Chapter 12 Saturn - stjohns-chs. · PDF fileChapter 12 Saturn Saturn,(a(huge ... Saturn also...

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Chapter 12 Saturn

Saturn, a huge ball of lightweight gas surrounded by a spectacular ring system of orbi8ng rocky debris, is a planet far different from Earth. This image combines visible, infrared, and ultraviolet data acquired by cameras aboard the Cassini spacecra@ as it cruised behind Saturn in 2005, revealing both the planet and its intricate rings in exquisite detail. Looking back toward the inner parts of the solar system in this view, we can see not only part of the bright Sun (boGom center), but also a dim and distant speck of reflected light (at the 8 o’clock posi8on)—planet Earth floa8ng in space. (JPL)


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

Units of Chapter 12


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




View of rings from Earth changes as Saturn orbits the Sun

Figure 12-‐1. Ring Orienta+on (a) Over 4me, Saturn’s rings change their appearance to terrestrial observers as the 4lted ring plane orbits the Sun. At some 4mes 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 1995 (boIom) to 2003 (top), showing how the rings change from our perspec4ve, from almost edge-‐on to more nearly face-‐on. See also Figure 12.2 for a close up image of its 4lted ring system rela4ve to Earth. (NASA)


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



12.2 Saturn’s Atmosphere This true-color image shows the delicate coloration of the cloud patterns on Saturn

Figure 12-‐2. Saturn This image, acquired in 2005 by the Cassini spacecraM while approaching Saturn, is actually a mosaic of many images taken in true color. Note the subtle colora4on of the planet and the detail in its rings. Resolu4on is 40 km. (NASA)


Similar to Jupiter’s, except pressure is lower

Three cloud layers

Cloud layers are thicker than Jupiter’s; see only top layer


Figure 12-‐3. Saturn’s Atmosphere The ver4cal 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.


12.2 Saturn’s Atmosphere Structure in Saturn’s clouds can be seen more clearly in this false-color image

Figure 12-‐4. Saturn’s Cloud Structure More structure is seen in Saturn’s cloud cover when computer processing and ar4ficial color are used to enhance the contrast of the image, as in these Voyager images of the en4re gas ball and a smaller, magnified piece of it. (NASA)


Wind patterns on Saturn are similar to those on Jupiter, with zonal flow


Figure 12-‐5. 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 varia4ons in wind speed.


Jupiter-style “spots” rare on Saturn; don’t form often and quickly dissipate if they do


Figure 12-‐6. Saturn Storms (a) Circula4ng 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 observa4ons 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 colora4on indicates regions where the atmosphere is rela4vely 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 whi4sh. Two of Saturn’s moons are also visible in the image, as noted. (NASA)



This image shows what is thought to be a vast thunderstorm on Saturn, as well as the polar vortex at Saturn’s south pole.

Figure 12-‐7. Storm Alley This false-‐color, near-‐infrared image from Cassini shows what is thought to be a vast thunderstorm on Saturn. This storm generated bursts of radio waves resembling the sta4c created by lightning on Earth. The inset shows the polar vortex at Saturn’s south pole. (NASA)


Interior structure similar to Jupiter’s



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



Saturn also has a strong magnetic field, but only 5% as strong as Jupiter’s

Creates aurorae


Figure 12-‐10. 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 1998. (NASA)


Saturn has an extraordinarily large and complex ring system, which was visible even to the first telescopes



Overview of the ring system


Figure 12-‐11. Saturn’s Rings Much fine structure, especially in the rings, appears in this image of Saturn taken with the Cassini spacecraM in 2005. The main ring features long known from Earth-‐based observa4ons—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)


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



Closest distance that moon could survive is called Roche limit; ring systems are all inside this limit


Figure 12-‐13. 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 approxima4on—the details depend on the internal structure of the planet, and Neptune’s Adams ring is actually consistent with Roche’s calcula4ons.


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 12-‐15. 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 4ny ringlets visible in the main rings. Earth is superposed, to proper scale, for a size comparison. The inset is an overhead view of a por4on of the B ring, showing the ringlet structure in even more detail; in fact the resolu4on here is an incredible 4 km. (NASA)



This backlit view shows the fainter F, G, and E rings

Figure 12-‐15. 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 12.11. 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 orbi4ng within the E ring; its erup4ons likely give rise to the ring’s icy par4cles. (NASA)


Voyager also found radial “spikes” that formed and then dissipated; this probably happens frequently


Figure 12-‐17. 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 par4cles 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)


•  Other edges and divisions in rings are also

the result of resonance

•  “Shepherd” moon defines outer edge of A

ring through gravitational interactions



Strangest ring is outermost, F ring; it appears to have braids and kinks


Figure 12-‐18. 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 in this enlarged view. (NASA)


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



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



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 12-‐18. 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 1980. 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 spacecraM in 2005. (b) In the infrared, as captured with the adap4ve-‐op4cs 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)


Trace chemicals in Titan’s atmosphere make it chemically complex


Figure 12-‐19. Titan’s Atmosphere The structure of Titan’s atmosphere, as deduced from Voyager 1 observa4ons. The solid blue line represents temperature at different al4tudes. 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)



Some surface features on Titan are visible in this Cassini infrared image

Figure 12-‐20. Titan Revealed Cassini’s infrared telescopes revealed this infrared, false-‐color view of Titan’s surface in late 2004. 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 ac4vity. The inset shows a surface feature thought to be an icy volcano, further sugges4ng some geological ac4vity on this icy moon’s surface. Resolu4on of the larger image is 25 km; that of the inset is 10 4mes beIer. (NASA/ESA)


12.5 The Moons of Saturn The Huygens spacecraft has landed on Titan and returned images directly from the surface

Figure 12-‐21. The View from Huygens (a) Ar4st’s concep4on of the Huygens lander parachu4ng through Titan’s thick atmosphere. (b) This photograph of the surface was taken from an al4tude of 8 km as the probe descended. It shows a network of channels reminiscent of streams or rivers draining from the light-‐shaded upliMed terrain (at center) into a darker, low-‐lying region (at boIom). Resolu4on is about 20 m. (c) Huygens’s view of its landing site, in approximately true color. The foreground rocklike objects are only a few cen4meters across. (D. Ducros; NASA/ESA)


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 12-‐23. Titan’s Interior Based on measurements of Titan’s gravita4onal 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)


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.


12.5 The Moons of Saturn This image shows Saturn’s mid-sized moons

Figure 12-‐24. Saturn’s Mid-‐Sized Moons Saturn’s six medium-‐sized satellites, to scale, as seen by the Cassini spacecraM. 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 boIom). 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 ac4vity. Dione and Tethys show possible evidence of ancient geological ac4vity. Enceladus appears to be volcanically ac4ve, presumably because of Saturn’s 4dal 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 leM. See also the full-‐page chapter opening photo for a closeup look at the fascina4ng moon, Enceladus. (NASA)


•  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




Surface of Enceladus seems oddly youthful

Figure 12-‐25. Enceladus In this puzzling image taken by the Cassini spacecraM, Saturn’s 4ny 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)


Masses of small moons not well known

Two of them share a single orbit


Figure 12-‐27. Orbit-‐Sharing Satellites The peculiar mo4on 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 loca4ons of the two moons at a few successive 4mes. 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.


Two more moons are at the Lagrangian points of Tethys


Figure 12-‐28. Synchronous Orbits The orbits of the moons Telesto and Calypso are 4ed to the mo4on of the moon Tethys. The combined gravita4onal pulls of Saturn and Tethys keep the small moons exactly 60° ahead and behind the larger moon at all 4mes, so all three moons share an orbit and never change their rela4ve posi4ons.


•  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

Summary of Chapter 12


•  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

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

Summary of Chapter 12 (cont.)

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