THE MARS WATER CYCLE - DETERMINING THE ROLE OF EXCHANGE WITH THE REGOLITH

The near-surface nighttime atmospheric water vapor concentrations infe rred by Ryan et al. (J. A. Ryan, R. D. Sharman, and R. D. Lucich 1982, J. Geophys. Res. 87, 7279-7284) from Viking Lander air temperature me asurements are a factor of 2-3 lower than the same quantities estimate d from daytime atmospheric column water vapor abundances observed from the Viking Orbiters. We show that a physical model of the atmospheric boundary layer and regolith can produce a nighttime depletion of this magnitude by diffusion of water into the regolith and adsorption onto regolith grains, Quantitative validation of the model is not possible at present due to the lack of direct measurements of the near-surface atmospheric water vapor concentration and by uncertainties regarding surface regolith and atmospheric boundary layer properties. However, i f the diurnal exchange of water vapor with the surface is as large as is suggested by the Viking Lander and Orbiter measurements, then the e xchange of water between the atmosphere and regolith also is important in the seasonal cycle of water vapor. Further characterization of the se processes can be made using measurements from the various landing s ite and atmospheric profiling experiments to be conducted by the Mars Pathfinder and Mars Surveyor Lander and Orbiter missions. (C) 1997 Aca demic Press.