Forming moon may have taken three big impacts early in Earth’s history

Multiple impacts on Earth might better explain our moon’s origin than a single giant impact 4.5 billion years ago – and could help solve one of its biggest mysteries.

Pinning down the origin of our moon has been hard. The leading idea is that it was formed early in the history of the solar system in the aftermath of an impact between Earth and a giant Mars-sized object known as Theia, which possibly originated closer to the sun than where our planet is. The impact would have thrown debris into space that eventually coalesced into the large natural satellite we see today, at a time when material was more jumbled up around the sun and impacts were common.

But Earth and the moon are surprisingly similar in composition, which makes this model a little difficult because the moon should have retained more material from Theia than Earth. “This is a big problem for the canonical model,” says Philip Carter at the University of Bristol, UK.

Instead, Carter and his colleagues propose that a chain of impacts on Earth over a few million years might better explain why Earth and the moon are so compositionally similar. They show that three or more large impacts on our planet in the early solar system, involving objects ranging from the moon’s current size to nearly the size of Mars, could explain the origin of the moon we see today.

In this scenario, each impact would produce a small moon, or moonlet, in Earth’s orbit. Over thousands of years, these moonlets would gradually combine together under their gravity, forming one large object. “They will attract and collide with each other,” says Carter. “It’s very unlikely you’d end up with a stable system with multiple large moonlets.”

Previous models have also invoked a multiple-impact origin of the moon, but they have required a much larger number of impacts on Earth, up to 20, compared with this latest model. “After three impacts, we put enough mass into orbit to make a full moon,” says Carter.

Robert Citron at the Southwest Research Institute in Colorado says that having fewer impacts “can be better” because the more impacts a model has, the more likely it is that existing moonlets would be kicked out of Earth’s orbit, preventing the moon from forming. However, invoking more impacts leads to a closer compositional similarity between Earth and the moon, better explaining what we see today. “When you have multiple impacts, you’re averaging more of these impactors,” says Citron.

Working out how the moon formed is important because the Earth-moon system is unusual. “It’s such a unique satellite,” says Citron. “It’s very big relative to Earth, whereas the Martian moons are very small compared to Mars, and the satellites of the gas giants are very small compared to those planets.”

More complex modelling is needed to work out which idea is correct, says Carter, including the ferocity of the impacts on Earth and the amount of material thrown into space. “To actually calculate everything in detail is still really hard to do,” he says. “Personally, I favour this multiple-impact model over the canonical single-impact model.”