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Crater Morphologies and the Distribution of Ejecta Deposits on the Dwarf Planet Ceres

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Crater Morphologies and the Distribution of Ejecta Deposits on the Dwarf Planet Ceres Dissertation zur Erlangung des akademischen Grades „doctor rerum naturalium“ (Dr. rer. nat.) Crater Morphologies and the Distribution of Ejecta Deposits on the Dwarf Planet Ceres vorgelegt von Dipl.-Geophysikerin Franziska Schulzeck im Fachbereich Geowissenschaften der Freien Universität Berlin Berlin, 2018 Erstgutachter: Prof. Dr. Ralf Jaumann Freie Universität Berlin Institut für Geologische Wissenschaften Fachrichtung Planetologie und Fernerkundung sowie Deutsches Zentrum für Luft- und Raumfahrt Institut für Planetenforschung, Abt. Planetengeologie Zweitgutachter: Prof. Dr. Lena Noack Freie Universität Berlin Institut für Geologische Wissenschaften Fachrichtung Geochemie, Hydrogeologie, Mineralogie Tag der Disputation: 19. November 2018 V EIDESSTATTLICHE ERKLÄRUNG Hiermit erkläre ich, dass ich die beigefügte Dissertation selbstständig verfasst und keine anderen als die angegebenen Hilfsmittel genutzt habe. Alle wörtlich oder inhaltlich übernommenen Stellen habe ich als solche gekennzeichnet. Ich versichere außerdem, dass ich die beigefügte Dissertation nur in diesem und keinem anderen Promotionsverfahren eingereicht habe und, dass diesem Promotionsverfahren keine endgültig gescheiterten Promotionsverfahren vorausgegangen sind. Berlin, 07. Dezember 2018 Franziska Schulzeck VII DANKSAGUNG Ich danke Herrn Prof. Dr. Ralf Jaumann und Frau Dr. Katrin Stephan für die Betreuung meiner Doktorarbeit und Frau Prof. Dr. Lena Noack für die Übernahme des Zweitgutachtens. Darüber hinaus danke ich allen Mitarbeitern der Abteilung Planetengeologie am DLR-Institut für Planetenforschung in Berlin für ihre Unterstützung. Für ihre fachliche Unterstützung danke ich speziell Katrin Krohn, Stefanus Schröder und Katharina Otto. Besonders danken möchte ich meinen beiden Zimmerkollegen Isabel von der Gathen und Martin Voelker. Auch danke ich meinem Onkel Jochen. Der größte Dank gilt meinem Freund, meiner Schwester und meinen Eltern. IX 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 X 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. XI ZUSAMMENFASSUNG Die vorliegende Dissertation kombiniert eine geologische Kartierung mit ballistischen Berechnungen von Auswurfmaterial zur Untersuchung der Oberflächenzusammensetzung und der oberflächenformenden Prozesse des Zwergplaneten Ceres. Die Analysen basieren auf Daten der Dawn-Mission. Nachdem sich die Raumsonde Dawn zwischen 2011 und 2012 im Orbit des Asteroiden Vesta aufhielt, trat sie im März 2015 in den Orbit des Zwergplaneten Ceres ein. Ziel der Dawn-Mission war es, sowohl die innere und äußere Zusammensetzung von Ceres und Vesta als auch endogene und exogene Prozesse zu erforschen, um mehr über die Entwicklung des Sonnensystems und die Entstehung von Protoplaneten zu erfahren (Russell and Raymond, 2011; Russell et al., 2016). Die vorliegende Arbeit trägt zu diesem Verständnis von Geologie, Evolution und Dynamik des Zwergplaneten Ceres bei. Das methodische Vorgehen zur Untersuchung der Entwicklung und Zusammensetzung der Oberfläche von Ceres lässt sich in zwei Schritte unterteilen. Zunächst wurde unter Einbeziehung von Klarfilteraufnahmen, Farbfilteraufnahmen und topografischen Daten das Quadrangel Sintana in Ceres‘ Südhemisphäre geologisch kartiert. Im Zuge dessen wurden geologische Einheiten und Strukturen untersucht und das Oberflächenalter bestimmt. Dadurch konnten die zu Grunde liegenden geologischen Prozesse identifiziert werden und eine Einordnung in die globale Stratigraphie vorgenommen werden. Ballistische Berechnungen veranschaulichen als zweite Methode die Verteilung von Auswurfmaterial des kleinen schnell rotierenden Zwergplaneten. Dies ist relevant, um den Ursprung geologischer Einheiten zurückverfolgen zu können. Um den Zusammenhang zwischen Auswurfmaterial und Sekundärkratern zu untersuchen, wurden darüber hinaus Partikelgrößen berechnet. Als Spezialfall wurde die Auswurfdynamik von Kratern mit großen Auswurfbrocken analysiert sowie die Größenverteilung und Ballistik der Brocken selbst. Die Ergebnisse der geologischen Kartierung weisen auf einen hohen Eisanteil im Untergrund hin und zeigen, dass hauptsächlich große Einschläge für die Umstrukturierung der Oberfläche verantwortlich sind. Die kartierten Kraterstrukturen ähneln denen auf den Eismonden. Das kartierte Gebiet zeigt zudem eine Vielfalt an Kraterformen und zahlreiche Sekundärkraterketten. Auch die Morphologie von Massenbewegungsablagerungen auf Ceres ist ein Indiz für eine eisreiche Zusammensetzung des Untergrundes. Hinweise auf endogene Aktivitäten in dem kartierten Teil der Oberfläche wurden allerdings nicht gefunden. Die Altersbestimmung deutet XII zusätzlich darauf hin, dass das kartierte Gebiet von Auswurfmaterial oder Impaktschmelze überprägt wurde, wahrscheinlich vom angrenzenden Kraterbecken Kerwan. Die Sintanaregion zeigt Sekundärkraterketten, Auswurfbrocken, die Überprägung durch große Einschlagskrater und deutet auf eine asymmetrische Ablagerung des Auswurfmaterials hin. Diese Strukturen und Prozesse sind noch ausgeprägter in anderen Regionen von Ceres zu finden. Ballistische Berechnungen haben den Ursprung dieser Beobachtungen untersucht. Die Ergebnisse dieser Berechnungen zeigen, dass sich das Auswurfmaterial durch die Rotation von Ceres asymmetrisch ablagert. Zudem ist eine oberflächenweite Kontamination mit Sekundärkratern sehr wahrscheinlich, was zu einer Verzerrung der Altersmessungen führt. Die geringe Gravitationsbeschleunigung erlaubt den Partikeln weite Strecken zurückzulegen, bevor sie wieder auf die Oberfläche treffen. Dies hat zur Folge, dass sich
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