SUMMARY

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 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 with 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 that contains 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 gap 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 a moon, tearing it 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. Beyond the F ring is the faint, narrow G ring, also discovered by Pioneer 11. Voyager 2 discovered the faint D ring, lying between the C ring and Saturn’s cloud layer, and the E ring, associated with the moon Enceladus. Planetary rings may have lifetimes of only a few tens of millions of years. If so, 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 single 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 midsized 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 Lagrangian points 60° ahead of and behind the orbit of a larger moon. The moon Hyperion tumbles in an unpredictable way as it orbits the planet.



SELF-TEST: TRUE OR FALSE?

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 atmosphere. HINT

4. Saturn probably does not have a rocky core. HINT

5. Saturn emits nearly three times more energy in the form of infrared radiation than it receives from the Sun. 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, at the cloud tops, is about as strong as Earth’s magnetic field. HINT

8. Saturn is the only planet with a ring system. HINT

9. A typical ring particle is 100 m 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



SELF-TEST: FILL IN THE BLANK

1. Saturn is best known for its spectacular _____ system. HINT

2. Low-lying features in the atmosphere of Saturn, as on Jupiter, are mostly hidden by the 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 is probably 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



REVIEW AND DISCUSSION

1. Seen from Earth, Saturn’s rings sometimes appear broad and brilliant but at other times seem to disappear. Why? HINT

2. What is a ring crossing? When will the next one occur? HINT

3. Why does Saturn have a less varied appearance than Jupiter? HINT

4. What does Saturn’s shape tell us about its deep interior? HINT

5. Compare and contrast the atmospheres and weather systems of Saturn and Jupiter, and tell how the differences affect each planet’s appearance. HINT

6. Compare the thicknesses of Saturn’s various layers (clouds, molecular hydrogen, metallic hydrogen, and core) with the equivalent layers in Jupiter. Why do the thicknesses differ? HINT

7. What mechanism is responsible for the relative lack of helium in Saturn’s atmosphere, compared to Jupiter? HINT

8. Is Saturn as a whole deficient in helium relative to Jupiter? HINT

9. When were Saturn’s rings discovered? When did astronomers realize what they were? HINT

10. What would happen to a satellite if it came too close to Saturn? HINT

11. What evidence supports the idea that a relatively recent catastrophic event was responsible for Saturn’s rings? HINT

12. What effect does Mimas have on the rings? HINT

13. What are shepherd satellites? HINT

14. When Voyager 1 passed Saturn in 1980, why didn’t it see the surface of Saturn’s largest moon, Titan? HINT

15. Compare and contrast Titan with Jupiter’s Galilean moons. HINT

16. Why does Titan have a dense atmosphere when other large moons in the solar system don’t? HINT

17. What is the evidence for geological activity on Enceladus? HINT

18. What mystery is associated with Iapetus? HINT

19. Describe the behavior of Saturn’s co-orbital satellites. HINT

20. 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



PROBLEMS Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website.

The number of squares preceding each problem indicates its approximate level of difficulty.

1. What is the angular diameter of Saturn’s A ring, as seen from Earth at closest approach? HINT

2. What is the size of the smallest feature visible in Saturn’s rings, as seen from Earth at closest approach with a resolution of 0.05"? HINT

3. What would be the mass of Saturn if it were composed entirely of hydrogen at a density of 0.08 kg/m3, the density of hydrogen at sea level on Earth? Assume for simplicity that Saturn is spherical. Compare your answer with Saturn’s actual mass and with the mass of Earth. HINT

4. 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

5. 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

6. Based on the data given in Sections 12.1 and 12.3 (Figure 12.8), estimate the average density of Saturn’s core. HINT

7. 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 are there? HINT

8. What is the orbital speed of ring particles at the inner edge of the B ring, 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

9. 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

10. Assuming a spherical shape and a uniform density of 2000 kg/m3, calculate how small an icy moon would have to be before a fastball pitched at 40 m/s (about 90 mph) could escape. HINT

11. Calculate the orbital radii of particles having the following properties: (a) a 3:1 orbital resonance with Tethys, that is, orbiting Saturn three times for every orbit of Tethys; (b) a 2:1 resonance with Mimas (two orbits for every orbit of Mimas); (c) a 3:2 resonance with Mimas (three orbits for every two of Mimas); (d) a 2:1 resonance with Dione. HINT

12. Compare Saturn’s tidal gravitational effect on Mimas with Mimas’s own surface gravity. HINT

13. Compare Saturn’s tidal gravitational effect on Titan with Titan’s own surface gravity. Based on these numbers and the corresponding numbers for Jupiter’s Galilean moons, would you expect significant internal heating in Titan? HINT

14. 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 corresponds to twice the particle’s orbital speed. A certain solar spectral line, of wavelength 656.112 nm, is reflected from the rings and observed on Earth. If the rings happen to be seen almost edge-on, what is the line’s observed wavelength in light reflected from (a) the approaching inner edge of the B ring? (b) the receding inner edge of the B ring? (c) the approaching outer edge of the A ring? (d) the receding outer edge of the A ring? HINT

15. Based on the data given in the text, estimate the difference in orbital radii between the two co-orbital satellites. HINT



COLLABORATIVE EXERCISES

1. Saturn’s Rings Scale Model. Using your tallest group member’s outstretched arms, create a complete scale model of all Saturn’s rings as listed on Table 12.1 using self-stick notes or tape labels. Measure the maximum distance from nose to fingertip and use this as the scale factor for the rings’ radii. For example, if the distance from nose to finger-tip was 40 cm and the outer radius of the E ring is 480,000 km, then the inner radius of the D ring is 67,000 km (40 cm/480,000 km) = 5.6 cm out from the nose.



RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 12 on the Companion Website for Astronomy Today 4/e.

1. Access the "Saturn Fact Sheet" from NASA and determine the current number of known natural satellites (moons) around Saturn.

2. Access the "Saturn Nomenclature" page and determine the size and name of the largest crater on Enceladus.

3. Access the "Saturn Pages in Planetary Collections" at The Nine Planets and determine which space probe was the first to arrive at Saturn and what the next space probe to arrive there is named and when it will arrive.



PROJECTS

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 flattened? 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? They line up with the rings; Titan is often the farthest out, and always the brightest. How many moons can you see? Use an almanac to identify each one you find.



SKYCHART III PROJECTS The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text.

1. Center on Saturn, and zoom in to a 1/6° field of view. Saturn is much farther than Jupiter, so a greater magnification is required to create the same angular size image of the planet. Deselect horizon mask; turn off stars, constellations, and grid lines. Set animation for 10-minute steps. It may be necessary to lower the fainter limits of displayed objects to bring up some of the moons. To bring up the fainter objects, use F3 or DRAW/Fainter Limits. Animate the image to observe the motion of the moons. Notice that they orbit in the same sense as Saturn spins—counterclockwise as seen from above the North Pole.

2. Center on Saturn and set parameters the same as for observing the motion of the moons. Set field of view to 1/20° and animate with one-week time steps. Observe the tilt of Saturn and its ring system. Explain why the planet and its ring should change their tilt so markedly. When will the ring system appear edge-on, and when after that will it next appear edge-on? What is this time interval? How is this time interval related to the period of Saturn’s orbit?

3. Galileo trained his telescope on Saturn and initially saw something that he interpreted as two companion planets. He was alarmed when he saw them diminish and disappear, and wondered whether Saturn had devoured its own children. From Rome in Italy in 1612, follow Saturn over a period of three years. Describe the views that Galileo might have had of Saturn.

4. The first person to propose the correct explanation for "Saturn’s children" or the appendages observed by Galileo was Christian Huygens. Aided by a better telescope, he was able to surmise the nature of these appendages. Observe Saturn in the four years leading up to 1659. What happens? The feat of Huygens was to propose a ring: the first heavenly object that had unmistakably a shape that was not spherical.

5. Observing from a point directly above the ecliptic plane, determine the period between successive superior conjunctions of Saturn. This determines the synodic orbital period. How long does it take for Saturn to complete one revolution around the Sun? This is the sidereal orbital period. What is the relationship between the synodic and the sidereal periods? It might be helpful to find an analogy in a watch, with the little hand representing Saturn and the large hand Earth.



In addition to the Practice Problems and Destinations modules, the Companion Website at http://www.prenhall.com/chaisson provides for each chapter an additional true-false, multiple choice, and labeling quiz, as well as additional annotated images, animations, and links to related Websites.