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 A

摘要

We report ultraviolet HST/GHRS observations of Io's spectrum at opposite orbital elongations in the wavelength band 2095-2135 A smoothed to an effective resolution of 3.7-4.1 A. Gaseous SO_2 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 x 10~(16) and 3.2 x 10~(16) SO_2 molecules cm~(-2), respectively. This is in qualitative agreement with the millimeter-wave observations and is opposite to the case for Io's SO_2 frost absorption, which is stronger over the leading hemisphere. The corresponding disk-averaged column abundances are 0.7 x 10~(16) and 0.5 x 10~(16) SO_2 molecules cm~(-2), respectively, with a 5% uncertainty. Lower limits to the fractional disk coverage of gaseous SO_2 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 SO_2 column abundance of 9.6 x 10~(16) cm~(-2) for each hemisphere (12% fitting uncertainty), substantially more diluted than the millimeter-wave results. Unidentified absorptions appear near 2114 A and greater than 2128 A which are unlikely to be SO_2 frost because they are not periodic, like the SO_2 gas manifolds, and are stronger over the trailing hemisphere.