SO2 DISTRIBUTIONS ON IO

We present an analysis of disk-resolved images of Io at 232.5, 260, an d 285 mn taken with the FOC (Faint Object Camera) of the Hubble Space Telescope. The images at 232.5 and 260 mn were acquired in 1993 in an effort to separate the surface and SO2 atmosphere contributions to the observed UV albedo. We combine these images to make UV color maps of the regions centered on the leading and trailing hemispheres. Between latitudes +60 degrees and -60 degrees we find that the UV colors are d ominated by three distinctive components, B-0, B-1, and B-2, one more than was found to be required to fit visible wavelength Voyager data a t these latitudes. The Voyager component ''B'' (McEwen, A. S., T. V. J ohnson, D. L. Matson, and L. A. Sonderblom 1988. Icarus 75, 450-478) a ppears to be a combination of two distinct spectral components: B-1 an d B-2. We find that B-1, the darker component, represents either a new compositional unit or patches of SO2 vapor overlying compositional un it B (= B-2). To distinguish between these two possibilities, we propo se two simple models of surface reflectivities and SO2 vapor curve of growth designed to allow a crude separation between the effects of the absorptions by surface materials and SO2 vapor. In Model 1, we recogn ize that all absorption bandmodels share a linear regime at the limit of small absorption path-lengths and assume that the SO2 vapor absorpt ion spectrum on Io is linearly dependent on absorption pathlength at a ll wavelengths without temperature dependencies. In this case Io's alb edo must be dominated by the surface reflectance and the spectral diff erences between B-1 and B-2 are the signature of different surface uni ts. In this model the two-way SO2 vapor column density is either below our detectability limit of similar to 4.10(17) mol . cm(-2) or is con fined to denser patches below our spatial resolution limit of similar to 250 km. In Model 2, we recognize that SO2 absorptions on Io may be non-linear at 285 nm (a local maximum in SO2 absorption cross sections ) even in the presence of significant transmission through the gas. We retain the assumption of linearity at 232.5 mn where the SO2 absorpti on cross sections are smallest and consider two variants of the model in which different assumptions are made about the underlying albedo of the surface materials. In variant A, which is characterized by a rela tively high assumed UV reflectivity for SO2 frost, the amounts of gas are inconsistent with mm-wave and UV spectroscopic observations, and w ith the cold temperatures found for the pervasive thermal reservoir un it in Veeder et al.'s (Veeder, G. J., D. L. Matson, T. V. Johnson, D. L. Blaney, and J. D. Goguen 1994. J. Geophys. Res. (Planets) 99, 17095 -17162) thermophysical model of the surface. In variant B, SO2 frost i s characterized by the lowest UV reflectivity consistent with the data . In this case there is no detectable SO2 vapor over SO2 frost rich re gions and the FOC UV images are consistent with the presence of SO2 va por in patches of column density N similar to 10(18) cm(-2) covering s imilar to 11-15% of Io's projected surface. This variant of Model 2 is found to be in agreement with both the disk integrated UV spectroscop ic and mm-wave observations and Veeder et al.'s thermophysical model. In particular the longitude distribution of the SO2 patches is similar to the longitude distribution of thermal anomalies in Veeder et al.'s model. The identification of the composition of the B unit remains pr oblematic. Polysulfur oxides (PSO) give a reasonable accounting of the UV reflectivities but may be too bright near 700 nm; Sulfur does not satisfy the UV albedos but cannot be ruled out because of uncertaintie s in its behavior under Io conditions. In any of the above models, reg ardless of the assumptions made concerning the curve of growth of SO2 vapor absorption, we find that the percentage coverage of SO2 frost in the regions of Io's surface that we observed is in the range of 50-60 %. This is a similar result to those found in earlier spectroscopic st udies of SO2 frost features near 2 and 4 mu m. (C) 1996 Academic Press , Inc.