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KEY CONCEPT The outer solar system has four giant planets. Sunshine State STANDARDS SC.E.1.3.1: The student understands the vast size of our Solar System and the relationship of the planets and their satellites. SC.E.1.3.2: The student knows that available data from various satellite probes show the similarities and differences among planets and their moons in the Solar System. VOCABULARY gas giant p. 758 ring p. 761 BEFORE, you learned NOW, you will learn • Planets formed along with the Sun • Vast distances separate planets • The gravity of a terrestrial planet may be strong enough to hold the heavier gases • About the four giant planets in the solar system • What the atmospheres of giant planets are like • About the rings of giant planets THINK ABOUT What is Jupiter like inside? Most of Jupiter’s huge mass is hidden below layers of clouds. Scientists learn about Jupiter by studying its gravity, its magnetic field, its motions, and its radiation. Scientists also use data from other space bodies to make models, from which they make predictions. Then they observe Jupiter to test their predictions. What might it be like under Jupiter’s clouds? The gas giants have very deep atmospheres. VOCABULARY Remember to draw a word triangle when you read a new term. You have already read about the four rocky planets in the inner solar system, close to the Sun. Beyond Mars stretches the outer solar system, where the four largest planets slowly orbit the Sun. The gas giants — Jupiter, Saturn, Uranus (YUR-uh-nuhs), and Neptune—are made mainly of hydrogen, helium, and other gases. When you think of gases, you probably think of Earth’s air, which is not very dense. However, the giant planets are so large and have such large amounts of these gases that they have a lot of mass. The huge gravitational force from such a large mass is enough to pull the gas particles close together and make the atmosphere very dense. Inside the giant planets, the gases become more dense than water. The outermost parts are less dense and more like Earth’s atmosphere. check your reading 758 Unit 6: Space Science Why are the gas giants dense inside? The atmosphere of a giant planet Interior of a Giant Planet is very deep. Imagine traveling into one. At first, the atmosphere is thin Jupiter and very cold. There may be a haze hydrogen—gas of gases. A little lower is a layer of and liquid clouds that reflect sunlight, just like hydrogen—liquid clouds on Earth. There are strong metal winds and other weather patterns. dense, hot core Lower down, it is warmer and there are layers of clouds of different materials. As you go farther, the atmosphere gradually becomes dense enough to call a liquid. It also gets thousands of degrees hotter as you get closer to the center of the planet. The materials around you become more and more dense until they are solid. Scientists think Jupiter that each of the four gas giants has a solid core, larger than Earth, Jupiter’s colorful stripes deep in its center. are produced by clouds at different levels in Jupiter’s deep atmosphere. Jupiter is a world of storms and clouds. Jupiter is the largest planet in the solar system. It is more than 10 times larger than Earth in diameter and more than 1200 times larger in volume. A jet plane that could circle Earth in about 2 days would take 23 days to circle Jupiter. If you could weigh the planets on a cosmic scale, all the other planets put together would weigh less than half as much as Jupiter. Mass 318 Earth masses Diameter 11 Earth diameters Average distance from Sun 5.2 AU Orbits in 12 Earth years Rotates in 9.9 hours Jupiter is more than five times farther from the Sun than Earth is. It moves more slowly through space than Earth and has a greater distance to travel in each orbit. Jupiter takes 12 Earth years to go once around the Sun. Even though it is big, Jupiter takes less than 10 hours to turn once on its axis. This fast rotation produces fast winds and stormy weather. Like Earth, Jupiter has bands of winds that blow eastward and westward, but Jupiter has many more bands than Earth does. Chapter 21: Our Solar System 759 Great Red Spot Stripes of cold clouds form along the bands. The clouds look white because they are made of crystals that reflect sunlight. The crystals in these high white clouds are frozen ammonia rather than frozen water, as on Earth. Between Jupiter’s white bands of clouds, you can see down to the next layer. The lower clouds are brown or red and made of different chemicals. Sometimes there are clear patches in the brown clouds, where the next layer of bluish clouds shows through. moon shadow This image shows one of Jupiter’s moons casting a shadow on Jupiter. If you were in that shadow, you would experience a solar eclipse. check your reading What are Jupiter’s white stripes? Storms can form between bands of winds that blow in opposite directions. Because Jupiter has no land to slow the storms, they can last for a long time. The largest of these storms is the Great Red Spot, which is twice as wide as Earth and at least 100 years old. Its clouds rise even higher than the white ammonia-ice clouds. Scientists are trying to find out which chemicals produce the spot’s reddish color. Saturn has large rings. reminder Density is the amount of mass in a given volume. An object of low density can still have a great total mass if it has a large volume. The sixth planet from the Sun is Saturn. Saturn is only a little smaller than Jupiter, but its mass is less than one-third that of Jupiter. Because there is less mass, the gravitational pull is weaker, so the gas particles can spread out more. As a result, Saturn has a much lower density than Jupiter. The storms and stripes of clouds form deeper in Saturn’s atmosphere than in Jupiter’s, so the details are harder to see. Saturn Saturn has an average density less than that of liquid water on Earth. The diameter of Saturn’s ring system is almost as great as the distance from Earth to the Moon. Mass 95 Earth masses Diameter 9 Earth diameters Average distance from Sun 9.5 AU 760 Unit 6: Space Science Orbits in 29 Earth years Rotates in 11 hours Saturn was the first planet known to have rings. A planetary ring is a wide, flat zone of small particles that orbit a planet. All four gas giants have rings around their equators. Saturn’s rings are made of chunks of water ice the size of a building or smaller. Larger chunks, considered to be tiny moons, orbit within the rings. Saturn’s main rings are very bright. The outermost ring is three times as wide as the planet, but it is usually too faint to see. Saturn’s rings have bright and dark stripes that change over time. gap between rings shadow You can use Saturn’s rings to see the planet’s seasons. Like Earth’s axis of rotation, Saturn’s axis is tilted. The angle is 27 degrees. When the image on this page was taken, sunlight shone more on the northern hemisphere, so the north side of the rings was bright. The shadow of the rings fell on the southern hemisphere. Winter started in Saturn’s northern hemisphere in May 2003 and will last more than seven Earth years. Saturn is almost ten times farther from the Sun than Earth is, so Saturn takes almost 30 Earth years to go around the Sun once. Sunlight shines from the upper right of this image. The rings cast shadows on Saturn‘s clouds. Giant Planets Why do Saturn’s rings seem to change size? SKILL FOCUS Observing PROCEDURE 1 Poke the stick through the plate and cut off the plate’s rim. Shape the clay onto both sides of the plate to make a model of a planet with rings. 2 Model Saturn’s orbit for your partner. Stand between your partner and the classroom clock. Point one end of the stick at the clock. Hold the model at the same height as your partner’s eyes. Have your partner watch the model with just one eye open. 3 Move one step counterclockwise around your partner and point the stick at the clock again. Make sure the model is as high as your partner’s eyes. Your partner may need to turn to see the model. MATERIALS • • • • ice-cream stick disposable plate scissors clay TIME 20 minutes 4 Continue taking steps around your partner and pointing the stick at the clock until you have moved the model all the way around your partner. 5 Switch roles with your partner and repeat steps 2, 3, and 4. WHAT DO YOU THINK? • How did your view of the rings change as the model planet changed position? • How many times per orbit do the rings seem to vanish? CHALLENGE How do Saturn’s axis and orbit compare with those of Earth? 761 Uranus and Neptune are extremely cold. The seventh and eighth planets from the Sun are Uranus and Neptune. These planets are similar in size—both have diameters roughly one-third that of Jupiter. Unlike Jupiter and Saturn, Uranus and Neptune are only about 15 percent hydrogen and helium. Most of the mass of each planet is made up of heavier gases, such as methane, ammonia, and water. As a result, Uranus and Neptune are more dense than Jupiter. Uranus is a smooth bluegreen in visible light. The small infrared image shows that the pole facing the Sun is warmer than the equator. Uranus looks blue-green, and Neptune appears deep blue. The color comes from methane gas, which absorbs certain colors of light. Each planet has methane gas above a layer of white clouds. Sunlight passes through the gas, reflects off the clouds, then passes through the gas again on its way out. The gas absorbs the red, orange, and yellow parts of sunlight, so each planet’s bluish color comes from the remaining green, blue, and violet light that passes back out of the atmosphere. rings pole Uranus Uranus is about twice Saturn’s distance from the Sun. The farther a planet is from the Sun, the more slowly it moves along its orbit. The greater distance also results in a larger orbit, so it takes Uranus 84 Earth years to travel around the Sun. Like the other gas giants, Uranus has a system of rings and moons around its equator. The ring particles and moons orbit Uranus in the same direction as the planet’s spin. Unlike the other planets, Uranus has an axis of rotation that is almost in the plane of its orbit. As a result, Uranus seems to spin on its side. During a solstice, one pole of Uranus points almost straight toward the Sun. Image not available for use on this CD-ROM. Please refer to the image in the textbook. Some scientists think that there was a large collision early in Uranus’s history. The result left the planet and its system spinning at an unusual angle. Uranus Each pole of Uranus experiences more than 40 years of sunlight and then more than 40 years of darkness as the planet orbits the Sun. Mass 15 Earth masses Diameter 4 Earth diameters Average distance from Sun 19 AU 762 Unit 6: Space Science Orbits in 84 Earth years Rotates in 17 hours Neptune Neptune orbits about 10 AU farther from the Sun than Uranus, so you would expect it to be colder. However, Neptune has about the same outside temperature as Uranus because it is hotter inside. Uranus is usually one smooth color, but light and dark areas often appear on Neptune. Clouds of methane ice crystals can form high enough in the atmosphere of Neptune to look white. Storm systems can appear in darker shades of blue than the rest of the planet. One storm, seen during the flyby of the Voyager 2 spacecraft in 1989, was named the Great Dark Spot. Unlike the huge storm on Jupiter, the Great Dark Spot did not stay at the same latitude. It moved toward Neptune’s equator. The winds there may have broken up the storm. Images of Neptune obtained a few years later with the Hubble Space Telescope showed no sign of the Great Dark Spot. check your reading What are the white patches often seen on Neptune? Neptune Neptune has a large moon that orbits in a direction opposite to Neptune’s rotation. Scientists think a giant collision might have occurred in Neptune’s past. Mass 17 Earth masses Diameter 4 Earth diameters Average distance from Sun 30 AU Orbits in 164 Earth years Rotates in 16 hours High clouds cast shadows on the layer below. cloud shadow KEY CONCEPTS CRITICAL THINKING 1. Which planet has a greater mass than all the other planets put together? 4. Compare and Contrast Why do Jupiter and Saturn show a lot of white, while Uranus and Neptune are more blue in color? 2. What do you see instead of a solid surface when you look at an image of a giant planet? 3. Which planets have rings? CHALLENGE 6. Apply If Uranus had areas of ice crystals high in its atmosphere, how would its appearance change? 5. Analyze Most of Saturn is much less dense than most of Earth. Yet Saturn’s mass is much greater than Earth’s mass. How can this be so? Chapter 21: Our Solar System 763