The Cassini mission and possible tests of theoretical models of planetary rings.
N. Gorkavyi (Greenwich Institute for Science and Technology)
Many mysteries of planetary rings can be solved during the Cassini mission. Is the mechanism destroying the ring particles collisional or tidal? Are there particles in the rings larger than tens of meters? Are the narrow elliptical rings independent stable formations or are they controlled by nearby shepherd-satellites? Is there a very thin - of a few hundred meters - stratification of the rings? Testable predictions from theoretical models for collisional, collective and resonance phenomena in planetary rings (A.M. Fridman, N.N. Gorkavyi, "Physics of Planetary Rings. Celestial Mechanics of Continuous Media", Springer, 1999):
1. Collisional break-up of the ring particles is the cause for the existence of rings and azimuthal brightness asymmetry. The density of the largest particles (5-10 m) is ~0.1-0.15 g/cm3 (loose snow); breaking strength is ~100 dyne/cm2; break-up energy is ~0.01 erg/cm3 (p.95-130).
2. Formation and stability of narrow Saturnian ringlets in the C-ring and the Cassini gap is the result of positive ballistic drift due to dust flows from the B-ring and negative drift due to satellite resonances (p.409, similar Uranian ringlets: p.233). Shepherds-satellites larger than tens of meters don't exist inside the Saturnian rings (excluding outermost part of the A-ring) or near elliptic ringlets (excluding the F-ring). Eccentricity of narrow ringlets without shepherds is the result of the ellipse-instability (p.184).
3. Resonant spiral waves must be located inside each "mesa-ringlets" (non-elliptical rings with width 100-200 km) in the C-ring (p.409). Relatively large satellite can be located near edge of the A-ring (possible orbit is 136,855 km) (p.416).
4. Diffusion/dissipative instabilities can cause invisible thin (hundreds meters) ringlets in the dense areas like the B-ring (p.168). Large-scale structure of the rings (~50-1000 km) is the result of the accretional instability (p.178) and can be more colored by dust.
5. F-ring clumps can be ephemeral analogs of permanent epitons inside Neptunian arcs (p.253).