The formation and structure of iron-dominated planetesimals

TL;DR

This study models the interior structures of iron-rich planetesimals under varying oxygen conditions to understand their formation and composition, providing insights relevant for interpreting meteorite data and upcoming space missions.

Contribution

It introduces differentiation models that evaluate how initial oxygen fugacity influences core composition and structure of 200 km planetesimals, highlighting potential signatures of formation environments.

Findings

Core fractions remain consistent under oxygen-poor conditions across different compositions.

Sulfur-rich cores can stay partly molten and sustain dynamos.

High carbon bodies can develop graphitic outer layers.

Abstract

Metal-rich asteroids and iron meteorites are considered core remnants of differentiated planetesimals and or products of oxygen-depleted accretion. Investigating the origins of iron-rich planetesimals could provide key insights into planet formation mechanisms. Using differentiation models, we evaluate the interior structure and composition of representative-sized planetesimals (approx. 200 km diameter) while varying oxygen fugacity and initial bulk meteoritic composition. Under the oxygen-poor conditions that likely existed early in the inner regions of the Solar System and other protoplanetary disks, core fractions remain relatively consistent across a range of bulk compositions (CI, H, EH, and CBa). Some of these cores could incorporate significant amounts of silicon (10-30 wt percent) and explain the metal fractions of Fe-rich bodies in the absence of mantle stripping. Conversely, planetesimals forming under more oxidizing conditions, such as beyond snow lines, could exhibit smaller cores enriched in carbon, sulfur (more than 1 wt percent), and oxides. Sulfur-rich cores, like those formed from EH and H bulk compositions, could remain partly molten, sustain dynamos, and even drive sulfur-rich volcanism. Additionally, bodies with high carbon contents, such as CI compositions, can form graphitic outer layers. These variations highlight the importance of initial formation conditions in shaping planetesimal structures. Future missions, such as NASA's Psyche mission, offer an opportunity to measure the relative abundances of key elements (Fe, Ni, Si, and S) necessary to distinguish among formation scenarios and structure models for Fe-rich and reduced planetesimals.