Planetary Science

Crashing to the Core

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Science  21 Jan 2011:
Vol. 331, Issue 6015, pp. 264
DOI: 10.1126/science.331.6015.264-c

Earth began to form through the accumulation and accretion of material in a disk of dust and gas that orbited the newly formed Sun. Once the so-called proto-Earth reached a certain size—through impacts with several other planetary bodies—gravity induced the differentiation of an internal structure with a metallic core as the planet continued to grow. An evolving composition of the impactors may explain Earth's internal structure and composition, but inconsistencies exist between various isotope systems and compositional estimates that are typically used to explain Earth's earliest history. A new model by Rubie et al., based on mass balances of average compositions of chondritic meteorites, attempts to reconcile Earth's accretion history with existing geochemical evidence. Among many simulations, the best-fit model predicts that the proto-Earth experienced a series of collisions with relatively large volatile-poor impactors of increasingly oxidized composition. This explanation requires minimal mixing between the metallic cores of the late impactors and silicate-rich magma on Earth before their heavier elements were incorporated into Earth's core as a consequence of equilibrium between metals and silicates at high pressures. According to this sequence of events, and a late addition of volatiles, the minor element composition of Earth's present-day iron-rich core is approximately 8% Si, 5% Ni, 2% S, and 0.5% O.

Earth Planet. Sci. Lett. 301, 31 (2011).

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