The Moon May Have Formed Intact Just Hours After a Giant Collision

Earth's moon is unusually large compared with other rocky planets in the solar system, and scientists still do not fully understand how it formed. A new study by researchers at the Southwest Research Institute (SwRI) and the University of Arizona, published in The Astrophysical Journal Letters, revisits the giant impact in which a Mars-sized body called Theia is thought to have struck the young Earth about 4.5 billion years ago.

The leading explanation for the Moon's origin is the giant impact theory: Theia crashed into early Earth, was destroyed, and its debris formed a disk that eventually became the Moon. A foundational 2001 study, led by Robin Canup of SwRI and Erik Asphaug of the University of Arizona, now a co-author of the new study, and many later models treated the colliding bodies as fluids, assuming the impact melted and vaporized much of both worlds.

Lead author Adeene Denton, a former postdoctoral researcher at the Lunar and Planetary Laboratory now at SwRI, tested that assumption using smoothed particle hydrodynamics (SPH) simulations, developed at the University of Arizona and the University of Bern in Switzerland, that for the first time include the two ancient bodies' temperature-dependent material strength. Denton said the idea came to her while working on an earlier study of the Pluto-Charon system's formation.

Temperature turned out to matter a great deal, since hotter bodies are mechanically weaker than colder ones. In some simulated conditions, the impact destroyed Theia and created a broad protolunar disk that gradually assembled into the Moon. But when Denton used the same parameters as the original impact models, including equal temperature structures inside both bodies, an intact Moon emerged within about five hours, rather than forming slowly from processed debris.

One long-standing puzzle remains unsolved: Earth and the Moon have remarkably similar compositions, something giant impact models have long struggled to explain, and the new simulations do not resolve that problem either.