The Earth and Moon, Products of Violent Upbringing

Proto-planet Theia collided with Earth in Moon forming event

New research has found the last piece of the puzzle for the accepted Moon formation model, the Giant Impact Theory, making it more water-tight than ever before.

The Giant Impact Theory was first brought forward in the 1970’s as a potential explanation for the formation of the Moon. It suggests that a Mars-sized proto-planet collided with the Earth ~4.5 Ga, generating an accretion disc that later coalesced into the lunar body.

When scientists originally began investigating, they simulated the Moon formation, and found that their model produced a Moon containing >60 wt% material from the impactor, and a smaller contribution from the Earth, ~20 wt%. Additionally, analysis was performed on lunar meteorites and Apollo samples, and results showed that the Earth and Moon were highly similar across a variety of isotopes, including O, Si, Ti and W. However, this posed a problem, as analysis of other solar system bodies had shown large differences in composition compared with the Earth, inferring that our ‘impactor’ and Earth already had a close familial relationship, which didn’t make sense.

However new data may be able to quell this discrepency. Researchers from Israeli and French Universities simulated the late accretion of the solar system using ~90 planetary embryos and 1000-2000 planetesimals, each given a specific 17O value that would vary linearly with distance from the Sun. After 100-200 million years, about 3-4 rocky planets were produced from collisions, each with their own unique isotopic signature, which the team used to infer compositional origin.

Their results showed that the compositions of giant impactors were statistically more similar to the planets they impact, compared to other simulated planets which were distinct. This suggested that assembling an impactor from a larger number of smaller pieces would tend to leave it on a less-elliptical orbit, perhaps reducing its incorporation of more distant, compositionally distinct material.

Although there are still a few minor discrepencies with geochemical data between us and our Moon, the gaping crater on the surface of the GIT has at least been accounted for, hopefully leaving it clear from future bombardment.

Read more: doi:10.1038/nature14333

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Martine Coates

Marketing Executive at Scarab4
Geologist hailing from the University of Southampton with a specialism in marine volcanic sedimentology and the subaqueous emplacement of subaerial volcanic mass flows. Now working for the LLW Repository Ltd in Cumbria, UK as part of the Environmental Monitoring and Site Characterisation team.

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