12.5 The Moons of Saturn
Saturn has the most extensive, and in many ways the most complex, system of natural satellites of all the planets. The planets 18 named moons are listed in Table 12.2. Observations of sunlight reflected from them 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.
The moons fall into three fairly natural groups. First, there are the many "small" moonsirregularly shaped chunks of ice, all less than 300 km acrossthat exhibit a bewildering variety of complex and fascinating motion. Second, there are six "medium-sized" moonsspherical bodies with diameters ranging from about 400 to 1500 kmthat offer clues to the past and present state of the environment of Saturn while presenting many puzzles regarding their own appearance and history. Finally, there is Saturns single "large" moonTitanwhich, at 5150 km in diameter, is the second-largest satellite in the solar system (Jupiters Ganymede is a little bigger). It has an atmosphere denser than Earths 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 Jupiters other satellites are smallno more than 200 km in diameter.
In 1995, researchers using the Hubble Space Telescope reported the sighting of two more moons (see Discovery 12-1). However, these observations now seem to have been in erroreasy to do when dealing with faint objects at the limits of detectability. More recently, in late 2000, astronomers working at observatories on Mauna Kea and at the VLT announced the discovery of 12 more moons orbiting Saturn, bringing the planets total (for now) to 30. As with the newly discovered satellites of Jupiter, these moons are all very faint (and hence small), and orbit quite far from the planet on rather inclined orbits, much like Saturns other "small" moons. (Sec. 11.5) These new moons are as yet unnamed, and their orbits are still unknown. If confirmed, they are most likely chunks of debris captured from interplanetary space after close encounters with Saturn.
Perhaps the most intriguing of all Saturns 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 moons reddish coloration is caused by something quite specialan atmosphere. So eager 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 Saturns 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.)
A Voyager 1 image of Titan is shown in Figure 12.19. Unfortunately, despite the spacecrafts close pass, the moons 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 spacecrafts view. Voyager 1 was able to provide mission specialists with detailed atmospheric data, however.
Titans atmosphere is thicker and denser even than Earths, and it is certainly far more substantial than that of any other moon. Prior to Voyager 1s 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 Titans atmosphere maintains steady (but trace) levels of hydrogen gas, the hydrocarbons ethane and propane, and carbon monoxide.
Based largely on Voyager measurements, Figure 12.20 shows the probable structure of Titans atmosphere. Despite the moons low mass (a little less than twice that of Earths Moon) and hence its low surface gravity (one-seventh of Earths), the atmospheric pressure at ground level is 60 percent greater than on Earth. Titans atmosphere contains about 10 times more gas than Earths atmosphere. Because of Titans 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 12.21). Below the haze the atmosphere is reasonably clear, although rather gloomy, because so little sunlight gets through.
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 Titans 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 Titans 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 Titans 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 Titans interior, these gases form the basis of the atmosphere we see today.
What is unusual about Titan?
SATURNS MEDIUM-SIZED MOONS
Saturns complement of midsized moons consists (in order of increasing distance from the planet) of Mimas (at 3.1 planetary radii), Enceladus (4.0), Tethys (4.9), Dione (6.3), Rhea (8.7), and Iapetus (59.1). They are shown, to proper scale, in Figure 12.22. All six were known from Earth-based observations long before the space age. The inner five move on circular trajectories, and all are tidally locked by Saturns 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.
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 Titans, primarily because their lower masses produce less compression of their interiors.
Inside Rheas 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. Diones trailing face has prominent bright streaks which are probably similar to Rheas 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 massonly 1 percent the mass of Rheaits 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 moons 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 shinyit reflects virtually 100 percent of the sunlight falling on itthat 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 Saturns 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 Enceladuss water volcanism in terms of tidal stresses. (Recall the role that Jupiters tidal stresses play in creating volcanism on Io.) (Sec. 11.5) However, Saturns 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 Ios volcanoes may not be present on Enceladus. For now, the mystery of Enceladuss internal activity remains unresolved.
The outermost midsized moon is Iapetus (Figure 12.22). It orbits Saturn on a somewhat eccentric, inclined orbit with a semimajor 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. Spectroscopic 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.
THE SMALL SATELLITES
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 Keplers 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 12.24, when the two get close enough to begin to feel each others weak gravity, they switch orbitsthe 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.
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 Saturns moons, its rotation is not synchronous with its orbital motion. Because of the gravitational effect of Titan, Hyperions 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.
Why do Saturns midsized moons show asymmetric surface markings?