Monte Carlo simulations of light scattering by composite particles in a planetary surface

Composite particles containing internal scatterers have been proposed as an explanation for the fact that most photometric studies of planetary surfaces based on Hapke's bidirectional reflectance model have found the planetary particles to exhibit moderately backscattering phase functions. However, an implicit assumption made in this explanation is that the scattering by composite particles containing multiple internal inclusions in a planetary surface can still be adequately computed using standard radiative transfer theory assuming the composite particles to be the fundamental scattering unit even though the particles are necessarily in close proximity to each other. This assumption was explored by J, K, Hillier (1997, Icarus 130, 328-335) using a Monte Carlo routine. However, in this initial study several simplifying assumptions were made. The internal scatterers were assumed to be isotropic and scattering off of the surface and absorption within the composite particle were ignored. While these assumptions are not very realistic, it was believed that the study could still provide insight into the light scattering by such surfaces, Here we relax these assumptions in order to examine the light scattering by more realistic particles. Almost all of the conclusions reached in the earlier paper remain valid, As before, we find that classical radiative transfer (assuming a random distribution of scattering particles) coupled with the assumption that the composite particle is the fundamental scatterer provides a good approximation in the high porosity limit, However, even for porosities as high as 90% the effects of close packing are clearly seen with the radiative transfer calculation underestimating the scattering by similar to 10% at high phase angles. In contrast to the earlier study we find that the radiative transfer calculation tends to overestimate, not underestimate, the scattering at high emission but moderate phase angles. As the porosity is lowered further, the discrepancy becomes more severe and can reach 100% or greater. In particular, our main conclusion remains intact: the parameters derived using the classical radiative transfer theory will yield results intermediate between those of the composite as a whole and those of the internal scatterers and thus one should exercise caution in interpreting the results of models based on classical radiative transfer theory in terms of the physical properties of the surface particles. (C) 2001 Academic Press.