DETECTION OF THE SO2 ATMOSPHERE ON IO WITH THE HUBBLE-SPACE-TELESCOPE

Observations of the trailing hemisphere of Io made with the Faint Obje ct Spectrograph of the Hubble Space Telescope in March 1992 have resul ted in the first detection of atmospheric SO2 absorption bands in the ultraviolet. These observations represent only the third positive mean s of detection of what is widely believed to be Io's primary atmospher ic constituent. Below approximately 2130 angstrom the geometric albedo of the satellite is dominated by SO2 gas absorption band signatures, which have been analyzed using models that include the effects of opti cal thickness, temperature, and spatial distribution. The disk-integra ted HST data cannot resolve the spatial distribution, but it is possib le to define basic properties and set constraints on the atmosphere at the time of the observations. Hemispheric atmospheres with average co lumn density N = 6-10 x 10(15) cm-2 and T(gas) = 110-500 K fit the dat a, with preference for temperatures of approximately 200-250 K. Better fits are found as the atmosphere is spatially confined, with a limit of approximately 8% hemispheric areal coverage and N almost-equal-to 3 x 10(17) cm-2 with colder 110-250 K temperatures. A dense (N greater- than-or-equal-to 10(16) cm-2), localized component of SO2 gas, such as that possibly associated with active volcanoes, can generate the obse rved spectral contrast only when the atmosphere is cold (110 K) and an extended component such as Pele is included. The combination of a den se, localized atmosphere with a tenuous component (N < 10(16) cm-2, ei ther patchy or extended) also fits the data. In all cases the best fit models imply a disk-averaged column density larger than exospheric bu t approximately 10-30 times less than the previous upper limit from ne ar-UV observations. (C) 1994 Academic Press, Inc.