The Moon May Have Formed in Just 5 Hours After a Giant Impact
About 4.5 billion years ago, Earth had a very bad, very short day. A Mars-sized body slammed into it hard enough to fling a ring of molten debris into orbit. New simulations suggest that debris may have pulled itself together into a fully formed moon before that day was even over.
Revisiting a 25-Year-Old Model
New modeling led by the Southwest Research Institute, in collaboration with scientists at the University of Arizona, shows fundamental differences in how the moon may have formed from the giant impact that created the Earth-moon system. The research, published in The Astrophysical Journal Letters, used state-of-the-art computational techniques that factor in the material strength of the two colliding bodies, something earlier models had left out entirely.
The giant impact scenario has been the leading theory of lunar formation since a foundational 2001 paper by Dr. Robin Canup, now vice president of SwRI’s Solar System Science and Exploration Division, and Dr. Erik Asphaug, a University of Arizona professor and co-author of the new study. That original research proposed that a Mars-sized body known as Theia struck the young Earth at just the right angle to throw debris into orbit, material that eventually became the moon. What that model and the giant impact simulations that followed left out was material strength, a factor long assumed to be irrelevant given the sheer violence of an impact on this scale.
Why Temperature Turned Out to Matter
Dr. Adeene Denton, formerly a NASA Postdoctoral Program fellow at SwRI and now a postdoctoral researcher there, led the team that decided to test that assumption. “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.”
“Models have evolved to include material strength, something that’s really important when you’re studying collisions between smaller bodies like asteroids, or for my previous paper about the formation of the Pluto-Charon system,” Denton said. “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.”
Hot Bodies, Weak Bodies
The physical principle behind the finding is straightforward: hotter bodies are weaker than colder ones. Since planets and protoplanets start out hot from their own formation and cool gradually with age, the internal temperature of both Earth and Theia at the moment of collision would have depended heavily on exactly when that collision occurred.
The team found that moon formation is highly sensitive to the temperatures of the two colliding bodies. Some scenarios in their simulations produced a fully intact moon within hours of impact. Others produced a protolunar disk of debris around Earth that only assembled into the moon gradually, over a much longer stretch of time. Because that outcome depends directly on how hot the two bodies were, and therefore on how long after the solar system’s formation the impact took place, the findings establish a new and testable connection between the timing of the giant impact and the physical state the moon started out in.
An Intact Moon in About Five Hours
“Depending on how hot the Earth and Theia 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 I 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.”
That kind of rapid, intact-moon outcome had shown up in prior simulations before, including NASA-led work in 2022. What makes this study different is that it’s the first to demonstrate that material strength and temperature play a central role in determining which outcome actually occurs, a rapidly assembled intact moon, or one built gradually from a longer-lived debris disk.
Two Very Different Scenarios
The two possible outcomes hinge on just how much Theia had cooled by the time of impact. If Theia was colder, meaning the collision happened later in the solar system’s history, perhaps 100 to 150 million years after the planets first formed, the resulting debris ring would have taken shape in a way that allowed the moon to accrete gradually. Because a colder Theia is also a stronger one, more of the impactor would have survived the collision intact, merging with Earth rather than being destroyed outright.
A warmer Theia, striking Earth less than 60 million years after the solar system’s planets formed, produces the opposite result: a scenario in which the moon can form very quickly while Theia itself is essentially obliterated in the process.
Independent Praise, and an Open Door to New Questions
Robin Canup, who was not involved in the new study, called the results both surprising and significant for the wider field. “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,” Canup said. “This in turn might help scientists better constrain when the moon-forming event occurred.”
In both scenarios modeled by Denton’s team, the moon ends up composed mostly of material from Theia’s mantle, with only a small contribution from Earth’s own mantle. That finding sits somewhat awkwardly alongside one of the giant impact theory’s long-standing puzzles: the moon’s composition actually looks remarkably similar to Earth’s mantle in several respects, even as isotopic measurements reveal subtle differences that current simulations, including this one, still can’t fully explain. As for the compositional relationship between Earth and its planetary neighbors, Denton offered a memorable way to think about it. “Because Earth and Mars formed in the same neighborhood of the solar system, they are like siblings,” she said. “The moon and Earth are more like fraternal twins.”
What This Could Mean for Moons Around Other Planets
The findings carry an unexpected implication for the search for exomoons, moons orbiting planets beyond our solar system. If large rocky moons really can form within hours of a giant impact, then the protolunar disks capable of producing them would be extremely short-lived, making them exceptionally difficult to actually catch and observe around distant terrestrial exoplanets.
Moons forming around gas giant exoplanets would follow an entirely different process, since those disks form from leftover material during planetary assembly rather than from a violent impact, meaning this particular observational challenge would be specific to rocky, Earth-like worlds.
A Method Built for Smaller Collisions, Now Applied to the Biggest One
The study relied on giant impact simulation methods originally developed at the University of Bern and the University of Arizona. Denton’s own prior use of material-strength modeling on the much smaller Pluto-Charon system helped establish the technique before it was ever applied to a collision on the scale of the moon-forming impact.
That such a well-established, decades-old theory could still yield genuinely new insight, simply by adding a factor once assumed to be negligible, underscores how much remains to be worked out about the violent, chaotic day that gave Earth its only natural satellite.
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