Understanding how the Moon might have formed

Supercomputer simulations could unlock the mystery of the Moon's formation.

The Moon is thought to have formed in a collision between the early Earth and Theia, which scientists believe might have been an ancient planet in our solar system about Mars’s size.

In an attempt to understand how the Moon might have formed out of a giant collision, scientists from Durham University, UK, ran supercomputer simulations on the DiRAC High-Performance Computing facility to send a Mars-sized planet—called Theia—crashing into the early Earth.

Their simulations produced an orbiting body that could potentially evolve into a Moon-like object.

Though, as scientists reported, it is not definitive proof of the Moon’s origin, it could be a promising stage in understanding the moon formation.

The simulations track material from the early Earth and Theia for four days after their collision. Later, scientists ran other simulations after spinning Theia like a pool ball.

The simulated collision with the early Earth produced different results depending upon Theia’s initial spin’s size and direction. At one extreme, the collision merged the two objects, while at the other, there was a hit-and-run grazing impact.

At one outrageous, the collision merged the two objects while there was a hit-and-run grazing impact. 

Critically, the simulation where no spin was added to Theia delivered a self-gravitating clump of material with a mass of around 80% of the Moon. At the same time, 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.

The simulated clump also has a small iron core, similar to that of the Moon, with an outer layer of materials from the early Earth and Theia.

Lead author Sergio Ruiz-Bonilla, a Ph.D. 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 an exciting place to continue looking.”

Co-author Dr. Vincent Eke, of Durham University’s Institute for Computational Cosmology, said: “We get several different outcomes depending upon whether or not we introduce spin to Theia before it crashes into the early Earth.”

“It’s incredibly fascinating that when no spin or very little spin is added to Theia that the impact with the early Earth leaves a trail of debris behind, which in some cases includes a body large enough to deserve being called a proto-Moon.”

“There may well be several possible collisions that have yet to be investigated that could get us even closer to understanding just how the Moon formed in the first place.”

Journal Reference:
  1. The effect of pre-impact spin on the Moon-forming collision, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa3385

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