Geologic Mapping and Geophysical Modeling of the Surface of Ceres: Insights into the Structural, Mechanical, and Compositional Properties of the Solar System’s Innermost Dwarf Planet
When NASA's Dawn mission arrived at Ceres on March 6, 2015 it made history by becoming the first spacecraft to enter orbit around a second extraterrestrial object after leaving the asteroid Vesta in September 2012. Dawn thoroughly investigated the surface and deep interior of the dwarf planet Ceres, the largest object in the main asteroid belt, through a series of successively lower mapping orbits until its end of mission on November 1, 2018. Prior to Dawn’s arrival Ceres was known to be the largest C-type asteroid, and was suspected of being rich in water ice and other hydrated materials. As a putative remnant of the earliest phases of rocky planet formation, Ceres was thought to contain clues as to how planetesimals accreted and how volatiles arranged themselves throughout the inner solar system during the tumultuous era of planet formation. The Dawn mission’s goals were to further elucidate the structure and composition of the early solar system, which would lead to an increased understanding of the conditions present during terrestrial planet formation, and to determine the chemical, geological, and structural nature of the largest surviving planetary embryos: Vesta and Ceres. At Ceres, this was accomplished by meticulously characterizing the surface geology, surface and near-surface geochemistry, and interior structure via a combination of photo geology; visible, infrared, and nuclear spectroscopy; and gravimetry. This dissertation contributes to the objectives of the Dawn mission by aiding in the global geologic mapping effort of Ceres, identifying and classifying geological features indicative of its compositional and physical properties, and then applying geophysical techniques to these features in order to estimate these properties, particularly the water ice content of the near-surface. Understanding the quantity and distribution of water ice in the upper layer of Ceres is paramount for understanding both its geochemical evolution and the nature of the early solar system. The investigations presented in this dissertation reveal that Ceres is ubiquitously covered in geologic features suggestive of significant quantities of near-surface ground ice, namely large constructional mountains and hills, and a broad spectrum of lobate flow and mass wasting deposits. The observed mass wasting features exhibits physical characteristics and runout efficiencies similar to many ground ice mediated flows on Earth, Mars, and Iapetus such as long run-out landslides and frozen debris flows. Additionally, many craters on Ceres were observed to emanate fluidized appearing ejecta similar to examples found on Mars, Ganymede, and other icy worlds. Analyzing the mobilities of these ejecta using a kinematic-dynamic sliding ejecta emplacement model indicated that the cerean crust is significantly weaker than competent silicate rock at impacting conditions, and that the frictional properties of its surface are consistent with a rock-ice mixture. Finally, a unique fractured terrain named Nar Sulcus was identified on Ceres’ southern hemisphere that displayed topography suggestive of elastic flexure. By applying a flexural-cantilever model to the observed topography, the flexural rigidity of the cerean crust was estimated to be similar to those of outer solar system moons such as Europa, Ganymede, and Enceladus, which are several orders of magnitude less rigid than the terrestrial planets. From the aforementioned observations and investigations, the ground ice content of the cerean crust is estimated to be ~30-70 vol %, although there is likely significant regional heterogeneity in its distribution. This result is significant as it independently supports the interpretation that Ceres is a water rich dwarf planet, and that large quantities of ice can be sequestered within massive C-type asteroids over geologically long time periods. This is particularly exciting as carbonaceous chondrites are the most spectrally similar meteorites to C-type asteroids, and their water is the most isotopically similar to the Earth’s oceans. In light of the results presented in this dissertation, and by a myriad of other authors, it increasingly appears that a significant portion of the Earth’s water was likely delivered by Ceres-like asteroids.