Exobiology and future Mars missions:  the search for Mars' earliest biosphere.
The primordial Mars may have possessed a thick carbon dioxide atmosphere, with liquid water common on the surface, similar in many ways to the primordial Earth.  During this epoch, billions of years ago, the surface of Mars could have been conducive to the origin of life.  It is possible that life evolved on Mars to be later eliminated as the atmospheric pressure dropped.  Analysis of the surface of Mars for the traces of this early martian biota could provide many insights into the phenomenon of life and its coupling to planetary evolution.
McKay
C P
CP
Solar System Exploration Branch, NASA/Ames Research Center, Moffett Field, CA 94035, USA.
D000538:Aluminum Silicates; D000864:Antarctic Regions; D001272:Atmosphere; D000077215:Clay; D003081:Cold Climate; D003889:Desert Climate; D018534:Earth, Planet; D018559:Exobiology / Q000639:trends*; D005118:Extraterrestrial Environment; D005580:Fossils; D019015:Geologic Sediments; D055593:Geological Phenomena; D005844:Geology; D018535:Mars; D000069497:Origin of Life; D013026:Space Flight / Q000639:trends
D000538:Aluminum Silicates; D000077215:Clay