The violet absorber on Io: Disulfur monoxide derivatives

Following the Voyager flybys of Jupiter it was generally accepted that the surface of Io is covered with SO2 frost and elemental sulfur. However, several difficulties have recently become apparent with this hypothesis. The S2O, which is readily formed under a variety of conditions from the dissociation of SO2, may be an important compound on Io, but at that time no reflectance data on the frost was available. The reflectance spectra of S2O/SO2 mixtures as low temperature frosts and their derivatives, polysulfur oxides, were measured at a variety of temperatures. Specific features possessed by these frosts and identified on Io are bands at 340 nm and 440 nm and a maximum at 800 nm, with decreasing IR reflectance. Thus, this material can account for many of the features in the spectrum of Io. So S2O rather than elemental S is probably the reddening agent on Io, and there is presently no good evidence for S on the surface.