Researchers at the Southwest Research Institute and the University of Arizona have conducted advanced computer simulations that challenge long-held assumptions about how the Moon formed. The study, published in The Astrophysical Journal Letters, examined the giant impact scenario in which a Mars-sized body collided with the young Earth approximately 4.5 billion years ago.
Previous models of this collision treated the two bodies as fluids because scientists believed the impact was energetic enough to melt and vaporize large portions of both worlds. The new research introduces a significant methodological shift by accounting for the material strength and temperature-dependent properties of rocky bodies. Using advanced smoothed particle hydrodynamics simulations developed at the University of Arizona and the University of Bern, researchers found that incorporating realistic geologic properties produces dramatically different outcomes than earlier models.
The simulations revealed that temperature plays a crucial role in determining how the Moon formed. When the colliding bodies were modeled as hotter, mechanically weaker objects with the same thermal structures as in original impact modeling, a fully intact Moon emerged within approximately five hours. In contrast, under other conditions, the collision destroys the impacting body and creates a broad disk of debris that gradually assembles into the Moon over time. This discovery establishes that material strength and temperature are central factors in determining whether the Moon formed largely intact or gradually assembled from processed debris.
These findings could help scientists better understand when the Moon-forming impact occurred by connecting the Moon’s present-day characteristics to the thermal state of Earth and the colliding body at the time of impact. However, a significant mystery remains: Earth and the Moon share remarkably similar compositions, a feature that giant impact models have historically struggled to fully explain. The new simulations do not resolve this compositional similarity puzzle, though researchers suggest both bodies may have formed from material in the same region of the early solar system.
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