The Surface of Mars

The Surface of Mars Our knowledge of Mars has grown enormously over the last decade as a result of the Mars Global Surveyor, Mars Odyssey, Mars Express, and the two Mars Rover missions. This book is a systematic summary of what we have learnt about the geological evolution of Mars as a result of these missions, and builds on the themes of the author’s previous book on this topic. The surface of Mars has many geological features that have recognizable counterparts on Earth. Many are huge in comparison to those on Earth, including volcanoes, canyons and river channels that are ten times larger than their terrestrial equivalents. The book describes the diverse Martian surface features and summarizes current ideas as to how, when, and under what conditions they formed. It explores how Earth and Mars differ and why the two planets evolved so differently. While the author’s main focus is on geology, he also discusses possible implications of the geological history for the origin and survival of indigenous Martian life. Up-to-date and richly illustrated with over two hundred figures, the book will be a principal reference for researchers and students in planetary science. The comprehensive list of references will also assist readers in pursuing further information on the subject. M I C H A E L C A R R is a Geologist Emeritus at the U.S. Geological Survey, and has over 40 years’ experience of planetary science research. In the early 1970s Dr. Carr was a member of the Mariner 9 team and leader of the Viking Orbiter Imaging team. He was co-investigator on the Mars Global Surveyor, the Mars Exploration Rovers, and the High Resolution Stereo Camera on Mars Express. He is a Fellow of the Geological Society of America, the American Geophysical Union, and the American Association for the Advancement of Science, and was awarded the 1994 National Air and Space Museum Lifetime Achievement Award for his work on Mars. He is also the author of The Surface of Mars (1981) and Water on Mars (1996).

Cambridge Planetary Science Series

Series editors: F. Bagenal, F. Nimmo, C. Murray, D. Jewitt, R. Lorenz and S. Russell Books in the series Jupiter: The Planet, Satellites and Magnetosphere F. Bagenal, T. E. Dowling and W. B. McKinnon Meteorites: A Petrologic, Chemical and Isotopic Synthesis R. Hutchinson The Origin of Chondrules and Chondrites D. W. G. Sears Planetary Rings L. Esposito The Geology of Mars: Evidence from Earth-Based Analogs M. Chapman The Surface of Mars M. Carr

The Surface of Mars MICHAEL H. CARR U.S. Geological Survey Menlo Park, CA

CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521872010 © Michael H. Carr 2006 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2006 ISBN-13 ISBN-10

978-0-511-27041-3 eBook (NetLibrary) 0-511-27041-0 eBook (NetLibrary)

ISBN-13 ISBN-10

978-0-521-87201-0 hardback 0-521-87201-4 hardback

Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

Contents

Preface Maps

page ix xi

1 Overview Telescopic observations Orbital and rotational motions Global structure and topography Atmosphere Surface temperatures Stability of water Global geology Meteorites Carbonaceous chondrites and chemical fractionation Martian meteorites

1 1 2 5 5 9 11 14 19 19 20

2 Impact craters Crater-forming objects Crater morphology Simple craters Complex craters Multi-ringed basins Crater formation Ejecta morphology Crater modification Crater size frequencies and ages Summary

23 23 24 24 25 26 27 31 34 36 41

3 Volcanism Basaltic volcanism Effect of Martian conditions Tharsis Tharsis Montes Olympus Mons Alba Patera Small Tharsis shields Elysium Lahars and dikes Cerberus
Amazonis Hellas
Hesperia Plains volcanism Volcano
ice interactions Summary

43 43 44 46 46 51 54 57 59 60 64 68 70 73 74

4 Global structure and tectonics Formation of the core Global dichotomy Thickness of the lithosphere Formation of Tharsis Surface indicators of stress Extensional structures Compressional structures Deformational features related to Tharsis

77 77 78 84 84 86 86 89 90

5 Canyons Physiography Canyon walls Landslides Interior layered deposits Formation of the canyons Summary

95 96 102 103 105 110 111

6 Channels, valleys, and gullies Outflow channels Circum-Chryse channels Description Mode of formation Tharsis Amazonis and Elysium Planitiae Description Mode of formation Utopia Planitia Hellas Argyre The poles Valley networks General description Drainage basins Origin Noachian valleys Post-Noachian valleys Gullies Summary

113 113 114 114 116 121 122 122 126 127 129 130 130 131 132 137 139 140 144 144 147

7 Lakes and oceans Paleolakes in the cratered uplands Argyre and Hellas

149 149 156

vii

Contents

viii

Northern oceans Shorelines Evidence for marine sediments Evidence for ice Possible fate of a northern ocean Summary

160 164 167 168 168 171

8 Ice The stability of ice Spectral evidence for ice Permafrost Ice-rich surficial deposits at high latitudes Fretted terrain Terrain softening Lobate debris aprons Lineated valley fill Origin of the fretted valleys Glaciers Other possible indicators of ground ice Crater ejecta patterns Polygonal fractures Thermokarst Summary

173 174 175 175 177 178 179 180 184 185 187 188 188 189 191 191

9 Wind Entrainment of particles by the wind Dust storms Wind streaks and tails Dunes, ripples, and drifts Regional eolian deposits Wind erosion Summary

193 193 195 197 198 203 204 205

10 Poles The present polar environments General description of polar terrains Northern polar deposits Upper unit Basal unit Southern polar deposits The Dorsa Argentea Formation The CO2 residual cap Summary

211 211 212 212 212 218 221 222 225 226

11 The view from the surface Vikings 1 and 2 Mars Pathfinder Mars Exploration Rovers Spirit Gusev crater regional context Gusev plains Columbia Hills Clovis class Wishstone class Peace class Watchtower class Backstay class Opportunity Regional context The Meridiani rocks and soils The Burns Formation Post-depositional alteration Groundwater movement Evaporitic sources Summary

229 229 231 231 232 232 235 238 239 240 241 241 242 244 244 246 246 252 253 254 254

12 Climate change Noachian climate Greenhouse warming Retention of a dense CO2 atmosphere Post-Noachian climate history Recent climate changes Summary

257 257 258 260 262 265 265

13 Implications for life The origin of life Habitability Survival ALH84001 Looking for life Summary

267 268 271 272 273 274 274

14 Summary

277

Reference Index

283 297

Preface

This book summarizes our knowledge of the morphology of the martian surface and speculates on how the surface evolved to its present state. During the last three decades our knowledge of Mars has increased dramatically. A succession of orbiting spacecraft (Table I) have observed the planet at ever-increasing resolution, rovers have traversed the surface, analyzing and scrutinizing rocks along the way, and ever more sophisticated techniques are being used to analyze increasing numbers of martian meteorites. The planet has had a complicated history. The aim of the book is to summarize our understanding of the nature and sequence of the processes that led to the present configuration of the surface. While the book is intended for the serious student or researcher, technical jargon is avoided to the extent that it is possible without compromising precision. It is hoped that the book will be readable to informed non-Mars specialists as well as those active in the field.

Sufficient documentation is provided to enable the reader to dig more deeply wherever he or she wishes. Heavy reliance is placed on imaging data. Other evidence is referred to where available, but at the present time, imaging is by far the most comprehensive global data set that we have in terms of areal coverage and resolution range. Exploration of Mars has captured world-wide interest. Mars is an alien planet yet not so alien as to be incomprehensible. The landscape is foreign yet we can still recognize familiar features such as volcanoes and river channels. We can transport ourselves through our surrogate rovers to a surface both strange and familiar and readily imagine some future explorers following in their paths. While past speculations about martian civilization may now seem absurd, the possibility that Mars may at one time have hosted some form of life remains plausible. It remains the strongest scientific driver of the Mars Exploration program. The life

Table I. Mars missions Mariner 4 Mariner 6 Mariner 7 Mars 2 Mars 3 Mariner 8 Mariner 9 Mars 4 Mars 5 Mars 6 Mars 7 Viking 1 Viking 2 Phobos 1 Phobos 2 Mars Observer Pathfinder Global Surveyor Odyssey Spirit Rover Opportunity Rover Mars Express Reconnaissance Orbiter

US US US USSR USSR US US USSR USSR USSR USSR US US USSR USSR US US US US US US Europe US

11/28/1964 2/24/1969 3/27/1969 5/19/1971 5/28/1971 5/8/1971 5/30/1971 7/21/1973 7/25/1973 8/5/1973 8/9/1973 8/20/1975 9/9/1975 7/7/1988 7/12/1988 9/22/1992 12/4/1996 11/7/1996 4/7/2001 6/10/2003 7/7/2003 6/2/2003 8/12/2005

Flew by 7/15/1965; first S/C images Flew by 7/31/1969; imaging and other data Flew by 8/5/1969; imaging and other data Crash landed; no surface data Crash landed; no surface data Fell into Atlantic Ocean Into orbit 11/3/1971; mapped planet Failed to achieve Mars orbit Into orbit 2/12/1975; imaging and other data Crash landed Flew by Mars Landed on surface 7/20/1976; orbiter mapping Landed on surface 9/3/1976; orbiter mapping Lost 9/2/1988 Mars and Phobos remote sensing Failed Mars orbit insertion Landed 7/4/1997; lander and rover Into orbit 9/11/1997; imaging and other data Into orbit 10/24/2001: imaging, remote sensing Landed in Gusev 1/3/2004 Landed in Meridiani 1/24/2004 In orbit 12/25/2003; imaging, remote sensing In orbit 3/10/2006; imaging, remote sensing

ix

x

theme is constantly in the background throughout the book. Impacts have implications for survival of any early life, and may have resulted in cross-fertilization of Mars and Earth. Large floods may have temporarily affected global climates and provided temporary refuges in the resulting lakes and seas. Volcanic activity may have created hydrothermal systems in which life could thrive. Conditions on early Mars may have been very similar to those on early Earth, at a time when life had already taken hold. Thus, while the book is not explicitly about life, almost every chapter has implications for the topic. The book is intended as a replacement for an earlier book (Carr, 1981) that summarized our understanding of the planet as it was shortly after completion of the Viking missions. This book is different from the original in several ways. The field was much less mature when the first book was written. I was able to read most of the literature and examine most of the imaging data. Neither of these tasks is possible any longer. Approximately 500 papers are published on Mars each year and the number is increasing. One can no more write a book about Mars and reference all the relevant papers, than one can about the Earth. Similarly, the book has been written without seeing most of the available imaging. Over 200,000 images have been taken just with the Mars Orbiter Camera on Mars Global Surveyor, and a comparable amount of imaging data has been acquired by THEMIS on Mars Odyssey, the High Resolution Stereo Camera on Mars Express, and the Mars rovers. In addition to the imaging there are vast amounts of other remote sensing data, as well as analytical data from the surface and from meteorites. Clearly, summarizing all this data has involved a great deal of simplification. The book is a snapshot of a moving picture. Following Viking there was almost a twenty-year drought during which barely any data was returned from the planet. But since the landing of Mars Pathfinder in 1996 and the insertion of Mars Global Surveyor into orbit in 1997, we have been receiving a steady stream. Along with the new data have come new ideas as to how the planet has evolved. The pace of change is rapid because our knowledge of the planet is still rudimentary and the data flux is high. It could be

Preface argued that the time is inopportune for a summary because of the rate of change. But change will continue. After two decades, new interpretations of the Viking data were still forthcoming. It will likely also take decades to digest the data currently being returned. I hope that there will never be a time when the field stabilizes and a good time to write a summary arrives. The book was written in 2005 and 2006. I had just retired after having participated in almost every mission to Mars since the late 1960s, including several months of Mars Exploration Rovers (MER) operations at Jet Propulsion Laboratory (JPL). The book has benefited significantly from the continuous informal science discussions that are part of participating in missions. The Mars Rover end-of-day discussions, when the scientists would gather and exchange ideas about any topic that had intrigued them, were particularly stimulating. The Mars Orbiter Laser Altimeter (MOLA) team on Mars Global Surveyor held regular meetings on different science topics that were always fun. Of course, the book has benefited mostly from the engineers who have built and operated the spacecraft that have flown all the science instruments to Mars in recent years. Without sound engineering there is no science. The engineers do most of the hard work acquiring the data. The scientists have the fun of interpreting it all. Two people deserve special mention for the help they provided. Phil Christensen, of Arizona State University, the THEMIS Principal Investigator, offered to make mosaics of areas of interest for illustrations. Some of the most spectacular images in the book are these THEMIS mosaics. Jim Head of Brown University is also a major contributor to the book. Jim has unusually broad expertise in planetary science, and is possibly the most prolific author in the field of planetary geology. He agreed to review all the chapters as they were written and provided numerous insightful comments that added greatly to the accuracy and comprehensiveness of the final product. Above all he provided encouragement to keep at it. Michael H. Carr U. S. Geological Survey Menlo Park, CA 94025, USA

xi

xii

Maps

Maps

xiii

xiv

Maps

1

Overview

This chapter has several goals. The first is to provide some general historical background on how Mars was perceived before spacecraft exploration started with the launch of Mariner 4 in 1964. The second goal is to provide an overview of what conditions are like on Mars today. Most of the book concerns the record of past events as preserved in the landscape and in the rocks at the surface. Although conditions may have been different in the past, those that prevail today provide strong constraints on how we interpret that past record. A third purpose is to give a brief overview of topics that are important for our understanding of the planet, but which are a little off the main theme of the book, which is to describe the major geological features of the planet and their origin. A brief description of the present atmosphere is included here, for example. We also include a section on martian meteorites. These are both huge topics with a vast literature, and no attempt is made in the book to treat them comprehensively. A fourth aim of the chapter is to provide a short geological overview so that the subsequent, more detailed chapters can be read in light of a general knowledge of the planet’s geology. Most geological topics are just touched upon here and referenced to later chapters.

changes according to where the planet is in its orbit. The spacing between oppositions therefore changes according to where Mars is at opposition. Eccentricity also affects the quality of the oppositions, the best being when Mars is at perihelion. This has caused a bias of telescopic observations toward the southern hemisphere, since at perihelion Mars’ southern hemisphere is tilted toward the Sun and hence toward Earth. No topography can be seen from Earth-based telescopes (Figure 1.1). What are seen are variations in the reflectivity (albedo) of the surface, including the polar caps, and changes in the opacity of the atmosphere. Although the surface markings may change in detail from opposition to opposition or over decadal time scales, the gross pattern has remained constant for the entire period of telescopic observation. The most prominent features outside the poles are dark markings in the 0
40°S latitude belt, although the most prominent dark feature on the planet, Syrtis Major, is outside this belt. The dark areas were originally thought to be seas and so were called maria. They are mostly areas that have been swept partly clean of the bright dust that covers much of the surface. Most dark markings do not correspond to topographic features, although some do. Some bright markings, such as Hellas and Nix Olympica, noted on some early maps, also correspond to topographic features, probably because of persistent clouds in these areas. The most famous features of the planet from the telescopic era, the canals, were portrayed on almost all twentieth-century maps until the mid 1960s. They are largely illusions on the part of observers straining to see markings at or below the limits of telescopic observation. Transient brightenings of part, or all, of the telescopic image of the planet were correctly attributed to clouds, of which two types were identified: yellow clouds interpreted as dust storms, and white clouds interpreted as condensate clouds. The yellow dust clouds were observed to occur mostly in the southern hemisphere in southern spring and summer. In some years such as 1956 and 1971 the dust storms became truly global. The classical markings disappeared and

Telescopic observations Mars is the fourth planet from the Sun. With a mean radius of 3389.5 km, it is intermediate in size between the Earth (6378 km) and the Moon (1738 km). As Earth and Mars move in their orbits around the Sun, telescopic viewing conditions change. When Earth and Mars are on opposite sides of the Solar System, they are close to 400 million km apart and Mars subtends an angle of only 3.5 arcsec. At closest approach (opposition) the distance between the two planets may be as small as 55 million km, and the planet subtends an angle of 25 arcsec. Telescopic viewing is thus best at opposition when features as small as 150 km across can be distinguished with the best ground-based telescopes. Oppositions are spaced roughly 780 days apart. The exact spacing varies because Mars’ orbit, unlike the the Earth’s, is distinctly eccentric. This results in an orbital velocity that

1

2

Overview

Figure 1.1. Comparison of martian terrain with what is seen in a telescopic image. The view on the left is a MOLA reconstruction of martian terrain from the same perspective as the telescopic image on the right. The MOLA image is of surface relief. The telescopic image was taken by Hubble Space Telescope in late northern spring when the planet was 103  106 km from Earth. It shows variations in the reflectivity of the ground and the atmosphere. Bright clouds are present in Hellas and Elysium, and the seasonal CO2 cap in the north has almost completely dissipated. The dark and light surface markings are only poorly correlated with the relief.

became re-established only after several months. White clouds were observed at places, such as Olympus Mons, Alba, Patera, and Sinai Planum, where we now know from spacecraft observation that water ice clouds are common. Two other phenomena deserve mention. The first is the wave of darkening, a progressive darkening of the dark markings that proceeded from pole to equator as the polar caps receded. It was variously interpreted as growing vegetation, release of water from the receding cap, and sweeping of dust from the area around the receding cap by strong off-pole winds. The second phenomenon was the appearance of blue clearings, times when the dark markings appear particularly crisp and clear. Neither phenomenon has been confirmed by spacecraft observations. For a comprehensive summary of Mars as viewed from the telescope see Martin et al. (1992). Orbital and rotational motions The orbital and rotational motions of the planet (Figure 1.2) affect how much insolation falls on the planet and how the amount changes with time of year and latitude. The motions, therefore, affect surface temperatures, atmospheric circulation, and climatic

conditions in general. Variations in obliquity (the angle between the spin axis and the orbit normal) are particularly important for Mars since the changes are large and can cause significant variations in atmospheric pressure and transfer of water between the poles and lower latitudes The Mars day is 24 hr 39.6 min and the year is 687 Earth days or 669 Mars days (sols). Instead of months, the areocentric longitude of the Sun (Ls) is used to denote time of year. This is the equivalent of the Sun-centered angle between the position of Mars in its orbit and the position of the northern spring equinox. At the start of northern spring Ls ¼ 0°, at northern summer solstice Ls ¼ 90° and so on. Mars’ rotation axis is tilted 25° with respect to the orbit plane so that the planet has seasons like the Earth. The orbit of Mars is, however, distinctly elliptical (eccentricity of 0.093), in contrast to the near-circular orbit of the Earth (eccentricity of 0.017), and this affects the length and intensity of the seasons. At closest approach to the Sun (perihelion), the Mars
Sun distance is 1.381 AU (One Astronomical Unit or AU is the mean Earth
Sun distance or 149.5  106 km.) At its furthest distance from the Sun aphelion, the Mars
Sun distance is 1.666 AU. Since the solar flux varies with the

Orbital and rotational motions

3

Figure 1.2. The orbits of Mars and the Earth compared. The Earth’s orbit is circular whereas Mars’ orbit is distinctly eccentric. The areocentric longitude of the Sun, LS, denotes the martian time of year as shown. (Adapted from Michaux and Newburn, 1972, NASA/JPL.)

square of the distance from the Sun, 45 percent more sunlight falls on the planet at perihelion than at aphelion. At present, perihelion occurs at the end of southern spring, so southern springs and summers are hotter than the same seasons in the north. They are also shorter because of the higher orbital velocity closer to perihelion. The eccentricity changes with time, mostly between values of 0 and 0.12, although over geological time values may have been as high

as 0.15 (Laskar et al., 2004) The oscillation has two periods, a 95,000
99,000 yr period with an amplitude of 0.04, and a 2.4 Myr period with an amplitude of 0.1 (Figure 1.3). These oscillations, coupled with precessional motions that control the timing of perihelion, cause the length and intensity of the seasons to change on time scales of 104 to 106 yrs. Precession is the slow conical motion of an axis of rotation such as observed with a spinning top.

4

Figure 1.3. Changes in eccentricity and obliquity projected back in time. The upper two panels show projections back 1 Myr. The lower two panels show projections back 10 Myr. Modulation of the 1.2  105 yr obliquity cycle has been modest for the last 0.4 Myr. From 0.4 to 4 Myr ago, obliquities ranged from 15° to 35° about a mean of 25°. From 4 to 10 Myr ago the mean was close to 35°. Prior to 10 Myr obliquities are chaotic and cannot be definitively predicted. Obliquity may affect a wide range of phenomena such as atmospheric pressure, the stability of water ice at the surface and in the ground, and the incidence of dust storms. The effects are most marked at the poles (Chapter 10). (From Laskar et al., 2002, copyright ß Nature Publishing Group.)

Precession causes a slow rotation of the line of equinoxes
the intersection of the equatorial plane and the orbit plane
with a 175,000 yr period and a rotation of the line of apsides
the line joining perihelion and aphelion
with a period of 72,000 yrs.

Overview The net result is that the longitude of perihelion, the angle between equinox and perihelion passage, changes with a period of 51,000 yrs. Thus, while today perihelion occurs in southern spring causing southern springs and summers to be short and hot, 25,000 yrs from now it will be the northern springs and summers that are short and hot. Changes in obliquity are likely to have much larger climatic effects than changes in eccentricity and the timing of perihelion. The present obliquity is 25.19° but it undergoes large changes (Figure 1.3). During the current epoch, it is thought to oscillate between 15° and 35°, about a mean of 24° (Laskar et al., 2004). The oscillations have a period of 1.2  105 yrs with an amplitude that is modulated on a 2 Myr cycle. Variations in obliquity have a particularly strong effect in the polar regions. At obliquities higher than 54° the average solar flux is higher at the poles than at the equator. Moreover, during polar summers at high obliquities, the pole is constantly illuminated, leading to high sublimation rates of any ice that may be present and deep penetration of a large annual thermal wave. During these periods water ice may be driven from the poles and accumulate at low to mid latitudes (Chapters 8 and 10). There are considerable uncertainties as to what past obliquities were (Ward, 1992; Laskar and Robutel, 1993; Touma and Wisdom, 1993; Laskar et al., 2004). Minute differences in the starting values for the calculations of past motions lead to large differences in the solutions when projected backward (or forward) in time, such that projections larger than 10 Myr are uncertain. Part of the problem concerns resonances. If the period of precession of the spin is commensurate with one of the periods of variation of the orbit, then spin-orbit resonances can occur. Excursions in obliquity significantly larger than are suspected from the current oscillations are then possible. These variations cause the obliquity to be chaotic, at least on time scales greater than 10 Myr. Laskar et al. (2004) ran a large number of simulations in order to estimate the distribution of obliquities over geological time. They found that the average obliquity is close to 40°, that there is a 63 percent probability of reaching 60° in the next 1 Gyr and 45 percent probability of exceeding 70° in 3 Gyr. In this respect Mars differs from the other terrestrial planets. The obliquities of Mercury and Venus have been stabilized by dissipation of solar tides, and that of the Earth by the presence of the Moon. Although the obliquity variations are chaotic on time scales longer than 10 Myr, calculations on the time scale of fewer than 10 Myr are reproducible (Laskar et al., 2004).

Atmosphere They indicate that obliquities were significantly higher prior to 3 Myr ago. Between 3 Myr and 10 Myr ago they oscillated between 25° and 46°, instead of the present 15
35°. Possible geological and climatic effects of the obliquity cycle are discussed in Chapters 8 and 10. Global structure and topography Mars, like the Earth, is differentiated into a crust, mantle, and core (Chapter 4). Because we have no seismic data, the size of the core is poorly defined but the radius is estimated to be between 1300 and 1500 km. From the partitioning and depletion of coreforming elements in the mantle, as indicated by the composition of martian meteorites, the core appears to be more sulfur-rich than the Earth’s (Treiman et al., 1986; Wa¨nke and Dreibus, 1988). Present-day Mars has no magnetic field so that the core is probably solid, but large remanent crustal magnetic anomalies indicate that the core was molten early in the planet’s history (Acuna et al., 1999). From relations between gravity and topography, the crust is estimated to range in thickness from 5 to 100 km, with a thicker crust in the southern hemisphere than in the north (Chapter 4). The crust is basaltic in composition. No crust analogous to terrestrial ‘‘granitic’’ continental crust has been detected. Two crustal compositions have been identified from Thermal Emission Spectrometer (TES) data (Bandfield, 2002). At low latitudes (