A new study suggests the Moon formed within five hours after a giant collision between Earth and a Mars-sized body called Theia, according to simulations that account for the material strength and temperature of the colliding worlds.
For decades, the giant impact hypothesis has dominated explanations for the Moon’s origin, positing that a Mars-sized body named Theia collided with the early Earth, scattering debris that coalesced into the Moon. A recent study led by Adeene Denton of the Southwest Research Institute (SwRI) and the University of Arizona challenges previous assumptions by revealing that the Moon may have formed far more rapidly than previously thought—within hours—depending on the thermal and geologic properties of the colliding planets.
New Simulations Rewrite Moon’s Formation Timeline
The study, published in The Astrophysical Journal Letters, used advanced computer simulations to model the collision, incorporating the structural strength of planetary materials for the first time. The simulations showed that if Earth and Theia were sufficiently hot, their collision could have produced a fully intact Moon within five hours, rather than a slow accumulation of debris.
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 explained. The research builds on a 2001 study led by Robin Canup, who previously modeled the impact without considering material strength. Based on our new results, however, we think that it is time to reconsider that,
Asphaug, a co-author of the new study, said.
Material Strength and Temperature: Key Factors in the Collision
The simulations revealed that temperature played a pivotal role. Hotter planetary bodies, which were likely the case for young Earth and Theia, are mechanically weaker and more prone to deformation. This property influenced how the collision unfolded. It turns out material strength is really important when you're studying collisions between smaller bodies like asteroids,
Denton noted. 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.
The study’s findings contradict earlier models that assumed the collision was so violent it would melt and vaporize the planets, treating them as fluids. 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.
Denton’s work, which began as part of a study on the Pluto-Charon system, led to this breakthrough. We weren't sure if it would matter for our moon or not,
she said. When we did the simulations, we found it actually matters quite a bit.
The results suggest that the Moon’s formation was not a gradual process but could have occurred almost instantaneously under the right conditions.
Implications for Planetary Science and Exomoons
The findings have broader implications for understanding planetary collisions beyond the Earth-Moon system. Models have evolved to include material strength, something that's really important when you're studying collisions between smaller bodies like asteroids,
Denton said.
The study also raises questions about the Moon’s composition. However, the new simulations highlight the complexity of this process.
Robin Canup, who was not involved in the study, noted that the results could help scientists better constrain when the Moon-forming event occurred. 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,
she said.
Unresolved Questions and the Path Forward
While the study provides a compelling new framework, uncertainties remain. The exact temperatures of Earth and Theia prior to the collision are unknown, making it difficult to pinpoint the precise conditions that led to the Moon’s rapid formation. We weren't sure if it would matter for the moon or not,
Denton said. When we did the simulations, we found it actually matters quite a bit.
However, the lack of direct evidence about the thermal state of the early solar system means the findings remain theoretical.
