New research suggests the Moon may have formed very shortly after the Earth

Although the Moon still holds many mysteries, there are two aspects of its origin that seem to be generally accepted by scientists: the satellite was born as the result of the collision between the Earth and a protoplanet they have called Theia. This is only a hypothesis, but several clues, such as the discovery of material from another planet in the Earth’s mantle, support it. The other thing on which there is relative consensus is that this collision must have happened somewhat before the 4.35 billion years that some of the lunar rocks brought back by the Apollo missions are aged at. However, a group of geophysicists maintain that the story was different: the Moon was formed up to 180 million years earlier than previously thought, shortly after the formation of the Earth.

What happened 4.35 billion years ago was a melting event on the forming Moon. That melting of its materials would have reset the crystallization process, resetting the clock and obfuscating the exact dating of most lunar samples. “At first, the Moon was close to Earth and its orbit was only concerned with what the planet was doing,” says Francis Nimmo, a geologist specializing in planetary formation and evolution at the University of California, Santa Cruz. “But the Moon slowly moved away from Earth and, as this happened, the influence of the Sun on its orbit became more important,” explains the researcher, lead author of this new study published in Nature. At some point, a kind of equilibrium point was reached, known as the Laplace plane transition (named after the French astronomer) that occurs when the influence of the Earth’s globe and the star become equal. “At that point, the Moon’s orbit is altered (it does not become circular) and tidal heating occurs,” he explains.

That is the event that took place and that would have complicated dating the Moon’s formation. Global in nature, with temperatures that could have exceeded 1,200 degrees, tidal heating occurs when a small body (the Moon) orbits a larger body (the Earth). “If the orbit is not completely circular, the distance between the two changes and, therefore, so does the gravity, so the satellite experiences changes,” explains Nimmo. “The result is that the Moon is compressed and stretched by the changing gravity of the Earth, heating up like a rubber ball when we compress and stretch it.”

After the impact with Theia, both Theia and Earth disintegrated and melted. Part of the ejected material grouped together to form a protomoon still in a magmatic state. This ocean of lunar magma crystallized as it cooled, following well-known chemical processes. From then on, these would be the lunar rocks brought to NASA laboratories. But what Nimmo and his colleagues argue is that the tidal heating event that occurred when the satellite adjusted its orbit re-melted a good part of the rocks, altering the isotopic composition (an isotope is a version of the same chemical element but with a different number of neutrons) that allowed them to be dated.

Using thermal evolution models, the authors posit that this explanation would indicate that the Moon’s formation occurred between 4.43 and 4.53 billion years ago, at the upper limit of previous age estimates. If the latter figure is correct, it would mean that the Moon formed a few million years after the Earth did. The researchers also say that the merger event would explain why there are fewer traces of the early lunar impacts that give it that chickenpox appearance, since they would have been erased during a warming event.

Domingo Gimeno, professor of petrology and geochemistry at the University of Barcelona, says that “the main petrological and geochronological contribution of this model would be to explain why there are zircon crystals (although not many, as far as we know) older than the rocks on the surface of the Moon that contain them, and for that they point to the hypothesis of lunar (re)melting, which would not only be on the surface, but also in the mantle.” The problem with this theory, Gimeno notes, is that “it is an intellectual exercise.” There is no new evidence or new materials, such as those brought by the Chinese Chang’e-5 mission, which allowed us to find out that the Moon maintained its volcanism until much more recently than previously believed.

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