What are Shepherd Moons?
‘Shepherd’ moons (or satellites) are small moons that orbit near the outer edges of planetary rings or within gaps in the rings. A planetary ring is a ring of dust and other small particles orbiting around a planet in a flat disc-shaped region. The most spectacular and famous planetary rings are those around Saturn, but all four of the solar system’s gas giant planets (Jupiter, Saturn, Uranus, Neptune) possess ring systems of their own. Examples of shepherd satellites include Saturn’s moons Prometheus and Pandora which shepherd its narrow, outer F ring. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon’s orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself. Peter Goldreich and Scott Tremaine first proposed the idea of shepherd moons in 1979 to explain why the rings of Uranus were so narrow. Adapted from:
Shepherd moons are moons which literally shepherd the contents of a planetary ring, giving it a crisper edge and more definition. The presence of such satellites was suggested by researchers in the 1970s who were struggling to understand planetary rings and the variations in size, gaps, and structure which could be seen in high resolution photographs. Saturn, a planet famous for its ring, has several shepherd moons, and upon close examination, it is possible to see that the disc of material which surrounds the planet is actually composed of numerous rings, rather than a single solid body. In a classic example, shepherd moons appear on either side of a planetary ring, although they can also appear on their own. The moon on the outside slows the particles in the ring, causing them to drop into a lower orbit. Outer moons push particles back into the ring, forcing the ring to have a clearly delineated border. If particles manage to push beyond this border, they end up being accreted onto t
Shepherd moons are small moons that appear to be responsible for defining the boundaries of some — and maybe most — of Saturn’s rings. Their gravitational fields keep the ring particles from straying, much like a shepherd keeps sheep from wandering away from the flock. One startling example is the F-ring, which is tended by two tiny moons, Prometheus and Pandora. They seem to be responsible not only for maintaining the slender ring, but for its apparent braids and kinks. Cassini sent us a picture of these moons and their ring: http://saturn.jpl.nasa.gov/cgi-bin/gs2.cgi?path=../multimedia/images/rings/images/PIA05387.
