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PLANETARY SCIENCE

Why the Moon is so like the Earth

Nature Geoscience volume 12, pages 402–403 (2019)Cite this article

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The Moon’s isotopic composition is uncannily similar to Earth’s. This may be the signature of a magma ocean on Earth at the time of the Moon-forming giant impact, according to numerical simulations.

For the past three decades, the answer to the question “Where did Earth’s moon come from?” was most likely to be “From the impact of a Mars-sized protoplanet”. Planetary scientists did not readily accept this surprising solution; initially, the giant impact proposal was regarded as both radical and unlikely. However, thanks to computer simulations, the idea was shown to better explain the Moon’s size, orbital angular momentum and overall composition than other hypotheses1. All these simulations predicted that the Moon-forming material is mostly derived from the impactor, rather than directly from the Earth. This prediction did not disagree with contemporary measurements of the chemical and isotopic composition of the lunar rocks, but as isotopic measurements have increased in precision, rocks from the Earth and Moon have continued to show near-identity2. This has created a modern crisis in the giant impact concept: if more than half of the Moon’s material came from the impactor, how can the Moon’s isotopes be nearly identical to the Earth’s? Writing in Nature Geoscience, Hosono et al.3 suggest a resolution to this crisis by showing that a much larger fraction of Earth material would have reached orbit if the Earth was partially molten at the time of the giant impact.

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Fig. 1: Snapshots of a simulation of the Moon-forming impact.

References

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  1. Department of Earth Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN, USA

    H. Jay Melosh

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  1. H. Jay Melosh
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Correspondence to H. Jay Melosh.

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Melosh, H.J. Why the Moon is so like the Earth. Nat. Geosci. 12, 402–403 (2019). https://doi.org/10.1038/s41561-019-0364-0

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