Moon May Have Formed in Just Five Hours After Giant Impact, Simulations Suggest

New simulations suggest the moon could have formed within hours of the giant impact between Earth and a Mars-size protoplanet called Theia, rather than over a much longer period, according to a team led by Adeene Denton of the Southwest Research Institute (SwRI). The giant-impact scenario has been the leading theory of the moon's formation since planetary scientist Robin Canup began refining computer simulations of it in 2001.

Denton's team ran detailed simulations of the collision that included material strength — how resistant a body is to deforming, which depends partly on how hot it is. "We weren't sure it would matter for the moon or not," Denton said. "When we did the simulations, we found it actually matters quite a bit." Warmer bodies are weaker than colder ones, and both Earth and Theia would still have been hot inside from their own formation at the time of impact.

The outcome depends on when the collision happened and how much Theia had cooled by then. A colder Theia — with the impact occurring 100 million to 150 million years after the planets formed — would leave a debris ring that let the moon accrete gradually, with more of Theia surviving intact. A warmer Theia, hitting Earth less than 60 million years after the planets formed, would be destroyed in the impact, and the moon could form very quickly. "Within around five hours, an intact moon emerged," Denton said of that scenario.

Robin Canup, who was not involved in the new study, said the results "imply a potential connection between the physical properties of the moon today ... and the thermal state of the Earth and Theia at the time of the giant impact." In both scenarios, the moon forms mostly from Theia's mantle material with only a little of Earth's mixed in, though differences in isotope levels between the moon and Earth's mantle remain unexplained.

The findings were published Sept. 1 in The Astrophysical Journal Letters.