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SUMMARY:The real thing: Saturn's ring - Burns\, J (Cornell)
DTSTART:20090819T094000Z
DTEND:20090819T100000Z
UID:TALK19495@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION: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 an
 d breakup have been inferred elsewhere\, and even non-gravitational forces
  are found to sculpt some regions. \n\nResonances 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 th
 e A and B rings. Density and bending waves\, initiated at resonances with 
 satellites\, are abundant in the outer A ring\, where they transfer angula
 r 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. \n\nSatellites with radii ~ 15km and ~ 4km open the Encke a
 nd Keeler gaps\, generating undulations along the gap edges that are remar
 kably persistent and surprisingly complex\; the A and B peripheries are al
 so complicated. It is unclear whether this morphology alters angular-momen
 tum 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\, per
 haps exhibiting smooth Type-I migration or stochastically scattering off d
 ensity clumps. \n\nSelf-gravity wakes develop in the A ring\, but the full
  agglomeration of moonlets is frustrated by Saturn's tides. These clumps f
 orm 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 ri
 ng is almost opaque (optical depth  ~ 5)\, concentric holes are occasional
 ly visible\; in places\, its  jumps repeatedly between two values over rad
 ial spans of hundreds of km. 
LOCATION:Seminar Room 1 Newton Institute
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