  Regolith on Mars exchanges water with the atmosphere on a diurnal basis and
this process causes significant variation in the abundance of water vapor at
the surface. While previous studies of regolith-atmosphere exchange focus on
the abundance, recent in-situ experiments and remote sensing observations
measure the isotopic composition of the atmospheric water. We are therefore
motivated to investigate isotopic water exchange between the atmosphere and the
regolith and determine its effect on the deuterium to hydrogen ratio (D/H) of
the atmosphere. We model transport of water in the regolith and
regolith-atmosphere exchange by solving a transport equation including regolith
adsorption, condensation, and diffusion. The model calculates equilibrium
fractionation between HDO and H2O in each of these processes. The fractionation
in adsorption is caused by the difference in the latent heat of adsorption, and
that of condensation is caused by the difference in the vapor pressure.
Together with a simple, bulk-aerodynamic boundary layer model, we simulate the
diurnal variation of the D/H near the planetary surface. We find that the D/H
can vary by 300 - 1400 per mil diurnally in the equatorial and mid-latitude
locations, and the magnitude is greater at a colder location or season. The
variability is mainly driven by adsorption and desorption of regolith
particles, and its diurnal trend features a drop in the early morning, a rise
to the peak value during the daytime, and a second drop in the late afternoon
and evening, tracing the water vapor flow into and out from the regolith. The
predicted D/H variation can be tested with in-situ measurements. As such, our
calculations suggest stable isotope analysis to be a powerful tool in
pinpointing regolith-atmosphere exchange of water on Mars.
