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

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