Laser-Driven Dynamic Compression of Planetary Materials

Abstract

This thesis describes the use of dynamic compression on nanosecond timescales to explore the properties of selected materials relevant to the deep interiors of Earth and other planets, especially recently discovered rocky exoplanets. High-powered laser systems are combined with pulsed X-ray diffraction capabilities to determine the structures, phase transitions, and equations of state of materials over a wide range of stresses. The results provide new constraints on the behavior of geological materials at extreme conditions of pressure, temperature, and strain rate.Forsterite, Mg2SiO4, is the magnesium end-member of the olivine solid solution. In this work (Chapter 2), I report the high-pressure behavior of polycrystalline and single-crystal forsterite under laser-based shock loading with in situ X-ray diffraction. My results provide direct determination of the crystal structure of forsterite and its dependence on starting material at shock stresses extending beyond 100 GPa. My results provide a new understanding of the Hugoniot behavior of magnesian silicates, showing that they may transform to metastable phases followed by amorphization at higher stresses. Ramp compression combined with in situ X-ray diffraction allows observation of the structural behavior, phase transitions, and kinetics of planetary materials at extreme conditions. In this thesis, laser-driven ramp-compression experiments with in situ X-ray diffraction were performed to explore the structural behavior and phase transitions in silicon carbide, SiC, (Chapter 3) and germanium dioxide, GeO2 (Chapter 4). For SiC, the rocksalt (B1) phase was observed from 140 - 1500 GPa. Using the equation of state of B1 SiC measured here I determine mass-radius curves for carbon-rich planets which are found to have a lower density (~10%) than Earth-like planets. iii For GeO2 which serves as an analog for SiO2, I have examined its crystal structure up to 882 GPa. My X-ray diffraction results show that GeO2 adopts the HP-PdF2-type structure under ramp loading from 154 GPa to 436 GPa. Above 436 GPa, I observe evidence for a post-HP-PdF2 phase in GeO2. The best candidate for this new phase is the cotunnite-type structure. These results offer a test of theoretical calculations as well as insights into possible high-pressure behavior of SiO2.