Shepherd Moons & Planetary Rings
Physics of Guardian Satellites & Saturn's Complex Ring Structure
The Voyager space probe to Saturn revealed incredibly detailed unexpected structure in Saturn's rings. Shepherd moons help cause this structure.
Saturn's Ring Structure
Prior to the space program we thought Saturn had only a few rings. The Voyager mission to Saturn sent back pictures of the rings that revealed tens of thousands of individual rings. Scientists scrambled to solve the puzzle of this structure.
Two of the most likely causes of Saturn's ring structure are resonances with Saturn's larger moons and small shepherd moons.
Shepherd Moons
Sometimes two small moons in the rings act together as shepherd (sometimes called guardian moons or satellites) moons. Working together the gravitational forces from these two shepherd moons confine the ring particles to a narrow band between the two shepherd moons. We get a small gap just inside and just outside a small confined ring structure.
How? These shepherd moons act in a way that is often counterintuitive. One would think that the outer shepherd moon would pull the ring particles outward and the inner moon would pull ring particles inward. The opposite however is true. The outer shepherd moon pushes ring particles inward and the inner moon pushes them outward.
There are two keys to understanding this counterintuitive behavior. First a satellite's speed and distance from Saturn (or any other orbited body) are related. Closer satellites orbit more rapidly, and more distant satellites have lower orbital speeds. Hence, changing the orbital speed of a ring particle will change its orbital distance.
The second key is understanding the energy of an orbit. If an orbiting particle is in a more distant orbit with a slower orbital speed, it has less kinetic energy. However the particle in a more distant orbit has more potential energy, just as lifting an object on Earth increases both its distance from Earth and its potential energy.
What about the total energy, which is the sum of the potential and kinetic energy? It turns out that in this case the change in potential energy is the greater effect. In a more distant orbit, a ring particle has more total energy; in a closer orbit it less total energy. Hence adding energy to an orbiting ring particle will cause it to move to a more distant orbit. Decreasing its energy will move it into a smaller orbit.
A shepherd moon orbiting just outside a narrow ring will orbit Saturn more slowly than the individual particles composing the ring. Because it orbits more slowly, its gravitational force will drag the ring particles backward and decrease their energy. Hence the outer shepherd satellite pushes ring particles inward to lower energy orbits. The inner shepherd satellite orbits Saturn more rapidly than the ring particles. It therefore pulls the particles along, increasing their energy. The inner shepherd moon therefore pushes ring particles outward to higher energy orbits.
The combination of the inner and outer shepherd moons confines the orbiting ring particles to a narrow ring between the two shepherd moons. A large number of shepherd moons in Saturn's large A and B rings contribute to the thousands of individual rings.
Taken together resonances and shepherd moons cause the fantastically detailed ring structures we observe around Saturn, the crown jewel of our solar system.
Further Reading
Chaisson, E, and McMillan, S. Astronomy Today 5th ed., Pearson, 2005.
Fowles, G.R., and Cassidy, G.L., Analytical Mechanics 6th ed., Saunders, 2004.
Beatty, J.K., O'Leary, B., and Chaikin, A. The New Solar System 2nd ed., Cambridge, 1982.
