An alternative scenario for the formation of the moon

That moonmoon formed after a titanic collision between young Earth, still in formation, and Theia, a small planetsmall planet the size of Mars. The cataclysm, which occurred about 4.5 billion years ago, is said to have shattered a vast amount of debris in space created by the decay of TheiaTheia and upper layers of the earth. By growing this debris orbitorbit would then have gradually come to a new satellite: the moon.

So much for the generally accepted hypothesis. However, scientists now offer a different scenario.

Gradual increase? Or quick workout?

Because the idea of ​​a accretionaccretion Progressive from a debris disk encounters certain observations. Analysis of the lunar rocks actually shows that their composition is quite similar to that of Earth’s mantle, a result that contradicts the notion that most of the debris would have come from Théia.

In an attempt to solve this mystery, scientists from Durham University have conducted numerical simulationsnumerical simulations Effects with a supercomputer. And the results lead to an alternative regarding the formation of the moon. By simulating hundreds of different impacts, including varying the angle and speedspeed the collision, but also the crowdscrowds of Earth and Theia, it appears that the impact could have created a body the mass of the Moon right in Earth orbit.

In fact, the parameters of the collision are still hotly debated today, and the simulations performed by the supercomputer show that certain variations in these parameters can lead to surprising results.

More interesting are the results of this study, published in the journal Astrophysical Journal Lettersexplain why the moon’s outer layers are rich in material of terrestrial origin, as demonstrated by samples recovered from the various Apollo missions.

In fact, the model suggests that the moon is thus 60% terrestrial material, versus only 30% for the classical debris disk theory.

Less molten rock

If the moon, or at least a large part of it, formed very quickly after the impact, that means there was less rock in the molten state during its formation. Certainly less than the amount contained in standard theories. And that would have major implications for the moon’s cooling rate, its composition, but also its structure. In fact, this new scenario assumes that only a fraction of that crustcrust had melted, the inside was quite cold and CelebrationCelebration. This little crowd rock in mergermerger would explain the composition of the thin lunar crust by the fact that the radial mixing between the different layers was then very limited, in contrast to a moon composed entirely of molten rock.

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


The most accepted scenario for the formation of the moon involves a collision during the Hadean between young Earth and a small planet the size of Mars called Theia. However, this hypothesis left cosmochemical mysteries. They may have just found their solutions, according to two groups of researchers.

Article by Laurent SackoLaurent Sacko published on October 24, 2012

  • Admire the moon in a slide show

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 surprising similarities in chemical composition with our planet, particularly in relation to isotopesisotopes from tungstentungstenChrome, siliconsilicon and oxygen, reminiscent of the composition of the earth’s mantle. Another certainty: the moon has an unusually small iron core and is very poor in water.

From this information about the composition of our satellite, along with considerations of celestial mechanics, the astronomersastronomers and cosmochemists had concluded that the most likely explanation for its formation involved a giant impact more than 4.4 billion years ago. So a small planet the size of Mars called Theia would have entered a tangential collision with Earth less than 100 million years after the birth of the solar system began. Under the impact, the ferrous core of Theia would have been captured by the (not yet) blue planet, part of the mantle of the two then found each other in the form of a accretion diskaccretion disk surrounding the earth where the moon would eventually be born.

How was the moon formed? Here’s the standard answer so far. Hubert ReevesHubert Reeves and Jean Pierre LuminetJean Pierre Luminetspecialists for cosmologycosmology Contemporary answers to all your questions. To learn more, visit www.dubigbangauvivant.com. © Dubigbangauvivant

Restriction of isotopes for collision scenarios

However, according to this scenario, the chemical composition of the moon should still be a little different from that of the earth, since a good part of the rocks of our satellite should come from the materialmaterial Initial of Theia. The analysis of meteoritesmeteorites indeed revealed remarkable isotopic variations, particularly for the titaniumtitanium and oxygen, in the rock bodies of the solar systemsolar system (although they also share similarities that hark back to a common origin, the fogfog protosolar). However, at the level of the isotopes of oxygen and titanium, the Earth and Moon appear as twins.

If the formation models of the Earth and the Moon reproduce their mass and their rotation speedrotation speed or several chemical properties, they therefore leave puzzles at the level of the isotopes. Attempts to solve these mysteries have hitherto introduced others to celestial mechanics.

The problem of the speed of rotation of the earth

For example, one might assume that the Earth was spinning rapidly before its collision with Theia, and that a larger quantity of mattermatter coming from our planet break awaybreak away due to this rotation, only to be captured in formation by the young Moon shortly after its collision. Unfortunately, the preservation of cinematic momentcinematic moment of the Earth-Moon system, if in fact it is possible to assume a rapidly rotating Earth, decreasing in speed over time under the influence of tidal forcestidal forces Moon, until now has not allowed the Earth’s initial rotation speed high enough to solve the Earth’s mysteries ChemistryChemistry from the moon.

It is this mechanism that caused the moon to move away from the earth and the durationduration of the terrestrial day and led to the fact that the moon always showed the same face to the earth (this is the phenomenon of rotation synchronoussynchronous). Indeed, based on current observations and reversing the direction of time to predict the past, we end up with a rotation of the Earth shortly after its collision with Theia of about 5 hours…which is too slow.

Researchers just published today in Science two articles proposing two new collision scenarios that seem to be able to solve all these mysteries.

Animated film depicting the birth of the earth and the moon. © TriSkull666 YouTube

Two planets of the same size collide

In one of these scenarios, the Earth, in the form of a very flattened ellipsoid, is in a very rapid rotation, with a day lasting only 2 to 3 hours. Under 2 hours, the Earth’s rotation speed would be such that the centrifugal forcecentrifugal force would burst it. At such a rate of rotation at the time of moon formation, enough material can be ejected from Earth’s mantle and incorporated into our satellite’s composition to account for its chemical proximity to Earth. This is the case, for example, when a Theia with half the mass of Mars hits the young Earth almost head-on at 20 km/s, as shown by numerical simulations.

It remained to reduce the speed of rotation of the earth. The researchers found that this was entirely possible when one mechanism was considered resonanceresonance gravity between movementmovement of the moon around the earth and of the earth around the earth SunSun. The same mechanism (evection resonance in English) makes it possible to correctly shape the moon after a head-on collision between two celestial bodies of almost identical mass, namely 4 to 5 times the mass of each planet, as suggested by the second scenario published in Science.

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