Evidence for sulphur implantation in Europa's UV absorption band

The International Ultraviolet Explorer (IUE) spacecraft has obtained observations of the galilean satellites over the past 2 years which fortuitously span the periods of the Voyager encounters with the jovian system. Our IUE observing programme is designed to determine the UV spectral characteristics of the galilean satellites as a function of orbital position, large-scale areal variability and temporal dynamics. During the past year we have concentrated on the albedo variations of each body1,2. We report here the discovery of an absorption feature at 280 nm in Europa's reflection spectrum. Observations with the IUE show that this absorption is strongest on Europa's trailing hemisphere (central longitude 270°). We identify the feature as an SO2 absorption band and hypothesize that SO2 may form when energetic jovian magnetospheric sulphur ions are injected into Europa's water-ice surface.