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 planets 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 planets overall butterscotch hue is due to cloud chemistry similar to that occurring in Jupiters 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 Jupiters, Saturns excess energy emission is the result of helium precipitation in the planets interior, where helium liquefies and forms droplets which then fall toward the center of the planet. This process is also responsible for Saturns observed helium deficit. Saturns 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 Jupiters. Saturns conducting interior and rapid rotation produce a strong magnetic field and an extensive magnetosphere that contains the planets ring system and the innermost 16 moons.
Saturns rings lie in the planets 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 planets 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 planets 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. Saturns 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 Saturns 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.
Saturns single large moon Titan is the second-largest moon in the solar system. Its thick atmosphere obscures the moons surface and may be the site of complex cloud and surface chemistry. The existence of Titans atmosphere is a direct consequence of the cold conditions that prevailed at the time of the moons 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 planets 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.
Saturns 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.
|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 Saturns A ring, as seen from Earth at closest approach? HINT
2. What is the size of the smallest feature visible in Saturns 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 Saturns actual mass and with the mass of Earth. HINT
4. How long does it take for Saturns equatorial flow, moving at 1500 km/h, to encircle the planet? Compare this with the wind-circulation time on Jupiter. HINT
5. If Saturns surface temperature is 97 K and the planet radiates three times more energy than it receives from the Sun, use Stefans 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 Saturns core. HINT
7. The text states that the total mass of material in Saturns 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 Titans surface gravity is about one-seventh of Earths, as stated in the text. What is Titans 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 Saturns tidal gravitational effect on Mimas with Mimass own surface gravity. HINT
13. Compare Saturns tidal gravitational effect on Titan with Titans own surface gravity. Based on these numbers and the corresponding numbers for Jupiters Galilean moons, would you expect significant internal heating in Titan? HINT
14. Sunlight reflected back to Earth from a particle in Saturns rings is Doppler-shifted twicefirst 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 particles 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 lines 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
1. Saturns Rings Scale Model. Using your tallest group members outstretched arms, create a complete scale model of all Saturns 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.
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 spinscounterclockwise 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 Saturns 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 "Saturns 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.