A primordial origin for the composition similarity between the Earth and the Moon

TL;DR

This study suggests that the similar compositions of Earth and Moon may originate from a primordial similarity in their building blocks, challenging the idea that the impactor was compositionally different.

Contribution

The paper introduces a simulation-based analysis showing impactors often share similar compositions with their target planets, explaining Earth-Moon similarity without requiring a unique impactor.

Findings

Impactors often have compositions similar to their target planets.

Many impactor-planet pairs have nearly identical compositions.

The Earth-Moon similarity could be due to primordial compositional similarities.

Abstract

Most of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic composition show them to be highly similar. In contrast, the compositions of other solar system bodies are significantly different than the Earth and Moon. This poses a major challenge to the giant impact scenario since the Moon-forming impactor is then thought to also have differed in composition from the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion, in order to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are systematically more similar to the planets they impact. A significant fraction of planet-impactor pairs have virtually identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.