Groundtruthing Diviner Lunar Radiometer Observations with Laboratory Measurements of Apollo Soils

Abstract:

Laboratory measurements of well-characterized samples under the appropriate near-surface environmental conditions are needed to best interpret the surface composition of planetary bodies using remote sensing observations, particularly at thermal infrared (TIR) wavelengths. Ideally those well-characterized samples are materials from the planetary body of interest (e.g., returned regolith or rocks, or meteorites). The Moon is the ideal planetary body to make this link between the lab and from orbit because we know the exact locations from which the Apollo astronauts collected rock and regolith samples and we now have high spatial resolution observations of those sampling sites from the Diviner Lunar Radiometer on board NASA’s Lunar Reconnaissance Orbiter. Thus, lab measurements of the Apollo samples can be used for ground truthing the Diviner observations and for constraining the lab conditions that best simulate the near surface environment of the Moon.

We present results of a comprehensive study to best replicate a lunar environment in the laboratory, evaluate the most appropriate sample and measurement conditions, collect TIR spectra of a representative suite of Apollo soils, and correlate these measurements with Diviner observations of the sampling sites. To simulate the lunar environment (SLE) we use a bespoke environment chamber (PASCALE) at the University of Oxford where we cool the vacuum chamber with liquid nitrogen (pressures < 10^-4mbar; temperatures < 125 K) and heat the sample from below with heaters and above with a lamp until the brightness temperature of the sample is similar to temperatures observed by Diviner.

We find that analyses of Diviner observations for individual sampling stations and lab measurements of the returned Apollo soils in a relevant environment generally show good agreement. Furthermore, the agreement was improved when the illumination geometry of the lab experiments and Diviner observations are similarly constrained. In contrast, comparisons between Diviner observations and TIR emission and reflectance measurements under terrestrial conditions or vacuum chambers heating without a lamp do not agree well. These analyses underscore the need for SLE measurements to validate TIR emission datasets from the Moon and other airless bodies.