SAN ANTONIO — September 1, 2026 — New Southwest Research Institute (SwRI) modeling, conducted in collaboration with scientists at the University of Arizona, shows fundamental differences in how the Moon may have formed from the giant impact that created the Earth-Moon system. The new results used state-of-the-art computational techniques that factor in the material strength of the two colliding planets. These impact simulations could change how researchers understand Moon formation and may help constrain the timing of the event. This research was published in The Astrophysical Journal Letters.
"We discovered that the preexisting geology of the Mars-sized proto-Moon matters," said Dr. Adeene Denton, formerly a NASA Postdoctoral Program fellow at SwRI and now a postdoctoral researcher in SwRI's Solar System Science and Exploration Division. "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."
Earlier studies of the giant impact scenario, including a foundational 2001 paper by Dr. Robin Canup, vice president of SwRI's Solar System Science and Exploration Division in Boulder, Colorado, and Dr. Erik Asphaug, a professor at the University of Arizona and co-author of the current study. That research found that a Mars-sized object "Theia" may have smashed into Earth to create the Earth-Moon system, but those simulations and subsequent giant impact modeling ignored material strength, which was thought to be insignificant for such high-energy events. Denton and her team revisited this hypothesis, using modern computational methods that incorporate temperature-dependent geologic strength for the first time.
"Models have evolved to include material strength, something that's really important when you're studying collisions between smaller bodies like asteroids or for my previous paper about the formation of the Pluto-Charon system, " 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."
Hotter bodies are weaker than colder ones, and the team found that Moon formation is sensitive to the temperatures of the colliding bodies. Some scenarios produce a fully intact Moon within hours of the impact, while others produce a protolunar disk around the Earth that ultimately forms the Moon over time. Because protoplanets generally start off hot and cool with age, this establishes an important new connection between the timing of the giant impact and the nature of the Moon's initial state and assembly.
"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."
While intact Moon outcomes have been seen in prior simulations, this work is the first to show that material strength and temperature play a central role in whether the Moon forms intact or is assembled from material processed within 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," says Canup, who was not involved in the study. "This in turn might help scientists better constrain when the Moon-forming event occurred."
As with prior models, explaining the close compositional makeup of the Earth and Moon remains an open scientific question. A possible explanation is that Theia and the proto-Earth formed from a common region of the protoplanetary disk, while Mars, which is compositionally distinct from the Earth and Moon, formed farther away.
"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."
The study used giant impact simulation methods developed at the University of Bern and the University of Arizona. The Astrophysical Journal Letters paper can be accessed at DOI: 10.3847/2041-8213/ae91e9.
Watch the Moon formation simulations: https://youtu.be/QAIRqc9dFHg .
