Saturn has the most extensive, and in many ways the most complex, system of natural satellites of all the planets. Saturn's 18 named moons are listed in Table 12.2. In 1995, researchers using the Hubble Space Telescope announced the discovery of two more moons (see Interlude 12-1). However, these observations now seem to have been in error, and the official moon count remains as listed in the table. The reflected light from Saturn's moons suggests that most are covered with snow and ice. Many of them are probably made almost entirely of water ice. Even so, they are a curious and varied lot.
| TABLE 12.2 The Moons of Saturn | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
*Indicates a retrograde orbit. |
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The satellites fall into three fairly natural groups. First, there are the "small" moons—irregularly shaped chunks of ice, all less than 300 km across—that exhibit a bewildering variety of complex and fascinating motion. Second, there are six "medium-sized" moons—spherical bodies with diameters ranging from about 400 to 1500 km—that offer clues to the past and present state of the environment of Saturn while presenting many puzzles in their own appearance and history. Finally, there is Saturn's single "large" moon—Titan—which, at 5150 km in diameter, is the second-largest satellite in the solar system (Jupiter's Ganymede is a little bigger). It has an atmosphere denser than Earth's and (some scientists think) surface conditions possibly conducive to life. Notice, incidentally, that Jupiter has no "medium" moons, as just defined. The Galilean satellites are large, like Titan, and all of Jupiter's other satellites are small—no more than 200 km in diameter.
Perhaps the most intriguing of all Saturn's moons, Titan was discovered by Christian Huygens in 1655. Even through a large Earth-based telescope, this moon is visible only as a barely resolved reddish disk. However, long before the Voyager missions, astronomers already knew (from spectroscopic observations) that the moon's reddish coloration is caused by something quite special—an atmosphere. So anxious were mission planners to obtain a closer look that they programmed Voyager 1 to pass very close to Titan, even though that meant the spacecraft could not then use Saturn's gravity to continue on to Uranus and Neptune. (Instead, Voyager 1 left the Saturn system on a path taking the craft out of the solar system well above the ecliptic plane.)
Scientists believe that Titan's internal composition and structure must have similarities to those of Ganymede and Callisto, because these three moons have quite similar masses and radii, and hence average densities (Titan's density is 1900 kg/m3). 
(Sec.11.5) Titan probably contains a rocky core surrounded by a thick mantle of water ice. However, in view of Galileo's discovery of significant differences between the internal structures of Ganymede and Callisto, the degree of differentiation within Titan is presently unknown. In addition to probing Titan's atmosphere and surface, another important objective of the Cassini mission on reaching Saturn in 2004 will be to make detailed measurements of the moon's interior. (Sec.6.6)
A Voyager 1 image of Titan is shown in Figure 12.17. Unfortunately, despite the spacecraft's close pass, the moon's surface remains a mystery. A thick, uniform haze layer, similar to the photochemical smog found over many cities on Earth, that envelops the moon completely obscured the spacecraft's view. Voyager 1 was able to provide mission specialists with detailed atmospheric data, however.
Figure 12.17 (a) Titan, larger than the planet Mercury and roughly half the size of Earth, was photographed in visible light from only 4000 km away as the Voyager 1 spacecraft passed by in 1980. All we can see here is Titan's upper cloud deck. For unknown reasons, the northern hemisphere appears slightly brighter than the southern. (b) In infrared, however, as captured by the Hubble Space Telescope, we can see large-scale surface features. The most prominent bright area shown here is nearly 400 km across, about the size of Australia.
Titan's atmosphere is thicker and denser even than Earth's, and it is certainly more substantial than that of any other moon. Prior to Voyager 1's arrival in 1980, only methane and a few other simple hydrocarbons had been conclusively detected on Titan. (Hydrocarbons are molecules consisting solely of hydrogen and carbon atoms; methane, CH4, is the simplest). Radio and infrared observations from Voyager 1 showed that the atmosphere is actually made up mostly of nitrogen (roughly 90 percent) and argon (at most 10 percent), with a small percentage of methane. In addition, complex chemistry in Titan's atmosphere maintains steady (but trace) levels of hydrogen gas, the hydrocarbons ethane and propane, and carbon monoxide.
Titan's atmosphere seems to act like a gigantic chemical factory. Powered by the energy of sunlight, it is undergoing a complex series of chemical reactions that ultimately result in the observed smog and trace chemical composition. The upper atmosphere is thick with aerosol haze, and the unseen surface may be covered with organic sediment that has settled down from the clouds. Speculation runs the gamut from oceans of liquid hydrocarbons, especially ethane, to icy valleys laden with petrochemical sludge. Future spacecraft exploration of Titan may present scientists with an opportunity to study the kind of chemistry thought to have occurred billions of years ago on Earth—the prebiotic chemical reactions that eventually led to life on our own planet (see Chapter 28).
Based largely on Voyager measurements, Figure 12.18 shows the probable structure of Titan's atmosphere. Despite the moon's low mass (a little less than twice that of Earth's Moon) and hence its low surface gravity (one-seventh of Earth's), the atmospheric pressure at ground level is 60 percent greater than on Earth. Titan's atmosphere contains about 10 times more gas than Earth's atmosphere. Because of Titan's weaker gravitational pull, the atmosphere extends some 10 times farther into space than does our own. The top of the main haze layer lies about 200 km above the surface, although there are additional layers, seen primarily through their absorption of ultraviolet radiation, at 300 km and 400 km (Figure 12.19). Below the haze the atmosphere is reasonably clear, although rather gloomy, because so little sunlight gets through.
Figure 12.18 The structure of Titan's atmosphere, as deduced from Voyager 1 observations.
Figure 12.19 The haze layers (blue) of Titan's atmosphere are visible in these false-color Voyager 1 images.
Titan's surface temperature is a frigid 94 K, roughly what we would expect on the basis of its distance from the Sun. At the temperatures typical of the lower atmosphere, methane and ethane may behave rather like water on Earth, raising the possibility of methane rain, snow, and fog and even ethane oceans! At higher levels in the atmosphere the temperature rises, the result of photochemical absorption of solar radiation.
Why does Titan have such a thick atmosphere, when similar moons of Jupiter such as Ganymede and Callisto have none? The answer seems to be a direct result of Titan's greater distance from the Sun. The moons of Saturn formed at considerably lower temperature than did those of Jupiter. Those conditions would have enhanced the ability of the water ice that makes up the bulk of Titan's interior to absorb methane and ammonia, both of which were present in abundance at those early times. As a result, Titan was initially laden with much more methane and ammonia gas than either Ganymede or Callisto. As Titan's internal radioactivity warmed the moon the ice released the trapped gases, forming a thick methane—ammonia atmosphere. Sunlight split the ammonia into hydrogen, which escaped into space, and nitrogen, which remained in the atmosphere. The methane, which was less easily broken apart, survived intact. Together with argon outgassed from Titan's interior, these gases form the basis of the atmosphere we see today.
Saturn's complement of midsized moons consists (in order of increasing distance from the planet) of Mimas (at 3.1 planetary radii), Enceladus (at 4.0), Tethys (at 4.9), Dione (at 6.3), Rhea (at 8.7), and Iapetus (at 59.1). They are shown, to proper scale, in Figure 12.20. All six were known from Earth-based observations long before the space age. The inner five move on circular trajectories and are tidally locked by Saturn's gravity into synchronous rotation (so that one side always faces the planet). They therefore all have permanently "leading" and "trailing" faces as they move in their orbits, a fact that is important in understanding their often asymmetrical surface markings.
Figure 12.20 Saturn's six medium-sized satellites, to scale. All are heavily cratered. Iapetus shows a clear contrast between its light-colored (icy) trailing surface, at the top and center in this image, and its dark leading hemisphere, at the bottom. The light-colored "wisps" covering essentially the whole central portion of this image of Rhea are thought to be water that was released from the moon's interior during some long-ago period of activity and then froze on the surface. Dione and Tethys show evidence of ancient geological activity of some sort. Enceladus appears to still be volcanically active, but the cause of the volcanism is unexplained. Mimas's main surface feature is the large crater Herschel, plainly visible in this Voyager 2 image; the impact that caused this crater must have come very close to shattering the moon.
Unlike the densities of the Galilean satellites of Jupiter, the densities of these six moons do not show any correlation with distance from Saturn. Their densities are all between 1000 and 1400 kg/m3, implying that nearness to the central planetary heat source was a less important influence during their formation than it was in the Jupiter system. Scientists believe that the midsized moons are composed largely of rock and water ice, as is Titan. Their densities are lower than Titan's, primarily because their lower masses produce less compression of their interiors.
The largest of the six, Rhea, has a mass only 1/30 that of Earth's Moon, and its icy surface is very reflective and heavily cratered (Figure 12.21). At the low temperatures found on its surface, water ice is very hard and behaves rather like rock on the inner planets. For that reason, Rhea's surface craters look very much like craters on the Moon or Mercury. The crater density is similar to that in the lunar highlands, indicating that the surface is old, and there is no evidence of extensive geological activity. Rhea's only real riddle is the presence of so-called wispy terrain—prominent light-colored streaks—on its trailing side. The leading face, by contrast, shows no such markings, only craters. Astronomers believe that the wisps were caused by some event in the distant past during which water was released from the interior and condensed on the surface. Any similar markings on the leading side have presumably been long since obliterated by cratering, which should be much more frequent on the satellite's forward-facing surface.
Figure 12.21 Rhea's north polar region, seen here at a superb resolution of only 1 km. The heavily cratered surface resembles that of the Moon, except that we see here craters in bright ice, rather than dark lunar rock.
Inside Rhea's orbit lie the orbits of Tethys and Dione. These two moons are comparable in size and have masses somewhat less than half the mass of Rhea. Like Rhea, they have reflective surfaces that are heavily cratered, but each shows signs of surface activity, too. Dione's trailing face has prominent bright streaks which are probably similar to Rhea's wispy terrain. Dione also has "maria" of sorts, where flooding appears to have obliterated the older craters. The cracks on Tethys may have been caused by cooling and shrinking of the surface layers or, more probably, by meteoritic bombardment.
The innermost, and smallest, medium-sized moon is Mimas. Despite its low mass—only 1 percent the mass of Rhea—its closeness to the rings causes resonant interactions with the ring particles, resulting most notably in the Cassini Division, as we have already seen. Possibly because of its proximity to the rings, Mimas is heavily cratered. The moon's chief surface feature, is an enormous crater known as Herschel, on the leading face. Its diameter is almost one-third that of the moon itself. The impact that formed Herschel must have come very close to destroying Mimas completely. It is quite possible that the debris produced by such impacts is responsible for creating or maintaining the spectacular rings we see.
Enceladus orbits just outside Mimas. Its size, mass, composition, and orbit are so similar to those of Mimas that one might guess that the two moons would also be very similar to each other in appearance and history. This is not so. Enceladus is so bright and shiny—it reflects virtually 100 percent of the sunlight falling on it—that astronomers believe its surface must be completely coated with fine crystals of pure ice, which may be the icy "ash" of water "volcanoes" on Enceladus. The moon bears visible evidence of large-scale volcanic activity. Much of its surface is devoid of impact craters, which seem to have been erased by what look like lava flows, except that the "lava" is water, temporarily liquefied during recent internal upheavals and now frozen again.
Although no geysers or volcanoes have actually been observed on Enceladus, there seems to be strong circumstantial evidence of volcanism on the satellite. In addition, the nearby thin cloud of small, reflective particles that makes up Saturn's E ring is known to be densest near Enceladus. Calculations indicate that the E ring is unstable because of the disruptive effects of the solar wind, supporting the view that volcanism on Enceladus continually supplies new particles to maintain the ring.
Why is there so much activity on a moon so small? No one knows. Attempts have been made to explain Enceladus's water volcanism in terms of tidal stresses. (Recall the role that Jupiter's tidal stresses play in creating volcanism on Io.) (Sec. 11.5) However, Saturn's tidal force on Enceladus is only one quarter the force exerted by Jupiter on Io, and there are no nearby large satellites to force Enceladus away from a circular trajectory. Thus, the ingredients that power Io's volcanoes may not be present on Enceladus. For now, the mystery of Enceladus's internal activity remains unresolved.
The outermost midsized moon is Iapetus (Figure 12.20). It orbits Saturn on a somewhat eccentric, inclined orbit with a semi-major axis of 3.6 million km. Its mass is about three quarters that of Rhea. Iapetus is a two-faced moon. The dark, leading face reflects only about 3 percent of the sunlight reaching it, whereas the icy trailing side reflects 50 percent. Spectral studies of the dark regions seem to indicate that the material originates on Iapetus, in which case the moon is not simply sweeping up dark material as it orbits. Similar dark deposits seen elsewhere in the solar system are thought to be organic (carbon-containing) in nature; they can be produced by the action of solar radiation on hydrocarbon (for example, methane) ice. But how the dark markings can adorn only one side of Iapetus in that case is still unknown.
Finally we come to Saturn's dozen or so small moons. Their masses are poorly known (they are inferred mainly from their effects on the rings), but they are thought to be similar in composition to the small moons of Jupiter. The outermost small moons, Hyperion and Phoebe, were discovered in the nineteenth century, in 1848 and 1898, respectively. The others were first detected in the second half of the twentieth century. Only the moons in or near the rings themselves were actually discovered by the Voyager spacecraft.
Just 10,000 km beyond the F ring lie the so-called co-orbital satellites Janus and Epimetheus. As the name implies, these two satellites "share" an orbit, but in a very strange way. At any given instant, both moons are in circular orbits about Saturn, but one of them has a slightly smaller orbital radius than the other. Each satellite obeys Kepler's laws, so the inner satellite orbits slightly faster than the outer one and slowly catches up to it. The inner moon takes about four Earth years to "lap" the outer one. As the inner satellite gains ground on the outer one a strange thing happens. As illustrated in Figure 12.22, when the two get close enough to begin to feel each other's weak gravity, they switch orbits—the new inner moon (which used to be the outer one) begins to pull away from its companion, and the whole process begins again! No one knows how the co-orbital satellites came to be engaged in this curious dance. Possibly they are portions of a single moon that broke up, perhaps after a meteoritic impact, leaving the two pieces in almost the same orbit.
Figure 12.22 The peculiar motion of Saturn's co-orbital satellites, Janus and Epimetheus, which play a never-ending game of tag as they move on their orbits around the planet. The labeled points represent the locations of the two moons at a few successive times. From A to C, satellite 2 gains on satellite 1. However, before it can overtake it, the two moons swap orbits, and satellite 1 starts to pull ahead of satellite 2 again, through points D and E. The whole process is repeated ad infinitum.
In fact, several of the other small moons also share orbits, this time with larger moons. Telesto and Calypso have orbits that are synchronized with the orbit of Tethys, always remaining fixed relative to the larger moon, lying precisely 60° ahead of and 60° behind it as it travels around Saturn (see Figure 12.23). The moon Helene is similarly tied to Dione. These 60° points are known as Lagrange points, after the French mathematician Joseph Louis Lagrange, who first studied them. Later we will see further examples of this special 1:1 orbital resonance in the motion of some asteroids about the Sun, trapped in the Lagrange points of Jupiter's orbit.
Figure 12.23 The orbits of the moons Telesto and Calypso are tied to the motion of the moon Tethys. The combined gravitational pulls of Saturn and Tethys keep the small moons exactly 60° ahead and behind the larger moon at all times, so that all three moons share an orbit and never change their relative positions.
The strangest motion of all is that of the moon Hyperion, which orbits between Titan and Iapetus, at a distance of 1.5 million km from the planet. Unlike most of Saturn's moons, its rotation is not synchronous with its orbital motion. Because of the gravitational effect of Titan, Hyperion's orbit is not circular, so synchronous rotation cannot occur. In response to the competing gravitational influences of Titan and Saturn, this irregularly shaped satellite constantly changes both its rotation speed and its rotation axis, in a condition known as chaotic rotation. As Hyperion orbits Saturn it tumbles apparently at random, never stopping and never repeating itself, in a completely unpredictable way. Since the 1970s the study of chaos on Earth has revealed new classes of unexpected behavior in even very simple systems. Hyperion is one of the few other places in the universe where this behavior has been unambiguously observed.
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