The Moon may have emerged largely intact within hours of the colossal impact that shaped its birth.
Earth has an unusually large moon for a planet of its size and position in the solar system, yet scientists still do not fully understand how that companion came to exist. Now, researchers from the Southwest Research Institute and the University of Arizona have used advanced computer simulations to uncover important differences in what may have happened when a Mars-sized object struck Earth about 4.5 billion years ago.
The findings, published in The Astrophysical Journal Letters, account for the material strength of the two colliding worlds for the first time. That addition could alter how scientists reconstruct the moon-forming impact and may eventually help narrow down when the collision occurred.
“We discovered that the preexisting geology of the Mars-sized proto-moon matters,” said Adeene Denton, a former postdoctoral researcher at Lunar and Planetary Laboratory who is now at SwRI. “When you simulate the Earth and the Moon as colliding bodies with geologic properties, it changes how the Moon forms out of that impact – that’s something we considered unnecessary before.”
Rethinking the Giant Impact
One of the foundations of the giant impact hypothesis came from a 2001 study by Robin Canup, vice president of SwRI’s Solar System Science and Exploration Division in Boulder, Colorado, and Erik Asphaug, a professor at the Lunar and Planetary Laboratory and co-author of the new research.
According to the giant impact scenario, a Mars-sized body known as Theia collided with the young Earth. The impact destroyed Theia and spread its material into a disk surrounding Earth, where that debris eventually assembled into the Moon.
Those early simulations, along with many later models, did not include the strength of the materials inside the colliding bodies. Scientists had assumed that this property would have little influence during an impact energetic enough to melt and vaporize enormous amounts of rock. Denton and her colleagues tested that assumption using newer computational techniques that include temperature-dependent geologic strength.
“Because the collision was considered violent enough to melt and vaporize large portions of Earth and Theia, those previous papers assumed that it is okay to approximate them as fluids,” Asphaug said. “Based on our new results, however, we think that it is time to reconsider that.”
Adding Realistic Rock Physics
To revisit the collision, Denton used a more advanced form of smoothed particle hydrodynamics simulations, or SPH, that accounts for the structural strength of the objects involved.
“It turns out material strength is really important when you’re studying collisions between smaller bodies like asteroids, dwarf planets and moons,” said Denton, who got the idea of applying the concept to Earth’s moon while working on a previous paper about the formation of the Pluto-Charon system. “We weren’t sure if it would matter for our moon or not. When we did the simulations, we found it actually matters quite a bit.”
The SPH approach used in the study was developed at the U of A and the University of Bern in Switzerland. Its strength model allows simulated planetary bodies to resist deformation in ways expected from realistic geological materials, including the rock and metals that would have composed Theia and the proto-Earth, or solid ice.
The Moon Could Form in Hours
Temperature turned out to be especially important. Hotter planetary bodies are weaker than colder ones, and the simulations showed that the temperature of Earth and Theia could dramatically alter the outcome of their collision.
In some scenarios, Theia is destroyed and its material forms a protolunar disk around Earth. The moon then assembles gradually from that debris. Under other conditions, however, the simulations produce a fully intact moon only hours after the impact.
Because protoplanets begin hot and cool as they age, this relationship creates a potential link between when the giant impact occurred and how the moon initially formed.
“Depending on how hot the Earth and moon 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 we 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.”
Previous simulations have also produced intact moons, but the new work is the first to demonstrate that temperature and material strength can be decisive in determining whether the moon emerges largely intact or forms later from material in a protolunar disk.
“These surprising and exciting new results imply a potential connection between the physical properties of the moon today, including perhaps its volatile content, and the thermal state of the Earth and Theia at the time of the giant impact,” said Canup, who was not involved in the study. “This in turn might help scientists better constrain when the moon-forming event occurred.”
A Major Mystery Still Remains
The new simulations do not solve every puzzle surrounding the moon’s origin. One longstanding problem is the remarkable similarity between the materials that make up Earth and the moon.
As with previous models, explaining why the two bodies are so compositionally alike remains an open question.
“Because Earth and Mars formed in the same neighborhood of the solar system, they are like siblings,” Denton said. “The moon and Earth are more like fraternal twins.”
One possibility is that Theia and the proto-Earth originated in the same region of the solar system and therefore formed from similar material. Mars, which has a composition distinct from both Earth and the Moon, may have developed farther away.
“We now know that the geophysical state of Earth and Theia play a fundamental role in shaping the outcome of the collision,” said study co-author Namya Baijal, a doctoral student in Asphaug’s group. “This gives us a new way to explore the conditions of the impact and what they might reveal about the Moon’s origin.”
Reference: “Collisional Capture of an Intact Moon Depends on Strength” by C. Adeene Denton, Erik Asphaug, Namya Baijal and Robert E. Melikyan, 32 August 2026, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ae91e9
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