Regolith variations on Io: Implications for bolometric albedos

Global maps of Io produced from two sets of Galileo images (solar phase angles 4°-14° and 71°-86°, respectively) reveal that this satellite's color and albedo patterns change dramatically with phase. At low phase the equatorial band is the brightest, whitest part of Io; that is, it is brighter than the polar regions at all wavelengths. At high phase, however, the equatorial band becomes a dark gray, exhibiting little contrast with the polar regions at violet and green wavelengths and appearing darker than the polar regions at red wavelengths. To quantify these phase-related changes, we derive global maps of the Henyey-Greenstein asymmetry factor g that show how the strength of backscattering by Io's regolith varies from region to region. In the green and red, where albedo patterns change most radically with phase, the equatorial band forms a well-defined unit of strongly backscattering material; material that is more weakly backscattering at these wavelengths, mainly in the polar regions, shows a broad continuum of different g values. In the violet the phase-related changes in albedo patterns are more subdued, and the scattering units are poorly defined. Using this information on g, we generate a global map showing variations in the bolometric Bond albedo A_B, the true ``energy balance albedo'' that governs insolation-based surface temperatures. The mean albedo of this map, A_B~0.52, is similar to A_B values computed previously for Io, but the distribution of albedos is markedly different. In previous Bond albedo maps the highest A_B coincided with the bright equatorial band seen at low phase; the new map, however, more closely resembles Io's high-phase albedo patterns (i.e., the band of high Bond albedos at the equator is absent). This change in A_B patterns has a significant effect on how insolation-based temperatures vary with latitude on Io; in particular, it increases the satellite's equator-to-pole temperature contrast.