Saturn's Mysterious Arc-Embedded Moons: Recycled Fluff?

Introduction: The satellites of Saturn less than 150 km radius occupy several dynamical niches: within rings (Pan, Daphnis, Atlas, Prometheus, Pandora), Coorbiting (Janus and Epimetheus), orbiting among faint rings or in ring arcs (Aegaeon, Methone, Anthe, Pallene) [1], and populating libration regions of larger moons (Telesto, Calypso, Polydeuces, and Helene). Hyperion orbits between Titan and Iapetus, and Phoebe is in a retrograde orbit well outside the orbits of the large satellites along with many other irregular satellites. The inner satellites groups are morphologically distinct from one another. All are low density: those for which masses and sizes have been well measured [2,3], have mean densities ρ of ~430-640 kg m -3 . Cassini images of the arc-embedded moons show these are far smoother and more ovoid than other small satellites, whether of Saturn or other planets (and compared to small asteroids and comets) (Fig. 1). Methone and Pallene have been imaged well enough to use standard limb-finding and ellipsoidal solutions (54 and 14 pixels/mean radius); Aegaeon is less well measured (3 pixels/radius); fit ellipsoid shapes are given in Table 1. Methone is particularly smooth; the best images have 27 m/pixel and still show no surface or limb topography.