Mid-Infrared Reflectance and Emissivity Spectra of High Porosity Regoliths

Mid-infrared (MIR; 5–35 μm) spectroscopy is often used for mineralogical identification on planetary surfaces. Laboratory spectra aiding remote sensing observations are typically performed in reflection geometries, while MIR spectra of planetary surfaces are typically obtained via emission. Here we explore the validity of Kirchhoff's Law in converting reflectance to emissivity spectra, focusing on the high-porosity regoliths found on airless bodies such as the Moon and asteroids. Specifically, we compared ambient reflectance, ambient emissivity, and simulated asteroid environment (SAE) spectra of fine-particulate olivine and pyroxene with varying regolith porosities, focusing on how spectral features, including the Christiansen feature (CF), reststrahlen bands (RBs), and transparency features (TF), changed under these different conditions. Our results indicate that Kirchhoff's Law can be effectively employed to interpret 19 MIR reflectance spectra of high-porosity samples, provided environmental spectral effects (i.e., spectral changes due to different pressure and temperature conditions) are considered.