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Supercomputer simulations could unlock mystery of moon’s formation

By Nina Massey
Astronomers say they are one step closer to understanding how the moon might have formed out of a giant collision between the early Earth and another massive object 4.5 billion years ago.

Scientists ran supercomputer simulations to send a Mars-sized planet called Theia crashing into the early Earth.
These simulations produced an orbiting body that could potentially evolve into a moon-like object.

While the researchers, led by Durham University, are careful to say this is not definitive proof of the moon’s origin, they add that it could be a promising stage in understanding how the celestial body formed.

The findings are published in the Monthly Notices of the Royal Astronomical Society journal.

The moon is thought to have formed in a collision between the early Earth and Theia.

Scientists believe Theia might have been an ancient planet in our solar system, about the size of Mars.

Researchers ran simulations to track material from the early Earth and Theia for four days after their collision.

They then ran other simulations after spinning Theia like a pool ball.

The simulated collision with the early Earth produced different results depending upon the size and direction of Theia’s initial spin.

At one extreme the collision merged the two objects together while at the other there was a grazing hit-and-run impact.

The simulation where no spin was added to Theia produced a self-gravitating clump of material with a mass of about 80% of the moon, while another moon-like object was created when a small amount of spin was added.

The resulting clump, which settles into an orbit around the post-impact Earth, would grow by sweeping up the disc of debris surrounding our planet.

It has a small iron core, similar to that of the moon, with an outer layer of materials made up from the early Earth and Theia, the astronomers found.

Recent analysis of lunar samples collected by the Apollo space missions suggests that a mixture of early Earth and impactor material might have formed the moon.

Lead author Sergio Ruiz-Bonilla, a PhD researcher in Durham University’s Institute for Computational Cosmology, said: “By adding different amounts of spin to Theia in simulations, or by having no spin at all, it gives you a whole range of different outcomes for what might have happened when the early Earth was hit by a massive object all those billions of years ago.

“It’s exciting that some of our simulations produced this orbiting clump of material that is relatively not much smaller than the moon, with a disc of additional material around the post-impact Earth that would help the clump grow in mass over time.

“I wouldn’t say that this is the moon, but it’s certainly a very interesting place to continue looking.”

The high-resolution simulations were run using the SWIFT open-source simulation code.

They were carried out on the DiRAC Memory Intensive service (“COSMA”), hosted by Durham University on behalf of the DiRAC High-Performance Computing facility.

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