Organic compounds in Saturn's E-ring and its compositional profile in the vicinity of Rhea

Abstract

The general topic of this dissertation is the analysis of impact ionization time-of-flight mass spectra of ice grains in Saturn’s E ring sampled in-situ by the Cosmic Dust Analyzer (CDA) onboard the Cassini-Huygens spacecraft. The source of these E ring ice grains is the subsurface ocean of Saturn’s cryo-volcanically active icy moon Enceladus. The Chemical Analyzer subsystem of CDA generated mass spectra of cations that form when the ice grains impinge onto the instrument’s target plate with high speed. The first aim of this work is a detailed compositional analysis of the organic material in the ice grains ejected from subsurface Enceladus’ into the E ring. Many of these ice grains carry diverse organic material that is characterized in this work. A laser-based analogue laboratory experiment is used to simulate the impact ionization CDA spectra of ice grains enriched in organic material. This experiment allowed to understand the varying cationic fragmentation patterns from organic material in a water ice matrix. Despite the relatively low mass resolution of the CDA, results of the analogue experiment allow to identify characteristic finger prints of certain classes of organic compounds in many CDA mass spectra. Three main categories are classified: (i) Amine-, (ii) Carbonyl-, and (iii) Aromatic-type mass spectra. Furthermore, some aromatic-type CDA spectra show features that correspond to breakup-products of larger complex aromatic species with masses above 200u. On the whole, the analysis of E ring ice grains in this work gives first insights into the largely varying and complex organic chemistry inside the ocean of Enceladus. The second aim of this thesis is to infer the compositional profile of ice particles in the E ring in the vicinity of Saturn’s moon Rhea, from a series of spectra recorded on Cassini’s Rhea flyby (R4) in 2013. No striking change in the frequency of different compositional types is observed along the spacecraft trajectory. However, a varying size distribution of different compositional populations of ice grains is observed and discussed. Sodium salts and organic compounds are more frequent in relatively large ice grains, whereas pure water ice particles become more abundant in smaller E ring grains. A generally higher number density of ice grains is observed in the close vicinity of Rhea, which might indicate either the presence of an ejecta cloud from Rhea’s surface or a general confinement of particles near the equatorial plane of the E ring.