23.2 the terrestrial planets section 23 -...

Touring Our Solar System 649

23.2 The Terrestrial Planets

Key ConceptsWhat are thedistinguishingcharacteristics of each terrestrial planet?

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

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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

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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

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Section 23.2 (continued)

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

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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

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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

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Section 23.2 (continued)

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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

Section 23.2 Assessment

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.

ASSESS

Evaluate 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.

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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.

Section 23.2 Assessment

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

23.2 the terrestrial planets section 23 -...

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