The resurgence in scientific interest in the potential for life on Mars began in the 1990s. It was recognized at that time that the types of extreme environments on Earth capable of supporting organisms, such as geothermal systems, hot springs, subfreezing environments, and the deep subsurface, likely existed on Mars and had the potential to support life there.1 The possibility of martian life gained visibility with both the science community and the public with the hypothesis of McKay et al. that evidence for past life could be found in the martian meteorite ALH 84001.2 Although that hypothesis has now been widely criticized, the ensuing discussion brought out the scientific value of incorporating astrobiology science goals into a broad exploration strategy for Mars.
In the late 1990s, the NASA Mars Exploration Program plan emphasized the study of Mars as an integrated system. The potential for life, the history of the climate, and the geological and geophysical evolution of the planet were recognized as all relating to the abundance, behavior, and history of water as the common intellectual thread. The strength of this approach was the ability to utilize cross-disciplinary information to address a common set of scientific questions. Although each of the major topics was considered important on its own, the questions about life were considered to be “more equal among equals.” The astrobiology goals were given central importance as a scientific driver for much of the space exploration program,3 and funding available for the Mars Exploration Program in particular was augmented in response to its enhanced emphasis on “life” questions.
Reflecting this emphasis, the Mars Exploration Program Analysis Group (MEPAG) defined the scientific goals and objectives for exploring Mars. Scientific goals were grouped according to their emphasis on life, climate, geology, and preparation for human missions. Although the goals were not prioritized across these categories, they did reflect the emergence, if not preeminence, of life-related questions. The initial goals and objectives4 have been updated several times, with the current version responding to measurements and discoveries made by the most recent round of spacecraft missions.5
The Vision for Space Exploration announced by President George W. Bush in February 2004 also emphasized astrobiology questions as being at the center of the program. The goals for exploration of the solar system put
forward by NASA in response to the Vision focused on astrobiology. For Mars, the objective was to “[c]onduct robotic exploration of Mars to search for evidence of life, to understand the history of the solar system, and to prepare for future human exploration,” and the goals across the solar system all had astrobiology objectives at their center.6 Irrespective of the central roles that astrobiology, in general, and the search for life on Mars, in particular, appear to play in NASA’s enunciated goals, budgetary decisions made in 2006 seemed to undercut this commitment.7 Fortunately, events in 2007 suggest that the downward trend in NASA’s astrobiology spending may have been reversed.
At approximately the same time as these policies were being developed, the National Research Council’s (NRC’s) decadal survey for solar system exploration emphasized that astrobiology goals, related to understanding the habitability of the planets and satellites and determining the distribution of life in our solar system, should be considered central to the underlying rationale for solar system exploration.8 The missions identified by the decadal survey as addressing high-priority science goals, although not identified as astrobiology missions per se, all addressed key astrobiology goals.
The closest preceding document related to a Mars astrobiology strategy was the 1995 NASA report titled An Exobiological Strategy for Mars Exploration.9 That report laid out the scientific objectives for martian astrobiology and identified the then-key points for implementing a successful astrobiology strategy. The basic approach outlined in that document was to carry out the exploration of Mars in discrete phases, each providing an increasingly detailed look at the planet. The phases described were the following:
Global reconnaissance in order to understand global processes and to identify sites for detailed in situ investigations;
Exploration of particular sites in detail using landed packages in order to understand their geology and history;
Deployment of astrobiologically relevant instruments onto the surface that focus on prebiotic chemistry, past life, and/or present life;
Robotic return of samples of the surface back to Earth for detailed analysis; and
Human exploration of the martian surface.
Remarkably, the Mars Exploration Program as actually implemented in the last decade closely follows the first three exploration phases recommended in the 1995 NASA report. Global reconnaissance has been carried out by Mars Global Surveyor, Mars Odyssey, and, now, Mars Reconnaissance Orbiter. The combination of high-resolution imaging; multispectral mapping in the visible, near-infrared, and thermal infrared; compositional mapping using gamma-ray and neutron techniques; and radar mapping of the subsurface has provided detailed information on the geological history of Mars and on processes that pertain to the potential for liquid water and for life. In situ analysis by the Mars Exploration Rovers Spirit and Opportunity investigated two sites for which there was evidence for significant water-related activity and confirmed that liquid water played an important role at both. Two missions planned for the rest of this decade, the 2007 Phoenix lander that will investigate the geology and chemistry of high-latitude ground ice and the 2009 Mars Science Laboratory that will investigate astrobiologically relevant chemistry and climate behavior, are both missions that address fundamental astrobiology science objectives.
It is in this context that the present scientific strategy for the astrobiological exploration of Mars has been formulated. Although the science issues had been discussed previously as components of prior NRC reports,10–12 this is the first time that an integrated astrobiology strategy has been constructed for Mars by the NRC. The construction of such a strategy recognizes the increased scientific importance of astrobiology, and of astrobiology for Mars in particular, within the overall space science community. Not only does Mars arguably have the best chance in the solar system (other than Earth) of having or having had life, but it is also the most accessible of the bodies that are important to the astrobiological study of the solar system.
All of these considerations have informed the committee’s effort to describe the science objectives for the subsequent astrobiological investigation of Mars. Although science objectives are emphasized in the committee’s strategy, they cannot be developed in isolation from knowledge of what instruments are or might be available to
make measurements, how they can be assembled into flight missions, and what the costs are relative to the amounts of funding likely to be available.
OUTLINE OF APPROACH
In developing a strategy, the committee followed some general guidelines given in previous reports and scientific discussions that include consideration of the following:
The breadth of astrobiology goals,
Lessons learned from past searches for life, and
The biochemical nature of life.
The Breadth of Astrobiology Goals
The committee’s general approach is that astrobiology goals are broader than determining whether life is (or has been) present or is absent. Although such a determination is a key goal for the astrobiological study of Mars, a proper understanding of the significance of the findings is broader. By understanding the nature of habitable environments and the history of habitability (Box 1.1) on Mars and how they differ from Earth’s, as well as the nature of the relationship between organisms and their planetary environment and how those relationships might differ between planets, researchers can apply the results to estimates of where life might exist in our solar system other than on Mars and what the distribution of life on extrasolar planets and throughout the galaxy might be. Furthermore, having the broader astrobiological context might be necessary to determine whether, in fact, life is present or absent.
Indeed, it is entirely possible that life never existed on Mars. A definitive conclusion, if such were possible, that Mars is, was, and always has been lifeless would not represent an astrobiological failure. Rather, such a finding would be just as important scientifically as a finding of life, in terms of constraining our views on the origin, evolution, and distribution of living organisms in the universe.
Lessons Learned from Past Searches for Life
The committee assumes, based on experience garnered from previous searches for martian life associated with the Viking mission in the 1970s and analysis of the martian meteorite ALH 84001 in the 1990s and 2000s, that a single mission will not necessarily be able to determine if martian life is or ever was present and to characterize the boundary conditions of martian habitability. Thus, the committee anticipates that a progression of missions will be required that have the following characteristics:
The missions will span the range of issues related to the habitability of Mars, the potential for martian life, and whether life actually is present; and
The missions will allow for a sufficiently detailed look at the organic geochemistry of Mars that scientists can actually determine whether life is present or absent and, if the former proves to be the case, investigate its characteristics.
The committee recognizes that habitability and the actual occurrence (or not) of life are inextricably linked.
The Biochemical Nature of Life
The committee’s charge explicitly asked that it address searching for life that might not be Earth-like, and the NRC report The Limits of Organic Life in Planetary Systems provides a convenient starting point for such a discussion.13 That report was prepared to examine the possibilities for unconventional forms of life and to address, in part, concerns that previous life-detection experiments had not considered the wider possibilities for life (i.e.,
The Concept of Habitability
In its strategy, the committee makes extensive use of the concept of habitability, as does the Mars science community as a whole. MEPAG has defined habitability as:1
A general term referring to the potential of an environment (past or present) to support life of any kind. In the context of planetary exploration, two further concepts are important: Indigenous habitability is the potential of a planetary environment to support life that originated on that planet, and exogenous habitability is the potential of a planetary environment to support life that originated on another planet.
In general, the concept of habitability does not relate to whether life actually exists or has existed on a planet. It refers instead to whether environmental conditions are such that life could exist, grow, and multiply, and whether resources are available that can support life. Such environmental conditions are discussed in Chapter 4 but might include, for example, temperatures between about 253 K and 304 K, a range in which liquid water can exist and life can function; salinity or pH within ranges in which life can exist; and the actual presence of liquid water. Although life can exist in a frozen, dormant state at liquid nitrogen temperatures, it can neither grow nor multiply, so that an environment at that temperature would not qualify as habitable. The concept of habitability is difficult to apply in that researchers have only terran life as a guide, and the range of conditions in which life in general could conceivably function is likely to be broader than the range in which terran life functions, but by an unknown degree; thus, the concept of habitability includes a theoretical component and has a significant degree of uncertainty.
Related to habitability is the concept of a habitat. As used in this report, the term “habitat” refers to an environment (defined in time and space) that is or was occupied by life.2 Although habitability and the occupation of a habitat by life are general concepts, the committee uses them more specifically to mean habitability by microbial organisms.
The question What is life? cannot be addressed easily by definitions. The reader is referred to discussions of this question in Box 1.2.
what the Limits report calls nonterran life) and had been too geocentric in concept and execution. The reader is referred to that report for an extended discussion of possibilities for nonterran life. The generic characteristics of life as we know it—terran life, in the terminology of the Limits report—can be summarized as the following:
Uses water as a solvent;
Is built from cells, and exploits a metabolism that focuses on the C=O carbonyl group;
Is a thermodynamically dissipative structure exploiting chemical energy gradients; and
Exploits a two-biopolymer architecture that uses nucleic acids to perform most genetic functions, and proteins to perform most catalytic functions.
What complicates the discussion is the fact that martian life might be terran or nonterran. If some variant of
What Is Life?
Any discussion of the environmental requirements necessary to support life or of the search for life on another planet begs the question, What is life? This is a difficult question to answer, and there is no wide agreement on a definition.1 Sagan described a number of characteristics of life on Earth, including the ability to take in nutrients and produce waste products, the ability to grow and reproduce and to pass on genetic information, the ability to respond to the environment, and the ability to evolve via Darwinian evolution.2 Each of these characteristics generally applies to life on Earth, but each has clear exceptions that preclude its use as a definition. Standard counterexamples include fire, which takes in nutrients and produces waste products and grows, and mules, which are incapable of reproducing or evolving via Darwinian evolution. Although researchers think that entities that meet most or all of Sagan’s criteria are alive, and that those that meet few are not alive, this set of requirements has been carefully tuned to human preconceptions of what is or is not alive.3
Cleland and Chyba have argued that it is not possible to arrive at a unique definition for life without first having a comprehensive theory of life. This situation is analogous to the difficulty of defining water without first having a comprehensive molecular theory that tells us that water consists of H2O.4
Efforts to define life may be subject to the kind of ongoing controversy associated with efforts to define a planet (in the context of asking whether Pluto is a planet or not). Planets, in fact, represent a subset of objects generally associated with stars, and for which there is a smooth continuum of characteristics that include size, composition, and location in their planetary system, as well as the discrete characteristic of whether they orbit another object that itself orbits the star. It may not be possible to define a unique boundary between planets and nonplanets with which all scientists would agree. Similarly, the boundary between living and nonliving may be gradational, and it may not be possible to draw a unique dividing line. Defining life may have similar problems.
This lack of a concrete definition does not preclude the study of life in the universe or efforts to detect it on other planets. Rather, the questions that it raises and how they are answered will provide us with a much better understanding of our own existence here on Earth.
the Panspermia hypothesis is correct, life on Mars and on Earth might have a common origin, and so martian life would be terran by definition. But if life arose independently on Earth and on Mars, it can be argued that martian life would likely be nonterran because of the vanishingly small possibility that the same basic biochemical architecture would have evolved on two different planets. However, it could also be argued that the terran model is so compelling that its adoption is virtually guaranteed by the laws of physics and chemistry.14 Without a general theory of the origin of life it is impossible to decide which, if any, of these arguments is likely to be correct. For more discussion, see Box 1.2.
Thus, the committee faced a dilemma. The simplifying assumption that martian life is terran was excluded because the committee was specifically asked to address a search for life that might be non-Earth-like in its characteristics. But scientists and science-fiction writers have been highly inventive in their speculations about the possibilities of “life as we don’t know it.”15 A systematic discussion of all of the hypothetical nonterran life
forms discussed in the scientific literature is beyond the scope of this study. Thus, to create a tractable problem, the committee made some specific assumptions about the likely characteristics that hypothetical martian life forms will display. These assumptions are as follows:
They are based on carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and the bio-essential metals of terran life.
They require water.
They have structures reminiscent of terran microbes. That is, they exist in the form of self-contained, cell-like entities rather than as, say, a naked soup of genetic material or free-standing chemicals that allow an extended system (e.g., a pond or lake) to be considered a single living system.
They have sizes, shapes and gross metabolic characteristics that are determined by the same physical, chemical, and thermodynamic factors that dictate the corresponding features of terran organisms. For example, metabolic processes based on the utilization of redox reactions seem highly plausible. But the details of the specific reactions, including the identities of electron donors and electron acceptors, will be driven by local conditions and may well not resemble those of their terran counterparts.
They employ complex organic molecules in biochemical roles (e.g., structural compounds, catalysis and the preservation and transfer of genetic information) analogous to those of terran life, but the relevant molecules playing these roles are likely different from those in their terran counterparts.
These characteristics are based on generalizations of the characteristics of terran life that might be applicable to life found elsewhere in the universe. There are additional characteristics of life on Earth that must be carefully considered when searching for life on Mars lest they become too strong a guide and, as a result, bias the search. A prime example of such a bias is the fact that most life on Earth is powered either directly or indirectly by the Sun. Photosynthesis is not likely to be a major driver of hypothetical martian life because of the difficulties that organisms would have in surviving on or near the planet’s surface.
Although each of these assumptions might conceivably be wrong, they represent a starting point and at the same time allowed the committee to have an initial basis of understanding about what will allow a search for non-Earth-like life and conditions. In the end, though, astrobiologists will be guided by what is actually found on Mars. Nonrandom groupings of elements, molecules, or larger structures will stand out in detailed analyses of surface materials; although they may not be interpreted as being indicative of life, they will point to issues requiring further study. Thus, in designing a strategy for Mars exploration, astrobiologists must start close to those things they currently understand about life and its context of the origin and evolution of planetary environments, but then retain sufficient flexibility to branch farther and farther afield as the data require.
1. P.J. Boston, M.V. Ivanov, and C.P. McKay, “On the Possibility of Chemosynthetic Ecosystems in Subsurface Habitats on Mars,” Icarus 95:300-308, 1992.
2. D.S. McKay, E.K. Gibson, Jr., K.L. Thomas-Keprt, H. Vali, C.S. Romanek, S.J. Clemett, X.D.F. Chillier, C.R. Maechling, and R.N. Zare, “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH 84001,” Science 273:924-930, 1996.
3. National Aeronautics and Space Administration, Astrobiology Roadmap: September 2003, National Aeronautics and Space Administration, Washington, D.C., 2003.
4. MEPAG (Mars Exploration Program Analysis Group), “Scientific Goals, Objectives, Investigations, and Priorities,” Science Planning for Exploring Mars, JPL Publication 01-7, 2001, available at http://mepag.jpl.nasa.gov/reports/index.html.
5. MEPAG, “Mars Scientific Goals, Objectives, Investigations, and Priorities: 2006,” white paper, J. Grant, ed., 31 p., February 2006, available at http://mepag.jpl.nasa.gov/reports/index.html.
6. National Aeronautics and Space Administration, The Vision for Space Exploration: February 2004, National Aeronautics and Space Administration, Washington, D.C., 2004.
7. National Research Council, An Assessment of Balance in NASA’s Science Programs, The National Academies Press, Washington, D.C., 2006, pp. 20-21.
8. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
9. National Aeronautics and Space Administration, An Exobiology Strategy for Mars Exploration, National Aeronautics and Space Administration, Washington, D.C., 1995.
10. See, for example, National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington D.C., 2003.
11. See, for example, National Research Council, Assessment of Mars Science and Mission Priorities, The National Academies Press, Washington D.C., 2001.
12. See, for example, National Research Council, Assessment of NASA’s Mars Architecture 2007-2016, The National Academies Press, Washington D.C. 2006.
13. National Research Council, Limits of Organic Life in Planetary Systems, The National Academies Press, Washington D.C., 2007.
14. See, for example, N.R. Pace, “The Nature of Biochemistry in the Universe,” and S.A. Benner, “Chance and Necessity in Biomolecular Chemistry: Is Life as We Know It Universal?” pp. 59-63 and 64-67 in National Research Council, Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques, The National Academies Press, Washington, D.C., 2002.
15. For a recent scientific discussion of some of the possibilities see, for example, D. Schulze-Makuch and L.N. Irwin, Life in the Universe: Expectations and Constraints, Springer, New York, 2004.