  We used 0.85 - 5.1 micron 2006 observations by Cassini's Visual and Infrared
Mapping Spectrometer (VIMS) to constrain the unusual vertical structure and
compositions of cloud layers in Saturn's south polar region, the site of a
powerful vortex circulation, shadow-casting cloud bands, and spectral evidence
of ammonia ice clouds without the lightning usually associated with such
features. We modeled spectral observations with a 4-layer model that includes
(1) a stratospheric haze, (2) a top tropospheric layer of non-absorbing
(possibly diphosphine) particles near 300 mbar, with a fraction of an optical
depth (much less than found elsewhere on Saturn), (3) a moderately thicker
layer (1 - 2 optical depths) of ammonia ice particles near 900 mbar, and (4)
extending from 5 bars up to 2-4 bars, an assumed optically thick layer where
NH4SH and H20 are likely condensables. What makes the 3-micron absorption of
ammonia ice unexpectedly apparent in these polar clouds, is not penetrating
convection, but instead the relatively low optical depth of the top
tropospheric cloud layer, perhaps because of polar downwelling and/or lower
photochemical production rates. We did not find any evidence for optically
thick eyewalls that were previously thought to be responsible for the observed
shadows. Instead, we found evidence for small step-wise decreases in optical
depth of the stratospheric haze near 87.9 deg S and in the putative diphosphine
layer near 88.9 deg S, which are the likely causes of shadows and bright
features we call antishadows. We found changes as a function of latitude in the
phosphine vertical profile and in the arsine mixing ratio that support the
existence of downwelling within 2 deg of the pole.
