The combined analysis of microwave temperature and water profiling of the Mars atmosphere indicates that low- to mid-latitude water vapor saturation 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). Temperatures profiles of the 0–60 km global Mars atmosphere are retrieved from microwave CO spectra around Mars aphelions in 1980, 1982, 1989, 1991, 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 measurements. Mars water profiling from very large array water and Kitt Peak water 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 atmosphere is corroborated by HST ultraviolet and violet cloud imaging of the Mars atmosphere in 1991, 1993, and 1995. These images display a previously unidentified, global belt of moderate opacity (τ ∼ 0.2–0.6) clouds covering the ∼10°S–30°N latitude region around Mars aphelion (solar longitude,Ls∼ 60°–100°) for three consecutive Mars years. The center of this low-latitude cloud belt corresponds to the region of upward advection within the summer solstice Hadley circulation.
These cold atmospheric temperatures, low altitudes of water vapor saturation, and low-latitude cloud belts are observed only around Mars aphelion, which presently occurs during northern late spring/early summer (Ls= 71°). This behavior reflects the highly elliptical Mars orbit in which global surface and atmospheric temperatures vary by 20 K with orbital distance from the sun. The perihelion of Mars (southern late spring/early summer,Ls= 251°) is recognized as the season of global dust storms, which result from the higher solar flux incident at perihelion (e.g., Zurek and Martin 1993). We argue that the aphelion period exhibits a similarly distinct climate (cloudy and cold), which was not as apparent during the unusually dusty Mars years of the Viking observations. We further argue that this aphelion climate may be the key to understanding the large north–south hemispheric asymmetries of Mars water vapor and the residual polar ice caps. The orbital dependence of the altitude of water vapor saturation can couple with the solstice Hadley circulations of the Mars atmosphere to create a non-linear atmospheric water pump toward the aphelion summer hemisphere. It is even possible that this process accounts for the origin of the polar layered deposits, as the hemispheric direction of this water pump alternates every ∼25,000 years due to the orbital progression of the season of Mars perihelion. We also point out that an increased importance for global cloud formation in the Mars atmosphere suggests important non-linear relationships between atmospheric water and dust in the current Mars climate, which may contribute to the extreme interannual variations of Mars dust storm behavior and the current albedo and compositional differences of the north and south polar ice deposits.