The Moon is usually pictured as a geologically dead body that cooled shortly after its birth about 4.5 billion years ago. However, samples delivered by the Chang'e-5 mission showed that some lunar rocks of volcanic origin formed only about 2 billion years ago. This fits poorly with classical models, since the formation of basalts requires high temperatures and pressure that should not exist on a cooled Moon.
A group of researchers led by Miao Chen of Chengdu University of Technology proposed an explanation in an article published in the journal Planet. In their view, the key role was played by tidal heating - a process in which a celestial body is compressed and stretched by the gravity of a larger neighbor as it moves along an elongated orbit. The best-known example in the Solar System is Jupiter's volcanically active moon Io.
The Moon does not experience such deformations now, but 2 billion years ago it was closer to Earth and orbited it in about 16 days instead of today's 27. Its orbit was noticeably more elongated, and Earth created powerful rhythmic tidal forces that acted on the lunar interior.
The authors built a three-dimensional model of the Moon's mantle, taking into account a layer of rock with extremely low viscosity between the core and the mantle. In the convection simulation (mixing of material due to temperature differences), it turned out that a significant part of the energy was directed into two symmetrical regions at the core-mantle boundary. The researchers called them zones of anomalous tidal heating (THAZ).
The calculations showed that about 3.2 billion years ago these zones, occupying only 2.5% of the mantle's volume, received 150 gigawatts of power - more than four times what the decay of radioactive elements provided. This could have maintained heat in the lunar core and explain why the Moon's magnetic field existed longer than previous models predicted. In addition, the heating probably generated mantle plumes - upward flows of hot material that rose toward the crust. Billions of years later, when the tidal "engine" had already faded, these solidified regions of the mantle melted and produced the basalts found in the Chang'e-5 samples.
It is important to emphasize that this is a theoretical model, not a proven fact. It elegantly explains a number of long-standing puzzles but needs verification. Data from future lunar missions may help finally clarify the geological history of our nearest neighbor.