Europa: Disk-Resolved Ultraviolet Measurements Using the Galileo Ultraviolet Spectrometer

The Galileo ultraviolet spectrometer has measured the first-ever disk-resolved spectral ultraviolet albedos of regions on Europa's trailing and antijovian hemispheres at spatial resolutions approaching 55 km. We find that the ultraviolet albedo (2400–3200 Å) increases with distance from the trailing hemisphere apex (270° W). Regions close to 270° W display an absorption feature centered near 2800 Å, which generally decreases with distance from the trailing hemisphere apex. These results substantiate the idea that magnetospheric bombardment darkens the surface of the entire trailing hemisphere and creates an absorber in the central portion of the trailing hemisphere. The absorption is related to sulfur, but is much darker than any laboratory spectra, likely due to the large grain size on Europa's trailing hemisphere. The disk-resolved observations, in combination with three disk-integrated observations, are used to derive the ultraviolet particle scattering function for Europa's trailing hemisphere.