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. Plain Language Summary Knowing the amount of water on Mars over the past several billion years is critical in trying to understand the climate and atmosphere of Mars today, especially in light of the recent increased interest in human exploration of the red planet. There are several processes that can affect the amount of water on the surface of a planet. For Mars over the past three billion years, the two dominant processes are (a) escape to space through stripping of the upper atmosphere which reduces the amount of available hydrogen and (b) release from the interior of the planet through volcanism in a process called outgassing, which increases the amount of water on the surface. These processes not only alter the amount of water but also affect the deuterium to hydrogen (D/H) ratio in the water, which can help quantify how much water was added or removed. In this study, we use the evolution of the D/H ratio in computer modeling simulations to help narrow down the range of the amount of water on Mars in the past three billion years, including how much water was added and removed. Key Points Since 3.3 Ga, escape to space and outgassing are the most likely dominant processes that affected the fractionation and amount of water on Mars To recreate the enrichment in D/H measured today, past escape ra