The aim of this study is to contribute to a better understanding of Io's surface chemical composition. For this purpose, we combined a thermochemical model of Ionian volcanic gases with a spectroscopic and chemical experimental study of low temperature condensed molecules.The thermochemical study is carried out using an improved volcanic gases model, in order to predict the most probable molecules ejected by Io's volcanoes and to follow them during their cooling and the condensation of some of them.The experimental study concerns disulfur monoxide, S2O, a major component predicted by the thermochemical model, and its polymer, polysulfur oxide. We reproduced S2O low temperature condensation, followed its polymerization and measured its infrared spectra in laboratory, under conditions of temperature, pressure, mixing with SO2 and UV-visible radiation simulating Ionian ones.On the one hand, this study improves our knowledge of S2O and of its polymerization mechanism and gives a better idea of polysulfur oxide's structure. On the other hand, our experiments and our spectroscopic results compared to Io's infrared and visible spectra lead us to the conclusion that S2O can not be responsible for red volcanic deposits on Io and that Io's surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S8 and sulfur polymer. To these dominant components, some polysulfur oxide could be added, possibly localized in more restricted volcanic areas.
