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W. D. Smythe, R. Lopes-Gautier (Jet Propulsion Laboratory), S. Doute (UCLA), S. W. Kieffer (Kieffer Science Consulting), R. W. Carlson, L. Kamp (Jet Propulsion Laboratory), F. E. Leader (UCLA)
Sulfur Dioxide is ubiquitous on Io's surface. The presence of SO2 on Io has been recognized since the early seventies and the NIMS instrument mapped its widespread presence on Io in the nineties, during the Galileo prime mission. This mapping, obtained at resolutions varying from about 150 to 400 km/pixel showed that the strength of the SO2 absorptions varies considerably across Io's surface. When the relative band strengths are converted surface abundance it becomes evident that there is poor correlation between areas having high visible albedo (white areas) and areas having the highest concentrations of SO2. This result was unexpected, since pure SO2 frost is visibly white, and has been interpreted as evidence that SO2 often may be co-deposited with other materials.
In contrast, an observation obtained recently by the NIMS and SSI instruments near the Chaac region, revealed an area that exhibits one of the highest abundances of SO2 seen to date, and that is visibly very bright. The shape of this area is clearly controlled by topography, suggesting that the SO2 in the area was not emplaced by deposition. The high visible brightness of the deposit suggests that this SO2 was not deposited directly from a plume. The shape, color, and high abundance of SO2 in this deposit suggest that it may have been emplaced by a flooding event. The frequency of occurrence for deposits similar to this cannot be estimated from the current Io data set, because the size of the feature, ~100 square kilometers, is below the spatial resolution of the available spectral maps of Io.
This work was supported by NASA through Galileo and the JSDAP program.
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