A new impact model finds that Theia’s crust strength and temperature could determine whether the Moon formed directly or from a debris disk.
Whether the Moon began as an intact satellite or as a ring of orbiting debris may depend on a detail that many impact simulations have smoothed away: the strength of Theia’s outer layer. A study in Astrophysical Journal Letters finds that a Mars-sized body with a solid crust could split during its collision with early Earth, leaving material that stabilizes as a proto-Moon. That possibility gives researchers another route to explain the Earth-Moon system, though it doesn’t settle which route occurred.
The broad account of the Moon’s origin is familiar to planetary scientists: early Earth collided with Theia, a planet roughly the size of Mars, and the impact supplied material for the Moon. The collision also helps explain why Earth and the Moon share strong geological similarities and why the Moon is so large relative to its planet. The unsettled question is how the wreckage of that collision became a satellite.
Many models treat both bodies as fluids during impact. Lead author Adeene Denton, a planetary scientist at the Southwest Research Institute in Colorado, wanted to test what happens when Theia’s crust is given material strength. Her earlier work on the formation of Pluto’s moon Charon informed that question. The new simulations suggest that a stronger Theia can behave differently from the fluid bodies assumed in many prior scenarios.
“All of these [prior] explanations have relied on the assumption that we can treat the Earth and Theia as fluids when they collide,” Denton said. In the study, Theia’s strength changes the possible aftermath: instead of producing only a debris disk, an impact can split the body, with one part joining Earth and another surviving as a proto-Moon.
Temperature changes the outcome
The model’s results hinge on when the collision took place and how hot Theia was at the time. A relatively warm body, still below the melting point at its surface, may have been hot enough inside to deform globally while retaining a cooler, stronger exterior. Under those conditions, the simulations produced a split: one remnant merged with Earth, while another passed close to the young planet and stabilized as an intact satellite.
A colder Theia would have had a sturdier crust. In those simulations, the impact did not simply turn both planets into fluid-like blobs. Theia could remain largely intact, recollide with Earth, and send some material into orbit as a debris disk. That outcome resembles the familiar model, but leaves the same difficult next question: how did the disk gather into the Moon?
“We don’t actually know when the collision took place,” Denton said. “If Theia is cold, then it’s likely going to be stronger because the hotter you are, the weaker you are. So a cold, strong Theia that’s been around a long time isn’t going to deform as readily as a hot, squishy Theia.” The collision date is therefore more than a point on the solar system’s timeline; it affects which physical behavior a simulation should represent.
“Just knowing that [the debris disk stage] could be skipped entirely is a really interesting result.”
The researchers have not shown that the Moon formed directly as a solid body, or that a debris disk was absent. Their result is narrower, but still consequential: the disk stage may not be required in every physically plausible impact scenario. Sarah Stewart, a planetary scientist at Arizona State University who did not take part in the work, stressed how hard it is to model what happens after a disk forms.
“Making the Moon from a disk is even harder than trying to model giant impacts,” Stewart said. She described the formation problem as “basically unsolved because everyone has to make some simplifying assumptions.” For her, the study’s interesting contribution is that “just knowing that [the debris disk stage] could be skipped entirely” opens up another possibility.
Giving Theia a solid exterior
Including material strength makes an already demanding simulation more complicated. Traditional Earth-Theia impact runs can take days, which has limited how many variations researchers can explore. Improvements in computing power over the past decade have made it easier to add such details and compare outcomes across a wider range of scenarios.
To keep the comparison focused, Denton and her collaborators used standard values for the other major parameters. Their proto-Earth had about 88% of Earth’s present mass; Theia had about 13%. They set surface temperatures near, but below, the melting point of surface rocks, consistent with the canonical impact setup. The main change was how the model represented Theia’s material strength.
Without that strength, the collision is effectively treated as a meeting of fluid bodies. Denton described the result as material going “bwah, like blobs in a lava lamp.” Adding a solid outer layer changes how the collision unfolds. “But if you implement material strength, you have a crunchy outer layer,” she said. A hot interior and cooler exterior can then deform in different ways, allowing Theia to split rather than flow as one mass.
The analogy has limits, but it captures the central modeling point: material properties can shape the large-scale result. With enough strength, Theia doesn’t readily become a fluid mass. With less strength, its hot interior and relatively cool surface can still deform together, producing the split that leaves a proto-Moon.
One model, several plausible histories
The strongest-crust cases don’t lead to a direct satellite. When Theia is cool enough, the impact can break it into smaller pieces, like granola, and produce orbiting debris. A second simulation depicts both bodies as fully cooled: Theia remains largely intact after the impact, then recollides with Earth and sends a fraction of its material into a disk.
Those alternatives reflect a broader challenge: the early solar system was complicated, and a giant impact was a chance event that cannot be replayed. Stewart said researchers remain limited by the computing needed to explore all the possible paths to an Earth-Moon system. “It is possible that we will not have one answer, and that’s okay,” she said.
The next step Denton identifies is to work with geochemists and see how chemical evidence can narrow the range of material-strength simulations. Such constraints may help researchers judge which of the modeled histories better fits the real Earth and Moon. For now, the study’s clearest contribution is to show that the Moon’s origin could depend on whether Theia arrived hot enough to deform or cold enough to break apart. That makes the young planet’s temperature and crust a central part of the formation story.
—Matthew R. Francis (@BowlerHatScience.org), Science Writer
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