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

摘要

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. Plain Language Summary Remote sensing in the mid-infrared wavelengths (~5 - 50 μm) has been used widely to interpret the mineralogy of planetary surfaces. This technique works well when the material (soil, sand, rock, etc.) of interest is large compared to the reflected or emitted wavelength, however interpretation of such data is complicated by the presence of fine materials with sizes similar to or less than the wavelength of light. In these cases, a substantial portion of light is diffracted and accurate interpretations of mineralogy from remote sensing data becomes very difficult. This has been a problem as planetary surfaces are often covered with fine regolith - closely packed particles with sizes frequently on the order of mid-infrared wavelengths. In an effort to resolve this problem, one area of on-going research is the modeling of the interaction of light with such particles using light scattering models. We contribute to this effort by investigating advanced light scattering models that more realistically incorporate the physical conditions of planetary regoliths. We demonstrate that our approach achieves improvements that have been mostly unattainable with previous methods as well as give a critical analysis of its effectiveness. This work advances the development of more effective light scattering models for planetary regoliths which is crucial for accurate mineralogical analyses from remote sensing data.