An Improved Instrument for Angular Scattering Measurements of Candidate Planetary Surface Regolith Materials at Extremely Small Phase Angles: Relevance to the Outer Solar System

Abstract:

The reflection variation and the polarization change with phase angle of radiation scattered from particulate materials has been studied for a century in efforts to understand the nature of clouds, aerosols, planetary ring systems and planetary regolith materials. The increase in reflectance as phase angle decreases, the ‘Opposition Effect’, has been well documented in astronomical observations and laboratory studies. Variations in linear polarization near small phase angles have also been well studied (e.g. Shkuratov et al.,2002, Rosenbush et al. 2015). While the phenomena have been well documented, a generally accepted physical explanation is still lacking despite many excellent theoretical modeling efforts.

We have undertaken a reductionist approach in deconstructing the process. We have fabricated a goniometer which permits us to present samples with discrete wavelengths of monochromatic light that is linearly polarized in and perpendicular to the scattering plane. We also can illuminate our samples with both right handed and left handed circular polarization senses. Silicon Avalanche Photodiodes record the reflected radiation from the sample after it has passed through linear and circular polarizing analyzers(Kroner et al.).

This reductionist approach permits us to measure the reflectance and polarization phase curves and the change in linear and circular polarization ratio (LPR and CPR) with phase angle between 0.056 and 17 degrees. LPR and CPR are found to be important indicators of the amount of multiple scattering in the medium (Hapke, 1990, Nelson et al, 1998, 2000;Hapke, 2012).

This approach provides a way to distinguish between suggested models and to gain greater insight into the process of the scattering of electromagnetic radiation in a variety of media.

This work was supported by NASA’s Cassini Science Program

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