Forging the Conditions of Planetary Interiors Using Ultra-fast Compression Techniques

Abstract

This thesis explores planet formation processes through the use of sub-microsecond dynamic compression techniques to briefly create the pressure-temperature conditions of deep planetary interiors. Rocky planet evolution is characterized by the growth of planetesimals resulting from high-energy impacts. These impacts impart sufficient energy to melt the interiors of planets in the early stages of planet formation (<50 Ma). As the planet cools, core segregation, chemical fractionation, and atmospheric formation proceed, yet are strongly influenced by the initial set of composition and thermodynamic conditions. Using dynamic compression, I present three studies where I explore the behavior of fundamental planetary building blocks (SiO2, iron oxides, and TiO2) at three snapshots in Earth’s evolution: 1) the magma ocean stage, 2) inner core solidification, and 3) late bombardment. The second chapter of this thesis investigates the shock response of SiO2 across the solid-liquid phase boundary and up to pressures of 154 GPa. SiO2 is a primary oxide component of rocky planet interiors and its melt properties strongly influence the thermal evolution of Earth. Despite this importance, SiO2 is poorly studied in the liquid state, especially at deep Earth pressures. I report new stress-density, sound speed, and the Grüneisen parameter determinations for liquid silica. These results provide an accurate determination of melting along the Hugoniot and provide the most precise Grüneisen parameter model for a liquid silicate for use in assessing magma ocean temperature gradients. The third chapter investigates the pressure-density response and crystal structure of two iron oxides (Fe3O4 and Fe2O3) dynamically compressed to pressures corresponding to the core-mantle boundary of a five-Earth-mass exoplanet. Above ~250 GPa, Fe3O4 transforms to the crystalline Th3P4-type phase, analogous to predictions for Mg-silicates. In contrast, Fe2O3 transforms to metastable orthorhombic structure above ~175 GPa and subsequently transitions to a disordered state above ~350 GPa at these nanosecond timescales. The pressure-density relationships determined from this study are then used to predict the light element concentration of Earth’s inner core as well as the density of iron-bearing silicates in the deep interiors of rocky exoplanets. The final chapter explores the crystal structure of TiO2, an important impact diagnostic and analog system for SiO2, during laser-driven shock compression and unloading. I observe direct transformation from the ambient rutile structure to a distorted-fluorite phase that has been previously predicted from theory, but never experimentally observed. At higher pressures, TiO2 transforms to the Fe2P phase at pressures significantly lower than the equilibrium phase boundary. These results highlight the ability of ultra-fast compression to synthesize novel materials under far from equilibrium conditions. Taken together, the latter two chapters highlight the importance of atomic-level probes for understanding the complex, and often unexpected, response of geological materials to rapid dynamic compression.