This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: We typically think of the moon as a static, floating object that has remained the same for basically its entire lifetime, with an occasional asteroid impact serving as an exception. That understanding is grounded in physics—the moon should have cooled rapidly after forming when a huge protoplanet hit the early Earth around 4.5 billion years ago.
But some lunar material returned from the Chang'e-5 mission was notably formed only around 2 billion years ago, yet appeared to be volcanic in origin. So how did a world that was supposed to have settled down without volcanism remain hot enough to create these materials more than 2 billion years after its violent birth? A new paper from Miao Chen of the Chengdu University of Technology and his colleagues, recently published in the journal Planet, offers an explanation—tidal heating.
Let's first set the stage of the moon's creation. Our best guess is that it was created when another early protoplanet, named Theia, slammed into Earth in a massive collision, creating a chunk of debris that coalesced into the moon. However, that amount of debris was relatively small—only about a quarter the size of the Earth itself, which meant the internal heat generated during its formation would rapidly radiate away, turning the moon itself geologically dormant.
But when Chang'e-5 returned some lunar basalts from Oceanus Procellarum back to Earth, laboratory dating showed them to be only around 2 billion years old. Basalts are only formed in active volcanoes or other environments with similarly high pressures and temperatures—environments that, at least according to traditional geological theory, shouldn't have existed on the moon 2 billion years ago. Despite that, scientists have tried offering other solutions.
Radioactive elements, such as potassium and uranium, emit some heat as they decay. But there wasn't enough undecayed material left on the moon to offer the amount of heat needed to form basalt 2 billion years ago. Asteroid strikes could cause localized high-impact thermal spikes, but the heavy bombardment period that gave the moon most of its craters had ended long before the basalt was formed.
So instead the authors turned to gravity—specifically to a phenomenon called tidal heating. This happens when a celestial body is squeezed, stretched and kneaded as it travels in an elliptical orbit around a larger parent body. Perhaps the most famous example of this in the solar system is Io.
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