The Global and Local Characters of Mars Perihelion Cloud Trails

Abstract:

We present the seasonal and spatial distribution of Mars perihelion cloud trails as mapped from Mars Reconnaissance Orbiter (MRO) MARCI (Mars Color Imager) imaging observations in 2 ultraviolet and 3 visible filters. The extended 2007-2013 period of MARCI daily global image maps reveals the widespread distribution of these high altitude clouds, which are somewhat paradoxically associated with specific surface regions. They appear as longitudinally extended (300-700 km) cloud trails with distinct leading plumes of substantial ice cloud optical depths (0.02-0.2) for such high altitudes of occurrence (40-50 km, from cloud surface shadow measurements). These plumes generate small ice particles (Reff~1 to <0.2 microns) that become entrained in the strong westward mesospheric zonal wind regime of the perihelion (southern summer ) season, although the specific dynamics and cloud microphysics for these plumes are not determined. They are clearly tied to maximum surface heating on Mars, in terms of season (perihelion), local time (early afternoon), and surface latitude/albedo/elevation. The most persistent expressions of these clouds are found in association with two Valles Marineris locations (Clancy et al., 2009). However, a broader survey of MARCI observations reveals many specific regions of occurrence over Ls=200-300. Furthermore, MRO CRISM limb observations indicate a haze of such fine water ice particles characterizes the full southern hemisphere mesosphere at this time. Hence, the behavior of perihelion cloud trails appears to reflect locally elevated mesospheric water ice formation that may impact the global expression of mesospheric water ice aerosols.