Hubble Space Telescope observations of the Martian aphelion cloud belt prior to the Pathfinder mission: Seasonal and interannual variations

The presence of a globally extant equatorial belt of water ice clouds on Mars is quantitatively investigated using data from three seasons of our Hubble Space Telescope synoptic monitoring program (1993-1997). A subset of the 1996-1997 images covers the landing site of the Mars Pathfinder including a set of images taken after touchdown. Using multicolor imaging from the Wide Field Planetary Camera and the Wide Field Planetary Camera 2, we characterize both water ice cloud and dust optical depths as a function of latitude at several local times for each observing epoch. The analysis technique models calibrated data using a multiple scattering radiative transfer code. Our results support the initial results of Clancy et al. [1996a] regarding changes between the aphelion and perihelion climate of Mars and provide a more detailed look at the development and decay of the cloud belt. Comparing our dust optical depths to those of the Viking landers for the same seasons, we note a trend toward lower dust loading in late northern winter and in spring. Our observations of the Pathfinder site in July 1997 reveal a dust opacity in good agreement with that reported by Pathfinder [Smith et al., 1997b]. In addition, the serendipitous occurrence of a dust storm in Valles Marineris in late June 1997 allows us to derive a set of dust single scattering albedos for use in more accurately modeling the dust's radiative properties and effects.