Scientists have proposed a fresh idea in the long-running debate about how the Moon was formed. What is certain is that some sort of impact from another body freed material from the young Earth and the resulting debris coalesced into today’s Moon. But the exact details of the impactor’s size and speed have remained debatable. In a report online to be published in Icarus, researchers suggest that the crash happened with a much larger, faster body than previously thought. Such theories need to line up with what we know about the Moon, about the violent processes that set off the creation of moons, and what computer simulations show about the more sedate gravitational “gathering-up” that finishes the job.

In recent years, scientists’ best guess for how the Moon formed has been that a relatively slowly moving, Mars-sized body called Theia crashed into the very young Earth. That would have heated both of them up and released a vast cloud of molten material, much of which cooled and clumped together to give rise to the Moon. That would suggest that the Moon is made up of material from both the early Earth and from Theia, which should be somewhat different from one another.