Image of a small object smashing in to a cutaway off the Earth.

Enlarge / An early version of the collision model, showing a head-on impact.

The Earth and its moon are unique in our Solar System. Earth is the only rocky planet with a large moon, and only the dwarf planet Pluto has a moon that’s so similar in size to its host planet. The Moon is also remarkably similar to the Earth in terms of its composition, suggesting they formed from the same pool of material instead of the Moon forming elsewhere and having been captured.

This collection of properties led to a number of ideas about how the Moon formed, all of which failed to fit the data in various ways. Eventually, however, scientists came up with an idea that seemed to get most of the big picture right: a collision between the Earth and a Mars-sized object happened early in the Solar System’s history, creating a cloud of debris that coalesced into the Moon.

While that got the major features of our two-body system right, there were still some subtle differences that weren’t resolved by the impact model. Now, a team of Japanese researchers say that there’s a way to tidy up some of these loose ends: having the impact take place while the Earth was covered in a molten magma ocean.

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