Early in the evolution of the Moon, towards the late stages of differentiation of the early lunar magma ocean, a relatively thin layer of melt would have remained and been enriched with thorium, water, hydroxyl, and other incompatible materials as a result of fractional crystallization of the lunar crust and mantle. This layer is known as KREEP (potassium, rare earth elements, phosphorus). Lunar crust that shows high concentrations of thorium and hydroxyl/water may indicate interaction with the KREEP layer. Using results from the Lunar Prospector Gamma Ray Spectrometer (LP-GRS), we selected thorium anomalies on the Moon in an effort to detect KREEP-rich or material using hyperspectral imagery. Four sites were chosen: Lassell Crater (15 S, 8 W), Hansteen Alpha (15 S, 50 W), Gruithuisen Domes (36 N, 40 W), and the Compton-Belkovich Thorium Anomaly (61 N, 100 E). Three of these sites are non-mare volcanic features within the Procellarum KREEP Terrane (PKT), the area on the lunar nearside which has a KREEP signature, while Compton-Belkovich is located on the lunar farside. The Moon Mineralogy Mapper (M3) hyperspectral imager was used to analyze the composition of these locations. The reflectance spectra gathered from these sites all show pronounced absorptions at ~2.8 microns, indicating the presence of hydroxyl/water. Maps of the 2.8-micron absorption show concentric patterns centered on these sites with the deepest absorptions at their centers. Digital elevation models and high-resolution imagery from the Lunar Reconnaissance Orbiter-Near Angle Camera (LRO-NAC) show that many of the 2.8-micron absorption maxima are associated with morphologies consistent with volcanic domes or vents. This association suggests that the volcanic features are associated with potential sources of magmatic volatiles and were the sites degassing those volatiles. In order to measure the concentration of hydroxyl/water associated with the 2.8-micron absorptions, we measure the area of a Gaussian curve fit to the absorption spectra. The area of the Gaussian curve, an assumed particle size of 45 microns, the density (3.0 g cm-3), and an integrated molar absorption coefficient are parameters used to determine the concentration of hydroxyl/water creating the absorption. To ensure that these absorption features are the result of KREEP-related water that is intrinsic to the Moon, and are not associated with solar wind implanted water, we compare available M3 images from different optical periods in the lunar day. Whereas the concentration of implanted water should vary on a diurnal basis as it migrates through the lunar regolith and exosphere, the nearside volcanic features show consistent 2.8-micron absorptions throughout the lunar day, a behavior consistent with intrinsic water. A magmatic water source would support the hypothesis that the lunar interior is more hydrous than previously thought; and it suggests that KREEP may underlie the far side highlands near CBTA and possibly other areas outside PKT.
