Page 26410.2 SaturnSaturn's Appearance and Physical Properties

Saturn is the second largest planet in our Solar System and lies about twice as far as Jupiter from the Sun,about 10 AU. Saturn bears the name of an ancient Roman harvest god. In later mythology, Saturn cameto be identified with Cronus (also spelled Kronos), whom the ancient Greeks considered the father of thegods.

Saturn's diameter is about 9.5 times larger than the Earth's. Saturn's mass is about 95 times that of theEarth, and its average density is very small—only 0.7 grams per cubic centimeter, which is less than thedensity of water. Such a low density suggests that Saturn, like Jupiter, is composed mostly of hydrogenand hydrogen­rich compounds. Spectra of the planet bear this out, and astronomers think that Saturn issimilar to Jupiter in its composition and internal structure, as depicted in figure 10.11.

Figure 10.11Internal structure of Saturn.

Saturn radiates more energy than it gains from the Sun, implying that, like Jupiter, it has an internal heatsource. Based on the predicted conditions inside Saturn, astronomers think that a major source of Saturn'sheat comes from deep beneath Saturn's cold clouds, where helium droplets condense in its atmosphere,much as water droplets condense in Earth's atmosphere. As the helium droplets fall toward Saturn's core,they release gravitational energy that heats the planet's interior.

If Jupiter and Saturn both have hot interiors and similar compositions, why do they look so differentexternally? In particular, why does Saturn show only faint cloud belts and markings, compared with thestriking patterns seen on Jupiter? Saturn's greater distance from the Sun and its consequently lowertemperature may provide an answer. Saturn's atmosphere is cold enough for ammonia gas to freeze intocloud particles that veil its atmosphere's deeper layers, making markings below the clouds indistinct. Byexaggerating the color differences in figure 10.12, we can see that Saturn has similar bands beneath itshazy outer layer.

Figure 10.12A high­contrast image of Saturn, made by Cassini, reveals complex cloud belts and storms in Saturn'satmosphere.Saturn's Rings

Saturn's spectacular rings, illustrated in figure 10.13, were first seen by the astronomer Galileo. Throughhis small, primitive telescope, however, they looked like “handles” on each side of the planet, and it wasnot until 1659 that Christiaan Huygens, a Dutch scientist, observed that the rings were detached fromSaturn and encircled it.

Figure 10.13Saturn and its rings as imaged by the Cassini spacecraft are shown at left. At right, a sequence of imagesfrom the Hubble Space Telescope shows the changing appearance of the rings over a quarter of Saturn's

orbit. These range from the south pole tilted most toward the Sun (top) to about 7 Earth years later whenthe Sun shines directly down on the equator and the rings are almost invisible (bottom).

The rings are very wide but very thin. The main band extends from about 30,000 kilometers above thetop of Saturn's atmosphere to a little more than twice the planet's radius (136,000 kilometers, or about84,000 miles), as illustrated in figure 10.13. Some faint inner rings can be seen even closer to Saturn, andfaint outer rings extend considerably farther from the planet. Yet despite the rings' immense breadth, theyare probably less than a few hundred meters thick—so thin that stars can be seen through them.

The British physicist James Clerk Maxwell, a pioneer in the study of electromagnetism, demonstratedthat the rings must be a swarm of particles. He showed mathematically that no material could plausiblybe strong enough to hold together in a solid sheet of such vast size. Spectra of the rings supportMaxwell's theory: the inner and outer parts orbit Saturn at different velocities, obeying Kepler's third law,as shown by their Doppler shift. Thus, the rings must be a swarm of individual bodies.

Astronomers have since discovered that the ring particles are relatively small, only a few centimeters to afew meters across. Although these particles are far too small to be seen individually with telescopes, theyreflect radar signals bounced off them. From the strength of the radar “echo,” astronomers can estimatethe particle sizes. More precise measurements were made with radio signals from the Voyager spacecraft.As Voyager transmitted data to Earth, the signals passed through the rings, scattering slightly from theparticles. From the amount of scattering of the Voyager signal and the radar waves, astronomers candeduce not only the size of the ring particles but also their composition. Better information about thecomposition of the rings, however, comes from analyzing the spectrum of sunlight reflected from them.Such analyses show that the rings are composed primarily of water­ice. However, the Cassini spacecraftpictures, such as figure 10.14, show that some parts of the rings have different colors than others,implying that the composition of the rings is not the same everywhere. Particles in the darker ringsegments may be rich in carbon compounds similar to those found in some asteroidal material. Figure10.14 also shows that the rings are not uniformly filled: they consist of numerous separate ringlets. Largegaps in the rings had been seen from Earth, but the many narrow gaps, forming “ringlets” in the rings,came as a surprise.

Figure 10.14Cassini image detailing the substructure of Saturn's rings. Slight color differences have been greatlyexaggerated over half the image to illustrate differences in the ringlets' composition.Page 265ANIMATION

A 2:1 resonance

As long ago as 1866, Daniel Kirkwood, an astronomer who also discovered gaps in the distribution ofasteroids (chapter 11), noticed that the largest gap—known as Cassini's division—occurs where ringparticles orbit Saturn in exactly one­third the orbital period of its moon Enceladus. Thus, any ringparticle that attempted to orbit in the gap would undergo a strong and repeated gravitational force fromEnceladus every third orbit. Kirkwood concluded that, over long periods of time, the cumulative effect ofEnceladus's force would pull particles from the gap. He therefore hypothesized that Enceladus'sgravitational attraction creates Cassini's division, just as Jupiter's creates the gaps in the asteroid belt. Afew years later, Kirkwood revised his theory of the gap to account for the action of the four largestSaturnian moons. Today astronomers think that Saturn's moon Mimas, whose period is twice that ofparticles in Cassini's division, causes that large gap, but they think the many narrow gaps apparent inFigure 10.14 have a different cause.

A 2 to 1 Resonance

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Narrow gaps in the rings probably arise from a complex interaction between the ring particles and thetiny moons that orbit within the rings. As these moonlets—only tens of kilometers or less in size—orbitSaturn, their gravitational attraction on the ring particles generates waves. These waves spread through

the rings much like ripples in a cup of coffee that is lightly tapped. Such ripples are circular in a cup oron a still pond, but in a planetary ring system they take a different form. Because the inner part of thering orbits faster than the outer part—a consequence of Kepler's laws—the spreading waves wrap into atightly wound pattern called “spiral density waves.” The crests of these density waves form the narrowrings.

Moonlets may generate gaps within the rings in yet another way. If two moonlets move along orbits thatlie very close together, their combined gravitational force may deflect ring particles into a narrow streambetween them (fig. 10.15). Such “shepherding satellites” can also produce twists and knots as theyinteract gravitationally with the ring particles. But what created planetary rings in the first place?

Figure 10.15Two shepherding satellites (the bright dots) and a portion of a narrow outer ring around Saturn that theyshape. Repeated gravitational interactions with the moons also produce twists in the ringlets.Origin of Planetary Rings

For centuries astronomers believed that the only Solar System planet with rings was Saturn. But in 1977,thin rings were detected around Uranus, and within a few years thin rings around Jupiter and Neptunewere discovered as well. Rings made of small or dark particles are often easier to see when viewed fromthe side of the planet opposite the Sun, as seen in figure 10.16, where faint outer rings of Saturn arevisible. The ring systems around other Jovian planets are shown to the same scale for comparison.

Figure 10.16Ring systems of the Jovian planets. The images of Jupiter's and Neptune's rings were made by theVoyager 2 spacecraft from the nightside of the planets. Saturn's rings were also imaged from thenightside by Cassini. The Uranus image was made by the Hubble Space Telescope.

Astronomers have long debated what is the age of Saturn's ring system. Some argued that planetary ringswere material left over from a planet's formation, perhaps matter that had failed to condense into asatellite. Others argued that the rings must be recent additions, because the ring particles are subject toforces in addition to gravity. For example, gas trapped in a planet's magnetic field may exert a frictionalforce on the ring particles, gradually causing them to spiral into the planet's atmosphere, as we discussedearlier about the material in Jupiter's ring system. This suggests that new material must be added to therings from time to time, for without such replenishment the rings would disappear in a few million years.One source of new material is the satellites orbiting the planet. A moon in a satellite system as complexas that of a Jovian planet is subject not only to the gravitational forces of the planet but also to that of theother moons. The cumulative effect of such forces alters the satellites' orbits and may lead to collisionsbetween them.

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Recent studies of Saturn's main rings suggest that they may be more stable than previously thought. Thematerial in these rings appears to alternately clump together and break apart. These large clumpsexperience much less drag than tiny dust particles, so they can survive for long times. This also helpsexplain why Saturn's rings can remain bright even if they are very old. Older surfaces in the Solar Systemgrow dark as they become covered with dust, but by clumping and breaking apart, fresh surfaces areexposed. Why these clumps never continued to combine to form a moon can be explained by a theoryfirst suggested more than a century and a half ago.

The Roche Limit

In 1849 the French scientist M. E. Roche (pronounced rohsh), while studying the problem of a planet'sgravitational effect on its moons, demonstrated mathematically that if a moon gets too close to its planet,the planet's gravity could rip the moon apart. This disruption occurs because the planet pulls harder onone side of the satellite than on the other. If the difference in this pull exceeds the moon's own internalgravitational force, the moon will be pulled apart, as shown in figure 10.17. Thus, if a moon—or anybody held together by gravity—approaches a planet too closely, the planet raises a tide so large it pullsthe encroaching object to pieces. Roche calculated the distance at which the tide becomes fatally largeand showed that for a moon and planet of the same density, breakup occurs if the moon comes nearer toits planet than 2.44 planetary radii, a distance now called the Roche limit. All planetary rings lie neartheir planet's Roche limit, suggesting that rings might be caused by satellite disruption. But moons arenot the only bodies that can stray into the danger zone. Asteroids or comets may occasionally pass tooclose to a planet, and shattered fragments of such bodies (fig. 10.18) may help keep rings filled. Theexistence side­by­side of ringlets with different compositions (some rich in ice, others rich in carbon) isadditional evidence that the rings were formed by the breakup of many different small objects.

Figure 10.17The Roche limit. A planet's gravity pulls more strongly on the near side of a satellite than on its far side,stretching it and, if strong enough, pulling it apart.

Roche break­up of a moon

ANIMATION

Roche breakup of a moon

Figure 10.18Comet Shoemaker­Levy 9. This image shows 20 or so of the fragments into which it was broken byJupiter's tidal force when it passed close to Jupiter in 1992. These pieces struck Jupiter in 1994.

The Roche limit applies only to bodies held together by gravity, however. Artificial satellites or smallbodies bonded together by chemical forces can pass safely through the Roche limit without effect.

Page 268Saturn's Moons

Saturn has one very large moon and 61 smaller ones that have been identified. The larger of these bodies,like Jupiter's moons, orbit in a flat “mini–Solar System” aligned with Saturn's equator. Saturn's moonshave a lower average density than the Galilean satellites of Jupiter, from which astronomers deduce thattheir interiors must be mostly ice. Moreover, all have about the same density, implying that they were notstrongly heated by Saturn as they formed or later on. By contrast, the Galilean satellites nearest Jupiterhave a higher density than those farther away, presumably a result of Jupiter's heating driving away muchof the icy material from the nearer bodies.

Figure 10.19 shows pictures of Saturn's seven largest moons taken by the Cassini spacecraft. Notice thatmost of these bodies are heavily cratered, implying that they have been extensively bombarded byinfalling bodies. Many of Saturn's satellites have bright and dark streaks. Iapetus (eye­AP­eh­tuss) isparticularly extreme, with the side of the moon that faces forward as it orbits Saturn covered with anextremely dark material (fig. 10.19). Scientists are unsure of the origin of this material, but it may havecome from eruptions on other satellites or collisions that scattered fine debris in space. This material wasthen swept up on Iapetus's leading side, coating the surface with a thin layer of dark dust. Subsequently,solar heating of the darker side could have served to evaporate the remaining ice and further concentratethe dust and other dark material that had already been present in the surface ice.

Figure 10.19Pictures of Saturn's seven largest moons shown to scale. Note the thin haze layer in Titan's upperatmosphere.

Q. Based on their surface features, which of these moons might be geologically active? Why?

answer

The smooth surface seen on Enceladus (en­SELL­ah­duss) is the result of water that has erupted from itsinterior, flooding old craters and drowning them as it freezes. The Cassini spacecraft has observed theseeruptions (fig. 10.20), which appear to be powered by tidal flexing (like the eruptions on Jupiter's satelliteIo), and are probably the source of material in Saturn's faint outer ring (see fig. 10.16).

Figure 10.20Cassini pictures of eruptions from the south polar region of Enceladus.

Titan, the largest Saturnian moon, has a diameter of about 5000 kilometers (3000 miles), making itslightly bigger in diameter than the planet Mercury and comparable in mass and radius to Jupiter's largemoons Ganymede and Callisto. Because Titan is farther from the Sun than these bodies, it is muchcolder. Thus, as gas molecules leak from its interior, they move relatively slowly and are unable toescape Titan's gravitational attraction. This immense moon therefore possesses its own atmosphere,

which spectra show to be mostly nitrogen. Clouds in Titan's atmosphere hide its surface, but duringseveral flybys of Titan by NASA's Cassini spacecraft, astronomers have mapped Titan's surface withradar and infrared cameras (fig. 10.21).

Figure 10.21Titan, as seen from the Cassini spacecraft. (A) Visible­light image shows Titan's thick clouds. (B)Thisfalse­color infrared image shows some surface features, glimpsed through the clouds. (C) Radar imagereveals rivers and lake. (D) Radar image of impact crater and dunes.Page 269

Astronomers had shown on the basis of calculations that, because of Titan's extreme cold (94 K, or about–290°F), its clouds would be composed of methane (CH4) and other hydrocarbons rather than water.Such models also predicted that liquid methane or ethane (C2H6) would “rain” from Titan's clouds andmight even form oceans on this frigid moon. No oceans have been found, but radar maps of Titan's polarregions (fig. 10.21C) clearly show lakes, which appear to be liquid methane and ethane, and repeatedimaging shows that they sometimes dry up.

In addition to the lakes and channels, other radar images show impact craters and rows of dunes,presumably built and aligned by winds (fig. 10.21D), and at least one “ice” volcano. But these seeminglyfamiliar surface features differ in composition from similar looking features here on Earth. On Titan, thebitter cold makes ordinary water­ice as hard as rock, so Titan's dunes may be ice crystals (not particles ofsand) and its river channels may have been cut by methane rain falling and eroding water­ice (not rock).

A small landing probe, carried by Cassini and built by the European Space Agency, parachuted intoTitan's atmosphere in 2005. This probe, named Huygens in honor of Christiaan Huygens, descendedthrough Titan's clouds and landed on the surface. As it drifted down to the surface, the probe tookpictures of river networks (fig. 10.22A and B). On the surface it sent back pictures of a rock­strewn plain(fig. 10.22C), the “rocks” probably made of water­ice.

Figure 10.22The landscape of Titan: (A) river networks carved by flowing liquid methane imaged by the Huygenslander from a height of 16 kilometers (10 miles); (B) panorama of the horizon made when the probe wasabout 8 km (5 miles) above the surface; (C) “rocks” of ice on the surface are about 10–15 cm (4–6inches) across.