Formation of Iron Rich Bodies via Giant Impacts

Abstract

Giant impacts represent a critical process in the later stages of planetary formation, fundamentally shaping both the architecture and compositional diversity of planetary systems. In this thesis, we examine the dynamics of giant impacts and their implications for the formation and composition of terrestrial planets, with particular emphasis on pathways leading to the emergence of super-Mercuries—planets characterized by exceptionally high core fractions. Extensive smoothed particle hydrodynamics (SPH) simulations are employed to investigate a wide range of impact scenarios, allowing us to disentangle the complex interplay of physical and thermodynamic processes that govern collisional outcomes. Our results indicate that, although giant impacts can significantly modify planetary densities and compositions, the formation of super-Mercuries through single impact events remains challenging due to the narrowly constrained conditions required, such as high impact velocities combined with low impact angles.

Our analysis further demonstrates that the collisional behavior of multilayered, differentiated planets is considerably more complex than that of undifferentiated bodies. This finding suggests that the universal scaling laws previously adopted may not fully capture the intricate dynamics of all collision processes. In response, we introduce novel scaling laws and machine-learning pipelines derived from our extensive SPH simulations, which enhance the predictive accuracy of giant-impact outcomes and offer valuable tools for integration into $N$-body simulation frameworks.

The study also highlights the significant influence of core vaporization on mantle stripping efficiency, identifying distinct mechanisms of momentum transfer and vaporization-induced ejection. By examining head-on collisions and the underlying physical processes, we provide a detailed exploration of the thermodynamic evolution and energy budgets involved in catastrophic planetary collisions. Our findings emphasize the complex interplay among gravitational forces, shock dynamics, and thermodynamic processes, which collectively determine the properties of post-collision remnants. This work underscores the non-uniform behavior of planetary collisions across different target masses and stresses the importance of accurately modeling phase boundaries to fully comprehend impact dynamics.

Building on these insights, we compile a comprehensive database comprising 20,000 simulations across a diverse parameter space. This dataset underpins the development of robust machine-learning models that predict both the mass and iron fraction of collision remnants with high precision. Our analysis reveals that the formation of massive super-Mercuries—whether through single or multiple giant impacts—is possible but needs constrained conditions. We therefore propose an alternative formation mechanism in which an inward-migrating planet accretes the dense remnants produced by energetic collisions in the inner disk, providing a viable substitute to the traditional giant impact erosion model.

Overall, the findings presented in this thesis advance our understanding of planetary accretion and the evolution of terrestrial worlds, establishing a foundation for future studies aimed at unraveling the complex processes that govern planetary diversity across varied planetary systems.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Philip Carter (Supervisor), Zoe M Leinhardt (Supervisor) & Andrew J Young (Supervisor)