Terrestrial magma ocean origin of the Moon

Abstract

A conceptual framework for the origin of the Moon must explain both the chemical and the mechanical characteristics of the Earth–Moon system to be viable. The classic concept of an oblique giant impact explains the large angular momentum and the lack of a large iron-rich core to the Moon, but in this scenario it is difficult to explain the similarity in the isotopic compositions of the Earth and Moon without violating the angular momentum constraint. Here we propose that a giant, solid impactor hit the proto-Earth while it was covered with a magma ocean, under the conventional collision conditions. We perform density-independent smoothed particle hydrodynamic collision simulations with an equation of state appropriate for molten silicates. These calculations demonstrate that, because of the large difference in shock heating between silicate melts and solids (rocks), a substantial fraction of the ejected, Moon-forming material is derived from the magma ocean, even in a highly oblique collision. We show that this model reconciles the compositional similarities and differences between the Moon and Earth while satisfying the angular momentum constraint.

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Fig. 1: Snapshots of an impact simulation.
Fig. 2: Fraction of various materials in the disk and its mass for an MO depth of 1,500 km.
Fig. 3: PT conditions of materials outside the proto-Earth in the early stages of a GI.
Fig. 4: Disk mass versus target fraction in the Moon.

Data availability

The data that support the findings of this study are available from the corresponding author on request.

Code availability

The codes used to generate these results are available from the corresponding author on request.

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Acknowledgements

We thank K. Ozawa and H. Nagahara for a discussion of the lunar composition and of geochemistry, and J. Melosh for helpful comments. We used computational resources provided by the RIKEN Center for Computational Science through the HPCI System Research project (ID: ra000008).

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N.H. performed numerical simulations and analysed the results. S.-I.K. developed a model of the terrestrial MO origin of the Moon and suggested the numerical simulations to test his model. N.H. and S.-I.K. interpretated the results and wrote the paper. J.M. and T.R.S. helped N.H. with the numerical schemes.

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Correspondence to Natsuki Hosono.

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Hosono, N., Karato, S., Makino, J. et al. Terrestrial magma ocean origin of the Moon. Nat. Geosci. 12, 418–423 (2019). https://doi.org/10.1038/s41561-019-0354-2

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