The Morphology of Volatile-rich Regoliths on Asteroids and Comets

Kurzfassung

Recent space missions to asteroids and comets, have revealed divers morphologic features on multiple scales that can be linked to the presence of volatiles in the regolith of small bodies. The research presented in this thesis investigates the physics of these volatilerelated morphologies addressing (1) the relationship between surface morphologies and the presence of volatiles, (2) the regolith's volatile content linked to morphologic features, (3) the difference of asteroidal and cometary outgassing, and (4) the role of volatiles in the evolution of the surfaces and interiors of small bodies. For a comprehensive understanding a combination of three analysis techniques was applied including the geologic interpretation of remote sensing and in situ space mission data, the investigation of sublimation processes on the morphology of volatile-rich analogue materials in the laboratory, and discrete element method modelling assessing the physical and mechanical properties of cometary regolith through the simulation of dynamic surface processes. While morphologic features of many small bodies are considered in this thesis, a focus lies on those observed by the Dawn, Rosetta and Hayabusa2 missions. They include concentric crater fractures on Ceres and their relation to a volatile-rich subsurface layer, pitted impact deposits in volatile-rich ejecta on Vesta, mass-wasting features in volatile-poor/rich regoliths on Vesta and Ceres, aeolian-like features created by volatile-driven particle redistribution on comet 67P/Churyumov-Gerasimenko, and pond-like features generated through volatile fluidisation of Vesta's regolith. In situ analyses of Ryugu and 67P/Churyumov-Gerasimenko revealed a link between volatile sublimation and surface roughness and the similarity of Ryugu's regolith and that of aqueously altered carbonaceous chondrites. In the laboratory, analogue materials and the effect of volatile content, particle shape, insolation and organic material on the morphologic evolution of cliffs were established. Discrete element method modelling extracted the mechanical and physical regolith properties by simulating boulder drops and cliff collapses on 67P/ChuryumovGerasimenko and helped constraining the formation process of aeolian-like features. The work shows that morphologic features give access to the composition, geology and evolution of small solar system bodies and paves the way for future research involving other planetary objects and new data sets.