Detection of Atomic Deuterium in the Upper Atmosphere of Mars

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Science  05 Jun 1998:
Vol. 280, Issue 5369, pp. 1576-1580
DOI: 10.1126/science.280.5369.1576

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High-resolution spectroscopy of Mars' atmosphere with the Hubble Space Telescope revealed the deuterium Lyman α line at an intensity of 23 ± 6 rayleighs. This measured intensity corresponds to HD/H2 = 1.5 ± 0.6 × 10–4, which is smaller by a factor of 11 than HDO/H2O. This indicates that fractionation of HD/H2 relative to that of HDO/H2O is not kinetically controlled by the rates of formation and destruction of H2 and HD but is thermodynamically controlled by the isotope exchange HD + H2O ↔ HDO + H2. Molecular hydrogen is strongly depleted in deuterium relative to water on Mars because of the very long lifetime of H2 (1200 years). The derived isotope fractionation corresponds to an estimate of a planetwide reservoir of water ice about 5 meters thick that is exchangeable with the atmosphere.

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