Thermal Infrared Spectra of Lunar Analog Mineral Mixtures Under Simulated Airless Body Conditions and Comparison to LRO Diviner Observations

Abstract:

On airless bodies, the uppermost portion of regolith, the “epiregolith”, represents the boundary layer between the surface and space that dominates spectral observations from far-ultraviolet reflectance to far-infrared emission. On the Moon, this layer is typically less than 2 mm in thickness and is estimated to be characterized by significant thermal gradients (~60K / 100 μm). These thermal gradients make spectral emission from the Moon wholly different from Earth and Mars (where the epiregolith is essentially isothermal) and complicate the interpretation of spectral emission remote sensing data. Therefore, thermal infrared (TIR) spectroscopy experiments and spectral libraries measured in ambient laboratory conditions are not comparable to remote sensing datasets of airless bodies that contain significant emission components.

We work to overcome this challenge by measuring TIR emission spectra in a chamber that illuminates and heats particulate samples under vacuum to generate a thermal gradient akin to that found in the epiregolith of airless bodies. Recent advancements in our experiments include measurements under two illumination/temperature conditions to isolate temperature-dependant spectral effects. Simulating the lunar environment allows us to measure TIR spectra that are directly comparable to remotely sensed TIR observations from the Diviner Lunar Radiometer (Diviner) instrument aboard the Lunar Reconnaissance Orbiter (LRO).

Here we characterize the TIR emission spectra of two- and three-component silicate mineral mixtures with the endmembers plagioclase, pyroxene, and olivine. These uniform composition and particle-size endmembers bound the typical mineral compositions of the lunar surface. By understanding the TIR characteristics of these mixtures measured under two different illumination conditions, we can better interpret Diviner and future TIR datasets and their implications for surface compositions on the Moon and other airless bodies. In addition, we will revisit compositional maps of the Moon normalized to illumination conditions similar to those produced in our laboratory experiment and compare them to mineral mixture and lunar soil spectra.