The accretion of planet Earth

Abstract

Earth’s origins are challenging to elucidate, given the lack of surviving terrestrial geology from the first 500 Myr of the Solar System. In this Review, we discuss breakthroughs in geochemistry and theoretical modelling that have advanced understanding of Earth accretion. Theory holds that solar nebula dust particles stuck together to form pebbles, concentrations of which gravitationally collapsed into ∼100-km-sized planetesimals, which in turn accreted to yield planets. Isotopic variations in meteorites indicate that pebbles formed within the first 100 kyr of the Solar System, planetesimals melted and differentiated within a few 100 kyr, and Mars accreted quickly within 5 Myr. Earth’s growth was more protracted, with >98% of its mass being accreted by the time of the Moon-forming Giant Impact at ∼70–120 Myr. Earth is more enriched in s-process nuclides than chondritic meteorites, with a chemical composition affected by condensation, melting and loss. Early volatiles acquired from the nebula largely escaped, with the remnant volatiles being diluted by main-stage Earth accretion, accompanied by loss of nitrogen to the core and/or space. Areas for further research should include assessing mixing during large collisions and investigating the origin of very early mantle isotopic heterogeneities, which might indicate mass transfer from core to mantle over time. The history of Earth’s formation can be unravelled from the compositions of meteorites, terrestrial and lunar rocks, and observations from space-based telescopes. This Review discusses advances in theories and evidence concerning the dynamical mechanisms and timescales for Earth’s accretion in the Solar System.

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