Thorium concentrations in the lunar surface. I - Regional values and crustal content

The reported investigation is based on data which have been obtained with the aid of a gamma-ray spectrometer which had been installed on the Apollo 15 and 16 spacecraft to map the composition of the overflown lunar regolith to a maximum depth of about 1 m. Two additional techniques for the analysis of orbital gamma-ray data have been developed. These techniques have provided basic confirmation for the Th results reported previously and one of them, the energy band method, has yielded results for Th with a sensitivity greater than previous analyses, particularly at low concentrations. Significant variations of radioactivity exist in highland areas. The lowest concentrations of radioactivity mapped by Apollo are found near the western limb, contrasting with concentrations in the eastern limb which run roughly a factor of two higher. An inverse relation has been found between Th concentration and crustal thickness. It is suggested that Th, once uniformly distributed, has been extracted from a zone of constant depth over much of the moon, and concentrated in crustal blocks of varying thickness.