A Primordial Atmospheric Origin Of Hydrospheric Deuterium Enrichment On Mars

By Keith Cowing
Press Release
September 22, 2022
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A Primordial Atmospheric Origin Of Hydrospheric Deuterium Enrichment On Mars
Permitted climate scenarios. The initial climate and the lifetime of the primordial greenhouse depend on the H2 inventory. Extreme ultraviolet (EUV) radiation from the young Sun can power escape on million year timescales. Calculating escape with the energy-limited approximation (Watson et al., 1981) results in a loss rate of »2.5 bars/Myrs. In such scenarios, earliest Mars experiences habitable conditions (<100°C) for several million years. The final deuterium-enrichment (fD/H) of the hydrosphere (»2.17-2.51) only weakly depends on the H2 inventory for the full range of endowments here considered (pH2=10-100 bars) and overlaps with the enrichment observed in the Martian isotopic record (»2-3x, cf. Fig. 1). Such a robust model outcome is evidence for the past presence of an H2 greenhouse on post-magmaocean Mars.

The deuterium-to-hydrogen (D/H or 2H/1H) ratio of Martian atmospheric water (~6x 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 >3 Gyr clays yields a D/H ratio ~3x SMOW, demonstrating that most enrichment occurs early in Mars’s history. As on Venus, Mars’s D/H enrichment is thought to reflect preferential loss to space of 1H (protium) relative to 2H (deuterium), but 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. We find that a ~2-3x hydrospheric deuterium-enrichment is produced if the Martian magma ocean is chemically reducing at last equilibration with the primordial atmosphere, making H2-CO the initially dominant species, with minor abundances of H2O-CO2. Reducing gases – in particular H2 – can cause greenhouse warming and prevent a water ocean from freezing immediately after the magma ocean epoch.

Moreover, the pressure-temperature conditions are high enough to produce ocean-atmosphere H2O-H2 isotopic equilibrium such that surface H2O strongly concentrates deuterium relative to H2, which preferentially takes up protium and escapes from the primordial atmosphere. The proposed scenario of primordial H2-rich outgassing and escape suggests significant durations (>Myr) of chemical conditions on the Martian surface conducive to prebiotic chemistry immediately following Martian accretion.

Kaveh Pahlevan, Laura Schaefer, Linda T. Elkins-Tanton, Steven J. Desch, Peter R. Buseck

Comments: 5 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2209.10635 [astro-ph.EP] (or arXiv:2209.10635v1 [astro-ph.EP] for this version)
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Submission history
From: Kaveh Pahlevan
[v1] Wed, 21 Sep 2022 20:05:34 UTC (1,313 KB)
Astrobiology, Astrochemistry,

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