Adsorptive fractionation of HDO on JSC MARS-1 during sublimation with implications for the regolith of Mars

A chamber was constructed to simulate the boundary between the ice table, regolith and atmosphere of Mars and to examine fractionation between H2O and HDO during sublimation under realistic martian conditions of temperature and pressure. Thirteen experimental runs were conducted with regolith overlying the ice. The thickness and characteristic grain size of the regolith layer as well as the temperature of the underlying ice was varied. From these runs, values for the effective diffusivity, taking into account the effects of adsorption, of the regolith were derived. These effective diffusivities ranged from 1.8 × 10−4 m2 s−1 to 2.2 × 10−3 m2 s−1 for bare ice and from 2.4 × 10−11 m2 s−1 to 2.0 × 10−9 m2 s−1 with an adsorptive layer present. From these, latent heats of adsorption of 8.6 ± 2.6 kJ mol−1 and 9.3 ± 2.8 kJ mol−1 were derived at ice-surface temperatures above 223 ± 8 K and 96 ± 28 kJ mol−1 and 104 ± 31 kJ mol−1 respectively for H2O and HDO were derived at colder temperatures. For temperatures below 223 K, the effective diffusivity of HDO was found to be lower than the diffusivity of H2O by 40% on average, suggesting that the regolith was adsorptively fractionating the sublimating gas with a fractionation factor of 1.96 ± 0.74. Applying these values to Mars predicts that adsorbed water on the regolith is enriched in HDO compared to the atmosphere, particularly where the regolith is colder. Based on current observations, the D/H ratio of the regolith may be as high as 21 ± 8 times VSMOW at 12°S and LS = 357° if the regolith is hydrated primarily by the atmosphere, neglecting any hydration from subsurface ice.

Research highlights
► The ice-table/regolith/atmosphere boundary of Mars was simulated experimentally.
► Without regolith, subliming ice deuterium/hydrogen is consistent with KIE.
► With regolith, deuterium becomes adsorbed on grains with long equilibration times.
► Latent heats consistent with physisorption above 223K and chemisorption below.
► Implies adsorbed water has D/H up to 21x VSMOW based on atmospheric values.