Detection of sulfur monoxide in Io's atmosphere

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 x 10(7) to 3 X 10(8) cm(2) s(-1). Alternatively, SO could constitute a tenuous, global atmosphere with column density in the range (2-6) x 10(14) cm(-2). Photochemistry in SO2 hydrostatic and/or volcanic plume atmospheres and horizontal transport can conceivably produce such an extended SO atmosphere.