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

Abstract We used 0.85–5.1 μm 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 evidence for ammonia ice clouds without lightning. Finding ammonia ice spectral signatures in polar regions is surprising because over most of Saturn an overlying haze layer of unknown composition but significant optical depth completely obscures it, unless penetrated by significant convection, confirmed by lightning, as in the Great Storm of 2010–2011 (Sromovsky et al. 2013, Icarus 226, 402–418), and in the storms of Storm Alley region (Baines et al. 2009, Planet. & Space Sci. 57, 1650–1658). This is clarified by our radiation transfer modeling of VIMS spectra of the south polar background and discrete features, using 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 NH3 ice particles near 900 mbar, and (4) extending from 5 bars up to 2–4 bars, an assumed optically thick layer where NH4SH and H2O are likely condensables. The ammonia layer is the main modulator of pseudo-continuum I/F in reflected sunlight. That layer has about one optical depth in background clouds, but about double that in the brightest clouds, and about half that in discrete dark clouds. What makes the 3-μm absorption unexpectedly apparent in these polar clouds is the relatively low optical depth of the top tropospheric cloud layer, which can be an order of magnitude less than in non-polar regions on Saturn, perhaps because of polar downwelling and/or lower photochemical production rates. We found changes in the PH3 vertical profile and AsH3 mixing ratio that support the existence of downwelling within 2° of the pole. We also found evidence for step-wise decreases in optical depth of the stratospheric haze near 87.9° S and in the putative diphosphine layer near 88.9° S, and evidence against the idea that deep convective eyewalls are responsible for the shadows observed near the same latitudes. In 752-nm Cassini images we identified moderately bright features extending from shadow-producing boundaries when those boundaries rotated to the opposite side of Saturn's pole. Under those observing conditions an illuminated eyewall should produce a bright feature extending towards the pole. Instead, the features extend away from the pole, as expected for what we call antishadows, which are bright features produced by light illuminating a translucent layer from below. This provides strong qualitative evidence that both shadows and antishadows are produced by small step changes in the optical depth of the overlying translucent aerosol layers.