New computer simulations suggest the Moon could have formed in as little as five hours following a catastrophic collision between early Earth and a Mars-sized protoplanet named Theia. Published on September 1 in The Astrophysical Journal Letters, the study highlights how the material properties of the two bodies may have accelerated the formation process.
Scientists widely accept the giant impact hypothesis, which posits that the Moon was created when Theia struck the young Earth roughly 100 million years after the Sun’s birth, about 4.5 billion years ago. This event was pivotal for life on Earth, potentially delivering key elements and stabilizing the planet’s environment. While lunar samples from NASA’s Apollo missions support this theory, determining the exact duration and mechanism of the Moon’s formation has remained difficult.
Adeene Denton, a geologist and planetary scientist at the Southwest Research Institute in Boulder, Colorado, and lead author of the study, noted that previous models focused primarily on the masses of the colliding bodies. However, this new research emphasizes the role of geological material strength, specifically surface hardness, which had been largely overlooked in planetary-scale impact simulations.
“When you simulate the Earth and the moon as colliding bodies with [different] geologic properties, it changes how the moon forms out of that impact — that’s something we considered unnecessary before,” Denton said.
The team found that because both Earth and Theia were extremely young and hot at the time of impact, their surfaces were likely soft or molten. This thermal state acted as a cushion, allowing debris to remain intact and coalesce rapidly. In contrast, earlier versions of the giant impact hypothesis suggested the collision occurred later, when the bodies would have cooled and become more brittle, resulting in a longer formation period due to a larger field of fragmented rock.
Under the most extreme thermal scenario modeled, where both bodies were at their hottest, the Moon took shape in just five hours. Researchers acknowledged that the exact temperature of the proto-Earth and Theia remains unknown, making this five-hour figure one possible outcome among many.
After its rapid formation, the young Moon was covered by a magma ocean that solidified over hundreds of millions of years. Initially close to Earth, it existed as a volatile, volcanic landscape before gradually moving away. Today, the Moon is geologically inactive and continues to drift farther from Earth, though it may eventually escape Earth’s gravity if the Sun does not destroy both bodies in approximately five billion years.
The findings underscore the importance of incorporating material strength into impact modeling, potentially reshaping understanding of other cosmic collisions, such as those that affected Jupiter’s moon Ganymede or formed Saturn’s moon Titan. Additionally, the research has implications for the search for exomoons; if debris fields dissipate quickly, as they did here, identifying alien satellites around distant planets may require different observational strategies.
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