Radiative-transfer modeling of spectra of planetary regoliths using cluster-based dense packing modifications.

In remote sensing of planetary bodies, the development of analysis techniques that lead to quantitative interpretations of datasets has relatively been deficient compared to the wealth of acquired data, especially in the case of regoliths with particle sizes on the order of the probing wavelength. Radiative transfer theory has often been applied to the study of densely packed particulate media like planetary regoliths, but with difficulty; here we continue to improve theoretical modeling of spectra of densely packed particulate media. We use the superposition T-matrix method to compute the scattering properties of an elementary volume entering the radiative transfer equation by modeling it as a cluster of particles and thereby capture the near-field effects important for dense packing. Then, these scattering parameters are modified with the static structure factor correction to suppress the irrelevant far-field diffraction peak rendered by the T-matrix procedure. Using the corrected single- scattering parameters, reflectance (and emissivity) is computed via the invariant-imbedding solution to the scalar radiative transfer equation. We modeled the emissivity spectrum of the 3.3 μm particle size fraction of enstatite, representing a common regolith component, in the mid-infrared (~5 - 50 μm). The use of the static structure factor correction coupled with the superposition T-matrix method produced better agreement with the corresponding laboratory spectrum than the sole use of the T-matrix method, particularly for volume scattering wavelengths (transparency features). This work demonstrates the importance of proper treatment of the packing effects when modeling semi-infinite densely packed particulate media using finite, cluster-based light scattering models.