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6
Mars: Evolution of an Earth-Like World
Mars has a unique place in solar system exploration: it holds keys to many compelling planetary science ques-
tions, and it is accessible enough to allow rapid, systematic exploration to address and answer these questions.
The science objectives for Mars center on understanding the evolution of the planet as a system, focusing on the
interplay between the tectonic and climatic cycles and the implications for habitability and life. These objectives
are well aligned with the broad crosscutting themes of solar system exploration articulated in Chapter 3.
Mars presents an excellent opportunity to investigate the major question of habitability and life in the solar
system. Conditions on Mars, particularly early in its history, are thought to have been conducive to the formation
of prebiotic compounds and potentially to the origin and continued evolution of life. Mars has also experienced
major changes in surface conditions—driven by its thermal evolution and its orbital evolution and by changes in
solar input and greenhouse gases—that have produced a wide range of environments. Of critical significance is
the excellent preservation of the geologic record of early Mars, and thus the potential for evidence of prebiotic and
biotic processes and how they relate to the evolution of the planet as a system. This crucial early period is when life
began on Earth, an epoch largely lost on our own planet. Thus, Mars provides the opportunity to address questions
about how and whether life arose elsewhere in the solar system, about planetary evolution processes, and about
the potential coupling between biological and geological history. Progress on these questions, important to both
the science community and the public, can be made more readily at Mars than anywhere else in the solar system.
The spacecraft exploration of Mars began in 1965 with an exploration strategy of flybys, followed by
orbiters, landers, and rovers with kilometers of mobility. This systematic investigation has produced a detailed
knowledge of the planet’s character, including global measurements of topography, geologic structure and pro-
cesses, surface mineralogy and elemental composition, the near-surface distribution of water, the intrinsic and
remanant magnetic field, gravity field and crustal structure, and the atmospheric composition and time-varying
state (Figure 6.1).1,2,3,4,5,6,7,8,9,10 The orbital surveys framed the initial hypotheses and questions and identified the
locations where in situ exploration could test them. The surface missions—the Viking landers, Pathfinder, Phoenix,
and the Mars Exploration Rovers—have acquired detailed information on surface morphology, stratigraphy, min-
eralogy, composition, and atmosphere-surface dynamics and confirmed what was strongly suspected from orbital
data: Mars has a long and varied history during which water has played a major role.
A new phase of exploration began with the Mars Express and the Mars Reconnaissance Orbiter (MRO),
which carry improved instrumentation to pursue the questions raised in the earlier cycles of exploration. Among
the discoveries (Table 6.1) is the realization that Mars is a remarkably diverse planet with a wide range of aque-
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138 VISION AND VOYAGES FOR PLANETARY SCIENCE
FIGURE 6.1 Examples of global data sets highlight major accomplishments from multiple recent missions. SOURCE: P.R.
Christensen, N.S. Gorelick, G.L. Mehall, and K.C. Murray, THEMIS Public Data Releases, Planetary Data System node,
Arizona State University, available at http://themis-data.asu.edu.
ous environments (Figure 6.2). The role of water and the habitability of the ancient environment will be further
investigated by the Mars Science Laboratory (MSL), scheduled for launch in the latter part of 2011, which will
carry the most advanced suite of instrumentation ever landed on the surface of a planetary object (Box 6.1).
The program of Mars exploration over the past 15 years has provided a framework for systematic exploration,
allowing hypotheses to be formulated and tested and new discoveries to be pursued rapidly and effectively with
follow-up observations. In addition, the program has produced missions that support one another both scientifically
and through infrastructure, with orbital reconnaissance and site selection, data relay, and critical event coverage
significantly enhancing the quality of the in situ missions.11,12,13 Finally, this program has allowed the Mars science
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MARS: EVOLUTION OF AN EARTH-LIKE WORLD
TABLE 6.1 Major Accomplishments of Studies of Mars in the Past Decade
Major Accomplishment Mission and/or Technique
Provided global mapping of surface composition, topography, remanant magnetism, Mars Global Surveyor, Odyssey, Mars
atmospheric state, crustal structure Express, Mars Reconnaissance Orbiter
Mapped the current distribution of near-surface ice and the morphologic effects of Odyssey
recent liquid water associated with near-surface ice deposits
Confirmed the significance of water through mineralogic measurements of surface Mars Exploration Rovers, Phoenix
rocks and soils
Demonstrated the diversity of aqueous environments, with major differences in Mars Express, Odyssey, Mars
aqueous chemistry, conditions, and processes Reconnaissance Orbiter, Mars
Exploration Rovers
Mapped the three-dimensional temperature, water vapor, and aerosol properties of Mars Global Surveyor, Odyssey, Mars
the atmosphere through time; found possible evidence of the presence of methane Express, Mars Reconnaissance Orbiter,
and ground-based telescopes
FIGURE 6.2 Examples of the diversity of Mars’s environments and their mineralogy and morphology. SOURCE: Adapted
from S. Murchie, A. McEwen, P. Christensen, J. Mustard, and J.-P. Bibring, Discovery of Diverse Martian Aqueous Deposits
from Orbital Remote Sensing, presentation from the Curation and Analysis Planning Team for Extraterrestrial Materials
Workshop on Ground Truth from Mars, Science Payoff from a Sample Return Mission, April 21-23, 2008, Albuquerque, New
Mexico, available at http://www.lpi.usra.edu/captem/msr2008/presentations/.
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140 VISION AND VOYAGES FOR PLANETARY SCIENCE
BOX 6.1
Mars Science Laboratory
Scheduled to launch in the fall of 2011, the Mars Science Laboratory (MSL) is an advanced rover
designed to follow Spirit and Opportunity—the highly successful Mars Exploration Rovers. The primary
focus of the MSL is on assessing the habitability of geochemical environments, identified from orbit, in
which water-rock interactions have occurred and the preservation of biosignatures is possible. The MSL,
weighing nearly a metric ton, carries a sophisticated suite of instruments for remote and in situ rock
and soil analysis, including x-ray diffraction, high-precision mass spectroscopy, laser-induced breakdown
spectroscopy, and alpha-proton x-ray spectroscopy, and a suite of cameras including microscopic imaging
at 10-micron resolution. This analysis suite will provide detailed mineralogy and elemental composition,
including the ability to assess light elements such as carbon, hydrogen, and oxygen and their isotopes. The
mission will also demonstrate the MSL’s Sky Crane precision entry, descent, and landing system, long-term
surface operations, and long-range mobility.
community to construct a logical series of missions each of which is modest in scope and systematically advances
our scientific understanding of Mars.
Over the past decade the Mars science community, as represented by the Mars Exploration Program Analysis
Group (MEPAG), has formulated three major science themes that pertain to understanding Mars as a planetary
system:
• Life—Understand the potential for life elsewhere in the universe;
• Climate—Characterize the present and past climate and climate processes; and
• Geology—Understand the geologic processes affecting Mars’s interior, crust, and surface.
From these themes, MEPAG has derived key, overarching science questions that drive future Mars explora-
tion. These include the following:
• What are the nature, ages, and origin of the diverse suite of geologic units and aqueous environments
evident from orbital and landed data, and were any of them habitable?
• How, when, and why did environments vary through Mars history, and did any of them host life or its
precursors?
• What are the inventory and dynamics of carbon compounds and trace gases in the atmosphere and surface,
and what are the processes that govern their origin, evolution, and fate?
• What is the present climate and how has it evolved on timescales of 10 million years, 100 million years,
and 1 billion years?
• What are the internal structure and dynamics, and how have these evolved over time?
The next decade holds great promise for Mars exploration. The MSL rover (see Box 6.1) will significantly
advance our knowledge of surface mineralogy and chemistry at a site specifically selected to provide insight into
aqueous processes. The MAVEN mission currently in development and the European Space Agency (ESA)-NASA
Mars Trace Gas Orbiter (TGO) will provide major new insights into the state and evolution of the Mars atmosphere.
Following these missions, the highest-priority science goal will be to address in detail the questions of habitability
and the potential origin and evolution of life on Mars.
The major focus of the next decade will be to initiate a Mars Sample Return (MSR) campaign, beginning with
a rover mission to collect and cache samples, followed by missions to retrieve these samples and return them to
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MARS: EVOLUTION OF AN EARTH-LIKE WORLD
Earth. It is widely accepted within the Mars science community that the highest science return on investment for
understanding Mars as a planetary system will result from analysis of samples carefully selected from sites that have
the highest scientific potential and that are returned to Earth for intensive study using advanced analytical techniques.
These samples can be collected and returned to Earth in a sequence of three missions that collect them, place
them into Mars orbit, and return them to Earth. This modular approach is scientifically, technically, and pro-
grammatically robust, with each mission possessing a small number of discrete engineering challenges and with
multiple sample caches providing resiliency against any failure of subsequent elements. This modular approach
also allows the sample return campaign to proceed at a pace determined by prioritization within the solar system
objectives and by available funding. The study of Mars as an integrated system is so scientifically compelling that
it will continue well beyond the coming decade, with future missions implementing geophysical and atmospheric
networks, providing in situ studies of diverse sites, and bringing to Earth additional sample returns that build on
the coming decade’s discoveries.
All three of the committee’s crosscutting themes for the exploration of the solar system include Mars, and
studying Mars is vital to answering a number of the priority questions in each of them. The building new worlds
theme includes the question, What governed the accretion, supply of water, chemistry, and internal differentiation
of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles
play? Mars is central to the planetary habitats theme, which also includes two questions that are key components
of the scientific exploration of Mars—What were the primordial sources of organic matter, and where does
organic synthesis continue today? and, Beyond Earth, are there modern habitats elsewhere in the solar system with
necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now?
The workings of solar systems theme includes the question, Can understanding the roles of physics, chemistry,
geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate
change on Earth? Mars has transitioned from having an early, warm, wet environment to its current state as a cold,
dry planet with a thin atmosphere; the study of Mars’s climate can shed light on the evolution, and perhaps future,
of Earth’s own climate. The planet most like Earth in terms of its atmosphere, climate, geology, and surface envi-
ronment, Mars plays a central role in the broad question, How have the myriad chemical and physical processes
that shaped the solar system operated, interacted, and evolved over time?
SCIENCE GOALS FOR THE STUDY OF MARS
The Mars science community, through MEPAG, has worked to establish consensus priorities for the future
scientific exploration of Mars.14,15,16,17 One overarching theme is to understand whether life arose in the past and
persisted to the present within the context of a differentiated rocky planet (deep interior, crust, and atmosphere)
that has been strongly influenced by its interior evolution, solar evolution, and orbital dynamics. Parallel inves-
tigations among multiple disciplines are required to understand how habitable environments and life might have
developed on a dynamic planet where materials and processes have been closely coupled. The Mars science goals
embrace this approach by articulating an interdisciplinary research program that drives a multi-decadal campaign
of Mars missions. These goals include multiple objectives that embody the strategies and milestones needed to
understand an early wet Mars, a transitional Mars, and the more recent and modern frozen, dry Mars. Ultimately
these efforts will create a context of knowledge for understanding whether martian environments ever sustained
habitable conditions and life.
Building on the work of MEPAG, the committee has established three high-priority science goals for the
exploration of Mars in the coming decade:
• Determine if life ever arose on Mars—Does life exist, or did it exist, elsewhere in the universe? This is
perhaps one of the most compelling questions in science, and Mars is the most promising and accessible place to
begin the search. If answered affirmatively, it will be important to know where and for how long life evolved, and
how the development of life relates to the planet’s evolution.
• Understand the processes and history of climate—Climate and atmospheric studies remain a major objec-
tive of Mars exploration. They are key to understanding how the planet may have been suited for life and how
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142 VISION AND VOYAGES FOR PLANETARY SCIENCE
major parts of the surface have been shaped. In addition, studying the atmosphere of Mars and the evolution of
its climate at various timescales is directly relevant to our understanding of the past, present, and future climate
of Earth. Finally, characterizing the environment of Mars is also necessary for the safe implementation of future
robotic and human spacecraft missions.
• Determine the evolution of the surface and interior—Insight into the composition, structure, and history of
Mars is fundamental to understanding the solar system as a whole, as well as to providing context for the history
and processes of Earth. Geological and geophysical investigations will shed light on critical environmental aspects
such as heat flow, loss of a global magnetic field, pathways of water-rock interaction, and sources and cycling of
volatiles including water and carbon species (e.g., carbon dioxide and hydrocarbons). In contrast to Earth, Mars
appears to have a rich and accessible geologic record of the igneous, sedimentary, and cratering processes that
occurred during the early history of the solar system. Geophysical measurements of Mars’s interior structure and
heat flow, together with detailed mineralogic, elemental, and isotopic data from a diverse suite of martian geologic
samples, are essential for determining the chemical and physical processes that have operated through time on this
evolving, Earth-like planet.
Subsequent sections examine each of these goals in turn.
DETERMINE IF LIFE EVER AROSE ON MARS
The prime focus of the first high-priority goal for the exploration of Mars in the coming decade is to determine
if life is or was present on Mars. If life is or was there, we must understand the resources that support or supported
it. If life never existed yet conditions appear to have been suitable for the formation and/or maintenance of life,
a focus would then be to understand why life did not originate. A comprehensive conclusion about the question
of life on Mars will necessitate understanding the planetary evolution of Mars and whether Mars is or could have
been habitable, using multidisciplinary scientific exploration at scales ranging from planetary to microscopic. The
strategy adopted to pursue this goal has two sequential science steps: (1) assess the habitability of Mars on an
environment-by-environment basis using global remote sensing observations and (2) then test for prebiotic pro-
cesses, past life, or present life in environments that can be shown to have high potential for habitability. A critical
means of achieving both objectives is to characterize martian carbon chemistry and carbon cycling.
Therefore, the committee’s specific objectives for pursuing the life goal are as follows:
• Assess the past and present habitability of Mars,
• Assess whether life is or was present on Mars in its geochemical context, and
• Characterize carbon cycling and prebiotic chemistry.
Subsequent sections examine each of these objectives in turn, identifying critical questions to be addressed
and future investigations and measurements that could provide answers.
Assess the Past and Present Habitability of Mars
Understanding whether a past or present environment on Mars could sustain life will include establishing the
distribution of water, its geologic history, and the processes that control its distribution; identifying and character-
izing phases containing carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS); and determining
the available energy sources.
Recent exploration has confirmed that the surface of Mars today is cold, dry, chemically oxidizing, and
exposed to intense solar ultraviolet radiation. These factors probably limit or even prohibit any life near the surface,
although liquid water might occur episodically near the surface as dense brines in association with melting ice. 18
The subsurface of Mars appears to be more hospitable than its surface. With mean annual surface temperatures
close to 215 K at the equator and 160 K at the poles, a thick cryosphere could extend to a depth of several kilometers.
Hydrothermal activity is likely in past or present volcanic areas, and even the background geothermal heat flux could
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MARS: EVOLUTION OF AN EARTH-LIKE WORLD
drive water to the surface. At depths below a few kilometers, warmer temperatures would sustain liquid water in pore
spaces, and a deep-subsurface biosphere is possible provided that nutrients are accessible and water can circulate. 19
Biotic and abiotic pathways for the formation of complex organic molecules require an electron donor closely
coupled to carbon in a form suitable to serving as an electron acceptor. On Mars, igneous minerals containing ferrous
iron and/or partially reduced sulfur (e.g., olivine and pyrrhotite) are potential electron acceptors for reduction of
carbon. The report of methane in the martian atmosphere contends that an active source is required to balance its
destruction (its photochemical lifetime is less than 300 years).20 Any sources would likely reside in the subsurface
and might include volcanic emissions, low-temperature rock-water reactions, microorganisms, or gas from the
thermal degradation of organic matter.
Climate changes in the recent geologic past might have allowed habitable conditions to arise episodically in
near-surface environments. For example, Mars undergoes large changes in its obliquity (i.e., the tilt of its polar
axis). At present the obliquity ranges from 23° to 27°, with values as high as 46° during the past 10 million years. 21
At these higher obliquities, the water content of the atmosphere is likely higher, ground ice is stable closer to the
equator, and surface ice may be transferred from the poles to lower latitudes. 22,23
Past Habitable Environments and Life
Recent observations confirm that conditions in the distant past were probably very different from present
conditions, with wetter and warmer conditions prior to about 3.5 billion years ago (the oldest definitive evidence
of life on Earth is at least 3.7 billion years old). This evidence includes valley networks with relatively high drain-
age densities, evaporites and groundwater fluctuations,24,25 clay minerals, hydrothermally altered rocks, deltas,
and large inferred surface erosion rates (Figure 6.3).26,27,28 Early Mars also witnessed extensive volcanism and
high impact rates. The formation of large impact basins likely developed hydrothermal systems and hot springs
that might have sustained locally habitable environments.29,30,31
Since approximately 3.5 billion years ago, rates of weathering and erosion appear to have been very low, and
the most characteristic fluvial features are outflow channels formed by the catastrophic release of near-surface
water.32 Groundwater is likely to be stable at greater depths, and it might sustain habitable environments. In all
epochs, the combination of volcanism and water-rich conditions might have sustained hydrothermal systems in
which life could have thrived.
Important Questions
Some important questions concerning the past and present habitability of Mars include the following:
• Which accessible sites on Mars offer the greatest potential for having supported life in the past? How did
the major factors that determine habitability—the duration and activity of liquid water, energy availability, physico-
chemical factors (temperature, pH, oxidation-reduction potential, fluid chemistry), and the availability of biogenic
elements—vary among environments, and how did they influence the habitability of different sites?
• Which accessible sites favor the preservation of any evidence of past habitable environments and life?
How did the major factors that affect the preservation of such evidence—for example, aqueous sedimentation and
mineralization, oxidation, and radiation—vary among these sites?
• How have the factors and processes that give rise to habitable conditions at planetary and local scales
changed over the long term in concert with planetary and stellar evolution?
Future Directions for Investigations and Measurements
Central to addressing habitability-related questions is searching for future landing sites that have high potential
for both habitability and the preservation of biosignatures (Box 6.2). The key here is identifying accessible rocks
that show evidence of formation in aqueous environments such as fluvial, lacustrine, or hydrothermal systems. 33,34
An additional requirement is to be able to place the rock exposures in a stratigraphic framework that will allow a
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144 VISION AND VOYAGES FOR PLANETARY SCIENCE
FIGURE 6.3 Diverse mineralogy, observed with Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data,
formed by water-related processes and indicative of potentially habitable environments. SOURCE: B.L. Ehlmann and J.F.
Mustard, Stratigraphy of the Nili Fossae and the Jezero Crater Watershed: A Reference Section for the Martian Clay Cycle,
presentation at the First International Conference on Mars Sedimentology and Stratigraphy, April 19-21, 2010, El Paso, Texas,
#6064, Lunar and Planetary Science Conference 2010. Lunar and Planetary Institute.
reconstruction of past environmental conditions.35 Another key aspect in understanding present and past habitability
is to characterize the current geologic activity of the martian interior. The long-term evolution of geologic pro-
cesses, habitable environments, and life on Earth have been closely linked. Accordingly, geophysical observations
that contribute to our understanding of the martian interior are important to the search for signs of martian life.
Ultimately, our best understanding of present and past habitability will await the return to Earth of carefully
selected samples from sites that have the highest science potential for analysis in terrestrial laboratories. Analyses
of returned samples in Earth-based laboratories are essential in order to establish the highest confidence in any
potential martian biosignatures and to interpret fully the habitable environments in which they were formed and
preserved.36,37,38,39,40
Key technological developments for surface exploration and sampling include modest-size rovers capable of
selecting samples and documenting their context. These rovers should include imaging and remote sensing spec-
troscopy adequate to establish local geologic context and to identify targets. Suggested capabilities include surface
abrasion tool(s), arm-mounted sensors, and a rock core caching system to collect suites of samples that meet the
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MARS: EVOLUTION OF AN EARTH-LIKE WORLD
BOX 6.2
Biosignatures
Life can be defined as essentially a self-sustaining system capable of evolution. To guide the search for
signs of life on Mars, however, requires a working concept of life that helps to identify its key characteristics
and its environmental requirements. Biosignatures are features that can be unambiguously interpreted
as evidence of life and so provide the means to address fundamental questions about the origins and
evolution of life. Types of biosignatures include morphologies (e.g., cells, and plant or animal remnants),
sedimentary fabrics (e.g., laminations formed by biofilms), organic molecules, biominerals (e.g., certain
forms of magnetite),1 elemental abundances, and stable isotopic patterns. Because some biosignatures
are preserved over geologic timescales and in environments that are no longer habitable, they are impor-
tant targets of exploration. It is not unreasonable to anticipate that any martian life might differ significantly
from life on Earth, although Earth’s environments have been more similar to those on Mars than to the
environments of any other object in the solar system. Moreover, Mars and Earth may have exchanged life
forms through impact ejecta. Any martian life may reasonably be assumed to have shared at least some
of its basic attributes with life as we know it, which implies that any martian life also requires liquid water,
carbon-based chemistry, and electron transfer processes.2,3
Our working concept of life should also identify environmental conditions that are most conducive
to life. A habitable environment must sustain liquid water at least intermittently and must also allow key
biological molecules to survive. The elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur
must be available, because they are essential for forming the covalently bonded compounds utilized by
all known life. Organic compounds are therefore key targets, with the caveat that martian and earthly life
might have employed different compounds. Energy drives metabolism and motility and must be available
from, for example, light or energy-yielding chemical reactions.4 Finally, the rates of environmental changes
must not exceed rates at which life could adapt.5
Even if habitable environments supported the origination and evolution of life on Mars, the right set of
environmental conditions would be required in order to preserve biosignatures. The study of fossilization
processes will be as important for Mars as it has been for Earth.6 The preservation of biosignatures is criti-
cally sensitive to the diagenetic processes that control preservation; paradoxically, the very characteristics
(water; gradients in heat, chemicals, and light; and oxidant supply) that make so many environments habit-
able also cause them to be destructive to biosignature preservation. There are, however, habitable environ-
ments with geochemical conditions favoring very early mineralization that facilitate spectacular preservation.
Authigenic silica, phosphate, clay, sulfate, and, less commonly, carbonate precipitation are all known to
promote biosignature preservation.7 The search for environments that have been both habitable and favor-
able for preservation can be optimized by pursuing an exploration strategy that focuses on the search for
“windows of preservation,” remembering that Mars may indeed have its own uniquely favorable conditions.
1 R.E. Kopp and J.L. Kirshvink. 2008. The identification and biogeochemical interpretation of fossil magnetotactic
bacteria, Earth Science Reviews 86:42-61.
2 For a detailed discussion of these assumptions see, for example, National Research Council, An Astrobiology
Strategy for the Exploration of Mars, The National Academies Press, Washington, D.C., 2007.
3 For a discussion of the possibilities opened by relaxing some of these assumptions see, for example, National
Research Council, The Limits of Organic Life in Planetary Systems, The National Academies Press, Washington, D.C.,
2007.
4 T.M. Hoehler. 2007. An energy balance concept for habitability, Astrobiology 7:824-838.
5 D.J. Des Marais, B.M. Jakosky, and B.M. Hynek. 2008. Astrobiological implications of Mars surface composition
and properties, pp. 599-623 in The Martian Surface: Composition, Mineralogy and Physical Properties (J.F. Bell III, ed.),
Cambridge University Press, Cambridge, U.K.
6 J.P. Grotzinger. 2009. Mars exploration, comparative planetary history, and the promise of Mars Science Labora-
tory, Nature Geoscience 2:1-3.
7 J.D. Farmer and D.J. Des Marais. 1999. Exploring for a record of ancient Martian life, Journal of Geophysical
Research 103:26977-26995.
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146 VISION AND VOYAGES FOR PLANETARY SCIENCE
appropriate standards.41,42 The in situ measurements used to select samples for return to Earth must go beyond
identifying locations where liquid water has occurred.43,44 They should also characterize the macroscopic and
microscopic fabrics of sedimentary materials, be capable of detecting organic molecules, reconstruct the history of
mineral formation as an indicator of preservation potential and geochemical environments, and determine specific
mineral and chemical compositions as indicators of organic matter or coupled redox reactions characteristic of life.
Also essential to a better understanding of the geochemistry of martian environments and the compositional and
morphologic signatures that these different environments produce is the continuation of a robust research and analysis
(R&A) program. Theoretical, laboratory, and terrestrial analog studies should develop models, analysis approaches,
and instrumentation to interpret ancient environments from orbital, in situ, and returned sample data. 45,46,47,48
Assess Whether Life Is or Was Present on Mars in Its Geochemical Context
and Characterize Carbon Cycling and Prebiotic Chemistry
Assessing whether life is or was present on Mars will include characterizing complex organics, the spatial
distribution of chemical and isotopic signatures, and the morphology of mineralogic signatures, and identifying
temporal chemical variations requiring life. Characterizing the carbon cycle will include determining the distribu-
tion and composition of organic and inorganic carbon species; characterizing the distribution and composition of
inorganic carbon reservoirs through time; characterizing the links between carbon, hydrogen, nitrogen, oxygen,
phosphorus, and sulfur; and characterizing the preservation of reduced carbon compounds on the near-surface
through time.
Organic and inorganic chemical reactions in early planetary environments pioneered the pathways that, on
Earth, ultimately led to the origins of life. Organic compounds may have formed on early Mars through energetic
reactions in reducing atmospheres, mineral-catalyzed chemical reactions, transient reactions caused by bolide
impacts, and delivery of comets, meteorites, and interplanetary dust. The challenge is first to find organic matter and
any redox-sensitive minerals and compounds and then to characterize the conditions and processes that determined
their composition. The Mars Science Laboratory is specifically designed to address many of these questions, and
it is expected that significant progress will come from the MSL results.
Important Questions
Some important questions concerning whether life is or was present on Mars and the characterization of carbon
cycling and prebiotic chemistry in a geochemical context include the following:
• Can evidence of past (or present) life in the form of organic compounds, aqueous minerals, cellular
morphologies, biosedimentary structures, or patterns of elemental and mineralogic abundance be found at sites
that have been carefully selected for high habitability and preservation potential?
• Do habitable environments exist today that may be identified by atmospheric gases, exhumed subsurface
materials, or geophysical observations of the subsurface? Does life exist today, as evidenced by biosignatures,
atmospheric gases, or other indicators of extant metabolism?
Future Directions for Investigations and Measurements
To address the key questions concerning life listed above, there must be a broad range of mineralogic, ele-
mental, isotopic, and textural measurements of a diverse suite of martian rocks from well-characterized sites that
have high potential for habitability. Deposits formed by aqueous sedimentation, hydrothermal activity, or aqueous
alteration are important targets in the search for life. These deposits typically contain assemblages of materials
that indicate geological (and, possibly, biological) processes. Accordingly, a sample suite is defined as the set of
samples required to determine the key processes that formed these samples and, in turn, required to assess any
evidence of habitable environments or life. Many of the specific investigations and measurements overlap with
those necessary to determine the geologic context and to understand the potential for habitability described earlier,
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MARS: EVOLUTION OF AN EARTH-LIKE WORLD
including the technological development of modest-size rovers capable of selecting samples and documenting their
context, along with the development of critical sample selection criteria and analysis instrumentation. Additionally,
the preparation for the return to Earth of carefully selected samples from sites with the highest science potential
will mandate establishing the curation methodologies needed to accommodate the contamination, alteration, and
planetary protection challenges posed by the complex martian returned samples.
A direct way to search for extant life is to map the distribution of atmospheric trace gases as will be done
by the ESA-NASA Mars Trace Gas Orbiter. Both biotic and abiotic processes involving water in subsurface
environments can produce gases that escape into the atmosphere. Measurements of the composition, abundances,
variability, and formation processes of atmospheric trace gases will allow the separation of potential geological
and biological sources.
Finally, the support of a robust R&A program is crucial to a better understanding of the interactions between
organisms and their geologic environments and their biosignatures. Terrestrial analog studies should test instru-
mentation, develop techniques for measuring biosignatures under martian conditions, and conduct technological
proof-of-concept studies.
UNDERSTAND THE PROCESSES AND HISTORY OF CLIMATE
The fundamental science questions that underlie the goal of understanding the processes and history of Mars’s
climate are how the climate of Mars has evolved over time to reach its current state and what processes have oper-
ated to produce this evolution. The climate history of Mars can be divided into three distinct epochs:
1. Modern, with the climate system operating under the current obliquity;
2. Recent past, operating under similar pressures and temperatures but over a range of orbital variations
(primarily obliquity); and
3. Ancient, when the atmospheric pressure and temperature may have been substantially higher than at present,
and liquid water may have been stable on the surface, either intermittently or for extended periods.
The committee’s specific objectives for pursuing the climate goal are as follows:
• Characterize Mars’s atmosphere, present climate, and climate processes under both current and different
orbital configurations; and
• Characterize Mars’s ancient climate and climate processes.
Subsequent sections examine each of these objectives in turn, identifying critical questions to be addressed
and future investigations and measurements that could provide answers.
Understanding the current climate includes investigating the processes controlling the present distributions of
water, carbon dioxide, and dust; determining the production and loss, reaction rates, and global distribution of key
photochemical species; and understanding the exchange of volatiles and dust between surface and atmospheric
reservoirs. Understanding past climates includes determining how the composition of the atmosphere evolved
to its present state, what the chronology of compositional variability is, and what record of climatic change is
expressed in the surface stratigraphy and morphology. The ancient climate can be addressed by determining the
escape rates of key species and their correlation with seasonal and solar variability, the influence of the magnetic
field, the physical and chemical records of past climates, and the evolution of the isotopic, noble gas, and trace
gas composition through time.
Mars’s current climate system is complex and highly variable because the atmospheric circulation is coupled
to three cycles:
• The dust cycle—dust lifted by the wind modifies the atmosphere’s radiative properties;
• The carbon dioxide cycle—the atmosphere condenses and sublimes at seasonal polar caps and causes
planetary-scale transport and pressure cycles; and
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164 VISION AND VOYAGES FOR PLANETARY SCIENCE
ADVANCING STUDIES OF MARS
Previously Recommended Missions
The NRC’s 2003 planetary science decadal survey143 contained recommendations relating to five Mars
missions—technology development to enable Mars sample return, the Mars Science Laboratory, a long-lived lander
network, an upper-atmosphere orbiter, and the Mars Scout program. Of these five missions, three have flown or
are in final development. The upper-atmosphere mission is being implemented as the Mars Scout MAVEN mis-
sion, with a planned 2013 launch. The MSL mission is planned to launch in 2011, and the Mars Scout program
has produced both the Phoenix lander (2008) and MAVEN. The MSL, which was described only in very general
terms in the 2003 report, grew substantially in capability beyond what the 2003 survey envisioned, and it will
achieve significantly more science than originally planned. The principal-investigator-led Scout program has been
incorporated into the Discovery program.
New Missions: 2013-2022
Mars Sample Return Campaign
The committee places as the highest-priority Mars science goal the addressing in detail of the questions of
habitability and the potential origin and evolution of life on Mars. A critical next step toward answering these
questions will be provided through the analysis of carefully selected samples from geologically diverse and well-
characterized sites that are returned to Earth for detailed study using a wide diversity of laboratory techniques.
Therefore, the highest-priority missions for Mars in the coming decade are the elements of the Mars Sample Return
campaign—the Mars Astrobiology Explorer-Cacher to collect and cache samples, followed by the Mars Sample
Return Lander and the Mars Sample Return Orbiter (Figure 6.7) to retrieve these samples and return them to Earth,
where they will be analyzed in a Mars returned-sample-handling facility.
MAX-C is the critical first element of Mars sample return. It should be viewed primarily in the context of
sample return rather than as a separate mission that is independent of the sample return objective. The MAX-C
mission, by design, focuses on the collection and caching of samples from a site with the highest potential to study
aqueous environments, potential prebiotic chemistry, and habitability. In order to minimize cost and to focus the
technology development, the mission emphasizes the sample system and deemphasizes the use of in situ science
experiments. This design approach naturally leads to a mission that has a lower science value if sample return does
not occur. However, exploring a new site on a diverse planet with a science payload similar in capability to that of
the Mars Exploration Rovers will significantly advance our understanding of the geologic history and evolution
of Mars, even before the cached samples are returned to Earth.
By implementing sample return as a sequence of three missions, the highest-priority Mars objective of advanc-
ing the search for evidence of life on Mars can be achieved at a pace that maintains solar system balance and fits
within the available funding. The architecture provides resilience for adapting to budgetary changes and robustness
against mission failures. Two caches will be collected and remain scientifically viable for up to 20 years on the
surface or in orbit about Mars, so that a failure of the MAV would not necessitate reflight of MAX-C, and neither
the MAV nor MAX-C would need to be reflown if the return orbiter failed to achieve orbit. A modular approach
also permits timely reaction to scientific discoveries, so that a follow-on rover mission could pursue a major new
finding, and it enables additional Mars sample return missions using these same flight elements.
Mars Astrobiology Explorer-Cacher
The MAX-C, the sample-collection rover, would be landed using a duplicate of the Sky Crane EDL system.
The baseline design is a MER-class (~350 kg), solar-powered rover with about 20 km of mobility over a 500-
sol mission lifetime. It will carry approximately 35 kg of payload for sample collection, handling, and caching,
and a MER-class (~25 kg) suite of mast- and arm-mounted remote sensing and contact instruments to select the
samples. The key new development will be the sample-coring, sample-collection, and sample-caching system.
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MARS: EVOLUTION OF AN EARTH-LIKE WORLD
FIGURE 6.7 Mars Sample Return architecture. SOURCE: NASA Planetary Science Division.
MAX-C will acquire about 20 primary and about 20 contingency rock cores, each 10 gm in mass, from rock targets
with a high likelihood of preserving evidence for past environmental conditions including habitability, and with
a high likelihood of the possibility of preserved biosignatures. These cores will be sealed in two separate caches
for redundancy and left on the surface for retrieval by a subsequent mission. The cache systems will be designed
to prevent cross-contamination between samples, prevent exposure to the martian atmosphere, keep the samples
within the temperature range that they experienced prior to collection, and preserve the samples in this condition
for up to 20 years.
Mars Sample Return Lander
The Mars Sample Return Lander (MSR-L) will also land using the Sky Crane system and will carry a fetch
rover, local regolith and atmosphere sample-collection system, and the MAV. The fetch rover will be capable of
reaching the cache from any point within the 11-km-radius landing error ellipse within 3 months. The strawman
MAV design is a solid rocket that is maintained in a thermally controlled cocoon while on the martian surface for
up to 1 Earth year. Following sample retrieval, the lander will place the cache in the orbital sample (OS) container,
collect regolith and atmospheric samples, and seal the container to meet the planetary protection requirements.
The MAV will insert the OS into a stable 500-km altitude near-circular orbit.
Mars Sample Return Orbiter
The Mars Sample Return Orbiter will consist of a Mars orbiter, the OS acquisition and capture system, the
sample isolation system for planetary protection, and the EEV. The orbiter will detect, track, and rendezvous with
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166 VISION AND VOYAGES FOR PLANETARY SCIENCE
the OS, then capture and seal it in the EEV. The orbiter will leave Mars and release the entry vehicle to Earth,
where it will enter Earth’s atmosphere and hard-land using a parachute-less, self-righting system.
Mars Returned-Sample-Handling Facility
The Mars returned-sample-handling facility will meet the planetary protection requirements and will be
based on practices and procedures at existing biocontainment laboratories, NASA’s Lunar Sample Facility, and
pharmaceutical laboratories.
Mars Trace Gas Orbiter
The Mars Trace Gas Orbiter is currently conceived as a joint ESA-NASA collaboration to study the temporal
and spatial distribution of trace gases, atmospheric state, and surface-atmosphere interactions on Mars. This mis-
sion builds on the reported discovery of methane in the martian atmosphere. 144 The committee could only evaluate
the science return of this mission in a general sense, because the payload had not been selected at the time of the
evaluation. In addition, no independent cost estimate for this mission was generated because it would have been
inappropriate to perform such a science and cost evaluation during the competitive instrument payload selection
that was underway at the time of this assessment. NASA-provided cost estimates were used instead.
Technology Development
One of the highest-priority activities for the upcoming decade will be to develop the technologies necessary
to return samples from Mars. The technology program also needs to continue a robust instrument development
program so that future in situ missions can include the most advanced technologies possible. The new develop-
ments needed for MAX-C are the sample-coring, sample-collection, and sample-caching system. The modest
technology development for these systems has begun and should be continued at a level necessary to develop
them to TRL 6 at the time that the mission is approved.
The major new sample return technology needed will be the MAV. Although this launch system will be based
on existing solid rocket motor designs, major development will be needed in thermal control, autonomous launch
operations, and ascent and guidance under martian conditions. It is essential that these elements receive major
investments during the coming decade in order to ensure that they will reach the necessary maturity to be used by
the end of the coming decade or early in the decade after that.
The second major technology development that will require attention is the tracking, rendezvous, and capture
of the OS. An initial demonstration of this technology has been preformed by the Defense Advanced Research
Projects Agency’s Orbital Express mission, which performed detection and rendezvous in Earth orbit under similar
conditions. The MSR capture-basket concept has been demonstrated on zero-gravity aircraft flights. However,
significant technology development will still be required to develop this system for application at Mars.
The third technology element development is the planetary protection component of MSR to ensure that the
back-contamination (contamination of Earth by martian materials) requirements are met. This system will require
isolating the Mars sample cache completely and reliably throughout the entry, retrieval, and transport process.
This work will require the development and testing of the technology elements and the development of methods
and procedures to verify the required level of cleanliness in flight.
Finally, the definition and architecture development of the Mars returned-sample-handling facility need to
be accomplished in the coming decade. Significant issues must be resolved and requirements must be defined
regarding the methods, procedures, and equipment that can verify the required level of isolation and planetary
protection and sample characterization.
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New Frontiers Missions
Mars Geophysical Network
High-priority Mars science goals can be addressed by a New Frontiers-class geophysical network. The pri-
oritized science objectives for a Mars Geophysical Network mission are as follows:
1. Measure crustal structure and thickness, and core size, density, and structure, and investigate mantle com -
positional structure and phase transitions.
2. Characterize the local meteorology and provide ground truth for orbital climate measurements.
A study of the Mars Geophysical Network was performed at the committee’s request (Appendixes D and G).
Two identical free-flying vehicles would be launched on a single Atlas V 401 independently targeted for Mars
entry 7 days apart; to meet the science objectives they would land at sites geographically distributed. Each node
of the network would carry a three-axis very broad band seismometer with a shield and an X-band transponder;
an atmospheric package with pressure sensor, thermistors, and hotwire anemometer; a deployment arm; descent
and post-landing cameras; and a radio science package.
The science payload would have a 1 martian year nominal mission with continuous operation. This instrumen-
tation would allow the determination of crustal and lithosphere structure by cross-correlation of the atmospherically
induced seismic noise and would locate the seismic sources from joint travel times and azimuth determinations.
No major new technologies are required. The selected EDL architecture for this study employs a powered descent
lander with heritage from previous Mars missions. Key technology development for the seismometer has been
conducted over the past two decades, culminating in a TRL 5-6 instrument developed for the ESA ExoMars mission.
Mars Polar Climate Mission
As a follow-on to Phoenix, the next step for in situ high-latitude ice studies is to explore the exposed polar
layered deposits (PLD). A mission study initiated at the committee’s request (see Appendixes D and G) addressed
science objectives, including an understanding of the mechanism of climate change on Mars and how it relates to
climate change on Earth; determination of the chronology, compositional variability, and record of climatic change
expressed in the PLD; and an understanding of the astrobiological potential of the observable water-ice deposits.
Both mobile and static lander concepts were explored and could answer significant outstanding questions with
spacecraft and instrument heritage from existing systems. These concepts will likely fall within the New Frontiers
mission size range.
Discovery Missions
NASA does not intend to continue the Mars Scout program beyond the MAVEN mission, but instead plans
to include Mars in the Discovery program. The Discovery program has utility for Mars studies. Discovery is not
strategically directed but is competitively selected, a process that has been highly effective at producing affordable,
scientifically valuable missions. Examples of potential Mars missions that could be performed in the Discovery
program, in no priority order, include the following:
• A one-node geophysical pathfinder station,
• A polar science orbiter,
• A dual satellite atmospheric sounding and/or gravity mapping mission,
• An atmospheric sample-collection and Earth return mission,
• A Phobos/Deimos surface exploration mission (see Chapter 4), and
• An in situ aerial mission to explore the region of the martian atmosphere and remanant magnetic field that
is not easily accessible from orbit or from the surface.
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168 VISION AND VOYAGES FOR PLANETARY SCIENCE
Summary
A combination of missions and technology development activities will advance the scientific study of Mars
during the next decade. Such activities include the following:
• Flagship missions—The major focus of the next decade should be to initiate the Mars Sample Return
campaign. The first and highest-priority element of this campaign is the Mars Astrobiology Explorer-Cacher.
• New Frontiers missions—Although the committee looked at both the Mars Geophysical Network and the
Mars Polar Climate missions (see Appendixes D and G), due to cost constraints neither was considered a high
priority relative to other medium-class missions (see Chapter 9).
• Discovery missions—Small spacecraft missions can make important contributions to the study of Mars.
• Technology development—The key technologies necessary to accomplish Mars sample return include the
following: the Mars ascent vehicle; the rendezvous and capture of the orbiting sample-return container; and the
technologies to ensure that planetary protection requirements are met. Continued robust support for the develop-
ment of instruments for future in situ exploration is appropriate.
• Research support—Vigorous research and analysis programs are needed to enhance the development and
payoff of the orbital and surface missions and to refine the sample collection requirements and laboratory analysis
techniques needed for Mars sample return.
• International cooperation—While Mars sample return could proceed as a NASA-only program, inter-
national collaboration will be necessary to make real progress. The 2016 Mars Trace Gas Orbiter mission is an
appropriate start to a proposed joint NASA-ESA Mars program.
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