The Outer Planets

C H A P T E R 8

The most completevisualization ofSaturn and its ringsystem, taken by

the Cassini spacecraft. Thisimage is a combination ofultraviolet, visible light, andinfrared observations. (CourtesyNASA/JPL-Caltech)

WEB LINK 8.1

WHAT DO YOU THINK?

4 Is Saturn the only planet with rings?

5 Are the rings of Saturn solid ribbons?

Answers to these questions appear in the textbeside the corresponding numbers in themargins and at the end of the chapter.

1 Is Jupiter a “failed star?” Why orwhy not?

2 What is Jupiter’s Great Red Spot?

3 Does Jupiter have continents andoceans?

We can pretty easilyimagine a trip toMars, exploring

its vast canyons and icy polarregions. Even Venus andMercury lend themselves tobeing compared to Earth and

the Moon. However, we find few similarities betweenEarth and the giant outer planets: Jupiter, Saturn,Uranus, and Neptune. Until recently, Pluto was alsoconsidered one of the outer planets. That has changedas described in Appendix I. Now Pluto is categorized asa dwarf planet and we explore Pluto in Chapter 9.

These four worlds lack solid surfaces and are somuch larger, rotate so much faster, and have such dif-ferent chemical compositions from our planet that uponseeing them one knows, to paraphrase Dorothy, “You’renot on Earth anymore.” There is nothing on Earthremotely similar to the swirling red and brown clouds ofJupiter, the ever-changing ring system of Saturn, or theblue-green clouds of Uranus and Neptune. The outer fourworlds collectively have at least 163 moons, with anamazing range of shapes, sizes, surfaces, and properties.Some of these moons have water interiors; some havevolcanoes; some have surface ice; one, Titan, has a thickatmosphere; and one, Hyperion, is arguably the mostbizarre-looking object in the solar system. We begin ourexploration of the outer planets with magnificent Jupiter.

its moons. As the high-resolution images in this chapterreveal, Jupiter is a world of breathtaking beauty.

Jupiter is the largest planet in the solar system:more than 1300 Earths could be packed into its volume.Using the orbital periods of its moons in Kepler’s laws,astronomers have determined that Jupiter is 318 timesmore massive than Earth. Indeed, Jupiter has more than21⁄2 times as much mass as all of the other planets com-bined. This huge mass has created the urban myth thatJupiter is a failed star, meaning that it has almostenough matter to shine on its own, like the Sun, whichwe will study in Chapter 10. The fact is that Jupiterwould have to be 75 times more massive than it is, togenerate energy as the Sun and therefore be classified asa star. Nevertheless, Jupiter emits approximately twiceas much energy as it receives from the Sun. Its extraenergy comes from radioactive elements in its coreand from an overall contraction amounting to less than10 cm per century.

8-1 Jupiter’s outer layer is a dynamic areaof storms and turbulent gases

Jupiter is permanently covered with clouds (see Figure 8-1). Because it rotates about once every 10 hours—thefastest of any planet—Jupiter’s clouds are in perpetualmotion and are confined to narrow bands of latitude.

In this chapter you will discover• Jupiter, an active, vibrant, multicolored world more

massive than all of the other planets combined

• Jupiter’s diverse system of moons

• Saturn, with its spectacular system of thin, flatrings and numerous moons, including bizarreEnceladus and Titan

• what Uranus and Neptune have in common andhow they differ from Jupiter and Saturn

JUPITERJupiter’s multicolored bands, dotted withovals of white and brown (Figure 8-1), giveit the appearance of a world unlike any of

the terrestrial planets. Viewed even through a small tel-escope (see Figure 2-11), you can also see up to four of

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Jupiter Earth

Planet symbol:

1

FIGURE 8-1 Jupiter as Seen from aSpacecraft This view was sent backfrom Voyager 1 in 1979. Features as

small as 600 km across can be seen in the turbulent cloud topsof this giant planet. The complex cloud motions that surroundthe Great Red Spot are clearly visible. Also, clouds at differ-ent latitudes have different rotation rates. The inset image ofEarth shows its size relative to Jupiter. (NASA/JPL; inset: NASA)

VIDEO 8.1

WEB LINK 8.2

Even through a small telescope, you can see Jupiter’sdark, reddish bands called belts, alternating with light-colored bands called zones. These belts and zones aregases flowing east or west, with very little north-southmotion. In contrast, winds on slower-rotating Earth wan-der over vast ranges of latitude (compare Figure 6-1).

Jupiter’s belts and zones provide a framework forturbulent swirling cloud patterns, as well as rotatingstorms similar in structure to hurricanes or cyclones on

Earth. These storms are known as white ovals andbrown ovals (Figure 8-2). The white ovals are observedto be cool clouds higher than the average clouds inJupiter’s atmosphere. The brown ovals are warmer andlower clouds, seen through holes in the normal cloudlayer. The various oval features last from hours to cen-turies. Computers show us how the cloud features onJupiter would look if the planet’s atmosphere wereunwrapped like a piece of paper (Figure 8-3).

The Outer Planets 217

a b

FIGURE 8-2 Close-ups of Jupiter’s Atmosphere The dynamicwinds, rapid rotation, internal heating, and complex chemicalcomposition of Jupiter’s atmosphere create its beautifuland complex banded pattern. (a) A Voyager 2 southern

hemisphere image showing a white oval that has existed forover 40 years. (b) A Voyager 2 northern hemisphere imageshowing a brown oval. The white feature overlapping the ovalis a high cloud. (NASA)

FIGURE 8-3 Jupiter Unwrapped Cassini images ofJupiter combined and opened to give a maplike rep-resentation of the planet. The banded structure is

absent near the poles. The Web link will take you to a movie

version of this and related images. In them, you will see that thelight and dark regions slide by one another, continually movingeastward or westward. (Courtesy NASA/JPL-Caltech)

White ovals

Great Red Spot Brown ovals

VIDEO 8.2

Jupiter’s most striking feature is its Great Red Spot(Figure 8-4), which is so large that it can be seenthrough a small telescope. It changes dimensions, and atpresent is about 25,000 km long by 12,000 km wide—large enough so that two Earths could easily fit side byside inside it. The Great Red Spot was first observedaround 1656, either by the English scientist RobertHooke or the Italian astronomer Giovanni Cassini.Because earlier telescopes were unlikely to have beenable to see it, the Great Red Spot could well haveformed long before that time.

The Great Red Spot is a hurricane or typhoon-likestorm of swirling gases. Heat welling upward frominside Jupiter has maintained it for more than three cen-turies. (Consider what life would be like for us if Earthsustained storms for such long periods!) Between 1998and 2000, three smaller, white storms on Jupiter merged(Figure 8-5a–d), creating a larger white storm thatbecame red in 2006 (Inset: Figure 8-5e). Named RedSpot Jr., it is similar but somewhat smaller than theGreat Red Spot and located at nearly the same latitude(Figure 8-5e). As with the Great Red Spot, the cause ofthe red color is still being studied.

In 1690, Cassini noticed that the speeds of Jupiter’sclouds vary with latitude, an effect called differentialrotation. Near the poles, the rotation period ofJupiter’s atmosphere, 9 hr 55 min 30 s, is 5 minuteslonger than at the equator. Furthermore, clouds at dif-ferent latitudes circulate in opposite directions—some

eastward, some westward. At their boundaries, theclouds rub against each other, creating beautifulswirling patterns (see Figure 8-2). The interactions ofclouds at different latitudes also help provide stabilityfor storms like the Great Red Spot.

Astronomers first determined Jupiter’s overallchemical composition from its average density—only1330 kg/m3. Recall from Section 5-5 that average den-sity is mass divided by volume. We determine Jupiter’smass using Kepler’s third law and the orbital periods ofits moons, while the trigonometry of Jupiter’s distancefrom Earth and angular size in our sky reveal its diam-eter and hence its volume. This low density implies thatJupiter is composed of primarily the lightweight ele-ments hydrogen and helium surrounding a relativelysmall core of metal and rock. It has no solid continents,islands, or water oceans on its surface.

Spectra from Earth-based telescopes and from theGalileo probe sent into Jupiter’s upper atmosphere in1995 give more detail about the chemistry of this giant

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FIGURE 8-4 The Great Red Spot Thistrue color image of the Great Red Spot,taken by Galileo in 1996, shows what

this giant storm would look like if you were traveling over it ina spacecraft. The counterclockwise circulation of gas in theGreat Red Spot takes about 6 days to make one rotation. Theclouds that encounter it are forced to pass around it, andwhen other oval features are near it, the entire systembecomes particularly turbulent, like the batter in a two-bladedblender. (Courtesy NASA/JPL-Caltech)

Merging white spots

FIGURE 8-5 Creating Red Spot Jr. (a–d) For 60 years prior to1998, the three white ovals labeled FA, DE, and BC traveledtogether at the same latitude on Jupiter. Between 1998 and2000, they combined into one white oval, labeled BA, which (e)became a red spot, named Red Spot Jr., in 2006. (a–d: NASA/JPL/WFPC2; e: NASA, ESA, A. Simon-Miller [NASA/GSFC], and I. dePater [University of California Berkeley])

WEB LINK 8.3VIDEO 8.3

Great RedSpot

Red Spot Jr.

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c

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e

world’s upper level. About 86% of its atoms arehydrogen and 13% are helium. The remainderconsists of molecular compounds, such asmethane (CH4), ammonia (NH3), and watervapor (H2O). Keeping in mind that different ele-ments have different masses, we can convert thesepercentages of atoms into the masses of varioussubstances in Jupiter’s atmosphere: 75% hydro-gen, 24% helium, and 1% other substances.Because the interior contains heavier elementsthan the surface, the overall mass distribution inJupiter has been calculated to be 71% hydrogen,24% helium, and 5% all heavier elements.

The descent of the probe from the Galileospacecraft into Jupiter’s atmosphere revealed windspeeds of up to 600 km/hr (375 mph), higher-than-expected air density and temperature, andlower-than-expected concentrations of water, heli-um, neon, carbon, oxygen, and sulfur. This prob-ably occurred because the probe descended into aparticularly arid region of the atmosphere called ahot spot, akin to the air over a desert on Earth.

Observations from spacecraft visiting Jupiter andits moons, combined with the scientific model ofJupiter’s atmosphere developed to explain the observa-tions of Jupiter’s clouds and chemistry, indicate it hasthree major cloud layers (Figure 8-6a). The uppermostJovian cloud layer is composed of crystals of frozenammonia. These crystals and the frozen water inJupiter’s clouds are white, so what chemicals create thesubtle tones of brown, red, and orange? The answer isas yet unknown. Some scientists think that sulfur com-pounds, which can assume many different colors,depending on their temperature, play an important role.Others think that phosphorus is involved, especially inthe Great Red Spot. The middle cloud layer is primari-ly ammonium hydrosulfide, and the bottom cloud layeris mostly composed of water vapor.

Below its cloud layer, Jupiter’s mantle is entirelyliquid. Here on Earth, the distinction between thegaseous air and the liquid oceans is very clear—jump offa diving board and you know when you hit the water.However, the conditions on Jupiter under which hydro-gen liquefies are different from anything we normallyexperience. As a result, on Jupiter there is no definiteboundary between the planet’s gaseous atmosphere andits liquid mantle. The hydrogen gradually gets denseruntil, 1000 km below the cloud tops, the pressure ishigh enough for the hydrogen to be what we would con-sider a liquid.

In introducing the solar system, we noted that theyoung planets heated up as they coalesced. After theyformed, radioactive elements continued to heat theirinteriors. On Earth, this heat leaks out of the surfacethrough volcanoes and other vents. Jupiter loses heateverywhere on its surface, because, unlike Earth, it hasno landmasses to block the heat loss.

Heated from deep within Jupiter, blobs of liquidhydrogen and helium move upward inside it. Whenthese blobs reach the cloud tops, they release their heatand descend back into the interior. (The same process,convection, drives the motion of Earth’s mantle and itstectonic plates, as well as liquid simmering on a stove;see Figure 6-9.) Jupiter’s rapid, differential rotationdraws the convective gases into bands of winds mov-ing eastward and westward at different speeds aroundthe planet.

Astronomers believe that Jupiter’s belts and zonesresult from the combined actions of the planet’s convec-tion and rapid differential rotation. Until recently, theybelieved that the light-colored zones are regions of hot-ter, rising gas, while the dark belts are regions of cooler,descending gas (Figure 8-7). However, high-resolutionobservations by the passing Cassini spacecraft on itsway to Saturn revealed numerous white clouds that aretoo small to be seen from Earth rising in the dark beltsof Jupiter, while the zones are sinking gas—just theopposite of the original model! Thecorrect explanation for the behaviorof gases in the belts and zones maybe provided by Cassini, when itstudies the analogous belts and

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NH3 clouds

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Temperature (°C) Temperature (°C)

Jupiter’s atmosphere Saturn’s atmosphere

(a) (b)

H2O clouds NH3 clouds

NH4SH clouds

H2O clouds

NH3: ammoniaNH4SH: ammonium hydrosulfideH2O: water

Saturn has weaker surfacegravity than Jupiter, soits cloud layers are morespread out.

Saturn has loweratmospherictemperaturesthan Jupiter.

FIGURE 8-6 Jupiter’s and Saturn’s Upper LayersThese graphs display temperature profiles of (a)Jupiter’s and (b) Saturn’s upper regions, as

deduced from measurements at radio and infrared wave-lengths. Three major cloud layers are shown in each, alongwith the colors that predominate at various depths. Data fromthe Galileo spacecraft indicate that Jupiter’s cloud layers arenot found at all locations around the planet; there are somerelatively clear, cloud-free areas.

WEB LINK 8.4

What are the two mostcommon elements in Jupiter’supper layers?

220 CHAPTER 8

Rockycore

Liquid“ices”

Helium and liquid metallic

hydrogen

Helium andmolecularhydrogen

Gaseousatmosphere

15,000 kmSaturn

Earth, to scale

Jupiter

FIGURE 8-8 Cutaways of Jupiter and Saturn The interiors ofboth Jupiter and Saturn are believed to have four regions: aterrestrial rocky core, a liquid “ice” shell, a metallic hydrogenshell, and a normal liquid hydrogen mantle. Their atmos-pheres are thin layers above the normal hydrogen, whichboils upward, creating the belts and zones.

Figure 8-9) and its tail streams outward to Saturn, over700 million km away.

For Earth-based astronomers, the onlyevidence of Jupiter’s magnetosphere is a hissof radio static, which varies cyclically over a

period of 9 hr 55 min 30 s, the planet’s internal rotationrate. Beginning with the two Pioneer and two Voyagerspacecraft that journeyed past Jupiter in the 1970s, andcontinuing with the recent Galileo and Cassini space-craft, the awesome dimensions of Jupiter’s magnetos-phere have been revealed: It is nearly 30 million kmacross. It envelops the orbits of many of its moons. IfJupiter’s magnetosphere were visible from Earth, itwould cover an area of the sky 16 times larger than thefull Moon.

Although the sequence of events in Jupiter’s for-mation is still being worked out, it appears that a ter-restrial (rock and metal) protoplanet formed at

North South

East/west flow of belts and zones

ZoneBelt

Cool materialsinking?

Warm materialrising?

Upperatmosphere

To Jupiter’s center

? ? ?

FIGURE 8-7 Original Model of Jupiter’s Belts and ZonesThe light-colored zones and dark-colored belts in Jupiter’satmosphere were believed, until recently, to be regions ofrising and descending gases, respectively. In the zones,gases warmed by heat from Jupiter’s interior were thought torise upward and cool, forming high-altitude clouds. In thebelts, cooled gases were thought to descend and undergoan increase in temperature; the cloud layers seen there areat lower altitudes than in the zones. Observations by theCassini spacecraft on its way to Saturn suggest that just theopposite may be correct (stay tuned)! In either case, Jupiter’srapid differential rotation shapes the rising and descendinggas into bands of winds parallel to the planet’s equator.Differential rotation also causes the wind velocities at theboundaries between belts and zones to move predominant-ly to the east or west.

zones found on Saturn (see Section 8-9) in greater detailin the next few years.

8-2 Jupiter’s interior has four distinct regions

Descending below Jupiter’s clouds, we firstencounter liquid molecular hydrogen andhelium. As in Earth’s oceans, the pressure in

this fluid increases with depth. The gravitational forcecreated by Jupiter’s enormous mass compresses andheats its interior so much that 20,000 km (12,500 mi)below the cloud tops, the pressure is 3 million atm.Below this depth, the pressure is high enough to trans-form hydrogen into liquid metallic hydrogen (Figure 8-8). In this state, hydrogen acts like a metal in its abilityto conduct electricity and heat, like the copper wiring ina house. Electric currents that run through this rotating,metallic region of Jupiter generate a powerful planetarymagnetic field (Figure 8-9). At the cloud level of Jupiter,this field is 14 times stronger per square meter thanEarth’s field is at our planet’s surface. Jupiter’s magnet-ic field has a region, like Earth’s Van Allen belts, wherecharged particles are stored (depicted in orange in

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a

b

WEB LINK 8.6

Jupiter’s orbit and attracted hydro-gen and helium to form its outer lay-ers. This terrestrial matter is nowJupiter’s core. This core is only 4%of Jupiter’s mass, which still amountsto nearly 13 times the mass of theentire Earth. Water, carbon dioxide,methane, and ammonia are likelyto have existed as ice in Jupiter’s ter-restrial protoplanet, and they werealso attracted onto the young andgrowing planet. When astronomerstalk in general terms about “ice,”they refer to any or all of these fourcompounds.

The tremendous crushing weightof the bulk of Jupiter above the core—equal to the mass of 305 Earths—compresses the terrestrial core downto a sphere only 10,000 km in radius(see Figure 8-8). By comparison,Earth’s diameter is 12,756 km. At thesame time, the pressure forced the lighter ices out of therock and metal, thereby forming a shell of these com-pounds between the solid core and the liquid metallichydrogen layer. Calculations reveal that the temperatureand pressure inside Jupiter should make these “ices” liq-uid there! The pressure at Jupiter’s very center is calcu-lated to be about 70 million atm, and the temperaturethere is about 25,000 K, nearly four times hotter thanthe surface of the Sun. The interior of Jupiter is summa-rized in Figure 8-8a.

8-3 Impacts provide probes into Jupiter’satmosphere

On July 7, 1992, a comet nucleus (a clump of rock andice a few kilometers across) passed so close to Jupiterthat the planet’s gravitational tidal force ripped it intoat least 21 pieces. The debris from this comet was firstobserved in March 1993 by comet hunters Gene andCarolyn Shoemaker and David Levy. (Because it was theninth comet they had found together, it was namedShoemaker-Levy 9 in their honor.) Shoemaker-Levy 9was an unusual comet that actually orbited Jupiter,rather than just orbiting the Sun. Calculations of thecomet’s orbit showed that the pieces (Figure 8-10a)would return to strike Jupiter between July 16 andJuly 22, 1994.

Recall from Section 5-3 that impacts were extreme-ly common in the first 800 million years of the solar sys-tem’s existence. From more recent times, two chains ofimpact craters have been discovered on Earth, consis-tent with pieces of comets having hit our planet withinthe past 300 million years. However, it is very uncom-mon for pieces of space debris as large as several kilo-

The Outer Planets 221

Solarwind

ToSun

Magnetic axisMagnetic axis

Rotation axis

Bowshock

Europatorus

Europa

Io

1,000,000 kmNeutralsheet

Io torus

Magnetic axis

FIGURE 8-9 Jupiter’s Magnetosphere Created by the planet’srotation, the ion-trapping regions of Jupiter’s magnetosphere(in orange, analogous to the Van Allen belts) extend into therealm of the Galilean moons. Gases from Io and Europa formtori in the magnetosphere. Some of Io’s particles are pulled bythe field onto the planet. Pushed outward by the Sun, the mag-netosphere reaches all the way to Saturn.

meters in diameter to collide withplanets today. Therefore, the discov-ery that Shoemaker-Levy 9 would hitJupiter created great excitement inthe astronomical community. Seeinghow a planet and a comet respond tosuch an impact would allowastronomers to deduce information about the planet’satmosphere and interior and also about the strikingbody’s properties.

The impacts occurred as predicted, with mostof Earth’s major telescopes—as well as those on severalspacecraft—watching closely (Figure 8-10b). At least 20fragments from Shoemaker-Levy 9 struck Jupiter, and15 of them had detectable impact sites. The impactsresulted in fireballs some 10 km in diameter with tem-peratures of 7500 K, which is hotter than the surface ofthe Sun. Indeed, the largest fragment gave off as muchenergy as 600 million megatons of TNT, far more thanthe combined energy that could be released by allnuclear weapons remaining on Earth. Impacts were fol-lowed by crescent-shaped ejecta that contained a varietyof chemical compounds. Ripples or waves spread outfrom the impact sites through Jupiter’s clouds insplotches that lasted for months.

Why is the terrestrial bodyinside Jupiter so small com-pared to what it would be inorbit without the outer layersaround it?

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Io

GreatRed Spot

Impactsites

Aurora

Aurora

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The observations suggested that the pieces ofcomet did not penetrate very far into Jupiter’s uppercloud layer. This fact, in turn, supports the beliefthat the pieces were not much larger than a kilometerin diameter. The ejecta from each impact included adark plume that rose high into Jupiter’s atmosphere.The darkness was apparently due to carbon com-

pounds that vaporized from thecometary bodies. Also detectedfrom the comet were water, sulfurcompounds, silicon, magnesium,and iron.

JUPITER’S MOONS AND RINGSJupiter hosts at least 63 moons. The four largest—Io,Europa, Ganymede, and Callisto—may have formed atthe same time as Jupiter from debris orbiting the plan-et, just as the planets formed from debris orbiting theSun (see Section 5-3). Jupiter’s other moons, as we willsee, are probably captured planetesimals and smallerpieces of space debris.

Galileo was the first person to observe the fourlargest moons, in 1610, seen through his meager tele-scope as pinpoints of light. He called them the Medicean

FIGURE 8-10 Comet Shoemaker-Levy 9 and itsEncounter with Jupiter (a) The comet was tornapart by Jupiter’s gravitational force on July 7,

1992, fracturing into at least 21 pieces. This comet originallyorbited Jupiter, and its returning debris, shown here in May1994, struck the planet between July 16 and July 22, 1994. (b)Shown here are visible (left) and ultraviolet (right) images ofJupiter taken by the Hubble Space Telescope after three pieces

of Comet Shoemaker-Levy 9 struck the planet. Astronomers hadexpected white remnants (the color of condensing ammonia orwater vapor); the darkness of the impact sites may have comefrom carbon compounds in the comet debris. Note the aurorasin the ultraviolet image. Auroras and lightning are common onJupiter, due, in part, to the planet’s strong magnetic fields anddynamic cloud motions. (a: H. A. Weaver, T. E. Smith, STScI andNASA; b: NASA)

WEB LINK 8.7

Why did the comet impactsof 1994 not create craters onJupiter?

The Outer Planets 223

stars to attract the attention of the Medicis, rulers ofFlorence and wealthy patrons of the arts and sciences. ToGalileo, the moons provided evidence supporting thethen-controversial Copernican cosmology; at that time,Western theologians asserted that all cosmic bodiesorbited Earth. The fact that the Medicean stars orbited

Jupiter raised grave concerns in some circles.To the modern astronomer, these moons

are four extraordinary worlds, differentboth from the rocky terrestrial planets and

from hydrogen-rich Jupiter. Now called collectively theGalilean moons or Galilean satellites, they are namedafter the mythical lovers and companions of the Greekgod Zeus (called Jupiter by the early Romans). From theinnermost moon outward, they are Io, Europa,Ganymede, and Callisto.

These four worlds were photographed extensivelyby the Voyager 1 and Voyager 2 flybys and by theGalileo spacecraft (Figure 8-11). The two inner Galileansatellites, Io and Europa, are about the same size as ourMoon. The two outer satellites, Ganymede and Callisto,are roughly the size of Mercury. Figure 8-11 presentscomparative information about these six bodies.

8-4 Io’s surface is sculpted by volcanic activity

Sulfurous Io is among the most exotic moons in oursolar system (Figure 8-12a). At first glance, its density of3570 kg/m3 seems to place its chemical compositionbetween that of the terrestrial and Jovian planets.However, its density has that value in large measurebecause its mass is so small that it cannot compress itsinterior nearly as much as can Earth or more massiveplanets. Allowing for its lower compression, astronomerscalculate that Io is mostly rock and iron, like Earth, ratherthan being composed of lighter elements, as is Jupiter.

Io zooms through its orbit of Jupiter once every1.8 days. Like our Moon, it is in synchronous rotationwith its planet. Images of Io reveal giant plumes emit-ted by volcanoes (Figure 8-12b) and geysers, similar toOld Faithful on Earth. Most of this ejected materialfalls back onto Io’s surface; the rest is moving fastenough to escape into space. These geysers are usuallyassociated with Io’s 300 or so active volcanoes.Observations indicate that these volcanoes emit 10trillion tons of matter each year in plumes up to 500km high. That is enough material to resurface Io to adepth of 1 m each century (Figure 8-12c). One erup-tion in 2002 occurred over an area the size of London,some 1600 sq km (620 sq mi). The volcanoes also emitbasaltic lava flows rich in magnesium and iron. Io’svolcanoes are named after gods and goddesses associ-ated with fire in Greek, Norse, Hawaiian, and othermythologies. Io also has numerous black “dots” on itssurface, which, apparently, are dormant volcanic

INSIGHT INTO SCIENCEWhat’s in a Name? Following up on the preconcep-tions that common words create (see Insight intoScience: Imagine the Moon in Chapter 7), objects withfamiliar names often have different characteristics thanwe expect. We tend to envision “moons” as inert, air-less, lifeless, dry places similar to our Moon. However,as we will see throughout this chapter, starting with Io,some moons have active volcanoes, atmospheres, andvast, underground, liquid water oceans.

Just before their discovery, the existence of activevolcanoes on Io was predicted from analysis of the grav-itational forces to which that moon is subjected. As itrapidly orbits Jupiter, Io repeatedly passes betweenJupiter and one or another of the remaining Galileansatellites. These moons pull on Io, causing it to slightlychange its distance from Jupiter. This change in distancecreates a change in the tidal forces (see Section 6-8) act-ing on it from the planet. As the distance between Ioand Jupiter varies, the resulting tidal stresses alternatelysqueeze and flex the moon. This ongoing motion of Io’sinterior generates heat through friction, creating asmuch energy inside Io as the detonation of 2400 tons ofTNT every second. Gas and molten rock from inside Ioeventually make their way to the moon’s surface, wherethey are ejected through the volcanoes.

Satellite instruments have identified sulfur and sul-fur dioxide in the material erupting from Io’s volcanoes.Sulfur is normally bright yellow. If heated and suddenlycooled, however, it forms molecules that assume a rangeof colors, from orange and red to black, which accountsfor Io’s tremendous range of colors (see Figure 8-12).Sulfur dioxide (SO2) is an acrid gas commonly dis-charged from volcanic vents here on Earth and, appar-ently, on Venus. When eruptions on Io release this gasinto the cold vacuum of space, it crystallizes into whiteflakes, which fall onto the surface and account for themoon’s whitish deposits.

The Galileo spacecraft detected an atmospherearound Io. Composed of oxygen, sulfur, and sulfurdioxide, it is only one-billionth as dense as the air webreathe. Io’s atmosphere can sometimes be seen to glowblue, red, or green, depending on the gases involved.Gases ejected from Io’s volcanoes have also beenobserved extending out into spaceand forming a doughnut-shapedregion around Jupiter called the Iotorus, about which we will say moreshortly.

ANIMATION 8.1

What creates most of the heatinside Io that causes it to havevolcanoes and geysers?

vents. Old lava flows radiate from many of these loca-tions, which are typically 10 to 50 km in diameter andcover 5% of Io’s surface.

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Iron core

Mixed ice-rock interior

CALLISTO

EUROPA

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Ocean?

Icy crust

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Rocky mantleOceanIcy crust

GANYMEDE

lo Europa Ganymede Callisto

IO

Moltenmantle

Rocky crust Iron core

Iron core

FIGURE 8-11 The Galilean Satellites The fourGalilean satellites are shown here to the samescale. Io and Europa have diameters and densi-

ties comparable to our Moon and are composed primarily ofrocky material. Ganymede and Callisto are roughly as big asMercury, but their low average densities indicate that each

contains a thick layer of water and ice. The cross-sectional dia-grams of the interiors of the four Galilean moons show theprobable internal structures of the moons, based on their aver-age densities and on information from the Galileo mission.(NASA and NASA/JPL)

The Galilean Moons, Mercury, and Earth’s Moon: Vital Statistics

MassMean distance Sidereal Diameter Mean density

from Jupiter (km) period (day) (km) (kg) (Moon ! 1) (kg/m3)

Io 421,600 1.77 3630 8.94 " 1022 1.22 3570Europa 670,900 3.55 3138 4.80 " 1022 0.65 2970Ganymede 1,070,000 7.16 262 1.48 " 1023 2.01 1940Callisto 1,883,000 16.69 4800 1.08 " 1023 1.47 1860Mercury — — 4878 3.30 " 1023 4.49 5430Moon — — 3476 7.35 " 1022 1.00 3340

WEB LINK 8.8

FIGURE 8-12 Io (a) This true-colorview was taken by the Galileo space-craft in 1999. The range of colors results

from surface deposits of sulfur ejected from Io’s numerous volca-noes. Plumes from the volcano Prometheus rise up 100 km.Prometheus has been active in every image taken of Io since theVoyager flybys of 1979. (b) Galileo image of an eruption ofPilan Patera on Io. (c) Photographed in 1999 and then 2000(shown here), the ongoing lava flow from this volcanic eruptionat Tvashtar Catena has considerably altered this region of Io’ssurface. (a: Moses Milazzo, PIRL, LPL, NASA; b: Galileo Project, JPL,NASA; c: University of Arizona/JPL/NASA)

The Outer Planets 225

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8-5 Europa apparently harbors liquid waterbelow its surface

Images of Europa’s ice and rock surface from Voyager 2and the Galileo spacecraft suggest that Jupiter’s second-closest Galilean moon contains liquid water (Figure 8-13). Europa orbits Jupiter every 31⁄2 days and, like Io, isin synchronous rotation. The changing gravitational tugof Io creates stress inside Europa similar to the magma-generating distortion that Io undergoes. These stressesapparently cause numerous cracks or fractures seenon Europa’s surface (Figure 8-14a). Furthermore,astronomers think this stress creates enough heat insideEuropa to keep the water in a liquid state just a few kilo-meters below the moon’s ice and rock surface.

Besides the cracks, numerous other features onEuropa support the idea that it has liquid water inside.Just as in the Arctic region on Earth, the movement ofsurface features on Europa creates ice floes, along withswirls, strips, and ridges, driven by circulating waterunderneath the moon’s surface (Figure 8-14b). Alsoindicative of liquid water is the moon’s reddish color. Thecoloring may be due to salt deposits left after liquid waterrose to the surface and evaporated (Figures 8-14a, c).Galileo spacecraft images suggest that some of Europa’sfeatures have moved within the past few million years,

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and perhaps are still in motion. Indeed, the chaotic sur-face revealed by Galileo is interpreted as having formedas a result of water volcanism on Europa, strengtheningthe belief that this moon still has a liquid water layer.Replenishment of the surface by tectonic plate motionwould explain why only a few small impact craters havesurvived.

Galileo also photographed red and white domescalled lenticulae on Europa (Figure 8-14c) thatastronomers think are rising, warmed ice mixed withother material, possibly including organic matter.These lenticulae are typically a 100 m high and 10 km(6 mi) wide. The rising material behaves like the blobsin lava lamps (Figure 8-14d), although the lenticulaeare calculated to take 100,000 years to reach the sur-face from the liquid ocean that is believed to existinside the moon.

Europa’s average density of 2970 kg/m3 is slightlyless than Io’s. A quarter of its mass may be water. It alsohas a metallic core of much higher density and a weakmagnetic field. In 1995, astronomers discovered anextremely thin atmosphere containing molecular oxygensurrounding Europa. This gas is about 10#11 times lessdense than the air we breathe. The oxygen may comefrom water molecules broken up on the moon’s surface

by ultraviolet radiation from the Sun.The underground ocean of

Europa is especially interesting toscientists because virtually all liquidwater locations on Earth support

life, and life may well have evolved in Europa’s oceans.If it exists, and if it evolved differently from life onEarth, then life-forms from Earth would likely contam-inate it, even wiping it out. This would be analogous tothe deadly impact of diseases on Earth, such as intro-ducing smallpox from Europe to the Americas. To pre-vent the expired spacecraft Galileo from contaminatingany life that may exist on Europa, NASA sent thatspacecraft into Jupiter’s thick atmosphere, where themachine was vaporized.

8-6 Ganymede is larger than MercuryGanymede is the largest satellite in the solar system(Figure 8-15). Its diameter is greater than Mercury’s,but its density of 1940 kg/m3 is much less than that ofMercury. It also has a permanent magnetic field that istwice as strong as Mercury’s field. Ganymede orbitsJupiter in synchronous rotation once every 7.2 days.Like its neighbor Europa, Ganymede has an iron-richcore, a rocky mantle, an underground liquid waterocean, a thin atmosphere, and a covering of dirty ice.

The existence of an underground ocean is impliedby the discovery of a second magnetic field aroundGanymede, which continually varies. This field has adifferent origin than the permanent one just mentioned;it is generated by Jupiter’s magnetic field. As Ganymedeorbits Jupiter, the planet’s powerful magnetic field cre-ates an electric current inside the moon, which, in turn,creates Ganymede’s varying magnetic field. (The sameprocess is used to create electric currents in power sta-tions here on Earth, where falling water or steam is usedto rotate powerful magnets at high speeds. Their mag-netic fields cut across wires, thereby pushing electronsin the wire. These moving electrons are the electric cur-rent we use.) The best explanation of why current flowsinside Ganymede is that liquid salt water exists there,and salt water is a good conductor of electricity. Thisimplies, of course, the presence of a liquid ocean.Furthermore, salts have been observed on Ganymede’ssurface. They were apparently carried upward anddeposited there as water leaked out and froze. As onEuropa, liquid water may imply the presence of life.

Like our Moon, Ganymede has two very differentkinds of terrain (Figures 8-15, 8-16). Dark, polygon-shaped regions are its oldest surface features, as judgedby their numerous craters. Light-colored, heavily groovedterrain is found between the dark, angular islands. Theselighter regions are much less cratered and thereforeyounger. Ganymede’s grooved terrain consists of parallelmountain ridges up to 1 km high and spaced 10 to 15 kmapart. These features suggest that the process of plate tec-tonics may have dominated Ganymede’s early history.But, unlike Europa, where tectonic activity still occurstoday, tectonics on Ganymede bogged down 3 billionyears ago as the satellite’s crust froze solid.

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FIGURE 8-13 Europa Imaged by the Galileospacecraft, Europa’s ice surface is covered bynumerous streaks and cracks that give the satel-

lite a fractured appearance. The streaks are typically 20 to40 km wide. (NASA/JPL)

What region on Earth doesthe surface of Europa mostresemble?

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Another mechanism that may have createdGanymede’s large-scale features is a bizarre propertyof water. Unlike most liquids, which shrink uponsolidifying, water expands when it freezes. Seeping upthrough cracks in Ganymede’s original crust, watermay have thus forced apart fragments of that crust.

This process could have produced jagged, dark islandsof old crust separated by bands of younger, light-col-ored, heavily grooved ice. Toppingoff the list of Ganymede’s variedfeatures is the discovery that auro-ras occur there.

Linear ridges(dark colorsare caused byminerals inthe ice)

500 km

Smooth ice plains witha network of fractures

a b

dc

Smooth areaformed by fluiderupting ontothe surface

Rugged patchcreated by asubsurfacedisturbance

Ridges produced byfolding and faultingof the surface5 km

Lenticulae

FIGURE 8-14 Surface Features on Europa (a) This falsecolor Galileo image of Europa combining visible and infraredobservations shows smooth plains of ice, mineral ridgesdeposited by upwelling water, and numerous fracturesbelieved to be caused by tidal stresses. (b) This region ofEuropa’s surface shows the jumbled, stressed features com-mon to the surface, as well as direct indications of liquid wateractivity underground. (c) Lenticulae attributed to risingwarmed ice and debris travel up from the moon’s interior by

convection, arriving at and then leaking out at the surface. Thewhite domes are likely to be rising material that has not yetreached the surface. (d) A lava lamp in which warmed mate-rial rises through cooler liquid. The rising material in Europais analogous to the rising motion of the blobs in a lava lamp,except that on Europa, the motion is through ice. (a, b:NASA/JPL; c: NASA/JPL/University of Arizona/University ofColorado; d: Bianca Moscatelli)

Which planet is smaller thanGanymede?

8-7 Callisto bears the scars of a hugeasteroid impact

Callisto is Jupiter’s outermost Galilean moon. It orbitsJupiter in 16.7 days, and, like the other Galileanmoons, its rotation is synchronous. Callisto is 91% asbig and 96% as dense as Ganymede, with a thinatmosphere of hydrogen and carbon dioxide. LikeGanymede, Callisto, apparently, harbors a substantialliquid water ocean. As with Ganymede, this ocean’spresence is also inferred by Callisto’s changing mag-netic field. The heat that keeps the ocean liquid,apparently, comes from energy released by radioactivedecay inside the moon.

Although numerous large impact craters are scat-tered over Callisto’s dark, ancient, icy crust, it has veryfew craters smaller than 100 m across (Figure 8-17).Astronomers speculate that the smaller craters have dis-integrated. Unlike Ganymede and Europa, Callisto hasno younger, grooved terrain. The absence of groovedterrain suggests that tectonic activity never began there:The satellite simply froze too rapidly. It is bitterly coldon Callisto’s surface. Voyager instruments measured anoontime temperature of 155 K (#180°F), and thenighttime temperature plunges to 80 K (#315°F).

Like Mercury, our Moon, and other bodies we willencounter shortly, Callisto carries the cold, hard evidenceof what happens when one astronomical body strikesanother. Voyager 1 photographed the huge impact basin,named Valhalla, on Callisto (see Figure 8-17a). An aster-oid-sized object produced Valhalla Basin, which is locat-ed on Callisto’s Jupiter-facing hemisphere. Like throwinga rock into a calm lake, ripples ran out from the impact

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Dark terrain (old,heavily cratered)

Bright terrain (young, fewer craters)

FIGURE 8-15 Ganymede This sideof Ganymede is dominated by thehuge, dark, circular region called

Galileo Regio, which is the largest remnant of Ganymede’sancient crust. Darker areas of the moon are older; lighterareas are younger, tectonically deformed regions. The lightwhite areas in and around some craters indicate the presenceof water ice. Large impacts create white craters, filled in byice from below the surface. (NASA/JPL)

50 kmLong, deep furrows

Dark terrain:more craters

Bright terrain:fewer craters

Long grooves

FIGURE 8-16 Two Surfaces of Ganymede The older,rougher, more heavily cratered parts of Ganymede, the darkterrain, are surrounded by younger, smoother, less cratered

bright terrain. The parallel ridges suggest that the bright ter-rain has been crafted by tectonic processes. (NASA)

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site along Callisto’s surface, cracking the surface andfreezing into place for eternity. The largest remnant ringssurrounding the impact crater have diameters of 3000km. In 2001, the Galileo spacecraft revealed spires80–100 m high on Callisto (Figure 8-17b). These are alsobelieved to have been created by an impact, perhaps thesame one. One thing that is missing from Callisto is thejumbled terrain on the side opposite of Valhalla Basin, asis seen opposite of large impact sites on Mercury and ourMoon. The smoothness of Callisto can be explained by amodel that shows that a liquid water interior woulddampen the impact shock and thereby prevent the oppo-site side from becoming disturbed.This supports the idea that Callistohas liquid water inside. The probableinteriors of the Galilean moons areshown in Figure 8-11.

8-8 Other debris orbits Jupiter as smallermoons and ringlets

Besides the four Galilean moons, Jupiter has at least 59other moons, a set of tenuous ringlets, and two dough-nut-shaped tori of electrically charged gas particles. Thenon-Galilean moons are all irregular in shape and lessthan 275 km in diameter. Four of these moons (Figure8-18) are inside Io’s orbit; all of the other known moonsare outside Callisto’s orbit. The inner moon Amalthea(Figure 8-18) is red-colored, has about the same densityas water, and is, apparently, made of pieces of rock andice barely held together by the moon’s own gravity. TheGalilean moons, along with the smaller moons closer toJupiter and six of the outer moons, orbit in the samedirection that Jupiter rotates (prograde orbits). Theremaining outer moons revolve in the opposite direction(retrograde orbits). The outer ones appear to be individ-ual, captured planetesimals, while the inner ones areprobably smaller pieces broken off a single larger body.

Jupiter is only the first of four planets with rings.As astronomers predicted, the cameras aboard Voyager1 discovered three ringlets around Jupiter. These arebands of particles, the brightest of which is seen inFigure 8-19a, a Voyager 2 image. They are the darkest,simplest set of rings in the solar system. They consist ofvery fine dust particles that are continuously pushedout of orbit by the impact of radiation from Jupiter andthe Sun. Therefore, these rings are being replenished bymaterial ejected volcanically from Io and knocked freeby impact from the other moons.

At least two doughnut-shaped regions of electricallycharged gas particles, called plasmas, orbit Jupiter. Oneis in the same orbit as Io (Figure 8-19b). The Io torusconsists of sulfur and oxygen ions (charged atoms)along with free electrons. These particles were ejectedby Io’s geysers, and they are held in orbit by Jupiter’sstrong magnetic field. Guided by the field, some of this

FIGURE 8-17 Callisto The outermost Galileansatellite is almost exactly the same size asMercury. Numerous craters pockmark Callisto’s

icy surface. (a) The series of faint, concentric rings that covermuch of this image is the result of a huge impact that createdthe impact basin Valhalla. Valhalla dominates the Jupiter-facinghemisphere of this frozen, geologically inactive world. (b) Thetwo insets in this Galileo mission image show spires that containboth ice and some dark material. The spires were probablythrown upward as the result of an impact. The spires erode asdark material in them absorbs heat from the Sun. (a: Courtesy ofNASA/JPL; b: NASA/JPL/Arizona State University)

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What features on our Moonand on Mercury are similarto Callisto’s feature Valhalla?

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matter spirals in toward Jupiter, thereby creating theaurora seen there (see Figure 8-10).

The other torus is in Europa’s orbit (Figure 8-19b). It consists of hydrogen andoxygen ions and electrons createdfrom water molecules kicked offEuropa’s surface by radiation fromJupiter. The mass of this ring ofgas is calculated to be about 6 "104 tons.

SATURNSaturn, with its ethereal rings, presents the most spectac-ular image of the planets (Figure 8-20). Giant Saturn has95 times as much mass as Earth, making it second inmass and size to Jupiter. Like Jupiter, Saturn has a thick,active atmosphere composed predominantly of hydro-gen, and it radiates more energy than it receives from theSun—in this case, three times as much. It also has astrong magnetic field. Ultraviolet images from theHubble Space Telescope reveal auroras around Saturndiffering from those of Jupiter in that Saturn’s are spirals(Figure 8-20), while Jupiter’s are rings (see Figure 8-10).

8-9 Saturn’s atmosphere, surface, and inte-rior are similar to those of Jupiter

Partly obscured by the thick, hazy atmosphere abovethem, Saturn’s clouds lack the colorful contrast visibleon Jupiter. Nevertheless, photographs do show faintstripes in Saturn’s atmosphere, similar to Jupiter’s beltsand zones (Figure 8-21a). Their existence indicates thatSaturn also has internal heat that transports the cloudgases by convection. Changing features in the atmos-phere show that Saturn, too, has differential rotation—ranging from about 10 hours and 14 minutes at theequator to 10 hours and 40 minutes at high latitudes.As on Jupiter, some of the belts and zones move east-ward, while others move westward. In 2006, the Cassini

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Adrastea To Jupiter

Metis 5000 km

Europa

Ganymede

Io

FIGURE 8-19 Jupiter’s Ring and Torus (a) Aportion of Jupiter’s faint ring system, pho-tographed by Voyager 2. The ring is probably

composed of tiny rock fragments. The brightest portion of thering is about 6000 km wide. The outer edge of the ring issharply defined, but the inner edge is somewhat fuzzy. A ten-uous sheet of material extends from the ring’s inner edge allthe way down to the planet’s cloudtops. (b) Quarter images ofIo’s and Europa’s tori (also called plasma tori because the gasparticles in them are charged—plasmas). Io is visible in itstorus (green), while Europa is visible in its torus (blue). Someof Jupiter’s magnetic field lines are also drawn in. Plasmafrom tori flow inward along these field lines toward Jupiter. (a: NASA/JPL, Cornell University; b: NASA/JPL/Johns HopkinsUniversity Applied Physics Laboratory)

Metis

Amalthea Thebe

Adrastea

Long Island, New York

100 km

FIGURE 8-18 Irregularly Shaped Inner MoonsThe four known inner moons of Jupiter are signif-icantly different from the Galilean satellites. They

are roughly oval-shaped bodies. Although craters have not yetbeen resolved on Adrastea and Metis, their irregular shapesstrongly suggest that they are cratered. All four moons arenamed for characters in mythology relating to Jupiter (Zeus, inGreek mythology). (NASA/JPL, Cornell University)

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a

b

What other objects that wehave already studied in thesolar system are most similarto the smaller moons ofJupiter?