Mars Water and D/H Evolution From 3.3 Ga to Present

The current deuterium to hydrogen ratio (D/H) on Mars is enriched by a factor of 5–6 relative to terrestrial values, suggesting that large amounts of H from water have been lost to space. Loss of H occurs more efficiently than loss of D because H atoms are lighter than D atoms, so the remaining gas becomes enriched in D. We constrain the history of water on Mars using D/H by tracking the supply and loss of H and D in the atmosphere. We examined the evolution of water and D/H from 3.3 Ga to the present, using the measured D/H in an ~3‐billion‐year‐old Gale crater mudstone and in the present atmosphere as constraints. We define the boundary conditions by the amount of water present at the surface early in history and the amount of water present today and incorporate the supply of water from outgassing and loss of H and D to space. The factor‐of‐2 enrichment in D/H in the last 3.3 Ga can be produced if loss to space outstrips outgassing. This corresponds to a present‐day 20‐ to 50‐m water global equivalent layer (GEL) that is a residual of an initial inventory at 3.3 Ga of 40‐ to 170‐m GEL, combined with 5‐ to 100‐m GEL outgassed and 20‐ to 220‐m GEL lost to space.