Detection of SO₂ on Callisto with the Hubble Space Telescope

Abstract

We have detected SO₂ in ultraviolet spectra of Callisto obtained with the Hubble Space Telescope's Faint Object Spectrograph. An absorption band centered at 280 nm appears in the spectrum of Callisto's leading hemisphere, but is not apparent in the spectrum of the trailing hemisphere. The band is similar to the SO₂ band on Europa's trailing hemisphere. Callisto's leading hemisphere spectrum can be well fit with models that include SO₂ ice absorption with N(SO₂) ≥ 6 × 10¹⁶ cm^-2. Callisto's leading hemisphere is modified by impacts with micrometeorites; this may directly or indirectly be a source of sulfur dioxide.