The accretion of planet Earth
Alex N. Halliday & Robin M. Canup
Nature Reviews Earth & Environment
Review Article, Published: 29 November 2022
“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.”
“Key points
- Terrestrial planet accretion commenced with disk grains and high-temperature condensates sticking together to form pebbles, which in turn gravitationally coalesced to form planetesimals up to hundreds of kilometres in size. Planetesimals with metallic cores, sampled today as iron meteorites, were present within the first million years of the Solar System.
- Planetesimals collided to form Moon-to-Mars-sized planetary embryos in the presence of the solar nebula. Nebular dispersal triggered an era of giant collisions among the embryos that established the inner Solar System’s architecture and, for Earth, culminated in the Giant Impact that produced the Moon.
- Although most of Earth’s nucleosynthetic makeup is closest to that of enstatite chondrites, earlier (<50% by mass) stages of accretion had an isotopic signature intermediate between enstatite and ordinary chondrites. However, Earth is more enriched in those nuclides formed by slow addition of neutrons in large stars compared with all meteorites, and is different chemically from chondrites, particularly enstatites.
- These chemical differences partly reflect early melting and condensation in the disk, which produced fractionated chemical and isotopic compositions, but also result from subsequent losses and additions, especially of volatile elements, during accretion.
- Most lunar origin models fail to provide a natural explanation for the identical isotopic composition of the bulk silicate Earth and Moon for non-volatile elements. This isotopic match is particularly problematic for tungsten, which is sensitive to the nature and timing of core formation and is unlikely to result from the Giant Impact unless there was post-impact mixing and isotopic equilibration between the silicate Earth and Moon.
- The discovery of mantle isotopic heterogeneities generated in the first 100 million years of Earth’s history has changed thinking on preservation of primordial reservoirs in the deep Earth, as well as the nature of Earth’s late veneer, which could partially reflect a long history of compositional fluxes from Earth’s core.”
