Homogeneous accretion of the Earth in the inner Solar System

Meteorites are classified as either non-carbonaceous- (NC) or carbonaceous (CC), representing bodies that likely formed in the inner- or outer solar system, respectively [1]. Despite its location in the inner solar system, the Earth is thought to contain minor- (~6 %, [2]) to substantial amounts (~40 % [3]) of outer solar system material. However, because neither scheme accounts for variations among all isotopic systems simultaneously, the provenance of the terrestrial planets remains equivocal. Here, we examine the variations in 10 nucleosynthetic isotope anomalies among planetary materials to show that the linear extension of an array defined by NC bodies in any two isotopic anomalies always intersects the observed isotopic composition of the bulk silicate Earth to within 1 standard deviation. The Earth therefore formed from material that originated exclusively in the inner solar system and whose contribution was, on average, uniform over the course of its accretion. On this basis, we identify a heliocentric gradient in the nucleosynthetic isotopic compositions among the terrestrial planets. Because their masses are distributed in an approximately Gaussian manner about ~0.9 au, we predict that the mean isotopic composition of the inner solar system is intermediate between Earth and Venus, with the low-mass tails of distribution, Mercury and Mars, exhibiting more extreme compositions.