Sunshine StateSTANDARDSSC.E.1.3.1: The studentunderstands the vastsize of our SolarSystem and the rela-tionship of the planetsand their satellites.SC.E.1.3.2: The studentknows that availabledata from various satel-lite probes show thesimilarities and differ-ences among planetsand their moons in theSolar System.SC.H.1.3.2: The studentknows that the studyof the events that ledscientists to discoveriescan provide informa-tion about the inquiryprocess and its effects.

Chapter 21: Our Solar System 749

VOCABULARYterrestrial planet p. 749tectonics p. 750volcanism p. 750

BEFORE, you learned

• Planets are closer together inthe inner solar system than inthe outer solar system

• Planets formed along with the Sun

• Gravity made planets round

NOW, you will learn

• How four processes change thesurfaces of solid planets

• How atmospheres form andthen affect planets

• What the planets closest to the Sun are like

KEY CONCEPT

The inner solar systemhas rocky planets.

The terrestrial planets have rocky crusts.Scientists study Earth to learn about other planets. They also studyother planets to learn more about Earth. The areMercury, Venus, Earth, and Mars—the four planets closest to the Sun.They all have rocky crusts and dense mantles and cores. Their insides,

surfaces, and atmospheres formed in similar ways and follow similarpatterns. One planet—Earth—can be used as a model to under-

stand the others. In fact, the term terrestrial comes from terra,the Latin word for Earth.

terrestrial planets

EXPLORE Surfaces

How does a planet’s mantle affect its surface?

PROCEDURE

Dampen a paper towel and place it on top of two blocks to model a crust and a mantle.

Move one block. Try different amounts of motion and different directions.

WHAT DO YOU THINK?• What happened to the paper towel?• What landforms like this have you seen?

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MATERIALS• 2 blocks• paper towel• newspaper

Earth

Mass 6 � 1024 kgDiameter 12,800 kmAverage distance from Sun 1 AU

Orbits in 365 daysRotates in 24 hours

Most of Earth’s rocky surface is hidden by water. More details about Earthand other planets are listed in the Appendix at the back of this book.

750 Unit 6: Space Science

Processes and Surface FeaturesAll terrestrial planets have layers. Each planet gained energy from thecollisions that formed it. This energy heated and melted the planet’smaterials. The heaviest materials were metals, which sank to the centerand formed a core. Lighter rock formed a mantle around the core.The lightest rock rose to the surface and cooled into a crust.

Four types of processes then shaped each planet’s rocky crust.The processes acted to different extents on each planet, depending on how much the crust and inside of the planet cooled.

Tectonics Earth’s crust is split into large pieces called tectonic plates.These plates are moved by Earth’s hot mantle. Mountains, valleys,and other features form as the plates move together, apart, or alongeach other. The crusts of other terrestrial planets are not split intoplates but can be twisted, wrinkled up, or stretched out by themantle. is the processes of change in a crust due to themotion of hot material underneath. As a planet cools, the crustgets stiffer and the mantle may stop moving, so this process stops.

Volcanism A second process, called occurs whenmolten rock moves from a planet’s hot interior onto its surface.The molten rock is called lava when it reaches the surface throughan opening called a volcano. On Earth, lava often builds up intomountains. Volcanoes are found on Earth, Venus, and Mars.Lava can also flow onto large areas and cool into flat plains like the lunar maria. When the inside of a planet cools enough, nomore molten rock reaches the surface.

Weathering and Erosion You have read about weathering onEarth and the Moon. Weather or small impacts break down rocks.The broken material is moved by a group of processes called erosion.The material may form dunes, new layers of rock, or other features.On Earth, water is important for weathering and erosion. However,similar things happen even without water. Wind can carry sandgrains that batter at rocks and form new features. Even on a planetwithout air, rock breaks down from being heated in the daylightand cooled at night. The material is pulled downhill by gravity.

Impact Cratering A small object sometimes hits a planet’s surfaceso fast that it causes an explosion. The resulting impact crater isoften ten times larger than the object that produced it. On Earth,most craters have been erased by other processes. Impact cratersare easier to find on other planets. If a planet or part of a planet iscompletely covered with impact craters, then the other processeshave not changed the surface much in billions of years.

check your reading What processes affect the surfaces of terrestrial planets?

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volcanism,2

Tectonics

1reading tip

Compare what you readabout each type of featurewith the pictures and diagrams on page 751.

RESOURCE CENTERCLASSZONE.COM

Find out more aboutimpact craters on Earthand other space objects.

The processes that shape features on a planet’s surface can be divided intofour types. The features can tell you different things about the planet.

Features of Rocky Planets

Chapter 21: Our Solar System 751

A small space object can hit a planet’s surfaceand leave a crater. Because the other processeson Mercury are weak,newer craters can be seenon a background of older,more eroded craters.

A hot mantle can move anddistort the crust above it.This system of mountains and valleys on Earthformed as the crust was stretched.

Hot, molten rock can flow or explode out onto the sur-face of a planet. At this volcano on Venus, lavaflowed onto the surface several times.

Volcanism2

Which two processes happen because of hot material beneath the surface?

Rock can be broken downand moved. In this region of Mars, material broken from a cliff wasmoved by erosion into new slopes and dunes.

Impact Cratering4

Tectonics1

Weathering and Erosion3

752 Unit 6: Space Science

How do the layers inside of planets form?In this model, the materials you use represent different rocks and metals thatmake up the solid planets.

PROCEDURE

Put pieces of gelatin into the container until it is about one-quarter full.

Mix in a spoonful each of sand and wax. Use the spoon to break the gelatininto small pieces as you mix. Remove the spoon.

Place the container in a bowl of hot tap water (about 70°C) and observe what happens as the gelatin melts.

WHAT DO YOU THINK?• What happened to each of the materials when the gelatin melted?

• How do the results resemble the core, mantle, and crust of Earth and other planets?

CHALLENGE How might you improve this model?

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LayersLayers

AtmospheresAtmospheres on terrestrial planets mainly formed from gases thatpoured out of volcanoes. If a planet’s gravity is strong enough, it pullsthe gases in and keeps them near the surface. If a planet’s gravity istoo weak, the gases expand into outer space and are lost.

Venus, Earth, and Mars each had gravity strong enough to holdheavy gases such as carbon dioxide. However, the lightest gases—hydrogen and helium—escaped into outer space. The atmospheres of Venus and Mars are mostly carbon dioxide.

An atmosphere can move energy from warmer places to coolerplaces. This movement of heat energy makes temperatures more uniform between a planet’s day side and its night side and between its equator and its poles. An atmosphere can also make a planet’swhole surface warmer by slowing the loss of energy from the surface.

After Earth formed, its atmosphere of carbon dioxide kept the surface warm enough for water to be liquid. Oceans covered most ofEarth’s surface. The oceans changed the gases of the atmosphere, andliving organisms caused even more changes. Earth’s atmosphere isnow mostly nitrogen with some oxygen.

check your reading Why is the solid Earth surrounded by gases?

SKILL FOCUSUsing models

MATERIALS• container• spoon• firm gelatin• sand• wax pieces• bowl of hot

tap water

TIME40 minutes

reminder

Remember that Earth’satmosphere is like a blanketthat protects Earth, as youlearned in grade 6.

Content ReviewFLORIDA

Craters cover the surface of Mercury.

Mercury, like the Moon, has smooth plainsand many craters. The processes at workon Earth also affected Mercury.

Tectonics Long, high cliffs stretchacross Mercury’s surface. Scientiststhink that Mercury’s huge core of ironshrank when it cooled long ago.The crust wrinkled up, forming cliffs,as the planet got a little smaller.

Volcanism Parts of the surface were covered with lava long ago. Large, smoothplains formed. The plains are similar to lunar maria.

Weathering and Erosion Small impacts and tempera-ture changes have broken rock. Gravity has moved broken material downhill.

Impact Cratering Round features cover much of the surface. Thesecraters show that the other processes have not changed Mercury’s surface very much for a long time.

Mercury has the longest cycle of day and night of the terrestrialplanets—three months of daylight and three months of darkness.There is no atmosphere to move energy from the hot areas to the cold areas. In the long daytime, it can get hotter than 420°C (about800°F)—hot enough to melt lead. During the long, cold night, thetemperature can drop lower than –170°C (about –280°F).

Check Your Reading How is Mercury similar to the Moon?

Craters of all sizes coverMercury’s surface, butthere are also flat lavaplains and cliffs from long ago.

Chapter 21: Our Solar System 753

no data

cliff

craters

Mercury

This map of Mercury wasmade from many imagestaken by one spacecraft.The blank patches showareas that were notmapped by the spacecraft.

Mass 6% of Earth’s mass

Diameter 38% of Earth’s diameter

Average distance from Sun 0.39 AU

Orbits in 88 Earth daysRotates in 59 Earth days

Volcanoes shape the surface of Venus.The planet Venus is only a little smaller than Earth and orbits a littlecloser to the Sun. As a result, Venus is sometimes called Earth’s sisterplanet. However, Venus is different from Earth in important ways.

Venus takes about eight months to turn just once on its axis.Unlike most other planets, Venus rotates and orbits in opposite direc-tions. The rotation and orbit together produce very long days andnights—two months of daylight followed by two months of darkness.

The atmosphere of Venus is very dense. Air pressure on Venus is90 times that on Earth. Venus’s atmosphere is mostly carbon dioxide.This gas slows the loss of energy and makes the surface very hot.The ground temperature on Venus is about 470°C (about 870°F). Theatmosphere of Venus moves energy around so well that the long nightsare as hot as the days and the poles are as hot as the equator. In addi-tion, there are droplets of sulfuric acid, a corrosive chemical, in theatmosphere. These droplets form thick white clouds that completelycover the planet and hide the surface.

Like Mercury, Venus is affected by the same four types of processesthat change Earth’s surface. Scientists think that tectonics and volcanismmay still be changing Venus’s surface today.

Tectonics Patterns of cracks and cliffs have formed as movements ofthe hot mantle have stretched, wrinkled, and twisted the surface.

Volcanism Most of the surface of Venus has been cov-ered with lava in the last billion years or so. Volcanoesand flat lava plains are found all over the surface.

MAIN IDEA AND DETAILSWhen you see a new heading, remember to takenotes about important ideas.

754 Unit 6: Space Science

Thick clouds make itimpossible to see Venus’ssurface in visible light.This inset shows a map ofVenus that scientists madeusing radio waves.

weathered and eroded rock

spacecraft

Venus

Venus is nearly the size of Earth but has a thicker atmosphereand is much hotter than Earth. The surface is rocky, as you cansee in the image below.

Mass 82% of Earth’s massDiameter 95% of Earth’s diameterAverage distance from Sun 0.72 AU

Orbits in 225 Earth daysRotates in 243 Earth days

Weathering and Erosion Venus is too hot to have liquid water, andthe winds do not seem to move much material. Erosion may be sloweron Venus than on Earth.

Impact Cratering Round craters mark the surface here and there.Older craters have been erased by the other processes. Also, Venus’sthick atmosphere protects the surface from small impacts.

check your reading Why is Venus not covered with craters?

Erosion changes the appearance of Mars.Mars is relatively small, with a diameter about half that of Earth.The orange color of some of the surface comes from molecules of iron and oxygen—rust. Mars has two tiny moons. They were probably once asteroids that were pulled into orbit around Mars.

Surface of MarsThe same processes that affect the other terrestrial planets affect Mars.

Tectonics Valleys and raised areas formed on Mars as the mantlemoved. One huge system of valleys, called Valles Marineris,is long enough to stretch across the United States.

Volcanism Most of the northern hemisphere hassmooth plains of cooled lava. Several volcanoesare higher than any mountain on Earth.The lava must have built up in the same spotfor a long time, so scientists have inferredthat the crust of Mars has cooled more thanEarth’s crust. On Earth, the tectonic platesmove, so chains of smaller volcanoes forminstead of single larger volcanoes.

Weathering and Erosion Fast winds carrysand that breaks down rocks. Wind and gravitymove the broken material, forming new featuressuch as sand dunes. There are also landformsthat look like the results of gigantic flash floodsthat happened long ago.

Impact Cratering Round craters cover much of the southern hemisphere of Mars. Many craters are very old and eroded. A few impact craters on the volcanoes make scientiststhink that the volcanoes have not released lava for a long time.

Chapter 21: Our Solar System 755

Mars

volcanoes

Valles Marineris

The atmosphere of Mars isthin but causes weatheringand erosion.

Mass 11% of Earth’s mass

Diameter 53% of Earth’s diameter

Average distance from Sun 1.5 AU

Orbits in 1.9 Earth yearsRotates in 25 hours

756 Unit 6: Space Science

Gases and Water on MarsThe atmosphere of Mars is mostly carbon dioxide. The air pressure isonly about 1 percent of the air pressure on Earth. The gas is not denseenough to keep the surface warm or to move much energy from coldareas to warmer areas. Therefore, temperatures may reach almost 20°C (about 60°F) in the daytime and –90°C (–130°F) at night.The large differences in temperature produce fast winds. The windscause gigantic dust storms that sometimes cover most of the planet.

Like Earth, Mars has polar caps that grow in winter and shrink insummer. However, the changing polar caps of Mars are made mostly offrozen carbon dioxide—dry ice. The carbon dioxide of the atmospherecan also form clouds, fog, and frost on the ground.

There is no liquid water on the surface of Mars today. Any waterwould quickly evaporate or freeze. However, there were floods in thepast, and there is still frozen water in the ground and in one polar cap.Water is important for life and will also be needed to make rocket fuelif humans are ever to make trips to Mars and back.

check your reading In what ways is Mars different from Earth?

The sky of Mars is madered by dust that the windpicks up and carries tonew places.

KEY CONCEPTS1. What are the four types of

processes that shape planets’surfaces? For each, give oneexample of a feature that theprocess can produce.

2. How can an atmosphere affect the temperature of aplanet’s surface?

3. Which terrestrial planet has theoldest, least-changing surface?

CRITICAL THINKING4. Compare and Contrast

Make a chart with columns forthe four types of processes andfor an atmosphere. Fill out arow for each planet.

5. Apply If a planet had a sur-face with craters but no otherfeatures, what could you sayabout the inside of the planet?

CHALLENGE6. Infer Describe how a hot

mantle can affect a planet’satmosphere. Hint: Which ofthe four processes is involved?