D/H fractionation during the sublimation of water ice
Résumé
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 10−1 mb. 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 be- tween 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