New proof that the Moon was born from the Earth

For millennia, the Moon has fascinated. Just like the question of its origin and its formation, which has animated scientific debates for almost five centuries. Today, the commonly accepted hypothesis is that of a giant impact between Earth and another major celestial body. Yet, despite the evidence that accumulates every year a little more, there are still some gray areas to fully support this theory.

In a new study published in Science Advanceshowever, scientists claim to bring the irrefutable argument of such an origin.

Moon fragments in Antarctica

It all starts with the analysis of several meteorites moons found in Antarctic. These meteorites consist of volcanic rocks and more specifically of basaltswhich come from the very rapid cooling of a magma formed between 200 and 400 km deep. It is this type of rock that forms the great lunar seas that can be seeneye naked from Earth.

Among the crystals forming the basalts, there are thus tiny particles of volcanic glass which have the particularity of having imprisoned the chemical imprint of the lunar mantle. A precious and rare signature because the meteorites analyzed have the particularity of coming from inside a flow basaltand not of the lunar surface permanently subjected to the erosive action of solar winds.

For a long time, the study of the formation of the Moon has indeed come up against the difficulty of finding samples of “fresh” rocks, whose isotopic composition has not been altered by the action of cosmic rays which permanently bombard the surface of our satellite. That’s the whole problem with the samples brought back by the various Apollo missions. To obtain rock samples representative of the initial composition of the Moon, it is therefore necessary to drill the lunar surface (easier said than done)… or to wait for a asteroid do the job.

Without the protection provided by a atmosphere, the surface of the Moon is in fact continuously subjected to asteroid impacts. However, sufficiently large impacts have the capacity to eject fragments of crust lunar coming not only from the surface, but also from deeper levels, hitherto protected from solar winds. These fragments then reached us in the form of small meteorites.

A noble gas composition not altered by solar winds

The isotopic analysis of the fine particles of volcanic glass contained in the lunar basalts has thus enabled scientists from ETH Zurich to identify the presence of several gas so-called noble: thehelium and the neon. However, these gases represent excellent tracers for tracing the source of the volatile elements that make up the planets, and in particular the Moon. Among their possible origin: the solar winds. But this hypothesis was quickly ruled out, the samples having been protected. For the researchers, the helium and neon contained in the basalt glass particles are therefore native and were acquired at the time of the formation of the Moon.

Scientists therefore turned to the composition of the terrestrial mantle primitive and compared the noble gas isotopic signature of the Earth and its satellite. The data supports the hypothesis of a giant impact at the origin of the Moon.

The Moon would thus have inherited the composition of the Earth’s mantle, particularly with regard to noble gases and other volatile elements.

Moon formation explained by a new collision model

For more than a century, the question of the formation of the Moon has tormented researchers. And while the giant impact theory sounds appealing, it struggles to explain some observations. But researchers now believe they have solved the riddle.

Article of Nathalie Mayer published on April 30, 2019

The theory of fission. that of coaccretion. Or that of capture. For more than a century now, the astronomers try to explain the formation of our moon. And the theory ofgiant impact, issued in the 1970s, seems the most attractive. The Moon would have been formed from fragments of the celestial object in question. An object that the researchers named Theia. But problem: the composition of the rocks reported by the missions Apollo does not match the model.

According to researchers from theyale university (United States), the error should be sought in the initial conditions more than in the theory itself. Thus, they tested a new model which considers a solid impacting object and a proto-Earth covered, for its part, by a magma ocean hot, as was indeed the case some 50 million years after the formation of the Sun.

Reconcile theory and observations

According to their calculations, the collision caused an overheating of the magma much more important than that undergone by Theia. As a result, the magma expanded and then was torn from the Earth to form the Moon. What reconcile the giant impact theory with observational data.

“Our model gives a Moon made up of about 80% prototerrestrial material. Earlier models lead to 80% materials from Théia”says Shunichiro Karato, geophysicist at Yale and specialist in the chemical properties of prototerrestrial magma. “Our model thus confirms the theory without the need to resort to unconventional collision conditions. » Assuming the timing is right…

Moon formation: titanium contradicts collision theory

The standard model of the formation of the Moon involves the collision of a small planet the size of Mars, Theia, with the Earth. One of his predictions, which implies that the Moon and the Earth cannot have an identical composition in titanium, has just been refuted. We must review the scenario of the birth of our satellite.

Article of Laurent Sacco published on 02/04/2012

• Admire the Moon in slideshow

The Moon has an abnormally small ferrous core and shares with the Earth surprising similarities in chemical composition, particularly at the level of isotopes of tungsten, chromium, silicon and oxygen. From this information on the composition of our satellite joined to considerations of celestial mechanics, the astrophysicists and cosmochemists had concluded that the most likely explanation for its formation involved a giant impact. A small planet the size of Mars, called Theia, would have entered in tangential collision with our planet less than 100 million years after the beginning of the birth of the Solar system. Under the impact, the ferrous core of Theia would have been captured by the Earth, part of the mantle of the two planets would have been found in the form of a accretion disk surrounding the Earth.

In a few centuries, this disc of debris and gas would then have gathered to give the Moon, incorporating at least 40% of the initial mantle of Theia, the rest coming from the Earth. In this way, the proximity of the chemical composition of the Moon to that of the terrestrial mantlea similarity difficult to explain by the hypothesis of the capture of a Moon formed in chemically different regions of the protoplanetary disc.

A recent publication of an article in Nature Geoscience has just turned this scenario upside down. We already suspected that it might be necessary to review the theory of formation of the earthbut according to cosmochemists, the same should be done with that of the Moon

The researchers were indeed interested in the abundances of titanium isotopes in the lunar rocks brought back by the missions Apollo. After correcting for the effect of cosmic rays on these abundances, they found to their surprise that they were almost identical to those of terrestrial rocks. A small fraction of meteorites with similar abundances, it had to be deduced that the vast majority of the Moon is made of materials from the Earth’s mantle.

The return of Darwin’s fission theory?

The enigma is not small because it is not easy to see how such a homogeneity of composition could have happened. There are however various possible scenarios of which here are some examples.

So, just after the impact of Theia, a giant magmatic ocean must have covered the Earth, possibly letting materials escape into space that may have changed the chemical composition of the protolunar disk, erasing the chemical memory of future rocks. lunar.

Ironically, researchers are even considering going back to an old hypothesis proposed more than a century ago by George Darwin, an English astronomer and mathematician, son of the famous British biologist Charles Darwin. Fallen into disuse, it involved a rapid rotation of the Earth in a fluid state, causing the ejection of a shred of matter a little like the formation of drops of liquid from the separation from a single drop.