  The composition of Europa's trailing hemisphere reflects the combined
influences of endogenous geologic resurfacing and exogenous sulfur radiolysis.
Using spatially resolved visible-wavelength spectra of Europa obtained with the
Hubble Space Telescope, we map multiple spectral features across the trailing
hemisphere and compare their geographies with the distributions of large-scale
geology, magnetospheric bombardment, and surface color. Based on such
comparisons, we interpret some aspects of our spectra as indicative of purely
exogenous sulfur radiolysis products and other aspects as indicative of
radiolysis products formed from a mixture of endogenous material and
magnetospheric sulfur. The spatial distributions of two of the absorptions seen
in our spectra -- a widespread downturn toward the near-UV and a distinct
feature at 530 nm -- appear consistent with sulfur allotropes previously
suggested from ground-based spectrophotometry. However, the geographies of two
additional features -- an absorption feature at 360 nm and the spectral slope
at red wavelengths -- are more consistent with endogenous material that has
been altered by sulfur radiolysis. We suggest irradiated sulfate salts as
potential candidates for this material, but we are unable to identify
particular species with the available data.
