Rings and moons are related in complex ways. Rings are thought to be transitory structures in solar system history, probably lasting no more than hundreds of millions of years. Moons, by contrast, can be stable over the age of the solar system, or they can be unstable and either break up under the gravitations forces of the planet or even crash into the planet itself. The formation of a giant set of rings such as Saturn's is not well understood. An existing moon may have broken up under the force of an impact, or a passing icy body may have been trapped in Saturn's gravitational field and broken. Now that they do exist, though, the rings and moons affect one another in complex ways. Shown in the lower color insert on page C-2, some of Saturn's rings are shaped and maintained by the gravitational fields of nearby moons, and other rings are created from particles shed from moons.
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