Remote sensing evidence for regolith water vapor sources on Mars

A survey of ground‐based and spacecraft observations at visible wavelengths of condensate activity in the Martian atmosphere is presented. The observations reveal that Solis Lacus has been a center of condensate activity throughout the observational record, along with sites in Noachis‐Hellespontus and in border regions of Syrtis Major. The condensate activity did not appear to be controlled by the distribution of aerosol dust, surface albedo, thermal inertia, or topography. Rather, the patterns of cloud and fog occurrence, together with observed and inferred surface winds, point to these sites as primary H 2 O vapor source regions for condensate development. Specifically, it is proposed (1) that the unusual distribution of condensates around Solis Lacus at the onset of the 1973 dust storm may have resulted from the transport of moisture from the central plume site in northern Solis Lacus (near Sinai Planum) by prevailing daytime winds and nighttime drainage winds, (2) that the recurrent winter Tharsis and Hellas condensate features may have developed from moisture derived primarily from the upwind Solis Lacus and Noachis‐Hellespontus source regions, respectively, and (3) that the recurrent summer Elysium clouds may have developed as a result of moisture from upwind Syrtis Major border sites. The estimated outgassing rate from Solis Lacus during 1973 was ∼10 −1 km 3 yr −1 , and a similar rate is suggested for the Noachis‐Hellespontus site. These rates would probably not have been high enough to be detected by the Viking MAWD experiment, but they extrapolate to the possible loss of a meter of ice from the sites in only 10 3 years. It is proposed that the ice is present in the surface layer and that it could undergo melting during a portion of the day over an appreciable fraction of the year. Radar, thermal, and water vapor data would be consistent with the possible occurrence of soil moisture at these sites.