The moon would have formed in a matter of hours

This article is taken from the monthly Sciences et Avenir – La Recherche n°910, dated December 2022.

4.5 billion years ago, a Mars-sized protoplanet collided with the early Earth. Pushed into space, it then gradually aggregated to form our natural satellite. This is the scenario scientists have favored since the 1980s to explain the origin of the Moon. But experts are struggling with the exact circumstances of this phenomenon.

New numerical simulations

“The models struggle to reproduce all the characteristics of the star, such as its precise orbit or the very large chemical similarities between terrestrial and lunar rocks. “, emphasizes Frédéric Moynier, cosmochemist at the Institute of Earth Physics in Paris. But numerical simulations carried out by planetary scientists from NASA and the University of Durham (UK) indicate that our celestial companion would have formed, not in a few months or even several years as previously assumed, but in just a few hours!

400 collisions tested in a supercomputer

The team used the Cosma supercomputer, whose power is equivalent to 40,000 personal computers. 100 million particles of the Earth-Moon system were considered, compared to 100,000 to 1 million in previous work. And 400 collisions were simulated with unprecedented resolution by varying parameters such as the mass, composition and trajectory of the impactor.

In doing so, the researchers were able to identify the modalities that best reproduced the properties of the Moon. Thus, the celestial body would have aggregated and stabilized in less than ten hours in a sufficiently distant orbit from the discs of matter ejected into space. These were to consist of 60% materials from the primitive Earth, the rest coming from the protoplanet. A larger proportion than we imagined, especially in the Moon’s surface layers, where the materials melted together before cooling to form a thin crust.

This new canvas thus explains the very great similarities between terrestrial and lunar rocks, as well as the orbit and structure of our satellite. “It is a seductive theory, Judge Frederic Moynier. However, it will have to be supported by the collection of new lunar samples, at the south pole and especially on the far side, as planned by American and Chinese missions in the coming years.

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