  Our analysis of Cassini/VIMS near-infrared spectra of Saturn's Great Storm of
2010-2011 reveals a multi-component aerosol composition comprised primarily of
ammonia ice, with a significant component of water ice. The most likely third
component is ammonium hydrosulfide or some weakly absorbing material similar to
what dominates visible clouds outside the storm region. Horizontally
heterogeneous models favor ammonium hydrosulfide as the third component, while
horizontally uniform models favor the weak absorber. Both models rely on water
ice absorption to compensate for residual spectral gradients produced by
ammonia ice from 3.0 microns to 3.1 microns and need the third component to
fill in the sharp ammonia ice absorption peak near 2.96 microns. The best
heterogeneous model has spatial coverage fractions of 55% ammonia ice, 22%
water ice, and 23% ammonium hydrosulfide. The best homogeneous model has an
optically thin layer of weakly absorbing particles above an optically thick
layer of water ice particles coated by ammonia ice. This is the first
spectroscopic evidence of water ice in Saturn's atmosphere, found near the
level of Saturn's visible cloud deck where it could only be delivered by
powerful convection originating from ~200 km deeper in the atmosphere.
