After the formation of the Sun and the planets from a interstellar cloud of gas, the matter that was left behind coalesced into the asteroids, meteors, and comets, and a halo of rocks beyond the orbit of Neptune (50AU from Sun), collectively known as the Kuiper Belt. Scientists have long suspected that a occasional nudge or gravitational instability (due to a passing star, or attraction by the larger planets such as Neptune and Uranus) could and should push one or more of these rocks towards the Sun. These rocks then end up as meteors, only to crash into the Sun or planets, or get captured as a moon of a large planet.

Phoebe (Courtesy: NASA)Recent observations by the Cassini craft orbiting the Saturnian system have now confirmed that the moon Phoebe was once a member of the Kuiper Belt. Such inward motion by Kuiper-Belt objects have been theorized before. Most notably, it is possible that the meteorite that helped kill off the dinosaurs some 65 million years ago, was indeed such an object. Similarly, an ancient interaction with Saturn left the moon Phoebe as its prisoner. Before Cassini flew by the moon, little if anything was known about it. During the encounter, scientists got the first detailed look at Phoebe, which allowed them to determine its makeup and mass. With the new information they have concluded that it has an outer solar system origin, like Pluto and other Kuiper Belt objects.
It seems that Phoebe has a totally different ratio of rock and ice, as compared to other Saturnian moons. Its density is about 1.6 g/cc, much lighter than most rocks but heavier than pure ice, which is about 0.93 g/cc. This suggests a composition of ice and rock similar to that of Pluto and Neptune's moon Triton. Whether Saturn's other moons are also prisoners from the Kuiper's Belt remains to be seen.