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.