The formation and structure of iron-dominated planetesimals

TL;DR

This work investigates how oxidation state, quantified by $\Delta\text{IW}$, and bulk composition shape the interior structure of ~200 km iron-dominated planetesimals. Using a differentiation model with metal–silicate partitioning and a fixed core-formation temperature of $T\approx1900$ K, the authors map core mass fractions (CMF) and light-element distributions across end-member and mixed bulk compositions, highlighting the role of $f\text{O}_2$ in core growth and composition. Key findings include robust CMFs under reducing conditions, Si-dominated cores at strong reduction, and the emergence of S- and C-bearing cores under oxidized conditions, potentially accompanied by graphite crusts and the possibility of core dynamos in sulfur-rich systems. These results provide a framework for interpreting future measurements from asteroid rendezvous missions like NASA's Psyche and for understanding the formation of dense rocky planets in exoplanetary systems.

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.

Figures (9)

• Figure 1: Top left: Modeled parent body core mass fractions (CMF) versus oxygen fugacity ($f\text{O}_2$) for the compositional cases considered here. The shaded horizontal bar represents CMF estimates for Psyche elkins2022distinguishing. The dashed vertical line indicates the redox below which metallic Fe dominates. Top right: CMF evolution with mantle FeO content for our cases. Bottom: FeO versus $f\text{O}_2$ from our results. Data from recent studies on iron meteorites are overplotted for comparison grewal2024accretionspitzer2025comparisonhilton2022chemical
• Figure 2: Modeled core light element compositions versus oxygen fugacity ($f\text{O}_2$) for the compositions considered. Metal-silicate partition coefficients used in the models are from boujibar2014steenstra2016chi2014Fischer2015steenstra2020a. The carbon concentration at graphite saturation (CCGS) for the metal fractions of each bulk composition is plotted for comparison steenstra2020asteenstra2018.
• Figure 3: Modeled minor elemental mantle compositions for the different initial bulk compositions as a function of $f\text{O}_2$. Compositions are obtained from a core-mantle differentiation model with experimentally determined metal-silicate partition coefficients boujibar2014steenstra2016chi2014Fischer2015steenstra2020a. The estimated SCSS is plotted for comparison namur2016. The FeO evolution path shown for CBa represents the trend for other compositions.
• Figure 4: Summary of the Fe/Ni ratios for the core and mantle fractions of the four pure end-member bulk compositions versus $f\text{O}_2$. The Fe/Ni ratios of Fe meteorites from grewal2024accretion are overlaid in panel 1. For the cores of Earth and Mars, Fe/Ni ratios range from 16 to 20 dreibus1996cosmochemicalkhan2018geophysical, and from hundreds to thousands in rocky mantles and differentiated stony meteoritespalme2014.
• Figure 5: Top: Core densities vs. $f\text{O}_2$ for compositions considered in this study compared to densities of minerals, meteorites, and Psyche. Bottom: Change in bulk density for different amounts of mantle stripping. Densities are calculated by mass balance. Sources of initial densities: meteoritic britt2002macke2011densityostrowski2019physical, Psyche farnocchia2024mass, and minerals dziewonski1981preliminaryhauck2019mercuryzhu2021density. A density of 2.5 $g/cm^{3}$ is the minimum plausible mantle density.