Nucleation and growth of iron pebbles explains the formation of iron-rich planets akin to Mercury

Abstract

The pathway to forming the iron-rich planet Mercury remains mysterious. Mercury's core makes up 70% of the planetary mass, which implies a significant enrichment of iron relative to silicates, while its mantle is strongly depleted in oxidized iron. The high core mass fraction is traditionally ascribed to evaporative loss of silicates, e.g. following a giant impact, but the high abundance of moderately volatile elements in the mantle of Mercury is inconsistent with reaching temperatures much above 1,000 K during its formation. Here we explore the nucleation of solid particles from a gas of solar composition that cools down in the hot inner regions of the protoplanetary disc. The high surface tension of iron causes iron particles to nucleate homogeneously (i.e., not on a more refractory substrate) under very high supersaturation. The low nucleation rates lead to depositional growth of large iron pebbles on a sparse population of nucleated iron nano-particles. Silicates in the form of iron-free MgSiO${}_3$ nucleate at similar temperatures but obtain smaller sizes due to the much higher number of nucleated particles. This results in a chemical separation of large iron particles from silicate particles with ten times lower Stokes numbers. We propose that such conditions lead to the formation of iron-rich planetesimals by the streaming instability. In this view, Mercury formed by accretion of iron-rich planetesimals with a sub-solar abundance of highly reduced silicate material. Our results imply that the iron-rich planets known to orbit the Sun and other stars are not required to have experienced mantle-stripping impacts. Instead their formation could be a direct consequence of temperature fluctuations in protoplanetary discs and chemical separation of distinct crystal species through the ensuing nucleation process.

arXiv commentAccepted for publication in Astronomy & Astrophysics

References in corpus (54)

• Rapid planetesimal formation in turbulent circumstellar discs
• A low mass for Mars from Jupiter's early gas-driven migration
• Using FU Orionis Outbursts to Constrain Self-Regulated Protostellar Disk Models
• A simple model for the evolution of the dust population in protoplanetary disks
• The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments
• Forming the cores of giant planets from the radial pebble flux in protoplanetary discs
• Dust in the Early Universe: Dust Formation in the Ejecta of Population III Supernovae
• Dynamics of Solids in the Midplane of Protoplanetary Disks: Implications for Planetesimal Formation
• Concentrating small particles in protoplanetary disks through the streaming instability
• Equilibrium chemistry down to 100 K - Impact of silicates and phyllosilicates on carbon/oxygen ratio
• Revisited Mass-Radius relations for exoplanets below 120 Earth masses
• Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548
• 2-D simulations of FU Orionis disk outbursts
• The radial dependence of pebble accretion rates: A source of diversity in planetary systems I. Analytical formulation
• The Hot Inner Disk of FU Ori
• Minimum Radii of Super-Earths: Constraints from Giant Impacts
• Thresholds for Particle Clumping by the Streaming Instability
• From stellar nebula to planets: the refractory components
• High-Temperature Ionization in Protoplanetary Disks
• How planetary growth outperforms migration
• An Earth-sized exoplanet with a Mercury-like composition
• 3D Radiation Non-ideal Magnetohydrodynamical Simulations Of The Inner Rim In Protoplanetary Disks
• Accretion and destruction of planetesimals in turbulent disks
• The Chemical Composition of Mercury
• A new class of Super-Earths formed from high-temperature condensates: HD219134 b, 55 Cnc e, WASP-47 e
• Formation of Dust in the Ejecta of Type Ia Supernovae
• Forming Mercury by Giant Impacts
• Dynamical rearrangement of super-Earths during disk dispersal I. Outline of the magnetospheric rebound model
• Low Mg/Si planetary host stars and their Mg-depleted terrestrial planets
• Turbulence Regulates the Rate of Planetesimal Formation via Gravitational Collapse
• Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites
• Dusty tails of evaporating exoplanets. I. Constraints on the dust composition
• Making the Planetary Material Diversity During the Early Assembling of the Solar System
• A Simple Calculation in Service of Constraining the Rate of FU Orionis Outburst Events from Photometric Monitoring Surveys
• Chemical Diversity of Super-Earths As a Consequence of Formation
• Exploring the conditions for forming cold gas giants through planetesimal accretion
• Rocklines as Cradles for Refractory Solids in the Protosolar Nebula
• Streaming Instability with Multiple Dust Species: II. Turbulence and Dust-Gas Dynamics at Nonlinear Saturation
• Streaming Instability of Multiple Particle Species in Protoplanetary Disks
• Formulation of Non-steady-state Dust Formation Process in Astrophysical Environments
• Evolution of the Water Snow Line in Magnetically Accreting Protoplanetary Disks
• The Elusive Origin of Mercury
• Seeding the Formation of Mercurys: An Iron-sensitive Bouncing Barrier in Disk Magnetic Fields
• Streaming instability of multiple particle species II -- Numerical convergence with increasing particle number
• Pure iron grains are rare in the universe
• Pebble Accretion in Turbulent Protoplanetary Disks
• Jumping Jupiter can explain Mercury's orbit
• The terrestrial planet formation paradox inferred from high-resolution N-body simulations
• Composition and Size Dependent Sorting in Preplanetary Growth: Seeding the Formation of Mercury-like Planets
• Growth of calcium-aluminum-rich inclusions by coagulation and fragmentation in a turbulent protoplanetary disk: observations and modelisation
• Explaining Mercury's Density through Magnetic Erosion
• Dynamical avenues for Mercury's origin II: in-situ formation in the inner terrestrial disk
• Effects of global gas flows on type I migration
• Composition of super-Earths, super-Mercuries, and their host stars

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