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A team of scientists at Durham University in the UK has flipped the chessboard of our longstanding consensus about the origin of the Moon.

It was a story made to capture my attention. In high school English classes, I often lightly plagiarized the last line of Walt Whitman’s “When I Heard the Learn’d Astronomer.” Whitman’s protagonist attends an astronomy lecture, grows bored of all of the numbers and technical jargon, and decides to go outside and stare at the night sky instead. Instead of looking up “in perfect silence at the stars,” I’d sub in the Moon, my favorite sky object.

Humans cannot help but moon over the Moon, and have done so forever, and probably always will. But ongoing research exposes how little we’ve actually penetrated its mystery.

In a recent paper, the Durham team shattered previous notions of the Moon’s origin story. Through supercomputer-powered high-resolution simulations, researchers continually reproduced an impact between a very young Earth and a Mars-sized planet called Theia, the resulting rubble of which is believed to have formed the Moon. Their findings indicated that, rather than the Moon having slowly coalesced from (mostly) Theia’s debris, the satellite formed quite quickly, drawing from a more heterogeneous mixture of Earth’s and Theia’s matter. This new outcome fits the present-day Moon’s qualities far more closely.

The Moon’s formation has long interested scientists. Prior to the Apollo mission, however, there was little information on which to base theories. As a result, origin theories have been quite varied: I’m personally most charmed by capture theory, the idea that the Moon was wandering aimlessly around the solar system before being suddenly captured by Earth’s gravity, locked into orbit for good. 

With the moon rocks collected from the Apollo mission, most of these wild speculations were put to rest, replaced by the “giant impact theory.” The Moon and Earth’s compositions were just similar enough such that scientists speculated that the satellite probably formed from an impact between a young Earth and something else. This “something else” eventually took on the name Theia, and researchers began modeling the crash, envisioning Theia as a young planet whose life was snatched away too quickly.

“This was an unusual bit of scientific research… We realized we had some new technology, and applied it to an old puzzle, to see what it would reveal.”

Despite the innumerable simulations that have been carried out since the giant impact theory rose to prominence, certain discrepancies between reality and the outcomes of models persisted. After the impact, most simulations suggested, the Moon would have formed from a “disk of debris” primarily composed of Theia-derived material. However, this does not align with the Moon’s actual chemical and isotopic composition, which are notably similar to Earth’s, particularly in the kinds of silicon molecules found. Scientists attempted to offer other explanations for this inconsistency, such as the unlikely possibility that Theia had an extremely similar composition to that of the Earth. But questions lingered.

Researchers with Durham University’s Institute for Computational Cosmology (ICC) took a stab at the issue through much higher-resolution simulations powered by supercomputers. 

“New technology applied to an old puzzle”

“This was an unusual bit of scientific research,” Richard Massey, one of the researchers on the ICC’s planetary giant impact team, told OnlySky. “We realized we had some new technology, and applied it to an old puzzle, to see what it would reveal.”

The program used to run these simulations was a tweaked version of SWIFT, which models the forces and fluid dynamics that affect how celestial objects form and interact. The code was previously used for larger-scale astronomical simulations such as the life cycles of stars, and even the Big Bang. 

What made these new simulations distinct was their incredible level of detail. The simulations’ resolution was hundreds of times higher than previous simulations of the impact, which surprisingly lead to remarkably different outcomes. 

Massey called this change in resolution “like a butterfly flapping its wings”: a reference to the butterfly effect, which details how tiny initial changes can have large effects on a system over time. 

Researchers varied qualities such as the initial masses and temperatures of the planets, as well as impact angles. Through this experimentation, it became clear that the Moon could have been formed in a very different way: the “immediate satellite scenario.” In this situation, a satellite with a mass that rivals the Moon’s current mass formed extremely quickly post-impact. Rather than the Moon being gradually cobbled together from bits and pieces from Theia, the outer layer of the Moon would be composed of proto-Earth material, while the inner areas would be from Theia.

This new version of the Moon’s birth would “solve a long-standing mystery,” said Massey, who is hopeful about the future possibilities provided by these simulations for both his own research and science in general. “A really neat side-effect of our work is that we’re gradually bringing (rather abstract) simulations of the distant universe close to home. The extra code… is just what is needed to simulate gases or fluids mixing on Earth,” he said. “We’ll see where that physics will be useful to improve life on Earth, too.”

Georgia Michelman is a reluctant recent Yale College graduate with backgrounds in physics, astronomy, and history. She is always searching for intersections between the worlds of science and the humanities....

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