PEBBLE ACCRETION EXPLAINS EARTH'S MAJOR ELEMENT COMPOSITION

Over the last several decades, impact-based accretion of chondrites has become a well-established paradigm for planet formation, yet there is no combination of chondrite types (including iron meteorites) that reproduce the major element composition of Earth. Pebble accretion offers an alternative explanation for planetary growth. Pebbles (e.g., chondrules) follow sub-Keplerian orbits in the presence of a nebular gas and drift radially towards the central star. Large planetesimals formed by streaming instabilities efficiently capture the inward drifting pebbles. Calculations reveal that the terrestrial planets can be produced in a few million years by pebble accretion, assuming standard solar nebula conditions. Notably, the pebble accretion model is an efficient method for capturing sub-mm to cm-size particles, while excluding micron-size dust, because the capture radius for dust-size particles is small. Using least squares and Monte Carlo methods, we show that pebble mixtures of chondritic components – metal grains, chondrules, calcium-aluminum-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs) – match Earth’s major element composition (Fe, Ni, Si, Mg, Ca, Al, O) within uncertainties, whereas mixtures of chondrites and iron meteorites do not. Using published average compositional data for E, C, O chondrules, refractory inclusions, and metal grains, our best fit to Earth is predominantly composed of C and O chondrules with ~10% refractory inclusions. Evidence for comparably high proportions of refractory materials is lacking in chondrites, presumably because they formed after the majority of early-formed refractory grains were either drawn into the Sun or incorporated into terrestrial protoplanets via pebble accretion, in agreement with a heterogenous accretion scenario for Earth. Our calculations also demonstrate that a few My of pebble accretion with these chondritic components yields a total protoplanet mass inside 1 AU exceeding the combined masses of Earth, Moon, Venus, and Mercury. Accordingly, we conclude that pebble accretion is a viable mechanism to build Earth and its major element composition from primitive chondritic components within the solar nebula lifetime.