The Moon is older than previously believed, according to a study by astronomers which uses celestial mechanics to confirm that Earth’s satellite was born very shortly after the formation of the solar system.
Almost no one now disputes the circumstances of its birth: the collision of the Earth, then barely formed, with another protoplanet, an impact that threw debris into space that agglomerated to form the Moon.
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However, his age remains a matter of debate.
It is usually calculated from rocks that crystallized when the original magma ocean cooled and formed the lunar crust. These rocks have been dated at 4.35 billion years old.
The Moon “seems to have formed very late, 200 million years after the beginning of the solar system,” 4.56 billion years ago, observes Alessandro Morbidelli, professor at the College of France and co-author of the study published in Nature.
“Too late,” objects this specialist in the formation and evolution of planetary systems.
This delay is problematic for two reasons: first, it does not fit well with planetary formation models; and second, it contradicts the presence of zircon crystals – the most resistant – dated to more than 4.5 billion years.
A discussion on this topic last year between the study’s two other authors, Thorsten Kleine of the Max Planck Institute and Francis Nimmo of the University of California, Santa Cruz, produced a “very simple idea”… at least in theory.
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“The Moon experienced a second fusion caused by the Earth,” explains Morbidelli.
In this scenario, the Moon formed approximately 55 million years after the start of the solar system, not 200. Shortly afterward, it acquired its first crust over the course of a few million years.
“A little crazy”
Later, as a result of celestial mechanics, the Moon began to gradually distance itself from its “mother”, the Earth. In this process, it went from an orbit aligned with the Earth’s equator to one aligned with the Earth’s orbit around the Sun.
During this transition, he went through a “somewhat chaotic and dynamic, a little crazy” phase, Morbidelli describes. This orbital change generated “enormous” tidal forces that affected the interior of the Moon.
Triggered by Earth’s gravity, these tidal forces caused the lunar mantle, located beneath the crust, to partially melt. This occurred approximately 200 million years after the formation of the solar system, causing the lunar crust to “lose some of its rigidity.”
The Moon then experienced volcanic eruptions that partially reshaped its surface, while some areas sank due to melting.
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This tidal force phenomenon is not unusual. For example, it continues to occur on Io, one of Jupiter’s moons, which suffers constant volcanic eruptions.
In the case of the Moon, the tidal forces were strong enough to cause a “partial remelting” of the crust, which “reset the radioactive clocks” of the rocks.
This explains, for example, why in this “remelted” crust basaltic rocks from different depths appear to be the same age when dated using methods based on the atomic disintegration of certain elements.
“When you remelt and recrystallize rocks, you can only measure the age of the last crystallization,” clarifies Morbidelli.
Impact on lunar history
According to the authors of the study, this phenomenon could also explain certain physical characteristics of the Moon. For example, a deficit of meteorite impact basins compared to what was predicted according to the models. These basins would have been filled by magma rises during the second melt.
In short, the researchers’ proposal seems like an ingenious solution, similar to the famous “Columbus egg.” However, Morbidelli emphasizes that reaching this conclusion required some “dynamic and thermal modeling.”