Large Carbonate Reservoir in Mars’ Crust

Geological evidence indicates that liquid water flowed on the surface of Mars more than 3.5 billion years (Ga) ago. To account for this early warmer climate, atmospheric models require at least 1 bar of CO ₂ . Liquid water in the presence of a CO ₂ atmosphere should have deposited abundant carbonate deposits, which conflicts with the < 0.04 bar Martian reservoir of carbonate minerals estimated from orbital spectroscopy data to date. Here, we revise the estimates of Mars’ carbonate reservoir by ground-truthing orbital data with recent surface mineralogical observations and seismological estimates of crustal porosity from Mars landers. Rover analyses detect between 5 and 35 wt% of carbonates in olivine weathering products that evaded detection by orbital spectroscopy. Furthermore, orbital data indicate that carbonate is globally associated with olivine-rich regions that occupy ~1% of Mars’ surface area. Extrapolating the rover detections of carbonate concentrations and their geological context to the global abundance of olivine-rich regions yields a minimum of 0.3-2.5 bars of CO₂ in the crust. Such a large carbonate reservoir is consistent with a CO ₂ atmosphere that was sufficiently dense to stabilize liquid water on early Mars as well as its subsequent removal by geological sequestration and atmospheric escape.