The Moon is getting a little dated with a study by astronomers that uses celestial mechanics to confirm that Earth’s satellite was born very shortly after the formation of the solar system.
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Almost no one anymore disputes the circumstances of its birth, with the impact of the Earth then barely formed by another nascent planet, and the remains of the impact agglomerated to form the Moon.
His age is another matter. It is normally calculated from that of the rocks that crystallized when, as the original magma ocean cooled, it formed the lunar crust. Rocks dated to 4.35 billion years old.
The Moon “appears to be formed very late, 200 million years after the beginnings of the solar system”, 4.56 billion years ago, notes to AFP Alessandro Morbidelli, professor at the Collège de France and co-author of the study published in Nature.
“Too late” even, according to this specialist in the formation and evolution of planetary systems.
Because this late age fits poorly with models of planet formation on the one hand. And on the other hand with the presence of zircon crystals – the most resilient there is – dated to more than 4.5 billion years ago.
A discussion on this topic last year with the study’s two other authors, Thorsten Kleine at Max Planck and Francis Nimmo at the University of California, Santa Cruz, produced a “very simple idea”… about the paper.
“The Moon experienced a second fusion by the Earth,” says Mr. Morbidelli.
In this scenario, the Moon is formed approximately 55 million years after the beginnings of the solar system, and no longer 200. It then gains its first crust, after a few million years.
“She’s a bit crazy”
And then, the fruit of celestial mechanics, it gradually distances itself from its mother Earth. As it moves away, it goes from an orbit based on the Earth’s equator to an orbit based on the Earth’s orbit around the Sun.
And in this transition, she goes through a phase that is “a little chaotic, dynamic, she acts a little crazy,” describes Mr. Morbidelli. This orbit transition induces “enormous” tidal forces on the lunar interior.
Generated by Earth’s gravity, these tidal forces cause the lunar mantle beneath the crust to melt. We are then 200 million years after the formation of the solar system and the lunar crust “loses a little of its rigidity”.
It undergoes volcanic eruptions which partially reshape its surface and melts which engulf parts of it.
This phenomenon of tidal forces is nothing extraordinary. For example, it is still at work on Io, a moon of Jupiter, which is constantly undergoing eruptions.
On the Moon, it was powerful enough to cause a “partial remelting” of the crust, with the effect of “resetting the radioactive timers.”
This explains, for example, that in this “remelted” crust, basalt rocks from different depths appear to be the same age when we date them with a method using the atomic disintegration of certain of their elements.
“Because if you re-melt the rocks and recrystallize them, you only measure the age of the last crystallization,” Morbidelli explains.
The phenomenon would also fit well, according to the authors of the study, with the physical characteristics of the star.
Like a deficit of meteorite impact basins, compared to those expected by the models, and which would have been filled by magma rising during the second melt.
In the end, the researchers’ proposal has everything of Christopher Columbus’ egg and his famous “Just think about it”, according to Mr. Morbidelli. Who does not fail to emphasize the requirement for “a little dynamic and thermal modeling” to get there.
