Moon May Have Formed in Just Five Hours, Simulations Find

Simulations suggest Earth's moon could have formed from impact debris in as little as five hours.

Researchers led by Adeene Denton of the Southwest Research Institute ran some of the most detailed simulations yet of the giant impact between early Earth and the Mars-size protoplanet Theia, published Sept. 1 in The Astrophysical Journal Letters. Unlike earlier models, these included material strength and the internal temperature of the colliding worlds — factors previously thought unimportant for a collision this energetic. According to Space, when the same uniform temperatures were used for both bodies, an intact moon emerged from the debris within around five hours, a scenario first hinted at by NASA-led simulations in 2022 and now bolstered by the new work.

The long-standing view held that the energy of a giant impact was so immense that Theia's geology could not influence the outcome. The new models overturn that assumption: hotter bodies are weaker and colder ones stronger, so Theia's thermal state changed how it deformed and absorbed momentum on impact. RBC-Ukraine reports that Denton found the mechanics of moon formation changed completely once Earth and Theia were simulated as bodies with specific geological properties — a detail earlier modeling treated as irrelevant.

Both publishers describe two distinct scenarios. Space reports that a warmer Theia striking less than 60 million years after the planets formed would be obliterated, producing a massive debris disk from which the moon could form within hours — while a colder Theia, hitting 100 to 150 million years after the planets' birth, would leave more of itself intact to merge with Earth and allow the moon to accrete gradually from a longer-lived ring. In both scenarios the moon forms mostly from Theia's mantle, though puzzling isotope differences between lunar rock and Earth's mantle remain unexplained by the simulations.

The stakes extend beyond Earth's history. According to Space, if our moon really did assemble in a matter of hours, moon-forming debris disks around rocky exoplanets would be extremely short-lived, making them a fruitless target for exomoon hunters — though disks around gas giants, which form from leftover planet-assembly material rather than impacts, would be a different matter. Robin Canup, the leading pioneer of giant-impact modeling who was not involved in the study, told Space that the results may connect the moon's present-day physical properties, including its volatile content, to the thermal state of Earth and Theia at impact.

Scientists hope the new temperature-sensitive models can help constrain when the moon-forming event actually occurred, tying the moon's composition to the solar system's earliest timeline. Interest in the moon remains high on other fronts: Starlust reports that NASA has asked Axiom Space to simplify its AxEMU spacesuit designs ahead of the Artemis IV lunar landing tentatively planned for 2028, after NASA's Office of Inspector General warned suit development could stretch into 2031. Whether humans return in 2028, the story of how the moon itself arrived may have just gotten dramatically faster.