Phase curves of selected particulate materials: The contribution of coherent backscattering to the opposition surge

We present angular scattering measurements of a suite of particulate materials of varying particle sizes and albedos over phase angles of 1 < Theta < 70 degrees. Our results provide strong support for the hypothesis that coherent backscattering can be an important contributor to the enhanced reflectance seen in planetary regolith materials when observed at small phase angles, commonly known in the planetary science community as the opposition effect. Eight samples were each presented with both senses of linearly and circularly polarized light and the reflectance and the circular polarization ratio as a function of phase angle 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 should decrease as the phase angle of the observation decreases, The more reflective materials exhibit stronger opposition surge amplitudes than 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 predicted by the shadow hiding model for the opposition effect. Our results are consistent with predictions of theoretical models of coherent backscattering from spherical particles and strongly suggest that coherent backscattering is the major contributor to the opposition effect. They 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 shadow hiding model. (C) 1998 Academic Press.