![]() As mentioned above Mimas creates a gap, the Cassini Division, in the rings of Saturn by sweeping material out of the region which has 1/2 the orbital period of Mimas. One example is Janus (pronounced Jane-uss). Often, it was the study of those effects that led to the discovery of the moons which caused them. Similarly, other moons which orbit in or near the ring system can have various effects on the rings. ![]() ![]() (Click on pictures for detailed discussions of each of these moons)Ītlas, which is one of the inner moons of Saturn, seems to control and sharpen the outer edge of the A ring. Most of the moons of Saturn are named after various Titans, the mythological sons and daughters of the god of the Heavens, Uranus, and the goddess of the Earth, Gaea, but many of the more recently discovered smaller moons have not yet received official names. More than 50 moons have already been discovered orbiting Saturn, but it is expected that even more small moons will be discovered with further study, particularly in the second and third groups. The captured objects in the third group, however, can have orbits which are substantially inclined to the rotational plane of Saturn, and in several cases don't even go around the planet in the same direction that it rotates. All of the moons in the first two groups orbit the planet in the same direction that it rotates, in almost exactly the same plane as its rotation and ring system, in nearly circular orbits. The third group, which extends outward from Phoebe, consists of small bodies, presumably mostly broken fragments of comets, which were captured by an interaction between Saturn's gravity and that of the Sun. The rings can also be affected by the inner moons in the first group of moons, especially Mimas, which causes the Cassini Division, but much of the fine structure of the rings is controlled by shepherd satellites. ![]() In this region gravitational interactions between the moons and the ring particles strongly control the distribution of material within the rings, creating ringlets and density waves within the rings and sharp edges at ring boundaries. This group includes not only large moons, but also some small ones which are co-orbital with larger moons in the same way that the Trojan asteroids are co-orbital with Jupiter, making a comparison with the Solar System even more striking.Ī second group, extending from Pan and other small moons which are actually inside the ring system to Janus and Epimetheus near the outside of the major part of the ring system, consists of so-called "shepherd" moons. These are objects formed within the dust and gases which swirled around Saturn as it gravitationally pulled nearly a hundred Earth masses of hydrogen and other gases into itself in the last stages of its formation, or pieces resulting from collisions between the original moons and other objects soon afterwards. One group, which extends from Mimas to Iapetus and includes all of the larger moons, is a miniature version of the Solar System. Most of the moons are shown to scale in comparison to each other and Saturn, but Pan, Atlas, Telesto, Calypso and Helene are enlarged by a factor of five. Moons closer to the planet are shown on the left, those further away are shown on the right. The relative sizes and appearance of 18 of Saturn's moons (Image credit JPL/NASA).
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