Layerings in the nucleus of comet 67P/Churyumov-Gerasimenko

The Rosetta mission delivered images of comet 67P's nucleus at unprecedented resolution which indicate the presence of a global layering system. By merging techniques of structural geology, statistical image processing, and solar system science, this thesis aims to contribute to the understanding of the formation of the layerings, and thus of cometary nuclei as a whole. I describe the two distinctive approaches to studying the layerings' orientation on comet 67P's nucleus. First, I mapped layering-related linear features on a 3D shape model of the nucleus, onto which I projected high-res OSIRIS images. I selected only lineaments of substantial curvature, and used a plane-fitting algorithm to find the normals to the layering planes represented by these lineaments. I used the normals to confirm previous authors' results, including that the layering systems on the comet's two lobes are geometrically independent from each other. My results rule out the proposal that 67P's lobes represent collisional fragments of a much larger, layered body. Second, I developed a Fourier-based image analysis algorithm to detect lineament structures at pixel-precision. I analysed the layering-related features exposed on the Hathor cliff on the comet's Small Lobe. I found my algorithm to be a broadly applicable, powerful tool for automating the detection of layerings in images where conventional edge-detection algorithms are not effective. When correctly configured to the target conditions, I found the algorithm to have a higher signal-to-noise detection sensitivity than a human, while reducing over-interpretation due to bias. In summary, I studied the layerings in the nucleus of comet 67P using several unconventional approaches and constrained their lateral extent, curvature, and to a degree also their thickness. Finally, I nominated two mechanisms that could have formed these layerings in cometary nuclei.Comment: Dissertaton, Gottingen, 11.09.2019. online version with abbreviated Appendix B.4, full version available under ISBN 978-3-947208-20-

Detecting and analysing geomorphological structures in images of comet 67P/Churyumov-Gerasimenko using Fourier transform

We present a method for automatised detection and analysis of quasi-periodic lineament structures from images at pixel-precision. The method exploits properties of the images' frequency domain found by using the Fourier transform. We developed this method with the goal of detecting lineament structures in an image of the Hathor cliff of comet 67P/Churyumov-Gerasimenko, which are caused by layerings and furrows in the nucleus material. Using our method, we determined the orientation and wavelength-range of these structures. The detected layering edges have similar orientations, spatial separations of 9-20 m, and are ubiquitous throughout the image. We suggest that the layerings are a global feature of the comet nucleus that provide information about formation and evolution of comet 67P. The furrows are non-uniformly distributed throughout the image. Their orientation is broadly parallel to the direction of the local gravity vector at the Hathor cliff, with spacings similar to that of the layering structures. The furrows are interpreted as signatures of local down-slope movement of cliff material. We demonstrate that the developed method is broadly applicable to the detection and analysis of various kinds of quasi-periodic structures like geological layering, folding and faulting, and texture analysis in general. In order to facilitate the application of our method, this paper is accompanied by a demo program written in Matlab

Analysis of layering-related linear features on comet 67P/Churyumov-Gerasimenko

We analysed layering-related linear features on the surface of comet 67P/Churyumov-Gerasimenko (67P) to determine the internal configuration of the layerings within the nucleus. We used high-resolution images from the OSIRIS Narrow Angle Camera onboard the Rosetta spacecraft, projected onto the SHAP7 shape model of the nucleus, to map 171 layering-related linear features which we believe to represent terrace margins and strata heads. From these curved lineaments, extending laterally to up to 1925 m, we extrapolated the subsurface layering planes and their normals. We furthermore fitted the lineaments with concentric ellipsoidal shells, which we compared to the established shell model based on planar terrace features. Our analysis confirms that the layerings on the comet's two lobes are independent from each other. Our data is not compatible with 67P's lobes representing fragments of a much larger layered body. The geometry we determined for the layerings on both lobes supports a concentrically layered, `onion-shell' inner structure of the nucleus. For the big lobe, our results are in close agreement with the established model of a largely undisturbed, regular, concentric inner structure following a generally ellipsoidal configuration. For the small lobe, the parameters of our ellipsoidal shells differ significantly from the established model, suggesting that the internal structure of the small lobe cannot be unambiguously modelled by regular, concentric ellipsoids and could have suffered deformational or evolutional influences. A more complex model is required to represent the actual geometry of the layerings in the small lobe