  Io's intense volcanic activity results in one of the most colorful surfaces
in the solar system. Ultraviolet and visible-wavelength observations of Io are
critical to uncovering the chemistry behind its volcanic hues. Here, we present
global, spatially resolved UV-visible spectra of Io from the Space Telescope
Imaging Spectrograph on the Hubble Space Telescope (HST), which bridge the gap
between previous highly resolved imagery and disk-integrated spectroscopy, to
provide an unprecedented combination of spatial and spectral detail. We use
this comprehensive dataset to investigate spectral endmembers, map observed
spectral features associated with SO$_2$ frost and other sulfur species, and
explore possible compositions in the context of Io surface processes. In
agreement with past observations, our results are consistent with extensive
equatorial SO$_2$ frost deposits that are stable over multi-decade timescales,
widespread sulfur-rich plains surrounding the SO$_2$ deposits, and the
enrichment of Pele's pyroclastic ring and the high-latitude regions in
metastable short-chain sulfur allotropes.
