Physics
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..122..273s&link_type=abstract
Icarus, Volume 122, Issue 2, pp. 273-287.
Physics
25
Scientific paper
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 SO_2 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 deg and -60 deg we find that the UV colors are dominated 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 at these latitudes. The Voyager component ``B'' (McEwen, A. S., T. V. Johnson, D. L. Matson, and L. A. Sonderblom 1988. Icarus 75, 450-478) appears to be a combination of two distinct spectral components: B_1 and B_2. We find that B_1, the darker component, represents either a new compositional unit or patches of SO_2 vapor overlying compositional unit B (= B_2). To distinguish between these two possibilities, we propose two simple models of surface reflectivities and SO_2 vapor curve of growth designed to allow a crude separation between the effects of the absorptions by surface materials and SO_2 vapor. In Model 1, we recognize that all absorption band models share a linear regime at the limit of small absorption pathlengths and assume that the SO_2 vapor 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 B_1 and B_2 are the signature of different surface units. In this model the two-way SO_2 vapor column density is either below our detectability limit of ~4.10^17 mol.cm^-2 or is confined to denser patches below our spatial resolution limit of ~250 km. In Model 2, we recognize that SO_2 absorptions on Io may be non-linear at 285 nm (a local maximum in SO_2 absorption cross sections) even in the presence of significant transmission through the gas. We retain the assumption of linearity at 232.5 nm where the SO_2 absorption 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 relatively high assumed UV reflectivity for SO_2 frost, 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 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, SO_2 frost is characterized by the lowest UV reflectivity consistent with the data. In this case there is no detectable SO_2 vapor over SO_2 frost rich regions and the FOC UV images are consistent with the presence of SO_2 vapor in patches of column density N ~ 10^18 cm^-2 covering ~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 Veeder et al.'s thermophysical model. In particular the longitude distribution of the SO_2 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 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 SO_2 vapor absorption, we find that the percentage coverage of SO_2 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 studies of SO_2 frost features near 2 and 4 mum.
Belton Michael J. S.
McGrath Melissa A.
Sartoretti Paola
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