Simulations that include rock strength change how the Moon may have formed

# What changed in moon-formation modeling

Researchers at the Southwest Research Institute and the University of Arizona reran giant-impact simulations including material strength that depends on temperature. Previous models approximated the colliding bodies as fluids, assuming the violence of the impact would melt or vaporize them. The new models use an advanced smoothed particle hydrodynamics (SPH) approach with a strength model developed at the University of Arizona and the University of Bern.

# How strength and temperature alter outcomes

The updated simulations treat rock and metal as materials that resist deformation, and make those properties weaken as temperature rises. That produces two qualitatively different outcomes depending on the thermal state of the bodies:

• If the colliding bodies are colder and therefore stronger, the simulations can produce an intact Moon within hours of the collision.

Adeene Denton, who led the work, summarized the shift: "We discovered that the preexisting geology of the Mars-sized proto-moon matters. 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."

# Why this matters for timing and composition

Protoplanets cool as they age. By making the Moon's formation sensitive to temperature and material strength, the new results connect the thermal histories of Earth and Theia to how and when the Moon formed. Canup, commenting on the study, said the results "might help scientists better constrain when the moon-forming event occurred."

The paper also raises a potential link between the Moon's physical properties today—such as volatile content—and the thermal state at impact. If the Moon formed largely intact, versus being processed in a hot disk, that could influence how much volatile material it retained.

# What remains unresolved

The longstanding problem that Earth and the Moon have very similar isotopic and compositional signatures is not solved by these simulations. The study changes the plausible mechanical pathways to Moon formation but does not by itself explain why Earth and Moon materials match so closely.

Erik Asphaug, a coauthor, argued the field should reconsider the fluid approximation: "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. Based on our new results, however, we think that it is time to reconsider that."

# What the simulations add to the debate

These findings update the giant impact scenario by:

• Reintroducing material strength as a meaningful factor at planetary scales when temperatures are not uniformly extreme.
• Showing that identical input parameters but different temperature structures can lead to very different outcomes—intact Moon versus protolunar disk.
• Providing a testable connection between the Moon's present-day properties and the timing/thermal state of the impact.

# Bottom line

Adding realistic, temperature-dependent rock strength to SPH impact models changes how the Moon could have formed. The models expand the range of credible outcomes and tether those outcomes to the thermal histories of Earth and Theia. They do not yet resolve compositional similarities between Earth and the Moon, but they provide a new lever—material strength and temperature—to refine constraints on when and how the Moon formed.