New simulations show the Moon may have formed intact hours after Theia impact

Researchers at the Southwest Research Institute and the University of Arizona have modelled the giant impact that created the Moon with material strength factored in for the first time. Published in The Astrophysical Journal Letters, the simulations show the collision could have produced an intact Moon within hours, not just a debris disk. The findings link the Moon's initial state to the temperature of the colliding bodies, which may help pin down when the impact happened.

Bottom line — The new model ties the Moon's formation to the thermal state of Earth and Theia 4.5 billion years ago, the researchers said.

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• The simulations used a version of smoothed particle hydrodynamics developed at the University of Arizona and the University of Bern in Switzerland, which models resistance to deformation for realistic geologic materials.
• Previous giant-impact models, including a foundational 2001 paper by Robin Canup and Erik Asphaug, treated the bodies as fluids, assuming the collision was violent enough to melt and vaporise them, according to the University of Arizona.
• Temperature matters: hotter bodies are weaker, and depending on the thermal state of Earth and Theia, the impact either destroys Theia into a debris disk or produces an intact Moon within about five hours, said Adeene Denton.
• The findings may help explain the compositional similarity between Earth and the Moon — 'fraternal twins' — since Theia and the proto-Earth may have formed in the same region of the solar system, Denton said.
• Robin Canup, who was not involved in the study, said the results could help scientists better constrain when the Moon-forming event occurred.

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