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DDP

Seminar

The real thing: Saturn's ring

Burns, J (Cornell)
Wednesday 19 August 2009, 10:40-11:00

Meeting Room 2, CMS

Abstract

Of all dense astrophysical discs, only Saturn's rings can be studied in detail. Cassini observations reveal examples of many processes that are likely relevant in the dynamical evolution of debris discs, such as the interactions of the disc's particles with one another, with local masses and with more distant masses via resonances. Material accretion and breakup have been inferred elsewhere, and even non-gravitational forces are found to sculpt some regions. Resonances account for much of the rings's architecture that is understood. Lindblad resonances with Mimas (2:1) and the co-orbital moons (7:6) constrain the exterior perimeters of the A and B rings. Density and bending waves, initiated at resonances with satellites, are abundant in the outer A ring, where they transfer angular momentum between the satellites and the rings. These waves indicate disc's physical properties, which vary smoothly across this region. Gaps may also be opened by resonances with a lumpy planetary gravity field or with non-uniform rings. Structures in dust-laden rings are visible at Lindblad resonances with the planetary spin rate, likely driven by electromagnetic interactions. Satellites with radii ~ 15km and ~ 4km open the Encke and Keeler gaps, generating undulations along the gap edges that are remarkably persistent and surprisingly complex; the A and B peripheries are also complicated. It is unclear whether this morphology alters angular-momentum transfer. “Propellers”, believed to be disturbances generated by unseen embedded moonlets (tens to scores of meters), are concentrated in three bands in the mid-A ring. Some very large propellers (from >100-m objects) are found in the outermost A ring; one's orbit is noticed to evolve, perhaps exhibiting smooth Type-I migration or stochastically scattering off density clumps. Self-gravity wakes develop in the A ring, but the full agglomeration of moonlets is frustrated by Saturn's tides. These clumps form ephemeral elongated structures with height-to-width ratios of ~1x10; regions between wakes are fairly clear. Close-in moons have low densities (~0.5 g/cc) and nearly fill their Hill spheres. Even though the dense B ring is almost opaque (optical depth ƒ ~ 5), concentric holes are occasionally visible; in places, its ƒ jumps repeatedly between two values over radial spans of hundreds of km.

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