Core segregation during pebble accretion

We explore the heating and differentiation of rocky planets that grow by rapid pebble accretion. Our terrestrial planets grow outside of the ice line and initially accrete 28% water ice by mass. The accretion of water stops after the protoplanet reaches a mass of 0.01 ME where the gas envelope becomes hot enough to sublimate the ice and transport the vapour back to the protoplanetary disc by recycling flows. The energy released by the decay of 26Al melts the accreted ice to form clay (phyllosilicates), oxidized iron (FeO), and a water surface layer with ten times the mass of Earth’s modern oceans. The ocean–atmosphere system undergoes a run-away greenhouse effect after the effective accretion temperature crosses a threshold of around 300 K. The run-away greenhouse process vaporizes the water layer, thereby trapping the accretion heat and heating the surface to more than 6000 K. This causes the upper part of the mantle to melt and form a global magma ocean. Metal melt separates from silicate melt and sediments towards the bottom of the magma ocean; the gravitational energy released by the sedimentation leads to positive feedback where the beginning differentiation of the planet causes the whole mantle to melt and differentiate. All rocky planets thus naturally experience a magma ocean stage. We demonstrate that Earth’s small excess of 182W (the decay product of 182Hf) relative to the chondrites is consistent with such rapid core formation within 5 Myr followed by equilibration of the W reservoir in Earth’s mantle with 182W-poor material from the core of a planetary-mass impactor, provided that the equilibration degree is at least 25–50%, depending on the initial Hf/W ratio. The planetary collision must have occurred at least 35 Myr after the main accretion phase of the terrestrial planets.

show abstract

“…We note that during revision of this paper a similar idea and model were published byOlson et al (2022).…”

mentioning

confidence: 83%

Anatomy of rocky planets formed by rapid pebble accretion

Johansen

Ronnet

Schiller

et al. 2023

A&A

View full text Add to dashboard Cite

show abstract

“…Initially, the envelope has the same composition as the surrounding protoplanetary disk and consists of a mixture of hydrogen and helium (H 2 /He). As the planet grows, it will melt and differentiate into an iron-rich core and a magma ocean (Olson et al 2022;Johansen et al 2023). The envelope is assumed to be in vapor equilibrium with this magma ocean at all times.…”

Section: Treatment Of Silicate Vapormentioning

confidence: 99%

“…In their model, an outer region saturated in SiO 2 resides on top of a possibly undersaturated region. However, this model does not take into account that rapidly formed rocky planets undergo a magma ocean phase during their formation by pebble accretion (Olson et al 2022;Johansen et al 2023). Furthermore, chemical equilibrium models show that the dominant gas species of Si in equilibrium with molten silicates (MgSiO 3 or Mg 2 SiO 4 ) is SiO and not SiO 2 (Schaefer et al 2012;Herbort et al 2020).…”

Section: Introductionmentioning

confidence: 99%

Vapor equilibrium models of accreting rocky planets demonstrate direct core growth by pebble accretion

Steinmeyer,

Johansen

2024

A&A

View full text Add to dashboard Cite

The gaseous envelope of an accreting rocky planet becomes hot enough to sublimate silicates and other refractory minerals. For this work, we studied the effect of the resulting envelope enrichment with a heavy vapor species on the composition and temperature of the envelope. For simplification, we used the gas-phase molecule SiO to represent the sublimation of silicate material. We solved the equilibrium structure equations in 1D for planets in the mass range of $0.1$ to $3\ The convective stability criterion was extended to take the stabilizing effect of the condensation of SiO clouds into account. We assumed that the envelope is both in hydrostatic equilibrium and in vapor equilibrium with the underlying magma ocean. This means that pebbles do not undergo sublimation in the envelope and therefore survive until they plunge into the magma ocean. We find that the emergence of an inner radiative region, where SiO condensation suppresses convection, increases the pressure and temperature in the inner envelope compared to pure H2 He envelopes once $ For $ the temperature and pressure close to the surface reach the supercritical point of SiO. The amount of SiO stored in the envelope is lower than the total planet mass for low mass planets. However, for $ all accreted pebble material must contribute to maintain the vapor equilibrium in the envelope. Therefore, the non-vapor mass of the planet ceases to increase beyond this threshold. Overall, our vapor equilibrium model of the planetary envelope allows for direct core growth by pebble accretion up to much higher masses than previously thought.

show abstract

“…2015; Badro et al. 2018; Olson, Sharp & Garai 2022). In particular, the cratering process is responsible for the initial dispersion and mixing of the impactors’ cores (Landeau et al.…”

Section: Introductionmentioning

confidence: 99%

“…Planetary impacts occur on terrestrial planets from the early stages of accretion to modern meteorite impacts. During planetary formation, thermal and chemical partitioning between the core and the mantle is influenced by the physical mechanisms of segregation between the metal of the impactors' cores and the silicates of the growing planet (Stevenson 1990;Rubie, Nimmo & Melosh 2015;Lherm & Deguen 2018), with major implications for the chemical, thermal and magnetic evolution of the planet (Fischer et al 2015;Badro et al 2018;Olson, Sharp & Garai 2022). In particular, the cratering process is responsible for the initial dispersion and mixing of the impactors' cores (Landeau et al 2021;Lherm et al 2022).…”

Section: Introductionmentioning

confidence: 99%

Velocity field and cavity dynamics in drop impact experiments

Lherm

Deguen

2023

J. Fluid Mech.

View full text Add to dashboard Cite

Drop impact experiments allow the modelling of a wide variety of natural processes, from raindrop impacts to planetary impact craters. In particular, interpreting the consequences of planetary impacts requires an accurate description of the flow associated with the cratering process. In our experiments, we release a liquid drop above a deep liquid pool to investigate simultaneously the dynamics of the cavity and the velocity field produced around the air–liquid interface. Using particle image velocimetry, we analyse quantitatively the velocity field using a shifted Legendre polynomial decomposition. We show that the velocity field is more complex than considered in previous models, in relation to the non-hemispherical shape of the crater. In particular, the velocity field is dominated by degrees 0 and 1, with contributions from degree 2, and is independent of the Froude and the Weber numbers when these numbers are large enough. We then derive a semi-analytical model based on the Legendre polynomial expansion of an unsteady Bernoulli equation coupled with a kinematic boundary condition at the crater boundary. This model explains the experimental observations and can predict the time evolution of both the velocity field and the shape of the crater, including the initiation of the central jet.

show abstract

Core segregation during pebble accretion

Cited by 13 publications

References 40 publications

Anatomy of rocky planets formed by rapid pebble accretion

Anatomy of rocky planets formed by rapid pebble accretion

Vapor equilibrium models of accreting rocky planets demonstrate direct core growth by pebble accretion

Velocity field and cavity dynamics in drop impact experiments

Contact Info

Product

Resources

About