Stress, on the Rocks: Thermally Induced Stresses in Rocks and Microstructures on Airless Bodies, Implications for Breakdown

Abstract

This dissertation investigates the role of thermomechanical processes in the production of regolith on airless body surfaces. Thermally induced breakdown may provide a significant contribution to their surface evolution, by breaking down rocks and degrading craters. In Chapter 1, we use the traditional terrestrial methodology of evaluating the efficacy of this process by modeling the rate of surface temperature change (dT/dt) on various airless surfaces, using a damage threshold of 2 K/min. We find that the magnitude of dT/dt values is primarily controlled by sunrise/set durations on quickly rotating bodies, such as Vesta, and by distance to the sun on slowly rotating bodies, such as Mercury. The strongest rates of temperature change occur on slopes normal to the sun when a sunrise or sunset occurs, either naturally or because of daytime shadowing. We find, however, that high dT/dt values are not always correlated with high temperature gradients within the surface. This adds to the ambiguity of the poorly understood damage threshold, emphasizes the need further research on this topic that goes beyond the simple 2 K/min criterion. We further investigate this shortcoming in the terrestrial literature in Chapter two by modeling stresses induced by diurnal temperature variations at the mineral grain scale on these bodies. We find that the resulting stresses are controlled by mismatches in material properties between adjacent mineral grains. Peak stresses (on the order of 100s of MPa) are controlled by the coefficient of thermal expansion and Young's modulus of the mineral constituents, and the average stress within the microstructure is determined by relative volume of each mineral. Amplification of stresses occurs at surface-parallel boundaries between adjacent mineral grains and at the tips of pore spaces. We also find that microscopic spatial and temporal surface temperature gradients do not correlate with high stresses, making them inappropriate proxies for investigating microcrack propagation. Although these results provide strong evidence for the significance of thermomechanical processes, more work is needed to quantify crack propagation and rock breakdown rates in order to understand their overall contribution to surface evolution on these bodies. In Chapter 4, we investigate macroscopic scale effects on thermally induced stress fields in boulders of varying sizes and find that macroscopic thermal gradients may play a role in crack propagation within boulder interiors.