T-matrix and radiative transfer hybrid models for densely packed particulates at mid-infrared wavelengths

Mid-infrared spectroscopy is a useful tool for remotely sensing the composition of Earth and other planets. Quantitative mineralogical investigations are possible using remotely sensed data; however, the difficulty in modeling complex interactions of light with particles that are on the order of the wavelength limits the usefulness of the remote sensing data. As part of an effort to develop a more effective treatment of light scattering in planetary regolith, we explore the ability of T-matrix and radiative transfer (RT) hybrid models to produce emissivity spectra that are consistent with laboratory measurements. Parameters obtained from T-matrix calculations are used in three different RT models to construct emissivity spectra of enstatite particles of different sizes. Compared to the widely used Mie/RT hybrid models, the T-matrix/RT hybrid models produce more consistent emissivity spectra for the finest particle size fraction (3.3 μm). Overall, T-matrix hybrid models produce improved emissivity spectra, but larger particle sizes are still difficult to model. The improvement observed in T-matrix/RT hybrid models is a result of the inclusion of multiple scattering in closely packed media, and it demonstrates the importance of the implementation of physically realistic factors in developing a more effective light scattering model for planetary regolith. Further development and implementation of this and similar hybrid models will result in an improvement in quantitative assessments of planetary particulate surfaces from mid-infrared spectra.