The Pla nets A Resource & Activity Guide for Teachers

Mer cury Named for the winged messenger of Roman mythology, Mercury orbits the sun more rapidly than any other planet — at a speed of 29.7 miles per second. Although it is usually obscured from view by the sun’s glare, Mercury can be seen crossing the sun’s disk about 14 times every 100 years. This occurs most often in May and November. Always near the sun, Mercury can sometimes be seen low in the sky before sunrise and after sunset. To an observer on Mercury, the sun would appear two-and-a-half times larger than it does to an observer on Earth. And the sun’s rays are seven times stronger on Mercury. Mercury’s density is nearly equal to Earth’s, meaning equal portions of the two planets weigh about the same. But because it is smaller, Mercury has less gravity. That means a 100-pound weight would weigh only 38 pounds on Mercury. Detailed photos were obtained in 1974 and 1975 by the U.S. space probe Mariner 10. The photos revealed that many of the craters scarring the planet’s surface have been smoothed over by ancient lava flows. This showed there was once volcanic activity on Mercury. The surface also features huge cliffs called “scarps” that may have formed when the planet cooled and shrank. Mercury is the first of the so-called “terrestrial planets” — Mercury, Venus, Earth, and Mars. These solid planets have hard, rocky surfaces and largely metallic cores. Mercury’s crust, or outer surface, is thin. Much of the early crust may have been blasted away long ago by a huge meteor. With no air or water, Mercury has no life. When facing the sun, its surface is hot enough to melt lead. At night, when the surface faces away from the sun, temperatures drop to 300 degrees BELOW ZERO!

2

Venus Venus passes closer to Earth than any other planet. At its nearest point, it is about 26 million miles away. It is brighter than any other planet or star in Earth’s sky. At times, Venus is the last bright object visible in the morning sky. At other times, it is the first bright object to appear in the evening sky. Because of this, astronomers once believed that Venus was actually two separate stars. When they discovered these stars were one planet, they named it Venus — after the Roman goddess of love and beauty. Like Earth’s moon, Venus appears in phases. When it is on the same side of the sun as Earth, it is sometimes invisible because its dark side is facing Earth. As it moves away, it looks like a crescent. When Venus is near the far side of the sun, it appears as a small bright disk. Venus is somewhat mysterious because thick clouds hide its surface from view. These clouds rise as much as 37 miles into the planet’s atmosphere. The large amounts of carbon dioxide in that atmosphere trap the sun’s heat and keep it from escaping. This so-called “greenhouse effect” keeps Venus continually as hot as Mercury ever gets -— even though Venus is nearly twice as far from the sun. Venus is very dry because its high temperatures boiled away any water billions of years ago. Its average temperature of 800 degrees rises to nearly 1000 degrees in areas facing the sun. Venus’s rotation is so slow that the atmosphere overheats, creating 200-mile-an-hour winds on the sun-facing hemisphere. NASA’s Magellan space probe surveyed Venus in 1990 and revealed that the planet had a relatively smooth surface. Recent volcanic activity probably erased many craters caused by meteors. And the planet’s dense atmosphere destroys most small meteors before they hit the ground.

2

Earth Earth is the fifth-largest planet in our solar system. Contrary to what many people believe, Earth is not perfectly round. It is slightly flattened at its north and south poles, causing the diameter from pole to pole to be about 26 miles shorter than the equatorial diameter. Earth’s greatest circumference, however, is slightly below the equator. Experts disagree about when and how Earth was formed. But most scientists agree it is more than four-and-a-half billion years old. Its core of iron and nickel is about 7,200 degrees. Earth’s tallest surface feature is Mount Everest, which stands 29,028 feet above sea level. The lowest land surface is the shore of the Red Sea at 1,300 feet below sea level. The deepest part of the ocean is found in the Pacific Ocean in the Mariana Trench, 36,198 feet below the water’s surface. Water covers about 140 million square miles of Earth’s surface. As the planet rotates, the pull of the moon’s gravity causes the tides in Earth’s oceans. Earth is the only planet known to have life on it. It is far enough from the sun to get the right amount of sunlight and warmth for life to exist. It has water and the kind of atmosphere that supports life. Parts of its atmosphere also help keep out some of the sun’s more harmful rays.

Activity Imagine that earthlings plan to colonize one of the planets in the next century. Select the planet that you think would be best suited for a settlement. Why would you select this one? For example, what kind of resources would make it easier to colonize? How would these resources be used by the people of Earth? What kind of equipment and shelter would you need?

3

Mars Because of its red color, Mars was named after the bloody Roman god of war. And its moons — Phobos and Deimos — were named for the horses that pulled his chariot. Mars is the only planet whose surface is seen clearly from Earth. The planet has three distinct surface areas. Parts of Mars are bright yellow-red and appear to be covered with dust. The reddish color comes from iron oxide — rust — in the soil. Green or blue-gray dark areas cover about one-third of Mars’s surface. Dust storms occur seasonally on Mars, changing the surface colors of the planet. This happens when winds blow light-colored dust over dark areas. Polar caps, which are probably frozen carbon dioxide or water, are the third distinct area on Mars. The planet’s surface also has some amazing features, such as a 16-milehigh volcano and a canyon as long as the United States. A thin atmosphere of carbon dioxide contains enough water vapor to form some clouds, small areas of fog, and patches of frost. The surface of Mars is in some ways quite Earth-like. But plants and animals from Earth could not survive on Mars. Its atmosphere is too thin, and the average temperature is about 80 degrees below zero. Mars also has very little water -— less than what would fill one of the Great Lakes on Earth. Some scientists, however, think they have found evidence of life in the form of microscopic fossils in a meteor from Mars. Other scientists dispute this claim.

Activity Mars is the only planet whose surface can be clearly seen through a telescope. Read more about one of the planets. Then pretend that you have a telescope strong enough to clearly see the surface of that planet. Draw a picture of what you might see through your powerful telescope.

4

Jupiter Named for the king of the Roman gods, Jupiter is indeed the king of the solar system — its diameter is 11 times that of Earth. Jupiter has two-and-a-half times as much mass as the other eight planets combined. Because of this, the planet’s strong gravity creates enormous pressure and very high temperatures in Jupiter’s core. Jupiter is not hot enough to be a star. However, it does give off more heat than it gets from the sun. This heat mostly escapes into space. Jupiter, Saturn, Uranus, and Neptune are “gas giants” — huge balls of hydrogen and other gases. Actually, Jupiter’s atmosphere is about 600 miles thick, and its surface is covered by hydrogen oceans that may be 29,000 miles deep! Jupiter’s rapid spinning causes clouds to form in red, brown, and white bands that make the planet appear striped. The planet’s Great Red Spot, which scientists believe to be a gigantic storm, varies in size from 8,000 miles wide and 16,000 miles long to nearly twice as large. The most interesting of Jupiter’s 16 known moons is Io, which has the highest-known level of volcanic activity in the solar system. Jupiter’s moon Ganymede is the largest moon in the solar system. The first close-up view of Jupiter came from the Pioneer 10 space probe in 1973, and a wealth of information was gathered by Voyager 1 and Voyager 2 in 1979. These probes discovered a faint ring system extending from the planet’s face. The probes also photographed volcanoes erupting on Io. The Galileo probe went into a two-year orbit around Jupiter in 1995. From Galileo, scientists have learned that there is very little oxygen and water in the planet’s atmosphere, and even the upper layers of the planet’s atmosphere are very hot and thick. Winds on the planet’s surface appear to reach more than 400 miles per hour. The winds are believed to be driven in large part by internal heat.

5

Saturn Saturn was named for the Roman god of agriculture. Saturn is nearly twice as far from the sun as Jupiter. It is the farthest planet that can be identified as a planet without a telescope. Saturn is pale yellow in color, but not nearly as bright as Mars. Like Jupiter, Saturn is a “gas giant.” Its diameter is nine times that of Earth. Beneath its thick atmosphere of hydrogen, helium, and other gases, it may have a solid core about two times the size of Earth. Because Saturn spins so rapidly on its axis, it is the most flattened of the planets. Its diameter is nearly 6,800 miles greater at its equator than at its poles. Saturn has a rocky core similar to Jupiter’s. Saturn’s interior is hot, and the planet gives off more heat than it receives from the sun. The heat escapes into space. Saturn’s most interesting feature is its system of rings. There are actually thousands of these rings. They are made up of ice and rock particles ranging in size from tiny specks of dust to boulders as big as cars. Some of the rings circling the planet are not circular in shape and appear to have dark spokes. At least two of the rings appear to be intertwined. In addition to its rings, Saturn has 18 named moons and dozens more that have been given provisional status. These moons may be “shepherds” that keep the rings in place. Most of Saturn’s moons are small and composed of rock and ice. Enceladus is the only moon that appears smooth and unmarked by craters. The largest of Saturn’s moons is Titan. It is the only satellite in the solar system with a sizable atmosphere. Scientists believe Titan may be a kind of frozen version of Earth before life appeared here. Like Jupiter, Saturn was studied by probes Voyager 1 and Voyager 2. These probes found a storm thousands of miles wide along the planet’s equator. Winds in the storm blow at 1,000 miles per hour. Other discoveries include a huge hydrogen cloud circling Saturn above its equator and seasonal differences between the northern and southern hemispheres.

6

Uranus Astronomer William Herschel discovered that Uranus was a planet in 1781. Before that time, it was thought to be a star. It was the first new planet discovered since ancient times. Uranus was named after the Greek god of the sky. Uranus has a cold, clear, 5,000-mile-thick atmosphere of hydrogen, helium, and methane gases. The methane is probably what gives the planet its blue-green color. Because Uranus has such a thick atmosphere, very little heat escapes from it. Uranus is composed primarily of rock and various ices. It appears the planet does not have a rocky core, but its materials are more or less uniformly distributed. Uranus is the seventh planet out from the sun. Little was known about Uranus until Voyager 2 flew past in 1986. The probe observed a mountain three miles high on the moon Oberon and a valley running across the entire surface of the moon Titania. It also studied the planet’s dark rings, which are quite different from the rings around Saturn. Uranus’s rings rotate around the planet every eight hours, and are made up mostly of boulders more than a yard wide. Scientists believe they may be the remnants of a large moon that shattered in a collision with an object as large as Earth. Some believe this collision toppled Uranus on its side, causing it to roll along in its orbit like a ball. Along with 11 rings, 21 moons circle the planet.

8

Neptune Neptune was first viewed in 1846, but scientists had long believed an eighth planet existed. After observing changes in the orbit of Uranus, astronomers used mathematical formulas to determine Neptune’s location. The planet is named after the Roman god of the sea. In many ways, Neptune is similar to Uranus. They are both “gas giants.” Like Uranus, Neptune has methane in its atmosphere, which gives the planet its blue color. A haze above its clouds causes a red rim. A major difference between the two planets is that Neptune radiates more heat than it receives from the sun and Uranus gives off no excess heat. Scientists learned much about the planet from Voyager 2 in 1989. The NASA spacecraft found six new moons orbiting the planet. Photographs of Neptune’s moon Triton revealed volcanoes that spew ice — the result of the moon’s 400degree-below-zero temperatures. Scientists already knew that Triton is the only satellite in the solar system that orbits in a direction opposite to that of its planet. It may have once been a large comet that was captured by Neptune’s gravity. In 1994, the Hubble Space Telescope revealed that large spots previously seen on Neptune had vanished, and a new one had appeared. Although scientists had once thought these spots were storms, they now think these unusual features are zones of clear gases, which allow darker layers of atmosphere to show through.

Activity The planets in the solar system were named for figures in Roman and Greek mythology. Choose one planet other than Earth. Then read more about the figure in mythology for whom it was named. You can find this information in an encyclopedia or a book about mythology. Using what you learn, draw a picture of that figure. Under the drawing, write a few sentences explaining why the planet was named after this figure.

8

Mor e Activities 1) Read more about the sizes and colors of the planets. Then collect nine round objects of different sizes, such as Styrofoam balls, ping pong balls, and rubber balls. Choose one round object to represent each planet. Paint each the same color or colors as the planet. Insert pins with colored heads into the planets that have moons. Attach the planets to a coat hanger with strings to make a mobile. 2) Many cities have planetariums. A planetarium is a building with special equipment that allows us to take a closer look at planets, stars, and the sun. Find out if there is a planetarium near where you live. If there is, plan a trip there. Make a list of questions to ask a planetarium worker about the planets. 3) When Christopher Columbus wanted to sail west across the Atlantic Ocean, he worked very hard to convince the king and queen of Spain to pay for his voyage. When he returned from his trip, he had found new lands and riches for Spain. Pretend you are a modern-day Columbus trying to convince the government to pay for your trip to Mars. Write a brief speech explaining what you think you will find on Mars and why you think you should be given the money to go. Give your speech in front of the class. 4) Stories about space and space exploration often appear in the newspaper. Clip several news stories about space. Make a list of all the space words you don’t know. Then use a dictionary to find the meaning of each word. 5) American astronauts use a special rocket called a space shuttle to travel into space and return to Earth. But the space shuttle cannot travel far distances to other planets. What kind of rocket or spaceship do you think would be needed to travel to another planet? Make a drawing of your idea. Then write a brief description of the kinds of equipment your spacecraft would need to have. 6) The United States began sending astronauts into space in the 1960s. The men and women who have worked as astronauts have helped us learn amazing things about space and our own world. Use encyclopedias, library books, almanacs, newspapers, and magazines to find out about these men and women. Then choose one astronaut and write a brief report explaining who that person is and what he or she did as an astronaut.

10

Bibliography The following books are suitable for readers in grades four through six: Asimov, Isaac. Mars: Our Mysterious Neighbor. Milwaukee, Wis.: Gareth Stevens, 1988. Asimov, Isaac. Uranus: The Sideways Planet. Milwaukee, Wis.: Gareth Stevens, 1988. Branley, Franklyn M. Mysteries of the Planets. New York: E.P. Dutton, 1988. Branley, Franklyn M. Uranus: The Seventh Planet. New York: Thomas Y. Crowell, 1988. Branley, Franklyn M. Venus: Magellan Explores Our Twin Planet. New York: HarperCollins, 1994. Dickinson, Terence. Other Worlds. Buffalo, N.Y.: Firefly Books, 1995. Gallant, Roy A. Macmillan Book of Astronomy. New York: Macmillan, 1986. Lampton, Christopher. Stars and Planets. New York: Doubleday, 1988. Lauber, Patricia. Journey to the Planets. New York: Crown Publishers, 1993. Rathburn, Elizabeth. Exploring Your Solar System. Washington, D.C.: National Geographic Society, 1989. Simon, Seymour. Mars. New York: William Morrow, 1987. Simon, Seymour. Our Solar System. New York: William Morrow, 1992. Simon, Seymour. Uranus. New York: William Morrow, 1987. Watters, Thomas R. Planets: A Smithsonian Guide. New York: Macmillan, 1995.

© 1997 Knowledge Unlimited®, Inc. • PO Box 52, Madison, WI 53701 1-800-356-2303 • www.knowledgeunlimited.com ISBN 1-55933-050-3