The Moon-forming Impact as a Constraint for the Inner Solar System's Formation

TL;DR

This paper argues that the Moon's formation constrains the terrestrial planets' formation theories, favoring chaotic collisional growth over pebble accretion due to the improbability and consequences of a suitable giant impact.

Contribution

It demonstrates that the Moon's origin challenges the pebble accretion model, supporting chaotic collisional growth as the primary formation mechanism for terrestrial planets.

Findings

Pebble accretion rarely produces the right impact conditions for Moon formation.

Giant impact simulations show perfect mixing, inconsistent with observed mantle heterogeneity.

Chaotic collisional growth better explains the Earth-Moon system's characteristics.

Abstract

The solar system planets are benchmarks for the planet formation theory. Yet two paradigms coexist for the four terrestrial planets: the prolonged collisional growth among planetesimals lasting $>100$ million years (Myr) and the fast formation via planetesimals accreting pebbles within 10 Myr. Despite their dramatic difference, we can hardly tell which theory is more relevant to the true history of the terrestrial planets' formation. Here, we show that the Moon's origin puts stringent constraints on the pebble accretion scenario, rendering it less favourable. In the pebble accretion model, the one-off giant impact between proto-Earth and Theia rarely (probability $<$ 1\textperthousand) occurs at the right timing and configuration for the Moon formation. Even if a potential impact happens by chance, giant impact simulations reveal perfect mixing between proto-Earth and Theia, leaving no room for the observed primordial Earth mantle heterogeneity and the compositional difference, though small, between Earth and the Moon. Thus, the Earth-Moon system along other terrestrial planets should preferably form from chaotic collisional growth in the inner solar system.