  Saturn's Great Storm of 2010 - 2011 produced a planet-encircling wake that
slowly transitioned from a region that was mainly dark at 5 microns in February
2011 to a region that was almost entirely bright and remarkably uniform by
December of 2012. The uniformity and high emission levels suggested that the
entire wake region had been cleared not only of the ammonia clouds that the
storm had generated and exposed, but also of any other aerosols that might
provide significant blocking of the thermal emission from Saturn's deeper and
warmer atmospheric layers. Our analysis of VIMS wake spectra from December 2012
provides no evidence of ammonia ice absorption, but shows that at least one
significant cloud layer remained behind: a non-absorbing layer of 3 - 4 optical
depths (at 2 microns) extending from 150 to ~400 mbar. A second layer of
absorbing and scattering particles, with less than 1 optical depth and located
near 1 bar, is also suggested, but its existence as a model requirement depends
on what value of the He/H2 ratio is assumed. The observations can be fit well
with just a single (upper) cloud layer for a He/H2 ratio of 0.064 in
combination with a PH3 deep volume mixing ratio of 5 ppm. At lower He/H2
ratios, the observed spectra can be modeled without particles in this region.
At higher ratios, in order to fit the brightest wake spectrum, models must
include either significant cloud opacity in this region, or significantly
increased absorption by PH3, NH3, and AsH3. As the exceptional horizontal
uniformity in the late wake is most easily understood as a complete removal of
a deep cloud layer, and after considering independent constraints on trace gas
mixing ratios, we conclude that the existence of this remarkable wake
uniformity is most consistent with a He/H2 mixing ratio of 0.055 (+0.010,
-0.015), which is on the low side of the 0.038 - 0.135 range of previous
estimates.
