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[29.10] Tentative identification of a Chlorine molecule at Io's surface

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