Compositional in situ analysis of dusty material stemming from Saturn’s main rings

During the final mission phase, the Cassini spacecraft travelled through the gap between Saturn and its innermost D ring. One goal of these highly inclined orbits was sampling the dust population, mostly made of impact ejecta from the main rings, in the vicinity of the planet. These in situ measurements were primarily carried out by the Cosmic Dust Analyzer (CDA) onboard the spacecraft, which provided time-of-flight mass spectra of individual ice and dust grains, mostly between about 10 and 50 nm in size. Here we present an update on the composition of the silicate dust fraction stemming from Saturn’s main rings, which makes up about 30 % of the observed particles with water ice being the remaining fraction [1].

Elemental analysis of the silicate spectra was performed using an updated deconvolution method, based on a technique originally applied to the interpretation of CDA interstellar dust measurements [2]. Neighboring spectral peaks due to mineral-forming ions such as Mg⁺, Al⁺ and Si⁺ are often unresolvable, because of CDA’s relatively low (m/dm = 20–50) mass resolution [3]. Therefore, application of a deconvolution technique is required to disentangle the peak interferences and derive valuable compositional information. The robustness of the applied method has been tested and optimized through comparison with an independent automated fit algorithm. In order to calculate elemental abundances within the particles, the derived ion abundances were combined with experimentally-determined relative sensitivity factors (RSFs) [4]. To provide context to the measured element ratios, we compared them with a variety of space-relevant materials. We find an overlap with chondritic material for Mg/Si and Fe/Mg ratios. The observed range within the element ratios, however, indicates the contribution of a variety of minerals such as olivine, plagioclase or pyroxenes. Although our results agree with realistic mineral compositions, the calculated abundances of Al⁺ ions are still relatively uncertain and can be seen as an upper limit.

Additionally, we present the results of a dynamical model, which allow us to derive the likely source region within the main rings of individually detected silicate grains. We find the C and B rings to be the most likely sources of the vast majority of grains with the D ring being only a minor source. Currently an analysis of compositional diversity between the different ring segments is under way.

 

References

[1] H.-W. Hsu et al. (2018) In situ collection of dust grains falling from Saturn’s rings into its atmosphere. Science 362.

[2] N. Altobelli et al. (2016) Flux and composition of interstellar dust at Saturn from Cassini’s Cosmic Dust Analyzer. Science 352, 312–318.

[3] R. Srama et al. (2004) The Cassini Cosmic Dust Analyzer. Space Science Reviews 114, 465–518.

[4] K. Fiege et al. (2014) Calibration of relative sensitivity factors for impact ionization detectors with high-velocity silicate microparticles. Icarus 241, 336–345.