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J.S. Kargel (U.S. Geological Survey), T. MacIntyre (Northern Arizona University), B. Dalton, R. Clark (U.S. Geological Survey)
Galileo SSI and NIMS data have returned a wealth of information on the surface composition of Io. Our analysis suggests that elemental sulfur and sulfur dioxide are nearly ubiquitous but that the ratio of these two materials varies widely. Some of the most spectacular concentrations of elemental sulfur occur near major volcanoes and hotspots. In these areas the color and spectral signature of sulfur and any other materials present varies widely, in some cases forming concentrically zoned deposits around calderas. The spectral variability of sulfur may be due to many mechanisms; inclusion of minor impurities is among the possible controls. We have made some tentative identifications of possible impurities-- iron (as opaque inclusions of FeS disseminated in sulfur) appears to cause the green coloration of some deposits on Io-- including Io's so-called "golf courses." Arsenic, selenium, and tellurium (dissolved in quenched sulfur polymers and/or bonded in stoichiometric compounds with sulfur) may contribute to reddish coloration of other deposits. As little as one percent of these impurities may produce marked changes in the spectral reflectance and color of sulfur. However, other mechanisms can account for Io's reds; Io's green deposits are harder to explain in the absence of pyrite or other metallic sulfide inclusions in elemental sulfur; the logical alternative, ultramafic silicates, has fallen out of favor with the Galileo team due to absence of a diagnostic band near 0.9 microns. These colored forms of impure sulfur have been observed both in nature (in terrestrial volcanogenic native sulfur) and in recent lab specimens quenched from the molten state and containing controlled quantities of impurities (cf abstract by MacIntyre et al., this volume). Evidence for impurities in Io's sulfur will be presented, and we will offer a glimpse into possible geological process implications of the distribution of sulfur on Io and the impurities that may be present in Io's sulfur.
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