Detection of the SO2 Atmosphere on Io with the Hubble Space Telescope

Observations of the trailing hemisphere of Io made with the Faint Object Spectrograph of the Hubble Space Telescope in March 1992 have resulted in the first detection of atmospheric SO₂ absorption bands in the ultraviolet. These observations represent only the third positive means of detection of what is widely believed to be Io's primary atmospheric constituent. Below ∼2130 Å 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 optical thickness, temperature, and spatial distribution. The disk-integrated HST data cannot resolve the spatial distribution, but it is possible to define basic properties and set constraints on the atmosphere at the time of the observations, Hemispheric atmospheres with average column density N = 6-10 × 10¹⁵ cm^-2 and T_gas = 110-500 K fit the data, with preference for temperatures of ∼200-250 K. Better fits are found as the atmosphere is spatially confined, with a limit of ∼8% hemispheric areal coverage and N ≈ 3 × 10¹⁷ cm^-2 with colder 110-250 K temperatures. A dense (N ≥10¹⁶ cm^-2), localized component of SO₂ gas, such ns that possibly associated with active volcanoes, can generate the observed spectral contrast only when the atmosphere is cold (110 K) and an extended component such as Pele is included. The combination of a dense, localized atmosphere with a tenuous component (N < 10¹⁶ cm^-2 either patchy or extended) also fits the data. In all cases the best fit models imply a disk-averaged column density larger than exospheric but ∼10-30 times less than the previous upper limit from near-UV observations.