D/H fractionation during the sublimation of water ice

Abstract

Experiments of sublimation of pure water ice have been performed in the temperature range −105 °C to −30 °C and atmospheric partial pressures ranging from 10−6 to 10−1mb. Sampling of both vapour and residual ice fractions has been performed with the use of a vacuum line designed for the extraction and purification of gases before the measurement of their D/H ratios. Sublimation was responsible for sizable isotopic fractionation factors in the range 0.969–1.123 for temperatures lying between −105 °C and −30 °C. The fractionation factor exhibits a cross-over at temperatures around −50 °C with the water vapour fraction being D-depleted relative to the residual ice fraction at T<−50 °C (αice-vapour= 0.969–0.995). This cross-over has implications for the understanding of the atmospheric water cycle of some terrestrial planets such as the Earth or Mars. The magnitude of deuterium enrichment or depletion between ice and water vapour cannot explain the differences in the D/H ratios amongst Jupiter comets and long–period comets families nor those that have been documented between Earth's and cometary water.