Lunar Heat Flow: Global Predictions and Reduced Heat Flux

Abstract

AbstractGeothermal heat flux of a body provides a unique look into its internal composition. The geothermal heat flux of the Moon is believed to vary primarily due to the distribution of radiogenic materials both spatially and vertically through the lunar crust. Here, we combine available global orbital, landed mission, returned sample, and meteorite data to produce a forward model of lunar heat flux consistent with present data. This provides a framework for future geothermal and geochemical measurements over the lunar surface, which can be tied back to global radiogenic and refractory element composition. We develop a model of the Moon as a single heat flux province represented by ∼60% of the Lunar Prospector Gamma‐Ray Spectrometer surface Th value or equivalently a 21.5 km thick layer with surface Th values. The zero‐thorium intercept, or “reduced heat flux,” of these models is approximately 5 m Wm−2, implying the net heat flux from the mantle is at or below this value. With an estimated Urey ratio between 0.65 and 0.73, this equates to a mantle thorium concentration of ∼19–∼25 ppb. This model suggests a bulk thorium concentration of approximately 50.5 ± 5.5 ppb. This value is based on the heat flux that would be obtained from a CTh/CU = 3.7, CK/CU = 2,000 = or equivalently 13.7 ± 1.5 ppb of uranium and 27.2 ± 3.0 ppm potassium.