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An incredible likeness of being

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Earth and the Moon share many puzzling chemical similarities. New analyses show that the last planet-sized body to hit Earth could have been similar enough to Earth to yield a Moon with an Earth-like composition. See Letter p.212

Some 4.5 billion years ago, our planet grew through a series of violent collisions with other planet-sized bodies, the last of which is thought to have produced the Moon. The impact of a roughly Mars-sized body with Earth can account for the Moon's mass and unusually small iron core, as well as Earth's rapid early spin rate, but it creates a Moon derived mainly from the impacting planet rather than from Earth1. It has been thought, mainly on the basis of the observed differences between Earth and Mars, that the impactor's composition would differ substantially from that of Earth2, and thus would most naturally produce a Moon distinct from Earth in its chemical composition. Instead, Earth and the Moon are nearly chemically identical in many respects. Now, in a paper on page 212, Mastrobuono-Battisti and colleagues3 estimate a substantial probability — of the order of 20% — that the giant impactor had an Earth-like composition, offering at least a partial solution to the 'isotopic crisis'4 facing the impact theory.

A key constraint on any hypothesis is that the proportions of different isotopes of a given element vary slightly across samples from the Moon, Earth, Mars and the main asteroid belt that lies between the orbits of Mars and Jupiter. Perhaps the best-studied example involves oxygen (Fig. 1). Earth and the Moon have almost identical oxygen isotope compositions, whereas the difference in oxygen composition between terrestrial rocks and meteorites from Mars or the large asteroid Vesta is considerable5,6. Earth and the Moon also share similar isotopic compositions for chromium, silicon, titanium and tungsten, elements whose isotopic abundances vary across meteorites from Mars and the asteroid belt7.

Figure 1: Oxygen isotopic similarities and differences.
figure1

The oxygen isotopic compositions of rock samples from Earth and the Moon12, and from meteorites from Mars5 and the asteroid Vesta17, define parallel lines on this graph of the isotopic-abundance ratio of 17O to 16O (δ17O) against that of 18O to 16O (δ18O); the ratios are shown as deviations in parts per thousand (‰) from a standard value. The various lines probably reflect different starting oxygen reservoirs that existed in the pre-planetary disk around the Sun, with the grey line indicating the 'terrestrial fractionation line' (TFL) along which Earth samples fall. The offset in the line for Mars compared with that for Earth is about 0.32‰ (ref. 5), whereas the Earth and Moon lines have an offset of only about 0.01‰ (ref. 6), essentially indistinguishable on the scale of this figure. Mastrobuono-Battisti et al.3 present analyses that explain the Earth–Moon isotopic similarity in the context of a giant impact of a planet-sized body with early Earth. (Figure based on data from refs 5, 12 and 17.)

How could the collision of two large and independently formed planets yield a chemically similar Earth and Moon, when Mars and most meteorites are so different? A canonical Moon-forming impactor contains about 15% of Earth's mass and produces a Moon with more than 80% of its material derived from the impactor1. For this to be consistent with the great similarity of Earth and the Moon in oxygen composition, the impactor would need to be nearly as similar to Earth in oxygen isotopic composition as is the Moon. Previous work2 suggests that this would occur for only a percentage or so of all giant impacts, making the conventional Moon-forming impact an uncomfortably rare event and motivating a search for alternative impact scenarios2,8,9.

Mastrobuono-Battisti and colleagues used data from simulations10 that follow the growth of the Solar System's inner planets from an initial distribution of several thousand smaller 'protoplanets' orbiting the Sun. The simulations track merging collisions between the protoplanets that produce a final system of, typically, three or four planets, with the largest comparable in mass to Earth. From these data, the authors extracted the orbital location of each initial protoplanet that contributed to a final planet and its last giant impactor. Following previous work2, they assumed that the protoplanets had oxygen compositions that varied linearly with distance from the Sun, with a distribution set so that the final third and fourth planets produced by the simulation are as different as Earth and Mars. They then calculated the compositional difference between each planet and its last giant impactor.

They found that about 20% of late giant impactors are close compositional matches to their target planets, which is consistent with an impactor-derived Moon having essentially the same composition as Earth. This is roughly ten times higher than previous estimates. In addition, the predicted compositional difference between a planet and its last giant impactor is found to be substantially smaller than the characteristic differences between the final planets. In other words, the same initial compositional mix of protoplanets implied by the Earth–Mars difference can still be consistent with the Earth–Moon similarity.

Two factors seem to have increased the likelihood of an impactor–planet compositional match. First, Mastrobuono-Battisti et al. used more-recent planet-formation simulations10, each with a tenfold higher resolution (that is, smaller initial protoplanets) than the simulation11 analysed previously. Trends in the new analysis suggest that, as the growth of the impactor itself is more finely resolved, predicted impactor–planet compositional differences decrease. All other factors being equal, 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. This suggests that higher resolutions than those of the simulations analysed by the authors might yield still smaller differences, a hypothesis testable by future work. Second, the present work relies on a recently reported difference in oxygen composition between Earth and the Moon, which although still very small6 (12 ± 3 parts per million), increases the probability of a successful impactor–planet match compared with the previous upper limit12 (less than 5 parts per million).

Mastrobuono-Battisti and co-workers' findings offer renewed support for the canonical impact by providing a plausible explanation for the compositional similarities between Earth and the Moon, apart from two possible exceptions: silicon and tungsten. Earth and the Moon are enhanced in the heavy isotopes of silicon, which has been attributed to the preferential incorporation of the light isotope of silicon into Earth's core13. Because this effect requires high pressures inside an Earth-sized planet, it would not occur in a smaller Mars-sized impactor, implying that a Moon derived from such a body would have a lighter silicon composition, inconsistent with that observed13. However, heavy silicon compositions among meteorites that are unlikely to have originated in a massive planet have now been identified14, suggesting that a Mars-sized impactor heavy in silicon might be possible as well.

A remaining puzzle is tungsten, whose isotopic composition is affected by the nature and timing of the formation of an object's iron core. Studies15,16 just published find a small difference in tungsten composition between Earth and the Moon, which is probably due to differing proportions of material accumulated by each object after the Moon-forming impact. Although the closeness of the Earth–Moon tungsten compositions could result from a canonical impact7, it seems a coincidence whose likelihood needs to be quantified. Even with such constraints, the conventional impact may ultimately prove more probable than alternatives that either require rarer types of impact or additional processes, or both2,8,9.Footnote 1

Notes

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Correspondence to Robin M. Canup.

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Canup, R. An incredible likeness of being. Nature 520, 169–170 (2015) doi:10.1038/520169a

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