Compositional evolution during rocky protoplanet accretion (1509.07504v1)

Abstract: The Earth appears non-chondritic in its abundances of refractory lithophile elements, posing a significant problem for our understanding of its formation and evolution. It has been suggested that this non-chondritic composition may be explained by collisional erosion of differentiated planetesimals of originally chondritic composition. In this work, we present N-body simulations of terrestrial planet formation that track the growth of planetary embryos from planetesimals. We simulate evolution through the runaway and oligarchic growth phases under the Grand Tack model and in the absence of giant planets. These simulations include a state-of-the-art collision model which allows multiple collision outcomes, such as accretion, erosion, and bouncing events, that enables tracking of the evolving core mass fraction of accreting planetesimals. We show that the embryos grown during this intermediate stage of planet formation exhibit a range of core mass fractions, and that with significant dynamical excitation, enough mantle can be stripped from growing embryos to account for the Earth's non-chondritic Fe/Mg ratio. We also find that there is a large diversity in the composition of remnant planetesimals, with both iron-rich and silicate-rich fragments produced via collisions.

• The paper uses N-body simulations to show that collisional erosion during rocky protoplanet accretion causes embryos to lose mantle material, which can explain Earth's non-chondritic Fe/Mg ratio.
• Simulations reveal collisions create diverse iron-rich and silicate-rich fragments, suggesting these dynamics explain the compositional variety seen in planets and smaller bodies.
• The study emphasizes that incorporating complex collision dynamics, like erosion and differentiation, is essential for accurately modeling planetary compositions, challenging uniform chondritic assumptions.

Compositional Evolution during Rocky Protoplanet Accretion

The paper "Compositional Evolution during Rocky Protoplanet Accretion" by Carter et al. discusses the non-chondritic composition of Earth in terms of refractory lithophile elements and explores how collisional erosion of differentiated planetesimals can account for these observations. By employing $N$-body simulations, the paper tracks the growth of planetary embryos from planetesimals, examining the phases of runaway and oligarchic growth within the Grand Tack model and absence of giant planets.

Key Findings

The simulations leverage a sophisticated collision model encompassing outcomes like accretion, erosion, and bouncing events to monitor the evolving core-mass fractions of planetesimals. Notably, the paper finds the following:

1. Embryo Core Mass Fraction: Embryos display varying core-mass fractions due to significant dynamical excitation. Specifically, enough mantle can be stripped from these growing embryos to align with Earth’s non-chondritic Fe/Mg ratio. This suggests that similar collisional processes may have influenced Earth's unique iron enrichment.
2. Collisional Dynamics: The simulations highlight a diversity in the composition of remnant planetesimals, resulting in both iron-rich and silicate-rich fragments through collisions. These processes may play a crucial role in explaining the compositional diversity observed in both planets and smaller bodies within our solar system.
3. Impact on Planet Formation Models: The variance in core fractions underpins the significance of collisional evolution in shaping planetary compositions. This challenges the assumption of uniform, chondritic beginnings for planetesimals and emphasizes the need to include complex collision dynamics in planetary formation models.

Implications

Practically, these findings impact our understanding of planetary formation by indicating that episodes of collisional erosion and core-mantle differentiation must be factored into models predicting terrestrial planet compositions. Theoretically, these results provide a foundation for refining accretion models, suggesting that non-chondritic compositions emerge naturally from dynamic planetary environments disturbed by migrating massive bodies like Jupiter under the Grand Tack scenario.

Additionally, these insights have ramifications for interpreting rocky exoplanet compositions. Given the increased understanding of compositional shifts due to accretion and erosion, researchers can improve predictive models of exoplanetary characteristics based on stellar compositions and potential dynamical histories.

Future Directions

The research presents several avenues for future exploration, particularly the transitions during the giant impact phase of planetary formation, where stochastic collisions might further alter elemental distributions. Furthermore, understanding these processes within the broader context of the solar nebula and subsequent volatile delivery provides an exhaustive narrative for planetogenesis.

Moreover, studying similar mechanisms in exoplanetary systems could elucidate whether such dynamic evolutionary paths are universal or particular to our solar system's history. Expanding on these models could enable accurate predictions about the habitability and geological evolutions of distant worlds.

In conclusion, Carter et al.'s work provocatively suggests that to comprehend the Earth’s and other terrestrial planets’ complex evolution, it is essential to consider not only initial chondritic compositions but also the dynamically rich and chaotic collisional histories that shape planetary cores and mantles.