More than one way to smash the Earth and build our Moon

UCSD.edu

The Moon is a bit of an enigma. In some ways, it's nothing like Earth, as its minerals contain few volatile chemicals and it has a relatively tiny core. But in other ways, it's nearly our twin, with many elements having isotopic signatures that are almost identical.

Currently, our best model for Moon formation involves having a Mars-sized object smack in to the early Earth. This could create a Moon that has some similarities to the Earth, but ends up with most of the iron from the impact being deposited in the Earth's core. The only problem with this is that anything as big as Mars probably originated from elsewhere in the Solar System, and thus would have a very distinct isotope ratio.

Today, Science is releasing two papers that take very different routes to tackling this problem. One models what would happen if, instead of a large size difference between the Earth and its impactor, the two bodies were of roughly equal size. Another models two differently sized bodies colliding, but assumes the proto-Earth was spinning much faster than it is now, with "days" on the order of 2.5 hours long. And, in a dilemma that may interest planetary scientists, both models produce the sort of distribution of materials we currently see.

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