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
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
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, atmospheric state, crustal structure||Mars Global Surveyor, Odyssey, Mars Express, Mars Reconnaissance Orbiter|
|Mapped the current distribution of near-surface ice and the morphologic effects of recent liquid water associated with near-surface ice deposits||Odyssey|
|Confirmed the significance of water through mineralogic measurements of surface rocks and soils||Mars Exploration Rovers, Phoenix|
|Demonstrated the diversity of aqueous environments, with major differences in aqueous chemistry, conditions, and processes||Mars Express, Odyssey, Mars Reconnaissance Orbiter, Mars Exploration Rovers|
|Mapped the three-dimensional temperature, water vapor, and aerosol properties of the atmosphere through time; found possible evidence of the presence of methane||Mars Global Surveyor, Odyssey, Mars Express, Mars Reconnaissance Orbiter, and ground-based telescopes|
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 exploration. 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
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 programmatically 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 environment, 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?
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 investigations 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 objective of Mars exploration. They are key to understanding how the planet may have been suited for life and
how 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.
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 processes, 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 characterizing 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
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 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 drainage 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.
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, physicochemical 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
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 processes, 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 spectroscopy 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
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 important 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 critically 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 habitable also cause them to be destructive to biosignature preservation. There are, however, habitable environments 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 favorable 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 Laboratory, 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.
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 distribution 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.
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, elemental, 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,
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 instrumentation, develop techniques for measuring biosignatures under martian conditions, and conduct technological proof-of-concept studies.
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 operated 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 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
• The water cycle—water vapor is transported by the atmosphere between surface reservoirs, allowing the formation of clouds, hazes, and frost.
The atmosphere is also the location of an active photochemistry coupled to these cycles and to the atmospheric dynamics, and it must be taken into account in order to understand the development of habitable near-surface environments. Photochemistry and dynamics are especially vigorous in the upper martian atmosphere (thermosphere and ionosphere), and an understanding of these processes is critical to understanding the loss of Mars’s upper atmosphere to space, which has probably controlled Mars’s long-term climate evolution, and to testing Earth-based theories in meteorology and aeronomy.
Observing and characterizing the present-day climate system are key for understanding the past. Two concepts of past climate (or paleoclimate) must be distinguished for Mars. For modern and transitional (recent) Mars time frames, it appears that the climate was periodically different from what it is today because of the oscillations of Mars’s orbit and rotation parameters. For ancient Mars the observations of the geology and mineralogy of the oldest surfaces provide evidence that there was abundant liquid water or brines on the martian surface either episodically or for extended periods of time.
Characterize Mars’s Atmosphere, Present Climate, and Climate Processes
Under Both Current and Different Orbital Configurations
A multi-year record of the seasonal cycles of water, carbon dioxide, and dust (including episodic hemispheric and global dust events) and of temperature is becoming available.49,50 The record reveals complex interannual variability but is not extensive enough yet to allow the identification of regimes and an understanding of the patterns and the controlling processes. Comparisons of these observations with numerical climate model predictions help to explain some processes but often raise questions. For instance, the reasons for the occurrence of global dust events some years and not others are not yet understood. Recent observations from Mars Global Surveyor, Mars Reconnaissance Orbiter, Mars Express, and Phoenix have also shown puzzling structures in the vertical profiles of airborne dust, unexpected distributions of water vapor, and surprising precipitating ice clouds.51,52
The carbon dioxide cycle itself is more complex than anticipated, with a condensation phase controlled by atmospheric precipitation, subsurface heating, and noncondensable gas enrichment and with a sublimation phase characterized by the formation of high-velocity carbon dioxide vents that erupt sand-size grains in jets able to form spots and control the polar cap albedo.53 The residual carbon dioxide ice cap near the south pole has been found to lie on a water-ice substrate but appears to be only a few meters thick.54,55 This discovery is surprising, because models suggest that this ice cap should either grow thick or disappear on a decadal timescale, unless it is the product of climate variations on such timescales; the discovery is also surprising because the thinness of the ice cap indicates that the readily available carbon dioxide reservoir may be smaller than previously thought.56 The atmospheric circulation has been observed using mostly remote measurements of the temperature at elevations between 0 and 60 km. Recent observations covering the middle martian atmosphere (60 to 130 km) by both Mars Climate Sounder (<90 km) and SPICAM (70 to 130 km, but with a very limited sampling) have revealed a very active dynamic atmosphere.57,58
Among the most striking recent findings on the Mars atmosphere is the report of the detection of methane.59 Its very presence would suggest an active subsurface source, as discussed in this chapter in relation to the goals of life and geology. Reported variations in space and time, still controversial, require a considerable source (even by Earth geologic standards) and the destruction of methane by very efficient chemical processes that must affect methane much more strongly than other known reactive species such as ozone or carbon monoxide.60
In the past decade, new studies based on geomorphology, neutron spectroscopy, radar sounding, and in situ observations, combined with numerical modeling of the global climate, have demonstrated that the same climate system observed today can transport volatiles back and forth between the polar and low-latitude regions in response to orbital and obliquity variations.61,62,63,64 These processes have created at most martian latitudes an array of glacial landforms, including debris-covered icy landforms, polar layered deposits, and a ground-ice mantle extending from the midlatitudes to the poles (Figure 6.4). Major aspects of the climatic processes
involved remain poorly understood. The apparent ages of the icy landforms are difficult to reconcile with or to relate to the polar stratigraphy and specific climate processes. The origin of the latitude-dependent ice mantle is still debated: Is it the remnant of past ice precipitation,65 or does it accumulate due to the diffusion of water vapor into ground pores?66
• What are the processes controlling the variability of the present-day climate? What is the four-dimensional wind structure of the martian atmosphere from the surface boundary layer to the upper atmosphere? What are the primary causes behind the occurrence of global dust events? What are the processes coupling the carbon dioxide, dust, and water cycles?
• What is the distribution of chemical species in the atmosphere, and what are their sources and sinks? Do unexpected short-lived trace gases indicate subsurface activity, or even the presence of life, currently or in the past? What was the role of volcanic gases and aerosols in controlling the atmospheric composition? What is the role of photochemical reactions? Are we missing key chemical or physical processes in our models?
• Is there an observable change in martian climate on the 10- to 1,000-year timescale? If so, what causes it? Which processes control the evolution and stability of the residual carbon dioxide ice cap?
• How do the climate, and especially the water cycle, vary with orbital and obliquity variations? What is the global history of ice on Mars? How and when did the polar layered deposits form? What is the origin of the latitude-dependent ice mantle?
Future Directions for Investigations and Measurements
To address these key questions, a set of investigations that relate to the atmosphere, upper atmosphere, and surface volatiles and that are achievable in the next decade have been identified. These investigations include the detection and mapping of possible trace gases and key isotopes, with the highest sensitivity achievable, as a window into underlying geological and possible biological activity—to be addressed by the ESA-NASA Mars Trace Gas Orbiter now under development. Fundamental advances in our understanding of modern climate would come from a complete determination of the three-dimensional structure of the martian atmosphere, from the surface boundary layer to the exosphere. This determination should be performed globally, ideally by combining measurements of wind, surface pressure, and temperature from landed and orbital payloads. Surface measurements are required in order to complement these measurements and to characterize the boundary layer and monitor accurately the long-term evolution of the atmospheric mass. On a global scale, a network of at least 16 meteorological stations would be ideal, and carrying a capable meteorological payload on all future landed missions to measure surface pressure, temperature, electrical fields, and winds would provide an excellent start to developing such a network.
These investigations should be complemented by the systematic monitoring of the three-dimensional fields of water vapor, clouds, and surface frosts. Isotopic signatures of volatiles (such as heavy water, HDO) should also be monitored to investigate the signature of ancient reservoirs and to study fractionation processes (e.g., cloud microphysics). Finally, research and analysis should continue in order to improve the numerical climate modeling of the key atmospheric processes and to support laboratory research, notably in relation to the properties of carbon dioxide ice and its behavior under martian conditions.
Characterize Mars’s Ancient Climate and Climate Processes
Recent analyses of the geomorphology and surface composition of ancient terrains have confirmed that the early Mars climate system was very different from today’s and that the global environment varied throughout this early period.70,71,72,73 Reconstructing early martian climates remains a challenge. Whether liquid water occurred
episodically or persisted over geologic timescales continues to be debated. The solar luminosity was 25 percent lower in early martian history than it is today, and climate modelers have difficulty understanding how Mars’s atmosphere greenhouse effect could have allowed sustained liquid water and precipitation consistent with the geologic records,74 although volcanic greenhouse gases75 or clouds76 or impact-induced warming77 have been suggested as explanations.
Some important questions concerning Mars’s ancient climate and climate processes include the following:
• What was the nature of the early martian climate? Were the conditions suitable for liquid water episodic or stable on longer timescales? What processes enabled such conditions?
• How and why did the atmosphere evolve? Which processes did and still do control the escape and the outgassing of the atmosphere?
Future Directions for Investigations and Measurements
Major progress in understanding the ancient martian climate can come from determining the rates of escape and outgassing of key species from the martian atmosphere, their variability, and the processes at work. It will also be crucial to investigate the physical and chemical record constraining past climates, particularly regarding the polar layered deposits.78 In order to follow up on scientific results and discoveries from the Phoenix and Mars Reconnaissance Orbiter missions, an in situ analysis of laterally or vertically resolved measurements of grain size, dust content, composition, thickness and extent of layers, elemental and isotopic ratios relevant to age (e.g., deuterium/hydrogen) and astrobiology (CHNOPS) should be performed.
Determining the composition, structure, and history of Mars is fundamental to understanding the planet as a whole, as well as to providing the context for virtually every aspect of the study of conditions of habitability and the potential for the origin and persistence of life.
The committee’s specific objectives for pursuing the geology goal are as follows:
• Determine the nature and evolution of the geologic processes that have created and modified the martian crust over time; and
• Characterize the structure, composition, dynamics, and evolution of Mars’s interior.
Subsequent sections examine each of these objectives in turn, identifying critical questions to be addressed and future investigations and measurements that could provide answers.
Determine the Nature and Evolution of the Geologic Processes
That Have Created and Modified the Martian Crust Over Time
The study of geologic processes will include investigating the formation and modification processes of the major geologic units, constraining the absolute ages of these processes, exploring potential hydrothermal environments, characterizing surface-atmosphere interactions, determining the tectonic history and structure of the crust, determining the present distribution of water on Mars, determining the nature and origin of crustal magnetization, and evaluating the effect of large-scale impacts. Despite our rich knowledge of martian surface properties, many questions remain about the nature of the surface and interior processes. Mars is the object in the solar system most similar to Earth, and insights into its history and evolution will inform our understanding of our planet’s origin and history.
Research over the past decade has resulted in a new integrated understanding of Mars as a dynamic geologic system that has changed significantly over time.79,80,81 In this context martian geologic history can be divided into ancient, transitional, and recent periods, with modern Mars, as observed today, providing insights into past processes and conditions.
A substantial fraction of the exposed terrain of Mars, unlike that of Earth, is inferred to be older than about 3.5 billion to 3.7 billion years (in Mars’s Noachian period). These terrains are represented by topographically high, extensively cratered surfaces that dominate the southern latitudes and provide a unique geologic record of the early stages of planet formation and possibly of the origin of prebiotic chemicals and life. Ancient Mars was marked by the presence of near-surface liquid water, with evidence for standing water, lakes, valley networks, and thick, layered sequences of sedimentary rocks with internal stratification.82,83 Secondary minerals on the surface of Mars, including iron-oxides/hydroxides/oxyhydroxides, hydrous sulfates, carbonates, phyllosilicates, and chlorides have been found by orbiters and surface missions and in martian meteorites. These minerals occur in thick, layered sedimentary units, in the soil, and in cements, veins, and rinds in individual rocks (Figure 6.5). The diversity of these mineral assemblages has been hypothesized to result from significant differences in the chemistry of the waters (brines), pH, and water availability.84,85,86 Whether or not these mineral assemblages display a general temporal trend87 or exhibit more complex relationships remains uncertain.88
In situ exploration has documented mineral assemblages and aqueous processes that are more diverse and complex than those seen from orbital observations. The hematite deposits discovered in Meridiani Planum from orbit89 were found by Opportunity to also include jarosite and other sulfates and a stratigraphic sequence inferred to indicate a sabkha-like environment that underwent wetting and drying cycles and diagenesis.90 The Gusev Crater landing site has revealed complex volcanic rocks, altered materials, carbonates, Fe3+-hydrated sulfates in the soils, halide enrichments, and silica-rich materials thought to have formed in a hydrothermal environment.91 The soil chemistry at the Phoenix landing site includes the presence of perchlorates and an inferred slightly basic pH92 in contrast to the inferred acid-sulfate aqueous systems that may have dominated the wet periods in Meridiani Planum and Gusev Crater.93
The diversity of minerals detected from orbit has increased significantly as the spectrometers have improved dramatically in spatial and spectral resolution. A myriad of hydrated minerals have been discovered, including iron and magnesium clays, chlorite, prehnite, serpentine, kaolinite, potassium mica, opaline, analcime, and magnesium carbonate (Figure 6.6).94,95,96,97,98 Exposures of the mineral serpentine have been identified in a variety of martian outcrops,99 with potential significance for the formation of the reported methane as a product in alteration of olivine to serpentine. Another key discovery is the occurrence of carbonate rocks in Mars’s ancient strata using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard Mars Reconnaissance Orbiter100 and the suite of instruments on the Spirit rover.101 Through most of its history Mars has had very acidic surface chemistry, rarely wet and usually dry. Finding old carbonates informs us that some ancient martian environments could have been less acidic and therefore more conducive to the emergence of life.
Ancient rocks also record processes in the planet’s interior, such as heat transfer from the mantle and magnetism in the core. Radiogenic isotopes in martian meteorites and data from magnetic regions show that Mars differentiated relatively quickly (over a period from 25 million to 100 million years). Hypotheses for how the martian dynamo formed (giant impact, degree-one convection, magma ocean cumulate overturn) have striking similarity to those proposed for the formation of the martian crustal dichotomy.102,103 Massive volcanic domes and escarpments (e.g., Tharsis and Elysium) indicate that large hot spots likely played a significant role in the geologic, tectonic, and thermal evolution of the planet, as well as in the surface history through the release of acidic volatiles to the martian atmosphere,104 the transport of aqueous fluids over immense distances, and the formation of hydrothermal deposits.105 It is likely that the cessation of the magnetic field had a major effect on the evolution of the early martian atmosphere.106,107 Thus, the history of the interior is closely connected to the atmosphere, surface mineralogy, and potential habitability, and the measurement of interior properties, identification of possible mantle phase transformations, and petrological and geochemical studies of martian meteorites would provide crucial constraints on magmatic processes on early Mars.
Recent Mars (post approximately 3.0 billion years) appears to have been less active than the planet was in its earlier history, with substantially reduced global aqueous modification and lowered erosion rates.108 Radiogenic
isotopes in martian basaltic meteorites show that the products of early differentiation may have remained isolated for most of the history of Mars. The young ages of martian meteorites have placed constraints on the igneous history of the planet and also on dynamic models for material transport from Mars to Earth. Orbital observations show clues for volcanic activity in the past hundred million years.
Secondary minerals in martian meteorites show a range of ages from 3.9 billion to 100 million years old, suggesting
that fluids were present in the martian crust for most of Mars’s history.109 Secondary minerals in martian meteorites also record isotopic signatures indicating interaction with the martian atmosphere, and impact glasses have trapped martian atmosphere and possibly regolith material.
We now have global maps of topography,110 mineral distribution,111,112,113,114 and morphology at scales of 6 to 18 meters,115 with local topography and texture at scales of less than 1 meter.116 Maps of gravity and magnetic fields show a thicker crust with isostatic compensation in the south and uncompensated gravity anomalies in the north.117 Orbital data have revealed active or recent processes of impacts, landslides, gully formation, wind, the formation of widespread midlatitude ice deposits, and changing carbon dioxide ice cover. The reported methane in Mars’s atmosphere may be related to active near-surface processes. In the polar regions, layered deposits dominated by ice and sedimentary deposits likely record geologically recent climate change. These results have significantly expanded the known water inventory and demonstrated that the surface and near surface constitute an active, changing environment, with water, particularly in the form of ice, apparently redistributed by climate changes on timescales of tens to hundreds of thousand years.
Some important questions concerning the nature and evolution of the geologic processes that have created and modified the martian crust over time include the following:
• How, when, and why did environments vary through Mars’s history, and were these environments habitable? What was the origin and nature of the diverse sedimentary units and inferred aqueous environments, what are their ages, and how did significant accumulations of layered sediments form? What is the mineralogy of the regolith, and how did it form?
• Are reduced carbon compounds preserved and, if so, in what geologic environments? What is the origin of the reported methane? What is the martian carbon cycle?
• What is the petrogenesis and character of the igneous rocks, how old are they, and what does this tell us about martian crustal and mantle processes and formation of the core? How do martian meteorites relate to the martian surface?
• What is the geologic record of climate change? How do the polar layered deposits and layered sedimentary rocks record the present-day and past climate and the volcanic and orbital history of Mars?
Future Directions for Investigations and Measurements
Key investigations to advance our understanding of geologic processes that have governed Mars’s evolution include understanding the origin and nature of the sedimentary units by applying physical and geochemical models, remote and in situ observations of diverse suites of sedimentary materials on Mars, and laboratory investigations of Mars analog materials to study the formation, transport, and deposition of sedimentary materials by fluvial, aeolian, impact, and mass wasting processes. Major advances will come from the investigation of the petrologic, mineralogic, isotopic, and geochronologic properties of rock suites in returned martian samples, martian meteorites, and Mars analog materials in order to understand environmental conditions and habitability over time; the history and timing of core separation and differentiation; past tectonic processes; Mars’s past and present geophysical properties; the bulk, mantle, and core compositions; and the relationship between martian meteorites and igneous rocks on Mars’s surface. Key investigations are needed for exploring the distribution and source of reduced carbon compounds in the surface and atmosphere. Better characterization of the distribution of carbon dioxide and water on a long-term scale and more detailed examination of the polar layered deposits and layered sedimentary rocks for the record of the present-day and the past climate will help to improve the understanding of volatile budgets and cycles.
Refined criteria need to be developed for selecting sample suites for return to Earth, including sample suites for sensitive analysis of biomarkers (e.g., CHNOPS elements); suites representing diverse sedimentary environments with possible rapid burial; suites showing chemical gradients formed through alteration, oxidation, neutralization, and precipitation; and those from aqueous alteration environments including hydrothermal suites that show potential for preserving biosignatures; igneous suites; regolith samples; and an atmospheric sample. Finally, advances in technologies are needed in order to better collect, handle, curate, analyze, and study martian materials, meteorites, and analog samples on all scales in a range of environmental conditions and in the context of new experimental and theoretical data, and planetary protection guidelines are needed.
Characterize the Structure, Composition, Dynamics, and Evolution of Mars’s Interior
The interior of Mars will be investigated by characterizing the structure and dynamics of the interior, determining the origin and history of the magnetic field, and determining the chemical and thermal evolution of the planet. Unfortunately, there has been little progress made toward a better understanding of the martian interior and the processes that have occurred. Probing the interior is best done through a network of geophysical stations, and such a network has not yet been implemented at Mars.
Some important questions concerning the structure, composition, dynamics, and evolution of Mars’s interior include the following:
• What is the interior structure of Mars? How are core separation and differentiation processes related to the initiation and/or failure of plate tectonic processes on Mars?
• When did these major interior events occur, and how did they affect the magnetic field and internal structure? What is the history of the martian dynamo? What were the major heat-flow mechanisms that operated on early Mars?
• What is Mars’s tectonic, seismic, and volcanic activity today? How, when, and why did the crustal dichotomy form? What is the present lithospheric structure? What are the martian bulk, mantle, and core compositions? How has Mars’s internal structure affected its magmatism, atmosphere, and habitability?
Future Directions for Investigations and Measurements
Major progress in understanding Mars’s interior requires obtaining key geophysical data through a network. 118,119 Seismic data will enhance the understanding of the martian interior structure, including current lithosphere/crust structure and thickness, the current seismic and volcanic activity, the depth of crustal magnetization, the basal structures of the crust under large topographic highs (e.g., Tharsis and Elysium) and lows (e.g., Hellas Basin); they will also place boundary conditions on models of the early thermal profiles, heat flows, and geologic evolution. Also we need to acquire other geophysical data (e.g., heat flow and magnetic sounding) to better constrain the mineralogic, density, and temperature structure of the martian interior.
Connections with Earth and the Terrestrial Planets
Mars is unique in solar system exploration because it has had processes comparable to those of Earth during its formation, interior evolution, surface modification, geochemical alteration, and atmospheric and climate evolution. Crucially, and perhaps uniquely, the martian surface preserves a record of the early solar system history on a planet with water and an atmosphere in which the conditions may have been similar to those on Earth when life originated. This record has been obliterated on Earth during crustal recycling related to plate tectonics. Mars records critical information that can provide a means to approach (and possibly answer) questions about the environmental conditions that may have accompanied the origin and evolution of life, short- and long-term climate change in comparison to that on Earth, and the early evolution and origin of the terrestrial planets.
When and how life began on Earth is not yet known. Evidence for early life on Earth has been reported in rocks at least as old as 3.7 billion years.120 The general processes by which the inventory of the basic building blocks of life was assembled, how those prebiotic components were chemically reorganized, and how replicating life forms originated and evolved all took place during the critical time period before 3.5 billion years ago. However, for the first billion years of Earth’s history, our ability to read the geologic record is either fragmentary or nonexistent. Mars has a number of characteristics that make it the most probable location for prebiotic processes to have occurred and for that record to have been preserved. Mars is in the Sun’s “habitable zone,” it likely had liquid water at some points in the past, and it might have had a thicker atmosphere that protected the prebiotic and biotic material from radiation. Mars today contains the essential ingredients to support and sustain life, and the geologic record shows numerous promising ancient habitable environments.121
The martian atmosphere is a simpler system than that of Earth, but it is also the most Earth-like of all the planetary atmospheres. This Earth-like character provides the opportunity to validate climate and atmospheric circulation models and to test these models of physical systems with different boundary conditions. Mars’s atmosphere has evolved significantly with time. It shows clear evidence for periodic climate change, which, combined
with calculations of the effects of large excursions in orbital parameters, points to significant changes in insolation driving major redistributions of water over the planet in cyclic episodes that are analogous to Earth’s ice ages.
The inferred origins and evolution of the four terrestrial planets are as varied as their surfaces and current environments. Mars is unique in the accessibility and comparative hospitability of its surface relative to Venus and Mercury. Its level of historical thermal and geologic activity, intermediate between the levels of Earth and the Moon, is ideally suitable both for elucidating the initial conditions of the terrestrial planets and for understanding subsequent processes such as accretion, the formation of magma oceans, differentiation, core convection, dynamo generation, partial melting, and volcanism. Mars, the Moon, Venus, and Mercury are linked by similar bombardment histories, and all contain evidence for volcanism, differentiation, and early crustal growth. Of these inner planets, Mars has evidence of an early dynamo process, which is absent on Venus and the Moon. Thus, Mars provides information on the early stages of planet formation and heat-loss mechanisms that are crucial for putting the differentiation history, bulk chemistry, and geophysical properties of all of the inner planets, including Earth, in context.
Connections with Extrasolar Planets
Mars provides a unique alternative to Earth as a leading potential example of a planet that has been habitable in the past, at least episodically. Therefore, it is especially interesting with regard to our understanding of the habitability of extrasolar terrestrial planets and the likeliness of life elsewhere. In particular:
• Mars has a low mass and radius relative to Earth and therefore expands the possible range of silicate extrasolar planet mass and radius values that may be targeted for habitability;
• The potential discovery of habitable regions on Mars forces us to expand the concept of the classic “habitable zone”122 within planetary systems;
• Ancient Mars was probably a case of a relatively dry planet (no global oceans) with an environmental regime fundamentally different from that of our planet, and which we can study through the geologic records; and
• The fact that Mars has lost its ability to sustain liquid water teaches us lessons about possible processes that may prevent many extrasolar planets from remaining habitable.
Connections with Human Exploration
Mars is the only planet in the solar system that is realistically accessible to human exploration; it has been proposed as a target for orbital flybys and future landing by human explorers. To reduce the cost and risk for future human exploration, robotic precursor missions would be needed to acquire information concerning potential resources and hazards, to perform technology and flight system demonstrations, and to deploy infrastructure to support future human exploration activities. The elements of the Mars Sample Return campaign, beginning with the Mars Science Laboratory, will provide crucial data for landing significant mass, executing surface ascent and return to Earth, and identifying potential hazards and resources.123
For the past three decades, the scientific community has consistently advocated the return of geologic samples from Mars. Summaries of the literature on this topic appear in the extensive writings of the National Research Council (NRC),124,125,126,127,128 several major recent reports by MEPAG,129,130,131 and a significant recent contribution by the International Mars Exploration Working Group.132 Numerous white papers submitted to the NRC decadal survey indicated substantial community support by way of signatories and addressed the importance and significance of Mars Sample Return as the keystone of future Mars exploration.133,134,135,136,137,138,139 (See Box 6.3.)
The committee, building on numerous community assessment groups, open discussions, and white papers, places as the highest-priority Mars science goal to address in detail the questions of habitability and the potential
Sample Return Is the Next Step
The analysis of carefully selected and well-documented samples from a well-characterized site will provide the highest science return on investment for understanding Mars in the context of solar system evolution and for addressing the question of whether Mars has ever been an abode of life.
The purpose and context of a Mars sample return has changed significantly since the early concepts that focused solely on reconnaissance. At that time the key questions centered on bulk planetary geochemistry, petrology, and geochronology, for which a wide range of sample types would be acceptable. Today the emphasis is on well-characterized and carefully selected rocks, with the recognition of the critical importance of sedimentary rocks that provide clues to aqueous and environmental conditions. Although it is widely accepted that we have samples of Mars on Earth in the form of the SNC meteorites, these meteorites are not representative of the diversity of Mars. They are all igneous in origin, whereas recent observations have shown the occurrence of chemical sedimentary rocks of aqueous origin as well as igneous, metamorphic, and sedimentary rocks that have been aqueously altered. It is these aqueous and altered materials that will provide the opportunity to study aqueous environments and potential prebiotic chemistry.
Two approaches to the study of martian materials exist—that using in situ measurements and that employing returned samples. The return of samples allows for the analysis of elemental, mineralogic, petrologic, isotopic, and textural information using state-of-the-art instrumentation in multiple laboratories. In addition, it allows for the application of different analytical approaches using technologies that advance over a decade or more and, most importantly, the opportunity to conduct follow-up experiments that are essential in order to validate and corroborate the results. On an in situ mission, only an extremely limited set of experiments can be performed because of the difficulty of miniaturizing state-of-the-art analytical tools within the limited payload capacity of a lander or rover. In addition, these discrete experiments must be selected years in advance of the mission’s launch. Finally, calibrating and validating the results of sophisticated experiments can be challenging in a laboratory and will be significantly more difficult when done remotely. The Viking and the ALH84001 martian meteorite experiences underscore the differences in these approaches. It has proven difficult to reach unique conclusions regarding the existence of possible extant life or organic materials from the Viking data because of the assumptions regarding the nature of the martian surface materials that were necessary in order to design the instrument payload. For example, recent analysis of the Phoenix data suggests that oxidizing compounds may have been present that would have destroyed any organics during the sample heating required for the Viking instruments, raising significant questions about the interpretation of the Viking results. In contrast, the ALH84001 experience underscores the tremendous value of being able to perform a large number of independent analyses and follow-on experiments. Multiple approaches in numerous laboratories have been possible for a decade because the samples were on Earth, and new experiments could be performed to test differing hypotheses. One essential lesson from ALH84001 is how involved sample studies can be, requiring multiple methods, complex sample preparation, and the collective capability of Earth’s research laboratories to evaluate complex questions. Finally, searching for evidence of extant life at Mars with a limited suite of experiments, with that constraint compounded by the uncertainty regarding the nature of possible martian life and issues of terrestrial contamination, would be difficult and carries very high scientific risk.
Discoveries by the MSL could provide additional justification for sample return but are unlikely to alter the basic architecture of sample return, in which the primary system variables—the sample site and the samples that are collected—are not constrained in the proposed architecture. Similarly, a lack of major new discoveries by the MSL would not impact the importance of getting samples back to Earth and might well increase the importance of collecting and studying samples in terrestrial laboratories where a much broader suite of measurements could be obtained.
Experience based on previous studies (e.g., of meteorites, the Moon, cometary dust, and the solar wind) strongly supports the importance of sample analysis. Such a diversity of techniques, analysis over time, improvements in sensitivity, and new approaches available in terrestrial laboratories are expected to revolutionize our understanding of Mars in ways that simply cannot be done in situ or by remote sensing.1,2
Site Selection and Context
The information needed to select a sample return site that would address high-priority science objectives has been, or can be, acquired with current assets.
Mars is a remarkably diverse planet with a wide range of aqueous environments preserved in its rock record. As a result of two decades of orbital and in situ exploration of Mars, a large number of excellent candidate sample return sites, where water played a major role in the surface evolution, have already been identified. Significantly, the geologic setting of these sites—as identified through mineralogic and stratigraphic mapping—indicates that there were major differences in water chemistry and temperature, weathering processes, and sediment transport and deposition processes across Mars, providing a diversity of environments from which to collect samples. The known sites also contain diverse sedimentary and igneous terrains within the roving range of existing spacecraft.
The site will be selected on the basis of compelling evidence in the orbital data for aqueous processes and a geologic context for the environment (e.g., fluvial, lacustrine, or hydrothermal). The sample-collection rover must have the necessary mobility and in situ capability to collect a diverse suite of samples based on stratigraphy, mineralogy, composition, and texture.3,4 Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated because the samples will be studied in detail on Earth.5,6
Selecting and preserving high-quality samples are essential to the success of the sample return effort. MEPAG identified 11 science objectives for Mars Sample Return (MSR) and specified the minimum criteria for a sample to meet these objectives.7,8 The collection of Mars samples will be most valuable if they are collected as sample suites chosen to represent the diverse products of various planetary processes (particularly aqueous processes), and addressing the scientific objectives for MSR will require multiple sample suites. A full program of science investigations is expected to require samples equal to or greater than 8 g for bedrock, loose rocks, and finer-grained regolith, and 2 g to support biohazard testing, each for an optimal size of 10 g.9,10 Textural studies of some rock types might require one or more larger samples of approximately 20 g. The number of samples needed to address the MSR science objectives effectively is 35 (28 rock, 4 regolith, 1 dust, 2 gas). In order to retain scientific value, returned samples must be fully isolated and sealed from the martian atmosphere, each sample must be linked uniquely to its documented field context, and rocks should be protected against fragmentation during transport. The encapsulation of at least some samples must retain any released volatile components.11,12
Technical Implementation and Feasibility
A three-element, step-by-step sample return campaign would reduce scientific, technical, and cost risks. It would build on technologies developed over the past decade of Mars exploration, although major technical challenges remain that must be addressed in a technology development effort that would be an integral part of the sample return campaign.
The proposed strategy would conduct sample return as a campaign with three separate steps:
1. A caching rover, the Mars Astrobiology Explorer-Cacher (MAX-C), followed by—
2. A Mars Sample Return Lander (MSR-L) that would include a rover to fetch the sample cache and an ascent vehicle to loft the cache into orbit for—
3. Rendezvous and return by a Mars Sample Return Orbiter (MSR-O).
This campaign would be scientifically robust, with the flexibility to return to a previously visited site (e.g., if motivated by an MSL discovery), to go to a new site, or to fly a second MAX-C rover if the first mission was unsuccessful for any reason. It would also be technically and programmatically robust, with a modular approach and multiple caches left on the surface by MAX-C to recover from a failure of either the MSR-L or MSR-O elements without requiring a reflight of MAX-C.
The Mars Exploration Program has made significant strides in developing the technologies needed for this multi-element sample return scenario. In particular:
• Mars Pathfinder and the Mars Exploration Rovers (MERs) have demonstrated surface mobility, and the MER has demonstrated much of the basic instrumentation needed to select high-priority samples.
• MER and Phoenix have provided valuable experience in sample handling and surface preparations; the MSL will go significantly farther.
• The MSL Sky Crane entry, descent, and landing system design will support Mars sample return. It can deploy a caching rover and can accommodate the MSR-L with a fetch rover and Mars Ascent Vehicle (MAV).
• Technologies that can be adapted to orbital rendezvous and sample canister capture have been demonstrated in Earth orbit.
• Sample return protocols and Earth-return entry encapsulation have been validated by the Stardust and Genesis missions.
Critical technologies still need to be developed.13 These include sample collection and handling, the Mars Ascent Vehicle, orbital acquisition, and back planetary protection. The MAV in particular is a system with significant development risk, pointing to the need for an early start to perform trade-off studies, retire technology risks, and develop and flight-test a flight-like engineering unit in a relevant environment. Key technology elements for the MSR-O include autonomously actuated mechanisms for orbital capture; optical sensors; orbital radio beacon; autonomous rendezvous guidance, navigation, and control; and ground validation tests.
1 Numerous previous studies have consistently pointed out the important contributions that sample return missions from planetary bodies can make to virtually every area of solar system exploration in general and to Mars exploration in particular. For general discussions of the importance of sample return missions, see the following and references therein: M.J. Drake, W.V. Boynton, and D.P. Blanchard, The case for planetary sample return missions: 1. Origin of the solar system, Eos 68:105, 111-113, 1987; J.L. Gooding, M.H. Carr, and C.P. McKay, The case for planetary sample return missions: 2. History of Mars, Eos 70:745, 754-755, 1989; G. Ryder, P.D. Spudis, and G.J. Taylor, The case for planetary sample return missions: 3. The origin and evolution of the Moon and its environment, Eos 70:1495, 1505-1509, 1989; T.D. Swindle, J.S. Lewis, and L.A. McFadden, The case for planetary sample return missions: 4. Near-Earth asteroids and the history of planetary formation, Eos 72:473, 479-480, 1991; National Research Council, Assessment of Mars Science and Mission Priorities, National Academy Press, Washington, D.C., 2001, pp. 83-88.
origin and evolution of life on Mars. The committee carefully considered the alternative of several rover missions instead of sample return. It is the opinion of the committee that sample return would have significantly higher science return and a much higher science-to-dollar ratio. Thus, a critical next step toward answering these questions would be provided through the analysis of carefully selected samples from geologically diverse and well-characterized sites that are returned to Earth for detailed study. Existing scientific knowledge of Mars makes it possible to select a site from which to collect an excellent suite of rock and soil samples to address the life and habitability questions, and the technology to implement the sample return campaign exists, or will be developed—including required entry, descent, and landing (EDL) and rover mobility systems. Existing and future analysis techniques developed in laboratories around the world will provide the means to perform a wide array of tests on these samples; develop hypotheses for the origin of their chemical, isotopic, and morphologic signatures; and, most importantly, perform follow-up measurements to test and validate the findings.
2 A Mars sample return mission has been an essential component of the Mars exploration strategies advocated by the National Research Council (NRC) for 30 years. For specific discussions, see the following NRC reports: Strategy for the Exploration of the Inner Planets: 1977-1987, National Academy of Sciences, Washington, D.C., 1978; Update to Strategy for Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990; International Cooperation for Mars Exploration and Sample Return, National Academy Press, Washington, D.C., 1990, pp. 1, 3, and 25; An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994; Review of NASA’s Planned Mars Program, National Academy Press, Washington, D.C., 1996, pp. 3, 26, and 29; Assessment of Mars Science and Mission Priorities, National Academy Press, Washington, D.C., 2001, pp. 3, 83-88, and 99-102; New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 85-87; An Astrobiology Strategy for the Exploration of Mars, The National Academies Press, Washington, D.C., 2007, pp. 8-9 and 105-107.
4 L.M. Pratt, C. Allen, A.C. Allwood, A. Anbar, S.K. Atreya, D.W. Beaty, M.H. Carr, A. Crisp, D.J. Des Marais, J.A. Grant, D.P. Glavin, et al. 2009. Mars Astrobiology Explorer-Cacher (MAX-C): A Potential Rover Mission for 2018. Final report from the Mid-Range Rover Science Analysis Group (MRR-SAG). Posted by the Mars Exploration Program Analysis Group (MEPAG). Available at http://mepag.jpl.nasa.gov/reports/.
6 L.M. Pratt, C. Allen, A.C. Allwood, A. Anbar, S.K. Atreya, D.W. Beaty, M.H. Carr, A. Crisp, D.J. Des Marais, J.A. Grant, D.P. Glavin, et al. 2009. Mars Astrobiology Explorer-Cacher (MAX-C): A Potential Rover Mission for 2018. Final report from the Mid-Range Rover Science Analysis Group (MRR-SAG). Posted by the Mars Exploration Program Analysis Group (MEPAG). Available at http://mepag.jpl.nasa.gov/reports/.
8 L.E. Borg, D.J. Des Marais, D.W. Beaty, O. Aharonson, S.A. Benner, D.D. Bogard, J.C. Bridges, C.J. Budney, W.M. Calvin, B.C. Clark, J.L. Eigenbrode, et al. 2008. Science priorities for Mars sample return. Astrobiology 8:489-535.
10 L.E. Borg, D.J. Des Marais, D.W. Beaty, O. Aharonson, S.A. Benner, D.D. Bogard, J.C. Bridges, C.J. Budney, W.M. Calvin, B.C. Clark, J.L. Eigenbrode, et al. 2008. Science priorities for Mars sample return. Astrobiology 8:489-535.
12 L.E. Borg, D.J. Des Marais, D.W. Beaty, O. Aharonson, S.A. Benner, D.D. Bogard, J.C. Bridges, C.J. Budney, W.M. Calvin, B.C. Clark, J.L. Eigenbrode, et al. 2008. Science priorities for Mars sample return. Astrobiology 8:489-535.
13 See, for example, the S. Hayati, Strategic Technology Development for Future Mars Missions, white paper submittedto the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
Perhaps the greatest driving force for planning and funding is the return of martian samples. Planning should begin on the requirements and needs for a facility to curate and analyze these unique samples and to preserve them in a Mars-like environment to prevent alteration.
Martian samples require screening for evidence of life and for biohazards, possibly necessitating robotic handling, temperature and atmosphere control, and strict biological isolation. They will also require special procedures beyond those in typical biosafety facilities. For example, most biosafety facilities maintain negative pressure,
driving outside air into the facility and only controlling the air exiting the facility. In the case of Mars samples, the outside air must also be carefully controlled to prevent contamination. No existing sample-handling facility currently meets the biosafety and environmental controls required for martian samples.140
The possibility of detecting life in martian samples and the attendant risk of terrestrial contamination require the preparation of extensive new analysis facilities, which will require a major planning and implementation process. Instruments will need to be sterile and isolated. Some instruments may also need environmental controls, particularly including temperature, and monitoring for gas release. Major instrumentation may need to include mass spectrometers, electron microscopes, and microprobes. Significant planning is required, along with updates to the NRC’s 2002 report The Quarantine and Certification of Martian Samples,141 with specific attention to facility and handling recommendations.
As Mars exploration moves toward sample return, surface networks, and sophisticated in situ analysis, it will require a suite of technology development efforts, primarily focused in the areas of sample acquisition and handling, Mars ascent, and orbital rendezvous. Improvements in instrumentation, ground-based infrastructure, and data analysis are also critical to the long-term success of the Mars exploration program. The highest-priority recommendations for the coming decade for Mars sample return are sample acquisition and processing technology funding to support the Mars Astrobiology Explorer-Cacher (MAX-C) mission, and sufficient technology development funding for the Mars Ascent Vehicle (MAV). Future technology development should focus on the Earth Entry Vehicle (EEV) and sample containment. No technology development is required for the 2016 Mars Trace Gas Orbiter mission. MAX-C will rely heavily on existing EDL technology and derivatives of existing remote sensing and contact instrumentation. The necessary investments in technology for MAX-C should focus on the continued development of tools to acquire and cache samples effectively and on the development and demonstration of high technology readiness level (TRL) sample selection instruments. (See Box 6.3.)
The Mars Ascent Vehicle, as part of the MSR-L element, is the greatest technology challenge for this decadal period. It must survive both the landing shock and prolonged exposure to the martian surface thermal environment. The risk of mass and cost growth must be mitigated through an early test program because of the currently low TRL of the MAV. Technology development for this element of the MSR-L mission (which is under consideration for the following decade) should begin in this decade.
The MAX-C rover may require improvement in the entry, descent, and landing precision—a landing ellipse semi-major axis reduction from 10 km to 6 to 7 km can be accomplished with more accurate inertial-unit handover at separation, and the use of a range rather than a velocity trigger for the deployment of the parachute. A straightforward approach must be developed for containing and efficiently transferring the samples from the sample-acquisition device to the storage medium and for effectively sealing the storage medium for planetary protection. The rover may require improvement in the onboard operations avionics to enable faster traverse mobility, in addition to the automation of target approach, measurement, and sample acquisition. If required, this technology development should begin immediately.
The Mars Sample Return Orbiter (MSR-O) element (under consideration for the following decade), which includes the flight of the EEV, requires the development of optical sensors, autonomous rendezvous guidance, and radio beacons for rendezvous with the orbiting sample container. Technology development for the MSR-O would need to begin in fiscal year (FY) 2017 to support an FY2022 launch. All of the component technologies for the EEV are available today, including the carbon phenolic heat-shield material that meets the planetary protection requirements. However, the EEV requires a rigorous ground-based test program and a systems-validation flight test to ensure sufficiently high reliability. This technology development would need to begin in FY2015 to support an FY2022 launch.
Science instruments for future missions will require increased funding beyond that provided by current levels of programs to mature instrumentation to TRL 6. In addition to the near-term development of miniaturized instruments, such as Raman, infrared, and elemental spectrometers, needed for the selection of samples for caching by MAX-C, the long-term development of instruments for follow-on in situ science should be supported, focusing on the most important future in situ measurements. Examples include isotopic characterization of a variety of biomarkers, identification of organic materials indicative of current or past biological systems, sensitive life-detection experiments, analysis of metastable minerals and organic compounds, and in situ geochronology experiments.
The addition of a relay payload with standardized protocols to each science orbiter provides an extremely cost-effective means for establishing a Mars orbiter relay network. Maintaining redundant relay assets whenever relay services are required is a goal, and the key to achieving this goal is attaining a long operational lifetime for each orbital relay asset. NASA’s Deep Space Network remains a critical part of the Mars exploration infrastructure. The continued development of onboard data-processing methods can alleviate mission bandwidth constraints for near-term missions. In the longer term both orbital and landed missions will greatly benefit from optical communication technologies.
Sample Curation and Laboratory Facilities
Sample return missions are unique in that they require a well-developed infrastructure and capabilities for the appropriate curation and analyses of the returned materials. Dedicated curation laboratories must be designed and constructed before samples are returned. Special requirements for the long-term preservation of ices, atmospheric samples, volatiles, and metastable materials are required, as are screening for life and biohazards in a dedicated sample-receiving facility. There is need for the continued development of advanced sample-processing and sample-preparation techniques. The recommendations of the NRC’s 2002 report The Quarantine and Certification of Martian Samples142 may need to be examined and updated as necessary based on the current plans for the nature and quantity of the returned samples.
Supporting Laboratory and Theoretical Studies
Relevant laboratory studies have in the past been deferred, due largely to their expense, but they are essential for supporting sample return. The development and maintenance of spectral reference libraries for atmospheric and surface composition studies, in ultraviolet, visible, infrared, and microwave spectral ranges, need to be undertaken. Materials must be measured at the appropriate temperatures, pressures, particle sizes, wavelength ranges, and viewing geometries for applicability to spacecraft observations of the martian surface and atmosphere. Theoretical model developments must also proceed in order to be able to link quantitatively flight and laboratory-based data sets. Laboratory studies are also needed to help determine the survival of organics under martian surface conditions. Support is required for basic laboratory research with potential in situ instrument development even at laboratory scale. Increased collaboration with the National Science Foundation, the National Institutes of Health, and other institutions addressing similar scientific and technological challenges related to microbial life at low temperatures will enhance this work.
Earth-based telescopic observations have been important for understanding the current and past conditions for the martian atmosphere and surface. For example, the reported detection of methane in the atmosphere has been a critical factor that has helped shape the plans for new orbital measurements. These observation types should continue and evolve to support spacecraft observations.
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 mission, 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 advancing 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.
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
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 mission 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.
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 developments 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.
New Frontiers Missions
Mars Geophysical Network
High-priority Mars science goals can be addressed by a New Frontiers-class geophysical network. The prioritized 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 compositional 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 instrumentation 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.
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.
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 development 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, international 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.
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