THE POTENTIAL FOR HABITABLE ENVIRONMENTS IN THE BASALTIC PLAINS AND COLUMBIA HILLS OF GUSEV CRATER, MARS

Habitable environments must provide, among other things, chemical building blocks, sources of biochemical energy and conditions that maintain liquid water at least intermittently. Gusev basalts resemble compositionally olivine basalts on Earth that can support life deep beneath the seabed. Mars' atmosphere provides C and N. The activity of liquid water during various times in the past can be evaluated by examining rocks, soils and surface dust. Surface dust documents relatively recent conditions whereas rocks can record conditions that existed billions of years ago. Laguna Hollow, a shallow crater on the basaltic plains, has received fine-grained deposits relatively recently. These deposits are a uniform mixture of silicates and soluble salts, indicating that they were deposited and stored under dry conditions that prevailed perhaps within the past few m.y. A trench in mature plains regolith (“The Boroughs”) revealed Mg- and S-rich soil horizons and locally low Cl/S values, consistent with aqueous mobilization of soluble salts perhaps over time scales ranging from m.y. to b.y. Fluids that could have mobilized these salts might have been too saline to sustain life. The known minimum required water activity is ~0.75 for haloarchea in NaCl brines and ~0.61 for fungi in high sugar media. Plains basalts exhibited alteration that ranged from minor void fills and surficial coatings to essentially total but nearly isochemical alteration of minerals. Isochemical alteration might indicate water/rock values that were too low to provide life's minimum required water activity. Rocks along the western slopes of Husband Hill have been extensively altered and chemical constituents have been added and/or removed. These indicate higher water/rock values that might have sustained life. Life can survive in subsurface darkness by obtaining energy from redox reactions such as iron oxidation. Spirit rover has not yet found ultramafic rocks, thus serpentization reactions, which produce reduced constituents and can sustain life, perhaps never occurred in Gusev crater. However ferrous iron has been totally oxidized in some mafic rocks that have been extensively altered. Such rocks indicate that at least some deposits in Husband Hill might have sustained habitable environments in the distant past.