Map of the abundance of the element thorium on the Moon made with data from the Lunar Prospector, a space mission launched in 1998, shows that most of this radioactive element is concentrated in a region on the Moon’s near side (left). But there is also a small hot spot called the Compton-Belkovich Thorium Anomaly (labeled C-B in the map) on the side of the Moon that faces away from Earth.

Credit: NASA/GSFC/ASU/WUSTL, Processing by B. Jolliff

Analysis of new images of a curious “hot spot” on the far side of the Moon reveal it to be a small volcanic province created by the upwelling of silicic magma. The unusual location of the province and the surprising composition of the lava that formed it offer tantalizing clues to the Moon’s thermal history.

One of the most spectacular features within the volcanic province is a large dome with a partially depressed summit plateau and partially collapsed rim to the left and upper left side, which appears to be an ancient volcano. It rises about a kilometer from base to summit. (The colors correspond to elevation relative to the lunar mean surface elevation.) The image and the digital terrain model were made by Tawny Tran, Arizona State University.

The Compton-Belkovich feature comes to life when an image from the LRO Wide-Angle Camera (WAC) is draped over a terrain model constructed from WAC data by F. Scholten of the Institute of Planetary Research in Berlin. The terrain at the center of the “hot spot” can be seen to be a low swelling with a variety of volcanic features within its boundaries.

As the magma ocean cooled, Jolliff explains, elements such as thorium were preferentially excluded from crystallizing minerals, forming pockets of KREEP-rich magma sandwiched between the crust and mantle.

One of these, which encompasses much of the mare basaltic volcanism on the Moon, is called the Procellarum KREEP Terrane, or PKT for short. This immense lunar “hot spot” contains high concentrations of thorium and other radioactive, heat-producing elements, such as potassium and uranium. [KREEP stands for potassium, (K), rare-earth elements (REE), and phosphorus (P).]

This simple picture of the Moon’s geology served for many years, but in 2000, Jolliff and his colleagues in the department of Earth and Planetary Sciences and WUSTL’s McDonnell Center for the Space Sciences introduced a concept in which they distinguished three different “terranes,” or regions of the Moon with distinctive geologic histories.

Procellarum KREEP Terrane It wasn’t so very long ago, Jolliff says, that scientists talked about the Moon as having two sorts of terrain, the dark maria, or “seas,” and the light terra, or highlands.

Earth’s continental crust, which reflects active geological processes such as subduction, magma intrusion and mountain building, includes many rocks whose compositions are intermediate between basalt and silica-rich rocks like granite, which are common on Earth. On the Moon, on the other hand, there are many basaltic rocks and only a small fraction of granite. Rocks of intermediate composition are all but missing.

Moreover, almost all of the volcanism on the Moon is basaltic rather than silicic, enriched in minerals containing the elements iron and magnesium rather than the elements silicon and aluminum.

One of the mysteries of lunar volcanism is the unequal distribution of these flood basalts. Nearly a third of the Moon’s near side is covered by ancient flood basalts but the Moon’s far side, where the crustal rocks are thicker, has much less.

The differentiation of the crust and mantle was followed by a wave of volcanic activity between about 3 to 4 billion years ago, when basaltic lavas erupted on the lunar surface, filling old impact craters and other low spots to form the lunar mare.

But because the Moon was small and had no atmosphere, the magma ocean cooled quickly, within perhaps 100 million years. Eventually lighter minerals such as feldspar crystallized out of the magma and floated to the top to create huge masses of feldspathic rock that formed the lunar highlands. Denser iron- and magnesium-rich minerals sank when they crystallized, forming the upper part of the Moon’s mantle.

Cameras aboard the Lunar Reconnaissance Orbiter, launched in 2009, showed that the center of the Compton-Belkovich Thorium Anomaly was relatively reflective in visible light compared to its surroundings. The high-resolution cameras also revealed unusual features in this bright area.

The Moon, thought to have been created when a Mars-size body slammed into Earth about 4.5 billion years ago, was originally a hellish world covered by a roiling ocean of molten rock some 400 kilometers deep

Volcanism on the Moon Lunar volcanism is very different from terrestrial volcanism because the Moon is a small body that cooled quickly and never developed rock-recycling plate tectonics like those on our planet.

“To find evidence of this unusual composition located where it is, and appearing to be relatively recent volcanic activity is a fundamentally new result and will make us think again about the Moon’s thermal and volcanic evolution,” he says.

The volcanic province’s very existence will force scientists to modify ideas about the Moon’s volcanic history, says Bradley Jolliff, PhD, research professor in the Department of Earth and Planetary Sciences in Arts & Sciences at Washington University in St. Louis, who led the team that analyzed the LRO images.

Recent observations, made with the powerful Lunar Reconnaissance Orbiter (LRO) optical cameras, have allowed scientists to distinguish volcanic features in terrain at the center of the bull’s-eye. High-resolution three-dimensional models of the terrain and information from the LRO Diviner instrument have revealed geological features diagnostic not just of volcanism but also of much rarer silicic volcanism.

The hot spot is a concentration of a radioactive element thorium sitting between the very large and ancient impact craters Compton and Belkovich that was first detected by Lunar Prospector’s gamma-ray spectrometer in 1998. The Compton-Belkovich Thorium Anomaly, as it is called, appears as a bull’s-eye when the spectrometer data are projected onto a map, with the highest thorium concentration at its center.



A concentration of heat-producing elements under the Procellarum KREEP Terrane may be partly responsible for the intensive mare volcanism there. The maria, Jolliff explains, were formed when the hot radioactive elements melted minerals deep in the Moon’s mantle, forming basaltic lava which erupted through fissures onto the Moon’s surface. Well over half the Procellarum KREEP Terrane was resurfaced by volcanism. A concentration of heat-producing elements under the Procellarum KREEP Terrane may be partly responsible for the intensive mare volcanism there. The maria, Jolliff explains, were formed when the hot radioactive elements melted minerals deep in the Moon’s mantle, forming basaltic lava which erupted through fissures onto the Moon’s surface. Well over half the Procellarum KREEP Terrane was resurfaced by volcanism. Although most of the volcanism was of the basaltic variety, resulting in the large, dark patches on the Moon visible to the unaided eye from Earth, a much rarer form of volcanism, one that produced lavas rich in silica, also occurred in the PKT. These volcanic deposits are known as “red spots” because of their spectral characteristics, and recent results from the LRO spacecraft confirmed their silica-rich compositions. The red spots include some with distinctive dome shapes, some quite large, and all within the boundaries of the PKT. A new volcanic province

Ever since the Lunar Prospector mission first revealed the thorium-rich bull’s-eye isolated on the far side of the Moon and distant from the Procellarum KREEP Terrane, Jolliff’s group has been curious as to what it was. “When the Lunar Reconnaissance Orbiter was launched in 2009, we were finally able to image it at high resolution,” he says

The nature of the terrain jumps out when orbiter camera images are draped over a digital terrain model created with those images. At the center of the province is an irregular depression that might well be a caldera and at its edges are domes with features that suggest they were formed by the intrusion of high-viscosity silicic lava, a type of lava rare on the Moon. Any model of the Moon’s thermal evolution must now be able to account for this volcanic province as well as the familiar mare.

Image credit: NASA/GSFC/ASU/WUSTL, processing by B. Jolliff, digital terrain model developed by F. Scholten, DLR.

PERSPECTIVE VIEW LOOKING TO THE NORTHEAST, GENERATED WITH A WAC 100-M-PER-PIXEL IMAGE DRAPED OVER THE GLD100 (100-M-PER-PIXEL) DIGITAL TERRAIN MODEL DEVELOPED BY F. SCHOLTEN, DLR. NASA/GSFC/ASU/WUSTL.



At the center of the thorium bull’s-eye is a small volcanic complex, 25 to 35 kilometers across, depending on the direction, nestled between the Compton and Belkovich craters, which are 162 and 214 kilometers across, respectively. Significantly, the Compton-Belkovich Thorium Anomaly lies about 900 kilometers from the northeastern extent of the Procellarum KREEP Terrane. At the center of the thorium bull’s-eye is a small volcanic complex, 25 to 35 kilometers across, depending on the direction, nestled between the Compton and Belkovich craters, which are 162 and 214 kilometers across, respectively. Significantly, the Compton-Belkovich Thorium Anomaly lies about 900 kilometers from the northeastern extent of the Procellarum KREEP Terrane. “In the initial LRO images, taken with the orbiter’s Narrow-Angle Cameras (NACs) during the commissioning phase of the mission, we could see lumpy terrain and collapse features that might be volcanic,” Jolliff says. But impacts that blast material out of craters and onto the surrounding surface can also produce lumpy or mountainous deposits. “To be sure we were seeing volcanic features, we looked more closely, using images from the NACs taken during the mapping phase once the orbiter had reached its 50-kilometer circular orbit. The NACs have telescopic optics and produce images with a 50-centimeter-per-pixel resolution when in a 50-kilometer-altitude orbit. “We mapped the same area more than once with the NACs,” Jolliff says. “We went over looking straight down at the feature we were studying, and then we tilted or ‘slewed’ the whole spacecraft on the next orbital pass so that we could image the same feature at a different angle. From those two views we built a three-dimensional model of the terrain — a digital terrain model, or DTM — that allows us to rotate the terrain in the computer.” (For more on the orbital images, see the slideshow to the right.) Among the diagnostic features revealed by these perspective views are a mountain whose features are commonly obscured by shadow, but when viewed with the terrain model, can be seen to have a depression at the summit, and what appears to be an area of the rim where a breach occurred together with subsequent collapse and mass wasting (downslope movement of rock or regolith).

Sunlight grazes a small dome in the central part of the terrain in this image, part of a larger frame captured by the narrow-angle camera on the orbiter. This dome was probably created by viscous lava, rare on the Moon and quite different from the fluid lava that created the familiar mare on the Moon’s near side.

Image credit: NASA/GSFC/ASU.