Where the Moon Came From

Moons are usually small next to their planet. Mars has two, both irregular chunks just a few dozen kilometers across. Jupiter's moons are large, but Jupiter itself is a giant. Yet above Earth hangs a sphere more than a quarter as wide as Earth. No other planet in the Solar System has a ratio like this.

Simple explanations were tested one by one. Capture: Earth pulled in a passing body. But to stay in orbit, the visitor would need to lose speed somewhere, and space around Earth is too empty for that. Co-formation: Earth and the Moon formed side by side from the same dust. Then their composition would be similar throughout, but the Moon has almost no iron core. Fission: Earth spun so fast that it flung off a piece of itself. For that to happen, a day would have had to last a couple of hours.

That left the option long considered too violent. In the mid-1970s, calculations showed that if a body roughly the size of Mars grazed the young Earth, the result looked much like what we see. This body is called Theia. The impact was glancing, so it blasted mostly material from the upper layers into orbit - rock with almost no iron.

The Apollo missions brought back about 382 kilograms of lunar rock. Every body in the Solar System has its own ratio of oxygen isotopes - Mars has one, asteroid meteorites another. In lunar samples, it matched Earth's. Add the Moon's low density: there is almost no heavy iron inside, but there is material like Earth's mantle.

But this neat picture has a downside. Theia came from somewhere else, so its isotopic signature should noticeably disrupt the pattern. Yet it isn't found. So the model is being refined: material mixed more thoroughly after the impact than first thought, or Theia formed nearby from the same material. The debate is not about the collision itself, but its geometry.

A scale like this is hard to wrap your head around: there is no familiar image for it. It helps to slow down and look at the Moon as a piece of familiar rock.