CONSTRAINING H2 PRODUCTION FROM THE NOACHIAN CRUST: ELEMENTAL COMPOSITION, WATER CAPACITY, AND IMPLICATIONS FOR HABITABILITY

Microbial activity is observed in the terrestrial crust to unexpected depths where organisms are sustained by H₂ produced through water-rock reactions in the form of serpentinization and radiolysis. Furthermore, modern production of H₂ in Archean greenstone belts is comparable to H₂ production of the oceanic crust. Tantalizingly, the Noachian crust of Mars contains the same reactants and potentially the conditions for sustained subsurface aqueous environments and H₂ production. Key unknowns for assessing the capacity of the Noachian crust for this form of habitability are the volume and accessibility of water, modal mineralogy of the crust and potential for serpentinizing reactions, and the concentration of radionuclides available for radiolysis. The existence of significant amounts of subsurface water is supported by the mineralogical evidence of pervasive clay mineral formation to depths in excess of 7 km, morphologic evidence of catastrophic water release to the surface in the Hesperian, and regional to global hydrological models for enormous volumes of circulating groundwater leading to sulfate deposition in upwelling regions. Here we present our quantitative analyses of the mineralogical and elemental composition of the Noachian crust, with emphasis on radionuclides, and a reassessment of the porosity of the Martian crust building from results of the GRAIL mission that showed unexpectedly high porosity in the lunar crust from impact processes. Building on these results we consider the porosity and water capacity of the martian crust. Combining these estimates brings us closer to understanding the possible extent of a martian crustal biosphere driven by water-rock reactions and H₂ production, perhaps the largest and longest habitable environment on Mars.