Earth's Pull May Have Powered Moon Volcanoes Two Billion Years After Birth

Earth's gravity may have kept the young Moon's deep interior hot long after it should have gone cold, according to a new theoretical model. The idea could explain why lunar rocks brought home by China's Chang'e-5 mission appear to have formed from volcanic activity only about two billion years ago, Universe Today reported.

The work, led by Miao Chen of the Chengdu University of Technology and colleagues, was published in the journal Planet. Universe Today stressed that it remains a theory and not a confirmed explanation.

The key numbers behind the Moon's long-lasting heat

• 4.5 billion years ago: Roughly when the Moon is thought to have formed, after a protoplanet named Theia hit the early Earth. Because the debris was relatively small, the Moon's internal heat should have faded quickly, according to Universe Today.
• Two billion years: The approximate age of the Chang'e-5 basalts from Oceanus Procellarum, as measured in laboratory dating. Basalts form in volcanic settings, which traditional theory says the Moon should no longer have had by then.
• Sixteen days versus twenty-seven: Two billion years ago the Moon circled Earth in sixteen days, compared with twenty-seven today. Its orbit was also far more oval-shaped, so Earth's tidal forces flexed its interior rhythmically.
• 2.5%: The share of the mantle's volume taken up by two symmetric regions near the core-mantle boundary that the team calls Tidal Heating Anomaly Zones.
• 150 gigawatts: The power the researchers calculate those zones received from tidal strain around 3.2 billion years ago.
• More than four times: How that tidal power compared with the heat produced by radioactive decay at the time.

How the model works

Tidal heating occurs when a body is squeezed and stretched as it moves along an elliptical orbit around a larger one. Universe Today pointed to Jupiter's moon Io, which erupts constantly because of this effect, as the best-known example.

Earlier studies, the outlet said, treated the Moon as a mostly uniform sphere. In reality, a zone of extremely low-viscosity rock lies between the lunar core and mantle. When the team put a more realistic three-dimensional mantle into a convection simulation, much of the tidal energy was channeled into the two regions at the core-mantle boundary.

According to the report, that heat could have kept the lunar core warm and sustained a magnetic field longer than models had predicted. That may also help with a puzzle: Apollo samples appeared to have formed in a strong magnetic field. The heat also would have driven large mantle plumes rising toward the crust. Billions of years later, after the tidal effect faded, leftover pockets of mantle could have melted to produce the Chang'e-5 basalts.

Other explanations that fell short

Scientists have tried other answers, Universe Today said. Radioactive elements such as potassium and uranium release heat as they decay, but not enough of that material was left on the Moon to melt rock two billion years ago. Asteroid impacts can cause local heat spikes, but the heavy bombardment that carved most lunar craters ended long before the basalts formed.

Why it matters for understanding the Moon

For general readers, the finding shows that the Moon, often pictured as unchanging, may have had a far more active interior for much longer than textbooks have said. It would also tie the Moon's long volcanic and magnetic history to the pull of Earth itself.

The result comes from a computer model, not a direct measurement of the lunar interior. Universe Today described the explanation as elegant but unproven, and noted that many lunar missions planned over the next decade could supply data that test it.

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Around Prague, based on reporting by Universe Today