The Distribution of Th on the Moon's Surface

Our present knowledge of the Th distribution at the lunar surface consists mainly of two bands of data taken by the Apollo 15 and 16 gamma-ray experiments, augmented by ground-truth measurements on Apollo and Luna samples and on lunar meteorites. Gamma-ray maps show a broad area of high Th concentration in the Imbrium-Procellarum region, which we refer to as the High-Th Oval Region (HTOR). The Imbrium impact occurred within this region. Here, we show that the global pattern of highland surface Th concentrations is consistent with that expected for Imbrium ejecta deposits. Outside of the HTOR itself, Imbrium ejecta produced deposits that are a mixture of Th-rich Imbrium ejecta and Th-poor, pre-existing crustal material. Inside the HTOR (including the Apollo 12 and 14 sites), Imbrium ejecta mixed with Th-rich crustal material. We suggest that except where younger Orientale ejecta diluted them, or mare basalt flows covered them, Imbrium ejecta deposits remain largely intact. Later gardening of their surfaces has not obscured the pattern of their Th concentrations.