Greenhagen, Benjamin - Compositional Ground Truth for the Diviner Lunar Radiometer: Comparing Apollo Sites and Soils

Apollo landing sites and returned soils afford us a unique opportunity to “ground truth” Diviner Lunar Radiometer compositional observations, which are the first global, high resolution, thermal infrared measurements of an airless body.  The Moon is the most accessible member of the most abundant class of solar system objects, including all SSERVI target bodies.  Additionally, the Apollo samples returned from the Moon are the only extraterrestrial samples with known spatial context.  Here we compare Diviner observations of Apollo landing sites and compositional and spectral laboratory measurements of returned Apollo soils.

Diviner, onboard NASA’s Lunar Reconnaissance Orbiter, has three spectral channels near 8 µm that were designed to characterize the mid-infrared emissivity maximum known as the Christiansen feature (CF), a well-studied indicator of silicate mineralogy.  It has been established previously that thermal infrared spectra measured in simulated lunar environment (SLE) are significantly altered from spectra measured under terrestrial or martian conditions, with enhanced CF contrast and shifted CF position relative to other spectral features.  Therefore only thermal emission experiments conducted in SLE are directly comparable to Diviner data.  Here we present data collected at the University of Oxford Simulated Lunar Environment Chamber (SLEC) and JPL’s Simulated Airless Body Emission Laboratory (SABEL).

With known compositions, Apollo landing sites and soils are important calibration points for the Diviner dataset, which includes all six Apollo sites at approximately 200 m spatial resolution.  Differences in measured CFs caused by composition and space weathering are apparent in Diviner data.  We find that analyses of Diviner observations  and SLE measurements for a range of Apollo soils show good agreement, while comparisons to thermal reflectance measurements under ambient conditions do not agree well, which underscores the need for SLE measurements and validates the Diviner compositional measurement technique.

Diviner observations of Apollo landing sites are also correlated with geochemical measurements of Apollo soils from the Lunar Sample Compendium.  In particular, the correlations between CF and FeO and Al2O3 are very strong, owing to the dependence on the feldspar-mafic ratio.  Our analyses, an extension of earlier work, support findings that Diviner data may offer an independent measure of soil iron content from the existing optical and gamma-ray spectrometer datasets.