WATER-VAPOR SATURATION AT LOW ALTITUDES AROUND MARS APHELION - A KEY TO MARS CLIMATE

The combined analysis of microwave temperature and water profiling of the Mars atmosphere indicates that low- to midlatitude water vapor sat uration typically occurs at much lower altitudes (below 10 km) during northern spring/summer than observed during this Mars aphelion season in the dusty, warm period of Viking observations (above 25 km). Temper atures profiles of the 0-60 km global Mars atmosphere are retrieved fr om microwave CO spectra around Mars aphelions in 1980, 1982, 1989, 199 1, 1993, and 1995. These microwave temperature retrievals are 15-20 K colder than the Viking temperature measurements at the same season in 1976 and 1978, implying dust-free, radiative-convective conditions for the global Mars atmosphere at the aphelions of the microwave measurem ents, Mars water profiling from very large array water and Kitt Peak w ater isotope spectra were obtained in the 1993 and 1995 Mars aphelion periods, Their analysis indicates that Mars water vapor at low to mid latitudes was confined to altitudes below 10 km during these aphelion periods, in agreement with the low altitude of water vapor saturation predicted by the cold microwave temperature profiles, The existence of such low-altitude water vapor saturation for the aphelion Mars atmosp here is corroborated by HST ultraviolet and violet cloud imaging of th e Mars atmosphere in 1991, 1993, and 1995, These images display a prev iously unidentified, global belt of moderate opacity (tau similar to 0 .2-0.6) clouds covering the similar to 10 degrees S-30 degrees N latit ude region around Mars aphelion (solar longitude, L(s) similar to 60 d egrees-100 degrees) for three consecutive Mars years, The center of th is low-latitude cloud belt corresponds to the region of upward advecti on within the summer solstice Hadley circulation, These cold atmospher ic temperatures, low altitudes of water vapor saturation, and low-lati tude cloud belts are observed only around Mars aphelion, which present ly occurs during northern late spring/early summer (L(s) = 71 degrees) . This behavior reflects the highly elliptical Mars orbit in which glo bal surface and atmospheric temperatures vary by 20 K with orbital dis tance from the sun. The perihelion of Mars (southern late spring/early summer, L(s) = 251 degrees) is recognized as the season of global dus t storms, which result from the higher solar flux incident at periheli on (e.g., Zurek and Martin 1993). We argue that the aphelion period ex hibits a similarly distinct climate (cloudy and cold), which was not a s apparent during the unusually dusty Mars years of the Viking observa tions. We further argue that this aphelion climate may be the key to u nderstanding the large north-south hemispheric asymmetries of Mars wat er vapor and the residual polar ice caps. The orbital dependence of th e altitude of water vapor saturation can couple with the solstice Hadl ey circulations of the Mars atmosphere to create a non-linear atmosphe ric water pump toward the aphelion summer hemisphere. It is even possi ble that this process accounts for the origin of the polar layered dep osits, as the hemispheric direction of this water pump alternates ever y similar to 25,000 years due to the orbital progression of the season of Mars perihelion. We also point out that an increased importance fo r global cloud formation in the Mars atmosphere suggests important non -linear relationships between atmospheric water and dust in the curren t Mars climate, which may contribute to the extreme interannual variat ions of Mars dust storm behavior and the current albedo and compositio nal differences of the north and south polar ice deposits. (C) 1996 Ac ademic Press, Inc.