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The impactor that struck the Earth to create the Moon was no bigger than Mars

It was in the mid-1970s that astronomers proposed the current theory of the Moon's formation: our satellite was created by a giant collision between the Earth and a Mars-sized protoplanet (about a tenth the size of Earth) called "Theia".

The impactor that struck the Earth to create the Moon was no bigger than Mars

Impact between a proto-Earth and Theia © NASA / JPL-Caltech

Publication date: 22/01/2018

Press, Research

Related themes : Origins

The collision produced a cloud of gas and debris, which rapidly condensed to form the Moon. However, this so-called “giant impact” hypothesis now faces a serious problem: it cannot explain why the Moon and the Earth are isotopically identical.

To remedy this, two different models have recently been proposed, which would explain the extent to which the Moon turns out to be a veritable clone of the Earth, but with radically different predictions for the size of the Theia impactor. In the first scenario, two half-Earths would have merged to form the Earth-Moon system. The resulting stars would be of identical natures due to the impactor-impacted symmetry. The second hypothesis suggests that Theia was a small projectile (a few per cent the mass of the Earth) that hit our planet at very high speed.

To distinguish between these two scenarios, a team from the Institut de Physique du Globe in Paris and the Ecole Polytechnique Fédérale de Lausanne set out to determine how each model affects the Earth’s global chemistry. Whether it was a small or large impactor, the latter must have left a chemical imprint in the Earth’s mantle, which differs according to the mass of Theia. The researchers therefore compared these results with geochemical measurements of the Earth’s mantle.

They carried out more than two million simulations of the impact and the resulting chemical equilibrium, varying the mass of Theia but also other parameters including the degree of melting of the Earth before and after the impact, the depth of penetration of the impactor core into the Earth’s mantle and the degree of equilibrium with the mantle. Whatever the scenario, it turns out that an impactor with a mass greater than 15% of the Earth’s – i.e. slightly heavier than Mars – is not consistent with the chemistry of the mantle; it systematically produces a mantle that is too enriched in metals such as nickel and cobalt. Consequently, the Moon must have formed as a result of a collision with a relatively small, high-energy impactor, guaranteeing a terrestrial mantle like the one we observe today and a lunar isotopic composition identical to that of the Earth.

Ref: “Geochemical constraints on the size of the Moon-forming giant impact” – H. Piet, J. Badro, P. Gillet (2017), Geophysical Research Letters, 44, 11,770, 14 December 2017, DOI: 10.1002/ 2017GL075225

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