Saturn was the outermost planet known to ancient astronomers. Its rings and moons were not discovered until after the invention of the telescope. Saturn is smaller than Jupiter but is still much larger than any of the terrestrial worlds. Like Jupiter, Saturn rotates rapidly, producing a pronounced flattening, and displays differential rotation. Strong radio emission from the planet's magnetosphere allows the rotation rate of the interior to be determined.
Weather systems are seen on Saturn, as on Jupiter, although they are less distinct. Short-lived storms are occasionally seen. Saturn has weaker gravity and a more extended atmosphere than Jupiter. The planet's overall butterscotch hue is due to cloud chemistry similar to that occurring in Jupiter's atmosphere. Saturn, like Jupiter, has bands, ovals, and turbulent flow patterns powered by convective motion in the interior.
Like Jupiter, Saturn emits far more radiation into space than it receives from the Sun. Unlike Jupiter's, Saturn's excess energy emission is the result of helium precipitation in the planet's interior, where helium liquefies and forms droplets, which then fall toward the center of the planet. This process is also responsible for Saturn's observed helium deficit.
Saturn's interior is theoretically similar to that of Jupiter, but it has a thinner layer of metallic hydrogen and a larger core. Its lower mass gives Saturn a less extreme core temperature, density, and pressure than Jupiter's. Saturn's conducting interior and rapid rotation produce a strong magnetic field and an extensive magnetosphere large enough to contain the planet's ring system and the innermost 16 moons.
Saturn's rings lie in the planet's equatorial plane, so their appearance from Earth changes as Saturn orbits the Sun. From Earth, the main visible features of the rings are the A, B, and C rings, the Cassini Division and the Encke gap. The Cassini Division is a dark region between the A and B rings. The Encke Division lies near the outer edge of the A ring. The rings are made up of trillions of icy particles ranging in size from dust grains to boulders, all orbiting Saturn like so many tiny moons. Their total mass is comparable to that of a small moon. Both divisions are dark because they are almost empty of ring particles.
When the Pioneer and Voyager probes reached Saturn, they found that the rings are actually made up of tens of thousands of narrow ringlets. Interactions between the ring particles and the planet's inner moons are responsible for much of the fine structure observed in the main rings.
The Roche limit of a planet is the distance within which the planet's tidal field would overwhelm the internal gravity of an orbiting moon, tearing the moon apart and forming a ring. All known planetary ring systems lie inside their parent planets' Roche limits.
Saturn's narrow F ring, discovered by Pioneer 11, lies just outside the A ring. It has a kinked, braided structure, apparently caused by two small shepherd satellites that orbit close to the ring and prevent it from breaking up. Voyager 2 probes also discovered the faint D ring, lying between the C ring and Saturn's cloud layer, and the E ring, associated with the moon Enceladus. Many scientists believe that planetary rings have a lifetime of only a few tens of millions of years. The fact that we see rings around all four jovian planets means that they must constantly be re-formed or replenished, perhaps by material chipped off moons by meteoritic impact or by the tidal destruction of entire moons.
Saturn's large moon Titan is the second-largest moon in the solar system. Its thick atmosphere obscures the moon's surface and may be the site of complex cloud and surface chemistry. The existence of Titan's atmosphere is a direct consequence of the cold conditions that prevailed at the time of the moon's formation.
The medium-sized moons of Saturn are made up predominantly of rock and water ice. They show a wide variety of surface terrains and are also heavily cratered. They are all tidally locked by the planet's gravity into synchronous orbits. The innermost mid-sized moon Mimas exerts influence over the structure of the rings. The Cassini Division, now known to contain faint ringlets and gaps, is the result of resonance between its particles and Mimas. The moon Iapetus has a marked contrast between its leading and trailing faces, while Enceladus has a highly reflective appearance, possibly the result of water "volcanoes" on its surface.
Saturn's small moons exhibit a wide variety of complex motion. Several moons "share" orbits, in some cases lying at the Lagrange points 60° ahead of and behind the orbit of a larger moon. The moon Hyperion undergoes chaotic rotation tumbling in an unpredictable way as it orbits the planet.
1. Saturn's orbit is almost twice the size of Jupiter's orbit. (Hint)
2. The rotation of Saturn is unlike that of Jupiter; it is slow and shows little differential rotation. (Hint)
3. Saturn's large axial tilt produces strong seasonal variations in the planet's appearance. (Hint)
4. Saturn probably does not have a rocky core. (Hint)
5. The atmosphere of Saturn contains only half the helium of Jupiter's atmosphere, but, overall, Saturn probably has a normal helium abundance. (Hint)
6. No storm systems have ever been seen on Saturn. (Hint)
7. The magnetic field of Saturn is much less than Jupiter's and, overall, is about as strong as Earth's magnetic field. (Hint)
8. Saturn is the only planet having a ring system. (Hint)
9. A typical ring particle is 100 meters in diameter. (Hint)
10. The composition of the ring particles of Saturn is predominantly water ice. (Hint)
11. Although Saturn's ring system is tens of thousands of kilometers wide, it is only a few tens of meters thick. (Hint)
12. Saturn has small and midsized moons, but no large moons. (Hint)
13. Water ice predominates in Saturn's moons. (Hint)
14. Titan's atmosphere is 10 times denser than Earth's. (Hint)
15. Titan's surface is obscured by thick clouds of water ice. (Hint)
1. Saturn is best known for its spectacular _____ system. (Hint)
2. Features in the atmosphere of Saturn, as on Jupiter, are mostly hidden by an upper layer of frozen _____ clouds. (Hint)
3. Saturn's cloud layers are thicker than those of Jupiter because of Saturn's weaker _____. (Hint)
4. Relative to Jupiter's atmosphere, Saturn's atmosphere is deficient in _____. (Hint)
5. Saturn's excess energy emission is caused by _____. (Hint)
6. As viewed from Earth, Saturn's ring system is conventionally divided into _____ broad rings. (Hint)
7. The Cassini Division lies between the _____ and _____ rings. (Hint)
8. The _____ ring of Saturn is the brightest. (Hint)
9. The rings exist because they lie within Saturn's _____. (Hint)
10. Two small moons, known as _____ satellites, are responsible for the unusually complex form of the F ring. (Hint)
11. The composition of Titan's atmosphere is 90 percent _____. (Hint)
12. Titan's surface may be covered with _____ sediment. (Hint)
13. Except for Titan, most of the moons of Saturn are heavily cratered, even though their surfaces are mostly water ice. The low _____ of the ice makes it hard as rock. (Hint)
14. Saturn's _____ ring may be related to volcanism on the icy moon Enceladus. (Hint)
15. When small moons share an orbit with a large moon, they are found at the _____ points of the large moon's orbit. (Hint)
1. Seen from Earth, Saturn's rings sometimes appear broad and brilliant but at other times seem to disappear. Why? (Hint)
2. Why does Saturn have a less varied appearance than Jupiter? (Hint)
3. What does Saturn's shape tell us about its deep interior? (Hint)
4. Compare and contrast the atmospheres and weather systems of Saturn and Jupiter,and tell how the differences affect each planet's appearance. (Hint)
5. Compare the thicknesses of Saturn's various layers (clouds, molecular hydrogen, metallic hydrogen, and core) with the equivalent layers in Jupiter. (Hint)
6. What is the relevance of the Roche limit to planetary rings? (Hint)
7. What evidence supports the idea that a relatively recent catastrophic event was responsible for Saturn's rings? (Hint)
8. What effect does Mimas have on the rings? (Hint)
9. When Voyager 1 passed Saturn in 1980, why didn't it see the surface of Saturn's largest moon, Titan? (Hint)
10. Compare and contrast Titan with Jupiter's Galilean moons. (Hint)
11. Why does Titan have a dense atmosphere when other large moons in the solar system don't? (Hint)
12. What is the evidence for geological activity on Enceladus? (Hint)
13. What mystery is associated with Iapetus? (Hint)
14. Describe the behavior of Saturn's co-orbital satellites. (Hint)
15. Imagine what the sky would look like from Saturn's moon Hyperion. Would the Sun rise and set in the same way it does on Earth? How do you imagine Saturn might look? (Hint)
1. What is the angular diameter of Saturn's A ring, as seen from Earth at closest approach? (Hint)
2. How long does it take for Saturn's equatorial flow, moving at 1500 km/h, to encircle the planet? Compare this with the wind circulation time on Jupiter. (Hint)
3. If Saturn's surface temperature is 97 K and the planet radiates three times more energy than it receives from the Sun, use Stefan's law to calculate what the surface temperature would be in the absence of any internal heat source. (Hint)
4. Based on the data given in Section 12.3, estimate the mass of Saturn's core and compare it with Earth's mass. (Hint)
5. The text states that the total mass of material in Saturn's rings is about 1015 tons (1018 kg). Suppose the average ring particle is 6 cm in radius (a large snowball) and has a density of 1000 kg/m3. How many ring particles would there be? (Hint)
6. What is the orbital speed of ring particles at the inner edge of the B ring in km/s? Give your answer in km/s. Compare this with the speed of a satellite in low Earth orbit (500 km altitude, say). Why are these speeds so different? (Hint)
7. Show that Titan's surface gravity is about one-seventh of Earth's, as stated in the text. What is Titan's escape speed? (Hint)
8. Calculate the orbital radii of ring particles having the following properties: (a) in a 2:1 orbital resonance with Mimas, that is, orbiting Saturn twice for every orbit of Mimas; (b) in a 3:2 resonance with Mimas (three orbits for every two of Mimas); (c) in a 3:1 resonance with Enceladus. (Hint)
9. Compare Saturn's tidal gravitational effect on Mimas with Mimas's own surface gravity. (Hint)
10. Sunlight reflected back to Earth from a particle in Saturn's rings is Doppler-shifted twice—first because of the relative motion of the source of the radiation (the Sun) and the ring particle, and then again by the relative motion of the particle and the observer on Earth (see Section 3.5). As a result, if Earth, Saturn and the Sun are roughly aligned (that is, Saturn is near opposition), the observed Doppler shift actually corresponds to twice the particle's orbital speed. A certain solar spectral line, of wavelength 656 nm, is reflected from the rings and is observed on Earth. If the rings happen to be seen almost edge-on, what is the line's observed wavelength in light (a) reflected from the approaching inner edge of the B-ring? (b) reflected from the receding inner edge of the B-ring? (c) reflected from the approaching outer edge of the A-ring— (d) reflected from the receding outer edge of the A-ring? (Hint)
1. Saturn moves more slowly among the stars than any other visible planet. How many degrees per year does it move? Look in an almanac to see where the planet is now. What constellation is it in now? Where will it be in one year?
2. Binoculars may not reveal the rings of Saturn, but most small telescopes will. Use a telescope to look at Saturn. Does Saturn appear oblate? Examine the rings. How are they tilted? Can you see a dark line in the rings? This is the Cassini Division. It once was thought to be a gap in the rings, but the Voyager spacecraft discovered that it is filled with tiny ringlets. Can you see the shadow of the rings on Saturn?
3. While looking at Saturn through a telescope can you see any of its moons? The moons line up with the rings; Titan is often the farthest out, and always the brightest of them. How many moons can you see? Use an almanac to identify each one you find.
