MIMAS

The density of Mimas (approximately 1.17) indicates that this moon is mostly composed of water ice with only a small amount of rock. The surface of Mimas is dominated by an impact crater which measures 130km across, or almost 1/3 the diameter of the moon itself. This crater is known as Herschel, and its walls are approximately 5km high, with its central peak rising 6km above the crater floor. The impact that made this crater must have nearly destroyed the moon, and there are even fractures on the other side of the moon that could be down to the same impact. The surface is saturated with impact craters, but there are no others that come close to the size of Herschel. This would suggest that early in its history Mimas was probably impacted by even larger bodies than the one that created Herschel which completed destroyed the new moon, but then the impact debris coalesced again to form the present day moon.

                                                    ENCELADUS

The surface of this moon is dominated by fresh, clean ice – and observations have identified five different types of terrain: craters, smooth planes, extensive linear cracks and ridges, with at least some of the surface being less than 100 million years old. This means that this moon must have been active until very recently, possibly through some type of water volcanism. This moon is however much too small to be heated by the decay of radio active material. Enceladus is locked in a 1:2 resonance with Dione, which is similar to the situation with Europa and Io. This may provide a heating mechanism but it is probably insufficient to melt water ice. On this premise the moon may there for be composed of some low melting point material rather than pure water.  There are also signs that Enceladus may be the source of the material in Saturn's tenuous E ring.

However two fly byes by the Cassini space craft have revealed that the moon has a significant atmosphere, and that the source may be due to volcanism, geysers or gasses escaping from its surface or interior. This is the first time since Cassini arrived in orbit around Saturn that an atmosphere has been detected around a moon of Saturn, other than its largest moon Titan. This moon is the most reflective object in the solar system, reflecting about 90% of the sunlight that hits it. If Enceladus does have ice volcanoes, the high reflectivity of the moons surface might result from continuous disposition of icy particles originating from the volcanoes.

                                                        TETHYS

Again this moon also has a low density, indicating that it is almost completely composed of water ice. The western hemisphere is dominated by a huge impact crater, known as Odysseus, and measures approximately 400km across, which is nearly 2/5 of the moon itself. The fact that such an impact did not shatter the moon completely indicates that it may have been liquid, or at least not very solid at the time. The second major feature seen on this moon is a huge valley (the Ithaca Chasma) which is 100 km wide and 3–5km deep, which runs 2000km, or 3/4 of the way round, the moons circumference. The point about the moon once being liquid would mean that impact craters from that era have been smoothed out. As it froze it expanded and the surface may have cracked to accommodate the extra volume producing the Ithaca Chasma. The smaller impact craters we see today are more recent.

                                                          DIONE

This is the densest of Saturn's moons (apart from Titan, whose density is increased by gravitational compression) which is compressed primarily of water ice, but also it must have a considerable fraction of denser material like silicate rock. Though it may be somewhat smaller than Rhea, it is otherwise very similar with regard to composition, albedo features and varied terrain. It is believed that shortly after its formation Dione was active. Some form of process (perhaps ice volcanism) resurfaced much of the moon, leaving the pattern of streaks. Later, after the internal activity and resurfacing ceased, a much less intense series of impacts occurred. This was concentrated on the leading hemisphere and wiped out the streak patterns but left them intact on the trailing hemisphere.

                                                         TITAN

It was long thought that this moon was the largest satellite in the solar system, yet recent observations have shown that Titan's atmosphere is so thick that its solid surface is slightly smaller than Ganymede's, yet nevertheless it is still larger in diameter than mercury and larger and more massive than Pluto. One of the principal missions of Voyager 1 was to study the moon, and it came within 4000km of the surface – and indeed we learned more in that few minutes of the fly by than in three hundred years. However in 2004 the Cassini orbiter sent further data back, and in 2005 the Huygen's probe actually landed on the surface of Titan and sent back images from the surface. This moon is similar to composition to Ganymede, Calisto and probably Pluto, or effectively half ice and half rocky material. It is probably differentiated into several layers with a 3400km rocky centre surrounded by several layers composed of different crystal forms of ice, yet its interior may still be hot! Though similar in composition to the rest of Saturn's moons, it is denser because it is so large that its gravity slightly compresses its interior.

Unlike all  other satellites in the solar system, Titan has a significant atmosphere. At the surface its pressure is more than 1.5 bar (50% higher than earth), yet its composed primarily of molecular hydrogen  (as is earth's) with no more than 6% argon and a few percent methane. However there are also trace elements of at least a dozen other organic compounds and water. The organics are formed as methane , which dominates in Titan's upper atmosphere , is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways these conditions are similar to the earths in its early stages of development when life was first getting started – but it is this atmosphere that makes it hard to see this moons atmosphere.

Titan has no magnetic field and sometimes orbits around Saturn's magnetosphere, and therefore it is directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere and may also drive some of its peculiar chemistry. At the surface its temperature is about –290F, and at this temperature ice does not sublimate, yet if this is the case what type of chemical action creates this smog? There are also scattered clouds in the atmosphere in addition to the haze. Prior to the arrival of Cassini it seemed likely that the clouds would produce a rain of ethane or methane onto the surface, perhaps producing an ocean up to 1000 metres deep. However at the present time this does not appear to be the case and there is little doubt that some active processes are occurring on Titan. There are few if any craters visible, indicating that the surface may be very young, but it may be that the lakes are slushy rather than liquid, or that the basins are not filled all of the time. Huygen's suggest that there is evidence for precipitation, erosion, mechanical abrasion and other fluvial activity.

By piecing all of the information we have together we can begin to form some understanding of this moon. Ground based observatories have shown a huge bright continent on the hemisphere of Titan that faces forward in its orbit, and some darker regions that are suggestive of oceans or lakes. Cassini's IR camera has also detected a strange bright spot, that as yet remains unexplained.

HYPERION

This is the largest non spherical object in the solar system, and it would be likely to assume that it is a fragment of a larger body that was broken off by a large impact in the distant past. Like most of Saturn's moons Hyperion has a low density, which indicates that it is composed of ice with a small amount of rock. However unlike most of Saturn's moons this moon has a low albedo, indicating that it is covered by at least a thin layer of dark material, which may be material from Phoebe. Voyager images and ground based observations reveal that Hyperion's rotation is chaotic, that is its axis of rotation wobbles so much that its orientation in space is completely unpredictable. There is only one other known body in the solar system (Asteroid 4179 Toutatis) that rotates chaotically but simulations seem to indicate that other irregular satellites may have done so in the past. Hyperion is unique in that it is very irregularly shaped, has a highly eccentric orbit and is near another large moon (Titan). These factors combine to restrict the set of conditions under which stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make chaotic rotation more likely.

IAPETUS

The leading and trailing hemispheres of Iapetus are radically different. The albedo of most of the leading hemisphere is about .04 as dark as lampblack, whereas the trailing hemisphere's albedo is .6 as bright as Europa. This difference is so striking that Cassini noted that he could see Iapetus only on one side of Saturn and not the other. One explanation of this is that the leading hemisphere is dusted with a coating of material knocked off of Phoebe or some other body. However the colour of the leading half of Iapetus and that of Phoebe don't quite match. Another possibility is that some active process within the moon is responsible – yet the puzzle is compounded by the two sides of the moon being sharply divided. In 2004 Cassini made its first close encounter with the moon, with images showing that the dark material overlays the topography, indicating that it is relatively young. This encounter also revealed another striking feature, a ridge 13 km higher than the surrounding terrain that extends at least 1300 km almost exactly parallel with the moons equator.

PHOEBE

Phoebe is the outermost of Saturn's known satellites and is almost four times more distant from Saturn than its nearest neighbour, Iapetus. Most of Saturn's moons are bright, but Phoebe's albedo is very low (.05) and so is dark. This moons eccentric, retrograde orbit and unusual albedo indicate that it may be a captured comet. Data from Cassini's flyby in 2004 reinforces this idea with the detection of carbon dioxide trapped within its rocks. This rules out the earlier idea that it might have been a captured asteroid and it appears that Phoebe is chemically similar to Pluto and Triton, and therefore it is probably very similar to the chemical composition of the outer solar system at the very beginning.