THE SURFACE ENVIRONMENT ON MARS AND HABITABILITY FOR EARTH-TYPE MICROBES: CLUES IN THE MARTIAN METEORITES AND COMPARISON TO OTHER MARS DATA

The known meteorites from Mars are fine-grained mafic and ultramafic igneous rocks. Most of them crystallized in shallow subsurface environments. Because of the proximity of the rocks to the martian surface and the presence of voids and fractures in the rocks, the meteorites have been affected by martian weathering. Although minor terrestrial weathering effects are common in the meteorites, unambiguously martian weathering features have been identified in several of the meteorites. Fracture surfaces on the primary igneous minerals contain evidence of typical aqueous dissolution. Traces to minor amounts of secondary silicates (phyllosilicates and amorphous silicates) of definite martian origin are common. The total degree of alteration of the silicates is small, however, indicating that temperatures were probably low (e.g., below ~100 deg C) and of short duration. Hot aqueous fluids (volcanic or impact-generated) could have been responsible but would have had to have been almost instantaneous in order to prevent more alteration of the silicates than is found. Other secondary minerals include traces of carbonates, sulfates, and halite. Martian carbonates and sulfates are present in most of the meteorites that have been studied. The salt assemblages in the Mars rocks are generally thought to have formed as the result of evaporation of small amounts of water with compositions much like marine water on Earth. Mars Rover data indicate that alteration in at least two locations has been strongly acidic. Many of the meteorites, however, contain carbonates, which do not commonly form in acidic environments. It is likely that different weathering environments have been present on Mars at different times. Individual meteorites may contain salts deposited during different episodes. The salts are scarce, so determining stratigraphic relationships between different salts each meteorite is a goal for helping to decipher the Mars weathering record. Martian weathering features are important indicators of environmental conditions on Mars, and can help us determine whether conditions are, or have been, suitable for life as we know it. The indications are that the shallow subsurface environment on Mars has been feasible for terrestrial-type life to survive, at least episodically, throughout the history of Mars.