SINCE the detection by Pioneer 10 of an ionosphere1 around Io, this moon of Jupiter has been known to possess a tenuous atmosphere. The local observation of gaseous SO2 and the upper limits on the concentrations of other gases deduced from Voyager data2, together with the identification of SO2 frost or adsorbate at the surface3,4 and upper limits on the global amount of atmospheric SO2 (refs 5–8), have led to several classes of models for Io's atmosphere9. Here we report the microwave detection of SO2 at 222 GHz, which constitutes the first direct global observation of Io's neutral atmosphere. The observations imply an SO2 surface pressure of 4–35 nanobars, covering 3–15% of the surface on both leading and trailing sides of Io when illuminated by the Sun. This supports atmospheric models in which the partial pressure of SO2 at the surface is determined by the local surface temperature8, favouring, in particular, the 'albedo cold-trap' models10,11. Our failure to detect H2S at 169 GHz suggests that the pressure of this gas is probably below 10 −10 bar. Our results, taken together with the Pioneer ionospheric data, suggest that an atmospheric gas other than SO2 is present. We propose that the locally buffered SO2 atmosphere coexists with a background atmosphere of oxygen with a partial surface pressure of ∼20 nanobars.
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