Section 23.1 23.1 The Solar System -...

23.1 The Solar System

Reading StrategyRelating Text and Diagrams As you read,refer to Figure 3 to complete the flowchart onthe formation of the solar system.

Key ConceptsHow do terrestrial planetsdiffer from Jovian planets?

How did the solar systemform?

Vocabulary◆ terrestrial planet◆ Jovian planet◆ nebula◆ planetesimal

The sun is the hub of a huge rotating system of planets, their satel-lites, and numerous smaller bodies. An estimated 99.85 percent of themass of our solar system is contained within the sun. The planets col-lectively make up most of the remaining 0.15 percent. As Figure 1shows, the planets, traveling outward from the sun, are Mercury, Venus,Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Guided by the sun’s gravitational force, each planet moves in anelliptical orbit, and all travel in the same direction. The nearest planetto the sun—Mercury—has the fastest orbital motion at 48 kilometers

per second, and it has theshortest period of revolu-tion. By contrast, the mostdistant planet, Neptune, hasan orbital speed of 5 kilome-ters per second, and itrequires 165 Earth-years tocomplete one revolution.

Imagine a planet’s orbitdrawn on a flat sheet ofpaper. The paper representsthe planet’s orbital plane.The orbital planes of sevenplanets lie within 3 degrees ofthe plane of the sun’s equa-tor. Mercury’s orbit isinclined by 7 degrees.

Figure 1 Orbits of the Planetsand Pluto The positions of theplanets and Pluto are shown toscale along the bottom of thediagram.

c. ?b. ?a. ?Cloud ofdust andgas beganrotating.

Mercury VenusEarth

MarsJupiter

Saturn

Uranus

Neptune

Pluto

SunAsteroid Belt

P N U S J MEVM

SUN

Kuiper belt

644 Chapter 23

644 Chapter 23

FOCUS

Section Objectives23.1 List the major differences

between the terrestrial andJovian planets.

23.2 Explain how the solar systemformed.

Build VocabularyWord Part Analysis Teach studentsthat terr- means “Earth,” and -ial and -ian mean “of, or related to” soterrestrial means “Earthly” or “Earth-like.” Terrestrial planets are those similarto Earth while Jovian planets are thosesimilar to Jupiter.

Advise students that nebula is related to the word nebulous, which means“hazy” or “unclear.” The word nebulais used to describe a “hazy mass of gases and dust seen among the stars.”Tell students that infinitesimal means“infinitely small,” and have them predict the meaning of planetesimal.(Planetesimals are small solid bodies thatcombine to form planets.)

Reading Strategya. The sun formed at the center of a disk.b. Matter collided to form planetesimals.c. Planetesimals eventually grow intoplanets.

INSTRUCTUse VisualsFigure 1 This diagram shows the orbitsof all 8 planets around the sun. Ask:Which planet is the closest to the sun?(Mercury) The asteroid belt is foundbetween which two planets? (Marsand Jupiter)

Direct students to observe the scalealong the bottom of the figure thatshows the scale distances from planet to planet. Tell students that the innerplanets are those found before theasteroid belt, and the outer planets arefound after the asteroid belt. Ask: Howdoes the distance between the innerplanets differ from the distancebetween the outer planets? (The innerplanets are much closer together than theouter planets.) Visual

L12

L2

L2

Reading Focus

1

Section 23.1

Touring Our Solar System 645

The Planets: An OverviewCareful examination of Table 1 shows that the planets fall quite nicelyinto two groups. The terrestrial planets—Mercury, Venus, Earth, andMars—are relatively small and rocky. (Terrestrial � Earth-like.)TheJovian planets—Jupiter, Saturn, Uranus, and Neptune—are huge gasgiants. (Jovian � Jupiter-like.)

Size is the most obvious difference between the terrestrial andthe Jovian planets. The diameter of the largest terrestrial planet, Earth,is only one-quarter the diameter of the smallest Jovian planet, Neptune.Also, Earth’s mass is only 1/17 as great as Neptune’s. Hence, the Jovianplanets are often called giants. Because of their distant locations fromthe sun, the four Jovian planets are also called the outer planets. Theterrestrial planets are closer to the sun and are called the inner planets.As we shall see, there appears to be a correlation between the positionsof these planets and their sizes.

Density, chemical makeup, and rate of rotation are otherways in which the two groups of planets differ. The densities of theterrestrial planets average about five times the density of water. TheJovian planets, however, have densities that average only 1.5 times thedensity of water. One of the outer planets, Saturn, has a density only0.7 times that of water, which means that Saturn would float if placedin a large enough water tank. The different chemical compositions ofthe planets are largely responsible for these density differences.

Compare the densities of terrestrial planets andJovian planets.

Table 1 Planetary Data

Average Distance from Sun Orbital Relative Average Number

Millions Period of Velocity Period of Diameter Mass Density of KnownPlanet AU of km Revolution km/s Rotation (km) (Earth � 1) (g/cm3) Satellites*

Mercury 0.39 58 88d 47.5 59d 4878 0.06 5.4 0

Venus 0.72 108 225d 35.0 244d 12,104 0.82 5.2 0

Earth 1.00 150 365.25d 29.8 23h 56m 04s 12,756 1.00 5.5 1

Mars 1.52 228 687d 24.1 24h 37m 23s 6794 0.11 3.9 2

Jupiter 5.20 778 12yr 13.1 9h 50m 143,884 317.87 1.3 63

Saturn 9.54 1427 29.5yr 9.6 10h 14m 120,536 95.14 0.7 56

Uranus 19.18 2870 84yr 6.8 17h 14m 51,118 14.56 1.2 27

Neptune 30.06 4497 165yr 5.3 16h 03m 50,530 17.21 1.7 13

Pluto** 39.44 5900 248yr 4.7 6.4d approx. 2300 0.002 1.8 3

*Includes all satellites discovered as of December 2006.

**Pluto is included for purposes of comparison.

For: Solar System activityVisit: PHSchool.comWeb Code: czp-7231

The Planets: An Overview Build Reading Literacy Refer to p. 642D, which providesguidelines for this reading strategy.Compare and Contrast Havestudents create a chart comparing thecharacteristics of the terrestrial planetsand the Jovian planets. Have them startwith what they observed about thedistances between planets in the UseVisuals activity on p. 644, and use the reading, tables, and figures on pp. 645–647. For example:

Terrestrial Planets Jovian Planets

Orbits are close Orbits are far aparttogether

Smaller diameter Larger diameter

More dense Less dense

Rotate slower Rotate faster

Thin or no atmosphere Thick atmosphere

Composed mostly of Mostly made of gases,rocky and metallic liquids, and ices, but substances, with few with rocky gases and ices and metallic materials

in their cores

Visual, Verbal

L1


Customize for Inclusion Students

Learning Disabled Help students completeCompare and Contrast activities by providingthem with scaffolding. Give them a chart to fillin that lists the categories they should be

comparing. For example, these students couldbe given a chart such as the one below to usefor the Compare and Contrast activity on thispage.

Answer to . . .

The terrestrial planetshave greater densities

than the Jovian planets.

Characteristic Terrestrial Planets Jovian PlanetsDistance from one planet to the nextDiameterDensityRotation rateAtmosphereComposition

For: Solar System activityVisit: PHSchool.comWeb Code: czp-7231Students can interact with the artabout the solar sytem.

646 Chapter 23

The Interiors of the Planets The planets(and Pluto) are shown to scale in Figure 2. The sub-stances that make up the planets are divided into threegroups: gases, rocks, and ices. The classification of thesesubstances is based on their melting points.1. The gases—hydrogen and helium—are those with

melting points near absolute zero (�273°C or 0 kelvin).

2. The rocks are mainly silicate minerals and metalliciron, which have melting points above 700°C.

3. The ices include ammonia (NH3), methane (CH4),carbon dioxide (CO2), and water (H2O). They haveintermediate melting points. For example, H2O hasa melting point of 0°C.The terrestrial planets are dense, consisting mostly of

rocky and metallic substances, and only minor amountsof gases and ices. The Jovian planets, on the other hand,contain large amounts of gases (hydrogen and helium)and ices (mostly water, ammonia, and methane). Thisaccounts for their low densities. The outer planets alsocontain substantial amounts of rocky and metallic mate-rials, which are concentrated in their cores.

The Atmospheres of the Planets TheJovian planets have very thick atmospheres of hydro-gen, helium, methane, and ammonia. By contrast, theterrestrial planets, including Earth, have meager atmos-pheres at best. A planet’s ability to retain an atmospheredepends on its mass and temperature, which accountsfor the difference between Jovian and terrestrial planets.

Simply stated, a gas molecule can escape from aplanet if it reaches a speed known as the escape veloc-ity. For Earth, this velocity is 11 kilometers per second.Any material, including a rocket, must reach this speedbefore it can escape Earth’s gravity and go into space.

A comparatively warm body with a small surfacegravity, such as our moon, cannot hold even heavygases, like carbon dioxide and radon. Thus, the moonlacks an atmosphere. The more massive terrestrial planets of Earth, Venus, and Mars retain some heavygases. Still, their atmospheres make up only a very smallportion of their total mass.

Mercury

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

Sun

Figure 2 The planets and Plutoare drawn to scale. Interpreting Diagrams How dothe sizes of the terrestrial planetscompare with the sizes of theJovian planets?

646 Chapter 23

Use VisualsFigure 2 Have students study Figure 2and answer the caption question. Ask:Which planet is the smallest? (Mercury)Which planet is the largest? (Jupiter)How does the size of the largestplanet compare to the size of the sun?(Jupiter is much smaller than the sun.)Visual

Many students think that the solarsystem and outer space are verycrowded. Help students overcome this misconception by using Table 1Planetary Data. Have students look atthe column describing the distance from the sun. Point out to them that the distances are given in millions ofkilometers. Tell students that if theychose a point in the solar system atrandom, it is unlikely it would be near a planet. Visual, Verbal

L2

L1

Section 23.1 (continued)

Why are the Jovian planets so much largerthan the terrestrial planets? According to thenebular hypothesis, the planets formed from arotating disk of dust and gases that surroundedthe sun. The growth of planets began as solidbits of matter began to collide and clumptogether. In the inner solar system, thetemperatures were so high that only the metalsand silicate materials could form solid grains. Itwas too hot for ices of water, carbon dioxide,and methane to form. Thus, the innermost(terrestrial) planets grew mainly from the high

melting point substances found in the solarnebula. By contrast, in the frigid out reaches of the solar system, it was cold enough for ices of water and other substances to form.Consequently, the outer planets are thoughtto have grown not only from accumulations ofsolid bits of metals and silicate minerals butalso from large quantities of ices. Eventually,the outer planets became large enough togravitationally capture the lightest gases(hydrogen and helium), and thus grow tobecome “giant” planets.

Facts and Figures


In contrast, the Jovian planets have much greater surface gravities.This gives them escape velocities of 21 to 60 kilometers per second—much higher than the terrestrial planets. Consequently, it is moredifficult for gases to escape from their gravitational pulls. Also, becausethe molecular motion of a gas depends upon temperature, at the lowtemperatures of the Jovian planets even the lightest gases are unlikelyto acquire the speed needed to escape.

Formation of the Solar SystemBetween stars is “the vacuum of space.” However, it is not a purevacuum because it is populated with regions of dispersed dust andgases. A cloud of dust and gas in space is called a nebula (nebula �

cloud; plural: nebulae). A nebula, shown in Figure 3A, often consists of92 percent hydrogen, 7 percent helium, and less than 1 percent of theremaining heavier elements. For some reason not yet fully understood,these thin gaseous clouds begin to rotate slowly and contract gravita-tionally. As the clouds contract, they spin faster. For an analogy, thinkof ice skaters—their speed increases as they bring their arms near theirbodies.

Nebular Theory Scientific studies of nebulae have led to a theoryconcerning the origin of our solar system. According to the neb-ular theory, the sun and planets formed from a rotating disk of dustand gases. As the speed of rotation increased, the center of the diskbegan to flatten out, as shown in Figure 3B. Matter became more con-centrated in this center, where the sun eventually formed.

Solarnebula

The Sun formsat the center ofa protoplanetarydisk.

Planetesimals form.

A

B

C

Figure 3 Formation of theSolar System A According to thenebular theory, the solar systemformed from a rotating cloud ofdust and gas. B The sun formed at the center of the rotating disk. C Planetesimals collided,eventually gaining enough massto be planets.

Formation of the Solar System

Speeding Up a Spinning Nebula Purpose Students will see howrotational speed would have increasedas the nebula contracted early in theformation of our solar system.

Materials a chair that can be spun inplace

Procedure One person sits in the chair,and the chair is spun. The seated personextends his or her arms out to the sides,which will cause the spinning to slow.Then the seated person pulls his or herarms in, causing the spinning rate toincrease. This activity can be repeatedwith multiple students.

Safety A lighter student will be easierto spin, however, you may prefer to bethe person in the chair. The person inthe chair should not move in any wayother than to put his or her arms in and out.

Expected Outcomes Students will seethat extended arms (representing theearly, wider nebula) result in a slowerspin. Pulling in the arms (representingthe contracting nebula) causes anincrease in spinning rate.Visual, Kinesthetic

L2


Answer to . . .

Figure 2 The terrestrial planets aremuch smaller than the Jovian planets.

648 Chapter 23

Section 23.1 Assessment

Reviewing Concepts1. Which planets are classified as terrestrial?

Which planets are classified as Jovian?

2. List the planets in order, beginning with theplanet closest to the sun.

3. How do the terrestrial planets differ fromthe Jovian planets?

4. What is a nebula?

5. How did distance from the sun affect thesize and composition of the planets?

Critical Thinking6. Summarizing Summarize the nebular

theory of the formation of the solar system.

7. Inferring Among the planets in our solarsystem, Earth is unique because water exists in all three states—solid, liquid, and gas—onits surface. How would Earth’s water cycle be different if its orbit was outside the orbit of Mars?

8. Jupiter is 6.3 x 108 (630 millionkilometers) from Earth. Calculate howlong it would take to reach Jupiter ifyou traveled at 1) 100 km/h (freeway speed); 2) 1,000 km/h (jetliner speed); 3) 40,000 km/h (rocket speed); and 4) 3.0 x 108 km/s (speed of light).

Planetesimals The growth of planets began as solid bits of matterbegan to collide and clump together through a process known as accre-tion. The colliding matter formed small, irregularly shaped bodiescalled planetesimals. As the collisions continued, the planetesimalsgrew larger, as shown in Figure 3C on page 647. They acquired enoughmass to exert a gravitational pull on surrounding objects. In this way,they added still more mass and grew into true planets.

In the inner solar system, close to the sun, temperatures were sohigh that only metals and silicate minerals could form solid grains. Itwas too hot for ices of water, carbon dioxide, and methane to form. As

shown in Figure 4, the inner planets grewmainly from substances with high meltingpoints.

In the frigid outer reaches of the solarsystem, on the other hand, it was cold enoughfor ices of water and other substances to form.Consequently, the Jovian planets grew not onlyfrom accumulations of solid bits of materialbut also from large quantities of ices.Eventually, the Jovian planets became largeenough to gravitationally capture even thelightest gases, such as hydrogen and helium.This enabled them to grow into giants.

Tem

per

atur

e (K

)

2000

500

0

Distance from Sun (AU)

1500

1000

0 5 10 15 20 25 30

Mercury

Venus

EarthMars

AsteroidsJupiter

Saturn Uranus Neptune

Tungsten

Aluminum oxide

Iron

Silicates

Carbon rich silicates

Ices

Figure 4 The terrestrial planetsformed mainly from silicateminerals and metallic iron thathave high melting points. TheJovian planets formed from largequantities of gases and ices.

648 Chapter 23

ASSESSEvaluate UnderstandingHave students create quiz questionsfrom this section and put them onflashcards. Then put the students in smallgroups where they will compete to seewho can answer the most questionscorrectly. Put the cards in the center ofthe table, and have students take turnsselecting a card and trying to answer it.If a student cannot answer the questionon the card he or she selects, it isreturned to the bottom of the pile.Students earn the card of each questionthey answer correctly. The winner is theone with the most cards at the end ofthe game.

ReteachHave students summarize the differencesbetween the terrestrial and Jovian planetsby using the figures and tables in thissection.

8. Show students how to use theequation: distance � rate / time toanswer these questions. Since they areasked to find time, the equation can berearranged as time � distance / rate.

Solutions(1) 6.3 � 108 km � 100 km/h �6,300,000 h � 24 h/day � 262,500days � 365 days/yr � 719 yrs (2) 6.3 � 108 km � 1000 km/h �630,000 h � 24 h/day � 26,250 days� 365 days/yr � 72 yrs (3) 6.3 � 108 km ÷ 40,000 km/h �15,750 h � 24 h/day � 656 days �365 days/yr � 1.8 yrs (4) 6.3 � 108 km � 300,000 km/s �2100 s � 60 s/ 1 min � 35 minutes

L1

L2

3


Thus, the inner planets grew mainly from substances with high melting points. In theouter reaches of the solar system, it was coldenough for ices of water and other substancesto form. Consequently, the Jovian planets grewnot only from accumulations of solid bits ofmaterial but also from large quantities of gasesand ices.6. According to the nebular theory, the sunand planets formed from a rotating disk ofdust and gases. As the speed of rotationincreased, the center of the disk began to


1. Terrestrial: Mercury, Venus, Earth, andMars; Jovian: Jupiter, Saturn, and Neptune2. Mercury, Venus, Earth, Mars, Jupiter,Saturn, Uranus, Neptune.3. The terrestrial planets are small and rocky.The Jovian planets are gas giants. 4. A nebula is a cloud of dust and gas in space.5. In the inner solar system, close to the sun,temperatures were so high that only metalsand silicate minerals could form solid grains.

flatten out. Matter became more concentrat-ed in this center, where the sun eventuallyformed.7. Sample answer: If Earth’s orbit were out-side the orbit of Mars, the extreme coldwould freeze all water and only ice wouldexist. With only frozen water, there would beno precipitation, runoff, or infiltration—thewater cycle and life itself would not exist.


23.2 The Terrestrial Planets

Key ConceptsWhat are thedistinguishingcharacteristics ofeach terrestrialplanet?

Reading StrategyUsing Prior Knowledge Copy the web diagram below. Before you read,add properties that you already know about Mars. Then add details abouteach property as you read. Make a similar web diagram for the otherterrestrial planets.

In January 2004, the space rover, Spirit, bounced onto the rocky sur-face of Mars, known as the Red Planet. Shown in Figure 5, Spirit andits companion rover, Opportunity, were on the Red Planet to studyminerals and geological processes, both past and pres-ent. They also searched for signs of the liquidwater—such as eroded rocks or dry stream channels onMars’s surface. For the next few months, the rovers sentback to Earth numerous images and chemical analysisof Mars’s surface. Much of what we learn about theplanets has been gathered by rovers, such as Spirit, orspace probes that travel to the far reaches of the solarsystem, such as Voyager. In this section, we’ll explorethree terrestrial planets—Mercury, Venus, and Mars—and see how they compare with Earth.

Mercury: The InnermostPlanetMercury, the innermost and smallest planet, is hardly larger thanEarth’s moon and is smaller than three other moons in the solarsystem. Like our own moon, it absorbs most of the sunlight that strikesit and reflects only 6 percent of sunlight back into space. This low per-centage of reflection is characteristic of terrestrial bodies that have noatmosphere. Earth, on the other hand, reflects about 30 percent of thelight that strikes it. Most of this reflection is from clouds.

The Red Planet a. ? b. ?

Mars

Figure 5 Spirit roved the surfaceof Mars and gathered data aboutthe Red Planet’s geologic pastand present.

FOCUS

Section Objective23.3 Describe the distinguishing

characteristics of eachterrestrial planet.

Build VocabularyVocabulary List Encourage studentsto keep a list of new terms theyencounter as they read this chapter.Have them use the context of each termto predict its definition. Go over theterms with the class. Some terms theymay select are rover, nonexistent,penetrate, veiled, summit, flank, and prominent.

Reading StrategySample answers:a. explored by roversb. numerous large volcanoes

Some students may think that scientificknowledge is only acquired fromcontrolled experiments. However, agreat deal of scientific knowledge is aresult of fieldwork and carefulobservations. In fact, a great deal ofwhat we know about the universe andour solar system, we learned strictlyfrom observation. To help studentsrealize this, ask these questions as youteach this section. Ask: What did welearn about Venus from the Magellanspacecraft? (Venus has variedtopography like Earth.) What did welearn about Mars from the orbitingspacecraft Mariner 9? (Mars hasvolcanoes and canyons.) What did welearn about Mars from the roversSpirit and Opportunity? (Mars hasevaporite minerals and evidence ofgeological processes caused by liquidwater; Mars has sand dunes and impactcraters.) What did we learn from MarsGlobal Surveyor? (Underground springsmay have existed on Mars.) Could wehave learned these things simply fromcontrolled experiments on Earth? (no)Verbal

L2

L2

L2

Reading Focus

1


Section 23.2

650 Chapter 23

Surface Features Mercury has cratered highlands, much like themoon, and some smooth terrains that resemble maria. Unlike the

moon, however, Mercury is a very dense planet, which implies thatit contains a large iron core for its size.Also, Mercury has very longscarps (deep slopes) that cut across the plains and craters alike.These scarps may have resulted from crustal changes as theplanet cooled and shrank.

Surface Temperature Mercury, shown in Figure 6,revolves around the sun quickly, but it rotates slowly. One full

day-night cycle on Earth takes 24 hours. On Mercury, one rota-tion requires 59 Earth-days. Nighttime temperatures drop as low

as –173°C, and noontime temperatures exceed 427°C—hot enoughto melt lead. Mercury has the greatest temperature extremes of

any planet. The odds of life as we know it existing on Mercury are almostnonexistent.

Venus: The Veiled PlanetVenus is second only to the moon in bril-liance in the night sky. It orbits the sunonce every 255 Earth-days. Venus issimilar to Earth in size, density,mass, and location in the solarsystem. Thus, it has been referredto as “Earth’s twin.” Because ofthese similarities, it is hoped that adetailed study of Venus will providegeologists with a better understandingof Earth’s history.

Surface Features Venus is covered in thick clouds that hide itssurface from view. Nevertheless, radar mapping by the uncrewedMagellan spacecraft and by instruments on Earth have revealed avaried topography with features somewhat between those of Earth andMars, as shown in Figure 7. To map Venus, radar pulses are sent towardthe planet’s surface, and the heights of plateaus and mountains aremeasured by timing the return of the radar echo. Data have con-firmed that basaltic volcanism and tectonic activity shape Venus’ssurface. Based on the low density of impact craters, these forces musthave been very active during the recent geologic past.

How does Mercury’s period of rotation comparewith Earth’s?

Figure 7 Venus This global viewof the surface of Venus iscomputer generated from twoyears of Magellan Project radarmapping. The twisting brightfeatures that cross the planet arehighly fractured mountains andcanyons of the eastern Aphroditehighland.

Figure 6 Mercury’s surface lookssomewhat similar to the far sideof Earth’s moon.

For: Links on extraterrestrialvolcanoes

Visit: www.SciLinks.org

Web Code: cjn-7232

650 Chapter 23

INSTRUCT

Mercury: TheInnermost Planet Build Reading Literacy Refer to p. 362D in Chapter 13, whichprovides guidelines for this readingstrategy.

Use Prior Knowledge Have studentsmake a web diagram for Mercury thatincludes information they already knowabout it. Have them add newinformation to their web as they read.Possible characteristics for web include:small, hot, closest to sun, has craters,very dense, revolves around sun quickly,rotates slowly, greatest temperatureextremes of any planet. Visual, Verbal

Venus: The Veiled Planet Build Science Skills Comparing and Contrasting Havestudents write a list of the similaritiesbetween Earth and Venus. (size, density,mass, location in solar system, clouds,plateaus and mountains, volcanoes, havefew impact craters) Then have studentscreate a chart contrasting Venus andEarth. For example:

Venus Earth

One year is 255 One year is 365Earth-days Earth-days

Covered in thick Thin atmosphereclouds

Very hot surface Surface temperaturetemperature allows liquid water

97 percent of Very little of the atmosphere is atmosphere is carbon dioxide carbon dioxide

Very little water Lots of water vapor vapor and nitrogen and nitrogen

Atmospheric pressure is 90 times Earth’s surface pressure

Verbal, Visual

L2

L1

2


Customize for English Language Learners

Have students use a thesaurus rather than adictionary to look up unfamiliar words. Thiswill help them learn the meaning of multiple

words simultaneously. It is likely that at leastone synonym listed is a word they know.

Download a worksheet on extra-terrestrial volcanoes for students tocomplete, and find additionalteacher support from NSTASciLinks.


About 80 percent of Venus’s surface consistsof plains covered by volcanic flows. Some lavachannels extend hundreds of kilometers—oneis 6800 kilometers long. Scientists have identi-fied thousands of now inactive volcanicstructures. Most are small shield volcanoes,although more than 1500 volcanoes greaterthan 20 kilometers across have been mapped.Figure 8 shows two of these volcanoes—one isSapas Mons, 400 kilometers across and 1.5 kilo-meters high. Flows from this volcano mostlyerupted from its flanks rather than its summit,in the manner of Hawaiian shield volcanoes.

Only 8 percent of Venus’s surface consists ofhighlands that may be similar to continentalareas on Earth. Tectonic activity on Venus seemsto be driven by upwelling and downwelling ofmaterial in the planet’s interior.

Surface Temperature On Venus, the greenhouse effect hasheated the planet’s atmosphere to 475°C. That’s hot enough to melt lead!Several factors contribute to what scientists have called Venus’s runawaygreenhouse effect.

The main reason for the runaway greenhouse effect on Venus is thatits atmosphere is 97 percent carbon dioxide, a greenhouse gas. Venuslacks oceans in which carbon dioxide gas could dissolve, thus removingit from the atmosphere. Scientists think that oceans on Venus may haveevaporated early in its history. Water vapor in the atmosphere then accel-erated the greenhouse effect. But the atmosphere eventually lost mostof its water vapor. The sun’s ultraviolet radiation broke down water mol-ecules into hydrogen and oxygen. These gases then escaped into space.

Mars: The Red PlanetMars has evoked great interest throughout history. Mars is easy toobserve, which may explain why so many people are fascinated by it.Mars is known as the Red Planet because it appears as a reddish ballwhen viewed through a telescope. Mars also has some dark regionsthat change intensity during the Martian year. The most prominenttelescopic features of Mars are its brilliant white polar caps.

Describe the composition of Venus’s atmosphere.

Figure 8 Sapas Mons andMaat Mons In this computer-generated image from Venus,Maat Mons, a large volcano, isnear the horizon. Sapas Mons isthe bright feature in theforeground. Comparing and ContrastingWhat features on Venus aresimilar to those on Earth? Whatfeatures are different?

Mars: The Red Planet Use Community Resources If possible, invite an astronomer orgeologist in your community to talk to students about the findings of therovers Spirit and Opportunity on Mars.Encourage students to list questionsthey have about Mars before thespeaker comes to visit. Verbal, Interpersonal

Integrate ChemistryPolar Ice Caps Inform students thatMars has polar ice caps that are mademostly of frozen carbon dioxide, withsome frozen water. Have advancedstudents brainstorm how these ice capscould be used to help make Marshabitable for humans. Verbal, Logical

L3

L2


Answer to . . .

Figure 8 Features on Venus that aresimilar to those on Earth include plains,highlands, mountains, and volcanoes.Features on Venus that are differentthan those on Earth include thickclouds, volcanic flows covering mostplains, thousands of volcanoes, noprocess of plate tectonics, and anatmosphere that can’t sustain life.

One full day-night cycleon Earth takes 24 hours.

On Mercury, it requires 59 Earth-days.

Venus’s atmosphere ismainly made of carbon

dioxide with traces of water vapor andnitrogen.

Mars has two natural satellites (moons), Phobosand Deimos. Although Mars is easy to observefrom Earth, these moons were not discovereduntil 1977. Perhaps this is because they areonly 24 and 15 km in diameter. Phobos is

closer to Mars than any other natural satellitein the solar system, and it requires just 7 hoursand 39 minutes for one revolution. Mariner 9found that both moons are irregularly shapedand have numerous impact craters.

Facts and Figures

652 Chapter 23

The Martian Atmosphere The Martianatmosphere has only 1 percent the density of Earth’s. Itis made up primarily of carbon dioxide with tinyamounts of water vapor. Data from Mars probes con-firm that the polar caps of Mars are made of water ice,covered by a thin layer of frozen carbon dioxide. Aswinter nears in either hemisphere, temperatures dropto �125°C, and additional carbon dioxide is deposited.

Although the atmosphere of Mars is very thin,extensive dust storms occur and may cause the colorchanges observed from Earth. Hurricane-forcewinds up to 270 kilometers per hour can persist forweeks. The composition of Mars’s atmosphere is sim-ilar to that of Venus. But Mars is very cold. Whydoesn’t the greenhouse effect warm Mars’s atmos-phere? The reason is that Mars’s atmosphere is

extremely thin compared with the atmosphere of Venus (or Earth).Scientists think that, early in its history, Mars had a thick atmospherewarmed by the greenhouse effect. But Mars’s gravity was too low forthe planet to keep its atmosphere. Most of the gases escaped into space,and the planet cooled.

Surface Features Mariner 9, the first spacecraft to orbit anotherplanet, reached Mars in 1971 amid a raging dust storm. When the dustcleared, images of Mars’ northern hemisphere revealed numerous large

inactive volcanoes. The biggest, Olympus Mons, is the size of Ohioand is 23 kilometers high—over two and a half times higher

than Mount Everest. This gigantic volcano and othersresemble Hawaiian shield volcanoes on Earth.

Most Martian surface features are old byEarth standards. The highly cratered southern

hemisphere is probably 3.5 billion to 4.5 bil-lion years old. Even the relatively “fresh”volcanic features of the northern hemi-sphere may be older than 1 billion years.

Another surprising find made byMariner 9 was the existence of severalcanyons that are much larger than Earth’sGrand Canyon. The largest, Valles Marineris,

is shown in Figure 10. It is thought to haveformed by slippage of material along huge

faults in the crustal layer.

Figure 10 Valles MarinerisMars’s Valles Marineris canyon system is more than 5000 kilometers long and upto 8 kilometers deep. The darkspots on the left edge of theimage are hugevolcanoes.

Figure 9 Many parts of Mars’slandscape resemble desert areason Earth.

Volcanoes

VallesMarineris

Build Reading Literacy Refer to p. 1D in Chapter 1, whichprovides guidelines for this readingstrategy.

Anticipation GuideAsk students to respond to the followingquestions in writing before they read the section on Mars. Have the studentscheck over their answers and makechanges as needed after they finishreading the section. Students shouldanswer True or False to the followingseries of statements: Mars’s polar icecaps are mostly made of water. (False)There are active volcanoes on Mars.(False) Mars often has dust stormswith hurricane force winds. (True)Mars has canyons that are muchlarger than Earth’s Grand Canyon.(True) There is evidence that liquidwater once flowed on Mars. (True)Liquid water currently flows on theMartian surface. (False)Verbal

L1


652 Chapter 23

Students may ask why the volcanoes on Earthare so much smaller than volcanoes on Mars.The reason is that Earth’s crust is tectonicallyactive, so the crust over a mantle plume isconstantly moving. This motion creates aseries of smaller volcanoes. Since Mars does

not have plates that move, a volcano was ableto grow larger and larger each time it erupted.Also, the lower gravity and surface air pressureon Mars allowed the volcanoes to grow tallerthan they do on Earth.

Facts and Figures


Reviewing Concepts1. Which inner planet is smallest?

2. How does Venus compare with Earth?

3. Identify one distinguishing characteristicof each inner planet.

4. What surface features does Mars have thatare also common on Earth?

Critical Thinking5. Making Judgments Besides Earth, which

inner planet may have been most able tosupport life? Explain your answer.

6. Relating Cause and Effect Why are surfacetemperatures so high on Venus?

Water on Mars Some areas of Mars exhibit drainage patternssimilar to those created by streams on Earth. The rover Opportunity,for example, found evidence of evaporite minerals and geologic for-mations associated with liquid water, as shown in Figure 11. In addition,Viking images have revealed ancient islands in what is now a drystreambed. When these streamlike channels were first discovered, someobservers speculated that a thick water-laden atmosphere capable ofgenerating torrential downpours once existed on Mars. If so, what hap-pened to this water? The present Martian atmosphere contains onlytraces of water.

Images from the Mars Global Surveyor indicate thatgroundwater has recently migrated to the surface. Thesespring-like seeps have created gullies where they emerge fromvalley and crater walls. Some of the escaping water may haveinitially frozen due to the average Martian temperatures thatrange between �70°C and �100°C. Eventually, however, itseeped out as a slurry of sediment, ice, and liquid that formedthe gullies.

Many scientists do not accept the theory that Mars oncehad an active water cycle similar to Earth’s. Rather, theybelieve that most of the large stream-like valleys were createdby the collapse of surface material caused by the slow meltingof subsurface ice. Data from Opportunity, however, indicatethat some areas were “drenched” in water. It will take scientistsmany months, if not years, to analyze the data gathered by thelatest Mars mission. Because water is an essential ingredientfor life, scientists and nonscientists alike are enthusiasticabout exploring this phenomenon.


Editorial A space mission to the moon orMars often costs millions of dollars. Yet, it ishoped that space exploration can give usvaluable knowledge about the solar system.Consider the pros and cons of space explo-ration. Then write an editorial statingwhether or not you believe the costs areworth the potential benefits.

Figure 11 These channels showthat liquid water once flowed onthe surface of Mars.

ASSESSEvaluate UnderstandingReview with the class by stating acharacteristic of one of the planets. Have students respond with the name of the planet having that characteristic.

ReteachHave students make a colored sketch ofeach planet. They should list each planet’scharacteristics next to their sketch. Thenhave students put their sketches in order(Mercury out to Neptune) and displaytheir work.

Remind students that writing an editorialmeans stating a position and backing upthat statement with factual evidence.

Student editorials should discuss both the costs and benefits of spaceexploration. Student opinions should be supported by facts.

L1

L2

3


water exists in all three states at the planet’ssurface. Mars experiences extensive duststorms and high winds.4. volcanoes, sand dunes, and large canyons5. Sample answer: Mars may have been themost able to support life because it may havehad liquid water on its surface.6. Venus’s atmosphere is very dense andmainly made up of carbon dioxide, whichtraps radiation so the heat cannot escape.


1. Mercury2. Venus is similar to Earth in size, density,mass, and location in the solar system. But itis covered in thick clouds and its surface tem-perature is much higher.3. Sample answer: Mercury has the greatesttemperature extremes of any planet. Venusshows evidence of recent volcanic and tec-tonic activity. Earth is the only place where

23.3 The Outer Planets (and Pluto)

Key ConceptsWhat characteristicsdistinguish each outerplanet?

Why is Pluto notconsidered a planet?

Vocabulary◆ dwarf planet

In 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 stun-ning 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 from

Cassini and explore the outer planets—Jupiter, Saturn, Uranus, andNeptune.

Jupiter: Giant Among PlanetsJupiter is only 1/800 as massive as the sun. Still, it is thelargest planet by far. Jupiter has a mass that is 2 1/2times greater than the mass of all the other planetsand moons combined. In fact, had Jupiter been about10 times larger, it would have evolved into a small star.Jupiter rotates more rapidly than any other planet,completing one rotation in slightly less than 10 Earth-hours.

When viewed through a telescope or binoculars,Jupiter appears to be covered with alternating bands ofmulticolored clouds that run parallel to its equator. The

most 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.

654 Chapter 23

Outer Planets Characteristics

Jupiter largest; most mass,Great Red Spot

a. b.

c. d. ??

??

Dark clouds (belts)

Bright clouds (zones)

Dark clouds (belts)Strong winds

Strong winds

Figure 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.

A

B

Figure 12 This artist’s renditionshows Cassini approaching Saturn.

Reading StrategySummarizing Make a table like the oneshown that includes a row for each outerplanet. Write a brief summary of thecharacteristics of each planet.

654 Chapter 23

FOCUS

Section Objective23.4 Describe the distinguishing

characteristics of each Jovianplanet.

23.5 Explain why Pluto is notconsidered a planet.

Build VocabularyVocabulary 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 Strategy Sample answers:a. Saturnb. largest ring system c. Uranusd. axis tilted more than 90°e. Neptunef. winds exceed 1000 km per hour

INSTRUCT

Jupiter: Giant Among Planets Build Reading Literacy Refer 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, Verbal

L1

2

L2

L2

Reading Focus

1

Section 23.3


Structure of Jupiter Although Jupiter is called a gas giant, it isnot simply a ball of gas. At 1000 kilometers below the clouds, the pres-sure is great enough to compress hydrogen gas into a liquid.Consequently, Jupiter is thought to be a gigantic ocean of liquid hydro-gen. 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 dark-colored 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.

Jupiter’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 gener-ated 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.

A B C D

Figure 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.

For: Links on the outer planets

Visit: www.SciLinks.org

Web Code: cjn-7233

Use VisualsFigure 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)Visual

Integrate Language Arts Mythological 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, Interpersonal

L2

L1



Have students create a concept map toorganize what they will learn about the outerplanets. Have them start with the mainconcept of the outer planets. Then have

branches 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.

656 Chapter 23

Jupiter’s Rings Jupiter’s ring system was one of the most unex-pected 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 ElegantPlanetRequiring 29.46 Earth-years to makeone revolution, Saturn is almosttwice as far from the sun as Jupiter.However, its atmosphere, composi-tion, 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 1500

kilometers 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.

Which Galilean moon is volcanically active?

Saturn

ABC

D

EF G

Figure 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.

656 Chapter 23

Saturn: The Elegant Planet Build Reading Literacy Refer to p. 124D in Chapter 5, whichprovides the guidelines for this ReadingStrategy.

Summarize Have students summarizethe major characteristics of Saturn asthey read. For example, Saturn haswind, storms, many moons, and rings. Verbal

Use VisualsFigure 15 This diagram shows therings of Saturn. Ask: How are rings Aand B different from ring C? (Ring C is 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.)Visual

L1

L1


Scientists 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, and

scientists 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.

Facts and Figures


Saturn’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 con-centric 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.

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 hydro-carbons. 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?

Figure 16 Saturn’s Moons Thisimage of Saturn shows several ofits moons.

Build Science Skills Inferring 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 Figures box below with the class. Visual, Logical

L2


There 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 asteroid

was 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.

Facts and Figures

Answer to . . .

Io is volcanically active.

Fifty-six moons havebeen discovered around

Saturn.

658 Chapter 23

Uranus: 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 pos-sess large, smooth areas on otherwise crater-riddled surfaces. Miranda,the innermost of the five largest moons, has a greater variety of land-forms than any body yet examined in the solar system.

Neptune: 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 van-ished 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.

What is unique about Uranus’s axis of rotation?

Figure 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?

Figure 17 The axis of rotation ofUranus is nearly parallel with theplane of its orbit. This photo alsoshows the planet’s ring system.

658 Chapter 23

Uranus: The Sideways Planet

Discovering the Rings of Uranus Purpose Students will experience asimulation of how the rings of Uranuswere discovered.

Materials meter stick, flashlight

Procedure 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.

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, Kinesthetic

L2


The 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, when

Neptune was discovered exactly where it hadbeen predicted. This discovery is an excellentexample of a hypothesis being tested not in a lab, but in outer space itself.

Facts and Figures

Pluto: Dwarf Planet Until 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 con-sidered 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.


Reviewing Concepts1. What is the largest planet? What is the

smallest?

2. What is Jupiter’s Great Red Spot?

3. 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?

Critical Thinking6. Relating Cause and Effect What may have

caused Uranus’s unique axis of rotation?

7. Making Judgments Should Pluto have beenreclassified as a dwarf planet? Explain youranswer.


Figure 19 This Hubble imageshows Pluto and its moon Charon.

Convection Currents Write a brief paragraph comparing and contrastingatmospheric convection currents on Jupiter and Earth.

Pluto: Dwarf Planet Build Science Skills Comparing and ContrastingChallenge students to find similaritiesand differences between Pluto and the planets (both terrestrial and Jovian).They may also want to compare Pluto tosome of the moons in our solar system. Verbal

ASSESSEvaluate UnderstandingPut students in cooperative groups, andhave them compare the summaries ofthis section that they made in responseto this section’s first Reading Strategy.

ReteachHave students make concept maps foreach outer planet that list its majorcharacteristics.

Before 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.

L1

L2

3

L2


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.


1. 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.

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 plane

of its orbit.

660 Chapter 23

23.4 Minor Membersof the Solar System

Reading StrategyBuilding Vocabulary Copy the table below.Then as you read the section, write adefinition for each vocabulary term in yourown words.

Key ConceptsWhere are most asteroidslocated?

What is the structure of acomet?

What is the origin of mostmeteoroids?

Vocabulary◆ asteroid◆ comet◆ coma◆ meteoroid◆ meteor◆ meteorite

In February 2001 an American spacecraft, NEAR Shoemaker, fin-ished its mission in spectacular fashion—it became the first visitor toan asteroid. This historic accomplishment was not part of NEARShoemaker’s original goal, which was to orbit the asteroid, takingimages and gathering data about these objects in space. With this mis-sion accomplished, however, NASA engineers wanted to see if theycould actually land a spacecraft on an asteroid. The data they wouldgather would be priceless. As an added benefit, NASA would gain valu-able experience that might help in the future to deflect an asteroid ona collision course with Earth.

Although it was not designed for landing, NEAR Shoemaker—shown in Figure 20—successfully touched down on the asteroid, Eros.It generated information that has planetary geologists both intriguedand perplexed. The spacecraft drifted toward the surface of Eros at therate of 6 kilometers per hour. The images obtained revealed a barren,rocky surface composed of particles ranging in size from fine dust toboulders up to 8 meters across. Researchers unexpectedly discoveredthat fine debris is concentrated in the low areas that form flat depositsresembling ponds. Surrounding the low areas, the landscape is markedby an abundance of large boulders.

Seismic shaking is one of several hypotheses being considered as anexplanation for the boulder-laden topography. This shaking wouldmove the boulders upward. The larger materials rise to the top whilethe smaller materials settle to the bottom, which is similar to what hap-pens when a can of mixed nuts is shaken.

Vocabulary Definition

asteroid a.

b. c.

d. e. ??

??

?

Figure 20 This artist’s renditionshows NEAR Shoemaker touchingdown on the asteroid Eros.

660 Chapter 23

FOCUS

Section Objectives23.6 Identify the location within

our solar system where mostasteroids are found.

23.7 Describe the structure of a comet.

23.8 Explain the possible origins for a meteoroid.

Build Vocabulary Concept Map Have students make aconcept map for the vocabulary terms inthis section. The center of the mapshould be “Minor members of the solarsystem,” the terms asteroid, comet, andmeteoroid should branch off of this topic,and students should expand their mapwith the other vocabulary terms as theylearn about each term.

Reading Strategy a. small rocky bodyb. cometc. body made up of rocky and metallicmaterials held together by frozen gasesd. comae. glowing head of a cometf. meteoroidg. small solid particle that travelsthrough spaceh. meteori. meteoroid that enters Earth’satmosphere and burns upj. meteoritek. meteoroid that reaches Earth’s surface

L2

L2

Reading Focus

1

Section 23.4


AsteroidsAsteroids are small rocky bodies that orbit the sun. The largest, the dwarfplanet Ceres, is about 1000 kilometers in diameter, but more than a mil-lion are greater than 1 kilometer across. By definition, asteroids are largerthan 10 meters in diameter. Most asteroids lie in the asteroid beltbetween the orbits of Mars and Jupiter. They have orbital periods ofthree to six years. Some asteroids have very elongated orbits and travelvery near the sun, and a few larger ones regularly pass close to Earth andthe moon as shown in Figure 21. Many of the most recent impact craterson the moon and Earth were probably caused by collisions with aster-oids. Inevitably, future Earth–asteroid collisions will occur.

Many asteroids have irregular shapes, as shown in Figure 22.Because of this, planetary geologists first speculated that they might befragments of a broken planet that once orbited between Mars andJupiter. However, the total mass of the asteroids is estimated to be only1/1000 that of Earth, which itself is not a large planet. What happenedto the remainder of the original planet? Others have hypothesized thatseveral larger bodies once coexisted in close proximity, and their colli-sions produced numerous smaller ones. The existence of several familiesof asteroids has been used to support this explanation. However, noconclusive evidence has been found for either hypothesis.

CometsComets are among the most interesting and unpredictable bodies inthe solar system. Comets are pieces of rocky and metallic materialsheld together by frozen water, ammonia, methane, carbon dioxide, andcarbon monoxide. Many comets travel in very elongated orbits thatcarry them far beyond Pluto. These comets take hundreds of thou-sands of years to complete a single orbit around the sun. However, afew have orbital periods of less than 200 years and make regularencounters with the inner solar system.

What is an asteroid?

Mars

Asteroid belt

Jupiter

Earth

Figure 22 Asteroid 951, alsocalled Gaspra, is probably thefragment of a larger body thatwas torn apart by a collision.

Figure 21 The orbits of most asteroidslie between Mars and Jupiter. Alsoshown are the orbits of a few near-Earthasteroids. Perhaps a thousand or moreasteroids pass close to Earth. Luckily, onlya few dozen of these are thought to belarger than 1 kilometer in diameter.

INSTRUCT

Asteroids:Microplanets Use Visuals Figure 21 This image shows the orbitalpaths of several asteroids in our solarsystem. Have students look carefully atthe figure and read the caption. Ask:Where are most asteroids found?(between Mars and Jupiter, in the asteroidbelt) Are most of the asteroids nearEarth found in the asteroid belt? (no)What is the shape of all of theasteroid orbits? (elliptical)Visual

Comets Build Reading Literacy Refer to p. 306D in Chapter 11, whichprovides guidelines for this readingstrategy.

KWL Create a KWL chart with studentsbefore reading the section on comets.Students will probably already knowthat comets are “dirty snowballs,” andwill be able to name at least one comet.What students want to know will vary.After teaching the section on comets,have students complete the chart onwhat they learned. Clarify learning asneeded. Verbal

L1

L1

2



Put beginning English language learners ingroups with native English speakers of averageto below-average ability. These groups willallow the conversations to be at appropriatelevels. Have students work together over a few

class periods to complete their concept mapsfor this section’s vocabulary words. Once ELLstudents reach the intermediate level, they canwork with average to above-average students.

Answer to . . .

An asteroid is a small,irregularly shaped solid

body in space.

662 Chapter 23

Coma When first observed, a comet appears very small. But as itapproaches the sun, solar energy begins to vaporize the frozen gases. Thisproduces a glowing head called the coma, shown in Figure 23. Asmall glowing nucleus with a diameter of only a few kilometers cansometimes be detected within a coma. As comets approach the sun,some, but not all, develop a tail that extends for millions of kilometers.

The fact that the tail of a comet points away from the sun in a slightlycurved manner led early astronomers to propose that the sun has arepulsive force that pushes the particles of the coma away, thus formingthe tail. Today, two solar forces are known to contribute to this forma-tion. One, radiation pressure, pushes dust particles away from the coma.The second, known as solar wind, is responsible for moving the ionizedgases, particularly carbon monoxide.You’ll learn more about solar windin the next chapter. Sometimes a single tail composed of both dust andionized gases is produced, but often two tails are observed.

As a comet moves away from the sun, the gases forming the comarecondense, the tail disappears, and the comet returns to cold storage.Material that was blown from the coma to form the tail is lost fromthe comet forever. Therefore it is believed that most comets cannotsurvive more than a few hundred close orbits of the sun. Once all thegases are expelled, the remaining material—a swarm of tiny metallicand stony particles—continues the orbit without a coma or a tail.

Kuiper Belt Comets apparently originate in two regions of theouter solar system. Those with short orbital periods are thought to orbitbeyond Neptune in a region called the Kuiper belt. Like the asteroids inthe inner solar system, most Kuiper belt comets move in nearly circu-lar orbits that lie roughly in the same plane as the planets. A chancecollision between two Kuiper belt comets, or the gravitational influenceof one of the Jovian planets, may occasionally alter the orbit of a cometenough to send it to the inner solar system, and into our view.

In which direction does the tail of a comet point?

Tail of ionized gases

Coma

NucleusTail composed of dust

Enlarged view

Orbit Sun

Figure 23 A comet’s tail alwayspoints away from the sun.

662 Chapter 23

Modeling a Comet’s Tail Purpose The demonstration will helpstudents understand why a comet’s tailalways points away from the sun.

Materials tabletop fan, light-weightpaper, tape

Procedure Have students create simplemodels of comets using a ball of paperor an actual ball as the coma with light-weight paper streamers as the tail. Turnon the fan, and have students hold theirmodels in the breeze. The streamers willalways point away from the fan’s breeze.

Expected Outcome Students will seehow radiation pressure and solar windwill cause a comet’s tail to always pointaway from the sun.Visual, Kinesthetic

L2


Many of the scientists who classified Pluto as adwarf planet consider it a Kuiper belt object.The highly eccentric orbit of Pluto, its size, and

its icy and rocky composition, give it thecharacteristics of many other objects orbitingthe sun in the Kuiper belt.

Facts and Figures


Oort Cloud Unlike Kuiper belt comets, comets with long orbitalperiods aren’t confined to the plane of the solar system. These cometsappear to be distributed in all directions from the sun, forming aspherical shell around the solar system called the Oort cloud. The grav-itational effect of another object in space is thought to send anoccasional Oort cloud comet into a highly eccentric orbit that carriesit toward the sun. However, only a tiny portion of the Oort cloudcomets pass into the inner solar system.

Halley’s Comet The most famous short-period comet isHalley’s comet, shown in Figure 24. Its orbital period averages76 years. When it passed near Earth in 1910, Halley’s comet haddeveloped a tail nearly 1.6 million kilometers long and was vis-ible during the daylight hours.

In March 1986, the European probe Giotto approached towithin 600 kilometers of the nucleus of Halley’s comet andobtained the first images of this elusive structure. We now knowthat the nucleus is potato-shaped, 16 kilometers by 8 kilometers.The surface is irregular and full of craterlike pits. Gases and dustthat vaporize from the nucleus to form the coma and tail appearto gush from parts of its surface as bright jets or streams.

MeteoroidsNearly everyone has seen a “shooting star.” This streak of light occurswhen a meteoroid enters Earth’s atmosphere. A meteoroid is a smallsolid particle that travels through space. Most meteoroids originatefrom any one of the following three sources: (1) interplanetary debristhat was not gravitationally swept up by the planets during the for-mation of the solar system, (2) material from the asteroid belt, or (3) the solid remains of comets that once traveled near Earth’s orbit.A few meteoroids are believed to be fragments of the moon, or possiblyMars, that were ejected when an asteroid impacted these bodies.

Some meteoroids are as large as asteroids. Most, however, are thesize of sand grains. Consequently, they vaporize before reaching Earth’ssurface. Meteoroids that enter Earth’s atmosphere and burn up arecalled meteors. The light that we see is caused by friction between theparticle and the air, which produces heat.

Occasionally, meteor sightings can reach 60 or more per hour.These displays, called meteor showers, result when Earth encounters aswarm of meteoroids traveling in the same direction and at nearly thesame speed as Earth. As shown in Table 2, some meteor showers areclosely associated with the orbits of some comets, strongly suggestingthat they are material lost by these comets. The Perseid meteor shower,which occurs each year around August 12, may be the remains ofComet 1862 III.

Figure 24 Halley’s Comet willreturn to the inner solar systemin 2061.

For: Links on comets andmeteor showers

Visit: PHSchool.comWeb Code: czd-7234

MeteoroidsBuild Reading Literacy Refer to p. 612D in Chapter 22, whichprovides the guidelines for this readingstrategy.

Think Aloud The first paragraph of theMeteoroids section is very important,but it contains many words that mayconfuse students. Read this paragraphout loud, and stop at the end of eachsentence to ask yourself (and the class) aquestion to check for comprehension.For example, after the first sentence youcan ask, “Have I ever seen a shootingstar?” The third sentence is completelyin bold, meaning it is really important soyou may want to ask multiple questionsabout that one, such as “What isinterplanetary debris? Where’s theasteroid belt? What would the remainsof a comet look like?” If your studentsare not able to answer your questions,encourage them to go back in thechapter to find the answers. Forexample, students may need to rereadthe section on comets to figure outwhat the remains of a comet are. Verbal, Intrapersonal

L1


Scientists have hypothesized that comets maycontain organic material, and the Giotto probeobserved what appeared to be organic materialon the surface of Halley’s comet. Since comets

occasionally come close the Earth, or evencrash into Earth’s surface, some people believethat life on Earth originated from organicmaterial carried to our planet by a comet.

Facts and Figures

Answer to . . .

away from the sun

Find links to additional activities andhave students monitor phenomenathat affects Earth and its residents.

664 Chapter 23


Reviewing Concepts1. Where are most asteroids located?

2. Describe the structure of a comet.

3. Where do short-period comets come from?What about long-period comets?

4. Meteoroids originate from what threesources?

Critical Thinking5. Comparing and Contrasting Compare

and contrast a meteoroid, meteor, andmeteorite.

6. Predicting What do you think would happenif Earth passed through the tail of a comet?

7. It has been estimated that Halley’scomet has a mass of 1 � 1011 metrictons. This comet is estimated to lose 1 � 108 metric tons of material eachtime its orbit brings it close to the sun.With an orbital period of 76 years,what is the maximum remaining lifespan of Halley’s comet?

A meteoroid that actually reaches Earth’s surface iscalled a meteorite. A few very large meteorites have blastedout craters on Earth’s surface, similar to those on themoon. The most famous is Meteor Crater in Arizona.Prior to moon rocks brought back by astronauts, meteoritessuch as the one in Figure 25 were the only extraterrestrialmaterials that could be directly examined.

Meteorites and the Age of the Solar SystemHow did scientists determine the age of the solar system? Theyused evidence from meteorites, moon rocks, and Earth rocks.Radiometric dating of meteorites found on Earth shows thatthe oldest meteorites formed more than 4.57 billion years ago.These meteorites are the oldest-known materials in the solarsystem. Some are made mostly of iron. Others, called stonymeteorites, contain silicates. Scientists think that the compo-sition of meteorites is similar to the composition of othermaterials in the inner solar system during its formation.

Moon rocks from the lunar highlands have a composition similar tothat of stony meteorites. These moon rocks date to about 4.5 billionyears ago, almost as old as the oldest meteorites. From these facts, sci-entists infer that the moon must be just slightly younger than theformation of the solar system, which occured 4.567 billion years ago.

The ages of the oldest known Earth rocks are consistent with thisconclusion. Scientists have dated rocks found in northwestern Canadaat about 4 billion years old. These are the oldest rocks found on Earthso far. In addition, some tiny crystals of the mineral zircon found insedimentary rocks in Australia are 4.4 billion years old.

Table 2 Major Meteor Showers

Shower Approximate AssociatedDates Each CometYear

Quadrantids Jan. 4–6

Lyrids Apr. 20–23 Comet 1861 I

Eta Aquarids May 3–5 Halley’s comet

Delta Aquarids July 30

Perseids Aug. 12 Comet 1862 III

Draconids Oct. 7–10 CometGiacobini-Zinner

Orionids Oct. 20 Halley’s comet

Taurids Nov. 3–13 Comet Encke

Andromedids Nov. 14 Comet Biela

Leonids Nov. 18 Comet 1866 I

Geminids Dec. 4–16

Figure 25 This meteorite is madeup mostly of iron.

664 Chapter 23

Build Reading Literacy Refer to p. 474D in Chapter 17, whichprovides the guidelines for this readingstrategy.

Monitor Your UnderstandingAfter students have read the paragraphson meteoroids, advise students to makesure they understand the differencebetween meteoroids, meteors, andmeteorites. Also, have them verify that they recognize the relationshipbetween meteor showers and comets.Recommend to students that they rereadthe passage if they did not understandthese ideas.Verbal, Intrapersonal

ASSESSEvaluate UnderstandingCheck for understanding by putting thestudents in groups and having thegroups write an answer for each KeyConcept question in the chapter.

ReteachReview the content in this chapter witha series of Venn diagrams or conceptmaps.

Solution7. 1 � 1011 tons of mass � 1 � 108 tonsof mass lost/orbit � 1 � 103 or 1000orbits remaining; 76 years/orbital period� 1,000 orbits � 76,000 years of liferemaining

L1

L2

3

L1


4. Most meteoroids originate from: (1) inter-planetary debris that was not gravitationallyswept up by the planets during the formationof the solar system, (2) material from theasteroid belt, or (3) the solid remains ofcomets that once traveled near Earth’s orbit.5. Meteoroids are small solid particles travelingthrough space. Meteors are meteoroids thatenter Earth’s atmosphere and burn up.Meteorites are meteoroids that strike Earth’ssurface. 6. Sample answer: There would be a hugemeteor shower.


1. Most asteroids lie between the orbits ofMars and Jupiter.2. A comet is made up of frozen gases andpieces of rocky and metallic materials. As thecomet approaches the sun, vaporizing gasesproduce a glowing head called a coma. Withinthe coma, a small glowing nucleus is some-times present. Most comets have long tails.3. Short-period comets come from the Kuiperbelt. Long-period comets come from the Oortcloud.


Is Earth on aCollision Course?

Ancient CollisionsDuring the last few decades, it has become increas-ingly clear that comets and asteroids have collidedwith Earth far more frequently than was previouslyknown. The evidence for these collisions is giantimpact structures. See Figure 26. More than 100 impact structures have been identified as shownon the map in Figure 27. Most are so old that theyno longer resemble impact craters. However, evi-dence of their intense impact remains. One notableexception is a very fresh-looking crater nearWinslow, Arizona, known as Meteor Crater.

Evidence is mounting that about 65 million yearsago a large asteroid about 10 kilometers in diame-ter collided with Earth. This impact may have causedthe extinction of the dinosaurs, as well as nearly 50 percent of all plant and animal species.

Close CallsMore recently, a spectacular explosion has beenlinked to the collision of our planet with a comet orasteroid. In 1908, in a remote region of Siberia, a“fireball” that appeared more brilliant than the sunexploded with a violent force. The shock waves rat-tled windows and triggered reverberations heard upto 1000 kilometers away. The “Tunguska event,” asit is called, scorched, de-limbed, and flattened treesup to 30 kilometers from the epicenter. However,expeditions to the area did not find any evidence ofan impact crater or metallic fragments. It is believedthat the explosion—which equaled at least a 10-megaton nuclear bomb—occurred a few kilo-meters above the surface. It was most likely the endof a comet or perhaps a stony asteroid. The reasonit exploded prior to impact remains unclear.

A reminder of the dangers of living with these smallbut deadly objects from space came in 1989 whenan asteroid—nearly 1 kilometer across—shot pastEarth. The asteroid came close to Earth, passing itby only twice the distance to the moon. It traveledat a speed of 70,000 kilometers per hour, and itcould have made an impact crater 10 kilometers indiameter and perhaps 2 kilometers deep.

The solar system is cluttered with meteoroids, aster-oids, active comets, and extinct comets. These

fragments travel at great speeds and can strike Earthwith the explosive force of a powerful nuclear weapon.

Impact structures

Figure 26Manicouagan,Quebec, is a 200-million-year-olderoded impactstructure. The lakeoutlines the craterremnant.

Figure 27 Major Impact Structures

Is Earth on a Collision Course? BackgroundThe solar system is cluttered withmeteoroids, asteroids, active comets,and extinct comets. These fragmentstravel at great speeds and can strikeEarth with the explosive force of apowerful nuclear weapon.

Teaching TipTell students to imagine they were close enough to the Tunguska event to see it. Have them write what theyexperienced—saw, heard, or smelled—during the event, how they felt, andwhat they thought it was. This can bewritten in the format of a newspaperarticle or diary entry. Verbal

L2


Date post:	31-Mar-2018
Category:	Documents
Upload:	hanhan
View:	222 times
Download:	1 times

Section 23.1 23.1 The Solar System -...

Documents