Moon may have formed intact within hours of giant collision, new simulations suggest

For decades, scientists have modeled the Moon's birth as a catastrophic collision between the young Earth and a Mars-sized body called Theia, imagining the impact so violent that both worlds behaved like fluids—melting, vaporizing, scattering debris into orbit. A new set of computer simulations is upending that picture. Researchers have found that when you account for the actual geological properties of rock and metal—how they resist deformation, how temperature affects their strength—the Moon's formation could have unfolded in radically different ways. Under some conditions, the familiar debris disk still forms, material gradually assembling into the Moon over time. But under others, something far more dramatic happens: a fully intact Moon emerges from the collision in roughly five hours.

Adeene Denton, a researcher now at the Southwest Research Institute, led the work with colleagues including Erik Asphaug at the Lunar and Planetary Laboratory. Their simulations, published in The Astrophysical Journal Letters, represent the first time scientists have incorporated temperature-dependent material strength into models of the Moon-forming impact. The breakthrough came from applying advanced computational methods originally developed at the University of Arizona and the University of Bern in Switzerland. These methods allow simulated planetary material to behave more like actual rock and metal, resisting deformation in ways that match real geology rather than treating everything as a fluid.

The difference this makes is striking. Temperature emerged as especially crucial. Hotter planetary bodies are mechanically weaker than cooler ones, and the simulations showed this property can completely reshape what happens after impact. When the proto-Earth and Theia were hot—as young protoplanets typically are—the collision could destroy Theia and create a broad disk of debris that gradually coalesces into the Moon. But when Denton and her team ran the same simulations using the original impact parameters, including equal temperature structures inside both bodies, an intact moon appeared within about five hours. Previous models had occasionally produced intact moons, but this is the first time researchers have demonstrated that material strength and temperature are central to determining whether the Moon forms largely whole or gradually assembles from processed debris.

This finding opens a new window onto one of planetary science's deepest mysteries: when exactly did the Moon form? Robin Canup, vice president of the Southwest Research Institute's Solar System Science and Exploration Division and a pioneer of giant impact theory, noted that the new results suggest a potential connection between the Moon's present-day properties—including perhaps its volatile content—and the thermal state of Earth and Theia at the moment of collision. If scientists can determine the Moon's physical state at formation, they may be able to work backward to constrain when the impact occurred, roughly 4.5 billion years ago.

Yet a major puzzle remains unsolved. Earth and the Moon have remarkably similar compositions, something that giant impact models have struggled to fully explain for years. The new simulations do not resolve this problem. One possibility is that Theia and the proto-Earth formed from material in the same region of the early solar system, making them geologically similar in a way that Mars, which formed farther away and has a different composition, was not. Denton described the relationship this way: Earth and Mars are like siblings born in the same neighborhood, while the Moon and Earth are more like fraternal twins.

What the new work does accomplish is fundamental. By showing that the internal condition of Earth and Theia could strongly influence the aftermath of their collision, the study gives researchers a new way to investigate how the Moon came to be. The geophysical state of both bodies—their temperature, their strength, their ability to hold together or flow apart—now emerges as central to understanding not just how the Moon formed, but when. That shift in perspective could reshape decades of assumptions about one of the solar system's most consequential events.