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B. Schmitt, S. Rodriguez (LPG)
Starting from the recent discovery of abundant chlorine ions in Io's plasma torus, we searched for evidence of Cl and S-bearing species at the surface of the satellite. The mid-infrared (2-25 \mum) transmission spectra of H2S, SO3, Cl2SO2, Cl2SO, Cl2S2, and Cl2S as pure solids and diluted in SO2 ice have been measured in our laboratory at several temperatures above 100 K. These laboratory data were compared to a whole set of Io spectra recorded by NIMS/Galileo, IRIS/Voyager, SWS/ISO and CGS4/UKIRT which still display a series of unidentified or controversial absorption and emission bands. Pure solid H2S and Cl2SO2 diluted in solid SO2 may both explain, or contribute to, the 2550 cm-1 band locally observed by the NIMS spectro-imager. Diluted Cl2SO2 is also the only molecule studied sofar that may explain the emission feature observed by Voyager around 587 cm-1. We discuss the very different physical states and abondances of these two candidate molecules as implied by their band positions and intensities. Their stability at Io's surface temperatures, strongly favouring Cl2SO2 trapped in SO2 ice over pure solid H2S, are also compared. We conclude on some possible implications on Io's chemistry. We acknowledge CNES and the French Programme National de Planétologie of CNRS (INSU) for their financial supports.
The author(s) of this abstract have provided an email address for comments about the abstract: Bernard.Schmitt@obs.ujf-grenoble.fr