From Mars Pathfinder to Mars Express, Mars Reconnaissance Orbiter, and the Mars Exploration Rovers Spirit and Opportunity, the recent spate of robotic missions to the Red Planet has led to a wealth of new information about the planet’s environment, including strong evidence of a watery past and the possible discovery of atmospheric methane. In addition, new developments in our understanding of life in extreme conditions on Earth suggest the possibility of microbial viability in the harsh martian environment. Together, these results have greatly increased interest in the search for life on Mars, both within the scientific community and beyond.
Such scientific interest achieved a new focus on January 14, 2004, when President George W. Bush announced the new Vision for Space Exploration, directing NASA to focus its efforts on robotic and human exploration of space, particularly of the Moon and Mars. Included in the Vision is an explicit directive to “[c]onduct robotic exploration of Mars to search for evidence of life ….”
Given the enhanced scientific and political interest in the search for life on Mars, it is surprising that NASA’s most recent end-to-end strategy for the detection of martian life, contained in the report An Exobiological Strategy for Mars Exploration, was published as long ago as 1995.
Against this backdrop, NASA’s Science Mission Directorate requested the Space Studies Board’s assistance in developing an up-to-date integrated astrobiology strategy for Mars exploration that brings together all the threads of this diverse topic into a single source for science mission planning. In particular, NASA asked that the strategy developed by the Committee on an Astrobiology Strategy for the Exploration of Mars address the following topics:
The characteristics of potential targets for Mars exploration particularly suited for elucidating the prebiotic and possibly biotic history of Mars, and methods for identifying these targets;
A catalog of biosignatures that reflect fundamental and universal characteristics of life (i.e., not limited to an Earth-centric perspective);
Research activities that would improve exploration methodology and instrumentation capabilities to enhance the chances of astrobiological discovery; and
Approaches to the exploration of Mars that would maximize the astrobiological science return.
THE SEARCH FOR LIFE ON MARS
Mars is the most logical place to look for life elsewhere in the solar system because it is the most Earth-like of all the other planetary bodies in terms of its geological environment and the availability of liquid water at or near the surface throughout time. Moreover, Mars is the most accessible planetary body other than the lifeless Moon. The finding of evidence for past or present life beyond Earth would have profound philosophical and scientific ramifications, and a finding either that life was present or that it was not would have dramatic implications for the prospects for life elsewhere in the universe.
The search for life on Mars requires a detailed understanding of the nature of life on Earth and how it functions in different environments. The search also requires a very broad understanding of Mars as an integrated planetary system. Such an integrated understanding requires investigation of the following:
The geological and geophysical evolution of Mars;
The history of Mars’s volatiles and climate;
The nature of the surface and the subsurface environments;
The temporal and geographical distribution of liquid water;
The availability of other resources (e.g., energy) that are necessary to support life; and
An understanding of the processes that controlled each of the factors listed above.
Although it is not the only possible emphasis for the Mars program, astrobiology provides a scientifically engaging and broad approach that brings together multiple disciplines to address an important set of scientific questions that are also of tremendous interest to the public.
Finding. The search for evidence of past or present life, as well as determination of the planetary context that creates habitable environments, is a compelling primary focus for NASA’s Mars Exploration Program.
The astrobiology science goals for the exploration of Mars extend beyond the search for present and past life to encompass an understanding of the geological and environmental context that determines planetary habitability; habitability is defined as a general term referring to the potential of an environment (past or present) to support microbial life of any kind. Such an undertaking entails understanding the geological and geochemical evolution of the planet, its internal structure, and the nature of its interaction with the space environment. Such a broad approach will likely be required to enable astrobiologists to determine which characteristics of martian materials result from nonbiological processes and which result from biological processes and so could be used as biosignatures.
Any comprehensive program focusing on the astrobiological exploration of Mars must be undertaken with the full understanding that the outcome is uncertain. It is entirely possible that surface water did not survive on Mars for a period of time sufficient for an origin of life, or that Mars never had life. Astrobiologists seek to explore Mars to better understand the nature of the planet, to assess its biological potential and habitability, and to determine how far chemical evolution proceeded and whether life was present. A finding of “no life” would be just as important scientifically as a finding of life, in terms of constraining our views of how life originates and spreads and of how widespread life might be in the universe.
NASA’s 1995 report An Exobiological Strategy for Mars Exploration took the approach of starting from the global perspective and focusing increasingly on the local perspective. This approach involved a series of steps:
Global reconnaissance that focused on the history of water and the identification of sites for detailed in situ analysis;
In situ analysis at sites that hold promise for understanding the history of water;
Deployment of experiments that address astrobiology science questions, including the nature of martian organic molecules and the presence of features indicative of present-day or prior life;
Return of martian samples to Earth for detailed study; and
Human missions that would provide the detailed geological context for astrobiology measurements and the detection of modern-day “oases” for life.
This reasoned and measured approach provides the best opportunity for determining the geological and geochemical context in which the most useful and appropriate astrobiological measurements can be determined, implemented, and then properly interpreted and understood. It combines a broad, interdisciplinary approach to understanding Mars as a whole with the detailed, focused investigations that allow researchers to understand the astrobiology of Mars.
Finding. The search for life and understanding the broad planetary context for martian habitability will require a broad, multidisciplinary approach to Mars exploration.
Finding. At the same time, the astrobiological science goals can best be addressed by an implementation that allows researchers to address increasingly focused questions that relate to astrobiology goals in particular.
An appropriate strategy for studies of Mars’s potential for life is to focus on the elements most relevant to life, especially carbon. This will require a determination of whether organic molecules are present on Mars and where, and of the chemical characteristics that will distinguish between meteoritic (nonbiological), prebiotic, and biological organic molecules. In addition, there is still much to be learned about the history and availability of water, and thus NASA should not abandon its current strategy of “following the water.”
Finding. The very successful intellectual approach of “follow the water” should be expanded to include “follow the carbon,” along with other key biologically relevant elements.
Any search for possible organic molecules within martian soil or rock samples must be undertaken in such a way as to avoid contamination by similar materials inadvertently transported from Earth. Similarly, life-detection experiments must avoid false-positive results caused by microbes inadvertently transported from Earth. Obtaining the desired science results demands that the contamination issues be addressed by appropriate planetary protection approaches. But it would be a mistake to not develop, because of concerns over planetary protection, appropriate procedures (e.g., new techniques for spacecraft cleaning and bioload reduction) that would allow access to the most promising sites for scientific discovery.
Finding. The desire to visit and sample the highest-priority astrobiological sites requires that future surface missions to Mars take the necessary and appropriate planetary protection measures.
Useful science analysis of martian surface samples can be carried out either in situ on the martian surface or in terrestrial laboratories with samples returned to Earth. Although in situ missions have many advantages, sample return offers the opportunity to carry out many more analyses on a sample than can be done in situ, to follow up exciting measurements with additional measurements that had not previously been anticipated, and to make measurements or observations using instruments that are not amenable to being accommodated on a lander or rover mission or that were not available at the time of mission development.
Finding. The greatest advance in understanding Mars, from both an astrobiology and a more general scientific perspective, will come about from laboratory studies conducted on samples of Mars returned to Earth.
Current astrobiology science goals for the exploration of Mars can be addressed via a series of robotic spacecraft missions in the near- to mid-term future. It is critical that any astrobiological evidence that might be present on Mars not be compromised by robotic or human activities before definitive measurements or sample return occur.
Finding. The scientific study of Mars, including return to Earth of astrobiologically valuable samples that can be used to address the questions being asked today, can be done with robotic missions.
CHARACTERISTICS OF POTENTIAL TARGET SITES
What are the characteristics of potential targets for Mars exploration that are particularly suited for elucidating the prebiotic and possibly biotic (and postbiotic) history of Mars, and what methods can be used to identify these targets? Current understanding of the interplay between organisms and their geological and planetary environment is strongly influenced by terrestrial experience. As such, the most relevant martian environments to investigate and the types of individual materials at those sites that should be studied are those with past or present associations with liquid water and having the potential to retain organic carbon.
Recommendation. Sites that NASA should target with highest priority to advance astrobiology science objectives are those places where liquid water might exist today or might have existed in the past and where organic carbon might be present or might have been preserved.
Sites pertinent to present-day or geologically recent water include the following:
The surface, interior, and margins of the polar caps;
Cold, warm, or hot springs or underground hydrothermal systems; and
Source or outflow regions associated with near-surface aquifers that might be responsible for the “gullies” that have been observed.
Sites pertinent to geologically ancient water include the following:
Source or outflow regions for the catastrophic flood channels;
Ancient highlands that formed at a time when surface water might have been widespread (e.g., in the Noachian); and
Deposits of minerals that are associated with surface or subsurface water or with ancient hydrothermal systems or cold, warm, or hot springs.
Although new measurements are likely to include major discoveries relevant to the identification of specific sites of relevance to astrobiology, a foundation of data is already available to identify exciting and appropriate sites for either in situ analysis or sample return.
Finding. Identification of appropriate landing sites for detailed analysis (whether in situ or by sample return) can be done with the data sets now available or imminently available from currently active missions.
Many of the types of sites listed above as being important for in situ investigation pertinent to ancient or recent life are not confined to the low latitudes and/or low altitudes accessible with current entry, descent, and landing technologies. Rather, many important sites are at high elevations or at polar latitudes. These include most of the ancient Noachian terrain that would tell researchers about the potential earliest life and polar regions where melting of ice (e.g., at relatively recent epochs of high obliquity) could provide liquid water to sustain life. Past and present Mars landers and rovers have not been able to access such sites because of technological limitations associated with their power supplies (e.g., sites in the polar regions) and entry, descent, and landing systems (e.g., sites at high altitudes). Technical advances are required if the most astrobiologically promising sites are to be accessible to future missions. Additional development of entry, descent, and landing technologies is, for example, especially important to enable landing within a readily traversable distance of a given point on Mars (e.g., to access small, high-value sites—such as hydrothermal vents—or to retrieve cached samples) or if high-mass payloads are to land at technically challenging sites, such as those at high elevation.
Recommendation. Future surface missions must have the capability to visit most of the martian surface, including Noachian terrains and polar and high-latitude areas, and to access the subsurface.
Exposure to strongly oxidizing environments or to high fluxes of radiation is not conducive to the preservation of biologically diagnostic carbon compounds. Earth-based experience suggests that the rock types best able to preserve biosignatures include fine-grained sedimentary rocks, evaporites, and hydrothermal deposits.
Recommendation. Selection of samples for analysis (either in situ or of samples returned from Mars to Earth) should emphasize those having the best chance of retaining biosignatures.
What biosignatures reflect fundamental and universal characteristics of life? Unfortunately, there is no single comprehensive or unique biosignature whose presence would indicate life and whose absence would uniquely indicate the absence of life. Experience from studies of the martian meteorite ALH 84001 and analysis of evidence relating to claims of the earliest life on Earth have demonstrated that the potential interplay between putative organisms and their geological environment is so complex that researchers may never be able to identify
a unique biosignature that would work in all environments and at all times. Rather, the sum total of all measurements on a sample, in the context of understanding of the origin and evolution of the martian environment, will be required.
Complicating the committee’s task was the specific injunction in the charge that the discussion of biosignatures not be limited by an Earth-centric perspective. As a result, the committee made some specific assumptions about the likely characteristics of martian life forms. These assumptions are as follows:
They are based on carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and the bio-essential metals.
They require water.
They exist as self-contained cell-like entities.
They have sizes, shapes, and gross metabolic characteristics of organisms found on Earth.
They employ complex organic molecules in biochemical roles.
The last assumption is particularly important because it implies that martian organisms will produce and use a wide range of small molecules and organic polymers that can serve as chemical biosignatures in their intact or fragmentary states. Experience with studies of terrestrial materials suggests that of all the various life-detection techniques available, analysis of carbon chemistry is the first among equals. In other words, organic analysis is likely to provide a more robust way to detect life than imaging technologies, mineral assemblages, isotopic measurements, or any one other single technique. This is the case because, on Earth, the patterns of biogenic carbon compounds reflect organized polymerization of smaller subunits, or precursors, and comprise mixtures with a limited range of atomic spatial arrangements very different from those made by abiological processes. However, organic analysis alone is insufficient to detect life. An ensemble of all of the relevant methodologies, combined with analysis of geological and environmental plausibility, will likely provide the best evidence for the presence or absence of life in a sample.
Recommendation. The lack of a comprehensive understanding of all of the potential biosignatures for Mars exploration means that NASA should employ a combination of techniques that utilize both Earth-centric and non-Earth-centric approaches that focus on the basic concepts in carbon chemistry, imaging, mineral assemblages, and isotopic measurements.
Specific aspects of carbon chemistry that should be investigated include the following:
The presence of polymers based on repeating universal subunits;
Patterns in the carbon isotopic compositions of organic molecules that reflect organized polymerization of smaller subunits or precursors;
Patterns in the carbon numbers of organic compounds; and
The presence of carbon compounds that have only a subset of the possible connectivities or atomic spatial arrangements (i.e., just a few structural isomers or stereoisomers and/or strong chiral preferences).
EXPLORATION METHODOLOGIES AND INSTRUMENTATION
What research activities would improve exploration methodology and instrumentation capabilities to enhance the chances of astrobiological discovery? Currently operating and planned Mars missions all are, or will be, returning scientific data that directly address astrobiology goals in substantive ways. Thus, if astrobiologists are to advance their science goals for the exploration of Mars, they must work with NASA to ensure that the upcoming missions proceed as scheduled, and then take advantage of the scientific data these spacecraft will collect. Ensuring the success of future missions will require attention to the following activities:
Research and analysis activities, and
Supporting activities such as studies of martian meteorites and Mars-analog environments on Earth.
Future Mars missions will require significant technical advances if they are to be carried out successfully. Technology development must occur both in mission-related areas (e.g., entry, descent, and landing systems, including a precision landing capability; sample-return technology; in situ sample processing and handling; and planetary protection) and in astrobiological science instrumentation development (so that the necessary next generation of instruments is ready to go).
Recommendation. The Mars Exploration Program must make stronger investments in technology development than it does currently.
Research and analysis (R&A) programs are the mechanism by which scientific results are extracted from the data returned by current missions and by which concepts for future missions are developed. R&A programs are the primary vehicle by which the Mars Exploration Program can maintain its vitality in response to new discoveries, and such programs represent a vital investment in nurturing the next generation of space scientists, engineers, and program managers.
Recommendation. Continued strong support of NASA’s basic research and snalysis programs is an essential investment in the long-term health of the Mars Exploration Program.
Analysis of martian meteorites has been central to the development of the current understanding of Mars, its potential for life, and the development of current ideas about detection of present or fossil life. It is especially important to search for martian meteorites that formed during early periods of Mars’s history, as well as meteorites of sedimentary origin. This is an important area of collaboration between NASA and the National Science Foundation.
Recommendation. Collection and analysis of martian meteorites must continue, even though biases in the compositions and ages of these meteorites, their uncertain provenance, and the effects of impact-ejection and transfer to Earth mean that they cannot take the place of samples returned from Mars.
As with the analysis of martian meteorites, studies of Mars-analog sites on Earth are essential to mission development and execution and to the training of the scientists, engineers, and managers engaged.
Recommendation. Terrestrial analog studies should include testing instrumentation, developing techniques for measuring biosignatures under Mars-like conditions, and conducting technological proof-of-concept studies.
MAXIMIZATION OF SCIENCE
What approaches to the exploration of Mars will maximize the astrobiological science return? The astrobiology science goals for Mars are extremely broad, and it can be argued that virtually any mission can return data of relevance to issues relating to the habitability of Mars. However, consideration of the data from past and current Mars missions and expectations for those currently in development for launch during the 2007 and 2009 opportunities suggest that the greatest increase in understanding of Mars will come from the collection and return to Earth of a well-chosen suite of martian surface materials. Given the Mars Exploration Rover experience and current understanding of the nature of materials on the martian surface, a “grab sample” obtained from a stationary lander is not likely to be sufficient to provide the necessary data.
Finding. Sample return should be seen as a program that NASA and the Mars science community have already embarked upon rather than as a single, highly complex, costly, and risky mission that is to occur at some future time.
Recommendation. The highest-priority science objective for Mars exploration must be the analysis of a diverse suite of appropriate samples returned from carefully selected regions on Mars.
Programmatically, sample return should be phased over three or more launch opportunities. That is, samples can be collected and cached on Mars by one or more missions. A selected cache can be retrieved by a subsequent mission and launched into orbit about Mars for collection and return to Earth at a later date. Sample caching could be carried out by each surface mission, utilizing a minimalist approach so as not to make sample caching a cost-or technology-driver. Such a strategy, accompanied by a reduction in the size of the landing error ellipse, should allow collection of diverse samples and mitigate the costs of sample-return missions.
Irrespective of the compelling scientific arguments for the return of martian samples to Earth, the implementation of a sample-return mission will be a technically challenging, high-risk, high-cost endeavor. Because it will be comparable in expense to the highest-priority activities proposed by other scientific communities, the decision to implement a Mars sample-return mission will hinge on factors beyond the scope of this study. As such, it behooves the astrobiology community to plan for the possibility that a Mars sample-return mission is not an integral component of current mission plans.
Recommendation. If it is not feasible to proceed directly toward sample return, then a more gradual approach should be implemented that involves sample caching on all surface missions that follow the Mars Science Laboratory, in a way that would prepare for a relatively early return of samples to Earth.
If a commitment is not made for sample return, then high-priority, astrobiologically relevant science still can be done on Mars with missions such as the Astrobiology Field Laboratory or the Mid Rovers, provided that they are instrumented appropriately. However, it must be recognized that the ability of these missions to make fundamental discoveries is much more limited than would be the case with a sample-return mission.
International collaboration has the potential to make expensive undertakings such as a Mars sample-return mission affordable. But the benefit has to be balanced against the political difficulties of working with multiple countries and multiple space agencies.
Recommendation. International collaboration in Mars missions should be pursued in order to make expensive missions affordable, especially in the areas of sample caching and sample return.