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J.K. Hillier, B.J. Buratti (JPL)
The vast majority of studies of planetary surfaces using Hapke’s photometric model have found moderately backscattering single particle phase functions. In contrast, most laboratory studies suggest that particles should exhibit strong forward scattering lobes. This has led to the suggestion that the backscattering behavior of the planetary particles is due to a complex particle structure and texture. One promising candidate is particles which contain internal scatterers such as inclusions, microcracks, and bubbles. We examined scattering by such composite particles in a close packed surface in an earlier paper (Hillier, 1997). However, several simplifying assumptions were made in that model. In particular, the internal scatterers were assumed to be isotropic and no scattering off of, or absorption within, the composite particle was allowed. Here, we have refined the model in order to relax these assumptions.
For scattering by isolated composite particles, we confirm the results of Mishchenko and Macke (1997): assuming more realistic forward scattering wavelength-sized internal scatterers, the phase function of the composite particle as a whole can be made backscattering (excluding diffraction), but only slightly so (asymmetry parameter > –0.1) for reasonable optical depths of internal scatterers. Our results for scattering in a close packed surface are qualitatively similar to our earlier results and our conclusions from that paper remain valid. Even at porosities as high as 90 with classical radiative transfer calculations significantly (10 our new results suggest a larger discrepancy at small scattering angles than before, though, as before, the discrepancy is most acute at moderate to high scattering angles.
Hillier, J.K. (1997). Icarus, 130, p. 328. Mishchenko, M.I. and Macke, A. (1997). J. Quant. Radiat. Trans., 57, p. 767.