For decades, scientists operated under a reassuring assumption: the moon was essentially a dead rock. We knew it formed around 4.5 billion years ago when a massive protoplanet named Theia collided with Earth, and we calculated that such a small body—only about a quarter the size of Earth—would cool rapidly and become geologically dormant. This made intuitive sense. It should have settled into permanent silence.
Then the Chang'e-5 mission returned lunar basalts from Oceanus Procellarum that upended everything. Laboratory dating revealed these samples were only around 2 billion years old, yet basalts can only form in environments with the intense heat and pressure of active volcanism. According to traditional geological theory, such environments simply shouldn't have existed on the moon 2 billion years after its violent birth. Scientists scrambled for explanations. Radioactive elements like potassium and uranium couldn't have generated enough heat. The heavy bombardment period that could have caused localized thermal spikes had ended long before.
Enter Miao Chen of Chengdu University of Technology and his colleagues with a more sophisticated answer: tidal heating. The concept isn't new—we've long known Jupiter's moon Io erupts constantly due to tidal forces. What changed is how researchers modeled the moon's interior. Rather than treating it as a uniform sphere, they incorporated a realistic 3D representation accounting for an ultra-low-viscosity zone between the moon's core and mantle. The results were striking: two symmetric tidal heating anomaly zones (THAZs) at the core-mantle boundary concentrated enormous energy. Around 3.2 billion years ago, these regions—comprising just 2.5% of the mantle's volume—delivered 150 gigawatts of power, more than four times the energy from radioactive decay.
The mechanism is elegant. Two billion years ago, the moon orbited Earth every 16 days in a much more eccentric path than today's 27-day circular orbit. This meant Earth's gravity relentlessly squeezed and stretched the moon's interior in a cosmic kneading process. The tidal heating kept the lunar core warm enough to sustain a magnetic field longer than models predicted—solving another puzzle from Apollo samples that showed evidence of forming in strong magnetic fields. Mantle plumes rose toward the crust, and their remnants eventually melted to produce the Chang'e-5 basalts.
This theory elegantly addresses persistent questions about lunar geology, yet Miao Chen's team rightfully emphasizes it remains theoretical. The numerous lunar missions planned over the next decade must test these predictions with concrete geological evidence. We should prioritize observations that could definitively confirm whether tidal heating zones actually exist and functioned as described. Understanding how gravity kept our moon's heart beating tells us not just about lunar history, but about how celestial mechanics shapes planetary evolution throughout the solar system.