Spectroscopy of Io's Pele plume against Jupiter by the Hubble Space Telescope in October 1999 revealed absorption due to S2 gas, with a column density of 1.0 ± 0.2 x 1016 per square centimeter, and probably also SO2 gas with a column density of 7 ± 3 x 1016 per square centimeter. This SO2/S2 ratio (3 to 12) is expected from equilibration with silicate magmas near the quartz-fayalite-magnetite or wustite-magnetite buffers. Condensed S3 and S4, probable coloring agents in Pele's red plume deposits, may form by polymerization of the S2, which is unstable to ultraviolet photolysis. Diffuse red deposits near other Io volcanoes suggest that venting and polymerization of S2 gas is a widespread feature of Io volcanism.
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