The Lunar Opposition Effect: A Test of Alternative Models

Abstract

We test the hypotheses that the lunar opposition effect is due to shadow-hiding, coherent backscatter, or some combination of the two phenomena. Hapke's photometric model is extended to include975726description of the coherent backscatter opposition effect. The model was fit simultaneously toV-filter (λ = 0.55 μm) whole-disk photometry of the Moon over many phase angles (2° ≤ α ≤ 143°) and to disk-resolved observations of the lunar surface over a broad range of incidence, emission, and phase angles. The shape of the Moon's opposition surge is accurately represented by the combination of a narrow coherent-backscatter peak whose effect is most strongly defined at α < 2° and a very broad shadow-hiding peak, best defined over α < 20°. The relative angular widths of the shadow-hiding and coherent backscatter contributions can be used to estimate the grain sizes most important for scattering at small phase angles. Scatterers with sizes comparable to the wavelength of light are required for coherent backscattering and are present in the smallest size fraction of lunar soil, but are predicted only for a model in which coherent backscatter causes the narrow component and shadow-hiding the wide one. The amplitude of the shadow-hiding opposition effect measures grain transparency and our solution implies that the lunar regolith particles responsible for the shadow-hiding contribution behave like opaque grains, consistent with complex structure of lunar regolith particles at much larger scales than those which control the coherent backscatter contribution. Most other airless planetary surfaces exhibit opposition surges whose broad components also appear to be caused by shadow-hiding. We propose that the submicron sized grains that control coherent backscatter do not contribute to the shadow-hiding opposition surge because the minute shadows cast by these grains are illuminated by their own forward-scattered light. We suggest that coherent backscatter cannot be responsible for the broad component of the Moon's opposition surge because such forward scattered light would illuminate all particle shadows and thereby eliminate the shadow-hiding contribution.