Isotopic fractionation through water vapor condensation: The Deuteropause, a cold trap for deuterium in the atmosphere of Mars

Recent observations of deuterium atoms at Lyman alpha with the Hubble Space Telescope have shown the puzzling result of a D/H ratio in the upper atmosphere of Mars to be 11 times smaller than the D/H ratio in HDO and H2O of the lower atmosphere [Krasnopolsky et al., 1998]. One factor to explain this vertical variation was proposed: the photo-induced fractionation effect (PHIFE), due to a lower absorption cross section of solar UV of HDO compared to H2O [Cheng et al., 1999]. Here we suggest that in addition to PHIFE, this vertical variation of D/H ratio is also the result of preferred condensation of HDO in rising air, a process already documented in the Earth's upper stratosphere. Results of a cloud model, including condensation, sedimentation, and sublimation of icy particles in the atmosphere of Mars, are presented, supporting the efficiency of the condensation/evaporation fractionation effect (CEFE) as an important factor controlling the D/H ratio in the upper atmosphere of Mars. For a typical H2O profile, PHIFE provides a depletion factor of 2.5, while CEFE induces a factor of 3.5. The combined effects of PHIFE and CEFE (depletion factor of 9.5) can explain the paucity of D atoms in the upper atmosphere of Mars, implying a very low escape rate of deuterium at present and in the past and, correspondingly, a smaller quantity of H2O in the past.