A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars

The deuterium-to-hydrogen (D/H or 2H/1H) ratio of Martian atmospheric water (similar to 6 & times; standard mean ocean water, SMOW) is higher than that of known sources, requiring planetary enrichment. A recent measurement by NASA & rsquo;s Mars Science Laboratory rover Curiosity of Hesperian-era (> 3 Ga) clays yields a D/H ratio similar to 3 & times;SMOW, demonstrating that most of the enrichment occurs early in Mars & rsquo;s history, reinforcing the conclusions of Martian meteorite studies. As on Venus, Mars & rsquo;s D/H enrichment is widely thought to reflect preferential loss to space of 1H (protium) relative to 2H (deuterium), but both the cause and the global environmental context of large and early hydrogen losses remain to be determined. Here, we apply a recent model of primordial atmosphere evolution to Mars, link the magma ocean of the accretion epoch with a subsequent water-ocean epoch, and calculate the behavior of deuterium for comparison with the observed record. In contrast to earlier works that consider Martian D/H fractionation in atmospheres in which hydrogen reservoirs are present exclusively as H2O or H2, here we consider 2-component (H2O-H2) outgassed atmospheres in which both condensing (H2O) and escaping (H2) components & ndash; and their interaction & ndash; are explicitly calculated. We find that a asymptotic to 2-3 & times; hydrospheric deuterium-enrichment is produced rapidly if the Martian magma ocean is chemically reducing at last equilibration with the primordial atmosphere, making H2 and CO the initially dominant species, with minor abundances of H2O and CO2. Reducing gases & ndash; in particular H2 & ndash; can cause substantial greenhouse warming and prevent a water ocean from freezing immediately after the magma ocean epoch. We find that greenhouse warming due to plausible H2 inventories (pH2 = 1 & minus; 102 bars) yields surface temperatures high enough (Ts = 290 & minus;560 K) to stabilize a water ocean and produce an early hydrological cycle through which surface water can be circulated. Moreover, the pressure-temperature conditions are high enough to produce ocean-atmosphere H2O-H2 isotopic equilibrium through gas-phase deuterium exchange such that surface H2O strongly concentrates deuterium relative to H2, which preferentially takes up protium and escapes from the primordial atmosphere. The efficient physical separation of deuterium -rich (H2O) and deuterium-poor (H2) species via condensation permits equilibrium isotopic partitioning and early atmospheric escape to be recorded in modern crustal reservoirs. The proposed scenario of primordial H2-CO-rich outgassing and escape suggests significant durations (> Myr) of chemical conditions on the Martian surface conducive to prebiotic chemistry immediately following magma ocean crystallization. (c) 2022 Elsevier B.V. All rights reserved. Superscript/Subscript Available

Automated summary

The ratio of deuterium-to-hydrogen in the Martian atmosphere suggests planetary enrichment and early hydrogen losses. The behavior of deuterium in Mars' primordial atmosphere can be explained by a model that considers water-ocean epochs and the interaction between condensing and escaping components. The findings support the notion of significant durations of chemical conditions conducive to prebiotic chemistry immediately following the crystallization of the Martian magma ocean.