<P> The composition of ring particles varies; they may be silicate or icy dust . Larger rocks and boulders may also be present, and in 2007 tidal effects from eight' moonlets' only a few hundred meters across were detected within Saturn's rings . The maximum size of a ring particle is determined by the specific strength of the material it is made of, its density, and the tidal force at its altitude . The tidal force is proportional to the average density inside the radius of the ring, or to the mass of the planet divided by the radius of the ring cubed . It is also inversely proportional to the square of the orbital period of the ring . </P> <P> Sometimes rings will have "shepherd" moons, small moons that orbit near the inner or outer edges of rings or within gaps in the rings . 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 . </P> <P> It is also predicted that Phobos, a moon of Mars, will break up and form into a planetary ring in about 50 million years, because its low orbit with an orbital period that is shorter than a Martian day is decaying due to tidal deceleration . </P> <P> Jupiter's ring system was the third to be discovered, when it was first observed by the Voyager 1 probe in 1979, and was observed more thoroughly by the Galileo orbiter in the 1990s . Its four main parts are a faint thick torus known as the "halo"; a thin, relatively bright main ring; and two wide, faint "gossamer rings". The system consists mostly of dust . </P>

What is the purpose of rings around planets