Lunar rocks returned by China’s Chang’e-5 mission pose a puzzle: some basalts from Oceanus Procellarum formed around 2 billion years ago, long after the Moon was expected to have cooled and become geologically quiet. A model described in a science news report offers a possible explanation: Earth’s gravity may have continued to heat parts of the Moon’s interior.
The report, by Andy Tomaswick for Universe Today, describes a paper by Miao Chen of Chengdu University of Technology and colleagues, published in the journal Planet. The paper proposes that tidal heating helped sustain the Moon’s internal heat. Tidal heating occurs when gravity repeatedly deforms a body as it travels in an orbit around a larger one. Jupiter’s moon Io is a well-known example of a world strongly heated this way.
The Moon’s orbit was different in the distant past. According to the report, around 2 billion years ago it was closer to Earth, took about 16 days to orbit the planet rather than today’s 27, and followed a more eccentric, oval-shaped path. The researchers modeled how Earth’s changing gravitational pull could strain the Moon’s interior over each orbit.
Earlier models treated the Moon as a mostly uniform sphere. The team instead used a three-dimensional representation that included a very low-viscosity zone of rock between the core and mantle, then ran a convection simulation. In that model, tidal energy concentrated in two symmetric regions along the core-mantle boundary. The researchers called these tidal heating anomaly zones, or THAZs.
The report says the modeled zones were small but received substantial energy. Around 3.2 billion years ago, they accounted for just 2.5% of the mantle’s volume, yet tidal strain delivered an estimated 150 gigawatts of power there—more than four times the heat attributed to radioactive decay. These are model results, rather than direct measurements of past lunar heating.
The proposed explanation could connect several chapters of lunar history. The team’s model suggests the heating helped keep the core warm and sustain a magnetic field longer than earlier models predicted. It also drove large mantle plumes toward the crust. The report says that, billions of years later, mantle pockets left behind by those plumes could have melted and produced the basalts found in the Chang’e-5 samples.
Other explanations have limits, according to the report. Radioactive elements such as potassium and uranium produce heat as they decay, but the report says too little remained to provide the heat needed for the younger basalts. Asteroid impacts can create localized heat, but the period of heavy bombardment had ended well before the samples formed.
The tidal-heating account is still a theory. It offers a way to reconcile young volcanic material and evidence of a longer-lived lunar magnetic field with a Moon expected to cool early, but the report says future lunar missions may provide more definitive data. Whether they support the predicted heating zones and their proposed role in the Moon’s evolution remains an open question.