Planetary scientists have discovered an extensive hunk of granite buried on the farside of the Moon. A research team analyzed measurements taken by a multinational array of lunar missions and found that the granite, located beneath the Compton-Belkovich region in the lunar highlands, likely caused the spot to be anomalously hot.
How the granite formed there, however, is a mystery. “All the textbook ways of making granite involve these processes that only happen on Earth,” said Matt Siegler, a planetary scientist at the Planetary Science Institute in Tucson, Ariz., and lead researcher on the discovery. “How do you re-create this process?”
A Hidden Batholith
On Earth, granite forms through partial melting of the lithosphere. Magma erupted from the mantle and cooled on Earth’s surface is mere basalt. When basalt-rich slabs of crust are subducted along the edge of a tectonic plate, pressure and heat cause the rock to release water, which lowers the melting point of the overlying lithosphere. What eventually emerges close to the surface is silica-rich granite enriched in thorium and uranium.
The process of making granite on Earth depends heavily on the recycling properties of plate tectonics and copious amounts of water. Both of those are rare throughout the solar system, and therefore, so is granite. Rare, but not impossible.
About 10 silica-rich features are known on the Moon, Siegler said. “They’re pretty rare, whereas 16% of the surface of the Moon is covered with basalt.” Apollo 12 astronauts discovered a small number of granite grains on the Moon’s nearside in the Procellarum region, and planetary scientists are still struggling to understand the grains’ origins.
So the researchers weren’t expecting to find evidence of granite on the Moon’s farside. They weren’t even looking for it.
They were initially trying to understand the origins of strangely high thorium concentrations in a small spot on the farside, explained coauthor Jianqing Feng a planetary scientist at the Planetary Science Institute and expert in remote sensing. The Compton-Belkovich anomaly has the highest thorium concentrations on the Moon and is also much hotter than the surrounding lunar highlands. The temperature within the microwave band is hotter than anywhere at the same latitude, Feng said.
Trying to peer beneath the surface of this anomaly, the team gathered new microwave data from instruments on board China’s Chang’e-1 and Chang’e-2 lunar orbiters and NASA’s Lunar Reconnaissance Orbiter, as well as past data from India’s Chandrayaan-1 and NASA’s Lunar Prospector, Gravity Recovery and Interior Laboratory (GRAIL), and Apollo missions. The combined remote sensing data from these missions helped the team precisely map the temperature around and beneath Compton-Belkovich.
The data revealed that the spot is about 20 times hotter than the surrounding highlands. The team’s geochemical models suggest that the temperature anomaly is caused by a large granitic system—a batholith about 50 kilometers across and 20 kilometers deep—heating the ground from below.
Its volume is likely similar to terrestrial batholiths such as the Andean Altiplano-Puna Magma Body. The researchers’ model approximated the batholith as two ellipsoids, but in reality, there might be several smaller chambers and sills woven together, empty remnants of the region’s volcanic past.
A Remelting Mystery
“This new finding of a large mass of granite on the Moon is incredibly interesting,” Stephen Elardo, a planetary geochemist at the University of Florida in Gainesville, said in a statement about the discovery. “We have tons of granite of different flavors all over Earth.…But geologically-speaking, it’s quite hard to make granite without water and plate tectonics, which is why we really don’t see that type of rock on other planets.”
Elardo, who was not involved with this research, added that understanding this discovery is “going to be massively important for how we think about the internal workings of other rocky bodies in the Solar System.”
Until now, lunar granite had been seen only in small quantities on the mare-rich nearside, and it’s theorized to have evolved via pathways that are less common on Earth such as rapid melting from a large impact. The Compton-Belkovich region shows no signs of such an event.
“Maybe there was a mantle plume underneath this feature…that baked this area again and again for millions of years,” Siegler speculated. “Or maybe once you get enough uranium and thorium concentrated, it heats and remelts itself almost like a natural nuclear reactor. Or maybe this little pocket of the Moon was especially wet, and so things didn’t have to get as hot here” to create granite.
The researchers proposed several theories to explain the formation of this granitic batholith, and they hope that this unsolved mystery will encourage others to look for similar features beneath the Moon’s numerous thorium anomalies.
For example, on the nearside, “there’s another place called Wolf Crater,” Feng said. “It’s an even larger place that has thermal anomalies, and maybe there is a larger granite body than Compton-Belkovich.” Discovering more granitic bodies on the Moon would help planetary scientists and geochemists better understand how they came to be.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
Correction, 16 August 2023: The description of Compton-Belkovich’s temperature anomaly has been updated.