Lyman-[alpha] imaging of the SO sub(2) distribution on Io

Imaging spectroscopy of Io in the ultraviolet (1160-1720 Aa) was carried out with the Space Telescope Imaging Spectrograph on HST on three dates in October 1997 and August 1998. Among the initial results was the observation of concentrated regions of Hi Lyman-[alpha] flux near the poles of Io that exhibited a morphology and temporal variability different from those of the atomic oxygen and sulfur emission regions seen near the equatorial limbs. We examine the suggestion that the primary source of Lyman-[alpha] emission is surface reflected solar radiation that penetrates the thin polar atmosphere, but is strongly absorbed by the thicker SO sub(2) atmosphere near Io's equator. Spectral and spatial analyses lead to derived SO sub(2) column densities that are in good agreement with those derived from earlier HST observations of Io's albedo in the 2000-2300 Aa wavelength range. The Lyman-[alpha] images clearly illustrate features of Io's atmosphere that have been deduced from previous observations and theoretical modeling: a non-uniformity with respect to the sub-solar point dominated by a freezing out of the SO sub(2) near the poles and variation with both longitude and time due to the variability of the sources of the atmospheric gas. Lyman-[alpha] imaging is demonstrated to be an extremely powerful and direct way to globally map the dynamic atmosphere of Io.