F Ring and Shepherd Satellites

The F-Ring, Prometheus (inner moon), and Pandora (outer moon). The A-Ring fills up the bottom half of the image with the Keepler Gap easily visible

Image Credit:

NASA/JPL/Voyager

 

Saturn's F-Ring is the outermost of the discrete rings discovered in 1979 by Pioneer 11. Compared to the other rings, it is very active with features changing in the structure of the ring on a timescale of hours. The F-Ring is 3,000 km from the A-Ring and is separated from the A-Ring by the Roche Division. Compared to the other discrete rings, the F-Ring is very narrow, only a few hundred kilometers thick. So how exactly does the F-Ring maintain its shape?

The ring is between the orbits of two satellites, Prometheus and Pandora. Prometheus orbits just inside the inner edge of the F-Ring and Pandor just outside the outer edge. These two satellites are able to use their gravitational influence on the ring to keep it stationary and in place. If these moons were not there, the F-Ring would have dissipated long ago.

Prometheus also creates kinks and knots in the ring from its orbit which show up in the ring when Prometheus is at apoapis (farthest distance from Saturn). Because Prometheus does have an elliptical orbit, at each successive apoapis, the knots and kinks are 3.2° ahead of the previous section.

Prometheus creating knots and streamers in the inner F-Ring

Image Credit:

Cassini/ISS/JPL/EPA/NASA