The simulations show the Moon may have coalesced within hours of the catastrophic Theia-Earth collision that took place billions of years ago.

Key takeaways

  • The simulations suggest the Moon formed within hours after a massive collision between Earth and a Mars-sized planet named Theia.
  • The traditional idea that debris from the Theia-Earth collision formed the Moon is supported by these new findings.
  • Advanced models using up to 100 million particles showed more detailed results, including a Moon with a large orbit and molten interior.
  • The simulations indicate the Moon could be made of 60% Earth material, helping explain its isotopic similarities with Earth.
  • NASA’s Artemis missions aim to collect more lunar samples to further understand the Moon’s origin and its connection to Earth’s history.

The moon is Earth’s continuous companion. But, as common as Luna may appear, Earth has not always possessed a moon.

A drifting Mars-sized planet dubbed Theia collided with a young, moonless Earth around 4.5 billion years ago, while the solar system was still emerging. Traditionally, it is assumed that the Theia-Earth impact sprayed debris about our planet, eventually coalescing to form the Moon. The Giant Impact Hypothesis is a hypothesis that explains how the moon arose.

However, there are a few issues with the Giant Impact Hypothesis as it now exists. The most obvious are the similarities between moon rocks and Earth’s mantle. When the Apollo astronauts returned samples from the Moon, scientists discovered that their isotopic fingerprints — chemical clues that indicate where and how they were formed — roughly (but not exactly) matched Earth. As a result, because no other body in the solar system resembles Earth’s rocks so closely, it is possible that much of the material that makes up the Moon originated on Earth.

However, under the Giant Impact Hypothesis’s debris-disk scenario, Theia-derived material made up the majority of the debris that finally formed the Moon. Alternative ideas often struggled to explain the Moon’s current orbit.

However, recent simulations from Durham University’s Institute for Computational Cosmology suggest a modified scenario in which the Moon originated shortly after the Theia impact. In fact, the simulations reveal that the Moon might have formed in just hours, which is much faster than previously imagined.

“This opens up a whole new range of possible starting places for the Moon’s evolution,” stated lead author Jacob Kegerreis in a NASA press statement. The study was published in the Astrophysical Journal Letters.

A puzzle of planetary proportions

The scientists simulated the Theia-Earth impact around 400 times, utilizing smoothed particle hydrodynamics in their numerical models. This approach, which is widely used to simulate massive collisions, allows scientists to model particles under the influence of both gravity and pressure. Previously, hundreds of thousands or millions of particles were employed to model the origin of the Moon. However, these simulations used up to one hundred million particles, making them some of the most detailed yet.

The increased computer capacity revealed that at lesser resolutions, researchers lose out on important behaviors that occur in such encounters. “We went into this project not knowing exactly what the outcomes of these high-resolution simulations would be,” Kegerreis told me. “So, on top of the big eye-opener that standard resolutions can give you misleading answers, it was extra exciting that the new results could include a tantalizingly Moon-like satellite in orbit.”

In their direct-formation simulations, the team was able to create a Moon with a large orbit and a partially molten interior. Together, these characteristics may help explain the Moon’s tilted orbit and thin crust.

Although fascinating, the models cannot explain what we know about the Moon. Specifically, the simulations were able to create a Moon made out of 60 percent Earth material. However, this alone cannot account for the significant isotopic similarities between Earth and the Moon.

“Even a clump with 60 percent protoEarth material, with the remainder from Theia, would still be expected to produce a much larger Earth-Moon isotopic difference than what we see,” Robin Canup, assistant vice president in the SwRI Space Science and Engineering Division, told Astronomy.

And, while the article argues that material from Theia and Earth may not have properly mingled in the rapidly developing Moon, resulting in a gradient of Earthlike material closer to the surface, Canup believes this is unlikely. “For any portion of the Moon that forms intact, there is no opportunity for mixing between the protolunar and post-impact Earth material to remove such compositional differences.”

Looking forward

To determine the intricacies of the Moon’s origin, additional examination of lunar rocks collected from undiscovered parts of the Moon will be required, which NASA’s planned Artemis missions aim to assist with. Scientists also want to collect materials from beneath the Moon’s surface. Researchers think that by combining models like this, they may be able to unravel the enigma of how the Moon originated around the young Earth 4.5 billion years ago.

As an added benefit, understanding more about our cosmic buddy will disclose more about Earth.

“The more we learn about how the Moon came to be, the more we discover about the evolution of our own Earth,” said Vincent Eke, a researcher at Durham University and a co-author of the new study. “Their histories are intertwined — and could be echoed in the stories of other planets changed by similar or very different collisions.”

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