The Multiscale Mechanics of Surface Modification Processes on Asteroids

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Johns Hopkins University
The solar system is sprinkled with detritus known as asteroids, with tens of thousands of these rocky masses clustered in the asteroid belt that lies between Mars and Jupiter. These rocky bodies are debris from early solar system formation and have witnessed a long history of surface modification processes. The presence of regolith (a layer of fine-grain loose and consolidated rocks) on such airless bodies is attributed to the reaccumulation of impact ejecta and to the gradual breakdown of boulders by micrometeoritic impacts. However, ejecta velocities for small kilometer-sized asteroids typically exceed the gravitational escape velocity. This greatly limits the amount of retained debris following a high-velocity impact event and suggests that other mechanisms could also be involved in the regolith generation process. Recently, it has been observed that airless bodies in the solar system show signs of a thermally driven process. Cracks in Martian boulders exhibited preferential orientations pointing towards solar induced thermal stresses, and ponds on asteroid (433) Eros imply the existence of an active mechanism that is capable of breaking down rocks in-place, without causing high-velocity ejecta. In this thesis, we develop techniques to bridge the varying timescales related to three surface evolution mechanisms on asteroids: thermal fatigue, mechanical disruption, and gravitational reaccumulation. The primary aim of this dissertation is to bridge the gap between material fragmentation experiments performed at lab scales, to material failure in the extreme thermomechanical environment of the solar system. The thermal fatigue mechanism is first considered, and a numerical model is formulated to capture the crack tip driving force throughout an asteroid's diurnal cycle (a few hours) until the complete fracture of a surface rock occurs (10,000 - 10,000,000 years). The efficiency of thermal fatigue is demonstrated and compared to breakdown estimates from mechanical erosion by micrometeorite impacts. A simple analytical scaling model is derived that allows the prediction of rock breakdown rates by thermal fatigue for different airless bodies in the solar system. Next, thermal cycling and mechanical characterization experiments are conducted on a meteorite sample to track the crack growth. The experiments revealed that cracks showed preferential extension along inclusion interfaces, which acted as stress concentration sites. Using an experimentally informed numerical model, insight into the thermally induced stress field in the meteorite is obtained. The heterogeneous mineral grains inside a meteorite are identified as key actors in the thermal fragmentation process. An examination of the interface bonding between the inclusion and matrix is then performed, showing that weaker interfaces typical of iron-rich meteorites could relax the thermal mismatch stresses and reduce the efficiency of thermal fatigue in small meteorites. Finally, attention is shifted to the timescales related to hypervelocity impacts onto asteroids. Two regimes with drastically different timescales are considered: the material mechanical response (from a few microseconds up to some tens of seconds), and the gravity response (from hours to days). A hybrid framework is developed to capture the mechanical response during the first seconds after impact through a multiscale material model implemented in a Material Point Method code, which is then coupled with an N-body gravity code to examine the fate of the fragmented material as the ejecta interacts with the asteroid's gravitational field during the subsequent hours. It is then shown that large asteroids (tens of km in diameter) may sustain higher impact energies than previously expected. This newly formulated hybrid approach is able to simulate a variety of asteroid impact events, such as angled impacts and spinning targets, from fragmentation to ejection and gravitational reaccumulation.
multiscale, micromechanics, geologic materials, fatigue, impact, fragmentation, gravity, asteroid, regolith