Spectroscopic Mapping of Io's Surface with HST/STIS: SO₂ Frost, Sulfur Allotropes, and Large-scale Compositional Patterns

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 ultraviolet-visible spectra of Io from the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, 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 data set to investigate spectral endmembers, map observed spectral features associated with SO₂ 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 SO₂ frost deposits that are stable over multidecade timescales, widespread sulfur-rich plains surrounding the SO₂ deposits, and the enrichment of Pele's pyroclastic ring and the high-latitude regions in metastable short-chain sulfur allotropes.