New simulations say the Moon assembled itself in about five hours

A new set of computer simulations suggests the moon could have snapped together almost instantly after the ancient collision that formed it, assembling into an intact body in roughly five hours rather than coalescing gradually from a ring of debris over months or years. The research, led by planetary scientist Adeene Denton at the Southwest Research Institute in collaboration with the University of Arizona, was published September 1 in The Astrophysical Journal Letters. The result does not settle how the moon formed, but it identifies a scenario the field had not previously modeled in this kind of detail, and one that depends heavily on how hot the colliding bodies were.

Revisiting a Collision First Modeled in 2001

The leading explanation for the moon’s origin, known as the giant impact hypothesis, holds that a Mars-sized protoplanet nicknamed Theia struck the young Earth roughly 4.5 billion years ago, and that debris from the impact eventually gathered into the moon. Foundational simulations of that scenario, published in 2001 by Robin Canup, now a vice president at SwRI’s Solar System Science and Exploration Division, and Erik Asphaug of the University of Arizona, helped establish the modern version of the theory. Lunar rock brought back by NASA’s Apollo missions supports the broad outline of that story, showing the moon is around 4.5 billion years old, meaning the collision happened roughly 100 million years after the sun itself formed. Denton’s team, which included Asphaug as a co-author, revisited that same impact scenario with a modeling approach earlier studies had not applied to a collision of this scale.

The Missing Variable: How Strong Was the Rock?

Earlier giant-impact simulations treated Earth and Theia largely as fluids, on the assumption that the energy of a planet-scale collision would be so extreme that the underlying material strength of the colliding rock wouldn’t matter. Denton’s team tested that assumption by building simulations that incorporate temperature-dependent geologic strength, an approach already used to study smaller collisions between asteroids and, in Denton’s earlier work, the formation of the Pluto-Charon system, but never before applied in this kind of detail to a planet-scale impact. “We weren’t sure if it would matter for the moon or not,” Denton said in a statement published by the Southwest Research Institute. “When we did the simulations, we found it actually matters quite a bit.”

Two Very Different Paths, Depending on Heat

The simulations showed that hotter bodies behave as weaker material, and that this single factor changes the outcome of the collision substantially. Because Earth and Theia were still young and likely retained soft or partially molten exteriors when they collided, that heat would have cushioned the impact and kept more debris intact rather than scattering it into a diffuse cloud. Denton reported that when she ran the model using the same basic parameters as the original 2001-style simulations, but with both bodies given matching, elevated temperature structures, an intact moon emerged within about five hours of the impact. Cooler, stronger starting conditions instead produced a protolunar disk, a ring of vaporized and molten rock encircling Earth that would have taken far longer, on the order of months to years, to gather into a single body. Because young protoplanets are generally hottest soon after they form and cool with time, the researchers argue this creates a genuine link between when the Theia impact happened in Earth’s early history and what kind of moon it produced.

A Possible Clue to the Moon’s Familiar Chemistry

The work also touches a long-standing puzzle: why lunar rock samples collected during NASA’s Apollo missions show isotopic signatures so similar to Earth’s, unlike material from Mars or elsewhere in the solar system. Denton suggested that Theia and the proto-Earth likely formed from the same region of the young solar system’s protoplanetary disk, while Mars, compositionally distinct from both, condensed farther out. “Because Earth and Mars formed in the same neighborhood of the solar system, they are like siblings,” she said. “The moon and Earth are more like fraternal twins.”

A Possible Wrinkle for Hunting Moons Around Other Planets

The finding also carries a secondary implication beyond the solar system’s own history. According to reporting on the study, astronomers searching for exomoons, the hypothesized natural satellites of planets orbiting other stars, have generally looked for lingering debris disks as a sign that a moon might be forming. If large impacts elsewhere in the universe can produce an intact moon within hours rather than leaving behind a disk that persists for years, those debris signatures could be far more short-lived and harder to catch than researchers had assumed, complicating one of the main strategies used to hunt for moons beyond the solar system.

Testing the Scenarios Against Real Lunar Samples

Canup, who was not involved in the new study, said the findings suggest a connection between the physical properties of today’s moon, including its volatile content, and the thermal state of Earth and Theia at the moment of impact, a link that could eventually help researchers narrow down when the collision occurred. Confirming which scenario actually played out will depend on comparing the simulations against real lunar material, including samples that NASA’s Artemis missions are expected to return from previously unsampled parts of the moon. Those rocks, drawn from deeper beneath the surface than the Apollo-era samples, could offer a more direct test of whether the moon assembled in hours or over a much longer stretch of time.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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