Researchers at the Southwest Research Institute and University of Arizona have conducted computer simulations examining the giant impact thought to have created Earth’s Moon approximately 4.5 billion years ago. The study, published in The Astrophysical Journal Letters, introduces a factor that previous modeling largely overlooked: the physical strength of the colliding bodies.
Earlier simulations of the Moon-forming collision treated the young Earth and the Mars-sized object known as Theia as fluid-like substances, an assumption based on the extreme energy involved in the impact. The new research challenges this approach by incorporating temperature-dependent geological properties into the models using advanced smoothed particle hydrodynamics simulations. The researchers found that material strength and temperature significantly influence the collision’s outcome, contrary to previous expectations.
The simulations revealed two distinct formation pathways depending on thermal conditions. In some scenarios, the collision destroys Theia and generates a broad disk of debris that gradually assembles into the Moon over time. In other cases, using parameters matching original impact models, a fully intact Moon emerged within approximately five hours. This dramatic difference hinges on how hot the proto-Earth and colliding body were at the time of impact, since hotter planetary bodies are mechanically weaker than cooler ones.
These findings potentially link the Moon’s current properties to the thermal state of Earth and Theia during the giant impact, which could help scientists better determine when the Moon-forming event occurred. However, the new simulations do not resolve a longstanding mystery: Earth and the Moon share remarkably similar compositions, something giant impact models have struggled to fully explain. One possibility is that Theia and proto-Earth formed from material in the same region of the early solar system, whereas Mars, with a different composition, may have originated from a more distant location.
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