Cassini observations and the history of Saturn's rings

Details Cassini observations and the history of Saturn's rings Cassini observations and the history of Saturn's rings

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.4701e&link_type=abstract

American Astronomical Society, DPS meeting #38, #47.01; Bulletin of the American Astronomical Society, Vol. 38, p.572

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Scientific paper

Cassini observations show unexpected ring variability in time and space.The rings are inhomogeneous,
with sharp gradients and edges, including structure on all scales. Compositional
gradients are sharper than expected, but nonetheless cross structural boundaries. This
is evidence for ballistic transport that has not gone to completion. The autocovariance
maximizes in the middle of the A ring, with smaller structure near the main rings
outer edge. Density wave locations have a fresher ice composition. The processes of
collisions, diffusion and transport should have homogenized the rings over the age of
the solar system. Instead, these differences persist. The mass density in the Cassini division
inferred from density waves is so low, that the material there would be ground to
dust in 100,000 years. The observed moons that cause such interesting structure in the
rings have short lifetimes against disruption by cometary bombardment and against
the angular momentum transfers that push them away from the rings.
These rapid processes evident in the Cassini data have been taken as evidence that
the rings were recently created, perhaps from a comet that passed too close to Saturn.
An alternative is that primordial material may have been re-used and recycled. In the
zone near the Roche limit where rings are found, limited accretion is possible, with
the larger bodies able to recapture smaller fragments. The propeller structures, the self-gravity wakes, and the size distribution of clumps in Saturn's F ring are all indications of the accretion process. Recycling could extend the ring lifetime almost indefinitely. The variety evident in the Cassini observations and the
low mass density of the largest bodies are both consistent with extensive
recycling of ring material to explain the apparent youth of Saturn's rings. Similar processes occur in the other ring systems and in the planet formation around other stars.

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