Moon May Have Formed Intact as New Models Give Colliding Planets Geological Strength

For decades, the prevailing story of the moon’s birth has been one of utter destruction. Roughly 4.5 billion years ago, a Mars-sized world called Theia slammed into the young Earth, and the collision shattered the impactor, scattering its molten remains into a swirling disk of debris orbiting our planet. From that ring of vaporized and molten rock, the moon was thought to have gradually assembled itself, particle by particle, over years or centuries. Now, a new modeling study from researchers at the Southwest Research Institute and the University of Arizona suggests that this catastrophic picture may be only half the story. By giving their simulated planets something earlier models lacked, realistic geological strength, the team found that under certain conditions the moon may have emerged from the impact essentially whole, an intact body forged within just a few hours of cosmic violence.

The study, published in The Astrophysical Journal Letters, is the first to incorporate temperature-dependent material strength into high-resolution simulations of the moon-forming giant impact. Previous simulations, including the foundational work published in 2001 by Robin Canup and Erik Asphaug, treated the colliding bodies as fluids. That assumption seemed reasonable at the time: the impact was believed to be energetic enough to melt and vaporize large portions of both Earth and Theia, and surely molten or vaporized rock cannot hold its shape. But the new research, led by Adeene Denton, a former postdoctoral researcher at the University of Arizona’s Lunar and Planetary Laboratory who is now at SwRI, demonstrates that this simplification was hiding something fundamental about how the collision actually unfolded.

“We discovered that the preexisting geology of the Mars-sized proto-moon matters,” Denton said. “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.” The insight grew out of Denton’s earlier work on the Pluto-Charon system, where she found that material strength played a crucial role in how the dwarf planet and its large moon came to be. She and her colleagues wondered whether the same physics might apply to Earth’s own moon, even though the moon-forming impact involved far larger bodies and far greater energies.

To test the idea, the team employed an advanced version of smoothed particle hydrodynamics, or SPH, a computational technique that represents planetary bodies as millions of interacting particles and tracks how they deform, heat, and flow during a collision. The version used in this study, developed at the University of Arizona and the University of Bern in Switzerland, includes a strength model that gives the simulated bodies the kind of resistance to deformation expected of real geologic materials, such as the rock and metal that would have composed Theia and the proto-Earth, or solid ice. In practical terms, the modeled planets were no longer infinitely deformable blobs; they had internal structure, rigidity, and a memory of their thermal state, and those properties shaped how the impact played out.

The results were striking. Hotter bodies are weaker than colder ones, and the team found that the outcome of the moon-forming impact is highly sensitive to the temperatures of the colliding bodies at the moment of contact. In some scenarios, the impact destroyed Theia and produced a massive protolunar disk of debris around Earth, which would then need to cool and coalesce into the moon over time, much as in the classic models. But when the researchers used the same parameters as the original impact modeling, including the equal temperature structures inside both bodies, something remarkable happened: within around five hours, an intact moon emerged directly from the collision. “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.”

While intact-moon outcomes have appeared in prior simulations under different assumptions, this work is the first to show that material strength and temperature are central controls on which formation pathway the impact takes. The distinction is far from academic. A moon that forms intact from the collision would preserve material in a very different state than one assembled from a disk of vaporized and molten debris, with potentially different histories of heating, differentiation, and loss of volatile elements. That means the physical and chemical properties of the moon we see today, including perhaps its volatile content, could act as a fingerprint of the thermal conditions prevailing at the moment of the giant impact.

“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 new study. “This in turn might help scientists better constrain when the moon-forming event occurred.” Because protoplanets generally begin their lives hot and cool as they age, the thermal states of Earth and Theia encode information about how long each body existed before the collision. If the moon’s characteristics can be tied to those thermal states, the moon itself becomes a kind of clock, offering a new avenue for dating one of the most consequential events in the solar system’s history.

The findings do not resolve every mystery surrounding the moon. One long-standing conundrum remains the striking similarity between the materials that make up Earth and the moon, which is difficult to explain if the moon formed largely from the remains of a distinct impactor. “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 possible explanation is that Theia and the proto-Earth formed from a common reservoir of material in the same region of the solar system, while Mars, which is compositionally distinct from both Earth and the moon, formed farther out. The new strength-aware simulations may ultimately help test such scenarios by tracking more precisely which material ends up where.

“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 at the Lunar and Planetary Laboratory. “This gives us a new way to explore the conditions of the impact and what they might reveal about the moon’s origin.” Asphaug, a co-author of both the original 2001 work and the current study, put the shift in perspective simply: because the collision was considered violent enough to melt and vaporize large portions of Earth and Theia, earlier papers assumed it was acceptable to approximate them as fluids. “Based on our new results, however, we think that it is time to reconsider that,” he said.

The broader lesson of the study extends beyond the Earth-moon system. Material strength, long dismissed as irrelevant for planetary-scale collisions, turns out to be essential for understanding impacts among smaller bodies such as asteroids, dwarf planets, and moons, and its importance for Earth’s moon suggests that thermal and structural properties deserve a place in every giant-impact model going forward. As computational techniques continue to improve, researchers can now ask not just whether a collision can build a moon, but what kind of moon it builds, and what that moon can tell us about the conditions under which it was born. For a companion world that has hung in our sky for four and a half billion years, the moon, it seems, still has plenty of secrets about its own origin story left to reveal.

Subject of Research: Giant impact simulations of moon formation incorporating material strength and temperature

Article Title: Soft and stretchy or rock solid? New modeling study sheds light on how the Earth got its moon

Article References: Soft and stretchy or rock solid? New modeling study sheds light on how the Earth got its moon. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: moon formation, giant impact, Theia, smoothed particle hydrodynamics, material strength, protolunar disk, proto-Earth, planetary science, lunar origin, computational modeling, Southwest Research Institute, University of Arizona