The Moon would have formed in a few hours

The moonmoon was formed following a titanic collision between the Earth, still in formation, and to little planetlittle planet the size of Mars called Theia. The cataclysm, which occurred about 4.5 billion years ago, is said to have thrown an enormous amount of debris into space, resulting from the disintegration of TeiaTeia and the upper layers of the Earth. Accusing, these fragments in orbitorbit it would then gradually form a new body, the Moon.

This is with regard to the most commonly accepted hypothesis. However, scientists now offer another scenario.

Gradual growth? Or a quick workout?

Why the idea of ​​a accretionaccretion progressive from a disc of debris collides with some observations. The analysis of the lunar rocks shows that their composition is quite similar to that of terrestrial mantleterrestrial mantlea result that contradicts the idea that most of the debris would come from Theia.

In an attempt to solve this conundrum, scientists from the University of Durham have played numerical simulationsnumerical simulations impact using a supercomputer. And the results lead to an alternative on the origin of the Moon. Simulating hundreds of different impacts, in particular by varying the angle and speed of the impact but also the massesmasses from Earth and Theia, it seems that the impact may have produced a body of the mass of the Moon, directly in Earth orbit.

The parameters of the collision are in fact still much debated today and the simulations carried out by the supercomputer show that some variations of these parameters can lead to surprising results.

Most interestingly, the results of this study, published in the journal Letters from the astrophysical diaryexplain why the outer layers of the Moon are rich in material of terrestrial origin, as evidenced by the samples recovered from the various Apollo missions.

In fact, the model suggests that the Moon would thus be made up of 60% material from the Earth, compared to only 30% of the classical theory of the debris disk.

Fewer molten rocks

If the Moon, or at least a large part of it, formed very quickly after the impact, it means that less rock was in the molten state during its formation. Less, in any case, than the quantity involved in standard theories. And this would have a significant impact on the Moon’s cooling rate, its composition but also its structure. Indeed, this new scenario assumes only a fraction of this CrustCrust it was loose, the inside was quite cold and solidsolid. This small amount of molten rock would explain the composition of the thin lunar crust by the fact that the radial mixing between the different layers was then very limited, unlike a Moon entirely composed of molten rocks.

This new scenario, which proposes that the Moon formed much faster than previously thought, from a large fragment, opens up new possibilities for thinking about the composition and structure of the Moon, but also about the characteristics of its orbit.

The most accepted scenario for the formation of the Moon involves a collision during theHadeanHadean between the young Earth and a small planet the size of Mars, called Theia. This hypothesis, however, left cosmochemical puzzles. They may have just found their solutions, according to two groups of researchers.

Article of Laurent SaccoLaurent Sacco published on 24 October 2012

  • See the moon in a presentation

Thanks to the Apollo program and the lunar rocks that Neil Armstrong and his colleagues brought back to Earth, we know that the Moon shares striking similarities in chemical composition with our planet, particularly in terms of isotopesisotopes from tungstentungsten, chromechrome, siliconsilicon And oxygenoxygen, which recall the composition of the earth’s mantle. Another certainty: the Moon has an abnormally small iron core and is very poor in water.

From this information on the composition of our satellite, together with considerations of celestial mechanics, the astronomersastronomers and the cosmochemists had deduced that the most likely explanation for its formation involved a gigantic impact more than 4.4 billion years ago. A small planet the size of Mars, called Theia, would thus have entered a tangential collision with the Earth, less than 100 million years after the birth of the Solar System. Under the impact, the iron core of Theia would have been captured by the (not yet) Blue Planet, part of the mantle of the two would then end up in the form of an accretion disk surrounding the Earth, in which the moon would eventually be born.

How was the moon formed? Here is the standard answer so far. Hubert ReevesHubert Reeves And Jean Pierre LuminetJean Pierre Luminetspecialists in cosmologycosmology contemporary answer all your questions. To find out more, visit © Dubigbangauvivant

Binding isotopes for collision scenarios

However, according to this scenario, the chemical composition of the Moon should still differ a little from that of the Earth because most of the rocks on our satellite should come from the MaterialMaterial initial of Teia. The analysis of meteoritesmeteorites in fact, it revealed notable isotopic variations, in particular for the titaniumtitanium and oxygen, in the rocky bodies of the Solar systemSolar system (although they also share similarities dating back to a common origin, i nebulanebula protosolar). However, the Earth and the Moon appear as twins at the level of the isotopes of oxygen and titanium.

If the formation models of the Earth and the Moon reproduce their mass and theirs rotation speedrotation speed in addition to different chemical characteristics, they therefore leave puzzles at the level of the isotopes. Attempts to remedy these mysteries have so far introduced others concerning celestial mechanics.

The problem of the speed of rotation of the Earth

For example, it could be assumed that the Earth, before its collision with Theia, was spinning rapidly on itself and that in greater quantities it mattersit matters coming from our planet had break awaybreak away due to this rotation, only to be captured by the young Luna in formation soon after her collision. Unfortunately the conservation of cinematic momentcinematic moment of the Earth-Moon system, although it allows starting from a rapidly rotating Earth whose speed decreases over time under the effect of the lunar tidal forces, has not until now allowed an initial speed of rotation of the Earth sufficiently high to solve the puzzles of the chemistrychemistry from the moon.

It is this mechanism that made the Moon migrate away from the Earth, increased the length of the Earth’s day and led the Moon to always present the same face to the Earth (this is the phenomenon of rotation synchronoussynchronous). Starting from current observations and reversing the direction of time to predict the past, we end up with a rotation of the Earth immediately after its collision with Theia of about 5 hours … which is too slow.

The researchers just published today in Science two articles that propose two new collision scenarios that seem able to solve all these puzzles.

Animated film representing the birth of the Earth and the Moon. © TriSkull666-YouTube

Two planets of the same size colliding

In one of these scenarios, the Earth, shaped like a very flattened ellipsoid, is rotating very rapidly, with a day lasting only 2 or 3 hours. Below 2 hours, the rotation speed of the Earth would be such that the centrifugal forcecentrifugal force it would blow it up. With such a speed of rotation at the time of the formation of the Moon, enough material from the Earth’s mantle can be ejected and enter the composition of our satellite to explain its chemical proximity to Earth. This is the case, for example, of a Theia with a mass equal to half that of Mars and which hits the young Earth almost head-on at 20 km / s, as demonstrated by the numerical simulations.

It remained to reduce the speed of rotation of the Earth. The researchers found that this was entirely possible if a mechanism of resonanceresonance gravity between movementmovement of the Moon around the Earth and of the Earth around the SunSun. This same mechanism (evection resonance in English) allows to correctly form the Moon following a frontal collision between two celestial bodies of almost identical mass, or from 4 to 5 times that of the mass for each planet, as proposed by the second scenario published in Science.

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