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The Compton–Belkovich Thorium Anomaly is a
volcanic complex A complex volcano, also called a compound volcano or a volcanic complex, is a mixed landform consisting of related volcanic centers and their associated lava flows and pyroclastic rock. They may form due to changes in eruptive habit or ...
on the
far side of the Moon The far side of the Moon is the hemisphere of the Moon that is facing away from Earth, the opposite hemisphere is the near side. It always has the same surface oriented away from Earth because of synchronous rotation in the Moon's orbit. C ...
. It was found by a
gamma-ray spectrometer A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution (or spectrum—see Gamma spectroscopy#Scintillation detectors, figure) of the intensity of gamma radiation versus the energy of each photon. The study and analysis of ...
in 1998 and is an area of concentrated
thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...
, a 'fertile' element. Lunar rock samples from the Apollo missions reveal that most lunar volcanism occurred around 3 to 4 billion years ago, but this feature could have formed as recently as 1 billion years ago due to the unknown history of the Moon's far side.


Description

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. It has unusually high
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
, identified by a visible imaging study that was carried out later by the Clementine spacecraft in a Clementine Visible Images study. High resolution images from the
Lunar Reconnaissance Orbiter The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric Polar orbit, polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic ...
LRO made it possible to analyze the surface features of the Compton–Belkovich Thorium Anomaly in 2011.


Location

The anomaly is between the Bel'kovich crater, which is wide, and the Compton crater, which is wide. 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).


Features

In the center of the elevated region is a depression; this is bounded by scarps and may be some kind of
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...
. Just to the north is a feature called Little Dome, in diameter. Further north is an elongated dome, oriented north-south, called Middle Dome. It is long and wide. Both Little Dome and Middle Dome have boulders on top that may be volcanic blocks. Big Dome is further to the north at the edge of the anomaly. It is in diameter with a depression in the top. An extension of the reflective material extends to the south-east from the elevated region by about . This may be a
pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
. 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 Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
or alkali feldspar is the major constituent. Explosive remains also appear scattered to the east for about 300 km covering an area of 70,000 km2.


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 The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...
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. The smoothness of the surface associated with the anomaly indicates that it could possibly have been formed in a more recent event. As the
lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
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

* Volcanism on the Moon


References


Further reading

* {{DEFAULTSORT:Compton-Belkovich Thorium Anomaly Thorium LQ06 quadrangle Lunar observation Magnetic anomalies Volcanoes on the Moon Extraterrestrial volcanic calderas