Hua-Shan Shi (Department of Space Science & Engineering, National Central University, Taiwan); Wing-Huen Ip (Institute of Astronomy, National Central University, Taiwan),
The “Ring Rain” effect with a large injection rate of nano-sized dust grains of ring origin falling into the Saturnian atmosphere, was detected by the Cassini Dust Analyzer (CDA), the Magnetospheric Imaging Instrument (MIMI), and the Ion Neutral Mass Spectrometer (INMS) during the Cassini Proximal mission. It was proposed that the collisional interaction between the entering dust grains and the exospheric gas particles will produce a thin disk of high-speed H-atoms and H2-molecules in eccentric or escaping orbits (Mitchell et al., 2018). Also, the gas particles so accelerated might be correlated with a narrow hydrogen gas plume feature emitted from Saturn’s near-surface. (Shemansky et al., 2009). In this work, we re-investigate the orbital motion of dust particles generated by the D68 ringlet by considering different sizes of the dust grains, and further explore the dynamical behavior of the gas particles. The production rates of the high-speed H-atoms and H2-molecules are estimated by using a series of Monte Carlo collision simulations. The collisions between nano-dust grains in Keplerian orbits and gas molecules of the corotating exosphere would lead to spiraling motion of the dust particles toward the planetary atmosphere, causing the dust particles to enter the atmosphere in less than 10 hours from ejection by the D68 ringlet. The highest collision frequency of dust with a radius of 3 nm occurs at an altitude of 2000-2500 kilometers from the 1-bar level of the Saturnian atmosphere. In addition, the maximum post-collision velocity of the gas particles is close to 40 km/s at 7000 kilometers altitude and the high-speed hydrogen molecules generated on the equatorial plane have a high probability of escaping the Saturnian system. The density and orbital distributions of fast hydrogen atoms and molecules of such dust collision origin will be examined.