Io's patchy SO₂ atmosphere as measured by the Galileo ultraviolet spectrometer

The Galileo ultraviolet spectrometer has observed Jupiter's volcanically active moon Io in the 2100–3200 Å wavelength range at 13.7 Å resolution. We find that both sulfur dioxide frost on the surface and sulfur dioxide gas in Io's atmosphere are detectable. Io's SO 2 atmosphere is not hemispherically homogeneous, but is patchy, with both thick and thin regions. During an observation near 120–150°W longitude, we find that a thick SO 2 atmosphere (column density N = 1.0 × 10 19 cm −2 ) covers ∼35% of the observed region, while a thinner SO 2 (N = 4.0 × 10 17 cm −2 ) atmosphere covers 25% of the observed region. The thick atmospheric component is significant in the 2350–2400 Å range, while the thinner component is important particularly at wavelengths shorter than ∼2300 Å. Likely sources of the SO 2 atmosphere are volcanic plumes and sublimation of SO 2 frost, as well as outgassing associated with the several hotspots in the field of view.