New supercomputer simulations suggest the Moon could have formed intact within five hours

For a quarter century, the leading explanation for how the Moon formed has rested on a simplifying assumption: that the ancient collision between the young Earth and a Mars-sized body known as Theia was violent enough to treat both worlds as fluids, sloshing and merging like liquids rather than behaving like solid rock. A new study led by planetary scientist C. Adeene Denton of the Southwest Research Institute, published this month in The Astrophysical Journal Letters, argues that assumption has been hiding something important — and that under the right conditions, the Moon could have condensed into an intact body within about five hours of the impact, rather than slowly accreting from a disk of debris over months.

What the new simulations found

Denton's team, working with Erik Asphaug and Namya Baijal at the University of Arizona's Lunar and Planetary Laboratory and Robert Melikyan, built the first major giant-impact simulations to incorporate the temperature-dependent mechanical strength of rock and metal, rather than modeling Theia and early Earth purely as fluids. That distinction turns out to matter more than expected. Using the same impact speed, angle and mass ratio as earlier landmark models, the researchers found that a hot-but-solid version of Theia resisted deformation on impact in a way a fluid model could not capture — and an intact, Moon-mass remnant was captured into orbit within roughly five hours. Run the same collision with a colder, mechanically stronger Theia, and the outcome flips: the impact instead produces the classic debris disk that has to slowly coalesce into a moon, the scenario most prior models assumed by default.

In the "with-strength" hot scenario, Earth directly absorbed about 84 percent of Theia's mass, with most of the remainder captured intact as the proto-Moon rather than scattered into orbiting rubble. The Southwest Research Institute's announcement of the findings frames the discovery in plain terms: the pre-impact temperature and geologic state of the two colliding bodies is a variable that determines whether the Moon was born whole in hours, or built up gradually from wreckage.

"The Moon and Earth are more like fraternal twins," said Denton, describing how differently the two origin stories play out depending on conditions that earlier models simply assumed away.

Why a 25-year-old assumption held for so long

Asphaug, a co-author and veteran of the field, said the fluid approximation was never arbitrary — it followed from the sheer violence of the event. "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," he said, according to the University of Arizona's account of the research. What the new work shows is that even amid partial melting and vaporization, the strength of the material that stayed solid still shaped how momentum transferred between the two bodies — a factor smoothed-particle hydrodynamics simulations going back to the field's 2001 foundational models had not built in. "We now know that the geophysical state of Earth and Theia play a fundamental role in shaping the outcome of the collision," said Baijal, a doctoral student on the project.

The result does not stand entirely alone. A separate 2022 study led by NASA researchers, using a different high-resolution fluid technique, had already proposed that some giant-impact scenarios could produce an intact Moon within hours rather than months — without invoking solid-body strength at all. The new SwRI-led work arrives at a similar timescale through a different physical mechanism, which researchers in the field say strengthens the case that "hours, not months" is a genuinely plausible outcome for at least some of the conditions the early solar system could have produced, rather than an artifact of one team's modeling choices. Space.com's write-up of the new paper noted that the two independent lines of research, arriving five years apart through different methods, is itself part of what has caught other planetary scientists' attention.

Why it matters beyond one collision

The stakes go beyond satisfying curiosity about a single ancient event. Planetary scientists have long struggled to explain why the Moon and Earth share nearly identical oxygen-isotope signatures — implying the two bodies mixed thoroughly during formation — while the Moon is simultaneously depleted in volatile elements, has almost no iron core, and is enriched in refractory elements relative to Earth. No single giant-impact model has cleanly reproduced all of those features at once. Denton's team suggests that whether the Moon formed intact or from a disk could eventually help researchers work backward to estimate when, in the solar system's early and chaotic history, the Theia collision actually happened, since a collision between hotter, still-forming bodies would behave differently than the same impact occurring after both had cooled.

That link between formation pathway and timing is the study's proposed contribution to a debate that has run since the Apollo era: not a final answer to how the Moon formed, but a new lever future researchers can pull, testing additional impact angles and velocities against the geochemical evidence Apollo and, eventually, NASA's Artemis missions have collected from the lunar surface itself.

What comes next

The authors are explicit that their model does not resolve the Earth-Moon compositional puzzle on its own. The next round of work, they say, will extend the strength-dependent approach to a wider range of impact angles and speeds, and attempt to match the results against the specific isotopic and compositional fingerprints scientists have already measured in lunar samples — the same test that has eluded fluid-only models for 25 years.

That next phase matters because the giant-impact hypothesis, for all its dominance, has never been a settled theory so much as the best available explanation among several imperfect ones. Rival models involving multiple smaller impacts, or a slower process of co-accretion, have each been proposed at various points specifically because no single-impact model has cleanly explained both the isotopic similarity and the compositional differences between Earth and its Moon. Denton's team is not claiming to have ended that debate. What the new simulations add is a previously unexamined variable — the temperature and strength of the colliding bodies themselves — that future models, including rival ones, will now need to account for rather than assume away.