The Opposition Effect in Simulated Planetary Regoliths. Reflectance and Circular Polarization Ratio Change at Small Phase Angle

Abstract We measured the change in reflectance and in circular polarization ratio with respect to phase angle of a suite of well-sorted, highly reflective, aluminum oxide powders for the purpose of understanding the contribution of the coherent backscattering phenomenon to the reflectance phase curves of planetary regoliths. The goniometric photopolarimeter that we used measured reflectance and circular polarization ratio over the range of phase angles from 0.05° to 5° where it has been suggested that coherent backscattering becomes important. The particle size of the samples varied from 0.1 to 30 μm. The wavelength of the illuminating radiation from a HeNe laser was 0.633 μm. We find that the reflectance at 0.05° is weakly dependent on particle size. We measured to good approximation the half-width half-maximum (HWHM) of the phase curves, and we find that there is only a weak qualitative agreement between our experimental results and the theoretical estimates of the HWHM for particles of comparable index of refraction and packing density as predicted by M. Mishchenko (1992, Astrophys. Space Sci. 194 , 327–333). For particle sizes that are close to the wavelength of the illuminating radiation we find that Mishchenko's model overestimates the HWHM of the phase curve by an order of magnitude. For particle sizes that are much larger or much smaller than the wavelength of the incident radiation the theoretical model underestimates the HWHM of the phase curve by more than an order of magnitude for the smallest particle sizes and factors of several for the large particle sizes. This disagreement between our experimental data and the predictions from theoretical modeling is not unexpected given the assumptions made in the theoretical models that the particles scatter light like isolated, perfect uniform spheres and that the variation in particle size distribution used by Mishchenko was remarkably small, smaller than that of the well-sieved particle sizes used in our experiment. For all samples the circular polarization ratio increased with decreasing phase angle consistent with the hypothesis that the coherent backscattering process is the principal contributor to the reflectance phase curve near 0° for highly reflective, multiply scattering, particulate media. We confirm our previously reported significant opposition surges for powders with sizes 50 times larger and 6 times smaller than the wavelength of the incident light (R. Nelson et al. 1998, Proc. Lurar Planet. Sci. Conf. 29th , Abstract 1146). This suggests that for cases where the wavelength of the incident radiation is much larger than the candidate regolith particle size, the photons behave as if they were interacting with ensembles of particles of size comparable to the wavelength of the incident radiation. For cases where the particle size is much larger than the wavelength of the incident light, the photons appear to be interacting primarily with wavelength-sized cracks, surface asperities, or other irregularities in the regolith particles rather than with the particle as a whole. This work, using sorted powders as planetary surface regolith analogues, has applicability for remote sensing studies of highly reflective planetary regoliths such as might be expected to be found on many the icy satellites of the outer Solar System. Many of these bodies exhibit sharp opposition surges that are most likely due to coherent backscattering.