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R. M. Nelson (JPL), B. Hapke (University of Pittsburgh), W. D. Smythe, L. J. Spilker (JPL)
The nature of the phase curve at small phase angles in a planetary regolith has been associated with important surface texture properties. We measured the circular polarization ratio as a function of phase angle from 0.05-5 degrees of a suite of well sorted aluminum oxide powders. The particle sizes were 0.1 to 30 microns. The samples were illuminated by a HeNe laser with a wavelength of 0.633 microns. The samples exhibit a pronounced increase in reflectance and an increase in circular polarization ratio with decreasing phase angle, consistent with a coherent backscattering induced opposition surge. We have previously reported the presence of significant opposition surge for powders with sizes 50 times larger and 6 times smaller than the wavelength of the incident light (Nelson et al, LPSC 98). Theoretical treatments of the opposition surge suggest that the size and shape of the circular polarization ratio phase curves is expected to change with particle size. The effects are expected to be greatest when the size of the scatterers approaches the wavelength of the incident radiation. The results of these experiments will provide a impotant test of these models. This work was performed at JPL under contract with NASA.