Dynamical analysis of mineral dust in the Saturnian system

Dust in the Saturnian system is dominated by ice grains stemming from the active moon Enceladus [1,2]. Moreover, a significant population of 1859 sub-micron sized mineral dust grains were detected by the Cosmic Dust Analyser (CDA) aboard the Cassini spacecraft. CDA inferred the composition of dust particles with an impact ionisation mass spectrometer via time of flight mass spectroscopy. The successful compositional characterization of 36 interstellar dust particles recorded by CDA showed the potential of this approach [3].

Our previous dynamic analysis [4] conservatively estimated orbital parameters of this non-icy particle population of which many dust particles occurred in confined time intervals (swarms).  Two main dynamic populations were identified: retrograde potentially endogenous and polar potentially exogenous swarms. Compositional analysis revealed two main types: iron-rich sulfide and oxide particles (58%) and Mg-rich silicates (34%) while a small share (8%) consisted of mixed type particles [4]. Retrograde swarms contained a significantly higher fraction of iron rich grains compared to exogenous swarms.

Here we present a refined approach to reconstruct orbital parameters. Considering the occurrence of certain element mass lines [5] within the mass spectra, we derive a minimum impact velocity. Assuming that the grains are bound to the Sun, we obtain as an upper bound for the impact speed onto CDA the escape velocity from the solar system at Saturn distance. Due to CDA's large field of view the impact direction is constrained within a range of 56°. We take into account the angular dependence of the sensitive area of the impact Target of CDA to derive the probability distribution of impact directions for a given detection. Combining both constraints we can determine a probability distribution density for the orbital elements which we use to evaluate the mean eccentricity and inclination for each swarm as well as for single detections.

Again, two disjoint dynamic populations are identified: almost certainly endogenous swarms with retrograde inclinations of about 170° and high-eccentricity exogenous swarms with nearly polar inclinations. The inclination of the retrograde endogenous particles is consistent with the previously suggested origin [4] from impact ejecta of Saturn's retrograde outer moons released by micro-meteoroid bombardment. In order to trace the origin of the exogenous particles their hyperbolic orbits in the Saturnian system are projected onto Saturn’s Hill sphere.

In this ongoing work we aim to identify potential sources like the Kuiper Belt and Oort Cloud or Centaur comets and present an updated compositional analysis with the potential to constrain the composition of the sources of the grains detected by CDA.

References

[1] Postberg F. et al (2008) Icarus 193, 438-454.

[2] Postberg F. et al. (2009) Nature 459, 1098-1101.

[3] Altobelli N. et al. Science 352, 312-318 (2016)

[4] Fischer C. et al. Poster EPSC 2018

[5] Fiege K. et al. Icarus 241, 336-345 (2014)