A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars

The deuterium-to-hydrogen (D/H or ²H/¹H) ratio of Martian atmospheric water (∼6× standard mean ocean water, SMOW) is higher than that of known sources, requiring planetary enrichment. A recent measurement by NASA's Mars Science Laboratory rover Curiosity of Hesperian-era (>3 Ga) clays yields a D/H ratio ∼3×SMOW, demonstrating that most of the enrichment occurs early in Mars's history, reinforcing the conclusions of Martian meteorite studies. As on Venus, Mars'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 – and their interaction – are explicitly calculated. We find that a ≈2-3× 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 – in particular H₂ – 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 (pH=21−10² bars) yields surface temperatures high enough (T=s290−560 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.