Evidence of SO₂ on Io from UV observations

Abstract

Gaseous SO₂ was discovered on Io, the innermost galilean satellite of Jupiter, by the IRIS IR Michelson interferometer¹ on board Voyager 1 in March 1979. This SO₂ is likely to be associated with eruptions of the seven active volcanoes discovered at the same time by the Voyager cameras². The UV spectrum of the solar light reflected by the surface of Io should be modified by the transmission S̄(λ) of a SO₂ atmosphere and by the reflectivity r (λ) of the surface. Both gaseous SO₂ and solid³ SO₂ present a deep absorption band around 290 nm. We have examined a published spectrum of Io taken in 1978 by the IUE Earth-orbiting telescope⁴, and found an absorption dip centred around 290 nm in the albedo curve. We attribute this dip to gaseous SO₂, with a possible contribution of solid SO₂. However, the absorption dip is smaller than if the gaseous SO₂ quantity measured by Voyager 1 in the IR were uniform. It can be better represented by an SO₂ frost controlled gaseous atmosphere and a temperature distribution of Io's surface compatible with observations.