The Rings of
Saturn


The ring system of Saturn is divided into 5 major components: the G, F, A, B, and C rings, listed from outside to inside (but in reality, these major divisions are subdivided into thousands of individual ringlets). The F and G rings are thin and difficult to see, while the A, B, and C rings are broad and easily visible. The large gap between the A ring and and the B ring is called the Cassini division.

The adjacent image is a rare view of Saturn's rings seen just after the Sun has set below the ring plane, taken with the Hubble Space Telescope on Nov. 21, 1995. This perspective is unusual because the Earth is slightly above and the Sun slightly below the rings. Normally we see the rings fully illuminated by the Sun.

Three bright ring features are seen: the F Ring, the Cassini Division, and the C Ring (moving from the outer rings to the inner). The low concentration of material in these rings allows light from the Sun to shine through them. The A and B rings are much denser, which limits the amount of light that penetrates through them. Instead, they are faintly visible because they reflect light from Saturn's disk (Ref).

Ring Structure and Composition

High resolution photographs from the Voyager missions indicate that the rings of Saturn are composed of hundreds of thousands of "ringlets", and that regions like the largest "gap" called the Cassini division, also contain fainter rings (adjacent image). The rings cannot be solid, because they lie inside the Roche limit. They presumably represent either a satellite torn apart by tidal forces, or (more likely) material that was never allowed to condense into moons because of the tidal forces. The evidence indicates that the rings are composed of particles that are mostly ice crystals, with sizes as large as centimeters or meters. The total mass in the rings is about the size of a medium mass moon, and the rings are only about 10 km thick.


Spokes and other Structure

It was expected that collisions between ring particles would tend to make the rings uniform, but Voyager I found changing structures in the radial direction that are termed "spokes". Some of this structure is shown in the adjacent animation. Here is a smoother and longer movie of the same phenomenon. It is thought that gravitational forces alone cannot account for the spoke structure, and it has been proposed that electrostatic repulsion between ring particles may play a role.


The Voyagers found that the rings were not necessarily circular, and even found rings that appeared to be braided (adjacent image). They found further that the outer ring was kept in place by the gravitational interaction of two small "shepherd moons" lying just inside and outside it, and that at least some of the other rings are kept narrow by similar small shepherding satellites. Generally, although we have increased immensely our knowledge of the rings of Saturn over the last 2 decades, we still do not fully understand their structure, dynamics, or origin.



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