The Moon produced volcanic material as recently as two billion years ago, far longer than physics predicted it should remain geologically active. Samples returned by China's Chang'e-5 mission showed this unexpected volcanism, even though the Moon should have cooled rapidly after forming 4.5 billion years ago when a protoplanet collided with Earth.
Miao Chen of Chengdu University of Technology and colleagues propose tidal heating as the explanation. Their paper, published in the journal Planet, suggests Earth's gravity may have kept the Moon's interior warm enough to drive volcanism billions of years after the Moon's violent birth.