Prometheus and rings. Credit: NASA/JPL/Space Science Institute
Shepherd moons are small moons that orbit close to the outer edges of planetary rings or within gaps that lie within the rings themselves. The gravitational forces of shepherd moons serve to maintain a sharply defined edge to the ring. These forces cause material that drifts close to a shepherd moon’s orbit to be deflected back into the body of the ring, ejected from the system, or accreted by the moon.
Shepherd moons are not a phenomenon limited to one planet. Nearly every ring around Saturn has its own moon. Several of Jupiter’s small inner moons are within Jupiter’s ring system. Uranus’ epsilon ring has two shepherds(Cordelia and Ophelia). Neptune has one shepherd moon(Galatea), but some astronomers believe that there are more waiting to be discovered. The gravitational perturbations of Galatea are thought to account for the appearance of Neptune’s rings. The planet’s rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but images from Voyager 2 showed them to be complete rings with bright clumps. The clumps are thought to be caused by shepherd moons.
Right now, the gas giants(Saturn, Jupiter, Neptune, and Uranus) are the only known bodies with ring systems, but that may change. Many scientists think that closer imaging of Pluto may reveal rings. The Martian moon Phobos will impact the planet’s surface in the next 50 million years and may create a ring. It is also thought that some of Saturn’s moons have rings of their own that are overshadowed by the planetary ring system.
As our imaging reach expands, it is very possible that our scientists will find additional ring systems through our the solar system. New ring systems opens the door for the discovery of new shepherd moons. We are no where near fully understanding our planet, so it is easy to believe that we may never understand the entirety of our solar system.
We have written many articles about shepherd moons for Universe Today. Here’s an article about Saturn’s irregular shepherd moon, and here’s an article about Pandora, another shepherd moon.
If you’d like more info on shepherd moons, check out an article from Wise Geek. You can also check out another nice article from csep10.phys.utk.edu.
We’ve also recorded an entire episode of Astronomy Cast all about Saturn’s Moons. Listen here, Episode 61: Saturn’s Moons.
Reference:
NASA
Comments on this entry are closed.