Section 23.3 23.3 The Outer Planets (and Pluto)

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Section 23.323.3 The Outer Planets (and Pluto)1 FOCUSReading StrategySection Objective23.423.5Describe the distinguishingcharacteristics of each Jovianplanet.Explain why Pluto is notconsidered a planet.Key ConceptsWhat characteristicsdistinguish each outerplanet?Vocabulary dwarf planetWhy is Pluto notconsidered a planet?Summarizing Make a table like theone shown that includes a row for eachouter planet. Write a brief summary ofthe characteristics of each planet.Outer PlanetsCharacteristicsJupiterlargest; most mass,Great Red Spota.?b.?c.?d.?Reading FocusBuild VocabularyL2Vocabulary Rating Chart Havestudents construct a chart with fourcolumns labeled Word, Can Define orUse It, Heard or Seen It, and Don’tKnow. Have students copy words as theyread the section into the first columnand rate their word knowledge byputting a check in one of the othercolumns. Ask how many studentsactually know each word. Have themshare their knowledge. Ask focusedquestions to help students predict textcontent based on the word, thusenabling them to have a purpose forreading. After students have read thesection, have them rate their knowledgeagain.Reading StrategyIFigure 12 This artist’s renditionshows Cassini approaching Saturn.Jupiter: Giant Among PlanetsA2 INSTRUCTBuild Reading Literacy654L1Refer to p. 392D in Chapter 14, whichprovides guidelines for this readingstrategy.Preview Have students preview thissection by skimming the headings andvisuals. This will help students toactivate their previous knowledge aboutthe outer planets and will likely makethem interested to read more abouteach planet.Visual, Verbal654Chapter 23dsg winStronFigure 13 A When photographedby Voyager 2, the Great Red Spotwas the size of two Earth-sizecircles placed side by side. B Thedark clouds are regions wheregases are sinking and cooling. Theconvection currents and the rapidrotation of the planet generatehigh-speed winds.Jupiter: GiantAmong Planetsds (beltsChapter 23)Jupiter is only 1/800 as massive as the sun. Still, it is thelargest planet by far.Jupiter has a mass that is 2 1/2nes)ds (zot cloutimes greater than the mass of all the other planetsBrighand moons combined. In fact, had Jupiter been aboutindsng wStroelts)10 times larger, it would have evolved into a small star.s (bducloDarkJupiter rotates more rapidly than any other planet,completing one rotation in slightly less than 10 EarthBhours.When viewed through a telescope or binoculars,Jupiter appears to be covered with alternating bands ofmulticolored clouds that run parallel to its equator. Themost striking feature is the Great Red Spot in the southern hemisphere,shown in Figure 13A. The Great Red Spot was first discovered more thanthree centuries ago by two astronomers, Giovanni Cassini (for whom thespace probe was named) and Robert Hooke. When Pioneer 11 movedwithin 42,000 kilometers of Jupiter’s cloud tops, images from the orbiterindicated that the Great Red Spot is a cyclonic storm.Dark clouL2Sample answers:a. Saturnb. largest ring systemc. Uranusd. axis tilted more than 90 e. Neptunef. winds exceed 1000 kmper hourn 2004, the space probe Cassini, launched seven yearsearlier, finally reached the planet Saturn. The mission ofCassini, shown in Figure 12, was to explore Saturn’s stunning ring system and its moons, including the uniquemoon Titan. In 2005, the Huygens probe, carried intospace by the Cassini orbiter, descended to Titan’s surface forfurther studies. In this section, we’ll take a clue fromCassini and explore the outer planets—Jupiter, Saturn, Uranus, andNeptune.

Use VisualsStructure of Jupiter Although Jupiter is called a gas giant, it isnot simply a ball of gas. At 1000 kilometers below the clouds, the pressure is great enough to compress hydrogen gas into a liquid.Consequently, Jupiter is thought to be a gigantic ocean of liquid hydrogen. Less than halfway into Jupiter’s interior, extreme pressures causethe liquid hydrogen to turn into liquid metallic hydrogen. Jupiter isalso believed to have a rocky and metallic central core.Jupiter’s hydrogen-helium atmosphere is very active. It containssmall amounts of methane, ammonia, water, and sulfur compounds.The wind systems, shown in Figure 13B, generate the light- and darkcolored bands that encircle this giant. Unlike the winds on Earth, whichare driven by solar energy, Jupiter itself gives off nearly twice as muchheat as it receives from the sun. Thus, the interior heat from Jupiterproduces huge convection currents in the atmosphere.For: Links on the outer planetsVisit: www.SciLinks.orgWeb Code: cjn-7233ABFigure 14 This diagram shows Jupiter’sfour largest moons. Ask: What do thesemoons have in common with eachother? (They are all round, and they orbitaround Jupiter.) Which of these moonshave craters? (Europa, Callisto, and Io)Could Europa have craters? (Yes, but itssurface is covered in ice.) Which of thesemoons has volcanoes? (Io)VisualIntegrate Language Arts L2Jupiter’s Moons Jupiter’s satellite system, consisting of 63 moonsdiscovered so far, resembles a miniature solar system. The four largestmoons, Io, Europa, Ganymede, and Callisto, were discovered by Galileoin 1610. Each of the four Galilean satellites is a unique geologicalworld. The moons are shown in Figure 14. The innermost of theGalilean moons, Io, is one of four known volcanically active bodies inour solar system. The other volcanically active bodies are Earth,Saturn’s moon Enceladus, and Neptune’s moon Triton. The heatsource for volcanic activity on Io is thought to be tidal energy generated by a relentless “tug of war” between Jupiter and the other Galileanmoons. The gravitational power of Jupiter and nearby moons pullsand pushes on Io’s tidal bulge as its orbit takes it alternately closer toand farther from Jupiter. This gravitational flexing of Io is transformedinto frictional heat energy and results in Io’s volcanic eruptions.L1Figure 14 Jupiter’s MoonsA Io is the innermost moon and isone of only four volcanicallyactive bodies in the solar system.B Europa—the smallest of theGalilean moons—has an icysurface that is crossed by manylinear features. C Ganymede isthe largest Jovian moon, and itcontains cratered areas, smoothregions, and areas covered bynumerous parallel grooves.D Callisto—the outermost of theGalilean moons—is denselycratered, much like Earth’s moon.CMythological Characters All of theplanets in our solar system, except forEarth, are named for characters or godsin Roman mythology. Have studentswork in groups. Each group shouldselect the name of one planet toresearch. They should find out whichmythological character or god theplanet was named after, learn about thecharacter, and determine why the namemay have been given to the planet. Forexample, Mercury was named after theRoman messenger god because it is theplanet with the fastest revolution ratearound the sun. Each group shouldpresent its findings to the class.Verbal, InterpersonalDTouring Our Solar System 655Customize for English Language LearnersHave students create a concept map toorganize what they will learn about the outerplanets. Have them start with the mainconcept of the outer planets. Then havebranches for Jupiter, Saturn, Uranus, andNeptune, which they will expand on by fillingin characteristics of each planet as they read.Download a worksheet on the outerplanets for students to complete,and find additional teacher supportfrom NSTA SciLinks.Touring Our Solar System 655

Section 23.3 (continued)Jupiter’s Rings Jupiter’s ring system was one of the most unexpected discoveries made by Voyager 1. By analyzing how these ringsscatter light, researchers concluded that the rings are composed of fine,dark particles, similar in size to smoke particles. The faint nature of therings also indicates that these minute fragments are widely dispersed.The particles are thought to be fragments blasted by meteorite impactsfrom the surfaces of Metis and Adrastea, two small moons of Jupiter.Saturn:The Elegant PlanetBuild Reading LiteracyL1Refer to p. 124D in Chapter 5, whichprovides the guidelines for this ReadingStrategy.Which Galilean moon is volcanically active?Summarize Have students summarizethe major characteristics of Saturn asthey read. For example, Saturn haswind, storms, many moons, and rings.VerbalUse VisualsSaturn: The ElegantPlanetFGL1Figure 15 This diagram shows therings of Saturn. Ask: How are rings Aand B different from ring C? (Ring Cis a darker color.) What do you think isthe cause of this difference? (Theyhave different compositions.) How arethe outer rings different from theinner rings? (The outer rings are muchthinner than the inner rings.) How is ringE different from the other rings? (RingE looks green, and is more diffuse.)VisualSaturnCBDFigure 15 Saturn’s RingsSaturn’s rings fall into twocategories based on particledensity. The main rings (A and B)are densely packed. In contrast,the outer rings are composed ofwidely dispersed particles.656AERequiring 29.46 Earth-years to makeone revolution, Saturn is almosttwice as far from the sun as Jupiter.However, its atmosphere, composition, and internal structure arethought to be remarkably similar toJupiter’s.The most prominentfeature of Saturn is its system ofrings, shown in Figure 15. In 1610,Galileo used a primitive telescopeand first saw the structures that werelater found to be the rings. Theyappeared as two small bodies adjacentto the planet. Their ring nature wasexplained 50 years later by the Dutchastronomer Christian Huygens.Features of Saturn In 1980 and 1981, flyby missions of theVoyagers 1 and 2 spacecraft came within 100,000 kilometers of Saturn.More information was gained in a few days than had been acquiredsince Galileo first viewed this elegant planet.1. Saturn’s atmosphere is very active, with winds roaring at up to 1500kilometers per hour.2. Large cyclonic “storms” similar to Jupiter’s Great Red Spot,although smaller, occur in Saturn’s atmosphere.3. Eleven additional moons were discovered.4. The rings of Saturn were found to be more complex than expected.More recently, observations from ground-based telescopes, theHubble Space Telescope, and Cassini have added to our knowledge ofSaturn’s ring and moon system. When the positions of Earth and Saturnallowed the rings to be viewed edge-on—thereby reducing the glare fromthe main rings—Saturn’s faintest rings and satellites became visible.Chapter 23Facts and FiguresScientists believe that liquid water is the key tothe development of life, so they carefully searchother planets and moons for this key ingredient.Studies of Jupiter’s moon, Europa, have revealedthat it is covered with a thick layer of ice, and656Chapter 23scientists have inferred that there may be liquidwater beneath this layer. This makes Europa aprime target for future space probes to look forevidence of life on this moon.

Build Science SkillsL2Inferring After reading the section onSaturn’s rings, have students take anotherlook at the image of Saturn’s rings inFigure 15. Ask students to use what theyread and observed, in addition to theirprior knowledge of how the solar systemformed, to infer how Saturn’s rings mighthave formed. Have students share theirideas with the class. Then share theinformation in the Facts and Figuresbox below with the class.Visual, LogicalSaturn’s Rings Until the discovery that Jupiter, Uranus, andNeptune also have ring systems, this phenomenon was thought to beunique to Saturn. Although the four known ring systems differ indetail, they share many attributes. They all consist of multiple concentric rings separated by gaps of various widths. In addition, eachring is composed of individual particles—“moonlets” of ice androck—that circle the planet while regularly impacting one another.Most rings fall into one of two categories based on particle density.Saturn’s main rings, designated A and B in Figure 15, and the bright ringsof Uranus are tightly packed and contain “moonlets” that range in sizefrom a few centimeters to several meters. These particles are thought tocollide frequently as they orbit the parent planet. Despite the fact thatSaturn’s dense rings stretch across several hundred kilometers, they arevery thin, perhaps less than 100 meters from top to bottom.At the other extreme, the faintest rings, such as Jupiter’s ring systemand Saturn’s outermost rings, are composed of very fine particles thatare widely dispersed. Saturn’s outermost rings are designated E inFigure 15. In addition to having very low particle densities, these ringstend to be thicker than Saturn’s bright rings.Figure 16 Saturn’s Moons Thisimage of Saturn shows several ofits moons.Saturn’s Moons Saturn’s satellite system consists of 56 moons,some of which are shown in Figure 16. Titan is the largest moon and isbigger than Mercury. It is covered with rivers and oceans of liquid hydrocarbons. Titan and Neptune’s Triton are the only moons in the solarsystem known to have substantial atmospheres. Because of its densegaseous cover, the atmospheric pressure at Titan’s surface is about 1.5times that at Earth’s surface. Another moon, Enceladus, is one of fourknown volcanically active bodies in our solar system. In 2006, the Cassinispace probe discovered liquid water geysers in the moon’s south polarregion.How many moons of Saturn have been discoveredthus far?Touring Our Solar System 657Facts and FiguresThere are several theories regarding the originof ring particles. Some scientists believe therings formed out of a cloud of gas and dustfrom which the planet formed. Others believethe rings formed later when a moon or asteroidwas pulled apart by the planet’s gravity. Stillothers believe a crash with a foreign bodyblasted apart one of the planet’s moon.Scientists hope that future missions to Saturnwill help them resolve this controversy.Answer to . . .Io is volcanically active.Fifty-six moons havebeen discovered aroundSaturn.Touring Our Solar System 657

Section 23.3 (continued)Uranus: The Sideways PlanetUranus:The Sideways PlanetA unique feature of Uranus, shown in Figure 17, is that it rotates “onits side.”Instead of being generally perpendicular to the plane ofits orbit like the other planets, Uranus’s axis of rotation lies nearlyparallel with the plane of its orbit. Its rotational motion, therefore,has the appearance of rolling, rather than the top-like spinning of theother planets. Uranus’s spin may have been altered by a giant impact.A surprise discovery in 1977 revealed that Uranus has a ringsystem. This find occurred as Uranus passed in front of a distant starand blocked its view. Observers saw the star “wink” briefly both beforeand after Uranus passed by. Later studies indicate that Uranus has atleast nine distinct ring belts.The five largest moons of Uranus show varied terrain. Some of themoons have long, deep canyons and linear scars, whereas others possess large, smooth areas on otherwise crater-riddled surfaces. Miranda,the innermost of the five largest moons, has a greater variety of landforms than any body yet examined in the solar system.Discovering the Ringsof UranusL2Purpose Students will experience asimulation of how the rings of Uranuswere discovered.Materials meter stick, flashlightProcedure Darken the classroom, andhave one student hold the flashlight atthe front of the room. Hold the meterstick horizontally to represent the ringsof Uranus. While the student holds theflashlight steadily, pass the meter stickslowly up and down in front of theflashlight so it appears to blink on andoff to the class. Explain to students thatthe flashlight represents the distant starwhich was blocked by Uranus in 1977.Help students understand that the ringswould have caused the light of the starto blink on and off a few times as Uranusand its rings passed in front of thedistant star.What is unique about Uranus’s axis of rotation?Figure 17 The axis of rotation ofUranus is nearly parallel with theplane of its orbit. This photo alsoshows the planet’s ring system.Expected Outcome Students will seethat Uranus’s rings would have causedthe occluded star to blink on and off afew times before and after the passingof Uranus’s body in front of the star.Visual, KinestheticFigure 18 The Great Dark Spotof Neptune (photographed byVoyager 2 in 1989) is visible in thecenter of the left of the image.Bright cirrus-like clouds thattravel at high speeds around theplanet are also visible.Identifying What was the GreatDark Spot?658Neptune: The Windy PlanetAs shown in Figure 18, Neptune has a dynamic atmosphere, much likethose of Jupiter and Saturn.Winds exceeding 1000 kilometers perhour encircle Neptune, making it one of the windiest places in thesolar system. It also had an Earth-size blemish called the Great DarkSpot that was reminiscent of Jupiter’s Great Red Spot. The Great DarkSpot was assumed to be a large rotating storm. About five years afterthe Great Dark Spot was discovered, it vanished, only to be replaced byanother dark spot in the planet’s northern hemisphere, which also vanished within a few years.Neptune has many surprising features. Perhaps most surprisingare the cirrus-like clouds that occupy a layer about 50 kilometers abovethe main cloud deck. The clouds are most likely frozen methane.Voyager images revealed that the bluish planet also has a ring system.Neptune has 13 known moons. Triton, Neptune’s largest moon, isnearly the size of Earth’s moon. Triton is the only large moon in thesolar system that exhibits retrograde motion. This motion indicatesthat Triton formed independently of Neptune and was gravitationallycaptured.Triton also has the lowest surface temperature yet measured on anybody in the solar system at 200 C. Its atmosphere is mostly nitrogenwith a little methane. Despite low surface temperatures, Triton displaysvolcanic-like activity.Chapter 23Facts and FiguresThe existence of Neptune was predictedbefore it was discovered. This prediction wasbased on irregularities in the orbit of Uranusand Newton’s Universal Law of Gravitation.Scientists were ecstatic, in 1846, when658Chapter 23Neptune was discovered exactly where it hadbeen predicted. This discovery is an excellentexample of a hypothesis being tested notin a lab, but in outer space itself.

Pluto: Dwarf PlanetPluto: Dwarf PlanetBuild Science SkillsUntil 2006, Pluto was considered to be one of the nine planets. But inAugust of 2006, the International Astronomical Union (IAU) redefinedthe word “planet” in a way that excluded Pluto.Pluto is not considered a planet, because it has not cleared the neighborhood aroundits orbit.Because Pluto was no longer a planet, the IAU also created a newterm to describe it. A dwarf planet is a round object that orbits thesun but has not cleared the neighborhood around its orbit. A planet'sgravity is strong enough for it to pull in smaller nearby bodies, thusclearing its orbital path. But a dwarf planet’s gravity is too weak toattract all the debris nearby. Therefore, a dwarf planet orbits in a zonealong with other small solar system bodies.Pluto is the most well known of the dwarf planets. However, it isneither the largest nor the first to be discovered. The dwarf planetCeres, which is in the asteroid belt, was discovered in 1801.And the dwarf planet Eris, just discovered in 2005, is slightlylarger than Pluto. All dwarf planets likely contain a mixture ofrock and ice, but can be found in very different parts of thesolar system. Pluto is unusual in that it has a moon, Charon,which is more than half its size and may be considered adwarf planet on its own. It is not yet known how manyobjects in the solar system will be considered dwarf planets.As new discoveries are made, this definition may be revisited.Comparing and ContrastingChallenge students to find similaritiesand differences between Pluto andthe planets (both terrestrial and Jovian).They may also want to compare Pluto tosome of the moons in our solar system.Verbal3 ASSESSEvaluateUnderstandingHave students make concept maps foreach outer planet that list its majorcharacteristics.Critical ThinkingWhat is the largest planet? What is thesmallest?6. Relating Cause and Effect What may havecaused Uranus’s unique axis of rotation?What is Jupiter’s Great Red Spot?Identify one distinguishing characteristicof each outer planet and Pluto.4. How are Saturn’s moon Titan and Neptune’sTriton similar?5. In what way is Io similar to Earth? What otherbody shows this similarity?7. Making Judgments Should Pluto have beenreclassified as a dwarf planet? Explain youranswer.2.3.L1ReteachSection 23.3 Assessment1.L2Put students in cooperative groups, andhave them compare the summaries ofthis section that they made in responseto this section’s first Reading Strategy.Figure 19 This Hubble imageshows Pluto and its moon Charon.Reviewing ConceptsL2Before students begin writing, reviewwith them how convection currents inEarth’s atmosphere work. Describe theircause (solar energy), what they look like(hot, less dense air rises while cooler,denser air sinks), and some results ofconvection currents (wind).Sample answer: On Earth, atmosphericconvection currents are driven by solarenergy. Jupiter, however, gives off nearlytwice as much heat as it receives fromthe sun. The interior heat from Jupiterproduces huge convection currents inthe atmosphere.Convection Currents Write a briefparagraph comparing and contrastingatmospheric convection currents onJupiter and Earth.Touring Our Solar System 659Section 23.3Assessment1. Jupiter is the largest planet. Mercury is thesmallest.2. a cyclonic storm3. Sample answer: Jupiter is the largest planet.Saturn has an amazing ring system. Uranus’saxis of rotation is nearly parallel with theplane of its orbit. Neptune is one of thewindiest places in the solar system. Pluto issmall and cold with a very eccentric orbit.4. Titan and Triton are the only moons in thesolar system with significant atmospheres.5. Io is volcanically active, just like Earth.Neptune’s moon Triton is also volcanically active.6. A giant impact may have changed Uranus’sspin.7. Sample answer: Further study is likelyrequired to determine if our definition of adwarf planet should apply to Pluto.Answer to . . .Figure 18 The Great Dark Spot isassumed to be a large rotating storm,much like Jupiter’s Great Red Spot.Uranus’s axis lies nearlyparallel with the planeof its orbit.Touring Our Solar System 659

the main rings—Saturn’s faintest rings and satellites became visible. Which Galilean moon is volcanically active? . The main rings (A and B) are densely packed. In contrast, the outer rings are composed of widely dispersed particles. 656 Chapter 23 Saturn: The Elegant Planet Build Reading Literacy Refer to p. 124D in Chapter 5, which .

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