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A computer simulation that traces the moon鈥檚 formation in high resolution may explain the mystery of why it is so chemically similar to Earth.

The conventional story for the moon鈥檚 origin is that a primordial planet named Theia smashed into Earth and spewed molten rock into space. This debris, primarily made up of Theia, then coalesced into the moon over tens of millions of years.

While this scenario accounts the moon鈥檚 observed angular momentum, it fails to explain the near-identical profile of the moon鈥檚 isotopes to those found on Earth. Isotopes are atoms of the same element that differ by the amount of neutrons they contain, and the ratio of different isotopes in a sample can be used to trace where it originated from.

at Durham University, UK, and his colleagues ran more than 400 high-resolution simulations of what might have happened when the early Earth was struck, using different initial conditions like impact angle and speed. The model simulated the behaviour of hundreds of millions of particles to map the collision, far more than previous attempts, which had used at most hundreds of thousands.

Many of the simulations produced satellites similar to our moon within hours 鈥� much more quickly than other models have suggested. They resulted in a moon with a similar angular momentum and isotopic make-up to Earth, which means that the moon contains more of Earth and less of Theia than previous models. The findings were presented at the Lunar and Planetary Science Conference in The Woodlands, Texas, on 10 March.

鈥淚t鈥檚 interesting that simulations in this work lead to more mixing between the impactor and proto-Earth than previous work suggested,鈥� says at the University of Rochester in New York. 鈥淭his would help explain the isotopic similarities between Earth and the moon.鈥�

One way to test the model鈥檚 validity would be by looking at the volatile elements that it predicts for the moon, says Nakajima. The moon contains less of these easily vaporisable elements like potassium and sodium that are found on Earth, which in conventional models is thought to be a result of the moon鈥檚 long coalescence from debris. 鈥淚t would be interesting to investigate whether their model can explain the observation that the moon is depleted in volatiles,鈥� says Nakajima.

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