Size-Dependent Scattering Properties of Planetary Regolith Analogs

The physics of the interaction of light with a particulate surface are important to understanding and analyzing remote sensing data from planetary surfaces. An examination of the angular scattering properties of powder samples with known compositions and particle sizes was undertaken to try and further understand the interaction of light with a closely packed particulate medium. The samples range in size from smaller than to larger than the wavelength(l) of incident light (0.05 - 30.09 µm in diameter, l=0.635 µm). Based on a rich history of both theoretical treatments and laboratory measurements, results would be expected to show any dependence of scattering parameters on composition and/or particle size. Scanning electron microscope analyses of the powders were done to characterize particle size, composition, and shape as the major contributors to observed trends in scattering parameters. Models currently in wide use to describe light scattering by planetary regoliths make two important assumptions: (1) the propagation of light through the medium can be described by the equation of radiative transfer, which treats the medium as if it were made up of a continuous distribution of independent scatterers and absorbers; and (2) these fundamental scatterers are the individual particles that make up the medium. Models based on the radiative transfer equation were found to provide good empirical descriptions of the light scattering properties of particulate media composed of complex particles, such as planetary regoliths. However, the results reported here show that changes in scattering parameters predicted by the assumption that the particles are the scatterers are not observed in these samples, and that such models do not accurately predict the transport mean free path, scattering coefficient, or extinction coefficient of such media. In particular, the transport mean free path shows remarkably little dependence on particle size over the size ranges studied, whereas the particle scattering assumption predicts a large variation. Theoretical models based on the second assumption should only be used with great caution when analyzing data taken on particulate surfaces