Discovery of Gaseous S 2 in Io's Pele Plume

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Abstract
Spectroscopy of Io's Pele plume against Jupiter by the Hubble Space Telescope in October 1999 revealed absorption due to S 2 gas, with a column density of 1.0 ± 0.2 × 10 16 per square centimeter, and probably also SO 2 gas with a column density of 7 ± 3 × 10 16 per square centimeter. This SO 2 /S 2 ratio (3 to 12) is expected from equilibration with silicate magmas near the quartz-fayalite-magnetite or wüstite-magnetite buffers. Condensed S 3 and S 4 , probable coloring agents in Pele's red plume deposits, may form by polymerization of the S 2 , which is unstable to ultraviolet photolysis. Diffuse red deposits near other Io volcanoes suggest that venting and polymerization of S 2 gas is a widespread feature of Io volcanism.