A Multiple-Impact Origin for the Moon.

• Position: PLANET EARTH

The formation of the moon has remained something of a puzzle. A leading theory proposes a cataclysmic impact involving a Mars-sized object and a young Earth, but there are some inconsistencies with this scenario. A study at the Weizmann Institute of Science, Rehovot, Israel, based on hundreds of simulations run on a computer cluster, suggests that a more plausible chain of events might involve a number of run-ins with smaller objects. This would have produced smaller moonlets that eventually would have coalesced into the single moon we have today.

Oded Aharonson, professor in the Department of Earth and Planetary Sciences, and research student Raluca Rufu point out that the accepted explanations for the formation of our moon rely on highly specific initial conditions--for instance, a collision with an object of a particular size traveling at a defined velocity and hitting Earth at a specific angle.

Furthermore, in a typical impact, different proportions of that object would have ended up in the Earth and the moon, leaving a detectable difference between the bodies. However, various chemical analyses of the moon's makeup, taken from samples returned by astronauts, reveal that it nearly is identical to that of Earth. In other words, there is no trace of the large body that supposedly hit Earth, and the theories, say the researchers, turn out to be improbable.

Aharonson and Rufu, together with Hagai...