6

Mars: Evolution of an Earth-Like World

Mars has a unique place in solar system exploration: it holds keys to many compelling planetary science questions, and it is accessible enough to allow rapid, systematic exploration to address and answer these questions. The science objectives for Mars center on understanding the evolution of the planet as a system, focusing on the interplay between the tectonic and climatic cycles and the implications for habitability and life. These objectives are well aligned with the broad crosscutting themes of solar system exploration articulated in Chapter 3.

Mars presents an excellent opportunity to investigate the major question of habitability and life in the solar system. Conditions on Mars, particularly early in its history, are thought to have been conducive to the formation of prebiotic compounds and potentially to the origin and continued evolution of life. Mars has also experienced major changes in surface conditions—driven by its thermal evolution and its orbital evolution and by changes in solar input and greenhouse gases—that have produced a wide range of environments. Of critical significance is the excellent preservation of the geologic record of early Mars, and thus the potential for evidence of prebiotic and biotic processes and how they relate to the evolution of the planet as a system. This crucial early period is when life began on Earth, an epoch largely lost on our own planet. Thus, Mars provides the opportunity to address questions about how and whether life arose elsewhere in the solar system, about planetary evolution processes, and about the potential coupling between biological and geological history. Progress on these questions, important to both the science community and the public, can be made more readily at Mars than anywhere else in the solar system.

The spacecraft exploration of Mars began in 1965 with an exploration strategy of flybys, followed by orbiters, landers, and rovers with kilometers of mobility. This systematic investigation has produced a detailed knowledge of the planet’s character, including global measurements of topography, geologic structure and processes, surface mineralogy and elemental composition, the near-surface distribution of water, the intrinsic and remanant magnetic field, gravity field and crustal structure, and the atmospheric composition and time-varying state (Figure 6.1).1,2,3,4,5,6,7,8,9,10 The orbital surveys framed the initial hypotheses and questions and identified the locations where in situ exploration could test them. The surface missions—the Viking landers, Pathfinder, and the Mars Exploration Rovers—have acquired detailed information on surface morphology, stratigraphy, mineralogy, composition, and atmosphere-surface dynamics and confirmed what was strongly suspected from orbital data: Mars has a long and varied history during which water has played a major role.

A new phase of exploration began with the Mars Express and the Mars Reconnaissance Orbiter (MRO), which carry improved instrumentation to pursue the questions raised in the earlier cycles of exploration. Among the discoveries (Table 6.1) is the realization that Mars is a remarkably diverse planet with a wide range of aqueous



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6 Mars: Evolution of an Earth-Like World Mars has a unique place in solar system exploration: it holds keys to many compelling planetary science ques- tions, and it is accessible enough to allow rapid, systematic exploration to address and answer these questions. The science objectives for Mars center on understanding the evolution of the planet as a system, focusing on the interplay between the tectonic and climatic cycles and the implications for habitability and life. These objectives are well aligned with the broad crosscutting themes of solar system exploration articulated in Chapter 3. Mars presents an excellent opportunity to investigate the major question of habitability and life in the solar system. Conditions on Mars, particularly early in its history, are thought to have been conducive to the formation of prebiotic compounds and potentially to the origin and continued evolution of life. Mars has also experienced major changes in surface conditions—driven by its thermal evolution and its orbital evolution and by changes in solar input and greenhouse gases—that have produced a wide range of environments. Of critical significance is the excellent preservation of the geologic record of early Mars, and thus the potential for evidence of prebiotic and biotic processes and how they relate to the evolution of the planet as a system. This crucial early period is when life began on Earth, an epoch largely lost on our own planet. Thus, Mars provides the opportunity to address questions about how and whether life arose elsewhere in the solar system, about planetary evolution processes, and about the potential coupling between biological and geological history. Progress on these questions, important to both the science community and the public, can be made more readily at Mars than anywhere else in the solar system. The spacecraft exploration of Mars began in 1965 with an exploration strategy of flybys, followed by orbiters, landers, and rovers with kilometers of mobility. This systematic investigation has produced a detailed knowledge of the planet’s character, including global measurements of topography, geologic structure and pro- cesses, surface mineralogy and elemental composition, the near-surface distribution of water, the intrinsic and remanant magnetic field, gravity field and crustal structure, and the atmospheric composition and time-varying state (Figure 6.1).1,2,3,4,5,6,7,8,9,10 The orbital surveys framed the initial hypotheses and questions and identified the locations where in situ exploration could test them. The surface missions—the Viking landers, Pathfinder, Phoenix, and the Mars Exploration Rovers—have acquired detailed information on surface morphology, stratigraphy, min- eralogy, composition, and atmosphere-surface dynamics and confirmed what was strongly suspected from orbital data: Mars has a long and varied history during which water has played a major role. A new phase of exploration began with the Mars Express and the Mars Reconnaissance Orbiter (MRO), which carry improved instrumentation to pursue the questions raised in the earlier cycles of exploration. Among the discoveries (Table 6.1) is the realization that Mars is a remarkably diverse planet with a wide range of aque- 137

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138 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 6.1 Examples of global data sets highlight major accomplishments from multiple recent missions. SOURCE: P.R. Christensen, N.S. Gorelick, G.L. Mehall, and K.C. Murray, THEMIS Public Data Releases, Planetary Data System node, Arizona State University, available at http://themis-data.asu.edu. ous environments (Figure 6.2). The role of water and the habitability of the ancient environment will be further investigated by the Mars Science Laboratory (MSL), scheduled for launch in the latter part of 2011, which will carry the most advanced suite of instrumentation ever landed on the surface of a planetary object (Box 6.1). The program of Mars exploration over the past 15 years has provided a framework for systematic exploration, allowing hypotheses to be formulated and tested and new discoveries to be pursued rapidly and effectively with follow-up observations. In addition, the program has produced missions that support one another both scientifically and through infrastructure, with orbital reconnaissance and site selection, data relay, and critical event coverage significantly enhancing the quality of the in situ missions.11,12,13 Finally, this program has allowed the Mars science

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139 MARS: EVOLUTION OF AN EARTH-LIKE WORLD TABLE 6.1 Major Accomplishments of Studies of Mars in the Past Decade Major Accomplishment Mission and/or Technique Provided global mapping of surface composition, topography, remanant magnetism, Mars Global Surveyor, Odyssey, Mars atmospheric state, crustal structure Express, Mars Reconnaissance Orbiter Mapped the current distribution of near-surface ice and the morphologic effects of Odyssey recent liquid water associated with near-surface ice deposits Confirmed the significance of water through mineralogic measurements of surface Mars Exploration Rovers, Phoenix rocks and soils Demonstrated the diversity of aqueous environments, with major differences in Mars Express, Odyssey, Mars aqueous chemistry, conditions, and processes Reconnaissance Orbiter, Mars Exploration Rovers Mapped the three-dimensional temperature, water vapor, and aerosol properties of Mars Global Surveyor, Odyssey, Mars the atmosphere through time; found possible evidence of the presence of methane Express, Mars Reconnaissance Orbiter, and ground-based telescopes FIGURE 6.2 Examples of the diversity of Mars’s environments and their mineralogy and morphology. SOURCE: Adapted from S. Murchie, A. McEwen, P. Christensen, J. Mustard, and J.-P. Bibring, Discovery of Diverse Martian Aqueous Deposits from Orbital Remote Sensing, presentation from the Curation and Analysis Planning Team for Extraterrestrial Materials Workshop on Ground Truth from Mars, Science Payoff from a Sample Return Mission, April 21-23, 2008, Albuquerque, New Mexico, available at http://www.lpi.usra.edu/captem/msr2008/presentations/.

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140 VISION AND VOYAGES FOR PLANETARY SCIENCE BOX 6.1 Mars Science Laboratory Scheduled to launch in the fall of 2011, the Mars Science Laboratory (MSL) is an advanced rover designed to follow Spirit and Opportunity—the highly successful Mars Exploration Rovers. The primary focus of the MSL is on assessing the habitability of geochemical environments, identified from orbit, in which water-rock interactions have occurred and the preservation of biosignatures is possible. The MSL, weighing nearly a metric ton, carries a sophisticated suite of instruments for remote and in situ rock and soil analysis, including x-ray diffraction, high-precision mass spectroscopy, laser-induced breakdown spectroscopy, and alpha-proton x-ray spectroscopy, and a suite of cameras including microscopic imaging at 10-micron resolution. This analysis suite will provide detailed mineralogy and elemental composition, including the ability to assess light elements such as carbon, hydrogen, and oxygen and their isotopes. The mission will also demonstrate the MSL’s Sky Crane precision entry, descent, and landing system, long-term surface operations, and long-range mobility. community to construct a logical series of missions each of which is modest in scope and systematically advances our scientific understanding of Mars. Over the past decade the Mars science community, as represented by the Mars Exploration Program Analysis Group (MEPAG), has formulated three major science themes that pertain to understanding Mars as a planetary system: • Life—Understand the potential for life elsewhere in the universe; • Climate—Characterize the present and past climate and climate processes; and • Geology—Understand the geologic processes affecting Mars’s interior, crust, and surface. From these themes, MEPAG has derived key, overarching science questions that drive future Mars explora- tion. These include the following: • What are the nature, ages, and origin of the diverse suite of geologic units and aqueous environments evident from orbital and landed data, and were any of them habitable? • How, when, and why did environments vary through Mars history, and did any of them host life or its precursors? • What are the inventory and dynamics of carbon compounds and trace gases in the atmosphere and surface, and what are the processes that govern their origin, evolution, and fate? • What is the present climate and how has it evolved on timescales of 10 million years, 100 million years, and 1 billion years? • What are the internal structure and dynamics, and how have these evolved over time? The next decade holds great promise for Mars exploration. The MSL rover (see Box 6.1) will significantly advance our knowledge of surface mineralogy and chemistry at a site specifically selected to provide insight into aqueous processes. The MAVEN mission currently in development and the European Space Agency (ESA)-NASA Mars Trace Gas Orbiter (TGO) will provide major new insights into the state and evolution of the Mars atmosphere. Following these missions, the highest-priority science goal will be to address in detail the questions of habitability and the potential origin and evolution of life on Mars. The major focus of the next decade will be to initiate a Mars Sample Return (MSR) campaign, beginning with a rover mission to collect and cache samples, followed by missions to retrieve these samples and return them to

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141 MARS: EVOLUTION OF AN EARTH-LIKE WORLD Earth. It is widely accepted within the Mars science community that the highest science return on investment for understanding Mars as a planetary system will result from analysis of samples carefully selected from sites that have the highest scientific potential and that are returned to Earth for intensive study using advanced analytical techniques. These samples can be collected and returned to Earth in a sequence of three missions that collect them, place them into Mars orbit, and return them to Earth. This modular approach is scientifically, technically, and pro- grammatically robust, with each mission possessing a small number of discrete engineering challenges and with multiple sample caches providing resiliency against any failure of subsequent elements. This modular approach also allows the sample return campaign to proceed at a pace determined by prioritization within the solar system objectives and by available funding. The study of Mars as an integrated system is so scientifically compelling that it will continue well beyond the coming decade, with future missions implementing geophysical and atmospheric networks, providing in situ studies of diverse sites, and bringing to Earth additional sample returns that build on the coming decade’s discoveries. All three of the committee’s crosscutting themes for the exploration of the solar system include Mars, and studying Mars is vital to answering a number of the priority questions in each of them. The building new worlds theme includes the question, What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles play? Mars is central to the planetary habitats theme, which also includes two questions that are key components of the scientific exploration of Mars—What were the primordial sources of organic matter, and where does organic synthesis continue today? and, Beyond Earth, are there modern habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now? The workings of solar systems theme includes the question, Can understanding the roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth? Mars has transitioned from having an early, warm, wet environment to its current state as a cold, dry planet with a thin atmosphere; the study of Mars’s climate can shed light on the evolution, and perhaps future, of Earth’s own climate. The planet most like Earth in terms of its atmosphere, climate, geology, and surface envi- ronment, Mars plays a central role in the broad question, How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time? SCIENCE GOALS FOR THE STUDY OF MARS The Mars science community, through MEPAG, has worked to establish consensus priorities for the future scientific exploration of Mars.14,15,16,17 One overarching theme is to understand whether life arose in the past and persisted to the present within the context of a differentiated rocky planet (deep interior, crust, and atmosphere) that has been strongly influenced by its interior evolution, solar evolution, and orbital dynamics. Parallel inves- tigations among multiple disciplines are required to understand how habitable environments and life might have developed on a dynamic planet where materials and processes have been closely coupled. The Mars science goals embrace this approach by articulating an interdisciplinary research program that drives a multi-decadal campaign of Mars missions. These goals include multiple objectives that embody the strategies and milestones needed to understand an early wet Mars, a transitional Mars, and the more recent and modern frozen, dry Mars. Ultimately these efforts will create a context of knowledge for understanding whether martian environments ever sustained habitable conditions and life. Building on the work of MEPAG, the committee has established three high-priority science goals for the exploration of Mars in the coming decade: • Determine if life ever arose on Mars—Does life exist, or did it exist, elsewhere in the universe? This is perhaps one of the most compelling questions in science, and Mars is the most promising and accessible place to begin the search. If answered affirmatively, it will be important to know where and for how long life evolved, and how the development of life relates to the planet’s evolution. • Understand the processes and history of climate—Climate and atmospheric studies remain a major objec- tive of Mars exploration. They are key to understanding how the planet may have been suited for life and how

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142 VISION AND VOYAGES FOR PLANETARY SCIENCE major parts of the surface have been shaped. In addition, studying the atmosphere of Mars and the evolution of its climate at various timescales is directly relevant to our understanding of the past, present, and future climate of Earth. Finally, characterizing the environment of Mars is also necessary for the safe implementation of future robotic and human spacecraft missions. • Determine the evolution of the surface and interior—Insight into the composition, structure, and history of Mars is fundamental to understanding the solar system as a whole, as well as to providing context for the history and processes of Earth. Geological and geophysical investigations will shed light on critical environmental aspects such as heat flow, loss of a global magnetic field, pathways of water-rock interaction, and sources and cycling of volatiles including water and carbon species (e.g., carbon dioxide and hydrocarbons). In contrast to Earth, Mars appears to have a rich and accessible geologic record of the igneous, sedimentary, and cratering processes that occurred during the early history of the solar system. Geophysical measurements of Mars’s interior structure and heat flow, together with detailed mineralogic, elemental, and isotopic data from a diverse suite of martian geologic samples, are essential for determining the chemical and physical processes that have operated through time on this evolving, Earth-like planet. Subsequent sections examine each of these goals in turn. DETERMINE IF LIFE EVER AROSE ON MARS The prime focus of the first high-priority goal for the exploration of Mars in the coming decade is to determine if life is or was present on Mars. If life is or was there, we must understand the resources that support or supported it. If life never existed yet conditions appear to have been suitable for the formation and/or maintenance of life, a focus would then be to understand why life did not originate. A comprehensive conclusion about the question of life on Mars will necessitate understanding the planetary evolution of Mars and whether Mars is or could have been habitable, using multidisciplinary scientific exploration at scales ranging from planetary to microscopic. The strategy adopted to pursue this goal has two sequential science steps: (1) assess the habitability of Mars on an environment-by-environment basis using global remote sensing observations and (2) then test for prebiotic pro- cesses, past life, or present life in environments that can be shown to have high potential for habitability. A critical means of achieving both objectives is to characterize martian carbon chemistry and carbon cycling. Therefore, the committee’s specific objectives for pursuing the life goal are as follows: • Assess the past and present habitability of Mars, • Assess whether life is or was present on Mars in its geochemical context, and • Characterize carbon cycling and prebiotic chemistry. Subsequent sections examine each of these objectives in turn, identifying critical questions to be addressed and future investigations and measurements that could provide answers. Assess the Past and Present Habitability of Mars Understanding whether a past or present environment on Mars could sustain life will include establishing the distribution of water, its geologic history, and the processes that control its distribution; identifying and character- izing phases containing carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS); and determining the available energy sources. Recent exploration has confirmed that the surface of Mars today is cold, dry, chemically oxidizing, and exposed to intense solar ultraviolet radiation. These factors probably limit or even prohibit any life near the surface, although liquid water might occur episodically near the surface as dense brines in association with melting ice. 18 The subsurface of Mars appears to be more hospitable than its surface. With mean annual surface temperatures close to 215 K at the equator and 160 K at the poles, a thick cryosphere could extend to a depth of several kilometers. Hydrothermal activity is likely in past or present volcanic areas, and even the background geothermal heat flux could

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143 MARS: EVOLUTION OF AN EARTH-LIKE WORLD drive water to the surface. At depths below a few kilometers, warmer temperatures would sustain liquid water in pore spaces, and a deep-subsurface biosphere is possible provided that nutrients are accessible and water can circulate. 19 Biotic and abiotic pathways for the formation of complex organic molecules require an electron donor closely coupled to carbon in a form suitable to serving as an electron acceptor. On Mars, igneous minerals containing ferrous iron and/or partially reduced sulfur (e.g., olivine and pyrrhotite) are potential electron acceptors for reduction of carbon. The report of methane in the martian atmosphere contends that an active source is required to balance its destruction (its photochemical lifetime is less than 300 years).20 Any sources would likely reside in the subsurface and might include volcanic emissions, low-temperature rock-water reactions, microorganisms, or gas from the thermal degradation of organic matter. Climate changes in the recent geologic past might have allowed habitable conditions to arise episodically in near-surface environments. For example, Mars undergoes large changes in its obliquity (i.e., the tilt of its polar axis). At present the obliquity ranges from 23° to 27°, with values as high as 46° during the past 10 million years. 21 At these higher obliquities, the water content of the atmosphere is likely higher, ground ice is stable closer to the equator, and surface ice may be transferred from the poles to lower latitudes. 22,23 Past Habitable Environments and Life Recent observations confirm that conditions in the distant past were probably very different from present conditions, with wetter and warmer conditions prior to about 3.5 billion years ago (the oldest definitive evidence of life on Earth is at least 3.7 billion years old). This evidence includes valley networks with relatively high drain- age densities, evaporites and groundwater fluctuations,24,25 clay minerals, hydrothermally altered rocks, deltas, and large inferred surface erosion rates (Figure 6.3).26,27,28 Early Mars also witnessed extensive volcanism and high impact rates. The formation of large impact basins likely developed hydrothermal systems and hot springs that might have sustained locally habitable environments.29,30,31 Since approximately 3.5 billion years ago, rates of weathering and erosion appear to have been very low, and the most characteristic fluvial features are outflow channels formed by the catastrophic release of near-surface water.32 Groundwater is likely to be stable at greater depths, and it might sustain habitable environments. In all epochs, the combination of volcanism and water-rich conditions might have sustained hydrothermal systems in which life could have thrived. Important Questions Some important questions concerning the past and present habitability of Mars include the following: • Which accessible sites on Mars offer the greatest potential for having supported life in the past? How did the major factors that determine habitability—the duration and activity of liquid water, energy availability, physico- chemical factors (temperature, pH, oxidation-reduction potential, fluid chemistry), and the availability of biogenic elements—vary among environments, and how did they influence the habitability of different sites? • Which accessible sites favor the preservation of any evidence of past habitable environments and life? How did the major factors that affect the preservation of such evidence—for example, aqueous sedimentation and mineralization, oxidation, and radiation—vary among these sites? • How have the factors and processes that give rise to habitable conditions at planetary and local scales changed over the long term in concert with planetary and stellar evolution? Future Directions for Investigations and Measurements Central to addressing habitability-related questions is searching for future landing sites that have high potential for both habitability and the preservation of biosignatures (Box 6.2). The key here is identifying accessible rocks that show evidence of formation in aqueous environments such as fluvial, lacustrine, or hydrothermal systems. 33,34 An additional requirement is to be able to place the rock exposures in a stratigraphic framework that will allow a

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144 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 6.3 Diverse mineralogy, observed with Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data, formed by water-related processes and indicative of potentially habitable environments. SOURCE: B.L. Ehlmann and J.F. Mustard, Stratigraphy of the Nili Fossae and the Jezero Crater Watershed: A Reference Section for the Martian Clay Cycle, presentation at the First International Conference on Mars Sedimentology and Stratigraphy, April 19-21, 2010, El Paso, Texas, #6064, Lunar and Planetary Science Conference 2010. Lunar and Planetary Institute. reconstruction of past environmental conditions.35 Another key aspect in understanding present and past habitability is to characterize the current geologic activity of the martian interior. The long-term evolution of geologic pro- cesses, habitable environments, and life on Earth have been closely linked. Accordingly, geophysical observations that contribute to our understanding of the martian interior are important to the search for signs of martian life. Ultimately, our best understanding of present and past habitability will await the return to Earth of carefully selected samples from sites that have the highest science potential for analysis in terrestrial laboratories. Analyses of returned samples in Earth-based laboratories are essential in order to establish the highest confidence in any potential martian biosignatures and to interpret fully the habitable environments in which they were formed and preserved.36,37,38,39,40 Key technological developments for surface exploration and sampling include modest-size rovers capable of selecting samples and documenting their context. These rovers should include imaging and remote sensing spec- troscopy adequate to establish local geologic context and to identify targets. Suggested capabilities include surface abrasion tool(s), arm-mounted sensors, and a rock core caching system to collect suites of samples that meet the

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145 MARS: EVOLUTION OF AN EARTH-LIKE WORLD BOX 6.2 Biosignatures Life can be defined as essentially a self-sustaining system capable of evolution. To guide the search for signs of life on Mars, however, requires a working concept of life that helps to identify its key characteristics and its environmental requirements. Biosignatures are features that can be unambiguously interpreted as evidence of life and so provide the means to address fundamental questions about the origins and evolution of life. Types of biosignatures include morphologies (e.g., cells, and plant or animal remnants), sedimentary fabrics (e.g., laminations formed by biofilms), organic molecules, biominerals (e.g., certain forms of magnetite),1 elemental abundances, and stable isotopic patterns. Because some biosignatures are preserved over geologic timescales and in environments that are no longer habitable, they are impor- tant targets of exploration. It is not unreasonable to anticipate that any martian life might differ significantly from life on Earth, although Earth’s environments have been more similar to those on Mars than to the environments of any other object in the solar system. Moreover, Mars and Earth may have exchanged life forms through impact ejecta. Any martian life may reasonably be assumed to have shared at least some of its basic attributes with life as we know it, which implies that any martian life also requires liquid water, carbon-based chemistry, and electron transfer processes.2,3 Our working concept of life should also identify environmental conditions that are most conducive to life. A habitable environment must sustain liquid water at least intermittently and must also allow key biological molecules to survive. The elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur must be available, because they are essential for forming the covalently bonded compounds utilized by all known life. Organic compounds are therefore key targets, with the caveat that martian and earthly life might have employed different compounds. Energy drives metabolism and motility and must be available from, for example, light or energy-yielding chemical reactions.4 Finally, the rates of environmental changes must not exceed rates at which life could adapt.5 Even if habitable environments supported the origination and evolution of life on Mars, the right set of environmental conditions would be required in order to preserve biosignatures. The study of fossilization processes will be as important for Mars as it has been for Earth.6 The preservation of biosignatures is criti- cally sensitive to the diagenetic processes that control preservation; paradoxically, the very characteristics (water; gradients in heat, chemicals, and light; and oxidant supply) that make so many environments habit- able also cause them to be destructive to biosignature preservation. There are, however, habitable environ- ments with geochemical conditions favoring very early mineralization that facilitate spectacular preservation. Authigenic silica, phosphate, clay, sulfate, and, less commonly, carbonate precipitation are all known to promote biosignature preservation.7 The search for environments that have been both habitable and favor- able for preservation can be optimized by pursuing an exploration strategy that focuses on the search for “windows of preservation,” remembering that Mars may indeed have its own uniquely favorable conditions. 1 R.E. Kopp and J.L. Kirshvink. 2008. The identification and biogeochemical interpretation of fossil magnetotactic bacteria, Earth Science Reviews 86:42-61. 2 For a detailed discussion of these assumptions see, for example, National Research Council, An Astrobiology Strategy for the Exploration of Mars, The National Academies Press, Washington, D.C., 2007. 3 For a discussion of the possibilities opened by relaxing some of these assumptions see, for example, National Research Council, The Limits of Organic Life in Planetary Systems, The National Academies Press, Washington, D.C., 2007. 4 T.M. Hoehler. 2007. An energy balance concept for habitability, Astrobiology 7:824-838. 5 D.J. Des Marais, B.M. Jakosky, and B.M. Hynek. 2008. Astrobiological implications of Mars surface composition and properties, pp. 599-623 in The Martian Surface: Composition, Mineralogy and Physical Properties (J.F. Bell III, ed.), Cambridge University Press, Cambridge, U.K. 6 J.P. Grotzinger. 2009. Mars exploration, comparative planetary history, and the promise of Mars Science Labora- tory, Nature Geoscience 2:1-3. 7 J.D. Farmer and D.J. Des Marais. 1999. Exploring for a record of ancient Martian life, Journal of Geophysical Research 103:26977-26995.

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146 VISION AND VOYAGES FOR PLANETARY SCIENCE appropriate standards.41,42 The in situ measurements used to select samples for return to Earth must go beyond identifying locations where liquid water has occurred.43,44 They should also characterize the macroscopic and microscopic fabrics of sedimentary materials, be capable of detecting organic molecules, reconstruct the history of mineral formation as an indicator of preservation potential and geochemical environments, and determine specific mineral and chemical compositions as indicators of organic matter or coupled redox reactions characteristic of life. Also essential to a better understanding of the geochemistry of martian environments and the compositional and morphologic signatures that these different environments produce is the continuation of a robust research and analysis (R&A) program. Theoretical, laboratory, and terrestrial analog studies should develop models, analysis approaches, and instrumentation to interpret ancient environments from orbital, in situ, and returned sample data. 45,46,47,48 Assess Whether Life Is or Was Present on Mars in Its Geochemical Context and Characterize Carbon Cycling and Prebiotic Chemistry Assessing whether life is or was present on Mars will include characterizing complex organics, the spatial distribution of chemical and isotopic signatures, and the morphology of mineralogic signatures, and identifying temporal chemical variations requiring life. Characterizing the carbon cycle will include determining the distribu- tion and composition of organic and inorganic carbon species; characterizing the distribution and composition of inorganic carbon reservoirs through time; characterizing the links between carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur; and characterizing the preservation of reduced carbon compounds on the near-surface through time. Organic and inorganic chemical reactions in early planetary environments pioneered the pathways that, on Earth, ultimately led to the origins of life. Organic compounds may have formed on early Mars through energetic reactions in reducing atmospheres, mineral-catalyzed chemical reactions, transient reactions caused by bolide impacts, and delivery of comets, meteorites, and interplanetary dust. The challenge is first to find organic matter and any redox-sensitive minerals and compounds and then to characterize the conditions and processes that determined their composition. The Mars Science Laboratory is specifically designed to address many of these questions, and it is expected that significant progress will come from the MSL results. Important Questions Some important questions concerning whether life is or was present on Mars and the characterization of carbon cycling and prebiotic chemistry in a geochemical context include the following: • Can evidence of past (or present) life in the form of organic compounds, aqueous minerals, cellular morphologies, biosedimentary structures, or patterns of elemental and mineralogic abundance be found at sites that have been carefully selected for high habitability and preservation potential? • Do habitable environments exist today that may be identified by atmospheric gases, exhumed subsurface materials, or geophysical observations of the subsurface? Does life exist today, as evidenced by biosignatures, atmospheric gases, or other indicators of extant metabolism? Future Directions for Investigations and Measurements To address the key questions concerning life listed above, there must be a broad range of mineralogic, ele- mental, isotopic, and textural measurements of a diverse suite of martian rocks from well-characterized sites that have high potential for habitability. Deposits formed by aqueous sedimentation, hydrothermal activity, or aqueous alteration are important targets in the search for life. These deposits typically contain assemblages of materials that indicate geological (and, possibly, biological) processes. Accordingly, a sample suite is defined as the set of samples required to determine the key processes that formed these samples and, in turn, required to assess any evidence of habitable environments or life. Many of the specific investigations and measurements overlap with those necessary to determine the geologic context and to understand the potential for habitability described earlier,

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147 MARS: EVOLUTION OF AN EARTH-LIKE WORLD including the technological development of modest-size rovers capable of selecting samples and documenting their context, along with the development of critical sample selection criteria and analysis instrumentation. Additionally, the preparation for the return to Earth of carefully selected samples from sites with the highest science potential will mandate establishing the curation methodologies needed to accommodate the contamination, alteration, and planetary protection challenges posed by the complex martian returned samples. A direct way to search for extant life is to map the distribution of atmospheric trace gases as will be done by the ESA-NASA Mars Trace Gas Orbiter. Both biotic and abiotic processes involving water in subsurface environments can produce gases that escape into the atmosphere. Measurements of the composition, abundances, variability, and formation processes of atmospheric trace gases will allow the separation of potential geological and biological sources. Finally, the support of a robust R&A program is crucial to a better understanding of the interactions between organisms and their geologic environments and their biosignatures. Terrestrial analog studies should test instru- mentation, develop techniques for measuring biosignatures under martian conditions, and conduct technological proof-of-concept studies. UNDERSTAND THE PROCESSES AND HISTORY OF CLIMATE The fundamental science questions that underlie the goal of understanding the processes and history of Mars’s climate are how the climate of Mars has evolved over time to reach its current state and what processes have oper- ated to produce this evolution. The climate history of Mars can be divided into three distinct epochs: 1. Modern, with the climate system operating under the current obliquity; 2. Recent past, operating under similar pressures and temperatures but over a range of orbital variations (primarily obliquity); and 3. Ancient, when the atmospheric pressure and temperature may have been substantially higher than at present, and liquid water may have been stable on the surface, either intermittently or for extended periods. The committee’s specific objectives for pursuing the climate goal are as follows: • Characterize Mars’s atmosphere, present climate, and climate processes under both current and different orbital configurations; and • Characterize Mars’s ancient climate and climate processes. Subsequent sections examine each of these objectives in turn, identifying critical questions to be addressed and future investigations and measurements that could provide answers. Understanding the current climate includes investigating the processes controlling the present distributions of water, carbon dioxide, and dust; determining the production and loss, reaction rates, and global distribution of key photochemical species; and understanding the exchange of volatiles and dust between surface and atmospheric reservoirs. Understanding past climates includes determining how the composition of the atmosphere evolved to its present state, what the chronology of compositional variability is, and what record of climatic change is expressed in the surface stratigraphy and morphology. The ancient climate can be addressed by determining the escape rates of key species and their correlation with seasonal and solar variability, the influence of the magnetic field, the physical and chemical records of past climates, and the evolution of the isotopic, noble gas, and trace gas composition through time. Mars’s current climate system is complex and highly variable because the atmospheric circulation is coupled to three cycles: • The dust cycle—dust lifted by the wind modifies the atmosphere’s radiative properties; • The carbon dioxide cycle—the atmosphere condenses and sublimes at seasonal polar caps and causes planetary-scale transport and pressure cycles; and

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164 VISION AND VOYAGES FOR PLANETARY SCIENCE ADVANCING STUDIES OF MARS Previously Recommended Missions The NRC’s 2003 planetary science decadal survey143 contained recommendations relating to five Mars missions—technology development to enable Mars sample return, the Mars Science Laboratory, a long-lived lander network, an upper-atmosphere orbiter, and the Mars Scout program. Of these five missions, three have flown or are in final development. The upper-atmosphere mission is being implemented as the Mars Scout MAVEN mis- sion, with a planned 2013 launch. The MSL mission is planned to launch in 2011, and the Mars Scout program has produced both the Phoenix lander (2008) and MAVEN. The MSL, which was described only in very general terms in the 2003 report, grew substantially in capability beyond what the 2003 survey envisioned, and it will achieve significantly more science than originally planned. The principal-investigator-led Scout program has been incorporated into the Discovery program. New Missions: 2013-2022 Mars Sample Return Campaign The committee places as the highest-priority Mars science goal the addressing in detail of the questions of habitability and the potential origin and evolution of life on Mars. A critical next step toward answering these questions will be provided through the analysis of carefully selected samples from geologically diverse and well- characterized sites that are returned to Earth for detailed study using a wide diversity of laboratory techniques. Therefore, the highest-priority missions for Mars in the coming decade are the elements of the Mars Sample Return campaign—the Mars Astrobiology Explorer-Cacher to collect and cache samples, followed by the Mars Sample Return Lander and the Mars Sample Return Orbiter (Figure 6.7) to retrieve these samples and return them to Earth, where they will be analyzed in a Mars returned-sample-handling facility. MAX-C is the critical first element of Mars sample return. It should be viewed primarily in the context of sample return rather than as a separate mission that is independent of the sample return objective. The MAX-C mission, by design, focuses on the collection and caching of samples from a site with the highest potential to study aqueous environments, potential prebiotic chemistry, and habitability. In order to minimize cost and to focus the technology development, the mission emphasizes the sample system and deemphasizes the use of in situ science experiments. This design approach naturally leads to a mission that has a lower science value if sample return does not occur. However, exploring a new site on a diverse planet with a science payload similar in capability to that of the Mars Exploration Rovers will significantly advance our understanding of the geologic history and evolution of Mars, even before the cached samples are returned to Earth. By implementing sample return as a sequence of three missions, the highest-priority Mars objective of advanc- ing the search for evidence of life on Mars can be achieved at a pace that maintains solar system balance and fits within the available funding. The architecture provides resilience for adapting to budgetary changes and robustness against mission failures. Two caches will be collected and remain scientifically viable for up to 20 years on the surface or in orbit about Mars, so that a failure of the MAV would not necessitate reflight of MAX-C, and neither the MAV nor MAX-C would need to be reflown if the return orbiter failed to achieve orbit. A modular approach also permits timely reaction to scientific discoveries, so that a follow-on rover mission could pursue a major new finding, and it enables additional Mars sample return missions using these same flight elements. Mars Astrobiology Explorer-Cacher The MAX-C, the sample-collection rover, would be landed using a duplicate of the Sky Crane EDL system. The baseline design is a MER-class (~350 kg), solar-powered rover with about 20 km of mobility over a 500- sol mission lifetime. It will carry approximately 35 kg of payload for sample collection, handling, and caching, and a MER-class (~25 kg) suite of mast- and arm-mounted remote sensing and contact instruments to select the samples. The key new development will be the sample-coring, sample-collection, and sample-caching system.

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165 MARS: EVOLUTION OF AN EARTH-LIKE WORLD FIGURE 6.7 Mars Sample Return architecture. SOURCE: NASA Planetary Science Division. MAX-C will acquire about 20 primary and about 20 contingency rock cores, each 10 gm in mass, from rock targets with a high likelihood of preserving evidence for past environmental conditions including habitability, and with a high likelihood of the possibility of preserved biosignatures. These cores will be sealed in two separate caches for redundancy and left on the surface for retrieval by a subsequent mission. The cache systems will be designed to prevent cross-contamination between samples, prevent exposure to the martian atmosphere, keep the samples within the temperature range that they experienced prior to collection, and preserve the samples in this condition for up to 20 years. Mars Sample Return Lander The Mars Sample Return Lander (MSR-L) will also land using the Sky Crane system and will carry a fetch rover, local regolith and atmosphere sample-collection system, and the MAV. The fetch rover will be capable of reaching the cache from any point within the 11-km-radius landing error ellipse within 3 months. The strawman MAV design is a solid rocket that is maintained in a thermally controlled cocoon while on the martian surface for up to 1 Earth year. Following sample retrieval, the lander will place the cache in the orbital sample (OS) container, collect regolith and atmospheric samples, and seal the container to meet the planetary protection requirements. The MAV will insert the OS into a stable 500-km altitude near-circular orbit. Mars Sample Return Orbiter The Mars Sample Return Orbiter will consist of a Mars orbiter, the OS acquisition and capture system, the sample isolation system for planetary protection, and the EEV. The orbiter will detect, track, and rendezvous with

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166 VISION AND VOYAGES FOR PLANETARY SCIENCE the OS, then capture and seal it in the EEV. The orbiter will leave Mars and release the entry vehicle to Earth, where it will enter Earth’s atmosphere and hard-land using a parachute-less, self-righting system. Mars Returned-Sample-Handling Facility The Mars returned-sample-handling facility will meet the planetary protection requirements and will be based on practices and procedures at existing biocontainment laboratories, NASA’s Lunar Sample Facility, and pharmaceutical laboratories. Mars Trace Gas Orbiter The Mars Trace Gas Orbiter is currently conceived as a joint ESA-NASA collaboration to study the temporal and spatial distribution of trace gases, atmospheric state, and surface-atmosphere interactions on Mars. This mis- sion builds on the reported discovery of methane in the martian atmosphere. 144 The committee could only evaluate the science return of this mission in a general sense, because the payload had not been selected at the time of the evaluation. In addition, no independent cost estimate for this mission was generated because it would have been inappropriate to perform such a science and cost evaluation during the competitive instrument payload selection that was underway at the time of this assessment. NASA-provided cost estimates were used instead. Technology Development One of the highest-priority activities for the upcoming decade will be to develop the technologies necessary to return samples from Mars. The technology program also needs to continue a robust instrument development program so that future in situ missions can include the most advanced technologies possible. The new develop- ments needed for MAX-C are the sample-coring, sample-collection, and sample-caching system. The modest technology development for these systems has begun and should be continued at a level necessary to develop them to TRL 6 at the time that the mission is approved. The major new sample return technology needed will be the MAV. Although this launch system will be based on existing solid rocket motor designs, major development will be needed in thermal control, autonomous launch operations, and ascent and guidance under martian conditions. It is essential that these elements receive major investments during the coming decade in order to ensure that they will reach the necessary maturity to be used by the end of the coming decade or early in the decade after that. The second major technology development that will require attention is the tracking, rendezvous, and capture of the OS. An initial demonstration of this technology has been preformed by the Defense Advanced Research Projects Agency’s Orbital Express mission, which performed detection and rendezvous in Earth orbit under similar conditions. The MSR capture-basket concept has been demonstrated on zero-gravity aircraft flights. However, significant technology development will still be required to develop this system for application at Mars. The third technology element development is the planetary protection component of MSR to ensure that the back-contamination (contamination of Earth by martian materials) requirements are met. This system will require isolating the Mars sample cache completely and reliably throughout the entry, retrieval, and transport process. This work will require the development and testing of the technology elements and the development of methods and procedures to verify the required level of cleanliness in flight. Finally, the definition and architecture development of the Mars returned-sample-handling facility need to be accomplished in the coming decade. Significant issues must be resolved and requirements must be defined regarding the methods, procedures, and equipment that can verify the required level of isolation and planetary protection and sample characterization.

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167 MARS: EVOLUTION OF AN EARTH-LIKE WORLD New Frontiers Missions Mars Geophysical Network High-priority Mars science goals can be addressed by a New Frontiers-class geophysical network. The pri- oritized science objectives for a Mars Geophysical Network mission are as follows: 1. Measure crustal structure and thickness, and core size, density, and structure, and investigate mantle com - positional structure and phase transitions. 2. Characterize the local meteorology and provide ground truth for orbital climate measurements. A study of the Mars Geophysical Network was performed at the committee’s request (Appendixes D and G). Two identical free-flying vehicles would be launched on a single Atlas V 401 independently targeted for Mars entry 7 days apart; to meet the science objectives they would land at sites geographically distributed. Each node of the network would carry a three-axis very broad band seismometer with a shield and an X-band transponder; an atmospheric package with pressure sensor, thermistors, and hotwire anemometer; a deployment arm; descent and post-landing cameras; and a radio science package. The science payload would have a 1 martian year nominal mission with continuous operation. This instrumen- tation would allow the determination of crustal and lithosphere structure by cross-correlation of the atmospherically induced seismic noise and would locate the seismic sources from joint travel times and azimuth determinations. No major new technologies are required. The selected EDL architecture for this study employs a powered descent lander with heritage from previous Mars missions. Key technology development for the seismometer has been conducted over the past two decades, culminating in a TRL 5-6 instrument developed for the ESA ExoMars mission. Mars Polar Climate Mission As a follow-on to Phoenix, the next step for in situ high-latitude ice studies is to explore the exposed polar layered deposits (PLD). A mission study initiated at the committee’s request (see Appendixes D and G) addressed science objectives, including an understanding of the mechanism of climate change on Mars and how it relates to climate change on Earth; determination of the chronology, compositional variability, and record of climatic change expressed in the PLD; and an understanding of the astrobiological potential of the observable water-ice deposits. Both mobile and static lander concepts were explored and could answer significant outstanding questions with spacecraft and instrument heritage from existing systems. These concepts will likely fall within the New Frontiers mission size range. Discovery Missions NASA does not intend to continue the Mars Scout program beyond the MAVEN mission, but instead plans to include Mars in the Discovery program. The Discovery program has utility for Mars studies. Discovery is not strategically directed but is competitively selected, a process that has been highly effective at producing affordable, scientifically valuable missions. Examples of potential Mars missions that could be performed in the Discovery program, in no priority order, include the following: • A one-node geophysical pathfinder station, • A polar science orbiter, • A dual satellite atmospheric sounding and/or gravity mapping mission, • An atmospheric sample-collection and Earth return mission, • A Phobos/Deimos surface exploration mission (see Chapter 4), and • An in situ aerial mission to explore the region of the martian atmosphere and remanant magnetic field that is not easily accessible from orbit or from the surface.

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168 VISION AND VOYAGES FOR PLANETARY SCIENCE Summary A combination of missions and technology development activities will advance the scientific study of Mars during the next decade. Such activities include the following: • Flagship missions—The major focus of the next decade should be to initiate the Mars Sample Return campaign. The first and highest-priority element of this campaign is the Mars Astrobiology Explorer-Cacher. • New Frontiers missions—Although the committee looked at both the Mars Geophysical Network and the Mars Polar Climate missions (see Appendixes D and G), due to cost constraints neither was considered a high priority relative to other medium-class missions (see Chapter 9). • Discovery missions—Small spacecraft missions can make important contributions to the study of Mars. • Technology development—The key technologies necessary to accomplish Mars sample return include the following: the Mars ascent vehicle; the rendezvous and capture of the orbiting sample-return container; and the technologies to ensure that planetary protection requirements are met. Continued robust support for the develop- ment of instruments for future in situ exploration is appropriate. • Research support—Vigorous research and analysis programs are needed to enhance the development and payoff of the orbital and surface missions and to refine the sample collection requirements and laboratory analysis techniques needed for Mars sample return. • International cooperation—While Mars sample return could proceed as a NASA-only program, inter- national collaboration will be necessary to make real progress. The 2016 Mars Trace Gas Orbiter mission is an appropriate start to a proposed joint NASA-ESA Mars program. REFERENCES 1 . D.E. Smith, M.T. Zuber, H.V. Frey, J.B. Garvin, J.W. Head, D.O. Muhleman, G.H. Pettengill, R.J. Phillips, S.C. Solomon, H.J. Zwally, W.B. Banerdt, et al. 2001. Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars. Journal of Geophysical Research 106:23689-23722. 2 . M.T. Zuber, S.C. Solomon, R.J. Phillips, D.E. Smith, G.L. Tyler, O. Aharonson, G. Balmino, W.B. Banerdt, J.W. Head, C.L. Johnson, R.G. Lemoine, P.J. McGovern, G.A. Neumann, D.D. Rowlands, and S. Zhong. 2000. Internal structure and early thermal evolution of Mars from Mars Global Surveyor topography and gravity. Science 287:1788-1793. 3 . M.C. Malin and K.S. Edgett. 2001. Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission. Journal of Geophysical Research 106:23429-23570. 4 . P.R. Christensen, J.L. Bandfield, V.E. Hamilton, S.W. Ruff, H.H. Kieffer, T. Titus, M.C. Malin, R.V. Morris, M.D. Lane, R.N. Clark, B.M. Jakosky, et al. 2001a. The Mars Global Surveyor Thermal Emission Spectrometer experiment: Inves- tigation description and surface science results. Journal of Geophysical Research 106:23823-23871. 5 . J.-P. Bibring, Y. Langevin, J.F. Mustard, F. Poulet, R. Arvidson, A. Gendrin, B. Gondet, N. Mangold, P. Pinet, F. Forget, and the OMEGA team. 2006. Global mineralogical and aqueous Mars history derived from OMEGA/Mars Express data. Science 312:400-404, doi:410.1126/science.1122659. 6 . S.L. Murchie, J.F. Mustard, B.L. Ehlmann, R.E. Milliken, J.L. Bishop, N.K. McKeown, E.Z.N. Dobrea, F.P. Seelos, D.L. Buczkowski, and S.M. Wiseman. 2009. A synthesis of martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter. Journal of Geophysical Research 114:E00D06. 7 . W.C. Feldman, W.V. Boynton, R.L. Tokar, T.H. Prettyman, O. Gasnault, S.W. Squyres, R.C. Elphic, D.J. Lawrence, S.L. Lawson, S. Maurice, G.W. McKinney, K.R. Moore, and R.C. Reedy. 2002. Global distribution of neutrons from Mars: Results from Mars Odyssey. Science 297:75-78. 8 . I. Mitrofanov, D. Anfimov, A. Kozyrev, M. Litvak, A. Sanin, V. Tret’yakov, A. Krylov, V. Shvetsov, W. Boynton, C. Shinohara, D. Hamara, and R.S. Saunders. 2002. Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey. Science 297:78-81. 9 . J.E.P. Connerney, M.H. Acuna, P.J. Wasilewski, G. Kletetschka, N.F. Ness, H. Reme, R.P. Lin, and D.L. Mitchell. 2001. The global magnetic field of Mars and implications for crustal evolution. Geophysical Research Letters 28(21):4015-4018. 10 . M.D. Smith. 2004. Interannual variability in TES atmospheric observations of Mars during 1999-2003. Icarus 167:148-165. 11 . R.E. Arvidson, C.C. Allen, D.J. Des Marais, J. Grotzinger, N. Hinners, B. Jakosky, J.F. Mustard, R. Phillips, and C.R. Webster. 2006. Science Analysis of the November 3, 2005 Version of the Draft Mars Exploration Program Plan. Available at http://mepag.jpl.nasa.gov/reports/index.html.

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