Infrared Observations of SO emission from Io's Atmosphere during Eclipse

Io, the volcanic moon of Jupiter, hosts an atmosphere dominated by SO₂ and SO, but the question of the direct source of these molecules is still debated. Many different approaches have been taken to establish a link between volcanic activity on Io and atmospheric effects, to distinguish whether the atmosphere is supplied by volcanic outgassing or ice sublimation. In the infrared, atmospheric emission lines are lost in reflected sunlight; observing Io in eclipse provides a unique opportunity to study infrared lines, during a time when most of Io's atmosphere may be frozen out in Jupiter's shadow. In 1999 the a¹Δ → Χ³Σ^- transition of SO at 1.707 μm was discovered by de Pater et al. (2002); Laver et al. (2007) made additional observations, which they fit with equilibrium models to infer a likely volcanic origin for the SO. Here we present additional high spectral resolution observations of the 1.707 μm SO line while Io is in eclipse. We model these observations with equilibrium and non-LTE models, and address implications for the origin of SO on Io.