Previous analyses of martian atmospheric water vapor measurements suggest that seasonal exchange of water occurs between the atmosphere and the regolith. This conclusion rests on the presumed physical properties of the regolith and on the comparison of models of the seasonal transport of water with the seasonal measurements. In addition, however, a direct indication of exchange can be seen in the comparison between orbiter measurements of atmospheric water vapor and measurements made from a lander. The Viking lander atmospheric temperature measurements determined the frost-point saturation temperature of the atmosphere. This was done by observing an inflection in the nighttime decline in temperature that resulted from latent heat effects. The temperature at which this occurred determines the near-surface number density of water molecules. This number density was a factor of 2-3 lower than expected based on the Viking MAWD measurements of the water vapor column. This depletion of the near-surface water is exactly what is expected when diffusion of water into the regolith occurs at night--the lower several hundred meters of the atmosphere becomes depleted of water vapor. The observed depletion is outside of the uncertainties of the two measurement techniques, and goes in the opposite direction from the behavior expected for other processes. Therefore, the measurements provide a direct indication that exchange is occurring diurnally. This conclusion provides guidance as to how to better define the martian water cycle with additional measurements: Global synoptic measurements from orbit can provide the 3-D water vapor field. Measurements of the polar regions allow inference of the polar-cap supply and loss of water vapor. And, measurements at discrete points on the surface can determine where and to what degree exchange with the regolith occurs. Upcoming Mars Pathfinder and '98 Surveyor Lander measurements will extend the analysis, and additional landers at key points can define the global and seasonal water cycle.
AAS/Division for Planetary Sciences Meeting Abstracts #28
- Pub Date:
- September 1996