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A. R. Hendrix, C. A. Barth, C. W. Hord, A. I. F. Stewart, K. E. Simmons (LASP, CU), W. K. Tobiska, A. L. Lane (JPL)
We report on Galileo Ultraviolet Spectrometer (UVS) observations of Europa from the initial orbits of the Galileo Europa Mission and from the final orbits of the nominal mission. These high spatial resolution observations (instantaneous field-of-view footprint size of 27 km x 109 km) reveal that Europa's trailing hemisphere absorber is more complicated than previously thought. Earlier global and lower spatial resolution observations indicated that Europa's 280 nm absorption becomes stronger approaching the trailing hemisphere apex (270 deg. W). The absorption has generally been thought to be due to implanted sulfur ions from the corotating magnetosphere being implanted into the water ice surface, creating an S-O bond that absorbs near 280 nm. However, these recent UVS spectra at higher spatial resolution show regions where the absorption-longitude correlation is not complete and the absorption appears to be stronger where Europa's surface is darker, for example in the mottled terrain and linea. These recent results may indicate that the absorber is endogenic, or that an additional ultraviolet absorber exists in Europa's dark regions.