How fast did the Moon form?

New simulations show the Moon could have formed as an intact body in approximately five hours if early Earth and Theia were cool enough at the time of impact. Previous models assumed gradual assembly from a debris disk over millions of years. The outcome depends critically on the temperature — and therefore material strength — of the colliding bodies.

Why does temperature matter in Moon formation simulations?

Temperature controls how rock responds to extreme stress. A hot planet behaves like a thick fluid and shatters easily into dispersed debris. A cooler planet retains structural strength and resists fragmentation. Previous Moon-formation simulations ignored this variable, systematically biasing results toward the debris-disk outcome.

Did scientists find proof that the Moon formed in five hours?

No. The five-hour result is a physically plausible alternative produced by more realistic simulations, not a confirmed history. The thermal state of early Earth and Theia at the moment of impact is not known with precision, so both fast and slow formation scenarios remain open. Future work will attempt to match these scenarios against the Moon's actual chemical record.

How does the Moon form without a debris disk?

When the colliding bodies are cool, their rock retains enough structural strength that the ejected material does not disperse into a diffuse disk. Instead, the most consolidated ejecta remains gravitationally coherent and forms a single bound object almost immediately — the intact Moon — rather than a cloud of fragments that must slowly accumulate over millions of years.

The Moon may have formed intact in just five hours after the giant impact

For decades, the Moon’s origin seemed settled in broad strokes: a Mars-sized body called Theia struck the early Earth, and the resulting cloud of molten debris gradually coalesced into the satellite we orbit today. New simulations overturn that gradual picture. When the calculations account for a physical property every previous model set aside — the temperature-dependent strength of rock — they produce an intact Moon within approximately five hours of the impact, not a debris disk that takes millions of years to assemble.

The difference between the two outcomes is not new data about the ancient solar system. It is a correction to the physics already in the models.

What the old models missed

The standard giant-impact picture has Earth and Theia colliding at high speed, the two bodies shattering and mixing, and a massive disk of material forming in Earth’s orbit. That disk, over time, draws together under gravity into the Moon. The picture is internally consistent — but it rests on a simplification: prior simulations modeled the colliding planets as though they behaved like fluids, ignoring how the temperature of the rock at the moment of impact affects its ability to hold together.

The team from the Southwest Research Institute and the University of Arizona, led by Dr. Adeene Denton, tested what happens when that assumption is dropped. In reality, planetary bodies that are still hot from their own formation behave more like viscous fluids — they flow and shatter easily under extreme stress. Bodies that have had time to cool retain structural integrity; they resist deformation rather than simply fragmenting. This distinction, which depends on the thermal state of early Earth and Theia at the moment of their collision, turns out to control the fundamental outcome of the impact.

How the new simulations work

The team ran dozens of simulations across a range of initial surface temperatures for the colliding bodies. When both Earth and Theia are modeled as hot — around 2,000 kelvin — the result is the classical picture: material is disrupted, a debris disk forms in orbit, and the Moon would assemble over an extended period. The warm-fluid assumption and the gradual-disk outcome go together.

At cooler temperatures — around 400 kelvin — the outcome changes completely. The lower temperature means higher material strength. When Theia strikes, the ejected material does not disperse into a diffuse disk. Instead, a coherent, gravitationally bound object forms almost immediately from the most consolidated ejecta. The simulation shows this intact body emerging within approximately five hours of the collision.

The five-hour result is not a product of slower assembly or a finer clock. It is the direct output of a physically more realistic model: when rock is strong enough to resist fragmentation, the Moon does not need to be built from scratch from dispersed debris — it forms as a unit.

What intact formation in five hours actually changes

The speed of formation matters because it affects composition. One of the long-standing challenges for any Moon-formation model is explaining why Earth and the Moon have nearly identical isotopic signatures — their oxygen and other light elements look almost the same. That similarity is hard to reconcile with a model in which Theia and Earth are thoroughly mixed in the disk, because Theia, formed elsewhere in the solar system, should have had a different isotopic composition.

Rapid intact formation might sidestep part of that problem: if the Moon coalesced quickly from material that was not fully mixed with Theia, it could preserve a more Earth-like composition. The researchers are careful not to overclaim here. The compositional similarity between Earth and Moon, they write, remains an open scientific question, and the new simulations do not resolve it. The intact-formation pathway opens a physically plausible scenario; it does not confirm that this is how our Moon formed.

What the simulations do not settle

The largest open question is the one the simulations cannot close on their own: the actual thermal state of early Earth and Theia at the time of impact. Nobody knows what temperature those bodies were at when they collided. The answer depends on when the impact happened relative to when each body formed — itself poorly constrained — and on how quickly planetary interiors cool after formation.

Both the hot scenario and the cooler one remain physically plausible. The new work does not eliminate the debris-disk model; it shows that the outcome depends critically on a variable that prior simulations assumed away. Which scenario matches the real early solar system is something future measurements and improved thermal histories will need to determine.

The simulations also simplify the internal structure of the impacting bodies. Real planets are compositionally layered, with dense metallic cores, silicate mantles, and crustal variations that no current simulation fully captures at planetary scale. The five-hour result is a robust signal within the model — it appears consistently across a range of cooler-temperature scenarios — but a model is still a model.

The work also leaves open how to connect the thermal state of early Earth and Theia to the timing of the impact. The team notes that these connections could eventually help constrain when the Moon-forming event occurred, but that link remains to be developed.

Common questions about Moon formation

What is the giant impact hypothesis?

The giant impact hypothesis proposes that the Moon formed when a Mars-sized body called Theia struck the early Earth roughly 4.5 billion years ago. The collision flung material into orbit, which eventually became the Moon. It remains the leading explanation for the Moon’s size, its orbital properties, and the similarity of its composition to Earth’s mantle.

What does it mean that the Moon formed ‘intact’?

In this context, intact means the Moon emerged as a single, gravitationally bound body almost immediately after the impact — not assembled gradually from a dispersed disk of orbiting debris. The new simulations show that when the colliding bodies are cool enough for rock to retain material strength, the Moon-like object forms as a coherent unit within hours rather than taking millions of years to build up from scattered fragments.

Why does the temperature of the colliding bodies matter?

Temperature controls how rock responds to extreme stress. A hot planetary body behaves like a thick fluid — it deforms easily and shatters into fine debris. A cooler body retains structural integrity and resists breakup. This difference determines whether the giant impact produces a disk of dispersed material or an intact body. Prior models treated the colliding planets as fluids regardless of temperature, which systematically biased the outcome toward the disk scenario.

Does this change when the Moon formed?

Not directly, but it opens a path toward better constraints on timing. If the thermal state at the moment of impact determines the formation mechanism, and if that thermal state can be tied to the age of the colliding bodies, future analysis may be able to narrow down when the giant impact occurred. The team identifies this as a direction for follow-up work.

How confident are scientists that the five-hour scenario is correct?

Not yet confident — it is a physically plausible alternative, not a confirmed history. The five-hour result is robust within the cooler-temperature simulations, but whether early Earth and Theia were actually at those temperatures when they collided is not known. Both the fast and slow formation scenarios remain open, and distinguishing between them will require connecting these simulations to the Moon’s actual chemical and isotopic record.

The next phase of the research aims to test whether specific thermal scenarios can be matched against the Moon’s chemical and isotopic record, which could eventually distinguish between the fast-formation and disk-assembly models with real observational evidence.

Reference: Denton et al., “Temperature-Dependent Material Strength Controls Giant-Impact Moon Formation Outcomes,” The Astrophysical Journal Letters, 2026. DOI: 10.3847/2041-8213/ae91e9

Tags: astronomy, moon, giant-impact-hypothesis, moon-formation, planetary-science, southwest-research-institute