THORIUM MEASUREMENTS
Thorium is one of the ten elements being mapped by the Gamma Ray Spectrometer (GRS). Blue indicates low concentrations, and red indicates high concentrations. The GRS is especially sensitive to this heavy, radioactive element. The distribution of thorium can be used to gain insight into the evolution of the moon's crust. Most of the measured thorium is found concentrated on the near side maria (regions of dark, smooth, young crust), particularly around Mare Imbrium. While it is known from Apollo return samples that some of the thorium south of Mare Imbrium was produced by volcanic activity, the Lunar Prospector measurements indicate that some of the thorium was spread on the lunar surface as a result of the impact that produced the Imbrium basin.
On the far side (at a longitude of 180 degrees), there is a much smaller concentration of thorium seen at the South Pole-Aitken Basin, the largest known impact basin in the solar system. This suggests that despite its size, the impact may not have dug deep enough to excavate much subsurface thorium. This is possibly because the lunar crust is thicker on the far side than it is on the near side.
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