The moon may have formed in just 5 hours, new simulations suggest

The moon may have formed in as few as five hours after the cataclysmic collision between Earth and a long-lost protoplanet, new simulations suggest. The surprising findings could help shed light on other historic impacts throughout our cosmic neighborhood and beyond.

Experts strongly suspect that the moon was created when a Mars-size space rock called Theia smashed into an early version of our planet, back when the solar system was young and extremely unstable. This colossal impact likely would have turned the once-neighboring worlds into spinning clumps of molten rock that eventually coalesced and cooled into the versions of Earth and the moon we see today. Without this collision, which also may have left chunks of Theia buried deep within our planet, many scientists think that life may have never existed on Earth.

Lunar rocks collected during NASA's Apollo missions have provided evidence to support this theory, dubbed the giant impact hypothesis. These samples also tell us that the moon is around 4.5 billion years old, meaning the collision occurred roughly 100 million years after the sun was born. But although scientists have previously simulated the impact event, it has been tricky to ascertain exactly how the moon formed and how long it took to emerge after the Earth-Theia collision.

In the new study, published Sept. 1 in The Astrophysical Journal Letters, researchers conducted a series of simulations focusing on the varying potential geological properties of both Earth and Theia prior to their collision. This factor has been relatively understudied until now.

"When you simulate the Earth and the moon as colliding bodies with [different] geologic properties, it changes how the moon forms out of that impact — that's something we considered unnecessary before," study first author Adeene Denton, a geologist and planetary scientist at the Southwest Research Institute in Boulder, Colorado, said in a statement.

Until now, simulations of the Earth-Theia impact have focused mainly on the masses of the colliding neighbors. But in the new study, the researchers were most interested in the material strength of the two bodies, particularly the hardness of their exteriors.

"Models have evolved to include material strength, something that's really important when you're studying collisions between smaller bodies like asteroids," Denton said. "We weren't sure if it would matter for the moon or not. When we did the simulations, we found it actually matters quite a bit."

The main factor in the strength of colliding bodies is temperature. Both Earth and Theia were very young when they smashed together, so they were still hot and had soft or molten surfaces. These properties would have cushioned the collision, allowing the debris to stay largely intact and enabling the moon to form more quickly. But when the giant impact hypothesis was first proposed (in 2001), researchers initially thought the collision occurred much later, meaning that the bodies would have cooled and become more brittle. In this case, their collision would have created a larger field of rocky debris that would have taken longer to coalesce.

Researchers simulated the collision using a range of potential temperatures and found that under the most extreme scenario — where the pair were at their hottest — the moon could have formed in just five hours. However, this is just one potential outcome, and there is no way of knowing exactly how hot — and, therefore, how strong — Earth and Theia really were when they smashed together.

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When the moon took shape, it was initially covered with a magma ocean that took hundreds of millions of years to solidify. Even then, its increased proximity to Earth meant that it was likely a volcanic hellscape. However, over time, it moved away from our planet, cooling further, and it is now assumed to be geologically dead. (The moon is continuing to slowly move away from us and could eventually escape our planet's gravity entirely, if the dying sun doesn’t destroy it and us first in around 5 billion years.)

The new simulations highlight the importance of material strength in planetary impact modeling, which could change what we know about other ancient collisions, like the one that knocked Jupiter's moon Ganymede off its axis or the one that birthed Saturn's moon Titan.

Additionally, the findings could have implications in the search for exomoons — the hypothetical moons of distant exoplanets, which have so far remained elusive, according to Live Science's sister site Space.com. Until now, one of the main ways astronomers have been hunting for these alien satellites is by looking for debris fields around exoplanets. However, if the moon is anything to go by, these debris fields could be very short-lived and difficult to identify.