Presentation Information
[PPS01-P21]Diffusion Process of Ejecta from Saturn's Moon Enceladus: Toward Estimating Accretion Rates into Saturn's Main Ring
*Takayuki Sakamoto1, Jun Kimura1, Taro Matsuo1, Sho Sasaki1 (1.The University of Osaka)
Saturnian moon Enceladus continuously ejects water from its south polar region. A fraction of which spreads around its orbit and forms Saturn’s E ring. Over long timescales, the ejecta are expected not only to remain within the E ring region but also to diffuse throughout the Saturnian system and to accrete onto other satellites and Saturn’s main rings. However, the long-term and large-scale transport of the ejecta has not yet been sufficiently studied. Quantitatively constraining this transport is crucial for understanding exogenic modifications of surface composition and morphology and for reconsidering estimates of their surface ages.
Most previous studies have focused on the dynamics of ejected water vapor. In contrast, the solid ice grains, which constitute approximately 10% of the ejecta, remains less well constrained, and the acting forces are often restricted to gravity. Ice grains moving through Saturn’s magnetosphere collect ambient electrons and ions from the magnetospheric plasma, become electrically charged, and experience the Lorentz force in Saturn’s magnetic field. For micron-sized grains, this perturbation is non-negligible, consequently, their orbital evolution is expected to differ fundamentally from that neutral water vapor. At the same time, energetic particles in the plasma environment, particularly hot electrons erode grain surfaces by sputtering, release of water molecules, and reduce grain mass. This mass-loss shortens grain lifetimes and may remove grains before they reach other bodies, including the main rings. Therefore, to accurately determine how far solid grains can travel, it is necessary to model electromagnetic forces in addition to gravity and to include the mass-loss processes that control survival.
In this study, we performed numerical simulations that trace the three-dimensional orbital evolution and mass variation of micron-sized ice grains ejected from Enceladus. Besides Saturn’s gravity, we included the Lorentz force determined by the grain charge state. The equilibrium potential was derived from the balance among ion and electron collection currents, photoelectron emission driven by solar radiation, and secondary electron emission, incorporating their spatial variations. We also included sputtering-driven mass loss caused by ions, cold electrons, and hot electrons. The simulations show that trajectories differ significantly from those under gravity alone. The Lorentz force associated with Saturn’s corotating plasma gradually transports grains inward toward Saturn. A grain with an initial radius of 1 µm released near Enceladus’ orbit at a distance of about 240,000 km from Saturn migrates to about 172,000 km within about 100 years. During this evolution, cold electron collection produces negative charging, and equilibrium surface potential approaches about -0.75 V. Concurrently, erosion by hot electrons causes substantial mass loss, and the grain radius decreases to about 1/100 of its initial value over the same period. These results suggest that micron-sized grains can drift inward to the orbit of Mimas, whereas many are likely to disappear before reaching the main rings. Nevertheless, uncertainties remain in the spatial and temporal variability of the plasma environment, in sputtering efficiencies, and in the assumed initial grain size. Under different conditions, delivery to the main rings may still occur. In this presentation, we will report the large-scale transport of plume grains under coupled effects of gravity, electromagnetic forces, and sputtering. We will present the fraction and accretion rate of grains that reache the main rings, and compare them with previously estimated vapor delivery and the influx of interplanetary dust. This comparison enables a quantitative discussion of how Enceladus' activity may influence interpretations of Saturn’s ring age.
Most previous studies have focused on the dynamics of ejected water vapor. In contrast, the solid ice grains, which constitute approximately 10% of the ejecta, remains less well constrained, and the acting forces are often restricted to gravity. Ice grains moving through Saturn’s magnetosphere collect ambient electrons and ions from the magnetospheric plasma, become electrically charged, and experience the Lorentz force in Saturn’s magnetic field. For micron-sized grains, this perturbation is non-negligible, consequently, their orbital evolution is expected to differ fundamentally from that neutral water vapor. At the same time, energetic particles in the plasma environment, particularly hot electrons erode grain surfaces by sputtering, release of water molecules, and reduce grain mass. This mass-loss shortens grain lifetimes and may remove grains before they reach other bodies, including the main rings. Therefore, to accurately determine how far solid grains can travel, it is necessary to model electromagnetic forces in addition to gravity and to include the mass-loss processes that control survival.
In this study, we performed numerical simulations that trace the three-dimensional orbital evolution and mass variation of micron-sized ice grains ejected from Enceladus. Besides Saturn’s gravity, we included the Lorentz force determined by the grain charge state. The equilibrium potential was derived from the balance among ion and electron collection currents, photoelectron emission driven by solar radiation, and secondary electron emission, incorporating their spatial variations. We also included sputtering-driven mass loss caused by ions, cold electrons, and hot electrons. The simulations show that trajectories differ significantly from those under gravity alone. The Lorentz force associated with Saturn’s corotating plasma gradually transports grains inward toward Saturn. A grain with an initial radius of 1 µm released near Enceladus’ orbit at a distance of about 240,000 km from Saturn migrates to about 172,000 km within about 100 years. During this evolution, cold electron collection produces negative charging, and equilibrium surface potential approaches about -0.75 V. Concurrently, erosion by hot electrons causes substantial mass loss, and the grain radius decreases to about 1/100 of its initial value over the same period. These results suggest that micron-sized grains can drift inward to the orbit of Mimas, whereas many are likely to disappear before reaching the main rings. Nevertheless, uncertainties remain in the spatial and temporal variability of the plasma environment, in sputtering efficiencies, and in the assumed initial grain size. Under different conditions, delivery to the main rings may still occur. In this presentation, we will report the large-scale transport of plume grains under coupled effects of gravity, electromagnetic forces, and sputtering. We will present the fraction and accretion rate of grains that reache the main rings, and compare them with previously estimated vapor delivery and the influx of interplanetary dust. This comparison enables a quantitative discussion of how Enceladus' activity may influence interpretations of Saturn’s ring age.