New computer simulations of the collision that produced Earth’s moon suggest the whole process, from a Mars-size body slamming into the young Earth to an intact moon settling into orbit, could have played out in about five hours rather than the months or years earlier models assumed. The result depends heavily on how hot the two colliding bodies were at the moment of impact, and the same simulations also produce a much slower, gradual outcome under different starting conditions. Researchers say the finding does not settle how the moon formed so much as sharpen the range of ways it plausibly could have.
The Giant-Impact Theory Gets a More Detailed Physics Model
The leading explanation for the moon’s origin holds that a Mars-size protoplanet known as Theia struck the proto-Earth roughly 4.5 billion years ago at an angle that threw a large volume of debris into orbit rather than simply merging the two bodies together. Planetary scientist Robin Canup has conducted and refined computer simulations of that basic scenario since 2001, and her work established the giant-impact hypothesis as the leading account of how Earth acquired its moon. A team led by Adeene Denton of the Southwest Research Institute has now added a factor those earlier models mostly left out: the material strength of the colliding rock, which changes depending on how hot the bodies were inside. “Models have evolved to include material strength, something that’s really important when you’re studying collisions between smaller bodies like asteroids,” Denton said in a statement carried by the Southwest Research Institute. “We weren’t sure it would matter for the moon or not. When we did the simulations, we found it actually matters quite a bit.”
Why a Warmer Planet Behaves Like Weaker Rock
Newly formed planets retain internal heat left over from their assembly, and warmer rock deforms more easily than colder rock under the same impact stress. Because that residual heat fades over time, how strong or weak Theia was during the collision depends on when, in the solar system’s first hundred million years or so, the impact actually happened. A warmer, weaker Theia and a warmer, weaker proto-Earth respond to a collision very differently than the same two bodies would if the impact came later, after both had cooled and stiffened.
Two Very Different Outcomes From the Same Model
Denton’s simulations produced two distinct scenarios depending on that timing. If the impact happened later, roughly 100 million to 150 million years after the planets formed, Theia would have cooled and strengthened enough that more of it survived the collision intact, and the resulting ring of debris around Earth would have lingered long enough for the moon to accrete gradually over an extended period, matching the slower picture earlier models favored. If the impact instead happened earlier, less than about 60 million years after formation, both bodies would still have been hot and weak, and the collision would have destroyed Theia outright while flinging its material into a massive debris disk.
An Intact Moon in About Five Hours
It is that second, earlier-impact scenario that produced the fast result. “Depending on how hot the Earth and Theia are prior to the collision, the impact can destroy Theia and produce this massive disk of debris that eventually forms the moon,” Denton said. “But when I used the same parameters as original impact modeling, down to the equal temperature structures inside both bodies, within around five hours, an intact moon emerged.” The finding builds on a 2022 NASA-led simulation that first suggested a rapid, hours-long formation was physically possible; Denton’s team arrived at a similar timescale using a more detailed treatment of the colliding material itself, lending the fast scenario independent support rather than proposing it from scratch.
A Composition Puzzle the Model Still Can’t Explain
In both the fast and slow scenarios, the simulations show the moon forming mostly from Theia’s mantle material with only a small admixture of Earth’s own mantle. That is notable because measurements of real lunar rock show the moon’s composition is remarkably close to Earth’s, a similarity these simulations, like earlier ones, still struggle to fully reproduce. Robin Canup, who was not involved in the new study, called the results surprising and exciting, saying they imply a potential connection between the physical properties of the moon today, including its volatile content, and the thermal state of Earth and Theia at the time of the giant impact. In her view, that connection might eventually help scientists better constrain when, during the solar system’s first hundred million years, the moon-forming collision actually took place.
What Rapid Formation Would Mean Beyond Earth
The distinction matters for more than Earth’s own history. If a rapid, hours-long formation turns out to be the more common outcome of giant impacts, then any debris disk capable of building a moon around a rocky exoplanet would likely be extremely short-lived, making such disks difficult to ever catch and observe directly. Moons forming around gas giants would not face the same limitation, since those satellites typically build up from leftover material during planet formation rather than from a single violent impact. The results were published September 1 in The Astrophysical Journal Letters, and the team’s reporting on the work notes the isotopic puzzle remains open even under the new modeling.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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