Europa : a New Look at Galileo Uvs Data

Introduction: Europa is unique among the icy galilean satellites, in that it is the one (with the exception of Io) that displays indications of recent or current surface activity. These indications include lineaments, triple bands, wedges, icebergs, as well as pits, spots and domes (e.g. [1], [2]). These regions of recent endogenic activity have been found to correlate with Galileo Near Infrared Mapping Spectrometer (NIMS) spectra in which the water ice absorption features at 1.5 and 2.0 μm are asymmetric [3]. There are two interpretations of NIMS Europa spectra and the asymmetric water ice bands that are measured. One interpretation is that the bands are produced by the presence of hydrated salts, such as MgSO4•7H2O, derived from an internal ocean [3]. A second interpretation is that the spectral features are the result of the presence of sulfuric acid hydrate [4] that is being continuously produced and destroyed by radiolysis. More recent laboratory data show that fits to NIMS data are achieved for mixtures of sulfuric acid with sodiumand magnesiumbearing sulfate salts [5]. Disk-integrated UV observations of Europa from the International Ultraviolet Explorer (IUE) [6] and Hubble Space Telescope (HST) [7] show an absorption feature centered at 280 nm on the trailing hemisphere. The absorption feature has been presumed to be due to an exogenic activity such as implantation of sulfur ions [6] and has been successfully compared with a laboratory spectrum of SO2 ice grown on H2O ice; the feature is likely related to an S-O bond. Low-resolution diskresolved UV observations from the Galileo Ultraviolet Spectrometer (UVS) [8] showed that the UV absorption feature decreases in strength with distance from the trailing hemisphere apex (270°W). Examples of two UV spectra of Europa are shown in Fig. 1. We now present the highest spatial resolution observations from Galileo UVS. Early analysis of highresolution Galileo UVS observations [9] showed a general correlation between the 280 nm absorption feature and the dark terrain on Europa’s trailing hemisphere. We now investigate and quantify that correlation by mapping the band strength for all UVS observations across the surface, and comparing the strength of the 280 nm absorption feature across the surface with the strength of the NIMS-measured asymmetric water ice bands. Fig. 1. Ultraviolet spectra for two disk-resolved regions on Europa, from the Galileo UVS [8]. At the apex of the trailing hemisphere (270° W), the 280 nm absorption band is prominent.