IT IS often argued1-3 that substantially more carbon dioxide and water were degassed from the martian interior than can be found at present in the atmosphere, polar caps and regolith. Calculations have shown that atmospheric escape cannot account for all of the missing volatiles4. Suggestions that carbon dioxide is stored as marine or lacustrine deposits 5, are challenged by Earth-based and spacecraft remote-sensing data6,7. Moreover, recent modelling of the martian atmosphere suggests that rainfall or open bodies of water are in any case unlikely to have persisted for extended periods of time8,9. Hydrothermal carbonates therefore provide a possible solution to this dilemma. Using an accessible terrestrial system (Iceland) as a guide to the underlying processes, and a host rock composition inferred from the least-altered martian meteorite10, we present a geochemical model for the formation of carbonates in possible martian hydrothermal systems. Our results suggest that an extensive reservoir of carbonate minerals-equivalent to an atmospheric pressure of carbon dioxide of at least one bar-could have been sequestered beneath the surface by widespread hydrothermal activity in the martian past.
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