Disk-resolved photometry of Io: I. Near-opposition limb darkening

We have determined the Minnaert limb-darkening parameters (B₀, k) for three color classes of regions on Io near opposition (solar phase angle = 4–5°). Bright (“white”) areas show k ≈ 0.6 independent of wavelength. k for Average (“orange”) regions starts slightly below 0.5 in the Voyager ultraviolet filter (≈0.35 μm), rises monotonically toward longer wavelengths, and reaches ≈0.6 in the orange (≈0.59 μm), a trend consistent with the globally averaged results of McEwen and Soderblom (NASA Technical Memorandum 86246, pp. 261–262 (1984)). For Polar (“brown”) materials, k lies in the range 0.6–0.7 in all filters longward of ultraviolet. The Bright and Average regions show significantly smaller limb darkening, and smaller wavelength dependence of k, than laboratory samples of sulfur, a discrepancy that might represent the influence of macroscopic surface roughness. From a study of the relative merits of two different techniques for determining limb darkening parameters, we find that the single-image method, involving a single spacecraft image and the choice of a group of “similar” regions, has serious consistency problems when applied to the multicolored surface of Io. In contrast, an adaptation of the two-image solution of McEwen and Soderblom, involving the use of two or more images and computation of the limb darkening for individual surface regions, gives consistent results for spectrally similar regions. We conclude that the two-image technique is the only way to accurately determine the limb darkening of Ionian surface materials.