Rm. Nelson et al., PHASE CURVES OF SELECTED PARTICULATE MATERIALS - THE CONTRIBUTION OF COHERENT BACKSCATTERING TO THE OPPOSITION SURGE, Icarus, 131(1), 1998, pp. 223-230
We present angular scattering measurements of a suite of particulate m
aterials of varying particle sizes and albedos over phase angles of 1
< Theta < 70 degrees. Our results provide strong support for the hypot
hesis that coherent backscattering can be an important contributor to
the enhanced reflectance seen in planetary regolith materials when obs
erved at small phase angles, commonly known in the planetary science c
ommunity as the opposition effect. Eight samples were each presented w
ith both senses of linearly and circularly polarized light and the ref
lectance and the circular polarization ratio as a function of phase an
gle was measured. The reflectances of the samples varied from 3 to 99%
, The circular polarization ratio was observed to increase in all the
samples as the phase angle of the observation decreased near 0 degrees
phase angle. The traditional explanation of the opposition surge, the
shadow hiding model, predicts that the circular polarization ratio sh
ould decrease as the phase angle of the observation decreases, The mor
e reflective materials exhibit stronger opposition surge amplitudes th
an the less reflective ones. We find a nonlinear relationship between
the slope of the opposition curve measured at 2 degrees and the single
scattering albedo of the sample which is the opposite of what is pred
icted by the shadow hiding model for the opposition effect. Our result
s are consistent with predictions of theoretical models of coherent ba
ckscattering from spherical particles and strongly suggest that cohere
nt backscattering is the major contributor to the opposition effect. T
hey provide an explanation of the opposition surges reported in highly
reflective particulate media, and also the high albedo icy satellites
of outer Solar System, both of which cannot be explained by the shado
w hiding model. (C) 1998 Academic Press.