A multiple-impact origin for the Moon

Abstract

The hypothesis of lunar origin by a single giant impact can explain some aspects of the Earth–Moon system. However, it is difficult to reconcile giant-impact models with the compositional similarity of the Earth and Moon without violating angular momentum constraints. Furthermore, successful giant-impact scenarios require very specific conditions such that they have a low probability of occurring. Here we present numerical simulations suggesting that the Moon could instead be the product of a succession of a variety of smaller collisions. In this scenario, each collision forms a debris disk around the proto-Earth that then accretes to form a moonlet. The moonlets tidally advance outward, and may coalesce to form the Moon. We find that sub-lunar moonlets are a common result of impacts expected onto the proto-Earth in the early Solar System and find that the planetary rotation is limited by impact angular momentum drain. We conclude that, assuming efficient merger of moonlets, a multiple-impact scenario can account for the formation of the Earth–Moon system with its present properties.

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Figure 1: Lunar formation in the multiple-impact scenario.
Figure 2: Impact simulation.
Figure 3: Disk properties in the angle–velocity phase space.
Figure 4: Final satellite mass and system angular momentum.
Figure 5: Planetary rotation.
Figure 6: Disk structure post impact.

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Acknowledgements

We thank S. Stewart and R. Citron for providing guidance on the computational code, as well as A. Mastrobuono-Battisti for providing the data used for the Monte Carlo simulations. This project was supported by the Minerva Center for Life Under Extreme Planetary Conditions as well as by the I-CORE Program of the PBC and ISF (Center No. 1829/12). R.R. is grateful to the Israel Ministry of Science, Technology and Space for their Shulamit Aloni fellowship. H.B.P. also acknowledges support from the Israel-US bi-national science foundation, BSF grant number 2012384, and the European union career integration grant ‘GRAND’.

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R.R. performed the SPH simulations and their analysis with guidance by O.A. H.B.P. suggested the multiple-impact idea. All authors contributed to discussions, interpretations and writing.

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Correspondence to Raluca Rufu.

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The authors declare no competing financial interests.

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Rufu, R., Aharonson, O. & Perets, H. A multiple-impact origin for the Moon. Nature Geosci 10, 89–94 (2017). https://doi.org/10.1038/ngeo2866

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