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 known sources, requiring planetary enrichment. A recent measurement by NASA's Mars Science Laboratory rover Curiosity Hesperian-era (>3 Ga) clays yields a D/H ∼3×SMOW, demonstrating most the enrichment occurs early in Mars's history, reinforcing conclusions meteorite studies. As on Venus, widely thought to reflect preferential loss space 1H (protium) relative 2H (deuterium), but both cause and global environmental context large hydrogen losses remain be determined. Here, we apply model primordial atmosphere evolution Mars, link magma accretion epoch with subsequent water-ocean epoch, calculate behavior deuterium for comparison observed record. In contrast earlier works consider fractionation atmospheres which reservoirs are present exclusively as H2O H2, here 2-component (H2O-H2) outgassed condensing (H2O) escaping (H2) components – their interaction explicitly calculated. We find ≈2-3× hydrospheric deuterium-enrichment produced rapidly if chemically reducing at last equilibration atmosphere, making H2 CO initially dominant species, minor abundances CO2. Reducing gases particular can substantial greenhouse warming prevent from freezing immediately after epoch. due plausible inventories (pH=21−102 bars) surface temperatures high enough (T=s290−560 K) stabilize produce an hydrological cycle through circulated. Moreover, pressure-temperature conditions ocean-atmosphere H2O-H2 isotopic equilibrium gas-phase exchange such strongly concentrates preferentially takes up protium escapes atmosphere. efficient physical separation deuterium-rich deuterium-poor species via condensation permits partitioning escape recorded modern crustal reservoirs. proposed scenario H2-CO-rich outgassing suggests significant durations (>Myr) chemical conducive prebiotic chemistry following crystallization.