Moon could have formed in hours – simulation reveals surprising course of collision between Earth and protoplanet Theia

Our moon could have formed much faster than thought – in a few hours rather than years or centuries. Evidence of this is provided by the simulation of the moon-forming collision from about 4.5 billion years ago, which has the highest resolution to date. According to them, the impact of the protoplanet Theia ejected a large, only half-molten piece from the primeval Earth, which then became the moon. This scenario could explain several quirks of the moon at once, according to the NASA team.

Earth’s moon was formed when the young Earth was rammed by a Mars-sized protoplanet about 4.5 billion years ago — there’s agreement on that. However, it is much less clear exactly how the moon came to be. According to the classic scenario, the debris from the protoplanet and the ejecta first formed a ring of debris around the damaged Earth, then the Earth’s satellite gradually agglomerated.

The history of the earth and the moon is inextricably linked. ©NASA

The only problem: This scenario doesn’t fit the inclined lunar orbit, nor does the nearly identical isotopic composition of the Earth and Moon. Because where is the rest of the protoplanet Theia? Some planetary researchers argue that the Earth also almost completely evaporated during the collision and that the current rock shell therefore also consists of the same isotopic mixture as the moon. Others suspect that the rest of Theia is hiding deep in the Earth’s mantle. However, these models cannot explain why the lunar orbit tilted and where the system got its initially relatively high angular momentum.

Collision with 100x higher resolution

So a team led by Jacob Kegerreis of NASA’s Ames Research Center has now re-analyzed the event using a simulation. “Most previous lunar simulations contained 100,000 to a million particles, but these resolutions cannot definitively map some core features of large impacts, such as the planetary rotation period or the mass of the ejected debris,” the researchers say. They therefore used 100 million particles, each 14 kilometers in size and ten trillion tons of mass, as a basis.

For their reconstruction, Kegerreis and his team allowed a virtual protoplanet to collide with the simulated primordial Earth in numerous, slightly different scenarios. Over the course of the approximately 400 passes, they varied the impact angle, masses, temperatures and rotation of the celestial bodies involved, as well as impact velocity. They observed the paths, shape and mass of the ejected crash debris.

Huge chunks instead of a ring of rubble

The surprising result: If the protoplanet hits Earth at an angle of about 45 degrees — which is in line with the common assumption — it can throw large, only half-melted chunks into space near Earth, as the simulations showed. A second chunk detaches from the first larger object and is catapulted into orbit as if by a catapult. “The inner calm transmits angular momentum to the satellite before it falls back to Earth itself,” report Kegerreis and his team.

Just five hours after the impact, the outer part stabilized in a near-circular orbit around the Earth at a distance of about seven Earth radii. Since it was thrown from the first clump, its orbit is inclined to Earth’s equator. The satellite formed in this way also has nearly 70 percent of the current lunar mass, explaining the unusual size of Earth’s moon, as the researchers explain.

This is how the moon could have been formed.© NASA Ames Research Center

iron core and terrestrial shell

The highlight here: Because the large chunks of debris are only half melted, their material is not evenly mixed. Although they consist of about 30 to 40 percent terrestrial material, this is mainly on the surface. “Most of the satellites generated in the simulations show a strong gradient in relation to their material origin,” the team reports. “The interior consists mainly of Theia material, which lies beneath a shell with an increasing proportion of proto-mantle rocks outwards.”

The resulting moon has an outer rock shell made up of about 60 percent Earth rock — that would explain the measured isotopic similarities to Earth. Depending on the simulation run, the iron content was between 0.1 and three percent. According to the team, that corresponds well with the actual mass fraction of the moon’s iron core of about one percent. Overall, the scenario could answer some of the previously unanswered questions about the moon’s quirks.

New moon samples may provide confirmation

“This opens up completely new possibilities for the formation and development of the moon,” says Kegerreis. “We started this project without knowing exactly what would come out of these high-resolution simulations. Not only do the results show that standard resolutions can give misleading answers, they also show how a moon-like satellite entered orbit.”

Future lunar missions as part of the Artemis program may show whether the new scenario is correct. During this one, astronauts will bring rock and drill samples from previously unsampled lunar regions back to Earth. Their composition could then reveal more about how Earth’s moon came to be. (The Astrophysical Journal Letters, 2022; doi: 10.3847/2041-8213/ac8d96)

Source: NASA

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