Volcanic Sulfur Products and Large-scale Compositional Patterns on Io from HST/STIS

Io is the most volcanically active body in the solar system and, as a result, one of the most colorful. Its surface features dark lava flows, red rings of volcanic plume fallout, widespread areas of yellow, orange, and brown, and regions coated in bright sulfur dioxide (SO₂) frost. Ultraviolet (UV) and visible-wavelength observations of Io have been key to understanding the composition behind these colors, but have relied on inference to bridge the gap between spatially resolved images, which lack spectral detail, and disk-integrated spectra, which lack the necessary spatial information to directly link composition with surface processes. To address this problem and complete our spectral understanding of Io’s colorful surface, we present global, spatially resolved UV-visible spectra of Io obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). We use this comprehensive dataset to investigate spectral end-members, map observed spectral features associated with SO₂ frost and other sulfur species, and explore compositional implications in the context of Io surface processes. In agreement with past observations, our results are consistent with low-latitude SO₂ deposits that are stable over decades, widespread sulfur-rich plains that are anti-correlated with SO₂ frost, and red plume deposits and high-latitude regions enriched in metastable short-chain sulfur allotropes.