THE NEW MARS: ASTROBIOLOGY OF A NEIGHBOR PLANET

Mars is the other planet in the solar system that had a climate most similar to that of Earth. The 1976 Viking mission documented extensive ancient streams and lakes. Later in martian history, interactions between volcanism, impacts, groundwater and ice sustained liquid water at least locally. On Earth, virtually everywhere we find liquid water we find life. Thus Mars is the prime exploration target for astrobiology.

Later missions found that liquid water had extensively altered the ancient crust. ESA’s Mars Express Orbiter found phyllosilicates and hydrated silicates. NASA’s Mars Reconnaissance Orbiter (MRO) found other geographically extensive phyllosilicates and sulfates. MRO also detected hydrous silica, carbonates and other aqueous minerals; some were associated with ancient deltas, crater lakes and canyons. The Mars Exploration Rover (MER) Opportunity found sulfates, hematite and other minerals that formed in playa lakes and aquifers. MER Spirit found deposits that had been altered by migrating aqueous fluids, and others that are aqueous precipitates, including carbonates and possibly silica. Spirit’s discovery of deposits rich in ferric sulfate indicates that the aqueous dissolution and/or alteration of olivine was associated with acidic conditions, as can occur during hydrothermal activity. The oxidation of iron and sulfur in aqueous environments can provide energy for life. Thus habitable environments might have existed in Gusev crater at least intermittently in the geologic past. Also, recent reports of atmospheric methane invoke a robust source(s) located in the subsurface, consistent with liquid water still present at depth.

The “follow the water” strategy that has successfully guided recent exploration should now change to “seek signs of habitable environments and life.” The 2011 Mars Science Laboratory will perform definitive measurements of diverse minerals, volatiles and any organic compounds at a site that experienced aqueous activity. The 2016 Mars Science Orbiter will identify atmospheric trace gases. Future missions will employ higher-resolution images, precision landings, and new rover-based measurements. Soil, rock and atmospheric samples will be returned to state-of-the-art laboratories on Earth to intensify and extend the search for evidence of life.