The effect of thermal cycling on the mechanical properties of rock and ice

Recent research has highlighted the importance of thermally induced stresses in the breakdown of rock, and its contribution to landscape evolution on airless bodies such as the Moon, Mercury, and asteroids. Investigations of thermal breakdown of lunar boulders have found that stresses induced at different locations within the rocks, at different times of day, work together to hasten disaggregation of surface material. These stresses are controlled by both the diurnal temperature range and boulder diameter. At the grain scale, heterogeneity in mineral properties also plays an important role in controlling stress amplitudes. Since ice and rock differ significantly in their thermal expansion behavior, this suggests that thermal stresses may also play an important role on objects made up of ice-rock mixtures. This emphasizes the broad significance this process has in our understanding of the surface evolution of icy airless bodies, including dwarf planets (e.g., Ceres), comets, and icy satellites. The proposed work focuses on understanding the effect of thermal cycling on ice and rock in planetary environments by developing a model relating their thermomechanical response to induced damage and changes in their bulk properties, and exploring the implications for surface evolution on rocky and icy airless bodies. We will take a three-stage approach to achieve this goal. 1) First, we will thermally cycle samples composed of ice, rock, and ice-rock mixtures in a vacuum chamber at a variety of temperature ranges, and monitor changes there Young’s moduli. 2) We will then conduct 3D simulations of the samples in the chamber to reproduce their thermomechanical response. In this way, we can relate the measured changes in Young’s modulus to the amount of simulated stress the samples experienced. This will allow us to develop a damage model that can constrain breakdown rates using existing studies of macroscopic stresses in rocks. 3) We will make the model more broadly applicable by imposing the macroscopic stress states determined from the 3D model as boundary conditions on 2D microstructures of mineral grains. In this way, we will investigate the grain-scale response to macroscopic thermal forcing, yielding a damage model independent of object size, shape, and thermal cycle. This can be used to estimate thermally induced damage in any object of the same composition by simulating that object’s grain scale stress. These 2D and 3D modeling techniques can be used to target specific bodies of interest, providing insight into their evolution and the relationship between micro-and macro-scale thermal processes on their surfaces. To put this work in context, we will relate the results to boulder size-frequency distributions on the Moon and compare our breakdown estimates with other models of lunar boulder lifetimes. The proposed work is directly relevant to the objectives of the Solar System Workings program, as defined in Appendix C.3 of ROSES-2018, including: characterize and understand the chemical, mineralogical, and physical features of planetary surfaces […] Develop theoretical and experimental bases for understanding these features in the context of the varying conditions through time of formation (Evolution and Modification of Surfaces item 6, C.3-2). Furthermore, this research addresses many key questions and specific objectives associated with the science goals outlined by the 2013-2022 NRC Planetary Science Decadal Survey, including: how have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time (p. 71, key question 10), and determine the effects and timing of secondary processes on the evolution of primitive bodies (p. 89, objective 3, science goal 1), and characterize planetary surfaces to understand how they are modified by geologic processes (p. 113, objective 3, science goal 1).