SO ₂Distributions on Io

Abstract

We present an analysis of disk-resolved images of Io at 232.5, 260, and 285 nm taken with the FOC (Faint Object Camera) of the Hubble Space Telescope. The images at 232.5 and 260 nm were acquired in 1993 in an effort to separate the surface and SO₂atmosphere 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° and -60° we find that the UV colors are dominated by three distinctive components,B₀,B₁, andB₂, one more than was found to be required to fit visible wavelength Voyager data at these latitudes. The Voyager component “B” (McEwen, A. S., T. V. Johnson, D. L. Matson, and L. A. Sonderblom 1988.Icarus75, 450-478) appears to be a combination of two distinct spectral components:B₁andB₂. We find thatB₁, the darker component, represents either a new compositional unit or patches of SO₂vapor overlying compositional unitB(=B₂). To distinguish between these two possibilities, we propose two simple models of surface reflectivities and SO₂vapor curve of growth designed to allow a crude separation between the effects of the absorptions by surface materials and SO₂vapor.

InModel 1, we recognize that all absorption bandmodels share a linear regime at the limit of small absorption pathlengths and assume that the SO₂vapor absorption spectrum on Io is linearly dependent on absorption pathlength at all wavelengths without temperature dependencies. In this case Io's albedo must be dominated by the surface reflectance and the spectral differences between B₁and B₂are the signature of different surface units. In this model the two-way SO₂vapor column density is either below our detectability limit of ∼4.10¹⁷mol·cm^-2or is confined to denser patches below our spatial resolution limit of ∼250 km.

InModel 2, we recognize that SO₂absorptions on Io may be non-linear at 285 nm (a local maximum in SO₂absorption cross sections) even in the presence of significant transmission through the gas. We retain the assumption of linearity at 232.5 nm where the SO₂absorption 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. Invariant A, which is characterized by a relatively high assumed UV reflectivity for SO₂frost, the amounts of gas are inconsistent with mm-wave and UV spectroscopic observations, and with the cold temperatures found for the pervasive thermal reservoir unit in Veederet 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. Invariant B, SO₂frost is characterized by the lowest UV reflectivity consistent with the data. In this case there is no detectable SO₂vapor over SO₂frost rich regions and the FOC UV images are consistent with the presence of SO₂vapor in patches of column densityN∼ 10¹⁸cm^-2covering ∼11-15% of Io's projected surface. This variant of Model 2 is found to be in agreement with both the disk integrated UV spectroscopic and mm-wave observations and Veederet al.'s thermophysical model. In particular the longitude distribution of the SO₂patches is similar to the longitude distribution of thermal anomalies in Veederet al.'s model.

The identification of the composition of theBunit remains problematic. 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 uncertainties in its behavior under Io conditions. In any of the above models, regardless of the assumptions made concerning the curve of growth of SO₂vapor absorption, we find that the percentage coverage of SO₂frost 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 studies of SO₂frost features near 2 and 4 μm.