Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water (1410.3509v1)

Abstract: In order to test planetary accretion and differentiation scenarios, we integrated a multistage core-mantle differentiation model with N-body accretion simulations. Impacts between embryos and planetesimals result in magma ocean formation and episodes of core formation. The core formation model combines rigorous chemical mass balance with metal-silicate element partitioning data. The primary constraint on the combined model is the composition of the Earth's primitive mantle, the composition of the Martian mantle, and the mass fractions of the metallic cores of Earth and Mars. The model is refined by least squares minimization with up to five fitting parameters that consist of the metal-silicate equilibrium pressure and 1-4 parameters that define the starting compositions of primitive bodies. This integrated model has been applied to 6 Grand Tack simulations. Investigations of a broad parameter space indicate that: accretion of Earth was heterogeneous, metal-silicate equilibration pressures increase as accretion progresses and are 60-70% of core-mantle boundary pressures at the time of each impact, and a large fraction (70-100%) of the metal of impactor cores equilibrates with a small fraction of the silicate mantles of protoplanets during each core formation event. Acceptable fits to the Earth's mantle composition are obtained only when bodies that originated close to the Sun, at <0.9-1.2 AU, are highly reduced and those beyond this distance are increasingly oxidized. The FeO content of the Martian mantle depends critically on the heliocentric distance at which the Mars-forming embryo originated. Finally, the Earth's core is predicted to contain 8-9 wt% silicon, 2-4 wt% oxygen and 10-60 ppm hydrogen, whereas the Martian core is predicted to contain low concentrations (<1 wt%) of Si and O.

• The paper utilizes six Grand Tack N-body simulations to integrate episodic core formation via magma ocean development.
• It finds metal–silicate equilibration pressures averaging 60–70% of core-mantle boundary values, with 70–100% impactor metals equilibrating during formation events.
• The study introduces a heterogeneous accretion model that links compositional gradients and water delivery to variations in heliocentric distance.

Analysis of Core-Mantle Differentiation in Terrestrial Planets

The paper explores an intricate integration of core-mantle differentiation and N-body accretion simulations to elucidate the formation and compositional evolution of terrestrial planets in the Solar System. Utilizing six Grand Tack N-body simulations, the research aims to assess accretion models while aligning chemical differentiation within the evolving celestial bodies.

The paper posits a multistage model where impacts between embryos and planetesimals lead to episodic core formation facilitated by magma ocean creation. A significant focus is placed on metal-silicate element partitioning under varied heliocentric distances, using Earth's primitive mantle composition as a primary constraint. Results suggest that Earth's accretion was heterogeneous and that metal-silicate equilibration pressures averaged 60-70% of core-mantle boundary pressures. Notably, 70-100% of impactor core metals equilibrate during each formation event, accentuating sensitivity to initial compositional models.

The research introduces three distinct composition-distance models to reflect varying degrees of axial heterogeneity in Solar System bodies' origin zones. One model suggests primitive bodies near the Sun were highly reduced, with increasingly oxidized compositions further out, yielding the most consistent results. At distances beyond 6-7 AU, bodies were largely oxidized and water-bearing. Key to these findings is the systematic variation of oxygen content across the heliocentric distances, influencing planetary body core-mantle differentiation.

For Earth, the fit offered a core composition contending with 8-9 wt% Si, 2-4 wt% O, and 10-60 ppm H, while Mars is seen to have significantly lower concentrations of these elements in its core. The results underscore the need to consider heterogeneity in accreted material, a pivotal factor sufficiently influenced by elements like Si dissolving into metallic phases during core formation, as presented in the alternative "step model."

Practical implications of this research lie in refining models that simulate accretionary and differentiation processes, especially given the sensitivity to initial elemental compositions. The paper also establishes a foundational approach for exploring volatile incorporation into planetary bodies, setting the stage for incorporating isotopic evolution and volatile elements into these models.

Speculative future directions for AI and computational techniques in this domain include enhancing predictive simulations with improved resolution on volatile pathways and core composition dynamics, and more robust statistical analyses facilitated by larger simulated datasets. Such advancements could underpin a refined understanding of planetary development scenarios across varied astrophysical contexts.