Accretion of the earliest inner solar system planetesimals beyond the water-snowline

TL;DR

This study uses meteorite geochemistry to determine that the earliest inner solar system planetesimals formed beyond the water-snowline, incorporating water-bearing materials and volatile elements, challenging previous formation models.

Contribution

It provides evidence that early inner solar system planetesimals accreted water-bearing materials, indicating formation beyond the water-snowline, based on Fe/Ni and Fe/Co ratios in meteorites.

Findings

Early NC planetesimals contained oxidized Fe, implying water incorporation.

Accretion likely occurred at or beyond the water-snowline, not the silicate condensation line.

Widespread formation of oxidized, water-bearing planetesimals in the early inner solar system.

Abstract

How and where the first generation of inner solar system planetesimals formed remains poorly understood. Potential formation regions are the silicate condensation line and water-snowline of the solar protoplanetary disk. Whether the chemical compositions of these planetesimals align with accretion at the silicate condensation line (water-free and reduced) or water-snowline (water-bearing and oxidized) is, however, unknown. Here we use Fe/Ni and Fe/Co ratios of magmatic iron meteorites to quantify the oxidation states of the earliest planetesimals associated with non-carbonaceous (NC) and carbonaceous (CC) reservoirs, representing the inner and outer solar system, respectively. Our results show that the earliest NC planetesimals contained substantial amounts of oxidized Fe in their mantles (3-19 wt% FeO). In turn, we argue that this required the accretion of water-bearing materials into these NC planetesimals. The presence of substantial quantities of moderately and highly volatile elements in their parent cores is also inconsistent with their accretion at the silicate condensation line and favors instead their formation at or beyond the water-snowline. Similar oxidation states in the early-formed parent bodies of NC iron meteorites and those of NC achondrites and chondrites with diverse accretion ages suggests that the formation of oxidized planetesimals from water-bearing materials was widespread in the early history of the inner solar system.