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Astrobiological Exploration of the Surface of Mars David J. Des Marais  NASA Ames Research Center, Moffett Field, CA  94035

   Mars is the other planet in the solar system that appears to have once had a habitable climate most similar to that of Earth. Early in Martian history, an active hydrological cycle included surface precipitation, streams and lakes. Later, liquid water was sustained at least locally by interactions between volcanism, impacts, groundwater, and ice. The Mars Exploration Rovers and a fleet of Mars orbiters found that liquid water has chemically altered the crust. Liquid water participated in rock weathering reactions, such as iron and sulfide oxidation, that created potential sources of energy for life. Methane recently discovered in the Martian atmosphere apparently requires a robust source(s) that probably resides in the subsurface, consistent with liquid water at depth. Conditions once existed that were capable of supporting life sometime in the past. Such conditions might exist even today in the subsurface. Our search for evidence of Martian life must deliver rovers to the most promising landing sites where they will perform sample measurements that might indicate the nature of ancient environments and any biomarkers. And we must carefully balance the agenda for exploration in situ versus the return of key samples to Earth-based laboratories.