630 Chapter 22

22.3 Earth’s Moon

Reading StrategySequencing Copy Copy the flowchartbelow. As you read, fill in the stages leadingto the formation of the moon.

Key ConceptsWhat processes createsurface features on themoon?

How did the moon form?

Vocabulary◆ crater◆ ray◆ mare◆ rille◆ lunar regolith

E arth now has hundreds of satellites. Only one natural satellite, themoon, accompanies us on our annual journey around the sun. Otherplanets have moons. But our planet-satellite system is unusual in thesolar system, because Earth’s moon is unusually large compared to itsparent planet. The diameter of the moon is 3475 kilometers, aboutone-fourth of Earth’s 12,756 kilometers.

Much of what we know about themoon, shown in Figure 18, comes fromdata gathered by the Apollo moon mis-sions. Six Apollo spacecraft landed on themoon between 1969 and 1972. Uncrewedspacecraft such as the Lunar Prospectorhave also explored the moon’s surface.From calculation of the moon’s mass, weknow that its density is 3.3 times that ofwater. This density is comparable to that ofmantle rocks on Earth. But it is consider-ably less than Earth’s average density,which is 5.5 times that of water. Geologistshave suggested that this difference can beaccounted for if the moon’s iron core issmall. The gravitational attraction at thelunar surface is one-sixth of that experi-enced on Earth’s surface. (A 150-poundperson on Earth weighs only 25 pounds onthe moon). This difference allows an astro-naut to carry a heavy life-support systemeasily. An astronaut on the moon couldjump six times higher than on Earth.

Figure 18 This is what themoon’s surface looks like fromEarth when viewed through atelescope.

c. ?b. ?a. ?Mars-size bodyimpacted Earth.

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FOCUS

Section Objectives22.7 Describe how the physical

features of the lunar surfacewere created.

22.8 Explain the history of themoon.

Build VocabularyVocabulary Rating Chart Havestudents construct a chart with fourcolumns labeled Term, Can Define orUse It, Heard or Seen It, and Don’tKnow. Have students copy the termscrater, ray, mare, rille, and lunar regolithinto the first column and rate their termknowledge by putting a check in one ofthe other columns. Ask how manystudents actually know each term. Havethem share their knowledge. Askfocused questions to help studentspredict text content based on the term,thus enabling them to have a purposefor reading. After students have read thesection, have them rate their knowledgeagain.

Reading Strategya. Huge quantities of crust and mantlewere ejected into space.b. The debris began orbiting Earth.c. Debris united to form the moon.

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The Lunar SurfaceWhen Galileo first pointed his telescope toward themoon, he saw two different types of landscape—dark lowlands and bright highlands. Because thedark regions resembled seas on Earth, they werelater named maria, which comes from the Latinword for sea. Today we know that the moon has noatmosphere or water. Therefore, the moon doesn’thave the weathering and erosion that continuallychange Earth’s surface. Also, tectonic forces aren’tactive on the moon, therefore volcanic eruptions nolonger occur. However, because the moon is unpro-tected by an atmosphere, a different kind of erosionoccurs. Tiny particles from space continually bom-bard its surface and gradually smooth out thelandscape. Moon rocks become slightly rounded ontop after a long time at the lunar surface. Even so, itis unlikely that the moon has changed very much inthe last 3 billion years, except for a few craters.

Craters The most obvious features of the lunarsurface are craters, which are round depressions inthe surface of the moon. There are many craters onthe moon. The moon even has craters withincraters! The larger craters are about 250 kilometersin diameter, about the width of Indiana. Mostcraters were produced by the impact of rapidlymoving debris.

By contrast, Earth has only about a dozen easilyrecognized impact craters. Friction with Earth’satmosphere burns up small debris before it reachesthe ground. Evidence for most of the craters thatformed in Earth’s history has been destroyed by ero-sion or tectonic processes.

The formation of an impact crater is modeled inFigure 19. Upon impact, the colliding object com-presses the material it strikes. This process is similarto the splash that occurs when a rock is dropped intowater. A central peak forms after the impact.

Most of the ejected material lands near thecrater, building a rim around it. The heat generatedby the impact is enough to melt rock. Astronautshave brought back samples of glass and rock formedwhen fragments and dust were welded together bythe impact.

Meteorite impact

Force ofcompression

Rebound

Ejecta

Figure 19 The energy of the rapidly movingmeteoroid is transformed into heat energy. Rockcompresses, then quickly rebounds. The reboundingrock causes debris to be ejected from the crater.

Formation of a CraterINSTRUCT

The Lunar SurfaceBuild Reading LiteracyRefer to p. 306D in Chapter 11, whichprovides the guidelines for KWL(Know/Want to Know/Learned) charts.

KWL Teach this independent study skillas a whole-class exercise.1. Draw a three-column KWL chart onthe board for students to copy.2. Have students complete the Knowcolumn with facts, examples, and otherinformation that they already knowabout the lunar surface.3. Tell students to complete the Want toKnow column with questions about themoon’s surface.4. Have students read pp. 631 and 632about the moon. As they read, havethem note answers to their questions inthe Learned column, along with facts,examples, and details they learned.5. Have students draw an InformationI Expect to Use box below their KWLchart. Have them review the informationin the Learned column and categorizethe useful information in the box.Verbal

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Customize for Inclusion Students

Visually Impaired Have students with visualimpairments feel the topography of the moon,using a relief globe or map of the moon.Assign visually-impaired students a partner to

guide their fingers and to read the map orglobe for them. Discuss with students whatthey have discovered about the surface ofthe moon.

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A meteoroid only 3 meters in diameter canblast out a 150-meter-wide crater. A few of thelarge craters, such asthose named Kepler andCopernicus, formed fromthe impact of bodies 1 kilometer or more in

diameter. These two largecraters are thought to be rela-

tively young because of the brightrays, or splash marks that radiate

outward for hundreds of kilometers.

Highlands Most of the lunar surface is made up of densely pitted,light-colored areas known as highlands. In fact, highlands cover thesurface of the far side of the moon. The same side of the moon alwaysfaces Earth. Within the highland regions are mountain ranges. Thehighest lunar peaks reach elevations of almost 8 kilometers. This isonly 1 kilometer lower than Mount Everest. Figure 20 shows highlandsand other features of the moon.

Maria The dark, relatively smooth area on the moon’s surface iscalled a mare (plural: maria). Maria, ancient beds of basaltic lava,originated when asteroids punctured the lunar surface, lettingmagma bleed out. Apparently the craters were flooded with layer uponlayer of very fluid basaltic lava somewhat resembling the ColumbiaPlateau in the northwestern United States. The lava flows are often over30 meters thick. The total thickness of the material that fills the mariacould reach thousands of meters.

Long channels called rilles are associated with maria. Rilles looksomewhat similar to valleys or trenches. Rilles may be the remnants ofancient lava flows.

Regolith All lunar terrains are mantled with a layer of gray debrisderived from a few billion years of bombardment from meteorites. Thissoil-like layer, called lunar regolith, is composed of igneous rocks, glassbeads, and fine lunar dust. In the maria that have been explored by Apolloastronauts, the lunar regolith is just over 3 meters thick.

What is lunar regolith?

Figure 20 Major topographicfeatures on the moon’s surfaceinclude craters, maria, andhighlands. Identifying Where are rilleslocated?

Highlands

Ejecta

Lavaflows

Large impactcrater

Rille

Rille

Old, lava-floodedimpact crater

Mare

Youthfulcrater

Youthful craterwith rays

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Build Science Skills

Using ModelsPurpose Students willmodel the surfacefeatures of the lunarsurface.

Materials foam poster board; softmodeling clay; toothpicks; narrow paperstrips; tape; pencil, pen, or marker

Class Time 30 minutes

Procedure Have students use Figure 20as a guide for their topographic map.First, instruct students to draw thephysical features on the foam posterboard. Next, have students use thesoft modeling clay to build the surfacefeatures. Then students should createflag-type labels for the physical features,using narrow strips of paper, toothpicks,and tape. Finally, have students use themarkers to identify the physical featureson the surface of their topographicalmap.

Expected Outcome Students learn themajor topographical surface features onthe lunar surface.Visual, Kinesthetic

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Origin of Modern Astronomy 633

Lunar HistoryThe moon is our nearest planetary neighbor. Although astronauts havewalked on its surface, much is still unknown about its origin. Themost widely accepted model for the origin of the moon is that whenthe solar system was forming, a body the size of Mars impactedEarth. The impact, shown in Figure 21, would have liquefied Earth’ssurface and ejected huge quantities of crustal and mantle rock froman infant Earth. A portion of this ejected debris would have enteredan orbit around Earth where it combined to form the moon.

The giant-impact hypothesis is consistent with other facts knownabout the moon. The ejected material would have been mostly iron-poormantle and crustal rocks. These would account for the lack of a sizableiron core on the moon. The ejected material would have remained inorbit long enough to have lost the water that the moon lacks. Despite thissupporting evidence, some questions remain unanswered.

Geologists have worked out the basic details of the moon’s laterhistory. One of their methods is to observe variations in crater density(the number of craters per unit area). The greater the crater density, theolder the surface must be. From such evidence, scientists concludedthat the moon evolved in three phases—the original crust (highlands),maria basins, and rayed craters.

During its early history, the moon was continually impacted as itswept up debris. This continuous attack, combined with radioactivedecay, generated enough heat to melt the moon’s outer shell and pos-sibly the interior as well. Remnants of this original crust occupy thedensely cratered highlands. These highlands have been estimated to beas much as 4.5 billion years old, about the same age as Earth.

Figure 21 The moon may haveformed when a large objectcollided with Earth. The resultingdebris was ejected into space. Thedebris began orbiting aroundEarth and eventually united toform the moon.

Diagram NOT drawn to scale

Lunar HistoryIntegrateLanguage ArtsMuch of the scientific information that isknown about the composition of thelunar surface is due to the Apollo SpaceProgram. Have students research thescientific discoveries that were made asa result of this program. Students shouldprepare a presentation about theinformation they have learned. Ifpossible, encourage students to makea computer presentation that includesactual photographs from the variousmissions.Verbal, Portfolio

Use VisualsFigure 21 Have students study thisfigure. Ask: What evidence suggeststhat the moon came from mostly iron-poor mantle and crustal rocks fromEarth? (The lack of a sizeable iron core onthe moon) Why doesn’t this ejectedmaterial contain water? (The materialhas been in orbit long enough to have lostany water that it may have had.)Verbal

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The United States was not the only countrywith a space program. The former Union ofSoviet Socialist Republics also had one. In fact,the U.S.S.R. was the first country to launch anartificial Earth satellite, Sputnik 1, in 1957.In early 1959, the Soviet spacecraft, Luna 1,became the first artificial object to orbit thesun. Later in 1959, Luna 2 crashed into the

moon to become the first artificial object onthe lunar surface. Also in 1959, Luna 3 tookthe first pictures of the moon’s far side. TheSoviet goal to send the first crewed spacecraftto the moon was never realized. In 1969, theUnited States became the first country thatsafely landed a person on the moon.

Facts and Figures

Answer to . . .

Figure 20 Rilles are associatedwith maria.

a soil-like layer ofigneous rocks, glass

beads, and fine lunar dust

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Section 22.3 Assessment

One important event in the moon’s evolu-tion was the formation of maria basins.Radiometric dating of the maria basalts putstheir age between 3.2 billion and 3.8 billionyears, about a billion years younger than theinitial crust. In places, the lava flows overlapthe highlands, which also explains the youngerage of the maria deposits.

The last prominent features to form werethe rayed craters. Material ejected from theseyoung depressions is clearly seen covering thesurface of the maria and many older raylesscraters. Even a relatively young crater likeCopernicus, shown in Figure 22, must be mil-lions of years old. If it had formed on Earth,erosional forces would have erased it long ago.If photographs of the moon taken several hun-dreds of millions of years ago were available,they would show that the moon has changedlittle. The moon is an inactive body wanderingthrough space and time.

5. Inferring Why are craters more common onthe moon than on Earth, even though themoon is a much smaller target?

Reviewing Concepts1. How do craters form?

2. How did maria originate?

3. What are the stages that formed themoon.

Critical Thinking4. Identifying On Earth, the four major spheres

(atmosphere, hydrosphere, solid Earth, andbiosphere) interact as a system. Which ofthese spheres are absent, or nearly absent, onthe moon? Based on your answer, identify atleast five processes that operate on Earth butnot on the moon.

Scientific Evidence Write a paragraphexplaining what evidence scientists use toreconstruct the history of the moon.

Figure 22 Rayed craters such asCopernicus were the last majorfeatures to form on the moon.

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Section 22.3 (continued)

4. Of the four spheres, the atmosphere,hydrosphere, and biosphere are absent, ornearly absent, on the moon. Because themoon lacks these spheres, processes such aschemical weathering, erosion, soil formation,weather in general, and sedimentation are allabsent.5. Erosion and subduction have removedmost craters from Earth’s surface.

Section 22.3 Assessment

1. Craters form from the impact of rapidlymoving debris.2. when asteroids punctuated the lunarsurface, letting magma bleed out3. A Mars-sized object collided with Earth.Huge quantities of crust and mantle wereejected into space. The debris began orbitingEarth and eventually united to form themoon.

ASSESSEvaluateUnderstandingHave students make flashcards onthe details found in this section. Tellstudents to write a question on one sideof the card and the answer on the other.Students can use these flashcards to quizeach other in class.

ReteachWrite the headings Lunar Surface andLunar History on the board. Ask studentsto give you facts from the section abouteach topic. Write the information underthe appropriate heading. Students cancopy this information from the boardand use it as a study aid.

Scientists observe variations in craterdensity. They also analyze materialgathered from space missions.

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Foucault’sExperiment

Earth rotates on its axis once each day to produceperiods of daylight and darkness. However, day andnight and the apparent motions of the stars can beaccounted for equally well by a sun and celestialsphere that revolve around a stationary Earth.

Copernicus realized that a rotating Earth greatly sim-plified the existing model of the universe. He wasunable, however, to prove that Earth rotates. Thefirst real proof was presented 300 years after hisdeath by the French physicist Jean Foucault.

The Swinging PendulumIn 1851, Foucault used a free-swinging pendulumto demonstrate that Earth does, in fact, turn on itsaxis. To picture Foucault’s experiment, imagine alarge pendulum swinging over the North Pole, asshown in the illustration on this page. Keep in mindthat once a pendulum is put into motion, it contin-ues swinging in the same plane unless acted uponby some outside force. Assume that a sharp point isattached to the bottom of this pendulum, markingthe snow as it swings. If we were to observe themarks made by the point, we would see that thependulum is slowly but continually changing posi-tion. At the end of 24 hours, the pendulum wouldhave returned to its starting position.

Evidence of Earth’s RotationNo outside force acted on the pendulum to changeits position. So what we observed must have beenEarth rotating beneath the pendulum. Foucault con-ducted a similar experiment when he suspended along pendulum from the dome of the Pantheon inParis. Today, Foucault pendulums can be found insome museums to re-create this famous scientificexperiment.

Apparent motion of pendulum

Rotation of Earth

Foucault’sExperimentBackgroundThe Foucault pendulum was constructedin 1851 by Jean Bernard Foucault andwas demonstrated for the first time atthe world’s fair in Paris. The pendulumconsisted of a 28-kg iron ball suspendedfrom the dome of the Pantheon by a67-m long steel wire.

Teaching TipsAfter students have read p. 635, ask:Once the pendulum is set in motion,it will continue to swing in the samedirection unless it is pushed or pulledby a force. Why is this true? (Thependulum’s motion will obey Newton’sfirst law of motion.)

What outside force is acting on thependulum? (There is no outside forceacting on the pendulum.)

Why does the pendulum changeposition over time? (The pendulum didnot change position, Earth moved beneaththe pendulum.)

How does this prove that Earth isrotating? (The pendulum continuedmoving in the same direction becauseno outside force acted upon it. Since thependulum did not change its motion,Earth beneath it must be moving.)Verbal

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