Scientific Investigations That Could Reduce Uncertainty
Uncertainties with regard to the possibility of extant martian life can be reduced through a program of research and exploration that might include data acquisition from orbital platforms, robotic exploration of the surface of Mars, the study of martian meteorites, the study of Mars-like or other extreme environments on Earth, and the study of returned samples. However, each returned sample should be assumed to contain viable exogenous biological entities until proven otherwise.
A number of avenues of scientific research could provide a context for understanding the uncertainty regarding the possibility of extant martian life. Research questions that could reduce uncertainties regarding the extent to which Mars is a possible abode for life include the following:
Are there locations on Mars where life could exist?—There are theoretical reasons to believe that the range of environments on Mars overlaps the range of habitats that living organisms can exploit. Direct evidence is not available, however. The search for potential habitats is closely related to other research goals, which may not necessarily be specifically directed to the search for life, such as the search for evidence of water, active volcanism, or the presence of nonequilibrium gases.
Are there environments on Mars that are inherently sterile?— It is conceivable that some environments on Mars are so inimical to life that organisms cannot survive there. If it could be shown that the physical and chemical properties of a particular martian environment preclude the existence of living organisms or dormant propagules (spores, cysts), such evidence could serve as a basis for reevaluating planetary protection criteria for that location.
Can meteorites carry living organisms between planets?—Living organisms might be dispersed between the terrestrial planets in debris launched into space by asteroid impacts (see Chapter 3). Direct evidence for the transfer of living organisms between planets is not currently available. The ecological consequences of such a phenomenon have not been fully explored but conceivably could be of consequence for planetary protection policy.
In addition to increasing our understanding of the limits and potential of life on Earth, other useful avenues of research include the search for, and investigation of, potential habitats for life on Mars and investigation of martian meteorites that have landed on Earth.
THE STUDY OF LIFE ON EARTH
Life on solar system bodies other than Earth, if any, would likely be similar, at some functional level, to microorganisms found on Earth, since the same geochemical constraints on energy transduction will apply. Understanding the limits of microbial life on Earth may yield clues to possible life on Mars. Studies of Earth ecosystems hypothesized to be analogous to putative martian ecosystems, such as the dry valleys of Antarctica or deep subsurface environments, could yield information useful to the search for life in samples returned from Mars.
If there is no feasible photosynthetic zone on Mars, any extant life must obtain energy from inorganic sources. Such sources are known to be utilized by Earth organisms (Jannasch, 1995; Stevens and McKinley, 1995), but the extent and ecology of such systems remain largely unknown. Further research would help determine the limiting factors in such model systems and the extent to which they are relevant to possible environments on Mars.
The martian surface is thought to be extremely oxidizing, extremely desiccated, and bathed in intense ultraviolet radiation, although there may be localized regions where conditions are less hostile to life. It is possible that the regolith, or pulverized rock debris that covers most of the surface, will prove to be uninhabitable by any living organism and inimical to organic carbon. However, highly resistant spores or cysts dispersed by putative organisms occupying more clement environments might possibly survive in the regolith. The study of the ability of terrestrial microorganisms and their resting states (spores, cysts) to withstand extreme conditions may shed light on this possibility.
There have been several proposals that particular assemblages of microorganisms with specific physiological capabilities could survive on Mars (e.g., Freidman and Ocampo-Freidman, 1984; McKay et al., 1992b; Boston et al., 1992; Stevens and McKinley, 1995). These proposals could be evaluated better if they were demonstrated under simulated Mars conditions, as defined by ongoing exploration. This would help determine whether the habitat requirements are met by known martian environments.
It may be possible that Mars harbored life at an earlier time when conditions on its surface were more favorable and that viable remnants are preserved in sedimentary mineral deposits or other precipitates. The ability of such deposits to shield living organisms or their resting states from the extreme conditions on the martian surface would be an appropriate subject for investigation. There have been reports of Earth organisms surviving up to 40 million years while encased in
amber (Cano and Borucki, 1995) and up to 100 million years while encased in halite crystals (Norton et al., 1993). Further investigation may increase our understanding of the ability of life to survive in a resting state for extended periods of time under adverse conditions.
The origin and validity of fossil features on Earth reported to be the remains of extremely small bacteria also may be appropriate subjects for additional research. Bacteria are known to cause or facilitate mineral precipitation around themselves in a number of settings, resulting in bacterial pseudomorphs composed of inorganic minerals (Beveridge et al., 1983; Ferris et al., 1994; Southam and Beveridge, 1994; Southam et al., 1995). Several investigators have proposed that certain mineral features found in various settings on Earth may represent fossilized remains of bacteria (Folk, 1993; Sillitoe et al., 1996).
Reliable methods for determining whether such features are truly biogenic would be useful in evaluating samples returned from Mars.
FURTHER EXAMINATION OF MARTIAN METEORITES
Continued and intensified study of martian meteorites could yield valuable data about physical and chemical conditions on Mars and the possibility of extinct or extant life there. Studies of the 12 known martian meteorites have already yielded information about hydrothermal rock alteration and the climatological history of Mars (Gooding, 1992). Some of these meteorites contain fractures filled with secondary minerals that are geologically similar to subsurface formations on Earth that are known to support microbial life (Stevens and McKinley, 1995; Kostelnikova and Pederson, 1996). Some researchers (e.g., McKay et al., 1996) have suggested that one of these meteorites contains evidence of past biological activity on Mars, although this has yet to be determined with certainty. Until samples from Mars are returned to Earth, the martian meteorites afford what is perhaps the best opportunity to explore the potential of Mars as an abode for life.
REMOTE AND IN SITU OBSERVATIONS OF MARS
NASA's An Exobiological Strategy for Mars Exploration (NASA, 1995) indicates that any sample-return mission should be an integral part of a comprehensive exploration program and should be preceded by a number of orbital and landed missions, the purpose of which is to conduct a systematic study of the martian environment. The exact nature of the orbital and landed missions that will be sent to Mars has yet to be determined, and the Space Studies Board task group does not have the requisite expertise to make specific recommendations in this area. However, the task group strongly endorses NASA's strategy as an effective means of characterizing the potential of Mars to harbor life. The use of remote sensing and in situ observations to identify and evaluate sites of potential