Detection of Sulfur Monoxide in Io's Atmosphere

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Abstract
Millimeter-wave observations of Io with the IRAM 30 m telescope have allowed the detection of two rotational lines of SO at 219.949 and 138.178 GHz in Io's atmosphere. The observations can be fitted by assuming that gaseous SO is collocated with SO2 on a restricted fraction of Io's surface. In this case, the SO/SO2 mixing ratio is 3%-10%, in agreement with predictions from one-dimensional photochemical models and suggesting a vertical eddy diffusion coefficient in the range 3 × 107 to 3 × 108 cm2 s-1. Alternatively, SO could constitute a tenuous, global atmosphere with column density in the range (2-6) × 1014 cm-2. Photochemistry in SO2 hydrostatic and/or volcanic plume atmospheres and horizontal transport can conceivably produce such an extended SO atmosphere.