A new model suggests the moon formed from three large impacts on early Earth rather than a single giant collision. This approach better explains the compositional similarities between Earth and the moon. Researchers propose that moonlets from these impacts merged over time to create our satellite.
The origin of the moon has long puzzled scientists, with the traditional view pointing to a massive collision between Earth and a Mars-sized body called Theia about 4.5 billion years ago. This event, occurring when the solar system was chaotic with frequent impacts, would have ejected debris that coalesced into the moon. However, the striking compositional similarities between Earth and the moon challenge this single-impact scenario, as the moon should contain more material from Theia.
Philip Carter at the University of Bristol, UK, highlights this issue: “This is a big problem for the canonical model.” In a study published in Monthly Notices of the Royal Astronomical Society (DOI: 10.1093/mnras/staf2084/8342140), Carter and colleagues propose an alternative: a series of three or more large impacts over a few million years. These involved objects ranging from the moon's current size to nearly Mars-sized, each generating a small moonlet in Earth's orbit.
Over thousands of years, these moonlets would gravitationally attract and collide, merging into one large moon. Carter explains: “They will attract and collide with each other. It’s very unlikely you’d end up with a stable system with multiple large moonlets.” Unlike earlier multiple-impact theories requiring up to 20 collisions, this model achieves sufficient orbital mass after just three impacts. “After three impacts, we put enough mass into orbit to make a full moon,” Carter notes.
Robert Citron at the Southwest Research Institute in Colorado views fewer impacts positively, as more could disrupt moonlet stability. Yet, additional impacts help average impactor compositions, aligning closer with observed Earth-moon similarities. “When you have multiple impacts, you’re averaging more of these impactors,” Citron says.
The Earth-moon system stands out in the solar system, with the moon unusually large relative to Earth compared to other planets' satellites. Carter favors this model but calls for advanced simulations to assess impact details and ejected material. “To actually calculate everything in detail is still really hard to do,” he adds. This theory underscores the moon's unique formation amid early solar system turmoil.