Phase curves of materials of Io: Interpretation in terms of Hapke's function

The photometric function developed by B. Hapke (1981,J. Geophys. Res.86, 3039–3054; 1984, Icarus59, 41–59) has been applied to near-opposition (α = 2–8°) disk-resolved phase curves for three color classes on Io, and the disk-integrated phase curve (α = 2–159°) of the satellite as a whole. Derived values of the Hapke compaction parameter h suggest that (1) a large percentage of the material on Io's surface has a porosity significantly greater significantly greater than 60%, supporting the estimate of high porosity made by D.L. Matson and D.B. Nash (1983,J. Geophys. Res.88, 4771–4783) and Nelson et al. (1984, Bull. Amer. Astron. Soc.16, 683–685; 1984,EOS65, 982–983); and (2) Average (“orange”) and Polar (“brown”) materials are significantly more porous than Bright (“white”) materials, a cottrast consistent with the Matson and Nash (1983) SO₂ cold trap model. The best-fit single particle phase function becomes more backscattering on moving from Polar to Average to Bright materials, with the surface of Io on average exhibiting significant backscattering comparable in magnitude to that of the lunar surface. For the color classes, and for Io as a whole, the degree of backscattering tends to increase toward longer wavelengths. The average macroscopic roughness of the Ionian surface, characterized by a mean slope angle of Ø ≃ 25°, is similar to that of other solid surface in the solar system. Consistency between observed limb darkening and that predicted by the Hapke model requires the presence of significant macroscopic roughness (Ø ≥ 20°) for the Average regions, but not necessarily for the Bright and Polar materials.