A 30-mile-long slab of granite may be hiding in an ancient volcano on the moon

Lunar researchers have announced the discovery of a 50-kilometer-wide boulder beneath the surface of the far side of the moon – and it’s made out of something commonly found in kitchens.

The team detailed their surprising discovery on Thursday, days before presenting the work at the Geochemistry Goldschmidt conference in Lyon, France. A series of satellites detected a heat source near an ancient lunar caldera, a sink formed when magma erupts from a volcanic chamber and collapses. Satellites determined that this location was about 10 degrees Celsius warmer than its surrounding area. They suspect that it came from a gigantic granite slab.

“We have discovered that at a spot on the moon that is thought to be a long-extinct volcano that last erupted over 3.5 billion years ago, additional heat is venting from the ground,” says Matt Siegler, senior researcher and chief scientist at the Planetary Science Institute in Arizona, said Thursday’s announcement. An article about the work was published in the magazine on Wednesday Nature.

This diagram shows the location of granite on the far side of the moon. On the left, the arrow points to the position of the large boulder of granite called basolith on a global view of the moon. In the center, a box shows the location and thermal gradient of the granite on a lunar surface map. On the right, a side view shows the thermal gradient.

Siegler et al., Nature July 2023

“It’s about 50 kilometers across, and the only solution we can think of that generates that much heat is a large body of granite, a rock formed when a body of magma — the unerupted lava — cools beneath a volcano “, he added.

Why is there granite on the moon?

According to the announcement, there is “almost no granite” in the solar system other than Earth, and “only small grains of granite” have been found in rock samples from the Apollo moon.

On Earth, granite is formed when water interacts with the tectonic plates of the Earth’s crust. According to the National Park Service, “granite rocks are found on continents around the world near active or past plate boundaries.” Granite is formed when magma rises close to the surface but does not erupt, and then slowly cools underground. Granite can become visible at the surface as the volcanic rock on top of it erodes over time. El Capitan and Half Dome in Yosemite National Park are good examples of granite slabs. Due to their enormous size, they are called batholiths.

But the moon has neither water nor plate tectonics in the traditional sense. However, it has experienced some volcanism and there are pockets of frozen water near its poles. Whether granite exists on the moon is still somewhat confusing. “This is more Earth-like than we imagined could form on the Moon, which lacks the water and plate tectonics that help form granites on Earth,” Siegler said.

A view of El Capitan while snow obscured Yosemite National Park in California on February 22, 2023.

Agency Anadolu/Agency Anadolu/Getty Images

Siegler also credits his wife, Rita Economos, a geochemist and associate professor at Southern Methodist University in Texas, for pointing out that the spacecraft’s observations found granite despite the Moon’s position.

“To be honest we were a bit confused when we found it: Luckily my wife, Dr. Rita Economos, the geochemist in the family, so with her guidance we were able to figure out the likely geological cause of the heat anomaly,” Siegler said.

Exploration of the solar system

The new work came about thanks to the surveying capabilities of four lunar spacecraft: the Chinese lunar orbiters Chang’e 1 and 2, the Lunar Prospector and NASA’s Lunar Reconnaissance Orbiter. The work shows that remote sensing can detect hidden features beneath the surface of extraterrestrial terrain, Siegler says.

“This will be useful when exploring other planetary bodies in the solar system,” he adds.

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