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T. M. McCollom (CU Center for Astrobiology and Laboratory for Atmospheric and Space Physics, U. Colorado, Boulder)
One of the primary reasons for society’s long-standing fascination with Mars is the possibility that life may be present there, or at least may have been present at some time in the past. Direct detection of extraterrestrial life on Mars is beyond the capabilities of current spacecraft missions, but these missions are providing crucial background data to evaluate the past and present habitability of Mars and to identify promising sites for future exploration.
Life as we know it has three basic requirements: liquid water, a complement of elemental nutrients such as carbon and nitrogen, and a source of metabolic energy. It now appears likely that each of these required ingredients have been present on Mars. There is increasing geologic evidence for persistent liquid water at the surface in the past, and there are probably significant subsurface reservoirs at the present. The atmosphere and rocky crust likely contain a sufficient complement of biological elements, such as C, N, S, O, P, and trace metals. And plausible sources of metabolic energy from chemical sources and light have been proposed. But was this sufficient for life to originate and persist on Mars? One open question is how long clement conditions and liquid water may have lasted on the surface, and whether such periods were continuous or sporadic (perhaps induced by impacts). Current theories of the origin of life on Earth suggest that life appeared rapidly once conditions allowing it were present, but the evidence for this belief are being vigorously reevaluated and the origin and early evolution of life on our planet may have been more gradual than currently thought. If it indeed takes a long time for life to initially evolve, was there time enough on early Mars for a separate origin of life?
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.