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Mars Sample Return: Issues and Recommendations
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