The research, published in the journal Nature, argues for an earlier formation date of the Moon, indicating that rock samples could be from a much earlier period but their "remelted" surfaces are hiding older features.
Traditionally, the Moon's age has been calculated from rocks that crystallized when the original magma ocean cooled and formed the lunar crust. Rock samples brought to Earth by the Apollo astronauts and unmanned missions indicated an age of 4.35 billion years for the cooling of the Moon's surface, which was previously considered the Moon's age. However, researchers found crystalline inclusions in the Moon rock, called zircons, that were older than 4.35 billion years, raising doubts about the previously assumed age, according to Space.com.
The Moon's formation is believed to have begun with a collision shortly after the primordial Earth formed about 4.5 billion years ago, when a Mars-sized celestial body named Theia collided with the early Earth. This impact resulted in both bodies disintegrating and melting, as reported by Sky News. The collision catapulted large amounts of molten rock from the crust and mantle of both celestial bodies into space, which eventually coalesced to form the Moon, N-TV explains.
"At first, the Moon was close to Earth and its orbit was only concerned with what the planet was doing," explained Francis Nimmo, a geologist at the University of California, Santa Cruz, according to Space.com. "But the Moon slowly moved away from Earth and, as this happened, the influence of the Sun on its orbit became more important," he added. As the Moon distanced itself, it underwent a "somewhat chaotic and dynamic" phase, experiencing orbital disturbances due to a gravitational tug of war between Earth and the Sun, as noted by La República.
"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," Nimmo explained. These tidal forces caused intense heating around 4.35 billion years ago, leading to large amounts of magma from the Moon's interior reaching the surface, as reported by Space.com. This heating event caused a remelting of the lunar crust, effectively "resetting" the ages of rocks on the surface.
This phenomenon might explain why the Moon exhibits a deficit of impact basins and craters compared to predictions from models. The abundant flowing lava from the remelting would have filled up old craters, erasing early impact basins, as El Periódico notes. The researchers propose that the age of 4.35 billion years reflects this remelting event rather than the Moon's initial formation.
"The Moon 'would seem to have formed very late, 200 million years after the beginning of the solar system, 4.56 billion years ago," observed Alessandro Morbidelli, a professor at the Collège de France and co-author of the study published in Nature. He explained that the tidal heating caused by Earth's gravity led to a "partial refusion" of the lunar crust, which "restarted the radioactive clocks" of the rocks, as stated in El Universal. "When you refreeze and recrystallize the rocks, you can only measure the age of the last crystallization," Morbidelli said.
"I read this paper with great interest and I find this is a really elegant hypothesis, an elegant model to reconcile all these ages," said Philipp Heck, a researcher with the University of Chicago and the Field Museum who has studied ancient lunar samples but wasn't part of the research team, as per Science. "We will see, when we get more samples, how this holds up," he said.
China's upcoming Chang'e 6 mission aims to return samples from the far side of the Moon, which could help reveal the true age of the Moon by analyzing these lunar samples, according to Space.com.
This article was written in collaboration with generative AI company Alchemiq