High Spatial and Spectral Resolution Observations of the Forbidden 1.707 mu m Rovibronic SO Emissions on Io: Evidence for Widespread Stealth Volcanism*

We present observations obtained with the 10 m Keck telescopes of the forbidden SO a(1)Delta -> X-3 sigma(-) rovibronic transition at 1.707 mu m on Io while in eclipse. We show its spatial distribution at a resolution of similar to 0.'' 12 and a spectral resolution of R similar to 2500, as well as disk-integrated spectra at a high spectral resolution (R similar to 15,000). Both the spatial distribution and the spectral shape of the SO emission band vary considerably across Io and over time. In some cases the SO emissions either in the core or the wings of the emission band can be identified with volcanoes, but the largest areas of SO emissions usually do not coincide with known volcanoes. We suggest that the emissions are caused by a large number of stealth plumes, produced through the interaction of silicate melts with superheated SO2 vapor at depth. The spectra, in particular the elevated wing of the emission band near 1.69 mu m, and their spatial distribution strongly suggest the presence of nonlocal thermodynamic equilibrium processes in addition to the direct ejection of excited SO from the (stealth and other) volcanic vents.