters and P-V-T-X properties of inclusion fluids. Econ Geol 78:535–542 Bodnar RJ (2000) Fluid inclusions in ALH 84001 and other Martian meteorites: evidence for volatiles on Mars. Lunar Planet Sci 30:1222 Bohlke JK, Irwin JJ (1992) Laser microprobe analyses of noble gas isotopes and halogens in fluid inclusions: analyses of microstandards and synthetic inclusions in quartz. Geochim Cosmochim Acta 56:187–201 Channer DM de R, de Ronde CEJ et al (1997) The Cl, Br, I composition of f3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry. Earth Planet. Sci Lett 150:325–335 De Ronde EJ, Channer DMD, Faure K, Bray CJ, Spooner ETC (1997) Fluid chemistry of Archean seafloor hydrothermal vents: implications for the composition of circa 3.2 Ga seawater. Geochim Cosmochim Acta 61:4025–4042 Dolomieu, Commandeur De´odat, de (1792) Sur l’huile de pe´trole dans le cristal de roche et les fluides e´lastiques tire´s du quartz. Obs Phys 42:318–319
Fluorescent Emission Dubessy J, Audeoud D, Winkins R, Kosztolanyi C (1982) The use of the Raman microprobe MOLE in the determination of the electrolytes dissolved in the aqueous phase of fluid inclusions. Chem Geol 37:137–150 Foriel J, Philippot P, Rey P, Somogyi A, Banks D, Me´nez B (2004) Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia. Earth Planet Sci Lett 228:451–463 Frantz JD, Mao HK, Zhang YG, Wu Y, Thompson AC, Underwood JH, Giauque RD, Jones KW, Rivers ML (1988) Analysis of fluid inclusions by x-ray fluorescence using synchrotron radiation. Chem Geol 69:235–244 Goldstein RH, Reynolds TJ (1994) Systematics of fluid inclusions in diagenetic minerals, vol 31, Society for Sedimentary Geology, Short Course. SEPM, Tulsa, 199 pp Gutzmer J, Banks DA, Luders V, Hoefs J, Beukes NJ, Von Bezing KL (2003) Ancient sub-seafloor alteration of basaltic andesites of the Ongeluk Formation, South Africa: implications for the chemistry of Paleoproterozoic seawater. Chem Geol 201:37–53 Heinrich CA, Pettke T, Halter WE, Aigner-torres M, Aude´tat A, Gunther D, Hattendorf B (2003) Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively coupled-plasma mass-spectrometry. Geochim Cosmochim Acta 67(18):3473–3496 Hollister LS, Crawford ML (1981) Short course in fluid inclusions: applications to petrology. Mineral Association of Canada, Special Publication, 304 pp Lowe DR, Byerly GR (2003) Ironstone pods in the Archean Barberton Greenstone Belt, South Africa: Earth’s oldest seafloor hydrothermal vents reinterpreted as Quaternary subaerial springs. Geology 31:909–912 Pasteris JD, Wopenka B, Seitz JC (1988) Practical aspects of quantitative laser Raman microprobe spectroscopy for the study of fluid inclusions. Geochim Cosmochim Acta 52:979–988 Philippot P, Me´nez B, Chevalier P, Gibert F, Legrand F, Populus P (1998) Absorption correction procedures for quantitative analysis of fluid inclusions using synchrotron radiation x-ray fluorescence. Chem Geol 144:121–136 Poty B, Leroy J, Weisbrod A (1976) A new device for measuring temperatures under the microscope: the Chaixmeca microthermometry apparatus. Bull Soc Franc Mineral Cristallogr 99:182–186 Roedder E (1984) Fluid inclusions. Mineralogical Society of America, Washington, DC, 644 pp Saylor J, Zolensky M, Bodnar R, Le L, Schwandt C (2001) Fluid inclusions in carbonaceous chondrites. Lunar Planet Sci 32:1885 Sorby HC (1858) On the microscopic structure of crystals, indicating the origin of minerals and rocks. Q J Geol Soc Lond 14:453–500 Vanko DA, Sutton SR, Rivers ML, Bodnar RJ (1993) Major-element ratios in synthetic fluid inclusions by synchrotron x-ray fluorescence microprobe. Chem Geol 109:125–134 Zolensky ME, Bodnar RJ, Gibson EK Jr, Nyquist LE, Reese Y, Shih CY, Wiesmann H (1999) Asteroidal water within fluid inclusion-bearing halite in an H5 chondrite, Monahans (1998). Science 285:1377–1379
Fluid-Bearing Micro Cavities ▶ Fluid Inclusions
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Fluorescence Synonyms Fluorescence spectroscopy; Spectrofluorometry
Fluorescent
emission;
Definition When an electron in a substance is excited to a higher energy state by a photon, it may return to the lower energy state via series of two or more intermediate energy states, assuming they exist. The photons which are reemitted have a lower energy than the exciting photon and are consequently of a lower frequency and longer wavelength. This phenomenon is known as fluorescence. In some instances, two electrons may be adsorbed, allowing for emission of radiation of a shorter wavelength. As in UV spectroscopy, compounds often have unique fluorescence spectra which can be used in determining structure. Fluorescence detectors are extremely sensitive, thus this is an excellent analytical technique for trace amounts of substances. The infrared emission bands assigned to ▶ Polycyclic Aromatic Hydrocarbons (PAHs) in a variety of astronomical environments have been attributed to fluorescence, with the exciting radiation being in the ultraviolet region of the spectrum.
See also ▶ Polycyclic Aromatic Hydrocarbons
Fluorescence Resonance Energy Transfer ▶ FRET
Fluorescence Spectroscopy ▶ Fluorescence
Fluorescent Emission ▶ Fluorescence
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Fluorimetry ▶ Fluorometry
fluorescamine. Many widely used DNA stains and cell stains such as 4’, 6-diamidino-2-phenylindole (DAPI) are fluorophore-containing molecules. In the interstellar medium, infrared emission attributed to ▶ polycyclic aromatic hydrocarbon molecules (PAHs) is thought to result from fluorescence, following the absorption of ultraviolet photons.
Fluorometry Synonyms Fluorimetry; Spectrofluorometry
See also ▶ Fluorometry ▶ Polycyclic Aromatic Hydrocarbons
Definition Fluorometry is a chemical technique for measuring or monitoring ▶ fluorescence. In fluorometry, the species is excited from its ground electronic state to an excited electronic state by absorption of a ▶ photon (typically from a UV source). The species returns to the ground electronic state by emitting a photon (fluorescence). The amount of fluorescence measured is used to determine the sample concentration by comparison with a standard or by using a calibration curve. Fluorometry is highly sensitive with up to femtomolar detection limits. In addition, fluorometry is considered a more specific technique than ▶ absorption spectroscopy, since fluorescent molecules (▶ Fluorophores) are less common.
See also ▶ Absorption Spectroscopy ▶ Fluorescence ▶ Fluorophore ▶ Photon
Fluorophore Synonyms Chromophore
Flux, Radiative Definition The radiative flux is the amount of electromagnetic power crossing one surface unit. In astronomy, it corresponds to the part of the power emitted by a celestial object received per area unit at the Earth. If the contributions at all the wavelengths are summed, one speaks of bolometric flux and the unit is Wm2. If only a small interval in frequency (or wavelength) is considered, one speaks of spectral flux (unit: Wm2 Hz1). In the radio domain, the unit currently used is the Jansky (1026 Wm2 Hz1).
See also ▶ Electromagnetic Radiation ▶ Magnitude
Fomalhaut b Synonyms a Piscis Austrinus b
Definition Definition A fluorophore is a molecule or a portion of a molecule that has fluorescent properties. While many compounds are inherently fluorescent as they contain fluorophores, there are many compounds that are not. For the sake of analyzing these latter types of molecules, a fluorophore is often introduced via chemical derivatization with a fluorescent reagent, for example, fluorescein isothiocyanate (FITC) or
Fomalhaut b is a massive planet orbiting roughly 115 ▶ astronomical units from the main sequence A-type star Fomalhaut, 25 light years from Earth. Paul Kalas, James Graham, and Mark Clampin discovered the planet in a series of images of the star made by the ▶ Hubble Space Telescope (HST), showing the object to orbit the star just inside a narrow ring of dust. The planet’s mass is constrained to the range 0.05–3 Jupiter masses based on
Formaldehyde
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Formaldehyde HENDERSON JAMES (JIM) CLEAVES II Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, USA
Synonyms Methanal; Methyl Paraformaldehyde
aldehyde;
Methylene
oxide;
Keywords Formose, prebiotic chemistry, strecker synthesis Fomalhaut b. Figure 1 A composite of two images obtained with HST in 2004 and 2006, showing the apparent motion of the planet on the sky in the expanded insert. The light of the central star has been suppressed, leaving only residual noise speckles
the shape of the dust ring, which the planet stirs gravitationally. The ring of dust around Fomalhaut tipped off observers in the 1980s that this star might host a planetary system. Recent images of this narrow ring with the HST showed that the ring is not centered on the star; its center is offset from the star by 15 astronomical units. This offset suggested the presence of a planet perturbing the ring, and led to the planet’s 2008 discovery. The planet remains mysterious; it is several times brighter than expected for a body with this mass, and it has so far escaped observers who have tried to image it using other instruments. Fomalhaut b is considered the first extrasolar planet directly imaged by the HST; it is roughly a billion times fainter than the star it orbits (Fig. 1).
See also ▶ Beta Pictoris b ▶ Direct-Imaging, Planets ▶ Exoplanets, Discovery ▶ GJ 758 b ▶ HR 8799 b, c, and d ▶ Hubble Space Telescope
Definition Formaldehyde (methanal, HCHO), the simplest ▶ aldehyde, is a one carbon molecule intermediary along the redox continuum between CO2 and CH4, at the same oxidation state (0) as ▶ graphite. Formaldehyde was first reported by the Russian chemist Butlerow in 1860 and was conclusively identified by von Hofmann. It exists transiently but prominently in the abiological ▶ carbon cycle (Cleaves 2008). It is an abundant interstellar molecule and is suggested to be a major constituent of cometary ices (Biver et al. 2002). It is readily produced in prebiotic simulation experiments from a variety of gas mixtures and energy sources. HCHO may have played an important role in the synthesis of organic molecules relevant to the origin of life. HCHO is likely a significant precursor for the prebiotic synthesis of ▶ glycine and other ▶ amino acids (Miller 1957). HCHO can react to form sugars under basic conditions (Butlerow 1861) via the so-called “▶ formose reaction,” a reaction of potential importance for the origin of an “▶ RNA World” or early nucleic acids based on alternative sugars in a potential “pre-RNA World” (Joyce et al. 1987). It has been argued that HCHO may in fact be the only one carbon C-, H-, and O-containing molecule capable of generating complex organic compounds for the origin of life (Weber 2002). Formaldehyde is believed by some to be the primary precursor for most of the complex organic material in the interstellar medium, including amino acids (Schutte et al. 1993).
Overview
Footballene ▶ Fullerenes
Prebiotic Sources of HCHO One important source of HCHO on the primitive Earth is atmospheric photochemical synthesis by the photoreduction of CO2 with H2O:
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CO2 þ H2 O $ HCHO þ O2
ð1Þ
Pinto et al. (1980) estimated that HCHO could be produced in yields of up to 1011 moles/year, reaching a steadystate oceanic concentration of 103 M in 107 years. Electric discharges acting on a variety of gas mixtures also produce HCHO in good yield (Stribling and Miller 1987), however photochemistry may have been more significant simply due to the greater energy flux. Photochemical production from atmospheric CH4 and/or CO could also have been a significant source of HCHO, though the production and rainout rates of HCHO are highly sensitive to energy source and atmospheric composition (Chang 1993). HCHO could also potentially be produced in ▶ hydrothermal vents (Ferris 1994), for example, by the reduction of aqueous formate, CO or CO2, or by the oxidation of methanol or CH4, although HCHO does not appear to be a stable redox state of carbon under hydrothermal conditions. HCHO may be an intermediate in the redox equilibration of other more stable compounds (Seewald et al. 2006).
Prebiotic Sinks for HCHO HCHO decomposes thermally in the gas phase above 300 C to give CO and H2 (Bone and Smith 1905) (Eq. 2): HCHOðgÞ $ COðgÞ þ H2 ðgÞ
ð2Þ
Long wavelength photochemical destruction was also likely a significant sink for HCHO (see below) (Calvert and Steacie 1951). (Sekine 2002) found that MnO2 catalyzes the oxidation of HCHO at room temperature to CO2 under atmospheric conditions. (Lo¨b 1906) found that electric discharges acting on HCHO in the presence of water vapor give CO, CO2, H2, and CH4 as products, which is essentially the reverse of the reaction demonstrated by (Miller 1953). There is reason to believe that there was a considerable flux of atmospherically synthesized and rained-out H2O2 to the primitive oceans, particularly if the atmosphere was non-reducing (Kasting and Brown 1998). Reaction of HCHO with aqueous hydrogen peroxide gives formate and H2 (Walker 1964) (Reaction 3): 2HCHOðaqÞ þ 2OH þ H2 O2 ðaqÞ $ 2HCOO ðaqÞ þ 2H2 O þ H2 ðgÞ
HCHO absorbs UV radiation strongly in the range of 250 350 nm, resulting in photolysis. The hydrate does not absorb light at these wavelengths, thus dissolution in water could be an important photo-protection mechanism for HCHO.
Oligomerization Chemistry: Polyoxymethylene Low molecular weight polymers, ▶ polyoxymethylene (POMs) (of formula HO(CH2O)nH), form readily in neutral concentrated aqueous HCHO solutions. The cyclic oligomers, trioxane and tetraoxane (Fig. 1), may also form where acid catalysis is available.
Oligomerization Chemistry: The Formose Reaction HCHO can be oligomerized under basic conditions into sugars via the so-called formose reaction (Butlerow 1861) (See formose reaction). The equilibrium constant for the dimerization of HCHO to give ▶ glycolaldehyde is not well known, but is of considerable interest, since while HCHO is volatile, glycolaldehyde is not, and could thus be concentrated by evaporation. HCHO reacts with ▶ acetaldehyde at much lower concentrations to give acrolein among other products (Cleaves 2003). The reaction of HCHO into formose is one limiting reaction pathway for the accumulation of HCHO in primitive waters, which would depend on the rate of HCHO production and delivery to the primitive oceans (Pinto et al. 1980). If HCHO became sufficiently concentrated, side reactions may have established the steady-state concentration regardless of the production rate. Given the observed photochemical synthesis of pentaerythritol (Fig. 3, left hand side) (Schwartz and De Graaf 1993), and its likely mechanism of synthesis, it seems possible that HCHO is converted to CH3CHO by UV light, and thus it appears unlikely that bulk oceanic concentrations of HCHO could have been much higher than 103 M.
ð3Þ
Prebiotic Solution Chemistry of HCHO An excellent monograph has been written on the chemistry of HCHO (Walker 1964). In aqueous solution HCHO is mostly present as the monohydrate, methylene glycol (Eq. 4). HCHO þ H2 O $ CH2 ðOHÞ2
ð4Þ
Formaldehyde. Figure 1 Cyclic oligomers of HCHO, trioxane and tetraoxane
Formaldehyde
HCHO is prone to photoreaction in the primitive oceans. For example, HCHO photochemically reacts to give mostly pentaerythritol (Schwartz and De Graaf 1993), among other non-sugar products (Fig. 2) (Shigemasa et al. 1977).
2HOCH2 SH $ HSCH2 OCH2 SH
ð6Þ
HOCH2 SH þ HSCH2 OCH2 SH $ HSCH2 SCH2 OCH2 SH þ H2 O
ð7Þ
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Aqueous HCHO reacts with SO2 to give methylolsulfonic acid (Walker 1964) (Eq. 8):
Reactions with Amines HCHO is converted to ▶ hexamethylenetetramine (HMT or 1, 3, 5, 7-Tetraazatricyclo [3.3.1.13,7] decane) (Fig. 3) by reaction with ammonia (NH3). The reaction of fairly dilute (0.003 M) aqueous HCHO and NH3 between 100–250 C produces amino acids, amines, and amino alcohols (Aubrey et al. 2009), and the reaction of somewhat more concentrated HCHO (0.02 M) with NH3 (0.04 M) has been shown to produce traces of amino acids at lower temperatures (40 C) (Weber 1998). Thus, for at least several simple amino acids, NH3 and HCHO may be sufficient for synthesis to occur.
Reactions with Sulfur Species HCHO and H2S react rapidly to produce trithiane and other oligomers (Fig. 4), which have been detected in hydrothermal vent effluent (Simoneit 1992), as well as in hydrothermal vent simulations (Cole et al. 1994) (Eqs. 5 7): H2 S þ H2 CðOHÞ2 $ HOCH2 SH þ H2 O
ð5Þ
H2 CO þ H2 O þ SO2 $ HOCH2 SO3 H
Formaldehyde. Figure 2 2-Hydroxymethylglycerol and pentaerythritol, major products of photochemical reactions of HCHO
N N
Mineral Interactions There have been few studies of the adsorption of HCHO to minerals, though the ones that do exist suggest that HCHO-clay adsorption equilibria are not especially high. This is generally also true for low molecular weight POMs (Parfitt and Greenland 1970). The adsorption equilibria may be more significant for clays such as illite and ▶ kaolinite (Chandra and De 1983). Clay adsorption may have led to significant concentration effects which may have facilitated reactions of HCHO such as oligomerization to formose products and redox reactions to ▶ methanol, formate, and CO2.
Reaction with HCN Aqueous HCHO reacts readily with HCN to give glycolonitrile (Henry 1890) (Eq. 9)
S
+ 6 H2O
N Hexamethylenetetramine
Formaldehyde. Figure 3 Hexamethylene tetramine (HMT) forms readily from NH3 and HCHO
ð9Þ
The Keq for the reaction is exceptionally high (Schlesinger and Miller 1973), thus any atmospheric composition which results in the production of the two compounds essentially results in the production of ▶ glycolic acid, after hydrolysis of the ▶ nitrile. (Schwartz and Goverde 1982) found that the addition of HCHO or glycolonitrile to HCN actually accelerates the formation of HCN tetramer, diaminomaleonitrile (DAMN). HCHO reacts readily with DAMN to form a crystallizable product (Koch et al. 2007).
S N
ð8Þ
The salts of this compound are nonvolatile, but decompose in dilute acid, liberating HCHO.
HCHO þ HCN $ HOCH2 CN
6 HCHO + 4 NH3
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S Trithiane
Formaldehyde. Figure 4 Trithiane is formed from H2S and HCHO
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Reaction of HCN, HCHO, and NH3 produces the amino acid glycine via the Strecker synthesis. Cannizzaro reactions may also be important loss channels for HCHO. If HCHO concentrations are high enough, HCHO disproportionates to form formate and methanol (Walker 1964), which reduces the concentration of HCHO available to form sugars. These reactions are acid-, base-, and metal-catalyzed (Walker 1964). As the reaction is apparently third order, it is unclear whether it would occur significantly in dilute solution.
Interstellar Formaldehyde Formaldehyde was the first polyatomic organic molecule detected in the interstellar medium by (Zuckerman et al. 1970). Since its initial detection it has been observed in many regions of the galaxy. As the gas-phase reaction that produces formaldehyde is too inefficient to produce the abundance of formaldehyde that has been observed, it has been proposed that HCHO is formed via the hydrogenation of CO in ice (Eqs. 10–11): H þ CO ! HCO
ð10Þ
HCO þ H ! H2 CO
ð11Þ
There are several side reactions that take place with each step of the reaction that depend on the nature of the ice on the grain (Woon 2002). The isotopic ratio of [12C]/ [13C] in HCHO in the galactic disk has been determined to be 50:1 (Henkel et al. 1985), while the terrestrial ratio is 100:1.
Formaldehyde in Biology Free formaldehyde is rarely found in living metabolism, as formaldehyde is mutagenic due to its reactivity with proteins and nucleic acids. The main biological HCHO carrier is tetrahydrofolate; however, in methanogenesis, HCHO is masked as a methylene group in methanopterin.
See also ▶ Aldehyde ▶ Formose Reaction ▶ Strecker Synthesis
References and Further Reading Aubrey AD, Cleaves HJ, Bada JL (2009) The role of submarine hydrothermal systems in the synthesis of amino acids. Orig Life Evol Biosph 39(2):91–108 Biver N, Bockele´e-Morvan D, Crovisier J, Colom P, Henry F, Moreno R, Paubert G, Despois D, Lis D (2002) Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths. Earth Moon Planet 90:323–333
Bone W, Smith H (1905) The thermal decomposition of formaldehyde and acetaldehyde. J Am Chem Soc 87:910–916 Butlerow A (1861) Formation synthe´tique d’une substance sucre´e. Comp Rend Acad Sci 53:145–147 Calvert A, Steacie E (1951) The vapor phase photolysis of formaldehyde at wavelength 3130. J Chem Phys 19:1976–1982 Chandra K, De S (1983) Adsorption of formaldehyde by clay minerals in presence of urea and ammonium sulfate in aqueous system. Indian J Agric Chem 16:239–245 Chang S (1993) Prebiotic synthesis in planetary environments. In: Greenberg JM, Mendoza-Gomez CX, Pirronello V (eds) The chemistry of life’s origins. Kluwer, Boston Cleaves H (2003) The prebiotic synthesis of acrolein. Monatsh Chem 134:585–593 Cleaves HJ (2008) The prebiotic geochemistry of formaldehyde. Precambrian Res 164(3–4):111–118 Cole W, Kaschke M, Sherringham J, Curry G, Turner D, Russell M (1994) Can amino acids be synthesized by H2S in anoxic lakes? Mar Chem 45:243–256 Ferris J (1994) The potential for prebiotic synthesis in hydrothermal systems. Origins of Life Evol Biosphere 24:363–381 Fox S, Windsor C (1970) Synthesis of amino acids by the heating of formaldehyde and ammonia. Science 170:984–986 Gabel N, Ponnamperuma C (1967) Model for origin of monosaccharides. Nature 216:453–455 Henkel C, Guesten R, Gardner FF (1985) [12C]/[13C] ratios from formaldehyde in the inner galactic disk. Astron Astrophys 143(1):148–152 Henry L (1890) Sur le nitrile gycolique et la synthe`se directe de l’acide glycolique. Comp Rend 110:759–760 Joyce G, Schwartz A, Miller S, Orgel L (1987) The case for an ancestral genetic system involving simple analogues of the nucleotides. Proc Natl Acad Sci USA 84:4398–4402 Kasting J, Brown L (1998) Setting the stage: the early atmosphere as a source of biogenic compounds. In: Brack A (ed) The molecular origins of life: assembling the pieces of the puzzle. Cambridge University Press, New York, pp 35–56 Koch K, Schweizer W, Eschenmoser A (2007) Reactions of the HCNtetramer with aldehydes. Chem Biodivers 4:541–553 Lo¨b W (1906) Studien u¨ber die chemische Wirkung der stillen elektrischen Entladung. Zeitschr fu¨r Elektrochem 15:282–312 Miller S (1953) A production of amino acids under possible primitive Earth conditions. Science 117:528–529 Miller SL (1957) The mechanism of synthesis of amino acids by electric discharges. Biochim Biophys Acta 23(3):480–489 Parfitt R, Greenland D (1970) The adsorption of poly(ethylene glycols) on clay minerals. Clay Miner 8:305–315 Pinto J, Gladstone G, Yung Y (1980) Photochemical production of formaldehyde in Earth’s primitive atmosphere. Science 210:183–185 Schlesinger G, Miller S (1973) Equilibrium and kinetics of glyconitrile formation in aqueous solution. J Am Chem Soc 95:3729–3735 Schutte W, Allamandola L, Sandford S (1993) An experimental study of the organic molecules produced in cometary and interstellar ice analogs by thermal formaldehyde reactions. Icarus 104:118–137 Schwartz A, De Graaf R (1993) The prebiotic synthesis of carbohydrates: a reassessment. J Mol Evol 36:101–106 Schwartz A, Goverde M (1982) Acceleration of HCN oligomerization by formaldehyde and related compounds: implications for prebiotic syntheses. J Mol Evol 18:351–353 Seewald JS, Zolotov M, McCollom T (2006) Experimental investigation of single carbon compounds under hydrothermal conditions. Geochim Cosmochim Acta 70:446–460
Formic Acid Sekine Y (2002) Oxidative decomposition of formaldehyde by metal oxides at room temperature. Atmos Environ 36:5543–5547 Shigemasa Y, Matsuda Y, Sakazawa C, Matsuura T (1977) Formose reactions II. The photochemical formose reaction. Bull Chem Soc Japan 50:222–226 Simoneit B (1992) Aqueous organic geochemistry at high temperature/ high pressure. Orig Life Evol Biosph 22:43–65 Stribling R, Miller S (1987) Energy yields for hydrogen cyanide and formaldehyde syntheses: the hydrogen cyanide and amino acid concentrations in the primitive ocean. Origins of Life Evol Biosphere 17:261–273 Walker J (1964) Formaldehyde, 3rd edn. Rheinhold, New York Weber A (1998) Prebiotic amino acid thioester synthesis: thioldependent amino acid synthesis from formose substrates (formaldehyde and glycolaldehyde) and ammonia. Orig Life Evol Biosph 28:259–270 Weber A (2002) Chemical constraints governing the origin of metabolism: the thermodynamic landscape of carbon group transformations under mild aqueous conditions. Origins of Life Evol Biosphere 32:333–357 Woon DE (2002) Modeling gas-grain chemistry with quantum chemical cluster calculations. I. Heterogeneous hydrogenation of CO and H2CO on icy grain mantles. Astrophys J 569:541–548 Zuckerman B, Buhl D, Palmer P, Snyder LE (1970) Observation of interstellar formaldehyde. Astrophys J 160:485–506
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See also ▶ Comet ▶ Molecules in Space
References and Further Reading Barks HL, Buckley R, Grieves GA, Di Mauro E, Hud NV, Orlando TM (2010) Guanine, adenine, and hypoxanthine production in UVirradiated formamide solutions: relaxation of the requirements for prebiotic purine nucleobase formation. Chem Bio Chem 11(9):1240 Mehringer D, Colom P, Benford D, Bockelee-Morvan D, Despois D, Paubert G, Germain B, Biver N, Crovisier J, Gautier D, Gerard E, Rauer H, Lis DC, Phillips TG, Moreno R, Davies JK, Dent WRF, Owens A, Oosterbroek T, Orr A, Parmar AN, Antonelli LA, Fiore F, Maccarone MC, Piro L (1997) Comet C/1995 O1 (Hale-Bopp). IAU Circular No. 6614, 1 Rubin RH, Swenson GW Jr, Benson RC, Tigelaar HL, Flygare WH (1971) Microwave detection of interstellar formamide. Astrophys J Lett 169:L39 Saladino R, Crestini C, Ciciriello F, Costanzo G, Di Mauro E (2006) About a formamide-based origin of informational polymers: syntheses of nucleobases and favourable thermodynamic niches for early polymers. OLEB 36:523–531
Formamine Formamide
▶ Hexamethylenetetramine
Synonyms Methanamide; NH2CHO
Definition Formamide (IUPAC name: methanamide) is the amide with the simplest structure (other amides have one, two, or three hydrogen atoms replaced by radicals). Formamide is the smallest molecule with a peptide bond. At room temperature and standard pressure it is a colorless liquid. It remains liquid between 3 C and 210 C: this large range is one of the reasons for which formamide was proposed as an alternative to water as the “ideal” solvent for life. Above 90 C it decomposes to HCN and water. Ammonium formate, itself a product of the reaction of the two simple species, formic acid, and ammonia, produces formamide when heated. Formamide is a potential starting material to create the RNA bases (Barks et al. 2010). Formamide has been found in interstellar medium and in ▶ comets.
History Formamide was detected for the first time in 1971 in the interstellar medium by Rubin et al. and in 1997 in the bright comet C/1995 O1 (Hale-Bopp) by Mehringer et al.
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Formic Acid Synonyms Methanoic acid
Definition Formic acid is a simple carboxylic acid, whose chemical O
formula is HCOOH
C H
. It partly dissociates in OH
water with a dissociation constant (pKa) of 3.74. It dissolves readily in water and is slightly dissolved in hydrocarbons. It mostly consists of dimers in hydrocarbon solvents and in the gas phase. It may also react as a reducing agent. It dissociates to carbon monoxide and water upon heating or by the addition of sulphuric acid. In an early report on the abiotic synthesis of organic compounds, Garrison et al. (1951) reported that formic acid was produced by helium ion irradiation of an aqueous solution containing CO2 and Fe2+. Formate is also produced by the hydrolysis of HCN or formamide. Melting point: 8.4 C, boiling point: 101 C, density: 1.22 g cm3.
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Formic Acid Methyl Ester
References and Further Reading Garrison WM, Morrison DC, Hamilton JG, Benson AA, Calvin M (1951) Reduction of carbon dioxide in aqueous solutions by ionizing radiation. Science 114:416–418
Formic Acid Methyl Ester ▶ Methyl Formate
Formonitrile ▶ Hydrogen Cyanide
Formose Reaction HENDERSON JAMES (JIM) CLEAVES II Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, USA
Synonyms Butlerow reaction
Keywords Autocatalysis, carbohydrate, formaldehyde, formose, ribose, RNA World
Definition The formose reaction, discovered by Butlerow in 1861, is a complex autocatalytic set of condensation reactions of formaldehyde to yield sugars and other small sugar-like molecules. The reaction is particularly noteworthy in the context of astrobiology and prebiotic chemistry in that it could serve as a potential abiotic source of carbohydrates, in particular ribose, which could be important for the origin of an RNA World.
Overview The formose reaction is an autocatalytic reaction discovered by Butlerow (1861). It involves the formation of sugars, polyols and hydroxy acids from ▶ formaldehyde in a series of carbon-to-carbon condensations, as opposed to carbon-to-oxygen condensations of HCHO to form ▶ polyoxymethylene. Formose is a contraction
of formaldehyde and the suffix -ose, denoting a sugar. In fact, many biological sugars have empirical formulas of the form (CH2O)n, for example glucose, (CH2O)6, and ▶ ribose, (CH2O)5. The formose reaction may be a mechanism for the prebiotic synthesis of sugars from formaldehyde, of relevance to the origin of an ▶ RNA World (Gesteland and Atkins 1993), early nucleic acids based on alternative sugars in a potential “pre-RNA World” (Joyce et al. 1987), as well as other sugar-based models for the origin of life (Weber 1997, 2001). The formose reaction is one of the few chemical systems proposed to be close to an autocatalytic proto-metabolic cycle (Orgel 2000). Both formaldehyde and ▶ glycolaldehyde have been observed spectroscopically in outer space in both the ▶ interstellar medium and in ▶ comets (Hollis et al. 2000). Formaldehyde and sugar-related compounds, such as ▶ glycerol, which could be derived from this reaction are abundant in some ▶ carbonaceous chondrites (Cooper et al. 2001), thus the formose reaction could be a cosmically common abiotic mechanism for sugar synthesis.
Reaction Mechanism The mechanism of the reaction has been studied intensively, and the complexity of the product mixture is notorious (Breslow 1959). The reaction is typically carried out from concentrated HCHO under alkaline conditions in the presence of divalent metal ions, such as calcium, magnesium, or lead, and is preceded by an induction period which can be shortened by the addition of glycolaldehyde or other aldehydes as initiators. Reaction does not proceed at an appreciable rate under neutral conditions at room temperature. At high pH, it is sluggish in the absence of catalysts such as divalent metal ions (Orgel 2000). It has been difficult to decipher whether the reaction requires divalent cations or higher aldehydes to initiate. It has been reported that carbohydrates and other trace impurities in commercial formaldehyde solutions or paraformaldehyde present in ppm quantities may be the cause of the autocatalysis observed in some of the formose reactions studied to date (Socha et al. 1980). For example, paraformaldehyde sublimed into Ca(OH)2 suspension was not transformed to sugars via the formose reaction, but only to methanol and formate by the base-catalyzed Cannizzaro reaction, and a 3 ppm trace of glycolaldehyde was sufficient to initiate conventional autocatalysis (Socha et al. 1980). However, at neutral or slightly basic conditions, redistilled formaldehyde solutions undergo the classical reaction (at 80 ) in the presence of several minerals, while no reaction is observed in their absence (Schwartz and de Graaf 1993a; see below).
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Formose Reaction
The reaction involves a series of intermediate steps including aldol reactions, reverse aldol reactions, and isomerization reactions. A number of intermediates have been identified including glycolaldehyde, glyceraldehyde, dihydroxyacetone, and tetroses. Figure 1 shows a simplified proposed mechanism for the reaction (Breslow 1959). The reaction is proposed to begin by the apparently kinetically slow reaction of two formaldehyde molecules to make glycolaldehyde. Glycolaldehyde then reacts via an aldol condensation with another formaldehyde molecule to yield DL-glyceraldehyde. Isomerization of glyceraldehyde forms dihydroxyacetone (DHA), which can react with glycolaldehyde to form a 2-ketopentose. This 2-ketopentose may then isomerize to yield various isomeric pentoses. DHA can also react with formaldehyde to produce a 2-ketotetrose which can isomerize to give an aldotetrose. At this step, the aldotetrose can react via a retro-aldol reaction to give two molecules of glycolaldehyde. Due to the generation of two molecules
HCHO
of glycolaldehyde from the input of one, the reaction becomes autocatalytic.
Mineral Interactions Mineral adsorption may have been an important sink or concentration mechanism for HCHO and sugars. Mineral surfaces and other inorganic species can also have significant effects on the course of the reaction, as both general and selective catalysts for synthesis and degradation for the reaction products. There have been relatively few studies of the adsorption of HCHO to minerals, though the ones that do exist suggest that HCHO-clay adsorption equilibria are not especially high, which is also true for low molecular weight POMs (Parfitt and Greenland 1970). Adsorption equilibria may be higher for clays such as illite and kaolinite (Chandra and De 1983). Many minerals appear to catalyze the formose reaction, for example, various clays, apatite, and calcite were found to be catalysts (Gabel and Ponnamperuma 1967; Reid and Orgel 1967; Cairns-Smith et al. 1972;
O
HO
∗
OH
O
HO
DL-Glyceraldehyde
HO
Dihydroxyacetone O
HO 2 HCHO
Glycolaldehyde HO
O
∗ HO HO
O
HCHO
OH
2-Ketotetrose
Glycolaldehyde
HCHO HO
OH
∗
∗
HO
HO
OH
HO
∗
∗
HO
Aldotetrose
HO
OH
∗
∗ O
HO
HO
∗ ∗
OH
∗ O
O
HO
601
OH
Branched aldotetrose
OH
2-Ketopentose
O
Aldopentose
Formose Reaction. Figure 1 A simplified scheme for the formose reaction. The stereochemistry of the asymmetric carbon atoms (marked with an asterisk in the diagram) is not specified, but in general all isomers are formed. Side reactions leading to branched-chain molecules complicate the cycle and divert molecules from it
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F
Formose Reaction
Schwartz and de Graaf 1993b). The reaction can be carried out at lower pH and in the absence of glycolaldehyde initiator in the presence of some minerals (Schwartz and de Graaf 1993a). Certain mineral types, in particular borates and silicates, stabilize intermediates in the reaction pathway at high pH and might significantly alter the product mixture under these conditions (Ricardo et al. 2004; Lambert et al. 2010). This may be particularly significant given the wide distribution of silicates in the solar system. Double layer hydroxide minerals have been shown to be catalysts for aldol condensations under very dilute conditions (Arrhenius et al. 1994). Mineral effects have generally been studied using high concentrations of HCHO (0.01 M or higher) or other aldehydes, it remains unknown whether minerals affect the reaction of more dilute HCHO (see below).
Competing Chemistry Cannizzaro Reactions At high concentrations HCHO undergoes a disproportionation reaction to form formate and methanol (Eq. 1) (Walker 1964), which reduces the amount of HCHO available to form sugars. These reactions are acid-, base-, and metal-catalyzed (Walker 1964). The reaction reportedly shows third order kinetics. It is not clear whether it would occur significantly in dilute solution. 2HCHO þ H2 O ! CH3 OH þ HCOOH
ð1Þ
Oligomerization to Polyoxymethylene A series of low molecular weight polymers, polyoxymethylenes (POMs) (of formula HO(CH2O)nH) as well small 6 and 8 membered cyclic oligomers, form readily in neutral concentrated aqueous HCHO solutions. This polymerization is highly concentration dependant. A 5 wt% (1.3 M) aqueous HCHO solution contains only 14% POM dimer, 3% trimer, and 0.5% tetramer. Polymerization equilibria would be much lower for the prebiotic oceanic concentrations of HCHO which have been estimated.
Photocatalysis and Reaction with CH3CHO The photochemical synthesis of formaldehyde from ultraviolet irradiation of CO2 and water is quite robust (Pinto et al. 1980). Although its hydrate has a low UV crosssection, HCHO may have been prone to photoreaction in primitive surface waters. Schwartz and De Graaf found that UV irradiation of dilute solutions of formaldehyde produced acetaldehyde (CH3CHO) in significant yield, as well as very high yields of pentaerythritol (Fig. 2) (Schwartz and de Graaf 1993a). Other nonsugar products have been observed from photochemical formose reactions by others (Shigemasa et al. 1977). Significant quantities of acrolein have also been detected in spark discharge experiments (van Trump and Miller 1972). The DG for the condensation of CH3CHO and HCHO in the gas phase was estimated at 4.44 kcal mol1 and the Keq as 1640 (Malinowski et al. 1963). Thus the overall reaction is thermodynamically favorable. Acrolein is an intermediate in the industrial synthesis of pentaerythritol from concentrated HCHO and CH3CHO in aqueous alkaline solution (Berlow et al. 1958). HCHO reacts with CH3CHO at much lower concentrations, to give acrolein among other products, than the formose reaction has been observed to occur (Cleaves 2003). The synthesis of acrolein from equimolar HCHO and CH3CHO was found to be fairly independent of pH between pH 7 and 11 and relatively independent of concentration between of 104 to 1 M. In contrast, glycolaldehyde and glyceraldehyde synthesis were found to be extremely concentration dependent. At 103 M concentration of equimolar HCHO and CH3CHO and below, neither glycolaldehyde nor glyceraldehyde were detected. The same is presumably true of the higher sugars. It has been demonstrated that acrolein is one of the few compounds with which the nucleobases of RNA/DNA will readily react (Nelsestuen 1980). Thus, acrolein could have been an important sink for the nucleobases in the prebiotic environment, and a significant hindrance to the start of an RNA World.
Environmental Limitations Sugar Degradation Under the basic reaction conditions which allow the formose reaction to occur, sugar decomposition into tars (Reid and Orgel 1967; Shapiro 1988), furans, and other low molecular weight degradation products (De Bruijn et al. 1986; Cooper et al. 2001) is also facilitated (Larralde et al. 1995). This has been shown to be less significant depending on the presence of high concentrations of inorganic species which may act as stabilizers (Ricardo et al. 2004; Lambert et al. 2010).
The efficiency of the formose reaction in a prebiotic environment is highly dependent on the rate of HCHO production and delivery to the primitive oceans (Pinto et al. 1980). The reaction has been observed to occur with HCHO as dilute as 0.01 M (Reid and Orgel 1967; Schwartz and De Graaf 1993a), although there are accounts of its reaction as dilute as 103 M (Gabel and Ponnamperuma 1967). These concentrations may have been difficult to achieve in the bulk early oceans. As HCHO became concentrated, side reactions may have established a low
Formose Reaction
F
603
hυ 2 HCHO
CH3CHO
HCHO
O
O O
OH
HCHO
H2O
OH
OH Acrolein
HCHO
OH
F OH
O
HO (H)
OH
OH HO
HO
Pentaerythritol
Formose Reaction. Figure 2 Scheme for the photochemical synthesis of CH3CHO, acrolein, and pentaerythritol from UV irradiation of HCHO
steady-state concentration regardless of the production rate. Given the observed photochemical synthesis of pentaerythritol described above, it may be unlikely that bulk oceanic concentrations of HCHO could have been much higher than 103 M. If the concentration and reaction of HCHO on the primitive Earth were difficult because of all of the potential competing geochemical sinks, sugars may have been derived from meteoritic input. Polyols such as glycerol and ribitol have been identified in the ▶ Murchison meteorite, although only actual sugar identified was DHA (Cooper et al. 2001). This can potentially be explained by the instability of sugars over the 4 billion years since the Murchison parent body formed and the observed organic synthesis presumably occurred. The initial synthesis to form sugars may have occurred relatively rapidly on the parent body, while only those compounds stable enough to survive until the present day are still detectable. The detection of these compounds does however present an interesting paradox for prebiotic chemistry. If the polyols and their precursors were generated by the same aqueous phase chemistry which produced the amino and hydroxy acids, as well as the purines and pyrimidines
which have been detected in the Murchison meteorite (Pizzarello 2004), apparently the suggested inhibition of HCHO chemistry by HCN and vice versa (Schlesinger and Miller 1973) is not a genuine problem. The ratio of glycolic acid to glycine in Murchison suggests that NH3 concentrations were fairly high in the parent body (Peltzer et al. 1984). These quantities also suggest that the reactions which form ▶ HMT and glycolonitrile are not limiting to either purine or sugar synthesis, and that slow kinetic effects may be more important than the initial rapid equilibrium obtained. The prebiotic oceanic concentrations of most precursor organic compounds such as HCHO and NH3 were also likely quite low (Stribling and Miller 1987). It is necessary to consider the geochemical processes which might have concentrated HCHO sufficiently to allow the formose reaction to occur. It is worth considering what happens to dilute solutions of more complex mixtures of HCN, HCHO and other aldehydes, NH3, urea, nitrate, nitrite, sulfides, ferrous iron, salts, etc., as they are evaporated in the presence of UV and visible light over mineral surfaces, or as they are frozen from dilute solution under visible and/or UV irradiation.
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Formose Reaction
Possible Geological Settings for the Formose Reaction Sugars are easily made by the reaction of HCHO under certain conditions (fairly high concentrations, and slightly basic to basic pH). Such conditions of pH and concentration are likely only attainable in certain geological environments such as in eutectics and in evaporative environments after HCHO is rendered nonvolatile by reaction with other aqueous species, for example, NH3, SO3, or H2S.
Eutectic Freezing Eutectic freezing can effectively concentrate dilute prebiotic reactants to form ▶ purines, ▶ pyrimidines, and ▶ amino acids (Sanchez et al. 1966; Levy et al. 2000; Miyakawa et al. 2002). Whether HCHO can also be concentrated via eutectic freezing to form sugars is unknown, but plausible. One significant advantage of this process is that sugars are generally much more stable at low temperatures (Larralde et al. 1995). The eutectic freezing and thawing of precursor ice-grain organics such as HCHO in carbonaceous chondrites may have allowed for the synthesis of sugars on these bodies (Cooper et al. 2001). The frozen clay model proposed by Lahav and Chang (1976) may warrant careful reconsideration.
Concentration by Evaporation Evaporative concentration is a plausible geochemical mechanism for concentrating nonvolatile species. The equilibrium constant for the dimerization of HCHO to give glycolaldehyde is not known, but is of considerable interest, since while HCHO is volatile, glycolaldehyde is not, and could thus be concentrated by evaporation. A rough calculation using the free energy of the aldol reaction of HCHO given by Weber (2002) suggests the equilibrium constant is 40, but this does not take into account various kinetic factors which may limit the reaction. This simple reaction is worthy of further investigation from the standpoint of prebiotic chemistry, especially given the apparent ubiquity of glycolaldehyde in interstellar space (Hollis et al. 2000).
Hydrothermal Vents Although some ▶ hydrothermal vent environments are characterized by rather alkaline conditions which would be ideal for formose chemistry, most modern vent systems are generally characterized by low organic concentrations and moderate to very high temperatures, and thus may not be optimal sites for formose chemistry. In addition to the thermal instability of sugars, HCHO does not appear
to be a stable one-carbon species under hydrothermal conditions (Osada et al. 2004; Seewald et al. 2006). Given the uncertainties regarding primitive Earth conditions, the existence of specialized conditions which would allow the formose reaction to occur cannot be ruled out. The synthesis of sugars in such environments from low initial HCHO concentrations under mild conditions might be extremely facile, and warrants further investigation, as does formose synthesis in the presence of congeners such as HCN, NH3, and inorganic compounds such as borate, sulfide, and sulfite, and the impact of UV and visible radiation on this chemistry. As mentioned above, the perceived importance of the formose reaction rests mainly on its role in sugar synthesis which is important in the context of abiotic nucleic acid synthesis. Sugar synthesis, of course, is merely one step on the way to nucleoside, nucleotide, and nucleic acid synthesis, which also may require certain specialized environmental conditions (Fuller et al. 1972), although there are proposed mechanisms for nucleotide formation which do not necessarily depend on direct condensation of the nucleic acid base with a sugar (Powner et al. 2009).
See also ▶ Carbohydrate ▶ Formaldehyde ▶ Ribose ▶ RNA World
References and Further Reading Arrhenius TG, Arrhenius et al (1994) Archean geochemistry of formaldehyde and cyanide and the oligomerization of cyanohydrin. Orig Life Evol Biosph 24(1):1–17 Berlow E, Barth RH, Snow JE (1958) The pentaerythritols. Reinhold Publishing, NY Breslow R (1959) On the mechanism of the formose reaction. Tetrahedron Lett 21:22–26 Butlerow A (1861) Formation synthe´tique d’une substance sucre´e. Comp Rend Acad Sci 53:145–147 Cairns-Smith A, Ingram P, Walker G (1972) Formose production by minerals: possible relevance to the origin of life. J Theor Biol 35:601–604 Chandra K, De S (1983) Adsorption of formaldehyde by clay minerals in presence of urea and ammonium sulfate in aqueous system. Indian J Agr Chem 16:239–245 Cleaves H (2003) The prebiotic synthesis of acrolein. Monatsh Chem 134:585–593 Cooper G, Kimmich N, Belisle W, Sarinana J, Brabham K, Garrel L (2001) Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth. Nature 414:879–883 De Bruijn J, Kieboom A, Van Bekkum H (1986) Reactions of monosaccharides in aqueous alkaline solutions. Sugar Tech Rev 13:21–52 Fuller W, Sanchez R, Orgel L (1972) Studies in prebiotic synthesis VII. J Mol Evol 1:249–257
Formyl Cation Gabel N, Ponnamperuma C (1967) Model for origin of monosaccharides. Nature 216:453–455 Gesteland R, Atkins J (1983) The RNA world: the nature of modern RNA suggests a prebiotic RNA world (Monograph/Cold Spring Harbor Laboratory, No 24) Gesteland RF, Atkins JF (1993) The RNA world: the nature of modern RNA suggests a prebiotic RNA world. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY Hollis J, Lovas F, Jewell P (2000) Interstellar glycolaldehyde: the first sugar. Astrophys J 540:L107–L110 Joyce G, Schwartz A, Miller S, Orgel L (1987) The case for an ancestral genetic system involving simple analogues of the nucleotides. Proc Nat Acad Sci USA 84:4398–4402 Lahav N, Chang S (1976) The possible role of solid surface area in condensation reactions during chemical evolution: reevaluation. J Mol Evol 8:357–380 Lambert JB, Gurusamy-Thangavelu SA, Ma K (2010) The SilicateMediated formose reaction: bottom-up synthesis of sugar silicates. Science 327:984–986 Larralde R, Robertson M, Miller S (1995) Rates of decomposition of ribose and other sugars: implications for chemical evolution. Proc Nat Acad Sci USA 92:8158–8160 Levy M, Miller S, Brinton K, Bada J (2000) Prebiotic synthesis of adenine and amino acids under Europa-like conditions. Icarus 145:609–13 Malinowski S, Basinski S, Szczepanska (1963) Ann Soc Chim Polonorum 37:977–982 Miyakawa S, Cleaves H, Miller S (2002) The cold origin of life: B. Implications based on pyrimidines and purines produced from frozen ammonium cyanide solutions. Orig Life Evol Biosph 32: 209–218 Nelsestuen GL (1980) Origin of life: consideration of alternatives to proteins and nucleic acids. J Mol Evol 15(1):59–72 Orgel LE (2000) Self-organizing biochemical cycles. PNAS 97(23):12503–12507 Osada M, Watanabe M, Sue K, Adschiri T, Arai K (2004) Water density dependence of formaldehyde reaction in supercritical water. J Supercrit Fluids 28:219–224 Parfitt R, Greenland D (1970) The adsorption of poly(ethylene glycols) on clay minerals. Clay Miner 8:305–315 Peltzer E, Bada J, Schlesinger G, Miller S (1984) The chemical conditions on the parent body of the Murchison meteorite: some conclusions based on amino, hydroxy and dicarboxylic acids. Adv Space Res 4:69–74 Pinto J, Gladstone G, Yung Y (1980) Photochemical production of formaldehyde in Earth’s primitive atmosphere. Science 210:183–185 Pizzarello S (2004) Chemical evolution and meteorites: an update. Orig Life Evol Biosph 34:25–34 Powner MW, Gerland B, Sutherland JD (2009) Synthesis of activated pyrimidines ribonucleotides in prebiotically plausible conditions. Nature 459:239–242 Reid C, Orgel L (1967) Synthesis of sugars in potentially prebiotic conditions. Nature 216:455 Ricardo A, Carrigan M, Olcott A, Benner S (2004) Borate minerals stabilize ribose. Science 303:196 Sanchez R, Ferris J, Orgel L (1966) Conditions for purine synthesis: did prebiotic synthesis occur at low temperatures? Science 153:72–73 Schlesinger G, Miller S (1973) Equilibrium and kinetics of glyconitrile formation in aqueous solution. J Am Chem Soc 95:3729–3735 Schwartz A (1983) Chemical evolution: the first stages. Naturwissenschaften 70:373–377
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Schwartz A, De Graaf R (1993a) The prebiotic synthesis of carbohydrates: a reassessment. J Mol Evol 36:101–106 Schwartz AW, de Graaf RM (1993b) Tetrahedron Lett 34:2201 Seewald JS, Zolotov M, McCollom T (2006) Experimental investigation of single carbon compounds under hydrothermal conditions. Geochim Cosmochim Acta 70:446–460 Shapiro R (1988) Prebiotic ribose synthesis: a critical analysis. Orig Life Evol Biosph 18:71–85 Shigemasa Y, Matsuda Y, Sakazawa C, Matsuura T (1977) Formose reactions II. The photochemical formose reaction. Bull Chem Soc Jpn 50:222–226 Socha RF, Weiss AH, Sakharov MM (1980) Autocatalysis in the formose reaction. React Kinet Catal Lett 14(2):119–128 Stribling R, Miller S (1987) Energy yields for hydrogen cyanide and formaldehyde syntheses: the hydrogen cyanide and amino acid concentrations in the primitive ocean. Orig Life Evol Biosph 17:261–273 Van Trump JE, Miller SL (1972) Prebiotic synthesis of methionine. Science 178(63):859–860 Walker J (1964) Formaldehyde 3rd edn. Rheinhold, New York Weber A (1997) Energy from redox disproportionation of sugar carbon drives biotic and abiotic synthesis. J Mol Evol 44:354–360 Weber A (2001) The sugar model: catalysis by amines and amino acid products. Orig Life Evol Biosph 31:71–86 Weber A (2002) Chemical constraints governing the origin of metabolism: the thermodynamic landscape of carbon group transformations under mild aqueous conditions. Orig Life Evol Biosph 32:333–357
Formyl Cation Synonyms HCOþ
Definition
The triatomic ion HCOþ is one of the most abundant molecular ions in dense interstellar clouds. It is a key intermediary in the production of ▶ carbon monoxide, CO, which is in turn the most abundant constituent of these regions after molecular hydrogen (H2). The abundance ratio of HCOþ to its deuterated counterpart, DCOþ, can provide a measure of the electron density in molecular clouds. HCOþ is a linear, closed shell molecule, so that its pure rotational spectrum consists of a series of harmonically related lines, with the lowest transition in the 3 mm wavelength band.
History In 1970 L. Snyder and D. Buhl reported the detection of an unidentified emission line in the spectra of several interstellar ▶ molecular clouds and referred to the unknown carrier as “X-ogen.” Laboratory measurements subsequently confirmed that X-ogen was HCOþ, in agreement
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Fo¨rster Resonance Energy Transfer
with the theoretical predictions of ion-molecule chemistry by W. Klemperer and E. Herbst that this ion should be abundant in such clouds.
See also ▶ Carbon Monoxide ▶ Deuterium ▶ Molecular Cloud
References and Further Reading Herbst E, Klemperer W (1974) Is X-ogen HCOþ. Astrophys J 188:255–256
Fossa, Fossae Definition Fossae are long, narrow depressions, often occurring in arrays (definition by the International Astronomical Union; http://planetarynames.wr.usgs.gov/jsp/append5. jsp). This is a descriptor term for naming surface features on the ▶ terrestrial planets, the ▶ Moon, and on icy ▶ satellites.
See also ▶ Moon, The ▶ Satellite or Moon ▶ Terrestrial Planet
Fo¨rster Resonance Energy Transfer ▶ FRET
Fossil Definition
Fortescue Group Definition The 2.77–2.63 Ga Fortescue Group is a well-preserved, thick (0.5–6 km) succession of continental flood ▶ basalts and interbedded sedimentary rocks deposited unconformably on basement granite-greenstone crust of the ▶ Pilbara craton, Western Australia. Together with the contemporaneous Ventersdorp basalts in South Africa, the volcanic rocks are the oldest known continental flood basalts (trapps). The 2.63–2.45 Ga Hamersley Group, containing thick deposits of ▶ banded iron formation, conformably overlies the group. Sedimentary rocks of the Fortescue Group contain several horizons of stromatolitic carbonates, deposited in freshwater lacustrine environments. Controversial filaments showing septa have also been observed and related to septate microbial microfossils.
See also ▶ Archean Traces of Life ▶ Banded Iron Formation ▶ Basalt ▶ Pilbara Craton ▶ Stromatolites ▶ Trapps
A fossil is the preserved remnant of an organism after its death. A fossil may be morphological or chemical. A fossil can have various sizes (microscopic to macroscopic) and composition (organic or mineral), and may represent a whole organism, part of an organism, colonial organisms, or the morphological or chemical imprint of an organism or of its activity. The processes of ▶ fossilization are complex, and they may lead to the preservation of organisms in their original composition, or partially or completely replaced by another (organic or mineral) material, or preserved as a mold or cast, or as traces of activity such as footprints, trails or burrows (“ichnofossil”).
See also ▶ Biomarkers, Morphological ▶ Biomineralization ▶ Dubiofossil ▶ Fossilization, Process of ▶ Microfossils ▶ Molecular Biomarkers ▶ Pseudofossil
Fossilization Processes ▶ Biosignatures, Effect of Metamorphism
Fossilization, Process of
Fossilization, Process of KARIM BENZERARA Institut de Mine´ralogie et de Physique des Milieux Condense´s, UMR 7590, CNRS, Universite´ Pierre et Marie Curie & Institut de Physique du Globe de Paris, Paris, France
Keywords Biomineralization, diagenesis, taphonomy
Definition Fossilization refers to the processes leading to the preservation of traces of life in the geological record. While Metazoans and single-cell eukaryotes leave undisputed traces in the geological record in the form of hard mineral parts, the fossilization of microorganisms such as Bacteria or Archaea or of viruses has long been debated due in particular to the difficulty to recognize unambiguously such fossils in old rocks. ▶ Biomineralization controlled or induced by microorganisms themselves seems to be a major process allowing preservation of cell structures and/or of organic molecules.
Overview The three domains of life can, in principle, be preserved as ▶ microfossils, depending on the conditions of preservation, and their original composition. Most fossils in the Phanerozoic are made of hard mineral parts or imprints of hard mineral parts of metazoans or eukaryote unicellular organisms (e.g., Diatoms, Foraminifers). However, we focus here on processes of fossilization of soft tissues and in particular of microorganisms which are more relevant to astrobiology. Interestingly, it has been widely suggested that microorganisms were often involved in the fossilization of metazoan soft tissues (e.g., Raff et al. 2006). There are undisputed fossils of prokaryotes in the geological record and some have been assigned taxonomic names such as Girvanella (Riding 2002). The abundance of these microfossils has varied through geological time. Absence of microfossils has sometimes been interpreted as evidence for abiotic conditions prevailing during the formation of a rock. For example, absence of microfossils in Archean stromatolites has been used to question their biogenicity (e.g., Grotzinger and Rothman 1996). Alternatively, the varying abundance of microfossils has often been related to variations of environmental conditions triggering or hindering fossilization (e.g., Arp et al. 2001).
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It is thus important to understand processes of fossilization in order to identify the main factors controlling the formation of microfossils and better read the paleontological record. When a microorganism dies, organic molecules and cellular structures are degraded very rapidly. Enzymes released by dying cells and/or other cells catalyze this process (lysis). This results in a rapid cycling of organic matter. Some organic molecules might be selectively preserved because they are more resistant to this chemical degradation such as polymers forming the cell wall of spores (e.g., sporopollenin) or molecules transformed secondarily by chemical reactions such as sulfurization of organic carbon (e.g., Lepot et al. 2009a; Damste et al. 1998) or formation of geopolymers (Vandenbroucke and Largeau 2007). Precipitation of minerals on cells, i.e., biomineralization, is another major process that allows fossilization of prokaryotes by armoring cellular structures against the lytic action of enzymes. Several mechanisms can favor mineral precipitation within and/or at the surface of cells and usually involve specific metabolic activities and environmental conditions. For example, it has been proposed that anoxic conditions are necessary for fossilization. The absence of O2 might indeed limit aerobic respiration, hence degradation or organic matter but is not a sufficient parameter (Allison 1988). In addition to this, anoxic environments are usually rich in metals (e.g., Fe, Mn) and/or sulfides which can precipitate on cells. Experimental taphonomy refers to fossilization experiments conducted in the laboratory or in the field. Although these studies may not all reflect the full complexity of natural environmental conditions, they reveal biotic patterns and fossilizable properties that have been overlooked so far. First, such studies which are still in their infancy have shown that fossilization of microorganisms can be achieved in few days or few weeks. For example, exposing bacterial cells to a solution rich in Ca, Fe, or Si can lead to their encrustation in few hours (Toporski et al. 2002; Benzerara et al. 2004a; Ferris and Magalhaes 2008; Miot et al. 2009). Moreover, these experiments help decipher what can be preserved in microfossils, regarding their chemistry as well as their ultrastructure. Cyanobacteria have received particular attention. Intra- and extracellular silicification has been observed. It has been shown that sheaths are preferentially preserved as compared to walls and cytoplasm (e.g., Bartley 1996). Some transformations could be observed including cell collapse, shrinkage and disappearance, trichome disarticulation, varying terminal cell morphology, and rupture of sheath (e.g., Toporski et al. 2002). If precipitation occurs in close connection with the
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microbial structures and if the minerals are small enough, very fine cellular details can be preserved. For example, a 40-nm thick cell wall of Gram-negative bacteria can be fossilized by calcium phosphates (e.g., Benzerara et al. 2004a) or by iron minerals (e.g., Miot et al. 2009). Embryos of eukaryotes could be preserved as well in the laboratory at different development stages by phosphatization, simulating what likely occurred during the fossilization of the famous ~630 Ma old Doushantuo embryos (e.g., Yin et al. 2007). While some organic carbon might be degraded during these processes, it has been shown that most of it can be preserved within resulting mineralized microfossils (e.g., Benzerara et al. 2004b). The diversity of organic moieties that can be preserved in fossils still seems poorly assessed. Biomarkers, i.e., highly resistant organic molecules comprising cell walls, are obvious examples (e.g., Derenne et al. 2008). Some organic geochemistry studies have however shown that additional molecules such as proteins could be preserved more than a 100 million years (Riboulleau et al. 2002). Although viruses may have been major players in life evolution (e.g., Forterre 2006), only little is known about their geological record. Interestingly, some taphonomy experiments have been performed on these “organisms.” Biomineralisation experiments on viruses show that dissolved iron can penetrate virus capsids and bind to internal sites (Daughney et al. 2004). As a result, virus capsids can serve as nuclei for the growth of iron oxide particles. The resulting morphology differs from abiotic iron oxides and organic molecules, which originally composed the capsids, can be efficiently preserved by the iron minerals that armor them, increasing the possibility of accurately identifying them (e.g., Kaiser & Guggenberger 2000). After the precipitation of minerals on organic structures, further degradation of morphology and organic molecules can take place. This stage is influenced by mechanical stresses, circulation of fluids, and metamorphism, e.g., an increase in temperature and pressure. Formation of carbonate nodules has been inferred as an efficient mechanism for fossil preservation (e.g., Muller 1985). Observations of natural samples have shown that microfossils can sometimes be preserved even after highgrade metamorphism (e.g. Bernard et al. 2007), suggesting that T and P might not be prominent factors in the disruption of microfossils. In contrast, mineral growth/ transformation has been shown to be highly damaging (e.g., Oehler 1976). Metamorphic processes take place over much longer timescales, so it is more difficult to simulate them in the laboratory. However, some recent studies provide an interesting way to address this issue, in particular, by noting that time and temperature are
inherently linked and that aging at low temperature over long timescales can be simulated by shorter aging at a higher temperature (Skrzypczak-Bonduelle et al. 2008). One key problem encountered in the study of microfossils is that relatively complex morphologies can be produced by purely abiotic processes (e.g., Garcia-Ruiz et al. 2003) and can thus make their identification difficult. Laboratory experiments can be of value to identify possible specific features that might be used to discriminate abiotic from biological objects. In addition to morphology, the composition of the organic matter produced by reactions such Fischer–Tropsch has been carefully scrutinized and compared with mature kerogens (e.g., McCollom and Seewald 2007). For example, a combination of infra-red spectroscopy (providing an aliphaticity index of the organic matter) and microscale measurements of the carbon isotopic compositions were used by Sangely et al. (2007) to distinguish between biology and Fischer–Tropsch-type reactions as genetic processes for the bitumen found in the Cretaceous uranium deposits of Athabasca. Known abiotic products that can mimic life morphologies or chemistries include vesicles made in the laboratory from meteoritic kerogen or in other prebiotic chemistry experiments (e.g., Deamer et al. 2006), fluid inclusions, carbonaceous filamentous shapes resulting from migrating organic matter (with carbon isotopic fractionation resembling life patterns) around minerals casts in hydrothermal environments (Brasier 2005; Brasier et al. 2006), aggregates of silica spheres and rods in silica-rich waters of hydrothermal springs, migration of carbonaceous materials along microfractures (VanZuilen et al. 2007), within or around silica (e.g., Jones and Renaut 2007; Lepot et al. 2009b). Finally, mineralized pseudofossils have been produced using a mixture of barium carbonate and silica in laboratory experiments (GarciaRuiz et al. 2003). The resulting auto-assembling segmented filaments contain organic matter.
Basic Methodology Characterization of field samples using analytical tools at multiscale, including scanning electron microscopy, transmission electron microscopy, Raman spectromicroscopy, confocal laser scanning microscopy. Laboratory experiments simulating fossilization, diagenesis and metamorphism.
Key Research Findings Fossilization is a very fast process. Fossilization can preserve very fine details and chemical heterogeneities down to the nanometer-scale.
Fossilization, Process of
Applications Search for ancient traces of life on Earth and elsewhere.
Future Directions Several directions remain widely open: first, an improved characterization of microfossils in ancient rocks which will allow in particular discriminating between genuine microfossils and microfossil-like abiotic features. Most of the oldest traces of life are indeed still debated. The advance of cutting edge technologies, such as Raman spectromicroscopy (Bernard et al. 2008; Schopf and Kudryavtsev 2009), Transmission Electron Microscopy (e.g., Lepot et al. 2008), Focused Ion Beam milling (Wirth 2009), synchrotron-based X-Ray microscopy (e.g., Lepot et al. 2009a; Obst et al. 2009), and NanoSIMS (e.g., Oehler et al. 2009), provides analyses of mineral and organic matter down to the nanoscale; this should help in achieving that goal. Second, experimental taphonomy can be developed further, inspired by studies of metazoans (e.g., Briggs and Kear 1993). A wide diversity of biomineralizing microbial systems is now available and can provide pre-fossils. Their extensive characterization down to the nanoscale will offer new insight into what can be potentially preserved in the geological record. In addition, a significant improvement in the design of protocols to simulate diagenesis (in particular, the use of fluids with an appropriate chemical composition), aging, and metamorphism would complement this approach.
See also ▶ Biomineralization ▶ Bioprecipitation ▶ Biomarkers, Morphological ▶ Microfossils
References and Further Reading Allison PA (1988) The role of anoxia in the decay and mineralization of proteinaceous macro-fossils. Paleobiology 14:139–154 Arp G, Reimer A, Reitner J (2001) Photosynthesis-induced biofilm calcification and calcium concentrations in phanerozoic oceans. Science 292:1701–1704 Bartley JK (1996) Actualistic taphonomy of Cyanobacteria: implications for the Precambrian fossil record. Palaios 11:571–586 Benzerara K, Menguy N, Guyot F, De Luca G, Heulin T, Audrain C (2004a) Experimental colonization and weathering of orthopyroxenes by the pleomorphic bacteria Ramlibacter tatahouinensis. Geomicrobiol J 21:341–349 Benzerara K, Yoon TH, Tyliszczak T, Constantz B, Spormann AM, Brown GE Jr (2004b) Scanning transmission x-ray microscopy study of microbial calcification. Geobiology 2:249–259 Bernard S, Benzerara K, Beyssac O, Menguy N, Guyot F, Brown GE Jr, Goffe´ B (2007) Exceptional preservation of fossil plant spores in high-pressure metamorphic rocks. Earth Planet Sci Lett 262:257–272
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Bernard S, Beyssac O, Benzerara K (2008) Raman mapping using advanced line-scanning systems: geological applications. Appl Spectrosc 62:1180–1188 Brasier MD (2005) Critical testing of earth’s oldest putative fossil assemblage from the similar to 3.5 Ga Apex chert, Chinaman creek, western Australia. Precambrian Res 140:55–102 Brasier M, McLoughlin N, Green O, Wacay D (2006) A fresh look at the fossil evidence for early Archaean cellular life. Philos Trans R Soc B 361:887–902 Briggs DEG, Kear AJ (1993) Fossilization of soft-tissue in the laboratory. Science 259:1439–1442 Damste JSS, Kok MD, Koster J, Schouten S (1998) Sulfurized carbohydrates: an important sedimentary sink for organic carbon? Earth Planet Sci Lett 164:7–13 Daughney CJ, Chatellier X, Chan A, Kenward P, Fortin D, Suttle CA, Fowle DA (2004) Adsorption and precipitation of iron from seawater on a marine bacteriophage (PWH3A-P1). Mar Chem 91:101–115 Deamer D, Singaram S, Rajamani S, Kompanichenko V, Guggenheim S (2006) Self-assembly processes in the prebiotic environment. Philos Trans R Soc B 361:1809–1818 Derenne S, Robert F, Skrzypczak-Bonduelle A, Gourier D, Binet L, Rouzaud JN (2008) Molecular evidence for life in the 3.5 billion year old Warrawoona chert. Earth Planet Sci Lett 272:476–480 Ferris FG, Magalhaes E (2008) Interfacial energetics of bacterial silicification. Geomicrobiol J 25:333–337 Forterre P (2006) The origin of viruses and their possible roles in major evolutionary transitions. Virus Res 117:5–16 Garcia-Ruiz JM, Hyde ST, Carnerup AM, Christy AG, Van Kranendonk MJ, Welham NJ (2003) Self-assembled silica-carbonate structures and detection of ancient microfossils. Science 302:1194–1197 Grotzinger JP, Rothman DH (1996) An abiotic model for stromatolite morphogenesis. Nature 383:423–425 Jones B, Renaut RW (2007) Microstructural changes accompanying the opal-A to opal-CT transition: new evidence from the siliceous sinters of Geysir, Haukadalur, Iceland. Sedimentology 54:921–948 Kaiser K, Guggenberger G (2000) The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils. Org Geochem 31:711–725 Lepot K, Benzerara K, Brown GE Jr, Philippot P (2009a) Organic matter heterogeneity in 2.72 Ga stromatolites: alteration versus preservation by sulphur incorporation. Geochim Cosmochim Acta 73:6579–6599 Lepot K, Philippot P, Benzerara K, Wang GY (2009b) Garnet-filled trails associated with carbonaceous matter mimicking microbial filaments in Archaean basalt. Geobiology 7:1–10 Lepot K, Benzerara K, Brown GE Jr, Philippot P (2008) Microbially influenced formation of 2, 724-million year-old stromatolites. Nat Geosci 1:118–121 McCollom TM, Seewald JS (2007) Abiotic synthesis of organic compounds in deep-sea hydrothermal environments. Chem Rev 107:382–401 Miot J, Benzerara K, Morin G, Kappler A, Bernard S, Obst M, Fe´rard C, Skouri-Panet F, Guigner JM, Posth N, Galvez M, Brown GE Jr, Guyot F (2009) Iron biomineralization by neutrophilic iron-oxidizing bacteria. Geochim Cosmochim Acta 73:696–711 Muller KJ (1985) Exceptional preservation in calcacerous nodules. Philos Trans R Soc Lond B Biol Sci 311:67–73 Obst M, Wang J, Hitchcock AP (2009) Soft x-ray spectro-tomography study of cyanobacterial biomineral nucleation. Geobiology 7:577–591 Oehler DZ, Robert F, Walter MR, Sugitani K, Allwood A, Meibom A, Mostefaoui S, Selo M, Thomen A, Gibson EK (2009) NanoSIMS: insights to biogenicity and syngeneity of Archaean carbonaceous structures. Precambrian Res 173:70–78
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Oehler JH (1976) Experimental studies in Precambrian paleontology – structural and chemical changes in blue-green-algae during simulated fossilization in synthetic chert. Geol Soc Am Bull 87:117–129 Raff EC, Villinski JT, Turner FR, Donoghue PCJ, Raff RA (2006) Experimental taphonomy shows the feasibility of fossil embryos. Proc Natl Acad Sci USA 103:5846–5851 Riboulleau A, Mongenot T, Baudin F, Derenne S, Largeau C (2002) Factors controlling the survival of proteinaceous material in late Tithonian kerogens (Kashpir oil shales, Russia). Org Geochem 33:1127–1130 Riding R (2002) Structure and composition of organic reefs and carbonate mud mounds: concepts and categories. Earth Sci Rev 58:163–231 Sangely L, Chaussidon M, Michels R, Brouand M, Cuney M, Huault V, Landais P (2007) Micrometer scale carbon isotopic study of bitumen associated with Athabasca uranium deposits: constraints on the genetic relationship with petroleum source-rocks and the abiogenic origin hypothesis. Earth Planet Sci Lett 258:378–396 Schopf JW, Kudryavtsev AB (2009) Confocal laser scanning microscopy and Raman imagery of ancient microscopic fossils. Precambrian Res 173:39–49 Skrzypczak-Bonduelle A, Binet L, Delpoux O, Vezin H, Derenne S, Robert F, Gourier D (2008) EPR of radicals in primitive organic matter: a tool for the search of biosignatures of the most ancient traces of life. Appl Magn Reson 33:371–397 Toporski JKW, Steele A, Westall F, Thomas-Keprta KL, McKay DS (2002) The simulated silicification of bacteria – new clues to the modes and timing of bacterial preservation and implications for the search for extraterrestrial microfossils. Astrobiology 2:1–26 Vandenbroucke M, Largeau C (2007) Kerogen origin, evolution and structure. Org Geochem 38:719–833 van Zuilen M, Chaussidon M, Rollion-Bard C, Marty B (2007) Carbonaceous cherts of the Barberton Greenstone belt, South Africa: isotopic, chemical and structural characteristics of individual microstructures. Geochim Cosmochim Acta 71:655–669 Wirth R (2009) Focused Ion Beam (FIB) combined with SEM and TEM: advanced analytical tools for studies of chemical composition, microstructure and crystal structure in geomaterials on a nanometre scale. Chem Geol 261:217–229 Yin LM, Zhu MY, Knoll AH, Yuan XL, Zhang JM, Hu J (2007) Doushantuo embryos preserved inside diapause egg cysts. Nature 446:661–663
with a diameter of 2.3 m. The spacecraft touches down in the border zone of Russia and Kazakhstan. Payloads have consisted of scientific and technological experiments with often significant contributions from Western Europe. Power is provided by batteries, the flight duration is therefore limited to 10–16 days. Orbital parameters are: inclination 63 , orbital shape near-circular, cruising altitude around 300 km. Foton has been frequently used for astrobiological space exposure studies (▶ Biopan) and reentry experiments (▶ Stone).
See also ▶ Biopan ▶ Stone
Fourier Transform Infrared Micro-spectroscopy ▶ Infrared Spectroscopy
Fourier Transform Infrared Spectroscopy ▶ Infrared Spectroscopy
Fractional Abundances ▶ Molecular Abundances
Fossilized Microbial Mats ▶ Stromatolites
Fractionation Synonyms
Foton Capsule (Spacecraft)
Differentiation; Separation
Disproportionation;
Partitioning;
Definition
Definition
Foton is an unmanned recoverable spacecraft, manufactured by TsSKB-Progress in Samara (Russia). Fifteen Fotons have been launched since 1985, initially from Plesetsk, later from Baikonur. The launch vehicle is a Soyuz-U. The reentry capsule is spherically shaped,
▶ Fractionation is the partitioning of a chemical species into two or more phases based on its chemical or physical properties. The chemical or ▶ isotopic composition of each phase may reflect an enrichment or depletion in one element or ▶ isotope with respect to the others,
Fractionation, Mass Independent and Dependent
which can yield information about the mechanism of formation. Fractionation can be caused by differences in mass, binding energy, and by biochemical reactions, which alter the ratio of one element or isotope to another. For example, fractionation of stable ▶ carbon isotopes can provide information on the biotic or abiotic origin of some organic compounds, as photosynthetic reactions process the lighter isotope, 12C, more rapidly than 13C, resulting in enrichment in 12C of its products.
See also ▶ Biomarkers, Isotopic ▶ Carbon Isotopes as a Geochemical Tracer ▶ Deuterium/Hydrogen Ratio ▶ Differentiation (Planetary) ▶ Distillation (Rayleigh) ▶ Fractionation, Mass Independent and Dependent ▶ Isotope ▶ Isotopic Exchange Reactions ▶ Isotopic Fractionation (Interstellar Medium) ▶ Isotopic Fractionation (Planetary Process) ▶ Isotopic Ratio ▶ Nitrogen Isotopes
Fractionation, Mass Independent and Dependent FRANCIS ALBARE`DE Ecole Normale Supe´rieure de Lyon, Lyon Cedex 7, France
Keywords Isotope, mass fractionation
Definition Isotope fractionation is referred to as mass dependent when observed isotopic abundances deviate smoothly and monotonically with the masses of the isotopes, from those in the reference material. Both kinetic and equilibrium processes are known to account for this most common form of isotope fractionation. Alternate patterns are referred to as mass independent and are known for O and S in some specific environments, and are often related to photochemistry in the solar nebula and in planetary atmospheres.
Overview Isotope fractionation varies approximately with the difference of the isotopic masses. Using ▶ oxygen isotopes as
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an example, the mass difference 18O 16O is +2 and d18O will be twice the d17O ðwhere; e:g:; d18 O is 18 therefore 16 ð O= OÞsample =ð18 O=16 OÞstd 1 Þ: In a d17O vs. d18O diagram, all terrestrial samples plot on the same fractionation line with a slope of 0.5. Different planets plot on parallel fractionation lines, except the Earth and the Moon, which plot on the same line, a strong argument in favor of a common origin for the two planetary objects (the Moon Giant Impact). Small variations of the slope reflect different conditions of fractionation, notably kinetic and distillation effects. Mass-independent isotope fractionation (MIF) is observed for some elements and the most noticeable effects occur for O and S. The d17O and d18O of calcium–aluminum-rich refractory inclusions (CAIs) and chondrules in chondrites tend to plot on a line with a slope of 1.0, instead of the normal slope of 0.5 expected from mass-dependent fractionation. Likewise, the oxygen of some trace atmospheric compounds, notably ozone, nitrate, and sulfate also plot on a line with a slope of 1. Pre-2.3 Ga sulfur also shows MIF of d33S with respect to d34S. Causes for MIF are not agreed upon, at least not for all the elements. Self-shielding calls for isotope-dependent shifts of absorption wavelengths in the UV range: dissociation in the gas phase is proportional to the abundance of a particular isotope and not to its mass. Self-shielding upon dissociation of a CO-rich gas is a popular interpretation for oxygen in the solar nebula but critics invoke fast re-equilibration between the reaction products. Symmetry is a critical property that defines reaction paths: the two ozone isotopologues (molecules that differ only in their isotopic composition) 16O 18O17O and 16O 18O16O have different symmetry and their reaction pathways involve different numbers of quantum states, which greatly affects the rate of the reactions in which ozone is involved. Ozone present in the modern stratosphere blocks solar UV radiation. Prior to the “Great Oxygenation Event” at 2.3 Ga, the abundance of O2 in the atmosphere was low and the ozone layer absent: photolysis by solar UV of the tropospheric SO2 released by volcanic activity is thought to have been an important cause of the MIF recorded in Archean sulfates. Finally, the finite volume of heavy nuclei and spin–orbit coupling may be additional minor causes of deviation of isotope fractionation patterns from the purely mass-dependent trends.
See also ▶ Isotopic Fractionation (Planetary Process) ▶ Oxygen Isotopes ▶ Sulfur Isotopes
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References and Further Reading Sharp Z (2007) Principles of stable isotope geochemistry. Prentice Hall, Upper Saddle River Thiemens MH (2006) History and applications of mass-independent isotope effects. Annu Rev Earth Planet Sci 34:217–262
Fragmentation (Interstellar Clouds) STEVEN STAHLER Department of Astronomy, University of California, Berkeley, CA, USA
Synonyms Star formation
Definition The ▶ dense cores that form stars through gravitational collapse are embedded in much larger and more rarefied expanses of gas. How the parent ▶ molecular cloud produces its substructure of dense cores is the problem of fragmentation. The traditional view is that the parent cloud breaks apart as it collapses in on itself. In numerical simulations, objects resembling dense cores are created in turbulent, collapsing clouds. However, there is little evidence that large clouds are indeed collapsing. If they are not, but are at least temporarily stable, then dense cores must be produced in another fashion, perhaps by the slow accretion of background gas.
Overview The interstellar clouds that contain young stars are relatively small structures embedded within more diffuse background gas. These dense cores have sizes of about 0.1 parsec, and masses comparable to that of the Sun. The diffuse parent bodies, known as dark clouds or clumps, have sizes larger by two orders of magnitude and masses of at least 1,000 times solar. It is the gravitational collapse of dense cores that creates the new stars we observe in them. But how are dense cores themselves created? The general, and as yet unsolved, issue of how these substructures arise within relatively large molecular clouds is the problem of fragmentation. A starting point for many theories is the observed fact that the parent cloud has far too much mass, and is far too cold, to be supported by internal pressure. In the absence of other forces, the object must collapse on itself.
The traditional view of fragmentation is that this collapse generates substructure. As the cloud contracts, it breaks apart, producing several daughter clouds. Each daughter, in turn, collapses and breaks up. After a number of generations, fragments with stellar-type mass are produced – the observed dense cores. Within the last few decades, many theorists have performed numerical simulations of collapsing clouds. The fragmentation hierarchy is not seen. However, if the parent cloud is also turbulent, it may directly create substructures resembling dense cores. Intriguingly, the simulated objects even exhibit the range of three-dimensional shapes of real cores. Unfortunately, there is little evidence that the larger bodies are undergoing collapse. Observations indicate that the typical age of dark clouds and clumps is 10 million years, an order of magnitude longer than the time for them to collapse. It thus appears that these entities are indeed supported, probably by their internal magnetic fields and turbulent motion. If the parent bodies are not collapsing, then dense cores must arise in another way. Rather than breaking off from the parent (the top-down view), they may accrete gas from their surroundings (the bottom-up view). The mass of the ▶ dense core increases until the object becomes unstable and undergoes collapse. This picture is in better accord with the observation that dense cores themselves appear to be in force balance, at least before they form stars.
See also ▶ Dense Core ▶ Interstellar Medium ▶ Molecular Cloud ▶ Star Formation, Observations ▶ Stellar Cluster
References and Further Reading Hoyle F (1953) On the fragmentation of gas clouds into galaxies and stars. Astrophys J 118:513 Offner S, Klein RL, McKee CF (2008) Driven and decaying turbulence simulations of low-mass star formation: from clumps to cores to protostars. Astrophys J 686:1174 Stahler SW, Palla F (1994) Chapter 12. In: The formation of stars. Wiley, Weinheim
Free Amino Acid Definition In chemistry, a free amino acid is an amino acid which is not covalently bound in a peptide or in another sort of
FRET
linkage, for example, in a melanoidin polymer, a ▶ HCN Polymer or by chelating an inorganic ion.
See also ▶ Amino Acid ▶ HCN Polymer ▶ Oligopeptide ▶ Protein
Free Energy JACQUES REISSE Universite´ Libre de Bruxelles, Brussels, Belgium
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Free-Fall Time Definition The time required for an astronomical object to collapse under the influence of self-gravity is called the free-fall time. A hypothetical gas sphere of uniform density and zero temperature collapses to a point at its center in a finite time. This time is inversely proportional to the square root of the initial density. The free-fall time of real objects can still be approximated from the idealized result. For stars, this time is roughly one hour. Such objects are normally supported by internal pressure and do not collapse. In this case, the free-fall time is approximately equal to the sound-crossing time and to the period of global oscillations.
See also Definition Free energy refers to the amount of energy in a system that can be converted to work when the reaction takes place at constant pressure. This amount of energy is necessarily less than the available enthalpy. For chemical reactions taking place at constant pressure (which is frequently the case), free energy refers to the Gibbs free energy. The thermodynamic definition of the Gibbs energy (G) leads to the familiar relation: G = H – TS, where H stands for enthalpy, T for temperature, and S for entropy.
▶ Collapse, Gravitational ▶ Fragmentation (Interstellar Clouds) ▶ Protostars
Free-free Emission ▶ Bremsstrahlung Radiation
See also ▶ Bioenergetics
French Space Agency ▶ CNES
Free Radical ▶ Radical
FRET Synonyms
Free Water ▶ Water Activity
Freefall ▶ Microgravity
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Fluorescence resonance energy transfer; Fo¨rster resonance energy transfer
Definition FRET is a technique used to measure the proximity of chemical groups in macromolecules or between host and guest molecules. It involves the use of a donor (in an excited electronic state) and acceptor ▶ chromophore. When they are in close proximity (usually < 10 nm), the donor may transfer its energy to the acceptor through nonradiative dipole–dipole coupling. This is known as
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“Fo¨rster resonance energy transfer.” When both chromophores are fluorescent, the term “▶ fluorescence resonance energy transfer” is used instead, although the energy is not actually transferred by fluorescence, but by nonradiative transfer.
See also ▶ Chromophore ▶ Fluorescence
Frost Line ▶ Snow Line
FTIR ▶ Infrared Spectroscopy
Fulleranes Synonyms Hydrogenated fullerenes
Definition ▶ Fulleranes are hydrogenated derivatives of ▶ fullerenes. It has been speculated that they may be present in the interstellar medium and in the envelopes of certain types of stars.
See also ▶ Fullerenes
Keywords Carbon Molecules, hydrogenation, spectroscopy
Definition Fulleranes are carbon molecules arranged in the form of hollow spheres. Any graphene sheet can be closed into a fullerene cage provided that 12 pentagons are inserted into the sheet of condensed hexagonal rings. Only fullerenes having the 12 pentagonal sites fully annealed by hexagonal rings are stable and have been isolated in macroscopic quantity. This is an important rule regulating the fullerene stability. The fullerenes isolated in macroscopic quantity are those following strictly the isolated pentagons rule: C60, C70, C76, C84, C90, C94. . . Each fullerene contains 2(10+z) carbon atoms corresponding to 12 pentagonal sites and z hexagons. This building principle is a consequence of Euler’s theorem. Fullerenes can have elements (metals, noble gases) trapped inside the cages and these molecules are known as endohedral fullerenes (Kroto et al. 1991).
History The structure of the most common fullerene, C60 was first hypothesized in 1970 by Eiji Osawa in Japan and was the object of topological and theoretical calculations by the Russians I.V. Stankevich, D.A. Bochvar, E.G. Gal’pern in 1973. Only in 1984–1985, using mass spectrometry was it possible to observe a series of carbon clusters that were recognized as fullerenes. H. Kroto, R. Curl, and R. Smalley were awarded the 1996 Nobel Prize in Chemistry for their role in the discovery of this new class of molecules. The production of fullerenes in microscopic quantities at first by W. Kraetschmer and D. Huffmann in 1990 created a major breakthrough in fullerene science. The fullerenes are named after Buckminster Fuller, an architect who designed polyhedral domes based on hexagonal and pentagonal faces.
Overview
Fullerenes FRANCO CATALDO1,2, SUSANA IGLESIAS-GROTH3 1 Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania, Catania, Italy 2 Actinium Chemical Research, Rome, Italy 3 Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain
Fullerene C60 is the most studied fullerene. It is composed by 60 sp2 hybridized carbon atoms arranged in a truncated icosahedron geometry resembling a soccer ball. The molecule contains 30 weakly conjugated double bonds located between the hexagons. The diameter of C60 is 700 pm. All carbon atoms in C60 are equivalent, giving a sharp single signal in the 13C-NMR spectrum located at a chemical shift of 143.2 ppm. The mean C–C distance is 141 pm, which is almost same as in graphite (Kroto et al. 1991).
Basic Methodology Synonyms Buckminsterfullerene; Buckyballs; Footballene
Fullerenes are produced by quenching carbon vapor in a helium atmosphere. Carbon vapor can be generated by
Fullerenes
Fullerenes. Figure 1 Stick and ball model of C60 fullerene
resistive heating of graphite or in a 40–60 Ampe´re carbon arc. The best yields of fullerenes are obtained at a helium gas pressure between 100 and 200 mbar. Under these conditions, a carbon soot is collected which, when extracted with benzene or toluene, releases 5–10% of its weight under the form of extractable colored matter consisting of a mixture of C60 and C70 fullerenes. C60 is by far more abundant than C70 in the mixture. The two fullerenes can be separated by common chromatographic procedures on an alumina column using hexane/toluene as eluents. Small amounts of higher fullerenes (C76, C84, C90, C94) can be further separated by high-performance liquid chromatography on reversed phase using acetonitrile/toluene as mobile phase. In the extracted soot, fullerenes > C100 remain, which can be further extracted with 1,2,4-trichlorobenzene (Taylor 1999). Fullerenes can be produced in relatively small yield also under controlled combustion conditions in sooting flames and extracted with solvents from the recovered soot. This method has achieved industrial applications (Murayama et al. 2004).
Key Research Findings Fullerenes have been predicted to be present in space (Hare and Kroto 1992). The main sources of carbon dust and molecules in the interstellar medium are the late-type carbon-rich stars (Ehrenfreund and Charnley 2000; Millar 2004), but there is a class of stars, which lies in the transition between the asymptotic giant branch (AGB) and the planetary nebula stage that is very promising as a source of fullerenes. The stars in this class are
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rather rare, are helium rich, and are extremely depleted in the hydrogen content of their gaseous shells, so that as the carbon vapor is ejected from the star, it cools and forms an ideal environment for fullerene formation (Kroto 2006). In fact, the presence of hydrogen is known to have negative effects on the formation of the fullerene cage and to favor the production of ▶ polycyclic aromatic hydrocarbons (PAHs) and other products (Goeres and Sedlmayr 1992; 1993). The prototype of such stars where fullerenes may be present is R Coronae Borealis and the corresponding class of RCrB stars (Unsold and Baschek 2002). These stars are pulsating like the Cepheids but very irregularly. Thus, it may happen that their magnitude drops suddenly from fifth to as faint as 14th magnitude, recovering sometimes quickly, at other times taking months. The dimming comes from irregular clouds of carbon dust ejected by the stars, perhaps as a result of pulsation (Kaler 2006). Once ejected into space, fullerenes are extremely reactive, for instance with the ubiquitous atomic hydrogen, leading to the formation of hydrogenated derivatives known as ▶ fulleranes. In the interstellar medium, C60 fullerene should be present as a neutral molecule or may undergo ionization to C60+. This cation may be responsible for some spectral features in the diffuse interstellar bands (DIBs), absorption bands detected in the spectra of our Galaxy and beyond (Foing and Ehrenfreund 1994). Fullerene cations may undergo multiple addition of atomic hydrogen, forming hydrogenated derivatives (Petrie et al. 1995). Neutral fullerenes also easily add atomic hydrogen at very low temperatures (Howard 1993), so that it is reasonable to think that fulleranes, the hydrogenated fullerenes derivatives, should be present in the interstellar medium (Petrie and Bohme 2000). Fulleranes can be produced in the laboratory under a variety of conditions (Cataldo and Iglesias-Groth 2010). Deuterated fulleranes have also been synthesized. Significant isotope effects both in the photolysis and in the thermal decomposition of fulleranes and perdeuterofulleranes have been observed (Cataldo 2009a; Cataldo and Iglesias-Groth 2010). An enrichment in the deuterium content in fulleranes is hence expected in the space environment. Additionally, fulleranes release molecular hydrogen under certain circumstances and are thought to play a role in molecular hydrogen formation in space, starting from atomic hydrogen (Cataldo and IglesiasGroth 2010). Calculated spectra for hydrogenated fullerenes have been published in comparison with the unidentified infrared emission bands (Webster 1991; Cataldo and IglesiasGroth 2010). The infrared spectrum of C60H36 has also been compared with the infrared features of astrophysical
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objects like the proto-planetary nebulae (Cataldo 2003). An inventory about fullerenes and hydrogenated derivatives in the interstellar medium and their theoretical spectral properties can be found in the recent works of Iglesias-Groth (2004, 2005, 2006). A connection between fullerenes and the strongest ▶ diffuse interstellar band (DIB) known in the optical has also been proposed (Iglesias-Groth 2007, 2008). An important property of molecules of astrochemical interest regards their stability to UV photons. A plethora of organic molecules are today known in different space environments (Ehrenfreund and Charnley 2000), some of them known to be the precursors of life. Photochemical processing may lead to the production in space of other interesting molecules from common precursors or may lock certain molecules into dust or dust-forming nanoparticles. In this context, the photophysical and photochemical properties of fullerenes and their hydrogenated counterpart, the fulleranes, are of interest, both for the detection of these species and to estimate their survival and their fate in the harsh space environment. The photophysical properties of fullerenes are well known, the photochemistry and the radiation chemistry of these molecules having been explored as well, reveal a very high stability of fullerenes to both high-energy photons and corpuscular radiation (Cataldo et al. 2009b).
Future Directions The search for fullerenes in various natural environments has been the object of a monograph (Rietmeijer 2006). Fullerenes have been found in the Cretaceous-Tertiary boundary layer, in meteorites, and in organic deposits. The search for fullerenes in space will be intensified (Sellgren et al. 2009; Iglesias-Groth 2007, 2008; Lambert et al. 2001; Clayton et al. 1995; Goeres and Sedlmayr 1993). It will be necessary to record the spectra in the infrared, the ultraviolet, and the visible of fullerenes and fulleranes, especially at low temperatures, in order to have available reference spectra useful for the research of this class of molecules in space. Moreover, future research in the laboratory will be directed toward higher fullerenes, which until now have been scarcely studied simply because they are not easily accessible. On July 2010 there was the announcement that fullerenes C60 and C70 are present around a young planetary nebula known as Tc1 (Cami et al. 2010). The discovery was made possible through the Spitzer infrared space telescope measurements. The detection of fullerenes in that astrophysical object comes as a surprise since it is known that fullerenes are formed only in environments
completely free from hydrogen. The fullerenes were detected in the inner core region of Tc1 which turned out to be carbon-rich, hydrogen-poor and dusty (Cami et al. 2010). Fullerene C60 has been also detected in the interstellar medium and more precisely in the reflection nebulae NGC 7023 and NGC 2023 (Sellgren et al. 2010).
See also ▶ Diffuse Interstellar Bands ▶ Fulleranes ▶ Polycyclic Aromatic Hydrocarbons
References and Further Reading Cami J, Bernard-Salas J, Peeters E, Malek S E (2010) Detection of C60 and C70 in a young planetary nebula. Science 329:1180–1182 Cataldo F, Iglesias-Groth S (2010) Fulleranes: The hydrogenated fullerenes. Springer, Berlin Cataldo F, Iglesias-Groth S, Manchado A (2009a) On the action of UV photons on hydrogenated fulleranes C60H36 and C60D36. Roy Astronom Soc 400:291–298, Monthly Notice Cataldo F, Strazzulla A, Iglesias-Groth S (2009b) Stability of C60 and C70 fullerenes toward corpuscular and gamma radiation. Roy Astronom Soc 394:615–623, Monthly Notice Cataldo F (2003) Fullerane, the hydrogenated C60 fullerene: properties and astrochemical considerations. Fullerenes Nanot Carbon Nanostruct 11:295–316 Clayton GC, Kelly DM, Lacy JH, Little-Marenin IR, Feldman PA, Bernath PF (1995) A mid-infrared search for C60 in R coronae borealis stars and IRC + 10216. Astronom J 109:2096 Ehrenfreund P, Charnley SB (2000) Organic molecules in the interstellar medium, comets and meteorites: a voyage from dark clouds to the early Earth. Annu Rev Astronom Astrophys 38:427–483 Foing BH, Ehrenfreund P (1994) Detection of two interstellar absorption bands coincident with spectral features of C60+. Nature 369:296–298 Goeres A, Sedlmayr E (1993) Hydrogen-blocking in C60 formation theories. Fullerene Sci Technol 1:563–570 Goeres A, Sedlmayr E (1992) The envelopes of R Coronae Borealis stars. I – A physical model of the decline events due to dust formation. Astronom Astrophys 265:216–236 Hare JP, Kroto HW (1992) A postbuckminsterfullerene view of carbon in the galaxy. Acc Chem Res 25:106–112 Howard JA (1993) EPR, FTIR and FAB mass spectrometric investigation of reaction of H atoms with C60 in a cyclohexane matrix. Chem Phys Lett 203:540–544 Iglesias-Groth S (2008) Fullerenes as carriers of extinction, diffuse interstellar bands and anomalous microwave emission. Proc Int Astron Union IAU Symp 251:57–62 Iglesias-Groth S (2007) Fullerenes and the 4430 A˚ diffuse interstellar band. Astrophys J 661:L167–L170 Iglesias-Groth S (2006) Hydrogenated fulleranes and the anomalous microwave emission of the dark cloud LDN 1622. Roy Astron Soc 368:1925–1930, Monthly Notice Iglesias-Groth S (2005) Electric dipole emission by fulleranes and galactic anomalous microwave emission. Astrophys J 632:L25–L28 Iglesias-Groth S (2004) Fullerenes and buckyonions in the interstellar medium. Astrophys J 608:L37–L40
Fungi Kaler JB (2006) The Cambridge encyclopedia of stars. Cambridge University Press, Cambridge, p 203 Kroto HW, Allaf AW, Balm SP (1991) C60: Buckminsterfullerene. Chem Rev 91:1213–1235 Kroto HW (2006) Introduction: Space-Pandora’s Box. In: Rietmeijer FJH (ed) Natural fullerenes and related structures of elemental carbon. Springer, Dordrecht, pp 1–5 Lambert DL, Rao NK, Pandey G, Ivans II (2001) Infrared space observatory spectra of R Coronae Borealis stars. I. Emission features in the interval 3–25 microns. Astrophys J 555:925–931 Millar T (2004) Organic molecules in the interstellar medium. In: Ehrenfreund P (ed) Astrobiology: future perspectives. Kluwer, Dordrecht, pp 17–31 Murayama H, Tomonoh S, Alford JM, Karpuk ME (2004) Fullerene production in tons and more: from science to industry 1536–4046. Fullerenes Nanotubes and Carbon Nanostructures 12(1):1–9 Petrie S, Becker H, Baranov VI, Bohme DK (1995) Repeated addition of atomic hydrogen to fullerene cations, dications and trications. Int J Mass Spectrom 145:79–88 Petrie S, Bohme DK (2000) Laboratory studies of ion/molecule reactions of fullerenes: chemical derivatization of fullerenes within dense interstellar clouds and circumstellar shells. Astrophys J 540:869–885 Rietmeijer FJH (2006) Natural fullerenes and related structures of elemental carbon. Springer, Dordrecht, pp 1–5 Sellgren K, Werner MW, Ingalls JG (2009) The 5–15 micron spectrum of reflection nebulae as a probe for fullerenes. American Astronomical Society, AAS Meeting 214, 402.12. Bull Am Astronom Soc 41:664 Sellegren K, Werner MW, Ingalls JG, Smith JTD, Carleton TM, Joblin C (2010) C60 in reflection nebulae. Astrophys J Lett 722:L54–L57 Taylor R (1999) Lecture notes on fullerene chemistry. A Handbook for Chemists. Imperial College Press, London, pp 56–70 Unsold A, Baschek B (2002) The new cosmos: an introduction to astronomy and astrophysics, 5th edn. Springer-Verlag, Berlin, p 247 Webster A (1991) Comparison of a calculated spectrum of C60H60 with the unidentified astronomical infrared emission feature. Nature 352:412–414
Fumarole
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See also ▶ Crater, Impact ▶ Cryovolcanism ▶ Earth ▶ Mars ▶ Mars Exploration Rovers
Functional Inhibitors ▶ Antibiotic
Fungi ALDO GONZA´LEZ Centro de Biologı´a Molecular, lab 104, CBMSO Consejo Superior de Investigaciones Cientificas Universidad Auto´noma de Madrid, Cantoblanco, Madrid, Spain
Keywords Aspergillus, carpophores, filamentous fungi, hiphae, mushrooms, penicillium, yeasts
Definition Fungi are nonphototrophic, heterotrophic eukaryotic microorganisms that contain rigid cell walls and produce spores. Fungi form a tight phylogenetic cluster.
Definition
Overview
Fumaroles (Latin fumus, smoke) are vents from which volcanic gas escapes into the atmosphere. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic (explosive ash) flows. They may persist for decades or centuries if they are above a persistent heat source or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools. Fumaroles are common on ▶ Earth; for example, there are an estimated four thousand fumaroles within the boundaries of Yellowstone National Park. The ▶ Mars Exploration Rover Spirit has identified possible fumarolic deposits in Gusev ▶ crater.
Defining fungi within the group of eukaryotes is a complex and difficult task and it is preferable to construct a definition based on their common properties rather than on the differences that separate them from the rest of the eukaryotic species. Their peculiar method of reproduction is perhaps one of the main axis on which it is possible to construct a phylogenetic classification, reflecting, in some way, the mechanisms of evolution of each of the groups of fungi represented on Earth. Many species behave according to Mayr’s speciesdefinition, presenting a sexual life cycle where two mycelia having sexually compatible cells join to produce a panmictic population that mates randomly after meiotic division generating a fertile population, while others are
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parasexual (i.e., involving nuclear fusion followed by gradual de-diploidization). In a high percentage of species, this mechanism implies an asexual reproduction system that gives rise to specialized cells known as conidiophores that produce dikaryotic cells by mitosis (only nuclear division) which are able to generate a new organism. When they are isolated in pure culture, the asexual strain, known as anamorphous (i.e., Aspergillus nidulans) is obtained by the mating of two anamorphous, while the sexual strain is known as teleomorphous (i.e., Emericella nidulans). One species has these two reproductive states (asexual and sexual). These two ways of representation in the world of living beings, make it necessary to create two systems of classification, one for each type of reproductive state, knowing beforehand that the one for the sexual cycle is close to a phylogenetic proposal and that the other, based on the forms of conidia production, is rather artificial. About 120,000 species of fungi have been described to date, although the total estimated number of species is around 1.5 million. The versatility of their “soma,” also named mycelia, which in the case of filamentous fungi are made up of a group of septate hyphae, allow them to be present in all possible ecosystems on Earth. Where they are a minority, it is because this environment has, in fact, been little studied yet. Recent studies of metagenomics report their presence in ecosystems under environmental pressure or in the sea. In recent studies of classification of all described species, six Kingdoms have been established. Fungi have been included in the Kingdom Mycota, in which seven monophyletic “true fungi” are included: Chytridiomycota, Zigomycota, Endomycota (Yeasts), Ustomycota, Ascomycota, Basidiomycota, and Deuteromycota, all grouped as Eumycota. The organisms known as “pseudofungi” are phylogenetically close to Protists, and contain the Oomycota and HyphochytridioLabrynthulomycota groups. According with their special way of life, all fungi can be defined by the following characteristics: (1) they are heterotrophs and their nutrition is by absorption. (2) Vegetative stage, they grow on or inside the substrate as hyphae forming a mycelium showing internal protoplasmic streaming. Reproductive stages with mobility may occur. (3) Cell wall is generally formed by glucans and chitin, in some cases, glucans and cellulose. (4) Nuclear status, Eucariota, uni- or multinucleate, the thallus may be homo or heterokaryotic, dikaryotic or diploid, the last usually of short duration with exceptions in some groups. (5) Life cycle, simple although in some cases complex. (6) Propagules, typical little spores produced in high
amounts. (7) Sporocarps or carpophores, they can be microscopic or macroscopic, with characteristic shapes. In some groups they are known as mushroom and are limited to differentiation tissues. (8) Habitat, ubiquitous in terrestrial, fresh water habitats and in lower numbers in marine waters, although fungal species have been less studied in this environment. (9) Ecology, they play important roles as saprotrophs, mutualistic symbionts, parasites or hyperparasites. (10) Pathology, they are the cause of serious pathologies to animals and plants. (11) Cosmopolitan distribution. (12) Degradation, they are important degraders of plant eliminate macro-polymers (i.e., cellulose, lignin); of fossil combustibles; “sequesters” of heavy metals and radioactive elements at high concentrations. In general they are responsible for 75% of the turnover of carbon (▶ carbon cycle), contributing to the degradation of biomass and facilitating its re-incorporation into the vital cycle of terrestrial and aquatic plants. At present, the objective is to promote a new taxonomy based directly on comparisons of selected DNA sequences that encode genes with a conserved biological function, instead of or in addition to phenotypic characteristics. These gene sequences should allow the construction of phylogenetic trees integrating microscopic, ultrastructural, and biochemical data leading to a fuller understanding of fungal taxonomy with monophyletic criteria. Nevertheless, the most reliable current criteria to define a given species continues to be the cross-linking of two sexually compatible monokaryon mycelia to form dikaryotic mycelia able, in turn, to produce carpophores.
See also ▶ Carbon Cycle (Biological) ▶ Eukarya ▶ Eukaryote ▶ Heterotroph ▶ Phylogeny ▶ Yeast
References and Further Reading Barnett JA, Payne RW, Yarrow W (1983) Yeasts: characteristics and identification. Cambridge University Press, Cambridge Boddy L, Coleman M (2010) From another kingdom – the amazing world of fungi. Royal Botanical Garden, Edinburgh Cavalier-Smith T (1998) A revised six-kingdom system of life. Biol Rev Camb Phil Soc 73:203–266 Cavalier-Smith T (2001) What are fungi? In: McLaughlin DJ, McLaughlin EG, Lemke PM (eds) The mycota VII A: systematics and evolution. Springer, Berlin, pp 3–37 Crous PW, Samson RA, Gams W, Summerbell RC, Boekhout T, de Hoog GS, Stalpers JA (2004a) CBS centenary: 100 years of fungal biodiversity and ecology. Stud Mycol 50(1):1–298
Fusion Crust Crous PW, Samson RA, Gams W, Summerbell RC, Boekhout T, de Hoog GS, Stalpers JA (2004b) CBS centenary: 100 years of fungal biodiversity and ecology. Stud Mycol 50(2):299–586 de Hoog GS (2005) Fungi of the Antarctic: evolution under extreme conditions. Stud Mycol 51:1–79 Ellis MB (1976) Dematiaceous hyphomycetes. CMI, Kew, Surrey Emmons ChW, Chapman YHB, UTZ JP, Kwon-Chung KJ (1977) Medical mycology, 3rd edn. Lea & Febiger, Philadelphia Kendrick B (ed) (1992) The fifth kingdom. Focus Publishing, R. Pullins Co, Newburyport Kinghorn JR, Turner G (1992) Applied molecular genetics of filamentous fungi. Blackie Academic & Professional, Glasgow Kirk PM, Cannon PF, Minter DW, Stalpers JA (2008) Ainsworth and Bisby’s dictionary of the fungi, 10th edn. CABI, Wallingford Kreger-van Rij NJW (ed) (1984) The yeasts, a taxonomy study. Elsevier Science Pub. B.V., Amsterdam Ryvarden L (1991) Genera of polypores. Nomenclature and taxonomy. Synopsis fungorum 5 fungiflora. Gronlands Grafiske A/S, Norway Sutton BC (1980) The coelomycetes. Fungi imperfecti with pycnidia, acervuli and stromata. CMI, Kew, Surrey Tavares II (1985) Laboulbeniales (fungi, ascomycta). Micologia/memoir, vol 9. J Cramer Publisher, Braunschweig Von Arx JA (1981) The genera of fungi sporulating in pure culture. J Cramer, Germany Webster J, Weber R (2007) Introduction to fungi, 3rd edn. Cambridge University Press, Cambridge Whittaker RH (1969) New concepts of kingdoms of organisms. Science 163:150–160 Zycha H, Siepmann R, Linneman G (1969) Mucorales. Eine beschreibung aller gattungen und arten dieser pilzgruppe. Verlag von J Cramer, Mu¨nchen
Furanose The term furanose denotes a five member cyclic structure containing four carbon atoms and one oxygen atom, and
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is generally reserved for the description of molecular framework in monosaccharides. The furanose ring can be a cyclic hemiacetal of an aldopentose (e.g., ribose in RNA and deoxyribose in DNA) or aldotetrose (for example threose) or a cyclic hemiketal of a ketohexose (e.g., fructose). There are two isomers possible for a furanose: the alpha (a) and beta (b) anomers depending on the arrangement at the anomeric carbon. The furanose form can exist in two types of conformations, envelope (E) and twist (T).
See also ▶ Aldose ▶ Carbon ▶ Ketose
Fusion Crust Definition A fusion crust is a feature of the external appearance of ▶ meteorites. It describes a glassy coating of the meteorite. The crust forms as a consequence of the frictional heating and melting of the most external layer of the meteorite as it passes through the atmosphere and descends to the ▶ Earth’s surface. Most of the melt is lost due to ▶ ablation. Thus, the fusion crust is mostly about 1 mm thick.
See also Definition
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▶ Ablation ▶ Earth ▶ Meteorites
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G Ga Synonyms Giga-annum; Gigayear; Gyr
Definition Ga is a common scientific abbreviation for Gigayears, 109 years, derived from the Latin Giga-annum. Note that the Latin accusative, annum, expresses an absolute age while English accusative, years, expresses a period of time. It is frequently used in fields such as astronomy and geology. Thus, the current best estimate for the age of the universe, from the Wilkinson Microwave Anisotropy Probe (spacecraft) data on the cosmic microwave background radiation, is 13.7 Ga = 13.7 Gy, and the age of the Earth, commonly taken as the time of the completion of accretion, is 4.56 Ga. In geology, it is often used to signify time before the present; for example, the time of the Moonforming impact is 4.51 Ga and the age of the oldest rocks at the Earth’s surface is about 4.03 Ga.
its extrusive equivalent basalt, is common on the Moon, Mars, many large asteroids, and probably Mercury and Venus.
See also ▶ Basalt ▶ Oceanic Crust ▶ Ophiolite
Gaia (Mission) ALESSANDRO SOZZETTI Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Torino, Pino Torinese, Italy
Keywords Astrometry, exoplanet, extrasolar planet, planetary systems
See also ▶ Earth, Age of ▶ Geochronology ▶ Ma
Gabbro Definition Gabbro is a common mafic intrusive magmatic rock, chemically equivalent to the ▶ basalt. A medium to coarse-grained, dark-colored rock, gabbro is composed of Ca-plagioclase and clinopyroxene, with or without olivine and orthopyroxene. The Earth’s oceans are underlain by gabbro which comprises a 3–5 km-thick layer of the ▶ oceanic crust, produced by crystallization of basaltic magma erupting at the mid-oceanic ridges. Gabbro, and
Definition The ▶ European Space Agency’s Gaia all-sky survey will monitor astrometrically, during its 5-year nominal mission lifetime, all point sources (▶ stars, ▶ asteroids, quasars, extragalactic ▶ supernovae, etc.) in the visual ▶ magnitude range V = 6–20 mag, a huge database encompassing 109 objects. It is due to launch in Summer 2012. Using the continuous scanning principle first adopted for ▶ Hipparcos, Gaia will determine the five basic astrometric parameters (two components of position, two of ▶ proper motion, and the ▶ parallax) for all objects, with end-of-mission precision exceeding that of ▶ Hipparcos by 1–2 orders of magnitude. Gaia astrometry, complemented by onboard spectrophotometry and (partial) radial-velocity information, will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical, and ▶ star formation evolution of the Milky Way Galaxy.
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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Overview The Gaia mission is the new global, all-sky, astrometric initiative of the European Space Agency with a launch possibility occurring in late Summer 2012. A Soyuz-Fregat launcher will take the Gaia module to a transfer orbit, which in 1 month will allow the satellite to reach its operational environment on a Lissajous orbit at Sun– Earth ▶ Lagrange point L2, 1.5 million kilometers away from Earth. During its 5 years of operational lifetime (with the possible extension of an extra year), Gaia will monitor all point sources in the visual magnitude range V = 6–20 mag, a huge database of 109 stars, a few million galaxies, half a million quasars, and a few hundred thousand asteroids. As for the observing strategy, Gaia’s mode of operation has adopted the principles successfully experimented with the Hipparcos mission (ESA 1997). In particular, it will continuously scan the sky, implying that all detected objects, irrespective of their magnitudes, are observed for the same amount of time during each field-of-view crossing, with mission-end observing time mainly depending on ecliptic latitude (Lindegren 2010). In this way, it is anticipated that Gaia will determine the five basic astrometric parameters (two positional coordinates, two proper motion components, and the parallax) for all objects, with end-of-mission (sky-averaged) precision between 7 and 25 mas (micro-arcseconds) down to the Gaia magnitude G = 15 mag and a few hundred mas at G = 20 mag, depending on color. Red objects are expected to have better astrometry, while that for extremely blue targets is estimated to degrade by a factor of two. The main partners inside the Gaia project are: (1) the European Space Agency (ESA), which has the overall project responsibility for funding and procurement of the satellite, launch, and operations. Of interest is the fact that in this case satellite procurement includes the payload and its scientific instruments, unlike ESA’s other science missions for which scientific instruments are usually Principal Investigator led and funded (or, at least, co-funded) by participating national space agencies; (2) EADS Astrium, who was selected in 2006 as the prime industrial contractor for designing and building the satellite according to the scientific and technical requirements formulated to fulfil the mission science case as approved at time of selection (ESA 2000); (3) the Gaia Data Processing and Analysis Consortium (DPAC), charged with designing, implementing, and running a complete software system for the scientific processing of the satellite data, resulting in the edition of the “Gaia Catalogue” a few years after the end of the operational (observation) phase.
DPAC was formed in 2006 in response to an “Announcement of Opportunity” issued by ESA. The Consortium lists nearly 400 individual members in more than 20 countries. Six data processing centers participate in the activities of the consortium, which is organized in eight “Coordination Units,” each responsible for the development of one part of the software (e.g., core astrometric processing, photometry). Most of the financial support is provided by ESA and by the various national space agencies through a legally binding longterm funding agreement, a real first for ESA run missions. There will be no proprietary periods for the scientific exploitation of the data. The final Gaia catalogue will be produced and immediately delivered to the astronomical community worldwide as soon as ESA and DPAC will agree on the processed data having reached the targeted (science) quality. This catalogue is expected to be ready 3 years after the end of operations. Finally, intermediate releases of some provisional results are planned after a few years of observations. More information can be found in Lindegren (2010), while other organizational details and the latest news on payload and satellite developments are available on the Gaia web pages at http://www.rssd.esa.int/ gaia/. A combination of an ambitious science case, wishing to address breakthrough problems in Milky Way astronomy, and lessons learned from the Hipparcos experience brought European astronomers to realize that Gaia astrometry needed to be complemented by onboard spectrophotometry and (only for objects brighter than G = 17) radial-velocity information. These data will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical and star formation evolution of our Galaxy. The broad range of crucial issues in astrophysics that will be addressed by the wealth of the Gaia data is summarized by, e.g., Perryman et al. (2001). One of the relevant areas on which the Gaia observations will have great impact is the astrophysics of planetary systems (e.g., Casertano et al. 2008), in particular when seen as a complement to other techniques for ▶ exoplanet detection and characterization (e.g., Sozzetti 2010).
Basic Methodology The problem of the correct determination of the astrometric orbits of planetary systems using Gaia data (highly nonlinear orbital fitting procedures, with a large number of model parameters) will present many difficulties. For example, it will be necessary to assess the relative robustness and reliability of different procedures for orbital fits, together with a detailed understanding of the statistical
Gaia (Mission)
properties of the uncertainties associated with the model parameters. For multiple systems, a trade-off will have to be found between accuracy in the determination of the mutual inclination angles between pairs of planetary orbits, single-measurement precision and redundancy in the number of observations with respect to the number of estimated model parameters. It will constitute a challenge to correctly identify signals with amplitude close to the measurement uncertainties, particularly in the presence of larger signals induced by other companions and/or sources of astrophysical noise of comparable magnitude. Finally, in cases of multiple-component systems where dynamical interactions are important (a situation experienced already by radial-velocity surveys), fully dynamical (Newtonian) fits involving an n-body code might have to be used to properly model the Gaia astrometric data and to ensure the short- and long-term stability of the solution (see Sozzetti 2005). All the above issues could have a significant impact on Gaia’s capability to detect and characterize planetary systems. For these reasons, a Development Unit (DU) has been specifically devoted to the modelling of the astrometric signals produced by planetary systems. The DU is composed of several tasks, which implement multiple robust procedures for (single and multiple) astrometric orbit fitting (such as Markov Chain Monte Carlo and ▶ genetic algorithms) and the determination of the degree of dynamical stability of multiple-component systems.
Key Research Findings Gaia’s mode of operation (a signal-to noise limited survey with uneven coverage, including time sampling and scanning geometry, depending on ecliptic latitude) is such that there cannot be any optimization to the case of extrasolar planets. The fundamental requirement, i.e., to have sufficient astrometric accuracy at magnitudes brighter than V = 13, was established at the time of the science case definition. Since little can be done with the photometric and spectroscopic capabilities aboard the satellite, which cannot compete with present and planned ground-based facilities for very high-precision radial-velocity measurements (Pepe and Lovis 2008) and space-borne observatories devoted to ultrahigh precision transit photometry (e.g., Sozzetti et al. 2010), the potential contribution of Gaia to exoplanets science must be purely gauged in terms of its astrometric capabilities. A number of authors have tackled the problem of evaluating the sensitivity of the astrometric technique required to detect extrasolar planets and reliably measure their orbital elements and masses (Sozzetti 2005, and
G
references therein). The two most recent exercises on this subject (Casertano et al. 2008; Traub et al. 2010) have revisited earlier findings using a more realistic doubleblind protocol. In this particular case, several teams of “solvers” handled simulated datasets of stars with and without planets and independently defined detection tests, with levels of statistical significance of their choice, and orbital fitting algorithms, using any local, global, or hybrid solution method that they judged was best. The solvers were provided no information on the actual presence of planets around a given target. A double-blind test campaign to estimate the potential of Gaia for characterizing planetary systems (Casertano et al. 2008) showed that the following could be accurately modelled: a) planets having an effect on the astrometric signature (a expressed in m as) with a value around 6 times the single-measurement error (s expressed in m as) and orbital periods shorter than the nominal 5 year mission lifetime, and b) favourable configurations of two-planet systems with well-separated periods (both planets with a period inferior to 4 year, a ratio a/s > 10, and redundancy over a factor of 2 in the number of observations). In the latter case it is possible to carry out meaningful coplanarity tests (relative inclination of more than 10 ). Overall, Casertano et al. concluded that Gaia astrometry could allow the discovery and measurement of massive ▶ giant planets (Mp > 2–3 MJ) with an orbit semi-major axis between 1 and 4 AU orbiting solar-type stars as far as the nearest star-forming regions. Saturn-mass planets with similar orbital semi-major axes around late-type stars within 30–40 pc (see Fig. 1) could also be detected. From these results, using Galaxy models and with the current knowledge of frequency of exoplanets, it is possible to infer the number of planets of given mass and orbital separation that can be detected and characterized by Gaia. Table 1 demonstrates that Gaia’s main strength will be its ability to accurately measure orbits and masses for thousands of giant planets, and to perform coplanarity measurements of a few hundred multiple systems with favourable configurations.
Applications Gaia’s main contribution to exoplanet science will be its unbiased census of planetary systems orbiting hundreds of thousands nearby (d < 200 pc), relatively bright (V < 13) stars across all spectral types, screened with constant astrometric sensitivity. The Gaia data have the potential to: (a) Significantly refine our understanding of the statistical properties of extrasolar planets: the predicted
623
G
G
Gaia (Mission)
104
103
102 Planet mass (ME)
624
101
100
10−1
10−2 0.01
0.10
1.00
10.00
Semi-major axis (Au)
Gaia (Mission). Figure 1 Exoplanets discovery space for Gaia, astrometry, Doppler, and transit techniques. Detectability curves are defined on the basis of a 3s (standard deviation) criterion for signal detection. The blue curves are for Gaia astrometry with sA = 15 mas, assuming a 1 MSUN G dwarf primary at 200 pc and a 0.4 MSUN M dwarf at 25 pc, respectively. The survey duration is set to 5 years. The radial-velocity curves (red lines) assume sRV = 3 m/s (upper curve) and sRV = 1 m/s (lower curve), M* = 1 MSUN, and a 10-year survey duration. For visible-light transit photometry (green curves), the assumptions are sV = 5 103 mag (upper curve) and sV = 1 105 mag (lower curve), S/N = 9, M* = 1 MSUN, R* = 1 RSUN, uniform and dense (>1,000 datapoints) sampling. Pink dots indicate the inventory of Doppler-detected exoplanets as of December 2008. Transiting systems are shown as light-blue filled diamonds, while the red hexagons are planets detected by microlensing. Solar System planets are also shown as green pentagons. The small yellow dots represent a theoretical distribution of masses and final orbital semimajor axes from Ida and Lin (2008)
database of several thousand extrasolar planets with well-measured properties will allow, e.g., to test the fine structure of giant planet parameters’ distributions and frequencies and to investigate their possible changes as a function of stellar mass, metallicity, and age with unprecedented resolution (b) Help crucially test theoretical models of gas giant planet formation and migration: for example, specific predictions on formation timescales and the role of varying metal content in the protoplanetary disk will be probed with unprecedented statistics thanks to the thousands of metal-poor stars and hundreds of young stars screened for giant planets out to a few AU
(c) Achieve key improvements in our comprehension of important aspects of the formation and dynamical evolution of multiple-planet systems: for example, the measurement of orbital parameters for hundreds of multiple-planet systems, including meaningful coplanarity tests will allow discrimination between various proposed mechanisms for dynamical interaction (d) Aid in the understanding of direct detections of giant extrasolar planets: for example, actual mass estimates and full orbital geometry determination for suitable systems will inform direct imaging surveys about the epoch and location of maximum brightness, in order
Gaia (Mission)
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Gaia (Mission). Table 1 Top: Number of giant planets that could be detected and measured by Gaia, as a function of increasing distance. Starcounts are obtained using models of stellar population synthesis (Bienayme´ et al. 1987), while the Tabachnik and Tremaine (2002) model for estimating planet frequency as a function of mass and orbital period is used. Bottom: Number of planetary systems that Gaia could potentially detect, measure, and for which coplanarity tests could be carried out successfully Dd (pc)
Da (AU)
Ns
DMp (MJ)
Nd
Nm
0–50
1 10
1.0–4.0
1.0–13.0
1,400
700
50–100
4
5 10
1.0–4.0
1.5–13.0
2,500
1,750
100–150
1 105
1.5–3.8
2.0–13.0
2,600
1,300
150–200
3 105
1.4–3.4
3.0–13.0
2,150
1,050
4
Case
No. of Systems
Detection
1,000
Orbits and masses (1 MeV react with 39K, which is in constant proportions with 40K, to produce 39Ar. The age is deduced from the 40Ar/39Ar ratio, in lieu of 40Ar/40K, after proper calibration of the yield of the nuclear reaction through a mineral standard of known 40K-40Ar age Tm referred to as the monitor: ð40 Ar=39 ArÞspl ð40 Ar=39 ArÞm
¼
e l40 K Tspl 1 e l40 K Tm 1
ð8Þ
A major advantage of this technique is that resetting of the chronometer by thermal reheating or weathering and the presence of excess Ar can be easily identified. Argon is released by laser ablation or, most commonly, by stepwise heating: undisturbed minerals show very large “plateaus” with 40Ar/39Ar remaining constant over a broad range of 39 Ar degassing, whereas, for perturbed samples, 40Ar/39Ar reflects old ages only for high-temperature steps. In the case of inherited Ar, the high-temperature steps may be anomalous. Figure 2 shows a 39Ar-40Ar dating of the famous Martian meteorite ALH84001 (Bogard and Garrison 1999), which has been suggested to harbor fossil
remains. A limitation of this technique is the presence of atmospheric 40Ar, which is corrected for by measuring 36 Ar and using (40Ar/40Ar)atm = 295.5. Argon loss also happens easily, because it is a rare gas with no ionic or covalent bonding in crystals: only the weak van der Waals forces bind Ar to other species.
“Poor” Chronometers: Isochrons For a second class of chronometers, the condition D0 D does not apply and we replace it by the principle of isotopic homogenization. Isotope fractionation (both natural and analytical) is simply corrected for by internal normalization against some arbitrary reference ratio of stable isotopes, e.g., 179Hf/177Hf = 0.7325 for Hf. In marine carbonates, the 87Sr/86Sr ratio is exactly the same in calcite as in the seawater from which it precipitated; as the mantle melts, 176Hf/177Hf is the same in the molten liquid as in the residue. Let us therefore divide Eq. 3 by the number D 0 of atoms of a stable isotope of the same element as the radioactive nuclide represented by D. Because the system is closed, D’ remains constant and therefore: D D P lt ¼ þ e 1 0 0 0 D t D 0 D t
ð9Þ
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Geochronology
5.5
39Ar-40Ar
plateau age of ALH 84001
5.0 Age Ga
652
4.3 Ga 4.5
4.0
3.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Cumulative fraction of 39Ar outgassed
Geochronology. Figure 2 39Ar-40Ar in the gas fractions released by stepwise heating of the Martian meteorite ALH 84001 (Bogard and Garrison, 1999), which has been suggested to host microfossils. By irradiation in a nuclear reactor, fast neutrons change the 39K of the sample (which is in constant proportion with 40K) into 39Ar. The 39Ar/40Ar is a substitute for the 40K/40Ar ratio. The ages are plotted as a function of the progressive release of 39Ar. A plateau age (here 4.3 Ga) indicates that large fractions of the sample kept the memory of the same chronological event. Anomalous ages at low temperatures (small fractions of 39Ar released) indicate partial resetting by weathering or reheating. Anomalous ages at high temperatures indicate trapping of anomalous Ar components
This is the standard “isochron” equation. Figure 3 shows how an isochron based on the system 176Lu-176Hf: 176
Hf 177 Hf
¼ t
176
Hf 177 Hf
176
Lu þ 177 Hf 0
e l176 Lu t 1
ð10Þ
t
and a similar isochron based on the system 147Sm-143Nd date 2.1 Ga old Birimian basalts from West Africa (Blichert-Toft et al. 1999). D/D 0 represents the ratio of the radiogenic nuclide to its stable isotope (e.g., 176 Hf/177Hf) and P/D 0 is the “parent/daughter” ratio, in most cases proportional to an elemental ratio (e.g., 176 Lu/177Hf). If two samples 1 and 2 formed at the same time from an isotopically homogeneous medium (ocean, magma), they share the same (D/D 0 )0 and, taking the 176 Lu-176Hf system as an example, the time is obtained from: ð176 Hf =177 Hf Þ2 ð176 Hf =177 Hf Þ1 1 t¼ ln 1 þ 176 ð Lu=177 Hf Þ2 ð176 Lu=177 Hf Þ1 l176 Lu ð11Þ This age dates the time at which the two samples last shared the same 176Hf/177Hf ratio and their
parent/daughter ratios (here Lu/Hf) were fractionated. This method is commonly used for parent/daughter systems with a long half-life, typically 147Sm-143Nd, 176 Lu-176Hf, and 187Re-187Os. The 87Rb-87Sr technique is broadly used to date medium-temperature metamorphic events, 147Sm-143Nd for the emplacement of ancient basalts and komatiites, 176Lu-176Hf in garnets for hightemperature (>600 C) metamorphic events, and 187 Re-187Os for the deposition of reduced sediments (black shales) and some ores. A particular application is the Pb-Pb method, which combines the two chronometers 206Pb-238U and 207 Pb-235U. Equation 6 for the former system, slightly modified as: 207
Pb 204 Pb
t
207
Pb 204 Pb
¼ 0
235 U l235 U t e 1 ð12Þ 204 Pb t
is divided by the equation for the latter: 206
Pb 204 Pb
t
206
Pb 204 Pb
¼ 0
238 U l238 U t e 1 ð13Þ 204 Pb t
Geochronology
G
653
0.2845 Birimian basalts (West Africa)
176Hf/177Hf
0.2840
.15
Ga
2 T=
0.2835 0.2830 0.2825 0.2820 0.2815 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.25
0.30
0.35
176Lu/177Hf
0.5150
143Nd/144Nd
0.5140 0.5130 0.5120 .10
Ga
2 T=
0.5110 0.5100 0.00
0.05
0.10
0.15
0.20
147Sm/144Nd
Geochronology. Figure 3 Two examples of isochrons, using the 176Lu-176Hf (top) and 147Sm-143Nd (bottom) systems on the same early Proterozoic basalts from West Africa (Blichert-Toft et al., 1999). Slopes of these isochrons give the age at which these volcanic rocks were extracted from the mantle
to give: ð Pb= PbÞt ð Pb= PbÞ0 ð206 Pb=204 PbÞt ð206 Pb=204 PbÞ0 207
204
207
204
Pb∗ 1 e l235 U t 1 l238 t ¼ 206 ∗ ¼ 137:88 e U 1 Pb 207
ð14Þ
where we used the observation that, today, the 238U/235U ratio is constant and equal to 137.88. The asterisk stands for radiogenic Pb. Here the two parent nuclides on the one hand, and the two daughter nuclides on the other hand are isotopes of the same element, which dispenses the analyst from measuring the U and Pb contents. In a plot of 207 Pb/204Pb versus 206Pb/204Pb, samples formed at the same time from the same reservoir define a straight-line (another isochron) (Fig. 4) and the formation age can be retrieved from the slope. Historically, this is how Clair C.
Patterson determined the age of the Solar System. For statistical reasons, it is today common practice to use a 207Pb/206Pb versus 204Pb/206Pb isochron plot instead, in which the intercept, and not the slope, of the isochron gives the 207Pb /206Pb ratio and therefore the age. Figure 4 shows an isochron representing the Pb-Pb data on chondrules, little spherical blebs of molten material, and calcium-aluminum-rich refractory inclusions (CAIs) from the carbonaceous chondrite Allende (Connelly et al. 2008), which represents one of the finest attempts to date an early sample of planetary material.
Extinct Radioactivities A third class of chronometers relevant to astrobiology is that of extinct radioactivities. These have a short half-life
G
G
Geochronology
The age of Allende carbonaceous chondrite Chondrules Refractory inclusion (CAI)
0.90 0.85 207Pb/ 206Pb
654
0.80 0.75 0.70 0.65
4,567 Ma 0
0.01
0.02
0.03
0.04
0.05
0.06
204Pb/ 206Pb
Geochronology. Figure 4 A Pb-Pb “inverse” isochron of refractory calcium-aluminum rich inclusions and chondrules (small blebs of molten silicates) (Connelly et al. 2008), which accounts for most of the material in the carbonaceous chondrite Allende. The age given by the intercept (no stable 204Pb, so Pb is purely radiogenic) gives 4,567 Ma, one of the oldest estimates of the age of the Solar System
(and therefore a large l). For large values of lt, P becomes negligible and therefore the closed-system condition reads: Dtoday ¼ Pt þ Dt
ð15Þ
Let us write this equation for a sample (spl) and divide it by the abundance (D 0 )t = (D 0 )today of a stable isotope of D: spl Earth Earth 0 spl D D P P ¼ þ 0 ð16Þ 0 0 D today D t P t D 0 today in which we introduce P 0, a stable isotope of the parent 0 0 nuclide P ðPt ¼ Ptoday Þ. Using (Eq. 9), this equation is equivalent to: spl Earth Earth D D P ¼ þ 0 D0 today D 0 today P t " ð17Þ 0 Earth # P 0 spl P 0 D0 today D today For the example of the 182Hf-182W chronometer (T1/2 = 8.9 Ma) (Fig. 5), this equation reads: 182 spl 182 Earth 182 Earth W W Hf ¼ 183 þ 180 183 W W Hf t today today " Earth # ð18Þ spl 180 180 Hf Hf 183 183 W today W today This is the equation of the extinct radioactivity isochron. The conventional isochron cannot be used when all the parent nuclides have decayed away. When 182W /183W
is plotted against 180Hf/183W (note that the two nuclides of the latter ratio are both stable), samples formed at the same time in isotopic equilibrium within the Earth (e.g., mantle and core) define a straight-line. The slope of this isochron varies with time as: spl
Earth
ð182 W=183 WÞtoday ð182 W=183 WÞtoday 180 Hf =183 WÞEarth ð180 Hf =183 WÞspl today today ð ð19Þ 182 spl ¼ Earth 182 Earth Hf Hf ¼ 180 ¼ 180 e l182 Hf t Hf t Hf 0
Extracting time from this equation requires that (182Hf/180Hf)0 of the Earth is known. However, dividing this equation for the core (182Hf 0) by the same equation for the mantle gives the age mantle-core separation. This method is employed for a number of “extinct” short-lived nuclides. The chronometers 26Al-26Mg (T1/2 = 700,000 a), 60Fe-60Ni (T1/2 = 2.6 Ma), 53Mn-53Cr (T1/2 = 3.7 Ma), 182Hf-182W (T1/2 = 8.9 Ma), and 129I-129Xe (T1/2 = 15.7 Ma) provide essential chronological information on the accretion of planetary material from the solar nebula, its thermal history, and metal-silicate (core-mantle) segregation, while the 146Sm-142Nd (T1/2 = 103 Ma) system proved essential to demonstrate the existence of very early crust.
Closure Temperatures What chronology dates is actually not discrete formation events. Upon cooling, minerals keep exchanging
Geochronology
G
655
ite
iso
ch
ro n
1.8525
182W/183W
Ch
on
dr
1.8520
1.8515
Bulk silicate earth
30 Ma
G
1.8510
1.8505
0 Solar nebula
10
20
30
40
180Hf/183W
Geochronology. Figure 5 Dating of core-mantle segregation by the 182Hf-182W method (Yin et al., 2002). The difference in slope between the alignment of the points representing the chondrites and that representing the silicate Earth is a measure of the last fractionation of the Hf/W ratio. Tungsten fractionates into the metal and Hf into the silicate. The point of intersection of the two lines represents the unfractionated solar nebula. The difference in slopes show that core-mantle separation in the Earth occurred some 30 Ma after the formation of the Solar System
radiogenic isotopes with their surroundings: the chronometric system is open. In most cases, what a chronometer records is therefore the time at which diffusion stopped (Fig. 6). Diffusion of a particular ion is a thermally activated molecular process of exchange between adjacent minerals or between a phase and the interstitial medium, and is characterized by the energy of activation E and the so-called pre-exponential term D0. If cooling is fast, the closure temperature Tc (in K) is given by Dodson’s (1973) implicit equation: E Tc ¼ ð20Þ R ln AD0 RTc2 =Ea2 ðdT =dt Þ in which R is the gas constant, a the grain size, d T/dt the cooling rate, and A a constant equal to 55. If cooling is fast, the radiometric system dates the time at which temperature in the rock dropped below a particular value. The transition is usually fast (a few tens of degrees). Typical closure temperatures vary from 1,000 C for U-Pb in zircons to 800 C for PbPb in pyroxene and Lu-Hf in garnet, and to 300 C for K-Ar in feldspar and Re-Os in sulfides. If cooling is very slow, the system remains ajar for a long time, the demise of diffusion is sluggish, and the significance of the date is blurred. Diffusion loss is one of the major issues of geochronology. Rocks from the sub-lithospheric mantle evolve for long times above the closure temperature and therefore
Open system
Closed system Amount of radiogenic nuclide remaining in the mineral
Temperature T λP Tc
Closure temperature
Cooling age, tc Time, t
Geochronology. Figure 6 Cooling age and closure temperature of a chronometric system with decay constant l. The cooling time and the corresponding cooling age tc are defined by linearly extrapolating the ingrowth of radiogenic nuclides at zero, while the closure temperature Tc is the temperature of the mineral at this time. The system is open and looses the daughter isotope for T > Tc and is closed for T < Tc. The slope of the daughter isotope evolution curve is the activity lP
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Geochronology
cannot be dated by isotopic chronometers. The closure temperature varies as 2 ln a, so chronometers in smaller systems, first and foremost isolated crystals, close at a lower temperature than in the larger ones and may be easily reset by mild thermal events. Isochrons of large whole-rock samples are often used to mitigate this problem.
Basic Methodology Choosing a particular chronological system to use for a given problem depends on the duration of the age or age difference to be assessed. Enough decay must have happened that it measurably affects the abundance of the daughter isotope. In practice, a chronometer gives no useful information if this interval exceeds 5–8 half lives. The elements for which the isotope composition is sought are isolated in a variety of ways. Gases are extracted by heating and purified in high vacuum enclosures. Solid samples are normally digested with strong acids (HF, HNO3, HCl) in pressurized vessels and the metals usually separated by ion chromatography. ▶ Isotope ratios are measured on ▶ mass spectrometers, which combine a source of ions, electrostatic acceleration, a magnetic filter, and a set of detectors. These instruments differ by the nature of their ion sources. Gas sources use the
bombardment of an electron beam to ionize atomic gases and are used for O, C, He, Ar, Xe, etc. Thermal ionization mass spectrometers (TIMS) produce ions by evaporative ionization at the surface of a hot filament and are used for Sr, Nd, Pb, and Os. Inductively-coupled plasma mass spectrometers (ICP-MS) (Fig. 7) use Ar ions produced in a coil submitted to rapidly fluctuating currents to ionize the other elements: this relatively new technique accounts for most measurements for Hf, W, Mg, and Fe. Sputtering of the sample by a 30 mm primary ion beam (SIMS, or ion probe) is the method of choice for the in situ measurement of U-Pb ages in zircons (Fig. 8). The precision of measurements is first and foremost limited by counting statistics: a mass spectrometer can be seen as a device counting ions arriving on a detector. The measurement can therefore be seen as another Poisson process, which for n “events” must be associated with pffiffiffi pffiffiffi a standard deviation of n and a relative error of 1/ n. Let us assume that we use an ion probe with a 30 mm beam size to measure the abundance of 207Pb in a zircon crystal with density of 4.65 g cm3 and which contains 100 ppm Pb. We also assume that the beam excites a hemispherical volume below the spot and a one percent ion yield. We must therefore divide the weight of sputtered material by the molecular weight of Pb and multiply by
The Nu 1700 ICP-MS at Ecole Normale, Lyon, France Electrostatic sector Magnet (mass filter)
ICP ion source
Multiple collectors
Geochronology. Figure 7 The Nu 1,700 large radius MC-ICP-MS (multiple-collector inductively-coupled-plasma mass spectrometer) of Ecole Normale Supe´rieure in Lyon. The essential parts of the mass spectrometer are shown. This instrument can measure the isotope composition of most ions on quantities as small as 10 nanograms (one billionth of a gram) with a precision of 0.005% or better
Geochronology
Magnet (mass filter)
G
657
Multiple collectors
Electrostatic sector
Primary ion beam
Sample chamber
G
Geochronology. Figure 8 The Cameca 1280 ion probe (Secondary Ion Mass Spectrometer) of the Centre de Recherches Pe´trographiques et Ge´ochimiques de Nancy
the isotopic abundance of 207Pb, and finally multiply by the Avogadro number to obtain 0.01 (2/3 3.14) (15 104)3 4.65 100 106/207.2 0.22 6.0 1024 = 2.1 108 counts (approximate numbers suffice), i.e., an excellent precision of 0.007%. Reducing the beam size to one micron would deteriorate the precision to 1% and make the measurement essentially worthless. The quest for ages on smaller and smaller objects is intrinsically limited by the desired precision. A second stringent limitation is contamination, which for many elements such as Pb forces the analyst to work in ultra-clean environments with filtered air and distilled reagents. Not all contamination and common Pb can be removed and hence constitutes a significant source of uncertainty on ages. A third problem affects the chronometers based on beta decay: the kinetic energy of some emerging electrons is so small that they go undetected, and the corresponding decay constant cannot be accurately determined. This is a limitation for the 87Rb-87Sr, 176 Lu-176Hf, and 187Re-187Os techniques, which can be mitigated to some extent by a cross-calibration with U-Pb ages. Finally, a general limitation arises when data expected to form an alignment actually scatter beyond analytical errors. This can be due to (1) a lack of isotopic homogeneity at t = 0; (2) to the failure of the closed-system assumption because of weathering or thermal resetting;
and (3) to a long duration of the event to be dated, e.g., in case of slow cooling. Literature on least-square regression with correlated or non-correlated errors is abundant but a standard reference is that of York (1969). Among all the codes used to evaluate alignments, the most broadly used software is ISOPLOT created by Ludwig (2003).
See also ▶ Earth, Age of ▶ Isochron ▶ Isotope ▶ Isotopic Ratio ▶ Mass Spectrometry ▶ Radioactivity
References and Further Reading Albare`de F (2009) Geochemistry: an introduction. Cambridge University Press, Cambridge, p 356 Allegre CJ (2008) Isotope geology. Cambridge University Press, Cambridge, p 512 Blichert-Toft J, Albare`de F, Rosing M, Frei R, Bridgwater D (1999) The Nd and Hf isotopic evolution of the mantle through the Archean. results from the Isua supracrustals, West Greenland, and from the Birimian terranes of West Africa. Geochim Cosmochim Acta 63:3901–3914 Bogard DD, Garrison DH (1999) Argon-39-argon-40 “ages” and trapped argon in martian shergottites, chassigny, and Allan hills 84001. Meteorit Planet Sci 34:451–473 Connelly JN, Amelin Y, Krot AN, Bizzarro M (2008) Chronology of the solar system’s oldest solids. Astrophys J 675:L121–L124
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Dickin AP (1999) Radiogenic isotope geology. Cambridge University Press, Cambridge, p 490 Dodson MH (1973) Closure temperature in cooling geochronological and petrological systems. Contrib Mineral Petrol 40:259–274 Faure G, Mensing TM (2004) Isotopes. Principles and applications. Wiley, New York, p 897 Ludwig K (2003) ISOPLOT: a geochronological toolkit for Microsoft Excel 3.00. In Berkeley Geochronology Center Special Publication No. 4, 2455 Ridge Road, Berkeley 94709 Wilde SA, Valley JW, Peck WH, Graham CM (2001) Evidence from detritical zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409:175–178 Yin Q, Jacobsen SB, Yamashita K, B-T J, Te´louk P, Albare`de F (2002) A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites. Nature 418:949–952 York D (1969) Least squares fitting of a straight line with correlated errors. Earth Planet Sci Lett 5:320–324
Geological Time Scale, History of PIERRE SAVATON Universite´ de Caen Basse-Normandie, Caen, France
Keywords Chronology, radiometric dating, stratigraphic scale
History Geology is the first historical Science. Concept of time is a key concept in geology. Even if some geological processes operate over time of a few seconds (e.g., earthquake waves), most of them operate over million to hundreds of million of years. Thus, to tell the Earth’s history, we need to build a geological time scale to place geological events and measure their length. This scale results from a combination of time and space relations between rock formations (relative age) and from the measure of spontaneous decay of radioactive nuclides contained in rocks (absolute age). The only record we have of past geological events is the rock preserved from erosion, alteration, and plate tectonics. The geological time scale was first a chronological one, without date. Study of sedimentary rocks had allowed first “geologists” to establish the simple basis for the stratigraphic scale. Danish naturalist Nicolas Steno (1638–1686) noted that gravels, sands, and clays were laid down in more or less horizontal layers, which he called strata. In 1669, in his Prodromus (De solido intra solidum naturaliter content dissertationis prodromus), he enunciated principles of stratigraphy: original horizontality, superposition, and lateral
original continuity of layers. Using these principles, it was possible to arrange layers in a chronological succession and some attempts were tempted and offered local descriptions of the pile of stratas. Steno identified in these successions some angular unconformities, which represented breaks in the stratigraphical record, but without understood their possible use in dating of geological events, even if he showed that “geology” revealed a history. By contrast, he recognized that fossils were the remains of once living organisms. In 1667, working in Tuscany, he noted that shark teeth bored a striking resemblance to some stony objects found embedded within rock formations, called “tongue stones.” English philosopher Robert Hooke (1635–1703) considered fossil’s shells as medals or monuments of Nature, as greatest and more lasting monuments and records of Antiquity. He claimed that animals living today might not have been alive in the past and animals presented in the past were no longer present today. He interpreted the differences between past and actual faunas by imagining a kind of transformation of species in time. This idea was taken back later by Leibniz before being developed and theorized by Jean-Baptiste Lamarck. But it was too premature for the 1600s. Hooke used for the first time the word “chronology” and suggested a powerful tool to date geological layers. These medals (fossils) allowed establishing continuity between layers separated by great distance or layers of different lithology. They allowed replacing geographical isolated layers in a succession. Robert Hooke might be considered a precursor of the idea of index fossil, that is, fossils of organisms that lived in a short time span but widely distributed geographically. But, the time-keeper aspect of fossil’s records remained marginal for a long time. German geologist and mineralogist Abraham G. Werner still had neglected them in his Kurze Klassifikation und Beschreibung der verschiedenen Gebu¨rgsarten (1783), a stratigraphical framework attempt that was widely adopted like scheme of the succession of geological times. Fossils were just used to characterize his formations, but no more. Faunal succession, as the lithologic succession, could be used directly to identify rocky formations. This approach was that used by British geologist William Smith to build his Geological Map of England and Whales and part of Scotland (1815). His Strata identified by organized fossils (1816–1819) paved the way for the development of the biostratigraphy. George Cuvier’s and Alexandre Brongniart’s map and Description ge´ologique des environs de Paris (1811) proposed to use fossils to distinguish marine from continental sediments and to explain succession of sedimentological events. They used them as
Geological Time Scale, History of
indicators of paleoecological conditions rather than strict stratigraphical tools. In a few decades in the early 1800s, the geologists who mapped the world established a time scale based on assemblages of fossils, far more complex than the earlier succession of rocks (Primary, Secondary, Tertiary) of the 1700s based on a lithological time scale. Time and history of the Earth was divided in time units chronologically distinguished both by their vertical spatial relationships and their fossils content. Fossils determined a major division between the early time of the Earth, without visible fossils and the Phanerozoic time (today the fourth eon) characterized by its fossil diversity. Phanerozoic was subdivided into Eras, Periods and Epochs. The development of a systematic palaeontology during the nineteenth century and studies of some of the less visible one revealed new fossils discontinuities in the time and justified new subdivisions. French naturalist Alcide d’Orbigny showed how microscopic fossils, especially Foraminifera, could be used to characterize Cenozoic stratas. Paleontologists who studied the Jurassic ammonites identified species with a very short time range and suggested to use them to define small biostratigraphic units (biozones). The theory of the Evolution of Darwin (1859) wasn’t necessary to think a biostratigraphic scale, but gave a new breath to the fossil’s tool. The succession of changing fauna could be explained by successive destructions and creations, migrations, or evolution. In the continuity of ▶ Cuvier thinking, most of the first stratigraphers favored extinctions. Proposing that new species emerged as a result of modifications to earlier species was more powerful. But the theory of Evolution obliged to reconsider the age of the Earth, because progressive changes required longer time. The Archbishop Ussher of Ireland wrote in 1664 that the Earth was created 4004 years before Christ. He had not been alone to interpret word for word the Bible to determine the antiquity of the Earth. All these biblical ages were very short. George Buffon, who believed in a cyclical history of the Earth, clearly longer than that given in the biblical accounts, published in his Epoques de la Nature (1778) an empirical calculation of the age the Earth. Following the idea that the Earth might originally have been molten, he measured the cooling time of iron balls of different sizes, and other materials, and concluded through this model that Earth should have been 75,000 years old. This duration was to great enough for many of his contemporaries. The uniformitarian’s ideas spread by British geologist Charles Lyell suggested estimating the actual age of the
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Earth using sediment accumulation or denudation rate as indicators of time. John Phillips transformed the unit of geological time into an equivalent term of years and proposed in 1860 in Life on the Earth an age of Phanerozoic time between 38 and 96 million of years. It wasn’t enough for explaining the Evolution theories of Darwin but compatible with ages calculated by British physicist William Thomson (better known as Lord Kelvin) through his model of a cooling Earth. Most of the geologists of the 1800s believed that Earth of the first Time had been molten. Solid today, still hot, it would lose heat by conduction, compatible with the increasing temperatures recorded in mines or boreholes. In 1862, using Fourier’s laws of cooling, Thomson estimated the Earth’s center temperature, to about 3,800 C, and the age of the Earth to about 20–400 million years. In 1897, Thomson, now Lord Kelvin, ultimately settled on an estimate that the Earth was 20–40 million years old. The discovery of radioactivity by Henri Becquerel in 1896 and the isolation of radium by Pierre and Marie Curie in 1898 changed the classic view of a cooling Earth and offered to Geology the tool it needed to measure time. The work of Rutherford and Soddy about radioactive decay series and the discovery that lead was the end product of the uranium decay chain by American chemist Bertram Boltwood in 1905 stated the fundamental basis of radiometric dating and allowed geologists to put absolute dates in their geological column. Using what he knew about radium–lead decay, Boltwood aged rocks from 250 million to 1.3 billion years old. Although his estimate of the Earth’s age was inaccurate, his paper, published in 1907 made clear that sediments from the same layers had similar uranium–lead ratios and that generally, samples from younger layers had a lower proportion of lead. In 1911, British geologist Arthur Holmes determined an age of 1,640 million years for Archean rocks, and published in 1913, in The Age of the Earth the first stratigraphic time scale with radioactive ages for the boundaries of the main units. He estimated the beginning of the Phanerozoic time at about 600 million years before present (today this limit is fixed at 542 Ma). Athur Holmes’s time scale established in 1947 was very similar than these of today. Working on lead isotopes in iron and stone meteorites, American geochemist Clair Patterson established, and published in 1956, an age of the Earth of 4,550 70 million years. With fossil’s chronology of sedimentary rocks and radioactive age of crosscutting igneous rocks, geologists have built the geological time scale. Earth history could finally be told.
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Geological Timescale
See also ▶ Cuvier’s Conception of Origins of Life ▶ Darwin’s Conception of Origins of Life ▶ Earth, Age of ▶ Geochronology ▶ Geological Timescale ▶ Lamarck’s Conception of Origins of Life
interpolates at 600 Ma, curiously close to modern estimates (542 0.3 Ma). The new approach was a major improvement over a previous “hour-glass” method that tried to estimate maximum thickness of strata per period to determine their relative duration but had no way of estimating rates of sedimentation independently.
Overview References and Further Reading Brush S (1996) Transmuted past: the age of the Earth and the evolution of the elements from Lyell to Patterson. Cambridge University Press, New York Jackson PW (2006) The chronologers’ quest. The search for the age of the Earth. Cambridge University Press, New York Lewis C, Knell S (eds) (2001) The age of the Earth: from 4004 BC to AD 2002. Geological Society Special Publication, London Oldroyd D (2006) Earth cycles, a historical perspective. Greenwood Press, London Rudwick M (2008) Worlds before Adam. The reconstruction of geohistory in the age of reform. University of Chicago Press, Chicago/London
Geological Timescale FELIX M. GRADSTEIN University of Oslo, Blindem, Oslo, Norway
Definition The Geologic Time Scale is the framework for deciphering and understanding the long and complex history of the Earth. Understanding the physical, chemical, and biological processes since the Earth formed requires a detailed and accurate time scale. The time scale is the tool “par excellence” of the geological trade (Gradstein et al. 2004; Ogg et al. 2008).
History British geologist Arthur Holmes (1890–1965) was the first to combine radiometric ages with geologic formations in order to create a geologic time scale. His book, The Age of the Earth (1913, 2nd edition 1937), written when he was only 22, had a major impact on those interested in ▶ geochronology. For his pioneering scale, Holmes carefully plotted four radiometric dates, one in the Eocene and three in the Paleozoic from radiogenic helium and lead in uranium minerals, against estimates of the accumulated maximum thickness of Phanerozoic sediments. If we ignore sizable error margins, the base of Cambrian
Calibration to linear time of the succession of events recorded in the rocks on Earth has three components: (a) the international stratigraphic divisions and their correlation in the global rock record; (b) geochronology, the means of measuring linear time or elapsed durations from the rock record; and (c) the methods of effectively joining the two scales, the stratigraphic one and the linear one. For clarity and precision in international communication, the rock record of Earth history is subdivided into a “chronostratigraphic” scale (Figure 1) of standardized global stratigraphic units, such as “Devonian,” “Miocene,” “Zigzagiceras zigzag ammonite zone,” or “polarity Chron C25r.” This chronostratigraphic calendar is not unlike a historical calendar in which civilization periods, such as the Minoan Period, Reign of Louis XIV, or American Civil War, are used as building blocks, devoid of a linear scale. Archeological relics deposited during these intervals, such as the Palace of Minos on Crete, Versailles, or spent cannon balls at Gettysburg, comprise the associated physical chronostratigraphic record. The chronostratigraphic scale is thus assembled from rock sequences stacked and segmented in relative units based on their unique fossil and physical content. Through correlation of the unique fossil and physical record to other sediment sections in outcrops or wells across the globe, this scale becomes meaningful and useful. The standard chronostratigraphic scheme, in downloadable graphic format available from the International Commission of Stratigraphy (ICS) at their website (http:// www.stratigraphy.org/), is made up of up of successive stages in the rock record, for example Cenomanian, Turonian, then Coniacian, etc., within the Cretaceous System. The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. The estimation comes from radiometric dating and astrochronology (orbital tuning of long sets of cycles) in the sedimentary rock record. In contrast to the Phanerozoic that has an agreed upon chronostratigraphic scale with formal stage boundary stratotypes, Precambrian stratigraphy is formally classified chronometrically (see ▶ Proterozoic, ▶ Archean).
140.2 ± 3.0
136.4 ± 2.0
130.0 ± 1.5
125.0 ± 1.0
112.0 ± 1.0
99.6 ± 0.9
93.5 ± 0.8
89.3 ± 1.0
85.8 ± 0.7
83.5 ± 0.7
70.6 ± 0.6
65.5 ± 0.3
61.7 ± 0.2
58.7 ± 0.2
55.8 ± 0.2
48.6 ± 0.2
40.4 ± 0.2
37.2 ± 0.1
33.9 ± 0.1
28.4 ± 0.1
23.03
20.43
15.97
13.65
11.608
7.246
5.332
3.600
2.588
1.806
0.781
0.126
0.0118
Meso zoic Phanerozoic
Age Ma
Berriasian 145.5 ± 4.0
Valanginian
Hauterivian
Barremian
Aptian
Albian
Cenomanian
Turonian
Coniacian
Santonian
Campanian
Maastrichtian
Danian
Selandian
Thanetian
Ypresian
Lutetian
Bartonian
Priabonian
Rupelian
Chattian
Aquitanian
Burdigalian
Langhian
Serravallian
Tortonian
Messinian
Zanclean
Piacenzian
Gelasian
Lower
Middle
Geological Timescale. Figure 1
Lower
Upper
Paleocene
Eocene
Oligocene
Miocene
Pliocene
Stage age
Upper
Bashkirian
Visean Tournaisian
Middle Lower
Upper Serpukhovian
Moscovian
Lower
Kasimovian
Gzhelian
Asselian
Sakmarian
Artinskian
Kungurian
Roadian
Wordian
Capitanian
Wuchiapingian
Changhsingian
Induan
Olenekian
Anisian
Ladinian
Carnian
Norian
Rhaetian
Hettangian
Sinemurian
Pliensbachian
Toarcian
Aalenian
Bajocian
Bathonian
Callovian
Oxfordian
Kimmeridgian
Tithonian
Stage age
Middle
Upper
Cisuralian
Guadalupian
Lopingian
Lower
Middle
Upper
Lower
Middle
Upper
Age Ma 345.3 ± 2.1 359.2 ± 2.5
326.4 ± 1.6
318.1 ± 1.3
311.7 ± 1.1
306.5 ± 1.0
303.9 ± 0.9
299.0 ± 0.8
294.6 ± 0.8
284.4 ± 0.7
275.6 ± 0.7
270.6 ± 0.7
268.0 ± 0.7
265.8 ± 0.7
260.4 ± 0.7
253.8 ± 0.7
251.0 ± 0.4
249.7 ± 0.7
245.0 ± 1.5
237.0 ± 2.0
228.0 ± 2.0
216.5 ± 2.0
203.6 ± 1.5
199.6 ± 0.6
196.5 ± 1.0
189.6 ± 1.5
183.0 ± 1.5
175.6 ± 2.0
171.6 ± 3.0
167.7 ± 3.5
164.7 ± 4.0
161.2 ± 4.0
155.7 ± 4.0
150.8 ± 4.0
145.5 ± 4.0
Stage age
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Stage 6
Stage 7
Paibian
Stage 9
Stage 10
Tremadocian
Stage 2
Stage 3
Darriwilian
Stage 5
Stage 6
Hirnantian
Rhuddanian
Aeronian
Telychian
Sheinwoodian
Homerian
Gorstian
Ludfordian
Lochkovian
Pragian
Emsian
Eifelian
Givetian
Frasnian
Famennian
Age Ma 542.0 ± 1.0
~ 534.6 *
~ 521.0 *
~ 517.0 *
~ 510.0 *
~ 506.5 *
~ 503.0 *
501.0 ± 2.0
~ 496.0 *
~ 492.0 *
488.3 ± 1.7
478.6 ± 1.7
471.8 ± 1.6
468.1 ± 1.6
460.9 ± 1.6
455.8 ± 1.6
445.6 ± 1.5
443.7 ± 1.5
439.0 ± 1.8
436.0 ± 1.9
428.2 ± 2.3
426.2 ± 2.4
422.9 ± 2.5
421.3 ± 2.6
418.7 ± 2.7
416.0 ± 2.8
411.2 ± 2.8
407.0 ± 2.8
397.5 ± 2.7
391.8 ± 2.7
385.3 ± 2.6
374.5 ± 2.6
359.2 ± 2.5
GSSP
Copyright © 2006 International Commission on Stratigraphy
This chart was drafted by Gabi Ogg. Intra Cambrian unit ages with * are informal, and awaiting ratified definitions.
Series 1
Series 2
Series 3
Furongian
Lower
Middle
Upper
Llandovery
Wenlock
Ludlow
Pridoli
Lower
Middle
Upper
Series epoch
GSSP
International commission on stratigraphy
Phanerozoic
Series epoch
Pleistocene
* proposed by ICS
Phanerozoic
Series epoch
GSSP
International stratigraphic chart
Paleo zoic
Eonothem eon Erathem era System period
Quaternary ∗
Neogene
Paleogene
Cretaceous
Cenozoic
Mesozoic
Eonothem eon Erathem era System period
Jurassic Triassic
Paleo zoic Carboniferous Permian
Holocene
Pennsylvanian Mississippian
Eonothem eon Erathem era System period
Devonian Silurian Ordovician Cambrian
Eonothem eon
Proterozoic
Erathem era
Eoarchean
Paleoarchean
Mesoarchean
Neoarchean
Paleoproterozoic
Mesoproterozoic
System period Lower limit is not defined
Siderian
Rhyacian
Orosirian
Statherian
Calymmian
Ectasian
Stenian
3,600
3,200
2,800
2,500
2,300
2,050
1,800
1,600
1,400
1,200
1,000
542
Age Ma
Ediacaran ~630 NeoCryogenian proterozoic 850 Tonian
GSSP GSSA
Precambrian
Subdivisions of the global geologic record are formally defined by their lower boundary. Each unit of the Phanerozoic (~542 Ma to Present) and the base of Ediacaran are defined by a basal Global Standard Section and Point (GSSP ), whereas Precambrian units are formally subdivided by absolute age (Global Standard Stratigraphic Age, GSSA). Details of each GSSP are posted on the ICS website (www.stratigraphy.org). International chronostratigraphic units, rank, names and formal status are approved by the International Commission on Stratigraphy (ICS) and ratified by the International Union of Geological Sciences (IUGS). Numerical ages of the unit boundaries in the Phanerozoic are subject to revision. Some stages within the Ordovician and Cambrian will be formally named upon international agreement on their GSSP limits. Most sub-Series boundaries (e.g., Middle and Upper Aptian) are not formally defined. Colors are according to the Commission for the Geological Map of the World (www.cgmw.org). The listed numerical ages are from 'A Geologic Time Scale 2004', by F.M. Gradstein, J.G. Ogg, A.G. Smith, et al. (2004; Cambridge University Press).
Archean
ICS
Geological Timescale
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Geomicrobiology
See also ▶ Archea ▶ Earth, Age of ▶ Geochronology ▶ Geological Time Scale, History of ▶ Hadean ▶ Proterozoic (Aeon)
References and Further Reading Gradstein FM, Ogg JG, Smith AG, Agterberg FP, Bleeker W, Cooper RA, Davydov V, Gibbard P, Hinnov L, House MR, Lourens L, Luterbacher HP, McArthur J, Melchin MJ, Robb LJ, Shergold J, Villeneuve M, Wardlaw BR, Ali J, Brinkhuis H, Hilgen FJ, Hooker J, Howarth RJ, Knoll AH, Laskar J, Monechi S, Powell J, Plumb KA, Raffi I, Ro¨hl U, Sadler P, Sanfilippo A, Schmitz B, Shackleton NJ, Shields GH, Strauss H, Van Dam J, Van Kolfschoten T, Veizer J, Wilson D (2004) A geologic time scale 2004. Cambridge, Cambridge University Press, p 589, 200 figs and tables International Commission of Stratigraphy (ICS) geological timescale at http://www.stratigraphy.org/ Ogg JG, Ogg G, Gradstein FM (2008) The concise geologic time scale. Cambridge, Cambridge University Press, p 177
Geomicrobiology Definition Geomicrobiology examines the role of microbes in geological processes. Examples of such processes are the weathering of rocks, soil and sediment formation and transformation, the genesis and degradation of minerals, and the genesis and degradation of fossil fuels. Due to its transdisciplinary essence and the subject of study, geomicrobiology is of special astrobiological interest.
Geothermal Flux ▶ Heat Flow (Planetary)
Geothermal Gradient Definition A geothermal gradient is the increase in temperature with increasing depth beneath the Earth’s surface. This gradient is due to outward heat flow from a hot interior. The Earth’s internal heat comes from a combination of residual heat from planetary accretion (20%) and heat produced through radioactive decay of U, Th, and K (80%). The magnitude of the geothermal gradient depends on the rate of heat production at depth, the dynamics of the system, and the conductivity of rocks. The highest gradients, 40–80 K km1, are measured at oceanic spreading centers (▶ mid-ocean ridges) or at island arcs where magma is close to the surface. The lowest gradients occur at ▶ subduction zones where cold lithosphere descends into the mantle. The gradient in old stable ▶ continental crust is 20–30 K km1. Upwelling parts of the mantle ascend nearly adiabatically (i.e., they lose little to no heat to the surroundings) and the gradient is very low, about 0.3 K km1.
See also ▶ Continental Crust ▶ Heat Flow (Planetary) ▶ Heat Transfer (Planetary) ▶ Mid-Ocean Ridges ▶ Subduction
See also ▶ Archea ▶ Bacteria ▶ Biogeochemical Cycles ▶ Chemolithotroph ▶ Deep-Subsurface Microbiology ▶ Environment ▶ Hot Spring Microbiology ▶ Iron ▶ Iron Cycle ▶ Lithotrophy ▶ Magnetotactic Bacteria ▶ Oxygenic Photosynthesis ▶ Prokaryote ▶ Sulfate Reducers ▶ Sulfur Cycle
Geothermobarometers JEAN-EMMANUEL MARTELAT LST UMR5570, Universite´ Claude Bernard Lyon 1, St Martin d’He`res, Grenoble, France
Keywords Geobarometer, geothermometer, metamorphic rocks, metamorphism
Definition Geothermobarometers refer to all type of reactions that are useful to estimate temperature and pressure recorded
German Aerospace Center
in a magmatic or a metamorphic rock when it crystallizes or recrystallizes.
Overview Minerals have different physical properties (e.g., volume, density, or entropy). When pressure increases, minerals like quartz (density 2.66 g/cm3) may be metamorphosed into high-pressure polymorphs (e.g., for quartz: coesite, density 3.0 g/cm3). This kind of reaction is called polymorphic: the chemical composition remains (SiO2) but the atoms are rearranged in a more compact manner. On the other hand, as temperature increases, mineral like kyanite (Al2SiO5) (entropy S 298 K 83 J/mol-K) may be metamorphosed into a high-temperature polymorph, sillimanite (Al2SiO5) (entropy S 298 K 93 J/mol-K), displaying a higher atomic disorder. Hence, a mineral or a mineral assemblage is sensitive to P–T conditions and is only stable within a given range of those conditions (see also entries ▶ “metamorphic rocks” and ▶ “metamorphism”: metamorphic facies). Moreover, mineral compositions corresponding to a specific chemical equilibrium give a quantitative indication of P–T conditions: this method is called geothermobarometry. We use the term geothermometer for metamorphic reactions (or chemical equilibrium) that are highly sensitive to temperature and geobarometer for reactions that are sensitive to pressure. Our ability to figure out the quantitative P–T conditions of a given rock depends on (1) the observation of the sample’s texture under the microscope, (2) the quality of the experimental calibration of the chemical reactions (stability field of mineral phases), (3) the accuracy of the thermodynamic properties of the minerals (database), and (4) the precision of chemical analyses on the minerals (e.g., with electron microprobe using a 1 mm spot resolution). On well-studied samples bearing relevant mineral assemblages, quantitative results can be obtained with an accuracy of 50 C and 1–1.5 kbars. Several chemical reactions can help finding out the temperature. One of the most broadly used geothermometer is based on the diffusional exchange of Fe2+ and Mg2+ between garnet and biotite. Fe2+ and Mg2+ cations are easily exchangeable since they have similar electron valence and atomic radii. This exchange occurs without change in size of both the garnet and the biotite. The reaction is Phlogopite þ almandine ¼ annite þ pyrope
Fe pure end-members of garnet, respectively. When temperature decreases, Fe/Mg ratio decreases in biotite and increases in garnet. The most critical assumption behind temperature and pressure obtained by geothermobarometry is that equilibrium reactions were frozen during cooling. As diffusivity depends on temperature and on the distance between the exchange zones in the biotite and garnet in contact, one can obtain temperature gradients from the rim to the core of the given mineral pairs. Other equilibriums can constrain temperature: e.g., polymorphic thermometer, solvus thermometer (e.g., feldspath exsolution), net transfer reaction thermometer (some minerals are consumed while others are produced), trace element thermometer, and stable isotope partitioning thermometer. Some other techniques, such as liquid– vapor homogenization points in fluid inclusion provide quantitative thermometry results. The best way to find out the quantitative T and P recorded in rocks is to combine several internally consistent thermobarometers. Various freeware available on Internet calculate all possible equilibrium with internally consistent thermodynamic database for a large number of minerals (e.g., TWEEQ, Berman 1991; THERMOCALC, Powell et al. 1998).
See also ▶ Geothermal Gradient ▶ Metamorphic Rock ▶ Metamorphism ▶ Plate Tectonics
References and Further Reading Berman RG (1991) Thermobarometry using multiequilibrium calculations: a new technique with petrologic applications. Can Mineral 29:833–855 Powell R, Holland TJB, Worley B (1998) Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC. J Metamorph Geol 16:577–588 Spear FS (1993) Metamorphic phase equilibria and pressure-temperaturetime paths. Mineralogical Society of America Monograph, New York, 799 pp
Germ ▶ Microorganism
KMg3 AlSi3 O10 ðOHÞ2 þ Fe3 Al2 ðSiO4 Þ3 ¼ KFe3 AlSi3 O10 ðOHÞ2 þMg3 Al2 ðSiO4 Þ3 where phlogopite and annite are the Mg and Fe pure endmember of biotite and pyrope and almandine the Mg and
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Giant Impact
Giant Impact Definition During planetary formation, a giant impact is a collision between two planetary embryos with masses of at least a lunar mass and often significantly larger. Planetary embryos are thought to grow by runaway and ▶ oligarchic growth from a swarm of small ▶ planetesimals primarily via embryo–planetesimal collisions. Once the local density of embryos and planetesimals is comparable, embryos can grow by embryo–embryo collisions. These so-called giant impacts are very energetic events whose consequences are not completely understood. In fact, most such impacts probably do not lead to perfect merging and probably generate significant amounts of collisional debris. The last giant impact on Earth is thought to have been a low-speed, off-center collision with a Mars-sized projectile, usually called Theia. The debris from this collision stayed in orbit around Earth and coalesced into the Moon.
See also ▶ Impact (Hit and Run) ▶ Oligarchic Growth ▶ Planetesimals ▶ Runaway Growth
Giant Planets THERESE ENCRENAZ LESIA, Observatoire de Paris, Meudon, France
Synonyms Jovian planets
Keywords Planets
Definition The four giant planets – ▶ Jupiter, ▶ Saturn, ▶ Uranus, and ▶ Neptune – are found in the outer solar system, at heliocentric distances ranging from 5 to 30 AU. Unlike the terrestrial planets – Mercury, Venus, the Earth, and Mars – located in the inner solar system, within 2 AU from the Sun, the giant planets are characterized by a great size,
a great mass, and a low density. They all have a ring system and a large number of satellites. Tables 1 and 2 summarize the main orbital and physical properties of the giant planets.
History Early Exploration As naked-eye objects, Jupiter and Saturn have been known since Antiquity. Their modern astronomical observation started in 1610 when ▶ Galileo Galilei used for the first time his new telescope to look at celestial bodies. Galileo discovered the four big satellites that orbit around Jupiter (later called Galilean satellites), the phases of Venus, the relief of the Moon, and the multitude of stars that populate the Milky Way. The discovery of the Galilean satellites had immense implications for the history of astronomy. It supported the heliocentric system proposed in 1543 by Nicolas Copernicus and promoted by Johannes Kepler, but still strongly attacked by the theologians of Rome. Later, Isaac Newton elaborated the universal gravitation law that provided the theoretical support of the heliocentric model and opened the era of celestial mechanics. Galileo also noticed the changing aspect of Saturn as the orientation of the rings vary with respect to the Earth, but could not provide an explanation. The answer was given by Christiaan ▶ Huygens in 1659, who also discovered Titan, Saturn’s biggest satellite. At the end of the seventeenth century, large observatories were built and planetary observations took place on a regular basis. Cassini observed the zone and belt structure of Jupiter and noticed its Great Red Spot. He also discovered a gap inside Saturn’s rings, now called the Cassini Division, and he identified several icy satellites around Saturn. Cassini also used the precise timing of the Galilean satellites’ occultations to define a universal time and allow the navigators to determine the longitude at sea. An even more spectacular result was the first measurement of the speed of light by Ole Ro¨mer at Paris Observatory, based on the precise timing of the Galilean satellites’ occultations as a function of the Jupiter–Earth distance. Thanks to improved techniques in the building of glasses and lenses, astronomical telescopes got more and more powerful. William Herschel, in particular, specialized in manufacturing a new generation of telescopes. In 1781, with an instrument of about 15 cm in diameter, he discovered a new planet, which he called Uranus. The new planet was twice farther from the Sun than Saturn. At the same time, as a natural consequence of Newton’s theory of universal gravitation, a new science, celestial mechanics, was developing rapidly. In 1821, the first
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Giant Planets. Table 1 Orbital properties of giant planets Name
Semimajor axis (AU)
Inclination over the ecliptic ( )
Eccentricity
Revolution period (years)
Jupiter
5.20
0.054
1.30
11.86
Saturn
9.54
0.047
2.48
29.42
Uranus
19.2
0.086
0.77
83.75
Neptune
30.1
0.008
1.77
163.72
Giant Planets. Table 2 Physical properties of giant planets Name
Mass (M )
Equatorial radius (R )
Density (g/cm3)
Rotation period (h)
Obliquity ( )
Jupiter
317.9
11.21
1.33
9.925
Saturn
95.16
9.45
0.69
10.656
26.73
Uranus
14.53
4.00
1.32
17.24
97.92
Neptune
17.14
3.88
1.64
16.11
28.80
precise calculations of the giant planets’ orbits showed that the orbit of the new planet was perturbed by another more distant object. Two astronomers, John Couch Adams in England and Urbain Le Verrier in France, independently determined the position of the new object. John Adams was the first one to find the solution, but could not convince his director about the importance of his discovery. In 1846, Le Verrier sent the position of the new planet to a German astronomer, Johannes Galle, who immediately found it within 1 of its predicted position. The planet was called Neptune; this discovery marked the triumph of celestial mechanics. Visual observations and drawings remained for long the only means for planetary cartography. Photographic plates were used from the beginning of the twentieth century until the apparition of the CCD digital cameras in the 1980s. In parallel, spectroscopic observations started to develop during the twentieth century. Spectroscopic observations are precious as they give information on the nature of the atmospheric constituents of the planets. In 1932, methane and ammonia were detected in Jupiter’s atmosphere; in spite of their low relative abundance with respect to hydrogen, they were detectable because they are very active spectroscopic agents. Methane was detected also on the other giant planets. Molecular hydrogen, although by far the dominant atmospheric species on all giant planets, was not detected before the 1960s. Since the 1970s, several other minor species have been detected from ground-based observations, thanks to the development of infrared and millimeter spectroscopy.
3.08
The Space Exploration of the Giant Planets The space exploration of the giant planets started in the 1970s with two programs, Pioneer and ▶ Voyager. Two spacecraft were first sent by NASA toward Jupiter and Saturn, Pioneer 10 and 11. Pioneer 10 was launched in 1972 and flew by Jupiter at the end of 1973; Pioneer 11 was launched in 1973, flew by Jupiter in 1974 and took advantage of Jupiter’s gravitational assistance to encounter Saturn in 1979. The Pioneer spacecraft were equipped, in particular, with a camera, ultraviolet and infrared photometers, a magnetometer, a plasma analyzer and particle detectors. The Pioneer mission provided us with the first spectacular images of Jupiter’s atmospheric structures and Saturn’s rings, and they explored for the first time the planets’ magnetospheres. The first discoveries by the ▶ Voyager mission appeared soon after the success of the Pioneer spacecraft. Two identical spacecraft, Voyager 1 and 2, were launched by NASA in 1977. Flybys of Jupiter took place in 1979, and those of Saturn occurred in 1980 and 1981, respectively. Voyager 1 approached Titan for a close encounter of Saturn’s satellite, while Voyager 2 used Saturn’s gravitational assistance to encounter the other giant planets. Voyager 2 flew by Uranus in 1986 and by Neptune in 1989. The Voyager mission has been a spectacular success. In particular, Voyager 1 discovered the complexity of Jupiter’s dynamical structure, the active volcanism of Io, the possible water ocean below the surface of Europa, the multiple structures of Saturn’s rings, the magnetosphere of Saturn, and the nature of Titan’s atmosphere. Voyager 2 discovered the magnetospheres of Uranus and Neptune,
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the unexpected tectonic activity of Uranus’ satellite Miranda, the fascinating blue color Neptune’s atmosphere, and the active cryovolcanism at Triton’s surface. The Voyager mission marked a major step in our understanding of the giant planets (Fig.1). After the flybys, a more in-depth exploration of the giant planets required the launch of orbiters and probes. In 1989, the Galileo mission was launched by NASA; it included an orbiter for successive flybys of the planet and the Galilean satellites, and a probe for in situ measurements of the Jovian atmosphere. In spite of a technical problem that prevented scientists from using the large antenna and imposed a very low telemetry rate, the Galileo mission was also a great success. On December 7, 1995, the Galileo probe entered the Jovian atmosphere and transmitted data down to a pressure level of 22 bars. The Galileo orbiter monitored the planet and the Galilean satellites until 2003. Unexpectedly, Galileo’s magnetometer discovered, in particular, an intrinsic magnetic field on Ganymede. After Jupiter, Saturn and its system also deserved an in-depth exploration. In 1997, the ambitious ▶ Cassini mission, jointly developed by NASA and ESA (European Space Agency), was launched. The orbiter, led by NASA, was designed for an in-depth monitoring of Saturn, its rings, and satellites through multiple flybys. It approached Saturn’s system in 2004. On January 14, 2005, the Huygens probe, led by ESA, successfully landed on Titan’s surface and sent to the world the first images of this new world. Among other spectacular results, the Cassini orbiter discovered lakes of hydrocarbons at the surface of Titan, cryovolcanism on Enceladus and a very complex
Giant Planets. Figure 1 The great red spot of Jupiter as revealed by the Voyager 1 spacecraft in 1979 (© NASA)
meteorology in the atmosphere of Saturn. The exploration goes on and will continue until 2017, with the extended Cassini mission (Fig.2). In addition to in situ exploration by dedicated planetary spacecraft, one should not forget the role of Earthorbiting observatories. Both the International Ultraviolet Explorer (IUE), launched in 1978, and the Hubble Space Telescope (HST), launched in 1989, obtained UV spectra of the giant planets and ▶ Titan. The HST, in addition, gave excellent quality images of the giant planets and the outer satellites, allowing a long-term monitoring of the disk morphologies. The ▶ Infrared Space Observatory, launched by ESA in 1995, sent infrared spectra that led, in particular, to the detection of an external oxygen source on all giant planets and Titan. ISO was followed by Spitzer, launched by NASA in 2003, and more recently by Herschel, launched by ESA in 2009, which will continue the far-infrared and submillimeter exploration of the outer solar system.
Overview The Formation of the Giant Planets The striking differences between the main properties of the giant planets, as described above (Tables 1 and 2), and the terrestrial ones suggest a different formation scenario for the two types of planets. Indeed, there is presently a general agreement within the scientific community
Giant Planets. Figure 2 Saturn’s rings, as seen by the Cassini spacecraft (© NASA)
Giant Planets
about a formation model of the solar system that does account for these specific properties. It is now generally accepted that the solar system formed from a rotating fragment of an interstellar cloud, which collapsed into a disk perpendicular to its rotation axis. This model, called “model of the primordial nebula,” was first proposed in the eighteenth century by Immanuel Kant and later by Pierre-Simon de Laplace. Their justification for such a model was the simple observation of planetary orbits: they all rotate around the Sun on nearly coplanar and concentric orbits, in the same direction (direct, i.e., counterclockwise as seen from the North ecliptic pole), which is also the direction of the Sun’s rotation. The baselines of this model are still valid today; later observations are totally consistent with this scenario. First, the chemical analysis of extraterrestrial samples (lunar samples and meteorites) has shown that all solar system bodies formed some 4.56 billion years ago, contemporaneously with the Sun itself. Second, observations of young nearby stars have revealed that the formation of a protoplanetary disk, following the collapse of an interstellar rotating cloud, is a common scenario of stellar formation. In addition, hundreds of extrasolar planets have been discovered around nearby stars, either in these protoplanetary disks or after the dissipation of these disks. The main lines of the solar system formation scenario can thus be summarized as follows. The protosolar disk, after the collapse of the protosolar cloud, is mostly composed of gas (hydrogen and, to a lesser extent, helium) with a small fraction of dust (metals, silicates). At the center of the disk, matter accretes to form the young Sun. The temperature is high at the center and decreases outward. The protosolar disk (see ▶ Solar Nebula) is very turbulent such that solid particles are permanently subject to mutual collisions. Because all particles revolve around the center, their relative velocities are small and collisions are not always disruptive; some of them lead to the formation of small aggregates. Following a mechanism that is not presently fully understood (possibly helped by the turbulence), some of these aggregates reach the kilometer size; they are called planetesimals. Then the biggest fragments are able to capture the surrounding particles by collisions, then by gravity. Numerical dynamical simulations show that after a few million years, a small number of big objects (of the size of terrestrial planets and satellites) are formed. Why do we find two classes of planets, the terrestrial and the giant ones? This separation is a result of the condensation sequence that takes place within the disk as a function of the heliocentric distance. Within the disk, the element abundances follow the universal cosmic rule: the lightest one
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(hydrogen) is the most abundant (75% per mass), followed by helium (23%), and the 2% left is made of all heavier elements. Within these 2%, the lightest (C, N, O) are relatively more abundant than the heavier ones (Si, Mg, metals). As planets were formed from solid material, the nature of the planet depends upon the nature of the solid material available to form the planetesimals. In the protosolar disk, the temperature decreases as the heliocentric distance increases. Two cases can be identified: Close to the Sun, within a few AUs, the temperature (above 200 K) was such that the only solid matter available was made of heavy atoms (metals, silicates). Because heavy elements are not abundant in the disk, the accretion process led to the formation of relatively small and dense planets: the terrestrial planets, also called the rocky planets. At larger heliocentric distance, the temperature (below 180 K) became low enough for the small molecules (H2O, CH4, NH3, H2S, CO2. . .) to condense. As these species were much more abundant than the heavier elements, they accreted into bigger cores, able to reach 10–15 terrestrial masses. At this point, models predict that their gravity field is sufficient for the surrounding nebula to collapse in a disk, in the equatorial plane of the planetary core. As the nebula is mostly made of hydrogen and helium, the planets formed after these collapses are very big, with a low density: they are the giant planets with their ring and satellite systems, formed within their equatorial disk. Between the terrestrial and giant planets, the line of ice condensation is called the snow line. Water plays a special role for two reasons: first, formed from two abundant atoms, H and O, water is, among the ices, the most abundant small molecule; second, it is the first molecule to condense as the temperature decreases. Other ices (NH3, CO2, CH4. . .) condense at greater heliocentric distances. Presently, water condenses at about 2 AU from the Sun, as shown by the cometary activity. At the time of planets’ formation, when the disk was warmer, the snow line was probably located at about 4–5 AU from the Sun. The “core accretion model” of the giant planet’s formation described above gives a natural explanation for the basic properties of the giant planets (size, mass, density, number of satellites). As will be discussed below, another observational fact supports this model: the elemental and isotopic abundances measured in the giant planets.
Gaseous Giants and Icy Giants Table 2 shows the giant planets fall into two distinct subclasses. With masses of about 300 and 100 terrestrial
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masses, Jupiter and Saturn are mostly composed of protosolar gas: they are called the gas giants. In contrast, Uranus and Neptune, with masses around 15 terrestrial masses, are mostly composed of their initial icy core; they are called the icy giants. What can be the reason for such a difference? No definite answer can be given presently, but a plausible explanation can be proposed. Jupiter, formed just beyond the snow line, benefited from a maximum of icy material and a maximum of surrounding gas. To a lesser extent, Saturn’s formation followed the same way. Jupiter and Saturn probably completed their formation within a few million years at most. In contrast, Uranus and Neptune, formed at larger heliocentric distances, needed a longer time to accrete their icy core. Possibly, they reached the critical mass of 10 terrestrial masses late enough for the protosolar disk to have dissipated, leaving little gaseous material for the planets to continue accretion. The lifetime of the protosolar disk, as inferred from the observation of protoplanetary disks around nearby stars, was probably not longer than 10 million years, which may have been the time required by the icy giants to accrete their icy core.
A Moderate Migration Recent discoveries of extrasolar planets suggest that planetary migration has been a common phenomenon in the planetary disks of nearby stars. Migration is usually generated by the interaction of the planet with the turbulent protoplanetary disk, and leads the planet to move inward, from the outer system to the close environment of its star. In the case of the solar system, dynamical simulations also suggest that a moderate migration might have taken place. According to the “Nice Model” developed by Alessandro Morbidelli and his colleagues, the interaction of the giant planets with the turbulent protosolar disk may have induced an inward motion of Jupiter toward the inner solar system, while the three other giant planets moved outward. During this motion, the Jupiter–Saturn system crossed the 2:1 resonance (Saturn’s revolution period being twice Jupiter’s one). According to dynamical simulation, this event generated a great perturbation in the inclinations and ellipticities of solar system small bodies. A clear signature would be the ▶ “Late Heavy Bombardment” (LHB), traces of which are observed on the surfaces of all bare solar system objects, from Mercury and the Moon to the outer satellites. Counting the crater rate on these objects shows that the event took place some 3.8 billion years ago, that is, about 800 million years after the planets’ formation. Comparison with other planetary systems raises a question: why was migration moderate in the solar
system, while it seems to have been very efficient around nearby stars? We have presently no answer to this question.
Atmospheric Composition of the Giant Planets Because the atmospheres of the giant planets are dominated by hydrogen, atmospheric species are expected to be found in reduced form. This is the case for methane and hydrocarbons produced by its photodissociation. In addition, NH3, PH3, H2O, GeH4, and AsH3 are observed in Jupiter’s and Saturn’s tropospheres; these species are not observable on Uranus and Neptune because they condense below the observable atmospheric levels (at pressure levels of tens of bars). Methane also condenses on Uranus and Neptune at a level of 1 bar (T = 80 K) but, especially on Neptune, its stratospheric content is sufficient to lead to the formation of hydrocarbons (in particular C2H2 and C2H6). The atmospheric composition of the giant planets is listed in Table 3. Other unexpected stratospheric species have been discovered. In 1992, ground-based millimeter observations of Neptune led to the detection of stratospheric CO and HCN, in abundances much larger than the predicted ones (those predicted values were actually observed in Jupiter’s and Saturn’s stratospheres). In 1997, the ISO satellite discovered the presence of H2O and CO2 on all giant planets and Titan (except CO2 on Uranus, which remained undetected). CO was later detected on Uranus from ground-based infrared spectroscopy. Because the thermal vertical profiles of the giant planets exhibit a cold trap at the level of the minimum temperature, at a pressure level of 0.1 bar (see below), the stratospheric oxygen species had to be of external origin. The source might be either local (rings and satellites) or interplanetary (comets or flux of micrometeorites). From the abundances of tropospheric species, elemental and isotopic ratios have been determined. These parameters are important as they provide precious constraints for the formation models of the giant planets. Indeed, according to the nucleation formation model described above, heavy elements are expected to be enriched with respect to protosolar values; the larger the mass fraction of the icy core, the larger the enrichment. A simple model based on an initial icy core of 12 terrestrial masses predicts enrichment in heavy elements by a factor of 4 for Jupiter, 9 for Saturn, and about 30 for Uranus and Neptune, which is in full agreement with the carbon enrichment measured from tropospheric methane in the four giant planets. It is also fully consistent with the enrichments measured in situ by the mass spectrometer
Giant Planets
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Giant Planets. Table 3 Giant planets: mixing ratios of atmospheric constituents (fractional number density per volume) Constituent
Jupiter
Saturn
Uranus
Neptune
1
Troposphere H2
1
1
1
He
0.157
0.13
0.18
CH4
2.110
3
4
3
4.410
210
2–410
PH3
6107
1.7106
H2O
1.4105
2107
GeH4
2109
10
710
210
4102
4
NH3
10
0.23 2
AsH3
310
2109
CO
1.5109
2109
3108
106
2.1103
4.4103
3105 – 104
7104
Stratosphere CH4 C2H2
8
310
9
210
7
7
210
3107 107
C2H4
710
a
C2H6
2106
3106
2106
8
5108
CH3 C3H4
510 a
610
10
7
C3H8
610
C4H2
a
91011 9
C6H6
210
H2O
3109 9
a
108 210
7109
9
CO
1.510
CO2
31010
31010
a
a
8
310
106 51010 310–10
HCN H3+
2109
a
a
Detected NB: Isotopic species have been also detected: HD, CH3D, 13CH4, 15NH3,12C13CH2
of the Galileo probe on several elements (C, N, S, Ar, Kr, Xe). These results provide a clear validation of the nucleation formation model. Still, the Galileo measurements on Jupiter raise a problem: all elements seem to have been equally trapped in ices; but N and Ar cannot be trapped unless at very low temperature (10 mm (or >100 mm) to 200 km/s), extreme pressures, and temperatures of these giants make their environments too harsh for known organisms (e.g., Atreya 1986). Other potentially habitable environments, like extrasolar giant planets’ moons (e.g., Europa or Titan-like environments) called the cryo-ecosphere (see, e.g., Pen˜a-Cabrera and DurandManterola 2004), are not discussed here because we do not have enough information on their potential habitability. That leaves us with the terrestrial planets. Mercury is a small rocky planet similar to our Moon without atmosphere or water. Venus and Mars demonstrate the limits of planetary habitability. Venus has a surface pressure of 92 bars and a surface temperature of 482 C, too extreme to sustain known life even though some work suggests possible habitats for life in the clouds (Morowitz 1967; Sagan 1967; Seckbach and Libby 1971; Cockell 1999; Schulze-Makuch and Irwin 2002; Schulze-Makuch et al. 2004). Venus has an atmosphere but no water. It is uncertain if it had water after its formation. If Venus had a similar water reservoir as Earth, it was converted to vapor due to the high surface temperatures of the planet. It remains an open question whether Venus lost its water on a “runway greenhouse” or a “moist greenhouse” (Kasting et al. 1984; Kasting 1988). Numerous space missions have explored Mars and its geological history. About 4 billion years ago Mars had an atmosphere thick enough to maintain liquid water on the Martian surface (e.g., McKay and Stoker 1989), which led
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to the hypothesis of life on early Mars. But today, Mars is a dry, frozen dessert that cannot sustain life on its surface. The Martian atmosphere was partly lost and became too thin to warm the planetary surface. The first half billion years of the Solar System were characterized by violent impacts; planets were subjected to what is called the ▶ Late Heavy Bombardment. During this epoch, the leftovers of the building blocks of planetary formation (asteroids and comets) had frequent collisions with the planets. For Mars, the result was supposedly catastrophic for its atmosphere. Large impactors evaporated the Martian atmosphere and the low gravity of the planet was not able to retain the gas in the hot plumes created by those impactors (Melosh and Vickery 1989). In about 1 billion years, Mars lost most of its atmosphere. Plate tectonics could have replenished the Martian atmosphere. Water and CO2 react with a planet’s crust forming carbonates, some parts of the crust are subducted and melted when they reach the mantle. Volatile compounds like H2O and CO2 are released into the atmosphere through volcanism. Plate tectonics results in the release of the remnant heat of the core from planet formation. If the planet is small, the heat will be released faster and the planet has a short period of volcanic activity but not necessarily plate tectonics. This seems to have been the case for Mars. Note that the size of the planet does not guarantee active plate tectonics; see, for example, Venus.
Future Directions Planetary size is crucial for its habitability, assuming plate tectonics is essential. In our Solar System, no planets exist with masses between Earth (1 ML) and Uranus (14.5 ML). The possible characteristics and habitability of such planets has to be derived from planet formation and geologic evolution models of terrestrial planets. The proposed mass limits for a habitable planet are 1 ML (e.g., Turnbull and Tarter 2003; Catanzarite et al. 2006) to 10 ML (e.g., Catanzarite et al. 2006). Larger planets could accrete gaseous atmospheres and smaller ones could lose their atmospheres after geological periods of time. The presence of a large moon that stabilized the obliquity of our planet (Laskar et al. 1993), as well as the abundance of life-forming elements like nitrogen and carbon (Gaidos et al. 2005 and references therein), could further constrain habitability for advanced life. Compared to other terrestrial planets in our Solar System, Earth is the perfect place to live; it has liquid water on its surface, an atmosphere that keeps it warm, and the right mass to maintain tectonics.
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See also ▶ Atmosphere, Structure ▶ Biomarkers, Spectral ▶ Biomarkers, Atmospheric (Evolution Over Geological Time) ▶ Europa ▶ Giant Planets ▶ Greenhouse Effect ▶ Habitable Planet (Characterization) ▶ Habitable Zone ▶ Late Heavy Bombardment ▶ Mars ▶ Planet Formation ▶ Protoplanetary Disk ▶ Super-Earths
References and Further Reading Atreya SK (1986) Atmosphere and ionospheres of the outer planets and their satellites. Springer, Berlin Brack A (1999) Life in the Solar System. Adv Space Res 24:417–433 Catanzarite J, Shao M, Tanner A, Unwin S, Yu J (2006) Astrometric detection of terrestrial planets in the habitable zones of nearby stars with SIM PlanetQuest. Publ Astron Soc Pac 118:1319–1339 Cernicharo J, Crovisier J (2005) Water in space: The water world of ISO. Space Sci Rev 119:29–69 Cockell S (1999) Life on Venus. Planet Space Sci 47:1487–1501 DesMarais DJ et al (2002) Remote sensing of planetary properties and biosignatures on extrasolar terrestrial planets. Astrobiology 2:153 Gaidos E, Deschenes B, Dundon L, Fagan K, Menviel-Hessler L, Moskovitz N, Workman M (2005) Beyond the principle of plentitude: A review of terrestrial planet habitability. Astrobiology 5:100–126 Ikoma M, Genda H (2006) Constraints on the mass of a habitable planet with water of nebular origin. Astrophys J 684:696–706 Jones EG, Lineweaver CH (2010) To what extent does terrestrial life “follow the water”? Astrobiology 10(3):349–361 Kasting JF, Pollack JB, Ackerman TP (1984) Response of earth’s atmosphere to increases in solar flux and implications for loss of water from Venus. Icarus 57:335–355 Kasting JF (1988) Runaway and moist greenhouse atmospheres and the evolution of earth and Venus. Icarus 74:472–494 Laskar J, Joutel F, Robutel P (1993) Stabilization of the earth’s obliquity by the moon. Nature 361:615–617 McKay CP, Stoker CR (1989) The early environment and its evolution on Mars: implications for life. Rev Geophys 27:189–214 Melosh HJ, Vickery AM (1989) Impact erosion of the primordial atmosphere of Mars. Nature 338:487–489 Miller SL (1953) The production of amino acids under possible primitive Earth conditions. Science 117:528–529 Morowitz H (1967) Life in the clouds of Venus? Nature 215:1259–1260 Oparin AI (1924) Proikhozndenie Zhizni. Izd., Moskowski Rabochi. Pen˜a-Cabrera GVY, Durand-Manterola HJ (2004) Possible biotic distribution in our galaxy. Adv Space Res 33:114 Sagan C (1967) Life on the surface of Venus? Nature 216:1198–1199 Sagan C, Salpeter EE (1976) Particles, environments, and possible ecologies in the Jovian atmosphere. Astrophys J Suppl Ser 32:737–755 Schulze-Makuch D, Irwin LN (2002) Reassessing the possibility of life on Venus: proposal for an astrobiology mission. Astrobiology 2:197–202
Schulze-Makuch D, Grinspoon DH, Abbas O, Irwin LN, Bullock MA (2004) A sulfur-based survival strategy for putative phototrophic life in the Venusian atmosphere. Astrobiology 4:11–18 Seckbach J, Libby WF (1971) Vegetative life on Venus or investigations with algae which grow under pure CO2 in hot acid media and at elevated pressures. In: Sagan C, Owen TC, Smith HJ (eds) Planetary atmospheres, international astronomical union. Symposium no. 40, Dordrecht, Reidel, p 62 Segura A, Kaltenegger L (2009) In: Basiuk VA, Navarro-Gonza´lez R (eds) Astrobiology: Emergence, search and detection of life. ASP, 2010 Turnbull MC, Tarter JC (2003) Target selection for SETI. I. A catalog of nearby habitable stellar systems. Astrophys J Suppl Ser 145:181–198
Habitable Planet (Characterization) LISA KALTENEGGER1,2, FRANCK SELSIS3 1 Harvard University, Cambridge, MA, USA 2 MPIA, Heidelberg, Germany 3 Universite´ de Bordeaux-CNRS, Bordeaux, France
Keywords Biomarkers, extrasolar planets, habitability, habitable zone, planetary atmospheres, spectroscopy
Definition The spectrum of a planet can contain signatures of atmospheric species, which create its spectral fingerprint. The presence and abundance of atmospheric species, in the context of the properties of the star and the planet, can elucidate the underlying physics and characterize a planetary environment. Here, we concentrate on characterizing a habitable planet and see what features might have a biological origin.
History Sagan et al. (1993) analyzed a spectrum of the Earth taken by the Galileo probe, searching for signatures of life, and concluded that the large amount of O2 and the simultaneous presence of traces of CH4 are strongly suggestive of biology. To characterize a planet’s atmosphere and its potential habitability, we look for absorption features in the reflection and transmission spectrum of the planet. On Earth, some atmospheric species exhibiting noticeable spectral features in the planet’s spectrum result directly or indirectly from biological activity: the main ones are O2, O3, CH4, and N2O. CO2 and H2O are in addition important as ▶ greenhouse gases in a planet’s atmosphere and potential sources for a high O2 concentration from photosynthesis.
Habitable Planet (Characterization)
Overview
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We discuss how we can read a planet’s spectrum to assess its habitability and search for the signatures of a biosphere. After a decade rich in giant exoplanet detections, observation techniques have now reached the ability to find planets of less than 10 MEarth (so-called ▶ Super-Earths) that may potentially be habitable. Extrasolar planet searches have shown an extraordinary ability to combine research by astrophysics, chemistry, biology, and geophysics into a new and exciting interdisciplinary approach to understanding our place in the universe.
characteristics, determining the composition of their atmospheres, investigating their capability to sustain life as we know it, and searching for signs of life. This can set our own planet, the only known habitat, in context with other rocky worlds and expand our statistics of 3 for extrasolar rocky planets that could potentially support life (▶ Habitability, of the Solar System). They have the capacity to investigate the physical properties and composition of a broader diversity of planets, to understand the formation of planets, and to interpret potential biosignatures (see Fig. 1).
Introduction
Basic Methodology
The current status of exoplanet characterization shows a surprisingly diverse set of giant planets. For a subset of these, some properties have been measured or inferred using observations of the host star, a background star, or the combination of stellar and planetary photons (▶ radial velocity planets (RV), ▶ microlensing planets, ▶ transiting planets, and ▶ astrometry). These observations have yielded measurements of planetary mass, orbital elements and (for transits) the planetary radius and, during the last few years, physical and chemical characteristics of the upper atmosphere of some of the transiting planets. The detection of an Earthlike planet is approaching rapidly thanks to radial velocity surveys (▶ HARPS), transit searches (▶ Corot, ▶ Kepler), and space observatories dedicated to their characterization that are already in development phase (▶ James Webb Space Telescope), as well as future large ground-based telescopes and dedicated space-based missions. Space missions like CoRoT (CNES; Rouan et al. 1998) and Kepler (NASA; Borucki et al. 1997) will give us statistics on the number, size, period, and orbital distance of planets, extending to terrestrial planets on the lower mass range end, while future space missions are designed to characterize their atmospheres. Future space missions have the explicit purpose of detecting other Earthlike worlds, analyzing their
Characterize a Habitable Planet A planet is a very faint, small object close to a very bright and large object, its parent star. In the visible part of the spectrum we observe the starlight reflected off the planet, while in the infrared (IR) we detect the planet’s own emitted flux. The Earth–Sun intensity ratio is about 107 in the thermal infrared (10 mm), and about 1010 in the visible (0.5 mm) (see Fig. 2). The trade-off between contrast ratio and design is not discussed here, but leads to several different configurations for space-based mission concepts. The suggested interferometric systems operate in the mid-IR (6–20 mm) and observe the thermal emission emanating from the planet. The coronagraph and occulter concepts detect the reflected light of a planet and operate in the visible and near infrared (0.5–1 mm). The viewing geometry results in different flux contributions of the overall detected signal from the planet’s bright and dark side for the reflected light, and the planet’s hot and cold regions for the emitted flux. The contrast ratio of hot extrasolar giant planets (EGP) to their parent stars’ flux is much smaller and can partially be observed with current telescopes. The contrast ratio of a rocky planet to a smaller parent star is much more favorable, making Earthlike planets around small stars very interesting targets for the immediate future.
Life ? H2O CH4
H2O CO2
H2O
H2O O3
What biota?
Atmosphere
What chemistry? What star? HZ?
Interior
Plate tectonics? Ocean/land?
CO2
What kind
Density? (radius/mass)
Formation
Formation models
Habitable Planet (Characterization). Figure 1 Connection between spectra and characterization of rocky exoplanets
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10–6 10–7 10–8 10–9 10–10
3.0
10–11
Star
10–13 10–14 10–15 10–16 10–17 10–18 10–19 10–20 0.1
J
J
V E M Z
E V
10
Sun
2.0
K2V
1.5
Non-active star Teff = 3100K GJ 643
1.0
AD Leo 0.5
M Z
1
F2V
2.5
10–12 Flux (W m–2 nm–1)
Iλ erg/(cm2 s mm)
712
100
λ (mm)
0.0
0
2000 1000 Wavelength (nm)
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Habitable Planet (Characterization). Figure 2 SAO model of our Solar system (left) (assumed here to be black bodies with Earth spectrum shown). Spectra of different host stars (Segura et al. 2005) (right)
Our search for signs of life is based on the assumption that extraterrestrial life shares fundamental characteristics with life on Earth, in that it requires liquid water as a solvent and has a carbon-based chemistry (see e.g., Brack 1993; Des Marais et al. 2002). Life on the basis of a different chemistry is not considered here because the vast range of possible life-forms might produce signatures in their planet’s atmosphere that are so far unknown. Therefore, we assume that extraterrestrial life is similar to life on Earth in its use of the same input and output gases, and that it exists out of thermodynamic equilibrium (Lovelock 1975). “▶ Biomarkers” is used here to mean detectable species, or a set of species, whose presence at significant abundance strongly suggests a biological origin (e.g., the couple CH4 + O2, or CH4 + O3, (Lovelock 1975)). Bio-indicators are indicative of biological processes but can also be produced abiotically. It is their abundance and detection along with other atmospheric species and in a certain context (for instance the properties of the star and the planet) that points toward a biological origin. The spectrum of the planet can contain signatures of atmospheric species, which creates its spectral fingerprint (▶ biomarkers, atmospheric; (evolution over geological time) and ▶ Biomarkers, spectral). Figure 3 shows observations and model fits to spectra of the Earth in three wavelength ranges (Kaltenegger et al. 2007). The data shown in Fig. 3 (left) is the visible Earthshine spectrum (Woolf et al. 2002), (right) is the nearinfrared Earthshine spectrum (Turnbull et al. 2006), and
(bottom) is the thermal infrared spectrum of Earth as measured by a spectrometer enroute to Mars (Christensen and Pearl 1997). The data are shown in black and the SAO model in red. In each case, the constituent gas spectra in a clear atmosphere are shown in the bottom panel, for reference. Both spectral regions contain the signature of atmospheric gases that may indicate habitable conditions and, possibly, the presence of a biosphere: CO2, H2O, O3, CH4, and N2O in the thermal infrared, and H2O, O3, O2, CH4, and CO2 in the visible to near-infrared in reflected (Fig. 4 left). The Earth’s and transmission spectra is shown in Fig. 4. The presence or absence of these spectral features (detected individually or collectively) will indicate similarities or differences with the atmospheres of terrestrial planets and their astrobiological potential (see Kaltenegger and Traub 2009 and Palle et al. 2009 for details on Earth’s transmission spectrum).
Key Research Findings Characterizing Planetary Environments It is relatively straightforward to remotely ascertain that Earth is a habitable planet, replete with oceans, a greenhouse atmosphere, global geochemical cycles, and life – if one has data with arbitrarily high signal-to-noise ratio and spatial and spectral resolution. The interpretation of observations of other planets with limited signal-to-noise ratio and spectral resolution, as well as absolutely no
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spatial resolution as envisioned for the first-generation search instruments, will be far more challenging and implies that we need to gather information on the planetary environment to understand what we will see. The following step-by-step approach can be taken to set the planetary atmosphere in context. After detection,
we will focus on the main properties of the planetary system: its orbital elements as well as the presence of an atmosphere, using the ▶ light curve of the planet or/and a crude estimate of the planetary nature from very low-resolution information (three or four wavelength channels). Then a higher-resolution spectrum will be
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used to identify the compounds of the planetary atmosphere, and to constrain the temperature and radius of the observed exoplanet. In that context, we can then test if we have an abiotic explanation of all compounds seen in the atmosphere of such a planet. If we do not, we can work with the exciting biotic hypothesis. O2, O3, and CH4 are good biomarker candidates that can be detected by a lowresolution (resolution < 50) spectrograph. Note that if the presence of sets of biogenic gases such as (O2/O3 + CH4) may imply the presence of a massive and active biosphere, their absence does not imply the absence of life. Life existed on Earth before the interplay between oxygenic photosynthesis and carbon cycling produced an oxygenrich atmosphere.
Temperature and Radius of a Planet Knowing the temperature and planetary radius is crucial for the general understanding of the physical and chemical processes occurring on the planet (tectonics, hydrogen loss to space). In theory, spectroscopy can provide some detailed information on the thermal profile of a planetary atmosphere (▶ Atmosphere, structure). This, however, requires a spectral resolution and a sensitivity that are well beyond the performance of a first-generation spacecraft. Here we concentrate on the initially available observations. One can calculate the stellar energy of the star Fstar that is received at the planet’s measured orbital distance.
The surface temperature of the planet at this distance depends on its ▶ albedo and on the greenhouse warming by atmospheric compounds. However, with a lowresolution spectrum of the thermal emission, the mean effective temperature and the radius of the planet can be obtained. The ability to associate a surface temperature to the spectrum relies on the existence and identification of spectral windows (regions of transparency) which allow probing the surface or certain atmospheric levels. Such identification is not trivial. For an Earthlike planet there are some atmospheric windows that can be used in most of cases, especially between 8 and 11 mm as seen in Fig. 4. This window would, however, become opaque at high H2O partial pressure (e.g., in the inner part of the ▶ Habitable Zone (HZ) where a lot of water is vaporized) and at high CO2 pressure (e.g., a very young Earth or the outer part of the HZ). The accuracy of the radius and temperature determination will depend on the sensitivity and resolution of the spectrum, the precision of the Sun–star distance, the cloud coverage, and also the distribution of brightness temperatures over the planetary surface. Assuming the effective temperature of our planet were radiated from the uppermost cloud deck at about 12 km would introduce about 2% error on the Earth’s radius derived from reflection and/or transmission spectra. For transiting planets, the accuracy of the radius of the planet depends on how well the host star is characterized.
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Potential Biomarkers Spectral ▶ Biomarkers are discussed in a seperate entry in this Encyclopedia. We simply add here Fig. 6 on the terrestria oxygen cycle. Note that possible abiotic sources of biomarkers must be carefully evaluated.
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estimated radius is more reliable. The radius can be measured at different points of the orbit and thus for different values of brightness temperature Tb, which should allow estimating the error made. Note also that a Venus-like exoplanet would exhibit nearly no measurable phase-related variation of its thermal emission, due to the fast rotation of its atmosphere and its strong greenhouse effect, and thus can only be distinguished through spectroscopy from habitable planets. The mean value of Tb estimated over an orbit can be used to estimate the ▶ albedo of the planet, A, through the balance between the incoming stellar radiation and the outgoing IR emission. The thermal light curve (i.e., the integrated infrared emission measured at different position on the orbit) exhibits smaller variations due to the phase (whether the observer sees mainly the day side or the night side) and to the season for a planet with an atmosphere, than does the corresponding visible light curve (see Fig. 5). In the visible ranges, the reflected flux allows us to measure the product A R2, where R is the planetary radius (a small but reflecting planet appears as bright as a big but dark planet). The first generation of optical instruments will be very far from the angular resolution required to directly measure an exoplanet radius.
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The measured IR flux can directly be converted into a brightness temperature that will provide information on the temperature of the atmospheric layers responsible for the emission. If the mass of a planet can be measured (by radial velocity and/or astrometric observations), an estimate of the radius of the rocky core can be made by assuming a bulk composition of the planet which can then be used to convert IR fluxes into temperatures. Important phase-related variations in the planet’s flux are due to a high day/night temperature contrast and imply a low ▶ greenhouse effect and the absence of a stable liquid ocean. Therefore, habitable planets can be distinguished from airless or Mars-like planets by the amplitude of the observed variations of Tb (see Fig. 5). The orbital flux variation in the IR can distinguish (in the detection phase) planets with and without an atmosphere (see also Selsis 2002; Gaidos and Williams 2004). Strong variation of the thermal flux with the phase reveals a strong difference in temperature between the day and night hemisphere of the planet, a consequence of the absence of a dense atmosphere which would transport heat between the hemispheres. In such a case, estimating the radius from the thermal emission is made difficult because most of the received flux comes from the small and hot substellar area. The ability to retrieve the radius would depend on the assumptions that can be made on the orbit geometry and the rotation rate of the planet (▶ Habitabe Zone; effects of tidal locking). In most cases, degenerate solutions will exist. When the mean brightness temperature is stable along the orbit, the
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Atmosphere 1,100,000 Gt-0 2nH2O* + nCO2 + hv Photosynthesis 190 Gt-0/yr
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Habitable Planet (Characterization). Figure 6 Oxygen cycle on Earth (Kaltenegger and Selsis 2010); Gt is gigatons
Cryptic Worlds, Surface Features and Cloud Features ▶ Clouds reduce the relative depths, full widths, and equivalent widths of spectral features, weakening the spectral lines in the whole described wavelength range (Kaltenegger et al. 2007). Earth has an average of 60% cloud coverage, which prevents easy identification of any surface features without knowing the cloud distribution. If one records the planet’s signal with a very high time resolution (a fraction of the rotation period of the planet) and individually high signal-to-noise ratio (SNR), one could determine the overall contribution of clouds to the signal (Cowan et al. 2009; Palle et al. 2008). During each of these individual measurements, one has to collect enough photons for a high individual SNR per measurement to be able to correlate the measurements to the surface features, which precludes this method for first-generation missions that will observe a minimum of several hours to achieve an SNR of 5–10. For Earth (Cowan et al. 2009; Palle et al. 2008), these measurements show a correlation to Earth’s surface features, because the individual measurements are time resolved as well as have an individual high SNR, making it a very interesting concept for future generations of missions. Assuming one had a planet with a cloud-free atmosphere or could distinguish the signal from the overall cloud distribution from that due to the surface reflectivity, one could discover continents and seas on an exoplanet is the daily variation of the surface albedo in the visible (Ford et al. 2001; Palle et al. 2008). On a cloud-free Earth, the diurnal flux variation at visible wavelengths caused by different surface features rotating in and out of view could be high, assuming hemispheric inhomogeneity.
When the planet is only partially illuminated, a more concentrated signal from surface features could be detected as they rotate in and out of view on a cloudless planet (William and Gaidos 2008). Our knowledge of the reflectivity of different surface components on Earth – like desert, ocean, and ice – helps in assigning the vegetation red edge (VRE) of the Earthshine spectrum to terrestrial vegetation. On Earth around 440 million years ago (Pavlov et al. 2003; Schopf 1993), an extensive land plant cover developed, generating the red chlorophyll edge in the reflection spectrum between 700 and 750 nm. While they efficiently absorb visible light, photosynthetic plants have developed strong infrared reflection (possibly as a defense against overheating and chlorophyll degradation) resulting in a steep change in reflectivity around 700 nm, called the red-edge. The primary molecules that absorb the energy and convert it to drive photosynthesis (H2O and CO2 into sugars and O2) are chlorophyll A (0.450 mm) and B (0.680 mm). The exact wavelength and strength of the spectroscopic “vegetation red-edge” (VRE) depends on the plant species and environment. Averaged over a spatially unresolved hemisphere of Earth, the additional reflectivity of this spectral feature is typically only a few percent (see also (Montanes-Rodriguez et al. 2005; Tinetti et al. 2006; Kaltenegger and Traub 2009). Several groups (Arnold et al. 2002; Christensen and Pearl 1997; Montanes-Rodriguez et al. 2007; Woolf et al. 2002; Turnbull et al. 2006) have measured the integrated Earth spectrum via the technique of Earthshine, using sunlight reflected from the non-illuminated, or “dark,” side of the Moon. Earthshine measurements have shown that detection of Earth’s VRE is feasible if the resolution is high and the cloud coverage is known.
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On planets around other stars different biota could develop and produce different reflective features (Kiang et al. 2007). On the Earth, photosynthetic organisms are responsible for the production of nearly all of the oxygen in the atmosphere. However, in many regions of the Earth, and particularly where surface conditions are extreme, for example, in hot and cold deserts, photosynthetic organisms can be driven into and under substrates where light is still sufficient for photosynthesis. These communities exhibit no detectable surface spectral signature. The same is true of the assemblages of photosynthetic organisms at more than a few meters depth in water bodies. These communities are widespread and dominate local photosynthetic productivity. There could be such very interesting Earth-analog worlds that could have habitats but not exhibit biological surface feature in the disk-averaged spectrum (Cockell et al. 2009). Earth’s hemispherical integrated vegetation rededge signature is very weak, but planets with different rotation rates, obliquities, land–ocean fraction, and continental arrangement may have lower cloud-cover and higher vegetated fraction. Knowing that other pigments exist on Earth and that some minerals can exhibit a similar spectral shape around 750 nm (Seager et al. 2005), the detection of the red-edge of chlorophyll on exoplanets, despite its extreme interest, will not be unambiguous.
Summary Any information we collect on habitability is only important in a context that allows us to interpret what we find. To search for signs of life we need to understand how the observed atmosphere physically and chemically works. Knowledge of the temperature and the planetary radius is crucial for the general understanding of the physical and chemical processes occurring on the planet. These parameters, as well as an indication of habitability, can be determined with low-resolution spectroscopy and low photon flux, as assumed for first-generation space missions. The combination of spectral information in the visible (starlight reflected off the planet) as well as in the midIR (planet’s thermal emission) allows a confirmation of detection of atmospheric species and a more detailed characterization of individual planets, and the opportunity to explore a wide domain of planet diversity. Being able to measure the outgoing shortwave and longwave radiation as well as their variations along the orbit, to determine the albedo and identify greenhouse gases, would in combination allow us to explore the climate system at work on the observed worlds, as well as probe planets similar to our own for habitable conditions.
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Future Directions The results of a first-generation mission will most likely include an amazing scope of diverse planets that will set planet formation and evolution, as well as our own planet, in an overall context.
See also ▶ Albedo ▶ Astrometry ▶ Atmosphere, Structure ▶ Biomarkers, Atmospheric (Evolution Over Geological Time) ▶ Biomarkers, Spectral ▶ Blackbody ▶ Clouds ▶ CoRoT Satellite ▶ Greenhouse Effect ▶ Habitability (Effect of Eccentricity) ▶ Habitability (Effects of Stellar Irradiation) ▶ Habitability of the Solar System ▶ Habitable Zone ▶ Habitable Zone, Effect of Tidal Locking ▶ HARPS ▶ James Webb Space Telescope ▶ Kepler Mission ▶ Lightcurve (Planetary Science) ▶ Microlensing Planets ▶ Radial-Velocity Planets ▶ Super-Earths ▶ Transiting Planets
References and Further Reading Arnold L, Gillet S, Lardiere O, Riaud P, Schneider J (2002) A test for the search for life on extrasolar planets. Looking for the terrestrial vegetation signature in the Earthshine spectrum. Astron Astrophys 392:231–237 Borucki WJ, Koch DG, Dunham EW, Jenkins JM (1997) The Kepler mission: a mission to detennine the frequency of inner planets near the habitable zone for a wide range of stars. In: Soderblom D (ed) ASP Conference Series, vol 119: planets beyond the solar system and the next generation of space missions. Baltimore, pp 153–162 Brack A (1993) Liquid water and the origin of life. Orig Life Evol Biosph 23(1):3–10 Charbonneau D, Brown TM, Noyes RW, Gilliland RL (2002) Detection of an extrasolar planet atmosphere. Astrophys J 568:377–384 Charbonneau D, Allen LE, Megeath ST, Torres G, Alonso R, Brown TM, Gilliland RL, Latham DW, Mandushev G, O’Donovan FT, Sozzetti A (2005) Detection of thermal emission from an extrasolar planet. Astrophys J 626:523–529 Christensen PR, Pearl JC (1997) Initial data from the Mars Global Surveyor thermal emission spectrometer experiment: observations of the Earth. J Geophys Res 102:10875–10880 Cockell CS, Kaltenegger L, Raven JA (2009) Cryptic photosynthesis – extrasolar planetary oxygen without a surface biological signature. Astrobiology 9(7):623–636
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Cowan NB, Agol E, Meadows VS et al (2009) Alien maps of an oceanbearing world. Astrophys J 700(2):915–923 Deming D, Seager S, Richardson LJ, Harrington J (2005) Infrared radiation from an extrasolar planet. Nature 435:740–741 Des Marais DJ, Harwit MO, Jucks KW, Kasting JF, Lin DNC, Lunine JI, Schneider J, Seager S, Traub WA, Woolf NJ (2002) Remote sensing of planetary properties and biosignatures on extrasolar terrestrial planets. Astrobiology 2:153–181 Ford E, Seager S, Turner EL (2001) Characterization of extrasolar terrestrial planets from diurnal photometric variability. Nature 412: 885–887 Forget P, Pierehumbert H (1997) Warming early Mars with carbon dioxide clouds that scatter infrared radiation. Science 278:1273–1274 Gaidos E, Williams DM (2004) Seasonality on terrestrial extrasolar planets: inferring obliquity and surface conditions from infrared light curves. New Astron 10:67–72 Grenfell JL, Stracke B, von Paris P, Patzer B, Titz R, Segura A, Rauer H (2007) The response of atmospheric chemistry on earthlike planets around F, G and K Stars to small variations in orbital distance. Planet Space Sci 55:661–671 Harrington J, Hansen BM, Luszcz SH, Seager S, Deming D, Menou K, Cho JY-K, Richardson LJ (2006) The phase-dependent infrared brightness of the extrasolar planet u Andromedae b. Science 314:623–626 Kalas P, Graham JR, Chiang E, Fitzgerald MP, Clampin M, Kite ES, Stapelfeldt K, Marois C, Krist J (2008) Optical images of an exosolar planet 25 light-years from Earth. Science 322:1345–1347 Kaltenegger L, Selsis F (2007) Biomarkers set in context. In: Rudolf D (ed) Extrasolar planets: formation, detection and dynamics. Wiley-VCH, Zurich, pp 79–87 Kaltenegger L, Traub W (2009) Transits of Earth-like planets. Astrophys J 698(1):519–527 Kaltenegger L, Traub WA, Jucks KW (2007) Spectral evolution of an Earth-like planet. Astrophys J 658:598–616 Kaltenegger L, Selsis F (2010) Characterizing Habitable Extrasolar Planets using Spectral Fingerprints, CRAS, EAS Publications Series, 41, pp 485–504 Kasting JF (1997) Habitable zones around low mass stars and the search for extraterrestrial life. Orig Life Evol Biosph 27(1/3):291–310, Kluwer Academic Publishers Kasting JF, Whitmire DP, Reynolds H (1993) Habitable zones around main sequence stars. Icarus 101:108–119 Knutson HA, Charbonneau D, Allen LE, Fortney JJ, Agol E, Cowan NB, Showman AP, Cooper CS, Megeath ST (2007) A map of the day-night contrast of the extrasolar planet HD 189733b. Nature 447:183–185 Lagrange A-M, Gratadour D, Chauvin G, Fusco T, Ehrenreich D, Mouillet D, Rousset G, Rouan D, Allard F, Gendron E´, Charton J, Mugnier L, Rabou P, Montri J, Lacombe F (2009) A probable giant planet imaged in the b Pictoris disk. VLT/NaCo deep L’-band imaging. Astron Astrophys 493:L21–L25 Lovelock JE (1975) Thermodynamics and the recognition of Alien biospheres. Proc R Soc Lond B Biol Sci 189(1095):167–180 Marois C, Macintosh B, Barman T, Zuckerman B, Song I, Patience J, Lafrenie`re D, Doyon R (2008) Direct imaging of multiple planets orbiting the star HR 8799. Science 322:1348–1350 Mayor M, Udry S, Lovis C, Pepe F, Queloz D, Benz W, Bertaux J-L, Bouchy F, Mordasini C, Segransan D (2009) The HARPS search for southern extra-solar planets. XIII. A planetary system with 3 superEarths (4.2, 6.9, and 9.2 ML). Astron Astrophys 493:639–644 Montanes-Rodriguez P, Palle E, Goode PR (2007) Measurements of the surface brightness of the Earthshine with applications to calibrate lunar flashes. Astrophys J 134:1145–1149
Montan˜e´s-Rodriguez P, Palle´ E, Goode PR, Hickey J, Koonin SE (2005) Globally integrated measurements of the Earth’s visible spectral albedo. Astrophys J 629:1175–1182 Palle´ E, Ford EB, Seager S, Montan˜e´s-Rodrı´guez P, Vazquez M (2008) Identifying the rotation rate and the presence of dynamic weather on extrasolar Earth-like planets from photometric observations. Astrophys J 676:1319–1329 Palle´ E, Zapatero Osorio MR, Barrena R, Montan˜e´s-Rodrı´guez P, Martı´n EL (2009) Earth’s transmission spectrum from lunar eclipse observations. Nature 459:814–816 Pavlov AA, Hurtgen MT, Kasting JF, Arthur MA (2003) Methane-rich proterozoic atmosphere? Geology 31:87–92 Rouan D, Baglin A, Copet E, Schneider J, Barge P, Deleuil M, Vuillemin A, Leger A (1998) The exosolar planets program of the COROT satellite. Earth Moon Planets 81(1):79–82 Sagan C, Thompson WR, Carlson R, Gurnett D, Hord C (1993) A search for life on Earth from the Galileo spacecraft. Nature 365:715 Schindler TL, Kasting JF (2000) Synthetic spectra of simulated terrestrial atmospheres containing possible biomarker gases. Icarus 145:262–271 Schopf JW (1993) Microfossils of the early Archean Apex Chert: new evidence of the antiquity of life. Science 260:640–642 Seager S, Turner EL, Schafer J, Ford EB (2005) Vegetation’s red edge: a possible spectroscopic biosignature of extraterrestrial plants. Astrobiology 5:372–390 Seager S, Kuchner M, Hier-Majumder CA, Militzer B (2007) Mass-Radius relationships for solid exoplanets. Astrophys J 669:1279–1297 Segura A, Kasting JF, Meadows V, Cohen M, Scalo J, Crisp D, Butler RAH, Tinetti G (2005) Biosignatures from Earth-like planets around M Dwarfs. Astrobiology 5:706–725 Segura A, Meadows VS, Kasting JF, Crisp D, Cohen M (2007) Abiotic formation of O2 and O3 in high-CO2 terrestrial atmospheres. Astron Astrophys 472:665–672 Selsis F (2000) Review: Physics of planets I: Darwin and the atmospheres of terrestrial planets. In Darwin and astronomy – the infrared space interferometer’, Stockholm, Sweden, 17–19 November 1999. ESA SP 451, Noordwijk, the Netherlands, pp 133–142 Selsis F (2002) Search for signatures of life on exoplanets. In: Foing B, Battrick B (eds) Earth-like planets and moons. Proceedings of the 36th ESLAB Symposium, ESA SP-514, ESTEC, Noordwijk, The Netherlands, 3–8 June 2002. ESA Publications Division, Noordwijk, pp 251–258 Selsis F, Despois D, Parisot J-P (2002) Signature of life on exoplanets: can Darwin produce false positive detections? Astron Astrophys 388:985–991 Swain MR, Vasisht G, Tinetti G, Bouwman J, Chen P, Yung Y, Deming D, Deroo P (2009) Molecular signatures in the near-infrared dayside spectrum of HD 189733b. Astrophys J 690:L114–L117 Tinetti G, Rashby N, Yung Y (2006) Detectability of red-edge-shifted vegetation on terrestrial planets orbiting M stars. Astrophys J Lett 644:L129–L132 Tinetti G, Vidal-Madjar A, Liang M-C, Beaulieu J-P, Yung Y, Carey S, Barber RJ, Tennyson J (2007) Water vapour in the atmosphere of a transiting extrasolar planet. Nature 448:169–172 Torres G, Winn JN, Holman MJ (2008) Improved parameters for extrasolar transiting planets. Astrophys J 677:1324–1330 Turnbull MC, Traub WA, Jucks KW, Woolf NJ, Meyer MR, Gorlova N, Skrutskie MF, Wilson JC (2006) Spectrum of a habitable world: Earthshine in the near-infrared. Astrophys J 644:551–559 Valencia D, O’Connell RJ, Sasselov DD (2006) Internal structure of massive terrestrial planets. Icarus 181:545–554
Habitable Zone Vidal-Madjar A, Dsert J-M, Lecavelier des Etangs A, Hbrard G, Ballester GE, Ehrenreich D, Ferlet R, McConnell JC, Mayor M, Parkinson CD (2004) Detection of oxygen and carbon in the hydrodynamically escaping atmosphere of the extrasolar planet HD 209458b. Astrophys J 604:L69–L72 William DM, Gaidos E (2008) Detecting the glint of starlight on the oceans of distant planets. Icarus 195(2):927–937 Woolf NJ, Smith PS, Traub WA, Jucks KW (2002) ApJ 574:4302
Habitable Zone LISA KALTENEGGER1,2, ANTIGONA SEGURA3 1 Harvard University, Cambridge, MA, USA 2 MPIA, Heidelberg, Germany 3 Instituto de Ciencias Nucleares, Universidad Nacional Auto´noma de Me´xico. Circuito Exterior C.U.A, Me´xico D.F, Mexico
Exoplanets, habitability, life
Definition The circumstellar Habitable Zone (HZ) is defined as the annulus around a ▶ main sequence star where a rocky planet similar to the Earth in composition and mass with an atmosphere can support liquid water on its surface. The inner edge is defined as the distance at which a planet undergoes runaway greenhouse conditions vaporizing the whole water reservoir and, as a second effect, inducing the photodissociation of water vapor and the loss of hydrogen to space. The outer boundary is defined as the distance from the star where a maximum ▶ greenhouse effect fails to keep the surface of the planet above freezing, or the distance from the star where CO2 gas starts condensing (see, e.g., Kasting et al. 1993). The width and distance of this annulus, using this definition, depends mainly on the stellar luminosity and spectral energy distribution. A planet in the Habitable Zone is not necessarily habitable. Multiple mechanisms exist by which a planet cannot attain or can lose habitability; for example, low mass and resulting atmospheric losses or changes in initial composition (e.g., no water).
History The Habitable Zone (HZ) concept was proposed for the first time by Huang (1959, 1960) and has been calculated by several authors after that (see e.g., Rasool and DeBergh 1970; Hart 1979; Kasting et al. 1993).
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Overview Future remote-sensing characterization of planetary environments can be used to test our understanding of the factors that contribute to planetary habitability. The HZ is usually defined for surface conditions only. Chemoautotrophic life, whose metabolism does not depend on the stellar light, can still exist outside the HZ, thriving in the interior of the planet where liquid water is available. Such metabolisms rely on very limited sources of energy (compared to stellar light) and electron donors (compared to H2O on Earth). They mainly catalyze reactions that would occur at a slower rate in purely abiotic conditions, and they are thus not expected to modify a whole planetary environment in a way detectable remotely.
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Basic Methodology The equilibrium temperature Teq of a planet may be found by equating the stellar energy flux absorbed to the thermal energy that is radiated back to space. The result is Teq ¼ ðSð1 AÞ=f sÞ4
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where A is the Bond ▶ albedo, S the stellar energy flux, s the Stefan–Boltzmann constant, and f a redistribution factor that accounts for the movement of energy around the planetary surface due to rotation (▶ Habitable Zone, effect of tidal locking), atmospheric circulation, etc. The Bond ▶ albedo is 0.29 for Earth and is defined as the fraction of power at all wavelengths that is scattered back out into space. If all the incident energy is uniformly distributed on the planetary sphere (e.g., by atmospheric circulation) f =4, if only the day hemisphere is so heated, f =2. If there is no redistribution, f =1/ cos(F) where F is the zenith angle. The last case gives good results for the surface temperature on the sunlit hemisphere of airless bodies with known albedo like the Moon and Mercury. Note that for a planet with a dense atmosphere Teq is not necessarily equal to the physical temperature at the surface, but may refer to a level in the atmophere. With albedos of 0.75, 0.3, and 0.25, respectively, and assuming f =4, Venus, Earth, and Mars have equilibrium temperatures of 231 K, 255 K, and 210 K, while their mean surface temperatures are 737 K, 288 K, and 215 K. The orbital semimajor axis in the middle of the habitable zone for a circular orbit, aHZ (in AU), is derived by scaling the Earth–Sun system using Lstar/Lsun =(Rstar/ Rsun)2 (Tstar/Tsun)4, so aHZ =1 AU (Lstar/LSun)0.5, and finally aHZ ¼ 1AU ððL=Lsun Þ=Seff Þ0:5
ð2Þ
This formula (Kasting et al. 1993) assumes that the planet has a similar ▶ albedo to Earth, that it rotates or
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redistributes the insolation as on Earth (▶ Habitability effects of tidal locking; ▶ Habitability Zone, effects of eccentricity), and that it has a similar greenhouse effect. Climate models (▶ Atmosphere (1D); ▶ Atmosphere (GCM)) are required to calculate the inner and outer limits of the HZ for a planet with a given atmosphere. In this model, Seff is 1.90, 1.41, 1.05, and 1.05 for F, G, K, and M stars (▶ Habitability, effects of stellar irradiation), Habitability, effects of stellar irraoliation respectively, for the inner edge of the HZ (where runaway greenhouse occurs), and 0.46, 0.36, 0.27, and 0.27 for F, G, K, and M stars, respectively, for the outer edge of the HZ (assuming a maximum greenhouse effect in the planet’s atmosphere; see Kasting et al. 1993 and ▶ Spectral Type, Star). We assume here that the planet is rocky and is dominated by a global carbonate-silicate cycle that stabilizes the surface temperature and the CO2 level, like on Earth (Walker 1977). This implies that the planet is geologically active and continuously outgasses CO2, and that carbonates form in the presence of surface liquid water, which may require continental weathering. Without this stabilization, the Earth would not be habitable through geological times (▶ Biomarkers, atmospheric (evolution over geological times)). This in turn affects the composition of the atmosphere at the outer edge of the Habitable Zone. As the amount of CO2 increases, the greenhouse effect increases, warming the surface and releasing more CO2 from carbonates and from that dissolved in oceans. The feedback only works up to the temperature where the increase in the albedo due to increasing in CO2 concentration (producing more ▶ Rayleigh scattering and ▶ clouds formation; see e.g., Forget and Pierrehumbert 1997) is stronger than the greenhouse effect of CO2. Because of the relation between the albedo of the planet and the effective temperature of the star, the limits of the
habitable zone cannot be simply scaled to the stellar luminosity (▶ Habitability, effects of stellar radiation). Note that a planet found in the HZ is not necessary habitable. Many factors may prevent surface habitability: lack of water or ingredients necessary for the emergence of life, gravity that is too small to retain a dense atmosphere against gravitational escape (▶ atmospheric escape), and a lack of an active geology replenishing the atmosphere in carbon dioxide. The planet could have accreted a massive H2–He envelope that would prevent water from being liquid by keeping the surface pressure too high, or it could have migrated to its current position (see, e.g., Valencia et al. 2006). In addition, the HZ assumes a terrestrial planet with a water content large enough so that the surface can host liquid water for any surface temperature between the temperature of the triple point of water, 273 K, and the critical temperature of water, TC = 647 K (Kasting et al. 1993).
Key Research Findings The limits of the circumstellar Habitable Zone (HZ) around a main sequence star are given in Table 1. However, the limits of the HZ are known qualitatively, more than quantitatively. This uncertainty is mainly due to the complex role of ▶ clouds and three-dimensional climatic effects not yet included in the modeling (▶ Atmosphere (1D); Atmosphere (GM)). Thus, planets slightly outside the computed HZ could still be habitable, while planets at habitable orbital distance may not be habitable because of their size or chemical composition. Given that stellar luminosity evolves during the star’s lifetime, the concept of a Continuously Habitable Zone (CHZ) has been introduced to mean the zone that remains habitable around a star during a given period of time (Hart 1979).
Habitable Zone. Table 1 Properties and Habitable Zone limits of the main-sequence stars of Spectral type F-M considered for the search of Earth analogs, assuming no other limits than runaway and maximum greenhouse for an Earth-analog planet – this model does not take increasing cloud fractions into account Habitable zone (AU)b
a
Spectral type
Effective temp. (K)a
Luminosity (L)a
Mass (M)a
Inner limitc
Outer limitd
F
6,100–7,200
2.0–6.5
1.4–1.6
1–1.80
2.1–3.8
G
5,300–6,030
0.66–1.1
0.9–1.05
0.7–0.9
1.4–1.7
K
4,900–5,250
0.1–0.42
0.67–0.79
0.3–0.6
0.6–1.2
M
2,600–3,850
0.0012–0.077
0.06–0.51
0.03–0.3
0.1–0.5
Data from Ostlie and Carroll 1996 Data from Kasting et al. 1993 c Runaway greenhouse limit calculated for planets around stars with spectral types F0 (Teff =7,200 K), G2 (Teff =5,700 K), and M0 (Teff =3,700 K) d Maximum greenhouse limit for planets around stars with spectral types F0 (Teff =7,200 K), G2 (Teff =5,700 K), and M0 (Teff =3,700 K) b
Habitable Zone, Effect of Tidal Locking
Applications The concepts of the HZ and CHZ help define an astronomical search zone for habitable planets with remotely detectable species in their atmosphere that indicate habitable conditions, such as surface liquid water. On a habitable planet where the carbonate-silicate cycle is at work, the level of CO2 in the atmosphere depends on the amount of greenhouse warming required to maintain habitability and so depends on the orbital distance: CO2 is a trace gas close to the inner edge of the HZ but a major compound in the outer part of the HZ (Forget and Pierrehumbert 1997). Earth-like planets close to the inner edge are expected to have a water-rich atmosphere and those at the outer edge of the Habitable Zone a carbon-dioxide rich one. This is one of the first theories we can test with a first-generation space mission. There may be other habitable environments further from the star than the classical radiative habitable zone, in what is known as the “cryo-ecosphere” (see, e.g., Pen˜aCabrera and Durand-Manterola 2004). This region could include environments made habitable by the delivery of tidal energy as would be the case for a subsurface ocean on a giant planet’s moon (e.g., ▶ Europa or Titan-like environments). These regions have not been discounted as habitable, but are extremely difficult to detect remotely, even for moons in our own Solar system, and so will not be targeted by first-generation extrasolar planet characterization missions. Only global atmospheric signatures are detectable remotely.
Future Directions The Habitable Zone concept is still evolving as we learn more about planetary formation and evolution, and as we continue to improve the radiative transfer, ▶ clouds and three-dimensional atmospheric models (▶ Atmosphere (GCM)) that allow more accurate calculations of a planet’s temperature profile (▶ Atmosphere, structure).
See also ▶ Albedo ▶ Atmosphere, Escape ▶ Atmosphere, Model 1D ▶ Atmosphere, Structure ▶ Biomarkers, Atmospheric (Evolution Over Geological Time) ▶ Biomarkers, Spectral ▶ Clouds ▶ Europa ▶ GCM ▶ Greenhouse Effect
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▶ Habitability (Effect of Eccentricity) ▶ Habitability (Effects of Stellar Irradiation) ▶ Habitable Planet (Characterization) ▶ Habitable Zone, Effect of Tidal Locking ▶ Main Sequence ▶ Planetary Migration ▶ Rayleigh Scattering ▶ Spectral Type
References and Further Reading Forget P, Pierrehumbert H (1997) Warming early Mars with carbon dioxide. Clouds that scatter infrared radiation. Science 278:1273–1274 Hart MH (1978) The evolution of the atmosphere of the Earth. Icarus 33:23–39 Hart MH (1979) Habitable zones about main sequence stars. Icarus 37:351–357 Huang SS (1959) Occurrence of life outside the Solar system. Am Sci 47:397–402 Huang SS (1960) Life outside the solar system. Sci Am 202(4):55 Kasting JF, Whitmire DP, Reynolds RT (1993) Habitable zones around main sequence stars. Icarus 101:108–128 Ostlie DA, Carroll BW (1996) An introduction to modern stellar astrophysics. Addison-Wesley, USA Pen˜a-Cabrera GVY, Durand-Manterola HJ (2004) Possible biotic distribution in our galaxy. Adv Space Res 33:114–117 Rasool SI, DeBergh C (1970) The runaway greenhouse and the accumulation of CO: in the Venus atmosphere. Nature 226:1037–1039 Segura A, Kaltenegger L (2010) Search for habitable planets. In: Basiuk VA, Navarro-Gonza´lez R (eds) Astrobiology: emergence, search and detection of life. American Scientific Publishers, NY, USA, pp 341–358 Valencia D, O’Connell RJ, Sasselov DD (2006) Internal structure of massive terrestrial planets. Icarus 181:545–554 Walke JCG (1977) Evolution of the atmosphere. Macmillan/Collier Macmillan, New York/London
Habitable Zone, Effect of Tidal Locking LISA KALTENEGGER Harvard University, Cambridge, MA, USA MPIA, Heidelberg, Germany
Keywords Biomarkers, habitability, habitable zone, M stars, resonance, tidal locking
Definition Tidal locking is a state of dynamical equilibrium between a planet’s spin and orbital angular momentum. Tidally-locked planets on circular orbits may become
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synchronous rotators, presenting the same solid-body hemisphere to the star.
Overview Tidally-locked planets on circular orbits may become synchronous rotators always presenting the same solidbody hemisphere to the star. If the planet’s orbit is eccentric, either due to a primordial condition, or maintained by interactions with other planets, then the rotation period will be less than the orbital period and the planet will not synchronously rotate. Planets interior to the tidal lock radius may also become trapped in spinorbit resonances, where they complete an integer ratio of rotations on their axis to orbits around their star. For example, Mercury is within the Sun’s tidal lock radius in a 3:2 spin-orbit resonance, completing 3 rotations on its axis every two revolutions (orbits) around the Sun. For an M dwarf (less massive and cooler than the Sun), the ▶ Habitable Zone is so close to the star that the planets are likely to become tidally locked within a relatively short time after they form (Dole 1964; Kasting et al. 1993), but several resonances are possible. Only a planet in a 1:1 resonance receives starlight on only one hemisphere. Detailed models have shown that this does not prevent habitability for planets with even modestly dense atmospheres (Haberle et al. 1996; Joshi et al. 1997; Joshi 2003). Especially if an ocean is present, even a tidally locked Earth should remain habitable, provided that atmospheric cycles transport heat from the dayside to the nightside.
Habitat Synonyms Ecosystem
Definition Habitat is the location in an ▶ environment which can be colonized by a biological population. In nature, organisms live in association with other organisms in assemblages called populations. The components and number of individuals in a habitat are governed by the resources and conditions that exist in the habitat. Habitats differ markedly in their characteristics, and a habitat that is favorable for the development of one organism may actually be harmful for another organism. From an astrobiological point of view, habitat is deeply related to the habitability concept which refers to the conditions that can support life in an adverse surrounding.
See also ▶ Biotope ▶ Environment
HAC ▶ Hydrogenated Amorphous Carbon
See also ▶ Biomarkers, Spectral ▶ Habitable Planet (Characterization) ▶ Habitable Zone ▶ Habitability of the Solar System ▶ Spectral Type
References and Further Reading Dole SH (1964) Habitable planets for man. Blaisdell Publishing, New York Haberle RM, McKay C, Tyler D, Reynolds R (1996) Can synchronous rotating planets support an atmosphere? In: Doyle LR (ed) Circumstellar habitable zones. Travis House, Menlo Park, pp 29–41 Joshi M (2003) Climate model studies of synchronously rotating planets. Astrobiology 3:415–427 Joshi MM, Haberle RM, Reynolds RT (1997) Simulations of the atmospheres of synchronously rotating terrestrial planets orbiting M dwarfs: conditions for atmospheric collapse and the implications for habitability. Icarus 129:450–465 Kasting JF, Whitmire DP, Reynolds RT (1993) Habitable zones around main sequence stars. Icarus 101:108–128
Hadean SIMON A. WILDE Department of Applied Geology, Curtin University of Technology, Perth, Western Australia, Australia
Synonyms Early earth; Priscoan
Keywords Earliest eon, Precambrian, zircon
Definition Hadean is the earliest eon of geological time, extending from the accretion of the Earth (4.567 Ga) to the
Hadley Cells
formation of the earliest known rocks, the Acasta gneisses in the Northwest Territories of Canada (4.03 Ga).
Overview The term Hadean was coined in 1972 by the American paleontologist Preston Cloud to cover that period of geological time prior to formation of the earliest known rocks. Another term in the literature for essentially the same period of time is “Priscoan,” introduced by British geologist Brian Harland in 1989. The term Hadean was derived from Greek mythology where “Hades” first referred to the God of the Underworld and later to the Underworld itself. In geological usage, it is commonly equated with “Hell,” based on the perception that the Earth was extremely hot and turbulent at that time. “Hadean” is, however, an informal term that has not been ratified by the International Commission on Stratigraphy. This is partly because of the diversity of views as to its precise duration and to recent work that has questioned whether much of this period of time on Earth was really “hell-like.” Its starting point has variously been taken as the formation of the planetary nebula, the onset of accretion or the time from when the Earth was largely solid. Its termination is commonly placed at 3.8 Ga, the age of the ancient gneisses in West Greenland, although more recent usage places this at 4.03 Ga, based on the age of the World’s oldest gneisses at Acasta (Slave Province, Canada): this is the date currently identified by the International Commission on Stratigraphy for the “informal” Hadean eon. Study of zircon crystals up to 4.4 Ga, particularly from Jack Hills in Western Australia, has led to the view that the Earth had cooled down sufficiently by 4.3 Ga to support oceans and stable continents. This is based on study of the oxygen and lithium isotopes in pristine zircon domains. If this proves to be correct, the term Hadean is best restricted to the period of time before the formation of oceans and continents, when the surface of the Earth was too hot and unstable to allow liquid water to form and rocks to survive. Some workers who do not support the use of “Hadean” prefer to extend the Eoarchaean back to cover this period of geological time. Planetary-scale geological processes took place during the Hadean eon, from the accretion of the Earth to its differentiation into a metallic core and a silicate mantle. Continental crust developed earlier than thought, if geological information retained by the Jack Hills zircons is correct. Several authors speculate that plate tectonics could also have started during the Hadean, yet geological evidence for this has not yet been found. A detail account for these processes is reported in Earth, formation and early evolution.
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See also ▶ Acasta Gneiss ▶ Archea ▶ Canadian Precambrian Shield ▶ Chronological History of Life on Earth ▶ Earth, Formation and Early Evolution ▶ Geological Timescale ▶ Jack Hills (Yilgarn, Western Australia) ▶ Late Veneer ▶ Zircon
References and Further Reading Cloud P (1972) A working model of the primitive earth. Am J Sci 272:537–548 Harland WB, Armstrong RL, Cox AV, Craig LE, Smith AG, Smith DG (1990) A geologic time scale 1989. Cambridge University Press, Cambridge, pp 1–28 http://en.wickepedia.org/wiki/Hadean International Commission on Stratigraphy: http://www.stratigraphy.org/ column.php?id=Chart/Time%20Scale Valley JW, Peck WH, King EM, Wilde SA (2002) A cool early earth. Geology 30:351–354
Hadley Cells Definition The Hadley cell on the Earth is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, a compensatory sinking motion in the subtropics, the poleward flow in the upper troposphere (7–20 km above the surface), and the equator-ward flow near the surface. The Hadley circulation cells, one on each hemisphere, cover about half of the Earth’s surface area. The Hadley cells carry heat and moisture from the tropics to the northern and southern mid-latitudes. Hadley cells are also present on other terrestrial bodies like Mars, Titan, or Venus. On a slowly rotating planet like Venus, the Hadley circulation can extend to the poles. Circulation cells like the Hadley cells probably exist also on other rocky exoplanets, and influence their climate substantially.
See also ▶ Atmosphere, Structure ▶ Scale Height
References and Further Reading Chamberlain J, Hunten D (1987) Theory of planetary atmospheres. Academic, Orlando
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Haeckel’s Conception of Origins of Life
Haeckel’s Conception of Origins of Life History Ernst Haeckel (1834–1919) was a German zoologist, great supporter of the evolution theory. He developed his own theory of evolution founded on Darwinism and also on some Lamarckian principles. His theoretical thought was closely connected to his monistic philosophical view. Regarding the origin of life, he proposed a scenario of ▶ abiogenesis based on protoplasmic theory and claimed that evolution of matter could lead to a form of albuminoidal protoplasm. He called Monera the primitive entities coming before cells.
See also ▶ Abiogenesis ▶ Darwin’s Conception of Origins of Life ▶ Huxley’s Conception on Origins of Life ▶ Protoplasmic Theory of Life
Haldane’s Conception of Origins of Life STE´PHANE TIRARD Faculte´ des Sciences et des Techniques de Nantes, Centre Franc¸ois Vie`te d’Histoire des sciences et des Techniques EA 1161, Nantes, France
Keywords Origins of life, prebiotic soup
Abstract In 1927, the British biologist J.B.S. Haldane published one of the most important scenarios about the origin of life on earth of the first part of the twentieth century.
History John Burdon Sanderson Haldane (1892–1964) was one of the famous British biochemists and geneticists of the first part of the twentieth century. He notably participated to the reflexion about evolution and was one of the actors of the synthesis. During his career, he spoke several times about the problem of the origins of life; however, he never worked as specialist of the topic. He published a very important
text on this matter in 1929, in The Rationalist Annual. This text is often presented as linked to Oparin’s one (1924), and it is often said to be Oparin–Haldane’s theory. However these two papers were independently published. Haldane gave a complete scenario describing primitive conditions on earth and steeps of chemical evolution from mineral to organic molecules. The main reactions occurred in the primitive sea, sort of complex chemical solution, named hot dilute soup by Haldane. His text began with an historical narrative about spontaneous generation and by an analysis of the nature of the bacteriophage, which he considered as “a step beyond the enzyme on the road of life, but it is perhaps an exaggeration to call it fully alife” (p.106). He suggested a complete scenario beginning with the primitive atmosphere contained little or no oxygen, but carbon dioxide, and claimed that ammonia could be formed by the action of water on nitride contained in the crust. Moreover, in the lack of ozone, ultraviolet rays could be chemically active. One of his most important suggestion is that “when ultraviolet light acts on a mixture of water, carbon dioxide, and ammonia, a vast variety of organic substances are made, including sugars and apparently some of the materials from which proteins are built.” He claimed that in the current nature, a such mixture would be eaten by microorganisms, but in the primitive conditions, in this “hot dilute soup”, big molecules could be synthesized. For him, “the first living or half-living thing” was capable of reproduction; in other words, they were the first genes (p. 107). Then he claimed that “when the whole sea was a vast chemical laboratory,” the condition for the formation of oily films and then primitive cells must have been favorable. Haldane’s proposal, with Oparin’s one, revealed the evolution of ideas on the origin of life at the beginning of the twentieth century. Haldane, according to current biology, used his biochemist and geneticist view to describe the steps of a chemical process producing the most simple living beings.
See also ▶ Miller, Stanley ▶ Oparin’s Conception of Origins of Life ▶ Urey’s Conception of Origins of Life
References and Further Reading Haldane JBS (1929) The origin of life, rationnalist annual. In: On being the right size and other essays. Oxford University Press, Oxford, 1991
Halophile
Half-Life Synonyms
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seawater. Halogenated organic compounds are also produced by living organisms. The halogens all form binary compounds with hydrogen, e.g., HF, HCl, HBr, and HI, which are strong Brønsted–Lowry acids.
Period (half-life period)
Definition The half-life T1/2 is the time required to halve the number of atoms of a particular radioactive nuclide. It relates to the ▶ decay constant l through T1/2 = ln 2/l 0.69/l.
See also ▶ Decay Constant ▶ Earth, Age of ▶ Geochronology ▶ Radioactivity
Halophile JOSEFA ANTO´N Departamento de Fisiologı´a, Gene´tica y Microbiologı´a, Universidad de Alicante, Alicante, Spain
Synonyms Salt loving organisms
Keywords
Half-Major Axis ▶ Semi Major Axis
Halogen Definition A halogen is a chemical element from Group 17 (in the IUPAC convention) (formerly VII, VIIA) of the periodic table, composed of fluorine, chlorine, bromine, iodine, and astatine. The man-made element 117 is predicted to be a halogen. The Swedish chemist Berzelius coined the term “halogen” from the Greek ha´ls, “salt,” and gen, meaning “come to be” – for an element that produces a salt with a metal. The halogens are the only periodic table group that contains elements in all three familiar states of matter at standard temperature and pressure. At room temperature and pressure, fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. The halogens show several trends as the atomic number increases, including decreasing electronegativity and reactivity, and increasing melting and boiling points. In their elemental form, the halogens exist as diatomic molecules, but these are relatively unstable. Due to their high reactivity and electron affinity, halogens are usually found in nature as molecular compounds or as ions. Halogen anions, known as halides, (e.g., Cl, Br, and F) and oxoanions such as iodate (IO3) are commonly found in many minerals and are major components of
Compatible solute, hypersaline environment
Definition Halophile is an organism that needs high salt concentrations for growth. A widely used definition is that of Kushner and Kamekura (1988) who classify organisms depending on the salt concentration needed for optimum growth. Thus, non-halophiles grow best in media containing less than 0.2 M salts while halophiles grow best in media containing from 0.2 to 5.2 M dissolved salts. Halophiles can be further divided into slightly halophilic (optimum growth between 0.2 and 0.5 M salt), moderately halophilic (0.5–2.5 M salt), and extremely halophilic (above 2.5 M salt).
Overview All three domains of life include halophilic microorganisms. Archaeal halophiles, all belonging to the Euryarchaeota, can be found among the methanogens and the members of the order Halobacteriales, that only includes extreme halophiles. Within this order, there is a single family (the Halobacteriaceae) that includes 28 genera, some of them of alkaliphilic organisms. The most halophilic organism as well as the ones most frequently found in hypersaline environments are included within this family. Bacterial phyla such as Cyanobacteria, Proteobacteria, Firmicutes, Actinobacteria, Spirochaetes, and Bacteroidetes are also halophiles (Oren 2008). Finally, some eukaryotic organisms – plants, fungi, cilliates, and flagellates – are halophilic. The unicellular green alga Dunaliella salina is the most halophilic known eukaryote. Normally, every halophile has a relatively narrow range of salt concentration that allows growth. The proteobacterium Halomonas spp., is
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an exception, being able to grow over a very wide range of salt concentration. Halophiles are usually found in phylogenetic trees that also include non-halophilic relatives, but three large phylogenetically coherent groups comprise only halophiles. Such is the case of the aerobic ▶ Archaea from the order Halobacteriales (also known as haloarchaea), the anaerobic fermentative ▶ bacteria from the order Halanaerobiales and the Gammaproteobacteria from the family Halomonadaceae. A group of phylogenetically related halophiles within the Bacteroidetes is recovered from hypersaline environments all over the world. Halophilic microorganisms use two different strategies to cope with the osmotic stress stemming from the high ionic concentration of their environments. Archaea of the order Halobacteriales, anaerobic Bacteria of the order Haloanaerobiales, and some members of the Bacteroidetes (i.e., Salinibacter ruber) accumulate high concentrations of inorganic ions (mostly potassium) in the cytoplasm. These three groups include the most extreme halophiles. The so-called “salt-in” strategy requires an adaptation of the whole intracellular machinery to work in highly saline environments. All other halophilic organisms keep their osmotic balance by producing and/or accumulating “compatible” low-molecular-weight organic solutes (see “compatible solutes”) whose concentration is regulated according to the salinity of the environment. Habitats for halophilic microorganisms include salt marshes, salt lakes that may be alkaline (Mono Lake, Wadi Natrum) or near neutral (e.g., the Dead Sea, Great Salt Lake, Tuz Lake, Chaka Lake, and some cold hypersaline lakes in the Antarctica), man-made salterns either coastal or inland, hypersaline deep sea masses, salted food, animal hides, saline soils, dessert salt crusts, and subterranean brines. These environments have been classified depending on whether they originated through the evaporation of seawater or not, thus talassohaline (e.g., solar salterns) and athalassoaline (e.g., the Dead Sea). Hypersaline environments are extreme habitats and very high salt is not the only condition restricting biodiversity in these systems. They can also have high pH or, depending on their geographical location, high or low temperatures. In addition, some hypersaline environments have very low nutrient availability or high concentrations of toxic compounds such as heavy metals. Accordingly, halophilic organisms may display additional extremophilisms. Alkaliphilic, phycrotrophilic, and thermopilic halophiles have been described (for instance, the thermophilic bacterium Halothermothrix orenii, the
archaeon Halorubrum lacusprofundi, and members of the archaeal genus Natronobacterium). Moderately halophilic bacteria inhabit hypersaline habitats with salinities around 10%. Aquatic environments with higher, near-saturation salt concentrations, may be densely populated with extremely halophilic microorganisms and this leads to bright red, orange, or purple colorations due to the pigments of these organisms. In some multipond solar salterns, microbial diversity is found to decrease at higher salinities. However, the number of microorganisms increases with salinity, reaching very high densities of up to 108 cells/ml at nearsaturation concentrations. Most of these cells correspond to organisms of the Archaea domain although in some environments Bacteria can account for up to 30% of the total counts. Hypersaline environments all over the world contain Archaea such as the square haloarchaeon Haloquadratum walsbyi and related populations, as well as Halorubrum representatives. In the Bacteria domain, the Bacteroidetes Salinibacter spp. and related phylotypes are frequently found in hypersaline settings close to saturation, although in some cases Proteobacteria constitute the bulk of extremely halophilic Bacteria detected in these systems. At near-saturation concentrations, both cell number and diversity of ▶ Eukarya decreases dramatically, leaving only Dunaliella and some extremely halophilic fungi in low numbers. At salinities above 10–15% large organisms disappear, with the exception of the brine shrimp Artemia salina and the larvae of the fly Ephydra. Hypersaline environments harbor the highest numbers of viruses reported so far for aquatic systems. Most dissimilatory prokaryotic metabolisms can function in hypersaline environments although a few of them have never been detected at salt concentrations above 10% (e.g., proton-reducing acetogens, methanogenesis from acetate, or autotrophic nitrite oxidations). For a detailed discussion of the energetics of halophilism, see Oren (1999). In addition to their evolutive and ecological interest, halophilic microorganisms have some biotechnological applications. Haloarchaea produce bacteriorhodopsin that can be used as photoactive material in optical devices as well as polyhydroxyalcanoates, exopolysaccharides, and halocins. Moderately halophilic bacteria are used for the commercial production of their compatible solutes ectoines and hydroxyectoines, which find application in the cosmetic industry or the biological treatment of saline industrial waste effluents. Halophilic eukaryotes such as Dunaliella are used as a natural source of beta-carotene in cosmetics and some food while the shrimp Artemia salina can be used in aquaculture.
Handedness
See also ▶ Alkaliphile ▶ Archea ▶ Bacteria ▶ Compatible Solute ▶ Eukarya ▶ Euryarchaeota ▶ Extreme Environment ▶ Extremophiles ▶ Halotolerance ▶ Osmolite
References and Further Reading DasSarma S, Arora P (2006) Halophiles. Encyclopedia of life sciences, Wiley, London Kushner DJ, Kamekura M (1988) Physiology of halophilic eubacteria. In: Rodrı´guez-Valera F (ed) Halophilic bacteria. CRC, Boca Raton Oren A (1999) Bioenergetic aspects of halophilism. Microbiol Mol Biol Rev 63:334–348 Oren A (2006) Life at high salt concentrations. In: Dworkin M (ed) The prokaryotes: a handbook on the biology of bacteria. Springer, New York Oren A (2008) Microbial salt at high salt concentrations: phylogenetic and metabolic diversity. Saline Systems 4:2 Ventosa A (2006) Unusual micro-organisms from unusual habitats: hypersaline environments. In: Logan NA, Lappin-Scott HM, Oyston PCF (eds) SGM symposium 66: prokaryotic diversity-mechanisms and significance. Cambridge University Press, Cambridge
Halotolerance JOSEFA ANTO´N Departamento de Fisiologı´a, Gene´tica y Microbiologı´a, Universidad de Alicante, Alicante, Spain
Synonyms Salt tolerance
Keywords Compatible solutes, ionic stress, salt
Definition Halotolerance is tolerance to ionic stress, or the ability of an organism to grow at salt concentrations higher than those required for growth. Halotolerant organisms are able to survive at high salt concentrations but do not require these conditions for growth.
2.5 M salt. A 0.9% NaCl would be considered an isotonic solution by most nonmarine and non-halophilic organisms. In nature, halotolerant organisms can be found in settings such as saline waters and soils that are inhabited by autochthonous ▶ halophilic microbiota, or even in association with animals. Such is the case of microbiota such as Staphylococcus species that live on human skin. Because of their exposure to the salts in sweat, they are very halotolerant and they can also grow well at NaCl concentrations as high as 15%. However, they can also grow in the absence of salt and are thus said to be halotolerant and not halophile. This property is exploited in the design of selective growth media. Halotolerant organisms have developed metabolic processes, such as the accumulation of ▶ compatible solutes that allow them to compensate the osmotic pressure and continue to live in hyperosmotic environments. Salt tolerance can vary depending on nutritional or environmental factors such as pH, temperature, and redox potential. Since high salt has been traditionally used as a way of preserving food, halotolerant microorganisms can be a cause of spoilage of salt-preserved food. The three Domains of life include many examples of halotolerant microorganisms such as the ▶ bacteria of the genera Alteromonas, Lactobacillus, Bacillus, Myxococcus, and Pediococcus, as well as many cyanobacteria, some methanogenic and termophilic archaea, and halotolerant fungi such as Debaromyces, Hansenula, Cladosporium, and Saccharomyces.
See also ▶ Adaptation ▶ Bacteria ▶ Compatible Solute ▶ Halophile ▶ Osmolite
References and Further Reading DasSarma S, Arora P (2006) Halophiles. Encyclopedia of life sciences. Wiley, London Oren A (2006) Life at high salt concentrations. In: Dworkin M (ed) The prokaryotes: a handbook on the biology of bacteria. Springer, New York Oren A (2008) Microbial salt at high salt concentrations: phylogenetic and metabolic diversity. Saline Systems 4:2 Roberts MF (2005) Organic compatible solutes of halotolerant and halophilic microorganisms. Saline Systems 1:5
Overview Halotolerance is a relative term that refers to the ability to survive or thrive at salt concentrations higher than those necessary for growth. A microorganism is considered extremely halotolerant if its growth range extends above
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Handedness ▶ Chirality
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Haphazardness
Haphazardness ▶ Chance and Randomness
Hapten Synonyms Partial antigen
Definition A hapten is a small molecule that can elicit an immune response when attached to a larger carrier molecule such as a ▶ protein. Neither the larger molecule nor the hapten may illicit the response by themselves.
See also ▶ Antibody
Hard Landing
Definition HARPS is a high-resolution spectrograph designed to achieve long-term stability enabling ▶ Doppler velocity measurements at the level of 1 m/s, in order to derive ▶ radial-velocity orbits for stars orbited by planets as small as the Earth. It is presently the only instrument scheduled on the 3.6-m telescope at the European Southern Observatory on La Silla in Chile. The spectrograph is fed by fibers and the optics are enclosed in a vacuum tank, to eliminate the effects of changes in the index of refraction at the echelle grating due to changes in the atmospheric pressure. The temperature of the instrument is stabilized at the level of 0.001 K, which is also critical for the velocity stability. The wavelength calibration is provided by thorium-argon hollow cathode lamps, but experiments are underway with the use of Fabry–Perot etalons and/or Laser Combs as calibration sources. HARPS has demonstrated the potential to reach a velocity precision of perhaps 20 cm/s. HARPS and ▶ HIRES on Keck 1 in Hawaii are now the premier instruments for determining the masses of small planets. A copy of HARPS for use in the northern hemisphere is under construction.
See also Definition The term “hard landing” describes the disposition of a spacecraft on another planetary body that is not controlled and could result in fragmentation of the hardware. A loss of control resulting in a hard landing is called a “crash”! For ▶ planetary protection purposes, a hard landing is assumed to result in the release of ▶ encapsulated bioburden as well as the spreading of the ▶ exposed surface bioburden.
See also ▶ Encapsulated Bioburden ▶ Exposed Surface Bioburden ▶ Planetary Protection
Hard Snowball ▶ Snowball Earth
HARPS Synonyms High Accuracy Radial-velocity Planet Searcher
▶ Doppler Shift ▶ Exoplanets, Discovery ▶ HIRES ▶ Radial-Velocity Planets
HAT ▶ HATNet
HATNet Synonyms HAT; Hungarian-made Automatic Telescope Network
Definition HATNet is a wide-angle ground-based photometric survey for transiting ▶ exoplanets operated by the Smithsonian Astrophysical Observatory. Large areas of the sky are monitored robotically using six small telescopes (11 cm in diameter) located at two different sites in Arizona and Hawaii, looking for planets that transit their host stars. An additional telescope is located in Israel. HATNet has
Hayabusa Mission
discovered a significant fraction of the known ▶ transiting exoplanets, mostly ones larger than Jupiter. An important exception is HAT-P-11b, which is a Hot Neptune in the field of view of NASA’s ▶ Kepler mission. An international extension of the project to the southern hemisphere is now operational, with two 18-cm telescopes located at each of three collaborating sites in Chile, Australia, and Namibia.
See also ▶ Exoplanets, Discovery ▶ Kepler Mission ▶ Transiting Planets ▶ TrES ▶ WASP
Hayabusa Mission A. C. LEVASSEUR-REGOURD UPMC Univ. Paris 6/LATMOS-IPSL, Paris, France
Synonyms MUSES-C
Keywords Asteroid, Regolith, Sample-Return
Definition Hayabusa (“peregrine falcon” in Japanese) is a scientific space mission developed by JAXA to explore a near-Earth asteroid and to return asteroidal soil, while validating new engineering technologies (propulsion, landing, and atmospheric re-entry). Launched in May 2003, Hayabusa-1 rendezvous with asteroid Itokawa was in September 2005. After 2 months of close observations, it descended and attempted to collect surface grains. It departed in 2007 and cruised back to Earth. The sample canister returned safely in July 2010. JAXA has confirmed the presence of tiny dust particles sampled on Itokawa. Hayabusa-2 is now planned, targeting the C-type asteroid 1993JU3 (launch in 2014).
Overview The Hayabusa-1 spacecraft consisted of a core (1 1.6 2 m) and two solar paddles (5.7 m width), with a total mass of 530 kg, including its propellant (for chemical propulsion) and Xenon (for electrical propulsion). It carried four scientific instruments, the AMICA imaging
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camera, the NIRS near-infrared spectrometer, the LiDAR laser ranging instrument, and the XRS X-ray fluorescence spectrometer. In addition, the probe embarked three instruments for navigation: an optical camera (ONC), an electromagnetic sensor (FBS), a laser emitter to control the attitude during descent (LRF). As an engineering spacecraft, Hayabusa-1 tested two new developments, xenon ion engines and an autonomous navigation system, which both performed successfully. The mission was quite complex and faced major issues. Hayabusa-1 was launched on May 9, 2003, from Kagoshima Space Center (now called Uchinoura), with a MV-5 solid propellant rocket. Soon after, it was hit by a large solar flare that damaged the solar cells, reducing the electrical power and the efficiency of the ion engines. After an Earth swing-by in May 2004, Hayabusa arrived close to the targeted asteroid (25143) Itokawa in mid-September 2005, and performed its thorough remote characterization. In November 2005, it released the Minerva mini-lander, which unfortunately escaped Itokawa’s gravitational pull and was lost in space. Hayabusa then attempted twice to touchdown the surface of the asteroid, after having deployed a sampling horn, later sealed after having tentatively collected asteroidal dust. Hayabusa-1 return maneuver started in April 2007, for a 3-year cruise back to the Earth. The return capsule, released from the main spacecraft 3 h before re-entering the Earth atmosphere, landed safely near Woomera, Australia, on June 13, 2010. It was transferred to the JAXA curation facility. Preliminary analyses have confirmed that most of the collected particles, which have sizes below 10 mm, have come from Itokawa. The scientific observations made while Hayabusa-1 was hovering above Itokawa have established that it is a very small body (major axes 530 290 210 m, with a density of (1,925 160) kg m–3), indicative of a rather high porosity of about 40%. This may suggest that it is built from loose-packed rocks, held together by their gravity. Itokawa presents a significant topographic diversity, with a variety of rough terrains scattered with boulders, as well as featureless central areas, which likely result from fine dust particles forming a smooth regolith that has accumulated in local gravitational lows. Its shape may suggest it is a contact binary. Such results are to be taken into account when considering the orbital deflection techniques that should and could be implemented if a similar asteroid was discovered on a collision course with the Earth. Following the overall success of this pioneering mission, Hayabusa-2 is now planned, for a launch in 2014, to rendezvous with a primitive C-type asteroid, 1993JU3,
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with similar goals and instruments. A major added contribution still under consideration is a hoping lander, provided by DLR and CNES, designed to monitor the potential magnetic field of the asteroid, and to perform in situ a microscopic characterization of the composition of the asteroid, at a grain scale.
HCN Polymer IRENA MAMAJANOV1, JUDITH HERZFELD2 1 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA, USA 2 Brandeis University, Waltham, MA, USA
See also ▶ Asteroid ▶ Itokawa Asteroid ▶ JAXA ▶ Near-Earth Objects ▶ Regolith (Planetary)
References and Further Reading Abe A et al (2006a) Near infra-red spectral results of asteroid Itokawa from the Hayabusa spacecraft. Science 312:1334–1338 Abe S et al (2006b) Mass and local topography measurements of Itokawa by Hayabusa. Science 312:1344–1347 Demura H et al (2006) Pole and global shape of 25143 Itokawa. Science 312:1347–1349 Fujiwara A et al (2006) The rubble-pile asteroid Itokawa as observed by Hayabusa. Science 312:1330–1334 Okada HT et al (2006) X-ray fluorescence spectrometry of asteroid Itokawa by Hayabusa. Science 312:1338–1341 Saito J et al (2006) Detailed images of asteroid 25143 Itokawa from Hayabusa. Science 312:1341–1344 Yano H et al (2006) Touchdown of the Hayabusa spacecraft at the Muses sea of Itokawa. Science 312:1350–1353
Haze Particle ▶ Aerosols
HC4H ▶ Diacetylene
HCCH ▶ Acetylene
HCl ▶ Hydrogen Chloride
Synonyms Azulmin; Hydrogen cyanide polymer
Keywords Addition polymer, aminomalononitrile, diaminomaleonitrile, hydrogen cyanide, ladder polymer, polyheterocycle, polyimine, protopeptide
Definition ▶ Hydrogen cyanide polymers – i.e., heterogeneous solids formed upon spontaneous polymerization of HCN – are likely to have been among the first macromolecules on prebiotic Earth.
Overview Hydrogen cyanide (HCN) polymerizes spontaneously in the neat liquid and in solution. Due to the abundance of hydrogen cyanide in the Universe (Debes et al. 2008) the potential role of hydrogen cyanide polymers in astrochemistry and the origin of life has provoked much speculation. Gas phase HCN is found both in interstellar molecular clouds and in comets in the solar system. The ▶ oligomerization of HCN was first observed by Proust (1806), and analyses of the soluble fraction date back to the late 1800s and early 1900s (Lange 1863; Wippermann 1874; Bedel 1923). Volker was the first to propose a structure for the insoluble ▶ polymer in 1960, a so-called “ladder” structure formed by repeats of the HCN dimer in the trans configuration (Volker 1960). The cis variation was subsequently offered by Umemoto et al. (1987). A more provocative model, from the standpoint of the origins of life, was suggested by Matthews, based on the observation of a-amino acids in polymer hydrolyzates; polymerization was proposed to occur via the HCN trimer aminomalononitrile and result in a structure with a -NCC- backbone that would form ▶ polypeptide upon contact with water (Matthews et al. 1977). However, Ferris has argued that the HCN tetramer, diaminomaleonitrile (DAMN) must be the “direct precursor” to HCN polymers, since dimers and trimers are not expected to accumulate (Ferris et al. 1981).
HCN Polymer
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HCN Polymer. Figure 1 Proposed structures for the HCN polymers and related compounds
The above structures have been found to be inconsistent with recently acquired 13C and 15N solid state NMR spectra of polymers formed in neat HCN. These data suggest that the polymers are formed by simple monomer addition, first in head-to-tail fashion to form linear, conjugated chains, and then laterally to form saturated twodimensional networks (Mamajanov and Herzfeld 2009a). This interpretation of the NMR spectra finds support in other information about the polymerization of neat HCN, including the presence of free radicals (Budil et al. 2003) and the fragmentation pattern in a ▶ GC-MS chemolysis study (Minard et al. 1998). A different kind of HCN polymer has been shown to form upon mild heating of DAMN. 13C and 15N solid state NMR spectra, as well as the optical properties and electrical conductivity of the product, are
consistent with a HCN trimer repeat unit. However, rather than forming a protopeptide, the subunits cyclize to form poly-[aminoimidazole] (Mamajanov and Herzfeld 2009b). Despite extensive effort, there is no scientific consensus concerning the potential role of the HCN polymer in prebiotic evolution.
See also ▶ Biopolymer ▶ Exopolymers ▶ Formamide ▶ GC/MS ▶ Heterocycle ▶ Hydrogen Cyanide ▶ Insoluble Organic Matter
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▶ Nitrile ▶ Oligomer ▶ Oligomerization ▶ Polymer ▶ Polypeptide ▶ Prebiotic Chemistry ▶ Radical ▶ Tholins
References and Further Reading Bedel C (1923) Sur un polymere de l’acide cyanhydrique. C R Acad Sci 176:168–171 Budil DE, Roebber JL, Liebman SA, Matthews CN (2003) Multifrequency electron spin resonance detection of solid-state organic free radicals in HCN polymer and a Titan tholin. Astrobiology 3(2):323–329 Debes JH, Weinberger AJ, Schneider G (2008) Complex organic materials in the circumstellar disk of HR 4796A. Astrophys J 673: L191–L194 Ferris JP, Edelson EH, Auyeung JM, Joshi PC (1981) Structural studies on HCN oligomers. J Mol Evol 17(2):69–77 Lange O (1863) Ueber eine neue Verbindung von der Zusammen-setzung der Cyanwasserstoffsaure. Berichte 6:99 Mamajanov I, Herzfeld J (2009a) HCN polymers characterized by solid state NMR: Chains and sheets formed in the neat liquid. J Chem Phys 130(13) Mamajanov I, Herzfeld J (2009b) HCN polymers characterized by SSNMR: Solid state reaction of crystalline tetramer (diaminomaleonitrile). J Chem Phys 130(13) Matthews C, Nelson J, Varma P, Minard R (1977) Deuterolysis of aminoacid precursors – Evidence for Hydrogen-Cyanide polymers as protein ancestors. Science 198(4317):622–625 Minard RD, Hatcher PG, Gourley RC, Matthews CNZ (1998) Structural investigations of hydrogen cyanide polymers: New insights using TMAH thermochemolysis/GC-MS. Orig Life Evol Biosph 28:461–473 Proust JL (1806) Ann Chim Physique 60(1):233 Umemoto K, Takahashi M, Yokata K (1987) Studies on the structure of HCN oligomers. Orig Life Evol Biosph 17(3–4):283–293 Volker TH (1960) Polymeric Hydrocyanic Acid. Angew Chem Int Ed 72(11):379–384 Wippermann R (1874) Ueber Tricyanwasserstoff, eine der blausaure polymere verbindung. Berichte 7:767
HCNO Isomers HOLGER S. P. MU¨LLER I. Physikalisches Institut, Universita¨t zu Ko¨ln, Ko¨ln, Germany
Keywords Isomer
Definition HCNO isomers are molecules having the same empirical formula (HCNO), but a different connectivity. These isomers cannot be transformed from one to another without breaking a bond.
History About 1825, Liebig and Gay-Lussac found out that silver fulminate had the same empirical formula as a compound called silver cyanate (more correctly referred to as silver isocyanate), which Wo¨hler had analyzed about a year earlier. The two compounds were clearly different and thus led to the concept of isomerism.
Overview To date, four molecules are known with the empirical formula HCNO. The lowest energy form is isocyanic acid, H–N=C=O; cyanic acid, H–O–C N, is somewhat higher in energy (103.2 kJ/mol); fulminic acid, H–C N–O, is considerably higher (295.9 kJ/mol); and isofulminic acid, H–O–N C, is higher still (352.2 kJ/mol). Fulminic acid is a linear molecule, the others are all planar with the heavy atoms being nearly linear and the H atom bent trans to the heavy atom chain. A review of their structural and spectroscopic properties and their energetics was given by Teles et al. (1989). More recently, high-level quantum chemical calculations of the energies have been performed by Schuurman et al. (2004). Single crystal X-ray crystallography has shown that solid salts considered to be cyanates are generally much better described as isocyanates. Consequently, adding strong acids to these salts releases isocyanic acid (HNCO), without clear evidence of formation of cyanic acid (HOCN). HNCO was prepared in its pure form in the nineteenth century. In contrast, it appears as if HOCN can only be generated in situ. Interestingly, even though fulminic acid is much higher in energy than cyanic acid, it is possible to prepare HCNO in its pure form. Fulminic acid has again to be generated in situ. Rotational spectra of all four isomers have been measured. Not surprisingly, fulminic acid was the last detected (Mladenovic´ et al. 2009). While HNCO was detected in space in 1972, HCNO and HOCN were only detected very recently (see Bru¨nken et al. 2010). HNCO can trimerize to cyanuric acid, a six-membered ring with alternating C and N atoms. The anion NCO– is neither well described as isocyanate with the negative charge at the N atom nor as cyanate with the negative charge at the O atom. Instead, the charge is delocalized to essentially equal amounts over both
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atoms. Thus, the CO bond has a formal bond order of 1.5 while that of the CN bond is 2.5. An infrared feature at 4.62 mm, observed in interstellar ices, coincides with the strong asymmetric stretching mode of the NCO– anion and has been attributed to this species. Unfortunately, features in solid-state spectra are not particularly specific. As a consequence, the assignment may well be correct, but is by no means certain.
See also ▶ Molecules in Space
References and Further Reading Bru¨nken S, Belloche A, Martı´n S, Verheyen L, Menten KM (2010) Interstellar HOCN in the Galactic center region. Astron Astrophys 516:A109 Mladenovic´ M, Lewerenz M, McCarthy MC, Thaddeus P (2009) Isofulminic acid, HONC: ab initio theory and microwave spectroscopy. J Chem Phys 131:174308 Schuurman MS, Muir SR, Allen WD, Schaefer HF III (2004) Toward subchemical accuracy in computational thermochemistry: focal point analysis of the heat of formation of NCO and [H, N, C, O] isomers. J Chem Phys 120:11586–11599 Teles JH, Maier G, Hess BA Jr, Schaad LJ, Winnewisser M, Winnewisser BP (1989) The CHNO Isomers. Chem Ber 122:753–766
HCO+ ▶ Formyl Cation
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for the solar-type star HD 114762 that implied an unseen companion with minimum mass ten times that of Jupiter. However, it violated conventional ideas concerning giant planets on three counts. The orbital period of only P 84 days placed it far inside the ▶ snow line, into a region where conditions were thought to have been too hot for a gas giant to form; the orbit was eccentric, with e 0.3, unlike any of the ▶ giant planets in the solar system; and the mass was at least ten times that of Jupiter, which was larger than could be made by the theoretical models of the time. Thus, most astronomers dismissed the unseen companion of HD 114762 as a star or brown dwarf in an orbit seen nearly face on. They assumed that the observed orbital amplitude of 0.5 km/s was small because of projection effects, not because the mass of the companion was small enough to be a planet. ▶ Exoplanets are now known that have shorter periods, more eccentric orbits, and larger masses. Like most of the ▶ radial-velocity planets, the orbital inclination of HD 114762B is still unknown, so its actual mass could still be too large to qualify as a planet.
See also ▶ Exoplanets, Discovery ▶ Giant Planets ▶ Radial-Velocity Planets ▶ Snow Line
References and Further Reading Latham DW et al (1989) The unseen companion of HD114762 – A probable brown dwarf. Nature 339:38–40
HCOOCH3 ▶ Methyl Formate
HCP ▶ Phosphaethyne
HD 149026B DAVID W. LATHAM Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
Keywords Core accretion model, metallicity, transiting planet
HD 114762B Definition HD 114762B is a companion to the star HD 114762; it may be a planet. In 1989, a radial-velocity orbit was published
Definition HD 149026b is a ▶ transiting planet with a mass similar to Saturn and an unusually high bulk density for its size. Models of the internal structure suggest that this is a gas giant with as much as 70 Earth masses residing in a core of high-density material.
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History Early results from radial-velocity surveys suggested that ▶ gas giant exoplanets were being found more frequently around stars with higher concentrations of heavy elements in their atmospheres than the Sun (e.g., Santos et al. 2001; Fischer and Valenti 2005). This motivated some teams to search for planets around samples of metal-rich stars using radial velocities and then to look for transits with follow-up photometry. One of these efforts, the N2K survey led by Debra Fischer, announced the discovery of a transiting hot Saturn, HD 149026b (Sato et al. 2005). The unusually high-bulk density of this exoplanet has been used to argue in support of the ▶ core accretion model for giant planet formation.
Overview The parent star HD 149026 is similar to the Sun, but slightly hotter with effective temperature 6,147 K and with more than twice the solar abundance of heavy elements in its atmosphere. The period of the circular orbit for HD 149026 b is only 2.8766 days long, and thus the planet is strongly heated by the radiation from the star. Measurements with the ▶ Spitzer Space Telescope of the amount of thermal radiation emitted at 8 mm show a drop of 0.04% when the planet disappears behind the star during secondary eclipse (Knutson et al. 2009). This implies a surface temperature of 1,440 K for the dayside of the planet. The thermal emission has been monitored over more than half an orbit, and the emission deduced for the nightside implies a surface temperature that is only about 500 K cooler than the dayside. This implies efficient heat transfer from the dayside to the nightside and thus dramatic weather in the atmosphere of HD 149026B.
See also ▶ Core Accretion (Model for Giant Planet Formation) ▶ Metallicity ▶ Spitzer Space Telescope ▶ Transiting Planets
References and Further Reading Fischer DA, Valenti JA (2005) The planet-metallicity correlation. Astrophys J 622:1102–1117 Knutson HA et al (2009) The 8-micron phase variation of the hot Saturn HD 149026b. Astrophys J 703:769–784 Santos N et al (2001) The metal-rich nature of stars with planets. Astron Astrophys 373:1019–1031 Sato B et al (2005) The N2K Consortium II. A transiting hot Saturn around HD 149026 with a large dense core. Astrophys J 633:465–473
HD 189733b DAVID W. LATHAM Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
Keywords Planetary atmosphere, transiting planet
Definition HD 189733b is a transiting ▶ Hot Jupiter that is especially well suited for studies of its atmosphere. Spectroscopic observations with the ▶ Hubble Space Telescope and ▶ Spitzer Space Telescope have revealed the presence of molecules such as water vapor and methane (Fig. 1).
History Early results from radial-velocity surveys suggested that gas giant ▶ exoplanets were being found more frequently around stars with higher concentrations of heavy elements in their atmospheres than the Sun (e.g., see Santos et al. 2001; Fischer and Valenti 2005). This motivated some teams to search for planets around samples of metal-rich stars using radial velocities and then to look for transits with follow-up photometry. One of these efforts, the ELODIE metallicity-biased search for transiting Hot Jupiters (da Silva et al. 2006), announced the discovery of HD 189733b (Bouchy et al. 2005). This planet has proven to be especially well suited for follow-up studies of its atmosphere because it is in a tight orbit around a nearby star that is somewhat cooler than the Sun, thus producing a favorable contrast between the emission from the planet compared to the star (Fig. 2).
Overview The parent star HD 189733 is significantly cooler than the Sun, with an effective temperature of 4,980 K, corresponding to an early K spectral type. The distance to the star is only 19.3 pc, which makes it quite bright, with an apparent visual magnitude of 7.67. The period of the circular orbit is only 2.2186 days long, and thus the planet is strongly heated by the incident radiation from the star. These factors all combine to make HD 189733b especially well suited for follow-up observations of the thermal emission from the planet’s atmosphere. Deming et al. (2006) first reported the detection of secondary eclipses, using the Spitzer Space Telescope at 16 mm to measure a drop of 0.55% in the total thermal emission when the
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HD 189733b. Figure 1 The light curve of HD 189733 observed with the Infrared Array camera on the Spitzer space telescope at 8 mm. More than half the orbit is covered. The deep event at phase 0.0 is the transit of the planet in front of the star. The shallower event at phase 0.5 is the secondary eclipse when the planet passes behind the star and its thermal emission is blocked. The depth of the secondary eclipse indicates that 0.34% of the total radiation of the system at 8 mm arises from the planet. This sets the dayside temperature at 1,212 K. The emission increases slowly with phase as the planet moves around in its orbit and more of the dayside comes into view. (b) Provides an expanded view of the upper portion of (a)
planet disappears behind the star. Subsequently, Knutson et al. (2007) monitored the emission at 8 mm over more than half the orbit and showed that the dayside temperature is 1,212 K, and the nightside is only 239 K cooler. This implies efficient heat transfer from the dayside to the nightside and thus dramatic weather in the atmosphere of HD 189733b. Spectroscopy of the wavelength dependence of the transit depth in the near infrared with the NICMOS instrument on the Hubble Space Telescope by Swain et al. (2008) gave dramatic confirmation for the earlier preliminary reports of the detection of water vapor in the H2-dominated atmosphere of HD 189733b and suggested in addition the detection of methane. Subsequent NICMOS spectroscopy of secondary eclipses added carbon monoxide and carbon dioxide to the list of proposed molecular detections (Swain et al. 2009).
See also ▶ Exoplanets, Discovery ▶ Gas Giant Planet ▶ Hot Jupiters ▶ Hubble Space Telescope ▶ Radial-Velocity Planets ▶ Spitzer Space Telescope ▶ Transiting Planets
References and Further Reading Bouchy F (2005) Astron Astrophys 444:L15 da Silva R et al (2006) Astron Astrophys 446:717 Fischer DA, Valenti JA (2005) Astrophys J 622:1102 Knutson HA et al (2008) Nature 447:183 Santos N et al (2001) Astron Astrophys 373:1019 Swain M et al (2008) Nature 452:329 Swain M et al (2009) Astrophys J 690:114L
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HD 189733b. Figure 2 The observed spectrum (black triangles) and two theoretical spectra of the predominantly molecular hydrogen atmosphere, showing the effects of small amounts of water (blue) and methane in combination with water (orange). The measured spectrum exhibits significant differences at 1.7–1.8 mm and at 2.15–2.4 mm from what is expected due to water vapor alone. These departures are interpreted as additional absorption features due to the presence of one or more other species in addition to water. When considering only water and methane, the theoretical spectrum best fitting the data was determined by binning the model (shown as white crosses) to the spectral resolution of the observations (Swain et al. 2008)
HD 209458b DAVID W. LATHAM Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
Keywords Transiting planet
Definition HD 209458b was the first exoplanet observed to transit its host star.
History Arguments based on simple geometry predict that about one out of ten planets orbiting stars similar to the Sun should be seen to ▶ transit their parent star if the orbital period is only a few days. HD 209458 was roughly the tenth star to show a radial-velocity orbit implying
a possible ▶ Hot Jupiter companion, so the discovery that the unseen companion transited the star came right on schedule. Follow-up photometry at times predicted for transits by the spectroscopic orbit with period 3.5247 days revealed dips in the light curve consistent with a ▶ gas giant planet 1.32 times the radius and 0.64 times the mass of Jupiter. For the first time it was possible to show that the bulk density of a Hot Jupiter was approximately as expected for a gas giant planet. This left little doubt that the growing population of radial-velocity planet candidates must in fact be planets. The discovery that HD 209458b transits is another example of simultaneity in science. Discovery papers from two independent teams were published simultaneously and back to back in the same issue of the Astrophysical Journal Letters (Charbonneau et al. 2000; Henry et al. 2000). The two teams based their transit time predictions on radial velocities measured independently with ▶ HIRES on the Keck 1 telescope and Elodie on the 1.9 m telescope at the Observatoire de Haute Provence. The follow-up photometry was accomplished with small
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HD 209458b. Figure 1 The light curve for the transit of HD 209458b obtained with the STIS instrument on the Hubble Space Telescope. Observations from several orbits were combined to produce full phase coverage of the transit light curve (Brown et al. 2001)
telescopes, in the case of Charbonneau et al. (2000) using a 4-in. lens and CCD camera located in the parking lot of the High Altitude Observatory in Boulder, Colorado. Subsequent observations of transits with the STIS instrument on the ▶ Hubble Space Telescope illustrated (Fig. 1) the spectacular quality of light curve that could be achieved with a large telescope in space (Brown et al. 2001).
Overview HD 209458 is a star very similar to the Sun in almost all respects. Because it is relatively nearby, only 47 pc from the Sun, it has the relatively bright apparent visual magnitude of 7.65. This makes it especially attractive for follow-up observations, such as spectroscopy of the atmosphere of the planet HD 209458b, although the planetary candidate ▶ HD 189733b is somewhat better in this regard because its host star is cooler. The same STIS spectra that yielded the spectacular light curve when integrated over all the available wavelengths (Brown et al. 2001) were subsequently reanalyzed at much finer spectral resolution, showing the presence of sodium in the planet’s atmosphere (Charbonneau et al. 2002). This was the first detection of the atmosphere of an exoplanet. Subsequently, hydrogen was detected by Vidal-Madjar et al. (2003). Although the bulk density of HD 209458b matches more or less the predictions of models for gas giant planets, the density is somewhat lower than expected.
Apparently the radius of the planet is puffed up compared to the models. For a while HD 209458b was anomalous in this regard, but as more transiting Hot Jupiters were discovered, additional examples of inflated gas giants were identified. Although a number of possible physical mechanisms for inflating the radii of these planets have been suggested, the phenomenon remains puzzling.
See also ▶ Exoplanets, Discovery ▶ Gas Giant Planet ▶ HD 189733b ▶ HIRES ▶ Hot Jupiters ▶ Hubble Space Telescope ▶ Transit ▶ Transiting Planets
References and Further Reading Brown TM et al (2001) Hubble space telescope time-series photometry of the transiting planet of HD 209458. Astrophys J 552:699 Charbonneau D et al (2000) Detection of planetary transits across a sun-like star. Astrophys J 529:45 L Charbonneau D et al (2002) Detection of an extrasolar planet atmosphere. Astrophys J 568:377 Henry GW et al (2000) A Transiting “51 Peg-like” Planet. Astrophys J 529:41 L Vidal-Madjar A et al (2003) An extended upper atmosphere around the extrasolar planet HD209458b. Nature 422:143–146
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Heat-stable DNA Polymerase
Heat-stable DNA Polymerase ▶ Taq Polymerase
Heat Flow (Planetary) TILMAN SPOHN German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Synonyms Geothermal flux; Heat flux
Keywords Radioactive heating
Definition The heat flow in the most general sense is the rate of ▶ heat transfer per unit area across a given surface. The heat transfer mechanism could be radiation, convection, or conduction. It is measured in W m2. In geophysics and in planetary geophysics the surface heat flow is more narrowly defined as the heat conducted through the ▶ crust across the planetary surface. In the (older) geophysical literature the heat flow unit (hfu) is sometimes used, with 1 hfu = 106 cal cm2 s1 = 41.84 mW m2.
Overview Heat is generated in the interior of planets and satellites by the decay of radioactive elements, by exothermic phase transitions and chemical reactions, and by the dissipation of mechanical energy upon, for example, planetary contraction or ▶ differentiation. The rate of heat loss of a ▶ planet or ▶ satellite provides constraints on the amount of heat being produced in the planet and the temperature distribution in the interior. The heat loss can – in principle – be measured from space with infrared radiometers as the heat radiated by the planet. The latter quantity is also called the planet’s intrinsic ▶ luminosity (similar to the luminosity of a ▶ star). In the ▶ Inner Solar System, however, the luminosity is dominated by the solar heat that is absorbed in the atmosphere and in surface ▶ rock and radiated back into space. (The contribution by the reflected sunlight can be separated in the electromagnetic spectrum.) Therefore, the heat flow out of the interior of a ▶ terrestrial planet must be measured at a depth
where perturbations due to the solar radiation and thermal perturbations in the atmosphere are sufficiently pffiffiffismall. ffi This depth is a few times the thermal skin depth kt p with k the thermal diffusivity and t the period of the thermal perturbation. The period t ranges from a planetary day (or ▶ sol) to centuries if climatic effects are taken into account. Usually, a depth of a few meters will be sufficient. Measuring the heat flow then requires measuring both the thermal conductivity and the temperature gradient. Many heat flow measurements have been made on the ▶ Earth and heat flow maps have been compiled (e.g., Pollack et al. 1993). The variation of the heat flow reflects volcanic and orogenic provinces and chains where heat flow is high and thick continental shields where heat flow is low. The average value is 84 W m2 and the variation is between a few tens to a few hundred mW m2. Heat flow has been measured on the ▶ Moon on two sites by the ▶ Apollo Missions. The values are 16 and 18 mW m2. No other values are available but heat flow measurements have been repeatedly proposed (e.g., Spohn et al. 2001). In the outer solar system the intrinsic luminosities of ▶ Jupiter, ▶ Saturn, ▶ Uranus, and ▶ Neptune have been measured. These are 5.44 0.43, 2.01 0.14, 0.042 0.047 (but not smaller than 0), and 0.433 0.046 W m2, respectively (e.g., Guillot and Gautier 2007 for a review). Dissipation of gravitational energy released upon contraction is the major source of heat in the interior of Jupiter. The heat flow values for the other ▶ Giant Planets, in particular, the difference in heat flow between Uranus and Neptune is not well understood. The intrinsic luminosity of the Jovian satellite ▶ Io has been measured to be about 2 W m2 (Spencer et al. 2000). It is widely agreed that the heat is due to tidal dissipation (compare ▶ Tides).
See also ▶ Apollo Mission ▶ Crust ▶ Differentiation ▶ Earth ▶ Giant Planets ▶ Heat Transfer (Planetary) ▶ Io ▶ Jupiter ▶ Luminosity ▶ Moon, the ▶ Neptune ▶ Planet ▶ Rock ▶ Satellite or Moon
Heat Transfer (Planetary)
▶ Saturn ▶ Sol ▶ Solar System, Inner ▶ Stars ▶ Terrestrial Planet ▶ Tides (Planetary) ▶ Uranus
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Heat Transfer (Planetary) TILMAN SPOHN1, LISA KALTENEGGER2,3 1 German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany 2 Harvard University, Cambridge, MA, USA 3 MPIA, Heidelberg, Germany
References and Further Reading Guillot T, Gautier D (2007) Giant planets. In: Spohn T, Schubert G (eds) Treatise on geophysics, vol 10. Elsevier, Amsterdam, pp 439–464 Langseth MG, Keihm SJ, Peters K (1976) Revised lunar heat flow values. P Lunar Planet Sci C 7:3143–3171 Pollack HN, Hurter SJ, Johnston SR (1993) Heat flow from the Earth’s interior. Rev Geophys 31:267–280 Spencer JR, Rathburn JA, Travis LD, Tamppari LK, Barnard L, Martin TZ (2000) Io’s thermal emission from the Galileo photopolarimeter– radiometer. Science 288:1198–1201 Spohn T, Ball A, Seiferlin K, Conzelmann V, Hagermann A, Ko¨mle NI, Kargl G (2001) A heat flow and physical properties package for the surface of Mercury. Planet Space Sci 49:1571–1577
Keywords Atmospheres, conduction, convection, planetary interiors, plate tectonics, radiation, stagnant lid
Definition Heat is transferred in planetary and ▶ satellite interiors and atmospheres from regions of high temperature to regions of lower temperature. The ▶ heat transfer mechanisms are heat conduction, convection, and radiation.
Overview
Heat Flux ▶ Heat Flow (Planetary)
Heat Shock Definition The term “heat shock” describes a fast and short heat treatment. For ▶ planetary protection and microbiology, this technique is used to count the ▶ spores, generally in a liquid sample. The combination of high temperature with a short exposure time (i.e., 80 C for 10 min) kills the vegetative forms of the mesophilic ▶ bacteria, leaving mesophilic and thermophilic spores alive. Within such a short time, the microorganisms have not enough time to produce new spores. When cooled, the solution is spread in or on a nutritive media to perform the count of the viable microorganisms left. The result of this standardized ▶ assay represents the spores which were present in the original sample.
See also ▶ Bacteria ▶ Endospore ▶ Microorganism ▶ Spore
In planetary interiors, heat is generated by the decay of the ▶ radiogenic isotopes235U, 238U, 237Th, and 40K. In addition, latent heat may be released and consumed through phase transitions. The dissipation of kinetic energy of ▶ planetesimals colliding with the proto-planets during planetary accretion has heated the planetary interiors, in some cases, up to the melting temperatures of their outer layers. The formation of the ▶ cores in ▶ terrestrial planets through differentiation has further heated the deep interiors. Since planetary surfaces are colder – their temperature being determined by the rate of solar radiation and the atmosphere ▶ greenhouse effect – heat is transferred to these surfaces from the deep interior. The resulting flow of heat may be associated with the conversion of heat into mechanical work and the conversion of heat into ▶ magnetic field energy. Moreover, the cooling may result in planetary contraction (Kelvin–Helmholtz cooling). The dominant heat transfer mechanisms in planetary interiors are heat conduction and convection (for a discussion of heat transfer in planetary interiors, see, e.g., Schubert et al. 2001). In a typical substantial planetary atmosphere, heat is transferred by convection or radiation in the lower atmosphere (more specifically, the ▶ troposphere and ▶ stratosphere) and heat conduction and radiation in the upper atmosphere (more specifically, the mesosphere and thermosphere). The pressure level at the radiative–convective boundary (the tropopause) in the lower atmosphere depends on the composition of the atmosphere, the ▶ opacity of the major atmospheric
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constituents: gravity and temperature. Above this level (i.e., at lower pressures), the outgoing energy is transported by radiation until heat conduction becomes important. The radiative–convective boundary occurs when the atmosphere becomes optically thin to thermal radiation. Usually this translates into a column optical depth, t, approximately equal to unity. For t approximately equal to unity or larger, the atmosphere is opaque and the radiative energy is reabsorbed by the surrounding media, hence convection is the dominant form of heat transfer. For t smaller than unity, the atmosphere is transparent and does not absorb efficiently. (see: Atmospheric structure, Phase Change: Latent Heat, Thermal inversion, opacity). Heat Conduction. In gases, heat is conducted via the ▶ diffusion of molecules. Molecules with high kinetic energy diffuse from regions of high temperature to regions of lower temperature. Interaction between molecules transfers energy and thereby heat. In solids, energy is transferred via lattice vibrations. In a perfect solid, vibrational energy would travel with sound speed and heat would be rapidly dissipated. Imperfections in the solid slow the process of energy transfer, such that heat flows analogous to the diffusion of matter. One model of conductive heat transfer associates vibrational energy with particles called phonons (it can be shown that vibrational energy must be quantized). The transfer of heat can then be modeled as diffusion in the phonon gas similar to diffusion in a real gas. In planetary atmospheres, heat conduction is largely unimportant (except for the uppermost layers) with
convection and radiation being more efficient heat transfer mechanisms. In planetary interiors, heat is conducted in regions that are immobile on planetary timescales. One example layer is the stagnant lithosphere of planets such as ▶ Mars. On ▶ Earth where the lithosphere is broken up in plates that move laterally, vertical heat transfer is still conductive while lateral heat transfer is convective. Other prominent layers of conductive heat transfer are the boundary layers of mantle convection (compare Fig. 1 below and Schubert et al. 2001). Mathematically, the (conductive) flow of heat ~ F can be described by ~ F ¼ krT
ð1Þ
where T is temperature and k is the thermal conductivity. ~ F is a vector quantity having a value and a direction. The thermal conductivity can be both a function of pressure and temperature. Radiation. Radiation is a heat transfer mechanism in regions that are transparent to electromagnetic waves and is particularly important in planetary atmospheres (see radiation in planetary atmospheres and Thomas and Stamnes 1999). For the highly absorbing planetary interiors, radiation is usually integrated into the mechanism of heat conduction thereby increasing the thermal conductivity at high temperatures. Convection. Thermal convection is a consequence of thermal expansion with the density of gases and liquids decreasing with increasing temperature. Consider a layer of fluid (or gas) sandwiched between a hot plate at the bottom and a cold plate at the top. Fluid near the hot plate
Heat Transfer (Planetary). Figure 1 Color coded temperature distributions in stagnant lid (a) and mobile lid convection (b) for strongly temperature-dependent viscosity after numerical calculations by Stein and Hansen (2008). Blue indicates low temperatures and red high temperatures. In stagnant lid convection the flow is confined to occur underneath a cold near-surface stagnant layer. In the stagnant layer, heat is transferred by conduction. In mobile lid convection, the cold near-surface layer participates in the convection. Arrows indicate the direction of flow. Mobile lid convection is a mode of convection largely analogous to plate tectonics. Courtesy of C. Stein and U. Hansen
Heat Transfer (Planetary)
will absorb energy and its temperature will rise. As a consequence of the density decreasing with temperature, the fluid will become increasingly buoyant with respect to the colder fluid above and eventually will rise if the buoyancy is large enough to overcome inertia and internal friction. As it rises, it will transport heat that it may exchange with the fluid through which it rises and – in particular – with the fluid near the cold top plate. The heat transfer is thus from the hot to the cold plate. In a similar thought experiment, heat can be generated within the fluid, cause buoyancy, and again be transported to the cold plate. The above is – in principle – the mechanism of heat transfer from the hot core through the mantle to the cold planetary surface, and the mechanism of the transfer of heat generated in the interior of the planet to the same cold surface. It is the same mechanism of heat transfer from the interior of the hot core to the (colder) core–mantle boundary and from the warm surface of the planet to the colder regions in the atmosphere. In the atmosphere, convection includes large- and small-scale rising and sinking of air masses and smaller air parcels. These vertical motions effectively distribute heat and moisture throughout the atmospheric column. The mechanism also applies in the interiors of ▶ giant planets and in oceans. It is termed free convection because the flow is not driven by external forces. Forced convection in planets may be driven by, e.g., ▶ tides and in the atmosphere by variations in solar irradiation. The convective flow of heat ~ F across a surface in a fluid is given by ~ F ¼ r ~ urcp T
ð2Þ
where ~ u is the velocity vector, r is the local density, and cP the heat capacity at constant pressure. Heat transfer by free convection can be characterized by a set of dimensionless parameters. The first and most important is the Rayleigh number Ra. For fluids heated from below, it is defined as Ra
gaDTd nk
3
ð3Þ
In addition to already defined quantities, g is the acceleration due to gravity, a is the thermal expansion coefficient, DT is the temperature difference between the top and bottom boundary, d is the layer thickness, n is the kinematic ▶ viscosity, and k (rcp) the thermal diffusivity. For internally heated fluids, DT is replaced with Qd 2/k, where Q is the heat production rate per unit volume. The term g aDT in the nominator of Eq. (3) measures the thermal buoyancy. Note that there are two diffusivities in the denominator. The thermal diffusivity k measures the loss of buoyancy due to thermal conduction and the
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kinematic viscosity n – the momentum diffusivity – measures the deceleration of the fluid due to momentum diffusion. The steady-state heat transfer rate by convection through a fluid heated from below is measured by the Nusselt number Nu Nu
qd kDT
ð4Þ
with q the heat flow across the top surface. The Nusselt number is the ratio between the surface heat flow q and the heat flow that would be transferred by conduction kDT/d. Since convection is an instability that sets in when the heat can be more efficiently transferred by convection Nu 1. In a fluid heated from within, the surface heat flow in the steady state equals Qd, and the Nusselt number as defined above is always unity. The efficiency of convective heat transfer is then measured by the value of the internal temperature: the lower the temperature at which the convection is operating, the more efficient is the internally heated convection. Experiments with fluids and numerical solutions of the field equations of fluid mechanics suggest that there is a power law relationship between the Nu and the Ra numbers Ra b ð5Þ Nu ¼ Racrit where Racrit is the critical Rayleigh number for the onset of convection and b is a constant, the value of which is between 0.2 and 0.33 depending on boundary conditions and the temperature and pressure dependence of material properties. Similarly, for internally heated convection, kT Ra g ¼ ð6Þ y Qd 2 Racrit where T is the internal temperature and g is between 0.2 and 0.25. The parameters a, n, and k are pressure and temperature dependent. Of particular importance for planetary mantles – as numerical calculations and laboratory experiments have shown – is the pressure and temperature dependence of the viscosity (see ▶ rheology for a discussion. Note that the solid mantles of terrestrial planets and satellites can be regarded as extremely viscous fluids for processes operating on geological timescales; that is on timescales millions of years or more). The extremely large viscosity in the mantle causes inertia forces (such as the Coriolis force) to be negligible and the flow to be laminar, although at very large Rayleigh numbers the flow tends to become increasingly chaotic (see, e.g.,
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Ricard 2007). Within a planetary interior and in particular through the lithosphere, the outer boundary layer of the planet, the viscosity may vary over many orders of magnitude. A strongly temperature-dependent viscosity can result in the formation of a stagnant lid (compare Fig. 1). The pressure dependence can be speculated to lead to a second stagnant layer in the deep interior of the planet. The formation of a mobile lid requires the variation of the viscosity to be sufficiently small. The formation of a plate-like mobile lid (the fluid dynamical representation of ▶ plate tectonics) may require a viscoelastic rheology in which the yield strength may matter (compare Fig. 2). But it is not completely understood how plate tectonics on the Earth works and what the exact conditions for the stability of plate tectonics are. The latter makes it, of course, difficult to predict plate tectonics on other planets. While the viscosity and its dependence on pressure and temperature is thought to be a most important
Yield stress Episodic
Stagnant lid Stagnant lid
Episodic
Mobile lid
Viscosity contrast Mobile lid Rayleigh number (Planetary radius)
Heat Transfer (Planetary). Figure 2 Qualitative phase diagram for convection of a fluid with viscoelastic rheology including strongly temperature-dependent viscosity. The figure is based on numerical calculations by K. Stemmer and colleagues. It qualitatively shows how the stability fields of the stagnant lid and mobile lid modes of convection depend on the Rayleigh number, the viscosity contrast across the fluid, and the yield strength for the transition from elastic to plastic deformation. There is a transitional regime termed episodic in which a stagnant lid episodically becomes mobile. Courtesy of K. Stemmer
parameter for mantle convection, the viscosity is of little importance for convection in the planetary atmosphere and the core and in the interiors of the giant planets. In these, inertia forces, in particular the Coriolis force, matter and the flow is mostly turbulent. The Coriolis force provides helicity (cork-screw-like notion) for the flow in the core which is a prerequisite for ▶ dynamo action. In the atmosphere, the Coriolis force causes the vortices that dominate the weather system on, e.g., the Earth. In the giant planets, the Coriolis force is again taken to be the reason for the observed vortices and layers (see ▶ Jupiter).
See also ▶ Core, Planetary ▶ Diffusion ▶ Dynamo (Planetary) ▶ Earth ▶ Giant Planets ▶ Greenhouse Effect ▶ Interior Structure (Planetary) ▶ Jupiter ▶ Magnetic Field ▶ Mars ▶ Opacity ▶ Planetesimals ▶ Planet Formation ▶ Plate Tectonics ▶ Radiogenic Isotopes ▶ Rheology (Planetary Interior) ▶ Satellite or Moon ▶ Stagnant Lid Convection ▶ Stratosphere ▶ Terrestrial Planet ▶ Tides (Planetary) ▶ Troposphere
References and Further Reading Pierrehumbert RT (2010) Principles of planetary climate. Cambridge University Press Ricard Y (2007) Physics of mantle convection. In: Bercovici D, Schubert G (eds) Treatise on geophysics, vol 7. Elsevier, Amsterdam, pp 31–88 Schubert G, Turcotte D, Olson P (2001) Mantle convection in the Earth and planets. Cambridge University Press, Cambridge, p 940 Seager S (2010) Exoplanet atmospheres: physical processes. Princeton University Press, New Jersey, USA Stein CA, Hansen U (2008) Plate motions and the viscosity structure of the mantle – insights from numerical modeling. Earth Planet Sci Lett 272:29–40 Thomas GE, Stamnes K (1999) Radiative transfer in the atmosphere and ocean. Cambridge University Press, NYC, USA
Hematite
Heavy Charged Particle ▶ HZE Particle
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Helium Nuclei ▶ Alpha Rays
Heavy Element Synonyms Metal (in the astrophysical context)
Definition In the astrophysical context, a heavy element is any element heavier than Helium. The term “▶ metal” is also used by astronomers. The definition stems from the fact that only Helium and Hydrogen (and small quantities of Lithium and Beryllium) were produced in the few first minutes after the Big Bang: all other elements were produced afterward, through stellar processing (nuclear reactions or supernovae).
See also ▶ Metal (in the astrophysical context) ▶ Metallicity
Heavy Hydrogen ▶ Deuterium
Hematite Synonyms Kidney ore
Definition Hematite is an iron oxide of chemical formula Fe2O3 (trigonal crystal system). Hematite forms in a large variety of geological environments and is mined as one of the main ores of ▶ iron on Earth. Hematite is the major iron oxide in the ▶ banded iron formations (BIFs), which were deposited during global oxidation events. It is also found in high-grade ore bodies in ▶ metamorphic rocks due to contact ▶ metasomatism, and occasionally as a sublimate on lavas. Hematite can be associated to liquid water, particularly in hot springs deposits. For this reason, hematite on planetary surfaces is a potential indicator of the past presence of water. Hematite spherules, called “blueberries” (Fig. 1) due to their blue hue in false-color images released by NASA, were found by the ▶ Mars rover Opportunity at Meridiani Planum. Embedded in a sulfate salt matrix or loose on the surface (Fig. 1), those hematite spherules might derive from accretion under water, though alternative formation processes such as meteoric impact or volcanic processes have also been advocated.
Heavy Ion ▶ HZE Particle
Heavy Nucleus ▶ HZE Particle
Heavy Primary ▶ HZE Particle
Hematite. Figure 1 Mosaic of photos taken by the Mars rover Opportunity showing hematite “blueberries” spherules spread over the Martian soil grains (Photo credit: NASA)
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See also
Definition
▶ Banded Iron Formation ▶ Goethite ▶ Great Oxygenation Event ▶ Hydrothermal Environments ▶ Iron ▶ Iron Cycle ▶ Jarosite ▶ Mars ▶ Metamorphic Rock ▶ Metasomatism ▶ Oxygenation of the Earth’s Atmosphere
These filters are manufactured and tested to ensure that filtered air has undergone a reduction in the level of particulate contamination. An HEPA filter, i.e., H14, is certified and tested to filter 99.975% of the particles with a minimal size of 0.3 mm. Such filters are used in space industry to isolate some volumes from a dusty or contaminated environment. For ▶ planetary protection they are used in ▶ clean rooms to filter the incoming air and in BSL four laboratories to filter both air inlets and outlets.
Heme
See also ▶ Biological Safety Level ▶ Clean Room ▶ Planetary Protection
Definition A heme is a ▶ protein prosthetic group that consists of an ▶ iron ion chelated in the center of a ▶ porphyrin. A substantial fraction of porphyrincontaining metalloproteins have heme as their prosthetic group. Heme-containing proteins have a number of biological functions including diatomic gas transport, chemical catalysis, and electron transfer. The heme iron serves as a source or sink of electrons during redox chemistry; however, in several peroxidases, the porphyrin molecule also serves as an electron source. In the transportation of diatomic gases, the gas binds to the heme iron, often inducing a conformational change in the surrounding protein. The original function of heme-containing proteins may have been electron transfer in primitive sulfur-based ▶ photosynthesis pathways in ancestral cyanobacteria before the evolution of oxygenic photosynthesis.
See also ▶ Iron ▶ Photosynthesis ▶ Porphyrin ▶ Protein
HEPA Filters Synonyms High efficiency particulate absorbing filters; High efficiency particulate air filters; High efficiency particulate arresting filters
Herschel GO¨RAN L. PILBRATT ESA Astrophysics and Fundamental Physics Missions Division, Research and Science Support Department, European Space Agency (ESA), AZ, Noordwijk, The Netherlands
Keywords Astrochemistry, infrared galaxies, infrared observatories, intersellar chemistry, interstellar dust, interstellar medium, interstellar molecules, molecular clouds, protostars, space observatories, star formation
Definition The Herschel Space Observatory (normally referred to as Herschel) is a European Space Agency (▶ ESA) astronomy facility offering unprecedented observational capabilities in the far-infrared and submillimeter spectral range 55–671 mm. It was launched on May 14, 2009, carries a 3.5 m diameter Cassegrain telescope and three science instruments whose focal plane units are housed inside a super fluid helium cryostat. Herschel has been designed to offer approximately 20,000 h observing time for astronomical observations, which are available to the worldwide astronomy community. The main science objectives address ▶ interstellar medium physics, ▶ star formation and evolved ▶ stars, and ▶ galaxy evolution.
History Herschel was launched on 14 May 2009, but has a long history. Conceived of as the Far Infra Red and
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Submillimeter Space Telescope (FIRST) and proposed to ESA in November 1982 in response to a call for mission proposals issued in July 1982, it was incorporated in the ESA “Horizon 2000” long term plan. Over the years it was the subject of several studies adopting a variety of mission designs, spacecraft configurations, telescopes, and science payloads. In November 1993 it was decided that FIRST would be implemented as the fourth “cornerstone” mission. The three science instruments were selected in 1997– 1999, the major industrial contractors in 2000–2001, and in April 2001 the industrial activities commenced. Meanwhile, in December 2000, FIRST was renamed Herschel in recognition of the 200th anniversary of the discovery of infrared light by William Herschel. The science instruments were delivered and integrated in 2008, final spacecraft integration, testing, and verification activities took place in ESA’s ESTEC Test Centre from January 2008 to January 2009, when the Herschel spacecraft was flown to Kourou, French Guiana, for the launch campaign.
Overview The Herschel Space Observatory is a European Space Agency (ESA) facility for far-infrared and submillimeter astronomy open to the general astronomical community. The Herschel spacecraft employs a superfluid helium
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cryostat reusing the successful ESA ▶ Infrared Space Observatory (ISO) technology. However, the mission concept is different; Herschel operates autonomously approximately 1.5 million km away from the Earth in a large amplitude quasi-halo orbit around the 2nd Lagrangian point (L2) in the Sun-Earth/Moon system, with a daily ground contact period of normally about 3 h. Compared to earlier infrared space missions with cryogenic telescopes, it offers a much larger telescope and extends the spectral coverage further into the far-infrared and submillimeter range, covering approximately 55–671 mm. The Herschel spacecraft (Fig. 1) provides the appropriate working environment for the science instruments, points the telescope with required accuracy, autonomously executes the observing timeline, and performs onboard data handling and communication with the ground. It has a modular design, consisting of the “payload module” (PLM) supporting the telescope, the sunshade/sunshield, and the “service module” (SVM). The mission lifetime is determined by the cryostat lifetime, required to be 3.5 years; the initial 6 months were nominally allocated to early mission phases. The PLM is dominated by the cryostat vacuum vessel (CVV) from which the superfluid helium tank is suspended, surrounded by three vapor-cooled shields to
Herschel. Figure 1 Left: Herschel being prepared for acoustic testing in the ESTEC Test Centre in June 2008, providing a good view of the telescope. Right: A computer cut-away image of the payload module showing the three science instrument focal plane units on the optical bench on top of the helium tank. Also shown is the “Russian doll” structure minimizing “parasitic” heat loads (ESA)
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minimize parasitic heat loads. The optical bench with the three instrument focal plane units (FPUs) is supported on top of the tank, which has a nominal capacity of 2,367 l. A phase separator allows a continuous evaporation of the liquid into cold gas, keeping the FPUs and their detectors at their required temperatures. The telescope is 3.5 m in diameter, as large as is possible with no in-flight deployable structures, and has an operating temperature of about 85 K, as cold as possible with passive cooling, and provides a total wave front error (WFE) of less than 6 mm in the focal surface interfacing to the instruments. It also has a low mass and the required mechanical and thermal properties. The SVM houses “warm” payload electronics on four of its eight panels and provides the necessary “infrastructure” for the satellite such as power, altitude and orbit control, the onboard data handling and command execution, communications, and safety monitoring. It also provides a thermally controlled environment, which is critical for some of the instrument units. Finally, the SVM also provides mechanical support for the PLM, the sunshield/ sunshade, a thermal shield to thermally decouple the PLM from the SVM, and it ensures the main mechanical load path during the launch. Herschel has three science instruments: the Photo detector Array Camera and Spectrometer (PACS), the Spectral and Photometric Imaging REceiver (SPIRE), and the Heterodyne Instrument for the Far Infrared (HIFI). Their main characteristics are provided in Table 1.
Basic Methodology Herschel is a facility open to the general astronomical community. The available observing time, in the nominal mission approximately 20,000 h are available for science, is shared between guaranteed time (GT, 32%) owned by
Herschel. Table 1 Herschel science instrument main characteristics Acronym Instrument
Principal investigator
PACS
Imaging camera and grating A. Poglitsch, MPE, spectrometer, spectral Garching (D) coverage 55–205 mm
SPIRE
Imaging camera and FTS spectrometer, spectral coverage 194–671 mm
M. Griffin, U. Cardiff (UK)
HIFI
Heterodyne spectrometer, spectral coverage 157–212 and 240–625 mm
F. Helmich, SRON, Groningen (NL)
contributors to the Herschel mission (mainly by the Principal Investigator instrument consortia), and open time (OT) which is allocated to the general community (including the guaranteed time holders) on the basis of competitive calls for observing time. The mission and science operations of Herschel are conducted in a decentralized manner. The operational ground segment comprises six elements: ● The Mission Operations Centre (MOC), provided by ESA ● The Herschel Science Centre (HSC), provided by ESA ● Three dedicated Instrument Control Centres (ICCs), one for each instrument, provided by the respective PIs ● The NASA Herschel Science Center (NHSC), provided by ▶ NASA The MOC conducts the mission operations, it performs all contact with the spacecraft, monitors all spacecraft systems, safeguards health and safety, and performs orbit maintenance. It generates skeleton schedules to be “filled in” with science, calibration, and engineering observations. It creates and uplinks mission timelines and receives, consolidates, and provides telemetry for distribution. Science operations are conducted in a partnership among the HSC, the three ICCs, and the NHSC. The HSC is the interface between Herschel and (prospective) investigators in the science community. It provides information and handles calls for observing proposals including supporting the Herschel Observing Time Allocation Committee (HOTAC). It performs scientific mission planning, systematic pipeline data processing, data quality control, populates the science archive, and offers user support related to all aspects of observing, including observation planning and proposing, observation execution, data processing and access. The ICCs are responsible for the successful operation of their respective instruments, developing and maintaining instrument observing modes, and for providing specialized software and procedures for the processing of the data generated. The NHSC provides additional support and offers science exploitation funding for investigators based in the USA. The observers apply for time on Herschel by responding to the calls for proposals, containing the science objectives, the definition of the actual observations being proposed, and additional information. All proposals are reviewed by the HOTAC, and checked technically by the HSC. The observations by the successful proposers are
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then carried out. A small amount of the open time can be allocated as discretionary time. Given that Herschel would not have the benefit of an all sky survey for much of its wavelength coverage, it was decided early on that large so-called “Key Programmes” (KPs) in the form of “large spatial and spectral surveys” should be selected and executed early in the mission so that the results could be followed up by Herschel itself. An initial call for observing proposals limited to such KP observing proposals was therefore issued. This process took place from February 2007 to February 2008. There were 21GT and 62OT proposals, out of which by coincidence also 21 were awarded observing time. In total 11,000 h of observing time was allocated to these 42 KPs. The total amount of observing time allocated to KPs constitutes 55% of the nominally available observing time. The remainder will be allocated in two additional calls for proposals. The first of these is underway since February 2010, and will be concluded in November 2010. The final call will take place approximately a year later. All scientific data are archived and are initially made available only to the data owners. After the proprietary time has expired for a given observation, the data are made available to the entire astronomical community in the same manner in which they were previously available only to the original owner.
Key Research Findings Herschel is all about the “cool universe.” Continuum emission in the Herschel spectral range originates from dust with temperatures in the range 10–50 K, and spectral emission from gas with temperatures of tens to hundreds of K. About half of the energy emitted (typically at ultraviolet/optical/near-infrared wavelengths) in the universe since the epoch of recombination (the creation of the cosmic microwave background) has been absorbed mainly by dust in the ▶ interstellar medium (ISM) in our ▶ Milky Way and other galaxies, and reradiated at much longer wavelengths. The integrated spectrum of the reradiated emission peaks in the 100–200 mm range. This is also true for the emission from pre-stellar objects in our own Galaxy and other galaxies, and infrared dominated galaxies with red shifts up to about z = 4, which corresponds to a look-back time of about 12 billion years (or in other words to galaxies that were less than 2 billion years old at the time of emitting the light we now observe with Herschel) covering the epochs of the bulk of the star formation in the universe. The prime science objectives of Herschel are intimately connected to the physics of and processes in the ISM in the widest sense: near and far in both space and time,
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stretching from solar system objects and the relics of the formation of the Sun and our solar system, over star formation in and feedback by evolved stars to the ISM, to the star-formation history of the universe, galaxy evolution and cosmology. The very first observational results from Herschel already show that it will have large impact on research in these fields, as will be exemplified by the following three observational results. The “Great Observatory Origins Deep Survey” (GOODS) covers an area that has been observed by many telescopes at a range of wavelengths, seen now by Herschel/SPIRE in submillimeter wavelengths (Fig. 2). This area of sky is devoid of foreground objects, such as stars within our Galaxy or any other nearby galaxies, which makes it ideal for observing deeper into space. Every fuzzy blob is a very distant galaxy, seen as they were 3–10 billion years ago when star formation was very widely spread throughout the Universe. The image is made from the three SPIRE bands, with blue, green, and red, corresponding to 250, 350, and 500 mm, respectively. Herschel’s view of a stellar nursery around 1,000 ▶ light-years away in the constellation Aquila (the Eagle) is shown in Fig. 3. This cloud, 65 light-years across, is so shrouded in dust that no infrared satellite has been able to see into it, until now. Thanks to Herschel’s greater sensitivity at the longest infrared wavelengths, astronomers have their first picture inside this cloud. Using Herschel’s PACS and SPIRE instruments at the same time, the image shows two bright regions where large newborn stars are causing hydrogen gas to shine. Embedded in the dusty filaments are 700 condensations of dust and gas that will eventually become stars. Astronomers estimate that about
Herschel. Figure 2 An area of GOODS-N as observed by Herschel/SPIRE. Inset: the famous previous JCMT/SCUBA observation (ESA/SPIRE/HerMES/S. Oliver)
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100 are “▶ protostars,” celestial objects in the final stages of formation. Each one just needs to ignite nuclear fusion in its core to become a true star. The other 600 objects are not developed sufficiently to be called protostars, but eventually they will become another generation of stars. Observing these stellar nurseries is a key programme for Herschel, which aims to uncover the demographics of
star formation and origins, or in other words, the quantities of stars that can form and the range of masses for these newborn stars. A part of a Herschel/HIFI spectral scan is shown in Fig. 4. The observation is toward the Orion Nebula, a relatively nearby star forming region, the “sword” in the constellation of Orion. A characteristic feature is the spectral richness: among the organic molecules identified in this spectrum are water, carbon monoxide, formaldehyde, methanol, dimethyl ether, hydrogen cyanide, sulfur oxide, sulfur dioxide, and their ▶ isotope analogues. It is expected that new molecules will also be identified. This spectrum is Herschel’s first glimpse at the spectral richness of regions of star and planet formation. It harbors the promise of a deep understanding of the chemistry of space, once the complete spectral surveys are available.
Future Directions
Herschel. Figure 3 An area in the stellar nursery in the constellation of Aquila (ESA/SPIRE & PACS/P. Andre´)
At the time of writing it is a year since the launch of Herschel. The current best estimate of the total mission lifetime – from the launch – is in the range 3.5–4 years. Most of the first year has been spent on performance optimization and verification of both the observatory itself and the large observing programmes – the Key Programmes – selected before the launch. Although the initial science results from Herschel are just appearing and
Herschel. Figure 4 A part of a Herschel/HIFI spectral scan (the white curve) overlaid on a background Spitzer Space Telescope image (ESA/HIFI/HEXOS/E. Bergin)
Hertzsprung–Russell Diagram
are very exciting, they represent only a very small fraction of what is still to come.
See also ▶ Evolution of Stars ▶ Infrared Space Observatory ▶ Interstellar Medium ▶ Lagrange Points ▶ Molecular Cloud ▶ Molecules in Space ▶ Protostars ▶ Spitzer Space Telescope ▶ Star Formation
References and Further Reading A list of Herschel publications is available at: http://herschel.esac.esa.int/ ScientificPublications.shtml The starting point for further reading about Herschel is the website of the Herschel Science Centre, the URL is http://herschel.esac.esa.int/
Hertzsprung–Russell Diagram THIERRY MONTMERLE1, SYLVIA EKSTRO¨M2 1 Institut d’Astrophysique de Paris, CNRS/Universite´ Paris 6, Paris, France 2 Faculte´des Sciences, Observatoire astronomique de l’Universite´ de Gene`ve, Universite´ de Gene`ve, Sauverny, Versoix, Switzerland
Synonyms Color-magnitude diagram; HRD; HR Diagram; Luminosity-temperature diagram
Keywords Clusters, effective temperature, luminosity, star formation, stars, stellar evolution
Definition The Hertzsprung–Russell Diagram plots the ▶ magnitude and color of stars, which are converted via theoretical models and distances into stellar luminosities and surface (“effective”) temperatures. Comparison with theoretical tracks, the “paths” in the Diagram corresponding to changes in observed properties as the star evolves, then allows the determination of the mass or age of a star or cluster. Note in the following the astrophysical use of the term “burning” to signify nuclear fusion (e.g., of hydrogen to helium). Since the beginning of the twentieth century, astronomers have been using the “Hertzsprung–Russell Diagram”
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(from the name of its discoverers, in the early 20th century; hereafter HRD) to classify stars and understand their evolution, from their formation to their very late stages.
Overview When plotted in the HRD, the stars do not appear randomly distributed (see Fig. 1). There are some regions that are more populated, and others that are almost empty. There is an evident relation between the density of a region and the duration of the ▶ stellar evolution phase that occurs in that region. During their evolution, stars pass from long-lasting hydrostatic burning (nuclear fusion) phases to quick structural readjustments. While it is easy to observe the stars in regions where the hydrostatic burnings occur, very few stars can be observed during their readjustment phases.
Basic Methodology Observationally, astronomers first determine the magnitude and ▶ spectral type of the stars. These numbers are then transformed into luminosity and temperature, which are physical quantities that can be compared with models. This assumes the capability (1) to know the distance (to convert magnitudes into luminosities), which is determined by various methods (to an accuracy of 20–30% in the case of young stars), and (2) to convert from spectral types to temperatures, which can be done with models of stellar photospheres. For “simple” stars like the Sun, the temperature determination is very precise (106 cm3) and elevated temperatures of both gas and dust (100–500 K). The gaseous chemical composition is distinct from cold ▶ molecular clouds, due to evaporation of ice mantles from dust grains, and contains many complex organic molecules. The high temperatures are produced by the luminosity of the protostar and the protostellar accretion disk. Shock waves may also play a chemical role in both heating the gas and in sputtering icy grain mantles back into the gas. Hot cores are chemically rich and display the products of interstellar grain-surface chemistry in the gas phase. For this reason, they can be used to probe catalytic processes on dust grains. Amongst the species found in them are methanol, ethanol, dimethyl ether, methyl ethyl ether, methyl formate, ethyl formate, ketene, formaldehyde, acetaldehyde, formic and acetic acid, glycolaldehyde, ethylene glycol, several nitriles, and possibly glycine. Some of these organics (e.g., the ethers) could also be formed in the hot gas from those intermediaries formed on and sublimated from the icy mantles on the dust. Hot cores eventually evolve into ultracompact ▶ HII regions as the UV radiation from the massive central star
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The name hot corinos has been given to the immediate environment of low-mass ▶ protostars in which the gas and dust has been heated by the protostellar radiation and shock waves. Hot corinos are characterized by large abundances of complex molecules and high ▶ deuterium fractionation ratios, both indicative of sublimation of ▶ interstellar ices.
History Hot corinos were so named by Cazaux et al. (2003) because of the similarity with the ▶ hot cores found around high-mass protostars.
See also ▶ Deuterium ▶ Hot Cores ▶ Interstellar Ices ▶ Isotopic Fractionation (Interstellar Medium) ▶ Protostars
References and Further Reading Cazaux S, Tielens AGGM, Ceccarelli C, Castets A, Wakelam V, Caux E, Parise B, Teyssier D (2003) The hot Core around the Low-Mass Protostar IRAS 16293-2422: Scoundrels Rule! Astrophys J 593: L51–L55
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Hot Jupiters
Hot Jupiters Definition Hot Jupiters is a designation that is often assigned to ▶ gas giant planets in tight orbits around their parent star. The characteristics of Hot Jupiters are not well defined, but the orbital period must be short enough (say less than 10 or 20 days) for the planet to be truly “hot,” and the mass must be large enough (say more than about one third of a Jupiter mass) for the planet to be a gas giant. It is commonly accepted that Hot Jupiters originally form much farther from their hot star, and then migrate inwards either by interaction with the circumstellar disk from which they formed, or by gravitational interactions with other planets in the system. For reasons that are not fully understood, there is a pile-up in the number of Hot Jupiters with orbital periods near 3 days around solar-type stars. The first Hot Jupiter was found in 1995, orbiting the star 51 Peg (Mayor and Queloz 1995).
See also ▶ Exoplanets, Discovery ▶ Gas Giant Planet ▶ Planetary Migration ▶ Radial-Velocity Planets
References and Further Reading Mayor M, Queloz D (1995) A Jupiter-mass companion to a solar-type star. Nature 378:355–359
Hot Molecular Cores ▶ Hot Cores
Hot Neptunes
“hot,” and the mass must be substantially smaller than that of Saturn (say less than a tenth of a Jupiter mass). It is commonly accepted that Hot Neptunes originally form much farther from their host star, and then migrate inward either by interaction with the circumstellar disk from which they formed, or by gravitational interactions with other planets in the system.
See also ▶ Exoplanets, Discovery ▶ Gas Giant Planet ▶ Planetary Migration ▶ Radial-Velocity Planets
Hot Spring Microbiology Definition Hot spring microbiology refers to microorganisms associated to hot springs. Hot springs are characterized by hydrothermal systems that constantly release geothermal gases and fluids from the subsurface due to water sources in contact with shallow magma formations. Many hot springs have temperatures at or near boiling point, thus microorganisms from these environments are hyperthermophilic or thermophilic and phylogenetically very diverse, including ▶ Archaea, Bacteria, and viruses. Water chemistry and pH can vary dramatically in these environments. ▶ Methanogens that belong to Archaeae group and grow at temperature between 80 C and 90 C are present in these systems. Cyanobacteria are often the dominant or sole photosynthetic organism present in hot springs. Species of Thermotoga capable of growing at 90 C have been isolated in hot springs and, marine hydrothermal vents. Thermophilic Crenarchaeota have been isolated from hot, sulfur-rich solfataras and some metanogenic archaea have also been identified in these environments. Viruses are represented by extreme thermophiles, bacteriophages, living up to 85–90 C. These microorganisms play important roles in the carbon, sulfur, and iron cycles in hot spring ecosystems.
Definition
See also
Hot Neptunes is a designation that is often assigned to planets with masses and radii similar to the ice giants Uranus and Neptune, but in tight orbits around their parent star. The characteristics of Hot Neptunes are not well defined, but the orbital period must be short enough (say less than 10 or 20 days) for the planet to be truly
▶ Archea ▶ Carbon Cycle (Biological) ▶ Crenarchaeota ▶ Extreme Environment ▶ Hot Vent Microbiology ▶ Hyperthermophile
HR 8799 b, c, and d
▶ Iron Cycle ▶ Methanogens ▶ Sulfur Cycle ▶ Thermophile
Hot Spring on the Seafloor ▶ Black Smoker, Organic Chemistry
Hot Vent Microbiology Synonyms Hydrothermal Vent Microbiology
Definition Hot vent microbiology refers to the microbial diversity associated to underwater hot springs known as hydrothermal vents. Chemical analysis of hydrothermal fluids shows large amounts of reduced inorganic materials, including H2S, Mn2+, H2, NH4, and CO. The system is based on chemolithoautotrophy (nitrifying metal and sulfur oxidizing microorganisms and ▶ methanogens). Hydrothermal vents combine several extreme conditions including temperature (up to 380 C), pressure (20 MPa at 2,000 m), and pH (plumes of pH 2.5), in the absence of sun light all of which determine that primary producers have to use chemical energy for CO2 fixation. The temperature gradient from the surrounding water that is heated up to 300– 400 C by magma that pours out through cracks in the lithosphere to seawater, which remains around 2 C, creates a very unstable environment. Chimney growth from mineral precipitation when hydrothermal fluids come into contact with cold sea water (black smokers) serve as a substrate to associated microorganisms that can metabolize sulfur and metal sulfide compounds. The sea floor vents have important astrobiological connotations because they are considered interesting models for the origin of life on Earth as well as potential habitats on other planetary bodies such as Jupiter’s moon, Europa.
See also ▶ Archea ▶ Bacteria ▶ Barophile ▶ Chemolithoautotroph ▶ Deep-Sea Microbiology ▶ Hyperthermophile
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▶ Methanogens ▶ Nitrification ▶ Sulfur Cycle ▶ Thermophile
Hotspot ▶ Mantle Plume (Planetary)
HPLC Synonyms High performance liquid chromatography; High pressure liquid chromatography; LC; UHPLC
Definition HPLC is an efficient method of liquid–solid column chromatography. Using a stationary phase composed of small (3–10 mm) beads increases separation efficiency, but requires pumps to force the mobile phase through the column at pressures >300 bar. Ultra-HPLC (UHPLC) uses 1,000 bar. Unlike gas chromatography, the composition of the mobile phase is often varied with time while the column temperature is held constant. Reverse phase HPLC uses a hydrophilic mobile and hydrophobic stationary phase, the opposite of normal phase HPLC. HPLC is suited for coupling with different detectors, particularly in so-called hyphenated techniques, such as HPLC coupled to mass spectrometry (▶ LC-MS).
See also ▶ Chromatography ▶ Ion-Exchange Chromatography ▶ Liquid Chromatography-Mass Spectrometry
HR 8799 b, c, and d Definition HR 8799b, c, d is a system of three planets and possibly ▶ brown dwarfs orbiting HR 8799, a young star of about 1.5 solar masses, located 129 light years away in the constellation Pegasus. Christian Marois and his team
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HRD
HRD ▶ Hertzsprung–Russell Diagram
HR Diagram ▶ Hertzsprung–Russell Diagram
HR 8799 b, c, and d. Figure 1 An image of the three planets around HR 8799, obtained with the Keck telescope in Hawaii. The light of the central star has been suppressed, leaving only residual noise speckles. The light reflected by the particles in the circumstellar disk is visible as an elongated ring
discovered the three companions to this star using ▶ adaptive optics systems on the Gemini and Keck telescopes in 2008. The objects b, c, and d orbit at distances roughly 68, 38, and 24 astronomical units, respectively, from the star (Fig. 1). This system of objects could represent the first multiple-planet system discovered via ▶ direct imaging. Mass estimates for the objects based on their luminosities and on the age of the star are roughly 7, 10, and 10 Jupiter masses for b, c, and d, respectively, but could be roughly 50% higher, which would make some of them ▶ brown dwarfs, not planets. However, dynamical arguments suggest that the masses are lower than these numbers, supporting the interpretation that they are all planets.
History Besides these planets, HR 8799 also hosts a disk of dust, discovered by the IRAS satellite in 1986. It hinted at the presence of a planetary system, making HR 8799 a popular survey target.
See also ▶ Adaptive Optics ▶ Brown Dwarfs ▶ Direct-Imaging, Planets ▶ Exoplanets, Discovery ▶ Fomalhaut b ▶ GJ 758 b
HST MICHEL VISO Astrobiology, CNES/DSP/EU, Paris, France
Synonyms Hubble Space Telescope
Keywords Exoplanet, planetary disks, star formation
Definition The Hubble Space Telescope (HST) is the only NASA space observatory to be serviced by the Space Shuttle. The ▶ European Space Agency contributes through various participation up to 15% of the total cost. Launched in 1990, the HST was serviced five times and will last up to 2014. It operates with a 2.5-m diameter mirror in ultraviolet, visible, and infrared light. The 11,000 kg observatory is placed in low Earth orbit at an altitude of about 575 km. The data archiving and the scientific operations are managed by the Space Telescope Science Institute in the name of the scientific consortium consisting of numerous universities and institutions (Fig. 1).
History From 1970 up to 1978 US astronomers and NASA deployed an intense lobbying effort toward US congressmen to have the Large Space Telescope funded. The decision was taken after the decision of the European Space Agency to support this effort. Originally slated to be launched in 1983, technical delays made it almost ready only by 1986. The Challenger accident then grounded the Space Shuttle fleet for almost 3 years. The telescope had to be kept in a clean room, powered up, and purged with
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nitrogen. These operations were costing about $6 million per month, pushing the overall costs of the project up to $2.5 billion by the time of the launch. During this time period, the engineers performed extensive tests, swapped out a possibly failure-prone battery, and made several technical improvements. The ground software controlling Hubble was just ready by the 1990 launch. Once in space, the first images were not as sharp as expected and demonstrated a default in the primary mirror curvature. This flaw was mainly impairing the observation of faint objects and the cosmology program. The origin of the flaw was clearly identified and a corrective instrument was designed. Awaiting for the repair by the shuttle mission in December 1993, HST performed many valuable observations for astronomers. Since then and with the following four maintenance flights, the HST has delivered unprecedented results in all fields of astronomy. The overall US expenditure for the HST is estimated at between $4.5 and $6 billion, with Europe’s financial contribution at about €600 million.
Overview The Hubble Space Telescope was sent in orbit April 24, 1990, by the space shuttle Discovery. This 11,000-kg telescope was serviced five times by the space shuttle on its working orbit at an altitude of about 575 km. The telescope was built by NASA with a significant contribution from ESA. It is operated by NASA and the data are archived at the Space Telescope Science institute in Baltimore, USA. The Hubble’s spectral range extends from the ultraviolet, through the visible, and into the near-infrared. Hubble’s primary mirror is 2.4 m in diameter, which is not large by
ground-based standards. The instruments as well as the avionics and hardware were upgraded and exchanged during the five successive servicing missions. Now, and up to the end of the mission planned for 2014, the HST is equipped with five instruments. The Advanced Camera for Surveys (ACS) was installed in 2002, improving the field of view and the light sensitivity. It is working with the newly installed Wide Field Camera (WFC3). The Cosmic Origins Spectrograph (COS) instrument is an ultraviolet spectrograph optimized for observing faint point sources with moderate spectral resolution. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is the infrared instrument able to see through interstellar gas and dust. Finally, the Space Telescope Imaging Spectrograph (STIS) separates light into component wavelengths, acting much like a prism. All these instruments are placed at the focal plane of the telescope.
Key Research Findings The HST was built and designed to image far, faint galaxies. This fossil light will reveal how the Universe looked in the remote past and how it may have evolved with time. The Hubble Deep Fields gave a clear glance back to the time when galaxies were forming. Deep field observations require long-lasting pointing accuracy to a selected region of the sky. Deeper observation requires longer exposure time while the fainter objects become visible on the images. The first deep fields gave new insight about the early Universe, revolutionizing modern astronomy. The HST has harvested outstanding results in many fields of astronomy, like the calculation of the Hubble constant, the evaluation of the age of the Universe, and the
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astrophysics of objects such as quasars and super massive black holes in the centers of galaxies. The systematic surveys were also used to detect gravitational lenses and map the distribution of dark matter in some portions of the Universe. For astrobiology, the Hubble Space Telescope has contributed to the imaging of the clouds of gas and dust where new stars are forming. Data collected from the Orion nebula are giving new insight into these processes. The detected dust disks around newborn stars could be the beginning of planetary systems. Hubble performed the first detection of an atmosphere around an extrasolar giant planet, which orbits a Sun-like star (▶ HD 209458b) located 150 light-years away in the constellation Pegasus. Hubble made also unique, repeatable observations of the surfaces of the planets and objects of our Solar System: imaging the dust storms on Mars, preparing for the flyby of asteroids like Vesta by the Dawn mission, or recording “live” the diving of the comet Shoemaker-Levy into the atmosphere of Jupiter (see Fig. 2).
See also ▶ European Space Agency ▶ HD 209458b ▶ JWST ▶ National Aeronautics and Space Administration
Hubble Space Telescope ▶ HST
Hungarian-made Automatic Telescope Network ▶ HATNet
Future Directions By 2014, the James Webb Space Telescope (▶ JWST), to be placed in an orbit at the Lagrangian point L2, will replace the Hubble Space Telescope.
Huronian Glaciation ANDREY BEKKER Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada
Synonyms Paleoproterozoic ice ages; Paleoproterozoic Snowball Earth
Keywords Paleoproterozoic, ice ages, snowball earth, rise of atmospheric oxygen
Definition
HST. Figure 2 This natural-color image of Jupiter displays at bottom right debris from a comet or asteroid that plunged into Jupiter’s atmosphere and disintegrated (Credit: NASA, ESA, Michael Wong [Space Telescope Science Institute, Baltimore, MD], H. B. Hammel [Space Science Institute, Boulder, CO] and the Jupiter Impact Team)
The Huronian ▶ glaciation is the oldest series of protracted climatic refrigeration events that extensively affected Earth between 2.45 and 2.22 Ga in association with the rise of the atmospheric oxygen. During these events, glaciers covered continents, extended to low latitudes, and reached there sea level. The ice ages were followed by a protracted time interval with greenhouse (warm and humid) conditions. The name is derived from the Huronian Supergroup, a glacio-marine to fluviodeltaic sedimentary sequence of dolostone, siltstone,
Huronian Glaciation
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0; Hofbauer and Sigmund 1998). Without losing generality, all fj-values are chosen to be equal, and the dynamical system has a stationary point in the middle of concentration space: x1 = x2 = x3 = . . . = xn = 1/n. The solution curves xj(t) show characteristic dependence on the size of the hypercycle (Fig. 2):
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1. For n = 2, the dynamical system converges monotonously to a unique and asymptotically stationary state at which both replicators X1 and X2 are present. 2. For n = 3 and n = 4, the stationary state is unique and asymptotically stable. The solution curves xj(t) show strongly damped oscillations n = 3 and weak damping for n = 4. 3. For n 5, the stationary state is unstable and the concentrations xj(t) oscillate. Independently of internal dynamics, the total concentrations of hypercycles as integral units, CðtÞ ¼ Pn i ¼ 1 Ni ðtÞ, grow stronger than exponential for unlimited resources. This hyperbolic growth is observed for a class of systems in which the population size has an enhancing effect on the growth rate. Examples of
Hypercycle. Figure 2 Hypercycle dynamics visualized in form of solution curves of eq. (1). Normalized concentrations of the members of hypercycles xj(t) are plotted as functions of time t for different sizes of the cycle. Individual members cooperate in the reproduction of the entire cycle. Different dynamics is observed for different sizes: n = 2, the concentrations approach their stationary values monotonously; n = 3, the concentrations approach the stationary values with strongly damped oscillations; n = 4, the concentrations approach the stationary values with weakly damped oscillations; n = 5, the concentrations oscillate without damping. For n > 5 hypercycle dynamics shows oscillations without damping as illustrated here for the n = 5 case. The following parameter values were chosen: f1 = f2 = f3 = f4 = f5 = 1; initial conditions: x1(0) = 0.95 and x2(0) = 0.05 for n = 2 and x1(0) = 0.9 and xk(0) = 0.1/(n1) for n = 3, 4, 5
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hyperbolic growth were observed in virology, in demography (growth of human population), and in economics (increasing returns). Systems with hyperbolic growth outgrow exponentially growing systems since in theory a hyperbolically growing population can reach infinity in finite times. Once a hypercycle has been established, it is very hard to replace it by another system: hypercycles are candidates for once-and-for-ever decisions. Examples for hypercycles in the real world are symbioses and other forms of co-operation within and between species.
See also ▶ Endosymbiosis ▶ Evolution (Biological) ▶ Evolution, Molecular ▶ Quasispecies ▶ Self Replication ▶ Symbiosis
References and Further Reading Eigen M (1971) Self-organization of matter and the evolution of biological macromolecules. Naturwissenschaften 58:465–523 Eigen M, Schuster P (1977) The hypercycle. A principle of natural selforganization. Part A: emergence of the hypercycle. Naturwissenschaften 64:541–565 Eigen M, Schuster P (1978a) The hypercycle. A principle of natural selforganization. Part B: the abstract hypercycle. Naturwissenschaften 65:7–41 Eigen M, Schuster P (1978b) The hypercycle. A principle of natural selforganization. Part C: the realistic hypercycle. Naturwissenschaften 64:341–369 Hofbauer J, Sigmund K (1998) Evolutionary games and replicator dynamics. Cambridge University Press, Cambridge, UK Maynard Smith J, Szathma´ry E (1995) The major transitions in evolution. WH Freeman, Oxford, UK Nowak MA (2006) Evolutionary dynamics. The Belknap Press of Harvard University Press, Cambridge, MA, Exploring the equations of life Phillipson PE, Schuster P (2009) Modeling by nonlinear differential equations. Dissipative and conservative processes. World Scientific, Singapore, pp 61–75 Schuster P (1996) How does complexity arise in evolution? Complexity 2(1):22–30
Hypersaline Lakes ▶ Soda Lakes
Hyperthermophile JOSE BERENGUER Centro de Biologı´a Molecular Severo Ochoa, UAM-CSIC, Madrid, Spain
Synonyms Superthermophile
Keywords High temperature, hot springs, hydrothermal vents, origin of life, thermal environment
Definition The word hyperthermophile (literally extremely heat loving) refers to a microorganism that has an optimum temperature for growth above 80 C. Most hyperthermophiles belong to the Archaea domain, with only few exceptions belonging to Bacteria.
History Thomas D. Brock (1978) isolated the archaeal genus Sulfolobus at Yellowstone National Park and showed that it grew optimally at 75 C with an upper temperature limit for growth at 85 C. Previous reports by the same author described prokaryotic microorganisms that were able to grow on microscopy slides inside boiling (92 C) hot springs. These organisms probably were true hyperthermophiles. However, the first hyperthermophiles isolated and characterized in the laboratory (Methanothermus fervidus and the Thermoproteus spp) were isolated from hot springs in Iceland and described simultaneously in 1981 in two articles coauthored by Karl O. Stetter and Wolfram Zillig (Zillig et al. 1981; Stetter et al. 1981). In the following two decades, further work led mainly by K. O. Stetter focused on submarine or subterranean thermal environments. This work allowed the isolation and description of more than 50 species of hyperthermophilic Archaea and a few Bacteria, some requiring temperatures above 90 C to grow, with optimum temperatures above 100 C. Phylogenetic analysis based on 16 S RNA revealed that all of them are slowly evolving groups, supporting an ancient origin for this type of microorganisms.
Overview
Hypertelescope ▶ Interferometry
Since the first descriptions of hyperthermophiles in 1981 (Zillig et al. 1981; Stetter et al. 1981), about 90 species of hyperthermophilic Archaea and Bacteria have been described from different terrestrial and marine thermal
Hyperthermophile
environments. Taxonomically, and despite the existence of yet unassigned isolates, most are grouped in ten orders (Archaea: Thermoproteales, Desulfurococcales, Pyrodictiales, Thermococcales, Archaeoglobales, Methanococcales, Methanobacteriales, Methanopyrales, Bacteria: Aquificales and Thermotogales). A few hyperthermophilic isolates are even unable to grow below 80–90 C. Despite this, it has been possible to isolate hyperthermophiles far from the nearest known thermal effluent, probably because they remain viable for years at low temperatures. At the upper part of the temperature range, there are hyperthermophiles that can tolerate temperatures normally used in sterilization processes (121 C) for 1 h (Pyrodictium occultum). Kashefi and Lovley (2003) reported residual growth of a hyperthermophilic isolate under such conditions. The use of different phylogenetic markers assigns the hyperthermophiles to the root of the corresponding evolutionary trees, leading to the hypothesis that this kind of microorganism is very ancient. Despite this generally accepted view of the phylogenetic tree, the slow evolving character of hyperthermophiles has been challenged by different studies in the last decade based on other phylogenetic approaches (Ciccarelli et al. 2006). According to this view, hyperthermophilic character could be an adaptation of moderate ▶ thermophiles to superheated environments, in some cases through lateral gene transfer. Adaptation to hyperthermophilic conditions is the result of a series of specific modifications of the structures and macromolecules of the cell, as simple metabolites and cofactors are basically the same as those of mesophiles. Membrane adaptation results from an increase in the hydrophobic lateral interactions between the major lipids, esters of glycerol and two fatty acids in Bacteria, and ethers of glycerol and two isoprenol derived alcohols in Archaea. In hyperthermophilic Archaea, bipolar tetraether lipids form membrane monolayers in which C40 isoprenols are frequently cyclized as a further thermal adaptation. A low DNA content (1.8–3 Mbp) and the presence of histonelike proteins have been described in some hyperthermophilic Archaea. The presence of type I topoisomerase that introduces positive coils (Reverse Gyrase) into DNA is the only trait common to hyperthermophilic Bacteria and Archaea, leading to the hypothesis that DNA stability is an important factor for life at high temperatures. However, Thermococcus kodakaraensis mutants lacking this activity show only a slight decrease in growth rate above 90 C, so it has been proposed that this enzyme could help to rewind ssDNA regions separated by thermal stress (Sandman 2008). Enzymes from Hyperthermophiles usually have an activity profile that fits the temperature
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growth range of the organism of origin as the result of a higher number of intra and intermolecular interactions (Vieille and Zeikus 2001). Most hyperthermophiles grow as obligate or facultative chemolitotrophs by using either hydrogen or sulfides (e.g., pyrite) as electron donors, and nitrate, CO2, sulfur, sulfate, or ferric iron as electron acceptors to gain energy. Low concentrations of oxygen are also used by some isolates, but for most hyperthermophiles oxygen is highly toxic at or near their temperature of growth. Carbon assimilation by hyperthermophiles growing chemolithotrophically takes place on CO2 or CO mainly through the reductive acetyl-coenzyme A cycle (acetyl-CoA or Wood–Ljungdahl pathway), like in many anaerobic Gram-positive bacteria, or through the reductive tricarboxylic acid cycle (rTCA, or Arnon cycle), as in green sulfur bacteria, although alternative modified pathways also exist. These cycles usually generate acetyl-coenzyme A, from which gluconeogenesis must start. The presence of an irreversible condensing fructose biphosphate aldolase/ phosphatase is a generalized trait within autotrophic hyperthermophiles that appears to be an essential step to guide these pathways towards gluconeogenesis (Say and Fuchs 2010). Facultative and obligate chemoorganothrophs also exist among hyperthermophiles that grow by anaerobic respiration, essentially with elemental sulfur (Sº) as electron acceptors, or, more rarely, by fermentation (i.e., Thermoproteus uzonensis). Data obtained from the analysis of thermophilic environments using molecular ecology methods (Page´ et al. 2008) suggests that the biodiversity of chemoorganothrophic hyperthermophiles is underestimated.
Basic Methodology The high sensitivity to oxygen of hyperthermophiles at their growth temperatures is an important challenge for their isolation. Environmental samples to be used for the isolation of hyperthermophiles (water, soil, rock, etc.) have to be taken under reducing conditions to prevent any contact of microorganisms with oxygen at least at high temperature, when its toxic effect is more severe. For terrestrial surface environments, taking samples does not require sophisticated equipment. In the absence of N2 gas, reductants such as sodium sulfide/sodium dithionite and an appropriate redox indicator such as resazurin can be added to sample glass containers, which have to be tightly closed with gas-impermeable butyl rubber stoppers. Samples can then be transported to the laboratory at room temperature. For undersea hot environments, perforation drills, water samplers, piston corers, grab samplers, and dredge samplers or even Deep Submergence Vehicles
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(DSVs) and Remotely Operative Vehicles (ROVs) are required. Numbers of whole microbial communities in different submarine hydrothermal samples range from 104 to 109 cells per gram at the surface of ▶ black smoker chimneys, sediments, and hydrothermal plumes. Superheated vent fluids can range from undetectable numbers up to 106 cells per ml (Nakagawa and Takai 2006). Once at the laboratory, isolation of new hyperthermophiles requires enrichment cultures subjected to conditions mimicking those of the environment at the sampling site regarding temperature, putative electron donors and acceptors, and carbon sources. Growth rate is a limiting factor for successful enrichment, and periods of days are usually required before attempting the isolation of individual cells. Due to obvious difficulties in keeping solid media under the prescribed conditions, most isolation procedures require liquid medium and use either dilution or physical methods such as optical tweezers. For high scale production, special fermenters have been designed to withstand the harsh growth conditions that most hyperthermophiles require, which in some case combine high temperatures and pressure in acidic media (Stetter 2006).
Key Research Findings The main interest of hyperthermophiles is based on two key observations. On the one hand, they were and still are regarded as ancestral organisms, giving an opportunity to analyze what are most likely “living fossils” from the Archaean era. This view is still the most accepted hypothesis, although alternative hypothesis have been proposed. On the other hand, a search for the temperature limits for life has driven the search quest for more thermophilic microorganisms. Today, the upper temperature limit for growth of a well established and described microorganism is 113 C for Pyrolobus fumarii (Blo¨chl et al. 1997), whereas description of an isolate able to grow at 121 C is still under discussion (Kashefi and Lovley 2003). A further area of research on hyperthermophiles is focused on the impressive thermostability of their enzymes and their possible use in industrial processes. This thermostability is also responsible for the ease with which their proteins and macromolecular complexes crystallize under laboratory conditions, raising additional interest that led to develop large structural genomics projects (Jenney and Adams 2008).
Applications The high stability of enzymes under high temperatures (there are enzymes active above 125 C) is associated with an increase in compactness that results in increased
resistance to additional stresses such as the presence in the reaction of detergents or organic solvents (Vieille and Zeikus 2001). Such combination of resistances fit specific applications in different fields. For example, DNA polymerases from hyperthermophiles (Pyrococcus sp, Thermococcus sp) are commonly used to amplify DNA sequences with high fidelity. Other enzymes from hyperthermophiles are or have the potential to be used in industrial processes for which high temperatures represent an added value, such as starch and cellulose hydrolysis and further sugar modification (endoglucanases, glucosidases, xylanases, amylases, glucoamylases, pullulanases, etc.), proteases for detergents, lipases and esterases for biocatalysis, oxidoreductases like alcohol or glutamate dehydrogenases for biotransformations, C-C bonding enzymes (aldolases, transketolases), nitrile degrading enzymes, etc. (Antranikian 2008). Additional applications in specific but robust biosensor devices or for nanotechnology are also in development. In addition to the enzymes, hyperthermophiles produce special kinds of compatible solutes such asmannosyl glycerate, cyclic 2,3-biphosphoglycerate, diglyceryl phosphate in relevant amounts, some of which have been shown to confer thermoprotection and protection against desiccation to mesophilic enzymes (Santos et al. 2008).
Future Directions Given the biological and applied interest of hyperthermophiles, future research in the field will develop several lines of interest. Their ancient nature and extreme form of living requires a comprehensive physiological study of at least a pair of model organisms, for which a reliable system for genetic manipulation should be developed. Present developments on Thermococcus kodakaraensis and Sulfolobus solfataricus are at the frontlines of this effort. The construction of efficient systems for the overexpression of genes in such models will widen the number of hyperthermophilic enzymes available for applications that at present are hidden because of the inability to produce them in mesophilic hosts. Such expression systems will also be required to crystallize some of these proteins and complexes. Also, the metagenomic approaches on hyperthermophilic environments will open up a new world of enzymes having biotechnological potential to help to optimize existing processes and develop new ones. Finally, molecular ecology methods in such environments will likely unveil the presence of new non-cultivable hyperthermophiles that could uncover some phylogenetic surprises. Hyperthermophiles are of special interest for Astrobiology because high temperature processes are important factors in the development of
Hypolithic
planetary bodies; thus, the scenarios in which life can develop in the universe increase correspondingly.
See also ▶ Autotroph ▶ Autotrophy ▶ Black Smoker ▶ Chemolithoautotroph ▶ Deep-Sea Microbiology ▶ Deep-Subsurface Microbiology ▶ Hot Spring Microbiology ▶ Hot Vent Microbiology ▶ Hydrothermal Environments ▶ Hydrothermal Vent Origin of Life Models ▶ Osmolite ▶ Thermophile
References and Further Reading Antranikian G (2008) DNA-binding proteins and DNA topology. In: Robb F, Antranikian G, Grogan D, Driessen A (eds) Thermophiles, biology and technology at high temperatures. CRC, Boca Raton, pp 113–160 Blo¨chl E, Rachel R, Burggraf S, Hafenbradl D, Jannasch HW, Stetter KO (1997) Pyrolobus fumarii, gen. and sp. nov., represents a novel group of archaea, extending the upper temperature limit for life to 113 C. Extremophiles 1:14–21 Brock TD (1978) Thermophilic microorganisms and life at high temperatures. Springer, Berlin/Heidelberg/New York Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P (2006) Toward automatic reconstruction of a highly resolved tree of life. Science 311(5765):1283–1287 Jenney FE Jr, Adams MW (2008) The impact of extremophiles on structural genomics (and vice versa). Extremophiles 12:39–50 Kashefi K, Lovley DR (2003) Extending the upper temperature limit for life. Science 301:934 Nakagawa S, Takai K (2006) The isolation of thermophiles from deep-sea hydrothermal environments. In: Rainey FA, Oren A (eds) Extremophiles. Methods in microbiology 35:55–91. Elsevier Page´ A, Tivey MK, Stakes DS, Reysenbach AL (2008) Temporal and spatial archaeal colonization of hydrothermal vent deposits. Environ Microbiol 10:874–884 Sandman K (2008) DNA-binding proteins and DNA topology. In: Robb F, Antranikian G, Grogan D, Driessen A (eds) Thermophiles, biology and technology at high temperatures. CRC, Boca Raton, pp 279–289 Santos H, Lamosa P, Faria TQ, Pais TM, Lopez de la Paz M, Serrano L (2008) DNA-binding proteins and DNA topology. In: Robb F, Antranikian G, Grogan D, Driessen A (eds) Thermophiles, biology and technology at high temperatures. CRC, Boca Raton, pp 9–24 Say RF, Fuchs G (2010) Fructose 1, 6-bisphosphate aldolase/phosphatase may be an ancestral gluconeogenic enzyme. Nature 464:1077–1081 Stetter KO (2006) History of discovery of the first hyperthermophiles. Extremophiles 10:357–362 Stetter KO, Thomm M, Winter J, Wildgruber G, Huber H, Zillig W, Janecovic D, Ko¨nig H, Palm P, Wunderl S (1981) Methanothermus fervidus, sp. nov., a novel extremely thermophilic methanogen
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isolated from an Icelandic hot spring. Zbl. Bakt Hyg I Abt Orig C2:166–178 Vieille C, Zeikus GJ (2001) Hyperthermophilic enzymes: sources, uses, and molecular mechanisms for thermostability. Microbiol Mol Biol Rev 65:1–43 Zillig W, Stetter KO, Scha¨fer W, Janekovic D, Wunderl S, Holz I, Palm P (1981) Thermoproteales: a novel type of extremely thermoacidophilic anaerobic archaebacteria isolated from Icelandic solfataras. Zbl Bakt Hyg I Abt Orig C2:205–227
Hypolithic CHARLES S. COCKELL Geomicrobiology Research Group, PSSRI, Open University, Milton Keynes, UK
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Synonyms Subliths
Keywords Communities, habitats
cyanobacteria,
extremophiles,
lithic
Definition Hypoliths are organisms or communities of organisms that live on the underside of rocks or at the rock–soil interface.
Overview In hot and cold deserts, the underside of rocks can provide a refugium for microorganisms, both photosynthetic (cyanobacteria and algae) and non-photosynthetic (Cameron and Blank 1965; Schlesinger et al. 2003). The organisms are referred to as “hypoliths.” The community is termed “hypolithon” (following the terminology for endoliths by Golubic et al. 1981). The photosynthetic components of hypoliths include organisms adapted to extreme rock habitats including Chroococcidiopsis and Gloeocapsa species. Hypoliths often display well-defined “bands” of growth on the underside of rocks or in the case of thin rocks, complete colonization of their underside. As photosynthetic microorganisms provide a source of carbon for heterotrophic microorganisms, the hypolithic colonization of translucent rocks by photosynthetic microorganisms can indirectly benefit nonphotosynthetic microorganisms (Smith et al. 2000). Hypolithic colonization of quartz stones has now been documented in many locations, for example: the Vestfold Hills, ▶ Antarctica (Smith et al. 2000), the Mojave Desert,
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USA (Schlesinger et al. 2003), the Negev desert, Israel (Berner and Evenari 1978), the Namib Desert, Africa (Budel and Wessels 1991), and the deserts of China (Warren-Rhodes et al. 2007). On the underside of rocks, organisms gain a range of advantages compared to surface organisms such as protection from UV radiation (Cockell et al. 2003). In the Antarctic polar desert, the stones warm the hypolithic biota and, during the summer, mitigate freeze–thaw (Broady 1981). Quartz stones in the Vestfold Hills, Antarctica could provide temperatures ten degrees in excess of ambient air temperatures (Smith et al. 2000). For photosynthetic organisms, the limiting factor for the thickness of rocks under which photosynthetic microorganisms can grow is set by the thickness that reduces light levels to below those required for ▶ photosynthesis. Investigations on the colonization of the underside of translucent flint in the Negev desert showed that the more translucent flint types were colonized in a greater number of instances compared to less translucent flints (Berner and Evenari 1978). Less than 0.01% of incident light penetrated to below 40 mm in quartz rocks in the Negev (Berner and Evenari 1978), and light was reduced to 0.08% of incident under 25 mm of quartz rock from the Mojave desert (Schlesinger et al. 2003). At these depths colonization became limited. Broady found similarly large attenuations under Antarctic quartz rock (Broady 1981). The moisture availability in the hypolithic habitat is also an important determinant of which rocks are colonized and their ecological distribution in the environment (Warren-Rhodes et al. 2007). Hypoliths are also found under opaque rocks in polar deserts (Cockell and Stokes 2004). Greater than 90% of rocks examined were
Surface
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colonized. Periglacial processes can cause movements in rocks, which in turn create openings around the edges of rocks that allow the penetration of photosynthetically active radiation to the underside of rocks. These processes allow for the colonization of rocks with productivity of these communities potentially as high as the aboveground productivity accounted for by plants (see Fig. 1).
See also ▶ Antarctica ▶ Cryptoendolithic ▶ Extremophiles ▶ Photosynthesis
References and Further Reading Berner T, Evenari M (1978) The influence of temperature and light penetration on the abundance of the hypolithic algae in the Negev Desert of Israel. Oecologia 33:255–260 Broady PA (1981) The ecology of sublithic terrestrial algae at the Vestfold Hills, Antarctica. Brit Phycol J 16:231–240 Budel B, Wessels DCJ (1991) Rock inhabiting blue-green algae from hot arid regions. Archiv fur Hydrobiology 92:385–398 Cameron RE, Blank GB (1965) Soil studies – microflora of desert regions VIII. Distribution and abundance of desert microflora. Space Programs Summary 4:193–202 Cockell CS, Stokes MD (2004) Widespread colonization by polar hypoliths. Nature 431:414 Cockell CS, Rettberg P, Horneck G, Scherer K, Stokes DM (2003) Measurements of microbial protection from ultraviolet radiation in polar terrestrial microhabitats. Polar Biol 26:62–69 Golubic S, Friedmann I, Schneider J (1981) The lithobiontic ecological niche, with special reference to microorganisms. J Sed Petrol 51:0475–0478 Schlesinger WH, Pippen JS, Wallenstein MD, Hofmockel KS, Klepeis DM, Mahall BE (2003) Community composition and photosynthesis by photoautotrophs under quartz pebbles, Southern Mohave Desert. Ecology 84:3222–3231 Smith MC, Bowman JP, Scott FJ, Line MA (2000) Sublithic bacteria associated with Antarctic quartz stones. Antarct Sci 12:177–184 Warren-Rhodes KA, Rhodes KL, Boyle LN, Poiting SB, Chen Y, Liu SJ, Zhuo PJ, McKay CP (2007) Cyanobacterial ecology across environmental gradients and spatial scales in China’s hot and cold deserts. FEMS Microbiol Ecol 61:470–482
Colonized underside
No colonization (Light extinguished)
Hypoxanthine Synonyms 6-Hydroxypurine
Hypolithic. Figure 1 An example of a community of photosynthetic hypoliths inhabiting the underside of opaque rocks in the Canadian High Arctic
Definition Hypoxanthine (C5H4N4O, molecular weight: 136.11) is a naturally occurring purine base. It is occasionally
HZE Particle
found as a constituent of nucleic acids such as tRNA. The half-life of hypoxanthine to hydrolysis is 12 days at 100 C, and 5,000 years at 0 C at pH 7. It has a UV absorption maximum at 249.5 nm (pH 7). It has been found in the ▶ Murchison meteorite and is synthesized by hydrolysis of ▶ adenine, in HCN polymerizations, and in discharge experiments using gas mixtures such as CO–N2–H2O. The ribonucleoside of hypoxanthine is named inosine.
See also ▶ Adenine ▶ HCN Polymer ▶ Hydrogen Cyanide ▶ Meteorite (Murchison) ▶ Nucleic Acids ▶ Purine Bases
HZE Particle Synonyms Heavy charged particle; Heavy ion; Heavy nucleus; Heavy primary
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Definition HZE particles are a component of cosmic radiation consisting of energetic heavy nuclei (atomic number 3 or greater), so named for their high (H) atomic number (Z) and high energy (E). They contribute to roughly 1% of the flux of galactic cosmic radiation, and to an even lesser fraction to ▶ solar particle events. Because of the difficulty of adequate shielding and the special nature of HZE particle-produced lesions, these particles are considered a major hazard to living beings in space, especially outside Earth’s magnetosphere, that is, human exploratory missions and interplanetary transfer of life by natural processes.
See also ▶ Biostack ▶ Cosmic Rays in the Heliosphere ▶ DNA Damage ▶ Ionizing Radiation (Biological Effects) ▶ Linear Energy Transfer ▶ Lithopanspermia ▶ Panspermia ▶ Radiation Biology ▶ Solar Particle Events
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I IAF Synonyms International Astronautical Federation
Definition In September 1950, Alexandre Ananoff from the “Groupement astronautique franc¸ais” convened the first International Astronautical Congress (IAC). Eighteen representatives from the astronautical societies of eight countries (Argentina, Austria, Denmark, France, Germany, Spain, Sweden, UK), decided to set up an association to secure future international cooperation. The International Astronautical Federation (IAF) was formally established at the second IAC organized by the British Interplanetary Society (BIS), in London in 1951. Every year since then, the Federation together with the International Academy of Astronautics (IAA) and the International Institute of Space Law (IISL), has organized the International Astronautical Congress, which encourages the advancement of knowledge about space. It is held in connection with a major space exhibition. IAC is now recognized as the main international conference on space activities. More than 1,600 technical papers on space programs and research activities around the world are presented in various sessions. The IAF maintains an online archive of past congress papers and presentations to provide members with a historical record of global space activities. In 2010, the IAF had 205 members from 58 countries. The organizations involved include astronautical and other professional societies, space agencies and international organizations, companies from the space industry, universities and other research institutions, and nonprofit organizations interested in space matters.
▶ Saturn. Its distance to Saturn is 3,560,000 km (or 59 Saturnian radii), and its diameter is 1,440 km. Its density is 1.02 g/cm3, indicating a very low rock/ice ratio. Iapetus is unique in its albedo distribution, which is very low on Iapetus’ leading side and ten times higher on the opposite side. It has been proposed that the dark material could be due to the permanent accretion of matter coming from the neighboring satellite ▶ Phoebe, which is also very dark. Another possible explanation is that the low albedo of Iapetus could be a result of organic deposits formed from methane ice impacted by dust particles.
See also ▶ Phoebe ▶ Saturn
IAU Synonyms International Astronomical Union
Keywords Astronomy, ICSU
Definition The International Astronomical Union (IAU) was founded in 1919 with the mission of promoting and safeguarding the science of astronomy in all its aspects through international cooperation. More than 10,000 professional astronomers (active in professional research and education) from all over the world, at the Ph.D. level and beyond, are IAU members. The IAU is financed by academies or equivalent institution of its 70 member countries.
Overview
Iapetus Definition Iapetus was discovered in 1671 by Giovanni Domenico Cassini; it is the outermost mid-sized icy satellite of
The scientific and educational activities of the IAU are organized by its 12 scientific Divisions and their more than 50 specialized Commissions (for example Commission 51 is devoted to Bioastronomy, Commision 34 to Interstellar Matter and Astrochemistry, and Commission 16 to the Physical Study of Planets & Satellites) that cover
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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the full spectrum of Astronomy. The long-term policy of the IAU is defined by the triennial General Assembly and implemented by the Executive Committee, while the dayto-day operations are directed by the IAU officers. The IAU Secretariat is hosted by the Institut d’Astrophysique de Paris, France. The key activity of the IAU is the organization of scientific meetings, including nine international IAU Symposia every year, with additional symposia and other meetings every 3 years at the General Assembly. The IAU is also responsible for the definition of fundamental astronomical and physical constants; unambiguous astronomical nomenclature; promotion of educational activities in astronomy; and informal discussions on the possibilities for future international large-scale facilities. Furthermore, the IAU serves as the internationally recognized authority for assigning designations to celestial bodies and surface features on them. The IAU is a member of the International Council for Science (ICSU).
Ice Definition In chemistry, ice refers to water in a solid crystalline phase, the common form of which encountered at normal terrestrial temperatures and pressures is known as Ice Ih. There are 15 known crystalline phases of water ice, most of which are only known to occur at very high pressures, pressures which may be encountered in the interiors of large icy planets or moons, although, one other phase, Ice XI has been observed in some Antarctic glaciers. Ice Ih is the only known nonmetallic substance which expands as it freezes, being some 9% less dense than liquid water. This is due to hydrogen bonding in the crystal lattice which forces the molecules into positions farther apart than normally occurs in the liquid phase. This change in volume contributes to water’s activity in the mechanical weathering of minerals, and also causes ice to float when frozen. In planetary science, and more generally in astronomy, an ice refers to a volatile (a species with a low boiling point, such as N2, water, CO2, NH3, H2, CH4, and SO2) with a melting point above 100 K. The solid phases of these ices can be glasses (which are technically extremely viscous liquid phases) or crystalline solids, depending on the temperature, pressure, and rate at which the ice forms. Ices observed or expected to be present on the mantles of interstellar dust grains include, in addition to water, CO, CO2, CH3OH, CH4, NH3, O2, and N2. Ices reported on the
surfaces of satellites in the outer solar system and of Kuiper Belt Objects include H2O, CH4, N2, CO, CH3OH, and perhaps other volatile organics such as ethane.
See also ▶ Comet ▶ Enceladus ▶ Europa ▶ Hydrogen Bond ▶ Interstellar Ices ▶ Kuiper Belt ▶ Volatile ▶ Water
References and Further Reading Klinger J (1983) Extraterrestrial ice. A review. J Phys Chem 87(21):4209–4214 Klinger J, Benest D, Dollfus A, Smoluchowski R (eds) (1985) Ices in the solar system. Springer, New York Hobbs PV (2010) Ice physics. Oxford University Press, New York
Ice-albedo Instability ▶ Snowball Earth
Ice Line ▶ Snow Line
IDPs ▶ Interplanetary Dust Particles ▶ Meteorites
IEC ▶ Ion-Exchange Chromatography
IEP ▶ Isoelectric Point
Imaging
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IKI is located in Moscow and has a staff of some 290 scientists in the fields of astrophysics, planetary sciences, solar-terrestrial physics, cosmic plasma physics, and geophysics. Among its successes relevant to astrobiology were the ▶ Vega spacecraft to ▶ Comet Halley.
Definition
See also
An igneous rock forms by solidification of magma. There are two main types: volcanic rocks (subaerial) that crystallize or solidify from erupted lava or fragmental (pyroclastic) material, and plutonic or intrusive rock that crystallizes from magma that did not reach the surface. The two types are distinguished by their grain size: intrusive rocks are coarser grained because they crystallize slowly in an thermally insulated setting whereas volcanic rocks are fine grained or glassy because they cool rapidly following eruption at the Earth’s surface. Common minerals in igneous rocks are olivine, pyroxene, amphibole, mica, feldspar, and quartz. Common intrusive rock types include peridotite, ▶ gabbro, and ▶ granite; their volcanic equivalents are komatiite, ▶ basalt, and rhyolite.
▶ Vega 1 and 2 Spacecraft
Igneous Rock Synonyms
See also ▶ Basalt ▶ Cryovolcanism ▶ Gabbro ▶ Granite ▶ Hydrothermal Environments ▶ Mafic and Felsic ▶ Mantle ▶ Mid-Ocean Ridges ▶ MORB ▶ Plate Tectonics ▶ Volcano
IKI Synonyms Russian Space Research Institute
Definition IKI, the Russian Space Research Institute (Russian: Инcтитут кocмичecкиx иccлeдoвaний Poccийcкoй Aкaдeмии Haук), is an institute of the Russian Academy of Sciences that was founded in 1965 as the Space Research Institute of the USSR Academy of Sciences and renamed in 1992. It is the leading Russian organization for space research and has frequently collaborated on missions with ▶ NASA, ▶ ESA, and other national space agencies.
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Imaging DANIEL ROUAN LESIA, Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot, Meudon, France
Keywords Adaptive optics, CCD, Camera, Direct detection, telescope
Definition Imaging is the set of techniques used for obtaining images of one or several astronomical bodies or fields using a twodimensional detector, computerized techniques, and smart optical techniques.
Overview In astronomy, imaging is in most cases the first observational approach to discovering a new object, the image being required to clarify structure and morphology, before using the more physical tool of spectroscopy. Shape, size, decomposition into elements, arrangement of subparts, measurement of displacements, and discrimination of photometric variations belong to the set of information provided by the image. From the planets in the solar system to the identification of gigantic filamentary structures or sheets in the universe, the image has played a major role in astronomy. Improving the quality of the image has been and remains today more than ever, a powerful drive in instrumental research: two-dimensional detectors, high angular resolution, ▶ adaptive optics, high dynamical range, and postprocessing techniques are fields of intense activity. One notes that the direct detection of exoplanets is certainly among the most demanding topics in terms of angular resolution and high contrast. Detectors: In the last 30 years, the photographic technique has been largely replaced by digital sensors such as ▶ CCDs and now CMOS chips (Complementary MetalOxide-Semiconductor, a technology used for constructing
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Imaging. Figure 1 The Megacam camera developed by CEAFrance and installed at the prime focus of the Canada-FranceHawaii Telescope, behind a wide field corrector. This huge camera includes 36 CCDs of 2,048 4,612 pixels each, arranged side by side, leading to a total of 340 Mpixels
integrated circuits). Giant cameras associating several tens of CCD chips can feature up to 500 Mpixels and cover extra large fields on the sky, as demonstrated for instance by Megacam, a camera installed at the Canada-FranceHawaii telescope (Fig. 1). CCDs provide numerical images, which can easily be processed by computers. They also offer a very good detection efficiency in a large domain of wavelength. Beyond l = 1 mm, infrared arrays based on the CMOS technique have about the same advantages. Recently large arrays of infrared bolometers have been produced, allowing imaging in even the submillimeter domain. A certain degree of computer correction for atmospheric and instrumental effects allows sharpening up the image and improving the contrast. Multiple digital images can also be combined to further enhance the signal to noise and suppress artifacts. Thanks to the adaptive optics technology that can negate to a large extent the atmospheric blurring, image quality can approach the theoretical resolution capability of the telescope, that is, angular details as small as y = l/D, where D is the diameter of the telescope and l the wavelength. For l = 2 mm and a D = 8 m, this corresponds to a length of 10 km on Venus’ surface, or the orbit of Mercury seen around a star at 300 light-years. In space, almost perfect conditions also provide excellent angular resolution, despite a more moderate telescope size, as illustrated by the stunning images of the Hubble Space Telescope. The future James Webb Space
Imaging. Figure 2 High contrast imaging. Thanks to the combination of dedicated optical devices (here a fourquadrants phase mask coronagraph) and imaging techniques (here differential rotation) it is possible to reach an extremely high contrast ratio at the laboratory: a fake planet 0.15 billion times fainter than an artificial star (behind the central occulted disk) is perfectly seen on this image obtained at LESIA, Observatoire de Paris
Telescope with its 6.5 m diameter telescope will soon provide similar exquisite image quality but in the infrared domain. As regards high contrast imaging, the recent pressure put by the search for exoplanets was the cause of an explosion of ideas and new concepts to mitigate the problem of the huge difference in brightness between a star and the planets orbiting it. New types of coronagraphs to hide the starlight, of wavefront control to reduce speckles, and of pupil shaping to suppress the wings of the point spread function have been proposed and tested. Also, differential techniques based on chromatic differences on image rotation or on polarization properties have been refined and greatly improve the detection capability. Today in the laboratory, a contrast of 109 is reached at an angular distance of a few l/D only (Fig. 2). The next decade will see those techniques implemented in ground-based or space instruments.
See also ▶ Adaptive Optics ▶ CCD ▶ Coronagraphy ▶ Telescope
Impact Basin
References and Further Reading Aime C (2007) Optical techniques for direct imaging of exoplanets. CR Phys 8:298 Howell SB (2006) Handbook of CCD astronomy: Cambridge observing handbooks for research astronomers Vol 5, 2nd edn. Cambridge University Press, Cambridge, UK, ISBN 0-52185-215-3 Le´na P, Lebrun F, Mignard F (1998) Observational astrophysics: Astronomy and astrophysis library. Springer, Berlin, Germany, XV þ 512 p. ISBN 3-540-63482-7 Starck J-L, Murtagh F (eds) (2006) Astronomical image and data analysis. Astronomy and astrophysics library. Springer, New York
2, 4-Imidazolelidinedione ▶ Hydantoin
Imidogen
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Impact (Hit and Run) Definition During terrestrial planet formation, so-called hit-and-run impacts are giant impacts that do not result in a net increase in the mass of at least one of the planetary embryos involved in the collision. Hit-and-run impacts generally imply high relative velocities between bodies and/or oblique impact angles. In such cases, the larger one does not accrete the smaller body, but the larger one could actually be eroded and sometimes severely traumatized by the encounter. A significant amount of debris may be created in hit-and-run impacts.
See also ▶ Giant Impact ▶ Late-Stage Accretion
References and Further Reading Asphaug E et al (2006) Nature
Synonyms NH; Nitrogen hydride
Definition This diatomic radical, containing nitrogen and hydrogen, has been detected in both the diffuse ▶ interstellar medium, where it is seen in absorption against the light of background stars, and in denser gas toward the center of our ▶ Milky Way galaxy. In the latter case the relative abundances of NH3, NH2, and NH have been interpreted in terms of low-velocity shock activity.
History NH was first detected at near ultraviolet wavelengths in 1991 by D. M. Meyer and K. C. Roth. The subsequent observations of the Galactic center were made with the Infrared Space Observatory (ISO).
See also ▶ Interstellar Medium ▶ Milky Way
References and Further Reading Goicoechea JR, Rodrı´guez-Ferna´ndez NJ, Cernicharo J (2004) The FarInfrared Spectrum of the Sagittarius B2 Region: Extended Molecular Absorption, Photodissociation, and Photoionization. Astrophys J 600:214–233
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Impact Basin Definition An impact basin is a large complex ▶ impact crater. The threshold diameter to distinguish between craters and basins is approximately 150–200 km. In general, basins are characterized by two or more concentric rings, which are ridges or scarps facing toward the basin. One of these rings is the main rim that borders the cavity from which material was excavated and ejected during the impact. Most basins are heavily degraded or have been covered by younger material. All known basins are old impact features created during the first 800 million years of the planet or satellite on which they are found. Examples are Caloris basin on ▶ Mercury, the Orientale basin on the ▶ Moon, and the Hellas basin on ▶ Mars.
See also ▶ Chronology, Cratering and Stratography ▶ Crater, Impact ▶ Facula, Faculae ▶ Mars ▶ Mercury ▶ Moon, The
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Impact Melt Rock
Impact Melt Rock
Impact, Probability
Definition
Definition
An impact melt rock is a type of rock typically found in impact structures. It is an ▶ impactite consisting of a mixture of solid and melt fragments all derived from the target rock, floating in a glassy, microcrystalline, or recrystallized matrix. The solid fragments may contain shocked minerals. An impact melt rock results from the cooling of the melt pool that formed within a crater because of the high temperature and pressure generated during the impact. The largest craters such as Sudbury (Ontario, Canada), ▶ Chicxulub (Mexico), Popigai (Siberia), or Manicouagan (Que´bec, Canada) contain a very large volume of impact melt. In the case of Sudbury, it also hosts important ore deposits of nickel.
For purposes of ▶ planetary protection, the probability of impact is the probability, determined by a sequential evaluation of possible spacecraft failure modes, that a spacecraft might impact a planetary body under nominal and off-nominal conditions.
See also ▶ Chicxulub Crater ▶ Crater, Impact ▶ Gunflint Formation ▶ Impactite
Impact Parameter Definition The impact parameter is the projected distance between the axis carrying the vector velocity of a projectile and the center of the target that the projectile is approaching (see Fig. 1).
Impactite Definition Impactite is a term used to describe rocks formed during a meteorite impact on a planetary body. The term covers the lithologies such as ▶ breccia, ▶ suevite, or ▶ impact melt-rock found within or at close proximity of an impact structure. Monomictic breccias contain fragments of a single rock type; polymictic breccias contain several rock types. Each is composed of chaotic assemblages of solid and shocked fragments, or previously molten fragments, of the target lithologies. Suevite is a breccia containing melt and solid particles surrounded by a fine clastic matrix, first defined in the Ries crater (Germany). Distal impactite, composed of fine ejected material (spherules, shocked minerals), may be widespread in the case of large impacts such as the one that formed the ▶ Chicxulub Crater.
See also ▶ Breccia ▶ Chicxulub Crater ▶ Crater, Impact ▶ Impact Melt Rock ▶ KT Boundary ▶ Suevite
In Vitro Evolution ▶ Evolution, In Vitro
Inactivation Definition Impact Parameter. Figure 1 Definition of the impact parameter of a projectile directed toward a target
In ▶ planetary protection, the term “inactivation” expresses the result of a process that renders microorganisms
Infrared Astronomical Satellite
incapable of re-entering a proliferative state. The parameters (time, temperature, concentration, dose, . . .) of a process which allow to inactivate 90% of the population of a given microorganism define the ▶ D-value.
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Indigenous ▶ Endogenicity
See also ▶ Bioburden Reduction ▶ Disinfection ▶ D-Value ▶ Pasteurization ▶ Sterilization
Inelastic Photon Scattering ▶ Raman Scattering
Inclination (Astronomy) Definition The inclination of an object in the solar system is the angle between the orbital plane of the object (planet, comet, or asteroid) and the plane of the ecliptic. In binary systems and especially in star/planet systems, it corresponds to the angle i between the orbital plane and the plane of the sky. This angle is especially important in the ▶ radial velocity technique used for detecting exoplanets, since the derived planetary mass is undetermined by a factor 1/sin (i): if the orbital plane is close to the plane of the sky, the actual mass can be much larger than the raw value.
See also ▶ Orbit ▶ Radial Velocity
Indeterminacy ▶ Chance and Randomness
Indian Space Research Association ▶ ISRO
Indian Space Research Organization ▶ ISRO
Infrared Astronomical Satellite Synonyms IRAS
Definition The Infrared Astronomical Satellite or IRAS was the first observatory to perform an all-sky survey at infrared wavelengths. It was equipped with a telescope of 60 cm diameter and with detectors operating at 12, 25, 60, and 100 mm with angular resolution of 30 arc sec and 2 arc min at 12 and 100 mm, respectively. Launched January 25, 1983 it was operating up to November 21, 1983. During its primary mission, which was to obtain a complete map of the sky, it discovered 350,000 point-like sources, increasing the number of cataloged astronomical sources by about 70%. IRAS discoveries included a disk of dust grains around the star Vega, six new comets, and very strong infrared emission from interacting galaxies, as well as wisps of warm dust called infrared cirrus which are found in almost every direction of space. IRAS also revealed for the first time the core of our galaxy, the Milky Way. In addition to the infrared sky maps, IRAS was also equipped with a low-resolution spectrometer that was used to observe thousands of objects in the midinfrared. The outstanding scientific results obtained by IRAS have opened a new view of the universe and motivated new infrared space facilities such as ▶ ISO, ▶ Spitzer, and ▶ Herschel.
History The project was initiated in 1975 as a joint program of the USA, the Netherlands, and the UK. Launched in January 1983, it produced infrared data for a period of ten months and covered more than 96% of the sky.
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See also
and satellites are often used to carry instruments. IRAS, ISO, and Spitzer are the space instruments that brought the most spectacular results in the field. The infrared domain is of special importance to study planets, since it’s where thermal emission peaks and molecules exhibit many spectral features. Note that the infrared range is generally sub-divided in three domains: near-infrared (near-iR): l = 1–5 mm; mid-infrared (mid-IR) l = 5–25 mm; Far-infrared (Far-IR): l > 25 mm.
▶ Herschel ▶ Infrared Astronomy ▶ ISO ▶ Spitzer Space Telescope
References and Further Reading http://irsa.ipac.caltech.edu/IRASdocs/iras.html http://www.sron.rug.nl/irasserver/irasserverman.html
See also ▶ Electromagnetic Spectrum ▶ Infrared Astronomical Satellite ▶ Infrared Space Observatory ▶ Infrared Spectroscopy ▶ Spitzer Space Telescope
Infrared Astronomy Synonyms Near-infrared (Near IR); Far-infrared (FAR IR)
Definition Infrared astronomy is the branch of observational astronomy which studies celestial objects through their infrared radiation, that is, at wavelengths longer than 0.75 mm and shorter than 400 mm. The domain is not strictly defined and some astronomers consider that it begins at 1 mm where CCDs, the most universally used detectors in the visible domain, are no longer sensitive. Infrared astronomy is considered as part of optical astronomy because technologies and methods (incoherent detection, classical optics, solid-state digital detectors) are globally the same as in the visible domain. One peculiarity of infrared astronomy is the strong background emission of the environment (atmosphere, telescope) that requires dedicated differential techniques. Another one is the absorption by the atmosphere (see Fig. 1) in a large fraction of the wavelength domain: this explains why balloons, airplanes,
Infrared Excess STEVEN STAHLER Department of Astronomy, University of California, Berkeley, CA, USA
Keywords Circumstellar disk, protostars, T Tauri star
Definition Many ▶ pre-main-sequence stars emit substantial amounts of energy at infrared and longer wavelengths. To the degree that the flux at each wavelength exceeds
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Infrared Astronomy. Figure 1 Transmission of the atmosphere in the infrared range. The Earth’s atmosphere absorbs the infrared light in the largest part of the wavelength domain: only in a few windows is the transmission good enough to allow observations from the ground, particularly with telescopes on the top of high mountains (Mauna Kea is a summit at 4,200 m on the island of Hawaii, the best infrared site today). At higher altitudes (aircraft), the conditions are improving and broader windows are open
Infrared Space Observatory
the corresponding blackbody value (corresponding to the temperature of the stellar photosphere), the star is said to have an infrared excess. At least half of T Tauri stars have infrared excesses. Much of this emission arises from heated dust grains embedded in circumstellar disks. Other T Tauri stars have no infrared excess. Presumably, these stars have no disks, although they are of similar age. All stars must lose their disks by the time they reach the ▶ main sequence, whose stars do not have an infrared excess.
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infrared and longer wavelengths. Some of these latter sources are believed to be true protostars, objects deriving their luminosity from the infall of cloud material onto the stellar surface.
See also ▶ Interstellar Dust ▶ Main Sequence ▶ Pre-Main-Sequence Star ▶ T Tauri Star
Overview
References and Further Reading
Stars radiate over a broad range of wavelengths. A plot of radiation intensity as a function of wavelength, when covering a substantial range in the latter, is known as a spectral energy distribution. In such a plot, each intensity is measured using a relatively broad filter, which admits radiation over a wavelength range of roughly 1,000 A˚ (100 nm). A main-sequence star radiates like a blackbody. That is, its spectral energy distribution resembles, at least approximately, the Planck curve corresponding to the star’s surface temperature. The spectral energy distributions of young stars, however, depart sharply from the Planck curve. These objects are often surrounded by considerable amounts of interstellar dust, which absorbs and reddens the radiation in a wavelength-dependent manner. The raw intensities must therefore be corrected, a procedure known as dereddening. Even after dereddening, many T Tauri stars exhibit substantial flux above a blackbody at infrared and longer wavelengths. In these so-called classical T Tauri stars, the total excess flux emitted in the infrared can be a substantial fraction of the total stellar luminosity. Other T Tauri stars, the so-called weak-lined class, have no such excess. T Tauri stars are pre-main-sequence objects of roughly solar mass and below. That is, they are contracting slowly and generating energy through that contraction, but not yet capable of fusing hydrogen into helium. Detailed analysis of the infrared excess emission from classical T Tauri stars shows that it arises from radiation by heated dust grains. These grains must be relatively close to the star. Most are embedded in circumstellar disks, which have been imaged directly in some cases. It is surprising that weak-lined T Tauri stars, which have similar ages as the classical ones, apparently lack these disks. Pre-main-sequence objects of larger mass, from about 2 to 10 solar masses, are known as Herbig Ae and Be stars. Many of these also exhibit large infrared excesses. There are also many stars, presumably of lower mass, which are so deeply embedded in dust that all their emission is at
Beckwith SVW (1999) In: Lada CJ, Kylafis ND (eds) The origin of stars and planetary systems. Reidel, Dordrecht, p 579 Bertout C (1989) Annual reviews of astronomy and astrophysics 27:351
I Infrared Microscopy ▶ Infrared Spectroscopy
Infrared Space Observatory MARTIN F. KESSLER European Space Agency (ESA), European Space Astronomy Centre (ESAC), Madrid, Spain
Synonyms ISO
Keywords ISO, infrared astronomy, organic molecules in space, space astronomy, satellite, water
Definition The Infrared Space Observatory (ISO), a project of the European Space Agency (▶ ESA) with the participation of ▶ NASA and ISAS (now ▶ JAXA), was the first cryogenic space-borne infrared observatory for astronomy. It operated from 1995 to 1998 and made detailed imaging, photometric, and spectroscopic observations at wavelengths from 2 to 200 mm of all kinds of astronomical objects from ▶ planets and ▶ comets in our own Solar System right out to distant extragalactic sources. Its contributions to astrobiology include the detection in space of ▶ water, ▶ complex organic molecules, and a rich variety of ices (Fig. 1).
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IR path Sunshield with solar cells
Payload module (cryostat)
Startrackers Service module (for electrical power, attitude control and telecommunication)
Interface with ariane
Superfluid helium tank
Telescope with scientific instruments + star sensor
Infrared Space Observatory. Figure 1 Schematic of ISO
Overview The Infrared Space Observatory (ISO), selected by ESA in 1983, made a detailed exploration of the infrared universe. Industrial design and development started in 1986 with Ae´rospatiale (now part of Thales Alenia Space) as prime contractor. An Ariane 44P vehicle launched ISO in November 1995 and it operated until May 1998 in an elliptical 24-h orbit with perigee/apogee of 1,000/70,000 km. ISO’s Payload Module was essentially a large cryostat, containing 2,300 l of superfluid liquid helium, cooling four scientific instruments and a telescope of RitcheyChre´tien design with a 60 cm diameter primary mirror to temperatures of 2–4 K. The two ▶ spectrometers (Short Wavelength Spectrometer [SWS], Long Wavelength Spectrometer [LWS]), a camera (ISOCAM), and an imaging photo-polarimeter (ISOPHOT) jointly covered wavelengths of 2.5–240 mm with spatial resolutions ranging from 1.5–90 arcs s, depending on wavelength. The Service Module provided the traditional spacecraft services including 3-axis attitude stabilization at the arc-second level (Kessler et al. 1996). The mission was a technical, operational, and scientific success with most satellite systems operating far better than specifications and with its scientific results impacting practically all fields of astronomy. During its 29-month operational lifetime, ISO made over 30,000 individual observations. These data are publically available at http://iso.esac.esa.int/ida/.
ISO’s scientific legacy is summarized in various overviews and compilations (e.g., First ISO Results 1996; Kessler and Cox 1999; Cesarsky and Salama 2005). The mission made several significant contributions to astrobiology. ISO found water everywhere in the universe – in comets, ▶ Mars, the four giant planets and ▶ Titan, the interstellar medium (towards protostars and molecular clouds), evolved stars (both oxygen- and carbon-rich), and ▶ galaxies (e.g., Cernicharo and Crovisier 2005; Encrenaz 2008). ISO’s wide and uninterrupted wavelength coverage, free of telluric contamination, permitted it to make a detailed inventory of interstellar ices (including not only the ubiquitous CO2 but also 13CO2, CO, H2O, H2CO, CH4, CH3OH, CH4, OCS, . . .) and gas phase organic molecules (including H2O, CH4, C2H2, CH3, HCN, CO2, OH, . . .) as well as addressing the rich variety of ▶ Polycyclic Aromatic Hydrocarbon features and making an inventory of the reservoirs of the major elements (C, O, N, . . .) (e.g., Ehrenfreund and Charnley 2000; van Dishoeck 2004). In brief, ISO showed conclusively that space is full of the building blocks of life.
References and Further Reading Cernicharo J, Crovisier J (2005) Water in space: the water world of ISO. Space Sci Rev 119:29–69 Cesarsky C, Salama A (2005) ISO science legacy. Springer, Dordrecht Cox P, Kessler MF (1999) The Universe as seen by ISO. ESA SP-427 Noordwijk: ESTEC 1090 pp
Infrared Spectroscopy Ehrenfreund P, Charnley SB (2000) Organic molecules in the interstellar medium, comets and meteorites: a voyage from dark clouds to the early Earth. Ann Rev Astron Astrophys 38:427–483 Encrenaz T (2008) Water in the solar system. Ann Rev Astron Astrophys 46:57–87 First ISO Results (1996) A&A 315:L27–L400 Kessler MF et al (1996) The Infrared Space Observatory (ISO) Mission. Astron Astrophys 315:L27–L31 Van Dishoeck EF (2004) ISO spectroscopy of gas and dust: from molecular clouds to protoplanetary disks. Ann Rev Astron Astrophys 42:119–167
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is defined as the ratio of infrared energy passing through a sample (I) and irradiated infrared energy (background; I0): T ¼ I=I0 The absorbance is defined as: A ¼ log10 T ¼ log10 ðI=I0 Þ: The absorbance is linearly correlated with the concentration of the component of interest, known as Lambert– Beer’s Law: A ¼ edC;
Infrared Spectroscopy YOKO KEBUKAWA Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
Synonyms Fourier transform infrared micro-spectroscopy; Fourier transform infrared spectroscopy; FTIR; Infrared microscopy
Definition Infrared spectroscopy is a molecular-vibration spectroscopy using infrared light (0.7–1,000 mm; 14,000–10 cm1). Most molecular bonds are characterized by the absorption of specific frequencies of infrared radiation. The 0.7–2.5 mm (14,000–4,000 cm1) region is often called the nearinfrared, the 2.5–25 mm (4,000–400 cm1) region is called the mid-infrared, and the 25–1,000 mm (400–10 cm1) region is called the far-infrared.
Overview Many organic and inorganic materials absorb certain frequencies of infrared light. The absorption is caused by vibrations or rotations of molecules, at frequencies corresponding to the infrared region of the electromagnetic spectrum. Therefore, information about molecular species and structure can be obtained by irradiating samples in the infrared frequency range and subsequently observing of the absorption spectra. Typical functional groups appear in the regions of 3,600–2,500 cm1 (OH stretching), 3,100–2,800 cm1 (CH stretching), 1,800– 1,650 cm1 (C=O stretching), and 1,600 cm-1 (aromatic C=C stretching) (Socrates 2001). The spectral intensity is usually reported as a transmittance (T) or absorbance (A). The transmittance
where e is the molar absorptivity, d is the thickness of the sample, and C is the concentration of the component of interest. The infrared absorption intensity (or integral absorption intensity), with appropriate baseline correction, can be used therefore for quantitative analyses. Today, Fourier transform infrared spectroscopy (FTIR) is more commonly used than traditional monochromatic infrared spectroscopy. The instrumentation of FTIR usually consists of a light source, an interferometer (instead of a monochromator), a detector, and a computer system. In FTIR, the time domain spectrum (interferogram) obtained by infrared light passing thorough an interferometer is converted to the frequency-domain spectrum by a Fourier transform. A microscope is often combined with an FTIR for micro-to-millimeter-sized samples and is useful for spatial characterization. Most recently, a synchrotron radiation light source has been used instead of a conventional ceramic light source. The greater brightness of a synchrotron source enables a better spatial resolution (close to the expected diffraction limit according to the wavelength) and a higher signal-to-noise ratio than ceramic sources (Holman and Martin 2006). Infrared spectroscopy can be applied to solid, liquid, and gas samples. Infrared spectroscopy is also used in astronomical observation in the solar system and interstellar space.
See also ▶ Infrared Astronomical satellite ▶ Infrared Astronomy ▶ Infrared Space Observatory
References and Further Reading Bellamy LJ (1975) The infrared spectra of complex molecules, 3rd edn. Chapman and Hall, London Colthup NB, Daly LH, Wiberley SE (1990) Introduction to infrared and Raman spectroscopy, 3rd edn. Academic, San Diego Griffiths PR, de Haseth JA (1986) Fourier transform infrared spectrometry. Wiley, New York
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Initial Mass Function
Holman HYN, Martin MC (2006) Synchrotron radiation infrared spectromicroscopy: A non-invasive chemical probe for monitoring biogeochemical processes. Adv Agron 90:79–127 Silverstein RM, Webster FX, Kiemle D (2005) Spectrometric identification of organic compounds, 7th edn. Wiley, New York Socrates G (2001) Infrared and Raman characteristic group frequencies, 3rd edn. Wiley, Chichester
Initial Mass Function LETICIA CARIGI Instituto de Astronomı´a, Universidad Nacional Auto´noma de Me´xico, Me´xico, D.F., Mexico
Keywords Chemical and photometric evolution of galaxies, low mass stars, massive stars, solar neighborhood, star formation
Definition The initial mass function is the relative number of stars, as a function of their individual initial mass, that forms during a single star forming episode.
Overview The Initial Mass Function (IMF) is usually expressed as a power law, IMF = constant ma, where m is the initial star mass and a is the slope of the logarithmic plot. The IMF is defined over a large interval of masses, from the most massive stars to the lowest mass stars, created in a single ▶ star formation burst. The majority of the empirical IMFs has been inferred from star counts observed in the solar neighborhood. This region has had a long history of star formation bursts, including stars of different ages. Hence, to determine the true IMF, it is necessary to correct the actual count, assuming how the star formation rate has changed over time, stellar ages, the number of binary or multiple stellar systems, the Galactic age, and some other factors. The first and still most popular IMF is that obtained by Salpeter (1955), with slope, a = 2.35, over the entire mass interval. It is important to mention that Salpeter’s original calculation was for stars between 0.4 and 10 M and the astronomical community has often extrapolated it without careful consideration. When Salpeter’s IMF is extrapolated to m < 0.4 M, the number of very low mass stars is one or two orders of magnitude larger than the observed value. In the literature, there are many different IMFs
(e.g., Kroupa et al. 1993), all of them with several slope values; that is, they suggest different slopes for different mass intervals. The disagreement is larger for the low mass stars, due to their intrinsically low luminosity. It is known that Nature tends to form much more low mass stars than high mass stars, and this can be seen for the values proposed for a (larger than +2). For instance, Kroupa et al. (1993) predict approximately 4,000 stars of 1 M and 40 stars of 10 M for each star of 100 M. Since stellar properties mainly depend on their initial masses, the IMF is fundamental to the chemical and spectral evolution of star clusters and galaxies. It is very important to note that the universality of IMF remains as an unsolved problem, and so does the behavior of the IMF for sub-stellar masses (m < 0.08 M).
See also ▶ Solar Neighborhood ▶ Star Formation ▶ Stellar Evolution
References and Further Reading Bricen˜o C et al (2002) The initial mass function in the taurus star-forming region. Astrophys J 580:317–335 Carigi L, Hernandez X (2008) Chemical consequences of low star formation rates: stochastically sampling the initial mass function. MNRAS 390:582–594 Kroupa P (2001) On the variation of the initial mass function. MNRAS 322:231–246 Kroupa P, Tout C, Gilmore G (1993) The distribution of low-mass stars in the Galactic disc. MNRAS 262:545–587 Salpeter EE (1955) The luminosity function and stellar evolution. Astrophys J 121:161–167
Insoluble Organic Matter HIROSHI NARAOKA Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan
Synonyms IOM; Kerogen-like matter
Keywords Alteration, extraterrestrial, insoluble, organic matter, unusual isotopic composition
Institut for Rumforskning og -teknologi pa˚ Danmarks Tekniske Universitet
Definition Insoluble organic matter (IOM) is a major component of organic matter found in primitive extraterrestrial materials such as carbonaceous meteorites. Most of this is organic carbon with lesser amounts of hydrogen, nitrogen, oxygen, and sulfur forming aromatic hydrocarbon cores (2–4 aromatic rings) and short-chain aliphatic hydrocarbons with various functional groups including nitrile, carboxyl, and ether. Unusual isotopic signatures such as extreme D- and 15N-enrichment are observed.
Overview Insoluble organic matter (IOM) is a major constituent of organic matter in extraterrestrial materials. Even though various organic compounds including amino acids, carboxylic acids, and polycyclic aromatic hydrocarbons (PAHs) have been identified in meteorites, particularly in carbonaceous chondrites, these solvent-extractable organic compounds are only 10–30% of the total organic carbon. Most meteoritic organic matter is insoluble in water, acids, or organic solvents. This insoluble organic matter used to be called as kerogen-like material (c.f. kerogen is a solventinsoluble organic matter in terrestrial rocks) or macromolecular organic matter. Now it is generally called IOM. For the ▶ Murchison meteorite, which is relatively enriched in soluble organic compounds, the IOM consists of more than 90% of meteoritic organic carbon with minor amounts of hydrogen, nitrogen, oxygen, and sulfur. Although the chemical structure of IOM has not been defined accurately yet, it has aromatic hydrocarbon cores (2–4 aromatic rings) with short-chain aliphatic hydrocarbons. IOM from various meteorites has a wide variation in chemical composition. The H/C ratio of chondritic IOM varies from < 0.1–0.7, the more metamorphosed meteorite having a lower H/C ratio. Therefore, the H/C ratio is a sensitive indicator for thermal processing during the meteorites’ history. 13C-NMR and ▶ XANES studies have also revealed various chemical bonding and functional groups including aromatic and aliphatic structures with carbonyl, carboxyl, and aldehyde as well as amide and nitrile groups. As the Murchison IOM generates abundant PAHs and acetic acid (ca. four times higher compared to solvent-extractable acetic acid) during hydrous pyrolysis, the IOM may be an important precursor for organic compounds in meteorites. The primitive IOM also possesses unusual isotopic signatures such as extreme D- and 15N-enrichment, which have not been observed in solar processes. Such isotopic behavior suggests that meteoritic organic matter may have originated from interstellar processes.
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However, the unusual isotope signals are lost during the thermal and aqueous alteration on meteorite parent bodies.
See also ▶ Carbonaceous Chondrite ▶ Carbonaceous Chondrites (Organic Chemistry of ) ▶ Isotopic Fractionation (Interstellar Medium) ▶ Murchison ▶ XANES
References and Further Reading Alexander CMO’D, Fogel M, Yabuta H, Cody GD (2007) The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter. Geochim Cosmochim Acta 71:4380–4403 Alexander CMO’D, Newsome SD, Fogel ML, Nittler LR, Busemann H, Cody GD (2010) Deuterium enrichments in chondritic macromolecular material – implications for the origin and evolution of organics, water and asteroids. Geochim Cosmochim Acta 74:4417–4437 Busemann H, Young AF, Alexander CMO’D, Hoppe P, Mukhopadhyay S, Nittler LR (2006) Interstellar chemistry recorded in organic matter from primitive meteorites. Science 312:727–730 Cody GD, Alexander CMO’D (2005) NMR studies of chemical structural variation of insoluble organic matter from different carbonaceous chondrites groups. Geochim Cosmochim Acta 69:1085–1097 Huang Y, Alexandre MR, Wang Y (2007) Structure and isotopic ratios of aliphatic side chains in the insoluble organic matter of the Murchison carbonaceous chondrites. Earth Planet Sci Lett 259:517–525 Kerridge JF, Chang S, Shipp R (1987) Isotopic characterization of kerogen-like material in the Murchison carbonaceous chondrite. Geochim Cosmochim Acta 51:2527–2540 Oba Y, Naraoka H (2006) Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite. Meteorit Planet Sci 41:1175–1181 Oba Y, Naraoka H (2009) Elemental and isotopic behavior of macromolecular organic matter from CM chondrites during hydrous pyrolysis. Meteorit Planet Sci 44:943–954 Remusat L, Derenne S, Robert F, Knicker H (2005) New pyrolytic and spectroscopic data on Orgueil and Murchison insoluble organic matter. A different origin than soluble? Geochim Cosmochim Acta 69:3919–3932 Remusat L, Palhol F, Robert F, Derenne S, France-Lanord C (2006) Enrichment of deuterium in insoluble organic matter from primitive meteorite: a solar system origin? Earth Planet Sci Lett 243:15–25
Institut for Rumforskning og -teknologi pa˚ Danmarks Tekniske Universitet ▶ DTU Space (Denmark)
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Interference Zone ▶ Spallation Zone
Interferometry PIERRE KERVELLA LESIA, Observatoire de Paris, Meudon, France
Synonyms Hypertelescope; Multiple aperture astronomy
Keywords Diluted apertures, extrasolar planets, high angular resolution, interferences, interferometer, optical interferometry, radio interferometry
Definition Astronomical interferometry is an observing technique that makes use of several separate sub-apertures instead of a single, large-aperture light collector for the observation of celestial objects. The separation between two individual sub-apertures is called the baseline, and defines, together with the observing wavelength, the resolving power of the instrument. The longer the baseline and the shorter the wavelength, the higher the resolving power. Astronomical interferometry is currently operational in the radio, infrared, and visible wavelength ranges. The presently available facilities give access to angular resolutions of the order of 1–10 millisecond of arc (milliarcsecond, abbreviated as mas), i.e., ten to hundred times smaller than that provided by the Hubble Space Telescope.
Overview Due to the diffraction laws of optics, the larger the telescope and the shorter the observing wavelength, the better the angular resolution (i.e., the smallest details on the sky that the telescope can distinguish). However, the cost of building a telescope increases rapidly with its size. Astronomical interferometry offers a relatively cost-effective way to achieve high angular resolution by using a cluster of comparatively small telescopes rather than a single, very expensive monolithic telescope. The light collecting surface of a multi-aperture interferometer is naturally much smaller than that of a single aperture with the same
diameter, thus limiting its sensitivity. Interferometers in the optical domain are promising tools for the detection and resolved observation of extrasolar planets (including spectroscopy). This is particularly true if these instruments can be installed in space, where the absence of atmospheric turbulence permits very-high-contrast interferometric nulling observations (See ▶ Nulling Interferometry).
Basic Methodology The basic principles behind interferometry are founded on the wave properties of light. The superposition of light waves collected by the different sub-apertures results in an interference pattern whose properties are mathematically related to the structure of the observed object (through the Zernike-Van Cittert theorem). The light waves that have the same phase produce constructive interferences, while the waves that are out of phase produce destructive interferences. The accurate measurement of the interference pattern then gives access to the spatial structure of the observed object, at very high angular resolution. In the optical, the sub-apertures are telescope mirrors, and the light is transported to the recombination point using a series of mirrors, or optical fibers. One particular difficulty of interferometry is that the optical path length between the observed source and the recombination point must be equal for the light beams going through the different sub-apertures. The acceptable uncertainty on the path length is of the order of the wavelength. This constraint is particularly stringent for interferometers with long baselines operating in the visible, as path lengths of up to several hundred meters must be controlled with an accuracy of less than a micron. A variant of classical interferometry is called intensity interferometry. This technique uses two light detectors, typically either radio antennas or optical telescopes. Both detectors are pointed at the same astronomical source, and high-frequency intensity measurements are transmitted to a central correlator facility. The intensity interferometer measures interferometric visibilities like all other astronomical interferometers. These measurements can be used to calculate the diameter and ▶ limb darkening coefficients of stars, but with intensity interferometers, aperture synthesis images cannot be produced, as the visibility phase information is not preserved. New generations of interferometric instruments are specifically developed for high-contrast observation of the surroundings of nearby stars. The principle of these instruments is to recombine the light collected by the subapertures in a destructive way. In other words, the in-phase light from the central star collected by one
Interferometry
sub-aperture is canceled by its superposition with 180 phase-delayed light from another sub-aperture. This technique, called interferometric nulling (the instrument being called a nuller), can be understood as the interferometric version of the coronagraph. It is particularly promising for the observation of extrasolar planets and exozodiacal disks, as it provides simultaneously high-contrast and high-resolution observing capabilities. An interferometric nuller is currently operational on the Keck Interferometer, and several projects exist for the development of an interferometric nuller in space.
Key Research Findings Interferometry’s application to astronomy was first implemented in the visible in the late nineteenth and early twentieth century (Lawson 1997), but its development as a mainstream observing technique effectively started in the 1950s in the radio domain, at wavelengths for which the constraints on the quality of the reflector surfaces and optical path length equalization are less stringent than in the optical. Several giant radio interferometers were built in the 1970s and 1980s. The present largest single-site radio interferometer is the Very Large Array (VLA, see Fig. 1), near Soccoro, New Mexico, USA, but intercontinental interferometric observations were obtained with antennas spread over baselines of thousands of kilometers (Very Long Baseline Interferometry). Such extremely long baseline observations are made possible in the radio domain by the fact that the amplitude of the electric field of the light can be recorded and recombined a posteriori, provided that accurate time signals are recorded with the data. This technique is generally not applicable in the optical. The international Atacama Large
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Millimeter Array (ALMA, see Fig. 2) will become the largest radio interferometer, as well as the largest ground-based astronomical observatory ever built, when completed in 2011. Its 64 antennas, 7–12 m in diameter, can be positioned on baselines of 150 m to 14 km, allowing radio imaging observations down to a resolution of 10 mas. The first measurement of the angular size of a star was obtained in 1921 by Albert A. Michelson and Francis Pease using interferometry in the visible. As a remark, optical interferometry is not formally restricted to the visible domain, but includes the application of interferometric techniques from the blue to the mid-infrared wavelengths, for which the light is manipulated using optics. In the 1960s, intensity interferometry in the visible was developed by Robert Hanbury Brown and Richard Q. Twiss in Australia. It was soon followed in the 1970s by the first multi-telescope observations in the visible, obtained by Labeyrie (Interfe´rome`tre a` 2 Te´lescopes, I2T, followed by the Grand Interfe´rome`tre a` 2 Te´lescopes, GI2T). Several optical interferometers with collecting telescopes of a few decimeters to a meter were built in the 1980s and 1990s, including the Mark III (visible), the Palomar Testbed Interferometer (PTI, near-infrared), the Navy Prototype Optical Interferometer (NPOI, visible), the Infrared Optical Telescope Array (IOTA, near-infrared), the Infrared Spatial Interferometer (ISI, mid-infrared), and the CHARA array (visible and near-infrared). In the early twenty-first century, two very large interferometers with collecting apertures of 8–10 m were inaugurated: the Keck Interferometer (KI, two 10-m telescopes, see Fig.3) and the Very Large Telescope Interferometer (VLTI, four 8-m telescopes, see Fig. 4). These large arrays include a variety of focal instruments that combine the light collected by the
Interferometry. Figure 1 View of the very large array radio interferometer (New Mexico, USA)
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Interferometry. Figure 2 Three antennas of the Atacama Large Millimeter Array (ALMA) interferometer, located at Chajnantor (Chile), whose completion is foreseen in 2011
Interferometry. Figure 3 The two 10-m Keck telescopes (Mauna Kea, Hawaii) can be used together as an interferometer with a nulling capability for the observation of exozodiacal disks and exoplanets
sub-pupils at different wavelengths in the near- and midinfrared domains (OLBIN; Wikipedia). There exist several projects to install interferometers in space, where the environment is very stable, and no
atmospheric turbulence perturbation of the wave fronts is present. Apart from the gravitational wave detectors (that use laser interferometry for metrology), the most advanced project for space interferometry is the Space
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Interferometry. Figure 4 The very large telescope interferometer (Cerro Paranal, Chile) can combine the light from up to four 8-m telescopes (in the background), or up to four 1.8-m telescopes (white spherical domes in the forefront)
I Interferometry Mission (SIM), that will provide microarcsecond astrometric measurements in the narrow-angle regime. Such an accuracy will permit the detection of the astrometric wobble induced on a star’s position by the presence of orbiting planets, down to masses of a few terrestrial masses. Other projects include interferometric coronagraphs optimized for the observation of extrasolar planets, as well as ultraviolet and X-ray interferometers for stellar physics. The only operating interferometer presently in space is the Fine Guidance Sensor (FGS) of the Hubble Space Telescope. This threeaperture instrument is primarily employed to guide the telescope during scientific exposures, but has also produced important scientific results, such as the measurement of trigonometric ▶ parallaxes of nearby Cepheid variable stars. Interferometric measurements can also be produced with a monolithic telescope by using the aperture masking technique. The principle of this technique is to insert a mask in the beam collected by the telescope, punched with a number of holes arranged in a nonredundant pattern. This pattern is defined so that each baseline (the vector separation between two sub-apertures) is uniquely defined on the mask. The focal image of the masked telescope is the same as that of a multi-aperture interferometer (i.e., is made of the superposition of fringe patterns). Compared to full-aperture imaging, aperture masking allows a more efficient recovery of the highest spatial frequencies (the finest details) observable by the telescope. This is achieved at the cost of a loss in sensitivity (since part of the telescope aperture is covered by the mask), and an increased complexity of the data analysis. As a remark, the extremely large telescopes currently under development all employ segmented primary
mirrors, and are therefore similar in principle to multiaperture Fizeau interferometers. Radio interferometry was used successfully to image the vicinity of black holes and quasars, the surfaces of nearby stars, and to obtain high-precision astrometric measurements. The position of quasars measured by radio interferometry is the basis of the definition of the celestial reference frame. Historically, optical interferometry produced important results essentially in the stellar physics field. The first optical interferometric images of the surfaces and close environment of stars of various classes were obtained recently, as well as high-resolution observations of a sample of active galactic nuclei in the near- and mid-infrared. Although the imaging capabilities of optical interferometers are progressing rapidly, they are presently used essentially to constrain model parameters (e.g., stellar angular diameters, limb darkening coefficients, circumstellar environment of young and evolved stars, and binary orbits) rather that reconstruct ab initio interferometric images, due to their relatively small number of apertures.
See also ▶ Exozodiacal Light ▶ Exoplanet, Detection and Characterization ▶ Limb Darkening ▶ Nulling Interferometry ▶ Parallax ▶ VLBI
References and Further Reading Lawson PR (ed) (1997) Selected papers on long baseline stellar interferometry (SPIE Press Book), Bellingham, Wash. : SPIE Optical Engineering Press, c1997, ISBN: 9780819426725 Optical long baseline interferometry news (OLBIN) web site. http://olbin. jpl.nasa.gov/ Accessed 26 Nov 2011
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Interior Structure (Planetary)
Interior Structure (Planetary) TILMAN SPOHN German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Keywords Core, crust, density, mantle, pressure, temperature
Definition Models of the interior structure of a ▶ planet or ▶ satellite give the variations of thermodynamic state variables as functions of radius (starting at the center) or depth (starting at the surface). The thermodynamic state variables are pressure, temperature, density (or specific volume), and the concentration of chemical species. In many cases chemical layering is described in more specific terms as ▶ crust, ▶ mantle, and ▶ core or in shells described by characteristic elements, ▶ minerals and phases. In other circumstances shells are characterized by their rheological or transport properties such as the lithosphere.
Interior Structure (Planetary). Figure 1 Interior structure of the Earth as a generic model for the interior structure of a terrestrial planet
Overview Knowledge of the interior structures of planets and satellites is a major prerequisite for understanding the evolution of planets and for models of how planets work. From a thermodynamic point of view, planets can be regarded as heat engines that convert thermal and gravitational energy into mechanical work and ▶ magnetic field energy and evolve by differentiating, cooling, and contracting. The process of converting thermal or chemical energy into magnetic field energy is the ▶ dynamo process. The process that converts energy into mechanical work in ▶ terrestrial planets and satellites is associated with mantle convection and is discussed in the section on ▶ heat transfer (planetary). Mantle convection is the most important heat transfer mechanism in terrestrial planets and satellites. Convection is also the likely dominant heat transfer mechanism in the interiors of the ▶ giant planets. Most planets and satellites are thought to be differentiated with the density increasing toward the center (for a recent review on the interior structure of terrestrial planets see, e.g., Sohl and Schubert 2007 and Sohl et al. 2009). In a terrestrial planet (compare Fig. 1), an earthlike or rocky satellite such as the ▶ Moon or the Jovian satellite ▶ Io, or a differentiated icy satellite such as ▶ Europa and ▶ Ganymede (compare Fig. 2), the central region is an iron-rich core. The core is the region where a planetary magnetic field may be generated by a dynamo. The core may be layered like in the Earth with a solid inner core and
Interior Structure (Planetary). Figure 2 Interior structure of Ganymede as a generic model for a differentiated icy satellite
a liquid outer core. Growth of the inner core may release gravitational energy that may power the dynamo. The core is overlain by the rocky ▶ mantle, which, in turn, is overlain by the rocky, mostly basaltic crust.
Interior Structure (Planetary)
The crust is the product of partial melting and ▶ differentiation of the mantle and is thought to have grown in thickness over time. The mantle and the crust may be chemically layered with discontinuities in density and other material properties. There may, in addition, be discontinuities caused by solid state phase change boundaries. It is further possible that regions of the mantle are partially molten causing an Asthenosphere. The core radius of the ▶ Earth is very well known from the inversion of geophysical data to be 3485 km on average. The thickness of the crust is up to a few kilometers in oceanic regions and a few tens of kilometers in continental areas. ▶ Venus’s interior structure is much less well known with a core radius of probably 0.5RP and a basaltic crust of between 20 and 50 km average thickness. ▶ Mars’s core is also about 0.5RP in radius and its average crust thickness is between 30 and 80 km. ▶ Mercury’s core radius is about 0.8RP and the crust thickness is estimated to be 100–200 km. Sohl et al. 2009 give a compilation of interior models of terrestrial planets, satellites, and icy satellites. The above generic model also likely applies to the Moon whose iron-rich core should be small, comprising only 0.3 planetary radii RP , and Io with a core radius of about 0.5RP . The average lunar crust thickness is 49 15 km and varies between 67 km underneath the far side and close to zero underneath the South Polar Aitken basin. Other large differentiated satellites such as Ganymede and Europa may have ice shells above their ▶ rock mantles and iron-rich cores (see, e.g., Hussmann et al. 2007 and Sohl et al. 2009). The ice shell on Ganymede may be about 700–900 km thick while on Europa a thickness of about 150 km is likely. The iron core radius for Ganymede varies between 0.2RP and 0.3RP depending on model details while for Europa the core radius varies between 0.3RP and 0.6RP . The Jovian satellite ▶ Callisto and the Saturnian satellite ▶ Titan are believed to be incompletely differentiated, with hundreds of kilometers thick ice shells overlying undifferentiated ice/rock/iron cores. The giant planets ▶ Jupiter and ▶ Saturn have a molecular gaseous atmosphere underlain by a molecular layer of mostly ▶ hydrogen and helium see (Fig. 3 and, e.g., Guillot and Gautier 2007 for a review). At a pressure of 0.5TPa hydrogen becomes metallic, the transition pressure depending to some extent on temperature. In Jupiter this transition occurs at about 0.75 RP ; in Saturn the transition occurs at 0.45RP . At the center of these planets there is a core with a mass of up to ten Earth masses and 0.2RP radius composed of heavier elements like iron, silicon, and magnesium, but also of ▶ water, ▶ ammonia, and ▶ methane. The latter are sometimes termed the planetary ices regardless of whether they are in a solid, liquid, or gaseous state.
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I Interior Structure (Planetary). Figure 3 Interior structure of Jupiter as a generic model for the interior structure of a giant planet
The interiors of ▶ Uranus and ▶ Neptune are also thought of as layered, although the transitions between the layers should be more gradual (see, e.g., Guillot and Gautier 2007). In these models, the outer layer extending down from 0.8 to 0.9 RP consists of mostly molecular H and He but also of water, ammonia, and methane. The shell below is composed of ice plus little H and He, and there is a rock/iron plus ice core component of a few Earth masses. Knowledge about the interior structure is derived by using the gravity and magnetic fields of a planet or satellite, its shape, and the propagation of elastic waves through the interior of the body. The latter is the basis of seismology, which is the best method provided one can have seismometers operating on the surface of a planet that is sufficiently seismically active. A similar method uses global oscillations of a giant planet and requires that the oscillations can be observed with telescopes or from satellites. In addition, an equation of state is needed to relate density to pressure and temperature. The parameters in the equation of state need to be determined through laboratory experiments. For the Earth gravity, magnetic and seismic data have been inverted to provide detailed models of the interior structure that even include lateral variations of density and elastic properties. The shape of a gravitational equipotential surface about a planet will reflect the distribution of mass in its interior (see, e.g., Wieczorek 2007 for a review). An equipotential surface can be traced by Doppler-tracking of an
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orbiting spacecraft. For a rotating, rotationally symmetric model planet in ▶ hydrostatic equilibrium, the shape of an equipotential surface will be a flattened spheroid of ▶ rotation (one of which will coincide with the physical surface, if in perfect equilibrium). The gravitational potential V (r, y) can be expanded in a series of Legendre Polynomials with ! 1 n X Rp m 1 V ðr; yÞ ¼ G Jn Pn ðcos yÞ r r n¼2 1 X n n X Rp ðCnm cos ml þ Snm sin mlÞ þ r n¼2 m¼1 Pnm ðcos yÞ ; where r is the distance to the observer from the center of the planet, y is the planetocentric polar angle (measured from the pole such that y = 90 at the equator), l is the longitude, G is the universal gravitational constant, m is the planet’s mass, and RP is the average radius. The Jn, Cnm, and Snm are coefficients and Pn(cos y) and Pnm(cos y) are Legendre polynomials and associated Legendre polynomials, respectively (for an introduction to spherical harmonic functions for geophysical applications see Stacey and Davis 2008). The coefficients Jn, Cnm, and Snm measure the mass distribution in the planet. In particular, mR2 P J2 ¼ C A; where C is the moment of inertia of the planet about the rotation axis and A is the average moment of inertia about an axis in the equatorial plane. The flattening f of the planet depends on J2 and the rotation rate via 3 1 f ¼ J2 þ c 2 2 with o2 RP2 : Gm For a constant density body C/mRP2 = 0.4, J2 ¼ 12 c and f ¼ 54 c. The latter is the maximum value of the flattening for a given value of c. With increasing concentration of mass toward the center, C/mRP2 will become smaller than 0.4 and the flattening will decrease as well. While the low-order terms are used to constrain the deep interior structure – degree 2 for the terrestrial and degrees 2– 6 for the giant planets that are closer to hydrostatic equilibrium – high-order terms can be used to constrain density variations closer to the surface such as thickness variations of the planetary crust. It must be noted that any interpretation of the gravity field is fraught with nonuniqueness problems. The reason is that for the same gravitational force or potential at the location of an observer, mass can be traded against distance. Therefore, interior models based on the gravity field c
need additional constraints. For Mars the chemistry of the ▶ SNC-Meteorites together with the assumption of an overall chondritic composition of the planet has been used to constrain the core radius. The variation of the thickness of the Martian crust has been derived from ▶ Mars Global Surveyor gravity data and the assumption of a constant crust density. The magnetic field recorded at a planet or satellite provides additional constraints on the properties of the interior. A near-dipole shaped magnetic field usually indicates a dynamo and that electrical currents are flowing deep in the interior. The closer the field geometry resembles a dipole, the deeper the source region of the field is usually to be expected. The Earth’s magnetic field is understood as being produced in its outer core. The Earth’s outer core must therefore be electrically conductive and fluid to allow for dynamo action. That the Earth’s outer core is liquid is proven beyond any doubt by the absence of seismic shear waves propagating through that layer. Through analogy it is concluded that there must be at least a liquid outer core shell in Mercury, the only other terrestrial planet that presently produces a magnetic field. The remnant magnetization of the Martian crust (see, e.g., Connerney et al. 2004) suggests that this planet once produced a magnetic field and that – again – at least an outer core layer was liquid. More evolved modeling of the planet suggests that the entire core is fluid, even today. The magnetic fields of Jupiter and Saturn are thought to be generated in the metallic hydrogen shells, again allowing for electric currents to flow and dynamo action. The fields of Uranus and Neptune are thought to be produced in ionic oceans relatively close to their surfaces. This is consistent with observation that their magnetic fields are rich in higher-order terms. Periodically varying magnetic signals have been recorded at Europa, Ganymede, and Callisto by the ▶ Galileo spacecraft. The orbits of all three satellites lie within the giant ▶ magnetosphere of Jupiter and the signals at Europa and Callisto have been found to vary with their orbital period. Ganymede has a static dipole magnetic field superimposed with a weak time variable field. Hence, it has been concluded that there must be an electrically conducting layer in the interiors where a magnetic field can be induced as the satellites move through the global magnetic field of their primary. Since the satellites have ice shells and salty water is a good electric conductor, the magnetic signals have been taken as evidence of oceans in these satellites. Little is known about the interior structures of the many small satellites and ▶ Kuiper belt objects in the outer ▶ Solar System. Gravity data taken from ▶ Cassini fly-bys at the Saturnian satellites have been interpreted to
International Astronautical Federation
suggest that ▶ Rhea may by differentiated. Interpretation of these data does rely on the assumption of equilibrium, which becomes even more questionable for small satellites. The about 500 ▶ exoplanets detected to date suggest that there may be planets that differ from the ones found in our solar system. Even if the three chemical components of our planets, gas (H, He), ice (H2O, NH3, CH4), and rock/iron were universally representative, the relative proportions may differ. Thus there may be Super-Ganymedes or Sub-Jupiters (e.g., Stevenson 2004). ▶ Transit observations and ▶ Radial Velocity observations allow the masses and radii of these planets to be measured. With these and mass-radius relations the interior structure of these planets can be estimated. Of course, these models do have even more severe uniqueness problems than models of the interior structure of the planets in our solar system. Major difficulties lie with the unknown equations of state of degenerate matter at extremely high pressures and temperatures and the unknown volumes of potential ultra high pressure phases of solids. The difficulties may be more severe for Super-Earths than for large gaseous planets, since Jupiter is close to the maximum radius of an H–He sphere anyway.
See also ▶ Ammonia ▶ Callisto ▶ Cassini ▶ Core, Planetary ▶ Crust ▶ Differentiation ▶ Dynamo (Planetary) ▶ Earth ▶ Europa ▶ Exoplanets, Discovery ▶ Galileo ▶ Ganymede ▶ Giant Planets ▶ Heat Transfer (Planetary) ▶ Hydrogen ▶ Hydrostatic Equilibrium ▶ Io ▶ Jupiter ▶ Kuiper Belt ▶ Magnetic Field ▶ Magnetosphere ▶ Mantle ▶ Mars ▶ Mars Global Surveyor ▶ Mercury ▶ Methane ▶ Mineral
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▶ Moon, The ▶ Neptune ▶ Planet ▶ Radial Velocity ▶ Rhea ▶ Rock ▶ Rotation Planet ▶ Satellite or Moon ▶ Saturn ▶ SNC Meteorites ▶ Solar System Formation (Chronology) ▶ Terrestrial Planet ▶ Titan ▶ Transit ▶ Uranus ▶ Venus ▶ Water
References and Further Reading Connerney JEP et al (2004) Mars crustal magentism. Space Sci Revs 111(1–2):1–32 Guillot T, Gautier D (2007) Giant planets. In: Schubert G, Spohn T (eds) Treatise on geophysics planets and moons, vol 10. Elsevier, Amsterdam, pp 439–464 Hussmann H, Sotin C, Lunine JI (2007) Interiors and evolution of icy satellites. In: Schubert G, Spohn T (eds) Treatise on geophysics planets and moons, vol 10. Elsevier, Amsterdam, pp 509–540 Sohl F, Schubert G (2007) Interior structure, composition and mineralogy of the terrestrial planets. In: Schubert G, Spohn T (eds) Treatise on geophysics: planets and moons, vol 10. Elsevier, Amsterdam, pp 27–68 Sohl et al (2009) Planetary interiors. In: Tru¨mper JE (ed) Landolt bo¨rnstein. Springer-Verlag, Berlin/Heidelberg, pp 200–224 Stacey FD, Davis PM (2008) Physics of the Earth. Cambridge University Press, Cambridge/New York/Melbourne Stevenson DJ (2004) Planetary Diversity. Physics Today (American Institute of Physics) pp 43–48 Wieczorek MA (2007) Gravity and topography of the terrestrial planets. In: Schubert G, Spohn T (eds) Treatise on geophysics planets and moons, vol 10. Elsevier, Amsterdam, pp 165–206
International Astrobiology Society ▶ ISSOL
International Astronautical Federation ▶ IAF
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International Astronomical Union
International Astronomical Union ▶ IAU
International Society for the Study of the Origin of Life ▶ ISSOL
International Space Science Institute
International Space Station. Figure 1 The European instrument Expose R placed outside the Zarya Module (Photo ESA)
▶ ISSI
International Space Station MICHEL VISO Astrobiology, CNES/DSP/EU, Paris, France
Synonyms ISS; MKS (Russian acronym)
Definition The International Space Station (ISS) is orbiting the Earth at an altitude between 335 and 460 km in an orbital plane inclined of 51.8 . The assembly of this internationally developed research facility began in 1998 with the Russian module Zarya to be almost completed in 2011. The ISS is the result of the merging of former projects in an international effort gathering, Russia, United States of America, Europe, Japan, and Canada. The intergovernmental agreement is planning operations until at least 2015, discussion already began to pursue up to 2020. The orbital complex is permanently occupied since November 2, 2000, and since May 2009 the ISS has the capability to host simultaneously up to six astronauts or cosmonauts. The astrobiology interest of the ISS is the permanent base for long lasting experiments. The scientists have the opportunity to expose for several months samples in instruments attached to external payload supports as the European
International Space Station. Figure 2 The International Space Station with the docked ATV in June 2008 (Photo NASA)
Technology facility (EuTEF) on Columbus (European module), the external wall of Zarya (Fig. 1) (Russian core module), or on the exposure facility of Kibo (Japanese Module). After the retirement of the shuttle in early 2011, the expeditions to the ISS are relying on the Russian soyouz spacecrafts. The servicing with cargo is made by the European Automated Transport Vehicles (Fig. 2), the Japanese H-II Transport Vehicles, the Russian Progress spacecraft, and planned privately operated US cargos (Dragon and Falcon).
See also ▶ Expose
Interplanetary Dust Particles
International Standard Organization ▶ ISO (Normative Organisation)
Interplanetary Dust Particles WILLIAM M. IRVINE Department of Astronomy, University of Massachusetts, Amherst, MA, USA
Synonyms Brownlee particles; IDPs
Keywords Dust, interplanetary, Zodiacal light, Gegenschein, Cometary, Isotopic anomalies
Definition In addition to the Sun, the planets plus their satellites, and smaller bodies including asteroids, comets, meteoroids, and ▶ Kuiper Belt Objects, the roughly defined plane of the solar system contains gas and dust, referred to as the interplanetary medium. The dust grains are, naturally, called interplanetary dust particles, although that nomenclature (and particularly the abbreviation IDPs) is often used to designate particles of presumed interplanetary origin collected on the Earth, primarily in the stratosphere but also from Antarctic ice. In the interplanetary medium, sunlight scattered by the dust is responsible for the phenomena of the zodiacal light (the “false morning” of poets, including Omar Khayyam) and the gegenschein (a very faint glow in the antisolar direction). In addition, interplanetary dust particles produce meteors (“shooting stars”) when they strike the Earth’s atmosphere and are incinerated as a result of friction.
Overview Because these small particles tend over time to be either expelled from the solar system by radiation pressure or to fall into the Sun as a result of ▶ Poynting–Robertson drag, there must be sources to replenish them. These sources are thought to be collisions among asteroids and Kuiper Belt Objects, and the release of particles trapped in cometary ices as the comets approach the Sun and the ices sublimate. These sources are confirmed by ▶ Infrared Astronomical Satellite (IRAS) observations of dust bands in the
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asteroid belt and by meteor showers that are dynamically related to the orbits of comets. The laboratory study of IDPs began in 1978 when D. Brownlee, using an ex-CIA high altitude U2 spy plane, collected particles in the stratosphere whose extraterrestrial origin was confirmed by the presence of cosmic ray tracks and isotopic anomalies, particularly deuterium enhancements. Interplanetary particles with diameters approximately in the range 1–100 mm are slowed sufficiently by friction to survive atmospheric entry; in fact, the Earth is estimated to accrete some 4 107 kg of such material annually, and this influx was certainly much greater early in the history of the solar system. The IDPs are composed predominantly of silicates, but there is also up to about 10% by mass of carbon, together with other minerals such as sulfides and magnetite. Evidence for solid carbonaceous compounds (such as HCN polymers) that progressively evaporate with decreasing solar distance in the interplanetary medium, at least in the 1.5–0.3 AU range, confirm that IDPs might have significantly contributed to the enrichment of the telluric planets in organics during the LHB (▶ late heavy bombardment) epoch. The so-called anhydrous cluster IDPs appear to be the least processed of these particles reaching the Earth, and they are usually thought to have a cometary origin. Their highly porous structure increases their deceleration and heat transfer in planetary atmospheres, and thus favors the survival of carbonaceous compounds reaching the Earth. These presumably cometary IDPs seem to contain preserved interstellar material, including GEMs and organic material with significant isotopic anomalies, for example, in D/H and 15 N/14 N. IDPs have also been collected in space by the ▶ Stardust mission to comet 81P/Wild 2 and by experiments on the International Space Station. Analysis of the organic matter in IDPs is a very active subject of current research.
See also ▶ GEMs ▶ Infrared Astronomical Satellite ▶ International Space Station ▶ Kuiper Belt ▶ Late Heavy Bombardment ▶ Micrometeorites ▶ Poynting–Robertson Drag ▶ Stardust Mission
References and Further Reading Davoisne C, Djouadi Z, Leroux H, d’Hendecourt L, Jones AP, Deboffle D (2006) The origin of GEMS in IDPs as deduced from microstructural evolution of amorphous silicates with annealing. Astron Astrophys 448:L1–L4
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Flynn GJ, Keller LP, Jacobsen C, Wirick S (2004) An assessement of the amount and types of organic matter contributed to the Earth by interplanetary dust. Adv Space Res 33:57–66 Jessberger EK, Stephan T, Rost D, Arndt P, Maetz M, Stadermann F, Brownlee DE, Bradley JP, Kurat G (2001) Properties of interplanetary dust: information from collected samples. In: Gru¨n E, Gustafson B, Dermott SF, Fechtig H (eds) Interplanetary Dust. Springer, Berlin, pp 254–294 Lasue J, Levasseur-Regourd AC, Fray N, Cottin H (2007) Inferring the interplanetary dust properties from remote observations and simulations. Astron Astrophys 473:641–649 Levasseur-Regourd AC, Lasue J, Desvoivres E (2006) Early inner solar system impactors: physical properties of comet nuclei and dust particles revisited. Orig Life Evol Biosph 36:507–514
Interplanetary Transfer of Life ▶ Panspermia
Interstellar Chemical Processes STEFANIE N. MILAM Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
dominate the various environments. In general, the chemistry found throughout the ▶ interstellar medium (ISM) involves low densities and extreme temperatures with respect to Earth, and varies due to the range of physical conditions found in specific environments. These environments can be divided into distinct regions, based primarily on temperature and density (see Table 1). In the diffuse interstellar medium (ISM), where densities (1–100 cm3) and temperatures (100 K) are low, the chemistry is dictated by the large flux of ultraviolet radiation coming from nearby stars. The growth of molecular complexity in these regions is minimal due to the limited stability of larger organic species in the presence of an ultraviolet (UV) field. Nonetheless, ▶ polycyclic aromatic hydrocarbons (PAHs) do thrive in these harsh conditions and are ubiquitous throughout the ISM (see review by Peeters). On the other hand, ▶ dense clouds, the cold, dense molecular regions of the ISM, tend to have a much richer chemistry. In these clouds, external UV radiation is insignificant in the inner regions, and cosmic rays are the dominant radiation source creating ions and electrons, as well as providing a source of heating. Within these dark clouds, not only does gas-phase chemistry play a key role, but grains and surface chemistry are also important. The molecular inventory of each region helps delineate the chemical processes that occur throughout the ISM and how they influence the cycle of material from stellar birth to death.
Synonyms
Basic Methodology
Interstellar chemistry
The methods by which interstellar processes are studied typically involve high-resolution spectral line observations and maps of molecular emission that trace different regions and/or processes that may be occurring (e.g., Evans 1999). Molecules have rotational and vibrational motions coupled to electronic states, which are quantized. Molecular transitions are characterized by their energy as either electronic (4 eV; 104 K), vibrational (0.1 eV; 1,000 K), or rotational (0.01 eV; 100 K), see Fig. 2. The visible and ultraviolet wavelengths are typically associated with electronic transitions, the infrared traces mostly vibrations, and millimeter/submillimeter wavelengths are dominated by rotational transitions. The large range of temperatures and densities throughout the ISM has led to the detection of numerous species at multiple wavelengths (see ▶ Molecules in Space; or the CDMS web site at http://www.astro.uni-koeln.de/cdms/ molecules). All of the 150 identified interstellar species are just minor constituents of the molecular material, which is dominated by H2 in dense regions. As can be noted from Table 1, most of the molecular material and
Keywords Anions, gas-phase, grain chemistry, ion-neutral reactions, neutral-neutral reactions, photochemistry, shocks, surface chemistry
Definition Interstellar chemical processes manipulate matter by either creating or destroying molecular material. These processes, acting on gas and dust, include heating, irradiation, shocks, and cosmic-ray bombardment, as well as chemical reactions such as ion-neutral and neutral-neutral reactions and surface chemistry.
Overview All interstellar material is cycled during various phases of stellar life (Fig. 1): from the diffuse medium, to the formation of a star and planetary system, to stellar death. The molecular complexity that arises during each phase can be attributed to the physical and chemical processes that
Interstellar Chemical Processes
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Interstellar Chemical Processes. Figure 1 Cosmic cycle of interstellar material (Image credit: Bill Saxton, NRAO/AUI/NSF)
Interstellar Chemical Processes. Table 1 ISM components (Adapted from Wooden et al. (2004). See ▶ Interstellar Medium) Temperature (K)
Density (cm3)
Region
Common name
Hot ionized medium (HIM)
Coronal gas
106
0.003
Warm ionized medium (WIM)
Diffuse ionized gas
104
>10
Warm neutral medium (WNM)
Intercloud H I
103–104
0.1
Atomic cold neutral medium (CNM)
▶ Diffuse clouds
100
10–100
Molecular cold neutral medium (CNM)
Molecular clouds, dense clouds, dark clouds
0–50
103–105
Hot molecular cores
Protostellar cores
100–300
>106
the atomic Cold Neutral Medium are traced by low-temperature, rotational transitions at millimeter/submillimeter wavelengths. Spectra obtained for simple, closed-shell (no unpaired electrons), neutral (no charge), diatomic species are simple, having one feature, or line, per
rotational transition. This clean spectrum rapidly becomes quite complex when asymmetry, charge, and/or unpaired electrons are present. A detailed review of ▶ spectroscopy and analysis at these wavelengths can be found in Townes and Schawlow (1955). The following will be a brief
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Molecular energy levels
Electronic ∼4 eV (30,000 cm−1)
Vibrational ∼0.1 eV (800 cm−1) Rotational ∼0.01 eV (80 cm−1)
Interstellar Chemical Processes. Figure 2 Molecular energy levels
overview of the various interstellar processes that occur in various regions of the ISM and are used to examine the chemistry and physical conditions.
Gas-Phase Chemistry Matter expelled from stellar atmospheres, particularly older stars, is in the form of molecules and dust. Most species tend to not survive the harsh conditions of the ISM and are photodissociated or fragmented into smaller molecules. Some species will remain in molecular form and become part of the natal material for new stars in diffuse and molecular clouds. In molecular clouds, significant chemistry will occur due to the higher densities and protection from external destructive UV photons. Numerous chemical models have been developed to simulate the chemistry that occurs throughout stellar evolution (see Millar 2006 and references therein). Most of these models employ large chemical networks involving over 5,000 reactions and hundreds of atoms and molecules. Most of these reactions are found in either the UMIST Database for Astrochemistry 06 database (Woodall et al. 2007; http:// www.udfa.net/) or the Ohio State site (Herbst and Wakelam 2008; http://www.physics.ohio-state.edu/eric/ research.html). The ISM is very diffuse with respect to terrestrial densities. Even the “dense” protostellar cores are rarified compared to the Earth’s atmosphere. In these low-density environments, typically only two-body collisions can occur. Temperatures vary from tens to thousands of Kelvin. UV and EUV radiation from nearby stars also plays a key role in the chemistry that occurs throughout the Galaxy (see Wolfire & Kaufman review).
Photochemistry Regions within the ISM that have a strong UV field from nearby hot massive (OB) stars or the general interstellar
radiation field are known as ▶ photodissociation regions (PDRs). The chemistry of PDRs is dominated by the formation and destruction of H2 (Hollenbach and Tielens 1999). Electron recombination reactions combined with charge transfer help maintain the ionization balance (see Table 2). ▶ Photoionization and photodissociation drive additional chemistry. The extent of chemistry that may occur in these objects depends on the gas density and pressure, intensity of the incident radiation field, dust scattering, and elemental abundances (Sternberg and Dalgarno 1995). PDRs have distinguishable molecular regions, commonly associated with the optical depth of the source from the radiation field. The most exposed region contains atomic hydrogen, helium, oxygen, and ionized carbon, and generally a higher temperature (100–1,000 K). The next region is defined by the formation of molecular hydrogen (H2), but carbon is still in its ionized form. Beyond that, carbon is converted into the molecular form of CO, and temperatures are lowered (10–100 K) due to rotational emission from this molecule. Photochemistry also plays a key role on interstellar grains and ices. This is briefly discussed below.
Ion-Neutral Chemistry Interstellar chemistry is highly influenced by the low temperatures, which require exothermic reactions, and the low densities that restrict these reactions to mostly two bodies. Reactions that involve ions, either positively or negatively charged species, typically meet this requirement and tend to dominate interstellar gas-phase chemistry (e.g., Herbst and Klemperer 1973). The formation of positive ions in the ISM is highly dependent on cosmic ray (CR) bombardment. Ionization of H2, via this process, to produce H3+ drives most ion-neutral chemistry in dense regions, leading to a variety of complex ions (Bergin 2009). These newly formed ions then typically undergo dissociative recombination (see Table 2) to form less protonated neutral fragments. Radiative association with H2 does occur, though these reactions tend to be much slower (Herbst and van Dishoeck 2009). At low temperatures, ion-neutral reactions may also influence interstellar isotope enrichments, known as fractionation, due to small differences in the zero-point vibration energies. This is most commonly observed via the deuterium exchange reaction with H3+, where HD is favored over H2: H3 þ þ HD ! H2 Dþ þ H2 H2D+ will then fractionate other species by deuteron transfer reactions (see review by Millar).
Interstellar Chemical Processes
Interstellar Chemical Processes. Table 2 Reaction categories (Adapted from Bergin 2009. Rates for reactions can be found on the UMIST database (http://www.udfa.net/) by Woodall et al., 2007) Reaction
Example
Ion-neutral
H3+ + O ! H2O+ + H
Neutral-neutral
OH + H2 ! H2O + H
Photoionization
H2O + hn ! H2O+ + e
Photodissociation
H2O + hn ! OH + H
Photodetachment
C6H + hn ! C6H + e
Radiative association
H2 + S+ ! H2S+ + hn
Dissociative recombination
H3O+ + e ! H2O + H
Associative detachment
C6H + H ! C6H2 + e
Other isotope exchange reactions, mostly ion-neutral, are also quite significant in the ISM and can enhance abundance ratios to extreme values. These mechanisms are typically of the form: 13
Cþ þ12 CO!12 Cþ þ13 CO
The reverse reaction barely proceeds at low temperatures, thereby enhancing one isotopologue, 13CO in this case, in cold regions (e.g., Langer et al. 1984).
Neutral-Neutral Chemistry The interaction between two uncharged species, or neutral-neutral reactions, also plays a role in interstellar chemistry. These reactions are typically endothermic with high activation barriers for the interaction between two non-radicals (species bearing no unpaired electrons). These reactions are insignificant in cold, diffuse interstellar chemistry. However, the presence of a radical or an atom can significantly lower the barrier and such reactions then become important to the formation and destruction of molecules (Smith et al. 2004). Reactions involving radicals still have a limited number of experimental studies, though a few have been tested at low temperatures and have proven the current theories (Smith et al. 2006; Sabbah et al. 2007; review by Smith).
Anions The recent discovery of anions, negatively charged species, in the ISM and in circumstellar envelopes has expanded the gas-phase molecular processes typically considered in these environments. The formation of these negative ions is by radiative association of an electron to a neutral parent species, which is fairly efficient for precursors with large electron affinities (Herbst and Osamura 2008), or through
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dissociative recombination of a neutral and electron. Anions are considered to be readily destroyed by three mechanisms: associative detachment, ion-neutral reactions, and photodetachment (Herbst and van Dishoeck 2009). Recently, models have incorporated anion chemistry and have found consistency with observed abundances in the ISM (Millar et al. 2007). These studies have verified the enhanced abundances of larger (long chain) anions compared to their shorter counterparts. This is due to the radiative association rate increase with the number of atoms (Roueff and Herbst 2009). However, these processes have only limited experimental data.
Shocks The interstellar plasma is highly compressible and wave motions can easily exceed characteristic signal speeds leading to the formation of either hydrodynamic or magnetohydrodynamic (MHD) shocks, depending on the magnetic field strength. The former have been classified as J-type (jump), or J-shocks, in which physical flow variables (e.g., density and temperature) undergo a discontinuous jump at the shock front. The latter case gives rise, in weakly ionized plasma, to C-shocks where the neutral and (ion-electron) plasma components behave as individual fluids and in which the flow variables are continuous (see ▶ Shocks, Interstellar). The principal chemical effects in shocked gas arise from the elevated gas temperatures. Temperatures in the range 1,000–4,000 K can drive endothermic reactions and exothermic processes possessing activation-energy barriers. In multi-fluid MHD C-chocks, the relative streaming motion between the ion-electron plasma and the neutral fluid imparts additional energy into endoergic ion-molecule reactions. Shock chemistry thus opens up molecular formation and destruction pathways that are not accessible in cold pre-shock gas leading, for example, to high abundances of sulfur-bearing molecules. Particularly important are reactions with H2, as in the reaction sequence that converts oxygen atoms into hydroxyl and then into water molecules in shock waves around newly formed stars. Apart from allowing new high-temperature pathways for two-body molecular chemistry in the post-shock gas, strong shocks produce a UV radiation field that can initiate photochemistry both upstream and downstream of the shock front. An important chemical process involving interstellar shock waves is dust processing. Refractory dust particles coated by molecular ices can be efficiently sputtered by high-energy collisions with gaseous atoms and molecules. This leads to the removal of the icy mantles and consequently high abundances of the constituent molecules in
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the gas (e.g., methanol). At higher shock speeds, the refractory core itself is sputtered, and this is the origin of the pronounced SiO emission observed in the outflows and winds from young protostars.
(Wooden et al. 2004). CO also follows the same process once accreted onto a surface to produce CH3OH (Charnley et al. 1997).
Radiation Effects
Surface Chemistry Dust and grains formed in the outflows of evolved stars are dispersed throughout the ISM. At low temperatures, atoms and small molecules will collide with and/or adhere to the surface of these grains, forming an ice layer. The formation of some molecules, particularly hydrogenated species, on the surfaces of grains that are ejected into the gas phase is a significant process for interstellar chemistry (van Dishoeck and Blake 1998). There are three major theories explaining surface reactions in interstellar grains: ▶ Langmuir–Hinshelwood, hot-atom, and ▶ Eley–Rideal (Fig. 3). Small molecules or atoms will accrete onto a binding site of a molecular surface with some amount of efficiency. They can then react with an atom or a molecule already on the grain, and the product can desorb by either thermal evaporation (sublimation) or various other nonthermal processes (Herbst and van Dishoeck 2009). Such gas-grain processes dominate the formation of molecular hydrogen (H2), which is not efficiently formed by two-body collisions in the gas. However, due to our limited knowledge of the composition and structure of interstellar grains, the exact formation mechanism of H2 is not understood, regardless of numerous theoretical and experimental studies (e.g., Cazaux and Tielens 2002, 2004, 2010; Habart et al. 2004; Pirronello et al. 1997, 1999, among others). Surface processes are also considered to be the dominant method for hydrogenation of C, O, N, and S forming CH4, H2O, NH3, and H2S, respectively
Langmuir-hinshelwood
Icy grain mantles are observed throughout the ISM and are subjected to ultraviolet radiation, cosmic rays, and temperature variations that will alter the surface composition (e.g., Bernstein et al. 1995; Moore and Hudson 1998). Such effects as amorphization, formation of residues, sputtering, and surface chemistry are produced (Hudson and Moore 2001). Most observed interstellar ices are composed of H2O, CH3OH, NH3, CO, and CO2 though they likely bear other minor constituents not detectable at trace concentrations beneath the strong infrared bands of the simple species (e.g., Gibb et al. 2000). These simple ice mixtures may then be exposed to external radiation (UV photons or cosmic rays), which will either sputter simple species from the mantle or stimulate molecular reactions to occur and products will desorb (Fig. 4; see ▶ Photodesorption). Photolysis of interstellar ices will produce radicals and ions that react on the surface as well as during thermal events. Numerous experiments have been conducted to gain further understanding of these reactions upon exposure to UV photons and H+ irradiation (radiolysis). Particular focus has been toward the hydrogenation reactions of CO ices, typically mixed with H2O, in forming CH3OH in observed quantities (e.g., d’Hendecourt et al. 1986; Watanabe et al. 2003; Hudson and Moore 1999). However, these experimental yields are not fully consistent with observed abundances in the ISM. Ice-grain photolysis chemistry has received much attention for its astrobiological implications. Multiple
Hot-atom
Interstellar Chemical Processes. Figure 3 Grain-surface reactions mechanisms
Eley-rideal
Interstellar Chemical Processes
Accretion by adsorption from gas phase
Grain provides a ‘catalytic’ surface with a weak H-bonded network
H2
Cosmic rays
UV photons
H2O CH3OH
Silicate core
H NH3 Heating effects Desorption products
H2CO
CO2
CH4
CO CO
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▶ Photodesorption ▶ Photodissociation ▶ Photodissociation Regions ▶ Photoionization ▶ Polycyclic Aromatic Hydrocarbons ▶ Radio Astronomy ▶ Shocks, Interstellar ▶ Spectroscopy
References and Further Reading Reaction products
H2O Surface diffusion brings adsorbed molecular species into close contact
Interstellar Chemical Processes. Figure 4 Illustration of the makeup of an icy dust grain in the ISM and the typical energetic processes to which it is exposed. (From Fraser et al. 2002)
studies have been conducted on the formation of prebiotic species including amino acids, nucleobases, amphiphiles, etc. (e.g., Bernstein et al. 2002; Nuevo et al. 2009; Dworkin et al. 2001). These studies suggest that prebiotic compounds are readily produced by energetic processing of interstellar ices and may therefore be ubiquitous throughout the ISM.
See also ▶ Chemical Reaction Network ▶ Dense Clouds ▶ Dense Core ▶ Diffuse Clouds ▶ Dust Cloud, Interstellar ▶ Electron Dissociative Recombination ▶ Electron Radiative Recombination ▶ Eley–Rideal Mechanism ▶ Gas-Grain Chemistry ▶ Interstellar Medium ▶ Isotopic Fractionation (Interstellar Medium) ▶ Langmuir-Hinshelwood Mechanism ▶ Molecular Abundances ▶ Molecular Cloud ▶ Molecular Depletion ▶ Molecules in Space ▶ Photochemistry
Bergin EA (2009) The chemical evolution of protoplanetary disks. In: Garcia P (ed) Physical processes in circumstellar disks around young stars. University of Chicago Press, Chicago Bernstein M et al (1995) Organic compounds produced by photolysis of realistic interstellar and cometary ice analogs containing methanol. Astrophys J 454:327 Bernstein M et al (2002) Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues. Nature 416:401 Cazaux S, Tielens AGGM (2002) Molecular hydrogen formation in the interstellar medium. Astrophys J 575:L29 Cazaux S, Tielens AGGM (2004) H2 Formation on grain surfaces. Astrophys J 604:222 Cazaux S, Tielens AGGM (2010) Erratum: H2 formation on grain surfaces. Astrophys J 715:698 Charnley SB, Tielens AGGM, Rodgers SD (1997) Deuterated methanol in the orion compact ridge. Astrophys J 482:L203 d’Hendecourt LG, Allamandola LJ, Grim RJA, Greenberg JM (1986) Time-dependent chemistry in dense molecular clouds. II - Ultraviolet photoprocessing and infrared spectroscopy of grain mantles. Astron Astrophys 158:119 Draine BT, McKee CF (1993) Theory of interstellar shocks. Annu Rev Astron Astr 31:373 Dworkin JP, Deamer DW, Sandford SA, Allamandola LJ (2001) Special feature: Self-assembling amphiphilic molecules: Synthesis in simulated interstellar/precometary ices. PNAS 98:815 Evans N (1999) Physical conditions in regions of star formation. Annu Rev Astron Astr 37:311 Fraser HJ et al (2002) Laboratory surface astrophysics experiment. Rev Sci Instrum 73:2161 Gibb EL et al (2000) An inventory of interstellar ices toward the embedded protostar W33A. Astrophys J 536:347 Habart E et al (2004) Some empirical estimates of the H2 formation rate in photon-dominated regions. Astron Astrophys 414:531 Herbst E, Klemperer W (1973) The formation and depletion of molecules in dense interstellar clouds. Astrophys J 185:505 Herbst E, Osamura Y (2008) Calculations on the formation rates and mechanisms for CnH anions in interstellar and circumstellar media. Astrophys J 679:1670 Herbst E, van Dishoeck EF (2009) Complex organic interstellar molecules. Annu Rev Astron Astr 47:427 Herbst E, Wakelam V (2008) The Ohio State University astrophysical chemistry group. http://www.physics.ohio-state.edu/eric/research. html Hollenbach DJ, McKee CF (1979) Molecule formation and infrared emission in fast interstellar shocks. I Physical processes. Astrophys J Suppl S 41:555 Hollenbach DJ, Tielens AGGM (1999) Photodissociation regions in the interstellar medium of galaxies. RvMP 71:173
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Hudson RL, Moore MH (1999) Laboratory studies of the formation of methanol and other organic molecules by water+carbon monoxide radiolysis: Relevance to comets, icy satellites, and interstellar ices. Icarus 140:451 Hudson RL, Moore MH (2001) Radiation chemical alterations in solar system ices: An overview. J Geophys Res 106:33275 Langer WD, Graedel TE, Frerking MA, Armentrout PB (1984) Carbon and oxygen isotope fractionation in dense interstellar clouds. Astrophys J 277:581 McKee CF, Hollenbach DJ (1980) Interstellar shock waves. Annu Rev Astron Astr 18:219 Millar TJ (2006) What do we know and what do we need to know? In: Lis DC, Blake GA, Herbst E (eds) IAU Symp. 231, Astrochemistry: recent successes and current challenges. Cambridge University Press, Cambridge, p 77 Millar TJ, Walsh C, Cordiner MA, Ni Chuimin R, Herbst E (2007) Hydrocarbon anions in interstellar clouds and circumstellar envelopes. Astrophys J 662:87 Moore MH, Hudson RL (1998) Infrared study of ion-irradiated water-ice mixtures with hydrocarbons relevant to comets. Icarus 135:518 Nuevo M et al (2009) Formation of uracil from the ultraviolet photoirradiation of pyrimidine in pure H2O ices. AsBio 9:693 Pirronello V, Liu C, Roser J, Vidali G (1999) Measurements of molecular hydrogen formation on carbonaceous grains. Astron Astrophys 344:681 Pirronello V, Liu C, Shen L, Vidali G (1997) Laboratory synthesis of molecular hydrogen on surfaces of astrophysical interest. Astrophys J 475:L69 Roueff E, Herbst E (2009) Molecular ions in astrophysics. JPhCS 192:012008 Sabbah H, Biennier L, Sims IR, Georgievskii Y, Klippenstein SJ, Smith IWM (2007) Understanding reactivity at very low temperatures: The reactions of oxygen atoms with alkenes. Science 317:102 Smith IWM, Herbst E, Chang Q (2004) Rapid neutral-neutral reactions at low temperatures: a new network and first results for TMC-1. MNRAS 350:323 Smith IWM, Sage AM, Donahue NM, Herbst E, Quan D (2006) The temperature-dependence of rapid low temperature reactions: experiment, understanding and prediction. Faraday Discuss 133:137 Sternberg A, Dalgarno A (1995) Chemistry in dense photon-dominated regions. Astrophys J Suppl S 99:565 Townes CH, Schawlow AL (1955) Microwave spectroscopy. McGraw-Hill, New York van Dishoeck EF, Blake GA (1998) Chemical evolution of star-forming regions. Annu Rev Astron Astr 36:317 Watanabe N, Shiraki T, Kouchi A (2003) The dependence of H2CO and CH3OH formation on the temperature and thickness of H2O-CO ice during the successive hydrogenation of CO. Astrophys J 588:L121 Woodall J, Agundez M, Markwick-Kemper AJ, Millar TJ (2007) The UMIST database for astrochemistry 2006. Astron Astrophys 466:1197 Wooden DH, Charnley SB, Ehrenfreund P (2004) Composition and evolution of interstellar clouds. In: Festou M, Keller HU, Weaver HA (eds) Comets II. University of Arizona Press, Tucson, p 33
Interstellar Chemistry ▶ Interstellar Chemical Processes
Interstellar Dust DOUGLAS WHITTET New York Center for Astrobiology, Rensselaer Polytechnic Institute, Troy, NY, USA
Synonyms Cosmic dust; Interstellar grain; Interstellar particles
Keywords Chemical reactions, Extinction, Infrared radiation, Interstellar medium, Polarization, Scattering, Solid particles, Substrate
Definition Interstellar dust consists of small solid particles with sizes in the nanometer to micrometer range, accounting for approximately 1% of the total mass of the ▶ interstellar medium. The particles are generally thought to be composed of a mixture of silicates and solid carbon (amorphous or graphitic carbon, aliphatic or aromatic hydrocarbons, and possibly ▶ kerogen-like organic material). They are responsible for the ▶ extinction and polarization of starlight, and for ▶ scattering that produces ▶ reflection nebulae around dust-embedded stars; they also act as efficient radiators in the infrared, providing an important cooling mechanism for interstellar clouds and protostellar envelopes. The surfaces of dust grains provide a substrate for important chemical surface reactions.
History The earliest evidence for the presence of dust in the interstellar medium was provided in the late nineteenth and early twentieth centuries by photographic images of dark nebulae, such as those reported by E. E. Barnard in his Atlas of the Milky Way, and by studies of the distribution and colors of stars by Wilhelm Struve, Jacobus Kapteyn, Robert Trumpler, and others (see Whittet 2003 for a review). It was found that the apparent number of stars per unit volume of space shows an anomalous decline with distance from the Sun unless it is assumed that starlight is dimmed by extinction in proportion to the distance traveled. The dust was also shown to redden starlight. Much subsequent work in the mid-twentieth century on the optical properties of the dust was motivated by the desire to correct data for its presence rather than by an intrinsic interest in the phenomenon. In more recent times, dust has been recognized as a vital and active ingredient of the
Interstellar Dust
interstellar medium and as a primary carrier of the elements needed to make planets and life to new solar systems.
Basic Methodology The study of interstellar dust involves astronomical observation over a wide range of wavelength, interpreted with the aid of mathematical modeling and laboratory data (e.g., Mathis 1996; Draine 2003; Whittet 2003; Witt et al. 2004). The primary observational phenomena arising from dust are extinction, polarization, and infrared emission. Extinction is the combination of two physical phenomena: absorption (whereby photons are removed from a transmitted beam and their energy converted to internal kinetic energy of the particle) and scattering (whereby photons are deflected from the transmitted beam). Scattering is primarily responsible for the reddening of starlight by selective removal of blue light from the transmitted beam, and scattered light is observed directly as bluish reflection nebulae in the vicinity of dustembedded stars, and as a weak background (the diffuse galactic light). The absorptive component of extinction may produce discrete spectral features, including a broad, strong absorption “bump” centered at 217 nm in the ultraviolet, and various features associated with vibrational modes of solids in the infrared. For grains in thermal equilibrium, the balance of energy absorbed at shorter wavelengths is reemitted in the mid-to-far infrared. The smallest (nano-sized) interstellar particles undergo substantial transient heating on absorption of individual energetic photons, causing them to exhibit both line and continuum emission in the near-to-mid infrared. The interaction of dust with radiation also results in polarization. This is observed in starlight, in scattered radiation produced by reflection nebulae, and in infrared radiation emitted by the dust. Polarization phenomena are generally attributed to non-spherical (flattened or elongated) particles that are partially aligned by the presence of ▶ magnetic fields. Other methods used to study interstellar dust include gas-phase abundance measurements for chemical elements that may be adsorbed onto dust (Whittet 2003, 2010), and laboratory investigations of presolar grains in primitive meteorites (Zinner 2007).
Key Research Findings The efficiency of the interaction between radiation and a solid particle is optimized when the dimensions of the particle are of the same order as the wavelength of the radiation. Because of this, spectral variations in interstellar extinction and polarization are highly sensitive to particle size. The observations show that particles with dimensions
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in the nanometer to micrometer range are present in the interstellar medium. Small particles greatly outnumber larger ones (the number of particles in a given size interval varies approximately with size to the power -3.5), but the larger particles nevertheless contain the bulk of the total mass. Dust has been shown to contain virtually all of the interstellar endowment of the abundant planet-building chemical elements Mg, Si, and Fe, together with substantial fractions of the key life elements C and O. In lowdensity (“diffuse”) regions of interstellar space, the dust is composed primarily of amorphous, anhydrous silicates such as olivine and pyroxene, and of carbon in various forms (amorphous or partially graphitized carbon, aliphatic or aromatic hydrocarbons, and possibly kerogenlike organic refractory matter). The smallest particles include a variety of ▶ polycyclic aromatic hydrocarbons responsible for a characteristic family of spectral emission lines in the infrared (Tielens 2008). The grains exchange material with the interstellar gas: gas-phase atoms and molecules become attached to the surfaces of the grains and may react there to form new molecules, including ices, which are widely observed in lines of sight that intercept relatively dense molecular clouds. Reactions on the surfaces of dust grains provide the only viable route to formation of H2, the most abundant interstellar molecule. The grains are destroyed on timescales of a few hundred million years by energetic shock waves that permeate interstellar space as the result of supernova explosions. The composition and evolution of interstellar dust in molecular clouds is of particular importance to astrobiology as the dust acts both as a carrier of key chemical elements needed to form planets and life, and as a crucible for chemical evolution of those elements (van Dishoeck 2004; Whittet 2010). Catalytic surface reactions on dust have been shown to be effective sources of liferelevant molecules (such as H2O, CH4, CH3OH, HCOOH, H2CO, and HCN) that are produced much less efficiently by gas-phase reactions. It is not yet clear to what extent such molecules survive the formation of new solar systems such as our own, to become incorporated into planetesimals and delivered to planetary surfaces. However, analyses of the isotopic compositions of meteorites and interplanetary dust particles (including cometary samples returned by the Stardust Mission) indicate that they contain some material of exotic (presolar) origin, although this material is usually highly refractory and does not include the life-relevant molecules. Interstellar dust also plays a key role in the physical process of star formation for Sun-like stars. Stars are born in dense cores within molecular clouds at initial temperatures of order 10 K. As a ▶ protostar begins to collapse,
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the release of gravitational potential energy causes it to become warmer, and the resulting thermodynamic pressure would halt the expansion in the absence of a mechanism to dissipate heat. Dust grains provide such a mechanism: they are warmed by collisions with the gas, and dissipate heat by radiating at far-infrared wavelengths, allowing the collapse to continue. The radiation field that permeates a region of active star formation is regulated by the presence of dust. The dust attenuates energetic radiation, providing shielded environments in which complex molecules can survive. This interaction of dust and radiation can also lead to the production of significant levels of circular polarization in regions containing luminous protostars, typically as the result of scattering by aligned grains. It has been proposed that ▶ photochemistry driven by circularly polarized radiation of a given handedness may lead to chiral selectivity and might explain the observed excess of L-enantiomers in amino acids extracted from carbonaceous meteorites in our Solar System.
Future Directions Important questions concerning the nature and evolution of interstellar dust and its relevance to astrobiology remain to be answered. Two key projects are summarized here. The life cycle of dust. A prime source of interstellar dust is condensation of solid particles in the ejecta of dying stars such as red giants, supergiants, and ▶ supernovae. However, it is estimated that dust grains are destroyed in interstellar shocks far more rapidly than they are replenished by stellar ejecta, implying that the bulk of the dust material must be formed in the interstellar medium itself. This is facile for ices, which readily condense (and survive) within the confines of molecular clouds, but an interstellar origin is problematical for the refractory dust that persists in harsher environments. New observations are needed to test the hypothesis that organic refractory matter, formed by energetic processing of the ices in regions of active star formation, is a significant constituent of the dust. Future observations of greater sensitivity will be needed (as will be possible, e.g., with the James Webb Space Telescope), together with new laboratory work on the spectra of analogue materials. The interstellar endowment of protoplanetary disks. Models for the origin of primitive bodies such as ▶ comets and icy planetesimals in our solar system span the range from simple assemblages of interstellar dust and ices to material that has been almost completely reprocessed and recondensed in the solar nebula. Whether interstellar matter has any direct influence on the inventory of liferelevant molecules delivered to planetary surfaces thus
remains an open question. Progress toward an answer to this question is likely to result from integration of research activities in several areas, including astronomical observations of ▶ protoplanetary disks around Sun-like stars and of comets in our solar system, mathematical modeling of processes in protoplanetary disks, and laboratory analyses of available samples of primitive solar-system material, including ▶ carbonaceous chondrites, interplanetary dust particles, and stardust samples.
See also ▶ Absorption Spectroscopy ▶ Alignment of Dust Grains ▶ Carbonaceous Chondrite ▶ Coagulation, of Interstellar Dust Grains ▶ Comet ▶ Dust Cloud, Interstellar ▶ Extinction, Interstellar or Atmospheric ▶ Interplanetary Dust Particles ▶ Interstellar Chemical Processes ▶ Interstellar Ices ▶ Interstellar Medium ▶ Kerogen ▶ Magnetic Field ▶ Molecular Cloud ▶ Photochemistry ▶ Polarized Light and Homochirality ▶ Polycyclic Aromatic Hydrocarbons ▶ Protostars ▶ Protoplanetary Disk ▶ Protostellar Envelope ▶ Reddening, Interstellar ▶ Reflection Nebula ▶ Refractory Organic Polymer ▶ Scattering ▶ Shocks, Interstellar ▶ Silicate Minerals ▶ Star Formation, Observations ▶ Stardust Mission ▶ Supernova
References and Further Reading Draine BT (2003) Interstellar dust grains. Annu Rev Astron Astrophys 41:241–289 Mathis JS (1996) Dust models with tight abundance constraints. Astrophys J 472:643–655 Tielens AGGM (2008) Interstellar polycyclic aromatic hydrocarbon molecules. Annu Rev Astron Astrophys 46:289–337 van Dishoeck EF (2004) ISO spectroscopy of gas and dust: from molecular clouds to protoplanetary disks. Annu Rev Astron Astrophys 42:119–167
Interstellar Ices Whittet DCB (2003) Dust in the galactic environment. Institute of Physics, Bristol Whittet DCB (2010) Oxygen depletion in the interstellar medium: implications for grain models and the distribution of elemental oxygen. Astrophys J 710:1009–1016 Witt AN, Clayton GC, Draine BT (eds.) (2004) Astrophysics of dust. Astronomical Society of the Pacific Conference Series, vol 309, ASP, San Francisco Zinner E (2007) Presolar grains. Treat Geochem 1:1–33
Interstellar Grain ▶ Dust Grain ▶ Interstellar Dust
Interstellar Ices DOUGLAS WHITTET Department of Physics, Applied Physics & Astronomy, New York Center for Astrobiology, Rensselaer Polytechnic Institute, Troy, NY, USA
Keywords Dust grains, ice mantles, interstellar medium, molecular clouds, surface chemistry, photochemistry
Definition Interstellar ▶ ices are volatile molecular solids that accumulate on the surfaces of ▶ interstellar dust grains. They are composed primarily of H2O (typically 60–70% by number); other molecules observed or predicted to be present include CO, CO2, CH3OH, CH4, NH3, O2, and N2. Laboratory analog experiments show that thermal and photochemical processing of these ice mixtures can lead to production of more complex organic species, including amino acids. Interstellar ices account for a major fraction of all the available CNO-group elements in star-formation regions and are presumed to be primary carriers of these elements to protoplanetary disks, where they may become incorporated directly into icy planetesimals or may sublimate and undergo further chemical processing in the gas phase.
History Ices were discussed as a probable ingredient of interstellar matter as early as 1935, soon after the presence of dust
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responsible for the ▶ extinction of starlight was confirmed (see Whittet 2003 for a review). It was first proposed by Lindblad that solid particles composed of H2O, NH3, and CH4 could nucleate and grow in interstellar clouds; Jan Oort and Henk van de Hulst subsequently demonstrated that a realistic size distribution of such particles could account for the observed extinction. Subsequent models developed by Mayo Greenberg and others proposed that refractory dust particles composed of graphitic carbon or silicates provide ▶ nucleation sites for growth of ices in dense clouds, leading to the concept of core-mantle particles. The existence of interstellar ices was confirmed when observational facilities for infrared spectroscopy became available in the 1970s. It has long been suspected, following development of the “dirty snowball” model for cometary composition in the early 1950s that ▶ comets may contain presolar ices from the solar birth cloud, but this has yet to be verified.
Basic Methodology Interstellar ices are detected and characterized by means of infrared absorption features that result from the vibrational motion of the molecules they contain. Whereas gas-phase molecules produce infrared “bands” in which a vibrational energy level is split into multiple lines by rotation, a solid produces a pure vibrational feature for each vibrational mode because rotation is suppressed. Solid-state features are generally much broader than gasphase lines, even at the low temperatures appropriate to interstellar ices. Most of the important spectral features identified with the ices lie in the wavelength range 2–20 mm; the most prominent of them is centered at 3 mm and arises from the O–H “stretching” mode of H2O. An example of an infrared spectrum displaying interstellar ices features is shown below. The ices are observed against a continuum provided by a normal background field star (Elias 16, a K-type red giant) serendipitously located behind a cold core within a dense molecule cloud (Fig. 1). Because an observed infrared feature is generally associated with the vibrational mode of a chemical bond rather than a specific molecule, identification is often ambiguous, especially for minor constituents. For example, all organic molecules contain C–H bonds that are active in the 3.3–3.6 mm region of the spectrum, and the precise position, width, and shape of the feature produced by a given species depends on both the structure of the molecule and the nature of the ice matrix in which it is embedded. The absorption profile also changes with the temperature and crystallinity of the ice. Laboratory data for interstellar analogues are therefore essential to aid interpretation of astronomical spectra.
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10 Groundbased ISO SWS Spitzer IRS SL Spitzer IRS SH
H2O Flux (Jy)
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H2O CO2 1
CO silicate
Elias 16 2
CO2 5 Wavelength (μm)
10
20
Interstellar Ices. Figure 1 Composite infrared spectrum of Elias 16, a normal red-giant star located behind a dense molecular cloud. Data from several instruments are included (the Short Wavelength Spectrometer of the Infrared Space Observatory, the Infrared Spectrometer of the Spitzer Space Telescope, and ground-based telescopes). Spectral features of H2O, CO, and CO2 ices and of silicate dust are labeled. Note that narrow gas-phase lines are unresolved in this spectrum
Key Research Findings Observations of the absorption features attributed to ice constrain the basic composition of the material and provide evidence of variations in both composition and structure with environment (e.g., Gibb et al. 2004). In the cold, starless cores within interstellar clouds (i.e., regions that are yet to form stars), the ice is dominated by three species: H2O, CO, and CO2. Of these, H2O is always the most abundant (typically 60–70%). The ubiquity of H2O requires that it must form in situ, primarily by reactions such as H + O => OH and H + OH => H2O occurring on the surfaces of dust particles, because H2O molecules formed by gas-phase reactions are insufficiently abundant to account for the ice mantles by adsorption (freeze-out) onto the grains. CO is the only condensible gaseous interstellar molecule that is abundant enough to contribute significantly to the ices by adsorption. CO molecules sticking to a grain may be oxidized to CO2 (typically by the surface reaction CO + OH => CO2 + H) or may be hydrogenated first to HCO and subsequently to H2CO (formaldehyde) or CH3OH (methanol). Ice mixtures dominated by H2O are generally referred to as “polar” ices (i.e., they have a significant dipole moment). However, in regions of highest density within molecular clouds, most of the available hydrogen is converted efficiently to H2, which is relatively unreactive and remains in the gas at interstellar temperatures. In this situation, hydrogenation reactions are inhibited and ice-mantle growth is dominated by direct freeze-out of gas-phase molecules, primarily CO. The CO-rich ice mixtures that
form in such regions are generally referred to as “nonpolar” ices (i.e., they have a very small dipole moment). Observations of (e.g.,) the 4.67 mm and 15.3 mm features of CO and CO2 allow discrimination between the polar and nonpolar components, which display distinct absorption profiles. Evolution of the ices in regions of active ▶ star formation is explored by observing ice features in the spectra of dust-embedded young stellar objects (i.e., protostars or pre-main-sequence stars) and comparing them with those observed in background field stars that probe quiescent regions of molecular clouds (van Dishoeck and Blake 1998; Whittet 2003; Gibb et al. 2004). Protostars are typically luminous at infrared wavelengths, and the radiation they emit warms interstellar material in their local environs. Widespread evidence for heating of the ices is found: typically, the abundance of the relatively volatile nonpolar ice is reduced relative to the more robust polar ice due to sublimation; changes in profile shape are also observed and attributed to partial crystallization and segregation resulting from increased mobility of molecules in the ice matrix. Heating may also drive chemical reactions: for example, CH3OH, which is potentially an important intermediary between CO and more complex organics, is found to be more abundant in protostellar envelopes than in starless cores, suggesting that its formation has a significant activation energy that is overcome only when the ices are warmed. Evidence for more dramatic changes driven by UV irradiation or proton bombardment is also observed in the spectra of a few luminous
Interstellar Medium
protostars, characterized by the appearance of a feature near 4.62 mm attributed to synthesis of CN-bearing (cyanate) molecules in the ices.
Future Directions Interstellar volatiles are important to astrobiology because they represent a major reservoir of raw materials needed to form planets and life (Ehrenfreund and Charnley 2000; Herbst and van Dishoeck 2009). An important goal for future research will be to elucidate key chemical pathways toward complex molecule formation in the ices and their sublimates. The relatively high abundance of CO2 observed in interstellar ices represents a potential bottleneck that may inhibit synthesis of more biologically interesting molecules: at interstellar temperatures, CO2 is an endpoint that sequesters carbon and oxygen in a chemical form that is relatively nonreactive and not easily processed to other forms. Moreover, the relatively high abundance of CO2 is difficult to explain with current astrochemical models. A key question appears to be the relative rates of the surface reactions CO + OH => CO2 + H and CO + H => CHO (of which only the latter leads to subsequent production of organics such as CH3OH and H2CO). Progress on this problem requires a synthesis of research in observational astronomy (tighter observational constraints on abundances), laboratory astrophysics (measurement of rate constants and activation energies for key reactions), and mathematical modeling. Development of robust models will allow the organic inventories of interstellar matter entering protoplanetary disks to be accurately constrained, and this will inform meaningful comparisons between interstellar matter and comets.
See also ▶ Absorption Spectroscopy ▶ Active Site ▶ Apolar Molecule ▶ Coagulation, of Interstellar Dust Grains ▶ Comet (Nucleus) ▶ Dust Cloud, Interstellar ▶ Extinction, Interstellar or Atmospheric ▶ Gas-Grain Chemistry ▶ Ice ▶ Infrared Space Observatory ▶ Interstellar Chemical Processes ▶ Interstellar Dust ▶ Interstellar Medium ▶ Molecular Clouds ▶ Nucleation of Dust Grains ▶ Photochemistry
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▶ Polar Molecule ▶ Protostars ▶ Protostellar Envelope ▶ Spitzer Space Telescope ▶ Star Formation, Observations
References and Further Reading Ehrenfreund P, Charnley SB (2000) Organic molecules in the interstellar medium, comets and meteorites: a voyage from dark clouds to the early earth. Annu Rev Aston Astrophys 38:427–483 Gibb EL, Whittet DCB, Boogert ACA, Tielens AGGM (2004) Interstellar ice: the infrared space observatory legacy. Astrophys J Suppl 151:35–73 Herbst E, van Dishoeck EF (2009) Complex organic interstellar molecules. Annu Rev Aston Astrophys 47:427–480 van Dishoeck EF, Blake GA (1998) Chemical evolution of star-forming regions. Annu Rev Aston Astrophys 36:317–368 Whittet DCB (2003) Dust in the galactic environment. Institute of Physics, Bristol
Interstellar Medium RONALD L. SNELL Department of Astronomy, 517 K Lederle Graduate Research Center University of Massachusetts, Amherst, MA, USA
Keywords Atomic gas, galaxies, interstellar dust, interstellar gas, milky way, molecular gas, star formation
Definition The interstellar medium refers to the tenuous gas and dust that fills the void between stellar systems in galaxies. This gas and dust is not distributed uniformly in interstellar space, but displays significant variations in density, temperature, and ionization state. The interstellar medium is held in place by the gravitational force of the stellar component of galaxies. The interstellar medium contains the raw materials out of which new stars are formed, and as stars die, material is returned to the interstellar medium, often violently, enriched by stellar nuclear processing. The cycling of material in and out of the interstellar medium plays a critical role in the evolution of galaxies.
Overview The space between stellar systems in galaxies is called interstellar space, distinct from interplanetary space (the space within stellar systems) and intergalactic space (the space between galaxies). Interstellar space is not
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empty but is filled with rarefied gas and dust, and this gas and dust is what is commonly referred to as the interstellar medium. Besides gas and dust, interstellar space is also filled with ▶ electromagnetic radiation covering a vast range of wavelengths, energetic particles called cosmic rays, and ▶ magnetic fields, and these components have a strong influence on the physical properties of the interstellar gas and dust. The elemental composition of the interstellar medium is dominated by hydrogen, with lesser amounts of helium, and only trace amounts of the elements heavier than helium. The most abundant of these trace elements, or what astronomers often refer to as the “heavy elements” or “metals,” are oxygen, carbon, and nitrogen. The state of the gas in the interstellar medium ranges from being a nearly fully ionized plasma to being almost entirely in molecular form. The dust in the interstellar medium consists of small solid particles with a range of sizes, composed of both silicate and carbon grains (see ▶ interstellar dust). By mass, dust represents only about 1% of the interstellar medium; however, about half the mass of the heavy elements is in the form of dust. The interstellar medium is best studied in the ▶ Milky Way, where, as in other spiral galaxies, we find most of the gas and dust concentrated to the Galactic mid plane. The interstellar medium is held in place by the combined gravitational force of the stars in the Galaxy. The thickness of the gas and dust layer, often called the scale height, is determined by a balance between the forces of gravity and gas pressure (both thermal and turbulent), and in the Milky Way this layer is relatively thin compared with the dimensions of the Galaxy. In the Milky Way, astronomers have identified a number of distinct “phases” of the interstellar medium. These phases of the interstellar medium are characterized by significant differences in the temperature, density, and ionization state of the gas. Six phases of the interstellar medium are recognized, which in decreasing temperature
order are (1) the hot ionized medium (HIM), (2) ▶ HII regions (where the hydrogen is photoionized, H+ regions), (3) the warm ionized medium (WIM), (4) the warm neutral medium (WNM), (5) the cold neutral medium (CNM), and (6) the molecular medium (MM). Table 1 summarizes the properties of the gas in each of these phases. The dominate element, hydrogen, is found in ionized, neutral atomic, and molecular forms in the interstellar medium. The ultraviolet radiation in interstellar space can both dissociate hydrogen molecules and ionize atomic hydrogen. However, most hydrogen-ionizing radiation is localized near HII regions, so neutral atomic hydrogen is an important component of the interstellar medium. Some of the trace elements have ionization potentials smaller than hydrogen (13.6 ev) and can be ionized even in the neutral atomic hydrogen regions. Thus elements such as carbon (ionization potential 11.3 ev) or iron (ionization potential 7.9 ev) are singly ionized in the CNM and WNM phases. In the densest regions, the molecular dissociating radiation cannot penetrate, and the rate of molecule formation is sufficiently rapid to form ▶ molecular clouds (the MM phase). The gas in each of these phases is roughly in thermal equilibrium, where the gas cooling rate per unit volume equals the gas heating rate per unit volume. However, the gas is not in thermodynamic equilibrium with the surrounding radiation field, dust, or cosmic rays. The dominate heating and cooling mechanisms for the gas are different for each of the phases. Since, in general, the cooling rates depend on the square of the gas density, while the heating rates depend linearly on gas density, the equilibrium temperature of the gas decreases with increasing gas density. In the Milky Way we have only a rough estimate of the volume occupied by the different phases and their contribution to the total mass of the interstellar medium. It is believed that by volume the interstellar medium is
Interstellar Medium. Table 1 Propertiesa of the gas in the different phases of the interstellar medium. Density is particle density and not mass density Phase
a
Temperature (K)
Density (cm3)
State of Hydrogen
Hot ionized medium (HIM)
6
10
0.006
Ionized atomic
HII regions
104
1–104
Ionized atomic
Warm ionized medium (WIM)
8,000
0.03
Ionized atomic
Warm neutral medium (WNM)
6,000–8,000
0.25
Neutral atomic
Cold neutral medium (CNM)
100
25
Neutral atomic
Molecular medium (MM)
10–20
103–106
Neutral molecular
Adopted from Ferrie`re (2001) and Lequeux (2005)
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dominated by the lowest density phases (HIM, WIM, and WNM), while by mass, the interstellar medium is dominated by the highest density phases (MM, CNM, and WNM). In the inner regions of the Galaxy (within 8 kiloparsecs [kpc] of the Galactic Center) the mass is dominated by the molecular medium (MM), while in the outer regions of the Galaxy the mass is dominated by the neutral atomic gas (CNM and WNM). The total mass of the interstellar medium in the Milky Way is uncertain, but is estimated to be about 5109 solar masses or about 5% of the stellar mass of the Milky Way (Lequeux 2005).
Basic Methodology A variety of techniques are used to detect the gas and dust in the interstellar medium. The gas component is probed by observations of ▶ spectral lines that can be either in emission or in absorption against a background source of continuous radiation. The gas in the interstellar medium covers a vast temperature range. Since the wavelength or frequency at which emission lines from the gas in the interstellar medium are produced depends on temperature, observations extending from x-ray wavelengths to radio wavelengths are routinely used to probe the various gas phases. We summarize below some of the most common observations of the gas. The earliest evidence for interstellar gas came from optical observations of bright nebulae. Spectroscopic observations of these bright nebulae by William Huggins in 1864 showed that the light was dominated by emission lines, indicating that these objects (which included both HII regions and planetary nebulae) were composed of diffuse hot gas. We now know that the gas in these bright nebulae is almost entirely ionized, and the prominent emission lines are the optical recombination lines of hydrogen and the forbidden lines of O+ (OII), O++ (OIII), S+ (SII), and N+ (NII). The hydrogen recombination lines are produced when hydrogen ions recombine with an electron to an excited electronic state, and this recombination is followed by a series of radiative transitions as the electron cascades down to the ground electronic state, producing an emission line spectrum. In addition, some ions have low lying electronic states (O+, O++, S+, and N+) that can be readily excited by collisions with electrons and, when they radiatively deexcite, produce observable emission lines at optical wavelengths. These emission lines are called “forbidden lines” since they are intrinsically weak magnetic dipole or electric quadrupole transitions and not the much stronger electric dipole transitions. The observed emission line ratios can be used to determine the density and temperature of the ionized gas in HII regions.
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The neutral atomic hydrogen phases of the interstellar medium (WNM and CNM) are too cold for collisions of the gas to excite electronic transitions. The electrons in these atoms are consequently all in their ground electronic state. This neutral component was first detected in the early part of the twentieth century as the interstellar gas absorbed the light of background stars, producing interstellar optical absorption lines in stellar spectra. However, only a few atoms have resonance lines (absorption lines that arise from the ground electronic state) at optical wavelengths; instead most of these lines are in the ultraviolet, so these early observations were limited to only a few chemical elements. The launch of the Copernicus satellite in 1972 provided high resolution ultraviolet ▶ spectroscopy that permitted astronomers to study the most abundant elements (such H, O, C, N) in the mostly neutral atomic interstellar medium. Nevertheless, these absorption line observations are limited to the lines of sight toward relatively nearby bright stars. The detection in 1951 of the emission from neutral atomic hydrogen at a wavelength of 21 cm provided a new avenue for studying the CNM and WNM phases of the interstellar medium. The ground electronic state of hydrogen is split into two very finely spaced sub-levels due to the magnetic interaction between the spin of the proton and the electron. The energy splitting is extremely small, so the upper of the two hyperfine levels is readily excited by collisions even in the coldest phases of the interstellar medium. The subsequent magnetic dipole transition, although very weak, is detectable by radio telescopes. The 21 cm line was the first radio spectral line observed, and it provides the ability to detect atomic hydrogen anywhere in the Galaxy. Emission from this transition is routinely used to map out the distribution of neutral atomic hydrogen in the Milky Way and other galaxies. The 21 cm line can also be seen in absorption against background radio sources. The combination of both emission and absorption observations allowed astronomers in the 1970s to determine the temperature of the hydrogen gas. These studies provided evidence that the neutral atomic gas in our Galaxy coexisted in both cold (100 K) high-density and warm (6,000–8,000 K) lower-density phases. Besides the absorption lines expected to be present due the neutral atomic phase of the interstellar medium, the Copernicus satellite also detected absorption lines from a very highly ionized state of oxygen, O+++++ (OVI). The high ionization potential of this ion (113 ev) excludes photoionization as an origin, leaving only collisional ionization in a plasma with a temperature greater than 300,000 K. About the same time a series of rocket flights
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(Wisconsin Survey) mapped the sky at x-ray wavelengths and detected diffuse soft x-ray emission from the interstellar medium. The soft x-ray emission is a combination of spectral line and bremsstrahlung (continuous) emission from a hot plasma. Both the ultraviolet absorption lines seen from high ionization species and the x-ray emission are evidence for a hot, low-density phase of the interstellar medium (HIM) reviewed in Spitzer (1990). Because the absorption line studies are limited to lines of sight toward relatively nearby bright stars, and because the soft x-ray emission is readily absorbed by gas in the interstellar medium, we know little about the widespread distribution of the HIM in the galaxy. Prior to the detection of carbon monoxide (CO) emission via its pure rotational transition at 2.6 mm in 1970, molecular gas was thought to be only a very minor component of the interstellar medium. Absorption lines from a few simple molecules were detected at optical and ultraviolet wavelengths, suggesting that there was a small fraction of molecules in the neutral atomic phases. However, it was assumed that the ultraviolet radiation would effectively dissociate any molecule that formed. The detection of widespread CO emission, however, changed this view and revealed a surprisingly new and important component of the interstellar medium that is largely molecular. The CO emission, although widespread, is associated with discrete molecular “clouds.” In these molecular clouds, nearly all of the hydrogen is in molecular form. These unexpected large ▶ dense clouds can screen dissociating radiation from the cloud interior, allowing for molecules to form. Observations of the rotational lines of various molecular species permit estimation of the density and temperature of these clouds. The molecular medium is the coldest and densest phase of the interstellar medium. Now well over 100 different molecular species have been detected in the molecular medium (MM) by their rotational transitions at centimeter, millimeter, and submillimeter wavelengths (see ▶ Molecules in Space). The presence of free electrons in interstellar space produces a frequency dependent delay in the pulsed radio signals from pulsars. The amount of time delay, called the pulsar dispersion measure, is directly related to the column density of electrons between the Earth and the pulsar. Observations in the 1970s of the pulsar dispersion measure provided evidence for a widespread distribution of free electrons in the interstellar medium (see review by Ferrie`re 2001). Later, the development of sensitive FabryPerot spectrometers permitted the detection of very extended but weak recombination line and forbidden line emissions at optical wavelengths (Reynolds 2004). These observations suggested the presence of a warm and
ionized component of the interstellar medium (WIM). Prior to these observations, it was thought that photoionized gas was restricted to small isolated regions (called Stro¨mgren spheres) surrounding luminous hot stars. However, these observations revealed that about 90% of the photoionized gas in the Milky Way is found in the more widespread WIM (Reynolds 2004). Finally, the dust component of the interstellar medium can be detected either directly by its thermal emission at infrared wavelengths or by the effects of the intervening dust on the extinction and reddening of starlight. See entry on interstellar dust.
Key Research Findings Studies over the past century have revealed a complex multiphase interstellar medium in the Milky Way.
HII Regions The oldest known component of the interstellar medium is HII (or H+) regions. These are regions of ionized hydrogen gas surrounding luminous hot massive stars (O and B spectral type stars) which produce copious amounts of hydrogen-ionizing photons. The theory of HII regions dates back to Stro¨mgren (1939), who showed that in ionization equilibrium (number of ionizations within a volume equal to the number of recombinations) the gas surrounding these hot stars will be nearly fully ionized out to a radius, now called the Stro¨mgren radius, and is completely neutral beyond that radius. The size of HII regions depends on the density of the gas being photoionized and the rate at which the central star is producing hydrogen-ionizing photons. The gas in the HII region is heated by the photoionization process and cooled by the forbidden line emission, and is typically around 8,000 K. HII regions are not an important mass or volume component of the interstellar medium.
Two-Phase Interstellar Medium The discovery of gas components (WNM, CNM) more pervasive than HII regions (see Methodology) gave rise to the first global theory of the interstellar medium. Field et al. (1969) examined gas in the interstellar medium in both thermal and pressure equilibrium. In their model the gas is heated by the energetic electrons produced by the cosmic-ray ionization of hydrogen. The gas is cooled through the collisional excitation of the fine-structure split levels of the ground electronic states of atoms and ions, such as C+ and O, followed by a radiative transition that returns the atom or ion to its ground state. The emitted photon carries away kinetic energy, thus cooling the gas. Because the cooling rate has a strong dependence
Interstellar Medium
on both density and temperature, Field, Goldsmith, and Habing found that the neutral atomic hydrogen gas could coexist in two stable phases in thermal and pressure equilibrium, one cold and dense (CNW) and a second warmer and less dense (WNM), in agreement with the combined hydrogen 21 cm emission and absorption observations. From this model developed the concept that the interstellar medium was composed of a “cloud” component (CNM) surrounded by an “intercloud” medium (WNM). Although the interstellar medium is much more complex than this early simple model, this concept is still useful for understanding the neutral atomic gas and the model has been recently updated by Wolfire et al. (2003).
Three-Phase Interstellar Medium The existence of a hot “intercloud” medium was first suggested by Spitzer (1956), who argued that such a medium was important for the pressure confinement of interstellar clouds. The UVabsorption studies and x-ray observations, discussed earlier, revealed the presence of a very tenuous and hot component of the interstellar medium (HIM) as predicted by Spitzer. These hot interstellar bubbles are created by high speed shocks produced by ▶ supernova blast waves that expand into the interstellar medium (Cox and Smith 1974). Because of the high temperature and low density of these supernova-created bubbles, Cox and Smith suggested that they would cool slowly, and thus be relatively long lived. McKee and Ostriker (1977) produced one of the first models of the interstellar medium to incorporate the HIM, and in this model the structure of the interstellar medium was regulated by supernova explosions. McKee and Ostriker suggested that a multi-component interstellar medium resulted, in which a large fraction of the volume of the interstellar medium was filled with these hot bubbles. The fraction of the interstellar medium filled with the HIM has been a source of great debate, estimates as low as 10% to as high as 70% have been suggested. The volume fraction depends on both the frequency and clustering of supernovae and their subsequent evolution in the interstellar medium (Ferrie`re 2001, Cox 2005). The model of McKee and Ostriker includes a diffuse ionized layer (WIM) to the WNM component, ionized by diffuse stellar ultraviolet photons. However, the WIM is observed to be much more extensive than pictured in this early model. The WIM is estimated to occupy approximately 20% of the volume of the interstellar medium within 1 kpc of the Galactic midplane (Reynolds 2004). Although the WIM is believed to be photoionized, why hydrogen-ionizing photons are so widespread is not yet well understood. In the traditional view,
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hydrogen-ionizing photons should all be contained within the Stro¨mgren radius surrounding hot O and B stars in the Galaxy. However, if the hot bubbles produced by supernova explosions are pervasive and interconnect, then it may be possible for hydrogen-ionizing photons produced by hot stars to travel large distances, creating the observed widespread diffuse ionized component to the interstellar medium (Reynolds 2004).
Molecular Clouds The molecular medium (MM) is the coldest and densest phase of the interstellar medium and, by definition, represents regions where the hydrogen is almost entirely in molecular form. This medium is sufficiently dense that the molecules that form by gas phase and grain surface chemical reactions are shielded against photo-dissociation by the interstellar ultraviolet radiation field. This gas can be probed by emission produced from excited rotational transitions of molecules that are excited by collisions. Since molecular hydrogen is a symmetric molecule it does not have a permanent electric dipole moment, and hence has only weak electric quadrupole transitions. In addition, molecular hydrogen is a light molecule (small moment of inertia) and therefore the rotational levels are widely spaced in energy, so even the lowest excited rotational levels of molecular hydrogen are rarely excited by collisions in these cold clouds. Therefore, the millimeter and submillimeter wavelength rotational transitions of heavier molecules with permanent dipole moments, such as CO (the next most abundant molecule after molecular hydrogen), are commonly used to measure the distribution of molecular gas in galaxies. Numerous surveys of CO emission have been obtained that have mapped out the distribution of the molecular gas in the Milky Way (Combes 1991). The molecular and atomic gas components of the Galaxy have different distributions; the molecular gas is much more concentrated toward the center of the Galaxy than the atomic gas. Although the molecular component contains a significant fraction of the total mass of the interstellar medium within 8 kpc of the Galactic Center, it occupies a negligible fraction of the volume. The molecular medium is made up discreet units commonly referred to as “molecular clouds.” These molecular clouds are sufficiently dense that self-gravity is important. In fact, it is believed the molecular clouds are close to gravitational equilibrium, where the internal pressure of the molecular gas (which is dominated by macroscopic motions called turbulence and not microscopic thermal motions) is balanced by a combination of the cloud’s self-gravity and the pressure of the external medium. The molecular clouds are observed to be
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grouped together to form molecular cloud complexes. The masses of these molecular clouds and molecular cloud complexes range from 5 to 2106 solar masses and have sizes that range from 0.2 to 80 parsecs (Goldsmith 1987). Most of the molecular mass resides in the most massive molecular clouds (Combes 1991). The clouds have a mean particle number density of approximately 1,000 cm3, while the average density of cloud complexes is lower. In the nearest of these molecular clouds, dust mixed with the gas absorbs the light of background stars, and these clouds appear dark – the so called dark clouds (reviewed by Bergin and Taffalla 2007) (see entry on interstellar ▶ dust clouds). Within molecular clouds are denser regions called molecular cloud cores with densities of order 104–106 cm3, and it is in these cores that the gas is gravitationally collapsing to form new stars. The theory of ▶ star formation is complex and was recently reviewed by McKee and Ostriker (2007). The molecular clouds are also of great interest because of their rich chemistry. Over 100 molecules have been identified in molecular clouds, many of them containing 5 or more atoms (See entry on Molecules in Space). These complex organic molecular species are of particular relevance for astrobiology, as they may survive the formation of the Solar System and later be delivered to the early Earth by cometary and asteroidal impacts.
Summary Astronomers believe they have identified all of the major phases of the gas and dust in the interstellar medium and have a reasonable estimate of the physical properties of each of these phases. The picture of the interstellar medium that has emerged is a multiphase interstellar medium in which cooler denser “clouds” (molecular clouds and the CNM) are embedded in a much warmer less dense “intercloud” medium (WNM, WIM, and HIM). The interstellar medium is very dynamic, as supernova explosions carve expanding hot bubbles in the interstellar medium that are relatively long lived. Many believe that supernovae play a dominate role in shaping the interstellar medium. In the atomic gas, both ordered and turbulence motions can produce over-dense regions (“clouds”) where self-gravity can begin to dominate. The densest regions of these clouds can collapse and form new stars. Thus the gas is constantly being cycled between phases of the interstellar medium and in and out of stars. However, astronomers are far from having a complete understanding of this multiphase interstellar medium in the Milky Way, let alone in other galaxies. Observationally, we need a much better three-dimensional picture of how the various interstellar medium phases are distributed in
interstellar space, particularly the CNM, WNM, and HIM phases. Although we know much about the detailed microscopic processes at work in the gas and dust, we lack a detailed theoretical model for the interactions of these phases and how the interstellar medium evolves with time, cycling between phases and ultimately forming stars. Much observational and theoretical work is needed to fully understand the interstellar medium and the star formation process in galaxies.
See also ▶ Absorption Spectroscopy ▶ Dense Clouds ▶ Diffuse Clouds ▶ Dust Cloud, Interstellar ▶ Electromagnetic Radiation ▶ HII Region ▶ Interstellar Dust ▶ Magnetic Field ▶ Milky Way ▶ Molecular Cloud ▶ Molecules in Space ▶ Radio Astronomy ▶ Shocks, Interstellar ▶ Spectral Line ▶ Spectroscopy ▶ Star Formation ▶ Supernova
References and Further Reading Bergin EA, Tafalla M (2007) Cold dark clouds: the initial conditions for star formation. Annu Rev Astron Astr 45:339–396 Combes F (1991) Distribution of CO in the milky way. Annu Rev Astron Astr 29:195–237 Cox DP, Smith BW (1974) Large-scale effects of supernova remnants on the galaxy: generation and maintenance of a hot network of tunnels. Astr J Lett 189:L105–L108 Cox DP (2005) The three phase interstellar medium revisited. Annu Rev Astron Astr 43:337–385 Dopita MA, Sutherland RS (2003) Astrophysics of the diffuse universe. Springer, Berlin, Heidelberg and New York Dyson JE, Williams DA (1997) The physics of the interstellar medium, 2nd edn. Institute of Physics Publishing, Bristol and Philadelphia Ferrie`re KM (2001) The interstellar environment of our galaxy. Rev Mod Phys 73:1031–1066 Field GB, Goldsmith DW, Habing HJ (1969) Cosmic-ray heating of the interstellar medium. Astr J Lett 155:L149–L154 Goldsmith PF (1987) Molecular clouds: an overview. In: Hollenbach DJ, Thronson HA (eds) Interstellar processes. D. Reidel Publishing Company, Dordrecht, Holland Lequeux J (2005) The interstellar medium. Springer-Verlag, Berlin Heidelberg McKee CF, Ostriker JP (1977) A theory of the interstellar medium–three components regulated by supernova explosions in an inhomogeneous substrate. Astrophys J 218:148–169
Intron McKee CF, Ostriker EC (2007) Theory of star formation. Annu Rev Astron Astr 45:565–687 Reynolds R (2004) Warm ionized gas in the local interstellar medium. Adv Space Res 34:27–34 Spitzer L (1956) On a possible interstellar galactic corona. Astrophys J 124:20–34 Spitzer L (1990) Theories of the hot interstellar medium. Annu Rev Astron Astr 28:71–101 Stro¨mgren B (1939) The physical state of interstellar hydrogen. Astrophys J 89:526–547 Tielens AGGM (2005) Physics and chemistry of the interstellar medium. Cambridge University Press, Cambridge Wolfire MG, McKee CF, Hollenbach D, Tielens AGGM (2003) Neutral atomic phases of the interstellar medium in the galaxy. Astrophys J 587:278–311
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(“▶ Larson’s Laws”) are consistent with the presence of turbulent density and velocity fields.
See also ▶ Larson’s Law ▶ Linewidth ▶ Magnetohydrodynamics ▶ Molecular Cloud ▶ Shocks, Interstellar ▶ Supernova
References and Further Reading Brunt CM, Heyer MH, MacLow MM (2009) Turbulent driving scales in molecular clouds. Astron Astrophys 504:883–890 Hollenbach DJ, Thronson HA Jr (eds) (1987) Interstellar processes. D. Reidel, Dordrecht
Interstellar Particles ▶ Interstellar Dust
Intervening Sequence Interstellar Turbulence STEVEN B. CHARNLEY NASA Goddard Space Flight Center, Solar System Exploration Division, Code 691, Astrochemistry Laboratory, Greenbelt, MD, USA
▶ Intron
Intron Synonyms
Keywords Interstellar medium, Star formation
Definition Interstellar turbulence is the compressible and hydromagnetic bulk flow of interstellar gas on many physical scales.
Overview Unlike the incompressible turbulent flows encountered in terrestrial physics, which are generally subsonic and hydrodynamic, turbulence in the interstellar medium is supersonic, highly compressible, and magnetic. The driving forces of interstellar turbulence are nonuniform and nonhomogeneous, involving, for example, ▶ supernovae blast waves and winds from young stars. Numerical simulations indicate that interstellar supersonic turbulence is dissipated in ▶ shock waves and forms gravitationally unstable density enhancements, which collapse to form stars. In dense ▶ molecular clouds, observed scaling relations between density and ▶ linewidth and size
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Intervening sequence
Definition In interrupted (split) ▶ genes, intron is a part of the sequence that is present in the primary transcript but absent in the mature ▶ RNA sequence. The maturation of the RNA transcript of a split gene implies the specific removal of introns and the fusion of ▶ exons throughout the splicing process. Although the splicing involves sophisticated enzymatic machineries, in some cases, the elimination of introns is an autocatalytic process (selfsplicing), in other words, occurs in the absence of proteins since the RNA is catalytic (▶ ribozymes of group I introns) in nature.
See also ▶ Exon ▶ Gene ▶ Ribozyme ▶ RNA
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Io
Io THERESE ENCRENAZ LESIA, Observatoire de Paris, Meudon, France
Keywords Galilean satellites
Definition Io, discovered by Galileo Galilei in 1610 and named by Simon Marius (1614), is the Galilean satellite that is closest to ▶ Jupiter. It orbits at a distance of 421,800 km (or about 6 Jovian radii) from the planet. With a diameter of 3,640 km and a density of 3.5 g/cm3, Io is the densest Galilean satellite. This property is due to its proximity to Jupiter, which generates a strong internal energy due to tidal effects. As a result, in contrast with all other Galilean satellites, Io has lost all its water content.
Overview Io’s surface was revealed by the images of the ▶ Voyager 1 spacecraft that flew over the satellite in March 1979 at a distance of only 20,500 km (or 11 satellite radii) and provided the first evidence for active volcanism outside the Earth. Io’s surface shows no impact craters but several active volcanoes that replenish it on a short timescale. Fifteen years later, Io was explored again with the Galileo orbiter. From these observations, combined with the ▶ HST images, a temporal study of the volcanic evolution has been possible (Fig. 1).
The temperature of the active volcanoes can reach 1,700 K, while the surrounding surface is at about 130 K. The main detected outgased product is sulfur dioxide (SO2), which freezes at the surface in white spots. Depending upon their temperatures, the sulfur compounds are black, red, or yellow. Silicate products are likely to be dominant as indicated by the high temperature of the volcanoes. Their presence is consistent with models of Io’s internal structure that predict a massive core of iron and (Fe, S) compounds, with a silicate mantle and crust. The proximity of Jupiter induces a mechanical deformation due to tidal effects which would be stable in the absence of other satellites. However, due to gravitational perturbations linked to ▶ Europa and ▶ Ganymede (in resonance with Io), Io’s deformation varies along its orbit, inducing a strong mechanical relaxation especially in the equatorial regions; this is where most of the volcanoes are found. Io’s volcanism produces a stable (although variable) atmosphere of gaseous SO2 which was first identified in a plume by Voyager 1 in the infrared range, and later confirmed from ground-based millimeter spectroscopy. The surface pressure ranges from 3 to 40 nanobars. SO and NaCl have been identified as minor atmospheric components by the same technique. Io’s atmosphere is partially ionized by the ultraviolet solar radiation and by high energy particles. This plasma is driven by the Jovian magnetic field which generates a torus around Jupiter at the distance of Io’s orbit; indeed, Io’s orbit crosses the Jovian magnetosphere. This torus was first detected through the Ly a line by the Pioneer spacecraft. Further UV measurements led to the detection of S and O ions, and Na was detected from visible ground-based observations. Io’s torus was also studied in detail by the ▶ Ulysses spacecraft as it flew over the Jupiter system in 1992.
See also ▶ Europa ▶ Galileo ▶ Ganymede ▶ HST ▶ Jupiter ▶ Ulysses Mission ▶ Voyager (Spacecraft)
References and Further Reading
Io. Figure 1 Io as observed by the Galileo spacecraft in 1995; note the volcanic plumes in the insets (© NASA)
Lopez RMC, Williams DA (2005) Io after Galileo. Rep Progr Phys 68:303–340 McEwen AS (2001) Io: volcanism and geophysics. In: Murdin P (ed) The encyclopedia of astronomy and astrophysics. Bristol, IoP Publishing, pp 1299–1304 Thomas N (2001) Io: plasma torus. In: Murdin P (ed) The encyclopedia of astronomy and astrophysics. Bristol, IoP Publishing, pp 1297–1299
Ion-Neutral Reactions
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There are two alternative ways of radiation damage to biological key substances: either by direct energy absorption (direct radiation effect), or via interactions with radicals, for example, those produced by radiolysis of cellular water molecules (indirect radiation effect). For direct radiation effect, the mean number of injured molecules, for example, DNA is directly proportional to the dose. For indirect effects, the number of changed molecules increases with increasing ▶ Liner Energy Transfer (LET). ▶ Mutations, cancer induction, and cell death are the most critical biological radiation effects.
Definition
See also
IOM ▶ Insoluble Organic Matter
Ion-Exchange Chromatography Synonyms
Ion-exchange chromatography (IEC) is a laboratory separation process that allows the separation of ions and polar molecules based on their charge. It can be used for a multitude of charged molecules, including large molecules, such as proteins and oligonucleotides, and small molecules, such as amino acids. The stationary phase commonly consists of a resin or gel with a covalently linked charged molecule. Depending on the charge IEC can be either anion- or cation-exchange chromatography. It is often used for protein purification, DNA/RNA oligomer analysis, and water analysis.
▶ Biostack ▶ Cosmic Rays in the Heliosphere ▶ DNA Damage ▶ DNA Repair ▶ HZE Particle ▶ Linear Energy Transfer ▶ Mutagen ▶ Mutagenesis ▶ Mutation ▶ Nucleic Acids ▶ Radiation Biology ▶ Radiation Dose
History Ion-exchange chromatography is one of the oldest separation processes described in literature. In 1850, H. Thompson and J. T. Way treated various clays (= stationary phase) with ammonium sulfate or carbonate in solution to release calcium. During World War II, ion-exchange chromatography was further developed and played a crucial role in the “Manhattan project” to enrich radioactive elements.
See also ▶ Affinity Chromatography ▶ Chromatography ▶ HPLC
Ionizing Radiation (Biological Effects) Synonyms
Ion-Molecule Reactions ▶ Ion-Neutral Reactions
Ion-Neutral Reactions IAN W. M. SMITH Chemistry Laboratory, University of Cambridge, Cambridge, UK
Synonyms Ion-molecule reactions
Keywords
Biological radiation effects
Chemical kinetics, interstellar medium, long-range forces, low temperatures, rate coefficients
Definition
Definition
Radiation interacts with matter primarily through ionization and excitation of electrons in atoms and molecules.
Ion-neutral reactions are chemical reactions that occur in binary collisions between one electrically charged species, an
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ion, and one electrically neutral species. As with ▶ neutralneutral reactions, the crucial properties that are sought in kinetic experiments are the rate coefficient and its dependence on temperature, denoted by k(T) and, in cases where two or more sets of products are possible, the branching ratio: that is, the fractions of the overall reaction that proceed via different channels. For a bimolecular reaction, say Aþ þ B ! C þ þ D, the rate of the loss of the ion Aþ, that is d[Aþ]/dt, is given by k(T) [Aþ][B], where the square brackets denote concentrations, usually expressed in cm3, so that the units of k(T) are cm3 s1.
History The occurrence of gas-phase ion-neutral reactions has been recognized since the development of mass spectrometry in the early years of the twentieth century. The ready occurrence of such reactions was shown by the detection of ions that could not be formed by the direct ionization of any neutral species that were known to be present. Indeed, such spurious signals could be viewed as a nuisance when the primary purpose of the experiments was to analyze the species present in a mixture of neutral species. Systematic studies of these reactions began in the early 1950s (Herman and Smith 1992). Most of the experimental work on ionneutral reactions has been on the reactions of positively charged ions (cations), which are known to play a key role in the chemistry of the ▶ interstellar medium. However, the recent discovery of negatively charged ions (anions) in the interstellar medium is already stimulating laboratory work on the reactions of these species.
Overview The existence of an electric charge on an ion causes an attractive force between it and a neutral molecule, which is strong and acts over long-range, compared with the interaction between two neutral molecules. This force arises between the charge on the ion and an electric dipole which it induces in the molecule and, if the molecule itself possesses a permanent electric dipole, between this dipole and the charge. The existence of these strong forces is frequently sufficient to cancel out any potential energy barrier resulting from rearrangement of the electrons as bonds make and break. Consequently, the rates of ionneutral reactions are generally controlled by the rate at which the reactants “capture” one another and are brought into close contact by the attractive interaction. When the neutral molecule does not possess a dipole moment, as is the case for H2, N2, CO2, and CH4, the strength of the attraction depends only on the distance apart of the two species and a simple treatment, usually attributed to Langevin, leads to the prediction that the rate
coefficient (using c.g.s units) is given by k(T) = 2pe (a/m)1/2, where e is the electronic charge on the singly charged ion, a is the polarizability of the neutral molecule, and m is the reduced mass, mAmB/(mA þ mB), of the colliding reactants. This formula is (a) easy to evaluate, and (b) predicts that the rate coefficient for ion-neutral reactions of this kind will not depend on the temperature. However, when the neutral molecule possesses an electric dipole moment, for example, in molecules like H2O and NH3, the treatment of the capture process becomes more complicated, because the force between the reactants now depends not only on their separation but also on the orientation of the dipole with the line joining the centers of the two species. Several theoretical approaches have been adopted. They agree that the average attraction becomes stronger as the temperature is lowered and consequently k(T) increases. Formulae fitting this effect have been proposed (Wakelam et al. 2010). Figure 1 displays the temperature-dependent rate coefficients that have been measured for several ionneutral reactions, comparing the results for two reactions between Nþ ion and the nonpolar molecules N2 and O2 with those for two reactions of Nþ with the polar molecules H2O and NH3. The rate coefficients for the first two reactions do not depend on temperature and are fairly close to the values predicted by the Langevin treatment, whereas the rate coefficients for the reactions of Nþ with H2O and NH3 increase as the temperature is reduced.
Basic Methodology Experiments designed to obtain the rate coefficients (and branching ratios) of ion-neutral reactions fall into two main categories. One is where combinations of electric and magnetic fields are employed to “trap” the ionic reactant and observe the rate of its loss by reaction when a neutral molecule is introduced to the reaction cell. In the second method, the ions are injected into a flowing gas and the loss of the ion is observed when the neutral coreactant is introduced to the gas flow. As both methods employ mass spectrometric detection, it is, at least in principle, possible to observe product ions and hence to obtain branching ratios. The earliest form of the trapping methods used ion cyclotron resonance, where ions are constrained by a combination of electric and magnetic fields. After the introduction of the neutral co-reactant and after a selected time delay, the ions are transferred to an “analyzer region” where their (relative) concentration can be monitored (McMahon and Beauchamp 1972). Ion cyclotron resonance experiments were responsible for many of the rate coefficients that were used in early models of the
Ion-Neutral Reactions
Key Research Findings Measurements of the rate coefficients for a large number of ion-molecule reactions (Anicich 1993a, b) by the methods described in the previous section have confirmed that, for the majority (but not all) of exothermic reactions,
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interstellar medium (Prasad and Huntress 1980). Although this method has largely been employed to determine rate coefficients at room temperature, as explained above, the dependence of the rate coefficients for ionneutral reactions on temperature is frequently straightforward to predict. Trapping methods to determine rate coefficients at very low temperatures have been developed over a number of years by Dunn (1995), Gerlich (1995, 2008) and their coworkers. These experiments, especially those of Gerlich using a 22-pole trap, enable ions to be stored for minutes, thereby allowing the rate coefficients for very slow processes, such as the radiative association of Cþ with H2 to form CH2þ at 10 K, to be measured. The so-called flowing afterglow method was pioneered by Ferguson and his coworkers in the 1960s (Fehsenfeld et al. 1966; Dunkin et al. 1968). It is similar to the flow tube methods used to study the kinetics of neutral-neutral reactions, though the gas flows are greater and the gas dynamics must be carefully analyzed to extract rate coefficients. Ions are created at the upstream end of the flow tube and changes in their concentration are monitored at the downstream end, as neutral reactants are added at one or more point along the flow tube. There have been two major developments to the basic flowing afterglow technique. The first, originating with Adams and Smith (1976), was the introduction of a quadrupole mass filter between the ion source and the main flow tube. This makes it possible to select the reactant ion and the method is generally known by the acronym SIFT for selected ion flow tube. The second development, which allowed the first measurements of rate coefficients below ca. 90 K, was the invention of the CRESU (Cine´tique de Re´action en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flows) technique by Rowe et al. (1984). Some details of this method are given in the entry on neutralneutral reactions. In the study of ion-neutral reactions, ions were generated by irradiation with an electron beam that crossed the supersonic flow of gas just downstream from the exit of the Laval nozzle. Although the CRESU method has been applied to a limited number of ionneutral reactions, the results have been invaluable in demonstrating how the temperature dependence of the rate coefficients differs when ions react with nonpolar and polar molecules (see Figure 1).
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Ion-Neutral Reactions. Figure 1 Examples of rate coefficients for ion-neutral reactions, comparing results for reactions of ions with non-polar and polar molecules with one another and with simple predictions. Experimental data for Heþ þ N2 ( ) and Nþ þ O2 ( ) are compared with the respective Langevin predictions, and . The rate coefficients for Nþ þ H2O ( ) and Nþ þ NH3 ( , ) are compared with predictions based on expressions recommended by Wakelin et al. (2010), and . The experimental data for Heþ þ N2 and Nþ þ O2 are taken from Rowe et al. (1985), those for Nþ þ H2O from Marquette et al. (1985) and those for Nþ þ NH3 (a) from Marquette et al. (1985) and (b) from Rowe et al. (1995)
the observed rate coefficients are close to those suggested by the simple capture theories, and that the rate coefficients remain approximately the same or increase as the temperature is lowered. The rate coefficients may be lower than the simple predictions in those cases where both the ion and neutral species have unpaired electrons, since then reaction can only occur in the fraction of collisions in which electrons – one from each species – pair up to form a chemical bond.
Applications Ion-molecule reactions play an important role in the chemistry of the upper levels of Earth’s atmosphere. However, and especially in the context of this encyclopedia, the principal application of the rate coefficients for ion-molecule reactions is in the modeling of the interstellar medium, especially the cold (ca. 10 K) cores of dense, dark interstellar clouds, where H2 is the dominant species present, and where many of the known interstellar molecules have been detected. The chemistry in these regions of the cosmos, which serve as the birthplace of stars, is initiated by the
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ionization of molecular hydrogen, H2, followed by the reaction of the H2þ ions that are formed with the relatively abundant H2 molecules to form H3þ: H2 þ cosmic ray ! H2 þ þ e and H2 þ þ H2 ! H3 þ þ H The proton affinity of H2 is not very high so the ion H3þ can undergo facile ion-molecule reactions in which a proton is transferred from it to a neutral reactant. This occurs with O atoms, C atoms, and N2, creating OHþ, CHþ, and N2Hþ, and initiating networks of (largely) ionmolecule reactions that lead, inter alia, to molecules such as H2O, CH4, and NH3.
Future Directions Recent modeling calculations (Wakelam et al. 2010), which identify the reactions that are key in determining the abundances of the major molecular species observed in dense interstellar clouds, have shown that reactions of ions with radical atoms are especially important. Such reactions have been studied recently at room temperature in SIFT experiments in which flow-discharge techniques (see section on neutral-neutral reactions) are used to generate measured concentrations of atoms. The results of these experiments – on reactions of both positive and negative ions – have been reviewed by Snow and Bierbaum (2008). The rate coefficients of such reactions may be lowered if the ion as well as the radical atom has unpaired electrons. Furthermore, although radical atoms cannot possess an electric dipole moment, additional attraction to an ion can arise if the electrons in the atom are not symmetrically distributed around its nucleus, and this can lead to an acceleration in the rate of reaction with an ion as the temperature is lowered. This effect is predicted for the important reaction of O atoms with H3þ ions (Bettens et al. 1999). Experimental confirmation of these predictions is desirable.
Anicich VG (1993b) Evaluated bimolecular ion-molecule gas-phase kinetics of positive-ions for use in modeling planetary-atmospheres, cometary comae, and interstellar clouds. J Phys Chem Ref Data 22:1469 Bell MT, Softley TP (2009) Ultracold molecules and ultracold chemistry. Mol Phys 107:99 Bettens RPA, Hansen TA, Collins MA (1999) Interpolated potential energy surface and reaction dynamics for O(3P) þ H3+(1A10 ) and OH + (3S) + H2(1S+g). J Chem Phys 111:6322 Dunkin DB, Fehsenfeld FC, Schmeltekopf AL, Ferguson EE (1968) Ionmolecule reaction studies from 300 to 600 K in a temperaturecontrolled flowing afterglow system. J Chem Phys 49:1365 Dunn GH (1995) Ion-electron and ion-neutral collisions in ion traps. Phys Scripta T59:249 Fehsenfeld FC, Schmeltekopf AL, Goldan PD, Schiff HI, Ferguson EE (1966) Thermal energy ion-neutral reaction rates. 1. Some reactions of helium ions. J Chem Phys 44:4087 Gerlich D (1995) Ion-neutral collisions in a 22-pole trap at very-low energies. Phys Scripta T59:256 Gerlich D (2008) The study of cold collisions using ion guides and traps. In Smith IWM (ed) Low temperatures and cold molecules. Imperial College Press, London Herman Z, Smith D (1992) Ion-molecule reactions – introduction. Chem Rev 92:1471 Marquette JB, Rowe BR, Dupeyrat G, Poissant G, Rebrion C (1985) Ionpolar-molecule reactions: a CRESU study of He+, C+, N+ + H2O, NH3 at 27, 68 and 163 K. Chem Phys Lett 122:431 McMahon TB, Beauchamp JL (1972) Versatile trapped ion cell for ioncyclotron resonance spectroscopy. Rev Scient Instrum 43:509 Prasad SS, Huntress WT Jr (1980) A model for gas phase chemistry in interstellar clouds: I The basic model, library of chemical reactions, and chemistry among C, N and O compounds. Astrophys J Suppl 43:1 Rowe BR, Dupeyrat G, Marquette JB (1984) Study of the reactions N2+ + 2N2 ! N4+ + N2 and O2+ + 2O2 ! O4+ + O2 from 20 to 160 K by the CRESU technique. J Chem Phys 80:4915 Rowe BR, Marquette JB, Dupeyrat G, Ferguson EE (1985) Reactions of He+ and N+ ions with several molecules at 8 K. Chem Phys Lett 113:403 Rowe BR, Canosa A, Le Page V (1995) FALP and CRESU studies of ionic reactions. Int J Mass Spect Ion Proc 149/150:573 Snow TP, Bierbaum VM (2008) Ion chemistry in the interstellar medium. Ann Rev Anal Chem 1:229 Wakelam V, Smith IWM, Herbst E et al (2010) Reaction networks for interstellar chemical modelling: improvements and challenges. Space Science Rev, published on the web 24 November 2010. doi 10.1007/ s11214-010-9712-5
See also ▶ Interstellar Medium ▶ Langevin Rate Coefficient ▶ Neutral-Neutral Reactions ▶ Reaction Rate Coefficient
References and Further Reading Adams NG, Smith D (1976) Selected ion flow tube (SIFT) – technique for studying ion-neutral reactions. Int J Mass Spectrom Ion Phys 21:349 Anicich VG (1993a) A survey of bimolecular ion-molecule reactions for use in modeling the chemistry of planetary-atmospheres, cometary comae, and interstellar cloud – 1993 supplement. Astrophys J Suppl 84:215
Ionosphere ▶ Magnetosphere
IRAS ▶ Infrared Astronomical Satellite
Iron Cycle
Iridium Definition Iridium is a hard, brittle, corrosion-resistant metal with the symbol 77Ir192. It is one of the platinum group elements (PGE - ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum), which are highly siderophilic and form strong bonds with iron. Most of the terrestrial PGE are concentrated in the core, leaving the mantle and crust highly depleted in Ir (ca. 3 and 2Þ H2 Sn ðaqÞ þ 8Fe3þ ðaqÞ ! S8 0 ðsÞ þ 8Fe2þ ðaqÞ þ 8Hþ ðaqÞ
ð3Þ
ð4Þ
In this case, there is no direct production of sulfate. The elemental sulfur can be further oxidized by sulfuroxidizing microorganisms generating sulfuric acid according to Eq. 5: S8 0 ðsÞ þ 3=2O2 ðgÞ þ H2 OðlÞ ! SO4 2 ðaqÞ þ 2Hþ ðaqÞ
ð5Þ
The critical role of iron-oxidizing microorganisms, such as L. ferrooxidans and A, ferrooxidans, in the bioleaching of metal sulfides is to maintain a high concentration of the oxidizing agent, ferric iron, Eq. 6: 4Fe2þ ðaqÞ þ O2 ðgÞ þ 4Hþ ðaqÞ ! 4Fe3þ ðaqÞ þ 2H2 OðlÞ
ð6Þ
Furthermore, it is now well established that iron can be oxidized anaerobically in the absence of oxygen using nitrate (a probably other compounds) as an electron acceptor, Eq. 7:
Fe2þ ðaqÞ þ NO3 ðaqÞ ! Fe3þ ðaqÞ þ NO2 ðaqÞ
ð7Þ
In addition, the hydrolysis of ferric iron further releases protons, a buffer reaction that is responsible for the constant pH observed in these systems, Eq. 8: Fe3þ ðaqÞ þ 3H2 OðlÞ $ FeðOHÞ3 ðaqÞ þ 3Hþ ðaqÞ ð8Þ
Iron Cycle
Oxic [O2]
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At. ferrooxidans At. thiooxidans At. caldus
At. ferrooxidans
SRB
(CH2O)n Acidiphilium spp. Acidimicrobium spp. Ferromicrobium spp.
2−
SO4 (CH2O)n
CO2
Acidiphilium spp. CO2 Fe2+
Fe3+ + H2O
Fe(OH)3 + H+
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Fe2O3
Iron Cycle. Figure 1 Geomicrobiology of the Rı´o Tinto basin, a geochemical terrestrial analogue of Mars, showing the microbial activities associated with the operation of the iron and sulfur cycles
These reactions explain most of properties observed in different extreme acidic environments, which are under the control of iron (Fig. 1).
characteristics, the iron and sulfur cycles are considered important model systems of astrobiological interest.
Iron Reduction
▶ Acidophile ▶ Aerobic Respiration ▶ Anaerobic Respiration ▶ Biogeochemical Cycles ▶ Chemolithotroph ▶ Extreme Environment ▶ Extremophiles ▶ Iron ▶ Lithotroph
Many microorganisms can use ferric iron as an electron acceptor (e.g., A. ferrooxidans, Acidiphilium spp. or Geobacter metallireducens, which in anoxic conditions can obtain energy respiring anaerobically). Ferric iron reduction is common in anoxic environments such as lake sediments and bioleaching heaps. Movement of iron-rich groundwater from anoxic waterlogged soils can result in the transport of large amounts of ferrous iron. When this iron-laden water reaches oxic environments, ferrous iron is oxidized chemically or by iron-oxidizing microorganisms, precipitating ferric compounds. Ferric hydroxide precipitates can interact with reduced organic compounds, like humic acids, to reduce ferric iron back to ferrous iron. Ferric iron can also form complexes with different organic compounds. In this way, iron becomes solubilized and once again available to ferric-reducing microorganisms as an electron acceptor. In Fig. 1, a display of the iron cycle operating in the Rio Tinto basin coupled to a sulfur cycle is shown. Due to their
See also
References and Further Reading Amils R, Gonza´lez-Toril E, Ferna´ndez-Remolar D, Go´mez F, Aguilera A, Rodrı´guez N, Malki M, Garcı´a-Moyano A, Gonza´lez-Faire´n A, de la Fuente V, Sanz JL (2007) Extreme environments as Mars terrestrial analogs: the Rı´o Tinto case. Planet Space Sci 55:370–381. doi: 10.1016/j_pss.2006.02.006 Benz M, Brune A, Schink B (1998) Anaerobic and aerobic oxidation of ferrous iron at neutral pH by chemoheterotrophic nitrate-reducing bacteria. Arch Microbiol 169:159–165 Colmer AR, Temple KL, Himkle HE (1950) An iron-oxidizing bacterium from the acidic drainage of some bituminous coal mines. J Bacteriol 59:317–328
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Ehrlich, HL 2002 Geomicrobiology, 4th edn. Marcel Dekker, New York Ferna´ndez-Remolar D, Morris RV, Gruener JE, Amils R, Knoll AH (2005) The Rı´o Tinto Basin, Spain: mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars. Earth Planet Sci Lett 240:149–167 Gonza´lez-Toril E, Llobet-Brosa E, Casamayor EO, Amann R, Amils R (2003) Microbial ecology of an extreme acidic environment, the Tinto River. Appl Environ Microbiol 69:4853–4865 Sand W, Gehrke T, Jozsa PG, Schippers A (2001) (Bio)chemistry of bacterial leaching- Direct vs. indirect bioleaching. Hydrometallurgy 59:159–175
Iron Isotopes FRANCK POITRASSON Geosciences Environnement Toulouse, CNRS, Toulouse, France
Keywords Biomarkers (Isotopic), Isotope geochemistry, Meteorites, Plasma source mass spectrometry, Prokaryotes
Definition Iron has four naturally occurring stable isotopes: 54Fe, 56 Fe, 57Fe, and 58Fe of respective abundances 5.80%, 91.72%, 2.20%, and 0.28%. They are increasingly used to determine the source of the iron present in geological or biological materials and/or to characterize the chemical reactions that may have lead to the form of iron under study. The isotopic signature of Fe is particularly sensitive to redox effects, including certain biological reactions such as bacterial dissimilatory iron reduction (DIR).
Overview Although less variable in abundances than the carbon or the oxygen stable isotopes, iron isotopes show analytically significant mass-dependent isotopic variations in nature. The geochemical community has investigated the range of Fe isotope compositions in both high and low temperature geological environments. Contrary to early expectations, there are no specific biological-type iron isotope signatures. On the other hand, the largest ▶ isotopic fractionation were observed for low temperature redox reactions, and biological DIR is one of those biological reactions that affect significantly iron isotope compositions. Another requirement for inorganic or organic reactions to leave an isotopic imprint is that the reaction exchange is not complete, that is not all of the original pool of iron has been consumed. Nature‐based and
experimentally based astrobiology studies have shown that iron isotopes are particularly useful to study ▶ traces of life and/or their environment when used in conjunction with other mineralogical, biological, and chemical indicators.
Basic Methodology The isotopic composition of iron in natural samples has been measured since the 1940s by electron impact ionization and more recently by thermal ionization mass spectrometry (TIMS) after sample dissolution and chemical purification using anion exchange chromatography. The level of precision reached relative to natural variations rendered the study of Fe isotope signatures of little interest until the end of the twentieth century. The advent of the second generation of multiple collector – inductively coupled plasma – mass spectrometers (MC-ICP-MS) led to an order of magnitude improvement in analytical precision (Belshaw et al. 2000). This opened the way to an exponential development of these “nontraditional isotopes” such as iron isotopes. Although the purification chemistry is straightforward and has been know for some time (Strelow 1980), MC-ICP-MS techniques vary among laboratories. Mass bias corrections range from the simple sample-standard bracketing approach to more sophisticated Cu- or Ni-doping methods. A few groups have also adopted the double spike technique, which remains the only way to determine precise Fe isotope composition by TIMS. Isotopic results are conventionally reported using the delta notation with reference to the IRMM 14 European reference material that has a composition close to that of chondrites. For example: ! 57 Fe=54 Fesample 57 1 103 d Feð‰Þ ¼ 57 54 Fe= FeIRMM14 and this notation can be equally defined for d56Fe. Some groups still report their Fe isotope compositions with reference to average of igneous rocks. Although being isotopically homogeneous, this convention should be abandoned since some igneous rocks depart significantly from this average value (e.g., some granites from the Earth’s continental crust; Poitrasson and Freydier 2005) and therefore, this may generate some confusion among nonspecialists.
Key Research Findings A first important finding is that the iron isotope compositions of igneous rocks that form the Earth’s crust vary very little. As a result, they make a baseline against which low-temperature, superficial biogeochemical processes
Iron Isotopes
that display larger isotope variations, may be compared (Beard et al. 2003; Poitrasson 2006). Various field-based, experimental and especially theoretical studies conducted over the past decade revealed that the main driving factors generating Fe isotope variations are redox and coordination chemistry changes. For example, the largest equilibrium Fe isotope fractionation among two coexisting iron reservoirs (4.5‰ in d57Fe) were observed between aqueous Fe2+ and Fe3+ species (Welch et al. 2003; Anbar et al. 2005). The importance of the bonding environment on Fe isotope signatures is best illustrated by theoretical prediction of Fe isotope fractionation between mineral phases holding iron in the same redox state, but with different network-forming anions, such as in sulfide and carbonate minerals (Polyakov and Mineev 2000; Blanchard et al. 2009). The finding that mass-dependent Fe isotope variation occurs in nature is interesting in itself. Even more important however, is the discovery that for the same rocks, the stable isotopes of iron show contrasted fractionation when compared to those of carbon (Matthews et al. 2004). Although not unexpected given the contrasted interatomic bonding environments of Fe and C, this illustrates that the stable isotopes of different elements can trace different processes and/or different properties of the studied reservoirs. As such, combining different isotopic tracers is certainly a promising approach for future research, notably to detect biological activity (Johnson et al. 2008; Severmann and Anbar 2009).
Applications Since astrobiology aims at looking for traces of life on the early Earth and on planets other than the Earth, one would ideally do precise Fe isotope measurements of extraterrestrial samples to determine signatures that could be tracking life, or at least environmental conditions suitable for to life. However, the need for high-precision mass spectrometry methods that were established in terrestrial laboratories over the past decade makes such delicate measurements on robotic missions impossible at present. Aside for the Moon that has never been prone to the development of life since the beginning of its history given its extreme temperatures, lack of liquid water and lack of atmosphere, we should wait for sample return missions from other planetary bodies. In the meantime, we have to rely on meteorites thought to come from planets where life may have existed, such as Mars. However, Martian meteorites that belong to the Shergotty, Nakhla, Chassigny (SNC) class are essentially igneous rocks. Hence, looking for traces of life on those rocks using iron isotopes was unsuccessful (Anand et al. 2006). This was not unexpected given that even if traces of life
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were searched on bulk terrestrial fresh igneous rocks, the igneous rock Fe isotope signature would have likely overwhelmed the biologically processed Fe. Another limitation is that despite initial expectation (Beard and Johnson 1999), there are, like for carbon isotopes, no unique Fe isotope biological signatures (Anbar et al. 2000). Indeed, inorganic processes may also produce strong Fe isotope fractionation leading to light products. Hence, the use of Fe isotopes to search for traces of biological activity should necessarily be conducted in conjunction with other mineralogical, chemical, and isotopic indicators. Astrobiology also looks at extreme terrestrial environments where life may occur despite hostile conditions. This is the case of acid mine drainage waters from the ▶ Rio-Tinto-Odiel sulfide deposit Basin, Southern Spain that is characterized by anomalously acidic waters (1.5 < pH < 3.9) containing elevated concentrations of dissolved Fe (>20,000 ppm). Despite these conditions, bacterial activity occurs. This geological setting is of interest to astrobiologists because it forms specific minerals, including ▶ jarosite that also occur at the Martian Meridiani Planum site. Iron isotopes were used by Egal et al. (2008) to study this site. Although the imprint of the biological activity could not be detected by Fe isotope compositions, this study combined mineralogical, aqueous geochemistry, and iron isotope determinations to put constraints on the mechanisms that led to the formation of such an extreme geochemical environment on Earth. Archean cherts ▶ Apex chert from Marble Bar, Western Australia is another interesting geological case study for astrobiologists since it has been claimed to contain the oldest fossils of microbes found yet on Earth, ca. 3.5 Ga years ago, although this finding generated a lot of debate. Pinti et al. (2007) used iron isotopes to study Fe-Mnoxyhydroxides containing C and N with a biological-like isotopic signature that occur in these cherts. They found d57Fe values compatible with oceanic hydrothermal precipitation, thereby strengthening the inference that C and N isotope compositions mark 3.5 Ga-old biological activity. Experimental investigations may also help to study processes that may have occurred on the early Earth. For example, Gronstal et al. (2009) examined experimentally how the first living organisms on Earth, that is, prokaryotes, could have extracted energy by oxidizing metallic iron from meteoritic material. Such a form of Fe, that is nearly nonexistent on the present Earth’s surface, was supposedly abundant at that time corresponding to the end of the ▶ Late Heavy Bombardment and there was an anoxic atmosphere. Interestingly, these authors found that whereas metal weathering occurred regardless of the
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Iron Isotopes
isotopic fractionation between phases is taken as a reference against which the observed natural variations may be interpreted. This is especially important in low temperature environments where kinetic isotope fractionations are more likely to be observed given the sluggishness of chemical reactions under such conditions, and therefore the large amount of time required to reach equilibrium. For this, more experimental and numerical determinations of the fractionation factors are required. It is important to conduct both approaches simultaneously since cross validating experimental, theoretical, and natural determinations is clearly needed (Blanchard et al. 2009). Another important development is the need for in situ data. Meteorites and very old terrestrial samples have had a protracted history that generated chemical and isotopic heterogeneity. Producing bulk sample data unveils only a portion of the message. Doing precise and accurate in situ Fe isotope determinations (Horn et al. 2006) is more likely to fully unfold the history that iron isotopes can tell.
Bio-met precipitates
Bio-met scraping
Abio-met precipitates
Abio-met scraping
Iron meteorite
−0.5
0.0
0.5
1.0
δ57/54Fe (per mil)
Iron Isotopes. Figure 1 Products of iron meteorite weathering in aqueous solutions with (Bio-met) and without (Abio-met) iron-oxidizing bacteria. Both the weathered layers at the surface of the meteorites (scraping) and precipitates elsewhere in the experimental cell (precipitate) were sampled. Iron isotope composition relative to the starting meteorite show contrasted Fe isotope signatures for these weathering products with and without bacteria, suggesting contrasted reaction pathways. (From Gronstal et al. 2009)
presence of iron-oxidizing bacteria, the weathering products clearly showed isotopically contrasted iron pathways during oxidation with and without bacteria. This suggests that the biological action in meteorite weathering could be highlighted with Fe isotopes (Fig. 1).
Future Directions Iron isotopes have not yet been applied frequently to astrobiology problems. More case studies therefore need to be investigated, either experimentally, and from natural examples of extreme terrestrial environments, or of the oldest remains of the early Earth. There are however more fundamental issues that need to be addressed first to be able to fully exploit the potential of iron isotope signatures in this endeavor. In a number of studies, equilibrium
See also ▶ Archean Traces of Life ▶ Banded Iron Formation ▶ Biomarkers, Isotopic ▶ Isotopic Fractionation (Interstellar Medium) ▶ Jarosite ▶ Late Heavy Bombardment ▶ Rio Tinto
References and Further Reading Anand M, Russell SS, Blackhurst RL, Grady MM (2006) Searching for signatures of life on Mars: an Fe-isotope perspective. Philos Trans R Soc Lond B Biol Sci 361(1474):1715–1720 Anbar AD, Roe JE, Nealson KH (2000) Nonbiological fractionation of iron isotopes. Science 288:126–128 Anbar AD, Jarzecki AA, Spiro TG (2005) Theoretical investigation of iron isotope fractionation between Fe(H2O)63+ and Fe(H2O)62+: implications for iron stable isotope geochemistry. Geochim Cosmochim Acta 69:825–837 Beard BL, Johnson CM (1999) High precision iron isotope measurements of terrestrial and lunar materials. Geochim Cosmochim Acta 63:1653–1660 Beard BL, Johnson CM, Skulan JL, Nealson KH, Cox L, Sun H (2003) Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem Geol 195:87–117 Belshaw NS, Zhu XK, O’Nions RK (2000) High precision measurement of iron isotopes by plasma source mass spectrometry. Int J Mass Spectrom 197:191–195 Blanchard M, Poitrasson F, Me´heut M, Lazzeri M, Mauri F, Balan E (2009) Iron isotope fractionation between pyrite (FeS2), hematite (Fe2O3) and siderite (FeCO3): a first-principles density functional theory study. Geochim Cosmochim Acta 73:6565–6578
Iron Oxyhydroxides Egal M, Elbaz-Poulichet F, Casiot C, Motelica-Heino M, Negrel P, Bruneel O, Sarmiento AM, Nieto JM (2008) Iron isotopes in acid mine waters and iron-rich solids from the Tinto-Odiel Basin (Iberian Pyrite Belt, Southwest Spain). Chem Geol 253(3–4):162–171 Gronstal A, Pearson V, Kappler A, Dooris C, Anand M, Poitrasson F, Kee TP, Cockell CS (2009) Laboratory experiments on the weathering of iron meteorites and carbonaceous chondrites by iron-oxidising bacteria. Meteorit Planet Sci 44:233–247 Horn I, von Blanckenburg F, Schoenberg R, Steinhoefel G, Markl G (2006) In situ iron isotope ratio determination using UV-femtosecond laser ablation with application to hydrothermal ore formation processes. Geochim Cosmochim Acta 70(14):3677–3688 Johnson CM, Beard BL, Roden EE (2008) The iron isotope fingerprints of redox and biogeochemical cycling in the modern and ancient Earth. Annu Rev Earth Planet Sci 36:457–493 Matthews A, Morgans-Bell HS, Emmanuel S, Jenkyns HC, Erel Y, Halicz L (2004) Controls on iron-isotope fractionation in organic-rich sediments (Kimmeridge Clay, Upper Jurassic, southern England). Geochim Cosmochim Acta 68:3107–3123 Pinti DL, Hashizume K, Orberger B, Gallien JP, Cloquet C, Massault M (2007) Biogenic nitrogen and carbon in Fe-Mn-oxyhydroxides from an Archean chert, Marble Bar, Western Australia. Geochem Geophys Geosyst 8:20 Poitrasson F (2006) On the iron isotope homogeneity level of the continental crust. Chem Geol 235:195–200 Poitrasson F, Freydier R (2005) Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem Geol 222:132–147 Polyakov VB, Mineev SD (2000) The use of Mo¨ssbauer spectroscopy in stable isotope geochemistry. Geochim Cosmochim Acta 64:849–865 Severmann S, Anbar AD (2009) Reconstructing paleoredox conditions through a Multitracer approach: the key to the past is the present. Elements 5(6):359–364 Strelow FWE (1980) Improved separation of iron from copper and other elements by anion-exchange chromatography on a 4% cross-linked resin with high concentrations of hydrochloric acid. Talanta 27: 727–732 Welch SA, Beard BL, Johnson CM, Braterman PS (2003) Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III). Geochim Cosmochim Acta 67(22):4231–4250
Iron Oxyhydroxides DAVID C. FERNA´NDEZ-REMOLAR Centro de Astrobiologı´a (INTA-CSIC), Torrejo´n de Ardoz, Spain
Synonyms Green rust
Keywords Aqueous environment, biominerals, Earth, geochemical cycles, mars, ▶ oxidation
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Definition Iron oxyhydroxides are chemical compounds that commonly form in aqueous environments with different content in iron cations (Fe2+ and Fe3+), oxygen, hydroxyl, water and some amounts of SO42, CO32 and Cl. They also are present in igneous and metamorphie rocks. They encompass up to 16 different species including oxides, mixed oxide-hydroxides and hydroxides (Cornell and Schwertmann 2003), which are characterized by differences in the ion content and the mineral structure. Three compounds like FeO (wu¨stite), Fe(OH)2 and FeIIFe2IIIO4 (magnetite) contain the divalent form of iron, whereas ferric mineralogies like F2O3 (hematite), FeOOH (▶ goethite), Fe5O7.5·4H2O (ferrihydrite) (jambor and Dutrizac 1998) and Fe8O8(OH)5.5(SO4)1.25 (schwertmannite) (Bigham et al. 1996) are much more abundant in the surface of the Earth. Oxides are chemically active compounds that participate in different processes like oxidation-reduction and adsorption (Cornell and Schwertmann 2003) which are essential to the biogeochemical cycling on Earth.
Overview Iron oxides are widespread minerals that result from aqueous processes at diverse redox and pH conditions (Cornell and Schwertmann 2003). They have been found in oceanic areas, lacustrine and fluvial systems, laterites and soils, hydrothermal and volcanogenic systems, subsurface environments or extraterrestrial material (Cornell and Schwertmann 2003; Li and Schoonmaker 2005; McCubbin et al. 2009). FeII-bearing oxides like wu¨stite and magnetite show a distribution that is restricted to low oxygen fugacity which is found in subsurface regions and subaqueous areas isolated from the oxic atmosphere of Earth. In this case, they are usually associated to other FeII minerals like ▶ siderite and ▶ pyrite. Ferric oxyhydroxides are highly insoluble and Fe3+ solubility is minimum at near neutral pH. The generation of ferric species follows a mineralogical trend linking different mineralogies with similar composition but different crystallinity. The first precipitating species are polymerized amorphous phases (Fig. 1) known as nanophase iron oxides (C20 acyclic isoprenoids from archaea. Isoprenoids are dominantly hydrocarbon skeletons with attached functional groups that are lost during diagenesis and thermal maturation leaving the hydrocarbon skeletons for potential preservation in the rock record.
number of protons (same charges) but a different number of neutrons (different mass numbers). Because their electronic shells must balance the nuclear charge, isotopes have the same electronic configuration and, hence, very similar chemical properties. Isotopes can be either stable or radioactive. Stable isotopes are those isotopes that do not spontaneously undergo radioactive decay. Radioactive isotopes are those with an unstable nucleus that stabilizes itself by emitting ionizing radiation (decay). Radiogenic isotopes are produced by the decay of radioactive isotopes. With few exceptions, even-number isotopes are more abundant than odd-number isotopes. The uncertainty principle requires the existence of a mass-dependent vibrational zero-point energy, which accounts for subtle differences in the chemical properties of the isotopes of a same element. This is the basis of the stable isotope geochemistry of light elements (H, C, N, O, S, etc.).
See also
See also
▶ Archaea ▶ Bacteria ▶ Biomarkers ▶ Eukarya ▶ Eukaryotes, Appearance and Early Evolution of ▶ Hopanes, Geological Record of ▶ Hydrocarbons ▶ Kerogen ▶ Membrane ▶ Molecular Fossils ▶ Steranes, Rock Record
▶ Biomarkers, Isotopic ▶ Carbon Isotopes as a Geochemical Tracer ▶ Hydrogen Isotopes ▶ Iron Isotopes ▶ Isotopic Ratio ▶ Isotopic Fractionation (Interstellar Medium) ▶ Nitrogen Isotopes ▶ Oxygen Isotopes ▶ Sulfur Isotopes
References and Further Reading Brocks JJ, Summons RE (2003) Sedimentary hydrocarbons, biomarkers for early life. In: Holland HD, Turekian K (eds) Treatise in geochemistry. Ch 8.03, pp 65–115 Eigenbrode JL (2007) Fossil lipids for life-detection: a case study from the early earth record. Space Sci Rev 135:161–185 Summons RE, Albrecht P, McDonald G, Moldowan JM (2007) Molecular biosignatures: generic qualities of organic compounds that betray biological origins. Space Sci Rev 135:133–157 Volkman JK, Barrett SM, Dunstan GA (1994) C25 and C30 highly branched isoprenoid alkenes in laboratory cultures of two marine diatoms. Organic Geochem 21:407–414
Isotope Biological Markers ▶ Biomarkers, Isotopic
Isotope Biosignatures ▶ Biomarkers, Isotopic
Isotope Definition Two nuclides, e.g., 16O and 18O, are isotopes of the same element (here oxygen), if their nucleus has the same
Isotope Paleothermometry ▶ Precambrian Oceans, Temperature of
Isotopic Fractionation (Interstellar Medium)
Isotopic Exchange Reactions
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Isotopic Fractionation (Interstellar Medium)
STEVEN B. CHARNLEY NASA Goddard Space Flight Center, Solar System Exploration Division, Code 691, Astrochemistry Laboratory, Greenbelt, MD, USA
THOMAS J. MILLAR Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, UK
Keywords
Keywords
Chemical reactions, Isotopes
Big Bang, Deuterium, Isotopes, Nucleosynthesis, Star formation
Definition Isotopic exchange reactions are ion-molecule reactions that can transfer heavy isotopes between molecules at low temperatures by virtue of the zero-point energy difference between reactants and products.
Overview An example of the isotopic exchange reactions that are very important in interstellar chemistry is the process H3+ + HD ! H2D+ + H2, which is ▶ exothermic in the forward direction by 230 K but ▶ endothermic by the same amount in the reverse direction. Therefore, at low temperatures, D nuclei from plentiful HD molecules, when incorporated in H2D+, can be transferred to other neutral molecules by ion-molecule reactions. Other ionmolecule reactions can proceed at low temperatures to fractionate astrophysical molecules in the rarer ▶ isotopes 13 C, 15N and 18O.
See also ▶ Endothermic ▶ Exothermic ▶ Ion-Molecule Reactions ▶ Isotope ▶ Isotopic Fractionation (Interstellar Medium)
References and Further Reading Duley WW, Williams DA (1988) Interstellar chemistry. Academic, London Hollenbach DJ, Thronson HA Jr (eds) (1987) Interstellar processes. D. Reidel, Dordrecht
Isotopic Fractionation, Fisher–Tropsch Effect ▶ Fischer–Tropsch Effects on Isotopic Fractionation
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Definition Isotopic fractionation in the interstellar medium refers to the result of chemical processes that selectively incorporate heavier isotopes into a molecule due to zero-pointenergy effects.
Overview Isotopes are variants of an atom that contain different numbers of neutrons, and hence have different mass. Thus, the element carbon has three isotopes, 12C, 13C, and 14C, containing six, seven, and eight neutrons, respectively. Naturally occurring isotopes are made through the processes that create the elements. In the Big Bang, which formed the lightest elements, H, He, Li, Be, and B, nucleosynthesis formed isotopes of all these, 1H, 2H (D), 3He, 4 He, 6Li, 7Li, etc. Indeed, the relative abundances observed for these isotopes, when allowance is made for their destruction since the universe was created, is one of the most powerful tests of Big Bang models. When incorporated into molecules, isotopes enable more accurate information to be deduced from astronomical observations of molecular transitions, particularly at radio, millimeter, and infrared wavelengths, as well as act as probes for stellar nucleosynthesis. Table 1 lists the isotopes of the six most common elements that have been detected in interstellar molecules, together with an estimate of their galactic abundance relative to the most abundant isotopic form. Note that these values are representative of the local interstellar medium, although some are known to depend on local star formation activity and thus vary with galactic radius, and can be significantly different from values measured in the solar system.
Basic Methodology Molecular line astronomy, that is, the observation of rotational and ro-vibrational transitions in molecules, is the main means by which we can probe the formation of stars deep in the interstellar clouds in which they are born.
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Isotopic Fractionation (Interstellar Medium). Table 1 Isotopic abundance ratios in the local interstellar medium (LISM). The abundances ratios are accurate to about 10% Element
Symbol
H
1
H
2
H (D)
C
13
1/77
14
1
15
1/450
16
1
17
1/2,000
18
1/550
28
1
29
1/20
30
1/30
32
1
33
1/138
34
1/22
N O O O
Si
Si Si Si
S
1/67,000 1
C
N
O
1
12
C
N
Relative Abundance in LISM
S S S
In particular, rotational transitions, which have energies equivalent to a few tens of degrees Kelvin, are well matched to the kinetic temperature of the molecular gas, 10–100 K. Very high resolution spectrometers, coupled with large surface area radio and submillimeter telescopes on dry mountain tops, such as the James Clerk Maxwell Telescope on Mauna Kea in Hawaii, have been used to detect thousands of transitions in around 150 molecules, not counting isotopologues. (An isotopologue, an “isotopic analogue,” is the name given to a molecule with one or more of its constituent atoms replaced by one of its less abundant isotopes). When several transitions of the same species can be detected at high spectral resolution, it is possible to derive temperatures, number densities and kinematics. These physical parameters can be most accurately determined when the emission (or absorption) line is optically thin, that is when a transition occurring anywhere in the molecular cloud can be observed. Unfortunately, this is often not the case. The lowest rotational transitions of the most abundant molecule observed in the millimeter range, carbon monoxide (CO), are often optically thick, meaning that emission/absorption arises in the surface layers of the cloud and not from the dense core where stars are born. Isotopes allow us to overcome, to a great extent, this “black veil.” Thus while the transitions of the most common CO isotope, 12C16O, can be very optically thick, those of 12C18O, commonly written C18O, which is around 550 times less abundant
(see Table 1) are optically thin and, despite its lower abundance, still observable in interstellar clouds. The use of isotopologues to derive physical parameters is widespread in molecular astronomy but is less useful in determining abundances of the main isotopic species due to a process called “isotopic fractionation,” which selectively incorporates heavier isotopes into a molecule due to zero-point-energy effects. We consider first fractionation of 13C in CO when the bulk of carbon is in singly ionized form, C+, as commonly occurs, for example, in the edges of ▶ molecular clouds. In this case the ion-neutral isotopic exchange reaction: 13
Cþ þ12 C16 O!13 C16 Oþ12 Cþ þ DE1
ð1Þ
is exothermic in the forward direction and endothermic in the reverse direction due to the fact that 13CO has a slightly lower zero-point-energy, by ΔE1/k35 K, than 12CO. In steady state, the abundance ratio of 13CO/12CO, R(13CO), can be written as: R 13 CO ¼ ðkf =kr ÞR 13 Cþ ¼ R 13 Cþ expðDE1 =kTÞ where T is the kinetic temperature of the molecular gas. Hence at the low temperatures, 20 K, expected in the outer regions of molecular clouds, the isotopic ratio in 13 CO can be enhanced by a factor of about 6, and at 10 K by a factor of about 30. In fact, such a large enhancement is unlikely to occur since the chemistry is more involved than described by the simple equation above. Most importantly, regions in which C+ is abundant must be pervaded by a UV radiation field sufficiently strong to keep the bulk of carbon in ionized form. This radiation field, however, also photodissociates CO and its isotopologues. Since photodissociation of CO proceeds via absorption of line photons, CO self-shields against destruction with the less abundant isotopologues less able to shield and therefore destroyed faster than the main isotopic form. The detailed ratio of CO to its minor isotopes thus depends on a competition between the isotope exchange reaction, which enhances the 13C content in CO, and photodissociation, which decreases it. Nevertheless, the fact remains that the 13C content of CO can be larger than the underlying “cosmic” ratio, although typically by a factor of less than 2. The enhancement of 13C in CO, and hence its depletion in C+, has consequences for other molecules that form from these species. Thus, molecules formed from CO, such as HCO+, will carry an enhanced ratio while molecules formed from C+, such as CH+, will carry a lower ratio. It is not just gas-phase species that can carry these anomalies. Molecules formed from CO when it freezes out on to dust grains, for example H2CO, CO2, and CH3OH, should also have enhanced isotopic ratios in 13C. The study of isotopic
Isotopic Fractionation (Interstellar Medium)
ratios in grain ices, or in molecules evaporated from such ices in hot molecular core regions, thus provides a potentially powerful probe of studying how individual molecules may form on grain surfaces.
Key Research Findings Since isotopic fractionation occurs due to zero-point energy differences, the biggest effect occurs in the exchange of D and H since this causes the largest increase in reduced mass in a molecular system. Indeed, enhancements of up to ten orders of magnitude are seen in some deuterated molecules, about 30 of which have been detected in interstellar clouds, including doubly and triply deuterated species. In cold dense clouds the reservoirs of D and H are HD and H2, with an underlying ratio R(HD)=2RD 2–3 105, where RD is the cosmic ratio of D to H, 1–1.5 105. The most important isotopic exchange reaction in these clouds is the reaction of H3+, formed by the cosmicray ionization of H2, with HD: H3 þ þ HD ! H2 Dþ þ H2 þ DE2 ;
ð2Þ
where the precise value of the exothermicity, ΔE2, has contributions from both enthalpy and entropy and depends on the rotational energies of the reactants. For all species in their ground states, ΔE2/k230 K. Assuming steady state, then the abundance ratio of H2D+/H3+ becomes: RðH2 Dþ Þ ¼ RðHDÞexpðDE2 =kTÞ: For a gas temperature of 10 K, one sees that the theoretical enhancement of the cosmic ratio, RD, becomes extremely large as the rate coefficient of the reverse reaction in Eq. 2 goes to zero. In fact, the enhancement does not grow exponentially large at low temperatures because other reactions destroy H2D+, including dissociative recombination with electrons, and proton or deuteron transfer with neutrals, and act to limit the ratio. Taking these reactions into account gives a crude estimate of the enhancement factor, S(T), as: SðTÞ ¼ 1=½expðDE2 =kTÞ þ ðke =kf Þf ðeÞ þ ðkM =kf Þf ðMÞ
where kf is the forward rate coefficient of reaction Eq. 2, ke is the dissociative recombination rate coefficient of H2D+, and kM the rate coefficient for reaction of H2D+ with neutral species M. Here, f(X) represents the fractional abundance of species X relative to molecular hydrogen. Typically, the value of ke is about 100 times larger than those of ion-neutral reactions. The enhancement obtained thus depends not only on the temperature but also on the relative abundances of electrons and reactive neutrals. Since the value of f(e) is two to three orders of magnitude less than that of f(M) in molecular clouds, it is reactions
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with M that determine the level of enhancement below 20 K. In fact because of its low proton affinity, H3+ (and H2D+) reacts with almost all neutral species in molecular clouds, with the exception of H, H2, He, N, and O2. For the case in which neutral abundances, f(M), have their maximum values, given by the elemental abundances of C, N, and O, enhancements of up to 1,000 can occur, while for abundances decreased by 100, to simulate freeze-out on to cold grain surfaces, enhancements greater than 10,000 can occur, that is, R(H2D+) can be larger than unity. For temperatures greater than about 25 K, the first term in the denominator dominates and the enhancement falls off exponentially with increasing temperature. Such large enhancements in the deuterium fractionation in H2D+ are transferred to other molecules through deuteron transfer, which is assumed to occur statistically in one-third of all reactive collisions. Thus, DCO+ and N2D+, for example, should show – and indeed do show – extremely large enhancements in cold, dense cores that have undergone some depletion of molecules, the most important of which is CO because of its large underlying abundance, on to grain surfaces. In cases where such depletion is enhanced the formation of doubly and triply deuterated molecules becomes possible. In the equation above for the enhancement of H2D+, the species M implicitly includes HD since H2D+ (and D2H+) can also react with this species. H2 Dþ þ HD ! D2 Hþ þ H2 þ DE3
ð3Þ
D2 Hþ þ HD ! D3 þ þ H2 þ DE4
ð4Þ
where the exothermicities have the same dependencies as ΔE2 and values of 187 K and 159 K, respectively. One sees that, in certain circumstances – very low temperatures and heavy depletion of neutral species – D3+ can be more abundant than H2D+, and therefore potentially a more important deuteron donor since it transfers a deuteron on every reactive collision. In regions characterized by such conditions, corresponding to gas in star-forming regions immediately before nuclear burning commences, observations of molecular line emission from H2D+ and D2H+, both of which have a small electric dipole moment, may be the only feasible means of studying the physics of star formation. Gas-phase chemistry initiated by these and other fractionation reactions drives very efficient enhancements in deuterium. For example, triply deuterated ammonia has been observed with an abundance relative to normal ammonia of 103, some 11 orders of magnitude greater than the statistical ratio. A similar effect is found in triply deuterated methanol, while the D2CO/H2CO abundance ratio has been measured to be as large as 0.4 in some clouds.
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While gas-phase reactions involving the deuterated versions of H3+ are important, grain surface reactions may also play a significant role in enhancing isotopic fractionation – it is noteworthy that the molecules showing the largest deuterium fractionation, NH3, H2CO, and CH3OH, are all thought to form efficiently in cold ice mantles on dust grains. The large fractionation in H3+ gives rise, upon dissociative recombination with electrons, to a large enhancement in atomic deuterium. Collisions of atomic D with cold grains then lead to an enhancement of D/H on grain surfaces and to enhancements in the species that form there.
See also ▶ Gas-Grain Chemistry ▶ Interstellar Chemical Processes ▶ Ion-Molecule Reactions ▶ Molecular Cloud
References and Further Reading Brown RD, Rice EHN (1986) Galactochemistry – Part two – Interstellar deuterium chemistry. Mon Not R Astron Soc 223:429 Langer WD, Graedel TE, Frerking MA, Armentrout PB (1984) Carbon and oxygen isotope fractionation in dense interstellar clouds. Astrophys J 277:581 Lis DC, Roueff E et al (2002) Detection of triply deuterated ammonia in the barnard 1 cloud. Astrophys J 571:L55 Millar TJ (2003) Deuterium fractionation in interstellar clouds. Space Sci Rev 106:73 Millar TJ, Bennett A, Herbst E (1989) Deuterium fractionation in dense interstellar clouds. Astrophys J 340:960 Roberts H, Millar TJ (2000) Gas-phase formation of doubly-deuterated species. Astron Astrophys 364:780 Roberts H, Herbst E, Millar TJ (2003) Enhanced deuterium fractionation in dense interstellar cores resulting from multiply deuterated H3+. Astrophys J 591:L41 Watson WD (1976) Interstellar molecular reactions. Rev Mod Phys 48:513 Watson WD, Anicich VG, Huntress WT (1976) Measurement and significance of the equilibrium reaction 13C+ + 12CO yields 12C+ + 13CO for alteration of the 13C/12C ratio in interstellar molecules. Astrophys J 205:L165
Isotopic Fractionation (Planetary Process) FRANCIS ALBARE`DE Ecole Normale Supe´rieure de Lyon, Lyon Cedex 7, France
Definition The term isotope fractionation refers to subtle variations of isotopic abundances among coexisting solids, liquids, and gases. It can both take place at equilibrium and reflect
kinetic effects. It can be mass dependent, when its amplitude is a monotonous function of the isotope masses, or else be mass independent. It is normally given in delta (d) units, which represent the relative deviation of a particular isotopic ratio in a particular sample with respect to this ratio in a reference material.
History American physical chemist Harold C. Urey (1947) and chemists J. Bigeleisen and M.G. Meyer (1947) must be credited for the first theoretical prediction of isotopic fractionation. J.M. McCrea (1950) calibrated the fractionation of ▶ oxygen isotopes between coexisting carbonates and liquid water.
Overview Isotope fractionation results from slightly different chemical properties among the isotopes of the same element. These variations are large enough (typically 0.1–10 ‰) to be measured by mass spectrometry. They occur both at equilibrium and under the effect of kinetics. The isotopic properties of a molecule reflect its share of the different sorts of energy: translational, rotational, and vibrational. Isotope fractionation results from the Heisenberg uncertainty principle: the separation distance between two bonding atoms (e.g., O and H in OH) cannot be that of the potential energy minimum as both the position and the velocity would be fixed. The minimum of vibrational energy is reached for the mass-dependent zero-point energy ½ hn, where h is the Plank constant and n the vibration frequency. There is no zero-point energy for rotational energy, and the incidence of zero-point translational energy is negligible. Because vibrational energy varies inversely with the squared-root of the mean mass of the vibrating pair (heavy atoms vibrate less rapidly than light atoms), such an effect is referred to as ▶ massdependent isotope fractionation. Heavy isotopes prefer the low-energy states, which explains why liquid water is preferentially enriched in 18O and 2H (also represented with the notation “D”) over 16O and H, respectively. Likewise, oxides (CO2, NO2, SO2) tend to concentrate the heavy isotopes at the expense of the corresponding reduced species (CH4, NH3, H2S). The symmetry of interacting molecules also plays a major role. Isotopic variations of H, O, C, N, and S are commonly reported in the delta (‰) notation, e.g., d18O, which is the relative excess of 18O over 16O in a particular sample with respect to a reference material (see Delta Notation entry). Isotope fractionation varies with temperature, e.g., 1000 ln a18O(quartz/water) d18Oquartz d18Owater = A/T 2 + B where T is in Kelvin, a18O is the ratio of
Isotopolog
O/16O in coexisting quartz and water, and A and B are constants (most often B is zero). Isotope fractionation is therefore most noticeable at ambient temperatures and is still significant in the crust (< 600 C). Isotope fractionation during analysis is often referred to as a mass bias. It may be greatly enhanced by ▶ distillation processes. Understanding biological and physical isotopic fractionation processes is fundamental for discriminating the isotopic signature of elements such as C, N, S, and Fe preserved in the rock record and deciphering whether these signatures have a biological or abiological origin. Isotopic fractionation is also important for discriminating sources of elements in the universe. A best astrobiological example is the D/H ratio (the isotopic ratio between ▶ deuterium and hydrogen), which varies in hydrogenated molecules preserved in planetary bodies pending the temperature at which water molecules exchanges between a vapor and a solid phase. The D/H ratio is a valuable marker of the carrier phase of water on Earth (comets vs. meteorites) being these different bodies formed and hydrated at different distances from the Sun.
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See also ▶ Biomarkers, Isotopic ▶ Carbon Isotopes as a Geochemical Tracer ▶ Deuterium/Hydrogen Ratio ▶ Distillation (Rayleigh) ▶ Fractionation, Mass Independent and Dependent ▶ Hydrogen Isotopes ▶ Iron Isotopes ▶ Isotopic Ratio ▶ Nitrogen Isotopes ▶ Oxygen Isotopes ▶ Sulfur Isotopes ▶ Urey’s Conception of Origins of Life
References and Further Reading Criss RE (1999) Principles of Stable Isotope Distribution. Oxford University Press, Oxford Sharp Z (2007) Principles of Stable Isotope Geochemistry. Prentice Hall, Upper Saddle River Valley JW, Cole DR (2001) Stable Isotope Geochemistry (Reviews in Mineralogy and Geochemistry vol. 43). Washington, Mineralogical Society of America
Isotopic Ratio Definition ▶ Isotopic ratio refers to the ratio of the atomic abundances of two isotopes of the same element, e.g., 18O/16O or 143Nd/144Nd. An advantage of using ratios rather than absolute abundances of a particular nuclide is a better precision. Comparing the two signals 143Nd and 144Nd can be done at the ppm (1 part in one million) precision level, i.e., two to three orders of magnitude more precisely than the counting of either nuclide individually. Isotopic ratios change as a result of (1) thermodynamic fractionation, (2) radioactive ingrowth (3) the presence of nucleosynthetic compounds, and (4) spallation reactions. Ratios of a radiogenic isotope (e.g., 143 Nd) to a stable isotope (e.g., 144Nd) are used in geochronology; ratios of stable isotopes provide information about temperatures and sources.
See also ▶ Biomarkers, Isotopic ▶ Carbon Isotopes as a Geochemical Tracer ▶ Fractionation, Mass Independent and Dependent ▶ Geochronology ▶ Hydrogen Isotopes ▶ Iron Isotopes ▶ Isochron ▶ Isotopic Fractionation (Planetary Process) ▶ Nitrogen Isotopes ▶ Oxygen Isotopes ▶ Sulfur Isotopes
Isotopic Traces of Life ▶ Biomarkers, Isotopic
Isotopolog Definition
Isotopic Isomer ▶ Isotopomer
Isotopolog are molecules that differ only in the isotopic composition of their constituent atoms; for example, HDO and H2O, where D (deuterium) is 2H. Astrochemists sometimes use isotopomer as a synonym for isotopologue,
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although chemists normally define isotopomers to mean ▶ isomers, which have the same number of each rare-isotopic atom, but with these atoms differing in their positions in the molecule.
See also ▶ Isomer ▶ Isotope
Isotopomer Synonyms Isotopic isomer
Definition Isotopomers (the term being a contraction of isotopic isomer) are chemical compounds (molecules, ions, radicals, . . .) having the same number of each isotopic atom (same isotopic formula) but where the positions of isotopes differ, being either constitutional isomers (e.g., CH3–CDH–CH3 vs. CDH2–CH2–CH3), or isotopic stereo-isomers (e.g., (R)– vs. (S)–CH3–CHDOH, or (E)– vs. (Z)–CH3–CH=CHD). Not more astrobiologically significant than isomery, but not to be confused with ▶ isotopologue (a compound with the same chemical structure but with a different isotopic composition); for International Union of Pure and Applied Chemistry official definitions, see Minkin (1999) Glossary of terms used in theoretical organic chemistry. Pure Appl Chem 71:1919–1981.
See also ▶ Isomer ▶ Isotopolog
Isovaline Definition Isovaline is an a-amino acid that rarely occurs in biochemistry that has been found in the ▶ Murchison and other meteorites, in many instances with a significant L-enantiomeric excess. It is an isomer of the
biologically important amino acid valine. It has been proposed that it is synthesized prebiotically via a ▶ Strecker synthesis from methylethylketone and ammonium cyanide. Since the a-proton in isovaline is replaced with a methyl group, the kinetics of aqueous ▶ racemization are extremely slow, thus it has been suggested that the ▶ enantiomeric excess of L-isovaline represents an initial ▶ chiral synthetic bias, which has subsequently been lost in the a-proton containing ▶ amino acids. Isovaline is, however, found in some fungal antibiotic peptides, thus it cannot be considered a strict marker of abiological synthesis.
See also ▶ Amino Acid ▶ Chirality ▶ Enantiomeric Excess ▶ Racemization ▶ Strecker Synthesis
ISRO Synonyms Indian Space Research Association; Indian Space Research Organization
Definition India began space activities in the early 1960s as a nonaligned country. Various committees were created to support the national effort in developing launchers and satellites. In 1969, ISRO was created as part of the Nuclear Energy Department and became an independent organization in 1972. In 1975, the program lauched Arabhata, the first Indian satellite, and since then it has produced the Polar Satellite Launch Vehicle (PLSV) to launch satellites for remote sensing. The Geosynchronous Satellite Launch Vehicle (GSLV) is able to send up to 2 t in geosynchronous transfer orbit. In 1984, an Indian astronaut flew on board the Russian Saliout 7 space station. India is now turning to space sciences and exploration, and current ISRO activities are spread amongst more than 20 laboratories or centers across India. In 2008, India sent the Chandrayaan-1 to orbit the moon. ISRO is planning to develop a manned flight program and aims to send Indian astronauts into earth orbit around 2015–2016. For further information:http://www.isro.org/
Isua Greenstone Belt
ISS ▶ International Space Station
ISSI Synonyms International Space Science Institute
Definition The international Space Science Institute (ISSI) was established in 1995 as an Institute of Advanced Studies where scientists from all over the world meet in a multiand interdisciplinary setting to reach out for new scientific horizons. The institute organizes workshops as well as working groups and forums. Thereby the institute offers to space scientists, ground-based observers, experimenters, theoreticians, and modelers opportunities to work together and to analyze and compare data from a multitude of sources. Because data are collected from a variety of spacecraft and ground-based facilities, interdisciplinary meetings enable scientists to extract or create more science with little additional expenditure. About 5,620 scientists from more than 45 countries have participated in ISSI activities during the first 15 years of its existence. The institute is managing several publications covering all the fields of space sciences, among others, the Space Sciences Series of ISSI (SSSI), and the ISSI Scientific Report Series. Strategies of Life Detection, O. Botta, J. Bada, J. Go´mez Elvira, E. Javaux, F. Selsis, R. Summons (Eds.), published in August 2008 and Geology and Habitability of Terrestrial Planets K.E. Fishbaugh, P. Lognonne´, F. Raulin, D.J. Des Marais, O. Korablev (eds.), published in September 2007 are two examples of activities of ISSI related with astrobiology. The institute is based in Bern (Switzerland). The staff is headed by an executive director and includes three additional scientific directors and five scientists. The present permanent staff is around 15 people. Further information: www.issibern.ch
International Astrobiology Society; International Society for the Study of the Origin of Life
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Keywords Astrobiology, origins of life
Overview Following the first international conference on the origin of life on Earth held in 1957 in Moscow organized by A. I. Oparin under the auspices of the International Union of Biochemistry, and the second international conference held at Wakulla Falls, USA, with the support of ▶ NASA and Institute for Space Biosciences of the Florida State University, during the 1970 conference that took place in Pont-a`-Mousson, France, the organizational meeting for the founding of the International Society for the Study of the Origin of Life (ISSOL) took place. Its first president was the Soviet scientist Alexander I. Oparine is work during the first half of the twentieth century opened the possibility of transforming the study of the origin of life from mere speculation into a workable research program capable of unifying disparate facts and observations from widely different fields within a coherent explanatory framework provided by a Darwinian view. Although numerous origin of life meetings had been held following the 1957 Moscow conference, most of them had a regional character and limited scope. Because of this, it was decided that first ISSOL meeting, which took place in 1973 in Barcelona, was also named the 4th International Conference on the Origin of Life. Recognition of the scientific significance the Astrobiology promoted by NASA and adopted by scientists in many countries, led in 2005 to a change of name which transformed the society into ISSOL: The International Astrobiology Society.
References and Further Reading Buvet R, Ponnamperuma C (eds) (1971) Chemical evolution and the origin of life, vol I: Molecular evolution. North-Holland, Amsterdam Dick S, Strick JE (2004) The living universe: NASA and the development of astrobiology. Rutgers University Press, New Brunswick Oro´ J, Miller SL, Ponnamperuma C, Young RS (eds) (1974) Cosmochemical evolution & the origins of life, vol I. Reidel, Dordrecht Strick JE (2004) Creating a cosmic discipline: the crystallization and consolidation of exobiology, 1957–1973. J Hist Biol 37:131–180 http://www.issol.org
ISSOL Synonyms
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Isua Greenstone Belt ▶ Isua Supracrustal Belt
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Isua Supracrustal Belt MINIK T. ROSING Nordic Center for Earth’s Evolution, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
Synonyms Isua greenstone belt; Isuakasia
Keywords Amitsoq, Archean, Earth’s early crust, early life, faint young sun, Isua, late heavy bombardement
Definition The Isua supracrustal belt is the largest contiguous complex of Eoarchean supracrust (rocks formed at or near the Earth’s surface) known on Earth. It is located ca 150 km NE of Greenland’s capital Nuuk, in West Greenland.
History Since the establishment of The Geological Survey of Greenland in 1946, geological investigations of West Greenland shifted gradually from investigations of individual mineral deposits towards mapping of the general geology. By the early 1970s, the main framework of West Greenland’s geol ogy was laid out. An orogenic belt was identified around 66 N, the so-called Nagssuqtoqidian orogen, characterized by Paleoproterozoic penetrative deformation and metamorphism of a pre-existing Precambrian basement, still preserved towards the south (Noe-Nygaard and Ramberg 1961; van Gool et al. 2002). South of 61 N, the old basement was again overprinted by deformation and metamorphism during the Paleoproterozoic and the accretion of a juvenile magmatic arc onto the old continent. This Ketilidian orogen also hosted the alkaline Gardar igneous province (Ussing 1912; Garde et al. 2002) emplaced around 1,300 Ma ago (Vanbreem and Upton 1972). The Archean cratonic basement sandwiched between these two orogenic belts is known as the Archean Block. The Achaean Block is dominated by Eoarchean and Meso-Neoarchean amphibolite and granulite-facies felsic gneisses (Bridgwater et al. 1970; Griffin et al. 1980). The gneiss complex around Nuuk (Previously Godtha˚b) can be divided into two domains based on the presence or absence of a suite of mafic dikes named Ameralik dykes (McGregor 1968) that contain characteristic plagioclase megacrysts. The so-called Amitsoq gneisses contain mafic dikes or mafic enclaves that represent deformed fragments
of such dikes. The other gneiss suite, named the Nuuk (or Nuˆk) gneisses, contains no Ameralik dykes and was inferred to have been emplaced later than the intrusion of the dikes (McGregor 1973). In 1971, a suite of the Amitsoq gneisses was dated and produced an age of 3,650 Ma (Black et al. 1971). This exceeded by far any age obtained on terrestrial material before, and established the Nuuk area as home to the oldest rocks known on Earth. The two gneiss domains turned out to be dominated by gneisses with ages centered around 3,600 Ma for the Amitsoq gneisses (Moorbath et al. 1972; Baadsgaard 1973, 1976) and around 3,000 Ma for the Nuuk gneisses (Pankhurst et al. 1973). Both gneiss complexes also contain inclusions and contiguous belts of supracrustal rock (rocks formed at or near Earth’s surface). In both terranes the supracrustal rocks appeared to predate the gneisses, because the granitoid precursors to the gneisses were commonly intrusive into the supracrustal rocks. The supracrustal assemblage in the Amitsoq gneisses was named the Akilia association and the supracrustal rocks within the Nuuk gneisses were called the Malene Supracrustals (McGregor 1973; Gill and Bridgwater 1976). During the late 1960s prospectors carried out aeromagnetic surveys of the West coast of Greenland and identified a strong magnetic anomaly at Isua near the margin of the Inland Ice 150 km NE of Nuuk (Kurki and Keto 1966) (Fig. 1). This anomaly turned out to be caused by a large deposit of quartz-magnetite banded iron formation (BIF). Mapping showed that the BIF formed part of a large arcuate belt of supracrustal rocks some 30 km in length and up to 4 km in width (Bridgwater and McGregor 1974; Allaart 1976). The Isua BIF gave an age of 3,770 Ma (Moorbath et al. 1973), the oldest age obtained for any rock on Earth at that time. From field relations, the Isua supracrustals and the Akilia association occupy the same chronostratigraphic position since both are older than the enveloping Amitsoq gneisses. Further isotopic age determinations supported this observation and showed that at least some of these supracrustal rocks are more than 3,800 Ma old (Baadsgaard et al. 1984; Nutman et al. 1984, 2002). The collage of tectonic panels of different ages and origins in the Nuuk region has been placed into a model of Archean terrane accretion by Nutman et al. (1989). The oldest assemblage of rocks, which included the Amitsoq gneisses and the Isua and Akilia association of supracrustal rocks, was defined as the Itsaq Gneiss Complex (Nutman et al. 1996, 2000, 2002). Eoarchean rocks with ages > 3,600 Ma have also been identified along the northern boundary of the Archean Block in the so-called Aasivik terrane (Rosing et al. 2001), but little is known about this terrane at present.
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Isua Supracrustal Belt. Figure 1 The Isua landscape is barren with excellent exposure due to the high elevation and the proximity to the Greenland Ice Cap. The mountain peak in the background is “Iron Mountain,” a large quartz-magnetite banded iron formation deposit
Overview The scientific attention paid to the Eoarchean complexes around Nuuk, and the Isua and Akilia supracrustals is probably unparalleled in the world of geology. The research has focused on two main avenues. One was the state of chemical differentiation of Earth by the Eoarchean and the other was the characterization of the early Earth’s surface environments and the search for traces of life on the young planet.
Early Earth Differentation Earth has undergone continued differentiation ▶ Earth, Early Evolution into distinct chemical reservoirs such as the granitic continental crust, the basaltic oceanic crust, the depleted mantle from which the crust was extracted and the atmosphere and the oceans where the volatile components from the depleted mantle are concentrated. The Amitsoq gneisses and the Isua and Akilia supracrustal rocks became a prime target for the development and use of isotopic tracers to document the timing and extent of differentiation of the young Earth. If radioactive mother isotopes and their radiogenic daughter isotopes have different compatibilities in the mantle during melting, the timing and extent of melting will be reflected in the isotopic composition of both the residual mantle and the
rocks formed from the melt and the isotopic dissimilarity will be more pronounced with time. Given this insight, a number of isotopic systems were applied ▶ geochronology on Isua rocks to establish the extent and history of differentiation of Earth ▶ Earth, age of by the earliest Archean (U-Th-Pb (Moorbath et al. 1975), 147Sm-143Nd (Hamilton et al. 1978), Lu-Hf (Blichert-Toft et al. 2000), La-Ce (Shimizu et al. 1988), 146Sm-142Nd (Boyet et al. 2003; Caro et al. 2003). The conclusions drawn from these isotope studies were, that at Isua time, there was still some chemical heterogeneity inherited from early differentiation of Earth preserved in the mantle. These heterogeneities could have been established during an early magma ocean episode, and subsequently progressively erased by mantle convection and back mixing of proto-crust into the mantle. Another conclusion was that a depleted mantle reservoir similar to the present day source of mid ocean ridge basalt was present even during the earliest Archean.
Traces of Life and Early Earth Surface Conditions Already in 1979 the German geochemist Manfred Schidlowski (Schidlowski et al. 1979) proposed that the ratio between the two stable isotopes of carbon 13C and
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C found in ancient sedimentary rocks could be used to identify the presence of life in Earth’s early oceans. This is because living organisms incorporate 12C more efficiently that 13C when they form organic matter from ambient CO2, and therefore leave the ocean and atmosphere enriched in 13C and organic-rich sediments depleted in 13 C. He used this principle on carbonate rocks and graphite form Isua under the assumption that the carbonate represented the carbon isotopic composition of the early Archean ocean and the graphite represented metamorphosed organic matter (Schidlowski et al. 1979). Later geologic studies at Isua, however, showed that the carbonates were formed from circulating brines during metamorphism, and that the graphite studied by Schidlowski was also secondary in origin (Rosing et al. 1996). The search for traces of early life in the Archean of West Greenland continued. In 1996, Mojzsis et al. reported the presence of 13 C depleted graphite inclusions from apatite crystals in the supracrustal rocks from Akilia Island, near Nuuk (Mojzsis et al. 1996). In 1999 Rosing reported that shales from Isua contained abundant sedimentary 13C depleted carbon (Rosing 1999; van Zuilen et al. 2003) (Fig. 2). There has been an ongoing and continued controversy
over the credibility of these claims for the presence of life during the earliest part of Earth’s history, and research into this topic is still ongoing along many different avenues of study. The Isua supracrustal sequence also carries information about the surface conditions on the young Earth. Most significantly, water lain sediments attest to the presence of a liquid hydrosphere, most probably the existence of the oceans from at least 3,800 Ma ago. The Isua supracrustal rocks have also been analyzed for traces of the so-called ▶ Late Heavy bombardment, which cratered the Moon, and possibly also affected earth in a dramatic way at the time of deposition of the Isua sediments. However, this search has given no clear evidence for extraterrestrial material in the Isua rocks (Schoenberg et al. 2002; Frei and Rosing 2005).
The Composition of the Isua Supracrustal Belt Soon after the discovery of the Isua supracrustal belt, several lithologic units were identified (Keto and Kurki 1967; Allaart 1976). The belt is outlined in the field by a prominent erosion resistant ridge, which is dominated
Isua Supracrustal Belt. Figure 2 Well-preserved Bouma sequence with alternating graphite-rich black shale and gray turbidite sediments. The black shale contains abundant fine particles of carbon with d13C 25, most likely derived from living organisms
Isua Supracrustal Belt
by a chloritic amphibolite unit characterized by a distinctive amphibole sheave texture. This unit was called the Garbenschiefer Formation, and its protolith was generally believed to be a sill-like mafic intrusion. The ▶ BIF that originally had given the supracrustal belt away forms a very large body at the Northeast termination of the belt under the Inland Ice, but this unit can also be traced more or less continuously along the full extent of the belt. In early maps, this BIF was lumped with various other lithologies into a “Quartzitic sequence.” Bordering on the BIF is a thick unit of carbonate and calc-silicate rich rock which often grades into carbonate-rich mica schists. This unit was called The Carbonate Formation, and was believed to represent a change in sedimentary facies in the same sedimentary basin that gave rise to the BIF Formation. Along most of the length of the belt, felsic schists and two-mica gneisses are common. At many localities, these rocks display a diamictic structure. At one locality large felsic “boulders” stand out from a slightly darker, carbonate-rich matrix. This rock was called the “bomb rock” and fiamme-like textures were described from the alleged felsic volcanic bombs. For this reason this Felsic Formation was believed to be dominated by volcanic rocks. Along the margin of the belt black amphibolite form the predominant lithology, and was often referred to as the Amphibolite Formation. The distribution of lithologies gave the impression of a sequence of lithologies that was repeated by folding along an arcuate axial plane tracing the centre of the supracrustal belt. It was assumed that the belt formed a rim syncline enveloping a central gneiss dome, following the style that was seen in the classical granitegreenstone terranes of Australia (Keto and Kurki 1967). Another prominent suite of lithologies is ultramafic rocks, which occur as ▶ serpentinites, talc-anthophyllitemagnesite schists, and dunites. These rocks occur as discontinuous layers and lenses within the sequence and do not appear as a single formation. In the Northeastern termination of the belt, the distinctive Garbenschiefer units (see above) also occur in a separate splay along the inner perimeter of the belt, and is separated from the rest of the belt by a distinctive topographic lineament. This was considered a separate segment of the belt, which also included a mica schist unit and a felsic unit. The formations of the main belt were allocated to a socalled “A sequence” and the rocks in the small separate domain were allocated to a “B sequence.” This understanding of the geology of Isua permeated the literature (Nutman et al. 1984) and formed the basis for the 1:40,000 geological map published by the Geological Survey of Greenland in 1986 (Nutman 1986). Many of the main
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features of the belt were surprisingly well understood in these very early works, and the main lithological subdivision has remained almost unchanged during the more than 40 years passed since the discovery of Isua. In terms of describing the makeup of the Isua belt, subsequent work has mainly proposed new interpretations of the protoliths to the present lithologies and increased the detail in their geochemical description.
Metamorphism, Deformation, and Metasomatism It was readily observed that the rocks at Isua were generally strongly deformed (James 1976) and had experienced penetrative amphibolite facies metamorphism (Allaart 1976; Gill and Bridgwater 1976; Nutman et al. 1984). The contact between the orthogneiss complexes that envelop the belt was identified as predominantly tectonic and it was concluded that a suite of mafic dikes that transects both the Isua supracrustals and the gneisses were equivalent to the Ameralik dikes defined in the Godthaabsfjord region. Boak and Dymek (1982) presented a detailed study of the metamorphic overprinting on the supracrustals. With increasing intensity of geochemical studies of the supracrustals and especially the advent of new isotopic tracers and age determination protocols, it became increasingly obvious during the 1970s and 1980s that many lithologies had not remained closed geochemical systems during deformation and metamorphism. It was first noted that alkali metals had been mobile (Gill and Bridgwater 1979; Baadsgaard et al. 1986), but also the Sm-Nd and Lu-Hf isotopic systems showed clear signs of disturbance (Gruau et al. 1996; Frei et al. 2002; Polat et al. 2003). Rosing et al. (1996) proposed that the Isua supracrustals had been pervasively metasomatized, and that the carbonate – calc-silicate formation was not a stratigraphic unit of sedimentary origin, but rather a wide variety of different lithologies transformed by transient silica-undersaturated carbonic fluids derived from devolatilization reactions in the ultramafic rocks (Rosing and Rose 1993; Rose et al. 1996). Inherent in this reinterpretation of the lithologic variation within the supracrustal belt was the breakdown of the interpretation of the Isua belt as a tight syncline folded into an open arc, as the symmetry of formations about a central axial plane trace was no longer apparent when the metasomatic overprinting was considered. Recent developments in the understanding the tectonic construction of the Isua belt includes the proposal that the Isua belt formed as an accretionary complex, which shares many similarities with the Phanerozoic
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Isua Supracrustal Belt. Figure 3 Pillow structure in metabasalt from Isua. The pillow basalts occur in association with sheeted dykes, gabbros, and ultramafic rocks in an assemblage similar to modern ophiolite complexes
accretionary complexes of Japan (Komiya et al. 1999). This suggestion allowed for the realization that the Isua belt might be composed of tectonic panels of slightly different early Archean ages and origins (Nutman et al. 1997) and that the package included ocean floor sequences similar to Phanerozoic ophiolites (Furnes et al. 2007) (Fig. 3). A new updated 1:20,000 geologic map constructed on the basis of a composite origin of the Isua supracrustal belt has been presented by Nutman and Friend (2009).
References and Further Reading Allaart JH (1976) The pre-3760 m.y. old supracrustal rocks of the Isua area, central West Greenland, and the associated occurrence of quartz-banded ironstone. In: Windley BF (ed) The early history of the earth. Wiley, London, pp 177–189 Baadsgaard H (1973) U-Th-Pb dates on zircons from early Precambrian Amitsoq Gneisses, Gogodthaab-District, West Greenland. Earth Planet Sci Lett 19(1):22–28 Baadsgaard H (1976) Further U-Pb dates on zircons from early Precambrian rocks of Godthaabsfjord Area, West-Greenland. Earth Planet Sci Lett 33(2):261–267 Baadsgaard H, Nutman AP et al (1984) The Zircon Geochronology of the Akilia Association and Isua Supracrustal Belt, West Greenland. Earth Planet Sci Lett 68(2):221–228 Baadsgaard H, Nutman AP et al (1986) Alteration and metamorphism of Amitsoq Gneisses from the Isukasia Area, West-Greeland – recommendations for isotope studies of the early crust. Geochim Et Cosmochim Acta 50(10):2165–2172
Black LP, Gale NH et al (1971) Isotopic dating of very early Precambrian Amphibolite Facies Gneisses from Godthaab District, West Greenland – Oxford Isotope Geology Laboratory. Earth Planet Sci Lett 12(3):245 Blichert-Toft J, Albarede F et al (2000) The Nd and Hf isotopic evolution of the mantle through the archean. Results from the Isua supracrustals, West Greenland, and from the birimian terranes of West Africa (vol 63, pg 3159, 1999). Geochim Et Cosmochim Acta 64(7):1329–1330 Boak JL, Dymek RF (1982) Metamorphism of the Ca-3800 Ma Supracrustal Rocks at Isua, West Greenland – implications for early archean crustal evolution. Earth Planet Sci Lett 59(1):155–176 Boyet M, Blichert-Toft J et al (2003) Nd-142 evidence for early Earth differentiation. Earth Planet Sci Lett 214(3–4):427–442 Bridgwater D, McGregor VR (1974) Field work on the early Precambrian rocks of the Isua area, southern West Greenland. Rapp Grønl Geol Unders 65:49–53 Bridgwater D, Escher A et al (1970) Development of Precambrian shield in South-Western Greenland, Labrador, and Baffin-Island. AAPG Bull 54(12):2472 Caro G, Bourdon B et al (2003) Sm-146-Nd-142 evidence from Isua metamorphosed sediments for early differentiation of the Earth’s mantle (vol 423, pg 428, 2003). Nature 424(6951):974–974 Frei R, Rosing MT (2005) Search for traces of the late heavy bombardment on Earth – results from high precision chromium isotopes. Earth Planet Sci Lett 236(1–2):28–40 Frei R, Rosing MT et al (2002) Hydrothermal-metasomatic and tectono-metamorphic processes in the Isua supracrustal belt (West Greenland): a multi-isotopic investigation of their effects on the Earth’s oldest oceanic crustal sequence. Geochim Et Cosmochim Acta 66(3):467–486
Isua Supracrustal Belt Furnes H, de Wit M et al (2007) A vestige of Earth’s oldest ophiolite. Science 315(5819):1704–1707 Garde AA, Hamilton MA et al (2002) The Ketilidian orogen of South Greenland: geochronology, tectonics magmatism and fore-arc accretion during Palaeoproterozoic oblique convergence. Can J Earth Sci 39(5):765–793 Gill RCO, Bridgwater D (1976) Ameralik Dykes of West Greenland, earliest known basaltic rocks intruding stable continental crust. Earth Planet Sci Lett 29(2):276–282 Gill RCO, Bridgwater D (1979) Early archean basic magmatism in West Greenland – geochemistry of the Ameralik Dykes. J Petrol 20(4):695–726 Griffin WL, Mcgregor VR et al (1980) Early archean granulite-facies metamorphism south of Ameralik, West Greenland. Earth Planet Sci Lett 50(1):59–74 Gruau G, Rosing M et al (1996) Resetting of Sm-Nd systematics during metamorphism of > 3.7-Ga rocks: implications for isotopic models of early Earth differentiation. Chem Geol 133(1–4):225–240 Hamilton PJ, Onions RK et al (1978) Sm-Nd isotopic investigations of isua supra-crustals and implications for mantle evolution. Nature 272(5648):41–43 James PR (1976) Deformation of Isua block, West Greenland – remnant of earliest stable continental crust. Can J Earth Sci 13(6):816–823 Keto L, Kurki J (1967) Report on the exploration activity at Isua 1967. Kryolitselskabet Øresund A/S Prospekting report, 20024 Komiya T, Maruyama S et al (1999) Plate tectonics at 3.8–3.7 Ga: field evidence from the Isua accretionary complex, southern West Greenland. J Geol 107(5):515–554 Kurki J, Keto L (1966) 1966 report on geological investigations at Isua 1966. Kryolitselskabet Øresund A/S Prospekting report, 20219 McGregor VR (1968) Field evidence for very old Precambrian rocks in the Godtha˚b are, West Greenland. Rapp Grønl Geol Unders 15:31–35 McGregor VR (1973) The early Precambrian Gneisses of the Godtha˚b district, West Greenland. R Soc Lond Philos Trans 273:343–358 Mojzsis SJ, Arrhenius G et al (1996) Evidence for life on Earth before 3, 800 million years ago. Nature 384(6604):55–59 Moorbath S, Gale NH et al (1972) Further rubidium-strontium age determinations on very early Precambrian rocks of Godthaab District, West Greenland. Nat Phys Sci 240(100):78 Moorbath S, Onions RK et al (1973) Early archean age for Isua iron formation, West Greenland. Nature 245(5421):138–139 Moorbath S, Onions RK et al (1975) Evolution of early Precambrian crustal rocks at Isua, West Greenland – geochemical and isotopic evidence. Earth Planet Sci Lett 27(2):229–239 Noe-Nygaard A, Ramberg H (1961) Geological reconnaisance map of the country between latitudes 69 N and 63 450 N, West Greenland. Meddr Grønland 123(5):9 Nutman AP (1986) The geology of the Isukasia region, southern West Greenland. Grønl Geol Undersøgelser Bull 154:1–80 Nutman AP, Friend CRL (2009) New 1:20, 000 scale geological maps, synthesis and history of investigation of the Isua supracrustal belt and adjacent orthogneisses, southern West Greenland: a glimpse of EoArchean crust formation and orogeny. Precambrian Res 172(3–4):189–211 Nutman AP, Allaart JH et al (1984) Stratigraphic and geochemical evidence for the depositional environment of the early archean Isua supracrustal belt, Southern West Greenland. Precambrian Res 25(4):365–396
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Nutman AP, Friend CRL et al (1989) Evolution and assembly of Archean gneiss terranes in the Godthabsfjord region, southern West Greenland – structural, metamorphic, and isotopic evidence. Tectonics 8(3):573–589 Nutman AP, McGregor VR et al (1996) The Itsaq gneiss complex of southern west Greenland: the world’s most extensive record of early crustal evolution (3900–3600 Ma). Precambrian Res 78(1–3): 1–39 Nutman AP, Bennett VC et al (1997) Similar to 3710 and 3790 Ma volcanic sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications. Chem Geol 141 (3–4):271–287 Nutman AP, Bennett VC et al (2000) The early Archean Itsaq Gneiss Complex of southern West Greenland: the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution. Geochim Et Cosmochim Acta 64(17):3035–3060 Nutman AP, McGregor VR et al (2002) 3850 Ma BIF and mafic inclusions in the early Archean Itsaq gneiss complex around Akilia, southern West Greenland? The difficulties of precise dating of zircon-free protoliths in migmatites. Precambrian Res 117(3–4):185–224 Pankhurst RJ, Moorbath S et al (1973) Mineral age patterns in Ca 3700 my old rocks from West-Greenland. Earth Planet Sci Lett 20(2):157–170 Polat A, Hofmann AW et al (2003) Contrasting geochemical patterns in the 3.7–3.8 Ga pillow basalt cores and rims, Isua greenstone belt, Southwest Greenland: implications for postmagmatic alteration processes. Geochim Et Cosmochim Acta 67(3):441–457 Rose NM, Rosing MT et al (1996) The origin of metacarbonate rocks in the Archean Isua supracrustal belt, West Greenland. Am J Sci 296(9):1004–1044 Rosing MT (1999) C-13-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from west Greenland. Science 283(5402):674–676 Rosing MT, Rose NM (1993) The role of Ultramafic rocks in regulating the concentrations of volatile and nonvolatile components during deep-crustal metamorphism. Chem Geol 108(1–4):187–200 Rosing MT, Rose NM et al (1996) Earliest part of Earth’s stratigraphic record: a reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence. Geology 24(1):43–46 Rosing MT, Nutman AP et al (2001) A new fragment of the early earth crust: the Aasivik terrane of West Greenland. Precambrian Res 105(2–4):115–128 Schidlowski M, Appel PWU et al (1979) Carbon Isotope Geochemistry of the 3.7 109-yr-old Isua sediments, West Greenland – implications for the Archean carbon and oxygen cycles. Geochim Et Cosmochim Acta 43(2):189–199 Schoenberg R, Kamber BS et al (2002) Tungsten isotope evidence from similar to 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth. Nature 418(6896):403–405 Shimizu H, Nakai S et al (1988) Geochemistry of Ce and Nd Isotopes and Ree Abundances in the Amitsoq gneisses, West Greenland. Earth Planet Sci Lett 91(1–2):159–169 Ussing NV (1912) Geology of the country around Julianehaab, Greenland. Medd Grønl 38:376 van Gool JAM, Connelly JN et al (2002) The Nagssugtoqidian orogen of West Greenland: tectonic evolution and regional correlations from a West Greenland perspective. Can J Earth Sci 39(5):665–686
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van Zuilen MA, Lepland A et al (2003) Graphite and carbonates in the 3.8 Ga old Isua supracrustal belt, southern West Greenland. Precambrian Res 126(3–4):331–348 Vanbreem O, Upton BGJ (1972) Age of some Gardar intrusive complexes, South Greenland. Geol Soc Am Bull 83(11):3381
Itokawa Asteroid Synonyms Asteroid 25143
Definition
Isuakasia ▶ Isua Supracrustal Belt
Itabirite ▶ Banded Iron Formation
Itokawa is an S-type Near-Earth asteroid, numbered 25143. This irregular object has been the first asteroidal target for a sample return mission: the ▶ JAXA Hayabusa-1 mission. It has established that Itokawa is very small (530 290 210 m) and has a density of about 1,900 kg/m3, leading to a rather high porosity. It presents rough terrains scattered with boulders, and smooth featureless areas, resulting from fine dust accumulated in local gravitational lows. Its shape suggests that it is a contact binary (as the nucleus of comet Hartley 2). Its density could indicate that it is built from loose-packed rocks, held together by their gravity.
See also
Italian Space Agency ▶ ASI
▶ Asteroid ▶ Hayabusa Mission ▶ JAXA ▶ Near-Earth Objects
J Jack Hills (Yilgarn, Western Australia) SIMON A. WILDE Department of Applied Geology, Curtin University of Technology, Perth, WA, Australia
Keywords Continental crust, ▶ Hadean, Neoarchean, oceans, Western Australia, ▶ zircon
Definition Jack Hills is the name of a low range of hills located in the Murchison District of Western Australia, approximately 800 km north of Perth. Since 1986, the area has been internationally known as the site of the world’s oldest minerals. These are detrital zircon grains up to 4,404 Ma old.
Overview A ▶ zircon grain with a portion recording an ▶ age of 4,404 8 Ma (2s) was extracted from a conglomerate on Eranondoo Hill in the central Jack Hills. Published in 2001 (Wilde et al. 2001), this age was 130 Ma older than the previous oldest known crystal on Earth, also reported from the same site in 1986 (Compston and Pidgeon 1986). Prior to that, the oldest known zircon crystals (4,150 Ma) were obtained from Mt Narryer, located 60 km southwest of Jack Hills (Froude et al. 1983). Both Jack Hills and Mt Narryer are located in the Narryer Terrane, which occupies an area of 30,000 km2 in the northwestern part of the Archean Yilgarn Craton of Western Australia (Spaggiari et al. 2007; Wilde and Spaggiari 2008). The Jack Hills belt is approximately 90-kilometers long, and it has a sigmoidal shape. Deformation is extensive and contacts of the belt with adjacent deformed granites and gneisses are all sheared, so that its original disposition is obscured. The oldest known rock in Australia (3,731 4 Ma) is tonalitic ▶ gneiss, collected 3 km south of the Jack Hills (Nutman et al. 1991). Overall, the ages of granitoids at Jack Hills closely match those
obtained from Mt. Narryer, with a spread of U-Pb dates from 3.70 to 3.30 Ga for the older components, and ages of 2.65 Ga for younger monzogranites that intrude the older granitoids. The latter may locally intrude the belt itself, although the contacts are now sheared. The older granitoids record discrete peaks at 3.75–3.65 Ga, 3.50 Ga, and at 3.30 Ga, interpreted to define major magmatic events. The oldest group of ages may record the time of extraction of the earliest ▶ tonalites from a garnet – amphibolite source. The presence of 3.3 Ga zircon rims around older cores in several rocks suggests multiple recycling of the earliest granitoids (Cavosie et al. 2004). Importantly, evidence for 3.3 Ga-old magmatic activity appears to be restricted to the southern side of the Jack Hills belt, suggesting that the belt may lie along a major suture zone within the Narryer Terrane. The belt itself consists of weakly metamorphosed sedimentary rocks that include ▶ banded iron formation (BIF), ▶ chert, siltstone, quartzite, sandstone, and conglomerate, together with thin units of ▶ mafic and ultramafic rocks. These units have been grouped into three packages or associations (Wilde and Pidgeon 1990): (a) chemical sedimentary rocks consisting of BIF, chert, together with ▶ mafic schist (▶ amphibolite), and minor ultramafic intrusions that are developed along both the northern and southern margins of the belt; (b) a pelitesemipelite association characterized by quartz-biotite, quartz-chlorite, and andalusite-bearing schists, with local mafic and ultramafic schists, which were possibly part of a turbidite sequence that now forms most of the central part of the belt; and (c) mature clastic sedimentary rocks that occur in a more restricted sequence comprising conglomerate, sandstone, quartzite, and siltstone, developed in the central and northern parts of the belt. Until 2002, all sedimentary rocks in the Jack Hills belt were thought to have been deposited in the late Meosoarchean to early Neoarchean, since no zircon younger than 3.1 Ga had been recorded from the samples. However, concordant Late Archean and Paleoproterozoic zircons were discovered in some of the clastic metasedimentary units with ages ranging from 2,736 6 to 1,576 22 Ma (Cavosie et al. 2004). This indicates that younger sedimentary units
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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Jack Hills (Yilgarn, Western Australia)
Jack Hills (Yilgarn, Western Australia). Figure 1 Simon Wilde (left) and John Valley (right) standing behind the metaconglomerate of the site W74 site, Jack Hills where the oldest detrital zircons on the Earth were discovered
are tectonically interleaved with the Archean units. At present, the full extent of Proterozoic rocks within the belt is unclear, since it is impossible to distinguish them from Archean metasedimentary rocks in the field, due to their lithological similarity and the effects of Proterozoic deformation. The main source of ancient zircons for which Jack Hills is internationally known is the Eranondoo Hill site (Fig. 1), where a specific conglomerate yields 12% zircon with ages >4 Ga, making it the richest source of Hadean material currently known on Earth. The zircon crystals are structurally complex, and a range of ages can commonly be obtained from different domains within a single crystal, attesting to their long and complex history. However, because oscillatory zoning of igneous origin defines pristine magmatic domains in many grains, the original source rocks are widely considered to be plutonic igneous rocks (Cavosie et al. 2006), possibly of the ▶ tonalite-trondhjemite-granodiorite series (TTG).
▶ Oxygen isotope data from such oscillatory zoned domains in several zircon grains (including the oldest) reveal elevated values (d18O up to 7.4‰), indicating that they crystallized from the melting of preexisting supracrustal material that had undergone interaction with low-temperature liquid water (Peck et al. 2001; Cavosie et al. 2005). Such zircons thus represent the earliest evidence for both ▶ continental crust and ▶ oceans on Earth. Furthermore, isotopic analyses reveal that some of these zircons have fractionated lithium isotope ratios that are much more variable than those recorded from fresh igneous rocks. Values of d7Li below 10‰ are present in zircons as old as 4,300 Ma, suggesting that highly weathered regolith was being sampled by these early magmas (Ushikubo et al. 2009). This provides further evidence that the parent magmas of the ancient zircons incorporated material that was itself derived from the surface weathering and erosion of preexisting rock in the presence of liquid water, supporting the hypothesis that continental crust and oceans existed at 4,300 Ma, within 250 Ma of the formation of Earth. This requires relatively cool temperatures at the Earth’ surface between 4,400 and 4,325 Ma (Valley et al. 2002; Valley 2005). Additional studies of lutetium and hafnium isotopes in ancient zircons reveal that the precursor material that gave rise to the melts in which the ancient zircons crystallized had a history extending back to 4.5 Ga, extremely close to the age of the Earth itself (Kemp et al. 2010). Collectively, these data point to the early development on Earth of both continents and oceans, and require fairly rapid cooling following accretion, melting, and crystallization of any magma ocean that may have been present. Certain zircon grains, some as old as 4.25 Ga, also contain minute inclusions of diamond and graphite (Menneken et al. 2007). The origin of these is uncertain, but they have characteristics similar to diamonds found in zircon that grew within continental crust undergoing ultrahigh pressure metamorphism in subduction zones, possibly suggesting thick crust on Earth at this time. The carbon isotope signature of the diamonds and graphite is strongly negative (median d13C = 31‰), a feature not inconsistent with derivation from a biogenic source, although it is not unambiguous evidence for life and could be the result of inorganic processes (Nemchin et al. 2008).
Basic Methodology Virtually all the key data acquired from the Jack Hills and Mt. Narryer that has relevance to the early Earth comes from isotopic analysis of individual zircon crystals. In the absence of any rocks on Earth older than 4.03 Ga (▶ Acasta Gneisses in the Northwest Territories of
Jack Hills (Yilgarn, Western Australia)
Acc.V Spot Magn 12.0 kV 7.0 400x
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50 µm
Det WD Exp CL 17.8 1
Jack Hills (Yilgarn, Western Australia). Figure 2 Photo of the oldest zircon at site W74, Jack Hills, Yilgarn Craton (Western Australia). The ancient crystal is imaged in cathodoluminescence prior to the second analytical session that identified the 4.404 Ga portion (in red) (Photo Simon Wilde)
Canada), these zircon crystals, together with a few from other localities around the world, are all that remains of the first 500 Ma of Earth’s history. With recent improvements in analytical techniques and instrumentation, it is now possible to determine in situ U-Pb, Lu-Hf, O, C, Ti, Li, and other trace elements with reasonable precision on portions of crystals only a few microns in diameter. Further advances can be expected in the coming years.
Applications The Jack Hills zircons constitute the main database for study of the first 500 Ma of Earth’s history (▶ Hadean; ▶ Earth, Formation and Early Evolution). As such, they represent a unique inventory. If, or when, other areas are discovered with abundant Hadean zircons, they will provide the standard that will be used for comparison.
Future Directions Key Research Findings Jack Hills in Western Australia contains the oldest known fragment of the Earth’s crust; a portion of a zircon crystal with a 207Pb/206Pb age of 4,404 8 Ma (2s) (Fig. 2). In addition, zircon crystals 4.3 Ga old show elevated d18O values and strongly negative d7Li values, indicating they grew during melting of a protolith that evolved through weathering and interaction with liquid water at the Earth’s surface, possibly in a CO2-rich atmosphere. Lu-Hf isotope data from the zircons suggest the earliest crust may have formed as early as 4.5 Ga, close to the age of the Earth itself (Harrison et al. 2005; Kemp et al. 2010). The presence of diamond/graphite inclusions in zircon as old as 4.25 Ga is difficult to explain, but may indicate thick continental crust on the early Earth. The negative carbon isotope signature of these minerals is not prima facie evidence for a biogenic origin, but neither does it exclude it. It attests to a light carbon reservoir on the early Earth.
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Further work at Jack Hills is required to provide answers to the following problems: (1) the exact nature of the hostrocks of the ancient zircon crystals; (2) whether any remnants of those rocks are still present in the area; (3) the extent and provenance of Proterozoic sediments in the Jack Hills; and (4) the source and origin of the diamond/ graphite inclusions in zircon.
See also ▶ Acasta Gneiss ▶ Amphibolite Facies ▶ Archean Tectonics ▶ Earth, Formation and Early Evolution ▶ Geochronology ▶ Hadean ▶ Mafic and Felsic ▶ Oceans, Origin of ▶ Oxygen Isotopes
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James Webb Space Telescope
▶ Regolith (Terrestrial) ▶ Shale ▶ Silicate Minerals ▶ Tonalite–Trondhjemite–Granodiorite ▶ Water, Delivery to Earth ▶ Zircon
References and Further Reading Cavosie AJ, Wilde SA, Liu DY, Weiblen PW, Valley JW (2004) Internal Zoning and U-Th-Pb chemistry of Jack Hills detrital zircons: a mineral record of early Archean to Mesoproterozoic (4348–1576 Ma) magmatism. Precambrian Res 135:251–279 Cavosie AJ, Valley JW, Wilde SA, EIMF (2005) Magmatic d18O in 4400–3900 Ma detrital zircons: a record of the alteration and recycling of crust in the early Archean. Earth Planet Sci Lett 235:663–681 Cavosie AJ, Valley JW, Wilde SA, EIMF (2006) Correlated microanalysis of zircon: trace element, d18O, and U–Th–Pb isotopic constraints on the igneous origin of complex >3900 Ma detrital grains. Geochim Cosmochim Acta 70:5601–5616 Compston W, Pidgeon RT (1986) Jack Hills, evidence of more very old detrital zircons in Western Australia. Nature 321:766–769 Froude DO, Ireland TR, Kinny PD, Williams IS, Compston W, Williams IR, Myers JS (1983) Ion microprobe identification of 4,100–4,200 Myr-old terrestrial zircons. Nature 304:616–618 Harrison TM, Blichert-Toft J, Mu¨ller W, Albarede F, Holden P, Mojzsis SJ (2005) Heterogeneous Hadean hafnium: evidence of continental crust at 4.4–4.5 Ga. Science 310:1947–1950 Kemp AIS, Wilde SA, Hawkesworth CJ, Coath CD, Nemchin A, Pidgeon RT, Vervoort JD, DuFrane SA (2010) Hadean crustal evolution revisited: new constraints from Pb-Hf systematics of the Jack Hills zircons. Earth Planet Sci Lett 296:45–56 Menneken M, Nemchin AA, Geisler T, Pidgeon RT, Wilde SA (2007) Discovery of oldest terrestrial diamond in zircon from Jack Hills, Western Australia. Nature 448:917–920 Nemchin AA, Whitehouse MJ, Menneken M, Geisler T, Pidgeon RT, Wilde SA (2008) A light carbon reservoir recorded in zircon-hosted diamond from Jack Hills. Nature 453:92–95 Nutman AP, Kinny PD, Compston W, Williams IS (1991) SHRIMP U–Pb zircon geochronology of the Narryer Gneiss Complex, Western Australia. Precambrian Res 52:275–300 Peck WH, Valley JW, Wilde SA, Graham CM (2001) Oxygen isotope ratios and rare Earth elements in 3.3–4.4 Ga zircons: ion microprobe evidence for high d18O continental crust and oceans in the early Archean. Geochim Cosmochim Acta 65:4215–4229 Pidgeon RT, Wilde SA (1998) The interpretation of complex zircon U-Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer Gneiss Terrane, Western Australia. Precambrian Res 91:309–332 Spaggiari CV, Pidgeon RT, Wilde SA (2007) The Jack Hills greenstone belt, Western Australia, Part 2: lithological relationships and implications for the deposition of >4.0 Ga detrital zircons. Precambrian Res 155:261–286 Ushikubo T, Kita NT, Cavosie AJ, Wilde SA, Rudnick RL, Valley JW (2009) Lithium in Jack Hills zircons: evidence for recycling of Earth’s earliest crust. Earth Planet Sci Lett 272:666–676 Valley JW (2005) A cool early Earth? Sci Am, Oct 2005, 58–65
Valley JW, Peck WH, King EM, Wilde SA (2002) A cool early Earth. Geology 30:351–354 Wilde SA, Spaggiari CV (2007) The Narryer Terrane, Western Australia: a review. In: Van Kranendonk MJ, Smithies RH, Bennett VC (eds) Earth’s oldest rocks, developments in Precambrian Geology, vol 15. Elsevier, Amsterdam, pp 275–304 Wilde SA, Pidgeon RT (1990) Geology of the Jack Hills metasedimentary rocks, In: Ho SE, Glover JE, Myers JS, Muhling J (eds), 3rd international Archaean symposium, Perth, excursion guidebook, vol 21. University of Western Australia Publication, Perth, pp 82–92 Wilde SA, Valley JW, Peck WH, Graham CM (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409:175–178
James Webb Space Telescope ▶ JWST
Japan Aerospace Exploration Agency ▶ JAXA
Jarosite Synonyms Utahite
Definition Jarosite is a secondary mineral of chemical formula KFe3(SO4)2(OH)6 (trigonal crystal system) belonging to the group of ▶ alunite. This ocher-yellow or brown ferric sulfate is formed in wet, oxidizing, and acidic environments by ▶ weathering of volcanic rocks in the presence of acidic, sulfur-rich fluids or by oxidation of sulfide minerals in acid drainage environments. Jarosite is preserved only in relatively arid climates because it rapidly decomposes in the presence of water to ferric oxyhydroxides. Jarosite has been discovered by the ▶ Mars Exploration Rover Opportunity at the Meridianum Planum landing site. It is within a sulfate salt matrix containing jarosite that hematite spherules (Martian “blueberries”), possibly
Jeans Escape
accreted under water, were discovered (see ▶ hematite). The presence of jarosite on planetary surfaces could be a signature of the past occurrence of water.
See also ▶ Alunite ▶ Hematite ▶ Mars ▶ Weathering
Jasper ▶ Chert ▶ Jaspilite
Jaspilite Synonyms Jasper
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all their activities in the aerospace field as one organization, from basic research and development to utilization. The independent administrative institution is the Japan Aerospace Exploration Agency (JAXA). This administration manages 17 facilities across Japan as well as overseas offices. From the Tanegashima Space Center, national rockets H-II are regularly launched. Through JAXA, Japan is a major partner of the international space station and contributes with the KIBO scientific module. It is currently preparing to deliver a cargo spacecraft, the H-II Transfer Vehicle (HTV). The first Japanese citizen to fly in space was a journalist, Toyohiro Akiyama, who flew on the Soviet Soyuz TM-11 in December 1990 while the expenses (around 14 million USD) were paid by a broadcasting company. JAXA has 10 professional astronauts of staff, who are supporting experiments onboard the ISS. Its missions with relevance to astrobiology include those to ▶ Halley’s Comet (Suisei and Sakigake) and to the asteroid 25143 Itokawa (Hayabusa).
See also ▶ Comet Halley
Definition Jaspilite is a banded compact siliceous rock consisting of interbanded jasper (red ▶ chert) and ▶ hematite. In Australia the term jaspilite was commonly applied to Precambrian iron formations of Western Australia, but ▶ banded iron formation (BIF) is now generally used.
See also ▶ Banded Iron Formation ▶ Chert ▶ Hematite
JAXA Synonyms Japan Aerospace Exploration Agency
Definition On October 1, 2003, the Institute of Space and Astronautical Science (ISAS), the National Aerospace Laboratory of Japan (NAL), and the National Space Development Agency of Japan (NASDA) were merged into one independent administrative institution to be able to perform
Jeans Escape Synonyms Thermal escape
Definition The Jeans escape phenomenon is one of the mechanisms by which a planet can loose gradually some constituents of its atmosphere. It is named after James Hopwood Jeans (1877–1946), an English astrophysicist. The process corresponds to the probability for a molecule to travel a distance larger than the atmospheric ▶ scale height without colliding with another molecule, at a speed larger than the planet’s escape velocity. The average thermal velocity of a molecule in a gas is proportional to the square root of the temperature and inversely proportional to the square root of its mass, while the escape velocity increases with the mass of the planet. One concludes that a planet will more rapidly lose its atmosphere if it is of a low mass and if its atmosphere is hot and made of light elements such as hydrogen and helium. In the solar system, giant planets which are massive and rather cold do not lose their hydrogen atmosphere, while small planets, close to the Sun and less massive, did lose their initial hydrogen.
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Jet Propulsion Laboratory
See also
See also
▶ Atmosphere, Escape ▶ Atmosphere, Model 1D ▶ Scale Height
▶ Color Index ▶ Hertzsprung–Russell Diagram ▶ Magnitude
Jovian Planets
Jet Propulsion Laboratory
▶ Giant Planets
▶ JPL
JPL
Johnson UBV Bandpasses
Synonyms
Definition
Jet Propulsion Laboratory
The Johnson UBV bandpasses (Fig. 1) are the bandwidths of the three filters called U (ultraviolet), B (blue), and V (visual) in the widely used astronomical photometric system defined by Harold Johnson (1921–1980) and William Morgan (1906–1994). Those three colors are especially useful to build the ▶ Hertzsprung–Russell (HR) diagram that permits to classify stars. The central wavelengths and bandwidths are given in the table: Filter name Central wavelength (nm) Bandwidth (nm) U
365
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B
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V
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The reference flux in each band is such that the U, B, and V magnitudes are identical for an A0 star such as Vega.
100 Transmission (%)
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Johnson UBV Bandpasses. Figure 1 The Johnson UBV bandpasses
Definition The Jet Propulsion Laboratory (JPL) is the follow-on of Von Karman’s laboratory established in the 1930s at the California Institute of Technology to work on aeronautics and its physics. Several students by this time were attempting to work on and launch rockets. Many members of this so-called suicide squad were the founders of the present Jet Propulsion Laboratory. The Jet Propulsion Laboratory (JPL) is a research and development center managed by the California Institute of Technology for the US National Aeronautics and Space Administration (▶ NASA). JPL’s primary function is the construction and operation of robotic planetary spacecraft, though it also conducts earth orbit and astronomy missions. It is also responsible for operating NASA’s Deep Space Network for communication with spacecraft. JPL has been involved with a very wide range of solar system and astronomy missions relevant to astrobiology. Among the current such missions are Cassini-Huygens, the Mars Exploration Rovers, and the infrared Spitzer Space Telescope. Beyond Mars, the JPL had a lot of achievements all along the robotic exploration of the solar system, from earth orbit (Explorer-1) to the Moon (Ranger and Surveyor probes) to the far end of the solar system (Voyager). More than 5,000 people are working at JPL plus few hundreds of permanent contractors.
See also ▶ Cassini–Huygens Space Mission ▶ MER, Spirit and Opportunity (Mars)
Jupiter
▶ MSL ▶ Spitzer Space Telescope ▶ Viking
Jupiter THERESE ENCRENAZ LESIA, Observatoire de Paris, Meudon, France
Keywords Giant planets
Definition Among the ▶ Giant planets, Jupiter is the most massive and also the closest to the Sun; it orbits at 5.2 ▶ AU from the Sun. Its mass is 318 times the terrestrial mass and its diameter is 11 times the terrestrial one, which corresponds to a density of 1.31 g/cm3. As the other giant planets, Jupiter is generally believed to have formed in the outer solar system from the accretion of an icy core followed by the collapse of the surrounding nebula, mostly composed of hydrogen and helium. As Jupiter probably formed just beyond the snowline, which marked the condensation of ices in the protosolar disk, its core accretion was fast and the planet could accrete a large amount of protosolar gas, which accounts for its large mass. Jupiter has a tenuous ring system and many satellites (63 known in 2007), both regular and irregular. The regular satellites were formed in the equatorial plane of the planet after the collapse of the surrounding subnebula. They include the four Galilean satellites (▶ Io, ▶ Europa, Ganymede, and Callisto) discovered by Galileo Galilei in 1610 with his new telescope. The irregular satellites are probably captured distant asteroids.
Overview Early Observations As one of the brightest objects in the sky, Jupiter has been known since Antiquity. Modern astronomical observations started in 1610 with the first observations by Galileo Galilei. A few decades after the discovery of the Galilean satellites, the band and zone structure of Jupiter, induced by its fast rotation period (9 h 55 min), was observed. In 1664, Robert Hooke identified the Great Red Spot (GRS) which was studied in detail by ▶ Cassini. Since then, the
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planet has been continuously monitored, showing, in particular, the stability of the GRS over more than three centuries. Atmospheric compounds were first identified by spectroscopy in the twentieth century: CH4 and NH3, minor components but strong spectroscopic absorbers, were identified by Wildt in 1932. Hydrogen, the dominant species but less active spectroscopically, was expected to be present on a theoretical basis, but was not detected until 1961. Since the 1970s, the development of ▶ infrared spectroscopy has led to the detection of a long list of minor constituents (PH3, GeH4, SiH3, CO, C2H2, C2H6. . .). It must be noted that most of these species have been detected from ground-based observations. In 1994, the collision of the ▶ Comet Shoemaker-Levy 9 with Jupiter provided astronomers with a unique opportunity to study in real time the response of an atmosphere to a major meteoritic impact. Many new molecules were formed by shock chemistry, some of them remaining detectable for the following years. Another major result of the ground-based exploration of Jupiter was the discovery, in the 1950s, of strong radioemission, both in the decimeter and decameter range. This nonthermal emission was interpreted as the signature of a magnetic field, a result which was later confirmed by space exploration.
The Space Exploration of Jupiter Jupiter and ▶ Saturn were first encountered by the two spacecraft Pioneer 10 and Pioneer 11 in 1973 and 1974, respectively. They sent the first high-resolution images of the atmospheric structure and the first in situ exploration of the Jovian magnetosphere. This first step was followed by the very successful ▶ Voyager mission, which consisted of two identical spacecraft which flew over the four giant planets between 1979 and 1989. Jupiter was visited by Voyager 1 and Voyager 2 in 1979. Among the many discoveries of the Voyager mission, one should quote the unexpected complexity of the Jovian cloud structure, the evidence for a tenuous ring system around the planet, the evidence for active volcanism on Io, and the surface diversity of the other Galilean satellites. Several other small satellites were also discovered. After the flybys, a more ambitious mission, including an orbiter and an atmospheric descent probe, was devoted to the exploration of the Jovian system. In 1995, the ▶ Galileo orbiter approached the planet and a probe was sent into its atmosphere. The probe transmitted data in the deep troposphere down to a pressure level of 22 bars. The orbiter monitored the planet’s atmospheric and cloud
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structure as well as the Galilean satellites until 2003. In 2000, Jupiter was also observed by the Cassini spacecraft on its way to the Saturn system.
Dynamical Atmospheric Structures The main cloud features of Jupiter – Great Red Spot, zones, and belts – have been known since the seventeenth century. The zone and belt structure can be interpreted, to first order, as a ▶ Hadley circulation with multiple cells generated by the fast rotation of Jupiter. Zones are regions of ascending motion and belts are regions of subsidence. Measurements by the Galileo probe in 1995 confirmed this convective pattern, but they also showed that the present circulation is much more complex than this simple scheme. The Coriolis forces induce zonal winds of opposite direction, north and south of each zone, generating multiple time-variable eddies. The biggest structure is the Great Red Spot in the southern hemisphere. Its size is about 10,000–14,000 km in the north–south axis and 24,000–40,000 km in the east–west axis. The GRS is believed to be a giant anticyclonic structure; however, its stability over several centuries is not yet completely understood. The composition of the GRS, in particular its red color, is still an open question; it might be due to sulfur or phosphorus compounds, but the exact chemical composition remains unknown.
Atmospheric Composition and Structure As all giant planets, the thermal vertical profile of Jupiter is characterized by a troposphere where the temperature decreases adiabatically with altitude, and a tropopause where the temperature is minimum (T = 110 K at P = 100 mbar). Above the tropopause, the temperature increases again with the altitude, due to the absorption of the solar infrared flux by methane and aerosols. At higher altitudes, other mechanisms contribute to a strong heating (planetary waves, high-energy particles). As hydrogen is the main atmospheric component, most of the other species are in a reduced state. A first group consists in tropospheric compounds: CH4, NH3, PH3, GeH4, AsH3, CO, and H2O. H2S, not detected by spectroscopy, was identified by the mass spectrometer of the Galileo probe. Most of these species condense at the level of the tropopause, where the temperature is minimum, the two exceptions being CH4 and CO. A second group contains the hydrocarbons produced by the photolysis of methane in the Jovian stratosphere (C2H2, C2H4, C2H6, CH3, C3H8, C6H6. . .). A third group includes oxygen species detected in the Jovian stratosphere (H2O, CO, CO2) which have
an external origin; indeed, any water coming from the interior would be trapped as ice at the tropopause. The external source of oxygen might be either local (from the Galilean satellites) or from an interplanetary flux of comets. The cloud structure of Jupiter can be inferred from models using thermochemical equilibrium. At a pressure of about 0.5 bar, ammonia clouds are expected, which are believed to be responsible for the white color of the Jovian zones. At deeper tropospheric level (about 2 bars), models predict clouds of ammonium hydrosulfide NH4SH, and at a pressure of a few bars, water clouds should be present. The presence of water clouds has been shown with the Near Infrared Mapping Spectrometer of Galileo; however, the spatial distribution of the Jovian cloud structure appears to be much more complex than expected from the models. There is an active convective circulation between zones and belts, as previously anticipated, with zones being upwelling regions, rich in clouds, and belts being regions of subsidence. However, the Galileo observations have shown that even within the belts, there are cloud-free, specific holes which are downwelling regions. In these regions, the radiation comes from deeper, warmer layers, which accounts for their name of “hot spots.” The Galileo probe entered one of these regions, free of clouds and very dry. As a result, the H2O cloud was absent and the very low measured water content is not representative of the whole planet. Apart from oxygen, many other elements were successfully measured by the mass spectrometer of the Galileo probe.
Elemental and Abundance Ratios in Jupiter The Galileo ▶ mass spectrometry measurements of the Jupiter atmosphere showed a general enrichment in all heavy elements with respect to hydrogen, as compared to the protosolar value. This enrichment factor, first estimated to 3 1, is now considered as being 4 2 after revision of the solar abundances. This result provided decisive support to the nucleation model for giant planet growth. According to this scenario, giant planets result from the accretion of a solid core (mostly made of ices) of about 10–12 terrestrial masses; then the gravity field of this accreted core was sufficient to induce the collapse of the surrounding nebula, mainly composed of hydrogen and helium. After the collapse phase and the vaporization of the ices of the core, the relative abundances of heavy elements (C, N, O, S and noble gases) must have been enriched with respect to their protosolar abundances. It can be shown that, if all elements were equally trapped in ices, an enrichment factor of 4 would be expected for
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Jupiter, which is in perfect agreement with the Galileo measurements. However, the general enrichment of Jupiter’s planetesimals raises another question: some species, like N and Ar, cannot be trapped in ices unless their temperature is very low (< 40 K). Their presence in Jupiter indicates that Jupiter’s planetesimals accreted at a temperature much lower than that simply deduced from the Sun– Jupiter distance. The origin of Jupiter’s ▶ planetesimals still remains an open question.
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in Jupiter’s interior, within the hydrogen metallic ocean, might also contribute to the internal energy. Based on these constraints theoretical models of Jupiter’s internal structure have been proposed. It would consist of a rocky core having a temperature of about 23,000 K while the pressure would range from 50 to 100 Mbars. This core is surrounded by a convective ocean of metallic hydrogen and helium, above which, at about 0.8 Jovian radii, a layer of molecular hydrogen is expected to extend up to the observable layers of the atmosphere.
Internal Structure We do not have direct measurements of Jupiter’s internal structure. The only known parameters are its diameter, mass, density, and gravitational moments. The Voyager infrared spectrometer IRIS showed that Jupiter radiates more heat than it receives from the Sun (1.67 times the absorbed solar energy). The origin of this internal energy could be the cooling of the planet, still in a slow contracting phase after the initial accreting phase. Helium condensation
The Jovian Magnetosphere The large magnetosphere of Jupiter, first identified by the Jovian synchrotron radiation and later explored by spacecraft, can be explained by several factors: the great size of the planet, its fast rotation, and the presence of a liquid metallic interior. The Jovian magnetic field is generated by a dynamo effect within the ocean of metallic hydrogen or within the core. The dipole moment is tilted by 10 with respect to the rotation axis of the planet. This magnetic field is 14 times as strong as the terrestrial one. As Jupiter is five times more distant from the Sun than the Earth is, its magnetosphere is considerably more extended than the terrestrial one. Still, its structure shows strong analogies with the Earth’s magnetosphere. A shockwave, a magnetopause, and a magnetosheath are present in the sunward direction. The main difference is the interaction of the Jovian magnetosphere with Io (and to a lesser extent with Europa) whose orbit crosses the Jovian magnetosphere (Fig. 1).
See also ▶ Cassini ▶ Comet Shoemaker-Levy 9 ▶ Europa ▶ Galileo ▶ Giant Planets ▶ Hadley Cells ▶ Io ▶ Mass Spectrometry ▶ Planetesimals ▶ Saturn ▶ Voyager (Spacecraft) Jupiter. Figure 1 The planet Jupiter as observed by the Cassini spacecraft in 2000. The Great Red Spot is clearly visible in the southern hemisphere, as well as the zone and belt structure all over the planet, showing evidence for a dynamical atmosphere © NASA
References and Further Reading (1979) Mission to Jupiter and its satellites (Voyager 1 encounter) Science 204:945–1008 (1996) Galileo at Jupiter: results from the orbiter. Science 274:377–412 (1996) Galileo at Jupiter: results from the probe. Science 272:837–860
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Bagenal F, Dowling T, McKinnon W (2007) Jupiter: the planet, satellites and magnetosphere. Cambridge Planetary Science, Cambridge University Press, Cambridge Noll KS, Weaver HA, Feldman PD (1996) The collision of Comet Shoemaker-Levy 9 and Jupiter. Cambridge University Press, Cambridge
JWST Synonyms James Webb Space Telescope; Next generation space telescope; NGST
Definition The James Web Space Telescope (JWST) is an infrared space telescope project led by ▶ NASA with contributions of several other space agencies (including the ▶ European Space Agency and the ▶ Canadian Space Agency). It is viewed as a successor to the Hubble Space Telescope (▶ HST). The JWST’s 6,200-kg telescope will be launched by a European Ariane V rocket in 2014. It will orbit around
the Sun–Earth L2 Lagrange point. Equipped with several cameras for various infrared wavelengths, it will pursue and augment the science carried out by HST. The ambitious scientific program is split into four main topics. Fundamental cosmology seeks to identify the first bright objects that formed in the early Universe and to follow the ionization history of the Universe. For astronomers, the JWST will study how galaxies and dark matter, including gas, stars, physical structures (like spiral arms) and active nuclei, evolved up to the present day, as well as to understand the birth and early development of stars and the formation of planets. For astrobiology, the main interest is in its capability to study the physical and chemical properties of solar systems (including our own) where the building blocks of life may be present.
See also ▶ European Space Agency ▶ HST ▶ Lagrange Points
K K/T Boundary ▶ KT Boundary
K-T Boundary ▶ KT Boundary
Kaapvaal Craton, South Africa
See also ▶ Archean Eon ▶ Barberton Greenstone Belt ▶ Barberton Greenstone Belt, Sedimentology ▶ Barberton Greenstone Belt, Traces of Early Life ▶ Crater, Impact ▶ Craton ▶ Granite ▶ Greenstone Belts ▶ Pilbara Craton
Kant–Laplace Cosmogonic Hypothesis
Definition The Kaapvaal ▶ craton in South Africa is, along with the ▶ Pilbara craton of Western Australia, the only areas where mid-Archean (3.6–2.5 Ga) volcanic and sedimentary rocks are relatively well preserved. The craton covers an area of about 1.2 106 km2 and is made up of several strongly deformed early Archean (3.0–3.5 Ga) ▶ greenstone belts intruded by tonalitic gneisses (ca. 3.6–3.7 Ga), and a variety of granitic plutons (3.3–3.0 Ga). The craton formed and stabilized between about 3.7 and 2.6 Ga when major granitoid batholiths intruded, and deformation thickened the crust. At the same time, a thick, stable harzburgitic (olivineorthopyroxene peridotite) keel formed the lower part of the lithosphere. Subsequent evolution from 3.0 to 2.7 Ga involved collision with surrounding cratons and the development of basins filled with thick sequences of both volcanic and sedimentary rocks. Similarities of rock records of the Kaapvaal and Pilbara cratons suggest that they were once part of a single continent called Valbara. Barberton, the largest greenstone belt in the craton, is well known for its komatiites (ultramafic volcanic rocks) and traces of early life preserved in cherts and other sedimentary rocks. Also present in the Kaapvaal craton is the Bushveld complex, an enormous mafic–ultramafic layered intrusion, and the 2.0 Ga Vredefort meteorite ▶ impact crater, the largest on Earth.
STE´PHANE LE GARS Centre Franc¸ois Vie`te, Universite´ de Nantes, Nantes, BP, France
Keywords Astronomy, cosmogony, galaxies, history, kant, laplace, nebula, origins, physics, solar system
Abstract Guided by Newton’s mechanics, Emmanuel Kant proposed in 1755 a hypothesis concerning the origin of the universe from the mutual attraction and the motion of scattered particles in an initial chaos. Without knowing Kant’s ideas, the French mathematician Pierre Simon Laplace proposed, at the end of the eighteenth century, a scenario close to the one imagined by Kant. Limiting his hypothesis to the solar system only and basing it on the probability theory, a reasoning of probability, Laplace gave a more reliable scientific basis to his cosmology, the basic ideas of which are still globally accepted today.
History In 1755, philosopher Emmanuel Kant brought out a work of his early youth, “The general history of nature and sky
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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theory,” in which he expressed his ideas concerning the solar system’s origin. His cosmogony was entirely guided by Newton’s mechanics: Kant assumed that at the origin matter was diffused all over space, where it formed a kind of chaos. The constituent particles of this chaos, initially at rest, came to attract each other under the effect of gravitation and developed rotational motion. This motion, accelerating, created a circular ring in which breaking up produced in its turn planets and their satellites. Kant, in his cosmogonic hypothesis, was prompted by an analogical principle and the search of a systematic organization. He proposed that every system has a center and is reproduced in a hierarchical way, passing analogically from the Earth–Moon system, to the solar system, then to the distribution of stars in the Milky Way, and lastly from the galaxies (still called at that time nebulae) around the center of the universe: Kant’s cosmogony aimed at stating a cosmic order. If Kant said his hypothesis was scientific, in reality it was impossible to be confirmed by observations at that time: it was only speculative. Moreover, it did not abide by certain laws of mechanics, such as the angular momentum conservation: How could particles shielded from all external effects begin a rotational motion? The French mathematician Pierre Simon Laplace proposed in 1796 a cosmogonic hypothesis close to Kant’s, without knowing the latter’s work. His cosmogony appeared immediately more scientific, especially because Laplace limited himself to consider the solar system’s structure, but not the universe’s organization. Furthermore, he avoided the problem of setting the primitive nebula in motion, considering it already in a uniform rotation. His approach was different from Kant’s because it was based more on a reasoning of probability (planets and satellite’s revolution and rotation direction, known at that time) than on an evolutionary viewpoint. Aspects of the cosmogonic hypothesis quickly called the “Kant–Laplace’s hypothesis” and discussed throughout the nineteenth century are still globally accepted today.
Laplace PS (1824) Exposition du syste`me du monde, 5th edn. Bachelier, Paris (1e`re e´dition 1796) POINCARE Henri (1911) Lec¸ons sur les hypothe`ses cosmogoniques. A. Hermann et fils, Paris Roger HAHN (2004) Le syste`me du monde. Pierre Simon Laplace. Un itine´raire dans la science, Gallimard
Kaolin ▶ Kaolinite
Kaolinite Synonyms Kaolin
Definition Kaolinite is an extremely common layered silicate clay mineral with the chemical composition Al2Si2O5(OH)4. It consists of one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra. The name is derived from Chinese Gaoling meaning “High Hill,” located in Jingdezhen, Jiangxi province, China. Kaolinite has a low cation exchange capacity. It is a soft, usually white mineral produced by the chemical weathering of aluminum silicate minerals such as feldspar. It is often colored reddish-orange by the presence of traces of iron oxides, but may range from white, yellow, or light orange colors when lesser amounts of iron are present. Kaolinite occurs abundantly in soils formed from chemical weathering of rocks in hot, humid climates.
See also ▶ Clay
See also ▶ Planet Formation ▶ Solar Nebula
References and Further Reading Charles W (1886) Les Hypothe`ses Cosmogoniques. Examen des the´ories scientifiques modernes sur l’origine des mondes. Suivi de la traduction de la The´orie du Ciel de Kant. Gauthier-Villars, Paris Emmanuel K (1910) Allgemeine Naturgeschichte und Theorie des Himmels. e´dition par l’Acade´mie des Sciences de Berlin Merleau-Ponty J (1983) La science de l’univers a` l’aˆge du positivisme. Etude sur les origines de la cosmologie contemporaine. Vrin, Paris Michel S (1975) Le retour e´ternel. Annales Economie Socie´te´s Civilisations 30(5):999–1006
Kepler Mission DAVID W. LATHAM Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
Keywords Asteroseismology, exoplanets, photometry, spectroscopy, transiting planets
Kepler Mission
Definition Kepler is a ▶ NASA mission to discover and characterize transiting ▶ planets around other stars (▶ exoplanets), with a special emphasis on rocky planets where water could be liquid on the surface and life as we know it might be comfortable. Kepler also supports programs for asteroseismology and general astrophysics.
History Serious thinking about the feasibility of using very precise photometry to detect small changes in brightness when planets transit in front of their stars dates back more than 25 years (e.g., see Borucki and Summers 1984). Subsequent proposals for space missions to search for small transiting planets, such as FRESIP (Borucki et al. 1996) in the 1990s, were viewed as technologically premature. However, this objection was answered by careful experiments with CCDs, which demonstrated that the photometric precision required to detect a planet like the ▶ Earth passing in front of a star like the ▶ Sun could be achieved in a laboratory simulation of the mission. Scientific interest in the mission was piqued by the discovery of the first transiting planet, HD 209458 (e.g., see Charbonneau et al. 2000), and Kepler was selected as NASA’s tenth Discovery class mission. Kepler was launched successfully on a Delta II from Cape Canaveral in March 2009 and has sufficient consumables to operate for a decade.
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The spacecraft is rotated to a new orientation four times a year in order to maintain illumination of the solar panels. The target area is more than 55 north of the ecliptic, and the focal plane is well protected from solar illumination by a sun shield. Figure 1 shows a diagram of the Kepler flight unit, and Fig. 2 shows a cutaway diagram of the photometer. To aid selection of an optimized list of targets similar to the Sun, the Kepler Input Catalog (KIC) was prepared before launch. The KIC contains all the known stellar objects in the Kepler field of view, based on existing catalogs. This information was supplemented by stellar classifications of more than two million stars bright enough to be good targets, based on more than 50 million measurements of accurate positions and magnitudes in a set of filter-defined wavelength bands in the Sloan Digital Sky Survey plus a custom gravity-sensitive filter (e.g., Athay and Canfield, 1969). These observations were obtained with KeplerCam on the 1.2-m telescope at the F. L. Whipple Observatory on Mount Hopkins in Arizona.
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Basic Methodology The design of Kepler was driven by the goal of discovering an Earth-like planet transiting a Sun-like star in an orbit that would allow water to be liquid on the surface of the planet. The goal of detecting planets with orbital periods as long as a year set the requirement that the mission duration should be at least 4 years, so that at least three transits could be detected. The probability that the inclination of a 1-year orbit is properly aligned for transits to occur is less than 1%, and this set the requirement that at least 100,000 solar-type stars should be monitored continuously in order to yield enough planet detections to provide a statistically meaningful result for the case where Earth analogs are rare. This led to the requirement for a telescope with a wide field of view, on the order of 100 square degrees. To achieve the photometric precision needed to detect reliably a dimming as small as an Earth transit, 84 parts per million, a meter-class telescope was required. The Kepler photometer uses a Schmidt telescope with an array of 42 CCDs in a curved focal plane. The spacecraft is in an Earth-trailing orbit, which provides a much more stable environment than a low Earth orbit.
CCD radiator
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Kepler Mission. Figure 1 The Kepler flight unit
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Sunshade Schmidt corrector 0.95 m dia.
Graphite-cyanate metering structure Local detector electronics
Focal plane array 42 CCDs, >100 sq. deg. FOV
Primary mirror, 1.4 m dia.
Kepler Mission. Figure 2 A cutaway diagram of the Kepler photometer
Because of telemetry limitations, full images are transmitted to the ground only after spacecraft maneuvers, nominally once per month, in order to confirm proper re-acquisition of the target field. In normal operation a limited number of pixels are saved around each target for later transmission to the ground for processing. This has allowed the development of the data reduction pipeline to continue during the mission, thus resulting in improved photometric performance as various subtle instrumental effects are understood and modelled. For most targets, the images are accumulated and stored on the spacecraft for an integration time of 30 min, but a limited number of targets can be sampled with a cadence of 1 min, thus providing better time resolution for the light curves of transient events, such as the ingress and egress of transits. After the first year of operation on orbit, the photometric performance of all the data obtained by Kepler is close to the specification of 20 parts per million for a V = 12 magnitude solar-type star over an integration time of 6.5 h.
Key Research Findings In the first year of operation, Kepler identified several hundred candidate transiting planets. Many of the transit light curves implied planets smaller than Neptune. A key part of the Kepler mission is an effort to confirm and characterize the candidates as true transiting planets, using a variety of ground-based and space-based observations. This includes reconnaissance spectroscopy aimed at the elimination of systems involving eclipsing binaries that masquerade as transiting planets, and to provide more precise classification of the stellar characteristics.
The second aspect is quite important, because the accuracy with which planetary parameters such as radius and mass can be determined is often set by uncertainties in those parameters for the host star. Another important step in the follow-up of Kepler candidates is deep imaging with very high spatial resolution, with techniques such as adaptive optics, speckle imaging, and even HST (Hubble Space Telescope) imaging. This can reveal nearby companions that are hidden in the blur of the Kepler images, due to the large pixels, which are 4 arcsec2 on the sky. The Kepler data also allow very precise astrometry, despite the large pixels, because of the extraordinarily large number of collected photons and continuous duty cycle. Motion of the image centroids during transit events can often be measured to better than 0.0001 pixels. When combined with the highresolution images, this is a powerful tool for detecting background eclipsing binaries on the one hand, or for confirming that the target star is the source of the transits on the other hand. In some cases, observations in the infrared with the Warm Spitzer Space Telescope can be used to test whether the depth of the dimmings are achromatic, as expected for transits. The gold standard for confirming and characterizing a transiting planet is a spectroscopic orbit for the host star that yields a mass for the unseen companion that is consistent with a planetary interpretation. This final step usually requires substantial time on precious resources such as the HIRES spectrometer on the Keck I telescope. Limited access to such facilities dictates that not every candidate can be fully confirmed. Indeed, the orbital amplitude of less than 10 cm 1 that the Earth induces in the Sun is beyond the capability of any existing radial-velocity instruments, and it is not yet clear that the stars themselves will be quiet enough (see ▶ Activity (magnetic)) to allow velocity measurements at this level. For the smallest planets, the strategy is to eliminate as many as possible of the astrophysical false positives that might mimic Earth-like transits. Nevertheless, preliminary estimates suggest that at least half of the Kepler candidates will eventually prove to be transiting planets. Based on follow-up observations of candidates identified in the first 43 days of Kepler operations in the science mode, five confirmed planets were announced in January 2010 (Borucki et al. 2010). Several dramatic breakthroughs in asteroseismology using Kepler data were reported in the same time frame (Gilliland et al. 2010).
Future Directions The main legacy of Kepler will be the frequency and characteristics of the population of extrasolar planets, from those with periods shorter than a few months to planets
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belonging to the same family as the Earth. This information is critical for the design of future, much more expensive missions for astrometric and direct detection of exoplanets, such as NASA’s Space Interferometry Mission and Terrestrial Planet Finder. Kepler’s continuous duty cycle is well suited for the study of transit time variations and transit duration variations, which can reveal the presence of additional planets that do not transit. The exquisite performance of the Kepler photometry should allow the detection of rings and moons orbiting transiting planets, and of subtle effects such as planetary oblateness due to rotation. Kepler will enhance our understanding of planetary systems if it can detect examples where more than one planet show transits. Finally, Kepler has already made major contributions to asteroseismology, with the promise of much more to come, not only in this field but also in other areas of stellar astrophysics such as eclipsing binaries.
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were presented in his Astronomia nova. In essence, they state that planets move in elliptical orbits, sweeping out equal areas in equal times. The powerful Third Law surfaces in his Harmonice mundi, where Kepler shows that the square of the planet’s orbital period is proportional to the cube of the orbit’s semimajor axis. Newton reformulated the problem in terms of masses, and showed how the masses of the orbiting bodies enter into the Third Law. Thus, it is Newton’s formulation of orbital motion that is used to determine masses throughout the Universe, and in particular the masses of planets and stars.
References and Further Reading Kepler J (1609) Astronomia Nova Kepler J (1619) Harmonices Mundi Newton I (1726) Philosophiae Naturalis Principia Mathematica (Third Edition)
See also ▶ Activity (Magnetic) ▶ Astrometric Planets ▶ CCD ▶ CoRoT Satellite ▶ Direct-Imaging, Planets ▶ Exoplanets, Discovery ▶ Radial-Velocity Planets ▶ Spectroscopic Orbit ▶ Spitzer Space Telescope ▶ TPF/Darwin ▶ Transiting Exoplanet Survey Satellite ▶ Transiting Planets
References and Further Reading Athay RG, Canfield RC (1969) Computed profiles for solar MG b- and NA D-Lines. Astrophys J 156:695–706 Borucki WJ, Summers AL (1984) The photometric method of detecting other planetary systems. Icarus 58:121 Borucki WJ et al (1996) Astrophys Space Sci 241:111 Borucki WJ et al (2010) Kepler planet-detection mission: introduction and first results. Science 327:977 Charbonneau D et al (2000) Astrophys J 529:45 Gilliland RL et al (2010) Publ Astron Soc Pac 122:131
Kerogen Definition The kerogen is the macromolecular organic matter that is insoluble in organic solvents (soluble portion is known as ▶ bitumen). Kerogen is present in ancient sedimentary rocks and meteorites. It is composed of smaller molecules that have covalently bound together during synthesis (including biosynthesis), diagenesis, and thermal alteration processes. In sediments, kerogen is the fossil organic matter source of oil and natural gas. The degree of hydrogen-loss and aromatization is often used as an indicator of thermal maturity. The degree of structural order verses disorder for aromatic sheets is a defining characteristic of different kerogens identifiable by Raman spectroscopy. Organic-walled fossils resistant to acid maceration (such as acritarchs, spores) are observed in some kerogens.
See also ▶ Acid Maceration ▶ Bitumen ▶ Microfossils ▶ Microfossils, Analytical Techniques
Keplerian Orbits Definition Johannes Kepler is famous for his three laws of orbital motion for the planets in the Solar System. The first two
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Ketose
Ketose Definition A ketose is a monosaccharide ▶ carbohydrate containing at least one ketone group per molecule. The simplest ketose is dihydroxyacetone (DHA), usually found in biochemistry as its phosphate ester. 2-Ketoses can isomerize into aldoses in water. Some biological ketoses of note besides DHA are sedoheptulose-7- phosphate, xylulose5-phosphate, and ribulose-5-phosphate, which are important intermediates in the pentose phosphate cycle.
See also ▶ Carbohydrate ▶ Dihydroxyacetone
Kidney Ore ▶ Hematite
Komatiite NICHOLAS ARNDT Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France
of northern Canada. The komatiites of Gorgona Island off the coast of Colombia are the sole well-documented example of Phanerozoic komatiite. The older, 3.5 Ga komatiites have different chemical compositions from younger komatiites, being characterized by high CaO/Al2O3 and low Al2O3/TiO2. Important ore deposits of Ni-Cu sulfides are found in komatiites in Australia and Canada. Komatiite normally erupts in subaqueous lava flows, differentiating into an upper layer with spinifex texture and a lower olivine cumulate layer. Spinifex, the texture emblematic of komatiite, consists of large (up to several centimeters long) bladed, skeletal grains of olivine in a matrix of fine clinopyroxene and glass (Fig. 1). The texture forms during crystallization of hot lava in the thermal gradient of the upper part of the flow. The eruption temperature of komatiite can be estimated from the MgO content of the parent liquid. The maximum MgO content, inferred from examples in South Africa and Australia, is greater than 30%, which corresponds to a temperature higher than 1,600 C, some 400 C hotter than that of typical basalt. Komatiite results from partial melting in unusually hot parts of mantle, most probably in ▶ mantle plumes. Their abundance in the ▶ Archean is a manifestation of high temperatures in the mantle at that time. The release of Ni from hydrothermally altered komatiite in the oceanic crust may have been important during the early evolution of life; the decrease in the abundance of komatiite may have provoked a decline in the activity of methanogenic bacteria which was linked to the rise of oxygen in the early Proterozoic.
Keywords Archean volcanic rock, mantle temperature
Definition Komatiite is a volcanic rock of ultramafic composition. It is distinguished from the more common basaltic magma by a higher content of MgO (>18%) and low contents of most other elements.
Overview Komatiite is relatively common in Archean greenstone belts, rare in Proterozoic belts, and virtually absent in younger regions. The type area is the 3.5 Ga ▶ Barberton greenstone belt of South Africa where the rock type was first discovered. Examples are now known in most Archean greenstone belts, and komatiitic basalt, the fractionation product of komatiite, occurs in Proterozoic belts
Komatiite. Figure 1 An outcropping komatiite with the characteristic bladed-shaped spinifex structure made of olivine grains in a matrix of fine clinopyroxene and glass
KT Boundary
See also ▶ Archean Environmental Conditions ▶ Archean Mantle ▶ Archean Tectonics ▶ Barberton Greenstone Belt ▶ Earth, Formation and Early Evolution ▶ Mantle Plume (Planetary)
References and Further Reading Arndt N, Lesher MC, Barnes SJ (2008) Komatiite. Cambridge University Press, Cambridge, p 488
Korarchaeota Definition Korarchaeota is a phylum of hyperthermophilic ▶ Archaea that branches closest to the archaeal root. The Korarchaeota were originally discovered by microbial community analysis of ribosomal RNA genes from environmental samples of a hot spring in ▶ Yellowstone National Park. No pure cultures from this phylum have been isolated yet, but the sequenced 16S ribosomal RNA genes belong to organisms that branch near the root of the archaeal ▶ phylogenetic tree. Stable enrichment mixed cultures, containing Korarchaeota identifiable by in situ hybridization, have been grown at thermophilic conditions in the laboratory.
See also ▶ Archea ▶ Hot Spring Microbiology ▶ Hyperthermophile ▶ Phylogenetic Tree ▶ Yellowstone National Park, Natural Analogue Site
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mechanism for exciting the eccentricity of a planet orbiting a star is perturbations from a binary stellar companion whose orbit is highly inclined with respect to the planet’s orbital plane.
Krebs Cycle ▶ Citric Acid Cycle
KREEP Definition KREEP is a component of lunar basalts. It is an acronym for K (potassium), REE (rare earth elements), and P (phosphorous). These basalts are unusually rich in K, P, and incompatible REE compared to the anorthosites and gabbros of the lunar highlands and the mare basalts. Ages of the KREEP basalts are from 3.8 to 3.6 Ga. When the Mars-size protoplanet ▶ Theia hit the Earth and formed the ▶ Moon, the large amount of energy liberated formed in the interior of the Moon, a magma ocean. As crystallization of this magma ocean proceeded, olivine and pyroxene precipitated and sunk to form the lunar mantle. Less-dense anorthositic plagioclase floated, forming an anorthositic crust. Residual melt was progressively enriched in incompatible elements (i.e., those that partition preferentially into the liquid phase) forming the “KREEP”-rich layer ultimately trapped at the base of the anorthositic crust. Subsequent melting of this layer produced KREEP-rich basalts.
See also ▶ Moon, The ▶ Theia
Kozai Mechanism Definition In planetary dynamics, the Kozai mechanism is a coupling between inclination and eccentricity that can drive large oscillations in both quantities. For low eccentricity and inclination, motion in the plane and out of the plane are independent, but if either value is large (especially if both are), angular momentum can be readily exchanged, and the two parameters could change significantly more than expected in the decoupled case. For example, one
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KT Boundary PHILIPPE CLAEYS Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium
Synonyms K/T boundary; K-T boundary
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Definition The Cretaceous/Tertiary boundary (KT), also often called the Cretaceous/Paleogene (K/Pg) boundary, marks the contact between the Mesozoic and Cenozoic eras 65 Ma ago. The KT boundary is characterized by the last major ▶ mass extinction in Earth history that saw the demise of the non-avian dinosaurs along with 50% to 60% of the Earth fauna and flora.
Overview Over most of the world, this stratigraphic interval is composed of a mm- to cm-thick clay layer containing evidence of a large-scale asteroid or comet impact on Earth. The KT clays contain a significant enrichment in platinum group elements (Pt, Ir, Os, Rh, Ru, Pd), better known as the KT ▶ iridium peak, amounting to several tens of ppb (parts per billion). This enrichment is clearly indicative of a meteoritic or extraterrestrial contribution to the sedimentation, being iridium rare on the Earth surface. Other impact markers such as ▶ shocked quartz and zircons grains indicative of the high pressures generated by the impact (>> 5 GPa) and highly oxidized Ni-rich magnesioferrite spinels (cosmic spinels) also concentrate in the KT clay layer. This clay unit forms the distal ▶ ejecta material spread all over the Earth after the ▶ Chicxulub impact in the Yucatan Peninsula, Mexico. Closer to the impact site, around the Gulf of Mexico region, the KT boundary thickens to several meters of high-energy sediments. This proximal succession is composed at the base of glass spherules similar in composition to the Chicxulub ▶ impactites. This ejecta unit is followed by thick coarse sandstone units, most likely deposited by high-energy tsunami waves triggered by the cratering event. The Ir anomaly occurs just above, diluted by the high sediment input generated all over the Gulf region. In marine settings, the abruptness of the mass extinction is clearly visible in the microplankton that disappears precisely at the moment of deposition of the KT clay layer. The extinction of the calcareous primary producers is widespread and affected the whole marine food chain. The decrease in microplankton was short-lived and oceanic biologic productivity recovered after the KT boundary. The layer directly above the KT boundary is characterized by opportunistic organisms, before the recovery phase indicated by the appearance of new Paleogene species. On the continent, the extinction of diverse vegetation and the destruction of forest also coincide with the ejecta deposition. A fern-spore spike occurs at the boundary, illustrating the demise of the vegetation.
The eruption of Deccan flood basalts (trapps) coincided with the KT boundary and may have contributed to the mass extinction.
See also ▶ Chicxulub Crater ▶ Crater, Impact ▶ Deccan Trapps ▶ Ejecta ▶ Evolution (Biological) ▶ Impactite ▶ Iridium ▶ Mass Extinctions ▶ Shocked Quartz
References and Further Reading Alvarez LW et al (1980) Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095–1108 Claeys P et al (2002) Distribution of Chicxulub ejecta at the CretaceousTertiary Boundary. In: Koeberl C, MacLeod KG (eds) Catastrophic events and mass extinctions: impacts and beyond. Boulder Colorado, Geological Society of America, Special Paper 356, pp 55–69 Schulte P et al (2010) The Chicxulub asteroid impact and mass extinction at the cretaceous-paleogene boundary 10.1126/science.1177265. Science 327(5970):1214–1218 Smit J (1999) The global stratigraphy of the cretaceous-tertiary boundary impact ejecta. Ann Rev Earth Planetary Sci 27:75–113
Kuiper Belt MARIA ANTONIETTA BARUCCI LESIA, Observatoire de Paris, Meudon Principal Cedex, France
Keywords Centaurs, composition, ices, organics, origin, planetesimals, transneptunian objects, volatiles
Definition The Kuiper Belt is the region in the solar system beyond the orbit of Neptune that contains icy bodies orbiting the Sun.
Overview In the last decades, the outer solar system has been found to be densely populated by a multitude of icy bodies called Kuiper Belt objects, Edgeworth–Kuiper objects, or ▶ Trans-Neptunian Objects. The discovery of these bodies
Kuiper Belt
revolutionized our understanding of the solar system and most ideas relative to the evolution of the protoplanetary nebula. The general interest in the outer solar system grew rapidly in the scientific community. In fact, these bodies located so far from the Sun are considered as the fossils of the ▶ protoplanetary disk, which, consequently, can provide important information on the processes that dominated the evolution of the early ▶ solar nebula as well as of other planetary systems around young stars. ▶ Trans-Neptunian objects as well as asteroids and the ▶ Oort’s cloud comets can be considered as the building blocks (▶ planetesimals) of the solar system, offering clues to the composition of the matter from which planets formed some 4.6 billion years ago. The investigation of the physical properties of these icy bodies, as remnants of the outermost planetesimal swarms, is essential to understanding the formation and the evolution of the whole population of solar system bodies. TNO science has rapidly evolved in recent years, linking together different research areas. The attempt to know this population’s properties and history is one of the most active research fields in planetary science. The first Trans-Neptunian object to be discovered was Pluto by Clyde Tombaugh in 1930 (later numbered 134340; see numbering convention described below). This discovery confirmed early ideas concerning the existence of solar system objects beyond the orbit of Neptune. In fact just a few months after the Pluto’s discovery, Frederick Leonard (1930) wrote in a popular article: "
... Now that a body of the evident dimensions and mass of Pluto has been revealed, is there any reason to suppose that there are not other, probably similarly constituted, members revolving around the Sun outside the orbit of Neptune?... As a matter of fact, astronomers have recognized for more than a century that this system is composed successively of the families of the terrestrial planets, the minor planets, and the giant planets. Is it not likely that in Pluto there has come to light the first of a series of ultraNeptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected?
Later on, in the years 1943 and 1950, Kenneth Edgeworth and Gerard Kuiper hypothesized the existence of small bodies beyond Neptune and Pluto. A more conclusive study by Ferna´ndez and Ip in 1982 argued for the existence of a source of short-period comets close to the ecliptic plane and beyond the known planetary orbits. Even if many researchers hypothesized the existence of such a population, only after more than 60 years, in 1992,
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Jewitt and Luu discovered a second transneptunian object 1992 QB1 (now number 15760; Jewitt and Luu 1993), if we do not take into consideration the discovery of Pluto’s moon Charon in 1978, because at that time it was considered a satellite of the ninth planet. This was an epochal astronomical discovery and within 2–3 years more than 100 distant objects were discovered. Determining their orbits was in general difficult because of their faintness and the necessity to observe them for several months. After discovery, these objects receive a provisional designation as asteroids (year of discovery followed by letters and number for the order of discovery). Only when the orbit is well known do they receive a number and a name. As activity is not visible during the discovery and observations, their number follows the order number of asteroids rather than comets. More than 1,350 transneptunian objects are known up to today, with well-constrained orbits with differing sizes, orbits, and surface characteristics. Because of their distance and their faintness, some transneptunian were lost after discovery. Satellites have been detected for a large fraction of these objects using optical methods of highresolution imaging. Most of these satellites have been found with the Hubble Space Telescope (Noll et al. 2008).
Dynamical Classification The population has been classified (Gladman et al. 2008) into several groups, following their distance from the Sun and their orbital characteristics: (1) classical objects, (2) resonant objects, (3) scattered disk, and (4) detached objects (Fig. 1). The first two groups are also known as the Kuiper Belt, containing objects with an average distance from the Sun between 30 and 55 AU with almost circular orbits and small inclinations. Resonant objects are trapped in ▶ mean-motion resonances with Neptune, and that dynamical configuration provides them a dynamical stability. Although more than 20 resonances exist, the 3:2 mean-motion resonance with Neptune (a = 39.4 AU), which hosts Pluto, is the most densely populated, and objects trapped into this resonance are called Plutinos. The origin of these resonant objects is closely related to Neptune’s migration: indeed, while Neptune was migrating outward, objects that followed the outward migration of Neptune were swept and captured in a mean-motion resonance. Classical objects are those having a low eccentricity and moderate inclination. Several authors refer to two distinct populations in the classical belt based on orbital inclination: dynamically “hot” (high inclination, > 5 ) objects, and dynamically “cold” (low-inclination, < 5 )
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Eris Classicals
Resonants 3:4 2:3
Scattered disk 1:2
i
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0 0.8
4:3
3:2
Sedna
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0.6 Detashed objects e
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Pluto
0.2 0 30
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40 a (AU)
45
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100
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300 a (AU)
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Kuiper Belt. Figure 1 The diagram illustrates the distribution of Transneptunian objects (resonant, classical, scattered disk, and detached objects) at different semimajor axes (a), eccentricities (e), and inclinations (i) (Updated by Morbidelli)
objects. Numerical simulation by Gomez (2003) showed that the hot population could find its origin in the migration of Neptune, which scattered the planetesimals originally formed inside 30 AU. The current classical belt would hence be the superposition of these bodies with the local population (cold objects), believed to have form in situ beyond 30 AU. The so-called Kuiper Belt has a characteristic edge at about 55 AU, near the location of the 2:1 mean-motion resonance. It is not clear how this edge formed and if it is real (or due to an observational bias effect). The position of this edge might depend on Neptune’s migration (Levison and Morbidelli 2003) or a close stellar passage as suggested by Ida et al. (2000). Scattered disk objects (SDOs) or scattered objects are considered those that have orbits with large eccentricities, and perihelion distances near Neptune’s location. A few objects classified as SDOs are in fact resonant. Detached objects are those with orbits with large eccentricities (e > 0.24), and with large perihelion distances out of Neptune’s influence. They have very long lifetimes under the influence of other planetary perturbations. Their origin is not well understood and several scenarios exist to explain their formation. The best example of this category is (90377) Sedna, which has a semimajor axis of 501 AU,
with perihelion and aphelion distances at 76 and 927 AU, respectively. The most distant known today is (87269) 2000 OO67 with perihelion at 667 AU and aphelion at 1,294 AU. It is probable that many more distant objects are waiting to be discovered. A large population in the distant region is predicted, but no surveys for fainter objects have yet succeeded in detecting such distant objects. Another group of objects with similar physical characteristics is called Centaurs, which orbit in the region between Jupiter and Neptune and interact strongly with all the giant planets. Planetary perturbations and mutual collisions in the Kuiper belt are probably responsible for the ejection of objects onto Centaur orbits. Because Centaurs cross the orbits of the outer planets, they are dynamically unstable with a short lifetime (106–107 years), after which they can be ejected from the solar system, impact the giant planets, or evolve into Jupiter-Family Comets (JFCs) or ▶ Near-Earth Objects (NEOs). Centaurs are widely believed to come from the transneptunian region and to be scattered into their present orbits by gravitational instabilities and collisions. Hence, like the transneptunian objects, Centaurs accreted at low temperature and large solar distances, and must still contain relatively pristine material.
Kuiper Belt
Basic Methodology The availability of very large telescopes, both groundbased (8–10 m) and orbiting the Earth (Hubble and Spitzer), has enabled observational studies of the physical properties of a significant number of objects, but the physical properties of smaller transneptunian objects remain still poorly known. Obtaining this information for the whole population is still a big challenge. The diameter of the TNOs can be estimated from their absolute magnitude, but only if their albedo is known, which can be determined when both the reflected light and the thermal radiation at long wavelengths are measured. These objects are very cold (30–50 K), their thermal radiation is very weak because of their distance from the Sun, and reaches its blackbody peak near 100 mm. The Spitzer Space Telescope has been used to observe approximately 40 Centaurs and transneptunian objects (Stansberry et al. 2008) and using both thermal emission and ground-based measurements of the visible radiation, their dimensions have been determined with a precision of 10–20%. The determined albedos range from about 3% to 85%, with most objects having a low albedo. For the largest ones, with very well known orbital parameters, diameters can be measured very precisely by occultation of stars (Pluto, Charon etc...). The largest object is Eris which has a diameter of about 2,400 100 km
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and a very high albedo (0.86 0.07). The second is Pluto (2,306 20 km in diameter and 0.49–0.66 albedo), followed by Makemake, Haumea, and Sedna (see Fig. 2). The largest objects have been recently classified by the IAU as dwarf planets. The TNOs’ density can be estimated only when satellites are detected and consequently the mass of the primary is available. When the orbital period and distance of the satellites can be determined, the mass of the primary body can be calculated from Kepler’s third law, and the mass of the satellite can also be estimated from the magnitude of the systems’ components. The mean density reflects the internal composition, particularly the relative fraction of ices, silicates, and metals, as well as the porosity. The up to now known densities span a wide range, from 0.5 to nearly 3 g/cm3. Low densities are believed to dominate for the small objects, but the highest density has been measured for the large body Haumea, 2.6–3.3 g/cm3, suggesting a core with a high non-ice content. Pluto has a density of about 2.0 g/cm3. The TNOs with densities less than 1 g/cm3 are presumed to be porous to varying degrees. The lowest measured density, as in the case of (47171) 1999 TC36, requires that about 50–70% of the interior of its body consists of void space. Table I shows some physical characteristics of these objects.
Largest known trans-Neptunian objects (TNOs) Dysnomia
Nix
Namaka Charon
Hydra Eris
Pluto
Hi’iaka Haumea
Makemake
Weywot
Sedna
Orcus
2007 OR10
Quaoar
Kuiper Belt. Figure 2 Picture of the biggest Transneptunian objects that are reported in scale with their estimated sizes and those of their satellites (© NASA Courtesy)
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The current uncertainty in the size distribution of the TNOs does not allow us to define the total mass of the population. Current estimation ranges from 0.01 to a few times 0.1 Msolar. A TNO’s rotational period can be determined by measuring its lightcurve. The few dozen known rotational periods span from a few hours to several days with a peak around 8.5 h, which appears to be significantly slower than the main-belt asteroids of similar size. From the rotational properties it is also possible to extract information on the collisional history, since the current spin properties are supposed to have been strongly affected by the mutual collisions experienced by these bodies. On the basis of considerations of the rotational stability, some indications of the density can be also constrained. Assuming that the amplitudes of the observed lightcurve are due to the body’s shape, with negligible albedo variations on the surface, an estimation of a lower limit to the axis ratio a/b on the hypothesis of an ellipsoid shape can be also deduced. Smaller objects appear to be more elongated. The statistics on binary TNOs can provide models of their formation. Angular moment and relative sizes of most binaries are consistent with formation by dynamical capture. The small satellites of the largest objects, in contrast, more likely result from collisions. More than 50 transneptunian binaries are known, but triple and quadruple systems have also been detected.
Composition While ▶ spectroscopy provides more details for surface composition investigation, photometry has been the most extensively used technique to explore the surface properties of these remote objects, since most TNOs are extremely faint objects. Many different photometric observational campaigns have been performed, particularly in the visible region, which provided data for a large number of objects. About 200 objects have been observed with photometric surveys revealing surprising color diversity; the TNOs exhibit a large range of surface optical colors. A large number of transneptunian objects have surface colors much redder than any other solar system bodies. Statistical analyses have been used in researching a wide range of possible correlations between optical colors and orbital parameters. The visible and near-infrared color correlation indicates that a single coloring agent is responsible for the wavelength dependence. In particular there is a well confirmed correlation (Doressoundiram et al. 2008) between color and orbit inclination. Highly inclined classical objects have diverse colors ranging from gray to red and have large sizes, while low-inclination classical objects
are generally smaller and mostly very red. This cluster of “cold” (low-eccentricity, low-inclination) classical objects composed of smaller sizes is supposed to be primordial. Results on the photometry of binary TNOs observed with the Hubble Space telescope demonstrate a very strong correlation between primary and satellite colors, concluding that the colors of binaries as a group are indistinguishable from that of the larger population of apparently single transneptunian objects. The color of binaries or single objects is one characteristic signature of surface compositional differences set in the solar nebula. Multicolor broadband photometry can provide only limited constraints on the surface composition of the population. In fact colors can be influenced by composition, but also by scattering effects in particulate regoliths and by viewing geometry. Colors cannot, in general, be used to determine composition, but they can be used to classify objects into groups. A taxonomy based on color indices (B-V, V-R, V-I, V-J, V-H, and V-K) has been obtained (Barucci et al. 2005) identifying four groups of objects with homogeneous color content: BB, BR, IR, and RR. The group BB contains objects with neutral colors and RR those with very red colors (the reddest among the solar system objects), while the two others have an intermediate behavior. The physical significance of color diversity is still unclear, though it is reasonable to assume that the different colors reflect intrinsically different compositions or compositions that differ due to different evolutional histories. The trend from neutral (BB) to very red (RR) groups indicates the possible sequence of alteration processes (collisions, resurfacing, craters, UV and/or energetic particle bombardment, etc.) and each group could represent an evolutionary stage of the population, and its number density gives an indication of the duration of the each phase representing the effects of different alteration processes. Cosmic-ray bombardment will be the cause for an exposed surface to become redder, and impacts excavating “fresh material” will cause differently colored surfaces. These as well as other evolutionary processes (e.g., outgassing, surface gardening, atmospheric interaction . . .) can also account for the observed differences. The Centaurs seem to have a different distribution of colors. The available data suggest that the Centaurs colors may have a bimodal distribution while the TNOs do not. This has been explained by past or present sublimation activity. In fact when these objects pass closer to the Sun, they can develop an active coma. The most detailed information on composition can be acquired only from spectroscopic observations, especially covering the wavelength range between 0.4 and 2.4 mm.
Relative Reflectance
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Kuiper Belt. Figure 3 Visible and near-infrared spectra of some TNOs (Sedna, Eris, and Orcus) and a Centaur Asbolus. The four objects have spectra with different behaviors: from flat to very red spectra, different absorption bands, and different signal-to-noise ratios
This wavelength range provides the most sensitive technique to characterize from the ground the major mineral phases and ices present on transneptunian objects (Fig. 3). Silicate minerals like feldspar, carbonaceous assemblages, organics, and water-bearing minerals have diagnostic spectral features in the visible and near-infrared spectral regions. In the near-infrared region there are also signatures from ices and hydrocarbons. Weakly active Centaurs or TNOs could also show fluorescent gaseous emission bands. Most of the known transneptunian objects are too faint for spectroscopic observations, even with the world’s largest telescopes, and so far only the brightest bodies have been observed by spectroscopy. The exposure time required is generally long, and as the objects rotate around their principal axis, the resulting spectra often contain information coming from both sides of the object. Visible spectra are mostly featureless, showing large variations of the spectral slope, thus confirming the wide range of colors from neutral to very red. The visible wavelength is important to constrain models and to infer information on the composition, particular for the ultrared objects. The ultra-red slope is assumed to indicate the presence of organic material on the surface, as in the case of the Centaurs Nessus and Pholus (the reddest objects known up to now). A few objects show signatures which are very similar to those due to water-altered minerals
found in some main-belt asteroids and meteorites (Barucci et al. 2008). How an aqueous alteration process could have occurred at such low temperatures far from the Sun is not yet well understood, but it cannot be excluded that hydrated minerals could have been formed directly in the early solar nebula; in fact, hydrous silicates have been detected in interplanetary dust particles (IDPs) and in micrometeorites. The near-infrared (1–2.4 mm wavelength) is the most diagnostic region to determine the presence of ices. Signatures of water ice, methane, and methanol can be easily detected if the spectra are of good quality. In a few cases some data at longer wavelengths have been obtained with the far IR space telescope Spitzer. To investigate the composition, radiative transfer models have been used to interpret the features and the spectral behavior using intimate or geographical mixtures of organics, silicate minerals, carbonaceous assemblages, ices, and/or hydrocarbons. The red slopes are in general well reproduced supposing the presence of organic compounds on the surface like kerogen (complex dark organic compound) and ▶ tholins (synthetic macromolecules produced by irradiation of gaseous mixtures). The results given by the spectral modeling have to be considered as only indicative. In fact, much of the information obtained from spectral modeling is non-unique, especially if the necessary constraints are not well
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established (▶ albedo, signatures with high sensitivity detections of fainter components. . .). The details on fractional coverage of different materials and even the presence of these materials are easily influenced by choices of model parameters. Nevertheless, the modeling can be used to pick out broad variations in TNO spectra.
Key Research Findings In recent years about 50 objects including Centaurs have been spectroscopically observed and analyzed. Following their spectral characterization four principal groups can be identified.
Water Ice Group Water ice should be the main component of transneptunian objects, and many have spectra showing moderate to deep absorptions due to water ice. Most spectra with high signal-to-noise (S/N) show the presence of a feature at 1.65 mm characteristics of crystalline water ice. It is still a matter of debate whether water ice was amorphous or crystalline in the protosolar nebula. While crystalline water ice is neither expected at these low temperatures nor should be stable against cosmic-ray and UV bombardment, crystalline water ice appears to be ubiquitous in the outer solar system, from the icy satellites of the giant planets to TNOs. The presence of crystalline water ice implies that ice has been heated to temperatures above 100/110 K. This heating could have occurred by impacts or generated in the deep interiors and the ice been emplaced onto the surface. The quality of most spectra is too poor to distinguish between amorphous or crystalline water ice. Nonetheless, when trying to model the spectra, the best fit model is usually obtained when using a combination of the two water ice states.
Methane Group The largest TNOs- Eris, Pluto, Sedna, Makemake- display spectra dominated by methane absorption, some objects showing spectra of methane dissolved in nitrogen. Pluto and probably Sedna and Eris share these properties, together with Neptune’s satellite Triton (supposed to be a transneptunian object captured by Neptune). Methane is present in these large objects because their gravity is high enough to retain such a volatile component, but in smaller quantity methane may be present also on smaller objects. A small amount of ethane (by-product of methane ice irradiation) has been also detected on Quaoar.
Methanol Group Some objects – Pholus, (55638) 2002 VE95, possibly Bienor – show spectra with methanol. Many objects have
a low signal-to-noise ratio, especially in the near-infrared region around 2.2 mm, but many objects show a decreasing slope after 2.2 mm implying a possible presence of methanol or similar organic molecules.
Featureless Spectra Group Many objects have featureless spectra in the near-infrared, with a wide range of colors. Obviously, many spectra are featureless when observed at low S/N, but many have sufficient S/N spectra to rule out the signature due to water ice. These objects could be mantled by a surface rich in organics or carbon. As all these objects are supposed to be composed of ices, irradiation processes have to be responsible for these properties. Laboratory experiments have showed that irradiated ices become redder with increasing irradiation. Molecules progressively lose their hydrogen atoms, which results in a polymerization of the surface layer and the formation of a crust. Ammonia or ammonia hydrate has been detected in the spectra of a few objects (Charon, Orcus). A firm detection of ammonia would have important implications on the composition of the primitive solar nebula in the region at low density, far from the Sun.
The Largest Objects The dynamical and physical characteristics of the largest transneptunian objects are summarized in Table 1.
Eris Eris has a highly eccentric orbit, with a perihelion at 37.9 AU, typical for scattered objects, and an aphelion at 96.7 AU. Its orbital period is about 557 years. Eris is possibly the largest transneptunian object, and it has one known satellite, Dysmonia, that is 500 times fainter than Eris. The spectrum is very similar to that of Pluto even if the visible part is less red than that of Pluto. No variation has been seen on Eris to an upper limit of 0.05 mag. The visible and nearinfrared spectra are dominated by absorption from methane. As shown by Merlin et al. (2009) the near-infrared spectrum shows no evidence for shifts of the methane absorption wavelength, while the visible part of the spectrum shows small shifts, implying that methane is dissolved in nitrogen. These observations suggest a stratification, with a layer of diluted methane ice between two layers of pure methane ice. The possibility to have depleted nitrogen in the deepest layers after sublimation processes that enrich adjacent layers during condensation processes can be also suggested. Eris seems to be covered by more pure methane ice than Pluto and Triton. The amount of nitrogen on Eris’ surface is probably smaller and less important than on the surfaces of the latter objects. The high albedo and the lack of
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Kuiper Belt. Table 1 Properties of the largest TNOs
Diameter (km)
Eris
Pluto
Makemake
Haumea
2,400 100
2,306 20
1,500 300
1,150
+250 100
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Quaoar
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1,300–1,800
1,260 190
950 70
800 200 a (AU)
67.8
39.6
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43.2
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39.3
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0.25
0.19
0.84
0.04
0.22
0.25
i (deg)
44.0
17.1
29.0
28.2
11.9
8.00
20.5
1.2
1.0
0.3
0.3
1.6
2.7
2.3
H2 O
CH4 + N2 + ?
H2O + C2H6 + ?
H2O + ?
73
15–30
93
20 3
H Surface ices
CH4 + ?
CH4 + N2 + CO
CH4 + C2H6
Albedo (%)
86 7
49–66
80
166 2
130.5 0.6
–
42 1
-
-
91
2.3 0.3
2.03 0.06
-
2.6–3.3
-
-
1.9 0.4
Mass(1020 kg) Density g cm
3
+10 20
rotation are consistent with a surface dominated by seasonal atmospheric cycling.
Pluto Pluto appears to be the slightly smaller twin of Eris. The main differences appear to be the visible redder color and the presence of dark areas on the surface. Its high albedo and its position at perihelion make it the brightest object of the transneptunian population and for this reason it was the first discovered and has been heavily observed. Pluto is surrounded by three known satellites (Charon, Nix, and Hydra). The formation of these satellites could be as a consequence of a grazing collision between the protoPluto and Charon. Recent investigation of the surface implies that it is largely covered by nitrogen, methane, and lesser amounts of CO ice and organic compounds. The methane is present in both pure and diluted states and ethane seems also present in its surface. The question on continuous volatile transport on the surface of Pluto is still under debate. Pluto has a seasonal atmosphere mainly of N2, with lesser amounts of CH4 and CO.
Haumea Haumea has a typical orbit for the classical dynamical group, with an orbital period of 283 years, and a perihelion at 35 AU. It has an unusual lightcurve, with large amplitude and small rotational period (3.92 h) implying an ellipsoid shape, estimated to be 1960 1518 996 km assuming an albedo of 0.75. The near-infrared spectroscopy reveals the deepest water ice absorption found in these objects, with surface composition similar to the surface of Pluto’s moon Charon. Its characteristics and those of its two satellites (Namaka and Hi’iake) all point to a collisional origin for the system (Brown et al. 2007). A large near-infrared survey showed that a number of TNOs having deep water ice absorption are all dynamically clustered near the dynamical position of Haumea, representing a dynamical family. It is probable that the objects of this family, the first family detected in the TNOs population, are the collisional fragments of a giant impact on the proto-Haumea.
Sedna Makemake Makemake has a classical orbit with an orbital period nearly 310 years, more than Pluto’s 248 years and Haumea’s 283 years. It is the brightest TNO after Pluto, and has a surface dominated by methane. The methane absorption features on its surface spectrum are deeper and broader than those on the other objects. A signature of ethane is also present. Ethane is one of the expected products from dissociation of both gaseous and solidstate methane. For its known characteristics, Makemake can be a transition between the larger-surface-volatile-rich objects and the smaller-surface-volatile-depleted objects.
Sedna is one of the most remote objects with a perihelion distance at 76 AU, an aphelion distance at 927 AU and an orbital period of about 11,000 years. It is also one of the reddest transneptunian objects known. Methane and possibly nitrogen seem to be present on the surface of this object and a similarity with Triton has been proposed, which could indicate that these two objects originated in the same zone. This would better account for the origin of Sedna and would support a capture hypothesis for Triton. During its perihelion passage, Sedna may have a temporary N2 atmosphere whose properties depend upon the rotational period.
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Quaoar Quaoar falls in the classical dynamical group and has a surface with intermediate albedo (0.1) with respect to the other TNOs. First estimations of its diameter were based on direct size measurements with the Hubble Space Telescope, giving a diameter of 1,260 190 km, while more recent observation with Spitzer Space Telescope in thermal infrared photometry gave an estimation of 840 200 km. The rotational period is approximately 17.68 h and it has one known satellite. The best obtained model (Dalle Ore et al. 2009) reported a surface made of crystalline water ice, methane, as well as C2H6 and organic materials. The presence of nitrogen as well as ethane seems to fit better the general behavior of its spectrum, but needs confirmation.
Orcus Orcus is in the 3:2 resonance with Neptune with an orbit shaped similarly to that of Pluto. A relatively big satellite has been discovered. A possible rotational period of about 10 h has been estimated. Crystalline water ice and possibly ammonia (Barucci et al. 2008) have been found on its surface from spectroscopic observations. The existence of such ices may indicate a renewal mechanism on the surface such as geological activity.
Physical Processes The TNO population is considered to contain the most pristine objects in the solar system, but over the 4.5 Gy of the solar system life, they have experienced various modifying processes. The structure and the chemistry of the surface of these objects is affected by bombardment by cosmic-ray and solar wind ions and/or micrometeorites, with the result that the molecular complexes are structurally changed and the molecular compositions of ice and minerals are altered over time. Laboratory experiments on a variety of appropriate materials irradiated by energetic particles, simulating the space environment, show molecular transformation that leads to the production of different components. Energetic particles dissipate their energy in a complicated cascade of interactions that results in the production of molecules that were not present in the initial composition. Laboratory experiment shows the formation of an irradiation mantle, breaking bonds in the ice molecules to allow the formation of radicals, leading to the escape of hydrogen and the formation of a carbon-rich layer with low albedo. This can easily mask the presence of volatiles. The depth to which material can be damaged is a function of the particle energy.
Hudson et al. (2008) reported the estimation of the irradiation at different distances from the Sun. TNOs orbiting in the classical belt (40–50 AU) received lower irradiation than those orbiting beyond the terminator shock (beyond about 85 AU); the latter have upper layers more deeply affected by irradiation-induced chemistry, forming a crust rich in carbon. The formed crust hides the real composition of this objects, and complex organic materials called “tholins” (produced by spark discharge in low pressure gases) or “kerogen” (found in terrestrial oil shales) are often used as analogues for interpreting the spectra of the surface. It is clear that collisions have played an important role in the evolution of this population. The consequence of collisions is not only the alteration of the surface properties, but also the modification of the internal structure of the targets. Collisions are relevant both to small and large objects. One example is Charon, a moon of Pluto, but completely different in composition from Pluto; it has been modeled to have formed from a disk of debris ejected during the collision of Pluto with a body of almost equal size, in a process similar to that of the formation of Earth’s Moon. It has been suggested that the collisional family associated with Haumea originated from a parent body having a high-density rocky core and a low-density icy mantle that suffered a giant collision, ejecting a large fraction of its mantle into space. In fact some objects with similar spectral characteristics (deep H2O ice band and neutral color) have been identified to be clustered around Haumea. The internal structure of TNOs can also display a great diversity. The detection of crystalline H2O ice or ammonia ice on the surfaces raises the question of internal activity. Laboratory experiments on water ice irradiation at the same conditions as TNOs indicates that crystalline water ice on the surface should be completely amorphized by irradiation in about 107 years; in fact, crystalline water ice is neither expected at these very low temperatures nor should be stable against cosmic-ray and UV bombardment. Both internal (▶ cryovolcanism) and external (micrometeoritic or larger impacts) surface renewal mechanisms have been considered to account for the presence of crystalline water on the surface of these objects, even if the understanding of the physics of the crystalline/amorphous phase transition is not clear. Laboratory experiments show the same behavior for the ammonia hydrates, which are easily destroyed by energetic radiation. Studies of the early evolution of transneptunian objects concluded that some bodies (depending of the orbit and size) should have lost all volatile ices, such as
Kuiper Belt
water ice crystallized because of the heat generated by accretion, short-lived radiogenic elements and differentiation (McKinnon et al. 2008). The presence of some ices on the large TNOs indicates that other mechanisms (not only space weathering and collisions) could be the cause of the surface properties. Some internal geological activities, for example, have been proposed as explanation for the surface characteristics of Orcus and Quaoar. Models of the interiors of TNOs show that cryovolcanism, which is considered the most probable mechanism to explain geological activity on some satellites of the outer planets, may be possible on the bigger transneptunian objects (diameter > 1,000 km). Outgassing from the interior is another important evolution process because of the high volatile content but also due to the possible high porosity of these objects. In fact the porosity can increase the conductivity of volatiles to the surface and the size of the reservoir that supports the escaping atmosphere or coma. The high albedos and the detection of volatiles on the surfaces on some transneptunians suggest a possible presence of an atmosphere, even as a transient phenomenon. Pluto has a seasonal atmosphere. The requirement for atmospheric formation on these low gravity objects requires the presence of gases or sublimating/evaporating materials on the surface and this implies a resupply mechanism such as internal activity or impactors. The detection of surface volatiles and high albedos on some TNOs thus indicates the possible existence of at least transient (e.g., seasonal) atmospheres on some of these objects. The definitive evidence for atmospheres can only come from occultations or direct spectroscopic detection with spacecraft.
Future Directions Many future surveys [the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) and the Large Synoptic Survey Telescope (LSST)] will cover the transneptunian region, allowing the discovery of many more objects, with orbital and physical characterization. The population of small and very faint objects can be discovered only through occultations of background objects; in fact this method could investigate down to the subkilometre-sized population of the TNO size distribution, as well as the outer region of the Kuiper Belt. Stellar occultations observed from ground will be also the most powerful tool available to determine the shape, size, and the atmosphere for the larger objects with well known precise orbits. In the future, telescopes such as ALMA (Atacama Large Millimeter/submillimeter Array), LMT (Large Millimeter Telescope), and CCAT
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(Cornell, Caltech Atacama Telescope) will allow better characterization of many of these objects, as well as the ESA Herschel mission with its far infrared observations. The NASA New Horizons mission, already on its way to Pluto and Charon, will encounter them in 2015 and hopefully a few others. The on board instruments, with the first in situ measurements, will reveal many properties of these objects.
See also ▶ Centaurs ▶ Cryovolcanism ▶ Mean Motion Resonance ▶ Near-Earth Objects ▶ Planetary Migration ▶ Planetesimals ▶ Protoplanetary Disk ▶ Solar Nebula ▶ Tholins ▶ Trans-Neptunian Object
References and Further Reading Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) (2008a) The solar system beyond Neptune. University of Arizona Press, Tucson Barucci MA, Belskaya IN, Fulchignoni M, Birlan M (2005) Taxonomy of centaurs and trans-neptunian objects. Astron J 130:1291–1298 Barucci MA, Merlin F, Guilbert A et al (2008b) Surface composition and temperature of the TNO Orcus. Astron Astrophys 479:L13–L16 Barucci MA, Brown ME, Emery JP, Merlin F (2008c) Composition and surface properties of Transneptunia objects and Centaurs. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond neptune. University of Arizona Press, Tucson, pp 143–160 Brown ME, Barkume KM, Ragozzine D, Schaller EL (2007) Discovery of an icy collisional family in the Kuiper belt. Nature 446:294–296 Dalle Ore CM, Barucci MA, Emery JP et al (2009) Composition of KBO (50000) Quaoar. Astron Astrophys 501:349–357 Doressoundiram A, Boenhardt H, Tegler SC, Trujillo C (2008) Color properties ad trends of the transneptunian objects. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 91–104 Fernandez JA, Ip WH (1982) Dynamical evolution of a cometary swarm in the outer planetary region. Icarus 47:470–479 Gladman B, Marsden BG, VanLaerhoven C (2008) Nomenclature in the outer solar system. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 43–58 Gomez RS (2003) The origin of the Kuiper belt high-inclination population. Icarus 161:404–418 Houston RL, Palumbo ME, Strazulla G et al (2008) Laboratory studies of the chemistry of transneptunian object surface materials. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 507–524
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Hudson RL, Palombo ME, Strazzula G, Moore MH, Cooper JF, Sturner SJ (2008) Laboratory study of the chemistry of transneptunian object surface materials. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 507–523 Ida S, Larwood J, Burkert A (2000) Evidence for early stellar encountersin the orbital distribution of Ededgeworth–Kuiper belt objects. Astrophys J 528:351–356 Jewitt D, Luu J (1993) Discovery of the candidate Kuiper belt object 1992 QB1. Nature 362:730–732 Leonard FC (1930) The new planet Pluto. Leaflet Astron Soc Pac August, 121–124 Levinson HF, Morbidelli A (2003) The formation of the Kuiper belt by the outward transport of bodies during Neptune’s migration. Nature 426:419–421 McKinnon WB, Prialnik D, Stern SA, Coradini A (2008) Structure and evolution of Kuiper belt Objects and Dwarf Planets. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 213–242
Merlin F, Alvarez-Candal A, Delsanti A et al (2009) Stratification of methane ice on Eris’ surface. Astron J 137:315–328 Noll KS, Grundy WM, Chiang EI et al (2008) Binaries in the Kuiper Belt. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 345–364 Stansberry J, Grundy W, Brown M et al (2008) Physical properties of Kuiper Belt and Centaur objects: constrains from the Spitzer Space Telescope. In: Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (eds) The solar system beyond Neptune. University of Arizona Press, Tucson, pp 161–180
Kuiper Belt Object ▶ Trans-Neptunian Object
L L-Amino Acids Definition L-amino
acids are the only enantiomer of the amino acids with a chiral carbon atom encoded by genetic molecules into the proteins of living organisms. There are some D-amino acids found in the proteins of living organisms, but these are added by post-translational modification reactions.
See also ▶ Amino Acid ▶ Enantiomers ▶ Homochirality ▶ Isomer
in 1780 by the Swedish chemist Scheele. It is a carboxylic acid with the chemical formula CH3CH(OH)COOH. It has a hydroxyl group adjacent to the carboxyl group, making it an a-hydroxy acid. Lactic acid is chiral and has two optical isomers. One is known as L-(+) or (S)-lactic acid, and the other, its mirror image, is D-( )- or (R)lactic acid. L-(+)-Lactic acid is the biologically important isomer. In biochemistry, L-lactate is produced via reduction of pyruvate in a process of fermentation or anaerobic respiration. Lactic acid can also be derived from the deamination of alanine.
See also ▶ Chirality ▶ Hydroxy Acid
Lacus Labyrinthus, Labyrinthi Definition A complex of intersecting valleys or ridges (definition by the International Astronomical Union; http://planetarynames. wr.usgs.gov/jsp/append5.jsp). It is used as a descriptor term for naming surface features on ▶ Venus and ▶ Mars.
See also ▶ Mars ▶ Venus
Lactic Acid Synonyms 2-Hydroxypropanoic acid
Definition Lactic acid is an organic chemical compound that plays a role in several biochemical processes. It was first isolated
Synonyms Lake
Definition The term lacus (lake) refers to a number of small low▶ albedo plains on the ▶ Moon and ▶ Titan and also to one on ▶ Mars. Lacus features usually exhibit a wellpronounced and sharp boundary and have a size of 100–200 km in diameter. While on the Moon lacu¯s are associated with ▶ Mare-features and caused by basaltic flood volcanism, lacu¯s on Titan refers to small dark areas possibly associated with liquid ▶ hydrocarbons.
See also ▶ Albedo ▶ Hydrocarbons ▶ Mare, Maria ▶ Mars ▶ Moon, the ▶ Oceanus, Oceani ▶ Palus, Paludes ▶ Titan
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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Lagrange Points
it will be pulled back. In astronautics, Lagrange points L1 and L2 of the Sun-Earth system are of peculiar importance because they are locations where a spacecraft can be sent and maintained, far from the disturbing environment of Earth, but staying close enough for communication. Several space observatories have already been put at L2. Celestial objects are also found at Lagrange points: that is, the case of the ▶ Trojan asteroids at L4 and L5 of the SunJupiter system.
Lagrange Points ▶ Lagrangian Points
Lagrangian Point Objects ▶ Trojans (Asteroids)
See also ▶ Gravitation ▶ Trojans (Asteroids)
Lagrangian Points Synonyms
Lake
Lagrange points
▶ Lacus
Definition The Lagrangian points are five points in the orbital plane of two ▶ gravitationally bound bodies, where a small mass stays in equilibrium with respect to the other two, that is, remains at constant distances relative to them, despite the orbital movement. They are named after the ItalianFrench mathematician Joseph Louis Lagrange who discovered those five specific points while he reformulated the Newtonian mechanics. Three of the points, called L1, L2, and L3, are located on the line joining the two massive bodies, one is inside the segment joining the two bodies and the other two are outside. The two last Lagrangian points, L4 and L5, lie symmetrically on each side of this segment, each at the top of an equilateral triangle built on the two main bodies (see Fig. 1); these locations are stable, that is, if the small mass at L4 or L5 is slightly displaced,
4
1
3
2
5
Lagrangian Points. Figure 1 This cartoon locates the five Lagrange points in a two massive bodies system
Lamarck’s Conception of Origins of Life STE´PHANE TIRARD Faculte´ des Sciences et des Techniques de Nantes, Centre Franc¸ois Vie`te d’Histoire des Sciences et des Techniques, EA 1161, Nantes, France
History The French naturalist Jean-Baptiste de Monet de Lamarck (1744–1829) suggested an evolutionary theory in 1802, which was later called transformism. In this theory, he claimed that species changed and produced series. Transformations appeared when the environment and the habits acted on organisms for a long time. In his theory, Lamarck gave a very important place to ▶ spontaneous generations. Indeed, for him, they were at the beginning of the series and furthermore, describing them, I gave a short model of his theory. Lamarck claimed that gelatinous matter could receive the “vital orgasm,” a sort of agitation of molecules opposed to universal attraction. “Uncontainable” fluids, caloric and electricity, could provoke this “vital orgasm.” Later, the containable fluids, gases, and liquids, especially water, crossed this matter and deposited particles. This process was the beginning of nutrition.
Laplace Resonance
Lamarck considered that this steep of transformation corresponded to the structure of the most simple “polyp,” a monad just capable of elasticity. The continuous flow of water, always passing the same way, produced a little tunnel, the first digestive tube. Two points have to be underlined. Firstly, in his description of the spontaneous generation process, Lamarck summarized how all the principal causes of transformism act on matter and produce changes in species. Secondly, Lamarck’s theory on spontaneous generations currently exists in nature, and therefore vegetal and animal series are continuously beginning. However, if Lamarck spoke of permanent spontaneous generation, he did not really deal with the primordial origin of life, which was difficult to conceive in the context of his theory, and especially regarding his own chemical conceptions; according to which all the matter on Earth was combined by living beings.
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Langevin Rate Coefficient Synonyms Collisional rate; Langevin value
Definition The Langevin rate coefficient is the theoretical ▶ reaction rate coefficient for an ion-molecule process, calculated classically (i.e., neglecting quantum mechanical effects), assuming that reaction occurs upon every collision. The basic principle is that the ion induces a dipole in the (nonpolar) molecule; the theory does not work well for the interaction of an ion and a polar molecule.
See also ▶ Reaction Rate Coefficient
See also ▶ Darwin’s Conception of Origins of Life ▶ Spontaneous Generation (History of)
Langevin Value ▶ Langevin Rate Coefficient
References and Further Reading Corsi P, Gayon J, Gohau G, Tirard S (2006) Lamarck: philosophe de la nature. Paris, Presses Universitaires de France Corsi P (1988) The age of Lamarck, evolutionary theories in France:1790-1830. University California Press, California
Langmuir-Hinshelwood Mechanism Definition
Laminated Microbial Ecosystems ▶ Microbial Mats
Landing Site Definition A landing site is the planned, designated, or reached location on a ▶ Planet, moon (▶ Satellite), ▶ Asteroid, or ▶ Comet to host a landing probe, be it a hard landing (impact probe) or a soft landing.
See also ▶ Asteroid ▶ Comet ▶ Planet ▶ Satellite or Moon
This is a model of bimolecular chemical reactions on solid surfaces in which two atoms or molecules are adsorbed on the surface, and at least one diffuses on the surface until both are close enough to interact. In the interstellar medium, formation of molecular hydrogen is thought to occur primarily through reactions on the surfaces of dust grains.
See also ▶ Adsorption ▶ Eley–Rideal Mechanism ▶ Interstellar Dust
Laplace Resonance Definition The Laplace resonance among orbiting bodies is a series of two consecutive 2:1 ▶ mean motion resonances, making
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Large Millimeter Telescope
a 4:2:1 period ratio among three bodies in ▶ orbit. The Galilean satellites Io, Europa, and Ganymede are in the Laplace resonance. This commensurability was first pointed out by Pierre-Simon Laplace, and now bears his name. This interaction prevents the orbits of the satellites from becoming perfectly circular (due to tidal interactions with Jupiter), and therefore permits tidal heating of Io and Europa. Recently, an extrasolar planetary system, Gliese 876, was found to contain a Laplace resonance among three orbiting planets.
See also ▶ Mean Motion Resonance ▶ Orbit
Large Millimeter Telescope Synonyms LMT
This relation holds for clouds ranging in size from about 0.1 to 100 parsecs. Subsequent studies have confirmed the law, with n currently measured as 0.5. The theoretical basis of Larson’s law is still not understood. The existence of this relationship may signify that the diffuse gas spawning molecular clouds is dominated by turbulent motion. Dense cores, the clouds that form individual stars, have sizes in the order 0.1 parsec and have internal velocities that do not obey the law.
See also ▶ Fragmentation (Interstellar Clouds) ▶ Molecular Cloud
Last Common Ancestor ▶ Last Universal Common Ancestor ▶ LCA
Definition The Large Millimeter Telescope (LMT) is a 50 m diameter single-dish radio telescope optimized for astronomical observations at millimeter wavelengths (0.85 mm < l < 4 mm). The project is a binational collaboration between Mexico and the U.S.A, led by the Instituto Nacional de Astrofı´sica, ´ ptica y Electro´nica and the University of Massachusetts, O Amherst. The LMT is an open-air telescope being completed at an altitude of 4,600 m on a mountain in the Mexican state of Puebla (latitude about +19 degrees). The LMT will begin commissioning observations in 2011; when completed, it will be the most sensitive single-dish radio telescope in the world operating at short millimeter wavelengths.
Last Universal Ancestor ▶ Last Universal Common Ancestor
Last Universal Common Ancestor LUIS DELAYE Departamento de Ingenierı´a Gene´tica, CINVESTAVIrapuato, Irapuato, Guanajuato, Mexico
See also ▶ Radio Astronomy
Synonyms Cenancestor; Last Common Ancestor; Last Universal Ancestor; LCA; LUCA
Larson’s Law Definition Larson’s law is an empirical relationship, discovered by Richard Larson in 1981, between the size of a ▶ molecular cloud and its internal velocity dispersion. Larson found that the average velocity dispersion within a cloud is proportional to its size raised to a power n.
Keywords Comparative genomics, early cellular evolution, lateral gene transfer, secondary gene loses, tree of life
Definition The last universal common ancestor (LUCA) is the most recent ancestor from which all currently living species have evolved.
Last Universal Common Ancestor
Overview The idea that all life is related by common ancestry can be traced to the publication of the Origin of the Species by ▶ Darwin (1859), where he wrote in the last chapter of his book “. . .I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.” However, it was not until the second half of the twentieth century, with the analysis of the available molecular sequences, that the modern discussion on the nature of LUCA began. Initially Woese and Fox (1977a) suggested that the ancestor of ▶ Bacteria and ▶ Eukarya (the main lines of descent known at that time), was a progenote, i.e., a primitive entity with an imprecise relationship between the ▶ genotype and the ▶ phenotype. Later on, with the discovery of ▶ Archaea as the third line of descent (with the use of the small subunit of rRNA as a phylogenetic marker), this definition was extended to include the three cellular ▶ domains (Bacteria, Archaea, and Eukarya). Therefore, Woese and Fox (1977b) suggested that the three domains evolved independently from the progenote. This picture of the last universal ancestor began to change when Gogarten et al. (1989) and Iwabe et al. (1989) used ▶ paralogous genes to find the root on the universal tree of life. Both analyzes showed that the root of the universal tree was placed on the branch leading to Bacteria, suggesting a bacterial-like universal ancestor. In addition, a compilation of shared traits among the three cellular domains indicated that the last ▶ common ancestor was already a complex organism more similar to present day bacteria than to progenotes (Lazcano et al. 1992). In an attempt to identify a minimal gene set for cellular life, Mushegian and Koonin (1996) compared the ▶ genomes of Haemophilus influenzae and Mycoplasma genitalium. The inferred minimal gene set contained 256 genes, most of them with eukaryotic and archaeal homologs. The exceptions were seven key proteins involved in DNA replication. This led to the proposal that the last universal common ancestor had an RNA genome (Mushegian and Koonin 1996). This proposal was contested by Becerra et al. (1997), arguing that caution should be taken in backtrack characterizations of the cenancestor when only complete ▶ genomes from one domain are used. As more genomes become sequenced from different lineages, it was generally assumed that ▶ phylogenies derived from universally conserved genes would mostly reflect the canonical division of Archaea, Bacteria, and
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Eukarya. However, a large number of phylogenies from different families of genes reflected branching patterns differing from that of the three domains, leading to the suggestion that in the long run, the process of ▶ lateral gene transfer dominates the history of life (Doolittle 1999). If this is indeed the case, then it would be impossible to reconstruct the gene complement of LUCA (Zhaxybayeva and Gogarten 2004). However, ▶ phylogenetic trees derived from gene content recovered the separation between Archaea, Bacteria, and Eukarya, suggesting that the history of life has not been completely dominated by lateral gene transfer (Tekaia et al. 1999). The level of lateral gene transfer along the history of life continues to be a matter of debate today. The similarities among extant cells suggest that LUCA was not a direct descent from the primeval soup. For instance, genes involved in transcription and translation (the so-called informational genes) are highly conserved among Archaea, Bacteria, and Eukarya and must have been present in LUCA. However, the nature, and the level of complexity of the last universal common ancestor has remained controversial. Proposals range from a progenote level of organization to highly complex cells similar in nature to extant prokaryotes. Other controversies include the nature of its genetic material, whether DNA or RNA or the role played by viruses on the evolution of LUCA.
Basic Methodology In principle, the nature of LUCA can be inferred by identifying those genes universally conserved among Bacteria, Archaea, and Eukarya (Fig. 1). Drawbacks to this methodology include secondary gene loses (which leads to underestimate LUCA’s gene content) and lateral gene transfer (which on the contrary, may lead to infer as ancestral genes of recent origin that have been transferred among distant species at high frequency). See Fig. 2.
Key Research Findings This is a (non-exhaustive) chronological list of key research findings in this field: ● The concept of the progenote as universal ancestor is suggested (Woese and Fox 1977a). ● The division of life in to three domains (Archaea, Bacteria, and Eukarya) is discovered by using the small subunit of rRNA as a phylogenetic marker (Woese and Fox 1977b). ● By using paralogous genes, the root of the tree of life is positioned in the bacterial branch (Gogarten et al. 1989; Iwabe et al. 1989).
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Last Universal Common Ancestor
Bacteria
Archaea
Eukarya Archaea
Bacteria
LUCA
LUCA Eukarya
a
b
Last Universal Common Ancestor. Figure 1 (a) Phylogenetic position of LUCA (in this case the root is shown in the Bacterial clade); (b) The gene complement of LUCA can be inferred from conserved homologous traits among Archaea, Bacteria and Eukarya
Underestimated LUCA gene content Secondary gene loses
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● The genetic core of the last common ancestor (as inferred from universally conserved genes showing three domain phylogenies) comprises mostly ribosomal proteins and a few proteins involved in DNA polymerization, (Harris et al. 2003). ● Most estimates of the gene complement of the last common ancestor comprise between 500 and 1,000 protein-coding genes (Mushegian 2008).
Applications Overestimated LUCA gene content
Understanding the nature of LUCA (its level of complexity, the chemical nature of its genome, its gene complement or its level of similarity to modern cell lineages, among other aspects of its biology) is clearly a central question in evolutionary biology.
Horizontal gene transfer
Last Universal Common Ancestor. Figure 2 Our accuracy to reconstruct the gene complement of LUCA is inversely related to the number of secondary gene loses and lateral gene transfers
● Comparison of homologous genes from the three domains of life indicates a last common ancestor similar in complexity to extant prokaryotes (Lazcano et al. 1992). ● By the comparison of the first two completely sequenced genomes with gene sequences available in databases, a LUCA with an RNA based genome is proposed (Mushegian and Koonin 1996). ● The first inference of the gene complement of the last common ancestor by using genomes from the three domains of life is made. An ancestor similar in complexity to extant prokaryotes is suggested (Kyrpides et al. 1999).
Future Directions There has been much debate on the level of complexity (number of genes) and the chemical nature (DNA or RNA) of the genome of LUCA. However, the debate is far from solved. Understanding the role played by lateral gene transfer along prokaryotic evolution, and whether it affects all kinds of genes or not, is central to backtrack characterizations of the cenancestor. More research on this direction is clearly needed. The chemical nature of membranes in LUCA is another aspect of its biology that deserves more attention (Pereto´ et al. 2004). Finally, a detailed database comprising proteins that could have been inherited from LUCA would be of invaluable help for researchers.
See also ▶ Archea ▶ Bacteria
Late Heavy Bombardment
▶ Common Ancestor ▶ Darwin’s Conception of Origins of Life ▶ Domain (Taxonomy) ▶ Eukarya ▶ Evolution (Biological) ▶ Genome ▶ Genotype ▶ Lateral Gene Transfer ▶ Origin of Life ▶ Paralogous Gene ▶ Phenotype ▶ Phylogenetic tree ▶ Phylogeny ▶ Prokaryote
References and Further Reading Becerra A, Islas S, Leguina JI, Silva E, Lazcano A (1997) Polyphyletic gene losses can bias backtrack characterizations of the cenancestor. J Mol Evol 45:115–118 Darwin C (1859) On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, 6th edn. John Murray, London Doolittle WF (1999) Phylogenetic classification and the universal tree. Science 284:2124–2129 Freeland SJ, Knight RD, Landweber LF (1999) Do proteins predate DNA? Science 286:690–692 Galtier N, Tourasse N, Gouy M (1999) A nonhyperthermophilic common ancestor to extant life forms. Science 283:220–221 Gogarten JP, Kibak H, Dittrich P, Taiz L, Bowman EJ, Bowman BJ, Manolson MF, Poole RJ, Date T, Oshima T, Konishi J, Denda K, Yoshida M (1989) Evolution of the vacuolar H+-ATPase – Implications for the origin of eukaryotes. Proc Natl Acad Sci USA 86:6661–6665 Harris JK, Kelley ST, Spiegelman GB, Pace NR (2003) The genetic core of the universal ancestor. Genome Res 13:407–412 Iwabe N, Kuma K, Hasegawa M, Osawa S, Miyata T (1989) Evolutionary relationship of archae, bacteria, eubacteria, and eukaryotes inferred from phylogenetic trees of duplicated genes. Proc Natl Acad Sci USA 86:9355–9359 Kyrpides N, Overbeek R, Ouzounis C (1999) Universal protein families and the functional content of the last universal common ancestor. J Mol Evol 49:413–423 Lazcano A, Forterre P (1999) The molecular search for the last common ancestor. J Mol Evol 49:411–412 Lazcano A, Fox GE, Oro´ J (1992) Life before DNA: the origin and evolution of early archean cells. In: Mortlock RP (ed) The evolution of metabolic function. CRC Press, Boca Raton, pp 237–295 Mushegian A (2008) Gene content of LUCA, the last universal common ancestor. Front Biosci 13:4657–4666 Mushegian AR, Koonin EV (1996) A minimal gene set for cellular life derived by comparison of complete bacterial genomes. Proc Natl Acad Sci USA 93:10268–10273 Pereto´ J, Lo´pez-Garcı´a P, Moreira D (2004) Ancestral lipid biosynthesis and early membrane evolution. Trends Biochem Sci 29: 469–477 Tekaia F, Lazcano A, Dujon B (1999) The genomic tree as revealed from whole proteome comparisons. Genome Res 9:550–557
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Vaneechoutte M, Fani R (2009) From the primordial soup to the latest universal common ancestor. Res Microbiol 160:437–440 Woese CR, Fox GE (1977a) The concept of cellular evolution. J Mol Evol 10:1–6 Woese CR, Fox G (1977b) Phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proc Natl Acad Sci USA 74:5088–5090 Zhaxybayeva O, Gogarten JP (2004) Cladogenesis, coalescence and the evolution of the three domains of life. Trends Genet 20:182–187
Late Heavy Bombardment PHILIPPE CLAEYS1, ALESSANDRO MORBIDELLI2 1 Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium 2 Observatoire de la Cote d’Azur, Nice, France
Synonyms LHB; Lunar cataclysm
Keywords ▶ Asteroids, comets, gas planets, impact craters, impact process, ▶ meteorites, ▶ Moon, small bodies, ▶ solar system dynamics
Definition The term Late Heavy Bombardment (or LHB) corresponds to an elevated frequency of collisions that affected the inner Solar System between 4.0 and 3.8 billion years ago. The Earth preserved no trace of these major impacts, but they can be found on the highly cratered surface of the Moon and other planets such as Mars, or in the age of the impact melt measured on meteorites originating from the asteroid belt. The LHB forms either the slowly decreasing tail end of planetary accretion or a localized cataclysmic event, perhaps triggered by a readjustment of the orbits of the large gas planets long after the formation of the Solar System.
Overview Origin and Duration of the LHB The current stratigraphic timescale places the Late Heavy Bombardment (LHB) period in the beginning of the Archean (in the Eoarchean Era), coeval, or slightly before the occurrence of the oldest metamorphosed terrestrial sediments outcropping in ▶ Isua (Greenland). The ancient highly cratered surface of the Moon displays
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the traces of intense collisions apparently concentrated between 3.9 and 3.8 Ga. The Lunar crust crystallized around 4.44 Ga ago, and the morphology of the highlands displays a dense concentration of impact craters excavated prior to the emplacement of the first volcanic flows of the mare plains after 3.8 Ga. The LHB is now clearly established all over the inner Solar System; it is recorded on the Moon, Mars, and in meteorites, whose parent bodies originated in the asteroid belt. However, the magnitude and frequency of the collisions taking place between the end of planetary accretion around 4.5 Ga ago and the start of the Archean at 4.0 Ga remains a matter of controversy. Two explanations compete (Fig. 1). The first hypothesis assumes that the frequency of impacts declined slowly and progressively after the end of the accretion, explaining why the bombardment frequency remained important around 3.9 Ga (e.g.,
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Archean 4.0 Age in Ga
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Late Heavy Bombardment. Figure 1 Schematic representation of the flux of projectile on Earth between 4.5 and 3.5 Ga (Modified after Kring 2003). The yellow curve shows a slow decrease of the bombardment while the blue curve illustrates the leading hypothesis of a rapid decrease of the flux followed by a peak, the late heavy bombardment (LHB), around 4.0–3.9 Ga. The duration of this high flux varies between 20 and 200 Ma; the slope of the decrease after 3.9 Ga is also poorly constrained. It cannot be excluded that other cataclysmic events took place during the Hadean, but then their traces have been fully erased by the complete resetting of the Earth surface caused by the last event
Hartmann et al. 2000). According to this view, the socalled LHB is not an exceptional event. Rather it forms the tail end of a 500 million year period, during which the Earth and the inner Solar System were subject to devastating collisions, with major consequences such as the chronic melting of the crust, the ejection of the atmosphere, and the vaporization of the oceans. The second hypothesis advocates a post-accretion rapid decline in the frequency of impacts down to a value only slightly higher than today (2x). This low impact rate apparently persisted throughout the entire Hadean. The period around 3.9 Ga represents thus an exceptional and cataclysmic event, characterized by a huge increase in the rate of collisions. Several arguments favor the latter hypothesis. Some 500 million years of continual impacts would leave an obvious trace in the Lunar cratering record. However, the isotopic dating of impact melt-rocks returned by the Apollo and Luna missions fails to identify craters older than 3.92 Ga. The melt phases in lunar meteorites confirm this age limit. They provide a particularly strong argument because meteorites likely originated from random locations on the Moon compared to the samples collected at a few landing sites. A complete resetting of all older ages all over the Moon remains possible, but highly unlikely considering the difficulties of completely resetting all isotopic ages at the scale of a planet. The U–Pb and Rb–Sr isochrones of lunarhighland samples indicate a single disruption by a metamorphic event at 3.9 Ga and/or between 3.85 and 4.0 Ga respectively, after the formation of the crust around 4.4 Ga. There is no evidence for a resetting of these isotopic systems as expected from intense collisions between 4.4 and 3.9 Ga. It is also difficult to explain the preservation of 4.4 Ga old ▶ anorthosites on the Moon’s surface, if it had to endure such a long and continuous intense bombardment. In addition, the old upper crustal lithologies of the Moon fail to show a significant enrichment in siderophile elements (in particular ▶ iridium), which would have been produced by repetitive meteorite impacts during the entire Hadean. Moreover, if the elevated mass accretion documented in the period around 3.9 Ga forms only the tail end of an extended period of collisions, the entire mass equivalent of the Moon should have accreted at about 4.1 Ga instead of 4.5 Ga. On Earth, the oxygen isotopic signature (d18O) of the oldest recovered zircons at Jack Hills (4.4 Ga) indicates formation temperatures compatible with the existence of liquid water. None of the Hadean age zircons analyzed so far displays evidence of shock metamorphism. This sequence of arguments is difficult to reconcile with an extended period of intense
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collisions during the Hadean. Consequently, many authors currently favor the occurrence around 3.9 Ga of a unique cataclysmic bombardment event, taking place all over the inner solar system. On planets and satellites such as the Moon, Mars, and Venus, whose surface was not intensely remodeled by erosion, sedimentation, and plate tectonics, the ancient impact structures easily exceed 1,000 km in diameter. The battered old surfaces of these planets witness past collisions whose frequency and scale were two to perhaps three orders of magnitude higher than the Phanerozoic values. It seems highly unlikely that a target the size of Earth would have been spared, even if active geological processes have obliterated the record of these past impacts. So far, the terrestrial record yields no traces of the LHB. The oldest lithologies preserved in Greenland do not contain indication of a meteoritic contamination. There is in Isua no elevated concentration in siderophile elements (such as Ir), or minerals affected by shock metamorphism. Several explanations account for the lack of impact signatures in Earth’s oldest lithologies. First, it can be due to the limited locations and small number of available samples. Second, the LHB and the rocks from Isua may not overlap. Even considering the commonly accepted age of 3.82 Ga for the metasediments in Isua, it is possible that the LHB had already ended when deposition took place. On the Moon, where the record of these collisions is best preserved, some 1,700 craters larger than 20 km in diameter are known; among them 15 reach sizes between 300 and 1,200 km. All of them date between 4.0 and 3.9 Ga; ages that correspond to the end of the Nectarian and the beginning of the Imbrian periods according to the Lunar stratigraphy. About 6,400 craters with diameters >20 km should have been produced on Mars. Based on scaling the Moon flux, the Earth would have been impacted by 13–500 times more mass than the Moon according to the size distribution of the projectiles. Using conservative values, the formation of 22,000 craters with a diameter over 20 km must be considered. The number of structures larger than 1,000 km would vary between 40 and 200, and it is not impossible that some would reach 5,000 km, that is the dimension of an entire continent. The duration of the LHB cataclysmic event is difficult to estimate. Based on the cratering record of the Moon, it varies between 20 and 200 Ma, depending on the mass flux estimation used in the calculation. According to the lowest estimation on Earth, a collision capable of generating a 20-km crater could take place every 10,000 years. This would correspond to the impact of a 1 km projectile, an event that presently occurs once
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every 0.350–1 Ma. If the highest estimation is considered, the frequency increases to such an impact every 20 years! Based on the Lunar cratering record, the impact frequency decreases after 3.8 Ga and stabilizes close to the present values. However, the slope of the decrease is difficult to estimate.
A Model for the Origin of the Cataclysmic Spike at 3.9 Ga To explain a cataclysmic spike in the bombardment rate, a massive reservoir of ▶ planetesimals must remain intact during the planet formation epoch and the subsequent 600 Ma, and then, all of a sudden, be destabilized. This seems to be in conflict with the classical view according to which the Solar System did not undergo substantial modifications since soon after its formation. Gomes et al. (2005; the ▶ “Nice model”) designed a scenario that explains the origin of the LHB. In their model, the giant planets form on quasi-circular, coplanar orbits, between approximately 5.5 and 15 AU. This orbital configuration is much more “compact” than the current one, where the planetary orbits are located from 5.2 to 30 AU. A massive disk of planetesimals surrounded the planetary system, extending from a few AUs beyond Neptune outward. The perturbations of the planets allow the planetesimals from the inner parts of the disk to slowly develop planetcrossing orbits. The encounters of these planetesimals with the planets drive a slow, progressive migration of the planets. Computer simulations show that ▶ Saturn, Uranus, and Neptune slowly moved outward, and ▶ Jupiter slowly moved inward. The rate at which the planetesimals could leak out of the disk governs the rate of this migration, which was slow because the disk was quite far from the closest planet. As a consequence of Jupiter and Saturn’s migrations in divergent directions, the ratio of the orbital periods of these two planets increased. This ratio is currently slightly smaller than 2.5, and thus it had to be lower in the past, perhaps less than 2. The simulations show that with reasonable initial conditions for the planets and the disk, the ratio of the orbital periods could reach the exact value of 2 (the 2:1 ▶ mean motion resonance in the jargon of celestial mechanics) quite late: after a time ranging from 350 Myrs to 1.2 Gyrs, an interval that embraces the LHB time, 600 Ma after planet formation. The 1:2 resonance crossing triggers the LHB. Indeed, when the ratio of orbital periods passes through the value of 2, the orbits of Jupiter and Saturn become eccentric. This abrupt transition temporarily destabilizes the system of the four giant planets, leading to a short phase of close encounters
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among Saturn, Uranus, and Neptune. As a result of these encounters, the orbits of the ice giants become eccentric and the furthest planet penetrates into the disk. This destabilizes the disk completely. All planetesimals are scattered in sequence by the planets. A fraction of them temporarily penetrate into the inner solar system. About 10 7 of the original planetesimals hit the Moon and 10 6 the Earth. About 10 3 of the original planetesimals probably remain trapped in the Kuiper belt. The interaction of the planets with the destabilized disk enhances planet migration and damps the planetary eccentricities and inclinations. A large number of simulations of this process show that, if the planetesimal disk contained about 35 Earth masses at the time of the LHB, the planets statistically end on orbits very close to those currently observed. A more massive disk would have driven Jupiter and Saturn too far apart, and a less massive disk would not have moved them enough. Interestingly, given this total disk mass, the total mass of the planetesimals hitting the Moon turns out to be consistent with that inferred from the counting of basins formed at the LHB time. The distant planetesimals are not the sole cause of the LHB, though. The migration of Jupiter and Saturn from the 1:2 resonance to their current position also destabilizes the asteroid belt. About 90% of the asteroids could develop orbits crossing those of the terrestrial planets, although this fraction is poorly estimated because it depends on the dynamical state of the asteroid belt before the LHB, which is unknown. Given the current mass of the asteroid belt (510 4 Earth masses) this implies that 510 3 Earth masses of asteroids become Earth crossers. Of these, about 210 4 hit the Moon, supplying again a mass of projectiles consistent with the LHB constraints. Thus, according to this model, the projectiles causing the LHB consist a mixture of comets (from the distant disk) and asteroids, in roughly equal proportions. The exact relative contribution of asteroids and comets to the LHB cannot be stated with precision from the model, at least at the current state. Chemical analyses on lunar samples collected in the vicinity of basins seem to indicate that asteroids of enstatite or more likely ordinary chondritic type participated in the process. The strength of the Gomes et al. (2005) model on the origin of the LHB is that it explains not only the main characteristics of the LHB (intensity, duration, abrupt start) but also several other puzzling properties of the Solar System: the orbital architecture of the giant planets (the mutual spacing, the eccentricities, and the mutual inclinations of the orbits), the population of Jovian Trojans (their orbital distribution and total mass), and the absence of numerous asteroid families formed at the LHB time.
See also ▶ Anorthosite ▶ Asteroid ▶ Impactite ▶ Iridium ▶ Isua Supracrustal Belt ▶ Jack Hills (Yilgarn, Western Australia) ▶ Jupiter ▶ Mean Motion Resonance ▶ Meteorites ▶ Moon, The ▶ Nice Model ▶ Planetesimals ▶ Saturn ▶ Shocked Quartz ▶ Siderophile Elements ▶ Solar System Formation (Chronology)
References and Further Reading Cohen BA, Swindle TD, Kring DA (2000) Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages. Science 290:1754–1756 Culler TS, Becker TA, Muller RA, Renne PR (2000) Lunar impact history from 40Ar/39Ar dating of glass spherules. Science 287: 1785–1788 Gomes R, Levison HF, Tsiganis K, Morbidelli A (2005) Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature 435:466–469 Hartmann WK, Ryder G, Dones L, Grinspoon D (2000) The timedependent intense bombardment of primordial Earth/Moon system. In: Canup RM, Righter K (eds) Origin of the Earth and Moon. University of Arizona Press, Tucson, pp 493–512 Koeberl C (2004) The late heavy bombardment in the inner solar system. Earth Moon Planet 92:79–87 Kring DA (2003) Environmental Consequences of Impact Cratering Events as a function of Ambient Conditions on Earth. Astrobiology 3:133–152 Kring DA, Cohen BA (2002) Cataclysmic bombardment throughout the inner solar system 3.9–4.0 Ga. J Geophys Res 107(E2):4-1–4-6. doi:10.1029/2001JE001529 Morbidelli A, Levison HF, Tsiganis K, Gomes R (2005) Chaotic capture of Jupiter’s Trojan asteroids in the early Solar System. Nature 435:462–465 Ryder G (1990) Lunar samples, lunar accretion and the early bombardment of the Moon. Eos Trans Am Geophys Union 71:313–323 Ryder G, Koeberl C, Mojzsis SJ (2000) Heavy bombardment on the Earth at 3.85: the search for petrographical and geochemical evidence. In: Canup RM, Righter K (eds) Origin of the Earth and Moon. University of Arizona Press, Tucson, pp 475–492 Stoeffler D, Ryder G (2001) Stratigraphy and isotopic ages of lunar geologic units: chronological standard for the inner solar system. In: The evolution of Mars. International Space Science Institute, Bern, Switzerland, pp 7–53. Space Sci Rev 96:1–4 Tsiganis K, Gomes R, Morbidelli A, Levison HF (2005) Origin of the orbital architecture of the giant planets of the Solar System. Nature 435:459–461
Late Veneer
Latent Heat RAY PIERREHUMBERT Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA
Keywords ▶ Clouds, latent heat, phase change
Definition When a substance changes from one form to another (e.g., gaseous carbon dioxide condensing into dry ice), energy is released or absorbed even if the temperature of the mass is unchanged after the transformation has taken place. This happens because the amount of energy stored in the form of intermolecular interactions is generally different from one form, or phase, to another. The amount of energy released when a unit of mass of a substance changes from one phase to another, holding temperature constant, is known as the latent heat associated with that phase change (J/kg in mks units).
Overview Condensable substances play a central role in the atmospheres of many planets and satellites. On Earth, it is water that condenses, both into liquid water and ice. On Mars, CO2 condenses into dry ice in clouds and in the form of frost at the surface. On Jupiter and Saturn, not only water but ammonia (NH3) and a number of other substances condense. The thick clouds of Venus are composed of condensed sulfuric acid. On Titan it is methane, and on Neptune’s moon Triton nitrogen itself condenses. Water has an unusually large latent heat; the condensation of 1 kg of water vapor into ice releases nearly five times as much energy as the condensation of 1 kg of carbon dioxide gas into dry ice. This is why the relatively small amount of water vapor in Earth’s atmosphere can nonetheless have a great effect on atmospheric structure and dynamics. Ammonia also has an unusually large latent heat, it is a similar to water’s. In both cases, the anomalous latent heat arises from the considerable energy needed to break hydrogen bonds in the condensed phase. When air rises, it cools by adiabatic expansion, and if it gets cold enough that one of the components of the atmosphere begins to condense or the partial pressure of one of the compressed atmospheric components surpasses the saturation vapor pressure, latent heat is released. This makes the rising air parcel warmer than the dry adiabat (no energy input to the parcel) would predict. If we assume further that the condensation is efficient enough that it keeps the system at
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saturation, the resulting temperature profile is commonly referred to as the moist adiabat, regardless of whether the condensing substance is water vapor (as on Earth) or, e.g., CO2 on Mars or methane on Titan.
See also ▶ Ammonia ▶ Ice ▶ Phase Transition ▶ Water
References and Further Reading Pierrehumbert RT (2010) Principles of planetary climate. Cambridge University Press, Cambridge
Late Veneer FRANCIS ALBARE`DE Ecole Normale Supe´rieure de Lyon, Lyon Cedex 7, France
Definition The term late veneer refers to the late accretion of asteroidal or cometary material to terrestrial planets. Iron and nickel segregation during core formation leaves the mantle of the planets depleted in siderophile elements, notably platinum-group elements. The modern abundances of these elements in the terrestrial mantle greatly exceed the level expected from such a wholesale removal of metal. It is therefore surmised that 0.5–1.5% of chondritic or cometary material was brought to the planet by the late veneer after core formation (Dauphas and Marty 2002; Maier et al. 2009). This hypothesis is germane to the issue of water delivery to the Earth.
History Ringwood (1966) pointed out that the abundances of siderophile elements such as Ni, Co, Pt, Os in the upper mantle are remarkably higher than the values indicated by low-pressure partitioning experiments of these elements between metal and silicate phases. Chou (1978) suggested that about one percent of material resembling ▶ carbonaceous chondrites was added to the Earth after core formation.
Overview The excess of siderophile elements in the terrestrial mantle (the crust allotment may safely be neglected) with respect to the level of abundance expected from metal/silicate equilibration in the course of core segregation stirred strong debates (e.g., Drake and Righter 2002) and
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generated abundant experimental work. There is nothing really contentious about highly siderophile elements, such as platinum-group elements (Pt, Os, Ir, Pd, Ru, Rh), for which the metal/silicate distribution coefficient is in the range 104–106 (Holzheid et al. 2000). The very existence of the excess was, however, challenged for transition elements such as Ni and Co (Li and Agee 1996) with the assumption that core segregation took place under pressures in excess of 35 GPa, which corresponds to mid-mantle depth (Wade and Wood 2005). The best estimate of the proportion of late veneer in the Earth is therefore given by the platinumgroup elements excesses and is about 0.7%. Dynamic models of planetary accretion can accommodate such a late delivery of chondritic material. At the end of the oligarchic stage of planetary accretion, a few tens of million years after the formation of the Sun, when only a few tens of Mars-size bodies were left to plow in disks of asteroids, the main effect of Jupiter attractions (resonances) was to knock asteroids off their orbits and sling them out of the Solar System or into the inner Solar System and the Sun itself (Morbidelli et al. 2000). The Earth’s capture cross-section is thought to have been large enough that some of these projectiles hit our planet. Precisely how many of these landed on Earth is not well understood, but the tail of the asteroid shower seems to have lasted well beyond the formation of the terrestrial core. The relevance of the late veneer to the delivery of water to the Earth is clear. Because most of its constitutive material accreted in the hot inner Solar System, our planet should be even more depleted in volatile elements, such as S, C, N, Cl, Zn, Pb, and, most significantly, water, than the presence of conspicuous ocean and atmosphere indicates. ▶ Comets were the preferred volatile carriers (Owen and Bar-Nun 1995) until it was realized that water from these objects was twice too rich in deuterium with respect to hydrogen compared to seawater. Attention then turned to chondritic asteroids from the outer Solar System, which carry water in excess of 10% (Morbidelli et al. 2000). Not everybody agrees that water came in late with the late veneer rather than early, but the record of a process that fractionated terrestrial volatile from refractory elements some 100 50 million years after the formation of the Solar System is clearly found in both U-Pb and the I-Pu-Xe chronometers (Albare`de 2009). The late veneer should not be confused with the ▶ Late Heavy Bombardment, which is dated at about 3.8 Ga on the Moon. The LHB it is responsible for the largest lunar craters such as Imbrium. Abundant and large impacts of similar age are observed on Mars, and inferred on some asteroids. The LHB is thought to be associated with the disruption of a large asteroid or to the perturbation of the
asteroid belt by the rapid migration of the giant planets (Gomes et al. 2005).
See also ▶ Carbonaceous Chondrite ▶ Comet ▶ Deuterium/Hydrogen Ratio ▶ Earth’s Atmosphere, Origin and Evolution of ▶ Late Heavy Bombardment ▶ Oceans, Origin of ▶ Siderophile Elements ▶ Water, Delivery to Earth
References and Further Reading Albare`de F (2009) Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461:1227–1233 Chou CL (1978) Fractionation of siderophile elements in the Earth’s upper mantle. Proc Lunar Planet Sci Conf 9:219–230 Dauphas N, Marty B (2002) Inference on the nature and the mass of Earth’s late veneer from noble metals and gases. J Geophys ResPlanets 107: doi:10.1029/2001JE001617 Drake M, Righter K (2002) Determining the composition of the Earth. Nature 416:39–44 Gomes R, Levison HF, Tsiganis K, Morbidelli A (2005) Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature 435:466–469 Holzheid A et al (2000) Evidence for a late chondritic veneer in the Earth’s mantle from high-pressure partitioning of palladium and platinum. Nature 406:396–399 Li J, Agee CB (1996) Geochemistry of mantle-core differentiation at high pressure. Nature 381:686–689 Maier WD, Barnes SJ, Campbell IH, Fiorentini ML, Peltonen P, Barnes S-J, Smithies RH (2009) Progressive mixing of meteoritic veneer into the early Earth’s deep mantle. Nature 460:620–623 Morbidelli et al (2000) Source regions and time scales for the delivery of water to the Earth. Meteorit Planet Sci 35:1309–1320 Owen T, Bar-Nun A (1995) Comets, impacts, and atmospheres. Icarus 116:215–226 Ringwood AE (1966) The chemical composition and origin of the earth. In: Hurley PM (ed) Advances in earth sciences. The M.I.T. Press, Cambridge, pp 287–356 Wade J, Wood BJ (2005) Core formation and the oxidation state of the Earth. Earth Planet Sci Lett 236:78–95
Lateral Gene Transfer PURIFICACIO´N LO´PEZ-GARCI´A Unite´ d’Ecologie, Syste´matique et Evolution, CNRS UMR8079, Universite´ Paris-Sud 11, Orsay cedex, France
Synonyms HGT; Horizontal gene transfer
Late-stage Chaotic Growth
Keywords Conjugation, transduction, transfection, transformation
Definition Lateral or horizontal gene transfer is the acquisition of genetic material from another organism without being its offspring, although it frequently refers to transfer from organisms belonging to another species. It contrasts with vertical gene transfer, which is the acquisition of genetic material from an ancestor.
Overview There are three known mechanisms of lateral gene transfer: ▶ transformation, ▶ transduction, and ▶ conjugation. Transformation implies the acquisition of naked DNA, for example, from lysed cells, by a recipient cell. Transduction implies the acquisition of ▶ DNA via ▶ virus intermediaries, which may infect one host, recombine with its ▶ genome and pick up some genes that can then be transferred to a second host. Conjugation involves the transfer of large DNA segments via specialized (conjugative) ▶ plasmids that are mobilized between cells through particular ▶ pili. Thought to be a relatively minor process some years ago, lateral gene transfer is known today to play and have played a very important role in both prokaryotic and eukaryotic evolution. In the case of eukaryotic cells, the nuclear genome acquired massive amounts of foreign genes from the endosymbiotic ancestors of organelles (mitochondria, primary and secondary plastids). In the case of prokaryotes, gene transfer is very active and plays a readily adaptive role under particular selective pressures (e.g., the transfer of antibiotic resistance genes when faced with antibiotic pressure). Lateral gene transfer in prokaryotes appears so widespread that some authors even question the possibility of reconstructing a tree of life proceeding by bifurcation, and think that organismal evolution would be better depicted by a “web of life” with anastomosing (reconnecting) branches. Other authors think, on the contrary, that although lateral gene transfer is important, it is not rampant. In that case, the reconstruction of a tree of life using a “core” of conserved genes would be possible.
▶ Conjugation ▶ Endosymbiosis ▶ Evolution (Biological) ▶ Genome ▶ Phylogenetic Tree ▶ Pili ▶ Plasmid
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▶ Recombination ▶ Transduction ▶ Transformation ▶ Virus
References and Further Reading Dagan T, Martin W (2006) The tree of one percent. Genome Biol 7:118 Delsuc F, Brinkmann H, Philippe H (2005) Phylogenomics and the reconstruction of the tree of life. Nat Rev Genet 6:361–375 Doolittle WF, Bapteste E (2007) Pattern pluralism and the tree of life hypothesis. Proc Natl Acad Sci USA 104:2043–2049 Gogarten JP, Townsend JP (2005) Horizontal gene transfer, genome innovation and evolution. Nat Rev Microbiol 3:679–687
Laterites ▶ Regolith (Terrestrial)
Late-stage Accretion Synonyms Late-stage chaotic growth
Definition Late-stage accretion refers to the final stage in the formation of terrestrial planets. At the start of this stage, the solid mass is roughly equally divided between planetary embryos and ▶ planetesimals. Embryos grow large enough to gravitationally perturb each other onto crossing orbits, triggering a phase of giant embryo-embryo collisions. In most cases, the chaotic growth stage is thought to start 1–10 million years after the start of accretion and to last for another 10–100 million years, generating amounts of collisional debris. The impact that formed the EarthMoon system is thought to have been the last giant impact on Earth. It occurred during the chaotic growth stage, 4.45 Gy ago (50–100 million years after the start of accretion).
See also See also
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Late-stage Chaotic Growth ▶ Late-stage Accretion
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Latite ▶ Shale
LC-MS ▶ Liquid Chromatography-Mass Spectrometry
Laurasia Definition Laurasia is the ▶ supercontinent composed of the landmasses of the northern hemisphere (Eurasia and North America) that was once part of the supercontinent ▶ Pangea. Pangea was assembled around 500 Ma ago and its breakup started 180–200 Ma ago when ▶ Gondwana split from Laurasia. North America split from Eurasia about 62 Ma ago with the opening of the North Atlantic Ocean.
LDEF ▶ Long Duration Exposure Facility
Lemaıˆtre’s Theory of Expanding Universe (History)
See also ▶ Gondwana ▶ Pangea ▶ Supercontinent
STE´PHANE LE GARS Centre Franc¸ois Vie`te, Universite´ de Nantes, Nantes, BP, France
Keywords
Lava Tube ▶ Rille ▶ Rima, Rimae
Law of Mass Action ▶ Thermodynamical Chemical Equilibrium
LC ▶ HPLC
LCA ▶ Last Common Ancestor ▶ Last Universal Common Ancestor
Astrophysics, Big Bang, cosmogony, cosmology, history, Lemaıˆtre
Abstract The abbot Georges Lemaıˆtre deeply unsettled, in the 1920s and 1930s, the knowledge in the cosmology and cosmogony fields. Associating Hubble’s recent works about galaxies with the young relativity and quanta theories, Lemaıˆtre proposed in 1927 the image of a finished and expanding universe. In 1931, he soundly worked out on the idea of a beginning of the universe, suggesting that the latter came from what he called a “primeval atom.”
History Today, Lemaıˆtre’s name is closely connected to the idea of Big Bang. Indeed, Lemaıˆtre’s works concerned the structure of the Universe and its origin, i.e., problems about cosmology and cosmogony. After his studies at the Jesuit College of Charleroi, in Belgium, he began studies at the Engineer School from the University of Louvain, but he gave them up after World War I to dedicate himself to physics and mathematics. After his Ph.D., he received a grant from the Belgian government which let him take a course with Arthur
Leucine
Eddington for 1 year and made possible for him to complete his attainment in astronomy and general relativity fields. In 1924, he went to the USA, where he was a student at the Harvard College Observatory and at the MIT. He discovered at this time Hubble’s recent works which showed that nebulas are distant galaxies, but not objects in our own galaxy: This discovery will be crucial for his further works. For this purpose, in 1927, 2 years after being back in Belgium, Lemaıˆtre published an article titled “Un univers homoge`ne de masse constante et de rayon croissant, rendant compte de la vitesse radiale des ne´buleuses extragalactiques” (“A homogeneous universe with a constant mass and a growing radius, accounting for the radial speed of the extragalactic nebulas”). In this article, Lemaıˆtre explained his innovating ideas: Unlike the static universes taken off the general relativity by Albert Einstein and Willem De Sitter, Lemaıˆtre imagined a finished and expanding universe. Then, he drew every inferences from this idea of expansion: As early as 1931, Lemaıˆtre suggested that the universe comes from a “primeval atom.” Taking his knowledges of the general relativity, the new quantum theories and the first microphysics results into account, but also the recent observation data about the galaxy’s speed removal, Lemaıˆtre was the first to establish significantly the idea of a beginning of the Universe. He even affirmed that cosmic rays are the relics of this first explosive stage of the cosmic history and emphasized the role of a nonzero cosmological constant. Both priest and scientist, Lemaıˆtre has never had problem to conciliate at a personal level science and faith, avoiding always assimilating his cosmogonist ideas to any divine creation of the world.
References and Further Reading Godart O, Heller M (1978) Un travail inconnu de Georges Lemaıˆtre. Rev Hist Sci 31(4):345–359 Kragh H (2004) Matter and spirit in the Universe. Scientific and religious precludes to modern cosmology. Imperial College Press, London Lambert D (2007) Un atome d’univers. La vie et l’œuvre de Georges Lemaıˆtre, Bruxelles, Racine/Lessius, 2000; L’itine´raire spirituel de Georges Lemaıˆtre (suivi de Univers et Atome, manuscrit ine´dit de G. Lemaıˆtre), Lessius. McVittie GC (1967) Georges Lemaitre. Q J R Astron Soc 8:294–297 Michael H (1996) Lemaıˆtre, Big Bang, and the quantum Universe. Pachart Publishing House, Tucson Narlikar JV (1994) Abbe´ Georges Lemaıˆtre : Father of the primeval atom. Curr Sci 67(12) Picolet Guy (1975) Georges Lemaıˆtre, L’hypothe`se de l’atome primitif. Rev Hist Sci 28(1):91–93 Vibert Douglas A (1967) Georges Lemaıˆtre, 1894–1966. J R Astron Soc Can 61:77–80
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Lenticula, Lenticulae Definition Lenticulae are small, dark pits, domes, and spots only found on ▶ Jupiter’s ▶ satellite ▶ Europa. These features could have been created by ▶ cryovolcanism, intrusion, or ▶ diapirism.
See also ▶ Albedo Feature ▶ Cryovolcanism ▶ Diapirism ▶ Europa ▶ Facula, Faculae ▶ Jupiter ▶ Macula, Maculae ▶ Satellite or Moon
LET ▶ Linear Energy Transfer
Leuchs’ Anhydride ▶ Amino Acid N-Carboxy Anhydride
Leucine Definition Leucine is one of the 20 protein amino acids. Its three-letter symbol is Leu, and the one-letter symbol is L. Its chemical formula is HO2CCH(NH2)CH2CH(CH3)2, which gives a molecular weight of 131.17, and its isoelectric point (pI) is 5.98. Among the 20 protein amino acids, leucine belongs to the group of amino acids whose side chains are simple hydrocarbons, together with glycine, alanine, valine, and ▶ isoleucine. Leucine (side chain: –CH2CH(CH3)2) and isoleucine (side chain: –CH(CH3)CH2CH3) are the isomers with six carbons, but there are in addition 29 non-protein ▶ amino acid isomers, such as norleucine (side chain: – CH2CH2CH2CH3). Many of them have been identified in extracts from the ▶ Murchison meteorite.
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See also ▶ Amino Acid ▶ Isoleucine ▶ Murchison ▶ Protein
LHB ▶ Late Heavy Bombardment
Libration Definition Libration is the oscillation of an angle about a fixed value (as opposed to circulation, meaning the angle cycles through all possible values). In celestial mechanics, libration of certain angles such as the ▶ apsidal angle or certain resonant angles means that the system is in a state with a particular configuration. In other words, it is the libration of one or more resonant angles which indicates if a system of planets is in resonance.
See also ▶ Apsidal Angle ▶ Orbit ▶ Mean Motion Resonance
Lichens LEOPOLDO G. SANCHO Universidad Complutense de Madrid, Facultad de Farmacia Departamento de Biologia Vegetal II, Madrid, Spain
Keywords Algae, extremophiles, fungi, poikilohydric, symbiosis
Definition Lichens are mutualistic symbiotic organisms composed of a fungus, usually an Ascomycete, which is intimately associated with a photosynthetic partner that is an alga or a ▶ cyanobacteria (also called blue-green ▶ algae), or sometimes both. The result is an autotrophic form of life that uses carbohydrates produced by the photosynthetic partner to live and grow. Lichen symbioses involve more
than 14,000 species of fungi and around 100 species of green algae and cyanobacteria (Nash 2008).
Overview In lichen symbioses, the fungal partner (mycobiont) makes up most of the structure of the lichen and absorbs water and nutrients from the surroundings as well as providing a suitable environment and protection for the photosynthetic partner (photobiont). Many of the lichen-forming ascomycetes reproduce sexually producing ascomata mostly on the upper surface of the lichens. A high percentage of lichens can asexually reproduce the symbiosis via symbiotic propagules such as soredia and isidia. Lichens are extraordinarily diverse in size, color, and morphology and are divided into three main forms, crustose (deeply attached to surfaces), foliose (flattened leaf-like), and fruticose (bushy lichens usually attached by one point to the substrate) (British Lichen Society web page). They show a wide ecological range, from the intertidal zones to the highest summits of the world and from the tropics to the poles, and can grow on soil, tree bark, leaves, mosses, and rocks as well as artificial substrates such as concrete and glass. In polar and alpine tundra lichens are often the dominant vegetation (Green et al. 2007). Lichens are known for their slow growth that ranges from an increment in diameter of 0.01 mm yr 1 in some Antarctic crustose species to several centimeters per year by foliose and fruticose species. Lichenometry takes advantage of the slow growth and longevity of crustose lichens to estimate the exposure age of stone surfaces (Armstrong and Bradwell 2010). Lichens are also widely used as bioindicators of the air quality due to their sensitivity to oxidant pollution, mainly SO2 (Cislaghi and Nimis 1997). Lichens are poikilohydric organisms, that is, they desiccate, and remain dormant, when their environment dries out, but can rehydrate when water becomes available again. Upon ▶ desiccation, the lichens become dehydrated to a degree that halts most biochemical activity. In this dehydrated state many lichens are highly tolerant to stress conditions and have even survived full exposure to space conditions (Sancho et al. 2007; de la Torre et al. 2010) and to simulated Mars atmospheres (de Vera et al. 2010). The reasons for this resistance are both molecular, special metabolites protect important molecules, and structural, where accumulation of lichen substances, especially in the upper cortex, account for the ability of these organisms to survive high radiation, including UV, and highvacuum. Many lichens can hydrate directly from the water vapor or from the snow crystals and, after rehydration, lichens from the most extreme habitats can resume their activity in minutes, even having been inactive for months or years. These features make lichens suitable material for experiments in astrobiology.
Life
See also ▶ Algae ▶ Biological Indicator ▶ BIOPAN ▶ Cyanobacteria ▶ Desiccation ▶ Epilithic ▶ Expose ▶ Exposure Facilities ▶ Extreme Environment ▶ Hypolithic ▶ Planetary and Space Simulation Facilities ▶ UV Climate
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“autonomy in evolution”: a complex network of selfreproducing autonomous agents whose individual (farfrom-equilibrium, metabolic) organization is instructed by material records generated through an open-ended, historical process, in which that complex (collectiveecological) network evolves. In a minimal – and more operational – sense, this involves that any living system should be ultimately based on the dynamic intertwinement of a semi-permeable boundary (membrane), an energy transduction apparatus, and, at least, two complementary types of macromolecules: (enzyme-like) catalysts and (gene-like) templates.
Overview References and Further Reading Armstrong R, Bradwell T (2010) Growth of crustose lichens: a review. Geografiska Annaler 92 A:3–17 British Lichen Society web page – www.thebls.org.uk Cislaghi C, Nimis PL (1997) Lichens, air pollution and lung cancer. Nature 387:463–464 de la Torre R, Sancho LG, Horneck G, de los Rı´os A, Wierzchos J, OlssonFrancis K, Cockell CS, Rettberg P, Berger T, de Vera J-PP, Ott S, Martinez Frı´as J, Melendi PG, Lucas MM, Reina M, Pintado A, Demets R (2010) Survival of lichens and bacteria exposed to outer space conditions – Results of the Lithopanspermia experiments. Icarus 208:735–748 de Vera J-P, Mo¨hlmann D, Butina F, Lorek A, Wernecke R, Ott S (2010) Survival potential and photosynthetic Aactivity of lichens under Mars-like conditions: A laboratory study. Astrobiology 10:215–227 Green TGA, Schroeter B, Sancho LG (2007). Plant life in Antarctica. In: Pugnaire FI (ed.) Handbook of Functional Plant Ecology. Marcel Dekker Inc, New York, pp 389–434 Nash TH (2008) Lichen Biology. Cambridge University Press Sancho LG, de la Torre R, Horneck G, Pintado A, Ascaso C, Wierzchos J, de los Rios A, Schuster M (2007) Lichen survive in the space. The BIOPAN-5 experiment. Astrobiology 7:443–454
Life KEPA RUIZ-MIRAZO1, ALVARO MORENO2 1 Department of Logic and Philosophy of Science and Biophysics Research Unit (CSIC-UPV/EHU), University of the Basque Country, Donostia, San Sebastia´n, Basque Country, Spain 2 Department of Logic and Philosophy of Science, University of the Basque Country (UPV/EHU), Donostia, San Sebastia´n, Basque Country, Spain
Definition Although it is rather controversial whether the term “life” will ever fit into a definition, it could be conceived as
In spite of its multifarious meanings and the intrinsic difficulty it has shown over the years to become a welldefined unanimous generalization, the concept “life” can still be regarded as a proper scientific target, with its own specific weight and implications, not only for biology (and closely related disciplines) but for other fields of knowledge and research. Acknowledging that it is a notion that needs to be refined, made more precise and possibly further developed in the future, we consider that progress in the understanding of life is a scientifically relevant goal. This position is in contrast to the always present skeptical voices, e.g., those who think that life is simply a wrong category, because it does not help as a demarcation criterion; or that life cannot be linked to any “natural kind,” arguing on grounds of an “apparent” arbitrary or conventional character. In any case, the discussion below will just highlight several open problems linked to an interpretation of this concept in positive terms; several open problems that the investigation in Synthetic Biology, Artificial Life, Origins of Life, and, in particular, Astrobiology should contribute to solve in years to come. The first problem is determining the universality of life. Although the planet Earth hosts a biosphere composed of an incredible variety of living forms, molecular biology has shown that they all share the same biochemical core, and the evolutionary theory proposes that they all derive from a common type of ancestral organism. Therefore, it becomes reasonable to wonder whether we are dealing with a single example of life, after all. Nobody has been able to synthesize alternative, completely different, living forms in the lab, so far, or to detect them elsewhere in the universe. And this poses a severe difficulty: how to discern the particular or contingent from the general and necessary attributes that characterize the living phenomenon. Comparative analyses across the huge diversity of organisms on Earth can be very helpful in that sense: they should give good indications of which are the
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indispensable set of mechanisms involved in the strikingly complex organization and way to change over time of living systems (in relation to non-living ones). Complexity sciences and, in general, bottom-up approaches coming from physics and chemistry should also add to that aim, showing that, whenever certain boundary conditions are met (during a suitable time span), matter tends to create organized structures; and only if these self-organized structures bring about additional mechanisms for their long-term persistence can a minimal biological threshold of complexity and robustness be crossed. A second problem, clearly connected with the first, regards finding out the adequate level of abstraction or specificity of the concept. In the last decades, the great advances in computational tools and simulation (in silico) techniques to analyze living systems, complementing traditional (in vitro and in vivo) experimental approaches, have raised the question of whether some biological features, or life itself, might be captured or even implemented at a symbolic, materially detached level. The strongest theses coming from the field of ▶ Artificial Life in the nineties pointed in that direction, proposing to conceive of life “as it could be” and not only as “we actually know it.” That debate is still open, but now shifting to a somewhat different arena: even if we acknowledge (as it is being generally acknowledged) that materiality plays an important role, what is the actual level of molecular specificity required for life? Are there chemical alternatives to ▶ RNA, ▶ DNA, ▶ protein, etc. (that could play similar functional roles, as templates, catalysts, etc.)? Can a cellfree (or compartment-free) organism be built? Equivalently, can a genome-free organism be put together? If ever possible, for how long? What properties would these new kinds of “quasi-biological” systems have/lack? The now emerging discipline of Synthetic Biology (together with other novel enterprises, like Systems Chemistry) will hopefully address some of these questions soon. And Astrobiology, from its foundational steps, has also included them (or similar ones) in its research agenda: why water, why carbon, etc. (as key ingredients for life)? What minimal size (i.e., minimal internal molecular machinery) could an organism have? In so far as science provides means to answer these open issues, possibly through the fabrication of intermediate or semi-synthetic systems, or through the detection of extra-terrestrial systems with some life-like properties, it will be giving new clues to advance in our way of handling and maturing the general “life” question. Finally, a third big difficulty of the concept “life” lies in its inescapable twofold dimension. All living phenomena relate, one way or the other, to the physically bounded
organization of individual organisms, but are also embedded in the temporally and spatially much wider network of systems and relationships comprising the whole history of the biosphere. The former involves fundamental features that we, as organisms, naturally recognize in other living systems, like metabolism and adaptive agency, whereas the latter reflects larger-scale (but no less important) aspects, like the capacity to change and evolve through generations, or the more global interactive dynamics at the ecological level. In reality, these two dimensions of life are deeply entangled. On the one hand, any known living being cannot exist but in the context of a collective network of similar systems. And this is clearly reflected in the fact that genetic components (which specify the metabolic machinery and organization of single biological entities), in order to be operational, must be shaped through a process that involves a great amount of individual systems and many consecutive generations, or reproductive steps. The unfolding of an evolutionary process by natural selection, based on heritable modular templates and genetic mechanisms, allows this peculiar form of organization we call “living matter” to explore many possible combinations and formulas to survive, providing extreme examples of adaptation that make any borderline within the living domain rather diffuse. Actually, evolution, beyond that intrinsic competitive/selective dynamics, weaves a collective network of increasingly complex and entangled cooperative relations among entities at different phenomenological levels and with different cohesive strength. But, on the other hand, the organismic dimension of life, namely, its expression in the form of individual agents, is also crucial to understand it. However embedded in evolutionary and ecological webs living systems might appear, their metabolic functioning still points to a basic organizational core that should be properly characterized. If the essence of biological organization is conceived as a web of diverse interactions among – rather than within – different bio-entities (extending the idea of living system to molecular replicators, viruses, organelles, parasites, symbionts, etc.), it will not be possible to determine whether organisms should be taken as a basic starting point (i.e., as a highly integrated and cohesive type of organization that gets progressively complex) or just as some occasional result of that ongoing dynamics of loose cooperative relations among a plurality of bio-entities. This is an important issue, because without such a highly integrated and cohesive individual organization, it would be very difficult to give a proper account of metabolism, development, agency, unit of selection, etc., or to make a clear-cut distinction between organisms and other forms
Life in the Solar System (History)
of cooperative or “ecological” networks. Furthermore, life seems to demonstrate, in the course of evolution, that increasingly complex kinds of individual agents have emerged and developed, bringing forth progressively sophisticated interactive capacities (cognitive ones included). Nevertheless, the apparent tension between these two aspects or conceptions of life can be channeled constructively. Indeed, what really matters is their deep entanglement, since a process of open-ended evolution cannot occur except in the context of a population of individuated metabolic systems (i.e., organisms); and, conversely, the unfolding of organisms and their long-term maintenance depend on their insertion in an open-ended evolutionary pathway. So it is really the integration of these two dimensions that provides a complete, rich enough picture of the phenomenon of life. Such a synthetic conception could be interpreted, in addition, as an attempt to bring together the main results and theoretical legacy of two different traditions in biology: the physiologicalbiochemical tradition, focused on immediate or “proximate” causes, and the evolutionary-historical tradition, interested in the final or “ultimate” ones.
See also ▶ Chronological History of Life on Earth ▶ Complexity ▶ Evolution (biological) ▶ Metabolism (Biological) ▶ Minimal Cell ▶ Origin of Life ▶ Replication (Genetics) ▶ Self Assembly ▶ Self Replication
References and Further Reading Benner SA, Ricardo A, Carrigan MA (2004) Is there a common chemical model for life in the universe? Curr Opin Chem Biol 8:672–689 Cleland CE, Chyba CF (2007) Does “life” have a definition? In: Woodruff TS III, John AB (eds) Planets and life: the emerging science of astrobiology. Cambridge: Cambridge University Press Dupre´ J, O’Malley MA (2009) Varieties of living things: life at the intersection of lineage and metabolism. Phil Theor Biol 1(e003):1–25 Ga´nti T (2003) The principles of life. Oxford University Press, Oxford Gayon J et al (eds) (2010) Special Issue on “Definining Life.” Orig Life Evol Biosph 40:119–244 Joyce GF (1994) “Foreword”. In: Deamer DW, Fleischker GR (eds) Origins of life: the central concepts. Jones and Barlett, Boston, pp xi–xii Keller EF (2002) Making sense of life. Harvard University Press, Cambridge, MA Koshland DE Jr (2002) The seven pillars of life. Science 295:2215–2216 Langton CG (1989) Artificial life. In: Langton CG (ed) Artificial life I (Proceedings of the first conference on artificial life, Los Alamos, September, 1987). Addison-Wesley, Redwood City, pp 1–47
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Lovelock J (1979) Gaia: a new look at life on Earth. Oxford Univ Press, New York Luisi PL (1998) About various definitions of life. Origin Life Evol Biosph 28:613–622 Margulis L, Sagan D (1995) What is life? Simon & Schuster, New York Maynard Smith J (1986) The problems of biology. Oxford University Press, Oxford Mayr E (1982) The growth of biological thought. Harvard University Press, Cambridge, MA Oparin AI (1961) Life: its nature, origin and development. Oliver and Boyd, Edinburgh Rizzotti M (ed) (1996) Defining life: the central problem in theoretical biology? Padova University Press, Padova Ruiz-Mirazo K, Pereto´ J, Moreno A (2004) A universal definition of life: autonomy and open-ended evolution. Origin Life Evol Biosph 34:323–346 Schro¨dinger E (1944) What is life? The physical aspect of the living cell. Cambridge University Press, Cambridge Shapiro R, Feinberg G (1990) Possible forms of life in environments very different from the Earth. In: Leslie J (ed) Physical cosmology and philosophy. McMillan, New York, pp 248–255 Varela FJ (1994) On defining life. In: Fleischaker GR, Colonna S, Luisi PL (eds) Self-production of supramolecular structures. Kluwer, Dordrecht, pp 23–31 Ward P, Brownlee D (2000) Rare Earth: why complex life is uncommon in the Universe. Copernicus, New York
L Life, Artificial ▶ Artificial Life
Life in the Solar System (History) FLORENCE RAULIN-CERCEAU Maıˆtre de Confe´rences, Centre Alexandre Koyre´ (UMR 8560-CNRS/EHESS/MNHN/CSI) Muse´um National d’Histoire Naturelle, Brunoy, France
Abstract This entry describes the broad diversity of assumptions and scientific works regarding the search for life in the solar system from the seventeenth to the nineteenth century.
History Since Copernicus demonstrated in 1543 that the Earth was not located at the center of the universe but was revolving around the ▶ Sun like the other planets, the Earth took the status of a simple planet among the others. Stars became suns and planets became earths that could be inhabited. The consequence of this view of the universe was that each
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planet of the solar system could be peopled with various inhabitants. In fact, this changing in the position of the Earth was not a rapid scientific revolution. The heliocentric theory of Copernicus produced a slow shift in the religious, philosophical, and scientific mentalities. The unfortunate pioneer of the extraterrestrial debate at the end of the sixteenth century, Giordano Bruno (1548– 1600), who supported the Copernican system, had to come up against the religious dogma and to try to remove a very deep-rooted belief about our place in the universe. A new epoch of observations was born in 1609 when Galileo Galilei (1564–1642) began to study in details the surface of the Moon thanks to his telescope recently devised. He also discovered four satellites orbiting ▶ Jupiter and the impact of this discovery was decisive to provide confirmation of the triteness of our planet among the celestial bodies of the solar system. The seventeenth century saw the definitive affirmation of the Copernican theory thanks to Kepler’s works demonstrating that planetary orbits were not circles but ellipses. Planetary motions were completely demystified and the position of the Earth as an ordinary planet going around the Sun could no more be challenged. ▶ Galileo’s observations and ▶ Kepler’s works led to a new debate about the plurality of worlds. Literary and scientific works flourished about the habitability of our neighbor the Moon in a context however still controlled by religious authority. Fascinated by the new paradigm of heliocentrism, Kepler (1571–1630) wrote a little book mixing didactic and fiction style, named Somnium, Sive Astronomia Lunaris (The Dream, or The Astronomy of the Moon). This book, which could have been considered as provocative during Kepler’s life, was published posthumously by his son in 1634 (whereas it was started in 1609). The interesting point was that Copernican astronomy was illustrated by a transposition in order to make clear the heliocentric viewpoint: how does the Earth look from the lunar surface for inhabitants called Selenites studying astronomy? The fiction story imagined by Kepler was strongly consolidated by astronomical data (scientific footnotes are more numerous than the text itself) and lunar habitability was discussed with scientific arguments. But this book was more a support of the Copernican theory than a convincing demonstration of the plurality of worlds. The second part of the seventeenth century led to accept the plurality of worlds with less and less hostility from a scientific viewpoint as well as a popular angle, in spite of a continuous reluctance from the Roman Catholic Church.
The first writing attempting to be liberated from this religious ascendancy came from the French philosopher Bernard le Bovier de Fontenelle (1657–1757). He published his Entretiens sur la pluralite´ des mondes (Conversations on the Plurality of Worlds) in 1686 which soon became very popular. In spite of this success, the book was still regarded as dangerous by the Roman Catholic Church, which placed it on the Index of Prohibited Books in 1687. The book presented a series of conversations between a gallant philosopher (Fontenelle himself) and a Marchioness and looked like a didactic entertainment about astronomy. The central question was dealing with the differences between the inhabitants of each planet, according to its proximity to the Sun. Fontenelle populated all the planets of the solar system and the Marchioness was at the same time filled with wonder and overwhelmed in front of this “unlimited number of inhabitants likely to be on all these planets.” She was also dubitative when she asserted, “it’s a lot of ignorance based on very little science.” How can we imagine these planet dwellers so various indeed, if nature is opposed to repetitions, questioned the Marchioness? Fontenelle enjoyed himself imagining that differences increased as the planets became more and more distant from the Sun. But he spent very little time on the case of ▶ Mars, a planet which seemed to be very similar to the Earth. According to him, Mars had nothing special and it was not worth mentioning it. Eventually, Fontenelle offered to the reader a very broad plurality of living worlds and its merit is to have been the first to popularize in a pleasant style the idea of diversity of life in the universe. At the very end of this century, the Dutch physicist, mathematician, and astronomer Christiaan Huygens (1629–1695), who discovered the largest moon of ▶ Saturn (▶ Titan), worked on a philosophical treatise entitled Cosmotheoros (or Treatise on the Plurality of Worlds), posthumously published in 1698. This essay dealt with the consequences of the Copernican system and proposed a model for the construction of the universe. Every star was considered as a world with other possible forms of life. ▶ Huygens speculated on the habitability of the solar system and thought that the other planets were the abode of creatures quite similar to those living on Earth. The differences were supposed to come, as for Fontenelle, from the more or less great distance from the Sun and its influence on planetary solar heating. One of the most significant centuries about the question of the habitability of the solar system was the nineteenth century. Developments in planetary
Life in the Solar System (History)
observation, especially the first publications of maps of the Martian surface, built a new scientific field named areography (or Martian geography). The surface of Mars was supposed to be covered by oceans and continents (the darker regions were assumed to be seas and the lighter parts continents). This planetary world, which seemed to be very similar to ours, appeared to be a very good candidate to be the abode of life. The British astronomer Richard A. Proctor (1837– 1888), famous for having produced one of the earliest maps of Mars in 1867, wrote many popular books. Among them, Other Worlds Than Ours, The Plurality of Worlds Studied Under The Light of Recent Scientific Researches, published for the first time in 1870, immediately attracted attention not only of the scientific world but also of a very wide audience. Proctor used a poetical description to show what astronomy taught us about the Sun and its planets and discussed the probability that other worlds could be inhabited. According to him, habitability would be the key argument able to answer the question of the possible life forms on other planets. Habitability could be defined in considering analogies with the Earth, i.e., parameters resembling those existing upon our planet. Proctor studied the habitability of every planet of the solar system. He suggested that the existence of organized forms of life depended on the conditions supposed to have an effect on the planetary surface, such as climate, seasons, atmosphere, geology, and gravity. For instance, the physical conditions of Venus seemed to show very close resemblances to the terrestrial ones. Arguments coming from analogy allowed him to conclude that this planet could be inhabited. But it clearly appeared that the best candidate to be the abode of life was ▶ Mars, “the miniature of our Earth” said Proctor. With seasons equivalent to terrestrial ones, water vapor in the atmosphere, and forms of vegetation growing abundantly (these last points were supposed to be possible), Proctor’s Martian world was perfectly fitted for complex life. Proctor admitted also life on ▶ Jupiter. This giant planet might be inhabited by “the most favored races existing throughout the whole range of the solar system,” thanks to the very symmetry and perfection of the Jupiterian system. He attempted to imagine what kind of forms of life could populate this huge planet. However Proctor wanted to stay under the control of exact knowledge. He thought we could only claim that “the beings of other worlds are very different from any we are acquainted with, without endeavoring to give shape and form to fancies that have no foundation in fact.”
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Numerous writings dealing with the habitability of ▶ Mars were proposed during this century, especially after the canals controversy launched in the 1880s following Schiaparelli’s observations. Among the most prolific writers on that topic, the French astronomer Camille Flammarion (1842–1925), who founded the Juvisy Observatory (France) in 1883 and the Socie´te´ Astronomique de France (SAF) in 1887, was well-known for his Pluralite´ des Mondes habite´s (Plurality of Inhabited Worlds) published in 1862, when he was only 20 years old. This book, translated in many languages, explained the conditions of habitability of earthlike celestial bodies discussed from the astronomical, biological, and philosophical viewpoint. A comparative study of the planets of our solar system led him to state that “the Earth was, considering its physical characteristics, a planet of medium kind, without anything remarkable.” Following this idea, life would have been present everywhere in the solar system. He studied the habitability of the planet Mars while the Martian canals controversy was still running. He published The planet Mars and its conditions of habitability (first tome in 1892 – second in 1909) in which he presented the whole collected observations about the surface of Mars and its possibilities to be the abode of life. According to Flammarion, the planet Mars was a “living world” in which the climate and the supposed surface characteristics (seas, canals, continents) offered enough analogies with our planet to authorize the existence of living species not so different from the terrestrial ones. He was very concerned by the canals hypothesis but remained cautious as regards its interpretation. As a final point, ▶ spectroscopic methods were developed by astronomers such as Janssen (1824–1907) or Huggins (1824–1910) during the end of the nineteenth century and gave new parameters related to habitability and led to reconsider the question of life elsewhere in the solar system.
See also ▶ Habitability of the Solar System ▶ Jupiter ▶ Mars ▶ Saturn ▶ Titan
References and Further Reading Dick SJ (1982) Plurality of Worlds – the origins of the extraterrestrial life debate from Democritus to Kant. Cambridge University Press, Cambridge, UK Dick SJ (1996) The biological universe – the twenthieth century extraterrestrial life debate and the limits of science. Cambridge University Press, Cambridge, UK
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Ligand
Ligand Definition In chemistry, a ligand is an ion or molecule that binds to a central metal atom forming what is called a coordination complex, which can be neutral, cationic, or anionic. The complex, along with its counter ions, is called a coordination compound. The bonding between metal and ligand generally involves donation of one or more of the ligand’s electron pairs. The strength of metal–ligand bonding can range from covalent to ionic. Ligands in an ▶ organometallic complex often govern the reactivity of the central metal atom. Ligands that are directly bonded to the metal form the first coordination sphere and are known as “inner sphere” ligands. “Outer-sphere” ligands are not directly attached to the metal, but are bonded to the first coordination shell. Heme is an example of a biological coordination complex: the central iron ion is coordinated at the center of a ▶ porphyrin molecule by the latter’s four pyrrole nitrogen atoms.
Light Variation ▶ Lightcurve (Planetary Science)
Light Year ▶ Light-Year
Lightcurve (Planetary Science) MARIA ANTONIETTA BARUCCI LESIA, Observatoire de Paris, Meudon Principal Cedex, France
Synonyms Light variation
Definition See also ▶ Organometallic ▶ Porphyrin
Ligase Definition In biochemistry, a ligase is a ▶ catalyst, which could be a protein ▶ enzyme or ▶ ribozyme, which joins together two molecules, for example, ▶ peptides or ▶ nucleic acids. Important examples of this in molecular biology include DNA ligase and self-splicing introns. Ribozymes may have been evolved to carry out this reaction as it is considered a potentially important means of constructing long RNA molecules before protein catalysts were available evolutionarily.
See also ▶ Intron ▶ RNA World
The lightcurve describes the temporal variation of the brightness of an object.
Overview The lightcurve shows the temporal variation of the brightness of a celestial body at a given wavelength. When due to the rotation of an object, the lightcurve generally has a sinusoidal periodic behavior with two maxima and two minima. In planetary science, the lightcurve is the most useful tool for investigating the rotational properties and the shape of a body, mainly of small ones: ▶ asteroids, satellites, ▶ comets, and ▶ transneptunian objects. Most small bodies cannot be resolved since the apparent angular size of the object is in general smaller than the observer’s angular resolution; therefore, analysis of the lightcurve can be used to determine the shape and the surface spots of the observed body. For an airless body each point of the lightcurve represents the fraction of the sunlight reflected by the illuminated portion of the body surface in the direction of the observer. The received flux depends on physical (shape, albedo, surface morphology) and geometrical (rotational phase, polar axis orientation) characteristics of the body. All this information can be extracted from a lightcurve by using a well established inversion technique (Kaasalainen et al. 2002).
Limb Darkening
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The difference between the maximum and the minimum brightness is called the amplitude of the lightcurve and it is measured in magnitudes. The amplitude, however, varies considerably depending on the aspect angle at which the object is observed. It will have, for a given object, the biggest value for an equatorial view and it will be flat (lightcurve with no variation) when the object is observed on a polar view (rotational axis towards the observer). The amplitude of a lightcurve is an indicator of a body’s elongation and allows the computation of the axis ratios of the osculating ellipsoids approximating the shape of the observed object. The light variations can also be due to the presence of a satellite around the primary body, as well as to complicated rotations, as in the case of tumbling asteroids or active comets. Moreover, seasonal variation of a lightcurve can be produced by activity depending on the heliocentric distance of the observed body. For example, from ▶ Pluto’s lightcurve, strong evidence for albedo changes over time has been inferred, which are interpreted as a systematic sublimation of frosts from the sunward pole (seasonal variation).
Definition
See also
Definition
▶ Asteroid ▶ Comet ▶ Comet (Nucleus) ▶ Kuiper Belt ▶ Pluto ▶ Trans-Neptunian Object
In astronomy the limb of an object such as the Sun, a planet, or a satellite is the object’s apparent edge as seen against the dark sky background. Thus, solar “limb darkening” refers to the fact that the Sun is less bright (because of radiative transfer effects) toward its visible edge than toward its center. The limb may be compared to the “terminator.” For a planet or satellite that is not observed at zero phase angle (i.e., at full phase), the terminator is the locus of points on the object where the line to the Sun (or other source of illumination) is tangent; it thus separates the day side from the night side.
References and Further Reading Kaasalainen M, Mottola S, Fulchignoni M (2002) Asteroid models from disk-integrated data. In: Bottke WF, Cellino A, Paolicchi P, Binzel RP (eds) Asteroids III. University of Arizona Press, Tucson, pp 139–150
The light-year is a unit of length used in astronomy: it corresponds to the distance traveled by light during 1 year in vacuum. A light-year (symbol: ly) is equal to 9.461 1015 m. It tends to be less and less used by professional astronomers who prefer the ▶ parsec (3.26 ly) and its derived units (kpc, Mpc), because they are more directly linked to observation.
See also ▶ AU ▶ Parallax ▶ Parsec
Limb (Astronomical) Synonyms Terminator
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Lightyear ▶ Light-Year
Limb Darkening Definition
Light-Year Synonyms Lightyear; Light year
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The limb darkening phenomenon is a decrease in brightness at the edge (the limb) of the surface of a star or planet. The effect is best studied on the Sun, but has also been put in evidence on other stars. It gives to an image of the Sun the appearance of a sphere illuminated by the observer with a gradual shading on the edge (see Fig. 1). For a star,
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Limestone
planets by the ▶ transit method and must be taken into account to derive the proper planet’s size (see Fig. 2).
See also ▶ Photosphere ▶ Radiative Transfer ▶ Stars ▶ Transit
Limestone ▶ Carbonate
Limonite, Needle Ironstone Limb Darkening. Figure 1 The limb darkening effect is clearly seen on the Sun: the surface brightness decreases toward the outer part of disk
▶ Goethite
Lindblad Resonance Definition Lindblad resonances are the source of spiral density waves in disks. For the case of a planet orbiting a star within a ▶ protoplanetary disk, Lindblad resonances are found at the location where the disk’s natural epicyclic frequency (the frequency at which a radially displaced fluid parcel will oscillate) is a commensurate multiple of the planet’s orbital period. This is the same case as for ▶ mean motion resonances, but altered by pressure support in the gaseous component of the disk. Density waves triggered at inner and outer Lindblad resonances act to torque the planet’s orbit and lead to (type 1) orbital migration for planets with masses between roughly 1 and 50 Earth masses. Limb Darkening. Figure 2 The limb darkening effect modifies the depth of the light decrease observed when a planet transits in front of the disk of a star. The effect depends on the precise trajectory of the planet across the disk, as illustrated by the two cases
it occurs because the light at the edge comes on average from cooler – and thus less bright – layers of the ▶ photosphere. Jupiter also exhibits a limb darkening effect. The limb darkening effect is actually observed when detecting
See also ▶ Corotation Torque ▶ Planetary Migration ▶ Protoplanetary Disk
Line-of-sight Velocity ▶ Radial Velocity
Linear Energy Transfer
Line of Sight
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Linea, Lineae
Definition
Definition
The line of sight is the imaginary straight line between an observer and the celestial object he is pointing at.
A linea is a bright or dark curved or linear surface marking with or without topographic expression. On the ▶ terrestrial planets, lineae primarily occur on ▶ Venus but the majority of features termed lineae are observed on ▶ Jupiter’s ▶ satellite ▶ Europa. Seen at high resolution, these lineae are ridges – either single, double, or complex – which were created by tectonism and/or ▶ cryovolcanism. Lineae on Venus and Europa can extend over several thousands of kilometers.
Line Profile Synonyms Line shape
Definition The line profile for a ▶ spectral line is a representation of its spectral intensity as a function of frequency or wavelength. The line may appear in emission or absorption toward an astronomical source. Various physical processes can act to modify the natural line profile, such as Doppler, turbulent, or collisional broadening.
See also ▶ Albedo Feature ▶ Cryovolcanism ▶ Europa ▶ Jupiter ▶ Satellite or Moon ▶ Terrestrial Planet ▶ Venus
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See also ▶ Doppler Shift ▶ Spectral Line
Linear Energy Transfer Synonyms
Line Shape ▶ Line Profile
Line Shielding Definition Line shielding refers to the situation in which molecules whose photo-destruction is controlled by absorption of radiation in spectral lines can have their destruction rates greatly reduced in interstellar clouds. This arises when there is sufficient material between the source of UV radiation and the cloud interior such that molecules in the exterior layers of the cloud preferentially absorb the dissociating photons, thus leading to molecules in the cloud interior experiencing a greatly attenuated photon flux. Line shielding is important for the survival of H2 in ▶ molecular clouds near energetic astrophysical sources.
See also ▶ Molecular Cloud
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LET; Stopping power
Definition During its passage through matter, a heavy charged particle loses its kinetic energy via a sequence of small energy transfers to atomic electrons in the medium. Most energy is deposited in a narrow region around the particle’s trajectory, the so-called particle track.The energy loss per unit length of particle track is termed the stopping power in nuclear physics and linear energy transfer (LET) in ▶ radiation biology. LET is expressed in units of keV/mm. Heavy charged particles are referred as “high LET radiation,” while x-rays, gamma-rays, and fast electrons are known as “low LET radiation.”
History The dependence of LET on the energy and electric charge of the flying particle was developed by H. A. Bethe and F. Bloch in the 1930s (Bethe–Bloch equation).
See also ▶ Biostack ▶ Cosmic Ray Ionization Rate
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Linewidth
▶ Cosmic Rays in the Heliosphere ▶ DNA Damage ▶ HZE Particle ▶ Ionizing Radiation (Biological Effects) ▶ Radiation Biology
broadening is due to chaotic movements (turbulence, thermal motion) in the emitting medium. Note that the spectrograph used to measure a spectral line must have a spectral resolution better than the linewidth to derive a proper value. Figure 1 illustrates the linewidth of a strong hydrogen line (Ha) seen in the solar spectrum.
See also
Linewidth
▶ Spectral Line ▶ Spectrometer ▶ Spectroscopy
Definition The expression linewidth designates the intrinsic width of a spectral line expressed in units of wavelength (A˚, nm, mm, etc.) or frequency (Hz). Generally, it is the full width at half maximum (FWHM) that is used. The linewidth can refer to a line in absorption or emission. The width of a line results in general from the contributions of several phenomena: for instance, the natural linewidth is linked to the lifetime of the transition, while the Doppler
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Lingula, Lingulae Definition A lingula is an extension of a plateau having rounded lobate or tongue-like boundaries (definition by the International
Solar atlas (after Delbouille et al., 1972, 1981)
Normalized intensity
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Linewidth. Figure 1 The linewidth of the hydrogen Ha line seen in absorption in the spectrum of the Sun at l = 656.6 nm. The arrows mark the width adopted here (FWHM). One notes here that the pedestal of the line (top, or bottom in the case of an emission line) can be much larger than the linewidth
Liquid Chromatography-Mass Spectrometry
Astronomical Union; http://planetarynames.wr.usgs.gov/ jsp/append5.jsp). Lingula is used as a descriptor term for naming surface features on ▶ Mars.
See also ▶ Mars
Linkage Map ▶ Genetic Map
Lipid ▶ Amphiphile
Lipid Bilayer DAVID DEAMER Department of Biomolecular Engineering, University of California, Santa Cruz, CA, USA
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nonpolar interior composed of hydrocarbon chains, lipid bilayers are relatively impermeable to simple ions or ionic compounds in solution. This allows them to act as ▶ permeability barriers, which in turn permits ion gradients to be produced and maintained by ion transport enzymes. Lipid bilayers are surprisingly stable, even though the amphiphilic molecules are not held together by covalent bonds. Instead, the associations between lipid molecules are stabilized by the hydrophobic effect that tends to exclude hydrocarbon chains from bulk water. This stability allows lipids to assemble into microscopic compartments also called lipid vesicles, or sometimes liposomes. The relevance to the origin of life is that self-assembling amphiphilic molecules were likely to be components of the mixture of organic compounds present in the early Earth environment and could therefore provide membranous compartments required for the first forms of cellular life.
See also ▶ Amphiphile ▶ Cell Membrane ▶ Permeability ▶ Self Assembly
References and Further Reading Peter Walde (ed) (2005) Prebiotic chemistry: from simple amphiphiles to protocell models. In: Current topics in chemistry, Vol 259. Springer, Berlin
Synonyms Membrane
Keywords Hydrophobic effect, permeability barrier, self-assembly
Liquid Chromatography-Mass Spectrometry
Definition
SHOHEI OHARA Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
Lipid bilayers are the primary structural components of all biological membranes, serving as a boundary between the cytoplasm and external environment.
Synonyms LC-MS
Overview Self-assembled structures of amphiphilic molecules occur as micelles, monolayers, and bilayers. Monolayers form when amphiphilic molecules accumulate as a monomolecular film at the air–water interface. In contrast, lipid bilayers assemble in bulk aqueous phases, consisting of two monolayers associated in such a way that the nonpolar hydrocarbon chains are directed inward, with hydrophilic polar or ionic groups on the outer surface. Because of their
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Keywords Atmospheric pressure chemical ionization, electrospray ionization, liquid chromatography, mass spectrometry
Definition An analytical technique that combines liquid chromatography (LC) as a separation technique with mass spectrometry (MS) as a detection technique.
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Liquidus
Overview
References and Further Reading
Liquid chromatography (LC) is an analytical technique that exploits the fact that different molecules partition to differing extents between a liquid mobile phase and solid stationary phase. LC is used to separate a very wide range of polar and nonpolar compounds, including low-molecular-weight organic and inorganic compounds to high-molecular-weight proteins and nucleic acids. The use of mass spectrometric detection after a chromatographic separation is a particularly powerful combination in the case of the analysis of complex mixtures where many compounds co-elute and molecule-specific discrimination is required for unambiguous detection and quantification. Mass spectrometry requires the ionization of the eluting molecules (either positively or negatively charged) followed by the acceleration and separation of the subsequently formed ions according to their dynamic response to either a fixed (magnetic sector) or oscillatory (quadrupole) magnetic field or simply as a function of time (timeof-flight) subject to the ionic mass-to-charge (m/z) ratio. One of the greatest challenges in interfacing LC with MS was how to introduce the liquid chromatographic eluant into the low vacuum environment required for mass spectrometry. The introduction of the atmospheric pressure ionization (API (Huang et al. 1990)) technique greatly expanded the number of compounds that can be analyzed by LC/MS. In API, aerosol droplets are ionized at the tip of a nebulizer at atmospheric pressure. The carrier liquid is stripped from the droplets by N2 gas during their passage from the nebulizer tip to the extraction cone of the MS during which time the charge is transferred to the eluting molecule. Common API techniques include electrospray ionization (ESI (Fenn et al. 1989)) and atmospheric pressure chemical ionization (APCI (Horning et al. 1974)). ESI is especially useful for analyzing large biomolecules such as proteins, peptides, and oligonucleotides, where the use of time-of-flight mass detectors allows for the analysis of high MW molecules. However, if tandem MS is desired (i.e., MS/MS), at least one of the mass detectors has to be a quadrupole. During ESI, large molecules often acquire more than one charge, thus LC/MS with quadrupole mass analyzers can analyze molecules as large as 150,000 amu even though a quadrupole mass detector typically has an m/z upper limit on the order of 3,000 (e.g., 150,000 amu / 50 z = 3,000 m/z). APCI is applicable to a wide range of polar and nonpolar molecules with MW lower than 1,500 amu.
Fenn JB, Mann M, Meng CK, Wong SE, Whitehous CM (1989) Electrospray ionization for mass spectrometry of large biomolecules. Science 246:64–71 Huang EC, Wachs T, Conboy JJ, Henion JD (1990) Atmospheric pressure ionization mass spectrometry: detection for the separation sciences. Anal Chem 62:713A–725A Horning EC, Carroll DI, Dzidic I, Haegele KD, Horning MG, Stillwell RN (1974) Liquid chromatograph-mass spectrometercomputer analytical systems: a continuous-flow system based on atmospheric pressure ionization mass spectrometry. J Chromatogr 99:13–21
See also ▶ HPLC
Liquidus Synonyms Liquidus temperature
Definition In chemistry, the liquidus is the highest temperature at which crystals can exist in a melt at thermodynamic equilibrium. Above the liquidus temperature the material is homogenous, below it two phases exist as crystals begin to form. Below the liquidus temperature homogeneous glasses may exist via rapid cooling and kinetic inhibition of crystallization.
See also ▶ Solidus
Liquidus Temperature ▶ Liquidus
Lithium Absorption Definition The presence of the element lithium acts as a chronometer for stars and brown dwarfs. Objects of solar mass and below which have lithium in their atmospheres must be relatively young, since the element is eventually destroyed by fusing with protons. The presence of the lithium absorption line at 6,708 A˚ has often been used to confirm
Lithosphere (Planetary)
the youth of suspected pre-main-sequence stars. Very lowmass stars and brown dwarfs evolve so slowly and are cold enough that their lithium may be intact even at a substantial age. Thus, the appearance of lithium below a certain mass has yielded the ages of ▶ stellar clusters such as the Pleiades.
See also ▶ Pre-Main-Sequence Star ▶ Stellar Cluster ▶ T Association ▶ T Tauri Star
Lithopanspermia Definition The term Lithopanspermia describes a scenario of interplanetary transport of microorganism by use of ▶ meteorites. It involves three basic steps: (i) the escape process, that is, removal to space of biological material, which has survived being lifted from surface to high altitudes by impact ejection; (ii) interim state in space, that is, ▶ survival of the biological material over time scales comparable with interplanetary or interstellar passage; (iii) the entry process, that is, nondestructive deposition of the biological material on another planet. All three steps of lithopanspermia are now accessible to experimental testing in ground simulation facilities as well as in exposure experiments in space.
History The possibility of meteorite-mediated interplanetary transport of life was already considered in the 1870s by von Helmholtz and Thomson (Lord Kelvin), who favored a version of ▶ Panspermia in which fragments of extraterrestrial rocks carrying microbes as blind passengers within their cracks, may transport life from one planet to the other. However, during their lifetime, no mechanism was known to accelerate rocks to escape velocities in order to leave their planet of origin; as a result Kelvin’s idea was discarded. After the discovery in 1983 that a certain group of meteorites (SNC meteorites) has originated from ▶ Mars, evidence was provided that rocks can naturally be transferred between ▶ terrestrial planets in conditions that could maintain trapped viable life systems, and as a consequence, Kelvin’s idea, now called Lithopanspermia, has been revisited.
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See also ▶ Crater, Impact ▶ Ejecta ▶ Exposure Facilities ▶ Mars ▶ Meteorites ▶ Panspermia ▶ Planetary and Space Simulation Facilities ▶ Spallation Zone ▶ STONE ▶ Survival ▶ Terrestrial Planet
Lithophile Elements Definition In the Berzelius-Goldschmidt classification, lithophile elements are those concentrated in the rocky part (lithos) of the terrestrial planets. They bond with oxygen in tetrahedral sites (Si, Al, P) or in octahedral and dodecahedral sites. These elements form either silicates and oxides (Ca, Mg, Fe, Al, Ti, Na, K) or solid solutions in these minerals. Among the metals, the alkali (Li, Na, K, Rb, Cs) and alkali-earth elements (Mg, Ca, Sr, Ba), the rare-earth elements and the actinides (U, Th), and the high fieldstrength elements (Ti, Zr, Hf, V, Nb, Ta) are broadly used in geochemistry. Halogens (F, Cl, Br, I) also have a lithophile behavior.
See also ▶ Chalcophile Elements ▶ Siderophile Elements
Lithosphere ▶ Earth, Formation and Early Evolution
Lithosphere (Planetary) Definition The lithosphere is the cold, rigid outermost layer of the Earth and other planets. The Earth’s oceanic lithosphere comprises an upper layer of mainly basaltic ▶ crust and a lower layer of ▶ mantle peridotite whose thickness
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Lithospheric Plate
increases from a few kilometers at mid-ocean ridges to about 100 km beneath the oldest parts of the oceanic crust. Continental lithosphere comprises the ▶ continental crust and a mantle layer whose thickness varies from tens of kilometers beneath active rifts to several hundred kilometers beneath old cratons. Heat transfer is conductive through the lithosphere and convective in the underlying ▶ asthenosphere. Martian lithosphere is thicker, more than 120 km, beneath Olympus Mons and other volcanic complexes. The shallow lithosphere of ▶ Venus consists of a region of brittle flow in the crust to a depth of 2–4 km, underlain by a weak lower crust and brittle to ductile upper mantle.
See also ▶ Asthenosphere ▶ Continental Crust ▶ Craton ▶ Crust ▶ Mantle ▶ Plate Tectonics
Lithospheric Plate ▶ Continental Plate
Lithotroph CHARLES S. COCKELL Geomicrobiology Research Group, PSSRI, Open University, Milton Keynes, UK
Keywords Inorganic compounds, electron donor, extreme environments, lithic habitats
Definition Lithotrophs are microorganisms that use inorganic compounds as electron donors to conserve energy for growth.
Overview A lithotroph is a microorganism that uses inorganic substrates as a source of electron donors to drive energy acquisition, using either organic carbon or carbon dioxide as a source of carbon for constructing cellular materials
(Ehrlich and Newman 2008). Microorganisms oxidize the electron donors to generate electrons that are channeled into electron respiratory chains to produce the energystoring molecule, ▶ ATP. These organisms can use a variety of electron acceptors to complete the respiratory process, including oxygen, sulfate, and other compounds. Lithotroph means rock (lithos) eater (troph) and representatives are found in both the ▶ Bacterial and ▶ Archaeal domains. No multicellular organisms are currently known that are able to use inorganic compounds as an ▶ energy source, although they can gain energy from symbioses with lithotrophs. Examples of lithotrophs include iron-oxidizing ▶ bacteria that metabolize reduced iron to oxidized iron, purple sulfur bacteria that transform sulfide into sulfur, nitrifying bacteria that use ammonia and convert it into nitrite or use nitrite to produce nitrate, hydrogen bacteria that oxidize hydrogen to water, and a range of sulfur bacteria that use oxidized sulfur compounds to produce sulfide (Widdel et al. 1993; Raven 2009). ▶ Methanogens are also lithotrophs and use hydrogen to reduce carbon dioxide to methane. The use of inorganic compounds in this way makes lithotrophs important in a wide variety of geological processes including the biogeochemical cycling of carbon, nitrogen, and sulfur, and the weathering of rocks both on the surface of the Earth and in the deep ocean crust, which releases nutrients into the ▶ biosphere and is responsible for drawing down carbon dioxide from the atmosphere (influencing long-term climate control) (Bach and Edwards 2003). Lithotrophs are not exclusively associated with extreme environments, but in physical and chemical extremes they tend to dominate. For example, Thiobacillus species, which are iron-oxidizing bacteria, are found in highly acidic environments where reduced iron is stable. Their activity in the environment contributes to the wellknown phenomenon of Acid Mine Drainage (AMD) (Baker and Banfield 2003), whereby oxidation of iron in ▶ pyrite and its further catalytic effect on pyrite oxidation produces sulfuric acid, generating extremely acidic environments in which the organisms thrive. Methanogens are found in the deep subsurface, where hydrogen that is generated by water–rock reactions in ultramafic settings is used as the electron donor (Lollar et al. 2006). On account of their ability to grow in extreme environments, lithotrophs are of great interest to astrobiologists since their ability to use inorganic compounds from rocks, or the products of chemical reactions involving rocks, makes their metabolisms potential analogues for metabolic processes that could sustain life in other planetary crusts (Onstott et al. 2006) and they provide insights into potential lithotrophic conversions on the early Earth
Long Duration Exposure Facility
(Canfield et al. 2006; Sleep and Bird 2007; Cockell 2010). Lithotrophs can be divided into a number of subgroups. Photolithotrophs use light to gain energy, the inorganic electron donors being used only in biological synthesis reactions. Photolithotrophs are common in anaerobic environments and include the green bacteria (e.g., Chlorobiacaea) and purple bacteria (e.g., Chromatiacaea), which use sulfide, sulfur, hydrogen, and iron as electron donors. The oxygenic cyanobacteria are also photolithotrophs. Lithoheterotrophs gain their energy from inorganic compounds but use organic matter or other organisms as a source of carbon. Lithoautotrophs use carbon dioxide as a source of carbon and mixotrophs are capable of gaining carbon either from carbon dioxide or from organic carbon.
See also ▶ Chemolithoautotroph ▶ Chemolithotroph ▶ Energy Sources ▶ Iron Cycle
References and Further Reading Bach W, Edwards KJ (2003) Iron and sulphide oxidation within the basaltic ocean crust: implications for chemolithoautotrophic microbial biomass production. Geochim Cosmochim Acta 67:3871–3887 Baker BJ, Banfield JF (2003) Microbial communities in acid mine drainage. FEMS Microbiol Ecol 44:139–152 Canfield DE, Rosing MT, Bjerrum C (2006) Early anaerobic metabolisms. Phil Trans R Soc 361:1819–1836 Cockell CS (2010) Life in the lithosphere, kinetics, and the prospects of life elsewhere. Phil Trans R Soc (in press) Ehrlich HL, Newman DK (2008) Geomicrobiology. CRC Press, Boca Raton Lollar BS, Lacrampe-Couloume G, Slater GF, Ward J, Moser DP, Gihring TM, Lin LH, Onstoitt TC (2006) Unravelling abiogenic and biogenic sources of methane in the Earth’s deep subsurface. Chem Geol 226:328–339 Onstott TC, McGown D, Kessler J, Lollar BS, Lehmann KK, Clifford SM (2006) Martian CH4: sources, flux and detection. Astrobiology 6:377–395 Raven JA (2009) Contributions of anoxygenic and oxygenic phototrophy and chemolithotrophy to carbon and oxygen fluxes in aquatic environments. Aquatic Microb Ecol 56:177–192 Sleep NH, Bird DK (2007) Niches of the pre-photosynthetic biosphere and geologic preservation of Earth’s earliest ecology. Geobiology 5:101–107 Widdel F, Schnell S, Heising S, Ehrenreich A, Assmus B, Schink B (1993) Ferrous iron oxidation by anoxygenic phototrophic bacteria. Nature 362:834–836
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Living Stromatolites ▶ Microbial Mats ▶ Stromatolites
LMT ▶ Large Millimeter Telescope
Local Restframe ▶ Local Standard of Rest
Local Standard of Rest Synonyms Local restframe
Definition In astronomy, the local standard of rest designates the coordinate frame of reference linked to the center of mass of stars belonging to the neighborhood of the Sun. This frame rotates with the Milky Way at about 220 km/s with respect to the Galactic Center.
Long Duration Exposure Facility GERDA HORNECK German Aerospace Center (DLR), Institute of Aerospace Medicine, Cologne, Germany
Synonyms LDEF
Keywords
Lithotrophy ▶ Chemolithotroph ▶ Chemolithotrophy
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Cosmic rays, long-term survival in space, material testing, space parameters
Definition The Long Duration Exposure Facility (LDEF) was a large nearly cylindrical structure designed to provide long-term
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Long Duration Exposure Facility
data on the space environment and its effects on space systems and operations. It was transported to space by the Space Shuttle in 1984 and remained in Earth orbit for nearly 6 years until it was retrieved by the Space Shuttle in 1990.
Overview The Long Duration Exposure Facility (LDEF) was a large open-grid cylindrical structure, on which a series of rectangular trays used for mounting experiment hardware were attached. LDEF was launched and deployed in Earth orbit by the Shuttle Challenger on April 7, 1984, where it stayed in a nearly circular orbit at an altitude of about 500 km and an inclination of 28.4 (Fig. 1). LDEF was attitude controlled, i.e., oriented with respect to a defined frame of reference, with one end always facing Earth, the other one always facing space. This was achieved by gravity gradient and inertial distribution to maintain three-axis stability in orbit. Propulsion or other attitude control systems were not required, and LDEF was free of acceleration forces and contaminants from jet
firings. LDEF was retrieved by the Shuttle Columbia on January 11, 1990. LDEF accommodated 57 experiments, including the following three biological experiments: ● Space Expose Experiment Developed for Students (SEEDS), which carried 12.5-million tomato seeds that after retrieval were offered to students to evaluate their survivability after storage in the space environment, and to determine possible mutants and changes in the mutation rate (Hammond et al. 1996). ● Free Flyer Biostack experiment (Fig. 1), which consisted of several ▶ Biostack units to study the biological effectiveness of HZE particles (particles of high atomic number Z and high energy) of cosmic rays (Wiegel et al. 1995; Mei et al. 1994; Horneck 1994; Zimmermann et al. 1996). ● Exostack experiment accommodated in two Biostack units facing space, which were open to space (covered by a perforated dome only) to study the long-term ▶ survival of ▶ spores of Bacillus subtilis in outer space. If shielded from the influx of solar extraterrestrial ▶ UV radiation and embedded in 5% sugar as a chemical protecting substance, 70–90% of the spores survived the nearly 6-year exposure to outer space on board LDEF (Horneck et al. 1994; Horneck 1998). With LDEF, the longest exposure of microorganisms to space has been achieved so far.
See also ▶ Biostack ▶ Cosmic Rays in the Heliosphere ▶ Exposure Facilities ▶ HZE Particle ▶ Space Vacuum Effects ▶ Spore ▶ Survival
References and Further Reading
Long Duration Exposure Facility. Figure 1 LDEF structure before release from the arm of the Space Shuttle. Biostack units are accommodated on the space pointing end (upper right corner) of LDEF
Hammond EC Jr, Bridgers K, Berry FD (1996) Germination, growth rates, and electron microscope analysis of tomato seeds flown on the LDEF. Radiat Meas 26:851–861 Horneck G (1994) HZE particle effects in space. Acta Astronaut 32:749–755 Horneck G (1998) Exobiological experiments in Earth orbit. Adv Space Res 22(3):317–326 Horneck G, Bu¨cker H, Reitz G (1994) Long-term survival of bacterial spores in space. Adv Space Res 14(10):41–45 Mei M, Qiu Y, He Y, Bu¨cker H, Yang CH (1994) Mutational effects of space flight on Zea mays seeds. Adv Space Res 14(10):33–39 Wiegel B, Minguet J, Schneider C, Rusch G, Heinrich W, Bu¨cker H, Reitz G (1995) Measurements of cosmic ray nuclei with energies of
Lys some hundred MeV/nucleon in the LDEF mission. Adv Space Res 15(1):53–56 Zimmermann MW, Gartenbach KE, Kranz AR, Baican B, Schopper E, Heilmann C, Reitz G (1996) Recent results of comparative radiobiological experiments with short and long term expositions of Arabidopsis seed embryos. Adv Space Res 18(12):205–213
Long Wavelength Astronomy ▶ Radio Astronomy ▶ Wavelength
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Lunar Cataclysm ▶ Late Heavy Bombardment
Lunar Geology ▶ Selenology
Ly a Lowest Taxonomic Category
▶ Lyman Alpha
▶ Species
Lyman Alpha LUCA ▶ Last Universal Common Ancestor
Luminosity Definition The luminosity of a celestial object is the total electromagnetic power it emits; this depends in most cases on the object’s surface temperature and surface area. Symbol: L; unit: W, or Solar Luminosity (3.839 1026 W). The luminosity can sometimes be defined for a restricted spectral domain: For x-rays, visible, infrared, radio, etc., one uses then the symbols LX, Lvis, LIR, Lradio, respectively.
See also ▶ Bolometric Magnitude ▶ Effective Temperature
Luminosity-Temperature Diagram ▶ Hertzsprung–Russell Diagram
Synonyms Ly a
Definition In the hydrogen atom, the Lyman series of lines is emitted or absorbed during transitions between an excited state and the fundamental (ground) state. Lyman alpha is the first of these lines and connects the electronic levels n = 2 and n = 1, where n is the principal quantum number. Most of stellar matter is hydrogen, hence Lyman alpha is a very prominent line in stellar spectra. At a wavelength of 121.5 nm, Lyman alpha photons are in the VUV (vacuum ultraviolet) domain and play an important role in the dissociation of interstellar or cometary molecules.
History These lines were discovered in 1906 by the American physicist Theodore Lyman.
See also ▶ Comet ▶ Interstellar Medium
Lys ▶ Lysine
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Lysine Synonyms Lys
Definition Lysine is one of the 20 coded protein amino acids; its structure is NH2CH(CH2CH2CH2CH2NH2)COOH. Its molecular weight is 146.19. Its three-letter symbol is Lys,
and the one-letter symbol is K (L is used for leucine). Lysine is a basic ▶ amino acid (pI is 9.74), as are ▶ histidine and ▶ arginine, but only lysine is a diamino acid (amino acid with two amino groups and one carboxylic group). Diaminoacids have been suggested to be key molecules for the abiotic construction of peptide ▶ nucleic acids (PNAs). Lysine and ▶ ornithine (with the side-chain CH2CH2CH2NH2) have not been detected in carbonaceous chondrites, though diaminobutyric acid ( CH2CH2NH2) and diaminopropionic acid ( CH2NH2) have been.
M Ma Synonyms Mega annum; Megayear; Myr
Definition Ma is a common scientific abbreviation for Megayears = 1 million (106) years derived from the Latin Mega-annum. Note that the Latin accusative, annum, expresses an absolute age (e.g., the Earth is 4,560 Ma old), while the English accusative, years, expresses a period of time (e.g., the Archean eon lasted for 2,000 Myrs). Ma is the most common unit of time used to describe events through most of the history of the Earth, the other planets, and meteorites.
See also ▶ Earth, Age of ▶ Ga ▶ Geochronology ▶ Geological Timescale
Macronutrient Synonyms Culture media; Nutrients
Definition Macronutrients are nutrients required in large amounts for cell growth. All cells require carbon. On a dry basis, a typical cell is about 50% carbon, and this element is the major element in all classes of macromolecules. Some organisms are autotrophs and use CO2 as the source of carbon. The rest are heterotrophs and they use reduced organic compounds as a carbon source. Heterotrophs require autotrophs for their carbon source. The next abundant element is nitrogen. A typical nitrogen content in a prokaryotic cell is about 12%. Nitrogen is found mainly in ▶ proteins and ▶ nucleic acids.
Nitrogen is available in both organic or inorganic forms. However, the bulk of available nitrogen is in inorganic form, either as ammonia (NH4), nitrate (NO3), or diatomic nitrogen (N2). Most organisms can use ammonia or nitrate as a nitrogen source but only a few are able to fix atmospheric N2 because of the strong triple bond. Other macronutrients are phosphorus, sulfur, potassium, magnesium, calcium, sodium, and ▶ iron. Phosphorus occurs in nature in the form of organic and inorganic phosphate and is required by the cell primarily for synthesis of nucleic acids and phospholipids. Sulfur is needed for the synthesis of two aminoacids, cysteine and methionine and is also present in vitamins and coenzyme A, an important biosynthetic cofactor. Most cellular sulfur originates from reduced (S2) or oxidized sources (SO42). All organisms require potassium. Different enzymatic activities require this cation specifically. Magnesium stabilizes different cellular structures such as membranes, ribosomes, and nucleic acids. Calcium, which is not essential for cell growth, helps to stabilize cell walls, and is also fundamental for endospore heat stability. Sodium requirements are related to the ionic strength of the environment in which the organisms grow. Iron plays a fundamental role in important cellular functions including respiration.
See also ▶ Alkaliphile ▶ Compatible Solute ▶ Endospore ▶ Halophile ▶ Iron ▶ Nucleic Acids ▶ Protein
Macula, Maculae Definition A macula is a dark spot or dark area on the icy ▶ Satellites ▶ Europa (▶ Jupiter), ▶ Titan (▶ Saturn), and ▶ Triton (▶ Neptune). Maculae have circular, elliptical, elongate,
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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or irregular shapes. Various processes that can create these features are in discussion, for example, ▶ cryovolcanism (e.g., Ganesa Macula on Titan).
See also ▶ Albedo Feature ▶ Cryovolcanism ▶ Europa ▶ Facula, Faculae ▶ Jupiter ▶ Neptune ▶ Satellite or Moon ▶ Saturn ▶ Titan ▶ Triton
Magma Definition Magma is the molten or partially molten rock from which ▶ igneous rocks are derived through crystallization. It forms through partial melting of parts of the ▶ mantle or the crust. On eruption, it forms volcanic rocks; when it solidifies before eruption, it produces intrusive or plutonic rocks. Its composition ranges from ultramafic (komatiite) through mafic (basalt) to felsic (granite) and rarely to carbonatitic (carbonatite). Its temperature ranges from ca. 1,600 C in erupted komatiite to less than 700 C in water-rich intrusive granitic magma. Viscosity ranges from about 1 Pa s (pascal-second) in carbonatite (that of water) through 10–100 Pa s in basalt (roughly equivalent to the viscosity of motor oil) to 109 in rhyolitic magmas.
See also
Mafic and Felsic Synonyms Basic and acid rock
Definition The adjective mafic refers to a silicate mineral or rock (magmatic, either intrusive or extrusive) having a chemical composition rich in iron and magnesium and relatively poor in silicium. It corresponds to the older term basic as in basic rock. During crystallization of magma at high temperatures, a magmatic rock tends to concentrate firstly the elements that are easily incorporated into a crystal structure and called “compatible elements,” such as iron and magnesium. Consequently, these rocks contain ferromagnesium minerals such as olivine, pyroxenes, amphiboles, or plagioclase (▶ basalts). Pursuing crystallization of the residual magma, the produced magmatic rocks will tend to concentrate the elements that are not easily incorporated into a crystal structure, or “incompatible elements” such as silicon, aluminum, potassium, and sodium. These rocks contain quartz, muscovite, orthoclase, and sodium-rich plagioclase (▶ granites). These minerals are referred to as felsic and the rocks are generally termed as felsic, or acid rocks in the older rock classification.
See also ▶ Basalt ▶ Granite
▶ Archean Mantle ▶ Asthenosphere ▶ Cryovolcanism ▶ Igneous Rock ▶ Mafic and Felsic ▶ Mantle ▶ Plate Tectonics
Magmatic Rock ▶ Igneous Rock
Magnetic Anomaly Definition A magnetic anomaly is a local departure in orientation or strength from the large-scale pattern of a planetary magnetic field. Stripes of alternating positive and negative polarity parallel to isochrons in the Earth’s oceanic crust recorded reversals of the magnetic field and they provided the first evidence of sea-floor spreading. The pattern of variation of field direction and intensity is recorded in volcanic or stratigraphic sequences and this recording provides the basis of magnetostratigraphy. More localized anomalies provide information about the subsurface structure and composition of the Earth’s crust and are an important index for the presence of certain metallic (generally ferruginous) ore deposits.
Magnetic Field
See also ▶ Magnetic Field ▶ Magnetic Field, Planetary ▶ Magnetic Pole ▶ Mid-Ocean Ridges ▶ Paleomagnetism ▶ Plate Tectonics
Magnetic Field CLAUDE CATALA LESIA, Observatoire de Paris, Meudon Cedex, France
Synonyms Magnetic induction; Magnetism
Keywords Dynamo, magnetism, planet formation, planetary evolution, planetary magnetism, star formation, stellar evolution, stellar magnetism, activity
Definition A magnetic field is a physical field that arises from an electric charge in motion, producing a force on another moving electric charge. Magnetic fields play a major role in star and ▶ planet formation, as well as at most stages of planetary system evolution.
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Importance of Magnetic Fields for Planet Formation, Evolution, and Their Habitability Magnetic fields are believed to play a crucial role in the formation and evolution of stars and planetary systems. Stars are formed from molecular clouds in the interstellar medium, which are threaded by magnetic fields. The gas and the magnetic field are intimately coupled during star formation, the collapsing gas dragging in the magnetic field lines, while the magnetic field tends to slow down the collapse. This has a significant impact on the distribution of matter which is left behind in the circumstellar nebula (▶ protoplanetary disk) after the collapse, out of which planets form. Magnetic fields are then an essential ingredient of the coupling between protostars or young pre-main sequence stars with their surrounding accretion disks. In these early phases of stellar evolution, this coupling is responsible for the transfer of angular momentum from the star to the disk, and prevents the contracting star from spinning up to breakup velocity. We can conclude that stars could not be formed in the absence of magnetic fields, hence that magnetic fields are crucial for the emergence of life, which definitely requires the energy provided by stars (Fig. 1). Moreover, stellar magnetic fields are coupled to the star’s rotation throughout the evolution of a star. For solar-type stars, they are generated by dynamo action inside the star, whose efficiency is a strong function of rotation. Additionally, stellar magnetic fields, through their coupling with the stellar wind, enhance and control the angular momentum loss during the star’s evolution. As a result, due to its magnetic field, a star will keep spinning down during its entire life, with a related
Overview Magnetic fields are present everywhere in the Universe, with intensities ranging from a fraction of a micro-Gauss (106 G) in the intergalactic medium, to about 1 G for the Earth’s magnetic field, and up to several peta-Gauss (1015 G) in some highly magnetized compact objects, such as neutron stars. They play an important role in most physical processes at work in the Universe, and are believed to be responsible for some spectacular behaviors, such as energetic collimated jets escaping from galactic nuclei or from protostars and young stars. They also constitute an essential ingredient of star and planet formation and their subsequent evolution. The Earth’s magnetic field has played and continues to play a major role in protecting our planet from impinging hard solar radiation, and therefore has had a crucial impact on the emergence and evolution of life.
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Magnetic Field. Figure 1 A model of a pre-main sequence star, magnetically coupled to its circumstellar accretion disk. Credit Klaus Strassmeier
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decrease of its level of activity. Because the level of stellar activity and its accompanying particle bombardment can have a major impact on the appearance and evolution of life on a planet, we conclude that stellar magnetic fields can definitely not be ignored in this process. This relationship between stellar activity and life on a planet is further impacted by the eventual presence of a magnetic field on the planet itself, which can act as a shield against stellar particle bombardment.
Stellar Magnetic Fields Why Study Stellar Magnetic Fields The first stellar magnetic field, that of the Sun, was discovered in 1908 by George Hale, who measured the field inside solar dark spots using the Zeeman effect on spectral lines (see section “How to Measure Stellar Magnetic Fields”). Since then, our understanding of the solar magnetic field has considerably progressed. We know that the solar magnetic field can be approximated to first order by a dipole, and that it controls most of the active phenomena the Sun displays: Sun spots, chromospheric plages, filaments, corona, wind, flares, etc. However, the ▶ dynamo mechanism by which the solar magnetic field is generated and sustained is far from being fully understood. Understanding the solar magnetic field is one of the motivations for studying the magnetic field of other stars, which offer a wide range of physical conditions (rotation, convection, temperature, etc.) and which can therefore be used as cosmic laboratories to study stellar magnetism. It is also very important to understand stellar magnetic fields because of their strong impact on stellar formation and evolution, as described earlier. This is why powerful techniques for measuring stellar magnetic fields have been developed in the last few decades.
How to Measure Stellar Magnetic Fields Stellar magnetic fields can be measured thanks to the Zeeman effect on spectral lines: In the presence of a magnetic field, spectral lines are split into multiple, closely spaced components, called the Zeeman components. There exist several ways of exploiting this property, the most efficient being the analysis of the light polarization induced by the Zeeman effect. Each one of the Zeeman components indeed is polarized in a particular way, either linearly or circularly, depending on the component and on the orientation of the magnetic field. Thus, the
strength and direction of the star’s magnetic field can be determined by a careful examination of the Zeeman polarization of spectral lines. The instrument used for this purpose is called a spectropolarimeter, and consists of a high-resolution spectrograph combined with a polarimeter. Very powerful such instruments were developed recently, in particular ESPaDOnS on the 3.6 m Canada-France-Hawaii telescope on Mauna Kea, and NARVAL on the 2 m Bernard Lyot telescope at Pic du Midi in the French Pyre´ne´es. The remarkable efficiency of these instruments allowed astronomers to measure the magnetic fields of dozens of stars of all types, providing valuable information on stellar magnetism. The Zeeman polarization measurement described above provides only the longitudinal or transverse component of the magnetic field (depending on the type of polarization, linear or circular, which is measured), averaged over all the visible hemisphere of the star at the moment of the observation. In order to reconstruct the magnetic field vector at all locations at the stellar surface, astronomers have developed the so-called ZeemanDoppler imaging technique, which consists of monitoring the star during its rotation. A magnetic region appearing at the limb on the side which is approaching the observer will produce a polarimetric signature at the blue edge of a ▶ spectral line, due to the Doppler effect. As the star rotates, the magnetic region will continue to approach the observer but with a decreasing velocity, and therefore its signature will move toward the center of the line. At some point, the meridian plane of the magnetic region will cross the line of sight and the polarimetric signature will have a zero velocity in the observer’s direction, then it will start moving toward the red wing of the line, until eventually the magnetic region disappears at the opposite limb. The excursion of the polarimetric signal across the line and the speed at which it evolves from blue to red depend on the latitude of the magnetic region and on the inclination of the star’s rotation axis with respect to the line of sight. Based on this principle, astronomers have developed powerful tools to reconstruct the topology and intensity of the magnetic field at the surface of a star, even though it cannot be resolved directly (Fig. 2).
The Magnetism of Cool Stars Magnetic fields of cool stars, such as the Sun, are believed to be produced within the star’s convective zone, as the result of convection coupled with rotation. Differential rotation inside the star wraps and amplifies the magnetic
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Sun, but can be considerably shorter or longer for other stars, depending on various parameters, in particular the amount of differential rotation.
The Magnetism of Massive Stars
Magnetic Field. Figure 2 The magnetic topology of SU Aur (a young star of T Tauri type located at 150 pc from the Sun), reconstructed by means of Zeeman-Doppler imaging. Credit JF Donati
field around the star, changing the seed poloidal field into an intense toroidal one. Convection and meridional circulation then regenerate the poloidal field from the toroidal one. Although the details of this mechanism still need to be worked out, it seems well established that this type of dynamo, which is responsible for the magnetic field of most cool stars including the Sun, is predominantly located at the base of the convection zone, near its interface with the underlying radiative zone, where the differential rotation is maximum (Fig. 3). However, spectropolarimetric observations of various classes of cool stars, with various rotation rates and depths of the convective envelopes, have revealed the existence of several regimes of dynamo generation of magnetic fields. In particular, red dwarfs, which are entirely convective, and hence do not have any radiative-convective interface where dynamo action is supposed to be most effective, still exhibit intense, predominantly dipolar magnetic fields, in contradiction with predictions of the above simplified model. Stellar magnetism therefore still preserves some of its mysteries. As a consequence of the dynamo regeneration of magnetic fields in cool stars, the major magnetic component reverses direction periodically. The cadence of these reversals is of the order of once every 11 years for the
In addition to cool stars with masses similar to that of the Sun, a small fraction of the massive stars (more than twice the mass of the Sun) also exhibit intense, well-ordered, dipolar magnetic fields. These fields are believed to be fossil in origin, remnants of the weak magnetic field present in the molecular cloud out of which the star was formed. During the contraction of the star toward the ▶ Main Sequence, the magnetic field lines are frozen in the contracting plasma, and are therefore pinched, resulting in a gradual enhancement of the magnetic field intensity and the evolution of its topology toward a dipole. The magnetic fields of these massive stars are thought to be responsible for a variety of properties, such as their usually very slow rotation, or the presence of patches of chemical abundance anomalies seen at their surface.
The Magnetic Field of the Earth Like the solar magnetic field, the magnetic field of the Earth is approximately a dipole, more or less similar to that of a bar magnet. The Earth’s magnetic axis is tilted by about 11 with respect to its rotation axis. The magnetic poles are therefore near (but not at) the geographic poles. The magnetic field is generated and sustained in the liquid outer core by a dynamo process involving convection of molten iron and rotation. The strength of the field at the surface of the Earth is approximately 0.5 G on average, but varies over the surface in the range from 0.3 G (in the area including South America and South Africa) to 0.6 G (around the magnetic poles in northern Canada and southern Australia, and in Siberia). The direction of the Earth’s magnetic axis is not constant in time. The geographic location of the magnetic poles has considerably varied over the centuries. Rock specimens of different ages found at the same location have different directions of permanent magnetization, indicating this long-term variation of the Earth’s magnetic field orientation. We have found evidence for 171 magnetic field reversals during the past 71 million years. The locations of the magnetic poles move as much as 15 km every year. The magnetic field extends to thousands of kilometers above the Earth into a gigantic ▶ magnetosphere which shields us against solar high energy charged particles, which otherwise might have prevented life on Earth. The Earth’s magnetosphere deflects most of these solar
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Magnetic Field. Figure 3 Dynamo generation of magnetic field in the Sun. Credit M. Dikpati
particles. Some of them are trapped in the so-called Van Allen radiation belt. Frequently, the Earth’s magnetosphere is hit by energetic solar flares causing geomagnetic storms, giving rise to boreal and austral aurorae. During these events, a small number of solar charged particles reach the Earth’s upper atmosphere and ionosphere in the polar zones, channeled along the magnetic field lines (Fig. 4).
Star–Planet Interactions: The Role of Magnetic Fields Special attention is being paid presently to the interaction of close-in Jupiter-sized exoplanets with their host stars. Evidence has been accumulated in recent years that largesize planets orbiting very close to their star can have a significant influence, through tidal effects, on the activity
behavior of their star. If the planet is itself magnetic and orbits a magnetic star, we can expect a complex interaction between the planet’s magnetosphere and that of the star, comparable to some extent to the well-known Io–Jupiter system interaction. It has been recently conjectured that the magnetic interaction between close-in giant planets and their host star could play a dominant role in the migration and evolution of these planets. One of the best examples of this kind of star–planet magnetic interaction is provided by the t Bootis system, in which a 4.4 MJup planet orbiting a solar-type star at the distance of 0.049 AU. The star t Bootis has been shown to be magnetic, with a magnetic field intensity of the order of a few gauss, and to have a strong surface differential rotation. The planet orbital motion is synchronized with the star’s rotation at intermediate latitudes. The magnetic
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Magnetic Field. Figure 4 This artist view shows the Earth’s magnetosphere, protecting us from solar charged particles, compressed on the dayside and extended on the nightside. The image is not to scale
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Magnetic Field. Figure 5 An artist view of the hot giant exoplanet orbiting t Boo, within the star’s magnetosphere. The planet is 4.4 times more massive than Jupiter and its separation with the star is only 0.049 AU. Credit David Aguilar
field reverses its polarity approximately every 2 years, i.e., much more often than the Sun. The potential magnetic interaction between this star and its planet has triggered a major interest in recent years (Fig. 5).
See also ▶ Activity (Magnetic) ▶ Io ▶ Magnetic Field, Planetary ▶ Magnetic Field, Stars and Planetary Systems Formation ▶ Magnetosphere
▶ Main Sequence ▶ Planet Formation ▶ Planetary Evolution ▶ Planetary Migration ▶ Protoplanetary Disk ▶ Spectral Line
Magnetic Field Generation ▶ Dynamo (Planetary)
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Magnetic Field, Planetary TILMAN SPOHN German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Synonyms Magnetic flux density; Magnetic induction
Keywords Core, dipole field, dynamo, induced field, magnetometer, remnant field
Definition The magnetic field of a ▶ planet can be measured by magnetometers onboard orbiting space craft. On a global scale planetary magnetic fields resemble dipole fields. There may also be fields of complicated topology on local scales. Planetary magnetic fields are generated by a ▶ dynamo mechanism in the iron-rich liquid ▶ core shells of ▶ terrestrial Planets and ▶ Satellites, in metallic ▶ hydrogen shells in ▶ Jupiter and ▶ Saturn, and in ionic oceans in ▶ Uranus and ▶ Neptune. Magnetic fields may be also induced in electrically conducting oceans in icy moons such as the Jovian moon ▶ Europa. Local magnetic fields may be caused by remnantly magnetized ▶ rock.
Overview Magnetic fields are – in general – detected by the force they exert on magnetic materials and electrical charges. This principle is used in magnetometers onboard spacecraft with which magnetic fields around planets are detected and measured. The magnetic field strength is a vector quantity and is measured in the SI unit tesla [T]. In geophysics and planetary physics the cgs units Gauss [G] and gamma [g] are still widely used with 1g equaling 103 G and 1 G equaling 104 T. The terrestrial planets ▶ Earth and ▶ Mercury are known to have global magnetic fields. The Earth’s field is known to vary in time, with short period and secular changes and polar wander (motion of
the North and South Poles with respect to the solid ▶ crust). The polarity of the field has changed over geological history on time scales of 106 a. The Earth’s magnetic field is about 30,000 nT. The magnetometer onboard the ▶ Mars Global Surveyor Mission discovered local magnetic fields on ▶ Mars, mostly in the southern hemisphere, that have been interpreted to be due to magnetized crustal rock provinces with varying strength and direction of magnetization. This observation indicates that Mars once had a global magnetic field, the polarity of which may have varied in time similar to the Earth’s field. Magnetized crustal rock units have also been discovered on the ▶ Moon. The planets of the outer Solar System Jupiter, Saturn, Uranus, and Neptune all have global magnetic fields. Periodically varying magnetic fields have been discovered around Europa, ▶ Ganymede, and ▶ Callisto, the icy satellites of Jupiter. These fields vary in time with the orbital periods of the satellites, which are on the order of days. In addition, Ganymede has a steady global magnetic field. This satellite is the only satellite known to posses such a field. Global magnetic fields of planets largely resemble dipole fields (the field of an ideal bar magnet). A simple representation of the global magnetic field is the offset tilted dipole. This dipole is characterized by a moment (a vector ! quantity) M and an offset vector r!0 measured from the figure center of the planet. The dipole moment is measured in Tm3 and the tilt is measured relative to the rotation axis. Table 1 collects dipole moments and tilts for the planets with known global magnetic fields. More detailed representations of global magnetic fields use spherical harmonic functions. The offset in terms of the planetary radius is usually small, suggesting that the magnetic field is generated deep in the interior. Exceptions are Jupiter with 0.11RP and even more so Uranus and Neptune with 0.33 and 0.55 RP , respectively (e.g., Connerney 2007). It is held that in these planets the dynamo is closer to the surface. A planet with an intrinsic magnetic field stands as an obstacle to the solar wind with an interaction region that resembles a set of nested conic sections (Fig. 1). The outer section is called the bow shock. It marks the shock wave
Magnetic Field, Planetary. Table 1 Mercury
Earth
Jupiter
Ganymede
Saturn
Uranus
Neptune
Dipole moment [104 nT R3 P ]
0.0230–0.0290
3.000
43.535
0.0719
2.108
2.284
1.424
Tilt [ ]
5–12
12
9.5
176
1
60
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Int
k oc Sh w Bo
ry ld eta Fie lan tic erp gne Ma
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Magnetic Field, Planetary. Figure 1 Solar wind interaction with the terrestrial magnetic field. Adapted from Baumjohann and Nakamura 2007
front that is created by the impingement of the supersonic solar wind on the planetary field. The slowed solar wind mostly flows around the obstacle within a region called the magnetosheath. The latter is bounded on the one side by the bow shock and on the other by the magnetopause. Within the magnetopause is a region called the ▶ magnetosphere that is mostly filled by the planetary field. The magnetosphere extends downstream into the magnetotail. In terrestrial planets and in Ganymede the global field is agreed to be produced in their fluid iron cores or core shells by a dynamo. In the ▶ Giant Planets Jupiter and Saturn the field is generated by dynamo action in their metallic hydrogen shells. In Uranus and Neptune the field is held to be produced in ionic oceans. Rock that contains sufficient amounts of ferri- and ferromagnetic materials can be magnetized in a planetary magnetic field. Part of the magnetization can be permanent or remnant at temperatures below the Curie temperature. The value of the Curie temperature varies between ▶ minerals but is typically between 500 K and 1000 K. The remnant and the induced magnetizations are vector quantities. The direction of the remnant magnetization reflects the direction of the magnetizing field at the time of the cooling below the Curie temperature. Therefore, remnant magnetization data can be used to reconstruct the evolution of the global magnetic field and the movement of crustal units relative to, for example, the poles of the magnetic field. Remnantly magnetized crustal units are known from the Earth, Mars, and the Moon. For the Earth, remnant magnetizations have been used to date
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the spreading of the oceans and to determine its rates, continental drift, and polar wander as well as the evolution of the magnetic field and its polarity. For Mars, the magnetization patterns have been interpreted as indicating a global magnetic field in the first 500–1000 Ma that may have experienced polarity reversals. This field would have been similar in strength to the present global field of the Earth. For the Moon, an early field has been suggested, but it has also been proposed that the magnetizing field may have been due to plasma that emanated from large impacts (Hood and Vickery 1984). The periodically varying fields of the icy moons of Jupiter have been interpreted as induced fields (Kivelson et al. 1999). If the satellites contain an electrically conducting layer of sufficient conductivity then their moving through the magnetic field of Jupiter along their orbits will induce a time-varying current. That current by the laws of induction will induce a magnetic field. Salty ▶ water will have a sufficiently strong conductivity. The geometry of the field also suggests that the conductor cannot lie deep in the satellite. The induced fields thus have been taken to suggest the existence of (ice-covered) oceans. The presently available data for Mercury interpreted together suggest a dipole tilted by 5–12 with a moment of 230–290 nT. RM3 but non-dipolar terms are found to be significant [Uno et al. 2009]. These could indicate crustal fields or significant dynamo action close to the core/ ▶ mantle boundary, which should be close to 0.8 RM. Missions to ▶ Venus have failed to detect a magnetic field of the planet. An upper limit for its dipole moment is 105 times the moment of the Earth or 0.35 nT-RV3. The first unambiguous detection of an intrinsic field at Mars was achieved by the Mars Global Surveyor (MGS) mission (Acuna et al. 1999). The field strength varies between 200 nT along the mapping orbit at 400 km height above the surface, the distance of the mapping orbit. These values suggest magnetizations of approximately 103 nT at the surface. It has been concluded that the Martian crust must be about 10 times more intensely magnetized than the Earth’s crust. The Moon does not have a dipole field at present, but large parts of the crust show remnant magnetization. The magnitude of the magnetization reaches values of up to 250 nT, much smaller than the magnetization of the Martian and the Earth’s crust.
See also ▶ Callisto ▶ Core, Planetary ▶ Crust
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▶ Dynamo (Planetary) ▶ Earth ▶ Europa ▶ Ganymede ▶ Giant Planets ▶ Hydrogen ▶ Jupiter ▶ Magnetic Field ▶ Magnetosphere ▶ Mantle ▶ Mars ▶ Mars Global Surveyor ▶ Mercury ▶ Mineral ▶ Moon, The ▶ Neptune ▶ Planet ▶ Rock ▶ Satellite or Moon ▶ Saturn ▶ Terrestrial Planet ▶ Uranus ▶ Venus ▶ Water
References and Further Reading Acuna MH, Connerney JEP, Ness NF et al (1999) Global distribution of crustal magnetism discovered by the Mars global SurveyorMAG/ER experiment. Science 284:790–793 Baumjohann W, Nakamura R (2007) Magnetospheric contributions to the terrestrial magnetic field. In: Kono M, Schubert G (eds) Treatise on geophysics, vol 5. Elsevier, Amsterdam, pp 77–92 Connerney JEP (2007) Planetary magnetism. In: Spohn T, Schubert G (eds) Treatise on geophysics, vol 10. Elsevier, Amsterdam, pp 243–280 Hood LL, Vickery A (1984) Magnetic field amplification and generation in hypervelocity meteoroid impacts with application to lunar paleomagnetism. J Geophys Res 89:C211–C223 Kivelson MG, Khurana KK, Stevenson DJ et al (1999) Europa and callisto: induced or intrinsic fields in a periodically varying plasma environment. J Geophys Res 104:4609–4626 Kono M, Schubert G (2007) Geomagnetism, treatise on geophysics, vol 5. Elsevier, Amsterdam, p 589 Schubert G, Anderson JD, Spohn T, McKinnon WB (2004) Interior composition, structure, and dynamics of the Galilean satellites. In: Bagenal F, Dowling TE, McKinnon WB (eds) Jupiter. the planet, satellites, and magnetosphere. Cambridge University Press, Cambridge, pp 281–306 Spohn T (2009) Magnetic fields. In: Tru¨mper J (ed) Solar system, Landolt-Bo¨rnstein numerical data and functional relationships, vol VI/4B, New Series. Springer Verlag, Berlin, pp 386–391 Stevenson DJ (2003) Planetary magnetic fields. Earth Planet Sci Lett 208:1–11 Uno H, Johnson CL, Anderson BJ, Korth H, Solomon SC (2009) Modeling mercury’s internal magnetic field with smooth inversions. Earth Planet Sci Lett 285:328–339
Magnetic Field, Stars and Planetary Systems Formation WAYNE G. ROBERGE, GLENN E. CIOLEK New York Center for Astrobiology, Rensselaer Polytechnic Institute, Troy, NY, USA
Synonyms Star formation
Keywords Habitability, Magnetic fields, Planet formation, Star formation
Overview Magnetic Fields and Star Formation Magnetic fields play a crucial role in the support of interstellar molecular clouds against ▶ gravitational collapse. These clouds consist primarily of gaseous H2 molecules and He atoms, along with much smaller relative abundances of other molecular species (such as CO and NH3). They also contain interstellar dust grains comprising 1% of the cloud mass. Clouds are extremely cold (10 K), with sizes from 1 pc to 100 pc, mean particle densities 103 cm3, and masses 1 M to 105M (1 M = 1 solar mass). Because the temperatures are so low, thermal pressure stresses are negligible compared to magnetic forces in clouds. Star formation in molecular clouds is observed to be very inefficient: less than 10% of a cloud’s mass is converted by gravitational collapse into stars (Zuckerman and Palmer 1974). This inefficiency is due to internal B fields, whose supporting forces prevent wholesale cloud collapse; consequently, B fields regulate the rate of star formation (Mouschovias 1978; Shu et al. 1987). This is quantified by a cloud’s mass-to-magnetic flux ratio (MtMFR), which is the ratio of its gravitational binding energy (proportional to the cloud mass squared) relative to its internal supporting magnetic energy (proportional to the square of the cloud’s mean magnetic field strength). If the MtMFR is less than a critical value (“subcritical”), magnetic forces can support a cloud against its selfgravity. Conversely, if a cloud’s MtMFR is greater than the critical value (“supercritical”), gravitational forces overwhelm magnetic forces and collapse takes place (Mouschovias and Ciolek 1999). Observations of Zeeman splitting of molecular OH lines (See ▶ Hydroxyl radical), as well as polarimetric observations of the fluctuations in the direction of magnetic field lines in clouds, yield field
Magnetic Field, Stars and Planetary Systems Formation
strengths and MtMFRs in molecular clouds that are closely clustered around the critical value (Crutcher 2004), confirming large-scale magnetic support of clouds. Even though molecular clouds are supported by internal B fields, star formation can still occur within them on small scales because clouds are weakly ionized. A plasma can have a frozen-in magnetic field, but how effectively magnetic forces, which act only on charged particles, are transmitted to the bulk neutral matter (through charged-neutral particle collisions) depends on the abundance xcp of charged particles relative to neutral particles (the plasma in molecular clouds is made up of cations such as Na+, Mg+, and HCO+, electrons, and charged dust grains, which capture electrons on their surfaces). The outer layers of a cloud have xcp 104, a value sufficient to ensure that collisions are frequent enough so that supporting magnetic forces on the plasma are readily transferred to the neutrals, counterbalancing the compressing gravitational forces exerted on the neutrals. But xcp < 108 deep within a cloud, where matter is shielded from ionizing external UV photons and galactic low-energy cosmic rays. For xcp values this low, neutral particles slip through the plasma and magnetic field lines (a process known as ▶ ambipolar diffusion (Mestel and Spitzer 1956)), increasing the mass in cloud interiors, but not the B field strength, as the field lines are “left behind” by the inwardly drifting neutrals. Hence, dense compact regions (mean densities 104 cm3, radii 0.1 pc) having MtMFRs greater than the critical value form inside globally supported clouds. These supercritical regions are protostellar cores, which dynamically collapse as a result of self-gravity. They are the localized sites within clouds where stars are born (Mouschovias and Ciolek 1999). MHD simulations of the formation and collapse of protostellar cores by ambipolar diffusion have been constructed for model molecular clouds, and their predicted magnetic field strengths and density and infall velocity profiles are consistent with multiwavelength observations of collapsing cores (Mouschovias and Ciolek 1999; Ciolek and Basu 2001). An alternative theory of star formation is that molecular clouds are short-lived objects formed by interstellar MHD turbulence. In this scenario, supercritical cores form randomly, via turbulent fluctuations, and collapse gravitationally to form stars (Mac Low and Klessen 2004).
Magnetic Fields and Planet Formation ▶ Angular momentum conservation prohibits the direct formation of a star by gravitational collapse. A protostellar core contracts first into a rotating protoplanetary disk or proplyd (Vicente and Alves 2005; Williams et al.
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2005). The central star is built up later via the migration of disk material inward, a process that requires friction (viscosity) between matter in adjacent orbits with slightly different speeds. It is now generally accepted that magnetic fields are required to produce this friction, via a mechanism called the magneto-rotational instability (Balbus 2010). Because field lines are dragged inward by collapsing core material, the B fields required by the MRI are plausible. The presence of dynamically significant B fields in protoplanetary disks is also indicated by observations of ▶ bipolar outflows, extended and highly collimated jets of material ejected parallel and antiparallel to the disk rotation axis (Reipurth and Bally 2001). According to magnetocentrifugal outflow models of bipolar outflows (Blandford and Payne 1982; Ko¨nigl and Pudritz 2000), centrifugal forces fling plasma outward along magnetic field lines, which direct and collimate the flow. Bipolar outflows are extremely supersonic, producing multifluid MHD shock waves wherever they impact ambient material. Magnetic forces on the charged particles cause large differential velocities between the ions and neutral molecules in such shocks, heating the plasma to 103 K over an extended region (Draine et al. 1983). Shock heating opens chemical pathways, which are closed in cold clouds, with unknown but potentially important consequences for prebiotic chemistry.
Magnetic Fields and Habitability It is interesting that the only planet known to be inhabited also has a large magnetic field. However, it is well known that Earth’s B field ameliorates or eliminates effects that would be harmful to life. For example, the Sun emits a high-velocity stream of charged particles, the solar wind, which flows through the solar system at hundreds of kilometers per second. A planetary atmosphere directly exposed to the solar wind tends to be eroded. However, Earth is protected from the solar wind by its magnetic field, which fills a region (the ▶ magnetosphere) that largely excludes the solar wind plasma (Lammer et al. 2010). The frozen-in magnetic field of the solar wind also protects the Earth, by sweeping galactic cosmic rays out of the solar system (Parker 1963).
See also ▶ Ambipolar Diffusion ▶ Fragmentation (Interstellar Clouds) ▶ Interstellar Medium ▶ Magnetic Field ▶ Magnetic Field, Planetary ▶ Planet Formation ▶ Protoplanetary Disk
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▶ Protostars ▶ Shocks, Interstellar ▶ Star Formation, Observations
References and Further Reading Balbus SA (2010) Magnetohydrodynamics of Protostellar Disks. In: Garcia P (ed) Physical processes in circumstellar disks around young stars. Chicago, University of Chicago Press (in press) Blandford RD, Payne DG (1982) Hydromagnetic flows from accretion discs and the production of radio jets. MNRAS 199:883 Ciolek GE, Basu S (2001) The razor’s edge: magnetic fields and their fundamental role in star formation and observations of protostellar cores. In: Montmerle T, Andre´ P (eds) From darkness to light: origin and evolution of young stellar clusters. ASP, San Francisco, pp 79 Cowling TG (1957) Magnetohydrodynamics. Interscience, New York Crutcher RM (2004) Observations of magnetic fields in molecular clouds. In: Uyaniker B, Reich W, Wiebelinski R (eds) The magnetized interstellar medium. Copernicus GmbH, Katlenburg-Lindau, pp 123 Draine BT, Roberge WG, Dalgarno A (1983) Magnetohydrodynamic shock waves in molecular clouds. Astrophys J 264:485–507 Heiles C, Crutcher R (2005) Magnetic fields in diffuse HI and molecular clouds. In: Wielebinski R, Beck R (eds) Cosmic magnetic fields. Springer, Berlin, pp 137 Ko¨nigl A, Pudritz RE (2000) Disk winds and the accretion-outflow connection. In: Manning V, Boss AP, Russell SS (eds) Protostars and planets IV. University of Arizona, Tucson, pp 759 Lammer H et al (2010) Geophysical and atmospheric evolution of habitable planets. Astrobiology 10:45 Mac Low M-M, Klessen RS (2004) Control of Star formation by supersonic turbulence. Rev Mod Phys 76:125–194 Mestel L, Spitzer L Jr (1956) Star formation in magnetic dust clouds. MNRAS 116:503 Mouschovias TC (1978) Formation of stars and planetary systems in magnetic interstellar clouds. In: Gehrels T (ed) Protostars and planets. University of Arizona, Tucson, pp 209 Mouschovias TC, Ciolek GE (1999) Magnetic fiels and star formation: A theory reaching adulthood. In: Lada CJ, Kylafis ND (eds) The Origin of stars and planetary systems. Kluwer, Dordrecht, pp 305–340 Parker EN (1963) Interplanetary dynamical processes. Interscience, New York Reipurth B, Bally J (2001) Herbig-Haro flows: probes of early stellar evolution. Ann Rev Astron Astrophys 39:403–455 Shu FH, Adams FC, Lizano S (1987) Star formation in molecular clouds. Ann Rev Astron Astrophys 25:23–81 Stevenson DJ (2010) Planetary magnetic fields: achievements and prospects. Space Science Reviews 152:651–664 Vicente SM, Alves J (2005) Size distribution of circumstellar disks in the Trapezium cluster. Astron Astrophs 441:195 Williams JP, Andrews SM, Wilner DJ (2005) The Masses of the orion proplyds from submillimeter dust emission. Astrophys J 634:495 Zuckerman B, Palmer P (1974) Radio radiation from interstellar molecules. ARAA 12:279
Magnetic Flux Density ▶ Magnetic Field, Planetary
Magnetic Induction ▶ Magnetic Field ▶ Magnetic Field, Planetary
Magnetic Iron Ore ▶ Magnetite
Magnetic Pole Definition Magnetic poles are two regions on the opposite ends of a magnet, where the magnetic intensity is the highest. Due to the presence of a solid metal core, the interior of the Earth behaves as a giant dynamo, giving the Earth a magnetic field. The Earth’s magnetic field is dipolar with an axis at a small angle to the rotational axis, making the magnetic poles slightly offset from the geographic poles. The magnetic polarity has changed with highly variable frequency through geological history. At times the reversals are frequent, every few hundred thousand years or less, but in the mid-Cretaceous the polarity remained constant for nearly 40 million years. The inclination of the magnetic field is recorded by magnetic minerals in rocks and provides information about the latitude at the time the rock formed. Apparent polar wander curves trace the movement of continents with reference to the nearly fixed position of the poles. Such information provides evidence of plate movement from as early as the Archean period.
See also ▶ Magnetic Anomaly ▶ Magnetic Field ▶ Magnetic Field, Planetary ▶ Paleomagnetism ▶ Plate Tectonics
Magnetism ▶ Magnetic Field
Magnetosome
Magnetite Synonyms Magnetic iron ore
Definition Magnetite is a black, cubic iron oxide of chemical formula Fe3O4 belonging to the spinel group. It is the most magnetic ▶ mineral on Earth. The formula of magnetite may also be written as (Fe2+Fe3+[Fe3+O4]), which indicates the occurrence of two iron ions having different valences that occupy specific sites in the crystal structure. This arrangement causes a transfer of electrons between the different irons generating the magnetic field. Magnetite occurs in almost all ▶ igneous and ▶ metamorphic rocks and in several ▶ sedimentary rocks, including ▶ banded iron formations. Crystals of magnetite are found in some bacteria called ▶ magnetobacteria (e.g., Magnetospirillum magnetotacticum).
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tension and pressure correlated with the curvature and spacing of the field lines, respectively.
See also ▶ Magnetic Field ▶ Magnetic Field, Planetary ▶ Magnetic Field, Stars and Planetary Systems Formation
Magnetosome ALFONSO F. DAVILA SETI Institute – NASA Ames Research Center MS 245-3, Moffett Field, CA, USA
Keywords Bacteria, biomineralization, magnetism, magnetofossil, magnetotaxis, magnetite, greigite, biomarker
See also ▶ ALH 84001 ▶ Banded Iron Formation ▶ Hematite ▶ Igneous Rock ▶ Iron Oxyhydroxides ▶ Magnetosome ▶ Magnetotactic Bacteria ▶ Metamorphic Rock ▶ Mineral ▶ Sedimentary Rock
Magnetohydrodynamics Definition Magnetohydrodynamics (“MHD”) describes the motions of conducting liquids and plasmas (Cowling 1957). The fluid motions dictate electrical currents and hence the B field. Since magnetic forces also affect the fluid motions, the dynamics are generally complex. Magnetic field lines map magnetic fields: at each point on a magnetic field line, the orientation of the line indicates the direction of B and the spacing between adjacent lines is inversely proportional to the magnitude of B. Under conditions where Ohmic dissipation is negligible, corresponding to large length scales and/or conductivities, (“ideal MHD”), the magnetic field lines are frozen into and move with the plasma. This frozen-in field exerts forces that can be visualized as
Definition Intracellular magnetic crystal typically surrounded by a lipid membrane synthesized by magnetotactic organisms. Magnetosomes are chemically pure, have a narrow size range with species-specific crystal morphologies, and are arranged in chains (Bazylinski 1995; Spring and Bazylinski 2000). Magnetosomes are usually composed of the iron oxide ▶ magnetite (Fe3O4), or the iron sulfide griegite (Fe3S4). Membrane bound magnetosomes form from invaginations of the inner membrane, and their position in the ▶ cytoplasm is supported by cytoskeletal filaments (Komeili et al. 2006). These features indicate that magnetosome synthesis is a biomineralization process with strict biological control. Chains of magnetosomes are responsible for the passive orientation of ▶ magnetotactic bacteria along ▶ magnetic field lines (Blakemore 1975) (Fig. 1).
Overview Bacterial magnetosomes have been a subject of intense research due to their relevance in such diverse fields as biomineralization, paleo-microbiology, technology, medicine, or astrobiology. Due to their small size and strong magnetic remanence, magnetosomes, have also been used in nano- and biotechnological applications. Iron biomineralization is likely an ancient trait of terrestrial life, and iron minerals are common among organisms in the ▶ Bacteria and ▶ Eukarya Domains. In particular magnetosomes are often found in the fossil record (magnetofossils) in marine and lake sediments.
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Magnetosphere Blakemore R (1975) Magnetotactic bacteria. Science 190:377–379 Komeili A, Li Z, Newman DK (2006) Magnetosomes are cell membrane invaginations organized by the actin-like protein MamK. Science 311:242–245 McKay DS, Gibson EK, Thomas-Keprta KL, Vali H, Romanek CS, Clemett SJ, Chillier XDF, Maechling CR, Zare RN (1996) Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001. Science 273:924–930 Spring S, Bazylinski DA (2000) In: Dworkin K et al (eds) The prokaryotes. Springer, New York
Magnetosphere Magnetosome. Figure 1 Chains of magnetosomes extracted from lake sediments. The size, shape, morphology, and composition of the crystals reflect the biogenic origin. Crystals of different sizes are likely related to different species of magnetotactic bacteria. Scale bar represents 100 mm
PHILIPPE ZARKA LESIA, Observatoire de Paris, CNRS, UPMC, Universite´ Paris Diderot, Meudon, France
Synonyms The traits that characterize magnetosomes: chemical purity, size, morphology, and chain arrangement, all contribute to maximize the magnetic moment of the crystals, and the capacity of magnetotactic organisms to swim along magnetic field lines, and search efficiently for chemical gradients. These properties also confer magnetosomes a strong potential as ▶ biomarkers, since non-biogenic magnetite and greigite crystals often fail to possess one or several of these traits. Biogenic minerals are also more resistant to chemical and physical weathering than organic biomarkers. For those reasons, magnetosomes have been proposed as a potential target in the search for life beyond Earth. Putative magnetite magnetosomes have been identified in the Martian meteorite ▶ ALH84001, and presented as evidence for relic biological activity on ▶ Mars (McKay et al. 1996), although these claims have been disputed.
See also ▶ ALH 84001 ▶ Biomarkers ▶ Biomineralization ▶ Cytoplasm ▶ Magnetic Field ▶ Magnetite ▶ Magnetotactic Bacteria
References and Further Reading Bazylinski DA (1995) Structure and function of the bacterial magnetosome. ASM News 61:337–343
Aurora; Ionosphere; Planetary magnetic cavity; Solar wind
Keywords Aurora, currents, ionosphere, Jupiter, magnetic fields, magnetospheric dynamics, Mercury, Neptune, plasma, Saturn, solar wind, Uranus
Definition The ▶ solar wind (SW) which flows across the solar system interacts with obstacles possessing a ▶ magnetic field, an atmosphere, or a high internal conductivity. In the first case, generally concerning planetary obstacles, the magnetic pressure creates a cavity in the SW, where ▶ plasma motions are dominated by the planetary field; this region is called the “magnetosphere.” This magnetic bubble acts as a shield preventing the bombardment and escape of the planetary atmosphere. It also generates an intense electrical activity, behaving as a natural charged particle accelerator. Planetary magnetospheres are contained in the Sun’s sphere of influence (heliosphere), and in turn contain the magnetospheres of some of their ▶ satellites.
History Auroras have been known since ancient times. In the nineteenth century, statistical measurements allowed localization of the northern auroral oval at 65–80 latitude, but its origin was not understood. Shortly after, Thomson discovered the electrons. This gave Birkeland the idea to use them to bombard a magnetized sphere in a rarefied atmosphere (vacuum chamber). He could
Magnetosphere
then create and observe polar auroral lights as early as 1895–1910. He proposed that “polar magnetic storms” exist in the auroral zone, due to electron beams from the Sun. In the 1930s, Chapman and Ferraro proposed that “magnetic storms” are caused by solar plasma clouds interacting with the Earth’s magnetic field. They proposed the existence of the dayside magnetopause (the outer boundary of the magnetosphere) at a few Earth radii based on the use of a reversed “image” of the terrestrial magnetic dipole to represent the compressed dayside terrestrial field. Thomas Gold proposed the name “magnetosphere” for the cavity, above the ▶ ionosphere, where the planetary magnetic field has dominant control over the motions of gas and fast charged particles. The concept of solar plasma clouds remained vague until the suggestion by Biermann in 1951 of the existence of a permanent solar “wind” (flow of electrons, protons, and heavier ions escaping permanently from the solar atmosphere), in order to explain the second cometary tail, blueish, straight, and always antisolar (now known to be a plasma tail). In 1958, Parker proposed the first theory of the SW, which not only escapes the solar atmosphere but is accelerated up to supersonic speed into the interplanetary space. The first observational confirmation came with spacecraft observations in 1960. Two years earlier, the Explorer 1 spacecraft discovered the Earth’s radiation belts (of highly energetic charged particles). In 1961, Dungey proposed the model of an open magnetosphere, still valid to describe the terrestrial magnetospheric dynamics, and involving elusive magnetic reconnection at the magnetopause. The magnetopause was actually observed as an abrupt magnetic field discontinuity in 1967. The auroral oval was first imaged from space in the 1980s. In parallel, the discovery in 1955 of Jupiter’s decameter radio emission gave the first indications on the existence and intensity of the Jovian magnetic field, further refined by the discovery in 1958 of the synchrotron emission from Jupiter’s radiation belts. In 1964, the statistical influence of Io on Jupiter’s decameter radio emission unveiled the first satellite–magnetosphere interaction.
Overview Solar Wind Flow and Planetary Magnetic Fields A central concept to the following discussion is that magnetic field (B) and plasma are “frozen” together, due to high plasma conductivity. Thus, in a frame where the magnetized plasma moves at velocity V, Maxwell’s equations show that an electric field E = V B appears.
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It is equal to zero in the plasma frame, consistent with quasi-neutrality. Also, the fact the E and B are orthogonal implies that magnetic field lines are electric equipotentials. Potential drops between field lines are thus conserved along these field lines. Being dominated by its bulk kinetic energy density, the SW carries away the solar magnetic field rooted in the Sun, generating the undulated surface (“ballerina skirt”) which separates the two magnetic hemispheres of the interplanetary medium. At Earth orbit, the SW has a speed V 400 km/s (largely supersonic), a density of protons and electrons N 5 cm3, a temperature T 200,000K, and a weak magnetic field B 3 nT. Its interaction with a planetary obstacle depends on the existence of an intrinsic large-scale magnetic field, an ionosphere, and/or a high internal conductivity of the obstacle. In the first case, an abrupt boundary limits the region of influence of the planetary magnetic field: the magnetopause. There are six magnetized ▶ planets in the Solar System: Mercury, Earth, and the four giants. Their magnetic fields have a strong dipolar component as well as multipolar ones (of order n = 2, 3, . . .). These components are known up to n 14 at Earth versus n 4 for the other planets. Especially remarkable are the very weak dipolar field of Mercury (compared to the very large field of Jupiter), the largely tilted dipoles of Uranus and Neptune, and the purely axisymmetric Saturnian field (Table 1).
Magnetospheric Regions and Boundaries The magnetosphere is the cavity bounded by the magnetopause, where the external SW pressure balances the internal planetary magnetic field pressure (Fig. 1). Electric currents circulate on the magnetopause, preventing magnetic field lines from crossing it (the field is tangential on the magnetopause). As a first approximation, there is no plasma exchange across the magnetopause. The dayside magnetopause radius lies between a fraction to several tens of planetary radii from the planetary obstacle (Table 1). This “standoff ” distance fluctuates with variations of the SW strength. Planetary magnetospheres are the largest structures in the solar system. If Jupiter’s with its 107 km radius was visible, it would appear larger than the full moon. Because the SW flow is supersonic, a stationary bow shock develops ahead of the magnetopause. The intermediate region is the magnetosheath, whose thickness is 15–40% of the standoff distance, and where the flow is slowed down, heated and deflected around the obstacle, and the SW magnetic field lines “pile up” and are draped around it. Due to the flow and draping, the nightside
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Magnetosphere. Table 1 Typical parameters of planetary magnetospheres. Comparison of the convection and corotation potential drops, as well as the ratio between plasmasphere radius and magnetopause standoff distance tells which type of plasma circulation dominates magnetospheric dynamics Mercury
Earth
Jupiter
Saturn
Uranus
Neptune
Planetary radius RP (km)
2,439
6,378
71,492
60,268
25,559
24,764
Orbital radius (AU)
0.39
1
5.2
9.5
19.2
30.1
Rotation period, Protation (h, min)
1407 h 30 min
24 h
9 h 55.5 min 10 h 39.4 min 17 h 14.4 min 16 h 6.6 min
Dipolar magnetic moment Mdip (G · km3)
5.5 107
7.9 1010 1.6 1015
4.7 1013
3.8 1012
2.2 1012
Equatorial magnetic field Be (G)
0.003
0.31
4.3
0.21
0.23
0.14
Dipole tilt (B, O) ( )
+14
+11.7
9.6
0.
58.6
46.9
Solar wind magnetic field BSW (nT)
10 (20)
4
0.8
0.4
0.2
0.13
Magnetopause radius RMP (RP) [calculated] 1.4
9
40
17
22
21
Magnetopause radius RMP (RP) [measured]
[1.5]
[10]
[90]
[20]
[18]
[23]
Convection potential drop Dfconv (kV)
7
46
900
90
17
14
Corotation potential drop Dfcorot (kV) Plasmasphere/magnetopause Radius RPS/RMP
0.002
90
0.01–0.02 0.3–0.8
CK
12,000
1,500
1,000
2–4
0.9–4
1–3
INTERPLANETARY MAGNETIC FIELD
O
W BO
400,000 4
SH
EATH ETOSH MAGN
BOUNDARY LAYER (MANTLE)
CUSP REGION 1 REGION 2 RING CURRENT SOLAR WIND
TRAPPED PARTICLES
MAGNETOTAIL RMP
PLASMA SPHERE
TAIL LOBES
PLASMA SHEET
j j
CO-ROTATION
CURRENT
CONVECTIVE FLOW
CROSSTAIL j
MAGNETOPAUSE MAGNETOPAUSE
Magnetosphere. Figure 1 2D and 3D views of the terrestrial magnetosphere
magnetosphere is stretched and forms a magnetospheric tail up to several AU long. A singular region, the polar cusp, separates closed planetary field lines anchored to both hemispheres on the dayside from open lines swept back to the tail. A small fraction of the SW plasma penetrating the outer magnetosphere across the magnetopause may directly reach the polar ionosphere via the cusp. The cusp does not exist for obstacles not possessing an intrinsic magnetic field, although an induced magnetosphere and a tail form due to magnetic draping.
Plasma Sources and Sinks Although more rarefied than the surrounding SW, a magnetosphere contains plasma that may have various origins: the SW itself (via the cusp and diffusion/ reconnection across the magnetopause), planetary ionospheres (through vertical diffusive equilibrium), satellite exospheres (atmospheric escape from Titan, Io’s volcanism, Enceladus’ geyser plumes), surfaces of Mercury or icy satellites (through sputtering by SWof magnetospheric energetic particles), rings (sputtering, photo-dissociation
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and ionization). These sources represent between a few hundred g/s to 1,000 kg/s of H, He, and heavy (N, O, S, H2O, . . .) ions of temperatures between 0.1 eV (ionospheric source) and 100 eV (SW source). The magnetospheric plasma is stored in various reservoirs including “boundary layers” (low-latitude on the dayside and tail mantle), plasma tori along satellite orbits, an equatorial plasma/current sheet at the center of the tail, and radiation belts. The total mass stored ranges from 107 kg at Earth to 1010 kg at Jupiter. Plasma sinks include absorption by the rings, satellites and auroral ionosphere, and interaction with neutrals atoms. At Saturn, the neutral nebula is 100 times denser than the magnetospheric plasma (dominated by H2O ions), while the neutrals/ions ratio at Jupiter is only 0.003.
A
B
C
D
E
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FG
V
N1
I J
A
B
C
G
H
D
N2
E
FG
Magnetosphere. Figure 2 Meridian sketch of the Dungey cycle of magnetospheric convection driven by the solar wind. Magnetic reconnection occurs in N1 and N2. The motion of a field line is followed from A to G along arrows ())
Plasma Circulation and Role of Ionosphere If the magnetosphere was electro-magnetically insulated from external conditions, its plasma would simply be dragged by friction of the SW flow along the magnetopause flanks (over a thickness of few ion Larmor radii), generating two convection cells with antisolar plasma motion on the edges and solarward plasma return inside the magnetosphere. Such a circulation pattern is indeed observed at Earth, together with other phenomena inconsistent with the above “closed” magnetosphere model: ● Presence of energetic plasma inside the magnetosphere ● Existence of a large-scale (dawn ! dusk) electric field ● Quasi-permanent circumpolar ▶ aurora at 65–80 latitude, brighter on the nightside ● Magnetospheric “activity” correlated with the direction of the interplanetary magnetic field (IMF) perpendicular to the ecliptic (Bz) Hence the concept of an “open” magnetosphere, in which reconnection of the IMF with the planetary field at the magnetopause allows plasma entry. Dayside reconnection occurs at the magnetopause nose when Bz is oriented in opposition to the planetary magnetic field, so that their sum cancels out. This causes magnetospheric field lines to “open,” and then be transported to the tail above the poles, where they close after a new reconnection. Closed field lines then “re-dipolarize” (retract) towards Earth while a “plasmoid” is ejected tailward. In this cycle, proposed by Dungey (Fig. 2), the circulation pattern is similar as above, but it is associated to the observed dawn ! dusk “convection” electric field (Econv / VVS BVS) whose equipotentials are the solar–antisolar circulation lines. The second reconnection corresponds to
a magnetic “substorm” during which energy is dissipated through charged particles heating and acceleration. Planetary rotation generates another large-scale, radial electric field (Ecorot / R O B), whose equipotentials are circles centered on the planet. Corresponding plasma circulation is “corotation.” Global magnetospheric circulation results from the superposition of convection and corotation, with a different relative importance at the various planets (Table 1). The internal, corotation-dominated region is the dense plasmasphere fed by the ionosphere, where planetary field lines never open. Its radius is compared to that of the magnetopause in Table 1. At Jupiter and Saturn, the dominant centrifugal force drives outward plasma transport and forms a plasma disk where million-Ampere current flows and stretches the equatorial magnetic field (Fig. 3). Corotation dominates so much at Jupiter that it modulates many magnetospheric phenomena (including radio emissions from which the planetary rotation period has been measured with 106 accuracy); a modified Dungey cycle exists, entirely driven by corotation (so-called “Vasyliunas cycle”). The magnetospheres of Earth and Mercury are convection-dominated, while Saturn is an intermediate case (Fig. 3). Interestingly, plasma circulation and the degree of corotational control are – at first order – independent of the planetary magnetic dipole tilt (except at Uranus and Neptune where this tilt is extremely large). Ionosphere–magnetosphere coupling ensures the closure of magnetospheric currents in a conducting spherical layer around the planet. Because magnetic field lines are electric equipotentials, the potential drops originating in the magnetosphere (with respect to the SW) are mapped
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to the ionosphere along field lines. The magnetospheric circulation thus causes the existence of high-latitude ionospheric convection cells. Due to converging field line geometry, ionospheric electric fields are larger than their magnetospheric counterparts, while plasma velocities are smaller. At Mercury, the nature of the process(es) ensuring magnetospheric current closure is still a puzzle.
Magnetopause
TERRE
−5 RT
10 RT 5 RT
Magne
e topaus
JUPITER
100 RJ
50 RJ −50 RJ
SE PAU
40
ETO
−100 RJ
−150 RJ
−200 RJ
SATURNE
GN MA
Current Generators Large-scale closed current circuits exist in planetary magnetospheres, each related to a specific generator: ● SW-driven convection drives high-latitude currents at the boundary between open and closed field lines that close into the polar ionosphere. ● In the Jovian system, outward transport of the plasma from Io generates a radial current (J), which closes into the ionosphere via magnetic-field-aligned (or “Birkeland”) currents. The Lorentz force (J B) associated to this radial current accelerates the magnetospheric equatorial plasma to corotation speed, while slowing down the ionospheric plasma. ● Ganymede’s intrinsic magnetic field is in permanent Dungey-like reconnection between with the Jovian field sweeping past the satellite. ● Unmagnetized satellites swept past a fast-rotating magnetosphere “see” part of the corotation electric field in their rest frame. When they have an ionosphere, like Io or Titan, a current is driven by this electric field that again closes into the planetary ionosphere via field-aligned currents (Fig. 4). All these currents dissipate energy in the magnetosphere and the ionosphere. Furthermore, when a strong current is forced through a rarefied plasma, it leads to electron acceleration. A fraction of the magnetospheric plasma is thus accelerated to keV/MeV energies through various processes.
Aurora Polar auroras are the most spectacular sign of magnetospheric activity (Fig. 5). They are produced by the radiative de-excitation of atmospheric atoms and molecules, emitting green and red oxygen lines on Earth, and ultraviolet H and H2 bands on Jupiter and Saturn. These atmospheric constituents are collisionally excited by
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Magnetosphere. Figure 3 Corotation-dominated plasmasphere (gray-shaded) and moons positions (red dots) in the magnetospheres of the Earth, Jupiter, and Saturn. Plasma corotation generates the equatorial current disk that stretches magnetic field lines. The tail X-line (where magnetic reconnection occurs) is located in the convection-dominated region at Earth, and in the corotation region at Jupiter, explaining why substorm-like events are related to the SW–magnetosphere interaction at Earth and to internal plasma mass-loading effects at Jupiter
Magnetosphere. Figure 4 Current generator associated to outward transport of the plasma produced by Io
Magnetosphere
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Magnetosphere. Figure 5 Earth auroras (top) seen from the ground (left), the space shuttle (center), and from a spacecraft (right). The auroral oval corresponds to the boundary between open and closed field lines. Jovian auroras (bottom): the oval corresponds to the corotation breakdown, the isolated spots to the magnetic footprints of Io, Ganymede, and Europa
energetic electrons (1–10 keV) accelerated in the magnetosphere by the above processes. Joule heating also produces infrared auroral emissions, while intense radio emissions are produced along magnetic field lines above the auroras. In a convection-dominated magnetosphere (as Earth’s), auroral ovals are located at the boundary of open and closed field lines. They are the magnetic projection onto the atmosphere of reconnection sites, especially the tail one, where accumulated energy and magnetic flux are suddenly released during substorms (typically several times a day). Note that auroras are by no means related to direct SW entry in the cusps. In a corotation-dominated magnetosphere (as Jupiter’s), auroras map the region where corotation breaks down due to insufficient momentum transfer from the ionosphere to the magnetosphere, in spite of maximum intensity field-aligned currents. Auroral-like emissions are also produced at the planetary magnetic footprints of satellites, either magnetized or possessing an ionosphere (Fig. 5). Auroras (visible, UV, IR, radio) are the image of magnetospheric activity projected along magnetic field lines onto the atmosphere as on a TV screen.
laboratories, displaying universal processes operating in very diverse environments (depending on object size, distance to Sun, magnetic field intensity, rotation, plasma sources, etc.). Each solar system magnetosphere has specific characteristics, which makes comparative study essential. Expanding these studies to exoplanet–star plasma interactions is very appealing. ▶ Hot Jupiters (close-in orbiting giant ▶ exoplanets), enduring high stellar wind pressure, might have compressed magnetospheres. Alternately, they might be interacting with their parent star, via reconnection if they are magnetized (like Ganymede with Jupiter, or interacting magnetic binaries) or via currents and Alfve´n waves (like Io with Jupiter) if they are unmagnetized. They are expected to produce intense electromagnetic emissions, especially in the radio range, which are being actively searched for.
Notations and Acronyms AU Astronomical Unit IMF Interplanetary Magnetic Field nT nanoTesla SW Solar Wind
See also Concluding Remarks and Future Directions Planetary magnetospheres are complex electrodynamic machines, both charged particle accelerators and shields protecting the atmospheres against cosmic rays, solar eruptions, and evaporation. They are also plasma physics
▶ Aurora ▶ Exoplanets, Discovery ▶ Hot Jupiters ▶ Ionosphere ▶ Magnetic Field
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▶ Planet ▶ Plasma ▶ Satellite or Moon ▶ Solar Wind
References and Further Reading Bagenal F, McKinnon W, Dowling T (eds) (2004) Jupiter: the planet, satellites, and magnetosphere. Cambridge University Press, New York Blanc M (2002–2003) Magnetospheric and plasma science with CassiniHuygens. In: The Cassini-Huygens mission Vol. 1, Space Sci. Rev., 104, pp 253–346 Brown R, Dougherty M, Esposito L, Krimigis T, Lebreton J-P, Waite JH (eds) (2009) Saturn after Cassini–Huygens. Springer, Dordrecht Celnikier LM (1989) Basics of cosmic structures. In: Gif-sur-Yvette (ed) Frontie`res Dessler AJ (ed) (1983) Physics of the Jovian magnetosphere. Cambridge University Press, New York Dungey JW (1961) Interplanetary magnetic field and the auroral zones. Phys Rev Lett 6:47–48 Encrenaz T, Bibring J-P, Blanc M, Barucci A, Roques F, Zarka P (2004) The solar system, 3rd edn. A&A Library, Springer, Germany Encrenaz T, Kallenbach R, Owen TC, Sotin C (eds) (2005) The outer planets and their moons. Space Sci Rev 116:1–2, ISSI series, Bern, Springer Kelley MC (1989) The earth’s ionosphere. Academic, San Diego Kivelson MG, Russell CT (eds) (1995) Introduction to space physics. Cambridge University Press, New York Knight S (1973) Parallel electric fields. Planet Space Sci 21:741–750 Lilensten J, Blelly P-L (2000) Du soleil a` la terre. EDP Sciences, Grenoble Prange´ R (1992) The UV and IR Jovian aurorae. Adv Space Res 12(8):379–389 Russell CT (2004) Outer planet magnetospheres: a tutorial. Adv Space Res 33:2004–2020 Slavin JA (2004) Mercury’s magnetosphere. Adv Space Res 33(11):1859–1874 Zarka P (2007) Plasma interactions of exoplanets with their parent star and associated radio emissions. Planet Space Sci 55:598–617
Magnetotactic Bacteria Definition Magnetotactic bacteria are motile, prokaryotic organisms characterized by their tendency to align with, and move along ▶ magnetic field lines. All magnetotactic bacteria synthesize intracellular magnetic crystals called ▶ magnetosomes, which can be composed of iron oxide magnetite or iron sulfide greigite. Magnetite-producing magnetotactic bacteria are typically microaerophilic, whereas greigite-producing magnetotactic bacteria are usually strict anaerobes. Their magnetotactic response allows this type of bacteria to navigate efficiently across chemical gradients. Magnetotaxis is not a taxonomic trait, and has been described in many phylogenetic groups
Magnetotactic Bacteria. Figure 1 Magnetotactic bacteria with an intracellular chain of magnetosomes, extracted from lake sediments. Scale bar represents 0.5 mm
comprising diverse morphologies, including coccoid, rod-shaped, spirilloid, and even multicellular (Fig. 1).
History Magnetotactic bacteria were first observed by Salvatore Bellini, from the University of Pavia, in 1963 in samples from drainage water. Richard P. Blakemore described them in marsh sediments and first coined the term “magnetotactic” (Blakemore 1975). Since then, magnetotactic bacteria have been observed in almost every aquatic environment, including polar lakes, and several strains have been grown in pure cultures in the laboratory. This has allowed for comprehensive genetic studies and the identification of the genes involved in the synthesis of magnetosomes. In 1996, researchers from NASA Johnson Space Center suggested that magnetic crystals in the ▶ ALH84001 Martian meteorite resemble those synthesized by magnetotactic bacteria and would therefore constitute relic evidence of biological activity on the planet (McKay et al. 1996). While these claims have been disputed, the origin of the magnetic crystals in the meteorite remains a matter of debate.
See also ▶ ALH 84001 ▶ Magnetic Field
Main Sequence
▶ Magnetosome ▶ Microorganism ▶ Prokaryote
References and Further Reading Blakemore R (1975) Magnetotactic bacteria. Science 190:377–379 McKay DS, Gibson EK, Thomas-Keprta KL, Vali H, Romanek CS, Clemett SJ, Chillier XDF, Maechling CR, Zare RN (1996) Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001. Science 273:924–930
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objects that very large telescopes are able to catch are typically of magnitude 27. The magnitude scale has several avatars: the ▶ absolute magnitude, the ▶ bolometric magnitude, and the ▶ color index. It is also used to measure the dimming by an absorbing medium, for instance, that introduced by the atmosphere or by the interstellar ▶ extinction. In this last case, the difference in magnitude is noted as AV.
See also
Magnitude DANIEL ROUAN LESIA, Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot, Meudon, France
▶ Bolometric Magnitude ▶ Color Excess ▶ Color Index ▶ Extinction, Interstellar or Atmospheric ▶ Flux, Radiative ▶ Johnson UBV Bandpasses ▶ Magnitude, Absolute
Keywords Brightness, flux, photometric system, star
Definition In astronomy, the magnitude is a measure of the brightness of a celestial object expressed on a logarithmic scale.
Overview The magnitude is usually defined in the optical to midinfrared wavelength range (300 nm to 20 mm). The measure is generally defined for a specific filter, that is, for a given wavelength and a given bandpass. A set of filters corresponds to a photometric system, one of the most widely used being the Johnson system. With no other indication, the term magnitude refers to the magnitude in the V band of the Johnson system (l = 550 nm; bandpass = 89 nm). The magnitude scale is a relative scale, the reference of brightness being the star Vega. It is thus a quantity with no units. The magnitude of a given object whose measured flux in a given filter X is FX, is given by the expression: mX = 2.5 log 10(FX/FX[Vega]). The coefficient 2.5 as a factor in the logarithm of the brightness was chosen by Norman R. Pogson on the nineteenth century, so as to link this scale to the one used by ancient Greek astronomers when estimating qualitatively the star’s brightness. Note that the lowest magnitudes refer to the brightest objects: for instance, Sirius, the brightest star, has a magnitude V = 1.46, while the faintest stars seen by a trained unaided eye is of magnitude 6 and the dimmest
Magnitude, Absolute Definition The absolute magnitude of a star or any celestial object is the apparent magnitude that the object would have if it were at a distance of 10 pc or 32.6 light-year from Earth. Within the Solar system, the definition differs; it is the magnitude an object would have when put at a distance of one Astronomical Unit from Earth.
See also ▶ Magnitude
Main Asteroid Belt ▶ Asteroid Belt, Main
Main Sequence Definition The region of the ▶ Hertzsprung-Russell diagram occupied by stars during their central H-burning phase is called the main sequence. It is a quasi-diagonal band, running from low to high values of stellar luminosities and effective
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temperatures. The position of a star in that band is mostly determined by its mass (through the ▶ mass–luminosity relation), and to a much smaller degree by its metallicity (where astronomers refer to all elements heavier than helium as “metals”; less metallic stars being hotter) and age (older stars being more luminous). Since H-burning (fusion of hydrogen to helium) is the longest period in a star’s life, about 90% of all stars are on the main sequence. In star clusters, the upper end of the main sequence is truncated since the more massive stars have evolved toward the red giant branch (or already died); the position of this truncation provides a means to infer the cluster’s age.
See also ▶ Hertzsprung–Russell Diagram ▶ Mass–Luminosity Relation ▶ Stellar Evolution
Makganyene Glaciation
gradients and thermodynamic potentials that help drive biosynthesis around hydrothermal vent biological communities.
See also ▶ Archean Mantle ▶ Asthenosphere ▶ Magma ▶ Mantle Plume (Planetary) ▶ Mid-Ocean Ridges ▶ Plate Tectonics
Mantle Plume (Planetary) IAN CAMPBELL Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia
Synonyms Hotspot
▶ Snowball Earth
Keywords Flood basalt, hotspot, mantle plume, mass extinctions, ocean island basalt
Mantle Definition The mantle is the part of the Earth or of other planets between the outer crust and the core. On Earth, it is mainly composed of silicate minerals, whose nature changes depending on the pressure (or depth). In the upper mantle, between the base of the crust and the transition zone at about 660 km, the principal rock type is peridotite composed of olivine, orthopyroxene, clinopyroxene, and an aluminous phase (plagioclase, spinel, or garnet). In the lower mantle, these minerals convert to denser phases such as Mg- or Ca perovskite and magnesiowu¨stite. The terrestrial mantle is solid except in shallow zones of partial melting beneath ▶ mid-ocean ridges or in hot spots, and perhaps at the core–mantle boundary. The mantle convects at different scales, one manifested by relative motion of tectonic plates, the other by the upwelling of ▶ mantle plumes from the mantle/core boundary (D00 zone) or from the upper/ lower mantle boundary at 660 km depth. The occurrence of mantle convection is extremely important because ▶ plate tectonics and the subsequent exchange of matter and energy at the surface of the Earth create chemical
Definition Mantle plumes are columns of hot buoyant ▶ mantle that originate from a thermal boundary layer deep in the Earth. The most likely source of plumes is the core–mantle boundary at a depth of 2,900 km deep, although it has been suggested that small plumes may originate from the boundary between the upper and lower mantle at 670 km deep.
History Canadian geologist Tuzo Wilson in 1963 suggested that the age progression in the Hawaiian Islands was due to the oceanic lithosphere moving over a stationary “hot spot” in the mantle. Seven years later, American geophysicist Jason Morgan recognized that Wilson’s hot spots were plumes of hot mantle that originated from the thermal boundary layer above the core.
Overview Plumes must originate from thermal boundary layers, which in the case of Earth’s mantle is the core–mantle boundary or perhaps the boundary between lower and upper mantle. The core is hotter than the overlying
Mantle Volatiles
mantle and it cools by conduction to the mantle, forming an approximately 100 km thick boundary layer of hot, buoyant material. Before the buoyant mantle can rise at an appreciable rate, it must acquire enough buoyancy to overcome the viscosity of the cooler overlying mantle. As a consequence, starting plumes have a large head. Upwelling continues along the narrower hightemperature, low-viscosity pathway of the tail. The narrow diameter of mantle plumes conduits (< 50 km) is a result of the strong temperature dependence of mantle viscosity. Some models predict that plume heads initially have a diameter of about 200 km but that they grow by entrainment as they rise, reaching a diameter of 1,000 km at the top of the mantle. Here, they flatten to form disks about 2,000 km across. Other models predict more complex geometry and dynamics. If a plume head is compositionally heterogeneous, it can stall at the upper mantle–lower mantle boundary and give rise to one or more smaller, secondary plumes. The plume heads melt to form flood ▶ basalts when they ascend beneath continents, or oceanic plateaus when they ascend beneath oceans. Flood basalts and oceanic plateaus have observed lateral dimensions of 2,000 2,000 km, similar to the predicted dimensions of a flattened plume head. The Siberian and ▶ Deccan Traps, which erupted at 250 and 65 Ma respectively, are examples of continental flood basalts, and the 122 Ma old Ontong Java plateau is an example of an ocean plateau. Eruption is usually rapid and in these three cases probably took less than 1 Myr. Flood basalts are the most voluminous eruptions in the geological record and the two largest examples, the Siberian and Deccan Traps, correlate with the two biggest mass extinctions, the Permian-Triassic and the Cretaceous-Tertiary extinctions. More recent flood basalts, like the Deccan Traps, are connected by a chain of volcanic islands (Chagos–Laccadive ridge) to the current position of a plume or hot spot, which is now situated below Reunion Island. The island chain is formed by melting the plume tail and produces a track across the sea floor because motion of the plate drags the flood basalt away from the current position of the plume. Large planetary volcanic structures, such as ▶ Olympus Mons on Mars and Artemis on Venus, have also been attributed to mantle plumes.
See also ▶ Basalts ▶ Deccan Trapps ▶ K/T boundary ▶ Mantle ▶ Olympus Mons
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References and Further Reading Campbell IH (2007) Testing the plume hypothesis. Chem Geol 241:153–176 Griffiths RW, Campbell IH (1990) Stirring and structure and mantle plumes. Earth Planet Sci Lett 99:66–78 Hill IR (1991) Starting plumes and continental break-up. Earth Planet Sci Lett 104:398–416 Morgan WJ (1971) Convective plumes in the lower mantle. Nature 230:42–43 Saunders AD (2005) Large igneous provinces: origin and environmental consequences: Elements 1:259–263. http://www.elements magazine.org/ White R, McKenzie D (1989) Magmatism at rift zones: the generation of volcanic continent al margins and flood basalts. J Geophys Res 94:7685–7729, S Wilson JT (1963) A possible origin of the Hawaiian islands. Can J Phys 41:863–870
Mantle Redox State ▶ Mantle, Oxidation of
M Mantle Volatiles BERNARD MARTY Institut Universitaire de France, Ecole Nationale Supe´rieure de Ge´ologie, Centre de Recherches Pe´trographiques et Ge´ochimiques (CRPG), CNRS, Vandoeuvre les Nancy Cedex, France
Keywords Terrestrial mantle, volatile elements, noble gases, water, carbon, nitrogen
Definition Volatile elements are elements that are in gaseous, liquid, or icy form at temperature and pressure conditions of the Earth’s surface. ▶ Mantle volatiles are those elements present in the terrestrial mantle. These are mainly hydrogen (water), carbon, nitrogen and noble gases. Sulphur species and halogens are also transferred from the mantle to the Earth’s surface in gaseous form and are considered as mantle volatiles. These elements were trapped in the solid Earth during terrestrial accretion and were exchanged with the Earth’s surface through mantle-derived volcanism and
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plate ▶ subduction. Some of them (for example, helium and neon) have never seen the surface and are called primordial volatiles.
Overview Mantle-derived basalts and volcanic gases contain volatile elements such as water, carbon, nitrogen, sulphur halogens, and noble gases, whose isotopic composition indicates that they do not originate from the local crust or atmosphere/ hydrosphere but come from the underlying mantle. In particular, helium in mantle-derived volcanic rocks and gaseous emanations is enriched in the rare isotope 3 He, otherwise abundant in extraterrestrial material. Likewise, mantle neon has an isotopic composition different from that of the Earth’s atmosphere, and comparable to values found in the Sun or in material irradiated by the solar wind. The stable isotopes of H and N indicate that volatile elements in Earth were trapped from a source sharing similarities with primitive ▶ meteorites. Some of the noble gas isotopes are formed from extant and extinct radioactive decays, allowing quantification of mantle degassing through time. Comparison of the relevant isotopic ratios of the mantle and of the atmosphere indicates that mantle degassing was very active during the first tens to hundreds of Ma, resulting in the development of a stable atmosphere early in Earth’s history. In contrast to noble gases, major volatiles (H, C, N, S) have been exchanged between the mantle and the Earth’s surface because they can be trapped as solids in minerals and re-injected into the mantle at subduction zones. Mantle degassing takes place at mid-ocean ridges and at mantle plume localities, during magma generation. Volatile elements originally trapped in mantle minerals enter preferentially the magmas and degas to the atmosphere and hydrosphere during volcanic eruptions. Mantle ingassing occurs at subduction zones where the hydrated and carbonated oceanic plates are plunging into the mantle. Some of volatile elements are expelled during dehydration of plates and released back to the surface through arc magmatism, but a significant fraction of volatile elements survives dehydration and returns to the mantle, possibly at the core-mantle boundary. The mantle is a well-degassed reservoir: It contains 103 times less volatiles than primitive (▶ chondrite) meteorites. This depletion is the result of a combination of processes: volatile element depletion of the Earth’s building blocks impacts degassing of accreting bodies and mantle degassing through convection and magmatism. It is possible that mantle volatiles were acquired during the late accretion (that is, after terrestrial differentiation) of
a small fraction of volatile-rich material, a process known as the “▶ late veener” accretion.
See also ▶ Chondrite ▶ Earth’s Atmosphere, Origin and Evolution of ▶ Late Veneer ▶ Mantle ▶ Meteorites ▶ Subduction ▶ Volatile
References and Further Reading Graham DW (2002) Noble gas isotope geochemistry of mid-ocean ridge and ocean island basalts: characterization of mantle source reservoirs. In: Porcelli D, Ballentine CJ, Wieler R (eds) Symposium on Noble Gases. Mineralogical Soc America, Davos, Switzerland Hilton DR, Fischer TP, Marty B (2002) Noble gases and volatile recycling at subduction zones. In: Porcelli D, Ballentine CJ, Wieler R (eds) Symposium on Noble Gases. Mineralogical Soc America, Davos, Switzerland Zahnle KJ (2006) Earth’s earliest atmosphere. Elements 2:217–222
Mantle, Oxidation of DANIELE L. PINTI GEOTOP & De´partement des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada
Synonyms Mantle redox state
Keywords Archean mantle, atmosphere composition, mantle, outgassing, redox conditions
Definition The oxidation state of the mantle generally refers to the redox conditions prevailing in the mantle. This is evaluated by estimating the ▶ oxygen fugacity of the magmas.
Overview Mantle oxygen fugacity has been evaluated in different ways, mainly by studying the distribution of the Fe3+/S Fe ratio in basalts from midocean ridges and ocean islands, spinel and garnet peridotites for the upper mantle, and perovskite for the lower mantle (see Frost
Mare, Maria
and McCammon 2008 for a review). Speciation of C–O–H–S phases in magmas depends on the oxygen fugacity in the mantle and thus can be used for evaluating the oxidation state. Oxygen thermobarometry in mantle peridotite suggests that most the upper mantle falls within 2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer (fO2 108.5) (Frost and McCammon 2008). Mantle redox conditions might have varied with time and it has been suggested that the ▶ Archean or ▶ Hadean mantle could have been more reduced with oxygen fugacity fO2 1012, equivalent to the iron-wustite buffer (Kasting et al. 1993) (but see below). The redox conditions of the mantle may have influenced the evolution of the atmosphere-hydrosphere. Modern volcanic gases are relatively oxidized: typical H2/H2O and CO/CO2 ratios are 0.01 and 0.03 while reduced species such as NH3 and CH4 are virtually absent (Holland 1984). Their composition is consistent with that predicted for equilibrium with a magma whose oxygen fugacity is close to that of the FMQ equilibrium. In the Archean, more reduced gases, predominantly H2 and CO in equilibrium with a magma whose oxygen fugacity was close to that of the IW buffer, might have dominated the atmosphere and acted as an important O2 sink (Kump and Kasting 2001). The oxygen sink, by oxidation of the reduced volcanic outgassing flux, could have overwhelmed the photosynthetic O2 source, contributing to maintain a reducing atmosphere up to the Archean/Proterozoic boundary. However, variations in the contents of V and other redox-sensitive elements in Archean lavas and peridotites suggest that the oxygen fugacity in the ▶ Archean mantle was little different (Berry et al. 2008) or more oxidizing than that of the modern mantle (Canil 2002).
See also ▶ Archean Mantle ▶ Earth’s Atmosphere, Origin and Evolution of ▶ Earth, Formation and Early Evolution ▶ Geothermobarometers ▶ Oxygen Fugacity ▶ Oxygenation of the Earth’s Atmosphere
References and Further Reading Berry AJ, Danyushevsky LV, O’Neill HStC, Newville M, Sutton SR (2008) Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle. Nature 455:960–963 Canil D (2002) Vanadium in peridotites, mantle redox and tectonic environments: Archean to present. Earth Planet Sci Lett 195:75–90 Frost DJ, McCammon CA (2008) The redox state of Earth’s mantle. Annu Rev Earth Planet Sci 36:389–420 Holland HD (1984) The chemical evolution of the atmosphere and oceans. Princeton University Press, Princeton, 582 pp
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Kasting JF, Eggler DH, Raeburn SP (1993) Mantle redox evolution and the oxidation state of the Archean atmosphere. J Geol 101:245–257 Kump LR, Kasting JF (2001) Rise of atmospheric oxygen and the “upsidedown” Archean mantle. Geochem Geophys Geosyst 2. doi:10.1029/ 2000GC000114
Mare Plains ▶ Mare, Maria
Mare, Maria STEPHAN VAN GASSELT Planetary Sciences and Remote Sensing, Institute of Geological Sciences, Free University of Berlin, Berlin, Germany
Synonyms Basaltic flood plains; Mare plains
Keywords Basaltic flood volcanism, lava, mercury (not as an official term), The Moon, Titan
Definition Maria are medium- to large-size mostly circular flatfloored low-albedo plains on the ▶ Earth’s moon and the Saturnian ▶ Satellite ▶ Titan. On the ▶ Moon maria are formed by basaltic flood volcanism and are predominant on the lunar nearside. On Titan, maria are postulated to be filled with liquid ▶ hydrocarbon compounds, in particular ▶ methane and ethane.
Overview The term mare originally referred to large-size low▶ albedo features on the Moon. Lunar mare are lava plains characterized by low-viscosity basaltic flood lava. The use of the term mare has been extended to other terrestrial bodies in the solar system, especially to Titan where maria are radar-dark features that are assumed to be large lakes of liquid hydrocarbon compounds. On the Moon, the term mare basin refers to an impact structure filled by flood lava. Although lunar maria and ▶ impact basins are spatially related, mare materials were emplaced later when compared to impact basins formed
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during the Nectarian heavy bombardment period, as radiometric ages of samples and chronostratigraphic relationships demonstrate. Radiometric sample ages as well as crater-size frequency model ages of lunare maria are primarily in the range of 3.16–4.2 Gyr but may be as young as 1.2–1.5 Gyr. Despite a homogeneous impact basin distribution on the Moon, maria are usually found on the near side, which is considered to be the result of the Moon’s crustal dichotomy and is often associated with high mass concentrations (MASCONs) and large gravitational anomalies. Maria are rich in geologic features characteristic of emplacement of flood lava, such as lava flows, lobate flow scarps, volcanic domes and cones, lava ponds, sinuous and arcuate ▶ rilles, and wrinkle ridges. Maria were sampled during the Luna and ▶ Apollo missions and the returned samples as well as remotely sensed spectra showed a mafic (Mg/Fe) composition with modal plagioclase contents of up to 50 wt% and high FeO contents of up to 20 wt%, as well as high Ca/Al ratios when compared to lunar highland ▶ rocks. They show varying amounts of Ti, reaching up to 15 wt% TiO2 for high-Ti mare basalts. They are considered to be produced by partial melting of mafic rock in the lunar ▶ mantle at depths of up to 500 km. On ▶ Saturn’s satellite Titan, maria refer to flat, lowalbedo, radar-dark areas investigated in the infrared and radar spectrum by the ▶ Cassini spacecraft in 2006/2007. They are considered to be formed by liquid ▶ Hydrocarbons, especially methane. Ligeia Mare has a diameter of 500 km and is currently in discussion for investigation by a joint NASA-ESA planetary splash-down probe called Titan Mare Explorer (TiME).
See also ▶ Albedo Feature ▶ Apollo Mission ▶ Cassini Mission ▶ Earth ▶ Hydrocarbons ▶ Impact Basin ▶ Lacus ▶ Mantle ▶ Methane ▶ Oceanus, Oceani ▶ Palus, Paludes ▶ Rille ▶ Rock ▶ Satellite or Moon ▶ Saturn ▶ Titan
References and Further Reading Basaltic Volcanism Study Project (1981) Basaltic volcanism on the terrestrial planets. Pergamon, New York, p 1273 Hartmann WK, Phillips RJ, Taylor GJ (eds) (1986) Origin of the moon. Lunar and Planetary Institute, Houston Heiken GH, Vaniman D, French BM (1991) Lunar sourcebook: a guide to the moon. Cambridge University Press, New York, xix + 736 p Jolliff BL, Wieczorek MA, Shearer CK, Neal CR (eds) (2006) New views of the moon. Rev Min Geochem 60, Min. Soc. Am. and Geochem. Soc. Wilhelms DE (1987) The geologic history of the moon. USGS Prof. Paper 1348, 302 p
Marinoan Glaciation ▶ Snowball Earth
Mars FRANC¸OIS FORGET1, ERNST HAUBER2 1 Institut Pierre Simon Laplace, Laboratoire de Me´te´orologie Dynamique, UMR 8539, Universite´ Paris 6, Paris Cedex 05, France 2 German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Synonyms Atmosphere, mars; Water, mars
Keywords Atmosphere, climate, Habitability, ice, interior, Mars, surface, tectonics, volcanism, water
Definition Mars is the fourth ▶ Planet from the ▶ Sun and one of the ▶ terrestrial planets. It is named after the Roman god of war.
Overview Mars differentiated within a few tens of millions years after the formation of the ▶ Solar System into a ▶ core, ▶ mantle, and ▶ crust, as indicated by isotope geochemistry (Lee and Halliday 1997) and the old, 4.5 Ga age of the Martian meteorite, ▶ ALH84001 (Nyquist et al. 2001). It is commonly thought that the bulk composition of Mars is “chondritic” (Dreibus and Wa¨nke 1985), i.e., similar to
Mars
the composition of ▶ Carbonaceous Chondrites, the most primitive materials in the Solar System. The Fe–Ni–FeS core is dense and metal-rich (Fig. 1), and its radius is about 50% of the surface radius, the exact value depending on its content of lighter elements like sulfur. The remnant magnetization of parts of the Martian crust indicates that core convection induced by energy from core cooling and/ or inner core growth might have produced a core magnetic dynamo and a global ▶ magnetic field (Stevenson 2001). Evidence for crustal magnetization is mainly found in very old terrain and is absent in the largest ▶ impact basins, so the ▶ dynamo likely ceased before their formation about 4 Ga ago (Acuna et al. 1999) although this conclusion has been challenged (Schubert et al. 2000) For a discussion of the Martian dynamo see Connerney et al. (2004). The dynamic range of geopotential topography (i.e., the topography after subtraction of the gravitational potential or areoid from planetary radii) on Mars is 29.5 km, the largest of all ▶ terrestrial planets (Fig. 2). The major features – the lowest and highest elevations – are represented by deep impact basins and large shield volcanoes, respectively. The lowest single point of the Martian surface is located in Hellas Planitia (8,180 m below the Martian datum), the highest point is the summit of Olympus Mons (+21,300 m). The current thick elastic lithosphere of Mars allows maintaining such huge surface loads
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(e.g., Smith et al. 2001). The surface shows a global dichotomy or asymmetry in topography as well as in morphology (Watters et al. 2007). The southern hemisphere (about two third of the planet) displays significantly higher elevations and a rougher morphology, dominated by heavily cratered terrain. The border between the two hemispheres is in large parts marked by a topographic scarp with heights of up to several kilometers. An overview of basic facts about Mars is given in Table 2.
Crust and Mantle The Martian crust has a mean thickness between 30 and 80 km (perhaps up to 100 km), as inferred from gravity and topography (e.g., Zuber et al. 2000; Neumann et al. 2004). It is thicker in the southern highlands and thinner in the northern lowlands. The bulk composition of the Martian crust is assumed to be basaltic (Fig. 3), although minor amounts of alteration products have been identified (overviews on surface composition are given in Bell 2008). Thermochemical evolution models (reviewed by Breuer and Moore 2007) suggest that most of the crust was produced during the first billion years. According to the widely accepted compositional model of Wa¨nke and Dreibus (1994), the silicate mantle of Mars is Fe-rich and contains radiogenic elements in terrestrial abundances. Converting this composition to a pressure-dependent
Crust of variable thickness (~100 km in south; 30 km in north)
1,300–1,500 km Convecting silicate mantle (may be layered)
Fe-S(-Si?) liquid outer core Possible solid Fe inner core
Possible transition to perovskite (equivalent to 660 km discontinuity in earth)
Mars. Figure 1 Tentative cutaway view of the Martian interior (From Stevenson 2001; reprinted with permission, © Nature Publishing Group)
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Mars. Figure 2 Topography of Mars (shaded and color-coded elevation model derived from the Mars Orbiter Laser Altimeter (MOLA) data; Mollweide projection)
Mars. Table 1 Successful Mars missions
Viking 2
September 09, 1975 Orbiter and Lander
Phobos 2
July 12, 1988
Mars Global Surveyor
November 07, 1996 Orbiter
Mars Pathfinder
December 04, 1996 Lander and Rover
2001 Mars Odyssey
April 07, 2001
Orbiter
mineralogy, Longhi et al. (1992) obtain a layered mantle structure with an olivine-rich upper part, a transition zone of silicate spinel, and a lower part consisting mainly of perovskite (see Sohl and Schubert 2007, for a discussion of interior structure models). The thickness of the elastic ▶ lithosphere is variable over space and time and it is currently up to three or four times thicker at the north pole (>300 km) than at the Tharsis volcanoes (Grott and Breuer 2010). The average surface ▶ heat flow estimated from the elastic lithosphere thickness variation over time decreased from higher values in the ▶ Noachian (50–100 mW m2) to current values of the order of 10–20 mW m2 (Zuber 2001).
Mars Express
June 02, 2003
Orbiter and Lander
Global Geology
Spirit (MER-A)
June 10, 2003
Rover
Opportunity (MER-B)
July 7, 2003
Rover
Mission
Launch date
Type
Mariner 4
November 28, 1964 Flyby
Mariner 9
May 30, 1971
Orbiter
Viking 1
August 20, 1975
Orbiter and Lander Orbiter/attempted Phobos Landers
Mars August 10, 2005 Reconnaissance Orbiter
Orbiter
Phoenix
Scout Lander
August 04, 2007
The geologic surface record of Mars spans more than 4 billion years (Ga). Following the accepted stratigraphic system for Mars, this time is subdivided into the Noachian (4.1 Ga to 3.6 Ga), the ▶ Hesperian (3.6 Ga to 3 Ga), and the ▶ Amazonian (3 Ga to present) periods (Tanaka and Hartmann 2008). The precise boundaries between the periods depend on the cratering model that is used to derive absolute ages from crater size frequency
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Mars. Table 2 Bulk properties of Mars and Earth Bulk parameters
Mars
Earth
Ratio (Mars/Earth)
Mass (1024 kg)
0.64185
5.9736
0.107
Volume (1010 km3)
16.318
108.321
0.151
Equatorial radius (km)
3,396.2
6,378.1
0.532
Polar radius (km)
3,376.2
6,356.8
0.531
Volumetric mean radius (km)
3,389.5
6,371.0
0.532
Core radius (km)
1,700
3,485
0.488
Ellipticity (Flattening)
0.00648
0.00335
1.93
Mean density (kg/m3)
3,933
5,515
0.713
3.71
9.80
0.379
Surface acceleration (m/s )
3.69
9.78
0.377
Escape velocity (km/s)
5.03
11.19
0.450
Surface gravity (m/s2) 2
6
3
2
GM ( 10 km /s )
0.04283
0.3986
0.107
Bond albedo
0.250
0.306
0.817
Visual geometric albedo
0.150
0.367
0.409
Visual magnitude V(1,0)
1.52
3.86
–
Solar irradiance (W/m2)
589.2
1,367.6
0.431
Black-body temperature (K)
210.1
254.3
0.826
Topographic range (km)
30
20
1.500
Moment of inertia (I/MR2)
0.366
0.3308
1.106
J2 ( 106)
1,960.45
1,082.63
1.811
Number of natural satellites
2
1
Planetary ring system
No
No
distributions (Hartmann and Neukum 2001). The preNoachian is not represented by surface outcrops, but the 4.5 Ga age of the Martian meteorite, ALH84001, indicates the existence of crustal rocks that predate the oldest observable surface. More than 40% of the surface is Noachian in age (Fig. 4), as well as the largest wellpreserved impact basins Hellas, Argyre, and Isidis (Solomon et al. 2005). Noachian rocks are primarily found in the southern highlands, whereas Hesperian surfaces are more widely scattered. Large portions of the ancient crust in the northern lowlands are overlain by a relatively thin Hesperian cover of volcanic and sedimentary materials. Amazonian rocks are concentrated in volcanic regions (see below). The geologic surface inventory of Mars is very diverse and reflects the action of both endogenic and exogenic processes (Carr 2006).
Volcanism and Tectonics Evidence for volcanism on Mars is abundant (Greeley and Spudis 1981). About 15 huge volcanic shields, including the largest volcano in the Solar System, ▶ Olympus Mons,
as well as hundreds of smaller edifices and spatially extensive volcanic plains attest to the significant magmatic activity of Mars. Volcanism was active over all of Mars’ history, but the intensity has decreased over time. After the Noachian, volcanic resurfacing mainly focused on the two largest volcanic provinces of the planet, ▶ Tharsis and Elysium (Fig. 4). Volcanic eruption products are mainly basaltic, based on the analogy to terrestrial basaltic landforms, the analysis of ▶ SNC Meteorites, the in situ investigation of rocks and soils by landed missions, and spectral measurements from orbit (Fig. 3) (e.g., Zimbelman 2000). The Martian environment, in particular the low atmospheric pressure, leads to different eruption conditions as compared to Earth (Wilson and Head 1994). Since magma contains volatiles, volcanism has released large amounts of water and other volatile species (e.g., CO2) into the Martian atmosphere (outgassing). The rates, timing, and total amount of outgassing are unknown. Mars, like the ▶ Moon or ▶ Mercury, is a one-plate planet. There is no unambiguous surface record of plate
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GRS assumes gusev average Na/K
9
Trachyte Basaltic trachyandesite
Tephrite Na2O + K2O (wt. %)
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Trachyandesite
Gusev Rock RAT Rock brush Soil
Trachybasalt
6
Foidite
Rhyolite
Pathfinder rock
Basalt GRS
Martian meteorites Basaltic shergottite Ol-phyric shergottite Lherzolitic shergottite Nakhlite
Meridiani soils 3
TES Picrobasalt
Dacite Andesite
Pathfinder soils
Basaltic andesite
TES point density
Bounce rock 0 35
45
55 SiO2 (wt. %)
1
65
10
20 30–50
75
Mars. Figure 3 Composition of Martian surface materials shown in a diagram of total alkalis (vertical axis) versus silica. The bulk of the Martian crust plots in the basaltic domain (From McSween et al. 2009; reprinted with permission, © Science Magazine). Data from Mars meteorite analysis, ▶ Mars Pathfinder, Mars Odyssey Gamma Ray Spectrometer (GRS), ▶ Mars Global Surveyor Thermal Emission Spectrometer (TES), ▶ Mars Exploration Rover (MER) investigation using soil sample, brushed ▶ rocks or rock processed by the Rock Abrasion Tool (RAT) are combined
tectonics. However, a very early phase of plate tectonics could help explain the dynamo activity and would have avoided early massive melting on Mars. Its morphological traces might have been overprinted by later resurfacing processes. The single largest tectonic feature on Mars is the global dichotomy, which separates the southern highlands from the northern lowlands. The origin of this global dichotomy remains unknown, despite attempts to explain it as a consequence of an early Magma Ocean, mantle convection, an early phase of plate tectonics, or an exogenic process involving one or more impacts (Solomon et al. 2005). It was long thought that the southern highlands are much older than the northern lowlands, because much fewer ▶ craters were visible on images of the latter. Highly accurate topographic data from laser altimetry showed a large population of craters in the northern lowlands that have a very subdued topographic expression, obscuring their visibility in imaging data. If these craters are taken into account, the crust in the northern lowlands seems to be as old as in the southern highlands (Frey et al. 2002).
Except for the dichotomy, the Tharsis rise in the western hemisphere and, less importantly, the formation of Isidis Planitia and the Elysium rise in the eastern hemisphere clearly dominate the tectonic map of Mars on a global scale (Banerdt et al. 1992). Tharsis, the key to the tectonic evolution of Mars, is often considered to be the possible expression of a hot spot or mantle ▶ plume. (For an alternative view, see Schumacher and Breuer 2007). Deformation modeling indicates that large-scale lithospheric (magmatic) loading over Tharsis, which is large relative to the radius of the planet, is dominated by membrane stresses, and generates concentric extensional stresses around the periphery and radial compressional stresses closer to the center of Tharsis, which were accommodated by the formation of radial grabens and rifts and concentric wrinkle ridges (Golombek and Phillips 2010). As for the volcanism, tectonic activity on Mars seems to have peaked early and has gradually waned with time, although punctuated by episodes of higher and lower intensity. In general, vertical deformation is dominant and seems to be mainly associated with impact basins and large volcanic provinces.
Mars
A polar layered deposits
H materials
N-EH volcanic materials
EA vastitas borealis unit
LN-EH knobby materials
N materials
LH-LA volcanic materials
LN-EH materials
EN massif material
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90°N
60°N
30°N
0°N
30°S
60°S
90°S
Mars. Figure 4 Simplified global geology of Mars. The three major physiographic features are the southern highlands, the northern lowlands, and the volcanic provinces of Tharsis and Elysium (From Nimmo and Tanaka 2005). Unit age abbreviations: N, Noachian; H, Hesperian; A, Amazonian; E, Early; L, Late
Exogenic Surface Features Impact craters are found on virtually every surface on Mars. Of particular interest are craters with unique morphologies that reveal the distribution of water or ice at the surface or in the subsurface (Martian crater morphologies are reviewed by Barlow 2010). Rampart craters (Fig. 5c) are only observed on Mars. It is thought that they penetrated the ground into an ice- or water-rich layer. The minimum (or onset) diameter of rampart craters, therefore, can be used to determine the maximum depth of the ice or water table at the time of the impact. Other impacts might have strengthened an ice-rich target against erosion (e.g., by covering the ice by ejecta and protecting it from sublimation), so the present crater and its ejecta are elevated with respect to their surroundings (“pedestal” craters). Wind-blown materials (sand and dust) are also widespread and can be observed on almost any highresolution image, where they build aeolian bedforms such as ripples and different types of dunes. Wind was (and most probably still is) also an erosional force, and can be considered to be the most active geological agent on
present-day Mars (Greeley and Iverson 1985). ▶ Valley Networks (Fig. 5a) and ▶ outflow channels (Fig. 5e) resemble erosional patterns carved by water and are discussed below. Many young (Amazonian) landforms on Mars that were probably formed by exogenic processes show a latitude-dependent geographic distribution. They include surface mantling, lobate debris aprons, lineated valley fill and concentric crater fill, viscous flow features, ▶ gullies, and patterned ground. Collectively, these landforms (many of which are morphologically analogous to glacial and periglacial landforms on Earth) are hypothesized to represent the surface records of Martian ice ages (Head et al. 2003) that were induced by astronomical forcing and associated climate changes (see below).
Atmosphere and Present-Day Climate The Martian atmosphere is primarily composed of CO2 (95%) with a little nitrogen, argon, and oxygen (Table 3). The atmospheric pressure at the surface ranges from 1 to 14 mbar depending on location and season (compared to 1,013 mbar on average at sea level on Earth). In spite of
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a
b
c
d
e
f
Mars. Figure 5 Evidence for liquid water at the Martian surface: (a) valley network; (b) deltas (c) lobate ejecta crater; (d) gullies; (e) outflow channel with streamlined “islands”; (f) fluvial delta with inverted channels. Images a, b, c, e, f are from the Mars Express High Resolution Stereo Camera (HRSC), and d from the Mars Reconnaissance High Resolution Imaging Science Experiment (HiRISE)
Mars
Mars. Table 3 Principal constituents of the atmospheres of Mars (volume mixing ratios), compared to the Earth Gas
Symbol
Mars
Earth
Carbon dioxide
CO2
95.32%
(Molecular) nitrogen
N2
2.7%
78%
Argon
Ar
1.6%
0.93%
0.035%
(Molecular) oxygen
O2
0.13%
20.6%
Carbon monoxide
CO
0.07%
0.00002%
Water
H2O
0.03%
0.4%
these differences, the Martian climate system is similar to the Earth climate system in many aspects. The two planets rotate with almost the same rate and a similar obliquity. The length of day is thus almost the same (24 h and 40 min on Mars) and the seasonal cycle is comparable. In such conditions, the general circulation is controlled by similar processes (Read and Lewis 2004): On both planets, the Hadley circulation (the process that generates the trade winds) is important at low latitudes, whereas “baroclinic” planetary waves (a succession of low- and high-pressure zones) dominate the weather system at midlatitudes. However, with such a thin atmosphere and a dry soil, diurnal and seasonal surface temperature variations are much more marked, with temperatures typically ranging between 20 C and 135 C). Briefly, the climate of Mars is “hyper-continental.” To first order, the Martian meteorology can thus be compared with what one would expect on a cold, dry desert-like Earth. However, just like the climate system on the Earth is not controlled only by temperature gradients and the atmospheric circulation because of the presence of oceans and water clouds, several phenomena make the climate system more complex on Mars. Firstly, as much as 30% of the ▶ carbon dioxide atmosphere condenses every winter at high latitudes to form ▶ polar caps, inducing surface pressure variations all over the planet (CO2 cycle). Secondly, a highly variable amount of suspended dust modifies the radiative properties of the atmosphere, with global dust storms sometimes totally shrouding the planet (dust cycle). Finally, a peculiar hydrological cycle occurs on Mars, with water vapor transported by the atmosphere and the formation of clouds, hazes, and frost (water cycle).
Water on Mars Today Water is almost everywhere on present-day Mars, in solid or gaseous state (Carr 1996). There are several known types of reservoirs:
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First, relatively pure, white ice is directly exposed at the surface of Mars in an area of about 1,000 km in diameter around the north pole. The thickness of this ice layer may vary from a few millimeters at its boundary up to a few meters or a few tens of meters in some locations near its center. This ice layer is in direct interaction with the atmosphere, and is the principal surface reservoir involved in the atmospheric water cycle: in summer, the ice is warmed by the sun and sublimes. For a few weeks, the northern polar region becomes a source of water vapor that is transported away within the atmosphere. During the rest of the year, most of this water ultimately comes back to the northern polar cap through various transport mechanisms. The cycle is closed and near equilibrium. The amount of water involved in the seasonal water cycle is small. If one could precipitate the water content of the atmosphere on the surface, a layer thinner than a few tens of micrometers would be obtained even in the “wettest” regions. In the cold Martian conditions, though, saturation is often reached. Therefore, clouds form in the atmosphere and frost can condense onto the surface. The atmospheric water vapor should also be able to diffuse into the porous subsurface. The surface ice near the northern pole appears to cover a much bigger structure made of thousands of layers of ice and dust that have accumulated over more than 3,000 m in thickness (see Clifford et al. 2000). These northern ▶ polar layered deposits are thought to be comparatively young, preserving a record of the seasonal and climatic cycling of atmospheric CO2, H2O, and dust over the past million years. Radar observations of the south polar region demonstrated that the thick southern polar layered deposits (about 1,000 km in diameter, and 2-km thick) accumulated around the pole are also composed of relatively pure water ice (Plaut et al. 2007), currently isolated from the atmosphere by a layer of dry sediments in most locations. Near the south pole, the small (300 km), white, bright residual cap is primarily composed of carbon dioxide ice, and for a long time it had been assumed that no water ice layer was present at the surface in the southern polar region. Observations of the imaging spectrometer Omega aboard ▶ Mars Express, however, revealed the presence of perennial water ice deposits at the edges of the CO2 ice bright cap, and even in large areas tens of kilometers away from it. Most likely, the permanent CO2 ice cap lies on an extended water ice layer (Bibring et al. 2004). In both the northern and southern hemisphere, poleward of about 55–60 latitude, the Mars Odyssey
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Gamma-Ray Spectrometer investigation has shown that where ice is not exposed at the surface, it is still present just beneath the surface (below an ice-free layer a few centimeters thick), typically in the form of a dirty ice layer more than 1-m thick and with no less than 50% of ice. Moreover, in these areas the unique morphology of the ground is consistent with the presence of a few meters thick ice layer coating the surface (see a synthesis in Head et al. 2003). In 2008, the NASA Phoenix mission landed near 69 N in order to confirm the presence of ice and better understand its origin and its environment. As expected, a layer of ice was found just a few centimeters below the surface (Fig. 6). In specific locations in midlatitudes and even in the tropics, there is geomorphologic and radar sounding evidence of buried ice landforms that are reminiscent of the rock-covered glaciers that can be found on Earth. The ice is thought to have been transported in the atmosphere, deposited, and then insulated and protected by a layer of sediments. The origins of these deposits could be related to climate changes due to time variations of orbital parameters (see below). Below these buried ice layers, and even at lower Martian latitudes, it is likely that the large pore volume of the Martian subsurface regolith might be partly filled by ice. Calculations of the thermodynamic stability of ground ice suggest that it can exist very close to the surface at high latitudes, but can persist only at substantial depth (several hundred meters) near the equator (Squyres et al. 1992). Observational evidence of the subsurface ice
might be the presence of impact craters with distinctive lobate ejecta (Fig. 5c) that are not found on the Moon, and that apparently owe their morphology to the melting and vaporizing of ground ice during the impact. The size frequency distribution of these craters is consistent with the depth distribution of ice inferred from stability calculations.
Liquid Water on Mars Today To form liquid water, H2O must be exposed to a pressure above 6.1 mbar and a temperature above the freezing point (0 C for pure water), and below the boiling point (which depends on pressure). Such conditions are not uncommon at the surface of Mars, for instance in the lower plains in summer in early afternoon. They last only a few hours at most, however, and only the first few millimeters below the surface can be heated above 0 C. Because of the low vapor pressure of water in the atmosphere, any ice that may be present at the surface (in the morning, for instance) sublimates into the atmosphere well before it can melt (e.g., Haberle et al. 2001). The existence of liquid water in a “metastable” state in some specific circumstances has been suggested, but remains unlikely. In some cases, dissolved salts could form a brine, which can be liquid at temperatures well below the freezing point of pure water (Faire´n et al. 2009). Liquid water may exist in the subsurface, more likely at several thousands of meters below the surface, where water present in the pore space could be heated by the geothermal flux (Squyres et al. 1992). The most “optimistic” scientists imagine that liquid water aquifers could exist at much shallower depth, up to a few tens of meters below the surface, as a consequence of local geothermal activities, but, again, this remains speculative. In particular, radar soundings have not yet been able to detect any liquid water aquifers, in spite of the fact that the liquid water dielectric properties should make it detectable in theory.
Climate Changes Due to Obliquity and Orbital Parameters Variations Mars. Figure 6 An image of the Martian surface beneath NASA’s Phoenix Mars Lander taken by Phoenix’s Robotic Arm Camera (RAC) on the eighth Martian day of the mission on June 2, 2008. A superficial dust layer, shown in the dark foreground, has been blown off by Phoenix’s thruster engines, exhibiting an ice-rich layer a few centimeters below the surface. The presence of the ice in Mars high latitudes had been remotely detected by the Mars Odyssey Gamma-Ray Spectrometer investigation in 2002 (Image: NASA/JPL)
As on Earth, the climate on Mars depends on the planet’s orbital and rotational parameters, and in particular its obliquity (inclination of Mars’ axis of rotation with respect to its orbit plane). For Earth, such oscillations are small (1.3 for the obliquity), but they are thought to have played a key role in the glacial and interglacial climate cycles. For the case of Mars, calculations have shown that Mars’ obliquity varied widely and somewhat erratically in the past, between 0 and more than 60 (the current obliquity is 25.2 ) (Laskar et al. 2004).
Mars
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These variations must have had a strong impact on the Martian climate. A quantified description of these past climates is not easy because one has to predict the response of a complex system that includes the atmospheric circulation and the coupled CO2, dust, and water cycles. Nevertheless, on the basis of theoretical considerations and with the help of numerical climate simulations, we can speculate about the major changes. During periods of low obliquity, with an obliquity below 20 , the seasonal cycle is weaker than today. The latitudinal extension of the seasonal polar caps is reduced. The polar regions experience a net decrease in solar heating (the sun is lower on the horizon) and thus a net cooling. It is likely that the mass of frozen atmosphere trapped at the poles (currently near the southern pole) increases at the expense of the total atmospheric mass. Models also show that in such conditions the water and dust cycle are much less active. The atmosphere is thinner and clearer. Mars is then a frozen world. During periods of high obliquity. Beyond 30 of obliquity, the seasonal cycle is stronger than today. For instance, the CO2 ice seasonal polar caps extend to the tropics in winter when the obliquity reaches 40 . In the polar regions, the increase of annual mean insolation leads to a warming of the relatively deep subsurface. In summer, the polar water ice reservoir is strongly heated and releases tens to hundred times more water than today, inducing a very active hydrological cycle (vapor, clouds, frost). Beyond obliquities of 35–45 , computer simulations of the Martian climate suggest that warming of any ice left at the poles would liberate such a large amount of water into the atmosphere that, in certain areas, water vapor would condense and precipitate out much more readily than it could sublime. In these areas, ice would therefore have accumulated (as long as there was still a source at the pole) and might even have formed glaciers (Forget et al. 2006). The remains of these glaciers (moraines) and possibly debris-covered glaciers have indeed been found where the models indicate they once existed. Today, it seems that a large amount of the ice that may have been transported in and out of the polar regions is back at the poles. If this scenario is confirmed, the polar layered deposits can probably be attributed to the past oscillations of the climate.
▶ Gullies (Fig. 5d) are hundreds of meters to kilometer-long structures usually found on steep slopes with an apron or fan of debris at the distal end. They seem to have been created by flowing water in geologically recent times: perhaps a few million years ago, perhaps much more recently. Since they were discovered in high-resolution images (Malin and Edgett 2000a), their origin is debated. Some researchers put forward the idea that there are aquifers below ground, warmed by energy from within Mars. These could flow at the surface intermittently. However, this scenario does not satisfactorily explain the distribution of the flow features: they seem unconnected with any volcanism. Neither does it address the fact that these features are observed on isolated peaks and even on dunes. An alternative scenario would be simply to assume that they were formed when ice deposited during period of high obliquity (see above) may have briefly melted, inducing debris flows and gullies. Outflow channels (Fig. 5e) are immense and deep valleys that appear to have been carved by flowing water, mud, or glaciers (Baker 1982). Their widths can exceed 100 km, and they can reach lengths of >1,000 km. Most are several billions years old, but seem to have been episodically active in several stages, and some outflow channels appear to be less than a few tens of millions of years old. Outflow channels show a peculiar morphology: they have no tributaries, and their geographical origin is quite localized. On their floors, large numbers of channels sweep around prominent obstacles such as impact craters, sculpting teardrop-shaped structures tapering in the direction of the flow. The geometry of these streamlined “islands” has led researchers to conclude that they were shaped by very turbulent liquid water, rather than by ice, lava or mud, and that only sudden, short-lived bursts of vast amounts of water can explain the characteristics of the outflow channels. The water could have been suddenly liberated from an underground water reservoir, trapped under pressure within the Martian ▶ permafrost, or in relation to the melting of permafrost due to lava rising through fractures in the crust. The formation of the outflow channels does not require the climate to have been very different from what is observed today.
Evidence for Transient Liquid Water on Mars in the Past Million Years: Gullies and Outflow Channels
Early Mars: A Potential Habitable Planet More Than 3.5 Billion Years Ago
Two kinds of very different geological landforms suggest that liquid water may sometime briefly flow on Mars in the relatively recent geological past (Baker 2001).
About half of the surface of Mars is extremely ancient, dating back from prior to 3.8 billion years ago, a time when the impact rate from infalling meteoroids and planetesimals was much higher than today. This part of the
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Martian surface, mostly in the southern highlands, has also kept the record of a period where the environment was much different from what it became afterward. There is evidence that, at this time, liquid water was present on the surface of Mars and could have formed rivers, lakes, and possibly an ocean. ● Valley networks (Fig. 5a). Valleys, with characteristics very similar to terrestrial river drainage valleys, were discovered in 1971 by the Mariner 9 orbiter. Smaller valleys coalesce downhill into larger valleys, forming a dendritic network. These valleys are observed on most of the surfaces older than 3.5–3.8 billion years (Carr 1996), but only very few have been identified on more recent terrain. This finding suggests that Mars enjoyed an environment suitable for the formation of these valleys more than ~3.5 billion years ago (at least periodically), but that such conditions rarely occurred later. The recent exploration era is revealing more and more valley morphologies that suggest that precipitation and runoff was involved. ● Lacustrine deposits? (Fig. 5b, f). High-resolution images provided new evidence of deposits, mostly within craters, that have an appearance similar to layered lacustrine deposits, deltas, sedimentary terraces, and shorelines (Malin and Edgett 2000b, 2003; Kleinhans 2010). Well-defined layering has been identified within these deposits, supporting the idea of standing water, although windblown sediments could account for some of the observations. ● An ancient ocean? Most outflow channels seem to end downstream in the northern lowlands, which would have acted as a huge sink for water and sediments. If the Noachian were warm enough to prevent the formation of a sufficiently thick ▶ cryosphere, which could trap pressurized groundwater beneath and maintain an elevated global water table, the formation of a primordial ocean in the lowest part of the global topography (e.g., Hellas, northern lowlands) would seem plausible, if not inevitable (Clifford and Parker 2001). According to this model, such an ocean would have frozen with time, and finally the water would have been assimilated in the crust in a thickening cryosphere. It has to be noted, however, that there is no unambiguous morphologic or mineralogic evidence for an ancient ocean on Mars, and its existence is still contentious. ● Erosion rate: A detailed analysis of the ancient terrains reveals that old impact craters larger than about 15 km in diameter have a substantially degraded appearance. Crater rims, ejecta blankets, and central peaks have
been removed in many cases and the crater interiors have been filled in with debris. Craters smaller than about 15 km have been erased entirely. However, craters formed in regions a few hundreds of millions years younger are much better preserved. This observation suggests that in the distant past, before 3.8 billion years ago, the erosion rate was orders of magnitude more efficient than later in the history of Mars (Jakosky and Phillips 2001). While this erosion could have many possible causes, it is generally thought that liquid water was the eroding agent. ● Mineralogy: On Earth, the composition of many rocks and soils points to their formation in the presence of liquid water (e.g., hydrated materials). On Mars, such evidence is rarer. The observations gathered by the thermal infrared spectrometer TES aboard Mars Global Surveyor and by the near infrared imaging spectrometer Omega aboard Mars Express suggest that most surfaces are either of volcanic origin, or covered by dust (Christensen et al. 2001; Bibring et al. 2005). However, in 1998, thermal emission spectrometer (TES) revealed the presence of three regions with an exposed surface enriched in the iron oxide mineral ▶ hematite, a weathering product inferred to have been precipitated from water flowing through the crust. One of these regions, Sinus ▶ Meridiani, was selected to be explored by Opportunity, one of the two Mars Exploration Rovers that began their mission in January 2004. Opportunity did confirm the presence of hematite, and made an even more interesting discovery: it detected the presence of the sulfate salts jarosite (KFe3(III)(SO4)2(OH)6) and magnesium sulfates like kieserite (MgSO4 • H2O) in the sedimentary outcrops of the area, with physical characteristics indicative of aqueous transport (Squyres et al. 2004). On this basis, the Opportunity team concluded that these rocks probably record episodic inundation by shallow surface water, evaporation, and desiccation. At the same time, OMEGA actually discovered and mapped hydrated sulfates – kieserite, polyhydrated sulfates, and even gypsum (CaSO4 • 2 H2O) – mostly in light-toned layered terrains located in Terra Meridiani, ▶ Valles Marineris, and Margaritifer Sinus. Even more interestingly, the OMEGA team discovered clays (▶ phyllosilicates, primarily iron/magnesium smectites) in several locations restricted to the most ancient terrains (Poulet et al. 2005), suggesting that clay formation may have taken place primarily during the earliest portion of Martian history. Clays were probably formed by the action of liquid water on volcanic rocks such as ▶ basalt, over tens of thousands
Mars
of years. Altogether, the mineralogical investigations suggest that Mars experienced, early on, a period when liquid water was probably abundant, and able to soak soils during episodes long enough for clays to form. Later, in a drier and more acidic environment, sulphated salts were more likely to form (Bibring et al. 2006). The fact that some alteration minerals can still be observed at present, although they formed several billion years ago and are easily transformed into other materials by, e.g., ▶ diagenesis, suggests that water was very limited on the Martian surface from soon after their formation until today (Tosca and Knoll 2009). The Early Mars Climate enigma. The possible existence of relatively warm conditions suitable for surface liquid water on Mars 3.8 billion years ago is unexpected. Most experts believe that at that time, the young sun was less intense than today and its luminosity 25% lower than at the present time. Since Mars is about 1.5 times more distant from the sun than the Earth, the solar energy available on Mars then was only one third of what we enjoy on Earth today. In such conditions, the radiative equilibrium temperature (▶ effective temperature) of the planet should have been 75 C, not taking into account the atmosphere. In such conditions, what could have made the climate so warm and wet? ● Atmospheric greenhouse effect: The composition of the early Mars atmosphere is not well constrained, but it was probably mostly composed of carbon dioxide with a surface pressure between a few hundreds of millibars to a few bars. Such amounts are consistent with the initial volatile inventory of a planet like Mars, and it is likely that large amounts of CO2 and H2O should have been released in the atmosphere by the substantial volcanism that occurred on early Mars, associated with the formation of the volcanic plains and the Tharsis bulge. Both CO2 and H2O are greenhouse gases (▶ greenhouse effect), but their ability to warm the planet through greenhouse effects is limited, and they may not have been able to solve the early Mars climate enigma by themselves (see review in Haberle 1998). Other greenhouse gases, such as NH3, CH4, or SO2 have been proposed to help solve the enigma, but they should have been photochemically unstable in the early Mars atmosphere and rapidly exhausted, unless produced by a surface or subsurface source (volcanism? life?). However, the presence of sulfate salts on Mars has motivated recent studies involving SO2 in the atmosphere. Another possibility is that Mars was warmed by CO2 ice clouds. These clouds tend to
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form in a thick CO2 atmosphere. Models show that they can reflect a significant part of the thermal infrared radiation emitted by the surface and warm the planet through an exotic “scattering greenhouse effect” (Forget and Pierrehumbert 1997). ● Role of geothermal flux. Some geological evidence related to liquid water on early Mars could be explained by the early large geothermal heat flux (estimated to be 5–10 times the present-time values on average) that contributed to warm the near subsurface and probably induced intense geothermal activity. Volcanic intrusions may have locally caused even larger heat flow values. ● Asteroid impacts. The large impacts that often occurred at the end of the heavy bombardment may have played a role episodically. For instance, impacts produced global blankets of very hot ejecta that could have warmed the surface, keeping it above the freezing point of water for periods ranging from decades to millennia (Segura et al. 2002).
Life on Early Mars? Life was probably present on Earth when Mars enjoyed habitable conditions more than 3.8 billion years ago. If life on Earth arose as a result of fundamental and repeatable physicochemical processes, there is no a priori reason why life might not also have emerged on the Red Planet, more than 3.5–4 billion years ago. A study of Mars shows that conditions at its surface were such that liquid water could have been present there for a limited period only – most likely during the first 1 billion years of Martian history. On Earth, the first multicellular organisms (and, a fortiori, the first animals and plants) appeared after more than 2 billion years of unicellular predecessors. If any life form evolved on Mars, this suggests that it was probably primitive. However, it is not clear if the long evolution time is needed for genetic evolution, or because physical and chemical conditions on Earth (e.g., amounts of oxygen) were incompatible with the existence of more complex creatures? Perhaps evolution did not proceed in the same way on Mars, and some optimistic scientists have speculated that more evolved forms of life could have existed on Mars at the same epoch.
Life on Mars Today? Mars today is cold and dry. If one assumes that Martian life requires the same necessary ingredients as life as we know it on Earth (Jakosky et al. 2007), the primary challenge for potential Martian organisms would be to find liquid water. This is especially true at the surface or near the surface. Moreover, the Martian surface is exposed to
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intense ▶ ultraviolet radiation, which should be lethal for most living organisms although there are microorganisms that have been reported to survive high radiation doses. Also, the upper few meters of the soil seem to have been sterilized by highly oxidizing compounds present in the atmosphere. In such conditions, assuming that life ever did get started on Mars, how could it have survived until today? If life has survived until the present day on Mars, we will have to look for it in those niches where liquid water is still present. One possibility is several kilometers underground, where the planet’s internal heat source ensures that temperatures remain above 0 C. “Interstitial” water could harbor life, as observed in deep Earth crust samples. Nearer to the surface, Martian volcanism, which, though on the decline, still might occur, probably creates warmer places. Just as on Earth, organisms might exploit the relatively rich chemical resources found in such locations. It must be stated, however, that for life to have held out this long on the planet, Martian organisms would have needed a stable environment in their “bioniches” for billions of years. The detection of life on the Martian surface was one of the main objectives of the ▶ Viking Landers (1976). An articulated arm, capable of digging into the surface, gathered samples of Martian soil. The samples were subjected to three investigations: the Pyrolitic Release, Gas Exchange, and Labeled Release experiments, which yielded negative or ambiguous results. However, the official verdict was finally negative (Klein et al. 1992), because the gas chromatograph coupled with a mass spectrometer, capable of detecting organic molecules at a concentration below one part per billion, found nothing. An alternative way to detect biological activity on Mars is to monitor the presence of gases possibly produced by biological activity and which could not be present at a given level without the presence of life, like molecular oxygen on the Earth. There is no such obvious biogenic gas on Mars. However, the reported discovery of ▶ methane since 2004 from Earth-based and orbital observations has reactivated this investigation. Methane should be unstable in the Martian atmosphere, so its presence requires the existence of a source. In theory, this source could be unrelated to life. For instance, some methane on Earth is produced by hydrothermal activity, which leads to the oxydoreduction reaction between ironbearing primary silicates (olivine and pyroxene) and water to form hydrogen and FeIII-bearing phyllosilicates, a process called ▶ serpentinization. The hydrogen can then be used to produce methane by biologic and
nonbiologic mechanisms. However, the intensity of the source of methane inferred from the observations (local bursts of methane have been reported) is such that it would be comparable to the production of methane by serpentinization along the entire Mid-Atlantic Ridge on Earth (50,000–130,000 t/year). On a ▶ planet where no or a little volcanic activity seems to occur at present time, this would be unexpected. Because most of the methane produced on Earth is biogenic, it is thus tempting to speculate on a possible subsurface biological production. Nevertheless, the current environment on Mars would constitute a great challenge for life as we know it. The temperature and atmospheric pressure commonly prevent liquid water to be stable at the surface, which is also chemically harsh and affected by intense radiation. It is doubtful that organisms can survive today at the Martian surface (Knoll and Grotzinger 2006).
Future Directions The exploration of Mars will continue with the next generation rover, NASA’s Mars Science Laboratory (MSL). The objective of MSL (recently named “Curiosity”) is to investigate the habitability of Mars, i.e., to assess whether Mars ever was, or is still today, an environment able to support microbial life. Possible landing sites are locations that display morphological and/or mineralogical evidence for the former presence of liquid water. The Mars Atmosphere and Volatile Evolution Mission (MAVEN), set to launch in 2013, will explore the planet’s upper atmosphere, ionosphere, and interactions with the sun and the solar wind. Scientists will use MAVEN data to determine the role that loss of volatile compounds, such as CO2, NO2, and H2O, from the Mars atmosphere to space has played through time, giving insight into the history of the Martian atmosphere and climate, liquid water, and planetary habitability. In 2016, a planned ESA/NASA orbiter will search for trace gases such as methane, and will try to map their variations over time and location in the Martian atmosphere. Further plans are currently under development, with NASA and ESA collaborating toward further common missions (e.g., a rover mission in 2018 and a later network mission), with the distant goal of a ▶ Mars Sample Return (MSR) mission.
See also ▶ ALH 84001 ▶ Basalt ▶ Carbon Dioxide
Mars
▶ Carbonaceous Chondrite ▶ Core, Planetary ▶ Crater, Impact ▶ Crust ▶ Cryosphere ▶ Diagenesis ▶ Dynamo (Planetary) ▶ Effective Temperature ▶ Evaporite ▶ Greenhouse Effect ▶ Gullies ▶ Heat Flow (Planetary) ▶ Hematite ▶ Impact Basin ▶ Jarosite ▶ Lithosphere (Planetary) ▶ Magnetic Field ▶ Mantle ▶ Mars Express ▶ Mars Global Surveyor ▶ Mars Pathfinder ▶ Mars Sample Return Mission ▶ Mars Stratigraphy ▶ MER, Spirit and Opportunity (Mars) ▶ Mercury ▶ Meridiani (Mars) ▶ Methane ▶ Moon, The ▶ Noachian ▶ Olympus Mons ▶ Outflow Channels ▶ Permafrost ▶ Phobos-Grunt ▶ Phyllosilicates (Extraterrestrial) ▶ Planet ▶ Plume ▶ Polar Caps (Mars) ▶ Polar Layered Deposits (Mars) ▶ Rock ▶ Serpentinization (Mars) ▶ SNC Meteorites ▶ Solar System Formation (Chronology) ▶ Sulfates (Extraterrestrial) ▶ Sun (and Young Sun) ▶ Terrestrial Planet ▶ Tharsis ▶ UV Radiation ▶ Valles Marineris ▶ Valley Networks ▶ Viking
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References and Further Reading Acuna MH, Connerney JEP, Ness NF et al (1999) Global distribution of crustal magnetism discovered by the Mars Global SurveyorMAG/ER Experiment. Science 284:790–793 Baker VR (1982) The channels of Mars. University of Texas Press, Austin Baker VR (2001) Water and the Martian landscape. Nature 412:228–236 Banerdt WB, Golombek MP, Tanaka KL (1992) Stress and Tectonics on Mars. In: Kieffer HH, Jakosky BM, Snyder CW, Matthews MS (eds) Mars. University of Arizona Press, Tucson, pp 249–297 Barlow NG (2010) What we know about Mars from its impact craters. Geol Soc Am Bull 122:644–657 Bell JF (2008) The Martian surface. Cambridge University Press, Cambridge Bibring J-P et al (2004) Perennial water ice identified in the south polar cap of Mars. Nature 428:627–630 Bibring J-P et al (2005) Mars surface diversity as revealed by the OMEGA/ Mars express observations. Science 307:1576–1581 Bibring J-P, Langevin Y, Mustard JF, Poulet F, Arvidson R, Gendrin A, Gondet B, Mangold N, Pinet P, Forget F (2006) Global mineralogical and aqueous Mars history derived from OMEGA/Mars express data. Science 312:400–404 Breuer D, Moore WB (2007) Dynamics and thermal history of the Terrestrial Planets, the Moon, and Io. In: Spohn T (ed) Planets and Moons: treatise on geophysics, vol 10. Elsevier, Amsterdam, pp 299–348 Carr MH (1996) Water on Mars. Oxford University Press, Oxford Carr MH (2006) The surface of Mars. Cambridge University Press, Cambridge Christensen PR et al (2001) Mars global surveyor thermal emission spectrometer experiment: investigation description and surface science results. J Geophys Res 106:23,823–23,872 Clifford SM, Parker TJ (2001) The evolution of the Martian hydrosphere: implications for the fate of a Primordial Ocean and the current state of the Northern Plains. Icarus 154:40–79 Clifford SM et al (2000) The state and future of Mars polar science and exploration. Icarus 144:210–242 Connerney JEP, Acuna MH, Ness NF et al (2004) Mars crustal magnetism. Space Sci Rev 111(1–2):1–32 Dreibus G, Wa¨nke H (1985) Mars: a volatile-rich planet. Meteoritics 20:367–382 Faire´n AG, Davila AF, Gago-Duport L, Amils R, McKay CP (2009) Stability against freezing of aqueous solutions on early Mars. Nature 459:401–404 Forget F, Pierrehumbert RT (1997) Warming early Mars with carbon dioxide clouds that scatter infrared radiation. Science 278:1273–1276 Forget F, Haberle RM, Montmessin F, Levrard B, Head JW (2006) Formation of glaciers on Mars by atmospheric precipitation at High obliquity. Science 311:368–371 Frey HV, Roark JH, Shockey KM, Frey EL, Sakimoto SHE (2002) Ancient lowlands on Mars. Geophys Res Lett 29:22–1, CiteID 1384, doi: 10.1029/2001GL013832 Golombek MP, Phillips RJ (2010) Mars tectonics. In: Watters TA, Schultz RA (eds) Planetary tectonics. Cambridge University Press, Cambridge, pp 183–232 Greeley R, Iverson JD (1985) Wind as a geological process on Earth, Mars, Venus and Titan. Cambridge University Press, Cambridge Greeley R, Spudis PD (1981) Volcanism on Mars. Rev Geophys Space Phys 19:13–41
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Grott M, Breuer D (2010) On the spatial variability of the Martian elastic lithosphere thickness: evidence for mantle plumes? J Geophys Res 115:E3, CiteID E03005, doi:10.1029/2009JE003456 Haberle RM (1998) Early Mars climate models. J Geophys Res 103:28,467–28,480 Haberle RM, McKay CP, Schaeffer J, Cabrol NA, Grin EA, Zent AP, Quinn R (2001) On the possibility of liquid water on present-day Mars. J Geophys Res 106:23,317–23,326 Hartmann WK, Neukum G (2001) Cratering chronology and the evolution of Mars. Space Sci Rev 96:165–194 Head JW, Mustard JF, Kreslavsky MA, Milliken RE, Marchant DR (2003) Recent ice ages on Mars. Nature 426:797–802 Jakosky BM, Phillips RJ (2001) Mars’ volatile and climate history. Nature 412:237–244 Jakosky BM, Westall F, Brack A (2007) Mars. In: Sullivan WT, Baross JA (eds) Planets and life. Cambridge University Press, Cambridge, pp 357–387 Kieffer HH, Jakosky BM, Snyder CW (1992) The planet Mars: from antiquity to the present. In: Kieffer HH, Jakosky BM, Snyder CW, Matthews MS (eds) Mars. University of Arizona Press, Tucson, pp 1–33 Klein HP, Horowitz NH, Biemann K (1992) The search for extant life on Mars. In: Kieffer HH, Jakosky BM, Snyder CW, Matthews MS (eds) Mars. University of Arizona Press, Tucson, pp 1221–1233 Kleinhans MG (2010) A tale of two planets: geomorphology applied to Mars´ surface, fluvio-deltaic processes and landforms. Earth Surf Process Land 35:102–117 Knoll AH, Grotzinger J (2006) Water on Mars and the prospects for Martian life. Elements 2:171–175 Laskar J, Correia ACM, Gastineau M, Joutel F, Levrard B, Robutel P (2004) Long term evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170:343–364 Lee D-C, Halliday AN (1997) Core formation on Mars and differentiated asteroids. Nature 388:854–857 Longhi J, Knittle E, Holloway JR, Wa¨nke H (1992) The bulk composition, mineralogy and internal structure of Mars. In: Kieffer HH, Jakosky BM, Snyder CW, Matthews MS (eds) Mars. University of Arizona Press, Tucson, pp 184–208 Malin MC, Edgett KS (2000a) Evidence for recent groundwater seepage and surface runoff on Mars. Science 288:2330–2335 Malin MC, Edgett KS (2000b) Sedimentary rocks of early Mars. Science 290:1927–1937 Malin MC, Edgett KS (2003) Evidence for persistent flow and aqueous sedimentation on early Mars. Science 302:1931–1934 McSween HY, Taylor GJ, Wyatt MB (2009) Elemental composition of the Martian crust. Science 324:736–739 Neumann GA, Zuber MT, Wieczorek MA, McGovern PJ, Lemoine FG, Smith DE (2004) Crustal structure of Mars from gravity and topography. J Geophys Res 109:E08002. doi:10.1029/ 2004JE002262 Nimmo F, Tanaka K (2005) Early crustal evolution of Mars. Ann Rev Earth Planet Sci 33:133–161 Nyquist LE, Bogard DD, Shih CY, Greshake A, Sto¨ffler D, Eugster O (2001) Ages and geologic histories of martian meteorites. Space Sci Rev 96:105–164 Plaut JJ et al (2007) Subsurface radar sounding of the South Polar Layered deposits of Mars. Science 316:92–96 Poulet F et al (2005) Phyllosilicates on Mars and implications for early martian climate. Nature 438:623–627 Read PL, Lewis SR (2004) The Martian climate revisited: atmosphere and environment of a desert planet. Springer, Berlin
Schubert G, Russell CT, Moore WB (2000) Geophysics – timing of the Martian dynamo. Nature 408(6813):666–667 Schumacher S, Breuer D (2007) An alternative mechanism for recent volcanism on Mars. Geophys Res Lett 34:L14202. doi:10.1029/ 2007GL030083 Segura TL, Toon OB, Colaprete A, Zahnle K (2002) Environmental effects of large impacts on Mars. Science 298:1977–1980 Sheehan W (1996) The planet Mars: a history of observation and discovery. University of Arizona Press, Tucson Smith DE and 23 co-authors (2001) Mars Orbiter Laser Altimeter: experiment summary after the first year of global mapping of Mars. J Geophys Res 106:23,689–23,722 Sohl F, Schubert G (2007) Interior structure, composition, and mineralogy of the Terrestrial planets. In: Spohn T (ed) Planets and Moons: treatise on geophysics, vol 10. Elsevier, Amsterdam, pp 27–68 Solomon SC, Aharonson O, Aurnou JM, Banerdt WB, Carr MH, Dombard AJ, Frey HV, Golombek MP, Hauck SA, Head JW, Jakosky BM, Johnson CL, McGovern PJ, Neumann GA, Phillips RJ, Smith DE, Zuber MT (2005) New perspectives on ancient Mars. Science 307:1214–1220 Squyres SW, Clifford SM, Kuz’min RO, Zimbelman JR, Costard FM (1992) Ice in the Martian regolith. In: Kieffer HH, Jakosky BM, Snyder CW, Matthews MS (eds) Mars. University of Arizona Press, Tucson, pp 523–554 Squyres et al (2004) The opportunity Rover’s athena science investigation at Meridiani Planum, Mars. Science 306:1698–1703 Stevenson DJ (2001) Mars´ core and magnetism. Nature 412:214–219 Tanaka KL, Hartmann WK (2008) Planetary time scale. In: Ogg JG, Ogg G, Gradstein FM (eds) The concise geologic time scale. Cambridge University Press, New York, pp 13–22 Tosca NJ, Knoll AH (2009) Juvenile chemical sediments and the long term persistence of water at the surface of Mars. Earth Planet Sci Lett 286:379–386 Wa¨nke H, Dreibus G (1994) Chemistry and accretion history of Mars. Philos Trans R Soc Lond A349:285–293 Watters TR, McGovern PJ, Irwin RP (2007) Hemispheres apart: the crustal dichotomy on Mars. Ann Rev Earth Planet Sci 35:621–652 Wilson L, Head JW (1994) Mars: review and analysis of volcanic eruption theory and relationships to observed landforms. Rev Geophys 32:221–263 Zimbelman JR (2000) Volcanism on Mars. In: Sigurdsson H (ed) Encyclopedia of Volcanoes. Academic, San Diego, pp 771–783 Zuber MT (2001) The crust and mantle of Mars. Nature 412:220–27 Zuber MT, Solomon SC, Phillips RJ et al (2000) Internal structure and early thermal evolution of Mars from Mars Global Surveyor topography and gravity. Science 287(5459):1788–1793
Mars Analogue Sites RICHARD LE´VEILLE´ Space Science and Technology, Canadian Space Agency, Saint-Hubert, QC, Canada
Synonyms Analogue sites
Mars Analogue Sites
Keywords Astrobiology, Mars, exploration, field, extremophiles, biosignatures, missions, extreme environments, comparative planetology
Definition Mars analogue sites are places on Earth that present one or more geological or environmental features similar to those found on Mars, either current or past.
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dry conditions are perhaps the most important presentday Mars-like conditions that are especially relevant to astrobiology. Mars-like geological features on Earth include volcanic systems (including lava tubes), salt and sand dunes, gullies and canyons, polar and alpine deserts, impact craters, as well as various hydrothermal and sedimentary systems (Farr 2004; Osinski et al. 2006; Chapman 2007, Le´veille´ 2009; Le´veille´ 2010b).
Basic Methodology History Studies of ▶ Mars analogue sites have been an integral part of comparative planetary science, space exploration, and astrobiology for over a half-century (Le´veille´ 2010a). Following the successful Mariner missions of the 1960s, planetary scientists began studying in more detail Mars-like features and environments on Earth, including volcanic and erosional systems, glacial and thermokarst features, dunes, playas, and impact craters. Mars analogue sites were later studied in order to plan for possible landing sites for the ▶ Viking mission. Similarly, Mars analogue sites were used to test ▶ NASA’s earliest life-detection instruments. Early microbiological studies of desert soils, including some from ▶ Antarctica and the ▶ Atacama, focused on the adaptations and survivability of microorganisms under Mars-like conditions. Since the Viking mission and more recent orbital and rover missions, new Mars analogue sites have been continually identified and studied as new data are returned to Earth and our understanding of Mars is deepened.
Overview Mars analogue sites are defined as places on Earth that share one or more physical, chemical, or geological similarities with Martian ▶ environments, either current or past (Farr 2004; Osinski et al. 2006; Le´veille´ 2009). Studies of analogue sites are an essential aspect of understanding remote or in situ robotic observations of Mars, and to ultimately better understand our solar system (Chapman 2007). With respect to astrobiology, analogue sites provide insight into the limits to life on Earth, adaptations of organisms to extreme conditions, and the diversity and distribution of habitable environments on Earth and possibly elsewhere in our Solar System (Rothschild and Mancinelli 2001; National Research Council 2007; Le´veille´ 2010b). Analogue sites are also indispensible to the study of the formation and preservation of microbial biosignatures, in both ancient and modern systems. Though no location on Earth is identical to any location on Mars, many terrestrial analogue sites do approximate conditions or features found on Mars. Extreme cold and
The term fidelity is used to describe the similarity of an analogue site with respect to the extraterrestrial environment to which it is compared (e.g., Snook et al. 2007; Le´veille´ 2009). Analogue sites with multiple attributes that are closely related to Mars are said to offer a high fidelity. Several different analogue sites may be studied in order to fully understand the data generated during a space mission or to explain an extraterrestrial process. In studying Mars analogue sites, it is important to take into account what is similar and what is different to Mars, and to ensure that differences do not affect the analysis for which the analogue site is used (National Research Council 2007). With respect to astrobiology and ▶ Mars, analogue sites can be used in at least three fundamental ways: 1. To better understand data returned from a Mars mission in order to assess the habitability potential of studied sites. 2. To predict and identify where past habitable environments may have existed on Mars. 3. To identify biosignatures (or biosignature suites) that may be searched for at a given location on Mars. Results from such analogue studies can directly assist in the formulation of scientific objectives, as well as help to define operational and instrumentation requirements for future planetary missions.
Applications Analogue sites have been used extensively to test hypotheses about possible extraterrestrial processes, including biosignature formation, degradation, and preservation. They can also be used to test exploration strategies, biosignature detection and measurement techniques, sample-handling protocols, and mission concepts in a variety of geological and environmental settings. Analogue sites can also be used to test and validate science instruments, robotic systems, and integrated hardware packages in challenging environments and under operating conditions representative of actual missions, at a relatively low cost and risk (Le´veille´ 2009).
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Studies of analogue sites can also help to establish baseline abiotic signatures or provide positive controls for life-detection instruments. Similarly, issues of forward contamination and cross-contamination, as well as mitigation strategies and cleaning protocols, can be incorporated into many types of field-based studies at analogue sites. Studies of analogue sites are also useful for the training of scientists and engineers (including remote science and engineering operations teams), mission planners, ground operations crews, and even astronauts, in a cost effective, low-risk way (National Research Council 2007). By studying analogue sites, science teams can learn how to remotely detect and identify planetary features and perfect decisional protocols before missions begin (Chapman 2007). Field-based projects also help to train students in the tools and knowledge needed to participate in future astrobiology research, payload development, and ultimately missions. Some important examples of analogue sites for astrobiology include the Haughton Impact Crater on ▶ Devon Island and the perennial springs of Axel Heiberg Island, Canada; the Antarctic Dry Valleys; the acidic river system Rio Tinto, Spain; acidic lakes of Australia; and the ▶ Atacama Desert, Chile. Other types of analogue sites showing promise with respect to astrobiology and Mars exploration include sites of active terrestrial serpentinization and exposed basaltic glass, lava tubes and caves, playa lakes, hydrothermal springs, as well as ▶ permafrost and ground-ice containing areas. Analogue missions refer to simulations of planetary surface missions that involve an integrated set of scientific and operational activities at an analogue site (Snook et al. 2007; Le´veille´ 2009). Analogue missions can specifically target operational requirements and produce lessons learned in order to reduce costs and risks associated with future missions. For example, analogue missions can help to develop and test sampling strategies and measurement requirements, make ground truth measurements, direct training in planning science operations, promote team building, and improve communications. Some examples of astrobiology-focused analogue missions include the MARTE drilling project at ▶ Rio Tinto, Spain; the Life in the Atacama Desert project; and the AMASE project at Svalbard.
Future Directions The US National Research Council has recommended that studies of analogue sites should be a fundamental component of Mars astrobiological research and the development of future robotic missions to Mars (National Research Council 2007). Testing Mars surface operations at
analogue sites is also a high priority of the planetary science community (Farr 2004; Osinski et al. 2006). Integrated scientific and operational studies at analogue sites will likely be increasingly critical for Mars sample return missions, for which coordination of highly advanced technologies will be required at very carefully preselected sites (Borg et al. 2008). Analogue sites will also continue to be essential for selecting landing and sampling sites, sampling and site characterization strategies, and for planning overall mission operations, as well as for developing novel technologies for Mars missions. Increased international coordination and integrated field and remote operations teams will help to address future target mission requirements and scientific objectives.
See also ▶ Analogue Sites ▶ Antarctica ▶ Atacama Desert ▶ Biomarkers, Isotopic ▶ Crater, Impact ▶ Devon Island ▶ Extreme Environment ▶ Extremophiles ▶ Hydrothermal Environments ▶ Permafrost ▶ Rio Tinto ▶ Soda Lakes ▶ Terrestrial Analogues ▶ Yellowstone National Park, Natural Analogue Site
References and Further Reading Borg LE, Des Marais DJ, Beaty DW, Aharonson O, Benner SA, Bogard DD, Bridges JC, Budney CJ, Calvin WM, Clark BC, Eigenbrode JL, Grady MM, Head JW, Hemming SR, Hinners NW, Hipkin V, MacPherson GJ, Marinangeli L, McLennan SM, McSween HY, Moersch JE, Nealson KH, Pratt LM, Righter K, Ruff SW, Shearer CK, Steele A, Sumner DY, Symes SJ, Vago JL, Westall F (2008) Science priorities for Mars sample return. Astrobiology 8:489–536 Chapman MG (2007) The geology of Mars: evidence from Earth-based analogs. Cambridge University Press, Cambridge Farr TG (2004) Terrestrial analogs to Mars: The NRC community decadal report. Planet Space Sci 52:3–10 Le´veille´ RJ (2009) Validation of astrobiology technologies and instrument operations in terrestrial analogue environments. CR Palevol 8:637–648 Le´veille´ RJ (2010a) A half-century of terrestrial analog studies: from craters on the Moon to searching for life on Mars. Planet Space Sci 58:631–638 Le´veille´ RJ (2010b) The role of terrestrial analogue environments in astrobiology. In: Gargaud M, Lopez-Garcia P, Martin H (eds) Origin and evolution of Life: an astrobiology perspective, chapter 30. Cambridge University Press, Cambridge, pp 507–522
Mars Express National Research Council (2007) An astrobiology strategy for the exploration of Mars. National Academies Press, Washington Osinski GR, Le´veille´ R, Berinstain A, Lebeuf M, Bamsey M (2006) Terrestrial analogues to Mars and the moon: Canada’s role. Geosci Can 33:175–188 Rothschild LJ, Mancinelli RL (2001) Life in extreme environments. Nature 409:1092–1101 Snook K, Glass B, Briggs G, Jasper J (2007) Integrated analog mission design for planetary exploration with humans and robots. In: Chapman M (ed) The Geology of Mars: Evidence from Earth-based Analogs. Cambridge University Press, Cambridge, pp 424–455
Mars Exploration Rovers ▶ MER, Spirit and Opportunity (Mars)
Mars Express JEAN-PIERRE BIBRING Institut d’Astrophysique Spatiale, Universite´ Paris-Sud, Orsay Cedex, France
Keywords ESA, Mars
Definition The Mars Express mission is the first planetary mission of the European Space Agency (▶ ESA). Decided in 1997, it was launched from Baı¨konur on June 2, 2003, and was inserted into ▶ Mars orbit December 25, 2003, after proper maneuvers transferring the spacecraft into an almost polar orbit. Mars Express started its orbital observations in early January 2004. The targeted duration of the mission was 1 Martian year (687 terrestrial days). However, in 2010, it is still operating 7 years after launch, and will possibly continue a few more years. Some days before arriving at Mars, Mars Express ejected the Beagle 2 lander, designed to land and to perform surface measurements. Unfortunately, the contact with ▶ Beagle 2 was lost and never recovered. However, the measurements and observations from orbit are making this mission the most successful one in the exploration of Mars.
History The Mars Express mission was conceived after the dramatic loss at launch of the Russian Mars 96 mission
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in November 1996. The goal was to develop a “small mission,” embarking some spare models of instruments developed for Mars 96.
Overview The 2003 launch opportunity was extremely favorable: a Soyouz rocket would be sufficient, and the overall cost could be capped at a very low level. To fit within the allocated mass of 100 kg, five instruments were selected, out of the available spares: HRSC (High-Resolution Stereo Camera), built in Germany (G. Neukum, PI); OMEGA (Observatoire pour la Mine´ralogie, l’Eau, les Glaces et l’Activite´, a visible and infrared hyperspectral imager), built in France, Italy, and Russia (J-P. Bibring, PI); SPICAM (Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars, an ultraviolet and infrared atmospheric spectrometer), built primarily in France and Russia (J-L. Bertaux then F. Montmessin, PI); PFS (Planetary Fourier Spectrometer), primarily built in Italy (V. Formisano, then D. Grassi, PI); ASPERA (Analyzer of Space Plasmas and Energetic Atoms, an analyzer of ions and electrons in the outer atmosphere), primarily built in Sweden (R. Lundin, then S. Barabash, PI). ESA decided to add a lander (Beagle 2 was selected), and an orbiter instrument to sound the subsurface: the selected one was named MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding, a subsurface and ionospheric radar sounder), built in cooperation between Italy and the USA (G. Picardi, PI). Finally, Mars Express would embark a Mars Radio Science Experiment (MaRS), with no specific hardware, using the radio signals to probe the planet atmosphere and surface (M. Pa¨tzold, PI). With this suite of instruments, Mars Express is capable of sounding and analyzing, possibly in a coupled way, all Martian envelopes: its outer (ionized) atmosphere, its neutral atmosphere, its surface and its subsurface, down to a few kilometers. The variety and diversity of achievements is impressive. They cover all prime themes of Mars science, offering clues to decipher Mars’ evolution at all timescales, from its seasonal changes to its climatic and geological variations.
Key Research Findings A list of ten top scientific discoveries has been issued by ESA, as follows: 1. The history of Mars has been profoundly revisited after the detection by OMEGA of alteration minerals and phyllosilicates in particular. Hydrated phyllosilicates were formed, at a planetary scale,
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when long-standing bodies of water were present: they record an ancient era of potential habitability. This happened before the end of the heavy bombardment, and while the dynamo was still active; the relevant sites, that future astrobiological missions should explore, are located within the cratered crust. Methane has been detected from orbit by the PFS spectrometer and its spatial and vertical distribution is being mapped. Its presence would indicate that Mars is still either geologically or biologically active. It paves the way for future dedicated missions. The HRSC has offered breathtaking views of the planet and provided new insights into the planet’s topography, allowing a much better understanding of the formation and evolution of the surface geological features. In particular, tropical and equatorial landforms have been identified, as well as possible glaciers active just a few thousand years ago. They likely trace the chaotic evolution of the planet obliquity (Fig. 1). The North and South Polar Layered Deposits consist of nearly pure water ice, as deduced from the MARSIS radar data. Unique maps of H2O ice and CO2 ice in the polar regions have been produced by OMEGA: the CO2 southern ices constitute a very thin veneer on top of a massive water ice glacier (Fig. 2). The combination of digital terrain models with HRSC coverage at high resolution (better than 20 m/pixel) indicates very young ages for volcanic
Mars Express. Figure 1 The caldera of Olympus Mons as taken from high-resolution stereo camera
Mars Express. Figure 2 The Marth vallis view from the high-resolution camera depicting in light blue, the hydrated minerals as identified by the Omega spectro-imager
Mars Global Surveyor
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processes, which may have persisted until recent times, a few million years typically. They could still be active near the North Pole: Mars is just reaching its geological death. The analyzer of space plasma and energetic atoms (ASPERA) has found that the solar wind is slowly stripping off the high atmosphere down to 270-km altitude, and has measured the current rate of atmospheric escape of planetary ion, which happens to be relatively low and occurs in bursts. The composition of the escaping plasma has been precisely measured. Auroras have been revealed by SPICAM, over midlatitude regions. They are produced by the precipitation of electrons, measured by ASPERA above regions with crustal paleomagnetic field. SPICAM has also detected airglows in the Martian nightside. A transient ionospheric layer, due to meteors burning in the atmosphere, has been identified by radiooccultation. Phobos, the largest Mars moon (27 km in “diameter”), has been intensively observed, by all instruments, during a few close flybys, down distances less than 100 km. It led to refined measures of its size, mass, density, interior and surface structure, and surface composition, with in particular no features associated with neither hydrated nor carbonaceous constituents. Very high-altitude (up to 80 km) mesospheric ▶ CO2 ice clouds have been detected and characterized in the equatorial region of Mars. This discovery enlightens the processes responsible for the cloud formation on Earth, as well as for the sustenance of surface liquid water during the early Mars History.
Although limited in cost and ambition, and started with the failure Beagle 2, the Mars Express mission has greatly contributed to the understanding of the planet Mars. It has launched ESA within the few major actors of planetary space exploration.
See also ▶ CO2 Ice Cap (Mars) ▶ CO2 Ice Clouds (Mars) ▶ Dust Devils ▶ Ice ▶ Mars ▶ Mars Stratigraphy ▶ MRO ▶ Polar Caps (Mars)
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Mars Global Surveyor FRANC¸OIS FORGET Institut Pierre Simon Laplace, Laboratoire de Me´te´orologie Dynamique, UMR 8539, Universite´ Paris 6, Paris Cedex 05, France
Synonyms MGS
Keywords Mars, NASA
Definition Mars Global Surveyor was a ▶ NASA orbiter that mapped and monitored planet ▶ Mars with five scientific instruments between its arrival at Mars on September 11, 1997, and November 2006. Mars Global Surveyor was the first fully successful mission around Mars since the ▶ Viking missions in 1976. It revolutionized our understanding of the red planet and ushered in a new era of Mars exploration.
History After the very successful Viking missions launched in 1976, it took more than 16 years for NASA to select, design, and launch a new orbital mission to Mars: Mars Observer. It carried an ambitious scientific payload with seven instruments. Unfortunately, contact was lost 3 days before orbit insertion on August 21, 1993, due to an unknown failure of the spacecraft. The smaller Mars Global Surveyor project was then initiated on the ashes of Mars Observer, with the objectives of using spare models or copies of five of its experiments.
Overview Mars Global Surveyor (MGS) was a 1,060 kg (full mass at launch) spacecraft launched on November 7, 1996 and inserted into Mars Orbit on September 11, 1997. By March 9, 1999, MGS had slowly circularized through aerobraking to a Sun-synchronous, near-polar orbit with an average altitude of 378 km, mapping Mars at solar local time close to 2 am–2 pm (Albee et al. 2001). It operated until a battery failure in November 2007. Its payload included five instruments that revolutionized our understanding of Mars geophysics and climatology, and provided many reference datasets on the red planet. By helping scientists understand the nature of water reservoir and Mars, and the ancient Mars possibly “wet” environment, many of their findings are of
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interest to exobiology. Mars Global Surveyor experiments were as follows: – The Mars Orbiter Laser Altimeter (MOLA), which created the most accurate global topographic map of any planet in the solar system, giving scientists elevation maps precise to within about 0.3 m in the vertical dimension with a global resolution of typically less than 2 km (Smith et al. 2001). The topography and geodesy data identified pathways for the flow of past water and the locations, sizes, and volumes of water flows and reservoirs like the polar caps. The Laser altimeter was also used to detect ▶ CO2 ice clouds in the polar night and monitor the amount and distribution of condensed carbon dioxide. In June 2001, part of the laser reached the end of its life, but a sensor continued to detect changes in surface brightness in the near infrared part of the spectrum. – The Mars Orbiter Camera (MOC) science investigation used three instruments: a narrow angle camera that obtained grayscale (black-and-white) high-resolution images (long strip 3 km wide with typically 1.5–12 m per pixel), a red-and-blue wide angle cameras for context (240 m per pixel) and a daily global imager (7.5 km per pixel). MOC operated in Mars orbit between September 1997 and November 2006. It returned more than 240,000 images. The highresolution images allowed numerous key discoveries on Mars geomorphology (Malin and Edgett 2001), including the presence of numerous layered deposits (Malin and Edgett 2000b) and even deltas, which appears to have been formed in liquid water environment (Malin and Edgett 2003); the presence of ▶ gullies probably created by flowing water in geologically recent times (e.g., a few millions years ago) (Malin and Edgett 2000a). – The Thermal Emission Spectrometer (TES) monitored the Martian surface and atmosphere using thermal infrared spectroscopy (from 6 to 50 mm with either 5 or 10 cm1 resolution) together with a bolometric visible-NIR (0.3–2.9 mm) measurement. TES systematically monitored the Martian climate (retrieval of the atmospheric temperatures, dust loading, clouds, ▶ polar caps, water vapor) from the beginning of the mission until a failure on August 31, 2004. It provided a reference climatology (Smith 2004) and an unprecedented understanding of Mars meteorology. Surface monitoring allowed the mapping of the ground albedo and thermal inertia, and the retrieval of the abundance of several minerals (Christensen et al. 2001a). In particular, in specific locations, TES
detected some crystalline ▶ hematite, an iron oxide, whose formation often requires the presence of liquid water (Christensen et al. 2001b). Hematite was detected in ancient sedimentary regions on Mars, such as ▶ Meridiani, south of Arabia Terra. Meridiani was later the chosen site for in situ exploration by the ▶ Mars Exploration Rover Opportunity, which discovered an interesting terrain in which liquid water is thought to have played a key role. – The Magnetometer combined with an electron spectrometer (MAG/ER) was designed to study the magnetic properties of Mars surface and its ionosphere (Acun˜a et al. 2001). It discovered that while Mars had no global ▶ magnetic field generated in the planet’s core, significant fields could be detected in small, particular areas of the ▶ crust in the most ancient terrains on the planet. These are the vestigial signal of an ancient and once powerful magnetic field, capable of magnetizing the surface. This ancient field may have protected the early atmosphere from gas escape induced by the ▶ solar wind, and contributed to the ancient Mars habitability. – The Radio Science Investigations (RS) used measurements of the Doppler shift of radio signals sent back to Earth to determine a model of the Mars gravity field. The analysis of the signal passing through the atmosphere allowed the monitoring of the density and temperature of the lower atmosphere and of the ionosphere properties.
See also ▶ CO2 Ice Clouds (Mars) ▶ Gullies ▶ Habitability of the Solar System ▶ Hematite ▶ Magnetic Field ▶ Mars ▶ Mars Exploration Rovers ▶ Polar Caps (Mars) ▶ Viking
References and Further Reading Acun˜a MH, Connerney JEP, Wasilewski P, Lin RP, Mitchell D, Anderson KA, Carlson CW, McFadden J, Re`me H, Mazelle C, Vignes D, Bauer SJ, Cloutier P, Ness NF (2001) Magnetic field of Mars: summary of results from the aerobraking and mapping orbits. J Geophys Res 106(E10):23403–23417 Albee AL, Arvidson RE, Palluconi F, Thorpe T (2001) Overview of the Mars global surveyor mission. J Geophys Res 106(E10):23291–23316 Christensen PR et al (2001a) Mars global surveyor thermal emission spectrometer experiment: investigation description and surface science results. J Geophys Res 106:23823–23872
Mars Pathfinder Christensen PR, Morris RV, Lane MD, Bandfield JL, Malin MC (2001b) Global mapping of martian hematite mineral deposits: remnants of water-driven processes on early Mars. J Geophys Res 106(E10):23873–23885 Malin MC, Edgett KS (2000a) Evidence for recent groundwater seepage and surface runoff on Mars. Science 288:2330–2335 Malin MC, Edgett KS (2000b) Sedimentary rocks of early Mars. Science 290:1927–1937 Malin MC, Edgett KS (2001) Mars global surveyor Mars orbiter camera: interplanetary cruise through primary mission. J Geophys Res 106(E10):23429–23570 Malin MC, Edgett KS (2003) Evidence for persistent flow and aqueous sedimentation on early Mars. Science 302:1931–1934 Smith MD (2004) Interannual variability in TES atmospheric observations of Mars during 1999–2003. Icarus 167:148–165 Smith DE et al (2001) Mars orbiter laser altimeter: experiment summary after the first year of global mapping of Mars. J Geophys Res 106: 23689–23722 Zuber MT (2001) The crust and mantle of Mars. Nature 412:220–227
Mars Odyssey FRANCES WESTALL Centre de biophysique mole´culaire, CNRS, Orle´ans cedex 2, France
Keywords Mars, NASA, orbital measurements, water
Definition 2001 Mars Odyssey, a name inspired by the works of the science fiction writer A.C. Clarke, is a ▶ NASA orbiter that was launched in 2001 to study the surface of ▶ Mars from orbit.
Overview Mars Odyssey is still active and holds the record as the longest lived spacecraft in martian exploration. The science objectives of the mission are wide ranging, but the theme “follow the water” is of particular importance. Other objectives include analysis of the surface elemental and mineralogical composition of the planet, indications of geological processes, such as volcanic activity, and measurement of the radiation environment in preparation for human exploration. These objectives were accomplished using the instruments THEMIS (The Thermal Emission Imaging System), the GRS (Gamma Ray Spectrometer- Neutron spectrometer), and MARIE (the Mars Radiation Environment Experiment). Mars Odyssey also serves as an important relay for the continuing ground-based rovers, Spirit and Opportunity, the Mars Exploration Rovers (MERs).
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The mission has been very successful. In particular subsurface water ice was detected in the form of elemental hydrogen in the near subsurface (below a few centimeters of dry sediments) in both hemispheres above about 60 latitude. Other areas of hydrogen concentration, for example, Arabia Terra, are most likely related to the presence of hydrated minerals or possibly to subsurface ice. Geomorphological evidence for the presence of water in the past (and possibly in more recent times) was revealed by the infrared camera THEMIS in the form of channels features cut by water and melted snow, as well as sediments deposited by water. THEMIS has also mapped mineralogical variation in the surface composition of Mars. Evidence for rocks with a granite-like composition, as well as olivine-rich rocks, were detected. Lavas with an evolved composition indicate a complex volcanic history for the planet. The latter indicate little alteration of the original basalts during much of Mars’ history. Importantly, the surface images were used to determine the landing sites of not only the MER rovers but also the polar lander Phoenix. THEMIS was also designed to detect “hot spots” on Mars which may have corresponded to local geothermal of hydrothermal activities. This research – of high interest with regard to the possibility of life in the near Martian subsurface – has been unsuccessful. Such spots may not exist or be very rare. MARIE measured both galactic cosmic rays and solar energy particles to determine the radiation environment. These data could be used to prepare human exploration.
See also ▶ Mars ▶ Mars Analogue Sites ▶ Mars Express ▶ Mars Global Surveyor ▶ Mars Pathfinder ▶ Mars Reconnaissance Orbiter ▶ Mars Sample Return Mission ▶ Mars Science Laboratory ▶ MER, Spirit and Opportunity (Mars)
Mars Pathfinder Definition Mars Pathfinder was the second discovery class mission of NASA. It was sent to planet ▶ Mars in order to demonstrate the capacity to deliver safely and to operate remotely a robotic equipment on the surface of Mars. Launched on December 4, 1996, it had successfully landed on July 4,
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Mars Pathfinder. Figure 1 Sojourner rover taking an APX-S measurement of the rock Yogi
1997, in an ▶ outflow channel called Ares Vallis (19,30 N, 33,52 W) and worked until the end of September 1997. It consisted of a lander equipped with a panoramic camera and a meteorology package, plus a small rover called Sojourner that carried another camera and the alpha, proton, and x-ray spectrometer APX (Fig. 1). The fully successful mission provided many images of the landing site, and information on the diurnal variations of the meteorological conditions and on the composition of soils and of a few rocks; these were basaltic rocks covered by wind-blown dust. It was a precursor for the subsequent ▶ Mars Exploration Rovers SPIRIT and OPPORTUNITY.
See also ▶ Mars ▶ MER, Spirit and Opportunity (Mars) ▶ Outflow Channels
Mars Reconnaissance Orbiter JEAN-PIERRE BIBRING Institut d’Astrophysique Spatiale, Universite´ Paris-Sud, Orsay Cedex, France
Synonyms MRO
Keywords MARS, NASA
Definition ▶ Mars Reconnaissance Orbiter (MRO) is a ▶ NASA mission analyzing ▶ Mars from a quasi-circular polar orbit, about 300 km in altitude. Launched August 12, 2005, it arrived March 10, 2006, and is operating, at least till the end 2010, much beyond its designed duration of 2 terrestrial years. Its prime goals are to perform high-resolution observations of the surface, the atmosphere, and the subsurface, as well as to characterize potential ▶ landing sites for future missions.
Overview Mars Reconnaissance Orbiter (MRO) embarks six instruments and is capable to transmit to Earth ten times more data than previous missions (Fig. 1). The six instruments are the following: HIRISE (High-Resolution Imaging Science Experiment), a camera with unprecedented resolving capability: the pixel size can be as low as 20 cm, some six times better than the MOC camera on the NASA/▶ MGS mission. The counterpart is the limited size of each image, a few kilometers at most. When the mission will end, several percents of Mars surface will be covered. CTX (Context camera), a wide-angle camera providing the optical context of the other instruments, in images 40 km in size, with a resolution of 8 m/pixel. MARCI (Mars Color Imager) produces each day a global image of Mars in five colors, to track changes in Martian weather (▶ clouds and atmospheric constituents). It monitors Mars atmosphere on diurnal, seasonal, and annual timescales. CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) is a hyperspectral imager: in each pixel,
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some 20 m in size, CRISM acquires the entire spectrum, in hundreds of contiguous spectral channels covering the visible and near infrared range (from 0.4 to 4 mm). This enables to identify both surface (ices and frosts, minerals) and atmospheric (gas, clouds, aerosols) constituents. Its spatial resolution is one order of magnitude better than that of the OMEGA instrument on the ESA/▶ Mars Express mission. The counterpart is that each image is less than 10 km large: a few percents of the global Mars surface will be mapped by CRISM in this high-resolution mode. SHARAD (Shallow Subsurface Radar), is designed to search for potential water (liquid or ice) down some hundreds of meters below the surface, using electromagnetic waves at 20 MHz. Following potential discoveries made by the MARSIS instrument on ESA/▶ Mars Express,
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capable of penetrating some kilometers with a reduced vertical resolution, SHARAD would finely characterize the aqueous layers with a tenfold better accuracy, of 10–20 m (Fig. 2). MCS (Mars Climate Sounder) is a ▶ spectrometer, primarily operating in the infrared (12–50 mm), designed to quantify the global water vapor and dust content of the atmosphere, as well as its temperature. When pointing at the limb, its vertical resolution is 5 km. Its diurnal maps enable a global monitoring of Mars atmospheric variation. Finally, the fine monitoring of the spacecraft orbit provides an investigation of Mars’ gravity, as a means to sound local variations of, for example, crustal or polar cap thickness.
See also ▶ Mars ▶ Mars Express
Mars Sample Return Mission DENIS J. P. MOURA Centre National d’Etudes Spatiales, Paris, France
Keywords Exobiology, Mars, mission design, sample return, system architecture
Definition
Mars Reconnaissance Orbiter. Figure 1 Hydrated minerals (phyllosilicates) colored along the slopes of mesas and canyons as seen by Mars Reconnaissance Orbiter (MRO)
The Mars Sample Return (MSR) mission represents a key milestone in the exploration and understanding of ▶ Mars, including the associated astrobiology questions and those regarding its potential habitability. The very ambitious scientific objectives, the ▶ planetary protection and non-contamination issues, as well as
Mars Reconnaissance Orbiter. Figure 2 Cross section of the north polar ice cap of Mars, derived from data acquired by the Mars Reconnaissance Orbiter’s Shallow Radar (SHARAD)
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the overall technical complexity and difficulties put a huge challenge on its design, development, and operations. The main high-level requirements and their translation into engineering concepts are presented through the definition of a reference architecture defined by an international working group, which considered a mission in the 2020 time frame.
Overview A space mission aiming to collect, select, and return to the ▶ Earth some Mars surface and sub-surface samples is one of the most challenging but exciting projects dealing with space science and robotics. After initial steps where this type of mission has been studied internally within the main space agencies, the complexity as well as the required resources have led to considering this mission as an international endeavor. In that scope, in the year 2008 a Working Group for an international Mars Architecture for the Return of Samples (iMARS) was chartered by the International Mars Exploration Working Group (IMEWG) to develop a plan for such a mission in the 2020 time frame.
Basic Methodology The Scientific Objectives Due to some unique characteristics, Mars has always attracted the attention of scientists and, even if this planet is now well documented thanks to several successful space missions, some points remain today open, such as: ● Determining if Life ever arose on Mars. ● Understanding the processes and history of Climate on Mars. ● Determining the evolution of the Surface & Interior of Mars. ● Preparing the future Human Exploration of Mars. To collect relevant information on these issues, a Mars Sample Return (MSR) mission is the single mission that would make the most progress toward the entire list of the associated scientific objectives. Indeed, returning samples, carefully selected and documented, for analysis in terrestrial laboratories provides three fundamental advantages: ● On Earth the most powerful instruments may be used. ● Complex preparation of the samples for these analyses is possible. ● Adaptability, thanks to human decision making and interaction, is possible. As seen here, the “quality” of the returned samples is fundamental for the performed science and results. It
impacts severely the choice of the sampling areas, the sample acquisition techniques, the sample selection, and the sample preservation.
Mission Requirements and Impacts on System Architecture To achieve the scientific and high-level requirements (see Table 1), particularly in terms of sample choice and diversity, preliminary studies have shown that the system architecture would require, with current and near-future technologies and launch vehicle capabilities, at least two distinct mission elements. One flight element, the Lander Composite, is required to acquire the samples from the surface of Mars and launch them to Mars orbit in a container. A second flight element, the Orbiter Composite, is required to acquire the sample container in Mars orbit and return it to Earth.
Mission Analysis A major engineering challenge for MSR lies in the orbital mechanics. Indeed, sequential phases, have to be addressed, each one having its own critical elements: ● The Earth to Mars phase, for which the launcher capacity is the main concern. ● The Mars surface landing, for which safety is the driver. ● The Mars surface phase, from the landing to the takeoff, including the sample collection, for which the main challenges concern time and energy. ● The return Mars to Earth phase, for which the critical points are the autonomous maneuvers and safety issues. Taking various assumptions on the launchers, the Mars entry and insertion orbit, the Earth re-entry, etc., the performed mission analysis has defined various scenarios for the considered time frame, according to the launch sequence and time (see Fig. 1). For illustration, in the last scenario, the launch of the Orbiter Composite is in June 2019, followed by the launch of the Lander Composite in July 2020. This scenario would require a data relay capability from another mission, for example, from a scientific spacecraft launched previously.
Landing Accuracy/Mobility Requirements Current landing accuracies are within about 100 km on the Martian surface, with unguided ballistic aero-entry systems and about 10 km with guided aero-maneuvering such as the ▶ Mars Science Laboratory (MSL). With improvements in parachute deployment timing, 3 km accuracy can be reached with this last option. Better
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Mars Sample Return Mission. Table 1 The main iMARS high-level requirements Category
Requirement
Sample types to meet science objectives
MSR would have the capability to collect samples of rock, granular materials (regolith, dust) and atmospheric gas
Sample mass
MSR would return a minimum of 500 g of sample mass
Sampling redundancy including contingency samples at landing site
MSR would have both a rover-based sampling system and a lander-based sampling system
Sample encapsulation
MSR would have the capability to encapsulate each sample in an airtight container to retain volatile components with the associated solid samples and protect samples from comingling
Cache retrieval
If Mars Science Laboratory or ExoMars ends its mission in an accessible location with a cached sample on board, MSR should be designed to have the capability to recover the cache(s)
Horizontal mobility to acquire diverse samples needed to meet science objectives
In order to sample various geological sites, MSR would have the ability to rove to the edge of its landing error ellipse (“go-to” capability), carry out a 2.5 km sample acquisition traverse, then return to the lander
Landing site latitude range
MSR would be able to access landing sites within +/ 30 latitude
Planetary protection
All MSR flight and ground elements would meet the planetary protection requirements established by COSPAR; a MSR mission is classified as category V, restricted Earth return
International cooperation
MSR mission planning would enable international cooperation
Timing
The launch of Lander Composite would be no later than 2020, and MSR would return the samples within 5 years after the launch of the first element
accuracy requiring active guidance, will have a strong impact on the payload mass. With a foreseeable 3 km landing uncertainty, the acquisition of Martian samples will be confined to regions near the safe landing site. The valuable surface time could then be used to either seek more scientifically interesting sites outside but close to the landing area or to retrieve samples already collected during a prior mission. Average traverse mobility rates, estimated from experiences with the Mars Exploration Rovers (MER), might approach 100 m per Martian day.
Applications Reference Architecture On the base of previous research results, a reference MSR architecture has been defined. It consists of two flight composites, launched separately to Mars, which work sequentially to return a single Mars sample container back to Earth (Fig. 2). The Lander Composite is launched on a direct trajectory to the surface of Mars, just like the Mars Exploration Rovers and the Mars Science Laboratory missions. After aero-entry through the Martian atmosphere, the Lander
platform, with both a surface rover and Mars Ascent Vehicle, soft-lands on the Martian surface. The rover will traverse away from the Lander platform to acquire samples from the surface, including encapsulated rock cores, and then return back to the Lander platform, which has mechanisms to load the sample container into the Mars Ascent Vehicle. The lander platform has a backup capability to acquire soil samples. The sample container is then launched by the Mars Ascent Vehicle into low-Mars orbit for retrieval. The Orbiter Composite is launched on a direct trajectory to Mars or with an Earth gravity assist strategy in case of mass problem, but achieves its orbit insertion around Mars by a propulsive maneuver and then an aero-braking phase, where the atmosphere itself is used to decrease the composite velocity. Then it detects, rendezvouses with, and retrieves the sample container. After jettison of its main propulsion module used for the previous maneuvers, it leaves the Mars gravity field for returning to Earth at the next low-energy opportunity (depending on the relative positions of Earth and Mars). Then it releases the sample container, now contained in an Earth-Entry Vehicle, to an entry and descent through the Earth’s atmosphere to the Earth’s surface.
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Mars Sample Return Mission. Figure 1 MSR Mission analysis scenarios and time events
Mars Surface Mars Sampling Rover
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Mars Sample Return Mission. Figure 2 iMARS reference architecture (version 2008)
Sample Receiving and Curation Facilities
Mars Science Laboratory
Both of these composites can be launched in the same launch opportunity, or they may be launched in sequence, one before the other, one or more opportunities apart. In the case where the Orbiter Composite launch follows the Lander Composite launch such as the previous scenario, a prior-launched orbital telecom element is strongly recommended to both survey the landing events and provide early detection of the sample in Mars orbit. In any event, the time from the launch of the last composite to the return of the sample to Earth is about 3 years.
Planetary Protection and Contamination Control Issues The MSR mission is facing difficult challenges in ▶ planetary protection (PP): (1) for protecting Mars from viable Earth organisms; (2) for protecting Earth from potentially bio-hazardous Martian material; and (3) for preventing false-positive identification of material thought to be Mars organisms in the returned samples (but actually originating from Earth), which could affect PP policy in the future. The last challenge also overlaps with science needs to control organic and inorganic contaminants in the returned sample, in some cases to levels as low as the part-per-billion range. Both science and PP require sterilization and/or thorough cleaning of any hardware that comes in contact with the samples and subsequent isolation from dirtier parts of the spacecraft to avoid recontamination. Both also benefit from ▶ inventory of materials and organisms on the spacecraft before leaving Earth as an additional mean of preventing false-positives for life-detection and ambiguity in science observations. Preventing contamination of Mars by Earth organisms associated with landed spacecraft requires reduction of the ▶ bioburden on surface materials by cleaning, and containment or ▶ sterilization of internal materials. Elements expected to crash on the surface (the lander cruise, entry and descent systems) must further reduce the bioloads of internal hardware, since containment cannot be assured, and/or demonstrate sufficient sterilization by atmospheric entry heating. Current MSR plans do not include landing at a site that has the potential of propagating Earth organisms (designated as Special Regions), which would mandate full system sterilization prior to launch. Preventing contamination of Earth by potentially bio-hazardous Martian material requires highly reliable sample containment and ultra-safe entry and landing at Earth, as well as breaking the chain-of-contact with Mars in a way that precludes return of Mars organisms outside of sample containment.
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Future Directions The planning activities clearly identified the most critical issues, particularly: ● The need to refine the scientific objectives according to the most recent results concerning Mars exploration. ● The Mars Ascent Vehicle. ● The autonomous navigation, rendezvous, and docking in Mars orbit. ● The Earth-Entry Vehicle. ● The non-contamination/planetary protection issues of the relevant space assets. ● The scientific instrumentation, onboard the space assets as well as in the Earth laboratories, including their overall optimization. ● The sample contamination control, management, and analyses on the Earth. On all these aspects, a vigorous Research & Technology plan is ongoing in various space agencies, in order to scientifically and technically consolidate the presented concepts and then to update the reference architecture so as to define the responsibilities of each of the partners of the real mission to develop and fly in the horizon 2020/2025.
See also ▶ ExoMars ▶ Mars ▶ Mars Science Laboratory ▶ Planetary Protection
References and Further Reading iMARS report http://esamultimedia.esa.int/docs/Aurora/iMARS_Report_ July2008.pdf Main contributors: Frank Jordan (JPL/NASA), Alain Pradier (ESA/ESTEC), Lisa May (NASA), Andrea Santovincenzo (ESA/ESTEC), Richard Mattingly (JPL/NASA), Stuart Kerridge (JPL/NASA), Michael Khan (ESA), Bruno Gardini (ESA), Doug McCuistion (NASA)
Mars Science Laboratory MICHEL CABANE LATMOS/IPSL B102/T45-46, Universite´ Pierre et Marie Curie UPMC-Paris 6, Paris Cedex 05, France
Synonyms Curiosity; MSL
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Keywords
Overview
Atmosphere, biomarkers, derivatization, GC-MS, in situ, Mars, Mars Science Laboratory, methane, MSL, organics, pyrolysis, regolith, rover
Ancient Martian history, from its formation up to about 3 billion years ago, looks close to Earth’s (▶ HadeanArchaean on ▶ Earth, ▶ Noachian on Mars). We may then ask, was it possible for life to appear on Mars? One faces three possibilities: (a) life appeared and is extinct, but organic remnants or ▶ biominerals can be found; (b) only ▶ complex organic molecules, “bricks of life,” were formed and some of them still exist; and (c) Mars has been absolutely sterile from the beginning.
Definition Mars Science Laboratory is a large instrumented vehicle (rover) built by ▶ NASA that is planned to land on ▶ Mars in 2012 (launch: end 2011) and to operate for at least 2 years. This rover, also called Curiosity, will characterize and interpret the geology of the landing region at all scales, assess the biological potential of at least one target, investigate planetary processes of relevance to past habitability, and characterize radiation arriving at Mars’ surface. Aboard MSL, an analytical laboratory is especially devoted to atmospheric analysis and search for organics in the ground.
Viking Results In 1976, NASA sent the ▶ Viking orbiters and landers to Mars. On landers Viking 1 and 2, instruments were devoted to the search for ▶ biosignatures and molecular analysis of the ground. They were (1) a gas-exchange experiment: nutrients were brought to the soil,
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variations in concentrations of O2, CO2, CH4, etc., were searched for (Oyama and Berdahl 1977); (2) two experiments using radioactive 14-C: CO2 exchanges between atmosphere, soil, and nutrient were monitored (Horowitz et al. 1977) (Levin and Straat 1977); (3) a pyrolysis-gas chromatograph-mass spectrometer (▶ GC-MS) experiment: Mars soil was heated and analyzed (Bieman et al. 1977). Ambiguous responses to the first three experiments were understood as due to the presence of nonbiological reactants rather than metabolic activity. One knows, now, that Mars’ atmosphere contains gaseous oxidants, such as H2O2, due to atmospheric photochemistry (Atreya et al. 2006, Zanhle et al. 2008), and that oxidizing compounds may be present in the soil (Hecht et al. 2009). The ▶ Pyrolysis-GC-MS experiment did not detect any organics; it was shown (Benner et al. 2000) that, in harsh environments, organic complex molecules degrade into refractory species (e.g., benzene, carboxylic acids) (Fig. 1). So, pyrolysis at moderate temperatures (500 C in Viking ovens) could not produce any volatile organic compound; this might explain why there was no observation of the micrometeoritic organic matter arriving at Mars’ surface (103 mg/cm2/year, Flynn 1996). Consequently, detection of organic matter by pyro-GC-MS needs higher temperatures, or some preparation before pyrolysis.
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Mars Science Laboratory. Figure 2 Color-enhanced image of the rim of Jezero Crater (40 km diameter). Ancient river (meander at left) carried phyllosilicates (shown in green) into a lake, forming a delta. CRISM instrument aboard MRO, 2008 (Credits: NASA/JPL/JHUAPL/MSSS/Brown University)
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Water on Mars Since Viking, many orbiters ▶ Mars Express, MGS, ▶ Mars Odyssey, MRO, etc. (Bibring et al. 2006; Murchie et al. 2009), observed Mars, and returned strong indications that water circulated on Mars’ surface during the Noachian period (Fig. 2). Spectral analyses show that, linked to what could be riverbeds or deltas, ▶ carbonates, hydrates, sulfates, and ▶ phyllosilicates are present (Wray et al. 2009), which could be associated with life or its beginning. Likewise, instrumented vehicles (rovers) have also explored Mars since 1997. Pathfinder-Sojourner (1997) and the ▶ MER Spirit and Opportunity (Mars Exploration Rovers; 2004-present), were mainly designed to identify geologic patterns (panoramic cameras, microscope) (Fig. 3), and to analyze rocks using APXS (Alpha Particle X-Ray Spectroscopy), Mo¨ssbauer spectrometry, TES (thermal emission spectrometry). Again, one can quote the detection of the same minerals as above (Squyres et al. 2004).
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Mars Science Laboratory. Figure 3 Cape St Vincent, on the rim of crater Victoria, as seen by MER Opportunity (June 2007), vertical height 8 m (Credits: MER Mission, Cornell, JPL, NASA)
Mars Science Laboratory From 1999, MEPAG (Mars Exploration Program Analysis Group, NASA) defined four goals linked by the motto “follow the water”: (1) Life, (2) Climate, (3) Geology, (4) Human exploration. In this frame, MSL (Mars Science Laboratory) will be launched at the end of 2011; in particular, it has for objectives: assess the past and present
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Mars Science Laboratory. Figure 4 Four landing sites under consideration for MSL: Eberswalde, Gale, and Holden craters, and Mawrth Vallis. Primary landing ellipses are 20 km by 25 km. False colors: altitudes (maximum to minimum: about 3,000 m, depending on site) (Credits: NASA, http://marsoweb.nas.nasa.gov/landingsites/)
habitability, characterize carbon cycling in its geochemical context, assess whether life is or was present. From numerous candidates for MSL landing sites (Roger and Banfield 2009), four possible locations were selected (Fig. 4) that satisfy both engineers and scientists; the definitive one will be decided in 2011.
Basic Methodology MSL for 2012 This rover (Fig. 5) is larger (2.75 3.05 2.13 m) and heavier (875 kg) than Sojourner (11 kg) or MERs (174 kg each); in 2012, after landing (Fig. 6), it will operate for 1 Martian year (669 sols 687 terrestrial days) and navigate some tens of kilometers. It is powered, as were Viking 1 and 2, by radioisotope thermoelectric generators (RTGs; 2.5 kW-h/sol) and is heavily instrumented. Its 75 kg science payload is composed of ten instruments. They are devoted to (1) observation: cameras or microscopes (MarDI, descent imager; MastCam, on the mast; MAHLI, at contact with the
samples); (2) environmental studies: an instrument (RAD) to characterize the broad spectrum of radiation near the surface and a pulsed neutron source and detector (DAN) for measuring ▶ hydrogen or ▶ ice and ▶ water at or near the Martian surface, a meteorological station and UV sensor (REMS); (3) mineralogy and geochemistry of rocks and soil: APXS (Alpha Particle X-Ray Spectroscopy), CheMin (X-Ray diffraction on sampled powders), ChemCam (UV spectroscopy of gases issuing from laser impacts on targets); (4) molecular analysis of atmosphere and soil samples: SAM for ▶ GC/MS, and IR spectroscopy. Some of these instruments are on the mast (MastCam, ChemCam), or at the end of the 1.9 m length Robotic arm (APXS, MAHLI). This arm also carries the scooping or drilling (5 cm) instruments that feed the sample preparation devices for SAM and CheMin, which are located inside the rover.
Exobiology Aboard MSL: SAM A large part of the MSL instruments is devoted to the examination of ground samples, and SAM (Sample Analysis
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Mars Science Laboratory. Figure 5 An artist view of MSL (Credits: NASA/JPL)
Mars Science Laboratory. Figure 6 Landing of MSL: at 20 m above the surface the rover is lowered on a tether from the descent stage, and placed directly on the Martian surface (Credits NASA/JPL-Caltech)
at Mars) is one of the most complex and the most relevant for astrobiology. It belongs to the family of analytical laboratories following Viking (1978) and Huygens (2005), such as COSAC (Rosetta, 2004–2014), GAP (Phobos-Grunt, 2011–2013), and MOMA (ExoMars, foreseen for 2018). This heavy (40 kg) laboratory is developed by NASA (Mahaffy 2008) and incorporates three scientific units (Fig. 7). These are a quadrupole mass spectrometer (QMS), a tunable IR laser spectrometer (TLS), and a gas chromatograph (GC) (Cabane et al. 2004). The goals of SAM are to (1) analyze the atmosphere, search for minor species (e.g., ▶ CH4, ▶ noble gases), abundances, and
isotopic ratios; (2) analyze ground samples, search for organics and their possible ▶ chirality, structural gases in mineral species (CO2, H2O, SO2, . . .); (3) measure isotopic ratios. Samples of soil can be heated up to 1,000 C (74 ovens possibly reusable, in the Sample Manipulation System, SMS), and exhausted gases are analyzed. In case of refractory species, some cups are equipped for wet chemistry (▶ derivatization): a chemical reagent transforms the molecule into a vaporizable one, while preserving the structure (Buch et al. 2009). Combustion of samples in O2 atmosphere may be also used to determine the isotopic ratios. Gas sampled in the atmosphere, or evolved from ▶ pyrolysis, wet chemistry or combustion are sent to GCQMS-TLS; on their path, getters, scrubbers, cold traps help to eliminate/separate some species (CO2, H2O) or to concentrate the sample. The GC columns separate in time the evolved gases, and are followed by specific detectors that can measure down to some 1010 mole of a given species in a He flow. Six different columns can be used, which cover a range from low mass permanent gases or low carbon number (C1–C5) hydrocarbons to high masses (> C15), with the possible use of a “chiral” column to search for enantiomers. The QMS precisely identifies molecular compositions: the mass measurement ranges from 2 to 535 Da, with a detection limit 1 ppm (1 pmole in a He flow) and isotope ratios down to 1 per mil for noble gases. The cell of TLS instrument can detect down to 2 ppb of CH4 and 2 ppm of water without enrichment, and measure down to 109 K, and are compact compared with the scales of the astrophysical objects with which they are associated (e.g., stars, galactic nuclei). For example, masers have sizes of order 107 m in comets, and masers in galactic cores can have scale sizes up to 1019 m (Gray 2011). Other observational features that indicate maser emission are narrow, sub-thermal spectral line widths and time variability. Lines from different transitions of the same molecular species may also display ratios inconsistent with thermal emission. Maser polarization is typically interpreted as due to the Zeeman effect in the presence of magnetic fields. Maser emission often involves molecular rotational transitions, with the masers typically pumped in warm, dense gas. For example, the best-studied SiO masers are those at 43 and 86 GHz (l = 7 and 3 mm, respectively) originating from the two lowest-energy rotational transitions in each of the first and second vibrational levels. They are pumped at gas densities of n(H2) = 1010 1 cm3 and kinetic temperatures of Tk 1,500 K (e.g., Elitzur 1992). The best-studied water masers are those at 22 GHz (l = 1.3 cm) originating from the 616 – 523 rotational transition. They are pumped in gas of n(H2) = 108 – 1010 cm3 and Tk = 400 – 1,000 K (e.g., Elitzur 1992). When astrophysical objects associated with masers are imaged at high angular resolution, such as at sub-milli-arc second resolution using Very Long Baseline Interferometry (▶ VLBI), multiple maser features are often observed toward the same object. In addition, masers from multiple transitions of the same molecular species can amplify along the same regions of gas, and masers at different frequencies are often detected toward the same source. Finally, at least in the case of evolved stars and starforming regions, masers from different molecular species are also often found toward the same object.
Key Research Findings The circumstellar envelopes of oxygen-rich evolved lowmass and Supergiant stars commonly harbor SiO, H2O, and OH masers. Radio VLBI observations indicate that SiO masers occur close to the star, typically within about
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four stellar radii (Diamond et al. 1994). SiO maser emission has been detected from v = 0 – 4, up to at least J = 8 – 7 (n = 344 GHz; l 1 mm) in evolved stars and SiO ▶ isotopologue masers are also observed (Soria-Ruiz et al. 2005). H2O masers at 22 GHz form further from the star at a radial distance of about ten stellar radii with several other H2O maser transitions also detected at frequencies up to 658 GHz. Main line (1,665 and 1,667 MHz) OH masers occur at similar radii to the 22 GHz H2O masers, but are believed to probe significantly less dense gas (Gray 2011). At much larger radii, typically 100 stellar radii, 1,612 MHz OH masers are observed. Polarized emission of the three molecular maser species has been interpreted to estimate circumstellar magnetic fields (Vlemmings 2007). Proper motion studies of the masers have been use to trace gas dynamics in the circumstellar envelope, with both outflow and infall detected in the SiO maser zone due to stellar pulsation. Toward the less common carbon-rich stars, HCN, CS, and SiS masers have been observed. Star-forming regions in ▶ molecular clouds support a variety of maser species, with methanol, water, and OH masers the most frequently detected. Maser emission is observed from low- and high-mass forming stars; however, stellar masses significantly >1 Msun are favored (Gray 2011). Water masers at 22 GHz are often seen in protostellar outflows associated with shocked gas, and are more rarely thought to be associated with accretion disks. Other water masers, including those at frequencies of 183, 321, and 325 GHz, have also been detected. Methanol masers in star-forming regions are divided into two types, Class I and Class II, depending on the transitions present. Class I methanol masers are primarily collisionally pumped, and may trace outflow interactions with dense molecular gas. Class II methanol masers tend to be associated with ▶ HII regions, with the best-studied transitions occurring at frequencies of 6.7 and 12.2. GHz. Methanol, OH (mainly 1,665 and 1,667 MHz lines), and H2O masers are often present in the same source, although water and methanol masers usually appear in different regions from each other. Again, maser polarization is used to determine magnetic field characteristics, with fields as high as 40 mG seen in OH masers (Fish 2007). Rarer masers in star-forming regions include ammonia, formaldehyde, and SiO (currently detected in 3 high-mass star-forming regions only). ▶ Supernova Remnant (SNR)/Molecular Cloud interactions also give rise to maser emission. OH 1,720 MHz masers have been detected from 20 galactic SNRs, that is, 10% of the galactic sources (Brogan 2007). Methanol maser emission at 95 GHz has also been detected toward
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one SNR. Hot stars MWC 349A and Eta Carina harbor H-atom recombination masers over a range of frequencies (see, e.g., Gray 2011). Extragalactic maser emission, in some cases referred to as megamaser emission, is most commonly detected from 22 GHz H2O and OH 1,665 and 1,667 MHz lines, although formaldehyde maser emission at 6 cm is also known. OH megamaser emission occurs from nuclear starburst activity, on scales of 100 pc, whereas 22 GHz water maser emission is observed on (sub-)parsec scales and traces the central engines of Active Galactic Nuclei (AGN) (Pihlstro¨m 2006). Over a hundred extragalactic water maser sources are now known, including a z = 2.64 detection magnified by gravitational lensing (Impellizzeri et al. 2008). Detections of extragalactic methanol masers are limited to the Large Magellanic Cloud and appear to be similar to Galactic methanol masers. In the Solar System, masers have been detected from cometary comas. Cometary OH masers have a UV pumping mechanism and have been observed from at least 16 comets, including ▶ Comet Hale-Bopp. Water masers at 22 GHz have also possibly been detected. Water maser emission was detected after the impact of Comet Shoemaker-Levy 9 with Jupiter, and has also been reported from the Saturnian moons Hyperion, Titan, Enceladus, and Atlas (Pogrebenko et al. 2009). Molecular laser features from CO2 at 10.33 mm were detected in the atmospheres of Venus and Mars. Searches have also been performed for molecular maser emission associated with exoplanets, but to date no emission sources have been reported.
Applications Masers are used as tools to probe the physical conditions, kinematics, and magnetic fields of astrophysical sources at high angular resolution. Maser line ratios constrain radiative transfer models to determine gas temperatures and densities. Proper motion measurements enable tracing of gas kinematics. Maser polarization, when attributed to the Zeeman effect, can be used to determine magnetic field strength and morphology characteristics. Masers can be used to measure dynamical masses and to provide distance estimates. In the case of water masers in AGN, this has led to their use for planned high-accuracy estimation of the Hubble constant, which may lead to constraint on the nature of Dark Energy (Braatz et al. 2009; Greenhill et al. 2009). OH masers are used to search for variations in the fundamental “constants” over cosmological time (Darling 2003). Galactic trigonometric parallax measurements trace the number and location of the spiral arms in the
Milky Way and the distance of the Sun from the Galactic Center (Reid et al. 2009).
Future Directions New instruments will revolutionize the use of masers as diagnostic probes. The Atacama Large Millimeter/ Submillimeter Array (▶ ALMA) will perform imaging up to 950 GHz, enabling observation of high-frequency maser lines at angular resolutions of better than 10 milliarc seconds. At low frequencies (foreseen 0.1–25 GHz), the Square Kilometer Array (SKA) will provide very high sensitivity, allowing for searches for distant masers in the early Universe.
See also ▶ ALMA ▶ Comet ▶ HII Region ▶ Isotopolog ▶ Linewidth ▶ Molecular Cloud ▶ Star Formation ▶ Supernova Remnants ▶ VLBI
References and Further Reading Braatz J, Reid MJ, Greenhill LJ et al (2009) Water masers in AGN accretion disks. Astron Soc Pacific Conf Ser 402:274 Brogan C (2007) OH (1720 MHz) masers: signposts of SNR/molecular cloud interactions. IAU Symp 242:299–306 Darling J (2003) Methods for constraining fine structure constant evolution with OH microwave transitions. Phys Rev Lett 91:1301–1304 Davies R, Rowson B, Booth R, Cooper A (1967) Measurements of OH emission sources with an interferometer of high resolution. Nature 213:1109–1110 Diamond P, Kemball A, Junor W et al (1994) Observation of a ring structure in SiO maser emission from late-type stars. Astrophys J 430:L61–L64 Dinh-V-Trung (2009) On the theory of astronomical masers – II. Polarization of maser radiation. Mon Not R Astron Soc 399:1495–1505 Elitzur M (1992) Astronomical masers. Kluwer, Dordrecht Fish V (2007) Masers and star formation. IAU Symp 242:71–80 Gray MD (2011) Maser sources in astrophysics. Cambridge University Press, Cambridge, UK Greenhill LJ, Kondratko PT, Moran JM, Tilak A (2009) Discovery of candidate H2O disk masers in active galactic nuclei and estimations of centripetal accelerations. Astrophys J 707:787–799 Impellizzeri V, McKean J, Castangia P et al (2008) A gravitationally lensed water maser in the early Universe. Nature 456:927–929 Moran J, Burke B, Barrett A et al (1968) The structure of the OH source in W3. Astrophys J 152:L97–L101 Pihlstro¨m Y (2006) Megamasers, Proceedings of the 8th European VLBI Network Symposium. September 26-29, 2006, Torun, Poland. Editorial Board: Baan Willem, Bachiller Rafael, Booth Roy,
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Mass Extinctions Charlot Patrick, Diamond Phil, Garrett Mike, Hong Xiaoyu, Jonas Justin, Kus Andrzej, Mantovani Franco, Marecki Andrzej (chairman), Olofsson Hans, Schlueter Wolfgang, Tornikoski Merja, Wang Na, Zensus Anton., p 34 Pogrebenko S, Gurvits L, Elitzur M et al (2009) Water masers in the Saturnian system. Astron Astrophys 494:L1–L4 Reid MJ, Menten KM, Zheng XW et al (2009) Trigonometric parallaxes of massive star-forming regions. VI. Galactic structure, fundamental parameters, and noncircular motions. Astrophys J 700:137–148 Soria-Ruiz R, Colomer F, Alcolea J et al (2005) First VLBI mapping of circumstellar 29SiO maser emission. Astron Astrophys 432:L39–L42 Vlemmings WHT (2007) A review of maser polarization and magnetic fields. IAU Symp 242:37–46 Watson W (2009) Magnetic fields and the polarization of astrophysical maser radiation: a review. Rev Mex Astron Astrofis 36:113–120 Weaver H, Williams D, Dieter NH, Lum WT (1965) Observations of a strong unidentified microwave line and of emission from the OH molecule. Nature 208:29–31 Weinreb S, Meeks M, Carter C, Barrett A, Rogers A (1965) Observations of polarized OH emission. Nature 208:440–441
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variations of oxygen concentration in seawater could explain mass extinctions, though the exact mechanisms remain relatively unknown. These biotic crises must be viewed as important steps in the evolution of life. During the post-event recovery, the vacant ecological niches are filled by new species, like the mammals that became dominant after the dinosaur extinction at the well-known ▶ KT boundary.
Overview Several major biotic crises or mass extinctions mark the evolution of life during the Phanerozoic (Fig. 1). During these events, and over a rather short timeframe, a large proportion of the existing species on land and in the oceans suddenly become extinct. A major and brutal decrease in the diversity and abundance of life characterizes these extinction levels. The development of new species follows.
Mass Extinctions All genera
NICHOLAS ARNDT1, DANIELE L. PINTI2 1 Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France 2 GEOTOP & De´partement des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada
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Synonyms Biotic crisis; Extinction event; Extinction-level event
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Keywords Bolide impacts, biological crisis, chemostratigraphy, glaciation, life, snowball, volcanic traps
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Definition A mass extinction or extinction event refers to an abrupt decrease in the number of species in a short span of geological time. The term is different from simple extinction that denotes in ecology the disappearance of an organism or group of taxa. While extinctions are quite common in nature, mass extinctions are relatively rare events. Five mass extinctions have been recorded in the last 500 Ma (Phanerozoic). It is now suggested that a sixth one, caused by increased anthropogenic pressure on the environment, is ongoing. Catastrophic agents, such as meteorite impacts, and environmental agents, ▶ volcanic eruptions, ▶ glaciations and global climatic changes, and
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Mass Extinctions. Figure 1 Genus diversity in the Phanerozoic time (542–0 Ma). The light gray plot shows the number of known marine animal genera versus time from Sepkoski (2002), converted to the 2004 Geologic Time Scale. The dark gray plot shows the same data, with single occurrence and poorly dated genera removed. The trend line is a third-order polynomial fitted to the data. After Raup and Sepkoski (1982) redrawn by M. Laithier, UQAM
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Mass Extinctions
In the last 500 Ma, there have been five large mass extinctions that eliminated more than half of the species inhabiting the Earth. The five recorded mass extinction events occur at the end of the Ordovician (444 Ma; Hirnantian stage), in the Late Devonian (370 Ma), at the Permian–Triassic boundary (251 Ma), at the Triassic–Jurassic boundary (200 Ma), and at the most studied Cretaceous–Tertiary boundary (often labeled as the KT boundary 65 Ma) (Raup and Sepkoski 1982; Hallam and Wignall 1997).
Late Ordovician Extinction A major global cooling of the Earth’s climate followed by rapid glaciations could have triggered the Late Ordovician extinction. Extinction has been linked with environmental changes accompanying the glaciation of the supercontinent Gondwana that comprised most of the landmasses at that time. 60% of genera disappeared from the geological record and it is estimated that 85% of genera inhabiting the Earth became extinct. This is the second largest mass extinction after that of end-Permian. The extinction occurred as two 2-m.y.-long phases, one at the beginning and the other toward the end of the Hirnantian stage. The first is associated with sea-level regression and climate deterioration at the onset of the glaciation of Gondwana, the second with sea-level transgression caused by continental flooding and accompanied by anoxia as the ice caps melted.
Late Devonian and the Permo-Triassic Extinction The processes leading to the Late Devonian and Triassic– Jurassic biotic crises are still poorly documented. The Devonian extinction took place in the Frasnian– Famennian boundary, about 364 million years ago and lasted 2–3 Ma. Glacial deposits in Brazil and in the PeruBolivia Altiplano of Devonian age suggested that another glaciation on the Gondwana supercontinent could have triggered an episode of global cooling. This is also supported by the fact that warm water marine species were the most severely affected by this glaciation. The greatest mass extinction at the Permian–Triassic boundary is often called the “Great Dying” because 70% of land and more than 90% of marine species disappeared. This period is characterized by some of the best evidence recorded in the European geological record of extensive sea-level fall quickly followed by rise, and these global changes no doubt contributed to the extinction. There is no evidence of a meteorite impact but the
event was synchronous with Siberian flood volcanism, the largest-known continental volcanic province. The magmas of this province intruded carbonates and sulfate-rich sedimentary rocks, which released large amounts of CO2 and SO2 when heated and the toxic cocktail of sediment- and magma-derived gases probably contributed to the extinction.
KT Boundary The most famous KT boundary saw the demise of the dinosaurs on land, and of the ammonites and the marine reptiles in the oceans. Across this interval, 60% of the fauna and flora on Earth went extinct, and in particular, the marine calcareous microplankton was drastically affected. The impact of a 10–12 km asteroid on the Yucatan peninsula is widely accepted as one of the major causes of this mass extinction. This impact formed the 200 km ▶ Chicxulub crater, and spread ejecta material all around the globe. Worldwide the KT boundary layer is marked by the characteristic positive iridium anomaly, generated by the presence of meteoritic material. The largest phase of Deccan flood volcanism in India erupted 67 Ma ago, synchronously with the K/T event. Single lava flows extended for more than 1,000 km and covered most of India of continental flood basalts and huge emissions of greenhouse and toxic gases were associated with the eruptions. Possibly, both impact and volcanism contributed to this mass extinction (Courtillot 1999).
Precambrian Mass Extinctions Precambrian mass extinctions have been proposed such as during the Vendian period (605–543 Ma). This extinction largely affected stromatolites and acritarchs and it has been correlated with a large glaciation event that occurred about 600 million years ago. This event was of such severity that almost all microorganisms were completely wiped out. Whether the Vendian event was a major extinction is currently under debate. Older extinctions are impossible to evaluate because the fossil record is less well preserved and abundant. It is likely that mass extinctions occurred during global climate events such as ▶ Snowball Earths and in earlier times during the Archean and Hadean, if life already inhabited the Earth. Indeed, if microorganisms were already present during the Hadean, intense meteoritic bombardment the catastrophic volcanism and rapid changes in the physicochemical conditions of oceans (acidity, temperature, and salinity) could have driven repeated mass extinctions.
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Basic Methodology Apparent extinction intensity, i.e., the fraction of genera going extinct at any given time, has been reconstructed from the fossil record. Abundance and biodiversity estimates are directly related to quantity of rock available for sampling from different time periods, and thus this methodological approach can create artifacts. Development of the “chemostratigraphy,” i.e., the study of the variation of chemistry within sedimentary sequences, particularly the variations in the stable isotopes of C, S, N contained in kerogens or radiogenic isotopes (Sr, Os), give a valuable support for determining mass extinction horizons (Marshall 1992; Brenchley et al. 1994). Carbon isotope values (d13C) from rocks, fossils, and biomarkers record global changes in the carbon cycle. This implies changes in carbon inputs (e.g., riverine vs. marine), outputs (e.g., methane hydrate emissions), organic productivity, and carbon burial. All five of the major Phanerozoic mass extinction events are matched by significant carbon isotopic excursions. The d13C data of both carbonate and organic carbon from numerous sections worldwide reveal a positive d13C excursion shortly after the first pulse of extinction at the end of the Ordovician. This positive d13C excursion is interpreted to have resulted from high primary productivity or from the weathering of carbonate platforms that were exposed when sea level fell during the glaciation. Negative excursions found associated with the end-Permian are interpreted to record collapse in primary productivity, massive release of methane from hydrates, or release of gases from sedimentary rocks heated by basaltic magmas (Ganino and Arndt 2009).
Future Directions Two main questions need to be addressed: 1. Can we identify mass extinctions in the earlier geological record of the Earth? 2. Are there common or multiple causes to mass extinctions? The first question is difficult to answer because the answers must be sought from a period when the fossil record is almost totally absent or rare. Chemostratigraphy is difficult to apply because the diagenetic effects on the isotopic signatures and because Precambrian biogeochemical cycles of C, N, S, and other elements were possibly different than in post-Cambrian times and thus the isotopic signature rather complicated to interpret. Several authors try to answer the second question by identifying possible periodicity in the extinction events and relate them to common geological or environmental
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changes. This is an old approach used since Cuvier’s theory of Catastrophism and still largely debated (see Hallam and Wignall 1997). Recent robust statistical estimates suggest a 62 3 Ma cycles (Rodhe and Muller 2005). This cycle could be related to the passage of the Solar System into molecular clouds increasing meteoritic or cosmic dust showers or to perturbations of the Oort cloud increasing the risk of cometary impact on the Earth. Other theories suggest a relation with the episodic rise of the global-scale mantle plumes that cause flood basaltic volcanism. These huge basaltic volcanic eruptions are synchronous with all known mass extinction levels (except for the end-Ordovician event), within the limit of error of modern dating techniques (Courtillot 1999).
See also ▶ Carbon Isotopes as a Geochemical Tracer ▶ Chicxulub Crater ▶ Deccan Trapps ▶ Ejecta ▶ Evolution (Biological) ▶ Glaciation ▶ Iridium ▶ K/T Boundary ▶ Mantle Plume (Planetary) ▶ Snowball Earth ▶ Trapps
References and Further Reading Brenchley PJ, Marshall JD, Robertson DBR, Carden GAF, Long DGF, Meidla T, Hints L, Anderson TF (1994) Bathymetric and isotopic evidence for a short-lived late Ordovician glaciation in a greenhouse period. Geology 22:295–298 Courtillot V (1999) Evolutionary Catastrophes: the Science of Mass Extinctions. University Press, Cambridge Ganino C, Arndt NT (2009) Climate changes caused by degassing of sediments during the emplacement of large igneous provinces. Geology 37:323–326 Hallam A, Wignall PB (1997) Mass extinctions and their aftermath. Oxford University Press, Oxford Marshall JD (1992) Climatic and oceanographic isotopic signals from the carbonate rock record and their preservation. Geol Mag 129:143–160 McLean DM (1985) Deccan traps mantle degassing in the terminal cretaceous marine extinctions; cret. Res 6:235–259 Rohde R, Muller R (2005) Cycles in fossil diversity. Nature 434:208–210 Raup D, Sepkoski J (1982) Mass extinctions in the marine fossil record. Science 215:1501–1503 Sepkoski JA (2002) Compendium of fossil marine animal genera. Jablonski D, Foote M (eds) Bulletins of American Paleontology No. 363. Paleontological research institution, Ithaca Wignall P (2001) Large igneous provinces and mass extinctions. Earth Sci Rev 53:1–33
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Imaging capability and chiral identification are promising features of mass spectrometry for astrobiology.
Definition
Overview
The mass loss rate quantifies the rate at which a star loses its matter as a result of various effects. It corresponds to an ejection of matter, usually called ▶ stellar wind, which happens all along the star’s life but especially in the final stage (post-AGB phase). Massive stars exhibit very large mass loss rates. Unit: kg s1 or more usually Msol yr1.
Mass spectroscopy; MS
Mass spectrometry (MS) is capable of analyzing any substance, once sample molecules are ionized. By this generic feature, which is superior to other methods of chemical analysis, MS has become a common instrument for astrobiology. Ordinary MS requires volatilization process prior to analysis. However, many bio-associated molecules are hard to volatilize even at relatively high temperature. The strong intermolecular interaction of biomolecules, originating in their strong polarity, tends to produce charring, not volatilization, at high temperature. The innovation of soft ionization, appropriate for such substances in principle, leads MS to be a highly powerful tool for astrobiology with many new applications, both on the ground and in space. The most common use of MS is to detect and characterize organic substances of astrobiology interest. It is essential for understanding prebiotic chemical evolution, and ultimately for getting direct evidence of extraterrestrial life, if any. Analysis of the isotopic composition of substances is a unique capability of MS. The isotopic composition of bioelements in fossils can provide evidence of activities of extinct living organisms, based on the isotope effects on biochemical reaction rates. Spatially resolved MS techniques have been developed recently. This also opens new horizons for astrobiology.
Keywords
Basic Methodology
Cassini-Huygens, desorption ESI, electron impact, electrospray ionization, ExoMars, GC/MS, Giotto, ion trap, ion cyclotron resonance, imaging MS, LC/MS, matrix-assisted laser desorption ionization, MOMA, MSL, polyaromatic hydrocarbons, quadrupole MS, Rosetta, SAM, secondary ionization MS, soft ionization, Stardust, tandem MS, TEGA, time of flight MS, Viking
Mass spectrometry consists of three steps, i.e., ionization, mass separation, and ion detection. Electron impact (EI) is a common ionization method. Electrons, with energies at 50–100 eV, hit sample molecules to ionize them. Molecules should be volatilized before EI. Most biogenic molecules are hard to volatilize, and require chemical modification to increase vapor pressure. Thermal ionization, inductively – coupled plasmas (ICP) and sputtering sources are also widely used to determine isotopic abundances. Another approach for biomolecules is soft ionization. Electrospray Ionization (ESI) and Matrix-Assisted Laser Desorption Ionization (MALDI) are typical soft ionization methods. ESI ionizes molecules by spraying fine charged droplets in a strong electric field. Desorption Electrospray (DESI), a derived method of soft ionization, produces ions directly from a surface. MALDI ionizes biological substances through desorption of molecules after energy deposition by a laser to the matrix. ▶ Photoionization can provide highly selective ionization, useful in case the ionization potential of the target
See also ▶ Stellar Evolution ▶ Stellar Winds
Mass Spectrometry MASAMICHI YAMASHITA Institute of Space and Astronautical Science/JAXA, Sagamihata, Kanagawa, Japan
Synonyms
Definition Mass spectrometry is an analytical method used to identify and quantify chemical species based on their molecular mass. It is also widely used to determine the isotopic abundances (▶ isotopic ratios) of various elements. The mass to charge ratio of ionized molecules is determined by their motion traveling through electromagnetic fields. Elemental composition and chemical structure of molecules are also estimated by the higher mass resolving power of the system, and interpretation of the mass spectrum of fragment ions. By choosing appropriate ionization methods, mass spectrometry provides information other than mass, such as the polarity of molecules.
Mass Spectrometry
molecule is lower than the energy of the irradiating photon, while that of other molecules is higher than the photon energy. Because aromatic compounds have low ionization potentials, polyaromatic hydrocarbons (PAH) are selectively ionized by ultraviolet lasers in this way. An imaging capability of MS has been developed for several ionization methods. Secondary Ionization MS (SIMS) is based on excitation of sample molecules by the incidence of primary ion beams. Scanning of the primary beam on the substrate creates spatially resolved MS. Ionization methods employing lasers are capable of imaging. DESI can analyze the spatial distribution of substances at less resolving power. In the second step of MS analysis, charged molecules can be separated by the electromagnetic force. In the early era of MS, deflection of traveling ions in a magnetic field has been the common principle of MS (magnetic sector MS). The trajectory of ions in a quadrupole electric field, either two dimensional (Quadrupole MS) or three dimensional (Ion Trap and Orbitrap), stays within a defined space, if the operational parameters of the quadrupole field are tuned to a specific m/z. Time of Flight MS (TOF MS) generates a mass spectrum by measuring the traveling time of ions after pulsed acceleration. Ion Cyclotron Resonance (ICR) is based on the cyclotron motion of charged particles in magnetic fields and its dependence on m/z. Tandem MS (MS/MS) is a popular configuration to obtain the spectrum of fragment ions. The fragmentation spectrum provides information about chemical structure, mostly the sequence of monomers. Ion detection is the final stage of mass analysis. A resonance method such as ICR detects ions by their image current, which is induced by the motion of the ions. Another type of mass analyzer uses a secondary electron multiplying device to gain high sensitivity for the detection of ions.
Key Research Findings Mass spectrometry in ground-based studies: Organic substances in a Martian meteorite, ALH84001, marked the first direct evidence of organic molecules brought from an extraterrestrial planet. It was analyzed by a combination of infrared and ultraviolet lasers. Evaporation of the sample was made by infrared radiation, and successive shots of UV laser light produced photoionized PAHs. It has been proposed that the aliphatic side chains of the PAHs suggest a biological origin of these PAHs, although others have claimed these are in fact contaminants introduced from Antarctic ice. An extraterrestrial amino acid, glycine, was found in the captured dust of a comet by gas chromatography/ mass spectrometry (GC/MS). The Stardust sample was
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hydrolyzed and chemically derivatized in order to increase the vapor pressure for GC/MS analysis. The isotopic abundance of 13C in the glycine supports its extraterrestrial origin. This result bears on the understanding of the chemical evolution in our solar system and the possible contribution of comets in supplying prebiotic organics to initiate life on Earth. Mass spectrometry employed in space astrobiology missions is summarized below. ▶ Mars: The two ▶ Viking landers (1976) had MS to analyze the Martian atmosphere and its isotopic composition. Even though the composition of the atmosphere can be estimated remotely from Earth, on-site MS analysis is essential to provide isotopic information. The observed data was also used to determine the origin of the Martian meteorite, which contained trapped samples of the Martian atmosphere. The history of Mars’ lost atmosphere can be elucidated from measured isotopic ratios. GC/MS also performed a biological experiment on Viking. Phoenix landed on a polar region of Mars in 2008. A Thermal and Evolved Gas Analyser (TEGA) was included in the scientific payload of Phoenix. A surface sample was heated in its oven up to 1000 C, and volatiles from the sample were analyzed by a magnetic sector MS. The dynamic range of this system would have been able to detect organic molecules at 10 ppb level. ▶ Titan: The ▶ Cassini-Huygens mission to Saturn included the Huygens probe that descended to Titan in 2005. The GC/MS on the Huygens probe analyzed the atmospheric composition of Titan, which is dominated by CH4 and N2. The isotopic ratios were analyzed for those atmospheric species. Comets: ▶ Giotto explored ▶ Comet Halley with MS in 1986. Chemical species, both neutral molecules and ions, were analyzed during its flyby. Comets are considered to be a source of prebiotic molecules. Other than the major components H2O, CO, and CO2, both H2CO and HCN were found.
Applications Mass spectrometry is a powerful tool to detect and characterize biogenic and prebiotic substances and thus to find signatures of life on extraterrestrial bodies, and to elucidate their environmental factors. The generic features and high sensitivity of MS make it a common science payload of many space missions. Isotopic information available by MS is essential to differentiate biotic or abiotic origin, and to understand a planetary environment and its history for astrobiology.
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Future Directions The following space missions with MS instrument are now ongoing or in preparation. ▶ Mars Science Laboratory (MSL) will be launched in 2011 to Mars. The scientific goal of Sample Analysis at Mars (SAM) will carry out an astrobiology survey with Quadrupole MS and GC similar to TEGA on the Phoenix lander. A part of the ▶ ExoMars mission will be launched in 2018. The Mars Organic Molecule Analyzer (MOMA) is a scientific payload of this portion of ExoMars. It aims to detect organic substances in deep (2 m) drilled samples. Ion Trap MS with a Laser Desorption ion source enables detection of biogenic or prebiotic organics without the heating and charring of samples experienced in former missions. Comets: Rosetta, launched in 2004, will descend its lander to a comet nucleus in 2014. The Ptolemy GC/MS is part of the scientific payload. It will analyze substances of the nuclear surface and the atmosphere. An Ion Trap is used in this miniaturized GC/MS system. Mass spectrometry can be further adapted for astrobiology in several ways. To gain higher sensitivity to detect biogenic and prebiotic substances is essential for astrobiology. Even if organics will not be found at all, the extended limit of detection will contribute to assessing working hypotheses for the origin of life. Choosing proper soft ionization processes for biogenic and prebiotic molecules will bring new findings through the high signal-to-noise ratio and increased sensitivity. In addition to the choice of ionization, preprocessing of samples is an important step in astrobiology MS. One solution is direct ionization from a substrate surface. Another approach is preprocessing to convert complex sample substances to their constituent molecules. ExoMars implements a payload, Urey, to detect organics on the Martian surface. The sample will be exposed to subcritical (near the critical point) aqueous extraction. Organics are leached out from the solid sample, and macromolecules are hydrolyzed to constituent molecules. Digested smaller molecules, if similarly processed, keep information specific to the original biological system, and can be mass analyzed by soft ionization. For astrobiology, a combination of Liquid Chromatography and MS (LC/MS) is by its nature a more suitable approach compared to GC/MS. The imaging capability of MS enforces its strength by high sensitivity. Compared to the ordinary bulk analysis, spatial information from MS provides structural
information on biological forms at scales from the microscopic cellular level to mesoscopic systems. The homochirality of amino acids and sugars, if found on extraterrestrial bodies, is a factor that would distinguish biogenic origin from abiogenic substance. Even separation of optical isomers can be made by liquid chromatography-MS. Mass spectrometry continues to be a powerful tool for astrobiology.
See also ▶ Cassini–Huygens Space Mission ▶ Comet Halley ▶ ExoMars ▶ Gas Chromatography ▶ GC/MS ▶ Giotto Spacecraft ▶ Isotopic Ratio ▶ Liquid Chromatography-Mass Spectrometry ▶ Mars ▶ Mars Science Laboratory ▶ Molecular Weight ▶ Photoionization ▶ Rosetta (Spacecraft) ▶ Stardust Mission ▶ Titan ▶ Viking
References and Further Reading Berkel GJV, Pasilis SP, Ovchinnikova O (2008) Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry. J Mass Spectrom 43:1161–1180 Karas M, Bachman D, Bahr U, Hillenkamp F (1987) Matrix-assisted ultraviolet laser desorption of non-volatile compounds. Int J Mass Spectrom Ion Proc 78:53–68 McKay DS, Gibson EK, ThomasKeprta KL, Vali H, Romanek CS, Clemett SJ, Chillier XDF, Maechling CR, Zare RN (1996) Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001. Science 273:924–930 Takats Z, Wiseman JM, Gologan B, Cooks RG (2004) Mass spectrometry sampling under ambient conditions with desorption electrospray ionization. Science 306:471–473 Yamashitat M, Fenn JB (1984) Electrospray ion source. Another variation on the free-jet theme. J Phys Chem 88:4451–4459
Mass Spectroscopy ▶ Mass Spectrometry
Materialism
Mass–Luminosity Relation Definition The mass–luminosity relation for ▶ stars is an empirical correlation observed between the mass M and the luminosity L of stars on the ▶ main sequence of the ▶ Hertzsprung–Russell diagram. It has the form L / MK, with K3.5 as an average value. The lowest mass stars (0.1 MJ) are about 2,000 times less luminous than the Sun, whereas the most massive ones (100 MJ) are 1,000,000 times more luminous. The relation is due to the fact that stars in hydrostatic equilibrium – supported against self-gravity by their internal pressure – are hotter when they are more massive; therefore, their material radiates more strongly than in the case of lower mass stars.
See also ▶ Hertzsprung–Russell Diagram ▶ Main Sequence ▶ Stars ▶ Stellar Evolution
Mated Bioburden Definition In ▶ planetary protection, the term “mated ▶ bioburden” is used to indicate the number of viable microorganisms that are carried on spacecraft surfaces that are in contact with each other and joined by fasteners rather than by adhesives.
See also ▶ Bioburden ▶ Planetary Protection
Materialism CHRISTOPHE MALATERRE Institut d’Histoire et Philosophie des Sciences et Techniques (IHPST), Universite´ Paris 1-Panthe´on Sorbonne, Paris, France
Synonyms Physicalism
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Keywords Dialectical materialism, ethical materialism, historical materialism, matter, metaphysical materialism, substance
Definition In philosophy, materialism is most often taken as a metaphysical thesis according to which everything that exists is matter or results from matter (including life and consciousness); this thesis offers a monist ontology that typically contrasts with idealism, dualism, and ▶ vitalism. Materialism also refers to an ethical doctrine according to which the material well-being (including wealth, health or pleasure) is to be sought after. In social sciences, historical materialism is the Marxian thesis that historical and social phenomena are determined by economic facts; it is a particular case of dialectical materialism that construes the universe as a whole within which objects cannot be understood in isolation of one another.
Overview The nature of what exists is an old question that goes back to ancient Greek philosophers such as Thales or Lucretius. In the seventeenth century, Hobbes and Gassendi typically represent the materialist tradition, in clear opposition to Descartes’ dualism. The word “materialism” is said to have appeared for the first time in writings of Boyle. The doctrine is further developed in the eighteenth century by several French enlightenment thinkers (e.g., Meslier, de la Mettrie, Baron d’Holbach, Diderot). Broadly, materialism holds that “everything is matter,” and that, as a consequence, all phenomena, including life and consciousness, result from matter and material interactions. Materialism offers a monist ontology in which matter is the only substance, in contrast with idealism or spiritualism, and with pluralist ontologies such as dualism or vitalism. The metaphysical thesis of materialism raises interpretation and truth questions: what does it mean to say that everything is matter? What is matter? Is materialism true? The nature and definition of matter, in particular, have been much controversial. Traditionally referring to the type of tangible substance that we are broadly familiar with, the concept of matter has considerably evolved with the physical sciences, in particular relativity and quantum field theory, possibly including “dark matter” and “dark energy.” Some prefer now to use the term ▶ “physicalism” instead of “materialism,” as exemplified by much of the contemporary debate in analytic philosophy, also linked to questions of supervenience, emergence, and ▶ reductionism, especially in philosophy of mind and philosophy of biology.
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Mean Free Path
In moral philosophy, materialism refers to a practical doctrine according to which material considerations are the most worthy in one’s life: material well-being, wealth, health, or pleasure. Such an ethical materialism is present in Baron d’Holbach’s System of Nature and Haeckel’s Natu¨rliche Scho¨pfungsgeschichte, and can be traced back to Epicurus. The concept of historical materialism, first articulated by Engels and Marx, refers to the thesis that change in human societies originates in the economic means of production of the necessities of life; social classes, political structures, and other noneconomic elements of a society are seen as an outgrowth of the economic activity. Historical materialism fits as a particular case of dialectical materialism, a doctrine that construes the universe as a whole, constituted by matter in movement and within which qualitative transitions result from quantitative changes and from interactions of objects with one another. Evolutionary biologists such as Gould and Lewontin have defended dialectical materialism as a heuristic methodology.
See also ▶ Chance and Randomness ▶ Reductionism ▶ Vitalism
References and Further Reading Baron d’ Holbach, Paul Henri Dietrich (1770) Syste`me de la nature ou des loix du monde physique et du monde moral, Paris, trad. System of Nature Bunge M (1991) Scientific materialism. Reidl, London Charbonnat P (2007) Histoire des philosophies mate´rialistes. Syllepse, Paris Engels F (1878) Herrn eugen du¨hrings umwa¨lzung der wissenschaft. Dietz, Stuttgart Gould SJ (1990) Nurturing nature. In: An Urchin in the storm: essays about books and ideas. Penguin, London Lange FA (1866) Geschichte des Materialismus und Kritik seiner Bedeutung in der Gegenwart. J. Baedeker, Iserlohn, trad. The history of materialism and criticism of its present importance Moser PK, Trout JD (1995) Contemporary materialism: a reader. Routledge, London
or of the same type. ▶ Scattering of photons in a dusty medium and ▶ diffusion of molecules are examples of phenomena where the notion applies. Unit: m.
See also ▶ Diffusion ▶ Radiative Transfer ▶ Scattering
Mean Motion Resonance Definition In planetary dynamics, a mean motion resonance refers to a commensurability between orbital frequencies. If the ratio of the orbital frequencies of two (or more) planets or satellites is close to the ratio of two small integers, mutual perturbations may cause the orbits to evolve in ways not expected from secular evolution. Mean motion resonances have the potential to be protective or destructive, depending on the relative orientation of the orbits. For example, Neptune and Pluto’s orbital period ratio is close to 2/3, and the orbits actually cross, but resonance forbids the two planets from ever meeting at this crossing. Note that mean motion resonances do not depend solely on the orbital period ratio of two objects but also on their orbital alignment and phase.
See also ▶ Apsidal Angle ▶ Orbit ▶ Planetary Migration
MEED Synonyms Microbial Ecology Evaluation Device
Definition
Mean Free Path
The Microbial Ecology Evaluation Device (MEED) is a device to expose different biological systems to selected parameters of outer space (▶ space vacuum, ▶ solar UV radiation, and/or cosmic radiation).
Definition The mean free path is the mean travel length of an elementary particle (photon, atom, molecule, ion) between two successive interactions with other particles of another,
Overview The Microbial Ecology Evaluation Device (MEED) is an exposure facility for a variety of ▶ microorganisms
Membrane
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MEED. Table 1 Biological assay systems of MEED (Taylor 1975) Biological system
Phenomenon studied
Assay system
Bacillus subtilis spores
Genome alteration
Spore production
Bacillus subtilis spores
UV- and vacuum-sensitivity
Colony formation
Bacillus thuringiensis
a, b, and d toxin production
Sarcina flava, house fly, silk worm, and crystal assay
Aeromonas proteolytica
Hemorrhagic factor production
Guinea pig and hemoglobin
Escherichia coli (T7 phage)
Bacteriaphage infectivity
Host lysis
Chaetomium globosum
Cellulotytic activity
Cloth fibers
Trichophyton terrestre
Animal tissue invasion
Human hair
Rodoturula rubra
Drug sensitivity
Antibiotic sensitivity in agar
Saccharomyces cerevisiae
(Table 1) that were transported with the Apollo 16 spacecraft to the Moon and back to the Earth. It is, so far, the only device that has exposed microorganisms to the outer space conditions beyond the Earth’s magnetic field. During the extravehicular activity phase of the transearth coast, that is, return trip to the Earth, MEED was deployed outside the Apollo 16 capsule on the distal end of a television boom, opened, and oriented perpendicular to the Sun’s rays. After 10 min of Sun exposure, the MEED was closed again and returned back to the Command Module for return to Earth. MEED was also equipped with UV dosimeters, and passive nuclear track detectors and thermoluminescent dosimeters for monitoring cosmic rays.
Mega Annum ▶ Ma
Megayear
M
▶ Ma
Membrane
See also ▶ Apollo Mission ▶ Cosmic Rays in the Heliosphere ▶ Exposure Facilities ▶ Microorganism ▶ Solar UV Radiation (Biological Effects) ▶ Space Vacuum Effects
References and Further Reading Bu¨cker H, Horneck G, Wollenhaupt H, Schwager M, Taylor GR (1974) Viability of Bacillus subtilis spores exposed to space environment in the M-191 experiment system aboard Apollo 16. Life Sci Space Res 12:209–213 Taylor G (1974) Space microbiology. Annu Rev Microbiol 28:121–137 Taylor GR (1975) The Apollo 16 microbial response to space environment experiment. In: Johnston RS, Dietlein F, Berry CA (eds) Biomedical results of Apollo, NASA SP-368. NASA, Washington, DC Taylor GR, Spizizen J, Foster BG, Volz PA, Bu¨cker H, Simmonds RC, Heimpel AM, Benton EV (1974) A descriptive analysis of the Apollo 16 microbial response to space environment experiment. Bioscience 24:505–511
IRMA MARI´N Departamento de Biologı´a Molecular, Universidad Auto´noma de Madrid, Madrid, Spain
Synonyms Cell membrane; membrane
Cytoplasmic
membrane;
Plasma
Keywords Bilayer, energy conservation, glycerol-ester lipids, glycerol-ether lipids, lipids, proton-motive force, transmembrane proteins, transport
Definition A membrane is a fine structure that encloses the ▶ cell and separates the cytoplasm from the environment. In eukaryotic cells, membranes compartmentalize organelles to segregate processes and components.
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Overview The cell membrane is a thin bilayer (5–8 nm) of complex lipids imbedded with proteins as depicted in the Fluid Mosaic Model. The bilayer is arranged so that the polar ends of the lipids form the outermost surface of the membrane while the nonpolar ends form the center of the membrane bilayer. ▶ Bacteria and ▶ Eukarya have membranes composed mainly of glycerol-ester lipids, whereas ▶ Archaea have membranes composed of glycerol-ether lipids. Archaeal lipids are based upon nonpolar isoprenoid side chains. The proteins of the membrane can be of two different types: transmembrane or integral membrane proteins, which are inserted into the ▶ lipid bilayer and in some cases bound to the lipids; and periferical membrane proteins, which are usually attached by ionic bonding to the protruding portion of some integral membrane proteins. Membrane structure and composition make them selectively permeable, allowing only certain substances to enter or leave the cell. Dissolved gases and lipid soluble molecules simply diffuse passively across the bilayer but all other molecules require carrier molecules to transport them through the membrane by active transport. In transport processes, cells use transport membrane proteins and metabolic energy to transport substances across the membrane against concentration gradients. Depending on the carrier protein, the energy is provided by ▶ proton-motive force, the hydrolysis of ATP, the breakdown of some other high-energy compound, or other substance concentration gradients. A number of functions associated with the internal membranes of eukaryotic organelles are carried out in bacteria by the cell membrane as well as some other functions of bacterial membranes including energy production, motility, ▶ endospores formation, binary fission, cell adhesion, cell signaling, and attachment point for cytoskeleton. The ▶ gram-negative bacteria cell envelope contains an additional outer membrane, the lipopolysaccharide (LPS) layer, composed of phospholipids and lipopolysaccharides, which face the external environment. Psychrophilic and psychrotolerant bacteria have different strategies to adapt to low temperature, one of which is the ability of the cell to modulate the fluidity of the membrane by altering the fatty acid composition by including high concentrations of lipids with unsaturated fatty acids in their membranes. Similarly, highly saturated fatty acids in the cell membrane of thermophiles maintain their integrity even at high temperatures. Hyperthermophilic Archaea have side chains of covalently bonded glycerol tetraethers forming a lipid
monolayer that resist melting at the high temperatures at which they grow. Some compounds can alter microbial cell membranes and are used to control bacterial infections. The formation of membranes to separate the contents of the cell from the environment is considered an important event in the early steps of the origin of life.
See also ▶ Archea ▶ Bacteria ▶ Bioenergetics ▶ Cell ▶ Cell Membrane ▶ Cell Wall ▶ Endospore ▶ Eukarya ▶ Extremophiles ▶ Gram Negative Bacteria ▶ Hyperthermophile ▶ Lipid Bilayer ▶ Proton Motive Force
References and Further Reading Madigan MT, Martinko JM, Dunlap PV, Brock DP (2008) Biology of Microorganism, 12th edn. Clark Benjamin Cumming, San Francisco Prescott, Harley, Klein (2007) Microbiology, 7th edn. Willey, McCraw Hill Science, New York
Membrane Potential DAVID DEAMER Department of Biomolecular Engineering, University of California, Santa Cruz, CA, USA
Synonyms Diffusion potential; Nernst potential
Keywords Concentration gradient, Nernst equation, polarization, selective ▶ permeability
Definition Membrane potentials are measured in units of volts or millivolts. Potentials are produced across membranes by ionic ▶ concentration gradients if the membrane barrier contains channels that preferentially allow either cationic or anionic solutes to diffuse through.
MER, Spirit and Opportunity (Mars)
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Overview
Definition
Living cells today have evolved a variety of transmembrane channel proteins that allow sodium or potassium ions to pass through the lipid bilayer barrier of membranes. Gradients of these ions are produced and maintained by the action of ion transport enzymes that use ATP as an energy source. Potassium ions, carrying a positive charge, tend to diffuse down their concentration through the ion specific channel, while sodium and chloride ions cannot. As a result, the membrane becomes polarized, positive outside and negative inside, and the polarization is measured as a membrane potential. The voltage is described by the Nernst equation, a simplified form of which is: Co ð1Þ V ¼ 0:059 log Ci
The term mensa, meaning table or plate, is used for a flattopped rise with cliff-like edges on ▶ Mars and ▶ Jupiter’s ▶ moon ▶ Io. These topographic features can be built by the action of fluids or lava resulting from the erosion of adjacent, less resistant material. On Mars the term mensae is also used for long scarps along the highland–lowland boundary as well as for flat-topped deposits of layered sediments (i.e., Interior Layered Deposits, ILDs) located in the chasmata of ▶ Valles Marineris.
where Co and Ci are the external and internal concentrations of an ion such as potassium. Typical gradients are approximately ten-fold, and from the Nernst equation, a ten-fold gradient produces a membrane potential of 0.059 V, or 59 mV. Membrane potentials can also be produced by the active transport of protons. The electron transport systems of bacteria are capable of pumping 1000-fold gradients of protons, equivalent to 180 mV. Mitochondria and chloroplasts are evolutionary descendents of bacteria, and also pump protons. The chemiosmotic theory showed that a membrane potential of protons is the essential energy source for ATP synthesis. The relation to early forms of life is that at some point the membranes of primitive bacteria were able to generate and maintain a membrane potential, so that chemiosmotic ATP synthesis could occur. How this could happen remains an open question for research on the origin of life.
See also ▶ Bioenergetics ▶ Cell Membrane ▶ Concentration Gradients ▶ Permeability
References and Further Reading Hille B (2001) Ion channels of excitable membranes, 3rd edn. Sinauer Associates, Sunderland, MA
Mensa/Mensae Synonyms Table mountain
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See also ▶ Io ▶ Jupiter ▶ Mars ▶ Valles Marineris
MER, Spirit and Opportunity (Mars) CLAUDE D’USTON Centre d’Etude Spatiale des Rayonnements, Toulouse, France
Synonyms Mars exploration rovers
Keywords Mars
Definition Mars Exploration Rover (MER) is a ▶ NASA mission composed of two identical robot geologists, SPIRIT and OPPORTUNITY, which were launched separately in June and July 2003, respectively. After their successful landing on the surface of ▶ Mars in January 2004, in areas near the equator, they started exploring their ▶ landing sites, searching for clues of past water activity on Mars. Both Mars Exploration Rovers were designed to last 90 ▶ sol (a Martian day = 1.027491 Earth days). They were still functioning 6 years after landing.
Overview The MER mission is part of the Mars exploration program that NASA has conducted regularly to decipher the history of this planet, to find whether life ever existed there, and to prepare a future human visit.
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MER, Spirit and Opportunity (Mars)
After the previous missions, both in orbit (▶ Mars global Surveyor [▶ MGS], ▶ Mars Odyssey) and on the surface of Mars (▶ Viking 1 & 2, ▶ Mars Pathfinder), it appeared likely that liquid water had been present in the past on the surface of Mars. Then, the questions were when in the history of Mars, and how long were those periods of liquid water. The MER mission consisted in sending to Mars two identical robotic rovers to explore areas of Mars that were selected because they presented likely indications of past activity of water. These landing sites were chosen among areas that satisfied the engineering constraints for a safe landing (Fig. 1). One is within the flat-floored Gusev crater (14,57 S, 175,47 E) at the end of the large valley Ma’adim Vallis, suggesting that the materials inside were deposited in a crater lake. The second one is in Meridiani Planum (354.47E, 1.95ºS), composed of basaltic sand and sparse outcroppings, where the identification of coarse-grained hematite in MGS Thermal Emission Spectrometer (TES) spectra and the geologic setting from Thermal Emission Imaging System (THEMIS) data could arguably indicate the influence of liquid water. Within just a few months of arriving on Mars, both Spirit and Opportunity fulfilled their mission objectives, and discovered evidence that large quantities of water used to be on the surface of Mars. Spirit discovered hints that water had acted on a ▶ rock called Humphrey, while Opportunity found layers of sedimentary rock that would have been formed by deposits in water. Both rovers continued to find additional evidence for the presence of water. Over the course of their mission on the surface of Mars, both rovers travelled several kilometers. Spirit climbed a small mountain, and Opportunity crawled
into a large crater to sample the walls for evidence of past water. And both rovers continued to perform quite well, for many years beyond their originally estimated life spans. In total, Spirit travelled 7.7 km, while OPPORTUNITY covered more than 25 km, and is still progressing. They both survived several Martian winters.
Basic Methodology The rovers and their operational capacities were designed according to the scientific objectives of the mission, which were defined following the general motto “Follow the water.” Its main objectives are to search for and characterize a variety of rocks and soils, in particular, samples that could have minerals deposited by water-related processes; and to determine their distribution in the surroundings of the landing sites in order to determine what geologic processes have shaped the local terrain and influenced the chemistry. The rovers should also search for geological clues to the environmental conditions that existed when liquid water was present, in order to assess whether those environments were conducive to life. Consequently, each rover is equipped with a suite of science instruments to read the geologic record at each site, to investigate what role water played there, and to determine how suitable the conditions would have been for life. The scientific instrument suite consists of the following: 1. A panoramic Camera (PANCAM) mounted 1.5 m high on the Pancam Mast Assembly, for characterizing the local terrains; it is a stereo pair with 8 possible filters for each eye. 2. A miniature infrared spectrometer for the thermal emission (miniTES) whose mirror is also mounted
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Opportunity Meridiani Planum
pla
nitia
Spirit Cratère Gusev
MER, Spirit and Opportunity (Mars). Figure 1 Location of the landing sites (credit NASA/JPL)
MER, Spirit and Opportunity (Mars)
3.
4.
5. 6.
7.
on the mast near the PANCAM, to determine the mineral composition of rocks and soil. A microscope camera imager (MI) at the end of the robotic arm to obtain close-up high resolution macroscopic images of target rocks and soil. An alpha particle and X-ray fluorescence Spectrometer (APXS), for close-up analysis of the abundances of elements that make up rocks and soils. A Mo¨ssbauer spectrometer (MB) for close-up investigations of the mineralogy of iron-bearing rocks and soils. A rock abrasion tool (RAT) for removing dusty and weathered rock surfaces and exposing fresh material for examination by the suite of instruments. Three sets of magnets for collecting magnetic dust particles. The particles are analyzed by the Mo¨ssbauer Spectrometer and X-ray Spectrometer to help determine the ratio of magnetic particles to nonmagnetic particles and the composition of magnetic material.
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The two rovers are identical six-wheeled, solarpowered robots with a mass of 180 kg (Fig. 2). Each Mars rover communicates with Earth using antennas that transmit information to either Deep Space Network (DSN) antennas on Earth or to any spacecraft orbiting Mars, which in turn relays the information to land-based stations on Earth. To move autonomously on the uneven surface of Mars, the rovers use three pairs of monochromatic cameras and navigation software which identifies obstacles; the pairs of cameras allow for 3D vision and are located at the front and at the rear under the solar panel deck, and on top of the mast to allow for a larger view used for planning the displacements. They are also equipped with a robotic arm called the instrument deployment device (IDD), which holds a grinding tool (RAT) and in situ instruments (MI, APXS and MB).
Navigation cameras Mini-thermal emission spectrometer (at rear)
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Panoramic cameras Low-gain antenna UHF antenna
Solar arrays
Calibration target
High-gain antenna
Magnet array (forward)
Alpha particle X-ray spectrometer Microscopic imager
Mo¯ssbauer spectrometer
Rocker-bogie mobility system
Rock abrasion tool
Mars Exploration Rover
MER, Spirit and Opportunity (Mars). Figure 2 Description of the main elements of the MER rover
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MER, Spirit and Opportunity (Mars)
Story Board of the Mission The main events that marked the exploration of either rover are listed below: – Landings occurred on January 4 for SPIRIT and on January 25, 2004, for OPPORTUNITY. At first, the Gusev area appeared as dry and devoid of past water activity, similar to Mars Pathfinder or Viking landing sites. On the other side of the planet, OPPORTUNITY found itself in the middle of a small depression caused by impact, the Eagle crater, which revealed rock outcrops showing layering and composition that could be sediments deposited by water (Fig. 3). – On March 11, 2004, Spirit reached Bonneville crater hoping for a view of underlying bedrock, but none was visible and it was decided to continue toward the Columbia hills. – On April 30, 2004, OPPORTUNITY arrived at Endurance crater and started to explore its inner slope until it exited on September 22 (Fig. 4).
– On June 15, 2004, SPIRIT arrived at the foot of the Columbia Hills and started observing very different rocks that suggested that they had undergone water alteration (Fig. 5). Then, while climbing, several dust devils were observed, some of them cleaning the solar panels, thus allowing for a supplement of electric power. – On August 21, 2005, SPIRIT reached the summit of Husband Hill. It then went on downhill, making more analyses of the different outcrops and soils. – On February 7, 2006, SPIRIT reached the semicircular rock formation known as Home Plate. It is a layered rock outcrop, and it is thought that its rocks are explosive volcanic deposits, though other possibilities exist, including impact deposits or wind-/waterborne sediment. – OPPORTUNITY arrived at Victoria crater on September 27, 2006, and started to observe from around its rim. After a large dust storm faded away,
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MER, Spirit and Opportunity (Mars). Figure 3 Magnified view of a portion of outcrop rock seen in Eagle crater, which is interpreted as sediments that were laid down in flowing water (credit NASA/JPL)
MER, Spirit and Opportunity (Mars). Figure 4 View of the “burncliff” formation within Endurance crater that shows stratification that was analyzed to decipher the emplacement history (credit NASA/JPL)
MER, Spirit and Opportunity (Mars)
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▶ Jarosite, a basic hydrous sulfate of potassium and iron. In the Columbia Hills, Spirit found also very typical minerals, in particular, pure silica and also ▶ goethite (FeO (OH)), and for the first time some long-sought-after ▶ carbonate. All those observations indicate that the past environment at these locations was wet, possibly favorable to life. The atmospheric temperature variations through several Martian years have been recorded; density and composition of atmospheric dust and water ice ▶ aerosols, together with the obscuring of solar light, have been observed by both rovers. Numerous dust devils were seen during spring and at the beginning of summer.
MER, Spirit and Opportunity (Mars). Figure 5 False-color image taken by the panoramic camera on the Mars Exploration Rover Spirit shows the rock dubbed “Pot of Gold” located near the base of the Columbia Hills in Gusev Crater (credit NASA/JPL)
OPPORTUNITY entered Victoria crater in September 2007 and analyzed the rock layers until exiting on August 29, 2008. – OPPORTUNITY continued its way toward the very large Endeavour crater 12 km further south; it is progressing through sand dunes fields, making detours to avoid the most risky paths. – Since May 2009, SPIRIT had been stopped with its wheels sunk in very loose sand on the west of Home Plate. After unsuccessful attempts to extricate the rover from this trap, NASA redefined the robot mission as a stationary research platform – End of March 2010, SPIRIT went into hibernation at her location on the west side of Home Plate due to low electric power production during Martian winter. Next Martian spring started in November 2010, and could possibly allow reactivation of the rover.
Key Research Findings Signs of the past presence of water indicated that there has been liquid water in large quantities and for long periods of time. Little spherules that contain ▶ hematite (Fe2O3), called “blueberries,” have been found by OPPORTUNITY everywhere on the Meridiani Planum; they are considered as an indicator of a long presence of liquid water. There have been also other signs in the fine outcrop layering, or in the presence of vugs (small to medium-sized cavities inside rocks) in the bedrock, and with the identification of
See also ▶ Basalt ▶ Carbonate ▶ Hematite ▶ Jarosite ▶ Landing Site ▶ Life in the Solar System (History) ▶ Mars ▶ Mars Global Surveyor ▶ Mars Pathfinder ▶ Meridiani (Mars) ▶ Rock ▶ Sol ▶ Sulfate Minerals ▶ Water Activity
References and Further Reading Arvidson RE et al (2006) Overview of the Spirit Mars Exploration Rover Mission to Gusev Crater: landing site to Backstay Rock in the Columbia Hills. J Geophys Res 111:E02S01. doi:10.1029/ 2005JE002499 Arvidson RE et al (2008) Spirit Mars Rover Mission to Columbia Hills, Gusev Crater: mission overview and selected results from the Cumberland Ridge to Home Plate. J Geophys Res 113:E12S33. doi:10.1029/2008JE003183 Bell JF III et al (2003) Mars Exploration Rover Athena Panoramic Camera (Pancam) investigation. J Geophys Res 108(E12):8063. doi:10.1029/ 2003JE002070 Bell JF III (ed) (2008) The Martian surface – composition, mineralogy, and physical properties. Cambridge University Press, Cambridge Crisp JA et al (2003) Mars Exploration Rover mission. J Geophys Res 108(E12):8061. doi:10.1029/2002JE002038 Cristensen PhR et al (2003) Miniature thermal emission spectrometer for the Mars Exploration Rovers. J Geophys Res 108(E12):8064. doi:10.1029/2003JE002117 Golombek MP et al (2003) Selection of the Mars Exploration Rover landing sites. J Geophys Res 108(E12):ROV 13-1. doi:10.1029/ 2003JE002074, CiteID 8072
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Gorevan SP et al (2003) Rock abrasion tool: Mars Exploration Rover mission. J Geophys Res 108(E12):8068. doi:10.1029/2003JE002061 Herkenhoff KE et al (2003) Athena Microscopic Imager investigation. J Geophys Res 108(E12):8065. doi:10.1029/2003JE002076 Klingelho¨fer G et al (2003) Athena MIMOS II Mo¨ssbauer spectrometer investigation. J Geophys Res 108(E12):8067. doi:10.1029/ 2003JE002138 Knoll AH et al (2008) Veneers, rinds, and fracture fills: relatively late alteration of sedimentary rocks at Meridiani Planum, Mars. J Geophys Res 113:E06S16. doi:10.1029/2007JE002949 Ming DWet al (2008a) Geochemical properties of rocks and soils in Gusev Crater, Mars: results of the Alpha Particle X-ray spectrometer from Cumberland Ridge to Home Plate. J Geophys Res 113:E12S39. doi:10.1029/2008JE003195 Ming DW et al (2008b) Aqueous alteration on Mars. In: Bell JF III (ed) The Martian surface: composition, mineralogy, and physical properties, chap. 23. Cambridge University Press, Cambridge, pp 519–540 Rieder R et al (2003) The new Athena Alpha Particle X-ray spectrometer for the Mars Exploration Rovers. J Geophys Res 108(E12):8066. doi:10.1029/2003JE002150 Squyres SW et al (2003) Athena Mars Rover science investigation. J Geophys Res 108(E12):8062. doi:10.1029/2003JE002121 Squyres SW et al (2006) Two years at Meridiani Planum: results from the Opportunity Rover. Science 313(5792):1403–1407. doi:10.1126/science.1130890 DOI:dx.doi.org Squyres SW et al (2009) Exploration of Victoria Crater by the Mars Rover Opportunity. Science 324:1058–1061
Mercaptomethane ▶ Methanethiol
Mercury JO¨RN HELBERT German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Keywords Atmosphere-less body, terrestrial planet
Definition Mercury is the innermost ▶ planet and the smallest of the four ▶ terrestrial planets. It is the only terrestrial planet except Earth to have a global magnetic field. Mercury shows the most extreme surface temperature variations of all terrestrial planets. The average surface temperature is 442.5 K, with dayside temperatures up to 700 K and
nightside temperatures down to 80 K. Mercury has a tenuous exosphere containing hydrogen, helium, oxygen, sodium, calcium, and potassium. This exosphere is not stable – atoms are continuously lost and replenished from a variety of sources.
Overview So far, only two spacecrafts have visited Mercury. The NASA mission Mariner 10 made 3 flybys in 1974 and 1975. The NASA mission MESSENGER made three flybys between 2007 and 2009 and will enter orbit around the planet in 2011. The ESA mission BepiColombo will observe the planet from orbit starting 2019. Observing Mercury from ▶ Earth is challenging. While it is bright, ranging from 2.3 to 5.7 in apparent magnitude, the greatest angular separation from the ▶ Sun is only 28.3 . Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse, Mercury can only be viewed in morning or evening twilight. With a radius of 2,439.7 km, Mercury is the smallest of the terrestrial planets and its evolution and formation can provide important clues to the formation of planetary systems in general. With 5.427 g/cm3, Mercury has the second highest density of all terrestrial planets (Earth: 5.515 g/cm3). It has the highest metal-to-silica ratio of all planets or ▶ satellites. The surface mineralogy seems to be dominated by plagioclase. The ▶ Magnetic field of Mercury has about 1.1% of the strength of the Earth’s magnetic field. Observations by the NASA spacecrafts Mariner 10 and MESSENGER have indicated that the strength and shape of the magnetic field are stable. The high density and the magnetic field combined lead to the assumption that Mercury has a large iron ▶ core occupying about 42% of its volume, compared to 17% for Earth. There are several competing theories for the origin of this large core during the formation of Mercury, ranging from a giant impact that striped the planet of its crust to inhomogenities in the early solar nebular leading to an enrichment of protoMercury in iron. Mercury has a mean heliocentric distance of 0.3871 AU. One orbit around the Sun takes 87.969 days; in this time, the planet performs 1.5 rotations. This 3:2 spin-orbit coupling has been explained by Guiseppe Colombo. Given the eccentricity of Mercury’s orbit, the 3:2 coupling minimizes the tidal torque (see ▶ tides) on the planet during perihelion passage. The exosphere of Mercury is formed by thermal desorption and sputtering of surface material, as well as captured solar wind particles. The NASA MESSENGER spacecraft detected indications for water in the Exosphere,
Metabolic Diversity
most likely formed from solar hydrogen and oxygen sputtered off the planet’s surface. Radar observation from Earth show bright regions close to Mercury’s poles. This could be an indication for water ice deposits in permanently shadowed ▶ craters.
See also ▶ Core, Planetary ▶ Crater, Impact ▶ Earth ▶ Magnetic Field ▶ Planet ▶ Satellite or Moon ▶ Sun (and Young Sun) ▶ Terrestrial Planet ▶ Tides (Planetary)
References and Further Reading Balogh A, Ksanfomality L, von Steiger R (2008) Space Science Series of ISSI Vol 76. Mercury, Springer, ISBN 978-0-387-77538-8 Solomon SC (2003) Mercury: the enigmatic innermost planet. Earth Planet Sci Lett 216(4):441–444 Sprague A, Strom R (2003) Exploring Mercury: the iron planet. Springer, Berlin
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▶ Mineral ▶ Planum ▶ Water Activity
Mesophile Definition The term mesophile is mainly applied to ▶ microorganisms, and it refers to microbes whose optimal development temperatures are moderate, ranging from 15 C to 45 C. They are probably the most abundant and conspicuous group of microbes and their habitats are widely distributed, from soils to water to animal niches. Most of the pathogens for humans (bacteria and viruses) belong to this group.
See also ▶ Bacteria ▶ Microorganism ▶ Prokaryote
M Meridiani (Mars) Definition Meridiani Planum is an extended plain on ▶ Mars centered at 357.5 E/0.2 N in the western part of an ▶ Albedo feature called Sinus Meridiani. Meridiani Planum was the landing site of the ▶ Mars Exploration Rover Opportunity in 2004. In Meridiani Planum cross-bedded sediments are exposed that contain crystalline ▶ hematite (Fe2O3), and sulfate-rich ▶ Minerals such as ▶ jarosite. The presence of the latter mineral indicates an acid and saline environment and a soil saturated with liquid water during a considerable period of time. Airy-0, a small crater of 0.5 km in diameter at 0 E/5.1 S defines the prime meridian of Mars. It is located within the larger (40 km) crater Airy in southern Meridiani Planum.
See also ▶ Albedo Feature ▶ Crater, Impact ▶ Hematite ▶ Jarosite ▶ Mars ▶ MER, Spirit and Opportunity (Mars)
Metabolic Diversity Definition Metabolic diversity mainly refers to the different metabolic strategies that organisms have evolved to obtain energy. Metabolic pathways evolved among prokaryotes before eukaryotes arose as the result of their interaction and coevolution with changing physicochemical environmental conditions. Metabolic diversity is life’s evolutive response enabling it to adapt to and use all available energy and organic and inorganic matter sources. Life, as we know it, follows common nutritional patterns that can be subdivided depending on energy or carbon requirements. The following organisms can be classified according to their energy requirements as: ▶ Phototrophs, if energy comes from solar radiation; or ▶ Chemotrophs, if the energy source is inorganic-(▶ Chemolithotroph) or organic-(▶ Chemoorganotroph) reduced compounds. According to the mechanism used, they can be classified as Photosynthetic (oxygenic and anoxygenic), Respirers (aerobic and anaerobic), and Fermentors. Depending on the carbon source, organisms can be ▶ Autotrophs or self-feeders, if carbon is fixed from CO2 by the organisms; or Heterotrophs or consumers, if the carbon source is an already existing organic matter.
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Combining energy and carbon sources, organisms can be classified into four groups : Photoauthotrops (green plants, algae and cyanobacteria), Photoheterotrophs (some ▶ Bacteria), Chemoautotroph (some Bacteria and ▶ Archaea, such as methanogens), and Chemoheterotrophs (animals, fungi, protists and most bacteria). Due to this adaptative response, metabolic diversity has tremendous astrobiological implications since such wide diversity could cover life’s adaptation to other planetary conditions, especially the photoautotrophy or chemolithoautotrophy. An interesting property of some ▶ microorganisms is their metabolic versatility, which enables them to select their metabolic mode according to environmental conditions. The most versatile microorganisms known so far are the Purple nonsulfur Bacteria, a group of anoxigenic photosynthetic microorganisms that in the absence of light can employ aerobic and ▶ anaerobic respiration and ▶ fermentation.
See also ▶ Aerobic Respiration ▶ Anaerobic Respiration ▶ Anoxygenic Photosynthesis ▶ Autotroph ▶ Bioenergetics ▶ Biosynthesis ▶ Chemoautotroph ▶ Chemolithotroph ▶ Chemoorganotroph ▶ Chemotroph ▶ Energy Conservation ▶ Fermentation ▶ Lithotroph ▶ Metabolism (Biological) ▶ Oxygenic Photosynthesis ▶ Photosynthesis ▶ Phototroph ▶ Respiration
Metabolic Zones ▶ Redox Zonation
Metabolism (Biological) MARI´A LUZ CA´RDENAS, ATHEL CORNISH-BOWDEN Centre National de la Recherche Scientifique, Unite´ de Bioe´nerge´tique et Inge´nierie des Prote´ines, Marseille Cedex 20, France
Synonyms Metabolic networks; Minimal metabolism
Keywords Compartmentation, enzymes, metabolic cascades, metabolic flux, organization, pathway, regulation
Definition Metabolism is the collection of chemical reactions that define a living organism and allow it to make its components and obtain the energy required for staying alive.
History Recognition that the processes occurring in a living organism are fundamentally chemical reactions came progressively with the decline in vitalism in the nineteenth century. The discovery of cell-free fermentation by Eduard Buchner sounded the death-knell of vitalism and the beginning of the unraveling of the major metabolic pathways that constitute metabolism. This process began with the understanding of ▶ fermentation and ▶ glycolysis, and by the end of the 1950s virtually all the major pathways were known.
Overview
Metabolic Networks ▶ Biological Networks ▶ Metabolism (Biological)
Metabolic Organization ▶ Autopoiesis
Although a living organism is identified and recognized by its physical appearance, and hence by its structure, its status as living is defined by its chemistry, and thus by its metabolism. This is a network of reactions that is responsible for synthesizing all of the molecules needed for the organism to survive, apart from those directly available from its environment, and for disposing of molecules that are harmful or no longer required. The entire network consists of thousands of connected reactions, and as many of these need to be maintained far from equilibrium, all organisms require and consume energy.
Metabolism (Prebiotic)
To a first approximation, the metabolism of all organisms is the same, but there are important differences that depend on whether it is aerobic, requiring ▶ dioxygen as the ultimate electron acceptor, or anaerobic, which can be facultative, with dioxygen tolerated, or obligate, with dioxygen completely avoided. Some organisms, including plants and some bacteria and archaea, are ▶ autotrophs that produce organic compounds from inorganic precursors (CO2 and carbonates being considered inorganic), whereas others, including all animals, fungi, and some bacteria and archaea, are heterotrophs, depending on other organisms as primary sources of organic materials. Aerobes use dioxygen as their primary source of oxidizing power, but anaerobes also need oxidizing precursors, such as sulfates, and can obtain oxygen from these. Heterotrophs use organic compounds ingested as food as their primary source of reducing power, but autotrophs do not require organic food. As they exist in an oxidizing environment, they cannot use inorganic reducing agents, but depend on ▶ photosynthesis for reducing power. Oxidation and reduction inevitably conflict with one another, and so metabolism is only possible with a very high degree of regulation, preventing the terminal oxidizing and reducing agents from reacting directly with one another, and similarly allowing synthesis (▶ anabolism) and degradation (▶ catabolism) to proceed without interference. Essentially no reactions must occur without catalysis, therefore, and the catalysts should be highly specific. With no thermodynamic prohibition of unwanted reactions, they can only be avoided by the absence of active catalysts, which is achieved by ▶ enzyme regulation and to some degree by compartmentation (Ginger et al. 2010), with physical barriers separating reactions that might conflict with one another: a cell is not just a bag of catalysts. Only proteins (and to a much more limited extent RNA) can provide the necessary specificity that allows efficient catalytic activity in very mild conditions, and virtually all the catalysts of metabolism are proteinbased enzymes. These cannot be harvested by an organism from its environment, but must be synthesized by the organism itself. Thus, although enzymes are conventionally regarded as different from metabolites, the chemical intermediates of metabolism, there is no fundamental distinction between them, both being produced internally from the same precursors, used internally, and broken down to the same end products. During the course of evolution, several mechanisms have appeared that allow fine regulation of metabolism, such as allosteric and cooperative interaction of effectors with appropriate enzymes, chemical modification of enzymes (metabolic cascades), and compartmentation.
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Although cyclic pathways, such as the tricarboxylate (Krebs) cycle, have sometimes been claimed to be too complex to be the result of natural selection, a convincing analysis of the steps that could produce such a cycle has been given (Mele´ndez-Hevia et al. 1996).
See also ▶ Anabolism ▶ Autotroph ▶ Bioenergetics ▶ Catabolism ▶ Chemotroph ▶ Citric Acid Cycle ▶ Dioxygen ▶ Enzyme ▶ Fermentation ▶ Glycolysis ▶ Photosynthesis ▶ Respiration
References and Further Reading Ginger ML, McFadden GI, Michels PAM (2010) Rewiring and regulation of cross-compartmentalized metabolism in protists. Phil Trans R Soc B 365:831–845 Horton R, Moran LA, Scrimgeour G, Perry M (2005) Principles of biochemistry, 4th edn. Prentice-Hall, Upper Saddle River, NJ Mele´ndez-Hevia E, Waddell TG, Cascante M (1996) The puzzle of the Krebs citric acid cycle: assembling the pieces of chemically feasible reactions, and opportunism in the design of metabolic pathways during evolution. J Mol Evol 43:293–303 Salway JG (2004) Metabolism at a glance. Blackwell, Malden, MA
Metabolism (Prebiotic) HAROLD MOROWITZ George Mason University, Fairfax, VA, USA
Definition Metabolism in contemporary biology and biochemistry refers to the aggregate of chemical transformations taking place in living organisms or in ecosystems. It consists of ▶ anabolism, the buildup of compounds, catabolism, the breakdown of compounds, and the alteration of compounds for energy transformation or usage. The word itself comes from Hellenic Greek, where it refers to change.
Overview The contemporary usage seems to go back to the 1870s where it occurs in physiology books in the sense of
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chemical change. In present day ecological systems, the overall metabolic activity consists of the incorporation of carbon, hydrogen, nitrogen, phosphorous, and sulfur driven by photosynthesis or chemosynthesis, the transformation of the inputs into metabolites, the buildup of these small molecules into macromolecules and other cell structures; and, ultimately, the decay due to ▶ respiration, predation, and decay. There are common features of metabolism across all taxa: the same ▶ amino acids, the same sugars, the same ▶ nucleotides, and the same cofactors are used. These are matched even to the level of ▶ chirality and the universality of the ▶ genetic code. Many of these intertaxonomic features were gathered together in the 1950s by Donald E. Nicholson in a chart of intermediary metabolism. This material was organized and codified in 1970, in the book “An Introduction to Metabolic pathways” by S. Dagley and Donald E. Nicholson. What is striking is the universality of adenosine triphosphate and its hydrolysis in bioenergy transfer processes, the universality of macromolecule synthesis, and the universality of the citric acid cycle in both energy utilization and anabolism. All anabolism comes from five compounds in the citric acid cycle and the anapleurotic loop. A number of cofactors are essential and are either synthesized or obtained as vitamins. The enormous overlap in all metabolism indicates either it is a rare accident, it is the best solution to biochemistry, or it is the only solution to biochemistry. The overlap of metabolic intermediates is a necessity for trophic ecology, for otherwise the predator or grazer would find its food undigestable or unusable. The universality question can potentially be answered by experimental astrobiology or a theory of living systems subject to experimental test. In any case, metabolism may be regarded as a feature of an organism or an ecosystem. Indeed, the failure to grow many of the microbes of an ecosystem in pure culture suggests difficulties in defining species metabolism and questions regarding the meaning of species at the microbial level. Energy for metabolism is derived from mineral energy sources in the environment, photoautotrophy, or oxidative heterotrophy. Biogenesis would require one of the first two. All metabolisms require the flow of energy from a high quantum-size source to a low quantum-size sink, making metabolism consistent with the laws of thermodynamics. Complete genome sequencing allows the construction of the metabolic chart in toto, since almost all reactions require enzymes that are encoded in the genome. This has led to a common autotrophic metabolic chart, most reactions of which are universal, again suggesting universal laws of metabolism.
One way to search for microbial life on other astronomical objects is to search for core molecules of metabolism. The total absence of such molecules would indicate no life or a life so metabolically different from terrestrial life that a reframing of the question of what life is might be required. For example, it is extremely unlikely that organisms metabolically different from those on Earth could be pathogenic. In any case, the full consequence of cross-contamination of spatial bodies should be viewed in terms of metabolism. The laws of metabolism are a necessary feature of the laws of life on Earth and must form a central feature of astrobiology.
See also ▶ Chirality ▶ Genetic Code ▶ Nucleotide ▶ Respiration
Metagenome VI´CTOR PARRO Molecular Evolution Department, Centro de Astrobiologı´a (INTA-CSIC), Torrejo´n de Ardoz, Madrid, Spain
Synonyms Community genome; Environmental genome; Population genome
Keywords ▶ Bioinformatics, ecogenomics, environmental microbiology, metabolic diversity, metabolic networks, metabolomics, metaproteomics, metatranscriptomics, microbial diversity
Definition The metagenome is the genetic content of a biological community. The term is mainly applied to microbial communities which can be somehow considered as a whole entity and, consequently, treated and studied as a single “meta-organism” with a single ▶ genome. It can also be applied to subpopulations like a viral population isolated from a whole microbial community. In such case it is called the ▶ metavirome (the genome of the virome). The term “meta” comes from Greek and means “next to” or “related to.”
Metallicity
History Phylogenetic studies with the DNA ▶ sequence of environmental rRNA genes revealed that most of the microbial diversity (up to 99%) remained unexplored when only culturing techniques were applied. This was one of the main conclusions after the pioneering work conducted by Norman R. Pace and colleagues, who used the amplification technique called ▶ polymerase chain reaction to explore the diversity of ribosomal RNA sequences (Lane et al. 1985; Pace et al. 1985). It was Jo Handelsman who in 1998 first suggested the term “metagenomics” for naming the idea that a set of ▶ gene sequences obtained from environmental samples could be analyzed as if they came from a single genome (Handelsman et al. 1998). Since then many works have been reported where the whole genetic content of a microbial community (the metagenome) has been used either for gene sequencing and screening for novel enzymatic activities (Healy et al. 1995), or for whole genome DNA sequencing (Tyson et al. 2004; Venter et al. 2004).
Overview The exploration of metagenomes has been tightly bound to the improvement and development of new cloning and ▶ amplification strategies and sequencing techniques, as well as advances in bioinformatics for sequence analysis. From the analysis of metagenomes important information can be obtained about microbial and metabolic diversity as well as the versatility of natural populations. Some applications include: environmental monitoring either to follow the effect of pollutants or bioremediation technologies; monitoring of industrial processes conducted by microbial consortia; gene inventory of a microbial community either for the search of new enzymatic activities (Zhang et al. 2009) or to explore the potential metabolic networks in the community; sequencing single genomes from uncultivable microorganisms (Tyson et al. 2004); exploring potential pathogens in the microbial flora of humans and animals (Sommer et al. 2009); exploring the genetic wealth of environmental viruses from extreme environments (Lo´pez-Bueno et al. 2009); exploration of the preferential gene expression in extremophilic communities for astrobiology purposes (Parro et al. 2007).
See also ▶ Amplification (Genetics) ▶ Biodiversity ▶ Bioinformatics
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▶ Gene ▶ Genetics ▶ Genome ▶ Metavirome ▶ Minimal Genome ▶ Polymerase Chain Reaction ▶ Sequence
References and Further Reading Handelsman J, Rondon MR, Brady SF, Clardy J, Goodman RM (1998) Molecular biological access to the chemistry of unknown soil microbes: a new frontier for natural products. Chem Biol 5:245–249 Healy FG, Ray RM, Aldrich HC, Wilkie AC, Ingram LO, Shanmugam KT (1995) Direct isolation of functional genes encoding cellulases from the microbial consortia in a thermophilic, anaerobic digester maintained on lignocellulose. Appl Microbiol Biotechnol 43:667 Lane DJ, Pace B, Olsen GJ, Stahl DA, Sogin ML, Pace NR (1985) Rapid determination of 16 S ribosomal RNA sequences for phylogenetic analyses. Proc Natl Acad Sci USA 82:6955 Lo´pez-Bueno A, Tamames J, Vela´zquez D, Moya A, Quesada A, Alcamı´ A (2009) High diversity of the viral community from an Antarctic lake. Science 326:858–861 Pace NR, Stahl DA, Lane DJ, Olsen GJ (1985) Analyzing natural microbial populations by rRNA sequences. ASM News 51:4–12 Parro V, Moreno-Paz M, Gonza´lez-Toril E (2007) Analysis of environmental transcriptomes by DNA microarrays. Environ Microbiol 9:453–464 Sommer MO, Dantas G, Church GM (2009) Functional characterization of the antibiotic resistance reservoir in the human microflora. Science 325:1128–1131 Tyson GW, Chapman J, Hugenholtz P, Allen EE, Ram RJ, Richardson PM, Solovyev VV, Rubin EM, Rokhsar DS, Banfield JF (2004) Insights into community structure and metabolism by reconstruction of microbial genomes from the environment. Nature 428:37–43 Venter JC, Remington K, Heidelberg JF, Halpern AL, Rusch D, Eisen JA, Wu D, Paulsen I, Nelson KE, Nelson W, Fouts DE, Levy S, Knap AH, Lomas MW, Nealson K, White O, Peterson J, Hoffman J, Parsons R, Baden-Tillson H, Pfannkoch C, Rogers Y, Smith HO (2004) Environmental Genome Shotgun Sequencing of the Sargasso Sea. Science 304:66–74 Zhang T, Han WJ, Liu ZP (2009) Gene cloning and characterization of a novel esterase from activated sludge metagenome. Microb Cell Fact 8:67
Metallicity Definition Metallicity specifies the relative amount of elements heavier than helium (collectively called metals by astronomers) in a star, galaxy, or some part of the interstellar medium. The metallicity of the Sun is Z = 1.5% by mass. The most metal rich ▶ stars in the inner Galaxy have two or three times that figure, whereas the most metal poor
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halo stars of the Milky Way have metallicities as low as 0.0001 Z. Usually, the abundance of some key element (like Fe or O) is used as a proxy for metallicity. Host stars of extra-solar giant planets (observed in the solar neighborhood) are more metallic than non-planet hosting stars.
See also ▶ Abundances of Elements ▶ Stars
Metal (in the astrophysical context) ▶ Heavy Element
Metamorphic Rock JEAN-EMMANUEL MARTELAT LST UMR5570, Universite´ Claude Bernard Lyon 1, St Martin d’He`res, Grenoble, France
Synonyms Gneiss
Keywords Metamorphic facies, Mineral assemblages, protolith, rock texture
Definition A metamorphic rock forms when igneous, sedimentary, or older metamorphic rocks are subjected to heat, pressure, and strain that result in changes of their texture and mineral assemblage. The new texture, commonly characterized by aligned mineral grains and the new mineral assemblage reveal the metamorphic origin.
Overview Characteristic metamorphic minerals are mica, garnet, aluminosilicates, chlorite, amphibole, and serpentine. To give the proper name of a metamorphic rock, the petrologist refers to the nature of protolith, i.e., the rock prior to ▶ metamorphism. Most metamorphism is isochemical, except for the gain or loss of water and other volatile components, and the metamorphic rock retains the
chemical composition of its precursor. When shale, aluminous sediment, is metamorphosed, clay minerals are converted to aluminous minerals like micas to produce a rock called schist or metapelite. As metamorphic conditions increase, a part of the micas converts to feldspar and other minerals. When limestone is metamorphosed, it is transformed to marble. Schist, some types of gneiss, and marble are examples of metasedimentary rocks. Metamorphism of basalt produces an amphibolite; metamorphism of granite produces orthogneiss. The structure (shape, size, and spatial arrangement of minerals in the rock) is an essential characteristic of metamorphic rocks. Minerals may be distributed randomly, but commonly they show a preferred orientation. When platy minerals such as micas are well aligned, the rock has a planar structure or schistosity. At high metamorphic grades and with strain, minerals segregate into bands to produce a gneissic texture. Elongated minerals like sillimanite needles or strained minerals could underline a linear feature named mineral lineation. The size and shape of grains can be used to identify the strain intensity and preferential direction of strain (finite strain ellipsoid). In some cases, the mineral association of a metamorphic rock is restricted to a specific P–T domain or metamorphic facies. A small number of metamorphic facies, around 10, have been related to the wide range of P–T conditions that occur in the crust and uppermost mantle. Consequently, a metamorphic rock is named using the type of protolith and the metamorphic facies recorded by a specific set of minerals: for example, “blueschistfacies metapelite.” The metamorphic facies may be related to a specific geodynamic context: ▶ subduction results in high P–low T metamorphism (blueschist facies). Another example, contact metamorphism, results when a shallow magmatic intrusion produces low P–high T metamorphism (hornfels metamorphic facies).
See also ▶ Geothermobarometers ▶ Greenschist Facies ▶ Igneous Rock ▶ Mafic and Felsic ▶ Metamorphism ▶ Metasediments ▶ Plate Tectonics ▶ Sedimentary Rock ▶ Subduction
References and Further Reading IUGS Subcommission on the Systematics of Metamorphic Rocks (SCMR) http://www.bgs.ac.uk/scmr/products.html
Metasediments
Metamorphism JEAN-EMMANUEL MARTELAT LST UMR5570, Universite´ Claude Bernard Lyon 1, St Martin d’He`res, Grenoble, France
Synonyms Protolith
Keywords Geotherm, metamorphic reactions, pressure, temperature
Definition Metamorphism is the process that modifies the texture and the mineralogy of a rock subjected to a change of physical conditions, usually an increase in temperature and pressure. Metamorphism is by definition a dominantly solid-state process, operating at all scales, from regional to micrometric, and within all types of rocks.
Overview The field of metamorphism begins somewhat arbitrarily when diagenesis (lithification of sediment) ends, at a temperature around 150 C. It ends when the rock becomes largely molten. The transition from metamorphism to magmatism is not straightforward; a rock is said to be metamorphic even when it contains a molten fraction if this fraction is localized and in a sufficiently small amount that the rock as a whole behaves as a solid. When material transfer is important (driven by melt, volatiles, or diffusion at high temperature), the system is chemically open and the bulk rock composition may be modified. This process is called ▶ metasomatism. Hydrothermal alteration is considered a part of metamorphism (e.g., hydrothermalism of oceanic crust). In contrast, lowtemperature weathering at the Earth’s surface is considered a sedimentary process. The changes induced by metamorphism are visible in the textures and types of rock-forming minerals. Prolonged heating induces crystal growth with the consequence that grain size increases systematically with increasing metamorphic grade. Deviatory stress produces strain and as strain increases, the mineral grains become more elongated and commonly acquire a preferred orientation. As pressure increases with depth, minerals like quartz (density = 2.66 g/cm3) may be transformed into highpressure polymorphs such as coesite (density = 3.0 g/cm3).
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This metamorphic reaction results in a volume decrease. The time needed for such a change is relatively long and controlled by fluid, strain, or temperature. At room temperature and atmospheric pressure, the reaction coesite to quartz operates, but it is extremely slow. Because the stable mineral under these low P-T conditions is quartz, the coesite is metastable. In such a way, metastable mineral assemblages that are preserved in ▶ metamorphic rocks on Earth surface can record earlier temperatures and pressures. They can help to define old geotherms (paleogeotherms), the record of how temperature varies with depth in the Earth’s crust and mantle. On Earth, the rate and manner of temperature changes depends on the tectonic setting. The geotherm varies with the type of crust (continental or oceanic) and with the geodynamic activity (stable craton, subduction, collision, or extension). Thus, metamorphic rocks are sources of information about specific geodynamic contexts. In some cases, metamorphic rocks record a series of metamorphic conditions through time, defining what is called a P–T–t (pressure– temperature–time) path.
See also ▶ Geothermobarometers ▶ Greenschist Facies ▶ Heat Flow (Planetary) ▶ Metamorphic Rock ▶ Metasediments ▶ Metasomatism ▶ Plate Tectonics
Metasediments Definition A metasediment is a rock of sedimentary origin that has been subjected to ▶ metamorphism. When the protolith (the sedimentary rock before metamorphism) is shale, metamorphism produces first schist and then paragneiss as temperature and pressure increase. As grain size increases, the rock acquires first a foliation (alignment of mineral grains), then a banding, and the mineral assemblage changes to include minerals such as mica, garnet, and aluminosilicates. Metamorphism of pure limestone produces marble and metamorphism of impure silica or clay-rich limestone produces calc-silicate rocks. Sandstone is transformed into quartzite. Most Archean terranes contain metasediments, which have been affected by at least low-grade metamorphism prehnite-pumpellyte facies.
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Metasomatism
See also ▶ Metamorphic Rock ▶ Metamorphism
Metasomatism BRUCE YARDLEY School of Earth and Environment, University of Leeds, Leeds, UK
Synonyms Metasomatosis
Keywords Chemical change, metamorphism
Definition Metasomatism is the term for processes whereby a rock changes chemical composition while it recrystallizes, remaining as a coherent solid throughout. It is closely related to ▶ metamorphism, which is an isochemical process, apart from loss of volatiles. Metasomatism can be subdivided into diffusion metasomatism, observed at the interface between contrasting rock types, and infiltration metasomatism, in which material is introduced in a fluid phase infiltrated from a remote source.
History The term metasomatism has been used in its general modern sense since the nineteenth century, but has been applied to a number of rock types in the past for which it is no longer considered appropriate. For example, at one time, many geologists believed that many or all granites were not igneous rocks but were derived by metasomatic modification of pre-existing sedimentary or ▶ metamorphic rocks.
can also result in addition of carbonate as a replacement or porosity-filling phase. At greater crustal depths, metasomatism of limestones or marbles intruded by granite results in the formation of skarns, while metasomatism has been invoked to account for the composition of some mantle rocks, especially potassic xenoliths (xenolith = a deep-seated rock fragment transported to surface during magma emplacement and eruption), where it may be ascribed to melt migration. D.S. Korzhinskii developed a theoretical understanding of metasomatism in the 1930s. The Korzhinskii Phase Rule relates the number of phases and independent degrees of freedom in a system to the number of immobile chemical components, rather than the total number of components. A rock that has experienced substantive chemical change by migrating fluids has a relatively small number of phases because the budget of some components was dominated by the fluid phase (many veins are of course monomineralic), while solid solution minerals in metasomatic rocks have rather uniform compositions, despite the potential for variation. Classically, metasomatism is considered to be the result of the movement of aqueous fluids and brines, but it is now accepted that metasomatic transformations can result from the migration of a wider range of fluids, including melts. At pressures typical of the Earth’s mantle, aqueous fluids and silicate melts become increasingly miscible as the second critical end point is approached, and so the distinction between the effects of melt and aqueous fluid migration is increasingly blurred. The case for metasomatism of some Martian meteorites is based on the presence of REE-enriched minerals that are out of equilibrium with the major silicate phases, suggesting they grew subsequently as REE were introduced, and failed to equilibrate.
See also ▶ Hydrothermal Environments ▶ Metamorphic Rock ▶ Metamorphism ▶ Rare Earth Elements
Overview The most widespread metasomatism occurs in permeable rocks, often at relatively shallow depths in the Earth’s crust. This includes the modification of sandstones by formation waters, notably albitization of plagioclase feldspar, albitization of fractured crystalline basement rocks as they return to the surface, and the alteration of submarine basalts to spilites (metabasalts that have anomalously high Mg-contents). Hydrothermal alteration in basalts
References and Further Reading Humphris SE, Thompson G (1978) Hydrothermal alteration of oceanic basalts by seawater. Geochim Cosmochim Acta 42:107–125 Korzhinskii DS (1959) Physicochemical basis of the analysis of the paragenesis of minerals. Consultants Bureau, New York Menzies M, Murthy VR (1980) Mantle metasomatism as a precursor to the genesis of alkaline magmas. Am J Sci 280:622–638 Wadhwa M, Crozaz G (1994) 1st evidence for infiltration in a Martian meteorite, ALH-84001. Meteoritics 29:545
Metavirome
Metasomatosis ▶ Metasomatism
Metavirome JOSEFA ANTO´N Departamento de Fisiologı´a, Gene´tica y Microbiologı´a, Universidad de Alicante, Alicante, Spain
Synonyms Viral metagenome
Keywords Virus, metagenome, environmental sample
Definition The metavirome is the ▶ metagenome (sum of genomes) of the ▶ viruses present in a sample, obtained by the extraction and sequencing of the viral genetic material present in the analyzed sample. It can also be defined as the metagenome of a viral assemblage.
History This term was coined by J.H. Paul and M.B. Sullivan in 2005 to refer to the studies on the uncultivated marine ▶ virus community ▶ genome (Paul and Sullivan, 2005).
Overview The first study on an environmental metavirome was published in 2002 (Breitbart et al. 2002), which analyzed the metagenome of uncultured marine virus communities. Since then, studies on the metaviromes from different environments, such as human feces, rice paddy soil, solar salterns, marine water and sediments, coral, freshwater, microbialites, hot springs, Antarctic lakes, fish, mosquitoes, reclaimed water, and grapevine, among others, have been carried out by several authors. Most of the studies have been focused on dsDNA metaviromes, although ssDNA and RNA viruses have also been analyzed using this approach. The analysis of metaviromes allows for the direct study of natural virus communities and circumvents the problems derived of the fact that the majority of viruses cannot be cultured because their hosts cannot be readily isolated. There are several protocols for constructing metaviromes that normally follow this scheme: first, cells
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are removed, and the viral assemblage is concentrated by tangential filtration and/or ultracentrifugation; then viral nucleic acids are extracted by means of different protocols (Thurber et al. 2009; Santos et al. 2010) and ▶ sequenced. In some cases, nucleic acids are cloned into plasmids and/ or fosmids (cloning vectors that allow for the cloning of large DNA fragments) prior to sequencing, or amplified using different protocols (Paul and Sullivan 2005; Thurber et al. 2009) and directly sequenced using high-throughput sequencing methods. Pros and cons of every approach are discussed in Thurber et al. (2009). Once the metaviromic sequences are available, they can be analyzed and compared with databases using several ▶ bioinformatic tools. Metavirome characterization allows the cataloging of the viruses present in a given environment as well as the descriptions of relative abundance of different virus types. Thus, the analysis of metaviromes, in addition of ascertaining the viral diversity in a sample, makes it possible to reconstruct the structure of uncultured viral communities (Edwards and Rohwer 2005), and also to identify viral encoded functions. However, it has the main drawback of the high number of unknown genes found when trying to identify the protein-coding regions or open reading frames (ORFs) present in the metaviromes. For this reason, methods for metavirome analysis (Angly et al. 2005; Willner et al. 2009) have been developed that do not depend on gene annotation and allow for the comparison of different viral metagenomes corresponding to different environments, as well as for functional metagenomic approaches (McDaniel et al. 2009).
See also ▶ Biodiversity ▶ Bioinformatics ▶ Genetics ▶ Genome ▶ Metagenome ▶ Sequence ▶ Virology ▶ Virus
References and Further Reading Angly F, Rodriguez-Brito B, Bangor D, McNairnie P, Breitbart M, Salamon P, Felts B, Nulton J, Mahaffy J, Rohwer F (2005) PHACCS, an online tool for estimating the structure and diversity of uncultured viral communities using metagenomic information. BMC Bioinformatics 6:41 Breitbart M, Salamon P, Andresen B, Mahaffy JM, Segall AM, Mead D, Azam F, Rohwer F (2002) Genomic analysis of uncultured marine viral communities. Proc Natl Acad Sci USA 99:14250–14255 Edwards RA, Rohwer F (2005) Viral metagenomics. Nat Rev Microbiol 3:504–510
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McDaniel L, Breitbart M, Mobberley J, Long A, Haynes M, Rohwer F, Paul JH (2009) Metagenomic analysis of lysogeny in Tampa Bay: implications for prophage gene expression. PLoS One 3(9):e3263 Paul JH, Sullivan MB (2005) Marine phage genomes: what have we learned? Curr Op Biotechnol 16:299–307 Santos F, Yarza P, Parro V, Briones C, Anto´n J (2010) The metavirome of a hypersaline environment. Environ Microbiol 12:2965–2976 Thurber RV, Haynes M, Breitbart M, Wegley L, Rohwer F (2009) Laboratory procedures to generate viral metagenomes. Nat Protoc 4:470–483 Willner D, Thurber RV, Rohwer F (2009) Metagenomic signatures of 86 microbial and viral metagenome. Environ Microbiol 11:1752–1766
Meteor ▶ Meteoroid
Meteorite (Allende) MATTHIEU GOUNELLE Laboratoire de Mine´ralogie et Cosmochimie du Muse´um (LMCM) MNHN USM 0205 – CNRS UMR 7202, Muse´um National d’Histoire Naturelle, Paris, France
Keywords CAIs, chondrite, meteorite, protoplanetary disk
a 110-kg sample was found. In total, 2 t of rocks were recovered (see Fig. 1). Allende belongs to the class of CV3 chondrites, an important group of carbonaceous chondrites, which represent roughly 0.6% of the meteorite falls. The fall of Allende was a landmark in ▶ cosmochemistry, because it led to the rediscovery of calcium-, aluminium-rich inclusions (CAIs). These white inclusions were first reported and described in 1968 by Mireille Christophe Michel-Le´vy in a French journal, which did not receive the attention it deserved (Christophe Michel-Le´vy 1968). The abundance of CAIs in Allende is roughly 10%, while the mean abundance in CV3 chondrites is 4.8%, explaining why CAIs had been overlooked until Allende fell, though ten other meteorites of that kind were known in 1969. CAIs’ mineralogy indicates that they were the first solids to form in the protoplanetary disk. This is confirmed by their old Pb-Pb age and their high abundance of short-lived radionuclides. CAIs can potentially reveal the physicochemical conditions in the protoplanetary disk, 4.568 Gyr ago (see ▶ CAIs). The high abundance as well as the large size (millimeter objects are common, centimeter objects are not rare) of Allende CAIs explain why the study of these key objects has been dominated by samples coming from the Allende chondrite for at least 30 years. In addition, it was widely believed that CAIs within Allende did not suffer any secondary event in its parent-body and that the record of the protoplanetary disk was kept intact within these samples.
Definition Allende is a meteorite that fell in Mexico in 1969. Roughly 2 t of that CV3 carbonaceous chondrite were found. Allende is one of the most studied meteorites, because it contains abundant and large ▶ CAIs, which are believed to be the first solids to have formed in the solar system.
Overview The Allende meteorite fell early in the morning of Saturday, February 8, 1969, 7 months before the fall of Murchison (CM2 chondrite rich in amino acid, see entry ▶ Murchison) and 4 months before the return of 22 kg of Moon rocks by the Apollo 11 crew. After a bright fireball that was seen in Texas and New Mexico, thousands of stones fell near the small village (pueblito) of Allende, south of Chihuahua, in northern Mexico. A large stone (15 kg) fell near a house and some pieces were brought to the office of the newspaper El correo de Parral the same day. News of the meteorite fall was published the same evening. Mexican and US scientists collected rocks the next weeks. Eight months after the fall,
Meteorite (Allende). Figure 1 A piece of the Allende meteorite kept at the Muse´um National d’Histoire Naturelle in Paris (sample 2870, 144 g). The nearest face is still covered by the fusion crust (see entry ▶ cosmochemistry). The white inclusion is a calcium-, aluminum-rich inclusion (see entry ▶ CAIs) (Picture C. Fie´ni [MNHN])
Meteorite (Murchison)
Numerous chemical, mineralogic, and isotopic studies were made over the last 30 years on these objects. In the mid-1990s, Sasha Krot and collaborators demonstrated, however, that Allende endured metamorphism and metasomatism in its parent-asteroid. This means that Allende CAIs are not as pristine as it was thought. Furthermore, it appears that many of the CAIs studied within Allende (type B CAIs) are absent from other chondrites and that most of our knowledge on CAIs is based on unrepresentative samples.
See also ▶ CAIs ▶ Cosmochemistry ▶ Geochronology ▶ Meteorite (Murchison) ▶ Meteorites ▶ Micrometeorites
References and Further Reading Christophe Michel-Le´vy M (1968) Un chondre exceptionnel dans la me´te´orite de Vigarano. Bull Soc fr Mine´ral Cristallogr 91:212–214 Clarke RS, Jarosewich E et al (1970) The Allende, Mexico, meteorite shower. Smithonian Contrib Earth Sci 5:1–53 Krot AN, Petaev MI et al (1998) Progressive alteration in CV3 chondrites: more evidence for asteroidal alteration. MAPS 33:1065–1085
Meteorite (Murchison) FRANC¸OIS ROBERT Laboratoire de Mine´ralogie et Cosmochimie du Muse´um (LMCM), Muse´um National d’Histoire Naturelle, UMR 7202 CNRS, Paris Cedex 05, France
Synonyms Carbonaceous meteorite; Murchison
Keywords Extraterrestrial matter, meteorites, organic matter, prebiotic molecules
Definition On 28 September 1969 at about 10:58 a.m., near the town of Murchison, Victoria in Australia, a bright fireball was observed that broke into three distinct fragments. About 30 s later, a tremor was heard. Many specimens of this Murchison meteorite were found over an area larger than 13 km and the total collected mass exceeds 100 kg.
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Overview This meteorite belongs to the CM group of ▶ carbonaceous chondrites. Like most CM chondrites, Murchison is petrologic type 2, which means that it experienced extensive alteration by water-rich fluids on its parent body, rapidly after its formation 4.56 billion years ago. These chondrites are rich in carbon and are among the most chemically primitive ▶ meteorites. Murchison is exceptionally rich in pre-solar grains. Like most other CM chondrites, Murchison contains centimeter size inclusions of refractory oxides and silicates (Calcium–Aluminumrich inclusions; the ▶ CAIs). These inclusions were condensed from a gas of solar composition at high temperature and are interpreted as the first stages of the cooling of the protosolar disk that eventually led to the planets of the solar system. Interestingly, this class of chondrite also contains molecules formed at low temperatures (i.e., between 30 and 300 K). The emblematic example of these low temperature molecules is the amino acids that are the basic components of life. Many more ▶ amino acids have been identified in Murchison than those actually involved in the living protein world. For example, glycine, alanine, and glutamic acid, as well as unusual ones like isovaline and pseudoleucine are present in Murchison. Complex mixtures of alkanes were also isolated. In the 1970s several studies reported that amino acids were racemic (that is, the ▶ chirality of their enantiomers are equally left- and right-handed), indicating that they are not terrestrial contamination. However, refinements of analytical chromatographic techniques have shown that enantiomeric excesses do exist in some amino acids. This finding has re-opened the Pandora’s box of possible terrestrial contaminants. The debate was finally closed owing to an outstanding hightech analytical technique where the nitrogen isotopic compositions of each enantiomer were measured individually. These isotopic compositions are out of the terrestrial range and the two enantiomers are equally enriched in nitrogen 15 relative to their terrestrial counterparts. Since the terrestrial contamination would have left its isotopic signature on the L-enantiomer, indubitably an extraterrestrial source favored L-enantiomers in the early Solar System. It is possible that the origin of the homochirality of life (i.e., the occurrence of only the left-handed amino acids and right-handed sugars) was triggered on Earth by the deposition of chiral molecules contained in meteorites. Following this view it has been shown that an amino acid like L-proline can catalyze the formation of chiral sugars.
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Meteorite (Murchison)
Several lines of evidence indicate that the interior portions of Murchison are well-preserved and are indigenous to the meteorite, i.e., not modified or contaminated on Earth. A 2010 study using the new high-tech analytical tools of organic chemistry has identified more than 14,000 different compounds, including 70 amino acids in a single sample (1 g) of the Murchison meteorite. These results demonstrate that various organic molecules that are now the components of life on Earth were synthesized during the formation of the solar system. They represent a potential molecular source for life. Judging from the numerous interpretations on the origin of these organic compounds that can be found in the literature, a conservative attitude is to consider that the organic composition of Murchison reflects a mixture from different origins. While some precursor compounds of the insoluble organic matter may have been formed in the circumsolar disk by photochemical reactions (such as the polycyclic aromatic hydrocarbons), others – the small and soluble molecules – were processed inside the planetesimals. In this framework, it is usually assumed that amino acids were formed on the parent body of carbonaceous chondrites during aqueous alteration, most probably by the Strecker-cyanohydrin synthetic mechanism. This interpretation is supported by the fact that the presence of glycine in interstellar space has not yet been confirmed, in spite of several attempts aimed at detecting its millimeter-wave or infrared signature. Laboratory data on the deuterium retention of amino acids during ▶ Strecker synthesis indicates that, with the exception of glycine, meteoritic amino acids can be formed by this mechanism from highly deuterated precursors. The origin of these deuterium-rich precursors (solar or interstellar) is still in debate. In addition, the isotopic fractionation of carbon and nitrogen occurring during Strecker synthesis does not account for the large enrichment in 13C and 15N observed among several families of organic molecules; the possible – and proposed – interpretations of such a huge fractionation have not been experimentally substantiated. The correlative consequence for the origin of life on Earth is based primarily on the observation that the planetary surfaces were heavily bombarded during the first 500 million years of their existence. Therefore the question now opened is: could meteorite bombardment on Earth have delivered the molecular building blocks of life? Although it is difficult to estimate the flux of extraterrestrial carbon to the early Earth from the present day observations, it has been calculated that between 109 kg (at 4.3 Gyr) and 5 109 kg (at 3.5 Gyr) of organic carbon
could have been delivered to the planet per year in the first billion years of its existence, mostly in the form of interplanetary dust particles (IDPs). This flux is 2–3 orders of magnitude higher than today’s. The problem associated with the delivery of organic compounds by micrometeorites is that, depending on their mass and size, they can suffer full-depth heating to temperatures ranging from 200 to 1,200 C during atmospheric entry deceleration for IDPs having a diameter of 100 mm during atmospheric entry deceleration. This means that the organic compounds present that are thermally unstable at these temperatures, such as amino acids, would be decomposed during this flash-heating event. It has also been suggested that sublimation could be a possible mechanism by which volatile organic compounds could survive atmospheric entry heating, by vaporizing off the surface of IDPs or even larger meteorites, before they are melted and destroyed. Although it is not fully understood why most of the amino acids, purines, and pyrimidines do not sublime from Murchison, experimental evidence suggests that the presence of kerogen-type organic polymers in Murchison enhances the survival in small to medium sized (few tens of meters diameter) meteorites on entry and impact. But the organic carbon fluxes from these objects are estimated to be about 5 orders of magnitude lower than for the IDPs. Theoretical studies about impact processes on Earth have suggested that most organic compounds contained in a big impactor such as an asteroid or a comet would be destroyed by the high temperatures produced during impact. This discussion about the input of extraterrestrial organic molecules on Earth has several ramifications. Even if it can be demonstrated that amino acids can “safely” land on the primitive Earth, would that prove the origin of the world of left-handed amino acids in which we live? What is the amount – or the concentration in a sterile ocean – of pre-biotic molecules necessary for a life ancestor to be assembled? Are high concentrations and a large molecular diversity of organic molecules required for life to emerge? The answers to these questions are largely unknown; they are related to a credible model of the chemical pathways that gave rise to life. This model does not exist.
See also ▶ Amino Acid ▶ Asteroid ▶ CAIs ▶ Carbonaceous Chondrite ▶ Chirality ▶ Meteorite (Allende)
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▶ Meteorite (Orgueil) ▶ Meteorites ▶ Micrometeorites ▶ Strecker Synthesis
Keywords
References and Further Reading
Meteorites are rocks of extraterrestrial origin which travel in interplanetary space and fall on Earth as they become trapped in the Earth’s gravity field. If their mass is greater than about 1 kg, they survive abrasion as they pass through the atmosphere, and the central part arrives on Earth as a solid body. Further reading can be found in several textbooks on meteorites, their compositions and mineralogical classification; among them: Hutchison (2007), McSween (1999), Zanda et al. (2001), and Gounelle (2010).
Botta O, Bada JL (2002) Extraterrestrial organic compounds in meteorites. Surv Geophys 5:411–467 Robert F, Epstein S (1982) The concentration and isotopic compositions of hydrogen, carbon and nitrogen in carbonaceous chondrites. Geochim Cosmochim Acta 16:81–95 Sephton MA (2002) Organic compounds in carbonaceous meteorites. Nat Prod Rep 19:292–311
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Definition
Meteorite (Orgueil) Overview Definition The Orgueil meteorite fell on May 14, 1864, near the village of the same name, in the south of France. With a mass of 14 kg, it is the biggest sample of the C I carbonaceous chondrite group, which characterizes the most primitive bodies of the solar system. Its composition is identical to that of the Solar photosphere, except for the lightest elements hydrogen and helium. An important discovery was the detection of a high abundance of isotopically anomalous xenon which could be the signature of presolar material. The Orgueil meteorite also contains many amino acids. Recent studies suggest that Orgueil has a cometary origin.
See also ▶ Amino Acid ▶ Chondrite ▶ Meteorite (Allende) ▶ Meteorite (Murchison) ▶ Meteorites
Meteorites FRANC¸OIS ROBERT Laboratoire de Mine´ralogie et Cosmochimie du Muse´um (LMCM), Muse´um National d’Histoire Naturelle, UMR 7202 CNRS, Paris Cedex 05, France
Synonyms Achondrites; Carbonaceous meteorite; Comets; IDPs; Planetesimals
Preliminary Definitions Meteorites are classified as finds or falls, when they are found by chance on the ground, and falls when they are collected subsequent to the observation of their fall. Finds and falls are named after the closest city to where they were discovered. When they are found in deserts or in Antarctica, they are designated by acronyms such as NWA (North West Africa) or ALH (Alan Hills mountains in Antarctica) followed by numbers standing for their discovery order. When their size is smaller than 1 mm, they are usually called ▶ micrometeorites. In the solar system, small bodies (sometimes referred to as planetesimals) are orbiting around the ▶ Sun along with the major planets of the ▶ solar system. These include ▶ Asteroids and ▶ comets, defined by families linked to their orbits; asteroids are mostly concentrated in belts between Mars and Jupiter, while comets came from beyond the giant gaseous planets ▶ Jupiter and ▶ Saturn. For statistical reasons, meteorites recovered on Earth mostly originated from the asteroids and are produced during dispersive collisions in these belts. In some rare cases, they also come from the surfaces of small planets such as the Moon or Mars, ejected into space by asteroidal impacts. A question often raised that has never received an unambiguous answer is: do we have meteorites of cometary origin in our collections? Whatever its planetary origin (planets, comets, asteroids), the original host of a meteorite is designated by the concept of ▶ parent body. The classification of the meteorites is based on a fundamental observation: some meteorites are more primitive – i.e., some were not modified by thermal events that have followed their formation, except by the
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circulation of fluids – while others were metamorphosed by heating yielding new phase relations between newly formed minerals (Fig. 1). Such meteorites are undifferentiated, whereas others were differentiated by melting. By definition, a differentiated rock has been melted; new minerals appeared in the melt and were separated during fractional crystallization. A metamorphic rock has not been melted, but the elevation in temperature mobilizes the volatile elements which migrate through the parent rock. For both differentiated and metamorphic meteorites, the chemical memory of their condition of formation has been partially or totally erased. Impacts have played a central role in the formation of the solar system. Planets likely grew by constructive collisions of a myriad of small planets formed around the Sun – the so-called planetesimals. Meteorites are the left-over debris of these gigantic collisions and many bear mineralogical and geochemical signatures of such a collisional period. Accordingly, a supplementary specific classification of meteorites has been proposed, based on the intensity of the collision suffered by their parent bodies and registered in their minerals. Besides these modern classification schemes, meteorites were traditionally divided into stony, stony-iron, and iron. In other words, some are rocky, others are metallic, and others contain a mixture of silicate and metallic minerals.
Most meteorites are rocks; more specifically the undifferentiated ones are ▶ chondrites. They contain chondrules that are sub-millimeter spherical associations of minerals. These minerals – olivine and pyroxene – are the common ingredients of rocks on Earth. In chondrules they are associated with silicate glass, metallic phases, and sulfides. The origin of chondrules has remained mysterious for a century. With the recent laboratory simulations by experimental petrologists we have learned that chondrules are silicate droplets formed in a hot gas and cooled rapidly – in a few hours. While they were hot, various chemical exchanges with the surrounding hot gas took place. After their formation, when cold, ▶ chondrules were then mechanically accreted together with a matrix – a fine grained association of silicates and a large variety of volatile rich components – to form the parent body of the chondrites. Some rare meteorites underwent minimal thermal transformation and alteration by fluids after accretion; they are designated in the classification as type 3.0, while the most thermally transformed are designated as type 6 or 7. Among meteorites the ▶ carbonaceous chondrites are the most primitive; they are designated by the first letter C (C for carbonaceous) while their types mostly lie between 1 and 3. Chemical groups are identified among ordinary and carbonaceous chondrites. For example, carbonaceous chondrites are sub-divided according
Iron
Primitive
Vesta (HEC: Howardites, Eucrites, Diogénites) Martian (SNC: Shergottites, Chassignites, Nakhlites) Angrites Ureilites Aubrites Lunar
Differentiated
Winnonaites Lodranites Acapulcoites Brachinites
Stony-Iron
Achondrites
Stony
Chondrites
Ordinary
H, L, LL
Enstatites
EH, EL
Rumurutites
Carbonaceous
CI, CM, CO, CV, CK, CH, CB
Meteorites. Figure 1 The classification of most studied meteorites. The different types of Iron and Stony-Iron are not indicated for simplicity
Meteorites
to their chemical composition, the size of their chondrules, the abundance of their refractory inclusions (i.e., inclusions made of oxides and silicates condensed from a gas of solar composition at temperatures up to 2,000 K). They are designated by CM, CO, CV, CR, CH, CB, CK, which stand for chemical and mineralogical families. These sub-classes are thought to represent different conditions of formation – differences in temperature, in oxidizing conditions, in chondrule formation rates, etc. – and therefore different parent bodies. However, since little is known about these conditions, the relevance of such a classification to illustrate the genetic relationships among these rocks is far from being established. But after all, classifications are also made for specialists, to ease their discussions by replacing lengthy descriptions of natural objects by acronyms. Carbonaceous chondrites remain, except for comets, the most primitive objects in the solar system and, in that respect, the heterogeneity in their chemical and mineralogical compositions must reflect in some way the heterogeneity in the composition or in the processes at work during the formation of the first solids of the early solar system. Differentiated meteorites are igneous rocks, likely differentiated in a chondritic parent body. When composed of silicates, they are called achondrites. Among achondrites, Howardites, Eucrites, and Diogenites – the so-called HED achondrites – were probably excavated by a recent giant impact on the surface of the asteroid Vesta, as revealed by telescope studies. Indeed, the mineralogical composition of the surface that has been recorded by infrared spectroscopy is similar to that of the HED family; hence the attribution of Vesta as the source of HED. Similarly Shergottites, Naklites, and Chassignites (the so-called SNC meteorites) likely originated from the surface of Mars, as attested by the chemical and isotopic compositions of the most volatile elements trapped in impact melt pockets. These melts are formed in minerals during the transient shock pressure caused by the impact. Some achondrites are nevertheless primitive in the sense that their chemical composition still reflects a chondritic source from which some magma has separated. For these rare rock residues, melt formation did not cause the major chemical modification generally observed in differentiated rocks. Iron meteorites are made of iron with a few percent of nickel. Whether they are the product of local melting at the surface of early ▶ planetesimals or a global melting of these planetesimals resulting in the gravitational segregation of a metallic core (as on Earth) varies from group to group. Some iron meteorites show clear evidence of fractional crystallization supporting the metallic core interpretation, but others do not.
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Between iron and stony meteorites are the stony-irons. Stony-irons are mixtures of metal and silicates. The occurrence of such objects clearly demonstrates that massive differentiation of planetesimals occurred immediately after their formation, yielding a silicate crust and mantle, and a metallic core. In some cases the solidification of the metallic melt took place faster than the gravitational decantation of the silicates, which were hence trapped in their metallic hosts. In other cases, impact processes remixed previously separated silicate and metal.
Meteorites on Earth Meteorites enter the Earth’s atmosphere with a velocity lying between 11 and 70 km/s. High velocities are rare because they correspond to bodies traveling on a retrograde orbit. The temperature increase of the meteorite surface caused by the friction with the atmosphere yields a fusion crust. This vitrified crust never exceeds a few millimeters, since the material heated up to 2,000 K is ablated during the fall in the atmosphere. Low mass meteorites (90% for chondrites and up to 99% for the most fragile carbonaceous chondrites. As a consequence, the statistical distribution of meteorite classes does not reflect the cosmic distribution of these objects. Large impacts (>10 km) yield tektites; tektites are melted centimeter-size fragments of the terrestrial target rocks that are ejected into the upper atmosphere of the Earth, where they can sometimes travel a few thousands of kilometers before falling back to the surface. Interestingly, tektite fields are well identified at the surface of the Earth while, in several cases, their corresponding impact crater is not recognizable. On Earth more than 175 relict craters having a diameter between 100 m and 200 km have been identified; for most of them fragments of the projectile, whose diameter is more than 10 times smaller than that of the crater, has not been found. Shooting stars (meteors) are caused by the entry of micrometeorites into the atmosphere. Their size is usually smaller than 1 mm and many land on the ground in the form of melted spherules, so-called cosmic spherules. However, a vast majority of micrometeorites are not melted and arrive in the upper atmosphere with velocities relative to the Earth close to zero. For extremely small micrometeorites (1 mm) the atmospheric journey to the ground lasts a few weeks. There are two main methods for collecting micrometeorites (1) by melting the ice accumulated in selected ultra-clean
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regions of the south polar cap, or (2) by trapping them in the stratosphere on a film of silicon oil carried by planes. In polar regions, extraterrestrial grains less than half a millimeter in size can represent up to 50% of all the rocky grains mixed with the ice – the rest being minerals from the closest geological formations or industrial debris. Meteorites are now collected in cold and hot deserts or in Antarctica. These open spaces offer two characteristics aiding the search of new samples (1) their surface can be as old as 50,000 years (2) dark extraterrestrial rocks are easily recognizable on the clear surfaces made of sand or snow. About ten falls are observed every year on Earth. This flux can be extrapolated to all sizes over the whole Earth; this gives 4,500 meteorites greater than 1 kg/year. An estimate of the terrestrial flux of extraterrestrial objects greater than 10 cm is 3 107 kg/year and 6 106 kg/year for micrometeorites. The arrival of a 1 km size object giving rise to a 10 km size crater occurs statistically every 200,000 years, while the probability of having an encounter with a 10 km size object is only one in 107 years. Most meteorites that fell on Earth during geological times have disappeared, destroyed by the rapid weathering that takes place at the Earth’s surface. It is possible to determine precisely the residence time of a meteorite on Earth – the terrestrial age – by measuring the radioisotopes that are formed during the journey of the meteorite in space. These radioisotopes (14C, 36Cl) are formed by the nuclear interaction between the energetic particles – mainly protons – emitted from the Sun or from other Stars and the chemical elements constituting the parent body of the meteorites. This type of nuclear reaction (▶ spallation reaction) takes place at depth (1 cm to few meters), inside the parent body. Terrestrial ages lie between a few years and 50,000 years. Meteorites are also found within geological formations. For example, in Finland, numerous chondrites originating from the same parent body – i.e., belonging to the same class of meteorites – have been found in sediments dated at 420 million years. Interestingly, this age coincides with the measured disruption age of the parent body of all the meteorites of this class. During this geological period, the flux of this class of meteorite has been estimated to be 3 times the present day flux. Such an increase in the flux is attributed to the arrival on Earth of the debris of a massive collision that occurred between asteroids around 450 million years ago. The products of this collision have been sought and identified in the present day asteroid belt by analyzing the movements of a family of objects that are still running away from a center of mass. Meteorites have also been found on Mars and on the Moon; but no meteorite originating on the Earth that has traveled through interplanetary space and then re-encountered our planet has been found on Earth,
although such an occurrence remains statistically possible (tektites did not go into a heliocentric orbit).
Where Do Meteorites Come From? Among the 35,920 meteorites officially classified and stored by national museums in Washington, London, Paris, New York, Vienna, and Berlin (listed here according to their number of samples registered as falls), only 124 come from the Moon and 80 from Mars. Where do the rest come from? Based on observed trajectories of falls, it is now certain that the vast majority of the meteorites come from the asteroid belt. However, except for HED meteorites (cf. above), no specific parent body in space can be attributed to the different classes of meteorites. It is nevertheless almost certain that a single parent body hosts a given family of meteorites. In some cases this parent body was affected by a thermal event causing observed differences in the metamorphic grade. Particularly in the case of the ordinary chondrites, this situation is usually referred to as “the Onion Shell Model,” where the thermal gradient corresponds to depth, with the least metamorphosed samples sitting at the surface of the parent body. It is important to point out that, according to the numerical simulations of the dynamical evolution of the small bodies in the solar system, the asteroid belt was refilled through time through discrete events that had destabilized the orbits of all the small bodies of the solar system. This view is difficult to reconcile with the apparent chemical gradient that could exist in the asteroid belt – as revealed by spectroscopic studies. It is also likely that comets, coming from the outer regions of the solar system, can be trapped in the asteroid belts via a gravitational deflection by Jupiter during their return journey from the Sun. This possible scenario implies that debris of extinct comets is presently in the meteorite collections, but merely not recognized. Eventually comets can also be intercepted by the Earth during their journey around the Sun. This scenario, based on observations from the ground reported by numerous witnesses of its fall in 1884, was proposed for ▶ Orgueil, one of the most emblematic meteorites. In the asteroid belts, after a collisional disruption between parent bodies, most debris cannot reach a gravitationally stable situation. As a consequence most of this debris finishes its life in the Sun, but some would eventually encounter the Earth on its way to the Sun. This residence time in space of the debris resulting from collisional disruptions in the asteroid belt is well documented through their exposure ages. Exposure ages are measured by the same principle that is used to measured terrestrial age (cf. here above): isotopes are produced by nuclear reactions caused by highly energetic atoms from
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the Sun or from the Galaxy. These reactions are registered a few centimeters below the new surfaces resulting from the disruption and exposed to these energetic cosmic rays. Unlike terrestrial ages that are determined by short-lived radioactive isotopes, exposure ages are determined by measuring the absolute amount of a rare but stable (or long lived) isotope produced by spallation reactions (such as 21 Ne, for example). The experimental determination in accelerators of the production rates of these isotopes allows the calculation of an exposure age. This exposure age represents the average duration for a transit between the asteroid belt and interception by the Earth. The debris can also fall on the Sun (or on another inner solar system planet) or can be involved in another disruptive collision, where new and fresh surfaces will be exposed to these energetic solar and cosmic rays. Interestingly, irons that are harder to break than rocky meteorites exhibit much older exposure ages ( 30 million years compared to 300 million years). The distribution of the exposure age in chondrites is reported in Fig. 2.
Chondrites As Witnesses of the Formation of the Solar System The comparison of new astronomical data on starforming environments and geochemical data obtained on meteorite minerals at high spatial resolution yields the present scenario for the formation of the solar system. For heuristic purposes, such a scenario is presented here as a dogmatic scheme, but it should be kept in mind that it relies on interpretations of a huge corpus of data. Many interpretations are still subjects of debates. We have thus
0.5
1 2 5 10 20 50 100 Cosmic Ray Exposure Age (Million years)
Meteorites. Figure 2 Histogram of the Cosmic Ray exposure age of type H chondrites. The peak at 5 million years is interpreted as the age of the collision in the asteroid belt that gave rise to the H family (After Crabb and Shultz 1981)
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chosen to select in the present text only the data for which a scientific consensus exists for their interpretation. The formation of the solar system can be divided into four periods: (1) the interstellar, (2) the disk, (3) the collisional, and (4) the planetary periods. During the first period that lasts 108–109 years, new chemical elements synthesized inside stars are injected into the interstellar medium via supernova explosions or intense stellar winds. In this medium, giant molecular clouds are formed, made of gas and sub-micrometer grains. A cold chemistry is triggered by ultraviolet light and energetic particles. This type of chemistry is unusual in our terrestrial laboratories. Indeed most chemical reactions stop at temperature below 300 K since they are too slow to proceed forward. But in the presence of ultraviolet light, electrons are sputtered away from molecules, creating ions, i.e., molecules carrying an electric charge. In these conditions, the electrically charged molecules can react rapidly with their environment because they act like small magnets attracted by solid surfaces. This type of unusual chemical reaction is at the origin of many of the interstellar molecules detected in molecular clouds. Among these molecules, the widest variety are the so-called organic molecules – i.e., molecules that involved carbon atoms bonded to hydrogen, nitrogen, sulfur, or oxygen (See ▶ Molecules in Space). Although most of them are not chemically stable and do not enter into the composition of the terrestrial biosphere, they are called “organic” with reference to the carbon chemistry that was discovered in the nineteenth century. It remains unclear if such a heritage of interstellar organic chemistry was preserved in the grains forming the early solar system, or if these interstellar molecules were entirely destroyed by the gravitational collapse of the interstellar cloud, though circumstellar condensate grains have been identified in meteorites. Molecular clouds collapse into the form of protostellar disks around a protostar. This is the second period that lasts a few million years. The origin of such a collapse is complicated but is fundamentally due to the fact that interstellar clouds are gravitationally unstable. During collapse angular momentum is conserved and the cloud eventually forms a disk. This is why many astronomical objects have a tendency to form disks: stars in Galaxies, planets around the Sun, rings around planets. In the center of the disk the matter accumulates – accretes is a more proper word – pressure and temperature keep increasing and, when the accreted mass reaches about 0.7 of the solar mass, the first thermonuclear reactions start and the protostar has become a star. In this situation, a strong coupling between the magnetic field of the rotating star and the electrically charged disk occurs.
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The newly forming Sun sheds 50% of its mass in the form of bi-polar flows and, as a consequence of the conservation of angular momentum, the rotation of the central star slows down. Accompanying these enormous jets, the Sun sprays its surrounding disk with energetic particles and high energy photons. This activity yields specific nuclear reactions in the disk and an unusual hot atom chemistry, likely similar to the present day photochemistry in the upper atmosphere of the Earth. The products of this type of chemistry in the circumsolar disk are largely unknown. The nuclear reactions are the spallation reactions that are discussed in this text above. However identifying the spallation products among the matter constituting the meteorites is a difficult task, and they are also largely unknown. It remains possible that some of the shortlived radioactive elements discovered in the chondrites result from this interaction between the newly forming Sun and its surrounding disk. Although it is now proven for some specific chemical elements that this mechanism did indeed take place at the beginning of the formation of the solar system, there is no scientific consensus on the exact amount of matter formed in this manner. Injection of newly formed chemical elements into the disk via supernova explosions of nearby stars is also a realistic interpretation of the isotopic data recorded in chondrites. This field is the subject of intense research in the international community. The third period – the collisional period – lasts 108 years. Solid bodies whose size is greater than 1 m are not sensitive to gas drag. Their mechanism of formation and accumulation is not well known, but in the last few years has become a subject of laboratory and theoretical research. The gas surrounding the Sun has now disappeared. Planetesimals formed by accretion in the disk circulate around the Sun in elliptical (quasi-circular) orbits and in a quasitransparent medium illuminated by the young Sun. Up to 1024 of such planetesimals formed but such a system is not gravitationally stable. Mutual gravitational interactions rapidly (in less than few million years) yield the formation of so-called embryos, i.e., 1,000-km size objects. After the formation of these embryos, the collisional cleaning process is much slower and continues for 50–100 million years. Through constructive collisions it yields the present day planetary system. Although this billiard ball phenomenon is in essence stochastic, numerical simulations are able to reproduce the sizes, the locations, and the orbits of the present day planets. In such a scenario, the most volatile elements constituting the present day atmosphere (including water and organic molecules) are carried by the embryos drifting from the cold regions of the solar system and
eventually captured by the inner solar system planets. This numerical result about the origin of the planetary atmospheres solves the old enigma of the occurrence of an oxidizing atmosphere on planets that are essentially made by materials condensed at high temperatures in highly reducing conditions. Carbonaceous chondrites are the remains of such cold planetesimals and their chemical and isotopic compositions allow us to test the scenario in great detail. The planetary period corresponds to the end of the planetary accretion and lasts 4.4 109 years. The major phenomenon is planetary differentiation that yields the formation of a magma ocean and the gravitational segregation of the metal in the core. Although in the case of the Earth, this planetary differentiation took place during the growth of the planet, a similar differentiation has taken place at the earliest stages in meteorite parent bodies. This is attested by iron and some basaltic meteorites that show among the oldest ages of the solar system. For unknown reasons, although water has likely always been present in the Earth’s atmosphere, no sedimentary rocks, with few exceptions like Isua turbidites and Banded Iron Formation, have survived the first billion years of the Earth life. The oldest true sediments are dated at 3.5 billion years and thus the chemical composition of the Earth’s earliest hydrosphere is largely unknown. In some of these rare oldest samples, fossils of microbiota resembling the present day associations of bacteria have been discovered in siliceous sedimentary rocks. To summarize we repeat the point we wish to emphasize: meteorites contain information on the four main periods of planetary formation that are inaccessible in any other known natural objects.
Chemical and Isotopic Compositions of Chondrites An enormous scientific literature exists on this subject. Here, we briefly survey some of the most remarkable observations along with their possible interpretation. One class of the most primitive carbonaceous chondrites – CI – exhibits a chemical composition close to that of the Sun. Although these samples have been heavily weathered by an intense hydrothermal alteration, the correspondence with the Sun is astonishingly accurate (cf. Fig. 3). This indicates that, unlike hydrothermal systems on Earth, massive transport of elements did not take place on the parent body of CI chondrites. Thus the alteration mechanism of CI has long remained an unclear issue. Taking this chemical composition at face value for the composition of the disk, all other classes of chondrites (and the Earth) show a systematic pattern (Fig. 4) with
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H He
108 O
C
N Sun
106 Ca 104
Na
K
Si Al
Mg Fe S
102 100
Li
100
102 104 Chondrites
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Meteorites. Figure 3 The chemical composition of the carbonaceous chondrite Orgueil (CI) versus the composition of the Sun
Lithophile 1
Chalcophile
Abundances Normalized to CI
Cr Ni
Siderophile
Li P
Pd
0.8
Au As
0.6 Mn
Rb 0.4
Cu Na Ga K Ag Sb
F Ge Se Sn
0.2
1400
1000
Te
Bi Zn S
Tl In
500
Condensation Temperature (K) at 10−4 bar
Meteorites. Figure 4 The relative abundance of the chemical elements (normalized to CI, i.e., to solar) in the carbonaceous chondrites CV3 as a function of their temperature of condensation. This pattern is some of the best evidence that the temperature of accretion of the planetesimals governed the chemical fractionation relative to the solar gas (After Palme and Boynton 1993)
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the most volatile elements being the least abundant. Such a pattern can result either (1) from a fractional condensation dictated by the condensation temperatures of the chemical elements or (2) from the loss by evaporation at high temperature from the precursor material of chondrites and planets. The solution (2) has to be abandoned since the clear isotopic signatures that must have been left during evaporation have never been found in a natural object and at any scale – with the exception of some rare refractory oxides that confirm the rule. Therefore Fig. 4 should be regarded as evidence that planetary matter condensed from a wide range of temperatures and from a gas having the solar chemical composition. The H-C-N-O-S isotopic compositions of the most volatile compounds (water, rare gases, organics, etc.) have been studied in great detail. They essentially revealed the occurrence of chemical reactions taking place at low temperature (down to 30 K), likely mediated by an intense solar irradiation that has illuminated the disk. The role of photochemistry is less clear as far as the high temperature of the mineral condensation is concerned. The origin of the so-called non-mass-dependent isotopic effects observed for most chemical elements is still a pending issue. This lack of understanding remains a crucial issue for any theory of formation of the solar system since oxygen, the most abundant chemical element forming the silicates, exhibits such an isotope effect at all scales: minerals, chondrules, chondrites, planets and comets. Although the interactions of the molten chondrules with the gas and the origin of their mineralogy are now well understood, the chemical pathway that gave rise to chondrules from a gas remains an open issue. Part of the solution of this problem is probably contained in the correct interpretation of the mass-independent isotope effects. This type of reaction probably characterizes photochemical reactions where the reactants are formed by a molecular UV photodissociation. The difficulty of reproducing these high fluxes of photodissociating light in the laboratory may be at the origin of our ignorance on isotope selection mechanisms. To summarize these considerations on isotopes, photochemistry seems to have played a major role in the formation of complex molecules, later assembled in the form of clusters, then sub-millimeter grains and then mineralogical assemblages. Radioactive isotopes give access to the age of meteorites. The significance of this age is precise (cf. }. Solar System Formation): it stands for the time elapsed since the last isotopic homogenization of the rock. This homogenization can take place during complete melting, or in a gas phase. Since most meteorites do not arise from
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melted bodies, the ages of their inclusions stand for the last phase change between gas and solid. Once in solids, the radioactive decay of the parent nuclei gives birth in situ to the daughter nuclei that rest in the same mineralogical site as their parent nuclei. These two conditions – isotopic homogenization and in-situ decay of the radioactive nuclei – can be verified from the data that give the age of a rock. In the case of meteorites, or their inclusions, this age (4.567 109 years) is regarded as the age of the solar system for three reasons: (1) all primitive chondrites give the same age within 2 million years, i.e., 0.002 years in units of 109 years; (2) all other known solar system objects are younger; and (3) this age corresponds approximately (within 0.2 109 years) to the age of the Sun calculated from nuclear astrophysics. In addition, the close similarity in the oldest ages of solar system objects demonstrates that, in order to satisfy the first condition for radiometric dating systems (homogenization), the disk was chemically well mixed over large distances from the Sun. Carbonaceus chondrites contain Calcium-AluminumRich Inclusions (▶ CAI). The chemical and mineralogical composition of the CAI is compatible with condensation from a gas phase. Numerous thermodynamical calculations and some rare experimental simulations have proven the validity of this view. Some CAI have also experienced intense evaporation episodes followed by cooling (days or years?) indicating that the medium from which they formed experienced one or more
transient thermal outbursts. These CAI exhibit three remarkable properties: (1) their chemical composition in refractory elements is solar (they are condensates) (Fig. 5), (2) they exhibit the oldest solar system ages, and (3) they contain several non-mass-dependent isotopic systems whose origin is still elusive (as discussed above). Carbonaceous chondrites contain also presolar grains. The isotopic compositions of these grains are difficult to reconcile with the solar composition. The least that can be said is that there is no known chemical or nuclear processes in the solar system that can account for the enormous variations in isotope compositions measured in these grains. Therefore these grains are attributed to other stars – hence the name presolar. The products of the thermonuclear reactions taking place inside the star are mixed by turbulence in the envelopes of the star. Grains condense while the upper envelopes expend and cool in the interstellar medium. The isotopic compositions of these grains should reflect the isotopic composition of these envelopes. Since nuclear reactions in other stars can yield isotopic compositions markedly different from those of the Sun, it is generally supposed that the isotopic compositions of the presolar grains reflect those of their mother star’s atmosphere. Although this interpretation raises several contradictions and difficulties, the nuclear interpretation to account for the isotopic compositions of these grains seems inevitable. It can in no way be supposed that these grains have
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Enrichment rel. to Solar
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15
10
5 Solar Ca
Solar Ba
Sr
La Sc
Sm Ce
Tb Eu
Yb Dy
Solar Zr
Lu
Ta Hf
U W
Ru Re
Ir Os
Meteorites. Figure 5 The relative abundance of the most refractory chemical elements in Calcium-Aluminum-rich inclusions (CAIs). Although systematically enriched in the most refractory chemical elements, CAIs exhibit no internal chemical fractionation relative to Solar composition. This pattern is the geochemical signature of a condensation at high temperature in the solar gas (After Grossman et al. 1977)
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been exposed to chemical exchange processes with the gas of a disk whose composition is solar. Therefore the circumsolar disk should be viewed as a dusty medium containing condensates and grains predating the formation of the disk and inherited from the interstellar medium. This medium is well mixed – gas and grains – at the scale of our planetary system but each presolar grain retains a chemical memory of its origin. Most of the recent observations on the chemical composition of comets and the giant gaseous planets are in agreement with this scenario.
See also
References and Further Reading
References and Further Readings
▶ Cosmochemistry
Meteorites, History of History Meteorites have been objects of reverence in many premodern cultures, for example, in North America and
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in Europe, when the local population believed that they had fallen from the sky (e.g., Nininger 1972). Meteoritic iron was the only source of that metal before the invention of iron smelting and the beginning of the Iron Age; early iron weapons were thus exceptionally valuable. It is at the end of the eighteenth century that meteorites obtained the status of scientific objects. In 1794, the German scientist Ernst Florens Friedrich Chladni explained the result of the observational comparisons he had made among lots of “fallen stones” and made a link between these stones and the fireballs observed for a long time in the atmosphere. This first systematic work on objects that were at that time despised induced him to ascribe an extraterrestrial origin to what were soon to be called “meteorites.” His results were confirmed at the beginning of the nineteenth century by chemical analysis and the discovery of the asteroid belt between Mars and Jupiter.
▶ ALH 84001 ▶ CAIs ▶ Chondrite ▶ Meteorite (Allende) ▶ Meteorite (Murchison) ▶ Meteorite (Orgueil) ▶ Microfossils ▶ Micrometeorites ▶ Molecules in Space ▶ Protoplanetary Disk ▶ Solar System Formation (Chronology) ▶ Sun (and Young Sun)
Crabb J, Shultz L (1981) Cosmic-ray exposure ages of ordinary chondrites and their significance for parent body stratigraphy. Geochim Cosmochim Acta 45:2151 Gounelle M (2010) Les me´te´orites. PUF, Paris Grossman L, Ganapathy R, Davis A (1977) Trace elements in the Allende inclusions-III. Coarse-grained inclusions revisited. Geochim Cosmochim Acta 41:1647 Hutchison R (2007) Meteorites: a petrologic, chemical and isotopic synthesis. Cambridge University Press, Cambridge McSween HY (1999) Meteorites and their parent planets. Cambridge University Press, Cambridge Palme H, Boynton B (1993) Protostars and planets III. University of Arizonona Press, Tucson, p 987 Zanda B, Rotaru M, Hewins RH (2001) Meteorites: their impact on science and history. Cambridge University Press, Cambridge
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Nininger HH (1972) Find a falling star (autobiography). Paul S Erikson, New York
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Meteoroid Synonyms Meteor
Definition A meteoroid is a solid object orbiting the Sun (i.e., in interplanetary space) that is smaller than an ▶ asteroid but considerably larger than a single atom. The size demarcation between asteroids and meteoroids is somewhat arbitrary, but is sometimes set at about 10 m (see ▶ Asteroid). When a meteoroid strikes the atmosphere of the Earth, it produces the phenomenon called a meteor (popularly known as a “shooting star”) as it is heated by friction and completely or partially incinerated. If part of the meteoroid reaches the surface of the Earth, the surviving object or objects are called ▶ Meteorite(s).
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Meter-Size Catastrophe
See also
Definition
▶ Asteroid ▶ Meteorites
Methane, CH4, is the simplest hydrocarbon molecule. Four hydrogen atoms are covalently bound to a carbon atom with a bond angle (H–C–H) of 109.5 . Its melting point is 91 K, and its boiling point is 112 K. Methane is a major component of natural gas, and is known as one of the major greenhouse gases. Both gaseous and solid methane have been detected in molecular clouds, and it has also been found in cometary comae. Methane is a major carbon species in the atmospheres of the outer planets such as ▶ Jupiter and Saturn. ▶ Titan, the largest satellite of Saturn, has a dense atmosphere containing methane, and methane can form lakes on the surface of Titan. A wide variety of organic compounds can be easily formed in a mixture containing methane, such as has been seen in the Titan atmosphere. In 1953 Miller reported that amino acids were synthesized by spark discharges in the gas mixtures containing methane and ammonia, since he postulated that methane was the major carbon species in the primitive Earth atmosphere. These days, however, most scientists do not support the idea that methane was a major component of the Earth’s primitive atmosphere. Some microorganisms oxidize methane to obtain energy, and others produce methane as a product of their metabolism.
Meter-Size Catastrophe History In a protoplanetary disk, dust and gas orbit the star at different rates: gas pressure slows the rotation rate of the gas molecules, but not the much more massive dust particles. Therefore, the faster moving dust particles experience a headwind and a corresponding drag force, which leads to orbital decay. Small particles (1 mm) behave essentially like the gas and do not feel a strong headwind. Large particles (>10–100 m) have sufficient inertia to minimize orbital changes due to gas drag. However, at roughly 1 cm to 1 m in size, the effect of gas drag on particle orbits is the strongest. Calculations show that particles about 1 m in size feel the most gas drag, and they may spiral into their star from 1 AU so quickly that they probably cannot merge and grow fast enough to form 1 km-sized planetesimals. Unless particles grow quickly through this phase, planets cannot form.
See also ▶ Gas Drag (Aerodynamic, Tidal) ▶ Orbit
Methanal ▶ Formaldehyde
See also ▶ Chemical Evolution ▶ Comet ▶ Hydrocarbons ▶ Interstellar Ices ▶ Interstellar Medium ▶ Jupiter ▶ Methanobacteria ▶ Methanogens ▶ Methanotroph ▶ Miller, Stanley ▶ Titan
Methanamide ▶ Formamide
Methanethial ▶ Thioformaldehyde
Methane
Methanethioaldehyde
Synonyms CH4
▶ Thioformaldehyde
Methanogens
Methanethiol
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Methanogens
Synonyms CH3SH; Mercaptomethane; Thiomethanol
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Methyl
mercaptan;
JOSE´ LUIS SANZ Departamento de Biologı´a Molecular, Universidad Auto´noma de Madrid, Madrid, Spain
Definition Methanethiol (its IUPAC official name) is a colorless gas under standard temperature and pressure, becoming liquid at 6 C. It is the simplest thiol (thiols are analogs to alcohols, with an SH group replacing OH). Methanethiol has a strong rotten cabbage smell, so that some is added to natural gas to ease leak detection. It is produced by decaying organic matter in marshes and by decomposition of an algal metabolite (DMSP; Dimethyl Sulfonio Proprionate). It is the main sulfur source of some marine bacteria and a possible substrate for methanogenesis. In prebiotic chemistry, its presence allows for the synthesis of the S-bearing amino acid ▶ methionine in Urey–Miller experiments (Miller 1974). Methanethiol has been found in interstellar space.
Synonyms Methanobacteria; Methanogenic archaea
Keywords Anaerobic respiration, Archaea, Euryarchaea, hydrogen, methane
Definition
▶ Comet ▶ Methionine ▶ Molecules in Space ▶ Thioformaldehyde
Methanogens are ▶ microorganisms whose metabolism generates ▶ methane. All methanogenic organisms belong to the Domain ▶ Archaea, phylum ▶ Euryarchaeota. They are very diverse, morphologically (rods, filaments, cocci, sarcina among others) as well as phylogenetically: they are distributed in four Classes and five Orders: Methanobacteriales, Methanococcales, Methanomicrobiales, Methanosarcinales, and Methanopyrales. Methanogenic archaea, also referred to as methanobacteria, are the living beings most sensitive to oxygen (see ▶ Anarobe), which is why they only develop in anaerobic and reducing environments (with redox potentials below 200 mV). Despite this limitation, their ecological niches are widely distributed: aquatic sediments (marshes, swamps), stagnant soil (peat bogs, ricefields), marine geothermal vents, the digestive tract of certain animals (ruminants or termites), sludge or wastewater digestors, etc.
References and Further Reading
Overview
History Methanethiol was detected in the interstellar medium toward the Galactic Center by Linke et al. (1979) (but not in ▶ comets up to now).
See also
Linke RA, Frerking MA, Thaddeus P (1979) Interstellar methyl mercaptan. Astrophys J Lett 234:L139–L142 Miller S (1974) The atmosphere of the primitive Earth and the prebiotic synthesis of amino acids. Orig Life Evol Biosph 5:139–151
Methanobacteria ▶ Methanogens
Methanogenic Archaea ▶ Methanogens
Methanogens derive energy exclusively through the production of ▶ methane (a process known as methanogenesis). Methanogenesis constitutes the last step in the degradation/mineralization of organic matter in the absence of oxygen, an essential link in the carbon cycle. There are few substrates for methanogenesis. All methanogenic Archaea (with only one exception) are able to reduce CO2 with H2 forming CH4 (hidrogenotrophic methanogenesis), fixing CO2 by the carbon monoxide pathway to synthesize their biomass. They are, therefore, chemolithoautotrophs. Many, but not all, hydrogenotrophic species can alternatively use formic acid. Another substrate that can be converted into methane is acetate (acetoclastic methanogenesis). Even though only two genera, Methanosarcina and Methanosaeta, are acetoclastic methanogens, they are of great importance,
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Methanoic Acid
which is reflected in the fact that two-thirds of the methane produced from complex organic matter comes from acetate. Certain members of Methanomicrobia can use C1-compounds (methanol or methylamines), but their relevance in the environment anaerobic full-scale digestors is limited. Many of the ▶ coenzymes involved in methanogenesis are present exclusively in methanogens. Some, like the electron transporter F420, emit blue-green fluorescent light, which is useful for identifying methanogens by fluorescence microscopy. Others are involved in the final step of all methanogenesis reactions (conversion of methyl groups into methane). Coenzyme M (HSCH2CH2SO 3 ) is among these and it is present exclusively in these organisms, which is why an analog of this coenzyme (BESA: bromo-ethanesulfonic acid) is used to specifically inhibit methanogenesis. From an energetic point of view, methanogenesis is a very inefficient process, generating only one ATP per mole of methane produced. The yield is therefore very small (only 2% of fixed CO2 becomes biomass), and the generation times of methanogens are long (1–7 days in optimum conditions). Both of these characteristics make methanogenesis a key step in anaerobic digestion. This is of particular relevance in anaerobic treatment of wastewaters and in biomethanization processes. Although methane can be produced abiotically, 80% of the Earth methane is biologically produced by methanogens. Methanogenesis is of astrobiological interest as a unique and very ancient mode of energy conservation and especially after the recent detection of methane in the Mars atmosphere.
See also ▶ Archea ▶ Euryarchaeota ▶ Methane ▶ Methanotroph
References and Further Reading Balch WE, Fox GE, Magrum LJ, Woese CR, Wolfe RS (1979) Methanogens: reevaluation of a unique biological group. Microbiol Rev 43:260–296 Grigoryan A, Voordouw G (2008) Microbiology to help solve our energy needs - Methanogenesis from oil and the impact of nitrate on the oilfield sulfur cycle. Ann NY Acad Sci 1125:345–352 Kral TA, Brink KM, Miller SL (1998) Hydrogen consumptions by methanogens on the early earth. Orig Life Evol Biosph 28(3):311–319 Madigan M, Martinko J, Dunlap P, Clark D (2009) Brock biology of microorganisms, 12th edn. Person Education, Benjamin Cummings, San Francisco, Chaps 17, 21, 36 Prescott, Harley, Klein (2008) Chap 20 In: Willey JM, Sherwood LM, Woolverton CJ (eds) Microbiology, 7th edn. McGraw-Hill, New York
Sowers KR (1995) Methanogenic archaea: an overview. In: Robb, Place, Sowers, Schreier, DasSarma and Flischmann (eds) Archaea: A laboratory manual, Methanogens. Cold Spring Harbor Laboratory Press, pp 3–13 van Lier JB, van der Zee FP, Tan NCG, Rebac S, Kleerebezem R (2001) Advances in high-rate anaerobic treatment: staging of reactor systems. Water Sci Technol 44(8):15–26
Methanoic Acid ▶ Formic Acid
Methanol Synonyms Methyl alcohol
Definition Methanol is the simplest ▶ alcohol, with the structural formula CH3OH. The International Union of Pure and Applied Chemistry (IUPAC) recommends using the term methanol rather than methyl alcohol. Methanol is a colorless, volatile, and flammable liquid under standard conditions. Its melting and boiling point are 97.8 C and 64.7 C, respectively. It is a product of anaerobic metabolism of certain groups of bacteria, and of the abiotic reduction of carbon monoxide with hydrogen. It is toxic to humans and other organisms since it is converted to formic acid in vivo, which many organisms have trouble metabolizing. Methanol is widely distributed in interstellar space, where it was identified in 1970. It is particularly abundant in star-forming regions such as ▶ hot cores and ▶ hot corinos. Methanol has also been identified in comets. Methanol has been used as one of the carbon sources in simulated ▶ interstellar ices, and amino acids are formed by irradiation with charged particles or ultraviolet light in the presence of a nitrogen source such as ammonia.
See also ▶ Alcohol ▶ Comet (Nucleus) ▶ Ethanol ▶ Hot Cores ▶ Hot Corinos ▶ Interstellar Ices ▶ Molecules in Space
Methyl Acetylene
Methanophiles ▶ Methanotroph
Methanotroph
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Methinophosphide ▶ Phosphaethyne
Methionine
Synonyms
Definition
Methanophiles
Methionine is a sulfur-containing ▶ amino acid with the side chain –CH2CH2SCH3. Among the 20 protein amino acids, only methionine and ▶ cysteine have ▶ sulfur. Its three-letter symbol is Met, and its one-letter symbol is M. The molecular weight of methionine is 149.21, and its isoelectric point (pI) is 5.74. In the standard genetic code table, only one codon AUG corresponds to methionine, and this codon is also an initiation codon. Eukaryotes and archaea, therefore, synthesize all their ▶ proteins with methionine at N-terminal (the amino acid with free –NH2 group at the end of proteins) at first. Most of N-terminal methionine is removed after the translation processes. Methionine is an essential amino acid for humans.
Definition Methanotrophs are organisms which are able to obtain energy by oxidizing ▶ methane (CH4). Methane, found widely in nature, is produced in strict anaerobic conditions by methanogenic Archaea (see ▶ Methanogens). It is the main gas in anoxic muds, marshes, lakes, rice paddies, and lanfield. Methane is the major constituent of natural gas and is also present in many coal formations. It can be used as an electron donor by methanotrophic bacteria. Methanotrophs can also use other one carbon compounds as a source of energy and carbon. Most of the known methanotrphs are aerobic, although, recently, methanotrophic activities in anoxic environments have been detected. All methanotrophs possess a specific enzyme, methane monooxygenase, which can insert an oxygen atom into the methane molecule generating methanol. The oxygen requirement for the oxygenation of methane explains their obligate aerobic character. The anoxic oxidation of methane requires a consortium of methanotrophic and sulfate reducing bacteria. Methanotrophs are widespread in both aquatic and terrestrial environments and are found wherever stable sources of methane are present. They play an important role in the carbon cycle.
See also ▶ Amino Acid ▶ Cysteine ▶ Protein ▶ Sulfur
Methoxymethane (IUPAC Name) ▶ Dimethyl Ether
See also ▶ Aerobic Respiration ▶ Anaerobic Respiration ▶ Carbon Cycle (Biological) ▶ Methanogens ▶ Sulfate Reducers
Methenamine ▶ Hexamethylenetetramine
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2-(Methylamino) Acetic Acid ▶ Sarcosine
Methyl Acetylene ▶ Propyne
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Methyl Alcohol
Methyl Alcohol ▶ Methanol
Methyl Aldehyde ▶ Formaldehyde
Methyl Formate Synonyms
Houde M, Mehringer D, Moreno R, Paubert G, Phillips TG, Rauer H (2000) New molecules found in comet C/1995 O1 (Hale-Bopp). Investigating the link between cometary and interstellar material. Astron Astrophys 353:1101–1114 Brown RD, Crofts JG, Godfrey PD, Gardner FF, Robinson BJ, Whiteoak JB (1975) Discovery of interstellar methyl formate. Astrophys J Lett 197: L29–L31 Ellde J, Friberg P, Hjalmarson A˚, Ho¨glund B, Irvine WM, Johansson LEB, Olofsson H, Rydbeck G, Rydbeck OEH, Gue`lin M (1980) On methyl formate, methane, and deuterated ammonia in Orion A. Astrophys J Lett 242:L93–L96
Methyl Mercaptan ▶ Methanethiol
Formic acid methyl ester; HCOOCH3; Methyl methanoate
Definition Methyl formate is the simplest ester and the condensation product of methanol and formic acid. It is liquid under standard pressure between 100 C and 32 C. It is a very volatile, highly flammable, and harmful compound. It is used as an insecticide and has been used formerly in refrigeration. It has been detected in the interstellar medium, where it is more abundant than its two isomers, ▶ acetic acid CH3COOH and ▶ glycolaldehyde CH2OHCHO. It is one of the larger organic molecules detected in comet Hale-Bopp.
History Brown et al. (1975) detected methyl formate in emission in the spectrum of the Galactic Center source Sagittarius B2. Several emission lines previously assigned to rotationally distorted methane in the Orion molecular cloud were correctly assigned to methyl formate by Ellde et al. (1980), from which it became apparent that the millimeterwavelength spectrum of molecular clouds typically contain a multitude of its rotational transitions. The detection of methyl formate in comet C/1995 O1 (Hale-Bopp) was reported by Bockele´e-Morvan et al. (2000).
See also ▶ Acetic Acid ▶ Comet ▶ Glycolaldehyde ▶ Molecules in Space
Methyl Methanoate ▶ Methyl Formate
Methyl Radical Synonyms CH3
Definition The methyl ▶ radical CH3 is stable enough to be observable under laboratory conditions in a dilute gas, although it readily dimerizes to ethane. It is an important intermediary in gas phase interstellar chemistry. The incorporation of a methyl group, -CH3, into organic compounds can have fundamental biological effects.
History The CH3 radical was detected in interstellar molecular clouds at wavelengths of about 16 microns using the ▶ Infrared Space Observatory (Feuchtgruber et al. 2000).
See also ▶ Infrared Space Observatory ▶ Radical
References and Further Reading
References and Further Reading
Bockele´e-Morvan D, Lis DC, Wink JE, Despois D, Crovisier J, Bachiller R, Benford DJ, Biver N, Colom P, Davies JK, Ge´rard E, Germain B,
Feuchtgruber H, Helmich FP, van Dishoeck EF, Wright CM (2000) Detection of interstellar CH3. Astrophys J 535:L111–L114
Methylidyne Cation
Methylcyanide ▶ Acetonitrile
Methylene Synonyms CH2
Definition Methylene (CH2) is the simplest ▶ carbene. The carbon is covalently bonded to two hydrogen atoms and bears two nonbonded electrons whose spins may be antiparallel (singlet state) or parallel (triplet state). It is an intermediary in the production and destruction of interstellar organic molecules.
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Definition The diatomic radical CH (methylidyne), containing carbon and hydrogen, was one of the first interstellar molecules to be identified, in 1937 by T. Dunham and also by P. Swings and L. Rosenfeld. It is widespread in the diffuse ▶ interstellar medium, where it produces absorption lines in the visible spectrum of background stars. It is an important intermediary in the production and destruction of interstellar organic molecules. Methylidyne is also present in the coma (atmosphere) of ▶ comets, where it is presumably a photodissociation product of organic molecules sublimating from the cometary nucleus.
History
See also
After the first detection of the interstellar OH radical by radio astronomers in 1963, several groups looked for radio emission from interstellar CH. The search was difficult, however, because no laboratory measurements of the lowest frequency transitions were available, and theoretical predictions of the relevant frequencies were not very accurate. Radio astronomers at the Onsala Space Observatory in Sweden, led by Director Olof Rydbeck, succeeded in 1973 after a special ▶ maser receiver designed specifically for the search was built at the Chalmers Institute of Technology for use at the Observatory.
▶ Carbenes ▶ Interstellar Chemical Processes
See also
History Interstellar methylene was detected at a wavelength of about 4 mm by Hollis et al. (1995).
References and Further Reading Hollis JM, Jewell PR, Lovas FJ (1995) Confirmation of interstellar methylene. Astrophys J 438:259–264
▶ Comet ▶ Interstellar Medium ▶ Masers
References and Further Reading
Methylene Oxide ▶ Formaldehyde
Methylethylene ▶ Propylene
Crovisier J, Encrenaz T (2000) Comet science: the study of remnants from the birth of the solar system. Cambridge University Press, Cambridge Hartquist TW, Williams DA (1995) The chemically controlled cosmos. Cambridge University Press, Cambridge Rydbeck OH, Ellder J, Irvine WM (1973) Radio Detection of Interstellar CH. Nature 246:466–468
Methylidyne Cation Synonyms CH+
Methylidyne Synonyms CH
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Definition The ▶ methylidyne ion CH+ was one of the first molecules detected in the ▶ interstellar medium, being identified in absorption at visible/near ultraviolet wavelengths in the spectra of background stars in 1941 by Douglas and
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Herzberg. CH+ emission at visible wavelengths is seen from the tails of ▶ comets. Pure rotational transitions have been observed at far infrared wavelengths, for example, from ▶ planetary nebulae.
See also ▶ Amphiphile ▶ Lipid Bilayer ▶ Self Assembly
See also ▶ Comet ▶ Interstellar Medium ▶ Methylidyne ▶ Planetary Nebula
Micro-g ▶ Microgravity
References and Further Reading Cernicharo J, Liu XW, Gonzalez-Alfonso E, Cox P, Barlow MJ, Lim T, Swinyard BM (1997) Discovery of far-infrared pure rotational transitions of CH+ in NGC 7027. Astrophys J Lett 483:L65–L68 Douglas AE, Herzberg G (1941) Note on CH+ in interstellar space and in the laboratory. Astrophys J 94:381
MGS ▶ Mars Global Surveyor
Micas ▶ Phyllosilicates (Extraterrestrial)
Micelle
Microbe ▶ Microorganism
Microbial Ecology Evaluation Device ▶ MEED
Microbial Mats LUCAS J. STAL1, NORA NOFFKE2 1 Department of Marine Microbiology, Netherlands Institute of Ecology NIOO-KNAW, Yerseke, The Netherlands 2 Ocean, Earth & Atmospheric Sciences, Old Dominion University, Norfolk, VA, USA
Definition Micelles are self-assembled aggregates of amphiphilic molecules such as soaps, surfactants, and detergents. These compounds are called amphiphiles because each molecule consists of a hydrophobic hydrocarbon chain with a hydrophilic polar or ionic head group such as hydroxyl, carboxylate, phosphate, or sulfate at one end. Amphiphiles exist in solution as individual molecules, but at a certain concentration (the critical micelle concentration, or cmc) they assemble into micelles that typically contain a few hundred molecules. Micelles in aqueous phases have hydrocarbon chains directed inward while hydrophilic head groups line the surface. Micelles are one of the simplest examples of a ▶ self-assembly process in an aqueous medium.
Synonyms Benthic mats; Cyanobacterial mats; Farbstreifen Sandwatt; Laminated microbial ecosystems; Living stromatolites; Modern stromatolites
Keywords Biogeochemistry, carbon cycle, ▶ cyanobacteria, microbial ecosystem, nitrogen cycle, purple sulfur bacteria, sulfate-reducing bacteria, sulfur cycle
Definition Microbial mats are laminated microbial communities that generally develop in aqueous environments under
Microbial Mats
conditions that exclude fauna. The biogeochemical cycles in microbial mats are usually largely closed, although small fluxes of elements are exchanged with the geo-, bio-, and atmosphere. An important feature of microbial mats is their carbon autotrophy, i.e., the fixation of inorganic carbon is a key process. In illuminated environments, therefore, the photoautotrophic ▶ cyanobacteria are the mat-building organisms, while in the deep sea or in some caves chemosynthetic ▶ Bacteria and ▶ Archaea are the primary producers.
History Mileposts in research on microbial mats in marine settings are the studies by Ginsburg 1991; Hardie and Garrett 1977; Cohen and Rosenberg 1989; Friedman et al. 1985; Stal and Caumette 1994; and many others. Especially in carbonate sedimentology, the significance of microbial mats has long been recognized.
Overview The term microbial mat originates from the macroscopic structure that these microbial ecosystems often adopt. Large filamentous microorganisms grow and form an entangled mass with the sediment particles on which they grow, stabilizing the sediment surface and themselves from eroding forces. Mature microbial mats can sometimes be lifted in large pieces from the sediment, just like a doormat. However, in the course of the research on microbial mats, the term has been used for any benthic microbial community that forms a macroscopic structured entity. An important driver for the study of microbial mats is that these systems are considered to be analogs of the Precambrian stromatolites, which have been recognized as the oldest known ecosystems on Earth and their study could lift the veil of the evolution of early life. Microbial mats often develop in environments and habitats that are characterized by extreme conditions that exclude metazoans that would otherwise graze on the microbes, preventing them from forming a microbial mat. Microbial mats are largely closed ecosystems. This means that the biogeochemical cycles are basically closed. Therefore, a basic characteristic of microbial mats is that they are carbon autotrophic, meaning that they grow by fixing inorganic carbon. This can be done in two principally different ways, namely by photosynthesis and by chemosynthesis. In the former, light is the source of energy and phototrophic organisms are the organisms that build the microbial mats. In chemosynthetic microbial mats the source of energy is the oxidation of a reduced compound, often sulfide or methane. In most illuminated environments the phototrophic cyanobacteria are the
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mat-building organisms. In dark environments such as in the abyssal of the oceans or in caves where hyperthermal springs, mud volcanoes and cold seeps provide reduced compounds produced by the Earth’s internal heat that drive chemosynthesis. Chemosynthetic mats may also develop in illuminated environments when for instance the decomposition of large amounts of organic matter in seawater produces high amounts of sulfide which is in direct contact with oxygen. Since Precambrian stromatolites are known to have developed in shallow, illuminated coastal environments, the study of coastal microbial mats deserves special attention. Microbial mats develop in coastal intertidal sandy sediments. These environments can be considered as extreme because they are periodically inundated, desiccated, hyper- or hyposaline, experiencing high and low temperatures, and as a rule are nutrient poor. Cyanobacteria thrive well under such conditions. As photoautotrophs they use light, water, and CO2 as source of energy, electrons, and carbon, respectively, which are abundantly available. Many cyanobacteria are also nitrogen fixers, i.e., they can utilize atmospheric N2, the most abundant form of nitrogen on Earth and after carbon, the second most important element for life. Cyanobacteria have low nutrient requirements and have high-affinity phosphate uptake systems and have a variety of storage compounds. The carbon fixed by the cyanobacteria is made available to the microbial community by a variety of different ways. In the light, photorespiration may result in the excretion of glycolate while in the dark the storage compound glycogen may be fermented to low-molecular compounds such as acetate, lactate, and ethanol. Cyanobacteria exude also exopolymeric substances and upon salinity down shock may release compatible solutes such as the carbohydrates trehalose and sucrose and the polyol glucosylglycerol. Finally, cyanobacteria may die and lyse, providing another source of organic matter and nutrients. A plethora of microorganisms will degrade these compounds but the sulfate-reducing bacteria deserve a particular notice in marine microbial mats. At night, when oxygenic photosynthesis ceases or in the deeper aphotic layers of the mat, oxygen is quickly consumed by the microbial community, including the cyanobacteria. This happens often within minutes. Sulfate-reducing bacteria are therefore responsible for the bulk of the degradation of organic matter using the abundantly available sulfate (28 mM) in seawater as electron acceptor, reducing it to sulfide. The sulfide is subsequently oxidized in two ways. In the light, anoxygenic phototrophic purple sulfur bacteria use sulfide as the
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electron donor for photosynthesis. They oxidize it to elemental sulfur and eventually back to sulfate. These Bacteria are basically anaerobic and form a pink layer below the cyanobacteria where they experience anaerobic conditions while sufficient light (particularly of the far red wavelengths) reach them. The other group of microorganisms that oxidizes the sulfide is the colorless sulfur bacteria. They use oxygen or nitrate as electron acceptor. As a result of the microbial activities, microbial mats are characterized by steep and fluctuating physicochemical gradients. Only during the day, oxygenic photosynthesis by cyanobacteria enriches and supersaturates the sediment with oxygen. During the night this oxygen is quickly respired. Sulfide accumulates during the night because light and oxygen (and usually nitrate as well) are lacking for its oxidation while the sulfate-reducing bacteria are continuing to produce it. During the day sulfide is oxidized. The day-night rhythm and the activities of the mat microorganisms result in the continuously shifting gradients of oxygen and sulfide. Together with that, the pH shows dramatic shifts. During the day the fixation and the consequent depletion of CO2 cause the pH to rise to high levels (>9), while during the night the decomposition of organic matter causes the level of CO2 and the pH to fall rise. The mat-forming organisms take their positions in these physicochemical gradients. The aerobic cyanobacteria forming a green layer at the surface while the anaerobic purple sulfur bacteria forming a pink layer right below them. Often a rusty layer separates them. This layer is supposedly composed of iron which forms a barrier for both oxygen and sulfide, which are toxic to the purple sulfur bacteria and cyanobacteria, respectively. Sulfide diffusing to the surface reacts with iron and oxidizes to elemental sulfur and forms insoluble iron sulfide and pyrite. Oxygen diffusing downwards reacts with the iron and oxides it, forming iron hydroxides. It is also possible that anoxygenic photosynthetic iron oxidizing bacteria thrive in this layer, but hitherto this has not been demonstrated. Sulfate-reducing bacteria are obligate anaerobic microorganisms, although some can tolerate low levels of oxygen or are even capable of respiring it, although they are unable to live by ▶ aerobic respiration. For these reasons it has been assumed that sulfatereducing bacteria are mostly present in the permanent anoxic layers of the microbial mat. This is not the case. Sulfate-reducing bacteria occur throughout the microbial mat although the different groups show a distinct depth distribution, particularly with respect to their oxygen sensitivity. Microbial mats have been termed laminated microbial ecosystems. The lamination is the result of the
vertically stratified communities of different functional groups, such as phototrophic microorganisms, notably the cyanobacteria and the purple sulfur bacteria (and in some cases also green sulfur bacteria), giving the mat a varicolored appearance. This lamination is also called an instantaneous biological lamination to distinguish it from a historical lamination which is produced by successive microbial mats, while the older mats are only partly degraded (compare with the rings of a tree). This is often seen under extreme conditions such as hypersaline or hyperthermal environments. This historical lamination is also preserved when mats turn to stone, i.e., when calcification takes place. In some areas, microbial life is thriving, nutrient supply is high, and the substrate is most favorable. In these conditions multicolored sand flats can be found (Gerdes and Krumbein 1987). Thick, epibenthic microbial mats form a digestive cooperation. The mature microbial mat in Fig. 1 is a fine example. Stromatolites are, by definition, laminated rocks and are known from the fossil record up to almost 3.5 billion years before present. They may represent the oldest form of life. While the oldest stromatolites may have formed by direct silicification, most have turned to stone through calcification and subsequent diagenesis. The majority of modern microbial mats do not turn to stone, although it is uncertain whether this has always been the case. The fossil record also indicates the presence of non-calcified microbial mats through features known as Microbial Induced
Microbial Mats. Figure 1 Transformation of sunlight to organic matter by a microbial mat. A vertical cut through microbial mat-overgrown sediment shows a stack of layers, each of different color. Each layer is comprised of a different group of bacteria. The metabolisms of the bacteria interact with each other forming a digestive cooperative. The energy of sunlight is transformed through photosynthesis into an organic substance. The organic substance is then decomposed by various chemoautotroph bacteria (From Noffke 2010)
Microbially Induced Sedimentary Structures
Sediment Structures (▶ MISS). It is not fully clear why some mats calcify and others apparently do not. It is hypothesized that extracellular polymeric substances (EPS) produced by the cyanobacteria bind calcium, thereby lowering the solubility product of calcium carbonate. When the EPS is degraded, calcium is locally released causing saturation of calcite. Modern marine stromatolites are rare and found only in the Exuma Cays (Bahamas) and in Shark Bay (Western Australia). Stromatolites with remarkable similarity of those of the Precambrian are found in alkaline environments which may hint to an alkaline Archean ocean. It has been shown that calcification occurs in microbial mats that do not lithify but the calcite in these mats dissolves as a result of microbial metabolism.
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Schopf JW (2000) Solution to Darwin’s dilemma: discovery of the missing Precambrian record of life. Proc Natl Acad Sci USA 97:6947–6953 Stal L, Caumette P (1994) Microbial mats: structure, development and environmental significance. Springer-Verlag, Berlin Stal LJ (1995) Physiological ecology of cyanobacteria in microbial mats and other communities. New Phytol 131:1–32 Stolz J (2000) Structure of microbial mats and biofilms. In: Riding R, Awramik S (eds) Microbial Sediments. Springer-Verlag, Berlin Stal LJ (2001) Coastal microbial mats: the physiology of a small-scale ecosystem. S Afr J Bot 67:399–410
Microbial Sediment Fixation ▶ Biostabilization
See also ▶ Archean Traces of Life ▶ Biofilm ▶ Colonization (Biological) ▶ Cyanobacteria ▶ Microfossils ▶ MISS ▶ Stromatolites ▶ Sulfate Reducers
References and Further Reading Cohen Y, Rosenberg E (1989) Microbial mats: physiological ecology of benthic microbial communities. American Society for Microbiology, Washington Des Marais DJ (2003) Biogeochemistry of hypersaline microbial mats illustrates the dynamics of modern microbial ecosystems and the early evolution of the biosphere. Biol Bull 204:160–167 Dupraz C, Reid RP, Braissant O, Decho AW, Norman RS, Visscher PT (2009) Processes of carbonate precipitation in modern microbial mats. Earth Sci Rev 96:141–162 Friedman G, Krumbein W, Gerdes G (1985) Hypersaline ecosystems: the Gavish sabkha. Springer-Verlag, Berlin Gerdes G, Krumbein W (1987) Biolaminated deposits. Springer-Verlag, Berlin Ginsburg RN (1991) Controversies about stromatolites: vices and virtues. In: Mu¨ller DW, McKenzie JA, Weissert H (eds) Controversies in modern geology. Academic Press, London, pp 25–36 Hardie L, Garrett P (1977) Sedimentation on the modern carbonate tidal flats of Northwest Andros Island, Bahamas. John Hopkins University Press, Baltimore Konhauser K (2007) Introduction to geomicrobiology. Blackwell, Boston Krumbein W (1983) Stromatolites – the challenge of a term in space and time. Precambrian Res 20:493–531 Noffke N (2010) Microbial mats in sandy deposits from the Archean era to today. Springer, Heidelberg, 175 p Noffke N, Gerdes G, Klenke T (2003) Benthic cyanobacteria and their influence on the sedimentary dynamics of peritidal depositional systems (siliciclastic, evaporitic salty, and evaporitic carbonatic). Earth Sci Rev 62:163–176
Microbialites ▶ Stromatolites
M Microbially Induced Sedimentary Structures NORA NOFFKE Department of Ocean, Earth & Atmospheric Sciences, Old Dominion University, Norfolk, VA, USA
Synonyms MISS; Siliciclastic stromatolites
Keywords Archean, astrobiology, early life, geobiology, microbial mats, microbial sediment, microbially induced sedimentary structures, MISS, precambrian, siliciclastic, stromatolites, tidal flats
Definition Microbially Induced Sedimentary Structures (MISS) result from the interaction of ▶ microbial mats with physical sediment dynamics in siliciclastic marine environments. Microbial mats affect sediments by ▶ biostabilization, baffling, trapping, and binding. Mineral precipitation does not play any role in the formation of the MISS. MISS do not resemble ▶ stromatolites.
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They come in 17 main types. Thin-sections under the microscope reveal textures that are related to, have been caused by, or represent biofilms or microbial mats. In response to the long-term hydraulic pattern of their environment, microbial mats form biofilm-catenae. Consequently, MISS are also not distributed at random, but form a lateral suite from low to high energy settings. MISS occur from the early Archean to the present. The morphology and distribution of the structures allow conclusions on ancient mat-forming microbiota.
History Early workers interpreted “elephant skin textures,” crinkled upper bedding planes in sandstones, as fossil microbial mats (e.g., Runnegar and Fedonkin 1992; Gehling 1999). Schieber (1989) described ancient mat lamina in Mesoproterozoic shales of Montana, USA. Hagadorn and Bottjer (1999) and Schieber et al. (2007) summarized these early observations on microbial mat-related structures. About the same time, when elephant skin textures were discussed, researchers started to work on modern microbial mats and the sedimentary structures they form (e.g., Cameron et al. 1985; Gerdes & Krumbein 1987; Gerdes et al. 1991). The term “microbially induced sedimentary structures (MISS)” was coined in 1996, based on quantitative analyzes on modern microbial mat-related structures in sandy tidal flats (Noffke et al. 1996). The latest classification is described in Noffke (2009, 2010).
Overview Systematic studies on biogenic sedimentary structures in sandstones have opened a new perspective in the investigation of life’s early evolution in the Archean era (Noffke 2010). Benthic microorganisms interact with their sandy substrate and leave traces behind. The traces can become fossils. MISS constitute such traces and trace fossils. Microbial mats grow especially well during quiet hydraulic conditions, the latencies. During episodes of erosion, biostabilization can be observed (Paterson 1994). Baffling and trapping is active sediment accumulation. Binding is the incorporation of mineral particles into the microbial mat fabrics over time. Due to their biotic-physical genesis, the structures have been placed as own category in the classification of primary sedimentary structures (Noffke et al. 2001). MISS have been described from modern tidal flats along the North Sea coast of Germany (Gerdes and Krumbein 1987; Noffke 2010). “Erosional remnants and pockets,” “multidirected ripple marks,” “gas domes,”
or “mat chips” occur abundantly. Vertical sections through the sediments reveal “sponge pore fabrics,” “microsequences,” “roll-ups,” “oriented grains,” and other structures and textures. Sabkha-like tidal flats along the coast of southern Tunisia include “petees,” “oscillation cracks,” or “tufts” (Noffke 2010). Examples for MISS are shown in Fig. 1. Microbial mats are not distributed randomly. Different mat types develop in correspondence to the hydraulic conditions that change from geomorphologically low to high sites. For such lateral successions of mat types, the term “biofilm-catena” was introduced (Noffke and Krumbein 1999; Noffke 2010). Specific mat types induce specific MISS. Consequently, these MISS form lateral suites. The knowledge on formation and distribution of MISS allows comparison of modern with fossil structures (Noffke et al. 2006b). Fossil MISS and their relation to their ancient habitat allow conclusions on prokaryotic evolution.
Basic Methodology A geological survey identifies MISS in a rock succession. The geometries and dimensions of these structures are measured and the data used for statistical analyses. The statistical data describe the morphologies of the MISS quantitatively. Thin-sections must demonstrate the presence of microbial textures in possible MISS. Because MISS occur in the modern environment as well, a comparison of modern and ancient MISS concludes on the evolution of prokaryotes.
Key Research Findings Studies in modern tidal flats are key to understanding fossil microbial life (Noffke et al. 2006b, 2008; Noffke 2010). Microbial influences in marine environments have long been understood as biogeochemical processes. However, research on MISS elucidated the significance of microbial-physical interaction as a cause of sedimentary structures (Noffke & Paterson 2008). A set of subsequent studies revealed that fossil MISS occur in tidal flat and shelf sandstones of Phanerozoic, Proterozoic, and Archean ages. It is believed that the MISS were caused by photoautotrophic microbial mats and it appears that their structure has not changed for at least 3.2 billion years (Noffke 2000; Noffke et al. 2002, 2003, 2006 a, b, 2008). The exceptionally well preserved MISS of the 2.9 Ga Pongola Supergroup, South Africa, document the existence of biofilm-catena, possibly similar to biofilms in as today’s tidal flats.
Microbially Induced Sedimentary Structures
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Microbially Induced Sedimentary Structures. Figure 1 Modern and fossil MISS in comparison: (a) and (b): Microbial mat chips; scale: 10 cm. (c) and (d): Erosional remnants and pockets; scale: 1 m. (e) and (f): Oriented grains; scale: 0.2 mm; (g) and (h): Polygonal oscillation cracks; scale: 25 cm
Applications MISS serve to deepen understanding of Earth’s prokaryotic history (Noffke & Paterson 2008; Noffke & Bottjer 2009). They help us interpret paleoclimate and on paleocurrent systems. One example is the “multidirected
ripple marks.” This chaotic-like pattern of ripple marks documents biostabilization of tidal sands by a biofilmcatena. Because the microbiota responds to individual storm events, the resulting ripple marks record frequencies and intensities of storms in a paleoenvironment.
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Future Directions At present, MISS are caused by benthic microbial communities dominated by ▶ cyanobacteria. Possibly, the MISS of the 2.9 Ga Pongola Supergroup, South Africa, may record the oldest cyanobacteria in Earth history. Future research will focus on MISS in older lithologies including chert or carbonates.
See also ▶ Biostabilization ▶ Cyanobacteria ▶ Microbial Mats ▶ Stromatolites
References and Further Reading Cameron B, Cameron D, Jones J (1985) Modern algal mats in intertidal and supratidal quartz sands, northeastern Massachusetts, USA. In: Curran A (ed) Biogenic structures: their use in interpreting depositional environments. SEPM, Tulsa Gehling J (1999) Microbial mats in terminal Proterozoic siliciclastics; Ediacaran death masks. Palaios 14:40 Gerdes G, Krumbein W (1987) Biolaminated deposits. Springer-Verlag, Berlin Gerdes G, Krumbein W, Reineck H (1991) Biolaminations—Ecological versus depositional dynamics. In: Einsele G et al (ed) Cycles and events in stratigraphy. Springer-Verlag, Berlin Hagadorn J, Bottjer D (1999) Restriction of a late Neoproterozoic biotope; suspect-microbial structures and trace fossils at the VendianCambrian transition. Palaios 14:73 Noffke N (2000) Extensive microbial mats and their influences on the erosional and depositional dynamics of a siliciclastic cold water environment (Lower Arenigian, Montagne Noire, France). Sed Geol 136:207–215 Noffke N (2010) Geobiology–microbial mats from the Archean era to today. Springer, Heidelberg, 193 Noffke N, Gerdes G, Klenke T, Krumbein WE (1996) Microbially induced sedimentary structures–examples from modern sediments of siliciclastic tidal flats. Zbl Geol Palaeont 1:307–316 Noffke N, Krumbein WE (1999) A quantitative approach to sedimentary surface structures contoured by the interplay of microbial colonization and physical dynamics. Sedimentology 46: 417–426 Noffke N, Gerdes G, Klenke T, Krumbein WE (2001) Microbially induced sedimentary structures—a new category within the classification of primary sedimentary structures. J Sed Res 71:649–656 Noffke N, Knoll AH, Grotzinger J (2002) Sedimentary controls on the formation and preservation of microbial mats in siliciclastic deposits: a case study from the upper Neoproterozoic Nama group, Namibia. Palaios 17:1–14 Noffke N, Hazen RM, Nhleko N (2003) Earth’s earliest microbial mats in a siliciclastic marine environment (Mozaan group, 2.9 Ga, South Africa). Geology 31:673–676 Noffke N, Beukes NJ, Hazen RM (2006a) Microbially induced sedimentary structures in the 2.9 Ga old Brixton formation, Witwatersrand supergroup, South Africa. Prec Res 146:35–44 Noffke N, Hazen RM, Eriksson K, Simpson E (2006b) A new window into early life: microbial mats in a siliciclastic early Archean tidal flat (3.2 Ga Moodies group, South Africa). Geology 34:253–256
Noffke N, Beukes N, Bower D, Hazen RM, Swift DJP (2008) An actualistic perspective into Archean worlds–(cyano-)bacterially induced sedimentary structures in the siliciclastic Nhlazatse section, 2.9 Pongola supergroup, South Africa. Geobiology 6:5–20 Noffke N, Paterson D (2008) An actualistic perspective: biotic-physical interaction of benthic microorganisms and the significance for the biological evolution of Earth. Geobiology Spec Issue, p 93 Noffke N (2009) Geobiology: the significance of microbial mats for Earth history. Earth Sci Rev Spec Issue, p 219 Noffke N, Bottjer D (2009) Microbial mats as biosignatures (ancient and modern). Astrobiology Spec Issue Paterson D (1994) Siliciclastic intertidal microbial sediments. In: Stal LJ, Caumette P (eds) Microbial mats. Springer, Berlin Runnegar B, Fedonkin M (1992) Proterozoic metazoan body fossils. The Proterozoic Biosphere 369–388 Schieber J (1989) Facies and origin of shales from the mid-Proterozoic Newland Formation, Belt Basin, Montana, USA. Sedimentol 36:203–219 Schieber J, Bose PK, Eriksson PG, Banerjee S, Sarkar S, Alterman W, Catuneanu O (eds) (2007) Atlas of microbial mat features preserved within the siliciclastic rock record. Elsevier, Amsterdam, p 311
Microfossils EMMANUELLE J. JAVAUX Department of Geology, Paleobotany-PaleopalynologyMicropaleontology Research Unit, University of Lie`ge, Lie`ge, Belgium
Keywords Acritarchs, Fossil, Fossilization, morphological fossils
Definition A microfossil is a microscopic morphological remain of an organism. It may represent a whole or part of a microscopic or macroscopic organism. The organism may be a unicellular or multicellular prokaryotic or eukaryotic organism, or a virus.
Overview Microfossils can have a variety of morphologies and chemical compositions, depending on their original properties and the conditions in which they are preserved. Microfossils can have a carbonaceous composition or a mineral composition. The mineral composition can be primary (produced or precipitated by the organism) or secondary (the original carbonaceous or mineral walls or envelopes being in that case partially or completely replaced by other minerals). Microfossils are studied by micropaleontologists and provide useful information on the evolution of the
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biosphere, for biostratigraphy (dating sedimentary rocks), and for paleoecology, through most of geological time. Possible microfossils have simpler morphologies and smaller size as we look further back in time through the early Earth rock record. Moreover, abiotic processes can produce structures resembling microfossils (▶ pseudofossils). Deciphering the ▶ biogenicity of an object is very difficult even using cutting-edge in situ techniques. Several criteria have been proposed in the literature to test thebiogenicity of microstructures preserved three-dimensionally in cherts or silicified carbonates or phosphorites, or flattened in two dimensions in fine-grained siliciclastic rocks. Most criteria underline the importance of a well-characterized geological context as the environmental conditions will determine the conditions of ▶ fossilization. Three criteria need to be reached in order to correctly interpret a particular structure as a microfossil: (1) ▶ endogenicity: the microstructure is within the rock and not a contaminant; (2) ▶ syngeneity: the microstructure has the same age as the rock and is not a younger fossilized contaminant such as an organism boring the rock or transported by fluids between pores or veins through the rock; (3) biogenicity: the microstructure has a biological origin. Not only must biogenicity be demonstrated, but all possible abiotic hypotheses must be excluded before a biological origin can be accepted. Several analytical techniques coupled to observations of the microfossil in situ in sections cut through the rock are used to prove the points above. Geobiological studies in recent and past environments and laboratory experiments can improve understanding of preservational environments and fossilization processes, and help to recognize morphological (and other) traces of life on early Earth and beyond Earth. This is essential to choose landing sites, instrumentation, and samples to return in exobiological missions.
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References and Further Reading Armstrong HA, Brasier MD (2005) Microfossils, 2nd edn. Blackwell, Oxford Brasier MD, McLoughin N, Green O, Wacay D (2006) A fresh look at the fossil evidence for early Archaean cellular life. Philos Trans R Soc B 361:887–902 Buick R (1990) Microfossil recognition in Archean rocks: an appraisal of spheroids and filaments from a 3500 M.Y. old chert-barite unit at North Pole, Western Australia. Palaios 5:441–459 Hofmann HJ (2004) Archean microfossils and abiomorphs. Astrobiology 4:135–136 Javaux EJ, Benzerara K (2009) Microfossils. In: Gargaud M, Mustin C, Reisse J, Vandenabeele-Trambouze O (eds) Traces de vie pre´sente ou passe´e: quels indices signatures ou marqueurs? Comptes rendus Acade´mie des Sciences, Paris; PalEvol 8:605–615 Konhauser KO (2007) Introduction to geomicrobiology. Blackwell, Oxford, 425 pp Lipps JH (ed) (1993) Fossil prokaryotes and protists. Blackwell, Boston/ Oxford Sugitani K, Grey K, Allwood A, Nagaoka T, Mimurae K, Minamif M, Marshall CP, Van Kranendonk MJ, Walter MR (2007) Diverse microstructures from Archaean chert from the Mount Goldsworthy-Mount Grant area, Pilbara Craton, Western Australia: microfossils, dubiofossils, or pseudofossils? Precambrian Res 158:228–262
Microfossils, Analytical Techniques KEVIN LEPOT De´partement de Ge´ologie, UR Pale´obotanique, Pale´opalynologie et Micropale´ontologie, Universite´ de Lie`ge, Lie`ge, Belgium
Synonyms Characterization of microfossils; Micrometer to nanometer scale analysis of fossil cells
See also
Keywords
▶ Biogenicity ▶ Biomarkers, Morphological ▶ Dubiofossil ▶ Endogenicity ▶ Eukaryote ▶ Fossil ▶ Fossilization, Process of ▶ Microfossils, Analytical Techniques ▶ Prokaryote ▶ Protists ▶ Pseudofossil ▶ Syngenicity
AFM, CLSM, EDXS, FIB, microanalysis, Raman, SEM, SIMS, spectromicroscopy, STXM, SXM, TEM, XANES, XRF
Definition Versatile in situ analytical techniques provide morphological, structural, chemical, and mineralogical signatures to recognize abiotic microfossil-like structures, identify fossil microorganisms (▶ taxonomy), demonstrate their antiquity, and decipher their activities and host environments. Most of these techniques rely on electromagnetic (X-ray, UV, visible, IR) or particle (electrons, ions, protons)
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beams to produce images and perform spectrometric analyses in situ down to the nanoscale. Spectromicroscopy images the intensity distribution of characteristic spectral features (bands, peaks) by extracting their intensity distribution from spectra recorded point by point or by imaging the radiation emitted by or transmitted through the sample at their diagnostic energy.
Basic Methodology Preparation: ▶ Microfossils are often analyzed in 30–100 mm-thick polished sections preserving their mineral matrix. Nanoscale Focused Ion Beam (FIB) abrasion produces ultrathin (usually 100 nm thick, 11, and (2) in Scanning Transmission X-ray Microscopes (STXM), using samples thinner than 1 mm for light elements. Although several types of bonds may absorb at the same energy near the ▶ binding energy “edge” of a given element, functional groups can be further determined by combined analyses at the C, N, O, SK-edges and SL-edge. The high spatial resolutions of SXM (500 nm) and STXM (40 nm) allow in situ analyses of microfossils and their surrounding minerals with minimal phase overlap.
Key Research Findings Mineral Artefacts Abiotic minerals mimicking cellular morphologies and organizations (Livage 2009) can be observed in rocks (Fig. 3). Analyses of the textures, distributions, and compositions of organic matter within and around cellular structures (e.g., Figs. 1, 2, 4) can be used to argue against cell-like artifacts. Continuous cellular structures encapsulated and/or filled by minerals (Oehler et al. 2006; Schopf et al. 2006) support a microfossil origin, as opposed to intermineral organic matter filling voids (e.g., triple junctions) at grain boundaries (Lepot et al. 2009).
Age Combined in situ characterization has been used to seek textural relationships demonstrating a near contemporaneous origin of microfossils with the earliest fabrics of the rocks and denying late colonization (Lepot et al. 2009). Characterizations by Raman spectroscopy (Beyssac et al. 2002) or HRTEM (Oberlin et al. 1980) have been performed on organic matter from diverse thermal alteration settings, and have been used to correlate the structure of organic fossils with the metamorphic history of host rocks, hence demonstrating antiquity. Antiquity has also been assessed using abundances of heteroatoms (H/C, N/C, O/C) and functional (carboxylic or aliphatic relative to aromatic) groups.
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Microfossils, Analytical Techniques. Figure 3 Abiotic cell-like minerals. (a) rod- and (b) sphere-shaped silica inclusions in carbonates (2.7 and 3.5 Ga, respectively), SEM images of fractured (a, SE image) and polished (b, BSE image) surfaces of rocks. Such simple cell-like minerals are often observed with this technique
? 2 μm
Carbon
XANES
S Type A
STEM Carbonate matrix
Normalized intensity
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Sulfur Ca
S-poor OM intermineral
Type B S-rich globules O- / S-bearing groups
Fe S-rich
Mn Mineral S
Carbonate matrix S-poor 280
a
b
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300 Energy (eV)
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Microfossils, Analytical Techniques. Figure 4 Organic matter heterogeneities in 2.7 Ga-old stromatolites. (a) STEM image and mappings of C, Ca, Fe, Mn, and S, performed on a FIB section. These show dense cell-like organic fossils polymerized by organic sulfur (type B, solid arrowheads), and sulfur-poor organic matter (type A, empty arrowheads, mostly intermineral). (b) XANES carbon K-edge spectra of the carbonate matrix and of the two organic pools, indicative of highly aromatic carbon in both pools and sulfur- and oxygen-bearing functional groups only in cell-like structures. Chemical structures shown in b symbolize aromatic and thiophene groups
Ultrastructure
Chemistry
Diverse cell-wall ultrastructures indicating a high eukaryotic diversity have been observed in Precambrian microfossils assemblages by TEM imaging of ultrathin sections (Javaux et al. 2004). AFM has revealed that the ▶ cell wall of a 650 Ma-old microfossil is composed of 200 nm platelets oriented parallel to the radius, which might represent a biosignature (Kempe et al. 2002).
FTIR spectroscopy has distinguished some microfossil species in demineralized samples based on functional group concentrations (Marshall et al. 2005). The CH3/ CH2 ratio has been analyzed in situ (only carbonate minerals interfere with these peaks) and proposed as a taxonomic tracer (Igisu et al. 2009). SXM-XANES revealed organic sulfur in concentrations similar to living
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microorganisms in 700–800 Ma-old fossil cell walls, suggesting that the initial biogenic sulfur may be exceptionally preserved (Lemelle et al. 2008). Combined FIBsectioning, STEM-EDXS, and STXM-XANES analyses (Fig. 4) revealed dense organic fossils that might represent 2,700 Ma-old cells preserved by polymerization (Lepot et al. 2009), a process observed in dense dinoflagelate microfossils (Versteegh et al. 2004). EDXS also revealed micron-scale heterogeneities showing that these 2,700 Ma-old fossils rich in organic sulfur were polymerized by H2S, probably as a result of bacterial sulfate reduction, whereas the intermineral condensed organic matter has a significantly different chemistry. Combined TEM and STXM analyses revealed that chemical heterogeneities could be preserved in the organic wall of fossil plant spores through high-pressure metamorphism (Bernard et al. 2007). NanoSIMS revealed distinct N/C ratios in filamentous versus spheroidal (850 Ma-old) microfossils that might reflect different biological precursor: polysaccharide sheaths of filamentous organisms versus actual walls of spheroidal cells (Oehler et al. 2006). Using SIMS, carbon ▶ isotopic ratios (13C/12C) have been measured for several microfossils assemblages and have been linked with the metabolic activity of the organisms (Orphan & House 2009).
Future Directions Cell morphologies, ultrastructures, and their respective chemical signatures analyzed in situ may prove the biogenicity of microfossils and decipher their origin. These signatures must be constrained by the characterization of similar microfossils submitted to different alteration grades or settings, as well as artificially altered microbes. These techniques permit parallel study of all organic phases and rock-forming minerals. Such combined characterizations are necessary to constrain the environmental and geological context were these fossils were formed, selectively preserved, and altered, so as to interpret their biogenic signatures. Studying the micro- to nanostructure of crystals associated with organic fossils using these techniques may provide new biosignatures (Banfield et al. 2001; Benzerara et al. 2006; Lepot et al. 2008).
See also ▶ Ablation ▶ Absorption Spectroscopy ▶ Aromatic Hydrocarbon ▶ Binding Energy ▶ Biomarkers
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▶ Biomarkers, Isotopic ▶ Biomarkers, Morphological ▶ Biomarkers, Spectral ▶ Biomineralization ▶ Biosignatures, Effect of Metamorphism ▶ Carbon Isotopes as a Geochemical Tracer ▶ Cell Wall ▶ Fossilization, Process of ▶ Infrared Spectroscopy ▶ Isotopic Fractionation (Interstellar Medium) ▶ Isotopic Ratio ▶ Microfossils ▶ Polycyclic Aromatic Hydrocarbons ▶ Pseudofossil ▶ Raman Spectroscopy ▶ Spectroscopy ▶ Sputtering ▶ Synchrotron Radiation ▶ Taxonomy ▶ XANES
References and Further Reading Banfield JF, Moreau JW, Chan CS, Welch SA, Little B (2001) Mineralogical biosignatures and the search for life on Mars. Astrobiology 1:447–465 Benzerara K, Menguy N, Lo´pez-Garcı´a P, Yoon T-H, Kazmierczak J, Tyliszczak T, Guyot F, Brown GE Jr (2006) Nanoscale detection of organic signatures in carbonate microbialites. P Natl Acad Sci USA 103:9440–9445 Bernard S, Benzerara K, Beyssac O, Menguy N, Guyot F, Brown GE, Goffe B (2007) Exceptional preservation of fossil plant spores in high-pressure metamorphic rocks. Earth Planet Sci Lett 262:257–272 Beyssac O, Goffe B, Chopin C, Rouzaud JN (2002) Raman spectra of carbonaceous material in metasediments: a new geothermometer. J Metamorph Geol 20:859–871 Czaja AD, Kudryavtsev AB, Cody GD, Schopf JW (2009) Characterization of permineralized kerogen from an Eocene fossil fern. Org Geochem 40:353–364 Fletcher IR, Kilburn M, Rasmussen B (2008) NanoSIMS mm-scale in situ measurement of 13C/12C in early Precambrian organic matter, with permil precision. Int J Mass Spectrom 278:59–68 Igisu M, Ueno Y, Shimojima M, Nakashima S, Awramik SM, Ohta H, Maruyama S (2009) Micro-FTIR spectroscopic signatures of bacterial lipids in Proterozoic microfossils. Precambrian Res 173:19–26 Javaux EJ, Knoll AH, Walter MR (2004) TEM evidence for eukaryotic diversity in mid-Proterozoic oceans. Geobiology 2:121–132 Kempe A, Schopf JW, Altermann W, Kudryavtsev AB, Heckl WM (2002) Atomic force microscopy of Precambrian microscopic fossils. Proc Natl Acad Sci USA 99:9117–9120 Lemelle L, Labrot P, Salome M, Simionovici A, Viso M, Westall F (2008) In situ imaging of organic sulfur in 700–800 My-old Neoproterozoic microfossils using x-ray spectromicroscopy at the SK-edge. Org Geochem 39:188–202 Lepot K, Benzerara K, Brown GE, Philippot P (2008) Microbially influenced formation of 2, 724 million years old stromatolites. Nat Geosci 1:118–121
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Lepot K, Benzerara K, Rividi N, Cotte M, Brown GE, Philippot P (2009) Organic matter heterogeneities in 2.72 Ga stromatolites: alteration versus preservation by sulphur incorporation. Geochim Cosmochim Acta 73:6579–6599 Livage J (2009) Chemical synthesis of biomimetic forms. CR Palevol 8:629–636 Marshall CP, Javaux EJ, Knoll AH, Walter MR (2005) Combined microFourier transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy of Proterozoic acritarchs: a new approach to palaeobiology. Precambrian Res 138:208–224 Oberlin A, Boulmier JL, Villey M (1980) Electron microscopic study of kerogen microtexture. Selected criteria for determining the evolution path and evolution stage of kerogen. In: Durand B (ed) Kerogen: insoluble organic matter from sedimentary rocks. Editions technip, Paris, pp 191–204 Oehler DZ, Robert F, Mostefaoui S, Meibom A, Selo M, McKay DS (2006) Chemical mapping of proterozoic organic matter at submicron spatial resolution. Astrobiology 6:838–850 Orphan VJ, House CH (2009) Geobiological investigations using secondary ion mass spectrometry: microanalysis of extant and paleomicrobial processes. Geobiology 7:360–372 Schopf JW, Tripathi AB, Kudryavtsev AB (2006) Three-dimensional confocal optical imagery of Precambrian microscopic organisms. Astrobiology 6:1–46 Versteegh GJM, Blokker P, Wood GD, Collinson ME, Sinninghe Damste JS, de Leeuw JW (2004) An example of oxidative polymerization of unsaturated fatty acids as a preservation pathway for dinoflagellate organic matter. Org Geochem 35:1129–1139
MicroFUN ▶ Microlensing Follow-Up Network
Microgravity DAVID M. KLAUS Aerospace Engineering Sciences Department, University of Colorado/429 UCB, Boulder, CO, USA
Synonyms Freefall; Micro-g; Weightlessness; Zero-G
Keywords Drop tower, gravity, gravitational acceleration, parabolic flight, spacecraft, Spaceflight
Definition “Microgravity” (also commonly referred to as “Zero-G”) is broadly accepted as describing the condition of
weightlessness experienced during spaceflight. More precisely, this unique physical state arises from an apparent lack of gravitational acceleration occurring relative to a local inertial reference frame, but does not necessarily refer to a reduced level of gravity in an absolute sense. The root term “gravity” in this case is actually a unit of acceleration, with Earth’s gravitational attraction of 9.8 m/s2 defined as 1 g. The prefix “micro” can specifically indicate a factor of 106 or, more generally, just imply being very small.
Overview “Microgravity” (also commonly referred to as “Zero-G”) is broadly accepted as describing the condition of weightlessness experienced during spaceflight, on a parabolic aircraft trajectory, or under free-fall in a vertical drop tower. More precisely, this unique physical state arises from an apparent (or real) lack of gravitational acceleration that exists on a given mass relative to a local inertial reference frame (e.g., the spacecraft). The root term “gravity” actually refers to a unit of acceleration, with Earth’s gravitational attraction of 9.8 m/s2 defined as 1 g. The prefix “micro” can specifically indicate a factor of 106 or, more generally, just imply being very small. Any multiple or fraction of this unit of acceleration can be expressed in relative terms of g, for example, an astronaut might experience 3 g’s of acceleration during launch or 0.17 g on the lunar surface. It is important to note that “microgravity” does not necessarily refer to a reduction of the presence of gravity itself or to the gravitational constant (G = 6.672 1011 N·m2/kg2), which is neither a force nor an acceleration per se, rather a physical constant used to dimensionally derive the force (F12) resulting from the attraction of a mass (m1) on another mass (m2) a distance (r) away, defined as: F12 ¼ ðGm1 m2 Þr 2 As a result of this force, gravity can produce two effects on an object – motion and/or weight. The familiar force (F) equation derived from this relationship, taking into account the acceleration (a) that acts on a given mass (m), reduces to: F ¼ ma When resisted by an equal and opposite reaction, this relationship is used to define weight. When an equal and opposite reaction is not present, however, such as during orbital spaceflight or linear freefall in a vacuum, gravity instead gives rise to effectively unimpeded motion governed by either centripetal or linear acceleration,
Microlensing Planets
respectively. As a consequence, no weight is imparted to the mass, and within its inertial reference frame, it experiences a state of weightlessness attributed to the lack of relative acceleration, termed microgravity. For a spacecraft on a hyperbolic escape trajectory sufficiently far (r) from a dominant mass (m1) such that no significant gravitational acceleration is affecting it, Newton’s Law dictates that it will continue moving at a constant velocity until acted on by an external force. In this unique case, the absolute presence of gravity theoretically approaches zero. The microgravity environment experienced in Earth orbit has been used since the early 1960s to conduct studies on living organisms ranging from plants and microbes to humans. A wide variety of biological responses to weightlessness have been characterized by this research.
See also ▶ Gravitation ▶ Gravitational Biology ▶ Planetary and Space Simulation Facilities ▶ Space Biology ▶ Space Environment
References and Further Reading Brinckmann E (ed) (2007) Biology in space and life on Earth. Wiley-VCH, Weinheim Cle´ment G, Slenska K (eds) (2006) Fundamentals of space biology – research on cells, animals, and plants in space. Space Technology Library with Microcosm Press and Springer, El Segundo Gamow G (2003) Gravity, Dover Publications, Dover Ed edition, unabridged reprint of Gravity. Anchor Books, Doubleday, New York, 1962 Horneck G, Klaus D, Mancinelli RL (2010) Space microbiology. Mol Microbiol Rev 74:121–156 Klaus DM (2004) Gravitational influence on biomolecular engineering processes. Gravit Space Biol Bull 17(2):51–65
Microlensing Follow-Up Network Synonyms MicroFUN
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astronomers. MicroFUN has contributed to the discovery of the majority of the planets detected by microlensing. In 2008, MicroFUN announced the discovery (along with other collaborations) of the first planetary system discovered by microlensing, ▶ OGLE-2006-BLG-109Lb,c. This system of two planets closely resembles a scaled analog of Jupiter and Saturn.
See also ▶ Exoplanets, Discovery ▶ Microlensing Planets ▶ OGLE-2006-BLG-109Lb,c
Microlensing Observations in Astrophysics Synonyms MOA
Definition MOA is a collaboration of astronomers from New Zealand and Japan that employs the microlensing technique to search for dark matter and ▶ exoplanets. The Principal Investigator of the MOA collaboration is Yasushi Muraki of Nagoya University. MOA contributed to the discovery of nearly all of the planets discovered by microlensing. The MOA 1.8 m telescope is located at Mt. John Observatory on the south island of New Zealand.
See also ▶ Exoplanets, Discovery ▶ Microlensing Planets
Microlensing Planets ANDREW GOULD Department of Astronomy, Ohio State University, Columbus, OH, USA
Definition MicroFUN is an informal consortium of observers dedicated to photometric monitoring of interesting microlensing events in the Galactic bulge. The primary scientific objective is to observe high-magnification microlensing events in order to detect the signatures of ▶ exoplanets orbiting the primary lens. MicroFUN is some what unusual in that over half its members are amateur
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Keywords Einstein radius, galactic bulge, lens, light curve, source
Definition Gravitational microlensing differs from all other planetsearch techniques in that it does not depend on light from
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either the planet or its host. Recall that General Relativity predicts that the path of a light ray is bent by the gravitational effect of a mass in its path, which can act as a lens. Thus, planets are detected when the planet–host system (the “lens”) passes close to the line of sight of a more distant star (the “source”) and so splits the source light into multiple images whose changing magnification generates a distinctive “light curve” (e.g., Gould and Loeb 1992). With one major exception, this central characteristic of microlensing accounts for all its key features, both its advantages and its challenges.
Overview Because microlensing does not depend on host-star light, it is about equally sensitive to planets around all host-star types, not only FGKM dwarfs, but also ▶ brown dwarfs, white dwarfs, neutron stars, and black holes (Gould 2000). More precisely, it is sensitive in proportion to the square root of host mass; of course, its utility depends on the frequency of the host stars, with M dwarfs being the most common type of host star. Indeed, microlensing is sensitive to free-floating planets (Di Stefano and Scalzo 1999), which some theories suggest are copiously ejected from planetary systems during and shortly after their formation (e.g., Ford and Razio 2008). The indifference of microlensing to host light also makes it sensitive to planets of extremely distant hosts, in other parts of the Milky Way, and even in other galaxies such as the Large Magellanic Cloud and M31 (e.g., Covone et al. 2000). While microlensing (like all techniques) is less sensitive to planets of lower mass, this decline is actually much shallower than for other techniques, with no real break point until reaching Mars-mass planets (Bennett and Rhie 1996). When combined, these three advantages make microlensing an excellent method to probe planetary systems over an extremely wide range of host masses, planet masses, and galactic environments. In particular, microlensing is sensitive to analogs of all Solar System planets except Mercury. By the same token, however, microlensing’s indifference to host light implies that the host is generally not detected. The problem is not simply that the host may be very faint: it also lies directly superposed on the (generally much brighter) source and only moves away at a few milliarcseconds (mas) per year, so that even with adaptive optics it cannot be directly observed until of order 10 years after the event. Thus, although lightcurve analysis routinely returns the planet–host mass ratio, as well as their projected separation in units of the so-called Einstein radius (see next paragraph), these cannot be translated into a planet mass and physical projected separation,
because the host mass and distance, as well as the angular Einstein radius remain unknown. At least this is what was predicted for microlensing planets when searches were proposed 2 decades ago. The one major feature that is not connected to microlensing’s indifference to host-star light is the characteristic size of the Einstein radius. This scale is extremely important because the physics of microlensing (outlined below) dictates that sensitivity peaks when the planet–host separation is near this scale. The Einstein radius is basically the geometric mean of the Schwarzschild radius of the host (the radius at which, if compressed to this extent, the host would become a black hole) and a characteristic distance (basically the lesser of the lens-source and lensobserver distances). In principle, this distance could have been anything. But, by pure chance, it is typically about twice the ▶ snow-line distance (Gould and Loeb 1992), meaning that microlensing is most sensitive to planets in just the region where the core-accretion paradigm predicts the most robust ▶ planet formation.
Basic Methodology The physics of microlensing planet detection is best understood by first considering an isolated lens (no planet) lying exactly along the line of sight from the observer to the source. By axial symmetry, the source light will be bent into a ring surrounding the lens, that is, the so-called Einstein ring. If the source is now displaced slightly from the axis, the ring will break up into two magnified images on opposite sides of the lens, one just outside and the other just inside the Einstein ring. As the source is further displaced, the inner image will approach the lens and shrink in size until it eventually vanishes, while the outer image will approach the source, in both position, size, and shape. That is, the images will return to their unmagnified state. So during a microlensing “event,” the images first grow and then shrink in size. Since their separation is generally too small to resolve, the only recognizable effect is a change in brightness, a symmetrical light curve that first rises and then falls. If the lens has a planet, and if one of the two images passes close to the planet, then the planet will further deflect the source light, breaking up that image into additional images and so changing the magnification. This is why the sensitivity peaks at the Einstein radius: a planet lying at this separation is pre-positioned to distort one of the images when it is at its largest and is most unstable. It also explains why microlensing retains some sensitivity at arbitrarily large (even infinite) separation: the outer image never gets smaller than the source even as it moves far
Microlensing Planets
outside the Einstein radius. And it also explains why microlensing is insensitive to Mercury analogs: as the inner image moves far inside the Einstein ring (where it would in principle be sensitive to Mercury), it becomes so small that its “corruption” by Mercury yields no noticeable effect (Fig. 1). The duration of the main event is proportional to the square root of the host mass and is typically of order a few weeks. The duration of the planetary perturbation to this light curve is typically proportional to the square root of the planet mass and is thus shorter than the event timescale by the square root of the planet/star mass ratio. That is, of order 1 day for Jupiters and a few hours for Earths. Microlensing therefore requires intensive round-theclock monitoring of promising events in order to catch these fleeting signatures (Gould and Loeb 1992). And hence it requires complex cooperation among dozens of astronomers around the world. Some organize huge surveys, primarily of the dense star fields toward the Galactic bulge. The alignment requirements of microlensing are so stringent that even with the extreme projected density of lenses toward the bulge, the chance that any given source lies within the Einstein ring of some lens is only about one part in a million (Kiraga and Paczyn´ski 1994). Hence, these survey teams originally spent all night covering the bulge, once per night, even though they were equipped with comparatively wide-field cameras. By viewing 108 stars per night, they were able to find of order 1,000 events per year (e.g., Udalski et al. 2000; Hamadache et al. 2006). But these surveys, by themselves, could not detect many planets because they would sample only of order one point
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Key Research Findings As of 2010, about 15 microlensing planets had been discovered, of which 10 had been published (Bond et al. 2004; Udalski et al. 2005; Beaulieu et al. 2006; Gould et al. 2006; Gaudi et al. 2008; Bennett et al. 2008; Dong et al. 2009b; Janczak et al. 2010; Sumi et al. 2009). These planets have both confirmed microlensing’s promise and, in some very unexpected ways, exceeded them. As expected, the hosts are late type stars, mostly M dwarfs. Three of the detected planets are “cold Neptunes,” that is, planets lying beyond the snow line with planet–host mass ratios of about 104, similar to the Neptune–Sun ratio (Beaulieu et al. 2006; Gould et al. 2006; Sumi et al. 2009). This class of planets could not have been discovered by any other method. One two-planet system has been discovered, which is a nearly perfect analog of the Sun/Jupiter/Saturn system, that is, the two planets have the same mass ratios as Jupiter and Saturn as well as the same ratio of orbital radii, and the same (inferred) equilibrium temperatures (Gaudi et al. 2008; Bennett et al. 2010). The only difference is that the whole system is scaled down according to the host mass of
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on the planetary perturbation. So other teams formed to choose favorable events and intensively monitor them, round the clock, using telescopes on several continents (e.g., Albrow et al. 1998; Tsapras et al. 2009). The lightcurve analysis can be quite complex, sometimes requiring 10,000 or even 100,000 processor hours. Large interacting teams have evolved to carry out data reduction, event prioritization, coordination of observations, and other tasks.
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0.5 solar masses. Yet another microlensing planet is a 3-Jupiter mass object orbiting a late M dwarf, which is difficult to explain with the dominant ▶ core-accretion theory of planetary formation (Dong et al. 2009a).
Applications As indicated by the descriptions of these results (and contrary to initial expectations), it has proved possible to measure the lens mass and distance in some cases and to strongly constrain them in most others. The ideal path toward doing this is to measure the Einstein radius projected on the sky (“angular Einstein radius”) and projected on the observer plane (“projected Einstein radius”). The product of these two quantities gives the lens mass while their ratio gives the lens-source relative parallax (Gould 1992, 1994). Since the sources almost always lie in the Galactic bulge (which surrounds the center of our Milky Way Galaxy), the source parallax is approximately known, meaning that the lens distance would also be known. Now, for planetary events (in strong contrast to “normal” events), the angular Einstein radius is almost always measured. This is because the planetary perturbations have structure that is of order the source size, so the resulting light curve automatically yields the ratio of the source radius to Einstein radius. Since the angular source radius is known from its measured color and magnitude, the Einstein radius is known also. The main difficulty, then, is measuring the projected Einstein radius, which is of order several astronomical units. This could be done routinely if interplanetary probes were directed to take a few images of planetary microlensing events (Gould 1995). Since the probes are typically several AU from Earth, the event would look substantially different as seen from the satellite and Earth, and from the magnitude of this difference one could measure the Einstein radius projected on the observer plane. In a few cases, it has been possible to make use of the moving platform of the Earth to measure this “microlens parallax” effect (Gould 1992), or even to exploit the tiny lightcurve differences in observations of the event from different locations on Earth (Holz and Wald 1996), or to extract the mass and distance from yet other effects. But routine mass measurements will require access to Solar-System-probe cameras.
Future Directions Microlensing is currently undergoing a revolution. Both major survey groups have started observations with very wide-field cameras (2.2 and 1.5 square degrees), which allows them to monitor the most prolific few square degrees several times per hour, and another 10 or more
square degrees once per hour. This means that all of the roughly 1,000 events that are discovered each year are monitored “continuously” for large fractions of each 24-h cycle, even without follow-up observations by other teams. Since the follow-up teams can monitor at most a few dozen events per year, this should lead to a big increase in planet detection. Future, already-funded telescopes will monitor 16 square degrees even more frequently, once per 10 min. For the further future, there are well-developed proposals for microlensing planet searches from space, which could increase the planet-detection rate by yet a further factor of 10 (Bennett and Rhie 2002). These could be stand-alone missions, or share time on a wide-field space camera that could also probe dark energy from weak-lensing measurements (e.g., the so-called WFIRST mission).
See also ▶ Brown Dwarfs ▶ Core Accretion (Model for Giant Planet Formation) ▶ Dwarf Stars ▶ Exoplanets, Discovery ▶ Microlensing Follow-Up Network ▶ Microlensing Observations in Astrophysics ▶ OGLE-2005-BLG-390Lb ▶ OGLE-2006-BLG-109Lb,c ▶ Optical Gravitational Lensing Experiment ▶ Planet Formation ▶ Probing Lensing Anomalies Network ▶ Snow Line ▶ Spectral Type
References and Further Reading Albrow M et al (1998) Astrophys J 509:687 Beaulieu JP et al (2006) Nature 439:437 Bennett DP, Rhie SH (1996) Astrophys J 472:660 Bennett DP, Rhie SH (2002) Astrophys J 574:985 Bennett DP et al (2008) Astrophys J 684:663 Bennett DP et al (2010) Astrophys J 713:837 Bond IA et al (2004) Astrophys J Lett 606:L155 Covone G et al (2000) Astron Astrophys 357:816 Di Stefano R, Scalzo RA (1999) Astrophys J 512:564 Dong S et al (2009a) Astrophys J 695:970 Dong S et al (2009b) Astrophys J 698:1826 Ford EB, Razio FA (2008) Astrophys J 686:621 Gaudi BS et al (2008) Science 319:927 Gould A (1992) Astrophys J 392:442 Gould (1994) Astrophys J Lett 421:L71 Gould A (1995) Astrophys J Lett 441:L21 Gould A (2000) Astrophys J 535:928 Gould A, Loeb A (1992) Astrophys J 396:104 Gould A et al (2006) Astrophys J Lett 644:L37 Hamadache C et al (2006) Astron Astrophys 454:185 Holz DE, Wald RM (1996) Astrophys J 471:64
Micrometeorites Janczak J et al (2010) Astrophys J 711:731 Kiraga M, Paczyn´ski B (1994) Astrophys J Lett 430:L101 Sumi T et al (2009) Astrophys J 710:1641 Tsapras Y et al (2009) Astron Nachr 330:4 Udalski A et al (2000) Acta Astron 50:1 Udalski A et al (2005) Astrophys J 603:139
Micrometeorites FRANC¸OIS ROBERT Laboratoire de Mine´ralogie et Cosmochimie du Muse´um (LMCM), Muse´um National d’Histoire Naturelle, UMR 7202 CNRS, Paris Cedex 05, France
Synonyms Interplanetary Dust Particles
Keywords Extraterrestrial matter, Interplanetary Dust Particles
▶ meteorites,
comets,
Definition Micrometeorites are micron-size fragments of extraterrestrial matter. It is now generally accepted that their origin is cometary.
Overview The advent on Earth of microscopic extraterrestrial grains has been discussed since the late nineteenth century with the discovery by oceanographic missions of cosmic spherules. These spherules were collected with a magnet by dragging the surface of marine sediments at depth in the oceans. The extraterrestrial origin of these spherules was nevertheless a subject of debate and was not accepted until the emergence in the late 1980s of isotopic mass spectrometers. On Earth an average of 40 t/day of extraterrestrial material falls on the ground. The Earth-falling particles (referred to as IDPs and standing for Interplanetary Dust Particles) are collected (1) in the Earth’s stratosphere using plate collectors under the wings of NASA airplanes, (2) in large surface deposits in Antarctica (or Greenland/Arctic), and (3) in deep-sea sediments. In the early eighties, Don Brownlee and Michel Maurette identified unambiguously the extraterrestrial nature of stratospheric particles and those collected in ice cores from Antarctica, respectively. Laboratory measurements of implanted rare gases, solar flare tracks, and isotope abundances have confirmed that
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the collected particles are indeed extraterrestrial and that, prior to atmospheric entry, they spent 104–105 years as small particles orbiting around the Sun. In space, dust detectors on planetary spacecraft have provided information on the flux of extraterrestrial matter, its distribution in size and its orbits. The large orbital velocities of particles in interplanetary space (10–40 km/s) prevent intact particle captures by space missions. ▶ IDPs collected in the stratosphere or in Antarctica are the most fine-grained meteoritic objects available for laboratory analyses. In contrast to meteorites, IDPs are derived from a broad range of bodies extending from the inner main belt of the asteroids to the ▶ Kuiper belt. These bodies produce dust by the continuous bombardment of (micro)meteorites on their surfaces and by the continuous implantation in their soils of energetic atoms emitted by the Sun – the ▶ solar wind. In that sense, IDPs reflect a sampling of the whole solar system, while meteorite sources are restricted to the asteroids belt located between Mars and Jupiter. The presence of cometary IDPs is now demonstrated. They are released in space by comets when they enter the inner solar system. Comets are made of up to 50% water ice that sublimates when they come closer and closer to the Sun, causing an increase in their surface temperature. This massive sublimation of water – along with many other volatile species – causes the injection of cometary dust into space. The chemical and mineralogical nature of this dust has been studied by different spectroscopic methods from the ground and from space. One space mission – ▶ StarDust – has collected these grains directly in the coma of a well-identified comet and has returned them to Earth. It was thus possible to study these cometary grains with laboratory instruments that have no counterpart in instruments flown on spacecraft. Unpredictably, these grains bear resemblances to some specific IDPs. This identification has opened the possibility of studying cometary matter in great detail, since the number and size of IPDs collected on Earth and available for chemical analyses is almost infinite compare to the limited mass of matter returned to Earth by StarDust. During their atmospheric entry, most IDPs are heated to their peak temperature for 1 s, but the smallest particles are the least strongly heated. Therefore, and contrary to meteoritic material, the most fragile IDPs can survive their entry into the atmosphere depending on their size, mass, entry angle, and relative velocity, which dictate their heating temperature. Surprisingly some fluffy, extremely fragile IDPs land on the ground with almost no alteration of their pre-atmospheric structure and composition.
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From a mineralogical survey of chondritic IDPs studied in detail using current electron microscopy techniques, it has been shown that some of these represent unique groups of extraterrestrial materials. These IDPs differ significantly in grain size and texture from carbonaceous chondrites, and they contain mineral assemblages that do not exist in any meteorite class. Electron microscopy has established that these IDPs existed in space as small objects, that they endured minimal alteration by planetary processes since their formation, and that they were never heated to temperatures above 600 C for more than 1 s on entering Earth’s atmosphere. Their probable sources are comets or asteroids that have not been sampled by any meteorites in the collections. These IDPs occur in two predominant mineral assemblages: (1) carbonaceous phases associated with phyllosilicates and (2) carbonaceous phases and olivines or pyroxenes. Both types of silicate assemblages are nevertheless sometimes observed together. The mineralogy of the IDP silicates is common to all carbonaceous chondrites: olivine, pyroxene, layer-lattice silicates, and carbon-rich phases. Individual mineral grain sizes range from micrometers (primarily pyroxenes and olivines) to nanometers, with the predominant size for all phases less than 100 nm. Mineralogical signatures provide an indication of processes consistent with condensation from a vapor phase at relatively low temperatures (200 million years before the first significant rise in atmospheric oxygen concentrations at 2.5 Ga. (5) The discovery of the steroid, 24-isopropylcholestane, a molecular fossil of demosponges, Neoproterozoic Maronian glacial cap carbonate rocks, pushes back the record of animals on Earth to 635 Ma, making it the oldest evidence for animals (Love et al. 2009).
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potential of extraterrestrial organic molecules. Moreover, organic geochemical studies provide the foundation for understanding sources (biogenic or abiogenic) as well as surface and subsurface alteration processes. A caveat to extraterrestrial studies will be confirmation that the molecules detected are indigenous and were not transported from Earth.
Future Directions Contemporary interdisciplinary studies that integrate lipid biomarkers, isotopic compositions, microbial physiology and ecology, and microbial genomics are advancing our understanding of molecular biomarker specificity for phylogenetic groups and environmental conditions. Concurrent research efforts that focus on the fate of molecular biomarkers from origin to deposition and preservation in the geological record are providing essential insight for detecting and improving the recovery of molecular fossil records. Both lines of study aim to revolutionize our understanding of evolution of the biosphere on Earth and will aid in the exploration, detection, and establishment of biogenetic confirmation criteria for molecular biomarkers and their fossils in extraterrestrial sediments should they exist.
Applications
See also
On Earth, molecular fossils are used to understand a variety of aspects about ancient life (cf. Gaines et al. 2009). They are particularly valuable in reconstructing ancient environmental conditions, community composition, and ecologies – all of which are useful in paleoclimate studies (e.g., Zachos et al. 2006), constraining biological evolution (e.g., Brocks et al. 1999 and Love et al. 2009), understanding catastrophic and global events (e.g., Grice et al. 2005 and Eigenbrode et al. 2008), and correlating oil source rocks with petroleum reservoirs (cf. Peters et al. 2007). Molecular fossils are particularly useful as markers for microbial contributions to sediments (cf. Brocks and Summons 2003), and their isotopic compositions can reveal associated metabolisms and biogeochemical cycling (e.g., Freeman et al. 1990; Pancost and Sinninghe Damste´ 2003). In extraterrestrial studies, the identification of molecular fossils as definitive biosignatures will be a particularly difficult challenge in the absence of a broad understanding of the biochemistry, physiology, and ecology of modern extraterrestrial life. However, molecular biomarker and molecular fossil studies on Earth reveal generic traits of life (Summons et al. 2007) and ecology (Eigenbrode 2007) that will be essential in evaluating the biosignature
▶ Archea ▶ Archean Traces of Life ▶ Aromatic Hydrocarbon ▶ Bacteria ▶ Biomarkers ▶ Cyanobacteria, Diversity and Evolution of ▶ Ecosystem ▶ Eukarya ▶ Eukaryotes, Appearance and Early Evolution of ▶ Fortescue Group ▶ Geological Timescale ▶ Hopanes, Geological Record of ▶ Hydrocarbons ▶ Isoprenoids ▶ Kerogen ▶ Kerogen-Like Matter ▶ Membrane ▶ Metabolism (Biological) ▶ Methanotroph ▶ Microbial Mats ▶ Photosynthesis ▶ Photosynthetic Pigments ▶ Phototroph ▶ Porphyrin
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▶ Steranes, Rock Record ▶ Syngenicity ▶ Tumbiana Formation (Pilbara, Western Australia)
References and Further Reading Adam P, Schmid JC, Mycke B, Strazialle C, Connan J, Huc A, Riva A, Albrecht P (1993) Structural investigations of nonpolar sulfur crosslinked macromolecules in petroleum. Geochim Cosmochim Acta 57:3395–3419 Brocks JJ, Summons RE (2003) Sedimentary hydrocarbons, biomarkers for early life. In: Holland HD, Turekian K (eds) Treatise in geochemistry., pp 65–115, Ch. 8.03 Brocks JJ, Logan GA, Buick R, Summons RE (1999) Archean molecular fossils and the early rise of eukaryotes. Science 285:1033–1036 Eglington G, Pancost R (2004) Immortal molecules. Geoscientist 14:4–16 Eigenbrode JL (2007) Fossil lipids for life-detection: a case study from the early Earth record. Space Sci Rev 135:161–185 Eigenbrode JL, Freeman KH, Summons RE (2008) Methylhopane biomarker hydrocarbons in Hamersley province sediments provide evidence for Neoarchean aerobiosis. Earth Planet Sci Lett 273:323–331 Fischer WW, Summons RE, Pearson A (2005) Targeted genomic detection of biosynthetic pathways: anaerobic production of hopanoid biomarkers by a common sedimentary microbe. Geobiology 3:33–40 Freeman KH, Hayes JM, Trendel JM, Albrecht P (1990) Evidence from carbon-isotope measurements for diverse origins of sedimentary hydrocarbons. Nature 343:254–256 Gaines SM, Eglinton G, Rullko¨tter J (2009) Echoes of life: what fossil molecules reveal about earth history. Oxford University Press, New York, 375 pp Grice K, Cao C, Love GD, Bo¨ttcher ME, Twitchett RJ, Grosjean E, Summons RE, Turgeon SC, Dunning W, Jin Y (2005) Photic zone Euxinia during the Permian-triassic superanoxic event. Science 307:706–709 Hayes JM, Freeman KH, Popp BN, Hoham CH (1990) Compoundspecific isotopic analysis–A novel tool for reconstruction of ancient biogeochemical processes. Org Geochem 16:1115–1128 Hebting Y, Schaeffer P, Behrens A, Adam P, Schmitt G, Schneckenburger P, Bernasconi SM, Albrecht P (2006) Biomarker evidence for a major preservation pathway of sedimentary organic carbon. Science 312:1627–1631 House CH, Runnegar B, Fitz-Gibbon ST (2001) Geobiological analysis using whole genome-based tree building applied to the Bacteria, Archaea, and Eukarya. Geobiol 1:15–26 Kvenvolden KA (2002) History of the recognition of organic geochemistry in geoscience. Org Geochem 33:517–521 Logan GA, Hayes JM, Hieshima GB, Summons RE (1995) Terminal proterozoic reorganisation of biogeochemical cycles. Nature 376:53–56 Love GD, Grosjean E, Stalvies C, Fike DA, Bradley AS, Bhatia M, Meredith W, Snape CE, Bowring SA, Condon DJ, Grotzinger JP, Summons RE (2009) Fossil steroids record the appearance of Demospongiae during the Cryogenian period. Nature 457:718–721 Pancost RD, Sinninghe Damste´ JS (2003) Carbon isotopic compositions of prokaryotic lipids as tracers of carbon cycling in diverse settings. Chem Geol 195:29–58 Pearson A, Budin M, Brocks JJ (2003) Phylogenetic biochemical evidence for sterol synthesis in the bacterium Gemmata obsuriglobus. Proc Natl Acad Sci USA 100:15352–15357
Peters KE, Walters CC, Moldowan JM (2007) The biomarker guide: biomarkers and isotopes in petroleum systems and earth history. Cambridge University Press, Cambridge, 704 pp Sinninghe Damste´ JS, Rijpstra WIC, Kock-Van Dalen AC, de Leeuw JW, Schenck PA (1989) Quenching of labile functionalized lipids by inorganic sulfur species – evidence for the formation of sedimentary organic-sulfur compounds at the early stages of diagenesis. Geochim Cosmochim Acta 53:1343–1355 Summons RE, Jahnke LL (1992) Hopenes and hopanes methylated in ring-A: correlation of the hopanoids of extant methylotrophic bacteria with their fossil analogues. In: Moldowan JM, Albrecht P, Philp RP (eds) Biomarkers in sediments and petroleum. Prentice Hall, Englewood Cliffs, pp 182–200 Summons RE, Volkman JK, Boreham CJ (1987) Dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum. Geochim Cosmochim Acta 51:3075–3082 Summons RE, Albrecht P, McDonald G, Moldowan JM (2007) Molecular biosignatures: generic qualities of organic compounds that betray biological origins. Space Sci Rev 135:133–157 Waldbauer JR, Sherman LS, Sumner DY, Summons RE (2009) Late archean molecular fossils from the transvaal supergroup record the antiquity of microbial diversity and aerobiosis. Precambrian Res 169:28–47 Welander PV, Hunter RC, Zhang LC, Sessions AL, Summons RE, Newman DK (2009) Hopanoids play a role in membrane integrity and pH homeostasis in Rhodopseudomonas palustris TIE-1. J Bacteriol 191:6145–6156 Welander PV, Coleman ML, Sessions AL, Summons RE, Newman DK (2010) Identification of a methylase required for 2-methylhopanoid production and implications for the interpretation of sedimentary hopanes. Proc Natl Acad Sci USA 107:8537–8542 Wolf YI, Rogozin IB, Grishin NV, Koonin EV (2002) Genome trees and the tree of life. Trends Genetics 18:472–479 Zachos JC, Schouten S, Bohaty S, Quattlebaum T, Sluijs A, Brinkhuis H, Gibbs S, Bralower T (2006) Extreme warming of mid-latitude coastal ocean during the Paleocene-Eocene thermal maximum: inferences from TEX86 and isotope data. Geology 34:737–740
Molecular Interactions ▶ Van Der Waals Forces
Molecular Line Cooling STEVEN B. CHARNLEY NASA Goddard Space Flight Center, Solar System Exploration Division, Code 691, Astrochemistry Laboratory, Greenbelt, MD, USA
Synonyms Radiative cooling
Molecular Line Surveys
Keywords Interstellar medium, molecules
Definition Molecular line cooling is the physical process whereby deexcitation of collisionally or radiatively excited molecules leads to the emission of a photon, followed by its escape from the local cloud volume, thus cooling the local gas.
Overview In astronomical environments where most of the hydrogen is molecular, the gas can cool through emission of the photons associated with transitions from higher to lower rotational and vibrational levels in molecules. In cold ▶ molecular clouds, the major molecular coolant is CO. In shock waves, H2, H2O, and CO dominate the molecular cooling. Other molecules that can act as relatively minor coolants are OH, HD, and CH.
See also ▶ Molecular Cloud ▶ Shocks, Interstellar
References and Further Reading Hollenbach DJ, Thronson HA Jr (eds) (1987) Interstellar processes. D. Reidel, Dordrecht Kwok S (2007) Physics and chemistry of the interstellar medium. University Science, Herndon Tielens AGM (2005) The physics and chemistry of the interstellar medium. Cambridge University Press, Cambridge
Molecular Line Maps Definition Molecular line maps are spatial representations of molecular emission from an astronomical source. They are normally represented in the astronomical coordinates Right Ascension and Declination, or in offsets from a defined position. Molecular lines are observed either on a predefined set of grid points or by continuous scanning (On-The-Fly mapping), and a map is constructed from the emission intensity, either as contours or on gray/ color scales.
See also ▶ Line Profile ▶ Spectral Line
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Molecular Line Surveys ANTHONY J. REMIJAN NRAO, Charlottesville, VA, USA
Synonyms Band scans; Spectral surveys
Keywords Chemistry, Green Bank Telescope, interstellar medium, molecules, prebiotic species
Definition Molecular line surveys are studies of the spectra of astronomical sources over a wide, approximately continuous, range of wavelengths, in order to determine the chemical composition and physical properties (temperature, density, internal motion) of such regions. Primarily the lower energy transitions of molecules of astrophysical interest are excited at the cold temperatures of ▶ molecular clouds, and these rotational transitions are typically at millimeter and submillimeter wavelengths, so that molecular line surveys have normally been carried out using the techniques of high-frequency radio astronomy.
History Until recently, radio receivers had instantaneous spectral bandwidths that covered only very small fractions of the wavelength regions of interest. Consequently, searches for and studies of interstellar molecules have traditionally been targeted toward the narrow regions around laboratory-measured transition frequencies of specific molecules. In contrast, spectral line surveys were very time-consuming and could only be carried out at those radio observatories that were willing to devote significant observing time to a survey project. The first spectral line survey was made during 1977–1982 toward the giant molecular cloud known as Sagittarius B2 (Sgr B2) near the center of the Milky Way Galaxy (Cummins et al. 1986), using an antenna at the Bell Laboratories, USA. The first major projects with the new 20-m-diameter radio telescope at the Onsala Space Observatory in Sweden were line surveys of the nearest region of massive star formation to the Sun, Orion-KL, and the envelope expelled by the evolved carbon star, IRC + 10216 (Johansson et al. 1984). This survey found striking chemical heterogeneity within the Orion molecular cloud. These studies were extended to both higher and lower frequencies in
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subsequent surveys (e.g., Blake et al. 1987; Turner 1991; Nummelin et al. 1998; Cernicharo et al. 2000; Kawaguchi et al. 1995; Ziurys and McGonagle 1993; Lee et al. 2001) and to additional sources in our Galaxy (e.g., Kaifu et al. 2004; Pardo et al. 2007; Helmich and van Dishoeck 1997; Macdonald et al. 1996; Van Dishoeck et al. 1995). More recently, the Orion molecular cloud has been surveyed at frequencies blocked by the Earth’s atmosphere, using the Odin satellite (Olofsson et al. 2007), and spectral line surveys have been extended to external galaxies (Martı´n et al. 2006).
Overview There are many specific reasons why deep spectral line surveys are important and timely and could significantly advance the field of astrobiology. Building up the chemical inventory of astronomical molecules is important to understand how organic matter is produced in the interstellar space. Surveys not only provide probes of physical and chemical conditions, but they also discover new interstellar species. Although significant progress has been made in understanding the chemical content and formation mechanisms in interstellar clouds, yet we still cannot predict with certainty which molecular species will be present. A number of species such as the 3-carbon sugar glyceraldehyde and the simplest amino acid, glycine, have not yet been confirmed in astronomical environments, even though models and the presence of precursor species suggest that these molecules should be present. Surveys offer the possibility of ensemble averages of transitions, which may be the only way to make detections of the largest, most complex species for which the population is distributed over many energy states. We concentrate in the following on the molecular line survey of Sagittarius B2(N) near the center of our Galaxy, which is currently underway with the Green Bank Telescope (GBT) at the US National Radio Astronomy Observatory. Sagittarius B2(N) is a preeminent source for the study of large complex interstellar molecules. Of the 160 astronomical molecules detected to date, more than half have been discovered in this region. The Sagittarius B2 complex contains compact hot molecular cores, molecular maser emitting regions, and ultracompact sources of continuum radiation surrounded by larger-scale continuum features, as well as more extended molecular material.
Key Research Findings Successful dedicated searches with the GBT over the last several years have led to the detections of interstellar aldehydes, namely cyanoformaldehyde (CNCHO; Remijan et al. 2008), propenal (CH2CHCHO), and
propanal (CH3CH2CHO; Hollis et al. 2004a); simple aldehyde sugars like glycolaldehyde (CH2OHCHO; Hollis et al. 2004b); the first keto ring molecule to be found in an interstellar cloud, cyclopropenone (c-H2C3O; Hollis et al. 2006a); the third organic imine to be detected in an interstellar cloud, ketenimine (CH2CNH; Lovas et al. 2006); and acetamide, the largest interstellar molecule detected with a peptide bond (CH3CONH2; Hollis et al. 2006b). Above all else, these detections have shown that a suite of receivers working in synergy is necessary to start investigating the questions of the formation, excitation, and distribution of complex organic molecules in astronomical environments.
Basic Methodology The survey of the Sagittarius B2(N) region currently underway with the GBT has been dubbed the PRebiotic Interstellar MOlecule Survey (PRIMOS). The data taken from the survey are made available quarterly. Dissemination of the fully calibrated data products are available either in simple ASCII text or available through the Spectral Line Search Engine (SLiSE), a dedicated data display java-based tool. SLiSE contains functions to overlay possible molecule identifications based on a current line catalog as well as overlaying H and He recombination lines. For a full description of the PRIMOS project and the SLiSE tool, visit http://www.cv.nrao.edu/˜aremijan/ PRIMOS/.
Future Directions Organic materials found in meteorites are considered to be indicative of the chemicals that are present in extraterrestrial sources, such as molecular clouds. A recent study of the Murchison meteorite by Cooper et al. (2001) finds sugars, sugar alcohols (polyols), sugar acids, and other related species with gas chromatography–mass spectroscopy techniques. The monosaccharide sugars have an empirical formula of CnH2nOn, where n is integer, and take on both aldehyde and ketone forms and can occur as chains or rings (for n = 4 and greater). Some of the sugars take on an important role in biochemistry of Earth. For example, ribanose (C5H10O5) is an important group in RNA and is obtained by the hydrolysis of RNA. In space, the simplest sugar, glycolaldehyde (CH2OHCHO) was detected in the Sagittarius B2(N) molecular cloud, but the next largest sugar, glyceraldehyde, has been sought but not thus far detected. Nonetheless, given the importance of sugars to both terrestrial and interstellar chemistry, the search for more complex sugars, including the base of RNA, will continue into the next generation of astronomical instruments.
Molecular Recognition
See also ▶ Interstellar Medium ▶ Molecular Cloud ▶ Molecules in Space
References and Further Reading Blake GA, Sutton EC, Masson CR, Phillips TG (1987) Molecular abundances in OMNC-1: the chemical composition of interstellar molecular clouds and the influences of massive star formation. Astophys J 315:621–645 Cernicharo J, Guelin M, Kahane C (2000) A l2 MM molecular line survey of the C-star envelope IRC + 10216. J Astrophys Astron Suppl 142:181–215 Cooper G, Kimmich N, Belisle W, Josh S, Brabham K, Garrel L (2001) Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth. Nature 414:879–883 Cummins SE, Linke RA, Thaddeus P (1986) A survey of the millimeterwave spectrum of sagittarius B2. Astrophys J Suppl 60:819–878 Helmich FP, van Dishoeck EF (1997) physical and chemical variations within the W3 star-forming region. J Astrophys Astron Suppl 124:205–253 Hollis JM, Jewell PR, Lovas FJ, Remijan A, Møllendal H (2004a) Green bank telescope detection of new interstellar aldehydes: propenal and propanal. Astophys J 610:L21–L24 Hollis JM, Jewell PR, Lovas FJ, Remijan A (2004b) Green bank telescope observations of interstellar glycolaldehyde: low-temperature sugar. Astrophys J 613:L45–L48 Hollis JM, Remijan AJ, Jewell PR, Lovas FJ (2006a) Cyclopropenone (c-H2C3O): A new interstellar ring molecule. Astophys J 642:933–939 Hollis JM, Lovas FJ, Remijan AJ, Jewell PR, Ilyushin VV, Kleiner I (2006b) Detection of Acetamide (CH3CONH2): the largest interstellar molecule with a peptide bond. Astophys J 643:L25–L28 Johansson LEB, Andersson C, Ellder J, Friberg P, Hjalmarson A˚, Ho¨glund B, Irvine WM, Olofsson H, Rydbeck G (1984) Spectral scan of Orion A and IRC + 10216 from 72 to 91 GHz. Ast Astrophys 130:227–256 Kaifu N, Ohishi M, Kawaguchi K, Saito S, Yamamoto S, Mikaji T, Miyazawa K, Ishikawa S-I, Noumaru C, Harasawa S, Okuda M, Suzuki H (2004) A 8.8-50 GHz complete spectral line syrvey toward TMC-1. Pub Ast Soc Japan 56:69–173 Kawaguchi K, Kasai Y, Ishikawa S-I, Kaifu N (1995) A spectral-line survey observation of irc + 10216 between 28 and 50 Ghz. Publ Ast Soc Japan 47:853–876 Lee CW, Cho S-H, Lee S-M (2001) A spectral line survey from 138.3 to 150.7 GHz toward Orion-KL. Astophys J 551:333–346 Lovas FJ, Hollis JM, Remijan AJ, Jewell PR (2006) Detection of ketenimine (ch2cnh) in sagittarius b2(n) hot cores. Astophys J 645: L137–L140 Macdonald GH, Gibb AG, Habing RJ, Millar TJ (1996) A 330-360 GHz spectral survey of G 34.3 + 0.15. J Astrophys Astron Suppl 119:333–367 Martı´n S, Mauersberger R, Martı´n-Pintado J, Henkel C, Garcı´a-Burillo S (2006) A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253. Astrophys J Suppl Ser 164:450–476 Nummelin A, Bergman P, Hjalmarson A˚, Friberg P, Irvine WM, Millar TJ, Ohishi M, Saito S (1998) A three-position spectral line survey of sagittarius B2 between 218 and 263 GHz. Astrophys J Suppl 117:427–529 Olofsson AOH, Persson CM, Koning N, Bergman P, Bernath PF, Black JH, Frisk U, Geppert W, Hasegawa TI, Hjalmarson A˚, Kwok S, Larsson B, Lecacheux A, Nummelin A, Olberg M, Sandqvist AA, Wirstro¨m ES
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(2007) A spectral line survey of Orion KL in the bands 486–492 and 541–577 GHz with the Odin satellite. I. The observational data. J Astron Astrophys 476:791–806 Pardo JR, Cernicharo J, Goicoechea JR, Guelin M, Ramos AA (2007) Molecular line survey of CRL 618 from 80 to 276 GHz and complete model. Astophys J 661:250–261 Remijan AJ, Hollis JM, Lovas FJ, Stork WD, Jewell PR, Meier DS (2008) Detection of interstellar cyanoformaldehyde (CNCHO). Astophys J 675:L85–L88 Turner B (1991) A molecular line survey of sagittarius B2 and orion-KL from 70 to 115 GHz. Astrophys J Suppl 76:617–686 Van Dishoeck EF, Blake GA, Jansen DJ, Groesbeck TD (1995) Molecular abundances and low-mass star formation. Astophys J 447:760–782 Ziurys LM, McGonagle D (1993) The spectrum of orion-kl at 2 millimeters (150–160 GHz). Astrophys J Suppl 89:155–187
Molecular Nitrogen ▶ Dinitrogen
Molecular Oxygen ▶ Dioxygen
Molecular Phylogenetics ▶ Evolution, Molecular
Molecular Recognition Definition In chemistry and biology, molecular recognition is the specific interaction between two or more molecules through complementary noncovalent bonding, for example, via ▶ hydrogen bonding; metal coordination; ▶ van der Waals forces; and pp, hydrophobic, or electrostatic interactions. Molecular recognition plays an important role in biology and mediates interactions between receptors and ligands, antigens and ▶ antibodies, ▶ nucleic acids and ▶ proteins, proteins and proteins, ▶ enzymes and ▶ substrates, and nucleic acids with each other. A number of artificial systems have also been
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synthesized that depend on molecular recognition, for example, ▶ peptide nucleic acid and other antisense oligonucleotides.
See also ▶ Hydrogen Bond ▶ Ligand ▶ PNA ▶ Van Der Waals Forces ▶ Weak Bonds
or negatively (▶ anions). Space refers here to all regions outside the solar system. On occasion, we restrict the discussion only to the interstellar medium and circumstellar envelopes, thus excluding stellar atmospheres as well as extragalactic sources. Molecules in space usually, but not exclusively, refer to those in the gas phase.
Overview
HNCO
The spectra produced by molecules in space are used to characterize the physical and chemical properties of certain media in space. For example, certain atomic and molecular absorption lines in the visible region observed in stellar atmospheres are used to classify the stars. Atomic and molecular lines play an important role in star formation, as they help to dissipate the energy of molecular clouds. Lines of specific molecules are used to probe the density, temperature, or ionization degree of the ▶ interstellar medium or of circumstellar envelopes. Complex molecules are of particular interest for astrobiology, as their abundances may provide clues to which extent interstellar chemistry may have played a role in the formation of life on Earth or possibly on other planets. The Molecules in Space web page (http://www.astro. uni-koeln.de/cdms/molecules) of the Cologne Database for Molecular ▶ Spectroscopy, CDMS (http://www.astro. uni-koeln.de/cdms/; short-cut: www.cdms.de) (Mu¨ller et al. 2001, 2005) features an up-to-date table with more than 150 molecular species detected in the interstellar medium or in circumstellar envelopes, as well as an up-todate table with almost 40 molecules detected in extragalactic sources. Tables 1 and 2 were initially intended as supplementary material, accurate as of May 2010. Another useful resource is the web page A HyperBibliography of Known Astromolecules (http://www. astrochymist.org/astrochymist_mole.html) of The Astrochymist (http://www.astrochymist.org/) with tables of interstellar and circumstellar molecules, cometary molecules, molecules on planetoids, planetary molecules, and molecules in brown dwarfs and stellar atmospheres.
Keywords
Basic Methodology
Anion, Cation, Circumstellar envelope, Extragalactic source, Interstellar medium, Ion, Molecule, Radio astronomy, Rotational spectroscopy, Stellar atmosphere
All identified molecules in space have been detected by various spectroscopic means, usually by high-resolution (rotationally resolved) gas phase spectroscopy. Molecules in stellar atmospheres have been identified initially by their electronic spectra in the visible region. Later, the observed spectra were extended to the (vacuum) ultraviolet region and to the near-infrared region. Light hydride species such as H2O, NH3, and CH4 may be detected more favorably by their vibrational spectra in the mid- or near-infrared regions. These relatively short wavelength
Molecular Sieves ▶ Zeolites
Molecular Weight Definition The molecular weight is the sum of the masses of the atoms making up a molecule expressed in atomic units of mass. One atomic unit of mass, or Dalton (Da), is equal to 1/12 the mass of an atom of the isotope 12C of carbon.
Molecules in Space HOLGER S. P. MU¨LLER I. Physikalisches Institut, Universita¨t zu Ko¨ln, Ko¨ln, Germany
Synonyms
Definition In this entry by “molecules” we refer to all types of (generally chemically) bound species consisting of two or more atoms. They may be chemically saturated as well as unsaturated, may contain unpaired electrons (i.e., are radicals), or may be charged positively (cations)
Molecules in Space
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Molecules in Space. Table 1 Moleculesa detected in the interstellar or circumstellar medium 2 atoms 3 atoms
4 atoms
5 atoms 6 atoms
H2
C3
c-C3H
C5
7 atoms
8 atoms
9 atoms
C5H
C6H
AlF
C2H
l-C3H
C4H
l-H2C4
CH2CHCN HCOOCH3
AlCl
C2O
C3N
C4Si
C2H4
CH3C2H
CH3COOH
C2
C2S
C3O
l-C3H2
CH3CN
HC5N
C7H
CH
CH2
C3S
c-C3H2
CH3NC
CH3CHO
H2C6
HC7N
CH+
HCN
C2H2
CH2CN
CH3OH
CH3NH2
CH2OHCHO
CH3C3N
HC9N CH3C6H
C2H5OCH3 ?
(CH3)2O
(CH2OH)2
C2H5OCHO n-C3H7CN
CH3CH2OH
CH3CH2CHO
NH3
CH3SH
c-C2H4O
HCCN
HC3N
HC3NH+
H2CCHOH CH2CHCHO (?) C8H
CO+
HCS+
HCNH+
HC2NC
HC2CHO
C6H
HNCO
HCOOH
NH2CHO
SiC
H2O
HNCS
H2CNH
C5N
HCl
H2S
HOCO+
H2C2O
l-HC4H (?)
KCl
HNC
H2CO
H2NCN
l-HC4N
NH
HNO
H2CN
HNC3
c-H2C3O
NO
MgCN
H2CS
SiH4
MgNC
+
NS
+
H3O
H2COH
N2H
c-SiC3
C4H
OH
N2O
CH3
HC(O) CN
PN
NaCN
C3N
SO
OCS
PH3
SO2
HCNO
SiN
c-SiC2
HOCN
SiO
CO2
HSCN
SiS
NH2
CS
H3+
HF
H2D+, HD2+
HD
SiCN
FeO ?
AlNC
O2
SiNC
CF+
HCP
SiH ?
CCP
PO
AlOH
SO
CH3C(O)NH2
C3H6
H2NCH2CN
H2CCNH (?) +
C5N
NaCl
+
(?)
C8H
HCO+
CH2CCHCN
HC11N
CH3C5N (CH3)2CO
HCO
HOC
>12 atoms
CH3CH2CN
CO
CP
12 atoms
CH3C4H
CN
l-HC6H (?)
11 atoms
C6H6
CH4
+
10 atoms
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AlO a
All species were detected by radio astronomy except molecules marked with one or two asterisks that were detected by means of their rovibrational or rovibronic spectra. Entries with a question mark are tentative some are presented as detections in the literature. Entries with a question mark in parentheses are likely detections, viewed as somewhat tentative because of the small number of (unambiguous) lines some are presented as detections in the literature. In some instances, detections are viewed as secure in spite of a small number of lines; available documentation gives further details. The l indicates a linear molecule or a molecule that contains a linear backbone; the c indicates a cyclic molecule or a molecule with a cyclic subunit. The questionable detections of aminoacetic acid, H2NCH2COOH, aka glycine, and 1,3-dihydroxypropanone, aka dihydroxyacetone do not appear in the table.
spectroscopic methods have also some importance for the detection of molecules in the interstellar medium, circumstellar envelopes, or in extragalactic sources, but the majority of detections and (routine) observations are made by
▶ radio astronomy, initially carried out in the radio frequency and microwave region below 40 GHz or with wavelengths longer than 7.5 mm. The millimeter region up to about 300 GHz became accessible approximately in
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Molecules in Space. Table 2 Molecules in Space. Moleculesa detected in extragalactic sources 2 atoms
3 atoms
4 atoms
5 atoms
6 atoms
7 atoms
OH
H2O
H2CO
c-C3H2
CH3OH
CH3C2H
CO
HCN
NH3
HC3N
CH3CN
H2
HCO+
HNCO
CH2NH
CH
C2H
H2CS ?
NH2CN
CS
HNC
HOCO+
CH+
N2H+
c-C3H
CN
OCS
H3O+
SO
HCO
SiO +
H2S
CO
SO2
NO
HOC+
NS
C2S
a
See footnote to Table 1.
1970, and around 1985 it was possible to carry out observations at submillimeter or far-infrared wavelengths. Already the upper millimeter region, and even more so the far-infrared region, are faced with the obstacle that the atmosphere is almost or completely opaque in these frequencies, mainly because of the water vapor. Building observatories at high altitude sites is a partial remedy; putting them on high-flying airplanes or even on satellites is even better because such observatories are (essentially) not affected by water in the line of sight. On the other hand, such airplane- or satellite-borne observatories are much more expensive to run, and the size of the telescope dish is much more limited.
Key Research Findings Molecules in Stellar Atmospheres It is important to keep in mind that the presence or absence of atomic or molecular absorption lines in the visible spectra of stars is used to classify stars. O, B, A, and F stars are too hot and their UV radiation is too strong for molecular species to survive. A rather large number of molecular species has been detected in the atmosphere of our sun, a G star, but they were detected mostly in sunspots, which are considerably cooler at the surface than is common. TiO can be observed in late K stars and, more strongly, in M and S stars, which are classified mainly by the strengths of TiO lines and in which other transition metal oxides can be seen, such as VO, ScO, CeO, ZrO, CrO, and FeO. ▶ Water can be seen in emission in late K and M giants, while C2 and SiC2 have been seen in emission in
carbon stars. In still smaller, less luminous, and cooler stars transition metal hydrides have been identified, such as CrH, FeH, and TiH along with NH3 and CH4 and, in at least one case, SH.
Molecules in the Interstellar Medium and in Circumstellar Envelopes Around 1935 some diffuse absorption lines, suspected to be of molecular origin, were detected in the visible spectra recorded toward certain stars. The diffuse nature of the lines was interpreted as evidence that the carriers of these lines were not associated with the stellar environment, but rather with the interstellar medium in the line of sight. Three lines were assigned to CN, CH, and NaH (McKellar 1940); the latter assignment still remains to be confirmed. It was noted that only transitions originating from the lowest rotational levels could be identified unambiguously, suggesting very low temperatures of the environment. In fact, the second rotational level of the CN ▶ radical is at 5.43 K. Shortly thereafter, three additional lines were assigned to CH+ (Douglas and Herzberg 1941). Even though radio telescopes had been available since the early 1930s, the first molecule to be detected by this technique, and the fourth interstellar molecule altogether, was the OH radical in 1963 (Weinreb et al. 1963). Only diatomic molecules had been detected thus far, and it was even speculated by several astronomers that polyatomic molecules, that is, molecules with more than two atoms, could not be formed in the interstellar medium. Therefore, it came as a big surprise that the next two molecules, NH3 (Cheung et al. 1968) and H2O (Cheung et al. 1969),
Molecules in Space
were polyatomic molecules. Moreover, the molecular lines were observed in emission toward hot-core sources, the rather warm and dense molecular clouds that enshroud young massive stars. Several further light hydride species, molecules consisting of one heavy atom besides H atoms, have been detected. These include SiH4, CH4, H3+, and CH3, which have been observed by ▶ infrared spectroscopy because the molecules do not have a permanent electric dipole moment and thus no radio spectrum. H2S, HCl, H3O+, CH2, NH, NH2, HF have been detected or observed employing radio astronomy. However, the strong transitions of these light hydride species occur in the terahertz or far-infrared region, which is partly difficult to access and in part simply not accessible from the ground, requiring radio telescopes on high terrestrial sites, on board of high-flying airplanes such as KAO, or on board of satellites such as ISO; these two missions were in operation in the 1990s. More recently, the Atacama Pathfinder EXperiment (APEX) telescope has been built in the Chilean Andes, and the ▶ Herschel satellite has been launched. The author is aware of four, maybe five, new cationic light hydride species whose detection will be publicly available soon. The first polyatomic molecule with two heavy atoms, ▶ formaldehyde, H2CO, was among the very early detected molecules, and it is rather ubiquitous (Synder et al. 1969). Related molecules H2CNH, H2CS, and H2CCO were detected soon thereafter. The isoelectronic diatomic molecules CO, SiO, CS, and SiS were also among the molecules detected very early in the interstellar medium. CO is particularly important as it is the second most abundant molecule in space after H2. Moreover, since H2 is very difficult to observe and the CO to H2 abundance ratio is nearly constant, CO transitions are usually observed to trace H2, and thus the density of molecules in space. SiO is usually viewed as an indicator of shocks. It is worthwhile mentioning that many atoms have more than one stable isotope. As a consequence, several isotopologs have been detected for molecules that are abundant in certain regions in space. In the case of CO, for example, all six stable ▶ isotopologs involving 12,13C and 16,17,18O have been detected. Eight isotopic species have been observed for SiS in the circumstellar envelope of CW Leonis (IRC +10216), all possible combinations involving 28,29,39Si and 32,34S along with 28Si33S and 28 36 Si S, thus providing important information on the nuclear synthesis in this late-type star. PO was the first molecule for which the first detection was described in the circumstellar envelope of an oxygen-rich star, namely VY Canis Majoris (Tenenbaum et al. 2007).
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HCN and HC3N are quite abundant in all types of region and were also among the molecules detected earliest. The longer chain molecules HC5N, HC7N, and HC9N have been detected in dark molecular clouds, such as TMC-1 (the Taurus ▶ Molecular Cloud No. 1), or in circumstellar envelopes, in particular of the carbon-rich star CW Leonis; HC11N is the longest cyanopolyyne detected in space thus far, and only in TMC-1. In several cases, more than one isomer of a certain formula has been detected. For example, besides ▶ cyanoacetylene, HC3N, isocyanoacetylene, HCCNC, and iminopropadienylidene, HNC3, have been detected. More recently, fulminic acid, HCNO, and cyanic acid, HOCN, were detected, which are isomers of the abundant isocyanic acid, HNCO. Some molecules were detected by radio-astronomical means prior to their laboratory spectroscopic identification. Formylium, HCO+ (Buhl and Synder 1970) was the first and most notable example. Others include HNC, CCS, HCS+, HOCO+, and c-C3H2. Several additional examples will be given below. Many of the cations observed in the interstellar medium or circumstellar envelopes can be obtained formally by protonation of rather stable, closed-shell species, such as CO, N2, CS, CO2, HCN, H2O, HC3N, H2, and H2CO, thus leading to comparatively stable, closed-shell cations, such as HCO+ and HOC+, N2H+, HCS+, HOCO+, HCNH+, H3O+, HC3NH+, H3+, and H2COH+. The remaining known cations are in part radicals, SO+ and CO+, or also closed-shell species, CH+ and CF+. The latter may seem peculiar to some as it contains the most electronegative element, fluorine, in a cation. However, one should keep in mind that it is isoelectronic with CO. Only a few cyclic species have been detected unambiguously thus far. Besides c-C3H2 these are c-SiC2, c-C3H, H3+, c-C2H4O, c-SiC3, and c-C2H3O. Aromatic molecules, which play an important role in biochemistry, are mostly too big to be observed. There has been a report on the infrared detection of benzene. In addition, certain infrared features are commonly believed to be due to polycyclic aromatic hydrocarbons (PAHs), but the exact nature of these species is not known. ▶ Methanol, CH3OH, is very ubiquitous and was the first complex molecule detected in space (Ball et al. 1970). A complex molecule in radio astronomy may be defined as an asymmetric rotor molecule having at least five atoms and which is usually fully saturated or nearly saturated. CH3OH as well as some other complex molecules such as dimethyl ether, vinyl cyanide, ethanol, and methyl formate are so abundant and have such a large number of observable transitions that they have been called interstellar
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weeds. More complex molecules are usually less abundant and the intensity of spectral features is distributed over many more lines; both these issues make it harder to detect larger complex molecules unambiguously. Ethyl formate and n-propyl cyanide are the largest complex molecules unambiguously detected thus far, and model calculations are compatible with their formation on grain surfaces (Belloche et al. 2009). The purported detection of interstellar ▶ glycine shall be mentioned in this context. A fairly recent paper described the detection of several transitions of glycine, aminoacetic acid, the simplest ▶ amino acid, in three hotcore sources. Such sources are particularly line rich. The detection has been disputed because the derived model required several additional lines to be observable. It is important to note that emission features assigned to a certain molecule may well be stronger than predicted by a model because of overlap, possibly by an unidentified species, but a line cannot be weaker than predicted if the model is correct. Consequently, only an upper limit is currently known for interstellar glycine. As in similar cases for other molecules, this simply means that the abundance of interstellar glycine in the gas phase is too low to be identified unambiguously at present. This situation may change with Atacama Large Millimeter Array (▶ ALMA), as its much greater sensitivity and its greater spatial resolution may be advantageous for the detection of glycine and other complex molecules. Of course, no statement can be made about glycine residing in the solid phase, for example, in interstellar ices or grains. The detection of molecules with low ▶ abundances in the solid phase is generally very difficult as spectral lines of solids, for example, in the infrared, are quite broad and rarely very distinctive. Nevertheless, a plethora of complex molecules have been identified through direct chemical analyses of the Murchison meteorite, including many amino acids. The molecules show no or only small chiral preference and include many which have not been identified in living beings, indicative of an abiotic origin. These findings suggest that at least some amounts of more complex molecules are formed in certain regions of space. It should be mentioned in this context that aminoacetonitrile, which is considered to be the most likely precursor of glycine, has recently be detected in the high-mass star-forming region Sagittarius B2(N-LMH). Soon after the interstellar detection of SiO in 1971, vibrationally excited SiO was detected with an unknown carrier (Snyder and Buhl 1974), but correctly identified soon thereafter. Very highly excited SiO (up to v = 4 with a vibrational energy of almost 7,000 K) or HCN (up to v1 = 1, v2 = 2 with a vibrational energy of about 6,800 K)
have been detected in circumstellar envelopes. In addition, vibrational satellites of many more molecules have been detected in hot-core sources or in circumstellar envelopes. They offer the opportunity to probe exclusively the hottest regions in the interstellar medium. In fact, Wyrowski et al. (2003) used lines of vibrationally excited HC3N and its 13 C isotopologs to characterize the physical properties of the proto-planetary nebula CRL 618. Pardo et al. (2007) showed that in their very sensitive line survey of that source almost 3/4 of the lines can be assigned to ground and excited state lines of HC3N, its minor isotopologs, and corresponding lines of the heavier HC5N homolog. Low temperatures favor exchange reactions of the heavier isotope in molecules with large differences in zero-point vibrational energies. Such differences are particularly large for D/H exchange. HDO was the first deuterated molecule to be detected in space, by Turner et al. (1975). Since then not only singly deuterated molecules such as DCN, DNC, and DCO+ have been detected, but also doubly deuterated ones, for example, D2O, D2S, D2CO, and D2CS, and even triply deuterated ones, namely ND3 and CD3OH. Molecules containing metals in the chemical sense (not in the very general astronomical sense: every element heavier than helium) remained elusive for quite a while. The first one to be detected was MgNC (Gue´lin et al. 1986) in the circumstellar envelope of CW Leonis, but its correct identification occurred only much later (Kawaguchi et al. 1993). In the meantime, NaCl, KCl, AlF, and AlCl had been detected in the same source. Later, NaCN, MgCN, and AlNC were also detected in that source. Some of these, MgNC, NaCN, AlF, and NaCl, were also detected in other circumstellar envelopes. AlO and AlOH were the first two metal-containing molecules that were detected in the circumstellar envelope of an oxygen-rich star, VY Canis Majoris. Even though several transition metal compounds have been identified in stellar atmospheres, the only report of this type of molecule in the interstellar medium or in circumstellar environments is the tentative detection of FeO in absorption toward Sagittarius B2(M). The detection is tentative because only one line has been observed. This would also be the first detection of a metalcontaining molecule in the interstellar medium by radio astronomy, although the tentative detection of NaH mentioned above should be kept in mind. The first molecular anion detected in the gas phase was C6H (Kawaguchi et al. 1995); however, it was identified unambiguously only much later (McCarthy et al. 2006). C4H, C8H, C3N, and C5N have also been detected. Interestingly, the latter was identified unambiguously even though no laboratory data have been available to
Molecules in Space
date. All these ions are closed-shell species, which can be derived from HC2n+2H or its isoelectronic HC2nCN by abstraction of a proton. A feature in solid state infrared spectra has been attributed to the OCN ion, and another such feature was assigned to CO2. Molecular oxygen, O2, was suggested to be abundant at least in certain regions of the interstellar medium. Its observation is difficult because it has no permanent electric dipole moment, and the magnetic dipole transitions are rather weak. Moreover, observations of O2 are not possible from ground-based radio telescopes because of the presence of large amounts of oxygen in Earth’s atmosphere. Satellite-based observations have produced upper limits almost exclusively, and the only potentially positive observation is based on a barely significant result. It seems thus possible that all of the oxygen is locked up in CO, H2O, MgO, SiO2, SO2, similar molecules, and the interstellar dust grains, leaving at best very little for the formation of the fairly reactive O2 molecule. One of the greatest and longest lasting mysteries in astronomy is the identity of the ▶ Diffuse Interstellar Bands (DIBs), which have been detected in absorption in optical spectra toward stars for the first time in 1922. To date, more than 200 of these are known in the UV, visible, and IR regions. A fairly recent review has been published by Herbig (1995). The nature of these absorption lines is not clear. Often, they form small groups. High-resolution contour studies suggest that at least some of the carriers are of molecular origin. Another interesting issue is the unidentified aromatic infrared emission bands, which are commonly ascribed to PAHs. It is generally accepted that these are the most likely carriers of these emission bands, but again, the exact nature is not clear, in particular because the features are caused by more than one species and may include neutrals as well as charged species. ▶ PAHs are also proposed as one possible group of molecules for the carriers of the DIBs.
Molecules in Extragalactic Sources The molecules detected in external galaxies are a subset of the molecules detected in the interstellar medium, simply because these objects are farther away than our galaxy. Great progress has been made in recent years in terms of investigations into the chemical complexity of external galaxies. As for molecular clouds, the chemical composition depends to a great extent on details of the galaxy, such as size, age, and star-formation activity. Starburst galaxies, such as the nearby NGC 253 or M82, show a particularly rich and varied chemistry. Omont (2007) has presented a review on molecules in galaxies quite recently.
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The first molecule to be detected in an external galaxy was OH as early as 1971 by Weliachew, toward the two galaxies NGC 253 and M82. Many other di- and polyatomic molecules have been detected, including CO, H2, CH, CH+, SiO, H2O, HCN, and up to the seven-atomic molecule propyne, CH3CCH. Particular noteworthy in this context was a molecular line survey by Martı´n et al. (2006) toward NGC 253. Several molecules were detected for the first time in an extragalactic source in the course of this study. In recent years there have been several reports on molecules in very distant, high red-shift galaxies, in particular of CO.
Future Directions As indicated already, new ground-based instruments, such as APEX, as well as the recently launched Herschel satellite, have detected several new light hydride molecules; corresponding publications should appear soon. In addition, they provide extensive and in part new opportunities to carry out observations in the terahertz (or far-infrared) region. The Stratospheric Observatory for Infrared Astronomy (SOFIA) should begin operations soon and will complement and supplement these instruments. The ALMA, which is currently under construction, will provide unprecedented sensitivity and spatial resolution. This should facilitate studies of complex molecules in dense molecular clouds and will surely be beneficial for the study of circumstellar envelopes or of extragalactic sources. It will also be possible to study comparatively small-scale structures in nearby galaxies. In addition, the increase in sensitivity will open new opportunities to study very distant galaxies routinely. The field of astrochemistry is thus expected to flourish tremendously in the next few decades.
See also ▶ Absorption Spectroscopy ▶ Abundances of Elements ▶ Alcohol ▶ ALMA ▶ Amino Acid ▶ Ammonia ▶ Anions ▶ Complex Organic Molecules ▶ Cyanoacetylene ▶ Cyanopolyynes ▶ Dense Clouds ▶ Deuterium/Hydrogen Ratio ▶ Diffuse Clouds ▶ Diffuse Interstellar Bands ▶ Formaldehyde ▶ Formyl Cation
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Monod’s Conception on the Origins of Life
▶ Glycine ▶ HCNO Isomers ▶ Herschel ▶ Hot Cores ▶ Hot Corinos ▶ Hydrogen Cyanide ▶ Infrared Spectroscopy ▶ Interstellar Medium ▶ Isomer ▶ Isotopic Fractionation (Interstellar Medium) ▶ Isotopolog ▶ Meteorite (Murchison) ▶ Methanol ▶ Methylidyne ▶ Methylidyne Cation ▶ Molecular Abundances ▶ Molecular Cloud ▶ Organic Molecule ▶ Polar Molecule ▶ Radical ▶ Radio Astronomy ▶ Spectral Line ▶ Spectroscopy ▶ Star Formation
References and Further Reading Ball JA, Gottlieb CA, Lilley AE, Radford HE (1970) Detection of methyl alcohol in sagittarius. Astrophys J 162:L203–L210 Belloche A, Garrod RT, Mu¨ller HSP, Menten KM, Comito C, Schilke P (2009) Increased complexity in interstellar chemistry: detection and chemical modeling of ethyl formate and n-propyl cyanide in sagittarius B2(N). Astron Astrophys 499:215–232 Buhl D, Synder LE (1970) Unidentified interstellar microwave line. Nature 228:267–269 Cheung AC, Rank DM, Townes CH, Thornton DD, Welch WJ (1968) Detection of NH3 molecules in the interstellar medium by their microwave emission. Phys Rev Lett 25:1701–1705 Cheung AC, Rank DM, Townes CH, Thornton DD, Welch WJ (1969) Delection of water in interstellar regions by its microwave radiation. Nature 221:626–628 Douglas AE, Herzberg G (1941) Note on CH+ in interstellar space and in the laboratory. Astrophys J 94:381 Gue´lin M, Cernicharo J, Kahane C, Gomez-Gonzales J (1986) A new free radical in IRC +10216. Astron Astrophys 157:L17–L20 Herbig GH (1995) The diffuse interstellar bands. Ann Rev Astron Astrophys 33:19–74 Kawaguchi K, Kagi E, Hirano T, Takano S, Saito S (1993) Laboratory spectroscopy of MgNC – the first radioastronomical identification of Mg-bearing molecule. Astrophys J 406:L39–L42 Kawaguchi K, Kasai Y, Ishikawa SI, Kaifu N (1995) A spectral-line survey observation of IRC +10216 between 28 and 50 GHz. Publ Astron Soc Jpn 47:853–876 Martı´n S, Mauersberger R, Martı´n-Pintado J, Henkel C, Garcı´a-Burillo S (2006) A 2 millimeter spectral line survey of the starburst galaxy NGC 253. Astrophys J Suppl Ser 164:450–476
McCarthy MC, Gottlieb CA, Gupta H, Thaddeus P (2006) Laboratory and astronomical identification of the negative molecular ion C6H. Astrophys J 652:L141–L144 McKellar A (1940) Evidence for the molecular origin of some hitherto unidentified interstellar lines. Publ Astron Soc Pac 52:187–192 Mu¨ller HSP, Thorwirth S, Roth DA, Winnewisser G (2001) The cologne database for molecular spectroscopy, CDMS. Astron Astrophys 370: L49–L52 Mu¨ller HSP, Schlo¨der F, Stutzki J, Winnewisser G (2005) The cologne database for molecular spectroscopy, CDMS: a useful tool for astronomers and spectroscopists. J Mol Struct 742:215–227 Omont A (2007) Molecules in galaxies. Rep Prog Phys 70:1099–1176 Pardo J, Cernicharo J, Goicoechea JR, Gue´lin M, Asensio Ramos A (2007) Molecular line survey of CRL 618 from 80 to 276 GHz and complete model. Astrophys J 661:250–261 Snyder LE, Buhl D (1974) Detection of possible maser emission near 3.48 millimeters from an unidentified molecular species in orion. Astrophys J 189:L31–L33 Synder LE, Buhl D, Zuckerman B, Palmer P (1969) Microwave detection of interstellar formaldehyde. Phys Rev Lett 22:679–681 Tenenbaum ED, Woolf NJ, Ziurys LM (2007) Identification of phosphorus monoxide (X 2Pr) in VY canis majoris: detection of the first P-O bond in space. Astrophys J 666:L29–L32 Turner BE, Fourikis N, Morris M, Palmer P, Zuckerman B (1975) Microwave detection of interstellar HDO. Astrophys J 198:L125–L128 Weinreb S, Barrett AH, Meeks ML, Henry JC (1963) Radio observations of OH in the interstellar medium. Nature 200:829–831 Weliachew L (1971) Detection of interstellar OH in two external galaxies. Astrophys J 167:L47–L52 Wyrowski F, Schilke P, Thorwirth S, Menten KM, Winnewisser G (2003) Physical conditions in the proto-planetary nebula CRL 618 Derived from observations of vibrationally excited HC3N. Astrophys J 586:344–355
Monod’s Conception on the Origins of Life MICHEL MORANGE Centre Cavaille`s, USR 3308 CIRPHLES, Ecole normale supe´rieure, Paris Cedex 05, France
Keywords DNA, Monod, Oparin, origin of life, RNA world
Abstract For Monod, the formation of living organisms was a highly improbable event, unique to Earth. Both scientific and philosophical reasons supported this conviction. Some of the obstacles on the way to life pointed out by Monod have been partially removed, and biologists are more optimistic today about the possibility of finding living organisms on other planets.
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References and Further Reading
Monod’s interest in the question of the origin of life came late in his career, when he tried to summarize the major conceptual transformations resulting from the rise of molecular biology, first in lectures, and then in his influential book Chance and Necessity. For Monod, the formation of the first living organisms had been a highly improbable event, and Earth was probably the only place in the Universe harboring life. The formation of living organisms can be explained by the laws of physics and chemistry, but it was unpredictable. This opinion was based on the results recently obtained in molecular biology, which showed the exquisite nature of the mechanisms involved in the transmission of genetic information through generations, and its use at each generation. An even more serious obstacle to the origin of life was the necessity for organisms to possess two different types of macromolecules: DNA, to store the genetic information; and proteins, the active agents of living cells. The origin of life meant the simultaneous formation of these two types of macromolecules. There are tight relations between these two macromolecules: DNA codes for proteins, and proteins are required for the use of the information encoded in the DNA. Not only did both types of macromolecules have to appear simultaneously, but they had to have from the beginning at least a part of their complex relations. Monod was probably philosophically happy with this conclusion. It justified scientifically his feeling of an absurd Universe and of the loneliness of human beings in it. Monod did not see in the scenarios elaborated by the Russian biologist Oparin a way to overcome these difficulties. Not only did these scenarios fail to take into account the most recent molecular descriptions, but their author was compromised by the support he gave to Lysenko under the Soviet regime. Monod distrusted Oparin and his models. Forty years later, the situation is very different. The discovery of the catalytic role of RNA has led to the hypothesis of the existence of an RNA world that preceded the present living world. The difficulties outlined by Monod have not been totally removed, but ways to overcome them are being explored. Most presentday biologists consider that the formation of simple forms of life was not so difficult, and that the search for these forms of life on other planets deserves to be supported.
Monod J (1971) Chance and necessity. Knopf, New York Morange M (2008) Life explained. Yale University Press, New Haven
See also ▶ Genetics, History of ▶ Oparin’s Conception of Origins of Life
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Monomictic Breccia Definition A monomictic breccia is a clastic sedimentary rock composed of angular clasts having a single origin. The term monomictic also refers to a specific process of brecciation produced from rock deformation by shearing and granulation. Shear stress is produced during tectonic events or meteoric impact.
See also ▶ Breccia ▶ Crater, Impact ▶ Impact Melt Rock ▶ Impactite
M Monophyletic Synonyms Holophyletic
Definition In classical taxonomic and phylogenetic literature, a monophyletic group, also called a clade, is defined as an assemblage that contains an ancestor and all its descendants. This term is most often applied to define groups of organisms that derive from a ▶ common ancestor, which form monophyletic taxa or clades (e.g., animals or cyanobacteria). However, it is used by molecular phylogeneticists also to define particular groups of DNA, RNA, or protein sequences in ▶ phylogenetic trees (e.g., different protein families derived from gene duplications). Monophyletic groups are based on the identification of synapomorphies, which are shared derived characters (e.g., a skull and a distinct head are synapomorphies of vertebrates). This criterion allows differentiating monophyletic groups from paraphyletic groups (those containing an ancestor and part of its descendants, such as the reptiles) and polyphyletic groups (those that do not contain the ▶ last common ancestor of their members,
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such as the flying animals, including disparate members like insects, birds, and bats).
See also ▶ Common Ancestor ▶ Evolution (Biological) ▶ Last Common Ancestor ▶ Phylogeny ▶ Phylogenetic Tree ▶ Taxonomy
gov/jsp/append5.jsp) to refer to mountain (mountains). It is used as a descriptor term for naming surface features on the ▶ terrestrial planets, the ▶ Moon, and the Jovian ▶ satellite ▶ Io.
See also ▶ Io ▶ Moon, The ▶ Satellite or Moon ▶ Terrestrial Planet
Monosaccharide Synonyms Simple sugar
Definition Monosaccharides are monomeric ▶ carbohydrate molecules. Examples include glucose, fructose, galactose, xylose, and ▶ ribose. 2-Deoxyribose and ribose are constituents of DNA and ▶ RNA, respectively. Many of the carbon atoms attached to hydroxyl groups are chiral centers, giving rise to a number of isomeric forms. Many monosaccharides are unstable over geological time scales. The half-lives for the decomposition of ribose, for example, are 73 min at 100 C, and 44 years at and 0 C at pH 7. They can be formed in the ▶ formose reaction from ▶ formaldehyde. Monosaccharide-like compounds have been found in carbonaceous chondrites, and one of the ketoses, dihydroxyacetone (HOCH2COCH2OH), was identified in the extract of Murchison meteorite.
See also ▶ Carbohydrate ▶ Carbonaceous Chondrite ▶ Chirality ▶ Deoxyribose ▶ Formaldehyde ▶ Formose Reaction ▶ Ribose ▶ RNA
Montmorilllonite GO¨ZEN ERTEM National Institutes of Health, Bethesda, MD, USA
Keywords Catalyst, cation exchange, clay minerals, oligomerization, origin of life
Definition Montmorillonite is a ▶ clay mineral of the smectite group, which also includes nontronite and saponite. It was first discovered in 1847 in Montmorillon in the Vienne region of France. It is usually white, gray, or pink with shades of green or yellow as a result of the metal oxide impurities it contains. It is often found in compact or lamellar masses, but never in large individual crystals. Its specific gravity varies between 1.7 and 3.0. It has a hardness of 1–2 in the Mohs scale (the Mohs scale ranges between 1 and 10, where 1 and 10 correspond to talc and diamond, respectively). Chemically, it is a hydrated aluminum silicate containing small amounts of magnesium, iron, sodium, calcium, and potassium as a result of isomorphic substitutions. The nature of these cations and their ratio within the mineral structure vary with the source of montmorillonite.
Overview
Mons, Montes Definition Mons (plural montes) is the term defined by the International Astronomical Union (http://planetarynames.wr.usgs.
Montmorillonite (Grim 1968) has a layer structure with 2:1 arrangement: Each layer is composed of three sheets. Two outside sheets, called tetrahedral sheets, contain silicon ions, Si4+, tetrahedrally coordinated to oxygens. The middle sheet, called the octahedral sheet, is made up of aluminum ions, Al3+, surrounded by oxygens in octahedral
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coordination. The general formula of montmorillonite may be represented as: h iVI ½Si4:0x Alx IV Al2:0ðyþzÞ Mgy Fez O10 ðOHÞ2 ðNa; CaÞ xþyþz ; where IV and VI denote the tetrahedrally and octahedrally coordinated cations, respectively. The term (x + y + z) is the total of substitutions in the octahedral and tetrahedral sheets. Theoretically, montmorillonite is assumed to have formed from pyrophyllite [(Al2) (Si4O10)(OH)2]. During the formation of clay minerals by weathering, some of the Si4+ ions in the tetrahedral sheet are replaced by cations of similar size (isomorphic substitution), usually Fe3+, and some of the Al3+ cations are replaced by Mg2+. These replacements result in excess negative charge, which is counter-balanced by so-called interlayer cations held between the layers. Since sodium is one of the most abundant cations in nature, most clay minerals contain Na+ as an exchangeable cation. The extent of isomorphic substitutions is defined as cation exchange capacity, CEC, of montmorillonite and expressed as milliequivalents of interlayer cation/100 g of clay. Most montmorillonites have a CEC of about 100. Because of its large surface area, about 800 m2/g, montmorillonite is widely used as an absorbent for toxins, hazardous chemicals, and heavy metal cations. Montmorillonites can absorb water and swell to about 20 times of their dry volume and give rise to permanent suspensions of gel-like masses. Hydration, or solvation, of interlayer cations causes the layers to expand. Due to this expansion, organic molecules, such as amines, alcohols, and polyalcohols, nucleic acid bases, nucleosides, and oligomers, can enter the interlayer of montmorillonite. This expansion capability, which is a result of the isomorphic substitutions, renders the montmorillonite its catalytic properties. Extensive studies have demonstrated that montmorillonite serves as an effective catalyst for the ▶ oligomerization of activated nucleic acid monomers (Ertem et al. 2010). These reactions are very significant in terms of ▶ Origin of Life studies as was first suggested by John D. Bernal (1949).
See also ▶ Bernal’s Conception of Origins of Life ▶ Clay ▶ Oligomerization ▶ Oligonucleotide ▶ Origin of Life ▶ Phyllosilicates (Extraterrestrial)
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References and Further Reading Bernal JD (1949) The physical basis of life. Proc R Soc Lond A 62:537–558 Ertem G et al (2010) Astrobiology 10 (7) (in print) and references therein Grim RE (1968) Clay mineralogy. McGraw Hill, New York
Moon, The RALF JAUMANN German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany Department of Earth Sciences, Institute of Geosciences, Remote Sensing of the Earth and Planets, Freie Universita¨t Berlin, Germany
Keywords Earth–moon system
Definition The Moon is the only large planetary object in the ▶ inner solar system to orbit and affect a ▶ terrestrial planet. It may have supported the Earth’s specific evolution as a habitable environment. As an integral part of the Earth–Moon system, it is witness to more than 4.5 Ga of solar system history, and it is the only planetary body except Earth for which we have samples from known locations. The Moon’s simple composition and its restricted geological activity provide insights into elementary planetary processes. The Moon is thought to be the product of an early planetary collision of a Mars-sized body with Earth (cf. Hartmann et al. 1986). Earth and Moon form a celestial system with a common center of mass located at about 1,700 km beneath the Earth’s surface (Fig. 1). With a diameter of 3,474 km the Moon is the fifth largest ▶ satellite in the Solar System. Its surface covers less than one-tenth that of the Earth’s surface, its volume is about 2%, and its mass only 1.2% of that of Earth. Its equatorial surface gravity is 1.622 m/s2, about 1/6 (17%) of that on Earth. The lunar ▶ rotation slowed down early in its history due to frictional effects associated with tidal deformations and became locked into a synchronous state, keeping the same face turned toward the Earth at all times. A complete orbit around the Earth takes 27.3 days (the orbital period), whereas periodic variations in the geometry of the Earth– Moon–Sun constellation are responsible for the phases of the Moon, which repeat every 29.5 days (the synodic period).
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Moon, The. Figure 1 Earth–Moon relations
The Moon’s density is 3.34 10 g/cm3 (=cubiccentimeter) and its ▶ rocks are of a relatively simple mineralogical composition. Compared to Earth the Moon is depleted in iron and other siderophile elements (cf. Heiken et al. 1991; Jolliff et al. 2006) and shows depletion in ▶ volatile elements, in particular H2O and OH. The lunar topographic relief ranges from 6,000 m in the South-Pole region to about +7,000 m on the far side.
Basic Methodology Our knowledge about the moon is based on telescopic observations from Earth, observations by spacecraft from the lunar orbit, measurements on the lunar surface by manned and unmanned landing missions, and the analyses of lunar samples brought to Earth by the Apollo (382 kg) and Luna (326 g) missions, and about 120 lunar ▶ meteorites (48 kg) that have been collected on Earth to date. Remote sensing using passive sensors in the optical wavelength range as well as active laser and radar detection technology supported by in situ field work yield information concerning the geological processes that formed the Moon, allowing its entire history to be studied with respect to its impact-related, volcanic, tectonic, and ▶ space weathering evolution. Spectral measurements in all wavelength ranges from high energy gamma-rays to the mid-infrared provide the overall surface composition and
major mineralogical content of rocks, whereas geochemical details and the origin, differentiation, and evolution of the lunar rocks have been deduced from the returned samples.
Key Research Findings Since the Apollo 11 astronauts returned the first samples, the Moon is known to be a differentiated planetary body as a result of fractional crystallization of a postulated ▶ magma Ocean (cf. BVSP 1981; Heiken et al. 1991; Jolliff et al. 2006) beginning immediately after its accretion at 4.527 0.01 Ga (Kleine et al. 2005). The lunar ▶ crust is composed of four major distinct rock types developed from geochemical ▶ differentiation as well as mechanical destruction and mixing. (1) Pristine highland rocks of igneous origin are composed of older (4.5–4.3 Ga) ferroan anorthosites and slightly younger (4.43–4.17 Ga) troctolites, norites, gabbronorites, and dunites (the Mg-suite) that mark the transition between magma-ocean-related magmatism and serial magmatism (cf. Heiken et al. 1991; Jolliff et al. 2006). (2) Pristine basaltic volcanic rocks are generally enriched in FeO and TiO2 and depleted in Al2O3 (cf. BVSP 1981; Heiken et al. 1991; Jolliff et al. 2006) resulting in olivine and pyroxenerich and plagioclase-poor mare ▶ basalts and pyroclastic deposits. Most of the lunar basalts erupted between
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3.9 and 3.0 Ga from partial melts of a non-tholeiitic anhydrous basaltic magma located at various depths, mostly >350 km (cf. BVSP 1981; Heiken et al. 1991; Jolliff et al. 2006). (3) Polymict clastic breccias were produced by single or multiple impacts and are a mixture of different rock types from different locations containing various fractions of clastic rock fragments and impact melt varying in texture, grain size, and composition (Sto¨ffler et al. 1980). The uppermost lunar crust consists of an unconsolidated debris layer called regolith of fine-grained pristine crystalline rock and mineral fragments and agglutinates – aggregates of small particles welded together by glass produced in micrometeorite impacts (cf. Heiken et al. 1991; Jolliff et al. 2006). On a macroscopic scale the lunar surface has been modified by three major processes: impact, volcanism, and tectonics. Impacts are geological events that eject, dislocate, and redistribute material on all scales, building up morphologies from small bowl-shaped to large flatfloored craters partly refilled by fallback material and mass wasting. With increasing impact energy the relaxation of the compressed crust piles up a central peak and even multi-ringed mountain chains. As the number of impacts on a surface unit is correlated with its exposure to the cosmic bombardment, the frequency distribution of ▶ craters is a measure of its age (cf. Neukum et al. 2001). The Moon is stratigraphically divided into five time units: Pre-Nectarian (>4.1 Ga), Nectarian (4.1–3.85 Ga), Imbrian (3.85–3.2 Ga), Eratosthenian (3.2–0.8 Ga), and Copernican (100 kg Murchison meteorite that fell in Australia in 1969 (Schmitt-Kopplin et al. 2010). Nucleic Acids. Figure 2 Polynucleotides of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The three types of building blocks, phosphates, ribose and bases, are highlighted
neutral; (iii) a nucleotide unit is longer, with more bonds and greater flexibility than an amino acid; (iv) the side chains of nucleic acids (the bases) are planar and uncharged, while the side chains of proteins vary in size, shape, flexibility, polarity, and charge; and (v) nucleic acids readily assemble by side chain–side chain interactions (base pairs), while proteins primarily assemble by backbone–backbone interactions (a-helices and b-sheets). In cells, DNA codes for messenger RNA (mRNA) that in turn codes for protein. This direction of the flow of genetic information is known as the central dogma of biology (Crick 1970), which is almost universally employed by extant life. There is support for abiotic synthesis (Oro and Kimball 1962) and ▶ extraterrestrial delivery (SchmittKopplin et al. 2010) of nucleic acid precursors to Earth.
Deoxyribonucleic Acid (DNA) Specific pairing of purine and pyrimidine bases in a double helix (Fig. 4) was first proposed by James D. Watson and Francis Crick in 1953. This “B-form” helical structure contains nucleic acid bases paired by hydrogen bonds. In the B-form, base pairs are centered on the helical axis, with the base pair normals coincident with the axis. DNA bases stack like pennies in a roll. The sugar-phosphate backbones fall on the outside to form an antiparallel, right-handed helix. Base pair stacking, rather than hydrogen bonding, makes the greatest contribution to duplex stability (Devoe and Tinoco 1962). The B-form helix, with a repeat of around 10.3 base pairs, contains a wide major groove and a narrow minor grove. Elucidation of the base pairing scheme was aided by Erwin Chargaff ’s observation that the ratios of A to T and C to G are very close to 1:1 in a variety of species (Zamenhof et al. 1952). In addition, Watson and Crick’s model incorporated important results of DNA fiber diffraction experiments carried out by Rosalind Franklin (Franklin and Gosling 1953).
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led to the development of recombinant DNA technology, in which targeted ▶ DNA sequences are essentially cut, copied, and pasted into other DNA sequences. It was later found that DNA polymerases and oligodeoxyribonucleotide primers (short pieces of DNA) can be used to amplify essentially any target sequence from genomic DNA using what is called the ▶ polymerase chain reaction (PCR) (Mullis and Faloona 1987).
Ribonucleic Acid (RNA)
Nucleic Acids. Figure 4 The B-form DNA double helix shown in reduced form for simplicity. The phosphate groups are not shown. The bases are colored by type, as in Fig. 3
The importance of Watson and Crick’s model, along with work by Linus Pauling and others on protein structure, is that it greatly facilitated a unification of chemical and biological sciences, leading to the creation of the fields of biochemistry and molecular biology. The structure of DNA allowed the incorporation of the basic concepts of chemistry – hydrogen bonding, van der Waals interactions (stacking), bond angles, and torsions – into the practice and understanding of biological sciences. As Watson and Crick noted, “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” The ease of manipulation of DNA has fostered its extensive use in the study of essentially all cellular components. Identification of specific DNA sequences (restriction sites) cleaved by restriction enzymes (Roberts 1976)
The ▶ RNA world hypothesis (Rich 1962), articulated in detail by Gilbert (Gilbert 1986), proposes primitive living systems in which a single type of molecule served as both genetic information carrier and catalyst. In this model for the origin of life, genetic RNA was replicated by catalytic RNA. A single polymer fulfilled essentially all of life’s requirements, avoiding the ▶ chicken and egg dilemma presented by extant biochemistry – which came first, catalytic proteins or genetic nucleic acids? The RNA world hypothesis provides a reasonable answer: RNA came first, with both catalytic and genetic functions. A smooth and continuous evolutionary pathway from catalytic RNA to protein enzymes was paved with mixed RNA/protein assemblies (Poole et al. 1998). Support for the RNA World hypothesis is found in observations that RNA performs catalytic functions in extant biology (Kruger et al. 1982; Guerrier-Takada et al. 1983). RNA, like DNA, can form right-handed antiparallel duplexes with stacked base pairs. RNA duplexes give A-form helices, in which the base pairs are displaced from the helical axis, and are rolled, so that their normals are not parallel to the helical axis. RNA’s A-form helix is thicker and more compressed along the helical axis than DNA’s B-form helix. Unlike DNA, RNA forms complex three-dimensional structures dictated by combinations of secondary and tertiary interactions between bases, and by cationmediated interactions between phosphate groups. The result is distinct folds, such as the L-shaped structure of transfer RNAs (tRNAs) (Fig. 5), or more complex structures such as ribosomal RNAs (rRNAs). RNA polymerase transcribes DNA into mRNA, which carries sequence information to the ribosome (Kornberg 2007). The ▶ ribosome translates the mRNA into protein with the assistance of tRNAs specific to each amino acid. Ribosomes are around 200 A˚ in diameter and are comprised of 65% rRNA and 35% ribosomal proteins. They can be either cytoplasmic or membrane bound, depending on the protein actively in synthesis. Ribosomes and their subunits are classified by their Svedberg unit (rate of sedimentation in centrifugation), which ranges
Nucleic Acids
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Nucleic Acids. Figure 5 The L-shaped structure of transfer RNA (tRNA). The bases are colored by type as in Figs. 3 and 4. Here U is blue. Close inspection of the right panel reveals that the base pairs are shifted away from the helical axis, as expected for an A-form helix
N from 35S to 100S. Prokaryotes, mitochondria, and chloroplasts have 70S ribosomes (comprised of a small 30S subunit and a large 50S subunit), and eukaryotes have 80S ribosomes (with a small 40S and large 60S subunit). The small 30S and 40S subunits have their own subunits consisting entirely of rRNA: the 16S or 18S subunit, respectively. It is believed that mitochondria and chloroplasts are bacterial endosymbionts. Until 1977, the generally accepted evolutionary relationship among known species was represented by two kingdoms: prokaryotes (bacteria) and eukaryotes. Carl Woese and George Fox used sequences of 16S rRNA to support reclassification of a portion of the prokaryotes into a third kingdom, later dubbed Archaea (Woese and Fox 1977). Woese and Fox found 16S rRNA genes to be among the most highly conserved in sequence over the ▶ phylogenetic tree. Their approach has been widely accepted and is a standard for phylogenic classification, so that more than 10,000 16S rRNAs have been sequenced to date. Other RNAs include miRNA and siRNA, responsible for defensive RNA degradation, and snoRNAs that guide ribonucleoproteins to target sites on rRNA for modification.
X-ray crystallography has been used to determine the three-dimensional structure of the atoms in molecules such as the ribosome. The 2010 Nobel Prize in chemistry was awarded to Ada Yonath, Tom Steitz, and Venki Ramakrishan for ribosomal structure determination. Superimposition of the atomic coordinates of crystallized 50S subunits from Haloarcula marismortui and Thermus thermophilus, species representing disparate regions of the evolutionary tree, along with other information, suggests that the core of the ribosome is an ancient molecular fossil that predates coded protein (Hsiao et al. 2009). In part, these findings have led to the search for the ancestral peptidyl transferase center (or “ancestral PTC”) of the ribosome, the minimal rRNA necessary for folding and general condensation to produce proto-proteins.
Basic Methodology ● ▶ Oligonucleotide synthesis. Many DNA oligonucleotides are readily obtained by automated chemical synthesis (often on solid support, with synthesis occurring in the 30 –50 direction) at lengths of up to 100 nucleotides. Chemical synthesis of RNA is more difficult but the technology has improved substantially
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over the last decade. Most labs outsource their synthesis to highly efficient commercial enterprises. DNA sequencing. ▶ DNA sequencing is the determination of the sequence of the nucleotides A, C, G, and T in a nucleic acid. DNA sequencing technology is highly mature. A modern, high-throughput method uses fluorescent dye labeled chain terminator dideoxynucleotide triphosphates in a single sequencing reaction. Restriction. DNA can be cut at defined positions in a sequence by commercially available restriction enzymes. Commonly utilized type II enzymes cleave DNAwithin a specific palindromic recognition sequence (compared to type I enzymes that cleave DNA far from the recognition sequence at random) producing DNA with blunt or dangling ends of known sequence. Ligation. DNA fragments can be linked by a ligase such as T4, which catalyzes the formation of a phosphodiester bond between a 30 terminal hydroxyl and 50 terminal phosphate in duplex DNA. Ligases are commercially available. Polymerase Chain Reaction. PCR is the amplification of DNA sequences from short oligonucleotides or target regions of plasmid or genomic DNA. Target sequences are amplified with primers containing a short sequence complementary to the target sequence and extraneous specific sequences coding for restriction sites and/or promoters. One primer anneals to the sense strand and the other to the antisense strand of the DNA. A DNA polymerase performs the amplification work by creating new strands of DNA, with loose dNTPs added to the reaction. Amplification with primers containing specific restriction sites allows for subsequent ligation into a bacterial plasmid also containing those restriction sites. ▶ Electrophoresis. A basic technique used to separate nucleic acids by length, shape, or protein association in a fluid covered matrix (such as agar or polyacrylamide) by an external electric field. RNA Transcription. DNA with a promoter region (such as T7) can be transcribed to produce singlestranded RNA in vitro with RNA polymerase. Crystallography. The coordinates of atoms in molecules such as DNA, RNA, or protein complexes of either can be resolved in three dimensions using X-ray diffraction crystallography, the technique of growing crystals in solution by slowly lowering the solubility of component molecules, mounting the crystals on a loop, and irradiating them with a beam of monochromatic X-rays through a 180 degree rotation.
See also ▶ Adenine ▶ Chicken or Egg Problem ▶ Cytosine ▶ DNA ▶ DNA Damage ▶ DNA Repair ▶ DNA Sequencing ▶ Electrophoresis ▶ Extraterrestrial Delivery (Organic Compounds) ▶ Genome ▶ Guanine (Gua) ▶ Nucleic Acid Base ▶ Nucleoside ▶ Nucleotide ▶ Oligonucleotide ▶ Phylogenetic Tree ▶ Phylogeny ▶ Polymerase Chain Reaction ▶ Prebiotic Chemistry ▶ Purine Bases ▶ Pyrimidine Base ▶ Ribonucleoside ▶ Ribonucleotide ▶ Ribose ▶ Ribosome ▶ RNA ▶ RNA World ▶ Thymine (T) ▶ Uracil (Ura) ▶ Watson–Crick Pairing
References and Further Reading Crick F (1970) Central dogma of molecular biology. Nature 226:561–563 Devoe H, Tinoco I Jr (1962) The stability of helical polynucleotides: base contributions. J Mol Biol 4:500–517 Franklin RE, Gosling RG (1953) The structure of sodium thymonucleate fibres. I. The influence of water content. Acta Crystallogr 6 (8–9):673–677 Gilbert W (1986) Origin of life: the RNA world. Nature 319(6055):618–618 Guerrier-Takada C, Gardiner K, Marsh T, Pace N, Altman S (1983) The RNA moiety of ribonuclease P Is the catalytic subunit of the enzyme. Cell 35(3 Pt 2):849–857 Hsiao C, Mohan S, Kalahar BK, Williams LD (2009) Peeling the onion: ribosomes are ancient molecular fossils. Mol Biol Evol 26(11):2415–2425 Kim SH, Sussman JL, Suddath FL, Quigley GJ, McPherson A, Wang AH, Seeman NC, Rich A (1974) The general structure of transfer RNA molecules. Proc Natl Acad Sci U S A 71(12):4970–4974 Kornberg RD (2007) The molecular basis of eukaryotic transcription. Proc Natl Acad Sci U S A 104(32):12955–12961 Kruger K, Grabowski PJ, Zaug AJ, Sands J, Gottschling DE, Cech TR (1982) Self-splicing RNA: autoexcision and autocyclization of the
Nucleoside ribosomal RNA intervening sequence of tetrahymena. Cell 31(1):147–157 Lescoute A, Leontis NB, Massire C, Westhof E (2005) Recurrent structural RNA motifs, Isostericity matrices and sequence alignments. Nucleic Acids Res 33(8):2395–2409 Mullis KB, Faloona FA (1987) Specific synthesis of DNA in vitro via a polymerase-catalyzed chain reaction. Meth Enzymol 155:335–350 Oro J, Kimball AP (1962) Synthesis of purines under possible primitive earth conditions. II. Purine intermediates from hydrogen cyanide. Arch Biochem Biophys 96:293–313 Poole AM, Jeffares DC, Penny D (1998) The path from the RNA world. J Mol Evol 46(1):1–17 Rich A (1962) In: Kasha M, Pullman B (eds) Horizons in biochemistry. Academic, New York, pp 103–126 Roberts RJ (1976) Restriction endonucleases. CRC Crit Rev Biochem 4(2):123–164 Saenger W (1984) Principles of nucleic acid structure. Springer, New York, p 556 Schmitt-Kopplin P, Gabelica Z, Gougeon RD, Fekete A, Kanawati B, Harir M, Gebefuegi I, Eckel G, Hertkorn N (2010) High molecular diversity of extraterrestrial organic matter in murchison meteorite revealed 40 years after its fall. Proc Natl Acad Sci U S A 107(7):2763–2768 Voet JG, Voet D (2003) Biochemistry. Wiley, New York Watson JD, Crick FH (1953) Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid. Nature 171(5451):737–738 Woese CR, Fox GE (1977) Phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proc Natl Acad Sci U S A 74(11): 5088–5090 Zamenhof S, Brawerman G, Chargaff E (1952) On the desoxypentose nucleic acids from several microorganisms. Biochim Biophys Acta 9(4):402–405
Nucleoid Definition The nucleoid is the area of the prokaryotic cells in which the genetic material, the ▶ chromosome, is localized.
See also ▶ Cell ▶ Chromosome ▶ Genome
Nucleon Definition In chemistry and physics, a nucleon is the collective name for the two subatomic particles the neutron and the proton. Nucleons are constituents of the atomic nucleus. They are composed of still smaller subatomic particles, the quarks.
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Nucleoside JOHN H. CHALMERS Scripps Institute of Oceanography Geosciences Research Division, University of California, San Diego, La Jolla, CA, USA
Synonyms Ribosylaminoadenine; Ribosylaminocytosine; Ribosylaminoguanine; Ribosylaminouracil
Keywords Adenosine, cytidine, ▶ deoxyribose, DNA, guanosine, ▶ nucleic acid base, purine, pyrimidine, ▶ ribose, RNA, thymidine, uridine
Definition Nucleosides are ribosides and deoxyribosides of purine and pyrimidine nucleic acid bases (nucleobases) formed via beta-glycoside linkage to ring nitrogen atoms. The canonical ribonucleosides in RNA are adenosine, cytidine, guanosine, and uridine. In DNA, deoxythymidine replaces uridine and deoxyribose replaces the ribose moieties of adenosine, cytidine, and guanosine. Chemically modified nucleosides are also found in RNA and DNA, where they play functional roles in regulation, translation, and protection from viral infection, as well as in tRNA and rRNA where they play structural roles. These species include methylated adenines and cytosines as well as hydroxymethylated, aminoacylated, and glucoslyated species.
Overview Plausible, robust prebiotic syntheses of the nucleosides in DNA and RNA are among the most pressing unsolved problems in ▶ origin of life research (Orgel 2004; Switzer 2009). The prebiotic synthesis of ▶ adenine was accomplished by Oro´ in 1960 from ammonium cyanide, and variations of the synthesis yielded guanine, xanthine, hypoxanthine, and diaminopurine (Orgel 2004). Similar mixtures of purines have also been made by Saladino and coworkers from formamide (Orgel 2004). Syntheses of ▶ uracil and ▶ cytosine from cyanoacetaldehyde, the hydration product of cyanoacetylene, and urea or guanidine are efficient and also produce small amounts of isocytosine and diaminopyrimidine (Robertson and Miller 1995a). Formaldehyde reacts readily with uracil to make 5-hydroxymethyluracil, which can be readily reduced to ▶ thymine with formate (Robertson and Miller 1995b).
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Nucleoside 5’-monophosphorimidazolide
The instability and low yields of ribose from the formose reaction have been considered serious barriers, but the addition of lead, borate, or silicate ions to the reaction increases the yield (Zubay 1998; Ricardo et al. 2004; Lambert et al. 2010). Deoxyribose is somewhat more stable and reactive than ribose (Larralde et al. 1995; Dworkin et al. 2003), but fewer studies have been done. Adenosine, guanosine, xanthosine, and inosine have been made in significant yields by heating dry ribose, Mg2+, and sodium metaphosphate with adenine, ▶ guanine, xanthine, and ▶ hypoxanthine, respectively (Orgel 2004; Switzer 2009). Unfortunately, analogous experiments with ▶ pyrimidine bases have been unsuccessful except with 2-pyrimidone, which readily forms zebularine (Bean et al. 2007). To get around this difficulty, attempts have been made to start with ribose and build up the pyrimidine ring de novo by Sanchez and Orgel (Orgel 2004; Switzer 2009). Alternatively, Sutherland and colleagues have reacted glycolaldehyde with cyanamide and constructed both the ribose and pyrimidine moieties together by sequential addition of glyceraldehyde, cyanoacetylene, urea, and pyrophosphate followed by UV irradiation (Powner et al. 2009). This procedure produces cytidine and uridine 20 -30 -cyclic phosphates. Progress is being made, but a great deal more research needs to be done to develop robust syntheses of nucleosides or ▶ nucleotides. Because of these difficulties, analogues in which either the sugars or the bases have been altered have been synthesized for origin of life research (Eschenmoser 1999; Dworkin et al. 2003), DNA sequencing, cancer and AIDS chemotherapy, and other applications (Sismour and Benner 2005). These compounds include C-glycosides like the naturally occurring pseudouridine.
References and Further Reading Bean HD, Sheng Y, Collins JP, Anet FAL, Leszczynski J, Hud NV (2007) Formation of a pyrimidine nucleoside by a free pyrimidine base and ribose in a plausible prebiotic reaction. J Am Chem Soc 129(31):9556–9557 Dworkin JP, Lazcano A, Miller SL (2003) The roads to and from the RNA world. J Theor Biol 22:127–134 Eschenmoser A (1999) Chemical etiology of nucleic acid structure. Science 284:2118–2124 Lambert JB, Gurusamy-Thangavelu SA, Ma K (2010) The silicatemediated formose reaction: bottom-up synthesis of sugar silicates. Science 327:984–986 Larralde R, Robertson MP, Miller SL (1995) Rates of decomposition of ribose and other sugars: implications for chemical evolution. Proc Nat Acad Sci 92:8158–8160 Orgel LE (2004) Prebiotic chemistry and the origin of the RNA world. Crit Rev Biochem Mol Biol 39:99–123 Powner MW, Gerland B, Sutherland JD (2009) Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Nature 459(7244):239–242 Ricardo A, Carrigan MA, Olcott AN, Benner SA (2004) Borate minerals stabilize ribose. Science 303(5655):196 Robertson MP, Miller SL (1995a) An efficient prebiotic synthesis of cytosine and uracil. Nature 327:724–772 Robertson MP, Miller SL (1995b) Prebiotic synthesis of 5-substituted uracils: a bridge between the RNA world and the DNA-protein world. Science 268:702–705 Sismour MA, Benner SA (2005) Synthetic biology. Expert Opin Biol Ther 5:1409–1414 Switzer C (2009) A missing prebiotic link: discovery of a plausible synthesis of pyrimidine nucleotides. Chembiochem 10:2591–2593 Zubay G (1998) Studies on the lead-catalyzed synthesis of aldopentoses. Orig Life Evol Biospheres 28(1):13–26
Nucleoside 5’monophosphorimidazolide ▶ Activated Nucleotide
See also ▶ Adenine ▶ Cytosine ▶ Deoxyribose ▶ Guanine (Gua) ▶ Hypoxanthine ▶ Nucleic Acid Base ▶ Nucleotide ▶ Phosphates ▶ Purine Bases ▶ Pyrimidine Base ▶ Ribonucleoside ▶ Ribose ▶ Thymine (T) ▶ Uracil (Ura)
Nucleoside Phosphoimidazolide KUNIO KAWAMURA Department of Applied Chemistry, Osaka Prefecture University, Sakai, Osaka, Japan
Synonyms Activated nucleotide
Keywords Chemical evolution, nucleotide, oligonucleotide, RNA
Nucleosynthesis, Explosive
Definition In chemical evolution and related fields, a nucleoside phosphorimidazolide is defined as “a nucleoside 50 monophosphorimidazolide,” where imidazole is a leaving group bound to the phosphate of a nucleoside 50 -monophosphate via an N-P bond. Nucleoside phosphorimidazolides are a kind of ▶ activated nucleotide. The leaving group supplies energy to form ▶ oligonucleotides from the nucleoside phosphorimidazolide. Prebiotic experiments have showed that nucleoside 50 phosphorimidazolide monomers form ▶ RNA oligomers up to 50 nucleotide units in length in the presence of metal catalysts, clay mineral catalysts, or a polynucleotide template.
Overview The accumulation of RNA molecules should have been an essential step for the emergence of life-like systems on the primitive earth. In modern organisms, polymerization of RNA monomers merely proceeds under the specific conditions, of which thermodynamics and kinetics allow the biochemical polymerization in organisms. Nucleoside 50 triphosphate monomers are only used to synthesize RNA polymers in organisms. The cleavage of phosphoester bonding within triphosphate moiety supplies energy for the formation of phosphodiester bonding of RNA oligomers. Furthermore, the polymerization of the nucleoside 50 -triphosphate is accelerated by an RNA polymerase and directed by a DNA template in organisms giving the 30 –50 isomers in a selective way. In the absence of an RNA polymerase or the absence of a DNA template, the polymerization of 50 -triphosphate does not proceed. In addition, it is known that RNA polymers are formed from nucleoside 50 -diphosphate in the presence of polynucleotide phosphorylase without a DNA template. The formation of DNA molecules also proceeds by similar mechanism. In these reactions using 50 -triphosphate or 50 -diphosphate, the phosphoester bonding within the 50 triphosphate or 50 -diphosphate moieties provides sufficient energy for the formation of phosphodiester bonding of RNA polymers. This indicates that high-energy nucleotide monomers should have been essential for the formation of RNA polymers under the primitive earth conditions. Thus, the pathways to form RNA polymers without an enzyme and a template molecule have been investigated under the simulated primitive earth conditions. Continuous investigations by Orgel and coworkers have identified nucleoside 50 -phosphorimidazolide, a candidate primitive activated nucleotide monomer for the first time (Lohrmann and Orgel 1973; Orgel and Lohrmann 1974).
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The formation of nucleoside 50 -phosphorimidazolides proceeds under the potential primitive earth conditions (Lohrmann 1977). The nucleoside 50 phosphorimidazolide and related compounds have been examined whether oligonucleotides could be formed under the primitive earth conditions. Successful studies including RNA oligomer formation in the presence of metal catalysts (Sawai 1976), clay mineral catalyst (Ferris and Ertem 1992; Ferris et al. 1996), and a polynucleotide template (Inoue and Orgel 1983) showed RNA oligomers up to 50 nucleotide units in length form under primitive earth conditions. The variations using different bases instead of imdazole also facilitate the formation of oligonucleotides (Huang and Ferris 2006).
See also ▶ Oligonucleotide ▶ RNA ▶ RNA World
References and Further Reading Ferris JP, Ertem G (1992) Oligomerization of ribonucleotides on montmorillonite: reaction of the 50 -phosphorimidazolide of adenosine. Science 257:1387–1389 Ferris JP, Hill AR Jr, Liu R, Orgel LE (1996) Synthesis of long prebiotic oligomers on mineral surfaces. Nature 381:59–61 Huang W, Ferris JP (2006) One-step, regioselective synthesis of up to 50-mers of RNA oligomers by montmorillonite catalysis. J Am Chem Soc 128:8914–8919 Inoue T, Orgel LE (1983) A nonenzymatic RNA polymerase model. Science 219:859–862 Lohrmann R (1977) Formation of nucleoside 50 -phosphoramidates under potentially prebiological conditions. J Mol Evol 10:137–154 Lohrmann R, Orgel LE (1973) Prebiotic activation processes. Nature 244:418–420 Orgel LE, Lohrmann R (1974) Prebiotic chemistry and nucleic acid replication. Acc Chem Res 7:368–377 Sawai H (1976) Catalysis of internucleotide bond formation by divalent metal ions. J Am Chem Soc 98:7037–7039
Nucleosynthesis, Explosive Synonyms Explosive Nucleosynthesis
Definition Explosive nucleosynthesis refers to nucleosynthetic processes occurring in stellar explosions (▶ supernovae, novae, X- and g-ray bursts, etc.). Because of the short timescale,
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Nucleosynthesis, Neutrino
no weak interactions (modifying the neutron/proton ratio N/Z) occur. Explosive Si-burning (fusion) in core collapse and thermonuclear supernovae produces mostly 56Ni (N=Z=28), which decays to stable 56Fe; the characteristic g-ray lines of the decay were detected in ▶ supernova SN1987A, confirming the theoretical predictions. The so-called r-nuclides are also produced by explosive nucleosynthesis, through the rapid capture of neutrons in the innermost regions of core collapse supernovae.
See also
See also ▶ Nucleosynthesis, Explosive ▶ Supernovae
Nucleotide ERNESTO DI MAURO, SAMANTA PINO Department of Biology and Biotechnologies “Charles Darwin”, University of Rome “Sapienza”, Rome, Italy
▶ R-process ▶ Supernovae
Definition
Nucleosynthesis, Neutrino Definition Neutrino nucleosynthesis refers to the non-thermonuclear mode of nucleosynthesis occurring during the explosion of a massive star. A fraction of the 1058 neutrinos released by the supernova has energies comparable to the binding energies of the heavy nuclei (several MeV/nucleon) which are found in the various supernova layers. Neutrinos knock out one particle from such nuclei creating lighter ones, e.g., 11 B from 12C or 19F from 20Ne.
Purine
Pyrimidine N
N N
N
N N NH2
O N
NH N
NH2
Nucleotides are glycosylated derivatives of purine and pyrimidine heterocyclic bases. The purine derivatives are adenine and guanine; the pyrimidine derivatives are ▶ cytosine, uracil, and thymine (acronyms A,G,C,U,T), as shown in Fig. 1. Thymine occurs only as a deoxyribonucleotide and uracil only as a ribonucleotide. The bases afford compounds termed nucleosides, or deoxynucleosides, by linking to a carbohydrate, ▶ ribose or 2-deoxyribose, respectively. In the case of a link to 0 D-ribose (or to its 2 -deoxy form), a b-N-glycosidic bond is formed between the N9 of a purine or the N1 of a pyrimidine and the anomeric carbon of the sugar. Nucleosides can be mono-, di-, or tri-phosphorylated affording the corresponding nucleotides (Fig. 2).
N
N
N
X
X
Guanine, G
Cytosine, C O
NH2 N
N N
O
N X
Adenine, A
O H3C
NH N
O
X Uracil, U
NH N
O
X Thymine, T
Nucleotide. Figure 1 The nucleic bases. X = H: base. X = ribose or deoxyribose: nucleoside. X = ribose phosphate: nucleotide
Nucleotide Oligomer
Base NH2 N Phosphate
N
O –
O P O CH2 O–
N N
●
O H
●
●
H
H
H OH
OH
Sugar
Nucleotide. Figure 2 The nucleic base adenine bound to D-ribose yields the biologically predominant anti-adenosine conformer. In the syn conformation, the base is oppositely oriented about the N-glycosidic bond. The monophosphate nucleotide (50 AMP) is shown
Overview
Phosphorylation can occur at the 20 , 30 or 50 positions and cyclic 20 –30 and 30 –50 nucleotides may form, many of which play important biological functions. These compounds are those more commonly encountered in biological systems. Numerous alternatives and/or complex derivatives have been described both for the base and the sugar moieties, and for the number of phosphate groups. Nucleotides are among the most important components of the cell and its metabolism, both in ▶ anabolism and catabolism. In addition to being the precursors of nucleic acids, nucleotides are allosteric effectors of enzyme activity, and thus exert numerous controls on enzymatic reactions. The major regulatory functions exerted often pertain to the pathways involved in the syntheses of the nucleotides themselves and to the reactions affording their precursors. Consequently, the pool of nucleotides in the cell is the result of strict anabolic equilibria. The sophisticated compensatory mechanisms present at every level of the biological kingdoms are explained in evolutionary terms by their double role of providers of energy for phosphate transfer reactions and of genetic building blocks. In fact: ● Purine triphosphate nucleotides exert a particularly relevant role, GTP being an essential component of the translocation step in protein synthesis. (One GTP is used for the addition of one amino acid unit in
●
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protein polymerization from a tRNA–amino acid complex whose formation requires one ▶ ATP.) The nucleotides ATP, GTP, CTP, TTP, and UTP are required in equilibrated concentrations for the polymerization of RNA and ▶ DNA. ATP is the predominant energy store for phosphate transfer reactions. Nucleotides are constituents of key coenzymes (FAD, coenzyme A, NAD+, NADP+) and are activated intermediates in important biosynthetic reactions, directly or in modified forms (such as S-adenosylmethionine and sugar-coupled nucleotides). Many nucleotide derivatives are metabolites which serve as alarmones, i.e., 30 -50 cyclic AMP (cAMP), ppGpp, pppGpp (guanosine tetra- and pentaphosphates), ApppA (diadenosine tetraphosphate), ZTP (5-amino 4-imidazole carboxamide riboside 50 -triphosphate). These compounds are in general formed under stress conditions such as the scarcity of a carbon source, amino acids, tRNA, or folate, or in the presence of oxidative stress, favoring alternative or escape metabolic responses.
See also ▶ Adenine ▶ Anabolism ▶ ATP ▶ Co-enzyme ▶ Cytosine ▶ Deoxyribose ▶ DNA ▶ Guanine (Gua) ▶ Heterocycle ▶ Nucleic Acids ▶ Nucleic Acid Base ▶ Nucleoside ▶ Pyrimidine Base ▶ Ribose ▶ RNA ▶ Thymine (T) ▶ Uracil (Ura)
Nucleotide Oligomer ▶ Oligonucleotide
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Nucleus
Nucleus
Synonyms Dark fringe interferometry
Definition
Keywords
In biology, Nucleus is a double-▶ membrane limited space in the eukaryotic ▶ cell containing ▶ chromosomes and nucleoli. The double membrane is perforated by the nuclear pores that allow the metabolic and genetic connection with the ▶ cytoplasm. The outermost envelope is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus (for example, as a result of fusion of several cells – syncytium – or by nuclear division without cell separation – coenocyte). Some protists have two types of nuclei (macro- and micro-nucleus), and in the arachniophyta, there is the remnant of an ancient nucleus from an engulfed alga (nucleomorph). Albeit the evolutionary origin of the nucleus remains unsolved, several authors suggest that the nucleocytoplasm is the result of an ancient symbiotic association between a bacterial cell and an archaeal cell.
▶ Coronography, ▶ exoplanet detection
See also ▶ Cell ▶ Chromosome ▶ Cytoplasm ▶ Membrane
Nuclide Definition In chemistry and physics, the term nuclide refers to an atom with a distinct number of protons and neutrons in its nucleus. Nuclides may be stable or unstable. Of the 3100 known nuclides, there are 256 that are so stable that they have never been observed to decay. Unstable nuclides are radioactive and are called radionuclides. Their daughter decay products are called radiogenic nuclides. Nuclides with the same number of protons (of the same chemical element), but differing numbers of neutrons, are called isotopes.
See also ▶ Isotope
Nulling Interferometry DANIEL ROUAN LESIA, Observatoire de Paris, CNRS, UPMC, Universite´ Paris-Diderot, Meudon, France
Definition Nulling ▶ interferometry is the name given to an instrumental technique based on interferences between several telescopes to detect directly exoplanets. The principle is to create a virtual “blind spot” at the exact location of a bright source, a star, in order to reveal the much fainter source, which is a planet orbiting it.
Overview This technique is a variant of coronagraphy and aims at reducing the extreme contrast of intensity that may exist between two objects very close one to the other, as a star–planet system, but here the solution lies in the interferometric recombination of radiation from several telescopes. Coronagraphy is used primarily in the visible and near infrared: the angular resolution of a large telescope is indeed 10 times better than the angle subtended by a planetary orbit of 1 AU at a distance of 10 pc; however, the star–planet contrast still remains very high (typically 1010). On the other hand, in the mid-infrared (l=5–20 mm), this contrast is much more favorable since it is around 106–5. Furthermore, a set of bio-signatures, less ambiguous than those observable in the visible, is found in this domain: ozone (band at 9.7 mm), carbon dioxide, and water vapor. It has been suggested that the simultaneous presence of these three signatures may constitute strong evidence of the presence of life. But an angular resolution of 0.1 arcsec at l=15 mm would require a telescope with a diameter exceeding 30 m which is not presently conceivable in space. However, the issue of blocking the glare from the star using several small telescopes found a solution with the proposal by Ronald Bracewell of black fringe interferometry (or nulling interferometry). The first step, illustrated in Fig. 1, is to produce interference fringes projected onto the sky by interferometric recombination of radiation from two telescopes separated by a distance D. The fringes alternatively transmit and block the light, so, if it is arranged that the star is put on a dark fringe and that half the inter-fringe spacing corresponds to the distance separating the star from the putative planet, then the contrast is optimal. In a conventional interferometer, only the bright central fringe, which corresponds to a zero path difference, is common to all wavelengths, while the fringe systems at different wavelengths overlap each other, leading to
Nulling Interferometry
Star
D. sinθ
T1
θ
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Planet
D
+π
T2
Recombination
1 arcsec λ = 10 μm, D = 10 m, θ = 0.1 arcsec
Nulling Interferometry. Figure 1 The principle of nulling interferometry. Left: the optical scheme. The light collected by two telescopes pointing in the same direction is recombined after an optical device (in blue) has introduced a phase shift of p on one arm of the recombination path. The resulting interference pattern is characterized by a central fringe, which is dark and not bright as in a conventional interferometer. Right: the fringe pattern. This is in fact a transmission map projected onto the sky. If the star (yellow) is put on the central dark fringe, it is masked, while the fringes’ separation can be adjusted (by changing the telescopes separation), so as to correspond to a maximum of transmission at the location of the planet (red)
a global blurring and no actual dark fringe. On the other hand, it is unthinkable to operate a nulling interferometer at a unique wavelength to detect an exoplanet, because the planet’s brightness is extremely low and, in addition, the needed spectral information is in a wide wavelength range (5–17 mm). The powerful idea of Bracewell was to insert an optical phase shift of p, not dependent on the wavelength and thus said to be achromatic, in one arm of the interferometer: The central fringe becomes black but is still common to all wavelengths. Putting the star on this fringe then leads in principle to an efficient rejection of its light without a limitation in spectral band.
Basic Methodology Designing a nulling interferometer supposes that two or more large telescopes, separated by a distance of at least 50 m, are recombined coherently and that a special device, called an achromatic phase shifter, is introduced in one of the beams before recombination. In addition, since the expected brightness of the exoplanet is extremely low, the usually strong background that characterizes observations in the infrared has to be maximally reduced: this means that the telescopes must be at a very low temperature, typically below 100 K. If we add that the atmosphere absorbs a large part of the useful wavelength range, one understands that the space environment is mandatory. Finally, a very precise control of wavefront defects and a precise matching of the interferometer arms are required
to achieve the required null depth of 105. For all those reasons, it appears that detecting and characterizing exoplanets using nulling interferometry is an extremely ambitious goal and will not come to fruition before probably one or two decades. Note that the measured signal is not only the sum of that from the planet(s) and some residual light from the star. There is also light corresponding to starlight scattered by the dusty disk of micron-sized particles, the remnants of the protoplanetary disk around the star. This disk is a powerful source of emission, the so-called exozodi light, which may be well above the emission from the planets. To distinguish the fraction of radiation specific to the planet, one can modulate its signal. Bracewell proposed to rotate the interferometer around the line of sight: the image of the planet then travels on the system of fringes projected onto the sky, becoming sometimes visible, sometimes extinct, so that only the signal modulated at the rotation frequency and consistent with the expected pattern, reflects the contribution of the planet.
Key Research Findings Several difficulties emerged when the study of nulling interferometers became more detailed. First, the angular size of the star is not infinitely small and photons at the edge of the stellar disk can escape the nulling. They represent only a tiny fraction of the starlight, but the contrast planet/star is so small that they are still
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predominant and the corresponding noise becomes a limitation. It may be shown that using more than two telescopes can overcome this limit. Second, how does one achieve the achromatic phase shift of p. Several solutions with good performances have been demonstrated in the laboratory: sets of transparent blades in each arm of the interferometer with proper materials and thicknesses, use of symmetrical pairs of periscopes, crossing of a focus in one arm, use of specific checkboard mirrors, use of light dispersion on a deformable mirror, etc. Third, the signatures between a planet and the exozodi seen partly edge-on (appearing thus elliptic) must be separated: the modulation by a simple system of fringes would be undistinguishable. With a smart choice of interferometer configuration one can, however, distinguish the modulated signal produced by a single object from the one due to a centrosymmetric structure such as the exozodi. Fourth, the cancellation of the stellar flux must have great temporal stability. This is, perhaps, the most serious difficulty: any deviation, even very small, from the p phase shift produces very large fluctuations in the starlight leakage, introducing an extra noise. This point is subject to particular attention in the laboratory experiments. Fifth, the free-flying interferometer concept should be evaluated. A nulling interferometer aiming at exoplanet detection is meaningful only in space, but the size of the baseline, larger than 30 m, makes unbelievable a system where telescopes would be physically connected. The concept which is being studied is one of several free-flying crafts, each carrying a telescope, plus one carrying the recombination station, with distances between them made constant through some servo-system using micro jet engines and micrometric distance measurement. The Darwin mission concept is based on this idea (Fig. 2).
Nulling Interferometry. Figure 2 Artist’s view of the most recent concept of the Darwin project studied by ESA and NASA. The arrangement of the four telescopes in a X pattern allows modulation in a special way of the light from the planet, so that it may be distinguished from the light emitted by a cloud of small particles (the exozodiacal disk) in the plane of the planetary system; this cloud is believed to surpass the planet’s brightness by a large factor
Applications
Future Directions
The main goal of this technique, which will takes many years to reach an appropriate technology readiness level, is the detection and characterization of objects analogous to the Earth, that is, small exoplanets in the habitable zone, and ultimately to detect the presence of life. The Darwin mission is the emblematic project, recently resubmitted to space agencies, which illustrates the best state of the art in terms of specific studies carried out over several years. The present concept relies on four cryogenic telescopes of 1 m diameter, arranged on a X-shaped pattern, and a recombining station which is at the common focus of the telescopes, avoiding thus the use of the complex delay lines usually found in interferometers. The distance
between telescopes can be changed and a clever phase modulation allows astronomers to distinguish the contribution of a planet from the one of exozodi. Another goal that appeared more recently is the possibility to use a nulling interferometer from the ground to detect the exozodi in several stars, this knowledge being important for preparing future space missions. Several experiments are being developed in that direction at the Keck and at the LBT telescopes in the USA. Studies to implement a dedicated experiment, Aladdin, on the Antarctic continent – at dome C – are also being conducted in Europe.
A vigorous program of research and development has been put in place, especially in the USA and in Europe, in order to find the most appropriate solutions to the various technical problems listed above. For several of them, the performance level reached in the laboratory proves that there should be no definitive technical blockade; however, it must be clear that the road will be a long one before a nulling interferometer mission can be actually launched.
See also ▶ Coronagraphy ▶ Darwin’s Conception of Origins of Life
Nuvvuagittuq (Porpoise Cove) Greenstone Belt
▶ Exoplanet, Detection and Characterization ▶ Exozodiacal Light ▶ Interferometry ▶ Space Environment
References and Further Reading Angel JRP, Woolf NJ (1997) An imaging nulling interferometer to study extrasolar planets. Astrophys J 475:373–379 Bracewell RN (1978) Detecting nonsolar planets by spinning infrared interferometer. Nature 274:780–781 Bracewell RN, MacPhie RH (1979) Searching for nonsolar planets. Icarus 38(1):136–147 Cockell CS, Herbst T, Le´ger A (2008) Darwin, an experimental astronomy mission to search for extrasolar planets. Exp Astron 46 Defre`re D, Absil O, Coude´ du Foresto V, Danchi WC, den Hartog R (2008) Nulling interferometry: performance comparison between space and ground-based sites for exozodiacal disc detection. Astron Astrophys 490:435 Lay OP (2004) Systematic errors in nulling interferometers. Appl Opt 43(33):6100–6123 Le´ger A, Mariotti JM, Mennesson B et al (1996) Icarus 123:249 Marc Ollivier (2007) Towards the spectroscopic analysis of Earthlike planets: the DARWIN/TPF project. CR Phys 8(3–4):408–414 Rouan D (2007) Ultra deep nulling interferometry using fractal interferoers. CR Phys 8(3–4):415–425 Rouan D, Pelat D (2008) The achromatic chessboard, a new concept of a phase shifter for nulling interferometry. I. Theory. Astron Astrophys 484:581 Serabyn E (2000) Nulling interferometry: symmetry requirements and experimental results. In: Le´na P, Quirrenbach A (eds) Interferometry in optical astronomy, Proc. SPIE, vol 4006, pp 328–339
Numerical Taxonomy ▶ Phenetics
Nutrient Cycles ▶ Biogeochemical Cycles
Nutrients ▶ Macronutrient
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Nuvvuagittuq (Porpoise Cove) Greenstone Belt JONATHAN O’NEIL Carnegie Institution of Washington, Washington, DC, USA
Synonyms Porpoise Cove greenstone belt
Keywords Banded iron formation, Early Earth, Eoarchean, Extinct radionuclides, Greenstone belt, Hadean, Primitive crust, Superior Province, Volcanic rocks
Definition The Nuvvuagittuq greenstone belt is a ▶ Hadean/ Eoarchean volcano-sedimentary sequence located on the Eastern shore of the Hudson Bay in northern Quebec. The belt’s main rock units are composed of meta-igneous (cummingtonite-amphibolite, meta (mafic and ultramafic intrusions), and sedimentary rocks (banded iron formation (BIF) and silicic rocks) Zircon ages give a minimum age of 3.8 Ga to the belt (Cates and Mojzsis 2007; David et al. 2009). The cummingtonite-rich amphibolite called the “faux-amphibolite” (so called because of its unusual light color compared to the common dark amphibolite containing hornblende) showed a deficit in 142Nd compared to the terrestrial Nd standard, suggesting they may have formed 4.28 billion years ago. This would make them the oldest rocks known on the Earth.
Overview The Nuvvuagittuq greenstone belt is located in the Northeastern Superior Province of Canada, approximately 35 km south of the Inuit municipality of Inukjuak on the east coast of Hudson Bay. The Nuvvuagittuq greenstone belt covers an area of 12 km2 and is composed of a Hadean to Eoarchean volcano-sedimentary sequence (O’Neil et al. 2007, 2008) comprising rocks possibly as old as 4.28 billion years ago, which would make them the oldest rocks known on the Earth (O’Neil et al. 2008) and the ideal place for studying early Earth environmental conditions to life.
Significance for Early Earth Environments The geochemistry of the dominant lithologies in the Nuvuaggittuq greenstone belt is consistent with its
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protolith being hydrothermally altered volcanic rocks comprising BIF precipitated from seawater. The Nuvuaggittuq greenstone belt would therefore represent a portion of Eoarchean/Hadean oceanic crust. The presence of a presumably 4.28 Ga BIF within the Nuvvuagittuq greenstone belt potentially could have a significant impact on the study of the timing of the origin of life on Earth. Although the causes of Fe precipitation throughout geologic time are poorly understood, the BIF that occurs in ▶ Archean greenstone belts must have been precipitated under anoxic conditions. It has been suggested that Fe2+ can be directly oxidized by microbial activity under anaerobic conditions (Konhauser et al. 2002). However, establishing the chemical sedimentary origin for a rock is a prerequisite to demonstrating potential biological activity involved in its formation. The trace element chemistry of the Nuvvuagittuq BIF is consistent with a marine exhalative origin with a distinct seawater signature (O’Neil et al. 2007). The Nuvvuagittuq BIF has heavier Fe isotopic compositions than the surrounding igneous rocks (Dauphas et al. 2007; O’Neil et al. 2007), a feature that is consistent with an origin as a chemical precipitate. Although the mechanism(s) responsible are not well understood, the Fe isotopic fractionation observed in the BIF of Nuvvuagittuq raises the possibility that life was already established on Earth at nearly 4.3 Ga. This transition is typical of younger Archean greenstone belt suggesting that the environments and geological processes responsible for the formation of younger greenstone belts were already active in the Eoarchean and perhaps in the Hadean. The Nuvvuagittuq BIF thus represents the ideal candidate to address the controversial question of the timing of the origin of life on Earth. The geochemistry of the mafic rocks is consistent with a transition from tholeites to calc-alkaline volcanics comprising chemical sediments.
Geochronology of the Nuvvuagittuq Greenstone Belt Recent geochemical studies on the Nuvvuagittuq greenstone belt showed that the faux-amphibolite have a deficit in 142Nd of 7–15 ppm compared to the terrestrial Nd standard (O’Neil et al. 2008). The negative 142Nd anomaly of the faux-amphibolite is interpreted to reflect its formation within the first 300 million of Earth’s history, while 146 Sm was still actively decaying (146Sm has a half-life of 103 million years). A correlation between the Sm/Nd ratios of the faux-amphibolites and their 142Nd isotopic composition has a slope corresponding to an age of 4280þ53 81 Ma, which would make them the oldest known rocks on the Earth (O’Neil et al. 2008) and the only known
remnant of Hadean crust preserved on Earth. The oldest U-Pb dates from the Nuvvuagittuq greenstone belt have been obtained in rare thin intrusive felsic bands of tonalitic composition. Zircons from these felsic bands have yielded U-Pb ages of 3817 16 (David et al. 2009) and 3751 10 Ma (Cates and Mojzsis 2007) that are interpreted to be their crystallization age. The 147 Sm-143Nd whole-rock isochron obtained on a highly deformed gneissic gabbro sills intruding the fauxamphibolite gives an age of 4,023 110 Ma, which supports an older age for the faux-amphibolite (O’Neil et al. 2008), which they intrude. The Nuvvuagittuq greenstone belt is surrounded by 3,660 Ma tonalite and is also intruded by 2686 4 Ma pegmatites (David et al. 2009). These geochronological constraints suggest that the Nuvvuagittuq greenstone belt might have recorded more than 1.5 billion years of Earth’s history (4.3–2.7 Ga).
The Geology of the Nuvvuagittuq Greenstone Belt The belt has been metamorphosed to at least upper amphibolite facies conditions reaching temperatures of >640 C (O’Neil et al. 2007; Cates and Mojzsis 2009). The Nuvvuagittuq greenstone belt has suffered multiple phases of deformation (O’Neil et al. 2007; David et al. 2009). The belt is essentially composed of three major lithological units: (1) amphibolite rich in cummingtonite (a metamorphic amphibole with the chemical composition (Mg, Fe)7Si8O22(OH)2) that is the predominant lithology of the belt, (2) ultramafic and mafic intrusions (sills) that intrude the amphibolites, and (3) chemical sedimentary rocks that comprise a BIF and a silicic formation. The Nuvvuagittuq belt is surrounded by 3.66 Ga old tonalites (David et al. 2009) interpreted as being derived from the melting of the belt’s rocks. The dominant lithology of the Nuvvuagittuq belt is a heterogeneous cummingtonite-rich amphibolite called the “faux-amphibolite.” The faux-amphibolite consists of gneisses ranging from cummingtonite-amphibolite to garnet-biotite schist composed of variable proportions of cummingtonite + biotite + quartz, plagioclase garnet anthophyllite cordierite. The proportion of cummingtonite and biotite in the faux-amphibolite varies widely, with lithologies ranging from amphibolite composed almost entirely of cummingtonite to garnet-biotite schist. The faux-amphibolites can be divided into two chemical groups that are stratigraphically separated by the BIF. At the base of the sequence, the faux-amphibolite is mainly basaltic in composition and generally has geochemical features that resemble those of tholeiitic volcanic suites,
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Nuvvuagittuq (Porpoise Cove) Greenstone Belt. Figure 1 Geological map of the Nuvvuagittuq greenstone belt. (O’Neil et al. 2007, 2008)
whereas the faux-amphibolite at the top of the sequence exhibits a wider range of composition from basalt to andesite and is characterized by a chemical composition similar to calc-alkaline and boninitic volcanic suites. The faux-amphibolite is intruded, mainly in the west of the belt, by numerous ultramafic and gabbroic intrusions. The ultramafic intrusions (sills) range from 5 to 30 m in width and are composed of serpentine + talc + hornblende + tremolite. The gabbro sills are mainly composed hornblende + plagioclase. The Nuvvuagittuq
greenstone belt also comprises what is interpreted to be volcanic rocks with possible pillow lava structure, consistent with their formation in an oceanic environment. A BIF 5–30 m in width can be traced continuously along the western limb of the belt and discontinuously along the eastern limb. The BIF is essentially a finely laminated quartz + magnetite + grunerite rock. A large silica-formation occurs in the eastern limb of the belt that reaches 100 m in width. It is composed almost entirely of massive recrystallized quartz with minor disseminated
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pyrite. At the southern-most edge of this limb, the silicaformation grades into BIF, suggesting that it may be a silica-rich facies of the BIF. The geochemistry of the BIF is consistent with its precipitation from seawater (O’Neil et al. 2007).
See also ▶ Archea ▶ Banded iron formation ▶ Continental Crust ▶ Earth, Formation and Early Evolution ▶ Hadean ▶ Radioactivity ▶ Radiogenic Isotopes
References and Further Reading Cates NL, Mojzsis SJ (2007) Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Quebec. Earth Planet Sci Lett 255(1–2):9–21
Cates NL, Mojzsis SJ (2009) Metamorphic zircon, trace elements and Neoarchean metamorphism in the ca. 3.75 Ga Nuvvuagittuq supracrustal belt, Quebec (Canada). Chem Geol 261(1–2):98–113, Special Issue SI Dauphas N, Cates N, Mojzsis SJ, Busigny V (2007) Identification of chemical sedimentary protoliths using iron isotopes in the >3750 Ma Nuvvuagittuq supracrustal belt Canada. Earth Planet Sci Lett 254:358–376 David J, Godin L, Stevenson RK, O’Neil J, Francis D (2009) U-Pb ages (3.8–2.7 Ga) and Nd isotope data from the newly identified Eoarchean Nuvvuagittuq supracrustal belt, superior Craton, Canada. Geol Soc Am Bull 121(1–2):150–163 Konhauser KO, Hamade T, Raiswell R, Morris RC, Ferris FG, Southam G, Canfield DE (2002) Could bacteria have formed the Precambrian banded iron formations? Geology 30(12):1079–1082 O’Neil J, Maurice C, Stevenson RK, Larocque J, Cloquet C, David J, Francis D (2007) The Geology of the 3.8 Ga Nuvvuagittuk (Porpoise Cove) Greenstone Belt, northern Superior Province, Canada. In: Van Kranendonk MJ, Hugh Smithies R, Bennett VC (eds) Earth’s Oldest Rocks, Development in Precambrain geology, Elsevier O’Neil J, Carlson RW, Francis D, Stevenson RK (2008) Neodymium-142 evidence for hadean mafic crust. Science 321:1828–1831
O O2 ▶ Dioxygen
See also ▶ Oceanic Crust ▶ Ophiolite ▶ Plate Tectonics ▶ Subduction
OB Association Definition An OB association is a loose grouping of several thousand stars, a small fraction of which are of spectral type O and B. These latter are the most massive members of the group and also the most luminous, allowing identification of OB associations out to great distances. In external spiral galaxies, they trace the spiral arms. OB associations range in size, with diameters from tens of parsecs to about 100 parsecs. It is thought that all the stars formed together in a tighter configuration, and observations of stellar velocities reveal that the groups are in a state of expansion. The oldest recognizable OB associations have ages of order 10 million years.
See also ▶ Elephant Trunks ▶ Pre-Main-Sequence Star ▶ Stellar Cluster ▶ T Association
Oberon Definition Oberon, discovered by Herschel in 1787, is the outermost of the five big satellites of ▶ Uranus. Its distance to Uranus is 583,500 km (or 23 Uranian radii), its diameter is 1,520 km, and its density is 1.63 g/cm3. Oberon has been observed by the ▶ Voyager 2 spacecraft at the time of its ▶ Uranus flyby in January 1986. Its surface is heavily cratered; the largest crater, Hamlet, has a diameter of 206 km. It is considered as being differentiated into a silicate core surrounded by an icy mantle. The low albedo (0.14) of the surface suggests that it consists of a mixture of ice and organic matter, possibly irradiated by highenergy particles coming from Uranus’ magnetosphere.
See also ▶ Giant Planets ▶ Uranus ▶ Voyager
Obduction Definition Obduction is the thrusting of segments of ▶ oceanic crust and mantle onto continental crust at convergent plate margins. A slice of obducted oceanic crust is called an ▶ ophiolite. Commonly, blueschist-facies metamorphic rocks are exposed in front of the obducted ophiolite. During periods of enhanced ridge push, while most of the oceanic plate is subducted, slices of more buoyant material are thrust onto small continental fragments or onto the margins of continents, particularly during the closure of ocean basins preceding continent–continent collision.
Obliquity and Obliquity Variations FRANC¸OIS FORGET Institut Pierre Simon Laplace, Laboratoire de Me´te´orologie Dynamique, UMR 8539, Universite´ Paris 6, Paris Cedex 05, France
Synonyms Axial tilt
Muriel Gargaud (ed.), Encyclopedia of Astrobiology, DOI 10.1007/978-3-642-11274-4, # Springer-Verlag Berlin Heidelberg 2011
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Definition In astronomy, the obliquity is the angle between an object’s (e.g., planet’s) axis of ▶ rotation and a line perpendicular to its ▶ orbit plane. The obliquity controls the variation of insolation with latitude and time, and thus influences the climate.
Overview In the Solar System, the present-day obliquities of the ▶ planets are diverse Table 1. Planetary obliquities vary with time because of the long-term gravitational perturbation by other planets (Laskar and Robutel, 1993). This effect is small on the obliquity of the outer planets (▶ Jupiter, ▶ Saturn, ▶ Uranus and ▶ Neptune), which can be considered as primordial: They have roughly kept the value they had at the end of the formation of the ▶ Solar System. In contrast, the very low obliquity of ▶ Mercury results from tidal interactions with the Sun. This is also the case for ▶ Venus, although the effect was different because of its thick atmosphere (the Sun heats the atmosphere at the subsolar point, inducing a redistribution of the mass of the atmosphere, which also induces a torque). For the ▶ Earth, the obliquity presents only small variations of about 1.3 around the mean value of 23.3 , with a dominating period of 42,000 years. In the absence of the ▶ Moon, theoretical calculations suggest that the situation would be very different, as multiple resonances then occur between the precession of the axis and the precession of the orbital plane. Earth’s obliquity would then have varied widely between nearly 0 and 85 . In fact, this is the case for Mars, which may have varied between 0 and up to more than 60 . In the past 5 million years, the Martian obliquity varied (with a pseudo-period near 120,000 Earth years) between
Obliquity and Obliquity Variations. Table 1 Obliquity of the eight planets of the Solar System and Pluto Planet Mercury Venus
See also ▶ Earth ▶ Jupiter ▶ Mercury ▶ Moon, The ▶ Neptune ▶ Orbit ▶ Planet ▶ Rotation Planet ▶ Saturn ▶ Solar System, Inner ▶ Uranus ▶ Venus
References and Further Reading Laskar J, Robutel P (1993) The chaotic obliquity of the planets. Nature 361:608–612 Laskar J, Correia ACM, Gastineau M, Joutel F, Levrard B, Robutel P (2004) Long term evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170:343–364
Occultation Definition
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about 15 and 35 . Between 5 and 20 million years ago, it oscillated between 25 and 45 . Before that, it is impossible to reconstruct the details of the obliquity variations, because of the chaotic nature of the problem (Laskar et al. 2004). On any planet, the obliquity controls the distribution of solar illumination with latitude, and the seasonal cycle. Increasing the obliquity enhances the seasonal variations (warmer summer, colder winter). The average annual insolation slightly decreases with obliquity equatorward of 45 latitude, and strongly increases with obliquity poleward of 45 latitude. At the poles, it is proportional to the sine of the obliquity.
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In astronomical usage, an occultation occurs when an object of larger angular size passes in front of an object of smaller angular size. A frequent example is the passage of the Moon between an observer on Earth and a star, producing an occultation of the star. The rarer occultations of stars by planets, satellites, or asteroids provide opportunities to probe the properties (atmosphere, size) of the occulting bodies. Such observations led to the discovery of the rings around the planets ▶ Uranus and ▶ Neptune. When the Earth passes through the equatorial plane of ▶ Jupiter or ▶ Saturn (which correspond to the
Ocean Planet
orbital planes of the major satellites of these planets), mutual occultations among these satellites can be observed. Occultation events have also been observed from unmanned space probes in the Jovian and Saturnian systems. The case when the foreground object has a smaller angular size than the more distant object is called a ▶ transit.
See also ▶ Eclipse ▶ Jupiter ▶ Neptune ▶ Saturn ▶ Transit ▶ Uranus
Ocean (on Early Venus) Definition Liquid water oceans may have existed on early ▶ Venus, when the solar luminosity was weaker than today. The ocean would have disappeared no later than about 1 billion years ago, the epoch of global Venus resurfacing. The existence of an ocean remains speculative, since no direct observational evidence is available. The observation of a 100-fold enrichment in ▶ deuterium in the Venus atmosphere has been frequently cited as indirect evidence of the presence of significant amounts of ▶ water on early Venus, though.
See also ▶ Deuterium ▶ Venus ▶ Water
Ocean Planet MARC OLLIVIER Institut d’Astrophysique Spatiale, CNRS, Universite´ de Paris-Sud, Orsay, France
Synonyms Water planet; Water world
Keywords Accretion, exoplanet, migration, planetary formation, snow line
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Definition Ocean planets are hypothetical objects, formed beyond the snow line of a planetary system (distance in the protoplanetary nebula at which the temperature is cool enough to allow the hydrogen compounds such as H2O, NH3, CH4 to condense, essentially when the gas temperature reaches the sublimation temperature of water, 170 K in the vacuum; see Hayashi 1981). Their mass between about 5 and 10 terrestrial masses would be made of volatiles and of silicate rocks and iron. In the prototype model, the proportion was 50%–50%, which corresponds to the maximum ratio between volatiles and rocks found in the Solar System objects (except for some water-dominated satellites of Saturn), but 1–10% of volatiles may be sufficient to form an ocean planet. Following orbital migration, the snow line is crossed by the planet and, after orbital stabilization within the ▶ habitable zone of the planetary system, part of the volatiles become liquid, forming a deep ocean at the surface of the planet.
History The concept of ocean planet was first discussed in 2003 and led to two publications (Kuchner 2003; Le´ger et al. 2004). Ocean planets are now considered as particular cases in the larger framework of rocky planets’ composition and structure (Valencia et al. 2007), as opposed to the gas giant planets.
Overview The concept of ocean planet is the result of a thought experiment. In the classical theory of planetary formation (e.g., Wuchterl et al. 2000), accretion of planets beyond the snow line is rapid because volatiles in solid form provide more quickly sufficient mass of material to build the core of the planet. Some of these formed planetary cores are massive enough to accrete gas and form giant planets. Some are not and lead to small frozen objects such as Pluto or icy satellites of giant planets in our Solar System. The threshold is estimated to be around 6–10 Earth masses; but this value depends on the disk density and the orbital distance. In some case, atmospheric effects such as trapping of the gas by a cold trap should also be considered. Let us consider one of these several Earth-mass frozen planet. Migration can bring this planet inward, trespassing the snow line (Goldreich and Tremaine 1980), and stabilizing the planetary orbit, possibly within its habitable zone. This new position leads part of the volatiles to melt. According to the planetary final semi-major axis, the authors of this theory proposed that part of the volatiles can stay in the liquid state at the surface of the planet, creating thus a huge and deep planetary ocean. However, several questions
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have been raised concerning the internal structure of such planets, the real existence of a huge ocean and its potential characteristics, and the composition of the atmosphere allowing its remote identification and study as an exobiology object. At present, only Le´ger et al. (2004) have published a specific study on these objects. As mentioned before, other studies have been done in a larger context of terrestrial-type planet structure (that thus includes ocean planets.) In this entry we will consider Le´ger et al.’s work. Several supplementary assumptions as limits of their model have been done (Le´ger et al. 2004): ● The mass range of planets is limited to 1–8 Earth masses. As biggest objects are easier to detect and characterize, the study focuses on 6–8 Earth massobjects. ● Planets are supposed to form beyond the snowline and migrate inward to about 1 AU (habitable zone of a G star) in about 1 Myr. ● Planets are mostly made of 50% of refractory materials (Fe, silicates) and 50% of ices according to the composition of protoplanetary nebula (90% of H2O, 5% of NH3, 5% of CO2). Most of the ice is thus made of water.
composition and structure of ice at such extreme pressure and temperature values. The structure of such an object is described in Fig. 1. The ocean, with a thickness of about 100 km, lies on a huge water ice layer (several thousands of kilometers). Because of the very high pressure, ice is differentiated. Because of its intrinsic weight compared to water ice, CO2 is mainly in the form of ice, at the interface between the mantle and the water ice layer. This helps prevent CO2 from degassing into the atmosphere, creating to a runaway greenhouse effect.
Ocean Depth The question of the depth of the ocean was studied considering a pure water ocean, whose role is to transfer the internal heat of the planet to its surface. Assuming the worst case where the temperature gradient is adiabatic (limit to convection), a surface temperature of the ocean of 7 C is obtained. The depth of the ocean is estimated at 72 km. This value is higher if the surface temperature of the ocean is higher and vice versa. Non-adiabatic dependence of the temperature with depth leads to a smaller ocean depth.
Atmosphere Composition and Spectrum Internal Structure Modeling of a 6-Earth mass-object has been done starting from a model developed to describe the Earth’s interior, taking into account the cooling of the planet, and the
Our understanding of the origin of atmospheres (including the terrestrial one) is too poor to make accurate predictions of what an ocean planet atmosphere would exactly contain. However, assuming the composition of
e ∼ 100 km R/R = 2.00 1.63 e ∼ 20 km
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Ocean Planet. Figure 1 Calculated internal structure of a 6-Earth mass-ocean-planet (left). The core (Fe), mantle (silicates), and ice + ocean masses are 1, 2, and 3 Earth masses, respectively. Comparison is made with a 6-Earth mass-fully rocky planet (centre) and the Earth (right) (From Le´ger et al. 2004)
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cometary material, it is not irrelevant to assume that volatiles are made of H2O (mainly), NH3, and CO2. In the atmosphere, NH3 reacts with EUV radiations and dissociates leading to H2 and N2. Because of high exosphere temperature, H2 escapes and N2 is mixed with the remaining atmosphere as a buffer gas. The amount of CO2 in the atmosphere is strongly limited compared to the planetary CO2, because most of the CO2 is in the form of ice under the water ice layer. Models of ocean planet atmospheres including photochemical evolution include thus the presence of H2O, CO2, and N2 that should be detectable in the atmosphere.
Exobiological Interest of such Planets Because of the massive presence of liquid water at their surface, ocean planets are good candidates for the detection of life, assuming the classical criteria for the potential emergence of life (definition of the habitable zone). A search for life on these planets requires first an unambiguous identification of their nature (see next section), coming from the determination of their mean density, and the confirmation of their habitability thanks to a spectral analysis of their atmosphere. However, because of the lack of O2 sinks (such as the Earth’s silicate surface), it appears possible to maintain a quantity of abiotic O2 coming from photodissociation of CO2 or H2O in the atmosphere. The CO2, O2, H2O criteria cannot thus be used. However, in that case, the formation of a large ozone (O3) layer is limited by perpetual reaction of O3 in the upper atmosphere with radicals coming from H2O and CO2 dissociation. The effective habitability of ocean planets could thus be confirmed by searching for a well-developed ozone layer, which implies the occurrence of a huge amount of biologically produced O2 in the atmosphere.
Key Research Findings No observational confirmation of the existence of ocean planets has yet been done. To confirm the existence of these objects, one must determine, with sufficient accuracy to exclude all the other models, the density of the object, which depends on its internal composition (volatiles, iron, and silicates). In addition, mass-radius degeneracies should be taken into account carefully. A planet with a dense core and a hydrogen-rich atmosphere cannot certainly be distinguished from a H2O-rich planet, but populations of such objects might be separated by the fact that even low-mass hydrogen-rich envelopes produce very large radii that cannot be attributed to ocean planets. The amount of H2 that can mimic an ocean planet may not be stable, assuming atmospheric escape.
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To determine the density of the planet, one must measure its radius and evaluate its mass. The first parameter can be obtained by the transit method. Observation from space by dedicated satellites (▶ CoRoT, ▶ Kepler, HST, etc) allows the determination of a planetary diameter at the level of a few percent, assuming simultaneously the knowledge of the stellar diameter with an accuracy of a few percent. In principle, the mass of the planet can be determined by ▶ radial velocity measurements (RV) based on high spectral resolution spectroscopy. However, the accuracy reached by the best RV instruments does not yet allow measuring the mass of such small objects at a level better than several Earth masses, preventing from determination of the accurate density of the object. In that context, it is thus very difficult to identify within a large range of telluric planet models which one corresponds to the observed planet, preventing the observer from identifying a real ocean planet (Selsis et al. 2007). Future generations of RV instrument should be more adapted to this quest. The knowledge of stellar diameter may also be improved by interferometric observations.
Future Directions The concept of ocean planet is now considered as a particular case of the bigger family of rocky planets (mass between 2 and 10 terrestrial masses), whose structure and evolution is driven by several parameters such as the initial composition, the formation location, and the evolution of orbital parameters. The observational study of ocean planets is thus considered within the framework of detection and characterization of telluric planets.
See also ▶ CoRoT Satellite ▶ Exoplanet, Detection and Characterization ▶ Habitable Planet (Characterization) ▶ Habitable Zone ▶ Kepler Mission ▶ Radial Velocity
References and Further Reading Goldreich P, Tremaine S (1980) Disk-satellite interactions. Astrophys J 241:425–441 Hayashi C (1981) Structure of the solar nebula, growth and decay of magnetic fields and effects of magnetic and turbulent viscosities on the nebula. Prog Theor Phys Suppl 70:35–53 Kuchner MJ (2003) Volatile-rich earth mass planets in the habitable zone. Astrophys J 596:L105–L108 Le´ger A, Selsis F, Sotin C, Guillot T, Despois D, Mawet D, Ollivier M, Labe`que A, Valette C, Brachet F, Chazelas B, Lammer H (2004) A new family of planets? “Ocean Planets”. Icarus 169:499–504
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Selsis F, Chazelas B, Borde´ P, Ollivier M, Brachet F, Decaudin M, Bouchy F, Ehrenreich D, Griessmeier J-M, Lammer H, Sotin C, Grasset O, Moutou C, Barge P, Deleuil M, Mawet D, Despois D, Kasting JF, Le´ger A (2007) Could we identify hot ocean-planets with CoRoT, Kepler and Doppler velocimetry. Icarus 191:453–468 Valencia D, Sasselov DD, O’Connell RJ (2007) Detailed models of super-earths: how well can we infer bulk properties? Astrophys J 665:1413–1420 Wuchtel G, Guillot T, Lissauer JJ (2000) Giant planet formation. In: Mannings V, Boss AP, Russell SS (eds) Protostars and Planets IV. University of Arizona Press, Tucson, p 1081
Ocean Salinity ▶ Ocean, Chemical Evolution of
Ocean, Chemical Evolution of DANIELE L. PINTI GEOTOP & De´partement des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada
Synonyms Ocean salinity
Keywords Major ions, ocean chemistry, oxygen levels, pH, salinity
Definition The chemical composition of the ocean is the relative compositions of major ions (Naþ, Mg2þ, Ca2þ, Kþ, Sr2þ, 2 Cl, SO2 4 , HCO3 , Br , CO3 , B(OH)3, B(OH)4 , F ) of seawater. The major ions content is considered relatively constant and defined as salinity, which is a measure of the total dissolved salts in seawater (Table 1). The chemical evolution of the ocean is the evolution of its chemistry (major ions, pH, and oxygen content) and the processes that controlled the composition of this terrestrial reservoir.
Overview Though the chemical composition of the terrestrial oceans is constant, a number of processes can cause the oceanic waters to be nonconservative. These processes include precipitation, dissolution, water–rock interactions, freezing, and oxidation processes (Millero 2003). The terrestrial oceans have certainly undergone an evolution of their chemistry during the last 4.5 Ga (Holland 1984, 2003) driven by the competition between two sources of salinity: weathering of continental masses versus water cycling through mid-ocean ridges (▶ MOR). Weathering of continental rocks and river waters mainly controls salinity of present-day oceans. Seawater cycling at MOR was possibly the dominant source of ions in the ▶ Hadean and ▶ Archean oceans. This would be a direct consequence of a much vigorous mantle convection favoring higher oceanic crust production at MOR (Bickle 1986) and the limited growth of continents (McCulloch and Bennett 1994). Several authors indicated the beginning of the Proterozoic era as the turning point in the dominance of salinity sources (hydrothermal cycling versus continental weathering; Veizer et al. 1989; de Ronde et al. 1997; Pinti 2005). The long geological history of seawater started in the Hadean, when liquid water became stable at the surface of the planet, 4.4–4.3 Ga. These ages correspond to those of the ▶ zircons of ▶ Jack Hills (Wilde et al. 2001). At present, little can be said about the composition of the Hadean ocean (4.5–3.8 Ga) and mostly is pure speculation. The chemistry of seawater in the early Hadean was possibly controlled by high-temperature water–rock interactions between the condensing CO2–H2O runaway greenhouse atmosphere and the primitive basaltic crust. When condensing water started to penetrate deeper in the oceanic basaltic proto-crust, dissolved chlorine (degassed form the mantle as HCl; Holland 1984) reacted with Na-bearing minerals of basalt to produce halite (NaCl). Sleep et al. (2001) calculated that a global basaltic layer only 500-m thick could account for the whole ocean NaCl budget. Initial salinity of the ocean is unknown and values of 1.2–2 present value have been proposed only for the Archean ocean (Holland 1984; Knauth 2005). These could be considered as minimum values for the Hadean. The presence of large amount of CO2 in the Hadean
Ocean, Chemical Evolution of. Table 1 Standard mean seawater chemical composition (Salinity = 35 g/L) reported as mol kg H2O1 (DOE 1994) Na+
Mg2+
Ca2+
K+
Sr2+
Cl
SO42
HCO3
Br
CO32
B(OH)3
B(OH)4 F
0.48616 0.05475 0.01065 0.01058 0.00009 0.56567 0.02927 0.00183 0.00087 0.00027 0.00033 0.00010
0.00007
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atmosphere and oceans and large quantity of Na in the basaltic proto-crust could have promoted their alkalinity by production of Na2CO3, as observed in ▶ soda lakes (Kempe and Degens 1985). However, thermodynamic calculations indicated that the whole terrestrial available CO2 budget (atmosphere–hydrosphere–mantle) is several orders of magnitude lower than the one needed to make a global soda ocean (Sleep et al. 2001). Seawater geological record started in the Archean, though fragmentary and often hampered by large hydrothermal alteration and metamorphism. The 3.8 Ga ▶ Isua metasedimentary sequences of West Greenland contain chemical sediments deposited in an oceanic environment without a significant sialic detrital component (Holland 2003), i.e., without continental inputs of ionic species. Compelling evidence for the existence of an ocean during deposition of Isua rocks are clastics that have been interpreted as marine turbidites and iron formations (IFs). These iron-rich layers are magnetite–quartz IFs and the molar ratio of Fe2O3/FeO is less than 1.0, which suggests that magnetite was the dominant ▶ iron oxide, and that hematite was absent or very minor in these IFs (Holland 2003). This in turn suggests that magnetite is not a replacement of oxyhydroxides precursors but a primary precipitating phase, thus that the oxygen level in the oceans was insignificantly low. The oxidation of Fe2þ to Fe3þ was possibly produced by UV-induced reactions between iron and H2O (Cairns-Smith 1978). The chemistry of Isua seawater is unknown. Appel et al. (2001) observed ▶ fluid inclusions in quartz globules from pillow breccia at Isua. The chemistry of the highly saline aqueous fluids (about 25 wt.% NaCl equivalent) bears a strong resemblance to present-day seafloor hydrothermal fluids that likely provoked the alteration of the pillow breccia and the precipitation of quartz. Occurrences of fluid inclusions in Archean metasedimentary rocks see Fluid Inclusions have been signaled in several terrains of ages spanning from 3.5 to 2.7 Ga (De Ronde et al. 1997; Channer et al. 1997; Foriel et al. 2004; Weiershauser and Spooner 2005). They have been interpreted as a pristine record of the chemical composition of Archan seawater. The chemistry of these fluid inclusions always pointed out to the mixing between at least two fluids, a saline fluid of Na–Ca–Cl composition (seawater) and a hydrothermal effluent, often richer in Ba and Fe (de Ronde et al. 1997; Foriel et al. 2004). The Na:Cl ratio is the same as present-day seawater but NaCl concentration ranges from 4% to 25% equivalent (against 3.5% equivalent in modern seawater). The salt enrichment and the chemistry (Na–Ca–Cl) points out to an evaporate seawater residue (Martin et al. 2006). Among the notable
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differences with modern seawater, de Ronde et al. (1997) and Channer et al. (1997) noticed that the Cl:Br and Cl:I ratios are higher than in modern seawater and much closer to the bulk Earth values. This could indicate that Archean seawater chemistry was buffered by the mantle through emission of dissolved chemical species and volatiles from mid-oceanic vents. An alternative hypothesis is the absence of planktonic organisms that presently use bromine and iodine as metabolites, scavenging them from the ocean. Holland (1984, 2003) used a different approach for constraining the Archean seawater composition, looking at the mineralogy of the deposited sediments. Calcite (CaCO3) and dolomite (CaMg(CO3)2) were the dominant carbonate minerals. ▶ Siderite (FeCO3) was only a common constituent of ▶ BIFs. These observations imply that the Archean ocean was supersaturated with respect to CaCO3 and CaMg(CO3)2. For the theoretical range of values of atmospheric pCO2 in the Archean, the pH of seawater was probably between 6 and 7 (Holland 2003; Pinti 2005). The scarcity of siderite in BIFs is also an indicator of a low Fe2þ/Ca2þ ratio in the ocean, i.e., close to the ratio of the solubility product of siderite and calcite (4 10–3; Holland 2003). Archean ocean should have been mainly anoxic as suggested by the mass-independent fractionation (▶ MIF) of the ▶ sulfur isotopes in pre-2.47 Ga sulfides and sulfates (Farquhar et al. 2000). In the absence of O2, solar UV interacts with SO2 and generates MIF of the sulfur isotopes in the reaction products. In the absence of atmospheric O2, sulfide minerals would not have been oxidized during weathering reducing the input of SO2 4 in the ocean. Another consequence of the low pO2 in the Archean was the low amount of trace elements such as U, Mo, and Re that are extremely mobiles in an oxygenated environment. The Proterozoic ocean was certainly characterized by an increasing O2 concentration, although it reached appreciable amounts only after 1.8 Ga. The presence of O2 certainly favored the scavenging of iron from the ocean as oxyhydroxides and the deposition of ▶ BIFs (Holland 1984) and the increasing amount of sulfate. The cessation of BIFs deposition after 1.8 Ga was probably due to a decrease in the flux of reductants Fe2þ, H2, and H2S to the oceans, which in turn promoted a net increase in the dissolved O2 content in the deeper ocean (Holland 2006). The end of the Proterozoic was a quite turbulent period of biological and climatic global changes in the ocean driven by very large glaciations that possibly developed in ▶ Snowball Earths. Phanerozoic is the period where the largest (but discontinuous) geological record of the chemical
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evolution of the ocean is available. First studies pointed out the near-constancy of the seawater chemical composition (e.g., Holser 1963; Holland 1972), which turned out to be off the mark. Indeed, Phanerozoic is characterized by large excursions in the concentration of several major ions, particularly Ca2þ, Mg2þ, and SO2 4 , which contrast with a nearly constancy of Kþ (Spencer and Hardie 1990; Hardie 1996; Horita et al. 2002). These variations (2–5 the present seawater) have been differently interpreted, yet a clear process is difficult to assess. They are correlated to sea-level changes (Holland and Zimmermann 2000). Hardie (1996) proposed that seawater chemical variations in Phanerozoic were determined by the mixing ratio between river water and its solution charge and midocean ridges solutions coupled with solid CaCO3 and SiO2 phases. However, the model fails to explain the near-constancy of the Kþ concentration that may be related to the processes of potassium uptake by riverine clays (Holland 2003). On the striking correlation between the sea-level stand and seawater chemical variations, Hardie (1996) suggested that the stand of the sea level reflects the rate of oceanic crust formation (younger, hotter, and rapid oceanic formation produces higher sea levels). This, in turn, determines the rate of seawater cycling through MOR and hence the mixing ratio of hydrothermally altered water versus river water. But one of the most complete records of the seawater composition changes is the curve of the Sr isotopic ratio (87Sr/86Sr) of Veizer et al. (1989). This curve, refined through the years shows chemistry cycles that could be related to changes in the type of rocks undergoing weathering during time (as also indicated by the 187Os/186Os ratios) indicating the present dominance of continental weathering as source of salinity of the oceans.
Applications A complete picture of the seawater geological history on Earth could be extremely useful in determining the existence of past oceans on other telluric planets such as Mars, through the study of “sedimentary” rocks and chemical precipitates of “marine” origin.
Future Directions The geological record of the seawater is still much incomplete and spotty. Bringing new evidences that pristine seawater fluid inclusions could have been preserved through eons and finding fluid inclusions containing non-evaporated seawater are the most challenging researches to carry out in the next decade to clearly asses the chemical evolution of the oceans.
See also ▶ Archean Eon ▶ BIF ▶ Earth, Formation and Early Evolution ▶ Fluid Inclusions ▶ Fractionation, Mass Independent and Dependent ▶ Hadean ▶ Iron Oxyhydroxides ▶ Isua Supracrustal Belt ▶ Jack Hills (Yilgarn, Western Australia) ▶ Mid-Ocean Ridges ▶ Oceans, Origin of ▶ Oxygenation of the Earth’s Atmosphere ▶ Pillow Lava ▶ Siderite ▶ Snowball Earth ▶ Soda Lakes ▶ Sulfur Isotopes ▶ Zircon
References and Further Reading Appel PWU, Rollinson HR, Touret JLR (2001) Remnants of an early Archaean (> 3.75Ga) seafloor, hydrothermal system in the Isua Greenstone Belt. Precambrian Res 112:27–49 Bickle MJ (1986) Implications of melting for stabilisation of the litosphere and heat loss in the Archaean. Earth Planet Sci Lett 80:314–324 Cairns-Smith AG (1978) Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 276:807–808 Channer DMD, de Ronde CEJ, Spooner ETC (1997) The Cl-Br-I composition of 3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry. Earth Planet Sci Lett 150: 325–335 de Ronde CEJ, Channer DMD, Faure K, Bray CJ, Spooner TC (1997) Fluid chemistry of Archean seafloor hydrothermal vents: implications for the composition of circa 3.2 Ga seawater. Geochim Cosmochim Acta 61:4025–4042 DOE (1994) Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. Version 2, A. G. Dickson & C. Goyet, eds. ORNL/CDIAC-74 Farquhar J, Bao H, Thiemens M (2000) Atmospheric influence of Earth’s earliest sulfur cycle. Science 289:756–758 Foriel J, Philippot P, Rey P, Somogyi A, Banks D, Menez B (2004) Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia. Earth Planet Sci Lett 228:451–463 Hardie LA (1996) Secular variation in seawater chemistry: an explanation for the coupled variation in the mineralogies of marine limestones and potash evaporates over the past 600 My. Geology 24:279–283 Holland HD (1972) The geologic history of seawater: an attempt to solve the problem. Geochim Cosmochim Acta 36:637–651 Holland HD (1984) The chemical evolution of the atmosphere and oceans. Princeton University Press, Princeton, 582 pp Holland HD (2003) The geologic history of seawater. In: Elderfield H (ed) Treatise on geochemistry, vol 6. Elsevier, Amsterdam, pp 583–625 Holland HD (2006) The oxygenation of the atmosphere and ocean. Trans Roy Soc B 361:903–915
Oceans, Origin of Holland HD, Zimmermann H (2000) The dolomite problem revisited. Int Geol Rev 42:481–490 Holser WT (1963) Chemistry of brine inclusions in Permian salt from Hutchinson, Kansas. In: Bersticker AC (ed) Symposium on salt (first). Northern Ohio Geological Society, Cleveland, pp 86–95 Horita J, Zimmermann H, Holland HD (2002) The chemical evolution of seawater during the Phanerozoic: implications from the record of marine evaporites. Geochim Cosmochim Acta 66:3733–3756 Kempe S, Degens ET (1985) An early soda ocean? Chem Geol 53:95–108 Knauth LP (2005) Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution. Palaeogeogr Palaeoclimatol Palaeoecol 219:53–69 Martin H, Claeys P, Gargaud M, Pinti D, Selsis F (2006) 6. Environmental context. Earth Moon Planet 98:205–245 McCulloch MT, Bennett VC (1994) Progressive growth of the Earth’s continental crust and depleted mantle: geochemical constraints. Geochim Cosmochim Acta 58:4717–4738 Millero FJ (2003) Physicochemical controls on seawater. In: Elderfield H (ed) Treatise on geochemistry, vol 6. Elsevier, Amsterdam, pp 1–21 Pinti DL (2005) The formation and evolution of the oceans. In: Gargaud M, Barbier B, Martin H, Reisse J (eds) Lectures in astrobiology. Springer, Berlin, pp 83–107 Sleep NH, Zahnle K, Neuhoff PS (2001) Initiation of clement surface conditions on the earliest Earth. Proc Nat Acad Sci USA 98:3666–3672 Spencer RJ, Hardie LA (1990) Control of seawater composition by mixing of river waters and mid-ocean ridge hydrothermal brines. In: Spencer RJ, Chou I-M (eds) Fluid–mineral interactions: a tribute to H.P. Eugster. Special Publication 2. Geochemical Society, San Antonio, pp 409–419 Veizer J, Hoefs J, Ridler RH, Jensen LS, Lowe DR (1989) Geochemistry of Precambrian carbonates: I. Archean hydrothermal systems. Geochim Cosmochim Acta 53:845–857 Veizer J, Ala D, Azmy K, Bruckschen P, Buhl D, Bruhn F, Carden GAF, Diener A, Ebneth S, Godde´ris Y, Jasper T, Korte C, Pawellek F, Podlaha OG, Strauss H (1999) 87Sr/86Sr, d13C and d18O evolution of Phanerozoic seawater. Chem Geol 161:59–88 Weiershauser L, Spooner E (2005) Seafloor hydrothermal fluids, Ben Nevis area, Abitibi greenstone belt: implications for Archean (2.7 Ga) seawater properties. Precambrian Res 138:89–123 Wilde SA, Valley JW, Peck WH, Graham CM (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409:175–178
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carbonate or siliceous sediment that disappear close to the mid-ocean ridges; Layer 2A consisting of 0.5 km thick glassy-to-finely crystalline basalt usually in the form of pillow basalt and 1.5 km thick sheeted vertical diabase dykes (Layer 2B); Layer 3 (5 km thick and of density 3.0 g cm3) made of gabbros (the intrusive form of basalt) and ultramafic cumulates. The crust forms through partial melting of upwelling mantle beneath mid-ocean spreading centers; the layering results from differentiation of the basaltic magma produced by this melting. On Earth, the age of oceanic crust ranges from zero at spreading centers to a maximum of about 180 Ma in the northwestern Pacific.
See also ▶ Crust ▶ Obduction ▶ Oceans, Origin of ▶ Ophiolite ▶ Plate Tectonics
Oceans, Origin of DANIELE L. PINTI1, NICHOLAS ARNDT2 1 GEOTOP & De´partement des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al, Montre´al, QC, Canada 2 Maison des Ge´osciences LGCA, Universite´ Joseph Fourier, Grenoble, St-Martin d’He`res, France
Keywords Comets, cool early Earth, D/H, Isua, Jack Hills zircons, meteorites, volatiles, water
Ocean, Temperature of ▶ Precambrian Oceans, Temperature of
Oceanic Crust Definition Oceanic crust is the outer layer of the solid Earth beneath the oceans. This ▶ crust is 6–9 km thick and comprises 3 main layers (from top to bottom): Layer 1, a thin layer (typically less than 500 m) of deepwater unconsolidated
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Definition The term ocean applies to the entire body of saltwater covering more than 70% of the Earth’s surface. From a planetary point of view, the term can be extended to the layer of liquid that partially or totally covers the surface or pervades the subsurface of a planet or satellite. In addition to water, the term includes other substances such as ethane, methane, or ammonia that are in a liquid state under the conditions of the planetary surface. An analogue could be the hypothetical ocean of ▶ Titan. The liquid body could be at the planetary surface in thermodynamic equilibrium with an atmosphere or separated by another layer. In the early history of the planets, it could be
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an ocean of magma. Examples of subsurface oceans are those predicted for Titan or beneath the smooth icy surface of the satellites ▶ Europa and Enceladus. The origin of the ocean entails the complex processes that led to the formation of a stable body of liquid water in thermodynamic equilibrium with an atmosphere on a planetary body. The following overview focuses on the formation of the terrestrial oceans.
Overview There is good evidence that the oceans were present on the ▶ Earth in the early ▶ Archean (3.8 Ga ago) after the ▶ Late Heavy Bombardment and plausible evidence that they were already present in the ▶ Hadean. The convincing evidence comes from the two oldest areas of volcanic and sedimentary rocks – the ▶ Isua Supracrustal Belt, West Greenland and ▶ Nuvvuagittuq, Quebec, Canada (formally Porpoise Cove). The ages of the rocks in both areas have been established at about 3.8 Ga (for the latter a date of 4.3 Ga has been suggested; see ▶ Nuvvuagittuq). In the Isua Supracrustal Belt, pillow basalts provide evidence of underwater eruption, and in both belts, metasedimentary rocks (▶ banded iron formations, metapelite, and ferruginous quartzite) are the products of erosion, fluvial transport, and subaqueous deposition. Although the setting of some of these rocks may have been lakes or shallow seas, it is far more probable that these rocks erupted or were deposited in ocean basins. The evidence for Hadean oceans is less direct, because the geological record has been mostly wiped out by the intense tectonic activity of the young Earth. Most evidence comes from 4.4 to 4.2 Ga old detrital zircons (ZrSiO4) from the Mt. Narryer quartzite and from ▶ Jack Hills metaconglomerate, Western Australia (Mojzsis et al. 2001; Wilde et al. 2001). ▶ Zircon is a common U-rich trace mineral in granitic magmas formed by melting of a hydrous source. Parental rocks of the Jack Hills zircons possibly formed from melting of subducted hydrated oceanic crust. The presence of such a crust requires interaction between basaltic lava and seawater as is suggested by the 18O/16O ratio (denoted as d18O) of the Jack Hills zircons which extends to 7.4 0.7 ‰. This value can be explained only by a mixing between the mantle source of the zircons (d18O ca. 5.0 ‰) and a source enriched in 18O compared to the mantle value and derived from weathering and low-temperature alteration of oceanic crust. The d18O values of Hadean, Archaean, and Proterozoic zircons show a secular increase to values up to 10‰. This can be explained by an increase of the amount of supracrustal, high-d18O material available for melting and assimilation, as the “wet” continental crust evolved and
matured during the Precambrian. The presence of d18O of 7.4 0.7‰ in a 4.4 Ga old detrital zircon of Jack Hills could thus be the indirect and oldest record of interaction of a magmatic source with liquid water at the surface of the Earth (Peck et al. 2001). The combination of these observations lead to the “▶ cool early Earth” hypothesis according to which temperatures were clement, and oceans existed, on the surface of the Earth as early as 4.4 Ga (Valley et al. 2002). The source of the oceanic water is debated. It is commonly accepted that any primitive solar-type atmosphere surrounding the Earth was rapidly blown off by UV and X-rays produced by the sun during its proto-(T-Tauri)star stage. This primitive atmosphere probably did not contain water because Earth accreted too close to the Sun. Earth was composed of “dry” raw material related to enstatite chondrite ▶ meteorites (Javoy 1997). The present hydrosphere probably was derived from meteoritic and cometary material that was added after the moonforming impact (Morbidelli et al. 2000). Similarities between the hydrogen isotopic ratio D/H of the whole Earth (149 – 153 106), ocean water (155.7 106), and the average D/H ratios of hydrated carbonaceous chondrites (149 6 106) and Antarctic micrometeorites (154 16 106) indicates that asteroids or asteroidal dust are the most likely candidates as water carrier (Maurette et al. 2000; Marty and Yokochi 2006). ▶ Comets apparently exhibit heavier D/H ratios of 290– 320 106 that are incompatible with oceanic water, although only a few comets have measured D/H ratios. Mass and isotopic balance calculations suggest that a maximum of 10–20% of cometary water could have contributed to the terrestrial water budget. The total amount of water delivered to Earth was certainly higher than the mass of the present-day oceans (1.4 1021 kg), because the losses of volatiles to space were important at the beginning. Pepin (1991) suggested that at least 50% of the water delivered to Earth could have dissociated to hydrogen and oxygen by the strong early UV radiation. Further losses were due to impact erosion of the atmosphere by arriving planetary bodies. Geophysical and high-pressure mineralogy experiments suggest that the modern mantle contains two to five times the ocean volume of water (Abe et al. 2000) and thus an amount of water much higher than that preserved in the oceans was added to the newly born Earth. Theories of how the ocean formed and stabilized on the surface of the Earth are based on geophysical modeling and speculation. Water, together with CO2 (now trapped at the Earth’s surface as carbonates), were vapor phases in the secondary (outgassed) atmosphere. They did not
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condense until the surface of the planet cooled down to less than 647 K, the critical point of water. Assuming that all oceanic water was present as a gas, the atmospheric pressure was 270 bars of H2O plus 40 bars of CO2 (Sleep et al. 2001; Pinti 2005). This runaway greenhouse atmosphere, refurbished by a high heat flow from the still molten interior of the Earth, kept the planet’s surface sufficiently hot for several millions years to prevent water from condensing on its surface. When a primordial crust formed, it reduced the heat flux from the interior and the surface of the Earth cooled down, allowing the condensation of water. It is difficult to assess when this occurred. Geophysical models developed by Sleep et al. (2001) suggest a few million years or less after the Moon-forming impact, depending on the parameters chosen. If the geochemistry of the Jack Hills zircons does indeed reveal the presence of liquid water at the surface, this means that between 4.4 and 4.3 Ga, oceans were already present, or, at least, that liquid water was stable at the surface of the Earth. The oceans became cold enough to assure the survival of the first living communities sometime between 4.3 Ga and the end of the meteoritic bombardment at 3.85 Ga. The chemistry of seawater in the early Hadean was likely controlled by high-temperature fluid-rock reactions between the hot CO2–H2O-rich atmosphere and oceans, and primitive basaltic crust. Weathering of Na-rich basalt reacting with chlorine saturated the seawater with NaCl. The acidity of water was possibly as high as pH = 5 because the ocean was in thermodynamic equilibrium with a CO2-rich atmosphere (Pinti 2005), though alternative models of alkaline oceans saturated with ▶ thermonatrite formed by weathering of the basaltic protocrust have been proposed (Kempe and Degens 1985).
Key Research Findings To help understand the origin of the oceans requires the integration of information from the Earth, where the ocean still exists and where a fragmentary record of its early state is preserved in the geological record, with information from other planetary bodies. Studies of the Martian surface have already provided evidence of the small shallow oceans that existed there early in the planet’s history, and the cold icy satellites provide a record of oceans of very different types. This information can potentially be exported to models for the formation of oceans on exoplanets. Le´ger et al. (2004), for example, has proposed the existence of a hypothetical family of “OceanPlanets” that are entirely covered by an ocean hundreds of kilometers deep.
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Future Directions Two main questions need to be addressed: (1) When did oceans form on Earth? (2) What is the origin of the water? The first question is difficult to answer because the answers must be sought from a period when the geological record is almost totally absent. Two decades ago the task seemed hopeless, but the remarkable results emerging from the Jack Hills zircons and other relicts of the oldest crust have radically changed our knowledge of this period. We need to search for and precisely date and analyze other minerals or rocks that could provide complementary evidence of interaction with liquid water. For an answer to the second question, we need to wait for future space missions to study and collect samples from the surface of comets (e.g., the Rosetta mission) to establish to what extent this material could have contributed to the terrestrial water budget.
See also ▶ Archea ▶ Banded Iron Formation ▶ Comet ▶ Cool Early Earth ▶ Degassing ▶ Earth, Formation and Early Evolution ▶ Europa ▶ Hadean ▶ Isotopic Fractionation (Interstellar Medium) ▶ Isua Supracrustal Belt ▶ Jack Hills (Yilgarn, Western Australia) ▶ Late Heavy Bombardment ▶ Meteorites ▶ Nuvvuagittuq (Porpoise Cove) Greenstone Belt ▶ Rosetta (spacecraft) ▶ Soda Lakes ▶ Thermonatrite ▶ Titan ▶ Water, Delivery to Earth ▶ Zircon
References and Further Reading Abe Y, Ohtani E, Okuchi T, Righter K, Drake MJ (2000) Water in the early Earth. In: Righter K, Canup RM (eds) Origin of the earth and moon. University of Arizona Press, Tucson, pp 413–433 Javoy M (1997) The major volatile elements of the Earth: their origin, behavior, and fate. Geophys Res Lett 24:177–180 Kempe S, Degens ET (1985) An early soda ocean? Chem Geol 53:95–108 Le´ger A et al (2004) A new family of planets? “Ocean-Planets”. Icarus 169 (2):499–504 Marty B, Yokochi R (2006) Water in the early Earth. Rev Mineral Geochem 62:421–450
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Maurette M, Duprat J, Engrand C, Gounelle M, Kurat G, Matrajt G, Toppani A (2000) Accretion of neon, organics, CO2, nitrogen and water from large interplanetary dust particles on the early Earth. Planet Space Sci 48:1117–1137 Mojzsis SJ, Harrison MT, Pidgeon RT (2001) Oxygen-isotope evidence from ancient zircons for liquid water at the Earth’s surface 4, 300 Myr ago. Nature 409:178–181 Morbidelli A, Chambers J, Lunine JI, Petit JM, Robert F (2000) Source regions and timescales for the delivery of water to the Earth. Meteorit Planet Sci 35:1309–1320 Peck WH, Valley JW, Wilde SA, Graham CM (2001) Oxygen isotope ratios and rare earth elements in 3.3–4.4 Ga zircons: ion microprobe evidence for high d18O continental crust and oceans in the Early Archean. Geochim Cosmochim Acta 65:4215–4229 Pepin RO (1991) On the origin and ealy evolution of terrestrial planetray atmospheres and meteoritic volatiles. Icarus 92:2–79 Pinti DL (2005) The formation and evolution of the oceans. In: Gargaud M, Barbier B, Martin H, Reisse J (eds) Lectures in astrobiology. Springer, Berlin Sleep NH, Zahnle K, Neuhoff PS (2001) Initiation of clement surface conditions on the earliest Earth. Proc Natl Acad Sci USA 98:3666–3672 Valley JW, Peck WH, King EM, Wilde SA (2002) A cool early Earth. Geology 30:351–354 Wilde SA, Valley JW, Peck WH, Graham CM (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409:175–178
Oceanus, Oceani Definition The term Oceanus refers to a large low-albedo feature – Oceanus Procellarum – on the ▶ Moon. Oceanus Procellarum (ocean of storms) is the only surface feature to which the term Oceanus is applied. It denotes the largest ▶ Mare area on the Moon. Oceanus Procellarum extends approximately 2,500 km across and covers a surface area of 4 Mio. km2. It is composed of extensive flood ▶ basalts but is not confined to an isolated impact ▶ crater or basin as it connects several ▶ Mare-type features. Oceanus Procellarum was sampled during the Luna and Surveyor missions and was visited by the Apollo 12 astronauts in late 1969.
See also ▶ Albedo Feature ▶ Apollo Mission ▶ Basalt ▶ Crater, Impact ▶ Impact Basin ▶ Mare, Maria ▶ Moon, The ▶ Palus, Paludes
OGLE ▶ Optical Gravitational Lensing Experiment
OGLE-2005-BLG-390Lb Definition OGLE-2005-BLG-390Lb is a cool, ▶ super-Earth that was discovered with the gravitational microlensing technique by the Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/Robonet), ▶ Optical Gravitational Lensing Experiment (OGLE), and ▶ Microlensing Observations in Astrophysics (MOA) collaborations. The mass and orbital separation of the planet are uncertain, as is the mass and distance of the host star from the Sun, but the most likely values are a mass of 5.5 Earth masses and projected separation of 2.6 AU for the planet, and a mass of 0.22 solar masses and a distance of 6.6 kpc for the host star. With an equilibrium temperature of 50 K, this low-mass planet is located in the cool, outer reaches of its planetary system; it is the first such planet discovered by any technique. The figure shows the observed light curve for the OGLE-2005-BLG-390 microlensing event, including data from the PLANET Danish, Perth, and Canopus telescopes, RoboNet Faulkes North telescope, OGLE telescope, and MOA telescope. The OGLE data over a wider span of time is shown in the top left inset, whereas the planetary 3
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OGLE-2005-BLG-390Lb. Figure 1 The figure shows the observed light curve for the OGLE-2005-BLG-390 microlensing event, including data from the PLANET Danish, Perth, and Canopus telescopes, RoboNet Faulkes North telescope, OGLE telescope, and MOA telescope. The OGLE data over a wider span of time is shown in the top left inset, whereas the planetary deviation is highlighted in the top right inset (Beaulieu et al. 2006).
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deviation is highlighted in the top right inset (Beaulieu et al. 2006).
See also ▶ Exoplanets Discovery ▶ Microlensing Observations in Astrophysics (MOA) ▶ Microlensing Planets ▶ Optical Gravitational Lensing Experiment ▶ Probing Lensing Anomalies Network
References and Further Reading Beaulieu JP et al (2006) Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing. Nature 439:437–440
OGLE-2006-BLG-109Lb,c Definition OGLE-2006-BLG-109Lb,c is the first multi-planet system discovered with the gravitational microlensing technique.
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Is was discovered by the ▶ Microlensing Follow-Up NETwork (MicroFUN) collaboration, in conjunction with the ▶ Optical Gravitational Lensing Experiment (OGLE), ▶ Microlensing Observations in Astrophysics (MOA), and Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/Robonet) collaborations. This system consists of two planets with masses similar to Jupiter and Saturn orbiting a star with roughly half the mass of the sun located roughly 1.5 kPc from the Earth. This system is analogous to our Jupiter/Saturn system, in that the two planets have the same mass ratios as Jupiter and Saturn as well as the same ratio of orbital radii, and the same (inferred) equilibrium temperatures (Gaudi et al. 2008; Bennett et al. 2010). The Fig. 1 shows the observed light curve for the OGLE-2006-BLG-109 microlensing event, including data from OGLE, various MicroFUN telescopes, MOA, PLANET, and RoboNet. Five features are labeled. Four of these are due to the Saturn-mass planet, whereas the fifth can only be explained by a second, Jupiter-mass planet.
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OGLE-2006-BLG-109Lb,c. Figure 1 The figure shows the observed light curve for the OGLE-2006-BLG-109 microlensing event, including data from OGLE, various MicroFUN telescopes, MOA, PLANET, and RoboNet. Five features are labelled. Four of these are due to the Saturn-mass planet, whereas the fifth can only be explained by a second, Jupiter-mass planet. The inset shows the path of the source through the caustic, with the locations of the source which give rise to the five features labelled.
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The inset shows the path of the source through the caustic, with the locations of the source which give rise to the five features labeled.
See also ▶ Exoplanets Discovery ▶ Microlensing Follow-Up Network ▶ Microlensing Observations in Astrophysics ▶ Microlensing Planets ▶ Optical Gravitational Lensing Experiment ▶ Probing Lensing Anomalies Network
References and Further Reading Bennett DP et al (2010) Astrophys J 713:837 Gaudi BS et al (2008) Discovery of a Jupiter/Saturn Analog with gravitational microlensing. Science 319(5865):927
Oligomer Definition The term oligomer is derived from the Greek “oligos,” meaning “a few.” In chemistry, an oligomer is a short polymer consisting of approximately five monomer units, although agreement as to the strict length cutoff is debated and varies between four and one hundred. Oligomers are formed by oligomerization, which involves linking the monomer units together in a chemical reaction. Unlike a ▶ polymer, if one of the repeating units of oligomer is removed, its chemical properties may be significantly altered. In biochemistry, the term oligomer is commonly used for short, single-stranded DNA fragments, used in ▶ hybridization experiments as ▶ primers. It can also indicate a protein made of two or more subunits.
See also
OH ▶ Hydroxyl Radical
▶ Hybridization ▶ Nucleic Acids ▶ Polymer ▶ Primer ▶ Protein
Oligomerization Oligarchic Growth Definition Oligarchic growth is the second-to-last stage of the formation of terrestrial planets and giant planet cores. During this stage, the largest planetary embryos grow quickly while the smallest grow slowly, leading to a bifurcated mass distribution with a number of lunar- to Mars-mass embryos embedded in a swarm of smaller planetesimals. Oligarchic growth transitions to chaotic growth when the planetary embryos become large enough to overcome eccentricity damping due to dynamical friction from smaller planetesimals. This leads to embryo–embryo collisions and signals the last stage of accretion of terrestrial planets and giant planet cores.
See also ▶ Late-Stage Accretion ▶ Planetary Formation ▶ Planetesimals ▶ Runaway Growth
Definition Oligomerization is a chemical process that links monomeric compounds (e.g., amino acids, nucleotides, or monosaccharides) to form dimers, trimers, tetramers, or longer chain molecules (oligomers). Examples are the conversion of nucleotides to oligonucleotides and amino acids to peptides. The boundary between what is considered an oligomer and a ▶ polymer is unclear, but it is usually accepted to be in the range of 10–100 monomer units. Prebiotic experiments have shown that ▶ activated nucleotides can be oligomerized using ▶ clay minerals as catalyst.
See also ▶ Activated Nucleotide ▶ Amino Acid ▶ Clay ▶ Mineral ▶ Monosaccharide ▶ Nucleotide ▶ Oligonucleotide
Oligonucleotide
▶ Oligopeptide ▶ Polymer ▶ Prebiotic Chemistry
Oligonucleotide KUNIO KAWAMURA Department of Applied Chemistry, Osaka Prefecture University, Naka-ku, Sakai, Osaka, Japan
Synonyms DNA; Nucleotide oligomer; RNA
Keywords Chemical evolution, nucleotide, oligonucleotide, RNA
Definition An oligonucleotide is a relatively short RNA or DNA polymer linked via phosphodiester bonds. Under prebiotic conditions, oligonucleotides of RNA can be formed from ▶ activated nucleotide monomers including different kinds of nucleotide bases: ▶ guanine, ▶ adenine, ▶ uracil, ▶ cytosine, and ▶ hypoxanthine. The oligonucleotides of RNA formed under the primitive earth conditions normally contain both 20 ,50 - or 30 ,50 - linkages and often contain a 50 -residue linked by a diphosphate linkage.
Overview The formation of oligonucleotides would have been an essential step for the emergence of life on the primitive earth. In modern organisms, polymerization of RNA or DNA monomers merely proceeds from nucleoside 50 -triphosphate monomers in the presence of DNA template and specific enzymes. On the contrary, the prebiotic formation of RNA with 50 nucleotide units in length proceeds from nucleoside 50 -phosphorimidazolides under simulated primitive earth conditions. However, the prebiotic formation of DNA does not proceed from such activated nucleotide monomers. Although the usage of condensation reagents is possible to form oligonucleotides from nucleotide monomers, the activated nucleotides are more effectively used to form long oligonucleotides with high yield. Continuous investigations have been carried out to identify the pathways for prebiotic formation of RNA oligonucleotide. Successful examples include the oligonucleotide formation from the activated nucleotide monomers in the presence of metal
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ions (Sawai 1976), that in the presence of montmorillonite clay catalyst (Ferris and Ertem 1992; Huang and Ferris 2006), that in the presence of a polynucleotide template (Inoue and Orgel 1983). Normally, the condensation of the activated nucleotide monomers provides oligonucleotides with an average length of 10 units in the presence of metal ion catalyst or clay mineral catalyst. Besides, spontaneous condensation of the activated nucleotide by continuous addition of the activated nucleotide in the presence of ▶ montmorillonite clay results in the formation of oligonucleotides 50-mer (Ferris et al. 1996). On the other hand, oligoguanylates up to 40-mer in length form by the condensation of guanosine 50 -phosphorimidazolide on a polycytidylate template with Zn2+ ion, where the Watson–Crick hydrogen bonding directs the activated monomers of guanosine nucleotide on the template polynucleotide. However, this template-directed reaction is not efficient for the condensation of adenosine, uridine, or cytidine 50 -phosphorimidazolide on the complementary polynucleotide template. This is probably due to the fact that the template-directed formation of oligonucleotides is mainly supported by the stacking interactions between the activated nucleotide monomers rather than the Watson–Crick hydrogen bonding between the activated nucleotide and the complementary template. At the same time, it has been confirmed that the replication of oligonucleotides partially proceeds from the mixture of four types of activated nucleotide monomers in the presence of mixed base template polynucleotides.
See also ▶ Activated Nucleotide ▶ Montmorilllonite ▶ Nucleic Acids ▶ Nucleoside Phosphoimidazolide ▶ RNA World ▶ Self Replication ▶ Template-Directed Polymerization
References and Further Reading Ferris JP, Ertem G (1992) Oligomerization of ribonucleotides on montmorillonite: reaction of the 50 -phosphorimidazolide of adenosine. Science 257:1387–1389 Ferris JP, Hill Jr AR, Liu R, Orgel LE (1996) Synthesis of long prebiotic oligomers on mineral surfaces. Nature 381:59–61 Huang W, Ferris JP (2006) One-step, regioselective synthesis of up to 50-mers of RNA oligomers by montmorillonite catalysis. J Am Chem Soc 128:8914–8919 Inoue T, Orgel LE (1983) A nonenzymatic RNA polymerase model. Science 219:859–862 Sawai H (1976) Catalysis of internucleotide bond formation by divalent metal ions. J Am Chem Soc 98:7037–7039
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Oligopeptide JEAN-FRANC¸OIS LAMBERT Laboratoire de Re´activite´ de Surface, Universite´ Pierre et Marie Curie, Paris, France
Synonyms Peptide
Keywords Activation, amino acids, peptides, proteins
Definition An oligopeptide is a short-chain ▶ peptide, i.e., a polymer of ▶ amino acids (AAs) connected by amide, or more precisely peptide, linkages. The term is usually limited to peptides with less than 20–25 amino acid residues.
Overview In “peptides-first” scenarios for the emergence of biomacromolecules, oligopeptides capable of specific catalysis of some biological reactions (primitive metabolism) are suggested to have appeared without the previous existence of genetic material. For instance, in his protometabolic model, de Duve (1991) calls these short peptides, “multimers.” Chemically, the emergence of oligopeptides from preexisting amino acids poses several problems. The first two are consequences of chemical thermodynamics. The condensation of an amide bond between two AAs is thermodynamically unfavorable in solution, thus the amount of oligopeptide in an amino acid solution at equilibrium falls quickly with the peptide chain length: for instance, a glycine solution with a total AA concentration of 0.5 M, at equilibrium, would contain only one molecule of 18-mer per hundred cubic kilometers! In addition, the formation of oligomers from a mixture of different amino acids would likely show little selectivity, i.e., all possible sequences would be randomly formed. Since the number of sequences increases exponentially with the peptide length, the probability of forming significant amounts of a specific oligomer is vanishingly low. The last problem concerns the slow kinetics of oligomerization: at room temperature and moderate pH, completion of the reaction would take several centuries due to high activation energy barriers. Several solutions have been proposed to allow the prebiotic emergence of oligopeptides with a reasonable probability:
– They could have formed in submarine hydrothermal vents: thermodynamic equilibria are displaced toward AA oligomerization in hydrothermal conditions, although probably not enough to give high peptide yields. – Oligomerization could have been coupled to a thermodynamically downhill chemical process. This would involve the input of “high-energy molecules” (more properly, molecules with high free enthalpies of formation) such as nitrogen oxides. – Alternatively, it may have occurred in the “adsorbed state,” i.e., after adsorption of the AAs on mineral surfaces (“polymerisation on the rocks,” Orgel 1998). This hypothesis connects well to “surface metabolism” scenarios (e.g., those of Wa¨chtersha¨user) and may solve the kinetic as well as the thermodynamic problem, since catalysis by surface groups is likely. The last two scenarios have been shown empirically to result in the formation of oligopeptides up to the pentamers at least, even though molecular details remain sketchy. It is noteworthy that they are only successful in fluctuating environments, i.e., when coupled with variations of the macroscopic conditions such as wetting and drying cycles.
See also ▶ Amino Acid ▶ Peptide ▶ Polypeptide ▶ Oligonucleotide
References and Further Reading Brack A (2007) From interstellar amino acids to prebiotic catalytic peptides: a review. Chem Biodivers 4:665–679 Cleaves HJ, Aubrey AD, Bada JL (2009) An evaluation of the critical parameters for abiotic peptide synthesis in submarine hydrothermal systems. Orig Life Evol Biosph 39:3109–3126 Commeyras A, Taillades J, Collet H, Boiteau L, Vandenabeele-Trambouze O, Pascal R, Rousset A, Garrel L, Rossi J-C, Biron J-P, Lagrille O, Plasson R, Souaid E, Danger G, Selsis F, Dobrije´vic M, Martin H (2004) Dynamic co-evolution of peptides and chemical energetics, a gateway to the emergence of homochirality and the catalytic activity of peptides. Orig Life Evol Biosph 34:35–55 de Duve C (1991) Blueprint for a cell: the nature and origin of life. Neil Patterson, Burlington, NC Lambert J-F (2008) Adsorption and polymerization of amino acids on mineral surfaces: a review. Orig Life Evol Biosph 38:211–242 Leslie E (1998) Orgel polymerization on the rocks: theoretical introduction. Ori Life Evol Biosph 28(3):227–234 Rode BM (1999) Peptides and the origin of life. Peptides 20:773–786 Zaia DAM (2004) A review of adsorption of amino acids on minerals: was it important for origin of life? Amino Acids 27:113–118
Oort Cloud
Oligosaccharide ▶ Carbohydrate
Olympus Mons Definition Olympus Mons is the largest known ▶ volcano on ▶ Mars and in the entire ▶ Solar System (height above the Martian topographic datum: 22 km; height above surrounding plains: 20 km; basal diameter: 640 km). The flanks are covered by tube-fed and channel-fed lava flows and have slopes of typically 3–5 , except for a steeper, up to 6 km-high basal scarp. The morphological analogy to terrestrial volcanoes (e.g., Hawaii, Galapagos) suggests that Olympus Mons is a basaltic shield volcano. A summit caldera has 80 km diameter and developed in multiple stages over the last several hundred million years (Ma). Some lava flows are probably even younger (10 Ma), indicating possible ongoing volcanism on Mars.
See also ▶ Basalt ▶ Mars ▶ Solar System, Inner ▶ Tharsis ▶ Volcano
Oort Cloud JACQUES CROVISIER LESIA - Baˆtiment ISO (n 17), Observatoire de Paris, Meudon, France
Keywords Comets, small bodies
Definition The Oort cloud is a spherical cloud of ▶ comets surrounding the Solar System and extending out to heliocentric distances greater than 100,000 AU.
Overview The idea of a distant reservoir of ▶ comets, extending to the verge of the ▶ Solar System, was already discussed by
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¨ pik (1932). The Giovanni Schiaparelli (1910) and Ernst O Dutch astronomer Jan Oort (1900–1992) pointed out in 1950 the need of such a reservoir to explain the continuous input of new long-period comets. He showed, from the data gathered by Van Woerkom (1948) that many longperiod comets have their aphelia at more than 20,000 AU. This reservoir is now known as the “Oort cloud.” The Oort cloud is supposed to be spherical, extending from 20,000 ▶ AU from the Sun to 200,000 AU (the limit of the gravitational influence of the Sun). It could comprise as many as 1012 comets. These comets cannot be directly observed in the Oort cloud, being too far from the Sun to manifest any activity. Following perturbations by a nearby star (or by a hypothetical distant planet), they may be transferred to orbits bringing them close to the Sun, where they become active. Such “Oort-cloud comets” have random inclination over the ecliptic and belong to the family of “nearlyISOTROPIC comets.” The most famous of them is 1P/ ▶ Halley. Comets stored in the Oort cloud were not formed there. They were formed in the inner Solar System (presumably in the Jupiter – Uranus region) and subsequently ejected to the outer Solar System following gravitational perturbations by the giant planets. The Oort cloud contrasts with the alternate reservoir of comets, the ▶ Kuiper belt, which is at the origin of Jupiter-family comets.
See also ▶ Comet ▶ Kuiper Belt ▶ Trans-Neptunian Object
References and Further Reading Barucci MA, Boehnhardt H, Cruikshank DP, Morbidelli A (2008) The solar system beyond Neptune. University of Arizona Press, Tucson Dones L, Weissman PR, Levison HF, Duncan MJ (2005) Oort Cloud formation and dynamics. In: Festou MC, Keller HU, Weaver H (eds) Comets II. University of Arizona Press, Tucson, pp 153–174 Festou MC, Keller HU, Weaver H (2005) Comets II. University of Arizona Press, Tucson Oort JH (1950) The structure of the cloud of comets surrounding the Solar System, and a hypothesis concerning its origin. Bull Astron Inst Neth 11:91–110 ¨ pik EJ (1932) Note on stellar perturbations of nearly parabolic orbits. O Proc Am Acad Arts Sci 67:169–183 Schiaparelli G (1910) Orbites come´taires, courants cosmiques, mZˇtZˇorites. Bull Astron Ser I 27:194–205 Van Woerkom AJJ (1948) On the origin of comets. Bull Astron Inst Neth 10:445–472
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Opacity
Opacity Synonyms Attenuation
Definition Opacity measures the property of a medium to attenuate light. The opacity depends on the composition of the medium, its density, and temperature, but also on the wavelength considered. The absorption coefficient an (with units [cm1]) enters into the definition of the ▶ optical depth. More often in astronomy, it is the mass absorption coefficient kn ¼ an =r (with units [cm2 g1]) which is used. The main sources of opacity in an astronomical context are: ● Electron scattering (at temperatures of the order 1 billion K) ● Electronic transitions (free-free, bound-free, or bound-bound at temperature of the order 10,000 K) ● Molecular or dust absorption (around or below 3,000 K) The opacity plays an essential role in radiative transfer, and thus in the energetic equilibrium of stars.
See also ▶ Mean Free Path ▶ Optical Depth
Opaline Silica Definition Opaline silica (SiO2 · nH2O) occurs at several locations on ▶ Mars, although the definitive identification is contentious. The confirmed finding would indicate past aqueous activity. Aqueous free silica is a product of ▶ basalt weathering, when the interaction of water with mafic (i.e., Mg- and Fe-rich, silica-poor) ▶ rock rapidly dissolves olivine, pyroxene, and glass. Opaline silica could have been precipitated in hydrothermal or lacustrine (related to a lake) evaporitic environments on Mars. On Earth, it rarely persists more than a few million years, because it rapidly transforms into microcrystalline quartz during ▶ diagenesis. The persistence of opaline silica on Mars over much longer geologic timescales (billions of years) indicates that water was not present for extended periods of time.
See also ▶ Basalt ▶ Chert ▶ Diagenesis ▶ Hydrothermal Environments ▶ Mars ▶ Rock ▶ Weathering
Oparin’s Conception of Origins of Life STE´PHANE TIRARD Faculte´ des Sciences et des Techniques de Nantes, Centre Franc¸ois Vie`te d’Histoire des Sciences et des Techniques, EA 1161, Nantes, France
History In 1924, Alexander Ivanovitch Oparin (1894–1980), a young scientist in vegetal physiology, published in Moscow a very important text about the origin of life. With this text, he began a very long career devoted to this topic. He wrote important books and developed very active experimental researches. He published this first text, entitled Origins of Life, in Russian. He described the evolution of earth and the transformation of matter from mineral molecules to organic molecules. Finally, according to him, little drops of organic matter could appear, which constituted the last step before becoming cells. In 1936, Oparin published an important book entitled Origins of Life. In an interdisciplinary approach, he suggested a broad scenario of the origins of life on Earth, describing all the steps of the evolution of the Earth, of matter, and of primordial life. This book knew a great success and was translated into several languages, and especially into English in 1938. This book is an exhaustive presentation of Oparin’s ideas, very close to his 1924 conception. However, he introduced a lot of current scientific data and two specific points have to be underlined. Firstly, Oparin introduced the notion of coacervate coming from Bungenberg de Jong’s work. Coacervates were microscopical vesicles formed in colloidal solutions and Oparin used them as a model of a primitive cell in his theory. Secondly, Oparin claimed that the primitive atmosphere did not contain CO2, and that it was reductive.
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After the Second War, this book still was a point of reference. For example, John Desmond Bernal (1951) quoted it. In 1952, ▶ Harold Urey, the American physicist, wrote a synthesis on the primitive conditions and referred to Oparin (1938) about the lack of CO2 in the primitive atmosphere and agreed with the Soviet scientist on this point. During the following three decades, Oparin, who had important academic positions in the USSR, continued to be the leader of this topic in his country. He particularly worked on primitive metabolism and developed a heterotrophic hypothesis on the origin of life on Earth. However, in the lyssenkoist context, he neglected to work on heredity molecules.
See also ▶ Bernal’s Conception of Origins of Life ▶ Calvin’s Conception of Origins of Life ▶ Haldane’s Conception of Origins of Life ▶ Miller, Stanley ▶ Monod’s Conception of Origins of Life ▶ Urey’s Conception of Origins of Life
References and Further Reading Deamer DW, Fleischaker GR (1994) Origins of life, the central concept. Jones and Bartlett, Boston Oparin AI (1938) The origin of life, Sergius Morgulis trans. Macmillan, New York
Open Cluster Definition An open star cluster is a loosely bound group of a few 103 stars formed from the same giant molecular cloud and thus having the same age and ▶ metallicity (chemical composition). This property makes them valuable tools for the study of stellar evolution. Contrary to ▶ globular clusters, open clusters are relatively young (less than a few 108 years old) and they are found in the plane of the Milky Way. About a thousand open clusters are known in the Galaxy, but their total number may be ten times higher. The best known example is the Pleiades (108 year). Various processes (tidal interactions with giant molecular clouds and spiral arms) tend to disperse its members and dissolve an open cluster.
See also ▶ Globular Cluster ▶ Metallicity
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Operon Synonyms Polycistronic transcript
Definition An operon is a group of ▶ genes whose products have related or complementary functions and which are transcribed as a unit. Polycistronic or multi-gene are then independently translated by ribosomes into the individual, functionally related proteins. Many bacterial and archaeal genes are organized into operons, while they are exceptional in eukaryotes. The operon model was proposed by F. Jacob and J. Monod in 1961 to explain the coordinated regulation of co-transcribed genes involved in the metabolism of lactose in Escherichia coli. The so-called Lac operon spans about 6,000 nts and includes a promoter, an operator, three adjacent structural genes coding for the enzymes required for lactose metabolism, and a terminator. Regulation of the Lac operon was the first complex genetic regulatory mechanism to be elucidated.
See also ▶ Archea ▶ Bacteria ▶ Gene ▶ Genome ▶ Transcription
Ophiolite Definition An ophiolite is a slice of ▶ oceanic crust and underlying mantle that is thrust onto a continent by ▶ obduction. The stratigraphic sequence observed in an ophiolite corresponds to layered oceanic crust: from top to bottom, an upper layer of oceanic sediment (siliceous or carbonate ooze); a layer of pillow basalt; a layer of sheeted nearvertical dykes; layers of ▶ gabbro and plagioclase and mafic cumulates; and a basal layer of “tectonite,” the
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deformed harzburgite (a type of ▶ peridotite) of the uppermost mantle. Many ophiolites contain only some of these lithologies. Debate surrounds the question of whether any ophiolite represents normal oceanic crust formed at a mid-oceanic spreading centre. The geochemical compositions of basalt rocks indicate that many ophiolites are segments of crust that formed in back-arc basins. Reactions between water and ophiolite at high temperatures (300 C) can produce low-molecular hydrocarbons by abiotic Fischer-Tropsch-Type mechanisms during serpentinization.
See also ▶ Fischer-Tropsch-Type Reaction ▶ Gabbro ▶ Obduction ▶ Oceanic Crust ▶ Peridotite ▶ Pillow Lava ▶ Plate Tectonics
Optical Gravitational Lensing Experiment Synonyms OGLE
Definition OGLE is a collaboration of primarily Polish astronomers that uses wide-field temporal monitoring to study a broad range of astrophysical phenomena. The original goal of the OGLE collaboration was to search for dark matter with the gravitational microlensing technique. However, since its inception, OGLE has also played a central role in the search for ▶ exoplanets with microlensing. In addition, OGLE identified the first transiting planets found by photometry (as opposed to radial-velocity planets), and has contributed to the study of variable stars, Galactic structure, strong gravitational lenses, and Kuiper belt objects. The Principal Investigator of the OGLE collaboration is Andrej Udalski from Warsaw University. The OGLE 1.3-m telescope is located at the Las Campanas Observatory in Chile.
See also
Optical Depth
▶ Exoplanets Discovery ▶ Microlensing Planets
Synonyms Optical thickness
Definition The optical depth is a dimensionless quantity, generally noted t, that measures how absorbing – or ▶ scattering – a slab in a medium is. Depending on whether t < 1 or t > 1, a slab of medium is said to be optically thin or optically thick. In a medium of constant density, the optical depth is proportional to the thickness of the slab. In an absorbing medium, the intensity of inward radiation decreases as exp(t). In an emitting medium, such as a stellar photosphere, or a planetary atmosphere in the infrared, the outward radiation is well approximated by assuming that it comes entirely from the layer of optical depth t = 1.
See also ▶ Extinction, Interstellar or Atmospheric ▶ Opacity ▶ Radiative Transfer ▶ Scattering
Optical Thickness ▶ Optical Depth
Optical Rotatory Power ▶ Rotatory Power
Orbit AVI M. MANDELL NASA Goddard Space Flight Center, Greenbelt, MD, USA
Keywords Gravity, Kepler, Newton
Orbit
Definition The orbit of a celestial body is its path around a central mass, caused by the acceleration due to the gravitational force between them. The planets and minor bodies such as asteroids and comets in the Solar System orbit the Sun, while any celestial body in the Solar System that orbits something other than the Sun is called a satellite (moon). Long-term orbital motion is affected by the gravitational forces due to all bodies in the vicinity, as well as to torques due to forces such as gas drag, dynamical friction, and radiation pressure.
History The concept of a celestial orbit can be generalized to include any motion of a celestial body around another body, and dates back at least to the earliest written texts. However, Johannes Kepler was the first to establish the true qualitative characteristics of astronomical orbits, formulating his three laws of planetary motion between 1609 and 1619. Analyzing the results from painstaking observations of the motions of astronomical bodies by Tycho Brahe over many years, he determined that these bodies travel on elliptical orbits (with the Sun at one focus), speeding up as they approach the Sun and then slowing down as they move farther away. As simple as they seem today, these two conceptual breakthroughs revolutionized the Western world’s view of the structure of the universe. Until this time, the majority view was that the Sun, planets, and star all revolved around the Earth; this theory is known as geocentrism, with the most well-known proponent being the second-century Greco-Roman philosopher Ptolemy. Heliocentrism, the idea that the orbits of the planets are centered on the Sun, had first been proposed by Aristarchos of Samos in the third century BCE, but was largely ignored until it was explored by Copernicus in 1543 and later supported by Galileo’s observations of the motions of Jupiter’s moons. However, these early models conceived the orbits as perfect circles, and therefore required many ad hoc additions to fit the observed motions of planets in the sky. Kepler’s realization that the orbits could be elliptical turned heliocentrism into a viable predictive theory. Kepler’s third law, that the square of the period of a planet’s orbit is proportional to the cube of the distance from the Sun (P2 / a3), provided the first mathematical relation between two orbital quantities and formed one of the primary drivers for Isaac Newton’s derivation of the inverse square law of universal ▶ gravitation (Fgrav / 1/r2). Newton was able to show that two gravitationally bound bodies will orbit the center of mass of the two-body system, and if the difference in mass is large enough the
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smaller body will essentially orbit the more massive body. This simple mathematical formulation (with modifications included for general relatively as proposed by Albert Einstein) forms the backbone of all of celestial mechanics and dynamical astronomy.
Overview As first envisioned by Newton, orbital shapes can be organized into three different categories based on how much energy the orbiting body has: elliptical (including circular), parabolic, and hyperbolic. Parabolic and hyperbolic orbits are unbound, in that they begin and end at an infinite distance from the center of mass; the difference between the two is that a body on a parabolic orbit has zero net energy, while a hyperbolic orbit requires a positive total energy and therefore has a nonzero beginning and ending velocity. Newton realized that these shapes correspond to sections of a cone (conic sections), with one focus of the orbit lying on the rotational axis of the cone. The conic section therefore confines the orbital motion to a two-dimensional plane, known as the orbital plane.
Basic Methodology It is important to start with a discussion of terminology, since the language of celestial mechanics (the name for the study of orbital dynamics) can become very confusing due to the many specialized terms used to describe the properties of an orbit. The general terms for the closest and farthest points of an orbit are the periapsis and apoaspsis; these terms can be specialized for individual situations (i.e., perigee for orbits around the Earth, perihelion for orbits around the Sun, and periastron for orbits around any star). The line that connects the periapsis and the apoapsis is called the line of apsides; this line naturally passes through the central body, and the direction of the periapsis can rotate to trace a circle if the orbit is perturbed (known as apsidal precession). The motion of an orbiting body can be described by six quantities known as orbital elements. These quantities can be coordinates of position and velocity, such as the Cartesian coordinates of a body in a specific reference frame; but more commonly orbits are described using the Keplerian orbital elements, since these are independent of a specific reference direction. The six Keplerian elements are the semimajor axis and eccentricity of the ellipse, and the inclination, argument of periapsis, longitude of the ascending node, and either the mean (temporal) or true (angular) anomaly (see Fig. 1). The most useful and general elements are semimajor axis, eccentricity, and inclination, which describe the size, shape, and tilt of the
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two bodies will lead to an excitation of the orbit of the smaller body (such as the scattering of comet-sized bodies into the Oort cloud by Jupiter) and a damping of the orbital excitation of the larger body (such as the damping of planetary eccentricities during ▶ planet formation due to interactions with small planetesimals).
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Orbit. Figure 1 A diagram of an orbit, with several orbital elements labeled (inclination, argument of periapsis, longitude of the ascending node, and true anomaly). The additional orbital elements not labeled are the semimajor axis (i.e., size of the orbit) and the eccentricity (i.e., ellipticity) of the orbit (WikiCommons)
orbit with respect to the reference orbital plane (the plane of reference in the Solar System is known as the ecliptic plane). The semimajor axis is defined as one half of the major axis of the orbital ellipse; note that this is different than the distance to the central body when the orbit is not a circle. The eccentricity describes how elliptical the orbit is, ranging from 0 for a circular orbit to 1 for a parabolic orbit. The inclination of the orbit is measured in degrees, ranging from 0 to 180 (values larger than 90 indicate a “retrograde” orbit that moves in the opposite direction around the star). The other elements are less intuitive and less frequently used, since they primarily define a single orbital position and average out to zero for a population of randomly oriented bodies (as opposed to eccentricity and inclination that have a characteristic distribution for a randomized population), and they will not be discussed in detail here. There are a number of processes that can lead to an evolution of the orbital parameters of a single body or a population of bodies; these effects are known as perturbations. These perturbations generally fall into two categories: gravitational perturbations due to interactions with other bodies in the system (such as secular perturbations or dynamical friction), or relatively steady-state forces that either add or remove energy from the orbit (such as gas drag or radiation pressure). Gravitational forces between
Applications The study of orbital dynamics is fundamental to much of planetary astronomy in our own Solar System, as well as the study of other stellar and planetary systems. We deduce the orbital motions and positions of bodies in our own system by studying the proper motion of objects as they move across the sky over time – the same method has been used since classical times, and we are still using it to discover ▶ Kuiper Belt objects at the far reaches of the Solar System today. In other planetary systems, we can measure the radial velocity of the central star and use that motion to calculate the orbital motion of surrounding planets even when they are impossible to detect directly. We can also explore the evolution of the orbits of bodies in planetary systems over time using computer simulations. Computational numerical simulations of planetary dynamics all aim to solve the same general problem: how to calculate the gravitational and nongravitational forces on a system of bodies accurately over long timescales (known as the n-body problem). The solution to the dynamical evolution of any system of three or more bodies cannot be solved exactly, but there are many different types of mathematical algorithms that have been developed to optimize the pursuit of an approximate solution (i.e., Runge–Kutta, symplectic integration, etc). Important applications for these tools include the study of the formation of planetary systems through long-term accretion of planetary embryos, as well as the dynamical history of populations of small bodies in our Solar System (such as the Kuiper Belt).
See also ▶ Apsidal Angle ▶ Gravitation ▶ Kuiper Belt ▶ Orbit ▶ Period ▶ Planet Formation
References and Further Reading Danby JMA (1992) Fundamentals of celestial mechanics. Willmann-Bell
Organelle
Orbital Period/Frequency Definition The orbital period (P) is the time required for a body to complete an orbit around a central mass. The orbital period can be calculated by the Kepler’s third law, which in the solar system takes the form P 2 ¼ a3 where P is in years and the orbit’s semimajor axis a is in AU. For different stellar masses the full equation that should be used is P2 ¼
4p2 a3 G Mstar þ Mp
where Mstar and Mp are the stellar and planetary mass, respectively, and G is the gravitational constant.
See also ▶ Mean Motion Resonance ▶ Orbit ▶ Secular Dynamics
Orbital Resonance JE´ROˆME
PEREZ Applied Mathematics Laboratory, ENSTA ParisTech, Paris Cedex 15, France
Keywords Gravitation, instability, period, orbits
Definition An orbital resonance is either a stabilizing or disrupting phenomenon produced by the cumulative effect of gravity between at least two bodies in periodic orbits, typically around a third, more massive body.
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gravitational interaction. For the two-body Keplerian problem, this is Kepler’s third law: The square of the orbital period is directly proportional to the cube of the semi-major axis of its elliptical orbit (see ▶ Gravitation). If several bodies are in orbit in a fixed gravitational potential and the ratio of the period of two of these particular motions is rational, a special configuration occurs periodically. The cumulative effects of this configuration could be favorable for reaching a stabilized state or, on the contrary, could lead to a destructive process of the system or of part of it, by ejection. The solar system contains plenty of orbital resonances. Between Mars and Jupiter, there exist thousands of small bodies that form the asteroid belt. In 1866, Kirkwood announced the discovery of gaps in the distances of these bodies’ orbits from the Sun. These gaps were located at positions where, if there were a body, its period of revolution around the Sun would be an integer fraction of Jupiter’s orbital period. In fact, these gaps were dug by orbital resonances. Likewise, Mimas, a minor satellite of Saturn, has a period that is twice the one of a body, which would be found in the Cassini division of the planet’s ring. Sometimes resonances can maintain rather than exclude bodies from their orbits. Io, Europa, and Ganymede, the first three moons of Jupiter, show a special kind of orbital resonance called Laplace’s resonance: their orbital period are in the ratio 1:2:4. In consequence, these three moons cannot be aligned together on the same side of Jupiter. This configuration could eject one of them.
See also ▶ Gravitation ▶ Lagrangian Points
References and Further Reading Carl DM, Stanley FD (1999) Solar system dynamics. Cambridge University Press, Cambridge
Organelle
Overview The motion of a body in a fixed gravitational field is described by an ordinary differential equation. The solution of this problem is entirely determined provided we know its initial conditions. In several physical cases, this solution has some periodic characteristics. The period of such a periodic motion depends essentially on a mean distance between bodies in
Definition An organelle is a specialized subunit inside a cell. It is separated from the surrounding cytoplasmic media by a lipid bilayer and plays a particular role or function within the cell similar to that of organs in an animal’s body. The absence of a ▶ nucleus, a membrane-enclosed organelle containing the genome, is one of the major
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features that differentiate prokaryotes from eukaryotes. The most notable examples of organelles are those involved in energy transduction reactions in eukaryotes which were originated by endosymbiont bacteria: ▶ chloroplasts present in plants cells, algae and some protists which are responsible for ▶ photosynthesis, and ▶ mitochondria in almost all eukaryotes where ▶ respiration for energy production takes place. Other important organelles are hydrogenosomes involved in energy generation in anaerobic conditions; the endoplasmic reticulum responsible for many functions, including the synthesis of new membrane material; lysosomes involved in protein degradation and amino acid recycling; peroxisomes responsible for the degradation of oxidative stress products, like hydrogen peroxide; microfilaments and microtubules responsible for the internal structure of eukaryotic cells; and flagella and cilia involved in cell movement.
See also ▶ ATP Synthase ▶ Chloroplast ▶ Eukarya ▶ Mitochondrion ▶ Motility ▶ Nucleus ▶ Oxygenic Photosynthesis ▶ Photosynthesis ▶ Respiration
Organic Cyanide ▶ Nitrile
Organic Material Inventory Definition The organic material inventory is the list of materials that contain organic compounds (typically carbon, hydrogen, oxygen nitrogen. . .) that are carried on spacecraft in quantities above a specified limit. The list comprises the denomination of each compound and the total amount present inside the spacecraft. This inventory is a component of required ▶ planetary protection documentation for missions to target objects for which there is an interest in studying organic compounds that could have contributed to the origin and evolution of life in the Solar System.
Organic Molecule HENDERSON JAMES (JIM) CLEAVES II Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
Definition An organic molecule is any member of a large class of molecules containing carbon, and then limited by a number of somewhat arbitrary restrictions. For historical reasons, a strict definition of an organic molecule is difficult. The word “organic” dates back to the ancient Greeks. For centuries, many in the West believed in the concept of vitalism: that certain “organic” compounds could only be synthesized by the action of a vital “life-force” possessed only by living organisms. This implied that “organic” compounds were fundamentally different from the “inorganic” compounds that could be obtained by laboratory manipulation.
Overview The first well-documented experimental synthesis of an “organic compound” occurred in 1828 when Wo¨hler synthesized ▶ urea from the “inorganic” compounds potassium cyanate and ammonium sulfate. Urea had been considered an “organic” compound, as it was only known to occur in the urine of living organisms. Since then, many thousands of biological molecules have been synthesized by organic chemists. Some sources define organic compounds as those containing C-H bonds; others include C-C bonds in the definition, still others merely require a molecule to contain carbon. This last definition would include compounds such as steel alloys and calcite, which most organic chemists would not consider part of their field of study. The C-H bond or C-C bond containing definitions would exclude urea, and many other compounds commonly accepted by chemists as “organic.” While the definition of what constitutes an organic compound can be ambiguous for a few small molecules such as urea, CO, and HCN, this is much less true for even slightly larger molecules. Indeed, as of 2010, many millions of “organic” compounds have been synthesized by chemists, and the theoretical number possible is infinite. For example, the number of small organic molecules in what is considered the pharmacologically relevant size range (molecular weight 2% relative to the terrestrial composition. They assigned the observed 16 O-rich oxygen to the solar-wind composition, concluding that the bulk solar nebula was 16O-rich. However, the situation was complicated by the finding of a distinct O-isotope composition at the surface of metallic grains from a different lunar soil sample, where O was enriched in 17,18O by 3% relative to the terrestrial composition (Ireland et al. 2006). Hashizume and Chaussidon (2009) confirmed existence of both the 16O- and 17,18O-rich components among the lunar metallic grains, but concluding that the 17,18O-rich component seems to be a nonsolar component. This conclusion came from their observation that the 17,18O-rich component was more than an order of magnitude too abundant to assign it to the solar-wind composition, whose amount was estimated from the implanted dose of solar noble gases to the lunar soil grains. This debate may come to the end in the near-future after the final result for the O-isotope determination of the solar-wind component (McKeegan et al. 2010) captured by the GENESIS space mission. The space-probe circled the Sun for several years to accumulate solar wind on several substrates, which were returned to Earth in 2004. The other important end-member compositions, which may serve as tests for several models, are the primordial compositions of water ice and organic matter. The models by Yurimoto and Kuramoto (2004) or Lyons and Young (2005) predict that water ice is the carrier of the 17,18 O-rich isotope signature, connecting the 17,18O-rich O atoms, first produced by the CO self-shielding in the gas medium, and the rocky materials at the mid-plane of the solar-nebula, which finally trapped the isotope signature. Sakamoto et al. (2007) discovered in a primitive carbonaceous chondrite Acfer094, a poorly characterized ironoxide mineral with O-isotope compositions as high as d17O d18O 180‰. They explained that the ironoxide mineral was produced by reactions between primordial water and the metallic Fe-Ni or iron-sulfides, and argued that their observation demonstrates the role of water ice as a carrier of the O-isotope signature produced by the CO self-shielding. Hashizume et al. (2011) recently discovered that an Antarctic carbonaceous chondrite
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contains pieces of organic particles that display striking enrichments in the 17O and 18O at almost paralleled degrees, by as high as 530‰ relative to the terrestrial composition. The oxygen isotope anomalies were associated with carbon isotope anomalies, roughly with a relationship of d17O 1.05 d18O 0.5 d13C. By the CO self-shielding, the enrichment of 13C is indeed expected to be associated with the 17,18O enrichment. However, the 13C-rich signature produced among the photodissociated products is considered to be compromised by a competing reaction. The ion molecule reaction (12CO + 13C+ ↔ 12C+ + 13CO + 35 K) is particularly active at low temperatures, whose isotope effect is considered to exceed the effect by the self-shielding below a gas temperature of 60 K (Visser et al. 2009). The positive correlation between the d17,18O and d13C observed among the organics suggests that the temperature where the CO photodissociation occurred was higher than the one for the typical molecular cloud core (10 K), which is the place proposed by Yurimoto and Kuramoto (2004) where the CO self-shielding might have occurred. Hashizume et al. (2011) argued that the organic formation associated with the 17,18O-enrichment probably occurred at the envelope of the solar nebula, a slightly warm (>40 K) and dense gas medium intensively illuminated by the proto-Sun.
Future Directions The non-mass-dependent O-isotope anomaly, broadly observed among the solid planetary materials, was originally considered to represent the origins or formation processes of a subset of the rocky planetary materials. However, recent studies suggest that the O-isotope anomaly is deeply connected to the origins of the lowtemperature condensates, that is, the volatiles, represented by the water ice and the organic matter, which are critically important materials in astrobiology. It is important to summarize the two advantages of O-isotope over other light element (H, C and N) stable isotopes as a tracer. (1) Oxygen has three isotopes. We may accurately discriminate the original mass-independent isotope signature from the mass-dependent fractionations that may occur by miscellaneous effects. (2) Oxygen is the major constituent in both low- and high-temperature condensates. The oxygen isotope may become one of the most important tools to trace the entire history of the important ingredients for a habitable planet: from their origins; their distribution processes in the solar nebula; accretion to planets/ asteroids; and the processes within the planets, namely, the rock–water–organics interactions, possibly including the life-formation process.
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See also ▶ Fractionation, Mass Independent and Dependent ▶ Interstellar Chemical Processes ▶ Isotopic Fractionation (Interstellar Medium) ▶ Isotopic Fractionation (Planetary Process) ▶ Isotopolog ▶ Photochemistry ▶ Photodissociation Regions ▶ Protoplanetary Disk, Chemistry ▶ Stellar Nucleosynthesis
References and Further Reading Clayton RN (1993) Oxygen isotopes in meteorites. Annu Rev Earth Planet Sci 21:115–149 Clayton RN (2002) Self-shielding in the solar nebula. Nature 415:860–861 Clayton RN (2005) Oxygen isotopes in meteorites. In: Davis AM (ed) Meteorites, comets and planets, vol 1, Treatise on geochemistry. Elesevier-Pergamon, Oxford, pp 129–142 Clayton RN, Grossman L, Mayeda T (1973) A component of primitive nuclear composition in carbonaceous meteorites. Science 182: 485–488 Hashizume K, Chaussidon M (2005) A non-terrestrial 16O-rich isotopic composition for the protosolar nebula. Nature 434:619–622 Hashizume K, Chaussidon M (2009) Two oxygen isotopic components with extra-selenial origins observed among lunar metallic grains – in search for the solar wind component. Geochim Cosmochim Acta 73:3038–3054 Hashizume K, Takahata N, Naraoka H, Sano Y (2011) Extreme oxygen isotope anomaly with a solar origin detected in meteoritic organics. Nat Geosci (in press) Ireland Isotopic enhancements of 17O and 18O from solar wind particles in the lunar regolith. Nature 440:776–778 Kitamura Y, Shimizu M (1983) Oxygen isotopic anomaly and solar nebular photochemistry. Moon Planet 29:199–202 Lyons JR, Young ED (2005) CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula. Nature 435:317–320 Marcus RA (2004) Mass-independent isotope effect in the earliest processed solids in the solar system: a possible mechanism. J Chem Phys 121:8201–8211 McKeegan KD, Kallio APA, Heber VS, Jarzebinski G, Mao PH, Coath CD, Kunihiro T, Wiens R, Allton J, Burnett DS (2010) Genesis SiC concentrator sample traverse: confirmation of 16O-depletion of terrestrial oxygen. Lunar Planet Sci 41:2589, CD-ROM Navon O, Wasserburg GJ (1985) Self-shielding in O2 – a possible explanation for oxygen isotope anomalies in meteorites? Earth Planet Sci Lett 73:1–16 Nguyen AN, Stadermann FJ, Zinner E, Stroud RM, Alexander C, O’D M, Nittler LR (2007) Characterization of presolar silicate and oxide grains in primitive carbonaceous chondrites. Astrophys J 656: 1223–1240 Sakamoto N, Seto Y, Itoh S, Kuramoto K, Fujino K, Nagashima K, Krot AN, Yurimoto H (2007) Remnants of the early solar system water enriched in heavy oxygen isotopes. Science 317:231–233 Thiemens MH (1999) Mass-independent isotope effects in planetary atmospheres and the early solar system. Science 283:341–345 van Dishoeck EF, Black JH (1988) The photodissociation and chemistry of interstellar CO. Astrophys J 334:771–802
Visser R, van Dishoeck EF, Black JH (2009) The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks. Astron Astrophys 503:323–343 Young ED, Russell SS (1998) Oxygen reservoirs in the early solar nebula inferred from an Allende CAI. Science 282:452–455 Yurimoto H, Kuramoto K (2004) Molecular cloud origin for the oxygen isotope heterogeneity in the solar system. Science 305:1763–1766
Oxygen Respiration ▶ Aerobic Respiration
Oxygenase Definition Oxygenases catalyze the transference of oxygen from molecular oxygen to an oxidized substrate. They are a group of ▶ enzymes that form a class of oxydoreductases (enzymes that catalyze the transference of electrons from one molecule (called reductant) to another (the oxidant)). There are two classes of oxygenases: monooxygenases, which transfer an oxygen atom from molecular oxygen to the substrate and reduce the other atom to water, and dioxygenases, which are able to transfer both oxygen atoms to the substrate.
See also ▶ Enzyme ▶ Oxidation
Oxygenation ▶ Oxygenation of the Earth’s Atmosphere
Oxygenation of the Earth’s Atmosphere DAVID C. CATLING Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA
Synonyms Atmospheric redox change; Earth’s atmosphere, oxygenation; Great oxidation event; Oxygenation; Rise of oxygen
Oxygenation of the Earth’s Atmosphere
Keywords Great oxidation event, rise of oxygen, oxygenation
Definition Earth’s oxygenation is an increase in the concentration of atmospheric molecular oxygen (O2) from levels of less than 1 ppmv before 2.45 Ga to 21% by volume today. Larger amounts of atmospheric oxygen became possible because of shifts in the competition between the production of oxygen derived from ▶ photosynthesis and the rate of consumption of oxygen by different geological processes. Evidence from ancient rocks suggests that oxygenation happened in steps, with a first rise of O2 at 2.45–2.32 Ga and a second around 0.75–0.58 Ga. The latter increase was a precursor to the appearance of macroscopic animals.
Overview The present atmosphere contains (by volume) 78.05% N2, 20.95% O2, 0.93% Ar, 0.038% CO2, and various trace gases. With the exception of argon, the concentrations of all of major gases are biologically modulated. Oxygen, in particular, is almost solely biogenic because it has no significant abiotic source. Consequently, before life existed, the atmospheric partial pressure of O2 is estimated to have been