S2O, polysulfuroxide and sulfur polymer on Ioʹs surface?

To settle the question of disulfur monoxide and sulfur monoxide deposition and occurrence on Ioʹs surface, we performed series of laboratory experiments reproducing the condensation of S2O at low temperature. Its polymerization has been monitored by recording infrared spectra under conditions of temperature, pressure, mixing with SO2 and UV–visible radiation simulating that of Ioʹs surface. Our experiments show that S2O condensates are not chemically stable under ionian conditions. We also demonstrate that SO and S2O outgassed by Ioʹs volcanoes and condensing on Ioʹs surface should lead to yellow polysulfuroxide deposits or to white deposits of S2O diluted in sulfur dioxide frost (i.e., S2O/SO2 < 0.1%). Thus S2O condensation cannot be responsible for the red volcanic deposits on Io. Comparison of the laboratory infrared spectra of S2O and polysulfuroxide with NIMS/Galileo infrared spectra of Ioʹs 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 S8 sulfur and SO2 reflectance spectrum, leads to a very good matching of Ioʹs visible spectrum between 330 and 520 nm. We conclude then that Ioʹs surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S8 and sulfur polymer. Some polysulfuroxide could also co-exist with these dominant components, but is probably restricted to some volcanic areas.