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We used 0.85 - 5.1 micron 2006 observations by Cassini's Visual and InfraredMapping Spectrometer (VIMS) to constrain the unusual vertical structure andcompositions of cloud layers in Saturn's south polar region, the site of apowerful vortex circulation, shadow-casting cloud bands, and spectral evidenceof ammonia ice clouds without the lightning usually associated with suchfeatures. 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 opticaldepth (much less than found elsewhere on Saturn), (3) a moderately thickerlayer (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 whereNH4SH and H20 are likely condensables. What makes the 3-micron absorption ofammonia ice unexpectedly apparent in these polar clouds, is not penetratingconvection, but instead the relatively low optical depth of the toptropospheric cloud layer, perhaps because of polar downwelling and/or lowerphotochemical production rates. We did not find any evidence for opticallythick eyewalls that were previously thought to be responsible for the observedshadows. Instead, we found evidence for small step-wise decreases in opticaldepth of the stratospheric haze near 87.9 deg S and in the putative diphosphinelayer near 88.9 deg S, which are the likely causes of shadows and brightfeatures we call antishadows. We found changes as a function of latitude in thephosphine vertical profile and in the arsine mixing ratio that support theexistence of downwelling within 2 deg of the pole.

Saturn's south polar cloud composition and structure inferred from 2006 Cassini/VIMS spectra and ISS images