Miao Chen of the Chengdu University of Technology and colleagues propose that tidal heating kept the moon volcanically active billions of years after its formation. The theory addresses a puzzle posed by lunar samples collected by China's Chang'e-5 mission, which showed basalt rocks formed around 2 billion years ago despite models predicting the moon should have cooled and become geologically dormant long before.
The researchers published their findings in the journal Planet. They argue that 2 billion years ago, the moon orbited Earth much closer than today, completing an orbit every 16 days instead of the current 27. Its orbit was also more elliptical, meaning Earth exerted strong tidal forces that squeezed and stretched the moon's interior.
Previous models treated the moon as a uniform sphere, but Chen's team incorporated a more realistic representation of the lunar mantle structure, including an ultra-low-viscosity zone between the core and mantle. Their 3D simulation revealed two symmetric regions along the core-mantle boundary, which they called tidal heating anomaly zones (THAZs). Around 3.2 billion years ago, these zones despite comprising only 2.5% of the mantle's volume delivered 150 gigawatts of power. This exceeded by more than four times the energy generated by radioactive decay.
The tidal heating sustained the moon's magnetic field longer than previously predicted and drove mantle plumes toward the crust. Billions of years later, pockets of this melted mantle material produced the basalts observed in Chang'e-5 samples, the authors propose.
The authors acknowledge their work remains a theory awaiting confirmation from future lunar missions planned for the next decade.