The wavelength dependence of phase coefficients

Abstract

The wavelength dependence (0.4 to 1.2 μm) of the phase coefficients of powdered samples of a basalt, the ordinary (L6) chondrite Bruderheim, and the carbonaceous chondrite Allende are shown to be qualitatively similar to the observed wavelength dependence of the phase coefficients of the Moon and S- and C-class asteroids, respectively. Model calculations of the surface roughness indicate that the systematic difference between the visual ( λ = 0.55 μm) phase coefficients of S- and C-class asteroids reflects primarily differences in composition and not necessarily differences in surface roughness. On the average the large scale surface roughness of both S and C asteroids is comparable to that of the Moon. The wavelength dependence (0.4 to 0.7 μm) of three particle size fractions of sulfur (diameters < 10 μm, < 150 μm, and 420–850 μm) show that particle size effects are important modifiers of the phase coefficient. Large scale surface roughness at least comparable to that of the Moon is required to explain the phase curve of Io if elemental sulfur is a dominant surface material.