S 2O, polysulfuroxide and sulfur polymer on Io's surface?

Abstract

To settle the question of disulfur monoxide and sulfur monoxide deposition and occurrence on Io's surface, we performed series of laboratory experiments reproducing the condensation of S 2O at low temperature. Its polymerization has been monitored by recording infrared spectra under conditions of temperature, pressure, mixing with SO 2 and UV–visible radiation simulating that of Io's surface. Our experiments show that S 2O condensates are not chemically stable under ionian conditions. We also demonstrate that SO and S 2O outgassed by Io's volcanoes and condensing on Io's surface should lead to yellow polysulfuroxide deposits or to white deposits of S 2O diluted in sulfur dioxide frost (i.e., S 2O/SO 2 < 0.1%). Thus S 2O condensation cannot be responsible for the red volcanic deposits on Io. Comparison of the laboratory infrared spectra of S 2O and polysulfuroxide with NIMS/Galileo infrared spectra of Io's surface leads us to discuss the possible identification of polysulfuroxide. We also recorded the visible transmission spectra of sulfur samples resulting from polysulfuroxide decomposition. These samples consist in a mixture of sulfur polymer and orthorhombic sulfur. Using the optical constants extracted from these measurements, we show that a linear combination of the reflectance spectra of our samples, the reflectance spectrum of orthorhombic S 8 sulfur and SO 2 reflectance spectrum, leads to a very good matching of Io's visible spectrum between 330 and 520 nm. We conclude then that Io's surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S 8 and sulfur polymer. Some polysulfuroxide could also co-exist with these dominant components, but is probably restricted to some volcanic areas.