A New Look at Water Ice Clouds on Mars

M. J. Wolff, R. T. Clancy (SSI), P. B. James (U. Toledo), S. W. Lee (LASP/Colorado), J. F. Bell III (U. Cornell)

A fundamental problem in the current understanding of the Martian atmosphere and climate is the role of water ice clouds. Until recently, the community's view had been primarily shaped by the observations obtained during the Viking missions. However, recent microwave temperature and water profiling measurements indicate that the warm, dusty period observed by Viking was quite possibly atypical. More specifically, Clancy et al. (1995, Icarus, 122, 36) find that the current atmosphere is approximately 15-20 K colder than the Viking temperature measurements obtained for the same seasons. A natural consequence of a dryer, cooler Martian climate would be the presence of a global mid-latitude cloud belt, which has in fact been observed in the limited analysis performed by Clancy et al. of several HST images (but not seen in the Viking data). It is not unreasonable to consider that the effects of low-to-mid latitude water vapor saturation at low altitudes may play a significantly more important role in the current climatic state than previously believed. The key to enhancing our knowledge is the long-term synoptic monitoring of atmospheric phenomena. The combination of poor spatial resolution and other telluric effects necessitates the use of spacecraft data. Fortunately, an excellent dataset already exists in data archives of the Hubble Space Telescope. We will present a radiative transfer analysis of a subset of these data (taken 1990-1995) in order to better characterize the spatial and temporal variability of atmospheric cloud processes, including interannual variations. We include a series of electromagnetic scattering calculations (for hexagonal and cylindrical prisms) in order to derive the optical depth sensitivity to nonspherical ice particle phase functions.