Mars Geological Enigmas: From the Late Noachian Epoch to the Present Day

By Susan Conway

Mars Geological Enigmas: From the Late Noachian Epoch to the Present Day presents outstanding questions on the geology of Mars and divergent viewpoints based on varying interpretations and analyses. The result is a robust and comprehensive discussion that provides opportunities for planetary scientists to develop their own opinions and ways forward. Each theme opens with an introduction that includes background on the topic and lays out questions to be addressed. Alternate perspectives are covered for each topic, including methods, observations, analyses, and in-depth discussion of the conclusions. Chapters within each theme reference each other to facilitate comparison and deeper understanding of divergent opinions.

• Offers a transchronological view of the geological history of Mars, addressing thematic questions from a broad temporal perspective
• Discusses outstanding questions on Mars from diverging perspectives
• Includes key questions and answers, as well as a look ahead to which puzzles remain to be solved

• Language: English
• Publisher: Elsevier Science
• Release date: May 23, 2021
• ISBN: 9780128202463

Book preview

Mars Geological Enigmas

From the Late Noachian Epoch to the Present Day

Edited by

Richard J. Soare

Geography Department, Dawson College, Montreal, QC, Canada

Susan J. Conway

CNRS UMR 6112 Laboratoire de Planétologie et Géodynamique, Université de Nantes, Nantes, France

Jean-Pierre Williams

Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, United States

Dorothy Z. Oehler

Planetary Science Institute, Tucson, AZ, United States

Contents

Cover

Title page

Copyright

Contributors

Why Mars: a prologue

Introduction

Chapter 1: Resolving Martian enigmas, discovering new ones: the case of Curiosity and Gale crater

Abstract

1. Introduction

2. Curiosity and Gale Crater

3. Resolving Mt. Sharp’s origin

4. Additional findings

5. Enigmas solved … and generated

Acknowledgment

Node I: Of frozen floods, unfathomable deeps; on ancient Mars, what fluid formed the enormous outflow channels and highland-margin contacts?

Chapter 2: Outflow channels on Mars

Abstract

1. Introduction

2. Channels on Mars

3. Associations between outflow channels and other Martian landforms

4. Chaotic terrain

5. Fissures

6. Spectrometry—geochemical evidence of aqueous environments in outflow channel source terrains

7. Summary and conclusions

Chapter 3: Mars northern plains ocean

Abstract

1. Introduction

2. Published tests of the shoreline hypothesis

3. Current observations

4. Discussion

5. Conclusions

Chapter 4: Dry megafloods on Mars: formation of the outflow channels by voluminous effusions of low viscosity lava

Abstract

1. Introduction

2. Overview of past aqueous interpretations of the outflow channels

3. Problems with aqueous hypotheses

4. The volcanic interpretation of the Martian outflow channels

5. Discussion

6. Conclusions

Acknowledgments

Node II: Can impact craters be used to derive reliable surface ages on Mars?

Chapter 5: Challenges in crater chronology on Mars as reflected in Jezero crater

Abstract

1. Introduction

2. Basic concepts in crater chronology

3. Challenges in crater chronology in Jezero crater

4. Discussion and conclusions

Acknowledgments

Chapter 6: The role of secondary craters on Martian crater chronology

Abstract

1. Introduction

2. Review of crater size-frequency distributions

3. Review of the debate over the effect of secondary craters

4. Constraining the flux of small primary craters

5. Production of secondaries by Martian primaries

6. Model of the global secondary SFD

7. Model of the spatial distribution of secondaries

8. Conclusions

Acknowledgments

Node III: The Perplexing Story of Methane on Mars

Chapter 7: Methane on Mars: subsurface sourcing and conflicting atmospheric measurements

Abstract

1. Introduction

2. Rationale for subsurface sourcing of methane

3. Subsurface methane origin—potential biotic and abiotic generation

4. Methane accumulation in the Martian subsurface

5. Methane release from the subsurface

6. Predicting locations of subsurface methane accumulation

7. Path forward

8. Remaining questions and uncertainties

9. Summary and conclusions

Acknowledgments

Chapter 8: A review of the meteor shower hypothesis for methane on Mars

Abstract

1. Introduction

2. Review of literature

3. Methods

4. Observations

5. Interpretations

6. Uncertainties/remaining questions

7. Conclusions

8. Final thoughts: the significance of methane on Mars

Acknowledgments

Node IV: Does water flow on Martian slopes?

Chapter 9: The role of liquid water in recent surface processes on Mars

Abstract

1. Present-day and recent surface conditions on Mars

2. Present-day reservoirs of water on Mars

3. What is the evidence for recent liquid water on Mars?

4. Synthesis and outlook

5. Conclusions

Acknowledgments

Chapter 10: Dry formation of recent Martian slope features

Abstract

1. Introduction

2. Challenges for liquid water

3. Observed slope features

4. Discussion and conclusions

Acknowledgments

Node V: Earth Analogs for Mars - a Plethora of Choice!

Chapter 11: The McMurdo Dry Valleys of Antarctica: a geological, environmental, and ecological analog to the Martian surface and near surface

Abstract

1. Introduction

2. Background: geologic and environmental context of Mars and Antarctica

3. Observations: Antarctic insights into Martian enigmas

4. Discussion

5. Summary and conclusions

Acknowledgments

Chapter 12: The Atacama Desert: a window into late Mars surface habitability?

Abstract

1. Introduction

2. Environmental conditions

3. Ecological successions along the latitudinal rainfall gradient

4. Expression and preservation of signatures of life

5. Implications for Mars habitability and the search for evidence of life

6. Concluding remarks and path forward

Acknowledgments

Chapter 13: Life analog sites for Mars from early Earth: diverse habitats from the Pilbara Craton and Mount Bruce Supergroup, Western Australia

Abstract

1. Introduction

2. Geological setting: the Pilbara Craton and Mount Bruce Supergroup

3. Investigated geological units

4. Inhabited environments—from sea to land

5. Discussion

Acknowledgments

Node VI: The freeze-thaw cycling of water at/near the Martian surface: present, past, and possible?

Chapter 14: Pingo-like mounds and possible polyphase periglaciation/glaciation at/adjacent to the Moreux impact crater

Abstract

1. Introduction

2. Methods

3. Observations

4. Glacial terrain and open-system pingos

5. Periglacial terrain and closed-system pingos

6. Two disparate mound-formation scenarios

7. Basement age-estimates and the relative geochronology of the post-basement landscapes

8. Discussion and conclusion

Acknowledgments

Chapter 15: Thermokarst-like depressions on Mars: age constraints on ice degradation in Utopia Planitia

Abstract

1. Introduction

2. Thermokarst-like morphologies on Mars

3. Age dating mantling units and thermokarst-like depressions in Utopia Planitia

4. Modern CO2 sublimation pits as an analog for mid-latitude thermokarst-like depressions?

5. Excess ice versus mantle ice?

6. Conclusions and open questions

Acknowledgments

Node VII: Hemispheres together: toward understanding the crustal dichotomy on Mars

Chapter 16: Forging the Mars crustal dichotomy: the giant impact hypothesis

Abstract

1. Introduction

2. Giant impact excavation of the northern lowlands

3. Alternative impact models

4. Timing and likelihood of a dichotomy-forming impact

5. Additional consequences of an early giant impact

6. Summary

Acknowledgments

Chapter 17: Endogenic origin of the Martian hemispheric dichotomy

Abstract

1. Introduction

2. Historical and geologic context

3. Endogenic formation methods

4. Hybrid formation mechanism

5. Orientation of the dichotomy

6. Summary

Acknowledgments

Index

Copyright

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Cover: Upper left shows Mars today, created from Viking Orbiter images by the United States Geological Survey (USGS)

Image Credit: NASA/USGS. Lower right is an artistic rendition of Mars in its early history, when the planet may have had an ocean and extensive clouds. This was created by Kevin M. Gill, using topographic data from the Mars Orbiter Laser Altimeter and cloud data from NASA Blue Marble. Image Credit: NASA/USGS/Kevin M. Gill. Cover design by Dorothy Oehler.

Publisher: Candice Janco

Acquisitions Editor: Peter Llewellyn

Editorial Project Manager: Chris Hockaday

Production Project Manager: Paul Prasad Chandramohan

Designer: Victoria Pearson

Typeset by Thomson Digital

Contributors

Rickbir Bahia,     European Space Research & Technology Centre (ESTEC), Noordwijk, The Netherlands

Raphael Baumgartner

Australian Centre for Astrobiology, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW

Commonwealth Scientific and Industrial Research Organization, Mineral Resources, Kensington, WA, Australia

Bruce G. Bills,     Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States

Tomaso R.R. Bontognali

Space Exploration Institute, Neuchatel

Department of Environmental Sciences, University of Basel, Basel, Switzerland

Robert I. Citron,     Department of Earth and Planetary Sciences, University of California Davis, Davis, CA, United States

Susan J. Conway,     CNRS UMR 6112 Laboratoire de Planétologie et Géodynamique, Université de Nantes, Nantes, France

Ingrid Daubar,     Brown University, Providence, RI, United States

Alfonso F. Davila,     NASA Ames Research Center, Exobiology Branch, Mountain View, CA, United States

Jocelyne DiRuggiero,     Johns Hopkins University, Baltimore, MD, United States

Tara Djokic,     Australian Centre for Astrobiology, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, Australia

Colin M. Dundas,     U.S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ, United States

Kenneth S. Edgett,     Malin Space Science Systems, Inc., San Diego, CA, United States

M. Ramy El-Maarry,     Department of Earth and Planetary Sciences, University of London, London, United Kingdom

Giuseppe Etiope,     Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy

Abigail A. Fraeman,     NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States

Marc Fries,     NASA Astromaterials Acquisition and Curation Office, NASA Johnson Space Center, Houston, TX, United States

Colman J. Gallagher

UCD School of Geography, University College Dublin, Dublin, Ireland

UCD Earth Institute, University College Dublin, Dublin, Ireland

James B. Garvin,     NASA Goddard Space Flight Center, Greenbelt, MD, United States

Shoichi Kiyokawa,     Earth and Planetary Science, Kyushu University, Fukuoka, Japan

David W. Leverington,     Department of Geosciences, Texas Tech University, Lubbock, TX, United States

Joseph S. Levy,     Department of Geology, Colgate University, Hamilton, NY, United States

Dorothy Z. Oehler,     Planetary Science Institute, Tucson, AZ, United States

David A. Paige,     Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, United States

Timothy J. Parker,     Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States

Tyler M. Powell,     Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, United States

James H. Roberts,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Lior Rubanenko

Department of Geological Sciences, Stanford University, Stanford, CA

Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, United States

Mark R. Salvatore,     Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ, United States

Richard J. Soare,     Geography Department, Dawson College, Montreal, QC, Canada

David E. Stillman,     Department of Space Studies, Southwest Research Institute, Boulder, CO, United States

Kenichiro Sugitani,     Ecology Laboratory, Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan

Martin J. Van Kranendonk,     Australian Centre for Astrobiology, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, Australia

Donna Viola,     NASA Ames Research Center, Mountain View, CA, United States

Malcolm R. Walter,     Australian Centre for Astrobiology, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, Australia

Kimberly Warren-Rhodes,     The SETI Institute, Mountain View, CA, United States

Jean-Pierre Williams,     Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, United States

Why Mars: a prologue

As the next wave of robotic exploration of Mars commences in the early 2020s, led by a multiplicity of space-faring nations and emerging commercial entities such as SpaceX and Blue Origin, the possibility of paradigm-shifting discoveries about the Red Planet is high. In this midst, the always-lurking question of "why Mars?" merits reconsideration. The answer ranges from seeking to enhance high-tech prowess and leadership, evolving our understanding of geology, chemistry, physics, and meteorology, to providing intellectual inspiration to a broad cross-section of the society.

To some, Mars is a state of mind whose enigmas are an opportunity to challenge a tangible frontier with the best of what New Space can offer, while building on a foundation that began in the late 1960s with the first wave of ground-breaking NASA-led Mars missions. Mars, today, engenders fascination because of the perplexing and still undiscovered aspects at each step of its exploration. There are just enough similarities between the Red Planet and the Earth to invite persistently novel if not beguiling interpretations and speculation, both with regard to current conditions and those of the deep past. At each step a new Mars emerges, connecting enigmas to key questions we often need to ask about our own planet.

Why Mars? embraces more than the simple question as stated. Like Earth, Mars operates as a system of systems, with time-variable and multi-scale interactions between its atmosphere, hydrosphere, lithosphere and, possibly, a biosphere. As with our continuing discoveries about Earth and its sensitive climate system, Mars appears to have experienced episodic environmental-upheavals coupled with its planetary evolution. In some ways, this is an ideal control experiment, with obliquity and associated orbital parameters driving its long-term climate in ways much more severe than on Earth. For example, absence of a strong internal magnetic field, Mars provides an end-member vantage point from which to investigate over four billion years of solar-wind stripping; this contrasts sharply with the Earth which remains cocooned from radiation by its fulsome magnetic field.

Scientifically, Mars offers a test-bed of planetary-scale experiments, including the unavoidable issue of life. Over the course of the past 20+ years, the quest for real or imagined signs of life beyond Earth has expanded to Mars and outwards, in ways unimagined less than a generation ago. NASA’s current strategic plan raises questions such as Are we alone? and Mars is the first and easiest place other than Earth to investigate this ennobling question.

Most recently, well-organized teams of investigators have developed new approaches for evaluating agnostic bio-signatures (Johnson et al., 2018) as part of NASA’s Research Coordination Networks that evolved from the 20+ years of investment in the NASA Astrobiology Institute (NAI). The search for agnostic bio- signatures embraces Mars as a critical challenge for scientists seeking to escape our terrestrial biases and seeking pathways for recognizing extant or preserved signs of biology, if the signatures are different from those on Earth. Why Mars? is the first test of the guiding principles in the search for agnostic signs of life and amplifies the potential scientific value of returning samples from Mars to Earth by the early 2030s.

As we explore the universe in the pursuit of grand challenges such as "Are we alone?," Mars becomes increasingly relevant. The Red Planet is one of the most accessible destinations within our solar system, making it a target for exploration via an ever-growing myriad of technological innovations. Mars comprises a concrete, tangible frontier for affordable experimentation, guided by questions that are connected to widely accepted concerns here on Earth, including rapid environmental change. By exploring Mars, we can evaluate aspects of our own planet that might otherwise not have been considered. Even in the 1970s, using Mars as a window into the geological evolution of Earth was conceived and seminal studies suggested that Mars was a one geological-plate planet, with much of its dynamical evolution in the long-distant past. Lacking definitive measurements from networks of surface seismic and meteorology stations, surface samples from targeted sample returns, and unambiguous information about the distribution of shallow sub-surface water ice, a great many testable hypotheses remain whose answers will continue to drive the compelling case for Mars. Even with definitive datasets about topography and key aspects of atmospheric chemistry, a myriad of foundational issues remain, just as for our own planet Earth.

Perhaps Mars figured prominently in the earliest waves of robotic exploration because it is there and technologies for visiting it were viable. Beyond the Moon, Mars was also the most observable astronomical object, even before the space age that commenced with Sputnik and Explorer-1. Yet, Mars as a frontier that we can reach has presented challenges, and it was not until the Viking Landers of the mid-1970s that investigations tied to "are we alone?" began in earnest.

After more than 60 years of spaceflight, our scientific portrait of Mars remains woefully incomplete. To some, Mars is a masterpiece as yet unfinished—in effect, a series of tiles not yet fitting a coherent and evocatively beautiful mosaic. To others, it represents the hope of discovering that we are not alone as living organisms in our countably infinite universe. Having leapt to conclusions about the workings of Mars only to find out later that our basic assumptions were unfounded, we are perpetually surprised, if not astonished, by the extent to which our understanding continues to evolve well beyond its previous confines.

With the realization that some meteorites discovered on Earth are from Mars, we have a randomized grab-bag of Martian samples to examine, and discoveries from these priceless emissaries have further demonstrated the key role water has played on the Mars we have come to know. Some would say that ever-broadening discoveries about the inventory and role of water in Mars evolution has been a major factor in the why Mars? story. Indeed, a strategy known as "follow the water" (FtW) was established in the late 1990s as a crosscutting scientific element of a community-based strategy for Mars exploration, which initiated NASA’s new Mars Exploration Program circa 2000. Although FtW may have advanced beyond simple detection and discovery of water in any context on Mars, it remains in vogue today as a key puzzle-piece in an integrated set of priorities associated with Mars, including those outlined in the US National Academy of Sciences Decadal Surveys (Visions and Voyages, 2011).

Water is being discovered in places where it was never believed to have been possible, such as ice deposits on the Moon and at Mercury (and as buried liquid oceans beneath the icy crusts of some Saturnian and Jovian moons); however, nowhere in the solar system other than the Earth is the connection between water and the geological evolution of a planetary or lunar body clearer than on Mars (Fig. 1). Recent discoveries from the Mars Science Laboratory (MSL) Curiosity rover [cf: A. Fraeman Chapter 1 in this volume] and from the Mars Reconnaissance Orbiter (MRO) have highlighted the changing but essential role water has played in the geological history of Mars. Even in its current environmental state, Mars comprises a water cycle with ever-increasing evidence of a massive, buried cryosphere. To some, Mars may be the embodiment of a cryo-ocean world in contrast to outer solar-system ocean worlds such as Europa where the liquid oceans are lurking under kilometers of ice-silicate crusts.

Figure 1   Perspective view of Mars surface in the Nephentes region, near the crustal dichotomy boundary showing small km-scale cones. The latter possibly comprise hydro-volcanic features as islands in a former state of Mars when persistent liquid surface waters were present. Why Mars is compelling because of the water-geology-biology connections Mars presents. Regions with sustained hydrothermal activity like this one are but one example of the sometimes-enigmatic association of water with other processes, all possibly linked to biological potential. This simulated view takes advantage of NASA MRO imaging in stereo that enabled digital elevation models (DEM) to be developed, which were then artificially flooded to showcase what might have been a former state of ancient Mars. Image developed at NASA GSFC by SVS under Cindy Starr with guidance by J. Garvin and others using MRO HiRISE DEM data, courtesy NASA/JPL/Univ. Arizona.

Thus, Mars is the most accessible and likely solar system body to have preserved evidence of biology and, potentially, to have extant subsurface (or within-ice) microbiological communities even today. To suggest this in 2020 after the apparent setbacks of the Viking biology experiments of the mid-1970s illustrates how far understanding has progressed from our first voyages of discovery to Mars. This sense of wonder as we explore, learn, discover, and connect has never waned with Mars, even after numerous mission failures. Perhaps our knowledge of Mars is dangerous enough to dare hope that it will offer us clues about life across the universe and that this may differ from our Earth-based paradigm. If only as a second data point in the quest for life, Mars is quintessential.

With a suite of missions set to launch and reach Mars in the early 2020s, including NASA’s Mars 2020 rover (and helicopter) and ESA’s ExoMars Rosalind Franklin rover (with a molecular bio-signature experiment known as MOMA), as well as sample-return missions planned for the late 2020s, the prospects of detecting elusive biomarkers from the rock record of Mars are expanding rapidly. It would not be surprising if there were viable bio-signatures discovered within a decade or so, at least for ancient microbial life.

Mars is far more than a convenient, accessible, partially explored planetary neighbor! It represents the hope that sometimes radical, innovation-driven exploration delivers, with discovery potential that will change how we view ourselves forever. To me, Mars is our destiny in ways far beyond popular Hollywood movies such as the Martian. Over the course of 50+ years of robotic exploration of Mars, each step has catalyzed future ones, so that we are now at a cusp in capabilities, understanding, and discovery. By ∼2030, the Mars we know and explore will connect more closely to new space business-models, offering prospects for commercial access and utilization, as well as inventive new approaches for exploration at all scales. Until we visit Mars for ourselves, with women and men representing the collective of humanity on Planet Earth, we will continue to ask why Mars? Thus, the always-expanding litany of rationales will keep us going until we see ourselves as the Martians.

Perhaps Ray Bradbury and his "Martian Chronicles" was right after all (in the 1950s)—we will either realize that once we are there as people that we are the Martians, or that the real Martians are already there, waiting to be discovered by the creative imagination of our exploration. Mars waits for no one; once we understand its seemingly magical workings we will not cease from wanting to explore its evocative frontiers further. Rising from the enigmas Mars currently presents to us is a planet where we must go, both vicariously via robotics and Immersive Virtual Reality, and in person. Why Mars? is not a question we can definitively answer at this time, because the question keeps changing, expanding, and embracing more aspects of what makes all of us explorers. Mars embodies "never wait to wonder" as humanity moves away from Earth and pursues planetary frontiers well beyond the planned or current ones. As is stated in today’s vernacular, Mars rocks!

James B. Garvin

January 2020

Goddard Space Flight Center, NASA

Goddard, Greenbelt, MD, United States

References

Johnson, S.J., et al., 2018. Fingerprinting non-terran bio-signatures. Astrobiology 18(17), 915–922. DOI: 10.1089/ast2017.1712.

Visions and Voyages for Planetary Science in the Decade 2013-2022, 2011. National Academies of Sciences, Engineering, and Medicine (NASEM) Planetary Decadal Survey, National Academies Press, Washington DC.

Introduction

The Curiosity rover and Gale crater enigmas, Chapter 1 by Fraeman, sets a contemporary and in situ backdrop for the investigation of numerous long-standing and geologically based questions and enigmas. For example, does Mars’ evolution track the linear loss of water from deep in its geological history through to current times; or, has water been present intermittently, in greater or lesser sums, throughout its history? Was/is Mars habitable, at least for simple life, either near the surface or to depth? Could the small but variable amounts of atmospheric methane detected by Curiosity be a life sign or the work of abiotic processes? Curiosity has begun to address and unravel some of these questions, leaving the answers tantalizing close to our intellectual reach but not always absent of equifinality or of multiple working-hypotheses.

Of frozen floods, unfathomable deeps; on ancient Mars, what fluid formed the enormous outflow channels and highland-margin contacts? juxtaposes disparate veins of thought on the origin and type of fluids associated with these channels and contacts. Chapter 2, by Gallagher and Bahia, identifies key morphologies that are most strongly suggestive of aqueous processes, particularly in outflow channel source-regions characterized by chaotic terrain. It also acknowledges the fundamental relationships, both spatial and genetic, between some outflow channels and contextual volcanic activity at their source. Chapter 3, by Parker and Bills, points to well-preserved terracing along the highland margin that resembles strandlines in terrestrial paleolakes produced through wave action, tsunami surges, or ice-pushing. Lobate flow-fronts that resemble lava or debris flows also are observed. These terraces are observed at a higher elevation than the northern-plains interior, suggesting that a very large volume of fluid receded subsequent to the terraces having been formed along the plains’ margins. Chapter 4, by Leverington, posits that the outflow channels need not have been fluvial in origin. They have lithological and geochemical properties aligned with volcanic systems formed by the voluminous eruptions of low-viscosity lavas on the Moon, Mercury, Venus, and Earth.

Can impact craters be used to derive reliable surface ages on Mars? describes and evaluates the primary method used to estimate surface ages on Mars and other solar system objects. This method, derived of the size-frequency distribution of surface craters, can provide chronometric information based on an understanding of the accumulation rate of craters over time. In addition to the formation of secondary craters, that is, craters formed by fragments ejected at the time of a primary impact that fall back to re-impact the surface, a suite of surface modification processes ranging from erosional to depositional makes the interpretation of crater-derived age estimates challenging. This has led to a strenuous debate within the planetary science community regarding the means by which absolute ages should be modeled, interpreted, or even used as a reliable indicator of surface ages. Chapter 5, by Rubanenko et al., explores various surface processes that can modify a crater population. They use the Jezero impact crater, the landing site of the Perseverance Rover mission, as a case study. Chapter 6, by Powell et al., studies the influence of secondary craters on the crater chronology method.

The perplexing story of methane on Mars explores the possible presence of atmospheric methane, seemingly detected by terrestrial telescopes, Mars orbiters, and the Curiosity Rover. Chapter 7, by Oehler and Etiope, identifies and discusses potential subsurface sources of methane and contraposes biotic versus non-biotic pathways of production. Differences between Mars and Earth that could affect methane’s subsurface generation, preservation, and release are presented, as are scenarios that reconcile the ExoMars Trace Gas Orbiter’s non-detection of methane with the numerous positive detections noted earlier. Possible release-locations also are illustrated on an annotated map. Chapter 8, by Fries, speculates that some, if not all, of the atmospheric methane could be attributed to UV-photolysis of macromolecular carbon delivered by meteor showers and outbursts. Future and follow-up tracks of inquiry on this and competing hypotheses also are discussed.

Does water flow on Martian slopes? summarizes arguments for and against liquid water being a recently active geomorphic agent on Mars. Mars’ current surface conditions rarely approach the triple point of water, meaning that its liquid form is transient at best. However, there are numerous landforms and landscapes that are very similar to those created by liquid water on Earth. Examples include, downslope propagating dark-streaks resembling water seeps in arid environments on Earth ("RSL"), tributary channel-fan systems resembling terrestrial gullies, and downslope-oriented lobate tongues of material akin to terrestrial solifluction lobes, among others. Hence, the central question of this node is: Is our understanding of climatic conditions on Mars flawed/incomplete or can other processes create liquid-water-like landforms? Knowing whether liquid water is present or absent at the Martian surface is critical for understating Mars’ habitability, climate, and geophysics. It also greatly influences the planning of future space missions to the planet.

Earth analogs for Mars—a Plethora of Choice! comprise three disparate geological-lenses on possible Earth analogs of Mars’ surface: the McMurdo Dry Valleys, Antarctica (Chapter 11, Salvatore and Levy); the Atacama Desert, Chili (Chapter 12, Davila, DiRuggerio and Warren-Rhodes); and, the Pilbara, Western Australia (Chapter 13, Van Kranendonk et al.). The Dry Valleys are exemplary in their aridity, low temperatures, glacial circumscription and, surprisingly, seasonally ephemeral meltwater systems. The Atacama Desert shows similar aridity, lower atmospheric pressure at its plateau elevation, scattered, and widespread salt deposits, and diverse surface/near-surface microbial communities that survive despite the relative unavailability of liquid water. The Pilbara comprises ancient terrain billions of years old whose origin and evolution could have run parallel to that of early Mars. No less importantly, its rock record reveals disparate types of geological niches and microbial refugia that also could have arisen a long time ago on Mars. Each of these locations, despite their differences, teasingly attests to the multiplicity of possible analogs for Mars, as it has migrated geologically through its deep history to the present day.

The freeze-thaw cycling of water at/near the Martian surface: present, past, and possible? probes whether the freeze-thaw cycling of water has contributed to relatively substantial landscape-revisions within the northern mid-latitudes during the Late Amazonian Epoch up to if not including very recent periods of time. Chapter 14, by Soare et al., weighs the possibility that small-sized mounds at/adjacent to the Moreux impact-crater (42.1o N; 44.4o E) could be pingos, that is, perennial ice-cored mounds formed by the hydrostatic (open-system) or hydraulic (closed-system) pressure of water. The crater is located in northern Arabia Terra and lies astride the Martian crustal-dichotomy. Depending on their open or closed origin, the mounds could have been formed much earlier in the Late Amazonian Epoch and avoided any apparent conflicts between current boundary conditions and the availability of surface or near-surface water. Chapter 15, by Viola, focuses on thermokarst-like depressions in Utopia Planitia and uses crater-size frequency analysis of the terrain incised by these depressions to suggest that the depressions could be relatively youthful and formed by sublimation.

Hemispheres together: toward understanding the crustal dichotomy on Mars explores one of Mars’ most prominent and ancient features, a hemispheric-scale asymmetry in crustal thickness and topography. The heavily cratered southern highlands have an average elevation ∼5 km higher than the relatively smooth and sparsely cratered northern lowlands. The crustal dichotomy appears to be the oldest observable feature on the planet and its origin has important implications for the planet’s early history and evolution. Chapters 16 and 17, by Citron and Roberts respectively, present the wide range of possible formation mechanisms with the former exploring impact formation hypotheses and the latter focusing on endogenic processes such as a large hemispheric-scale mantle upwelling, an early episode of plate tectonics, and long-wave length overturn of a solidifying magma ocean.

Chapter 1: Resolving Martian enigmas, discovering new ones: the case of Curiosity and Gale crater

Abigail A. Fraeman    NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States

Abstract

The Mars Science Laboratory’s Curiosity rover is one example of how in situ exploration has been used to address questions about Mars that were not resolvable from orbital data alone. Data from the rover have been interpreted to show that its landing site in Gale crater once hosted habitable environment in the form of a long-lived lake that contained all of the chemical elements needed to support life. Abundant late-stage diagenetic features cross cut the lacustrine sediments, demonstrating that the history of liquid water and perhaps habitability at Gale crater extended beyond the duration of the lake environments. Beyond Mt. Sharp the Curiosity team has also reported the detection of small but variable amounts of methane in the Martian atmosphere. The example of Curiosity’s success gives us hope that we will able to solve some of the major Martian enigmas raised in this book through our continued exploration by orbiters, landers, rovers, and potential returned samples and human missions in the future.

Keywords

Mars

habitability

Curiosity

sedimentology

mineralogy

1. Introduction

The chapters within this book detail some of the most perplexing and long-standing questions we have about ancient and modern Mars. Some of these mysteries will be solved using data collected by the fleet of orbiters, landers, and rovers that are already at or have been on the Red Planet. Some questions, however, might only be answered by future missions carrying new scientific payloads to Mars orbit or to key sites on the planet’s surface, or through the return of carefully selected Martian samples to Earth laboratories. The Mars Science Laboratory’s Curiosity rover is an excellent example of how advanced instrumentation sent to a carefully selected site is being used to answer one of the biggest Martian mysteries: did the planet ever host environments that would have been capable of supporting and preserving evidence of life?

2. Curiosity and Gale Crater

By our Earth-centric standards, a habitable environment is broadly defined as one that has liquid water, a source of carbon, and a source of energy (Grotzinger et al., 2012). Curiosity’s scientific payload was selected for its ability to document the presence of carbon and other elements that could power microbial metabolisms, and to place these chemical measurements within a recognizable geologic and environmental framework.

Curiosity landed in the ∼155-km diameter Gale crater (5.37°S, 137.81°E). This impact crater is located on the dichotomy boundary between Mars’ younger northern lowlands and the ancient southern highlands (Fig. 1.1). Channels lead from the rim of the ∼3.6–3.8-billion-year-old impact crater to its interior, some of which terminate in alluvial fans on the crater’s floor (Anderson and Bell, 2010). Most notably, a ∼5-km tall mound of material informally named Mt. Sharp (formally Aeolis Mons) sits inside Gale crater (Fig. 1.1). Mt. Sharp had been a target of interest ever since images from the Mars Orbiter Camera showed it is composed of layered rock that was possibly sedimentary in origin (Malin and Edgett, 2000). Years later, data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) also revealed that there are minerals in the lower strata of Mt. Sharp that form in the presence of liquid water, including phyllosilicates, hydrated silica, sulfates, and crystalline iron oxides (Fraeman et al., 2016; Milliken et al., 2010).

Figure 1.1   (Color inset, upper left) Gale crater and Mt. Sharp context. (large gray image) Detail on a High Resolution Imaging Science Experiment (HiRISE) mosaic showing Curiosity’s traverse (yellow line) from the floor of Gale crater up Mt. Sharp through mission sol 2664 and key locations discussed in this text.

Orbital data alone were not sufficient to uniquely demonstrate how Mt. Sharp formed, however. Before Curiosity arrived at Gale crater, hypotheses about the geologic setting(s) in which the mound grew ranged from fluviodeltaic and lacustrine (Anderson and Bell, 2010; Cabrol and Grin, 1999; Thomson et al., 2011), volcanic (Malin and Edgett, 2000), spring deposits (Rossi et al., 2008) or eolian (Kite et al., 2013a), potentially followed by sedimentation and alteration due to ice or snow melt (Kite et al., 2013b; Niles and Michalski, 2009). Distinguishing among these hypotheses required detailed information about the sedimentary textures, grain sizes, and bedding orientations of Mt. Sharp’s rocks that could only be collected through in situ observations. Better knowledge of the chemical and mineralogical composition of the mound from in situ measurements was also needed both to support the depositional interpretations and to provide critical information about geochemical conditions in past environments.

Despite the uncertainty about the mound’s origin, Gale crater and Mt. Sharp were still an exceptional location to send Curiosity to answer questions related to Mars’ past habitability and environmental evolution (Golombek et al., 2012; Grotzinger et al., 2012). The kilometers’ thick section of sedimentary rock that composed the mound provided a favorable setting for preserving biosignatures (Grotzinger et al., 2012), and even in the absence of any biosignatures, the diverse mineralogy observed from orbit suggested that the mound would contain an extensive record of Mars’ changing environments (Milliken et al., 2010).

3. Resolving Mt. Sharp’s origin

When Curiosity landed on the Gale crater plains adjacent to the base of Mt. Sharp, the rover team discovered sandstones and conglomerates that formed from a network of braided rivers that originated at Gale crater’s northern rim and flowed south, into the crater (Grotzinger et al., 2015; Rice et al., 2017; Stack et al., 2016; Williams et al., 2013) (Fig. 1.2). Based on the size and distribution of the clasts within the conglomerates, these rivers likely had water depths between 0.03 and 0.9 m (Grotzinger et al., 2015; Williams et al., 2013). The rivers also fed delta mouth bars, which formed clinoforms that dipped south toward Mt. Sharp (Grotzinger et al., 2015). At Yellowknife Bay the Curiosity team found fine-grained, parallel-laminated mudstones, indicating a lake environment (Grotzinger et al., 2014). This fluvial-deltaic model predicted that Curiosity would encounter more lake deposits as it travelled basinward across the plains toward the mound.

Figure 1.2   Curiosity Mast Camera (Mastcam) images showing (A) conglomerates on the floor of Gale crater (sol 27), (B) inclined sandstone on the floor of Gale crater (sol 596), (C) parallel, thickly laminated sequence at the base of Mt. Sharp (sol 712), and (D) thin-laminated mudstones at the base of Mt. Sharp (sol 792). All images credit: NASA/JPL/MSSS and available from https://www.nasa.gov/mission_pages/msl/images/index.html.

Indeed, when the rover reached the base of Mt. Sharp, at an area informally known as Pahrump Hills, the Curiosity team discovered thinly laminated and low-angle cross-stratified mudstone, cross-stratified sandstone, and thickly laminated mudstone–sandstone that most likely were deposited in a lake (Grotzinger et al., 2015; Stack et al., 2019) (Fig. 1.2). In the 5 years after leaving Pahrump Hills, Curiosity climbed more than 370 vertical meters of lower Mt. Sharp strata that were composed of rocks with sedimentological characteristics that suggested they were deposited in a lacustrine and lacustrine–margin environment (Fedo et al., 2019). When considered in light of typical sediment depositional rates on Earth, the total thickness of this deposit suggests that the lacustrine environment(s) that emplaced these sediments could have existed (perhaps intermittently) for millions to tens-of millions of years (Fedo et al., 2019). The discovery of a long-lived lake was an exciting result in the search for the habitable environments, but what was the chemistry of the lake waters and would they have had all of the necessary ingredients to support life?

To answer these questions, the Curiosity team measured the mineralogy of the lacustrine sediments in lower Mt. Sharp and at Yellowknife Bay. Both locations were dominated by varying abundances of plagioclase, felsic igneous minerals, mafic igneous minerals, Fe-oxide minerals, phyllosilicates, sulfate minerals, and X-ray amorphous materials (Rampe et al., 2020 and ref. therein). These compositions reflect a combination of the compositions of the detrital material coming into the lake, the original lake water chemistry, and the chemistry of secondary diagenetic fluids. Overall, the compositional data from Mt. Sharp suggest that the lake was characterized by near-neutral and oxidizing conditions, although some intervals may record periods of reducing conditions (Bristow et al., 2018; Hurowitz et al., 2017; Rampe et al., 2017, 2020). Curiosity’s mass spectrometer also directly detected nitrates and reduced organic molecules in mudstones from Yellowknife Bay (chlorobenzene and dichloroalkanes) and Pahrump Hills (thiophenes, two isomers of methylthiophene methanethiol, and dimethylsulfide) (Eigenbrode et al., 2018; Freissinet et al., 2015). Combined detections of CO and CO2 from drilled samples are also consistent with the presence of oxidized organics in much higher concentrations (Leshin et al., 2013; Stern et al., 2018; Sutter et al., 2017). These molecules could be indigenous to Mars and formed by igneous, hydrothermal, atmospheric, or biologic processes, or they could be exogenous and sourced from meteorites, comets, or interplanetary dust particles. Combined, these findings provide strong evidence for past habitable environments.

The Curiosity team also found that the history of liquid water and perhaps habitability at Gale crater extended beyond the duration of the lake environments. Pervasive veins and nodules appear to have formed during multiple interactions with fluids during early and late diagenetic events (Frydenvang et al., 2017; Kronyak et al., 2019; Nachon et al., 2017; Sun et al., 2019; Yen et al., 2017) (Fig. 1.3). Higher up Mt. Sharp than the Curiosity team has explored to date, orbiters have spied decameter-scale veins in a boxwork pattern that are interpreted as early diagenetic features which formed through a large volume of groundwater (Siebach and Grotzinger, 2014). Diagenetic processes also left imprints on the bedrock of the mountain that affect how we interpret it from orbital data. This is most notable at a feature called Vera Rubin ridge, a ∼6-km long and ∼200-m wide topographic rise on the side of Mt. Sharp that is associated with a uniquely strong CRISM signature of red crystalline hematite (Fraeman et al., 2013). Based on this observation, the ridge was originally thought to be a distinct hematite-bearing stratum in lower Mt. Sharp that preserved a redox interface due to its apparently uniquely oxidized composition (Fraeman et al., 2013). However, the Curiosity team has found that Vera Rubin ridge is composed of lacustrine sediments similar in depositional setting and composition to the underlying strata. Ground data revealed that the orbital observations of the ridge were strongly influenced by the combination of changes in hematite grain size and crystallinity that were caused by diagenetic processes (Fraeman et al., 2020). These same processes also locally hardened the ridge and subsequently differentiated it from surrounding terrain by erosion.

Figure 1.3   Mastcam images showing an impressive vein complex at a site called Garden city on lower Mt. Sharp. The diversity of vein materials within this image provides evidence for multiple groundwater events. Numbered labels correspond to (1) thin, dark-toned fracture filling material, (2) thick, dark-toned vein material in large fractures, and (3) light-toned vein material, deposited last. Image credit: NASA/JPL-Caltech/MSSS and available at https://www.nasa.gov/image-feature/jpl/pia19922/garden-city-vein-complex-on-lower-mount-sharp-mars.

4. Additional findings

Not all of lower Mt. Sharp formed in a lacustrine/lacustrine–margin environment, however. While driving across an area of the mound’s base known as the Naukluft plateau and within a region called the Murray buttes, the rover encountered a ∼10-m thick section comprised of cross-bedded sandstone (Fig. 1.4). Detailed analyses of the grain sizes and 3D geometries of the beds showed rocks within this unit likely represented a dry, eolian dune field (Banham et al., 2018). The juxtaposition of the two contrasting environments, dry eolian and wet lacustrine, combined with detailed mapping of their stratigraphic relationships has been interpreted as evidence of an unconformity between the two that recorded a significant break in time, with the eolian sandstone being much younger (Banham et al., 2018; Grotzinger et al., 2015). This observation indicates that Mt. Sharp has undergone multiple episodes of burial followed by exposure from erosion, a perhaps uniquely Martian long-term landscape evolution process consistent with many observations planetwide (Malin and Edgett, 2000).

Figure 1.4   Sol 1087 Mastcam mosaic of a section of cross-bedded sandstone that sits unconformably on top of the lacustrine mudstone which makes up most of lower Mt. Sharp. This sandstone was deposited in a predominantly dry eolian setting. Image credit: NASA/JPL-Caltech/MSSS and available at https://www.nasa.gov/image-feature/jpl/msl/pia19818/vista-from-curiosity-shows-crossbedded-martian-sandstone.

Beyond studying the geology and composition of Mt. Sharp, the Curiosity team has also reported the detection of small (parts per billion level) amounts of methane in the Martian atmosphere (Webster et al., 2015, 2018). This discovery was directly linked to the question of Martian habitability because of the possibility that methane could be generated by subsurface methane-producing bacteria. However, many abiotic factors could also produce this gas, including the release of ancient atmospheric methane trapped in the subsurface or ongoing subsurface water–rock interactions (Yung et al., 2018). Intriguingly, the Curiosity team observed that the methane abundance had a strong, and repeatable, seasonal cycle. Nighttime methane abundances measured at Gale crater vary seasonally (Webster et al., 2018), although some disagree with this conclusion (Gillen et al., 2020). In addition to the reported seasonal methane variations, the Curiosity team has also reported occasional transient increases in methane abundances (Webster et al., 2015, 2018). These methane spikes are hypothesized to come from localized methane sources released from the Martian surface or subsurface (Webster et al., 2018). One explanation for the seasonal trends is that methane from these transient spike events, or methane supplied from below by microseepage, adsorbs onto and diffuses through the Martian regolith seasonally (Moores et al., 2019a). Interestingly, the European Space Agency’s Trace Grace Orbiter team have not reported any methane during that mission’s daytime measurements of the Martian atmosphere (Korablev et al., 2019). This discrepancy might be because methane accumulates close to the ground during the Martian nighttime (when Curiosity measurements are made), a result of an inhibition of atmospheric mixing at night (Moores et al., 2019b). Two chapters in this book provide more detail on the methane enigmas.

Finally, the Curiosity team also completed the first in situ exploration of an active eolian dune field on another planet at the Bagnold Dunes (Bridges and Ehlmann, 2018; Lapotre and Rampe, 2018). Data from this exploration resolved several outstanding Martian enigmas of eolian processes, including (1) an explanation of how distinctive, meter-scale sinuous bedforms unique to Mars form (high kinetic viscosity regime of the Mars’ thin atmosphere) (Lapotre et al., 2016), (2) how saltation on Mars can occur even at wind speeds below the fluid threshold (Sullivan and Kok, 2017), and (3) how volatiles are stored in sand versus dust-sized fractions of Mars soil (Ehlmann et al., 2016).

5. Enigmas solved … and generated

The Curiosity mission has solved the primary enigma that it was designed to investigate, and we can now conclusively say that Gale crater was a habitable environment in its past (Grotzinger et al., 2014, 2015). The rover team has documented geologic evidence of a long history of liquid water at Gale crater, and this history perhaps extended even longer in subsurface environments. Compositional data suggest that at least some of these aqueous settings had low salinity and near-neutral pH, making them favorable environments for potential biological activity (Rampe et al., 2020). The Curiosity team also discovered all the chemical ingredients necessary to support microbial life (C, H, N, O, P, S) at various points along the rover’s traverse, along with minerals in various states of redox that could have provided energy for chemolithotrophic or heterotrophic organisms (Grotzinger et al., 2015; Hurowitz et al., 2017; Rampe et al., 2020; Stern et al., 2015; Sutter et al., 2017).

Although Curiosity’s exploration has answered questions about the origin and evolution of Mt. Sharp, many new questions have also arisen. What new environments will be discovered as the rover continues to climb the mound, and how do they relate to the previously explored environments? When did the habitable lake environment cease, and why? How long were habitable environments maintained in the subsurface, where liquid water was present both during and after the lake era, as evidenced by the extensive early- and late-stage diagenetic alteration features across the mound? How can we reconcile climate models that predict a cold icy early Mars with Curiosity’s evidence for a long-lived standing body of water at the Martian surface? Does Mars have a low-density felsic crust that is the source of the felsic float rocks the Curiosity team discovered on the plains of Gale crater (Sautter et al., 2015)? What is the source of the atmospheric methane measured by the rover?

The example of Curiosity’s success gives us hope that we will able to solve some of the major Martian enigmas raised in this book through our continued exploration by orbiters, landers, rovers, and potential returned samples and human missions in the future. Curiosity’s story also suggests that as we continue to explore new destinations on Mars, our discoveries will undoubtedly lead to new questions and intriguing enigmas, opening new avenues of research and enquiry.

Acknowledgment

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration © 2020. California Institute of Technology. Government sponsorship acknowledged.

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