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.