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Regolith on Mars exchanges water with the atmosphere on a diurnal basis andthis process causes significant variation in the abundance of water vapor atthe surface. While previous studies of regolith-atmosphere exchange focus onthe abundance, recent in-situ experiments and remote sensing observationsmeasure the isotopic composition of the atmospheric water. We are thereforemotivated to investigate isotopic water exchange between the atmosphere and theregolith and determine its effect on the deuterium to hydrogen ratio (D/H) ofthe atmosphere. We model transport of water in the regolith andregolith-atmosphere exchange by solving a transport equation including regolithadsorption, condensation, and diffusion. The model calculates equilibriumfractionation between HDO and H2O in each of these processes. The fractionationin adsorption is caused by the difference in the latent heat of adsorption, andthat of condensation is caused by the difference in the vapor pressure.Together with a simple, bulk-aerodynamic boundary layer model, we simulate thediurnal variation of the D/H near the planetary surface. We find that the D/Hcan vary by 300 - 1400 per mil diurnally in the equatorial and mid-latitudelocations, and the magnitude is greater at a colder location or season. Thevariability is mainly driven by adsorption and desorption of regolithparticles, and its diurnal trend features a drop in the early morning, a riseto the peak value during the daytime, and a second drop in the late afternoonand evening, tracing the water vapor flow into and out from the regolith. Thepredicted D/H variation can be tested with in-situ measurements. As such, ourcalculations suggest stable isotope analysis to be a powerful tool inpinpointing regolith-atmosphere exchange of water on Mars.

Predicted Diurnal Variation of the Deuterium to Hydrogen Ratio in Water at the Surface of Mars Caused by Mass Exchange with the Regolith