During the Grand Finale orbits, the Cassini spacecraft orbited Saturn with high inclinations and perikrones near the ring plane crossings. The Cosmic Dust Analyzer sampled ice and mineral dust particles during these orbits. In previous work, time-of-flight mass spectra of ring debris close to the ring plane have been analyzed, with the main finding that the detected submicron mineral dust particles consist mostly of Fe-depleted silicates. Here we investigate silicate particles detected at unexpectedly high latitudes of up to 3 Saturnian radii ( R _S ) distance above and below the ring plane. We find striking compositional similarities to those detected close to the main rings, suggesting that these silicates could likewise originate from there. The spatial distribution of lifted main ring material can roughly be constrained to the radial extent of the rings themselves. From there outward water-ice particles become dominant, indicating a vertical E ring extension to 3 R _S distance to the ring plane here. Using dynamical simulations, we examine conditions under which main ring particles could be ejected to these surprisingly high latitudes. We identify a plausible scenario by assuming ejection velocities of >25 km s ^–1 and radii <20 nm, allowing for trajectories bent by electromagnetic forces. Such high ejection velocities may be explained if the observed particles form from condensation of a fast vapor phase after micrometeoroid impact onto the rings, a mechanism that has recently been suggested to indeed eject preferably silicate material in the observed size range from the water-ice-dominated rings of Saturn.

During the Grand Finale orbits, Cassini's Cosmic Dust Analyzer (CDA) recorded in situ mass spectra of ice and mineral nanodust grains ejected from Saturn's main rings falling into the planet's atmosphere. We present a compositional analysis of the mineral dust fraction employing a spectral deconvolution method to determine the elemental composition of these grains. The results indicate a relatively homogenous composition of exclusively Mg-rich silicates, with Mg, Si, and Ca close to CI chondritic abundances but a significant depletion in Fe and only traces of organic material at best. The Fe depletion becomes even more pronounced when compared to Fe-rich interplanetary dust particles encountered by CDA in the Saturnian system, which are assumed to contaminate and darken the main rings over time. We discuss potential explanations for the depletion, from which we favour compositional alteration of the infalling dust grains by impact-triggered chemistry in combination with dynamical selection effects and instrumental bias as the most plausible ones. This might cause an accumulation of Fe in the main rings over time, most likely in the form of oxides.

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.

ABSTRACT Before the end of its mission, the Cassini spacecraft orbited Saturn in a series of highly inclined elliptical ‘Ring-Grazing’ orbits (RGO). During the RGO, the spacecraft passed repeatedly through the ring plane outside the F ring, near the orbits of Janus and Epimetheus, at an average relative speed of ∼20 km s–1. For the first time, Cassini’s Cosmic Dust Analyser (CDA) directly sampled dust particles from this region. Here, we analyse the compositions of dust grains sampled within ±15 min relative to nine ring plane crossings of the RGO. The compositions of most analysed RGO grains are similar to those of E ring ice grains, implying that the E ring extends to within at least 2.45 Saturn radii (RS) of Saturn. The compositional distribution of these grains point at a similar average period (decades) since ejection from Enceladus as of particles in the outer E ring (beyond 8 RS). Higher fractions of larger grains are found near the orbits of Janus and Epimetheus, which probably represent ejecta from these moons. Most of these grains have compositions similar to the background E ring grains, indicating that E ring material is coating the surfaces of Janus and Epimetheus. We also report the detection of several types of mineral grains on prograde orbits, one of which, a water ice/silicate mixture, has never been observed by CDA elsewhere. These mineral grains appear to have a different origin from the E ring, and may arise from nearby moons, the F ring, or main rings.

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.

Measurements during the final phase of the Cassini mission revealed the composition of individual dust particles, ejected by micrometeoroid impacts from Saturn’s main rings. These measurements were conducted with the in situ time-of-flight mass spectrometer of the Cosmic Dust Analyzer (CDA) [1] during the close planet encounters of the Grand Finale Orbits from April to September of 2017.Here we present the compositional analysis of silicate bearing nanoparticles (about 20–100 nm in radius), that, according to our dynamical models [2], are ejected mainly from the B and C rings by micrometeoroid impacts. With an observed ice-to-silicate particle ratio of 2:1 [2], we see a much higher silicate abundance in these ring segments, compared to values constrained by remote sensing techniques [3,4,5].In order to assess the elemental composition of individual particles, application of a deconvolution technique to the CDA mass spectra is required. This technique is based on an approach to constrain the composition of Interstellar Dust Particles (ISDs), also detected with CDA [6]. After application of the deconvolution and Relative Sensitivity Factors (RSFs) [7], elemental abundances for the individual particles are derived.We find Mg, Si and Ca similar to cosmic abundances (ISD and CI chondritic). Fe, however, is significantly depleted, for the Fe/Mg ratio on average by a factor of 2.3 compared to cosmic abundances. This observation contrasts with Fe-rich (≈ 5 w.r.t. cosmic abundances) exogenous material (IDPs), observed in the Saturnian system by CDA [8]. This drastic discrepancy in composition between ring silicates and IDPs at Saturn seems difficult to reconcile with IDPs being the main factor in polluting and darkening the rings over time [4,5,9,10]. We review several scenarios, how these compositional differences could be explained.References[1] R. Srama et al. (2004), Space Science Reviews 114, 465–518.[2] H.-W. Hsu et al. (2018), Science 362.[3] E. Epstein et al. (1984), Icarus 58, 403–411.[4] Zhang et al. (2017a), Icarus 281, 297–321.[5] Zhang et al. (2017b), Icarus 294, 14–42.[6] N. Altobelli et al. (2016), Science 352, 312–318.[7] K. Fiege et al. (2014), Icarus 241, 336–345.[8] C. Fischer et al. (2022), this conference.[9] J. Cuzzi and P. Estrada (1998), Icarus 132, 1–35.[10] J. Cuzzi et al. (2009), Springer, Dordrecht, 459–509.

Here we highlight recent advances in our knowledge about Saturn's ring system and bring forward the outstanding science issues that could be addressed by studying the ring systems of the ice giants.We focus on interactions between planetary rings and other elements in the system, including the moons, host planet, and its magnetosphere, and conclude that ring science investigations, in accordance with magnetospheric and atmospheric science disciplines, are essential in advancing our knowledge of solar system evolution, the origin and evolution of the moons and Ocean Worlds, as well as contemporary phenomena observed in the ice giant systems.We request that the study of ice giant ring systems to be considered a top priority for all future ice giant explorations.

Here we highlight recent advances in our knowledge about Saturn's ring system and bring forward the outstanding science issues that could be addressed by studying the ring systems of the ice giants. We focus on interactions between planetary rings and other elements in the system, including the moons, host planet, and its magnetosphere, and conclude that ring science investigations, in accordance with magnetospheric and atmospheric science disciplines, are essential in advancing our knowledge of solar system evolution, the origin and evolution of the moons and Ocean Worlds, as well as contemporary phenomena observed in the ice giant systems. We request that the study of ice giant ring systems to be considered a top priority for all future ice giant explorations.