1. Chapter 12 Saturn
Chapter 12 opener. The number of known moons in the solar system increased rapidly during the late 1990s. Better telescopes enabled astronomers to take a more complete inventory of objects in our cosmic neighborhood. But the more closely we examine these alien worlds, the more complex they seem to be. In this puzzling image taken by the Cassini spacecraft in 2005, Saturn’s tiny ice-covered moon Enceladus (only 500 km in diameter) shows evidence of a youthful terrain in the south where craters are mostly absent. The long blue streaks (about 1 km wide) are likely fractures in the ice through which gas escapes to form a thin but real atmosphere. (JPL)

2. Units of Chapter 12 12.1 Orbital and Physical Properties
12.2 Saturn’s Atmosphere
12.3 Saturn’s Interior and Magnetosphere
12.4 Saturn’s Spectacular Ring System
12.5 The Moons of Saturn
Dancing Among Saturn’s Moons

3. 12.1 Orbital and Physical Properties
Mass: 5.7 × 1026 kg
Radius: 60,000 km
Density: 700 kg/m3—less than water!
Rotation: Rapid and differential, enough to flatten Saturn considerably (oblate)
Rings: Very prominent; wide but extremely thin

4. 12.1 Orbital and Physical Properties
View of rings from Earth changes as Saturn orbits the Sun
Figure Ring Orientation (a) Over time, Saturn’s rings change their appearance to terrestrial observers as the tilted ring plane orbits the Sun. At some times during Saturn’s 29.5-year orbital period the rings seem to disappear altogether as Earth passes through their plane and we view them edge-on. Numbers along Saturn’s orbit indicate the year. The roughly true-color images (inset) span a period of several years from the mid-1990s (bottom) to nearly the present (top), showing how the rings change from our perspective, from almost edge-on to more nearly face-on. See also Figure 12.2 for a closeup image of its tilted ring system relative to Earth. (NASA)

5. 12.2 Saturn’s Atmosphere
Saturn’s atmosphere also shows zone and band structure, but coloration is much more subdued than Jupiter’s
Mostly molecular hydrogen, helium, methane, and ammonia; helium fraction is much less than on Jupiter

6. 12.2 Saturn’s Atmosphere
This true-color image shows the delicate coloration of the cloud patterns on Saturn
Figure Saturn This spectacular image, acquired in 2005 by the Cassini spacecraft while approaching Saturn, is actually a mosaic of many images taken in true color. Note the subtle coloration of the planet and the detail in its rings. Resolution is 40 km. (NASA)

7. 12.2 Saturn’s Atmosphere
Similar to Jupiter’s, except pressure is lower
calmerFewer storms
Three cloud layers
Cloud layers are thicker than Jupiter’s; see only top layer
Figure Saturn’s Atmosphere The vertical structure of Saturn’s atmosphere contains several cloud layers, like Jupiter, but Saturn’s weaker gravity results in thicker clouds and a more uniform appearance. The colors shown are intended to represent Saturn’s visible-light appearance.

8. 12.2 Saturn’s Atmosphere
Structure in Saturn’s clouds can be seen more clearly in this false-color image
Figure Saturn’s Cloud Structure More structure is seen in Saturn’s cloud cover when computer processing and artificial color are used to enhance the contrast of the image, as in these Voyager images of the entire gas ball and a smaller, magnified piece of it. (NASA)

9. 12.2 Saturn’s Atmosphere
Wind patterns on Saturn are similar to those on Jupiter, with zonal flow
Note the speed
3x faster! Than Jupiter
Figure Saturn’s Zonal Flow Winds on Saturn reach speeds even greater than those on Jupiter. As on Jupiter, the visible bands appear to be associated with variations in wind speed.

10. 12.2 Saturn’s Atmosphere
Jupiter-style “spots” rare on Saturn; don’t form often and quickly dissipate if they do
Figure Saturn Storms (a) Circulating and evolving cloud systems on Saturn, imaged by the Hubble Space Telescope at approximately 2-hour intervals. (b) This infrared image, displayed in false color and created for greater contrast by combining observations made at three different wavelengths—1.0 μm (blue), 1.8 μm (green), and 2.1 μm (red)—enhances the belt structure and storms near the equator. Blue coloration indicates regions where the atmosphere is relatively free of haze (see Figure 12.3); green and yellow indicate increasing levels of haze; red and orange indicate high-level clouds. Two small storm systems near the equator appear whitish. Two of Saturn’s moons are also visible in the image, as noted. (NASA)

11. 12.2 Saturn’s Atmosphere
This “dragon storm” was first spotted in 2004; it is believed to be a long-lived phenomenon but is usually hidden under the clouds
Figure Saturn’s “Dragon Storm” This false-color, near-infrared image from Cassini shows what is thought to be a vast thunderstorm on Saturn. Called the “Dragon Storm,” it generated bursts of radio waves resembling the static created by lightning on Earth. (NASA)

12. 12.2 Saturn’s Atmosphere
As expected for a planet with an atmosphere, there is a vortex at Saturn’s south pole.
Figure Saturn’s Polar Vortex This infrared Cassini image shows a huge swirling polar vortex at Saturn’s south pole. The swirling, hurricane-like storm has a well-developed eye wall and calm winds at its center. The size of this vortex is slightly larger than the entire Earth, and its winds are about twice that of a category 5 hurricane on Earth. (NASA)

13. 12.3 Saturn’s Interior and Magnetosphere
Interior structure similar to Jupiter’s
Figure Saturn’s Interior Saturn’s internal structure, as deduced from Voyager observations and computer modeling.

14. 12.3 Saturn’s Interior and Magnetosphere
Saturn also radiates more energy than it gets from the Sun, but not because of cooling:
Helium and hydrogen are not well mixed; helium tends to condense into droplets and then fall
Gravitational field compresses helium and heats it up

15. 12.3 Saturn’s Interior and Magnetosphere
Saturn also has a strong magnetic field, but only 5% as strong as Jupiter’s
Magnetic Field Strength
1000 x earths’
Surface of Atmosphere (10 Earth Radii)
= Earth Surface
Creates aurorae:
Figure Aurora on Saturn An ultraviolet camera aboard the Hubble Space Telescope recorded this image of a remarkably symmetrical (orange) aurora on Saturn during a solar storm in (NASA)

16. 12.4 Saturn’s Spectacular Ring System
Saturn has an extraordinarily large and complex ring system, which was visible even to the first telescopes

17. 12.4 Saturn’s Spectacular Ring System
Overview of the ring system:
Figure Saturn’s Rings Much fine structure, especially in the rings, appears in this image of Saturn taken with the Cassini spacecraft in The main ring features long known from Earth-based observations—the A, B, and C rings, the Cassini Division, and the Encke gap—are marked and are shown here in false color in order to enhance contrast. The other rings listed in Table 12.1 are not visible here. (NASA)

18. 12.4 Saturn’s Spectacular Ring System
Ring particles range in size from fractions of a millimeter to tens of meters
Composition: Water ice—similar to snowballs
Why rings?
Too close to planet for moon to form—tidal forces would tear it apart

19. 12.4 Saturn’s Spectacular Ring System
Closest distance that moon could survive is called Roche limit; ring systems are all inside this limit
Figure Jovian Ring Systems The rings of Jupiter, Saturn, Uranus, and Neptune. All distances are expressed in planetary radii. The red line represents the Roche limit, and all the rings lie within (or very close to) this limit of their parent planets. Note that the red line is just an approximation—the details depend on the internal structure of the planet, and Neptune’s Adams ring is actually consistent with Roche’s calculations.

20. Roche Limit
The external tidal forces on an object become greater than the internal forces that hold it together.

21. 12.4 Saturn’s Spectacular Ring System
Voyager probes showed Saturn’s rings to be much more complex than originally thought
(Earth is shown on the same scale as the rings)
Cassini Division has several ringlets
Figure Saturn’s Rings, Up Close Cassini took this close-up, true-color image of Saturn’s dazzling ring structure just before plunging through the planet’s tenuous outer rings. The ringlets in the B ring, spread over several thousand kilometers, are resolved here to about 10 km. Note the large number of tiny ringlets visible in the main rings. Earth is superposed, to proper scale, for a size comparison. The inset is an overhead view of a portion of the B ring, showing the ringlet structure in even more detail; in fact the resolution here is an incredible 4 km. (NASA)

22. 12.4 Saturn’s Spectacular Ring System
This backlit view shows the fainter F, G, and E rings
Figure Back-Lit Rings Cassini took this spectacular image of Saturn’s rings as it passed through Saturn’s shadow. Note how the back-lit main ring system differs from the front-lit view shown earlier in Figure The normally hard to see F, G, and E outer rings are clearly visible (and marked) in this contrast-enhanced image. The inset shows the moon Enceladus orbiting within the E ring; its eruptions likely give rise to the ring’s icy particles. (NASA)

23. 12.4 Saturn’s Spectacular Ring System
Voyager also found radial “spikes” that formed and then dissipated; this probably happens frequently
Figure Spokes in the Rings Saturn’s B ring showed a series of dark temporary “spokes” as Voyager 2 flew by at a distance of about 4 million km. The spokes are caused by small particles suspended just above the ring plane. Cassini, too, has seen spokes, although (so far) they have not been as prominent as those seen by Voyager 25 years earlier. (NASA)

24. 12.4 Saturn’s Spectacular Ring System
Other edges and divisions in rings are also the result of resonance
“Shepherd” moon defines outer edge of A ring through gravitational interactions
Confine a narrow ring

25. 12.4 Saturn’s Spectacular Ring System
Strangest ring is outermost, F ring; it appears to have braids and kinks
Figure Shepherd Moon (a) Saturn’s narrow F ring appears to contain kinks and braids, making it unlike any of Saturn’s other rings. Its thinness can be explained by the effects of two shepherd satellites that orbit near the ring—one a few hundred kilometers inside, the other a similar distance outside. (b) One of the potato-shaped shepherd satellites (Prometheus here roughly 100 km across) can be seen at the right of this enlarged view. (NASA)

26. 12.4 Saturn’s Spectacular Ring System
F ring’s oddities probably caused by two shepherd moons, one of which can be seen here:
Figure F Ring Structure Kinks, waves, and other substructure in the F ring can be seen in this high-resolution Cassini image. A back-and-forth dance of the shepherd moons (Pandora shown here) gravitationally sculpts scalloped-shaped clumps in the core of the rings. (NASA)

27. 12.4 Saturn’s Spectacular Ring System
Details of formation are unknown:
Too active to have lasted since birth of solar system
Either must be continually replenished, or are the result of a catastrophic event

28. 12.5 The Moons of Saturn
Saturn’s many moons appear to be made of water ice
In addition to the small moons, Saturn has:
Six medium-sized moons (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus)
One large moon (Titan) which is almost as large as Ganymede

29. 12.5 The Moons of Saturn
Titan has been known for many years to have an atmosphere thicker and denser than Earth’s; mostly nitrogen and argon
Makes surface impossible to see; the upper picture at right was taken from only 4000 km away
Figure Titan (a) Larger than the planet Mercury and roughly half the size of Earth, Titan, Saturn’s largest moon, was photographed in visible light from only 4000 km away as Voyager 1 passed by in Only Titan’s upper cloud deck can be seen here. The inset shows a contrast-enhanced image of the haze layer (falsely colored blue). (b) In the infrared, as captured with the adaptive-optics system on the Canada–France–Hawaii telescope on Mauna Kea, some large-scale surface features can be seen. The bright regions are thought to be highlands, possibly covered with frozen methane. The brightest area is nearly 4000 km across—about the size of Australia. (NASA; CFHT)

30. 12.5 The Moons of Saturn
Trace chemicals in Titan’s atmosphere make it chemically complex
Figure Titan’s Atmosphere The structure of Titan’s atmosphere, as deduced from Voyager 1 observations. The solid blue line represents temperature at different altitudes. The inset shows the haze layers in Titan’s upper atmosphere, depicted in false-color green above Titan’s orange surface in this Voyager 1 image. (NASA)

31. 12.5 The Moons of Saturn
Some surface features on Titan are visible in this Cassini infrared image:
Figure Titan Revealed Cassini’s infrared telescopes revealed this infrared, false-color view of Titan’s surface in late The circular area near the center may be an old impact basin and the dark linear feature to its northwest perhaps mountain ranges caused by ancient tectonic activity. The inset shows a surface feature thought to be an icy volcano, further suggesting some geological activity on this icy moon’s surface. Resolution of the larger image is 25 km; that of the inset is 10 times better. (NASA/ESA)

32. 12.5 The Moons of Saturn
The Huygens spacecraft has landed on Titan and is returning images directly from the surface
Figure The View from Huygens (a) Artist’s conception of the Huygens lander parachuting through Titan’s thick atmosphere. (b) This photograph of the surface was taken from an altitude of 8 km as the probe descended. It shows a network of channels reminiscent of streams or rivers draining from the light-shaded uplifted terrain (at center) into a darker, low-lying region (at bottom). Resolution is about 20 m. (c) Huygens’s view of its landing site, in approximately true color. The foreground rocklike objects are only a few centimeters across. (D. Ducros; NASA/ESA)

33. 12.5 The Moons of Saturn
Based on measurements made by Cassini and Huygens, this is the current best guess as to what the interior of Titan looks like:
Figure Titan’s Interior Based on measurements of Titan’s gravitational field during numerous flybys, Titan’s interior appears to be largely a rocky-silicate mix. Most intriguing is the subsurface layer of liquid water, similar to that hypothesized on Jupiter’s Europa and Ganymede. (NASA/ESA)

34. Discovery 12-1: Dancing Among Saturn’s Moons
The Cassini spacecraft uses multiple “gravitational slingshots” to make multiple close passes around Saturn’s moons. Precise orbits are decided on the fly.

35. 12.5 The Moons of Saturn This image shows Saturn’s mid-sized moons
Figure Saturn’s Mid-Sized Moons Saturn’s six medium-sized satellites, to scale, as seen by the Cassini spacecraft. All are heavily cratered and all are shown here in natural color. Iapetus has a ridge around its middle and shows a clear contrast between its light-colored (icy) surface (at top in this image) and its dark cratered hemisphere (at bottom). The light-colored wisps on Rhea are thought to be water and ice released from the moon’s interior during some long-ago period of activity. Dione and Tethys show possible evidence of ancient geological activity. Enceladus appears to be volcanically active, presumably because of Saturn’s tidal influence. Mimas’s main surface feature is the large crater Herschel, plainly visible at the center in this image. For scale, part of Earth’s Moon is shown at left. See also the full-page chapter opening photo for a closeup look at the fascinating moon, Enceladus. (NASA)

36. 12.5 The Moons of Saturn
Mimas, Enceladus, Tethys, Dione, and Rhea all orbit between 3 and 9 planetary radii from Saturn, and all are tidally locked—this means they have “leading” and “trailing” surfaces
Iapetus orbits 59 radii away and is also tidally locked

37. 12.5 The Moons of Saturn Global maps of Rhea and Dione, from Cassini:
Figure Mid-Sized Moons, Close Up Global maps of Rhea (a) and Dione (b), based on many images taken by Cassini. The suspected snow and ice cliffs are the whitish streaks throughout these flat maps. (NASA)

38. 12.5 The Moons of Saturn Masses of small moons not well known
Two of them share a single orbit:
Figure Orbit-Sharing Satellites The peculiar motion of Saturn’s co-orbital satellites Janus and Epimetheus, which play a neverending game of tag as they move around the planet in their orbits. The labeled points represent the locations of the two moons at a few successive times. From A to C, moon 2 gains on moon 1. However, before it can overtake it, the two moons swap orbits, and moon 1 starts to pull ahead of moon 2 again, through points D and E. The whole process then repeats, apparently forever.

39. 12.5 The Moons of Saturn
Two more moons are at the Lagrangian points of Tethys
Figure Synchronous Orbits 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 all three moons share an orbit and never change their relative positions.

40. 12.5 The Moons of Saturn
Hyperion, orbiting between Titan and Iapetus, is so affected by their gravitational fields that its orbital speed and the orientation of its axis are constantly changing in a never-repeating pattern.
This tumbling is an example of chaotic motion!
Figure Hyperion The small moon Hyperion is irregularly shaped and deeply pitted with dark-floored craters. Its spongy appearance is likely caused by dark material, trapped within the moon’s many craters, that melt ever deeper into this low-density object. Its eccentric orbit and irregular shape means that Saturn’s gravity causes it to tumble in a chaotic, unpredictable way. (NASA)

41. Summary of Chapter 12
Saturn, like Jupiter, rotates differentially and is significantly flattened
Saturn’s weather patterns are in some ways similar to Jupiter’s, but there are far fewer storms
Saturn generates its own heat through the compression of “helium raindrops”
Saturn has a large magnetic field and extensive magnetosphere

42. Summary of Chapter 12 (cont.)
Saturn’s most prominent feature is its rings, which are in its equatorial plane
The rings have considerable gross and fine structure, with segments and gaps; their particles are icy and grain- to boulder-sized
Interactions with medium and small moons determine the ring structure
The rings are entirely within the Roche limit, where larger bodies would be torn apart by tidal forces

43. Summary of Chapter 12 (cont.)
Titan is the second-largest moon in the solar system
Titan has an extremely thick atmosphere, and little is known about its surface or interior
Medium-sized moons are rock and water ice; their terrains vary
These moons are tidally locked to Saturn
Several of the small moons share orbits, either with each other or with larger moons