Particle Size Effects on Mid-Infrared Spectra of Lunar Analog Minerals in a Simulated Lunar Environment

Mid-infrared spectroscopic analysis of the Moon and other airless bodies requires a full accounting of spectral variation due to the unique thermal environment in airless body regoliths and the substantial differences between spectra acquired under airless body conditions and those measured in an ambient environment on Earth. Because there exists a thermal gradient within the upper hundreds of microns of lunar regolith, the data acquired by the Diviner Lunar Radiometer Experiment are not isothermal with wavelength. While this complication has been previously identified, its effect on other known variables that contribute to spectral variation, such as particle size and porosity, have yet to be well characterized in the laboratory. Here we examine the effect of particle size on mid-infrared spectra of silicates common to the Moon measured within a simulated lunar environment chamber. Under simulated lunar conditions, decreasing particle size is shown to enhance the spectral contrast of the Reststrahlen bands and transparency features, as well as shift the location of the Christiansen feature to longer wavelengths. This study shows that these variations are detectable at Diviner spectral resolution and emphasizes the need for simulated environment laboratory data sets, as well as hyperspectral mid-infrared instruments on future missions to airless bodies.