A Moon May Have Taken Shape Within Hours of Earth’s Giant Collision

A new set of computer simulations suggests the Moon may have formed in a surprisingly short time after the collision that created the Earth–moon system—possibly emerging as an intact body within about five hours. The modeling, led by researchers at Southwest Research Institute in collaboration with the University of Arizona, revisits the widely studied theory that a Mars-sized object known as Theia struck the early Earth. The study was published in The Astrophysical Journal Letters. Earlier versions of giant-impact simulations generally treated the colliding planets as lacking material strength, an assumption researchers considered reasonable for such an energetic event. The new work instead included temperature-dependent geological strength in both bodies. That change produced substantially different outcomes. In some simulations, the impact destroyed Theia and created a disk of debris around Earth. Over time, material from that disk assembled into the Moon. In other runs, the collision produced a largely intact moon soon after the impact. The researchers found that the temperatures of Earth and Theia were especially important. Hotter planetary bodies are weaker, while colder ones are stronger. When the simulations used the same temperature structures as earlier impact models, but included material strength, an intact moon appeared within approximately five hours. Other temperature conditions led instead to a protolunar debris disk. According to the researchers, the result could connect the Moon’s initial formation process with the thermal condition of the two planets when they collided. The timing of the impact may therefore have influenced whether the Moon formed directly from the collision or gradually from orbiting debris. The findings could also help researchers investigate properties of the present-day Moon, potentially including its volatile content, although the study does not resolve those questions. Robin Canup of Southwest Research Institute, who helped establish earlier giant-impact modeling but was not involved in this study, said the results may help constrain when the moon-forming event occurred. One major issue remains open: why Earth and the Moon have such similar compositions. The researchers note that Earth and Theia may have formed in the same region of the early solar system, but the simulations do not settle that question.