Experimental Modeling of Opposition Effect and Negative Polarization of Regolith-Like Surfaces

Abstract

We present results of laboratory photometric and polarimetric measurements of samples simulating the structure of planetary regoliths at small phase angles. We study a suite of samples that are characterized by a variety of structures and albedos. Metallic powders usually exhibit a wide branch of negative polarization independent of particle size. Unlike dielectric powders, metallic particulate surfaces show negative polarization even when they are placed in water. We demonstrate a strong particle-size dependence of the negative branch of polarization for powdered dielectric surfaces. For dielectric powders, both the opposition effect and negative polarization become more prominent when the samples are compressed. We show the opposition effect and negative polarization for rough and polished solid surfaces with microcrystalline structure. Our measurements show that the degree of polarization for dielectric surfaces depends on the surface tilt; even at small phase angles the polarization degree for tilted surfaces can be nonzero. The negative polarization depends strongly on micro- and macrostructured optical inhomogeneities of scattering surfaces. Double-minimum branches of negative polarization are found in this series of measurements.