How did Earth get its own Moon? For decades, the most popular origin story has been the "Big Splat" theory, which says an object the size of Mars impacted the Earth more than three billion years ago, flinging material into space which became the Moon.

But now there's new evidence that the Big Splat theory may be incorrect — and the impact that created the Moon may have been more like a hit-and-run accident.

The Big Splat theory claims a planet the size of Mars struck Earth — and it's still with us today. The planet, known as Theia, merged with Earth, like two balls of bread dough, and most of Theia's mass sunk into the center of the Earth. Meanwhile, the impact of the collision threw an enormous amount of material was flung into space. This eventually coalesced into the body that became our Moon.


The Big Splat theory is supported by the fact that while the Moon has an iron core like the Earth, it does not have the same fraction of iron... just what the would be expected from the Theia impact. The Moon should also have a composition a little different than that of the Earth.

Unfortunately, samples taken from the Earth and from the Moon show that the ratio of the Earth's and the Moon's oxygen isotopes is nearly identical. And a fresh analysis of lunar samples taken by the Apollo missions show that the Moon and the Earth share a very similar isotope ratio of the metal titanium... All which should not be the case, if the Big Splat theory is correct. Maybe, it was suggested, the Moon was somehow split from the earth itself, like a sea anemone budding.


Andreas Reufer, of the Center for Space and Habitability in Bern, Switzerland, and his colleagues have proposed a variation on the Big Splat. They have suggested that instead of a collision with a slow-moving body like Theia, a much larger and faster-moving body hit the earth a glancing blow. Unlike Theia — which would have been largely sacrificed in the creation of our Moon — this new body would have lost only a small amount of material in the collision. After smacking the Earth, the planet would have continued on its way, like a hit and run driver fleeing the scene of an accident.

The result of this glancing blow would have been a much hotter disc of debris circling the Earth, but with still enough material to have coalesced into a body the size of the Moon. Since most of this material would have originated from the Earth, this would neatly explain the similarities between the isotope fractions.

One of the originators of the original Big Splat theory, Dr. William K. Hartmann, says, "An interesting thing is that the higher the velocity, the smaller the projectile needs to be. And the smaller the projectile, the higher the percentage of Earth material in the final swarm; this could help explain why the lunar material looks so Earth-like. On the other hand, it may be harder to preserve the tiny amounts of water that are turning up in lunar volcanic glasses, in a high-energy collision."

Regarding the new "hit and run" theory, Hartmann feels that it helps vindicate and expand upon the earlier one rather than entirely replace it. The original idea, he says, "explains why the isotope ratios of chemicals in earth and moon seem to be virtually exactly the same (as we posited the ideal projectile would be a planetesimal growing in the same general zone as earth, at about 1AU, so it would have an Earth-like composition. The new paper is also talking about a planetesimal hitting Earth, so the basic premise is the same. No need to step too far aside; I'm happy to applaud from the shadows while modestly clearing my throat occasionally. Ahem, ahem."

Read the full study in Nature Geoscience.