Although the Moon still retains many mysteries, there are two aspects about its origin that seem generally accepted by scientists: the satellite was called Theia. It is just a hypothesis, but several clues, such as the discovery of , support it. The other thing in which there is relative consensus is that that crash must have happened somewhat before the 4.35 billion years that some of the lunar rocks brought back by the Apollo missions are. However, a group of geophysicists maintains that the story was different: the Moon formed up to 180 million years earlier than thought, shortly after the formation of the Earth.
What happened 4.35 billion years ago was a fusion event of the forming Moon. That melting of its materials would have reset the crystallization process, setting the clock to zero and obfuscating the exact dating of Selene. “In the beginning, the Moon was close to Earth and its orbit only cared about what the planet did,” says Francis Nimmo, a geologist specializing in the formation and evolution of planets at the University of California, Santa Cruz. “But the Moon slowly moved away from the Earth and as this happened, the influence of the Sun on its orbit became more important,” explains Nimmo, first author of this . At one point a kind of equilibrium point was reached known as the Laplace plane transition (in honor of the French astronomer), which occurs when the influence of the 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 details.
That is the event that happened and that would have complicated the dating of the formation of the Moon. Global in nature, with temperatures that could exceed 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 them changes and, therefore, the gravity, so the satellite experiences changes,” explains Nimmo. “The result is that the Moon is compressed and stretched by the Earth’s changing gravity, warming like a rubber ball as we compress and stretch it.”
After the impact with Theia, both it and the Earth disintegrated and melted. Part of the ejected material gathered together to form a protomoon still in a magmatic state. This lunar magma ocean crystallized as it cooled following well-known chemical processes. From then on it would be the lunar rocks brought to NASA laboratories. But what Nimmo and his colleagues defend is that tidal heating event that occurred when the satellite was adjusting its orbit, melted a good part of the rocks again, altering the isotopic composition (an isotope, a version of the same chemical element but with a different number of neutrons) that allowed them to be dated.
Based on thermal evolution models, the authors postulate that this explanation would indicate that the formation of the Moon occurred between 4,430 and 4,530 million ago, at the upper limit of previous age estimates. If the last figure is correct, it would mean that the satellite formed a few million years after the formation of its planet. The researchers also say that the melting event would explain why there are fewer traces of the first lunar impacts that give it that chickenpox look, since they would have been erased during a warming event.
The professor of petrology and geochemistry at the University of Barcelona, Domingo Gimeno, says that “the main petrological and geochronological contribution of this model would be to explain why there are zircon crystals (even if they are few, as far as we know) older than rocks. of the surface of the Moon that contain them, and for that they point to the hypothesis of lunar (re)fusion, which would not only be the surface, but also the mantle. The problem with this, Gimeno recalls, is that “it is an intellectual exercise.” There is no new evidence or new materials, such as , that allowed us to find out that the Moon maintained its volcanism until much more recent times than was believed.
For the deputy director of the Institute of Astrophysics of the Canary Islands, Eva Villaver, the proposal by Nimmo and her colleagues “is a theoretical hypothesis that would make all the observational data fit together: that heating by tidal forces caused by a specific type of resonance could explain the evidence about the age of the moon. And it’s not so theoretical. Io, one of Jupiter’s moons, as was the case with the Moon in the past, is very close to its enormous planet and is the satellite with the highest seismic activity in the solar system. As Nimmo says, “it is currently believed to be partially melted inside as a result of tidal warming.” The Moon of the past could have resembled the Io of the present.
