High-resolution spectroscopy of Mars' atmosphere with the Hubble Space
Telescope revealed the deuterium Lyman alpha line at an intensity of
23 +/- 6 rayleighs. This measured intensity corresponds to HD/H-2 = 1.
5 +/- 0.6 x 10(-4), which is smaller by a factor of 11 than HDO/H2O. T
his indicates that fractionation of HD/H-2 relative to that of HDO/H2O
is not kinetically controlled by the rates of formation and destructi
on of H-2 and HD but is thermodynamically controlled by the isotope ex
change HD + H2O <-> HDO + H-2. Molecular hydrogen is strongly depleted
in deuterium relative to water on Mars because of the very long lifet
ime 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.