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J.R. Spencer (Lowell Obs.), K.L. Jessup, G.E. Ballester (U. Michigan), M.A. McGrath (STScI), R. Yelle (Northern Arizona U.)
2100-3000 Å\ spectroscopy of Io's Pele plume against Jupiter by HST/STIS on October 9 1999, at the time of the Galileo I24 flyby, revealed absorption due to S2 gas, with a column density of 1.0 ±0.2 \times 1016 cm-2, and probably also SO2 gas with a column density of 7 ±3 \times 1016 cm-2 This SO2/S2 ratio (3 to 12) is expected from equilibration with silicate magmas near the quartz-fayalite-magnetite or wüstite-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 and thermal breakdown. Diffuse red deposits near other Io volcanoes suggest that venting and polymerization of S2 gas is a widespread feature of Io volcanism. For full details see Spencer et al. 2000, Science 288, 1208--1210.
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