  Remote sensing data from orbiter missions have proposed that ground ice may
currently exist on Mars, although the volume is still uncertain. Recent
analyses of Martian meteorites have suggested that the water reservoirs have at
least three distinct hydrogen isotope compositions (D/H ratios): primordial and
high D/H ratios, which are approximately the same and six times that of ocean
water on Earth, respectively, and a newly identified intermediate D/H ratio,
which is approximately two to three times higher than that in ocean water on
Earth. We calculate the evolution of the D/H ratios and the volumes of the
water reservoirs on Mars by modeling the exchange of hydrogen isotopes between
multiple water reservoirs and the atmospheric escape. The D/H ratio is slightly
higher in the topmost thin surface-ice layer than that in the atmosphere
because of isotopic fractionation by sublimation, whereas the water-ice
reservoir just below the exchangeable topmost surface layer retains the
intermediate D/H signature found in Martian meteorites. We propose two possible
models for constraining the volume of the ground ice considering the observed
D/H ratios and geomorphological estimates of Paleo-oceans. The first assumes
that the atmospheric loss is dominated by the Jeans escape. In this case, the
volume of ground ice should be larger than the total volume of the observable
surface ice that mainly occurs as polar layered ice deposits. The other model
assumes diffusion-limited atmospheric loss in which the interactive evolution
of the multiple water reservoirs naturally accounts for the observed D/H
ratios. In this scenario, a large volume of ground ice does not necessarily
exist currently on Mars as opposed to the perspective view proposed on the
basis of recent orbiter missions.
