[P-PS21_29PM1] Planetary Sciences

[PPS21-19] Gravitational accretion of particles onto moonlets embedded in Saturn's rings

*Yuki YASUI¹, Keiji OHTSUKI¹, Hiroshi DAISAKA² (1.Department of Earth and Planetary Sciences, Kobe University, 2.Graduate School of Commerce and Management, Hitotsubashi University)

Keywords:gravitational accretion, moonlet, Saturn's rings

Collision and gravitational accretion of particles is an important issue related to the origin of ring-satellite systems of giant planets in the solar system. The Hill radii of Pan, Daphnis, Atlas, and Prometheus are found to be within 15 % of the observed long axes of these satellites given by the best-fit model ellipsoids. Also, the densities of these satellites (0.4 - 0.6 g cm^-3) are very low compared to the density of water ice and all approximately equal to the critical density at that distance, which is defined as the density of a body that entirely fills its Hill sphere. From these results, the small satellites within the orbit of Pandora are thought to be formed by accretion of small porous ring particles onto large dense cores, and further accretion seems to have been suppressed when the density of the satellite reaches the critical density at that distance. Local N-body simulations also demonstrated that a Hill sphere-filling body is produced by accretion of small porous particles onto a large dense core. However, it has not been studied how the degree of particle accretion onto moonlets in the inner parts of Saturn's rings depends on the distance from Saturn.The shapes of these small ringmoons would also provide clues to the dynamical evolution of Saturn's rings. The fact that the shapes of these ringmoons approximately match those of their associated Hill sphere suggests that the moonlet cores were surrounded by a number of particles when they were formed. On the other hand, Pan and Atlas have the characteristic shapes with equatorial ridges, and are thought to be formed by two stages. First, their precursors whose shapes are similar to their Hill sphere without equatorial ridges were formed when the rings were thick. Then, equatorial ridges were formed through particle accretion onto the equatorial planes of the above formed objects after the rings became sufficiently thin and also before ring particles diffused. However, effects of dynamical properties of the rings on the shaping of moonlets formed by particle accretion have not been examined in detail.Propeller-shaped structures have also been found in Cassini images of Saturn's rings. These propeller-shaped features are explained by gravitational interaction between ring particles and unseen embedded moonlets. From these observations, the sizes and orbital distributions of these unseen embedded moonlets are obtained, and such information provide us with clues to the evolution of the ring-satellite system. The propeller-shaped structures are mainly observed in the A ring. Recently, observations of similar structures have also been reported for the Cassini Division, and the B and C rings. Although some of these moonlets either may be collisional shards resulting from the breakup of a bigger icy progenitor ring body or may have formed by accretion of small low-density ring particles onto larger dense fragments, the origin of these moonlets is not clear. Using local N-body simulation, we examine gravitational accretion of ring particles onto moonlets in Saturn's rings. We find that gravitational accretion of ring particles onto moonlets is unlikely to occur at radial locations interior to the outer edge of the C ring, unless the density of the moonlets is much larger than that of water ice or non-gravitational cohesive forces play a major role. Detailed analysis of accretion process of individual particles onto moonlets shows that particle accretion onto high-latitude regions of the moonlet surface occurs even if the rings' vertical thickness is much smaller than the moonlet's radius. The degree of particle accretion in outer rings is found to depend significantly on rings' vertical thickness and optical depth. Our results suggest that large boulders recently inferred from observations of transparent holes in the C ring are likely to be collisional shards, while propeller moonlets in the A ring would be gravitational aggregates formed by particle accre