Lunar Hotspot Points to Granite on the Moon

There’s a massive cache of granite on the Moon—but there isn’t supposed to be. A team of scientists from across the United States have located a large body of granite on the Moon’s “dark side,” thanks to a lunar hotspot that suggests the presence of an old underground volcano.

Matthew Siegler, a researcher with the Planetary Science Institute and professor at Southern Methodist University (SMU), led a team of scientists in measuring the heat simmering below the Moon’s surface. They used NASA’s Lunar Reconnaissance Orbiter—known recently for snapping a photo of the Moon that looked like a teddy bear—to capture microwave-wavelength measurements of whatever heat could be found underground. When they compared their data with that from China’s Chang’e-1 and Chang’e-2 probes, they confirmed the presence of a hotspot with a peak heat flux of 180 milliwatts per square meter.

The far side of the Moon is typically 20 times cooler, indicating the presence of a unique heat source. This aligned with what researchers already knew about the Compton–Belkovich volcanic complex, a region discovered shortly after the Lunar Reconnaissance Orbiter’s debut. But despite the size of the complex’s namesake, the 200-kilometer-diameter Belkovich Crater, few suspected the Moon’s volcanic systems to be so large. Siegler’s study suggests the Moon’s underground volcanic workings cover a 50-kilometer-diameter region. 

Part of the Compton–Belkovich volcanic complex.
Credit: NASA/GSFC/Arizona State University

Perhaps more shocking was that the radioactive elements under the surface could only exist on the Moon as granite. While granite is relatively common here on Earth, the Moon lacks the mechanisms necessary to create it—at least, not in the ways we’re familiar with. Granite owes its Earthly presence to water and plate tectonics, neither of which the Moon has. As a result, researchers have previously only gleaned tiny crumbs of granite from sediment retrieval missions. 

Siegler and his team believe this cache of lunar granite formed when the Moon had active volcanoes, meaning it’s existed for roughly 3.5 billion years. “If you don’t have water it takes extreme situations to make granite,” Siegler said in a SMU statement. “So, here’s this system with no water, and no plate tectonics—but you have granite. Was there water on the Moon—at least in this one spot?  Or was it just especially hot?”