THE LUNAR OPPOSITION EFFECT - A TEST OF ALTERNATIVE MODELS

We test the hypotheses that the lunar opposition Effect is due to shad ow-hiding, coherent backscatter, or some combination of the two phenom ena, Hapke's photometric model is extended to include M. I. Mishchenko 's (1993, Astrophys. J., 411, 351-361) description of the coherent bac kscatter opposition effect, The model was fit simultaneously to V-filt er (lambda = 0.55 mu m) whole-disk photometry of the Moon over many ph ase angles (2 degrees less than or equal to alpha less than or equal t o 143 degrees) 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 combi nation of a narrow coherent-backscatter peak whose effect is most stro ngly defined at alpha < 2 degrees and a very broad shadow-hiding peak, best defined over alpha < 20 degrees. The relative angular widths of the shadow-hiding and coherent backscatter contributions can be used t o estimate the grain sizes most important for scattering at small phas e angles. Scatterers with sizes comparable to the wavelength of light are required for coherent backscattering and are present in the smalle st size fraction of lunar soil, but are predicted only for a model in which coherent backscatter causes the narrow component and shadow-hidi ng the wide one. The amplitude of the shadow-hiding opposition effect measures grain transparency and our solution implies that the lunar re golith particles responsible for the shadow-hiding contribution behave like opaque grains, consistent with complex structure of lunar regoli th particles at much larger scales than those which control the cohere nt backscatter contribution. Most other airless planetary surfaces exh ibit opposition surges whose broad components also appear to be caused by shadow-hiding, We propose that the submicron sized grains that con trol coherent backscatter do not: contribute to the shadow-hiding oppo sition surge because the minute shadows cast by these grains are illum inated by their own forward-scattered light, We suggest that coherent backscatter cannot be responsible for the broad component of tile Moon 's opposition surge because such forward scattered light would illumin ate all particle shadows and thereby eliminate the shadow-hiding contr ibution. (C) 1997 Academic Press.