In Situ Analysis of Silicate Particles Originating from Saturns Main Rings

During the Grand Finale Orbits Cassini traveled through the gap between Saturn and its innermost D ring. Sampling the dust population, mostly impact ejecta from the main rings, in the vicinity of the planet was one objective of these orbits. The in situ measurements were carried out by the Cosmic Dust Analyzer (CDA), providing time-of-flight mass spectra of individual ~1050 nm sized ice and dust grains. Here we present an update on the composition of the silicate dust fraction originating from Saturns main rings, which makes up about 30 % of the observed particles with water ice grains being the remaining fraction [1]. Based on a technique originally used to interpret CDA interstellar dust detections [2], we applied a manual deconvolution to infer the elemental composition of the silicate particles. This is necessary as CDAs relatively low mass resolution (m/dm = 2050) [3], means neighboring spectral peaks of important mineral-forming ions such as Mg+, Al+ and Si+ are often unresolvable. The robustness of the deconvolution technique has been optimized through comparison with an independent automated algorithm. The derived spectral ion abundances were combined with experimentally-determined relative sensitivity factors (RSFs) [4], to calculate elemental abundances within the particles. In order to place the derived element ratios in context, we compared them with those of a selection of space-relevant materials, finding similarities with cosmic abundances for Mg, Si and Ca, but with Fe being significantly depleted. An analysis of implications for the formation of Saturns rings is ongoing. Additionally, we present intermediate results of an evolving dynamical model, from which the likely main ring source regions of individual silicate grains can be derived. An analysis of the compositional diversity between the different ring segments is under way. References [1] H.-W. Hsu et al. (2018), Science 362. [2] N. Altobelli et al. (2016), Science 352, 312318. [3] R. Srama et al. (2004), Space Science Reviews 114, 465518. [4] K. Fiege et al. (2014), Icarus 241, 336345.