Past And Current Searches For Life On Mars
S.W. Squyres (Cornell U.)
For more than a century, Mars has been considered as a possible abode for life. Early telescopic studies investigated "canals" presumed to have been built by intelligent life, and a "wave of darkening" thought to perhaps result from changing vegetation cover. Subsequent flyby and orbital spacecraft disproved these ideas, but also provided evidence for warmer, wetter conditions on ancient Mars. The Viking landers tested the hypothesis that microbial life exists today in near-surface martian soil, and did not produce a convincing confirmation. Controversial evidence for ancient martian microbial life has been argued for in an ancient martian meteorite, and more recent spacecraft studies have focused primarily on better understanding the habitability of ancient Mars. These studies have produced strong evidence for ancient water on Mars, including subsurface aquifers, surface flow, hydrothermal systems, and a wide range of aqueous alteration and precipitation processes. All of these could have provided habitable niches on Mars, although some conditions, particularly salinity and acidity, would have posed challenges to life. Widespread sulfate deposits suggest largely acidic conditions over much of martian history, while phyllosilicate deposits indicate that some martian waters had a more neutral pH. Very recent telescopic data have revealed methane in the martian atmosphere, suggesting either recent geologic or biological activity on the planet. Planned missions to Mars will search for ancient biomarkers, including organic carbon, in the planet's rocks and soils.