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 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.