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Cooling of the Earth and core formation after the giant impact


Kelvin calculated the age of the Earth to be about 24 million years by assuming conductive cooling from being fully molten to its current state1. Although simplistic2, his result is interesting in the context of the dramatic cooling that took place after the putative Moon-forming giant impact, which contributed the final 10 per cent of the Earth's mass3,4. The rate of accretion and core segregation on Earth as deduced from the U–Pb system5 is much slower than that obtained from Hf–W systematics6,7,8, and implies substantial accretion after the Moon-forming impact, which occurred 45 ± 5 Myr after the beginning of the Solar System. Here we propose an explanation for the two timescales5,9. We suggest that the Hf–W timescale reflects the principal phase of core-formation before the giant impact. Crystallization of silicate perovskite in the lower mantle during this phase produced Fe3+, which was released during the giant impact10, and this oxidation resulted in late segregation of sulphur-rich metal into which Pb dissolved readily, setting the younger U–Pb age of the Earth. Separation of the latter metal then occurred 30 ± 10 Myr after the Moon-forming impact. Over this time span, in surprising agreement with Kelvin's result, the Earth cooled by about 4,000 K in returning from a fully molten to a partially crystalline state.

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Figure 1: Application of the growth model to the determination of the time of the Moon-forming impact.
Figure 2: Calculated effect of perovskite crystallization on the Fe3+ content and hence oxygen fugacity ( f O 2 ) of a magma ocean of peridotite composition.
Figure 3: Results of modelling 11 estimates of the Pb isotopic composition of the BSE with late stage loss of Pb5.

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Discussions with J. Wade and M. Walter helped to clarify our arguments. B.J.W. acknowledges the support of a Max-Planck Research Award.

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Correspondence to Bernard J. Wood.

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Wood, B., Halliday, A. Cooling of the Earth and core formation after the giant impact. Nature 437, 1345–1348 (2005).

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