Detection of Atomic Deuterium in the Upper Atmosphere of Mars

Abstract

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/H 2 = 1.5 ± 0.6 × 10 –4 , which is smaller by a factor of 11 than HDO/H 2 O. This indicates that fractionation of HD/H 2 relative to that of HDO/H 2 O is not kinetically controlled by the rates of formation and destruction of H 2 and HD but is thermodynamically controlled by the isotope exchange HD + H 2 O ↔ HDO + H 2 . Molecular hydrogen is strongly depleted in deuterium relative to water on Mars because of the very long lifetime of H 2 (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.