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P.E. Geissler, A.S. McEwen (University of Arizona), C. Phillips (SETI Institute), D.P. Simonelli (Cornell University), R. Lopes-Gautier (J.P.L.), S. Douté (U.C.L.A.)
Io's visible appearance changes dramatically with solar phase angle: the polar regions and some plume deposits near active volcanic centers become comparatively bright with increasing phase angle, while the equatorial band grows relatively dark. This photometric behavior greatly complicates studies of surface changes due to volcanic activity on the satellite [1,2,3].
To quantify the scattering properties of ionian surface materials we fit a photometric model to the brightnesses observed in a selection of green filter SSI images acquired under a variety of illumination and viewing conditions. The disk-resolved directional reflectance properties of the surface were determined by deriving the parameters of Hapke's equation [4,5] from a set of 42 images spanning a range of phase angles from 0.5 to 123 degrees. Because of incomplete data coverage at very low and very high phase angles, we assumed constant values  for the opposition effect magnitude and width, and allowed the macroscopic roughness to vary only after solving for the single scattering albedo and Henyey-Greenstein asymmetry parameter.
The result is a set of global maps of Io's photometric parameters at a scale of 1 degree/pixel (32 km/pixel at the equator). We find that the photometric behavior of Io's surface is largely independent of its low-phase color. The areas that appear unusually bright at high phase correspond to isotropic to slightly forward-scattering materials that are concentrated in the higher latitudes above 30 degrees or else associated with active volcanic centers. Comparison with high resolution infrared spectral maps from NIMS  suggests that the isotropic to forward-scattering materials have relatively strong 4.1 micron absorptions indicative of fine-grained SO2. We hypothesize that that these regions are coated by thin frosts of SO2 that are transparent under normal illumination. The equatorial band is strongly back-scattering in contrast, has a noticeably larger opposition effect magnitude, and is somewhat rougher than the poles.
 Phillips et al., LPSC 30, 1448, 1999.  Phillips et al., submitted to JGR, 2000.  Geissler et al., Icarus, 140, 265-282, 1999.  Hapke, B., J.G.R., 86, 4571-4586, 1981.  Hapke, B., Icarus, 59, 41-59, 1984.  McEwen et al., Icarus, 75, 450-478, 1988.  Lopes-Gautier et al., Science, 288, 1201-1204, 2000.