Predicting the Strength of Planetary Surfaces

Citation

Harmon, John Michael (2022) Predicting the Strength of Planetary Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/r9zm-kp19. https://resolver.caltech.edu/CaltechTHESIS:10152021-195617065

Abstract

Our curiosity and spirit for exploration has fueled advancements towards visiting Earth's neighbors in the solar system. Environments outside of Earth are extreme, however, and it is far from guaranteed that landing and operating on the surface of these bodies is an easy task. Conditions such as reduced gravity, extreme temperatures and sparse atmospheres play a role in the compressive, shear, and tensile strength of a surface. These environmental factors make experiments that work to inform design decisions for spacecraft-surface interaction difficult and expensive. In order to better ensure successful mission operations in the future, this thesis focuses on the development of a platform of numerical modeling for planetary surface interaction.

Dry regolith and water ice are two surface materials that are pervasive in the solar system. For each, the mechanical properties are heavily reliant on features at the microscale that are insufficiently modeled. The first part of the thesis will focus on crushable dry regolith. There will be two chapters on this topic, the first of which discusses the development of the modeling capability to capture both the highly irregular particle shapes and the brittle nature of regolith. The second chapter on regolith will focus on the validation of this method on a crushable sand sample experiment. This model demonstrates excellent predictive capability for the constituitive relationship, the evolution of particle sizes, and the evolution of particle shape in the sample. Further, evidence from the forces between the particles shows that despite larger particles being weaker on average, many survive due to two reasons. One, the surviving particles are generally on the stronger side of the particle strength distribution, and second, that larger particles have a higher coordination number producing a more isotropic stress state in the particle.

Unlike dry regolith, distinct neighboring water ice particles will sinter together over time at varying rates depending on their environment. This leads to a large amount of the water ice surfaces that are of interest to future missions, having a highly varied and many times unknown levels of strength. The contact interaction between water ice particles at the microscale will be handled the same as regolith, however a modification was added to account for sintering. The strong cohesiveness sintering generates is modeled by placing massless bonds where sinters would form. The cross-sectional area of the bond represents the amount of sintering that has taken place and can be thought of as a representation of the neck geometry that early stage sintering is described as. The bonds used are linear elastic and breakable in order to capture the crushable nature of porous ice. Three chapters are dedicated to ice modeling. First, the model development will be shown with verification examples for its use. Second, the model will be used to predict cone penetration tests on ice that were previously conducted in experiment. Comparisons show that the model can produce similar stresses and qualitative features observed in the experiment. A sensitivity analysis is conducted and shows that the most important controlling parameters are the ice’s critical strength and the sinter's neck thickness. The relation of the bond characteristics to the sintering process is discussed. In the third chapter on water ice, the landing of a footpad on the surface of Enceladus is modeled. The model predicts that a lack of sintering could result in catastrophic sinkage, however even moderate sintering provides enough strength to support a lander. Also the model predicts landing on inclined surfaces and shows that landing could be possible at angles as high as 20 degrees.

Thesis Files