A New Look at Photometry of the Moon

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Scientific paper

We test 2 very different models for light scattering by the lunar regolith: 1) TRLA - a Topographically Rough surface layer comprised of Low Albedo particles, and 2) RTE - a flat surface layer with the published size distribution and optical constants of Apollo soil particles, Mie calculation of the mean particle scattering matrix and rigorous solution of the Radiative Transfer Equation for all orders of scattering (Mishchenko et al 1999, JQRST 63, 409). For a typical lunar highland location (-17.21, 20.01), ROLO photometry (Kieffer & Stone 2005, Astron. J. 129, 2887) shows a distinct difference between the radiances measured before and after full moon (BFM, AFM) at the same 47.5 ± 2.5 degree phase angle, corresponding to incidence angles of 30 and 70 degrees, respectively. The TRLA model requires rms slopes of 34.1 ± 2.6 degrees to reproduce this BFM/AFM radiance difference, similar to previously published results. Analysis of Apollo ALSCC stereo images of in situ lunar regolith (Helfenstein and Shepard 1999, Icarus 141, 107) shows that such large rms slopes pertain to a sub-mm horizontal distance scale. The RTE model also reproduces the same BFM/AFM radiance difference, but as a direct consequence of multiple scattering between particles within the flat (zero topographic roughness) regolith layer. Approximately 80% of the radiance in the RTE model is contributed by photons scattered more than once. We conclude that topographic roughness of the regolith surface and multiple scattering between regolith particles can have a nearly identical effect on this BFM/AFM difference in the lunar radiance. Roughness and multiple scattering are strongly correlated parameters in the sense that any approximations regarding one of them will influence the value deduced for the other. A complete treatment of this work is published in Goguen et al 2010, Icarus 208, 548.

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