The Compton–Belkovich Thorium Anomaly was found in 1998 by the Gamma Ray Spectrometer (GRS) instrument on board the
Lunar Prospector (LP) and subsequently identified as a hotspot, located around . The estimated thorium concentration reaches (5.3
micrograms per gram) while the surrounding highland basalts only contain between . Compared to the Earth's thorium concentration of , the Compton–Belkovich's is very high. High resolution images from the
Lunar Reconnaissance Orbiter LRO made it possible to analyze the surface features of the Compton–Belkovich Thorium Anomaly in 2011. The region as a whole is wide and long. The center of the region is a volcanic complex, to across, between the Bel'kovich and the
Compton craters. It is from the extent of the northeastern
Procellarum KREEP Terrane (an area which has high abundances of
KREEP, a geochemical component of some lunar rocks). An extension of the reflective material extends to the south-east from the elevated region by about . This may be a
pyroclastic flow. This more highly reflective area also matches an area that shows a
Christiansen feature with shorter wavelength. It reflects more strongly in the 7.1 to 7.5 μm range, which indicates
quartz or
alkali feldspar is the major constituent.
Volcanic slope Volcanic features provide information about the composition of the lava that formed the Compton–Belkovich Thorium Anomaly. On average, many volcanoes on the Moon have slopes of lower than 7 degrees. However, the Compton–Belkovich Thorium Anomaly has a slope which reaches 25 degrees at the highest. This suggests that the region was formed by more viscous lava.
Composition Using infrared reflectance data from Clementine at 750 nm and 950 nm, the level of iron oxide was determined to be about 3% by mass.
Formation A direct analysis of
Apollo program samples revealed that most lunar volcanism occurred around 3 to 4 billion years ago. However, volcanic activity on the unsampled lunar back side could have occurred around 1 billion years ago. As the
lava cooled, it would have
crystallized to produce a
silicate structure;
incompatible elements such as thorium would have been excluded from the process and formed thorium-rich pockets in the remaining liquid rock. The eruption associated with the thorium anomaly could have created the elevated features to the west and the low and broad area to the east. The latest possible eruptions of lava would have made domes with steeper slopes, and also would have caused small bulges, as they would barely reach the surface. ==See also==