The Gaseous Sulfur Dioxide Abundance over Io's Leading and Trailing Hemispheres: HST Spectra of Io's C 1B 2--X 1A 1 Band of SO 2 near 2100 Angstrom

Astronomy and Astrophysics – Astronomy

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Molecular Processes, Planets And Satellites: Individual Io, Ultraviolet: Solar System

Scientific paper

We report ultraviolet HST/GHRS observations of Io's spectrum at opposite orbital elongations in the wavelength band 2095-2135 Å smoothed to an effective resolution of 3.7-4.1 Å. Gaseous SO2 absorption over Io's trailing sunlit hemisphere was found to be 37% stronger than over the leading sunlight hemisphere. The effective two-way path of SO_{2 }gas averaged over the disk is 4.4 × 1016 and 3.2 × 1016 SO2 molecules cm-2, respectively. This is in qualitative agreement with the millimeter-wave observations and is opposite to the case for Io's SO2 frost absorption, which is stronger over the leading hemisphere. The corresponding disk-averaged column abundances are 0.7 × 1016 and 0.5 × 1016 SO2 molecules cm-2, respectively, with a 5% uncertainty. Lower limits to the fractional disk coverage of gaseous SO2 are found to be 0.40 and 0.35, respectively. The extreme case where the gas is restricted to a circular region centered at the subsolar point implies minimum fractional hemispherical coverages of 0.23 and 0.20, respectively, and a maximum SO2 column abundance of 9.6 × 1016 cm-2 for each hemisphere (12% fitting uncertainty), substantially more diluted than the millimeter-wave results. Unidentified absorptions appear near 2114 Å and greater than 2128 Å which are unlikely to be SO2 frost because they are not periodic, like the SO2 gas manifolds, and are stronger over the trailing hemisphere.

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