Compositional analysis of rocky material in Saturn’s C and D Rings as seen by CDA during Cassini’s Grand Finale Orbits

The gap between Saturn and its innermost D ring is populated with dust particles which primarily originate from the main rings. These particles were sampled by Cassini’s Cosmic Dust Analyzer (CDA) during the final mission phase, providing individual high-speed time-of-flight mass spectra. Compositionally, two main groups were observed, water ice and silicate particles. The relative frequencies of these spectral types vary with distance to Saturn’s ring plane (Hsu et al. 2018).

In this study we present an approach to infer the composition of the silicate particles, which make up nearly 30% of the evaluated spectra, with the aim of deriving the elemental composition of minerals stemming from Saturn’s C and D rings. Owing to CDA’s relatively low mass resolution, spectral peaks from important mineral-forming ions such as Mg⁺, Al⁺ and Si⁺ are often unresolvable individually, forming a single broad peak. To remove this effect, we apply manual deconvolution, enabling the interferences to be disentangled and allowing the relative abundances of the different constituents to be measured. After combining the results of the deconvolution with experimentally-determined relative sensitivity factors (RSFs), converting ion abundances to elemental abundances (Fiege et al. 2014), we are able to infer elemental ratios within the particles and thus quantitatively measure their compositions. The grain detection locations allow C or D ring sources to be discriminated between, and this work represents the first in situ analysis of the rocky fraction in Saturn’s main rings, which until now was only accessible via remote sensing.

 

 

Fiege, K., Trieloff, M., Hillier, J. K., Guglielmino, M., Postberg, F., Srama, R., Kempf, S., Blum, J.: Calibration of relative sensitivity factors for impact ionization detectors with high-velocity silicate microparticles, Icarus 241, 2014.

 

Hsu, H.-W., Schmidt, J., Kempf, S., Postberg, F., Moragas-Klostermeyer, G., Seiß, M., Hoffmann, H., Burton, M., Ye, S.-Y., Kurth, W. S., Horányi, M., Khawaja, N., Spahn, F., Schirdewahn, D., O’Donoghue, J., Moore, L., Cuzzi, J., Jones, G. H., Srama, R.: In situ collection of dust grains falling from Saturn’s rings into its atmosphere, Science 362, 2018.