What are shepherd moons?

Question posed by Jennie.


The most well-known planetary ring system is that of Saturn. Its large, bright, easily observed ring system makes it instantly recognisable by budding astronomers of any age and at any level of education. The rings themselves consist of bands of colour punctuated by dark gaps. The edges of some of the rings are very sharply defined: there is a distinct cut-off point, whereas most rings tend to thin out and fade away gradually.


Shepherd moons
The reason for these sharply defined edges to certain rings lies in the hands of 'shepherd' moons (or shepherd satellites). Shepherd moons are just ordinary moons: there is nothing special about them other than being in the right place to cause this effect. Shepherd moons orbit their parent planets close to either the inner or outer edge of a ring, and the effect of their gravity helps to maintain a sharply defined edge to the ring. This can happen in one of three ways:

1. The moon 'Hoovers up' dust and debris that escapes from the ring, incorporating it into itself.
2. The moon's gravitational influence speeds up particles, which has the effect of pushing them into a higher orbit and back into the main body of the ring. This generally occurs on the inside of a ring.
3. The moon's gravitational influence slows down particles, which has the effect of pushing them into a lower orbit, and back into the main body of the ring. This generally occurs on the outside of a ring.

Uranus's moons Cordelia and Ophelia can be used an examples of the second two items in this list: Cordelia, a fast-orbiting satellite on the inside of the planet's brightest ring has the effect of speeding dust particles up and 'shepherding' them into a higher orbit. Cordelia's slower-moving sister, Ophelia, orbits on the outside of the same ring and has the effect of slowing down particles, shepherding them into a lower orbit. Between the two, Cordelia and Ophelia have created a narrow, bright and sharp-edged ring between them.

The picture on the left was taken by Voyager 2 and shows nine of Uranus's rings. The brightest, outermost ring is the one Shepherded by Cordelia and Ophelia, and is known as the ε ring*.

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

* ε is the lower case form of the Greek letter 'epsilon'.