Compositional structure within the lunar crust as constrained by Lunar Prospector thorium data

Lunar Prospector gamma-ray spectrometer data for surface thorium indicate t hat craters greater than about 60 km in diameter tend to excavate into mate rials less Th-rich than the surface regolith. For the largest 12 craters an d basins amenable to this type of analysis (diameters between 90 and 930 km ), the ratio of crater (or basin) Th concentration to regional background T h averages 0.76 +/-0.13. This trend implies a pronounced diminution in aver age Th concentration with increasing depth (down to roughly 30 km deep) in the lunar crust. Assuming deep-origin Orientale ejects was strongly diluted by turbulent basin-modification mixing, an even more pronounced depth-Tn a nticorrelation would be indicated by extrapolation of the trend of the 30-3 00 km craters to higher diameters, i.e., deeper excavations. In any case, t he overall lunar crust, and by extension the bulk Moon, is probably far les s Th-rich than a simple averaging of surface compositions would suggest.