In situ collection of dust grains falling from Saturn's rings into its atmosphere

INTRODUCTION During the Cassini spacecraft’s Grand Finale mission in 2017, it performed 22 traversals of the 2000-km-wide region between Saturn and its innermost D ring. During these traversals, the onboard cosmic dust analyzer (CDA) sought to collect material released from the main rings. The science goals were to measure the composition of ring material and determine whether it is falling into the planet’s atmosphere. RATIONALE Clues about the origin of Saturn’s massive main rings may lie in their composition. Remote observations have shown that they are formed primarily of water ice, with small amounts of other materials such as silicates, complex organics, and nanophase hematite. Fine-grain ejecta generated by hypervelocity collisions of interplanetary dust particles (IDPs) on the main rings serve as microscopic samples. These grains could be examined in situ by the Cassini spacecraft during its final orbits. Deposition of ring ejecta into Saturn’s atmosphere has been suggested as an explanation for the pattern of ionospheric H 3 + infrared emission, a phenomenon known as ring rain. Dynamical studies have suggested a preferential transport of charged ring particles toward the planet’s southern hemisphere because of the northward offset of Saturn’s internal magnetic field. However, the deposition flux and its form (ions or charged grains) remained unclear. In situ characterization of the ring ejecta by the Cassini CDA was planned to provide observational constraints on the composition of Saturn’s ring system and test the ring rain hypothesis. RESULTS The region within Saturn’s D ring is populated predominantly by grains tens of nanometers in radius. Larger grains (hundreds of nanometers) dominate the mass density but are narrowly confined within a few hundred kilometers around the ring plane. The measured flux profiles vary with the CDA pointing configurations. The highest dust flux was registered during the ring plane crossings when the CDA was sensitive to the prograde dust populations (Kepler ram pointing) (see the figure). When the CDA was pointed toward the retrograde direction (plasma ram pointing), two additional flux enhancements appeared on both sides of the rings at roughly the same magnetic latitude. The south dust peak is stronger and wider, indicating the dominance of Saturn’s magnetic field in the dynamics of charged nanograins. These grains are likely fast ejecta released from the main rings and falling into Saturn, producing the observed ionospheric signature of ring rain. We estimate that a few tons of nanometer-sized ejecta is produced each second across the main rings. Although this constitutes only a small fraction ( CONCLUSION Our observations illustrate the interactions between Saturn and its main rings through charged, nanometer-sized ejecta particles. The dominance of nanograins between Saturn and its rings is a dynamical selection effect, stemming from the grains’ high ejection speeds (hundreds of meters per second and higher) and Saturn’s offset magnetic field. The presence of the main rings modifies the effects of the IDP infall to Saturn’s atmosphere. The rings do this asymmetrically, leading to the distribution of the ring rain phenomenon. Confirmed ring constituents include water ice and silicates, whose ratio is likely shaped by processes associated with ring erosion processes and ring-planet interactions.