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

Abstract

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.