dc.description.abstract
The following thesis describes a combined approach of geologic mapping and ballistic calculations of ejected material to investigate the surface composition and the processes that shaped the dwarf planet Ceres. The analyses are based on data of the Dawn mission.
After orbiting the asteroid Vesta from 2011 to 2012, the Dawn spacecraft reached Ceres’ orbit in March 2015. The main goals of the Dawn mission were to investigate the geology, interior composition and past and present endogenic and exogenic processes of Ceres and Vesta to describe their evolution and to gain a broader understanding of the early solar system and protoplanet growth (Russell and Raymond, 2011; Russell et al., 2016). This thesis will contribute to the understanding of the geology, the evolution and the dynamics of Ceres.
This work utilized a two step procedure to analyze the composition and geologic history of the surface of Ceres. First, including clear filter, topographic and color data, geologic mapping was conducted for the Sintana quadrangle in the southern hemisphere of Ceres to identify geologic processes and to help to assemble a global stratigraphy for Ceres. In the course of the mapping, geologic units and features were analyzed and absolute ages of geologic units were determined. Second, ejecta deposition of the small and fast rotating dwarf planet was investigated by analytical ballistic calculations in order to understand the origin and distribution of geologic units. To examine the relation between ejecta and secondary cratering, particle sizes were calculated. Moreover, the special case of ejecta dynamics of boulder craters was analyzed, as well as the ballistics and size frequency distributions of the ejecta blocks themselves.
The geologic mapping reveals an ice rich subsurface that is predominantly resurfaced by large impact events. The crater shapes in the Sintana quadrangle resemble those on icy satellites with a great variety of morphologies. Moreover, secondary crater chains cross the quadrangle. In addition, the morphology of blocky landslides is indicative of an ice rich subsurface. The mapping region shows no evidence of endogenic activity, but the absolute age of the cratered terrain background unit hints to a resurfacing event that was possibly induced by impact ejecta/melt of the close by Kerwan basin.
Features and processes that were identified in the Sintana region, such as secondary crater chains, boulders, resurfacing by large impact craters and asymmetric ejecta blankets, are even more distinct in other regions of Ceres. Ballistic calculations were used to investigate the origin of those observations. The results of the ballistic calculations show that the rotation of Ceres leads to a specifically asymmetric ejecta deposition and that a planet wide contamination with secondary craters that bias surface ages is very likely. The low gravity regime results in a long range transport of particles; hence ejecta might be deposited far away from its primary crater. The particle sizes of high velocity particles could be large enough to produce distant secondary craters and can therefore explain the secondary crater chains that are observed all over Ceres. The analysis of boulder crater ejecta deposits shows that they are as asymmetric as the ejecta deposits of larger impact basins on Ceres. The investigation of boulders demonstrates that the deposition of low velocity ejecta is also influenced by the Coriolis effect. Steep slopes of boulder size frequency distributions are in agreement with the idea that such blocks produce secondary craters with a similar distribution (McEwen and Bierhaus, 2006). As already revealed by geologic mapping, the ballistic analysis confirms that a high contamination of the surface by secondary impacts is likely, for instance, in the shape of non radial crater chains. Secondary craters, in turn, pose problems in the dating of geological units, because they can increase the estimated age of a surface.
In short, this work is the first extensive study about the rotational effects on ejecta deposition on the dwarf planet Ceres.
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