Validity of Regolith Reflectance Models Based on the Radiative Transfer Equation and Coherent Backscatter Theory

Details Validity of Regolith Reflectance Models Based on the Radiative Transfer Equation and Coherent Backscatter Theory Validity of Regolith Reflectance Models Based on the Radiative Transfer Equation and Coherent Backscatter Theory

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.6204h&link_type=abstract

American Astronomical Society, DPS meeting #38, #62.04; Bulletin of the American Astronomical Society, Vol. 38, p.605

Physics

Geophysics

1

Scientific paper

Most regolith reflectance models are based on two assumptions: (1) radiative transfer (RT) theory is valid, and (2) the fundamental scatterers of the medium are its particles, which scatter independently (the fundamental scatterer assumption - FSA). These assumptions give good results when applied to suspensions of well-separated particles comparable to or smaller than the wavelength Î≫. However, serious questions have arisen regarding their applicability to close-packed media of particles larger than Î≫. In a major continuing study we have measured the bidrectional reflectances of several well-characterized powders using the JPL goniometric photopolarimeter, which is capable of measuring reflectances in linearly and circularly polarized light between 0.05 - 22 °. From our results we infer the following:
1. RT-FSA models give good results when used to calculate the reflectance functions of close packed particulate media at phase angles greater than a few degrees. Evidently coherent effects between particles can be neglected.
2. For media of perfect spheres with diameters > Î≫, models of the coherent backscatter opposition effect (CBOE) peak based on the RT-FSA assumption predict photon transport mean free paths L consistent with laboratory results.
3. For media of large irregular particles L's deduced from the measured CBOE widths are much smaller than predictions of RT-FSA models. Exactly what the CBOE is measuring in these materials is unclear, although scattering by small, closely-spaced surface and interior imperfections is a good candidate.
4. Much of the light that contributes to the CBOE is scattered from the top layer.
5. The diffraction peak of a large isolated particle does not exist when the particle is in a powder.
These conclusions have strong implications for understanding planetary surfaces and Saturn's ring particles. The latter are discussed in a companion paper.
Rresearch sponsored by the Planetary Geology and Geophysics program of NASA.

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