Anatomy of rocky planets formed by rapid pebble accretion I. How icy pebbles determine the core fraction and FeO contents

Abstract

We present a series of papers dedicated to modelling the accretion and differentiation of rocky planets that form by pebble accretion within the lifetime of the protoplanetary disc. In this first paper, we focus on how the accreted ice determines the distribution of iron between the mantle (oxidized FeO and FeO${}_{1.5}$) and the core (metallic Fe and FeS). We find that an initial primitive composition of ice-rich material leads, upon heating by the decay of ${}^{26}$Al, to extensive water flow and the formation of clay minerals inside planetesimals. Metallic iron dissolves in liquid water and precipitates as oxidized magnetite Fe${}_3$O${}_4$. Further heating by ${}^{26}$Al destabilizes the clay at a temperature of around 900 K. The released supercritical water ejects the entire water content from the planetesimal. Upon reaching the silicate melting temperature of 1,700 K, planetesimals further differentiate into a core (made mainly of iron sulfide FeS) and a mantle with a high fraction of oxidized iron. We propose that the asteroid Vesta's significant FeO fraction in the mantle is a testimony of its original ice content. We consider Vesta to be a surviving member of the population of protoplanets from which Mars, Earth, and Venus grew by pebble accretion. We show that the increase in the core mass fraction and decrease in FeO contents with increasing planetary mass (in the sequence Vesta -- Mars -- Earth) is naturally explained by the growth of terrestrial planets outside of the water ice line through accretion of pebbles containing iron that was dominantly in metallic form with an intrinsically low oxidation degree.

Version accepted for Astronomy & Astrophysics

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Cited by in corpus (16)

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• Stirred but not shaken: a multi-wavelength view of HD 16743's debris disc
• Anatomy of rocky planets formed by rapid pebble accretion III. Partitioning of volatiles between planetary core, mantle, and atmosphere
• Thermal processing of primordial pebbles in evolving protoplanetary disks
• The cosmochemistry of planetary systems
• Potential Interior Structures and Habitability of Super-Earth Exoplanets LHS 1140 b, K2-18 b, TOI-1452 b and TOI-1468 c
• Can tidal evolution lead to close-in planetary bodies around white dwarfs II: volcanism and transits
• Post-main sequence thermal evolution of planetesimals
• Exploring the conditions for forming planetesimals by the streaming instability and planetary systems by pebble accretion
• Diversity of Exoplanets
• A Validated Low-to-Intermediate Mass Planetary Interior Structure Model and New Mass-Radius Relations
• Evolution of gas envelopes and outgassed atmospheres of rocky planets formed via pebble accretion
• Interstellar Ices as Carriers of Supernova Material to the Early Solar System
• Coupling dynamical accretion and chemical differentiation: A unified framework for the diversity of Earth and Mars