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Y. L. Yung, D. M. Kass (Caltech)
Water is a key component of the Martian climate, both at present and over the history of the planet. The current atmosphere averages ~10 pr-\mum of water. Its D/H ratio is enriched by a factor of five relative to terrestrial water, presumably due to water loss by escape. Based on H loss rates, H2O loss is 10-3 pr-\mum y-1. At this rate, the atmospheric reservoir would be depleted in 104 years, but there are several other reservoirs of water (the permanent northern polar cap, high latitude ground ice, and adsorbed water in the regolith) that should keep the atmospheric reservoir constant since its size climatologically controlled.
Recently, Krasnopolsky {\it et al.} (1998) measured the D/H ratio in HD in the upper atmosphere. Their measurement implies that the fractionation factor, F (efficiency of D escape relative to H), is ~0.02. Mars has lost ~ 80 m of water via fractionating processes. This implies that Mars has an exchangeable reservoir equivalent to a ~13 m global layer, which represents a polar cap 2000 km in diameter (the Martian northern permanent cap is about 1200 km). Thus, while the northern permanent cap may be a significant water reservoir, it is probably not the only one.
One of the major issues is the time-scale and mechanism for exchangeable reservoirs to buffer the atmosphere. During periods of high obliquity, high latitude water may be forced to migrate and equilibrate with the atmosphere in the process. But the low F value, combined with the rapid loss, implies that there has to be at least partial exchange on shorter time-scales to avoid extremely high D/H values. Over the last 4 \times 105 years, since the last period of high obliquity, 40 pr-\mum of water have been lost. In order to keep this from causing more than a factor of 2 change in the atmospheric water D/H, at least 40 pr-\mum needs to have been involved in the hydrological cycle. This requires cycling through the atmosphere the equivalent of a 2 cm thick layer of ice covering the northern permanent polar cap.