Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Saturn Link for sharing this page on Facebook
(also see Pictures of Saturn, The Rings of Saturn)
(Most of the discussion on this page was written in 2004, before the Cassini spacecraft reached Saturn, but most of it is general information that will not be changed even when brought up to date, and most of the pages linked from this page have already been partially or completely brought up to date.)

The relative sizes and appearance of 18 of Saturn's moons (Image credit JPL/NASA). Moons closer to the planet are shown on the left, those further away are shown on the right. 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.

      Saturn has three groups of moons. One group, which extends from Mimas to Iapetus and includes all of the larger moons, is a miniature version of the Solar System. 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. 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.
      A 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. 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. 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.
      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. 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 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.
      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.
      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.

The Miniature Solar System Moons
(Click on pictures for detailed discussions of each of these moons)












The Inner, Shepherd Moons
Atlas, which is one of the inner moons of Saturn, seems to control and sharpen the outer edge of the A ring. Similarly, other moons which orbit in or near the ring system can have various effects on the rings. Often, it was the study of those effects that led to the discovery of the moons which caused them.
      One example is Janus (pronounced Jane-uss). 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. There are several other much fainter gaps in the rings, some of which can be explained by other orbital interactions with Mimas, but in the 1960's it was realized that some faint gaps must require a previously unknown moon to account for them. The search for such a moon actually resulted in the discovery of two moons, Janus and Epimetheus. These two moons are co-orbital, meaning they share the same orbit. At most times one moon orbits about 30 miles inside the other one. Over a period of about four years the inner moon gradually gains on the outer one and finally laps it. When this happens the outer moon tugs the inner one outwards and vice versa, and the two moons exchange orbits. Each four years the process repeats, and the moons change positions again and again. Because the two moons share the same orbit it is possible that they are fragments broken from a single older moon, but the extensive cratering of Epimetheus indicates that its surface must be quite old, so such a break-up would have to have taken place very early in the history of the Saturnian system, while the oddly saucer-shaped appearance of Atlas suggests it may have a different history entirely.
      A similar search for an object which could create the Encke Gap within Saturn's A-ring resulted in the discovery of the innermost known moon of Saturn, Pan. This small presumably icy body is not likely to be more than 10 to 12 miles in diameter, and is barely visible even in a detailed view of the ring system.
      The last of the known shepherd satellites are Pandora and Prometheus. They were discovered as the result of a search for shepherding satellites which could maintain the structure of Saturn's F ring, which is a very thin slightly wavily kinked ring which lies between the orbits of the two moons. The ring material may well be debris knocked off of these two satellites, but whatever its source the two moons control the position of the debris, as discussed below.
      S/2005 S 1 (Daphnis) is a newly discovered moon (one of the 20 moons discovered since S/2000 S12) inside the Keeler Gap, a very thin gap in the outer portion of the rings.

How Satellites Shepherd Rings

How Pan Creates the Encke Gap
      This diagram shows how Pan creates the Encke gap. Pan is shown as the large dot on the middle circle, which represents its orbit. Stray ring particles are shown outside and behind it on the right, and ahead of and inside it on the left. Black arrows show the direction of the net force exerted on the stray ring particles by Pan, while red dotted arrows show the paradoxical result of that force. If only Pan and the particles existed, the particles would move in the direction of the black arrows, but because all of the objects are orbiting Saturn inside a region where Saturn's gravity is much stronger than Pan's gravity, the particles actually move in the direction of the red arrows.
      The ring particles inside Pan's orbit are orbiting Saturn faster than Pan, so as they lap Pan they experience a net outward and backward force (outward because its orbit is outside theirs, backwards because it is going slower than they are). The backward force slows down the particles, which makes their orbits smaller and causes them to fall towards Saturn. As they fall inward they pick up speed, and as a result end up moving forward and inward relative to Pan.
      Meanwhile, exactly the opposite thing happens to ring particles which are outside Pan's orbit. Since Pan is orbiting Saturn faster than they are, they experience a net inward and forward force. The forward force speeds up the particles, which makes their orbits bigger and causes them to move away from Saturn. As they move outward they slow down, and as a result end up moving backwards and outwards relative to Pan.
      The effect of this is that although Pan is trying to pull the particles toward itself, the interaction of Saturn's gravity with Pan's gravity causes the particles to actually be pulled away from Pan, so the area near it is swept clear of particles, creating the Encke Gap.

How Pandora and Prometheus Shepherd the F-Ring
      In this diagram, Prometheus is shown as the large black dot on the inner circle and Pandora as the one on the outer circle while particles in the F-ring orbit near the middle circle. Stray particles are again shown as small black dots, the forces exerted on them by the two shepherd moons as black arrows, and the paradoxical motion of the particles by red arrows.
      Since the inner moon Prometheus orbits Saturn faster than particles in the ring, it pulls them inward and forward, which speeds them up, causing their orbits to become larger and as in the discussion of the Encke Gap above, they slow down and move outward and backward relative to Prometheus. Similarly, the ring particles orbit Saturn faster than Pandora, and are pulled outward and backward by that moon, which slows them down, causing their orbits to become smaller and speeding them up, so they move inward and forward relative to Pandora. The interactions of the moons and the ring particles are exactly the same as in the case of Pan and the Encke Gap particles, but since there are two moons, one pulling the particles one way and the other pulling the particles the other way, the particles become trapped between the two moons, creating the F ring.

The Outer Captured Moons
Phoebe (pronounced PHEE-bee) is a relatively small moon, only about 130 miles in diameter. It is almost four times further from Saturn than its nearest neighbor, Iapetus, and has an eccentric retrograde orbit (meaning that it orbits Saturn in the opposite direction from that of the planet's rotation) that is closer to the Ecliptic (the orbital plane of the planets) than to the plane of Saturn's orbit. This suggests that it is either an asteroid or cometary object that was captured by Saturn, and not one of the original moons of the planet. Phoebe is very dark, in fact almost as dark as lampblack, and it is thought that collisions between it and other small objects (e.g., micrometeoroids) is probably the source of the dark material which coats the leading edge of Iapetus, though the color of its surface is slightly different from that of the dark material on Iapetus.
      Voyager 2 didn't come very close to Phoebe, so only very large features would have shown up in its photographs, and current (2004) estimates of its mass are quite uncertain, varying by nearly a factor of two. As the Cassini spacecraft approached Saturn it passed less than 30,000 miles from Phoebe, and took images that showed hundreds of times more detail than the Voyager photographs (as a result, the linked page for Phoebe shows far more information than this brief discussion would suggest). Once analysis of the gravitational interaction with Phoebe is complete we should have a much more accurate measurement of its (very small) gravitational field. One interesting early result from the Cassini flyby is that the surface of Phoebe is covered by a very dark material (as already known), but the interior appears to be much brighter and may consist of relatively clean ices. This is revealed by the fact that many of the extremely numerous craters pockmarking its surface show a variation in brightness which is most easily explained by a dark overlayer sitting on top of a lighter underlayer.

     S/2000 S 1 to S 12 and Later Discoveries are several dozen small moons discovered in 2000 - 2007. They are all small starlike dots in the discovery photographs, and nothing can be determined of their sizes except by estimation (brighter ones being presumed to be bigger than fainter ones). Assuming that they are all as dark as Phoebe and other icy bodies in the far Solar System, their differing brightnesses yield diameters between four and twenty miles. It isn't likely that they are much darker and larger than this, but if any of them are relatively bright, they could be even smaller than these estimates.
      Some of these moons orbit Saturn in a direct motion (the same way that the planet rotates), and some in a retrograde motion (the opposite way), and their orbital planes vary from the rotational plane of the planet by as much as 45 degrees. This implies that they are probably captured comets, or more likely, since their orbital motions seem to naturally group into a small number of similar orbits, broken pieces of such objects.