Deformation and its surface expression in stressed planetary materials

Abstract

This thesis investigates the response of planetary materials to changing stress fields, and resultant signatures of stress in geophysical properties observable from planetary surfaces. When forces change within rocky and icy layers of planetary bodies, constituent materials of these layers adjust on the microscale; energetically favorable alignment of microstructural materials builds across scales to result in deformation, preferred directions for material transport and wave propagation, and heat release. This work therefore explores the relationship between microstructure and stress conditions in order to connect geophysical observations to the underlying forces on subsurface materials, using both experimental and computational methods. The first two chapters investigate two-phase deformation, where a partial melt phase is present between grains of solid materials such as olivine (Chapter 2) or ice (Chapter 3). Chapter 2 finds that in partially molten rocky materials, microstructural melt aligns parallel to the maximum applied stress direction quickly over geological time, while crystallographic orientations require significant strain intervals to reset. This shows that we can use the melt-induced changes to properties in the deforming Earth, for example, as an indicator of short-term stress fields. Chapter 3 applies these findings to the evolution of icy systems through simulated deformation of ice-melt aggregates, suggesting that current seismic studies which do not correct for the orientation of melt may misinterpret deformation at the base of warm ice sheets. The final two chapters center on deformation mechanisms that may shape the properties of icy outer Solar System satellites as they orbit their host planets. Chapter 4 provides novel experimental constraints on meteoritic materials relevant to the cores of icy moons, finding that microstructural brittle deformation, and resultant energy release, occurs even at very small differential stresses. Acoustic emissions associated with this brittle deformation are also more energetic at lower confining pressures, indicating that smaller, lower-pressure icy moons might receive enhanced heat from core deformation. The final chapter (Chapter 5) investigates crustal processes on Titan, Saturn’s largest moon. This work models how tidal stresses interact with local topographic stresses to create fracture across Titan’s crust, creating pathways for sediment generation and fluid transport.