1. Chapter 12

2. Chapter 12 Saturn
Saturn, a huge ball of lightweight gas surrounded by a spectacular ring system of orbiting rocky debris, is a planet much different from Earth. This image combines visible, infrared, and ultraviolet data acquired by cameras aboard the Cassini spacecraft as it cruised behind Saturn in 2005, revealing both the planet and its intricate rings in exquisite detail. We can also see, since this photo looks back toward the inner solar system, part of the bright Sun (bottom center) and a speck of reflected light from a dim and distant planet Earth (at the 8 o’clock position just outside the brightest ring). (JPL)

3. 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
Discovery 12.1 Dancing Among Saturn’s Moons

4. 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
Rings: Very prominent; wide but extremely thin

5. 12.1 Orbital and Physical Properties
View of rings from Earth changes as Saturn orbits the Sun
Figure Ring Orientation Over time, Saturn’s rings change their appearance to terrestrial observers as the tilted ring plane orbits the Sun. The roughly true-color images (inset) span a period of several years from 2003 (bottom) to nearly the present (top) and show how the rings change from our perspective on Earth, from almost edge-on to more nearly face-on. See also Figure 12.2 for a close-up image of its tilted ring system relative to Earth. (NASA)

6. 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

7. 12.2 Saturn’s Atmosphere
This true-color image shows the delicate coloration of the cloud patterns on Saturn
Figure Saturn This image, acquired in 2005 by the Cassini spacecraft while approaching Saturn, is actually a mosaic of many images taken in true color. Note the bland coloration of the gas ball and the detail in the planet’s rings. Resolution is 40 km. (NASA)

8. 12.2 Saturn’s Atmosphere
Similar to Jupiter’s, except pressure is lower
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.

9. 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)

10. 12.2 Saturn’s Atmosphere
Wind patterns on Saturn are similar to those on Jupiter, with zonal flow
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.

11. 12.2 Saturn’s Atmosphere
Jupiter-style “spots” rare on Saturn; don’t form often and quickly dissipate if they do
Figure Saturn Storms Circulating and evolving cloud systems are evident on Saturn, imaged by the Hubble Space Telescope at approximately 2-hour intervals (left to right). (NASA)

12. 12.2 Saturn’s Atmosphere
This image shows a huge storm on Saturn; it is expected to last at least two years. Smaller storms dissipate more quickly due to extremely high wind speeds.
Figure Storm Alley This huge storm on Saturn was observed by the Cassini spacecraft in While churning its way through the northern hemisphere, it seems to leave behind a “tail” wrapped around the planet. (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 here from Voyager observations and computer modeling, can be compared with similar properties noted for Jupiter in Figure

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
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
Figure Sketches of Saturn’s Rings Three artist’s re-renderings of sketches of Saturn’s rings, made (a) by Galileo in 1610, (b) by Galileo in 1616, and (c) by Huygens in 1655.

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 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 All of the distances to the rings of Jupiter, Saturn, Uranus, and Neptune 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.

20. 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)
Figure Saturn’s Rings, Up Close Cassini took this true-color image of Saturn’s dazzling ring structure just before flying through the planet’s tenuous outer rings. Earth is superposed at right, to proper scale, for a size comparison. The inset at left 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)

21. 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. The normally hard to see F, G, and E outer rings are clearly visible 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)

22. 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)

23. 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

24. 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 is caused by two shepherd satellites that orbit near the ring—one a few hundred kilometers inside the ring and the other a similar distance outside it. (b) One of the potato-shaped shepherd satellites (Prometheus, here roughly 100 km across) can be seen in this enlarged view. (NASA)

25. 12.4 Saturn’s Spectacular Ring System
Details of formation are unknown:
Probably too active to have lasted since birth of solar system
Not all rings may be the same age
Either must be continually replenished, or are the result of a catastrophic event

26. 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), almost as large as Jupiter’s Ganymede

27. 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 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, true-color image of the haze layers in Titan’s upper atmosphere, taken by the Cassini spacecraft in (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)

28. 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)

29. 12.5 The Moons of Saturn
Some surface features on Titan are visible in this Cassini infrared image
Figure Titan Revealed Cassini’s telescopes captured this infrared, false-color view of Titan’s surface in late The semicircular 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 circular 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)

30. 12.5 The Moons of Saturn
The Huygens spacecraft has landed on Titan and returned images directly from the surface
Figure Titan’s Surface (a) This photograph of the surface was taken from an altitude of 8 km as the Huygens probe descended. It shows a network of dark channels reminiscent of streams or rivers (at center) draining from the light-shaded uplifted terrain into darker, low-lying regions (at bottom). (b) Huygens’s view of its landing site, in approximately true color. The foreground icy “rocks” are only a few centimeters across. (NASA/ESA)

31. 12.5 The Moons of Saturn
Based on measurements made by Cassini and Huygens, this is the current best understanding 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 rock-ice mixture. Most intriguing is the subsurface layer of liquid water, similar to that hypothesized on Jupiter’s Europa and Ganymede.

32. 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.

33. 12.5 The Moons of Saturn This image shows Saturn’s mid-sized moons
Figure Saturn’s Midsized Moons Saturn’s six medium-sized satellites as seen by the Cassini spacecraft and compared to Earth’s Moon, all to scale. All are heavily cratered and all are shown here in natural color. (UC/Lick Observatory; NASA)

34. 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

35. 12.5 The Moons of Saturn
Surface of Enceladus seems oddly youthful; inset shows active geysers
Figure Enceladus In this puzzling image taken by the Cassini spacecraft, Saturn’s tiny ice-covered moon Enceladus shows evidence of youthful terrain in the south where craters are mostly absent. The long blue “tiger stripe” streaks (about 1 km wide) are fractures in the ice through which gas escapes to form a thin but real atmosphere. The inset shows some of the jets that are launched from geysers on the limb of this peculiar moon. (JPL)

36. 12.5 The Moons of Saturn Masses of small moons not well known
Two of them share a single orbit
Figure Orbit-Sharing Satellites Saturn’s co-orbital satellites Janus and Epimetheus play a never-ending game of tag while moving around the planet in their orbits. Their peculiar motions are depicted here by the labeled points that represent the locations of the two moons at a few successive times. The whole process then repeats, apparently forever.

37. 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.

38. 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

39. 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

40. 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