A primordial origin for the compositional similarity between the Earth and the Moon

Abstract

Most of the properties of the Earth–Moon system can be explained by a collision between a planetary embryo (giant impactor) and the growing Earth late in the accretion process1,2,3. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor1,4,5. However, analysis of the terrestrial and lunar isotopic compositions show them to be highly similar6,7,8,9,10,11. In contrast, the compositions of other Solar System bodies are significantly different from those of the Earth and Moon12,13,14, suggesting that different Solar System bodies have distinct compositions. This challenges the giant impact scenario, because the Moon-forming impactor must then also be thought to have a composition different from that of the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion15, to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact. A large fraction of planet–impactor pairs have almost identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.

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Figure 1: The distribution of planetesimals composing the planet and the impactor.
Figure 2: The cumulative distribution of the absolute Δ17O differences between planets and their last giant impactors (blue), compared with the differences between planets in the same system (red).

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Acknowledgements

H.B.P. acknowledges support from BSF grant number 2012384, the Minerva Center for Life under Extreme Planetary Conditions, the ISF I-CORE grant number 1829/12 and the Marie Curie IRG 333644 ‘GRAND’ grant. S.N.R. acknowledges funding from the Agence Nationale pour la Recherche via grant ANR-13-BS05-0003-002 (project MOJO). We thank O. Aharonson for remarks on an early version of this manuscript. We thank N. Kaib and N. Cowan for helpful discussions on their related work.

Author information

A.M.-B. analysed the simulation data and produced the main results; H.B.P. initiated and supervised the project and took part in the data analysis. S.N.R. provided the simulation data used for the analysis. The paper was written by A.M.-B. and H.B.P. with contributions from S.N.R.

Correspondence to Alessandra Mastrobuono-Battisti or Hagai B. Perets.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The cumulative distribution of the planetesimals composing the planet (red) and the impactor (blue).

All planet–impactor pairs in Table 1 are shown, cases 1–12 (panels al), including the cumulative distributions corresponding to the histograms in Fig. 1.

Extended Data Figure 2 The cumulative distribution of the planetesimals composing the planet (red) and the impactor (blue).

All planet–impactor pairs in Table 1 are shown, cases 13–20 (panels mt).

Extended Data Figure 3 The cumulative distributions of the compositions of planets and last impactors assuming 0% mixing between Earth and Moon material.

The cumulative distribution of the absolute Δ17O differences between planets and their last giant impactors (blue), compared with the differences between planets in the same system (red), assuming 0% mixing between Earth and Moon material. From the top left panel (a) to the bottom right panel (f) we consider all the systems, regardless of the number of particles that contributed to their formation, and planets and last impactors composed of a minimum of 10, 20, 40 and 50 particles. Only last impactors with mass >0.5MMars have been taken into account.

Extended Data Figure 4 The cumulative distributions of the compositions of planets and last impactors assuming 20% mixing between Earth and Moon material.

The cumulative distribution of the absolute Δ17O differences between planets and their last giant impactors (blue), compared with the differences between planets in the same system (red), assuming 20% mixing between Earth and Moon material. From the top left panel (a) to the bottom right panel (f) we consider all the systems, regardless of the number of particles that contributed to their formation, and planets and last impactors composed of a minimum of 10, 20, 40 and 50 particles. Only last impactors with mass >0.5MMars have been taken into account.

Extended Data Figure 5 The cumulative distributions of the compositions of planets and last impactors assuming 40% mixing between Earth and Moon material.

The cumulative distribution of the absolute Δ17O differences between planets and their last giant impactors (blue), compared with the differences between planets in the same system (red), assuming 40% mixing between Earth and Moon material. From the top left panel (a) to the bottom right panel (f) we consider all the systems, regardless of the number of particles that contributed to their formation, and planets and last impactors composed of a minimum of 10, 20, 40 and 50 particles. Only last impactors with mass >0.5MMars have been taken into account.

Extended Data Table 1 The mean fraction of last impactors with a composition compatible with the planet they impact
Extended Data Table 2 The mean fraction of planet–impactor consistent pairs

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Mastrobuono-Battisti, A., Perets, H. & Raymond, S. A primordial origin for the compositional similarity between the Earth and the Moon. Nature 520, 212–215 (2015). https://doi.org/10.1038/nature14333

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