Hydrogen sulfide on Io - Evidence from telescopic and laboratory infrared spectra

Evidence is reported for hydrogen sulfide on Io's surface. An infrared band at 3.915 (+ or - 0.015) micrometers in several ground-based spectra of Io can be accounted for by reflectance from H2S frost deposited on or cocondensed with sulfur dioxide frost. Temporal variation in the occurrence and intensity of the band suggests that condensed H2S on Io's surface is transient, implying a similar variation of H2S abundance in Io's atmosphere.