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Saturn's Great Storm of 2010 - 2011 produced a planet-encircling wake thatslowly transitioned from a region that was mainly dark at 5 microns in February2011 to a region that was almost entirely bright and remarkably uniform byDecember of 2012. The uniformity and high emission levels suggested that theentire wake region had been cleared not only of the ammonia clouds that thestorm had generated and exposed, but also of any other aerosols that mightprovide significant blocking of the thermal emission from Saturn's deeper andwarmer atmospheric layers. Our analysis of VIMS wake spectra from December 2012provides no evidence of ammonia ice absorption, but shows that at least onesignificant cloud layer remained behind: a non-absorbing layer of 3 - 4 opticaldepths (at 2 microns) extending from 150 to ~400 mbar. A second layer ofabsorbing and scattering particles, with less than 1 optical depth and locatednear 1 bar, is also suggested, but its existence as a model requirement dependson what value of the He/H2 ratio is assumed. The observations can be fit wellwith just a single (upper) cloud layer for a He/H2 ratio of 0.064 incombination with a PH3 deep volume mixing ratio of 5 ppm. At lower He/H2ratios, the observed spectra can be modeled without particles in this region.At higher ratios, in order to fit the brightest wake spectrum, models mustinclude either significant cloud opacity in this region, or significantlyincreased absorption by PH3, NH3, and AsH3. As the exceptional horizontaluniformity in the late wake is most easily understood as a complete removal ofa deep cloud layer, and after considering independent constraints on trace gasmixing ratios, we conclude that the existence of this remarkable wakeuniformity 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 previousestimates.

Cloud clearing in the wake of Saturn's Great Storm of 2010 - 2011 and suggested new constraints on Saturn's He/H2 ratio