What is a Shepherd Moon?

Shepherd moons are small bodies sequestered within planetary rings. These diminutive moons, which can resemble asteroids, play an important role in keeping the rings of the four gas giants (Saturn, Neptune, Jupiter, and Uranus) neat and tidy.

One of the most notable shepherd moons, Prometheus, so named for the Titan in Ancient Greek mythology, is an elongated, icy rock skirting the inner edge of Saturn’s F-ring.

Prometheus, along with another shepherd moon, Pandora, helps define the F-ring. The gravitational pull of the two bodies removes loose dust and ice from the gaps between the rings, either by consuming it, returning it to the rings, or pushing it out of the system.


>>above: Prometheus (bottom) and Pandora. The F-ring, estimated to be just a few hundred kilometres wide, is between the two shepherd moons. Image by NASA.

Shepherd moons can also affect the structure of planetary rings directly by ‘stealing’ material from them. This action can gouge dark bands and channels in the ring, and cause orderly lines of material to twist and distort.

Astronomers believe that all of Saturn’s rings are governed by shepherd moons to some extent. Equally, Uranus and Jupiter both have ring sculptors in their midst, while Neptune has at least one shepherd moon, the namesake of a living statue in Greek mythology, Galatea.

>>above: Prometheus ‘steals’ material from Saturn’s F-ring.

Galatea, a moon with arguably the worst passport photo in astronomy, has been cited as the cause of an as-yet unexplained phenomena in Neptune’s rings; that of large arcs in the Adam’s ring.

These arcs, which manifest as bright patches along the length of the planet’s rings, indicate that material is concentrated in one particular area, possibly due to the gravitational pull of Galatea.

So, there you have it; the answer to a question you never asked.


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