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Name CHAPTER 27 Class Date Planets of the Solar System 3 The Inner Planets SECTION KEY IDEAS As you read this section, keep these questions in mind: • What are the basic characteristics of the inner planets? • What are some similarities and differences between the inner planets? • What planetary features allow Earth to sustain life? What Are the Inner Planets? The inner planets are the planets closest to the sun. These planets are Mercury, Venus, Earth, and Mars. They are also called terrestrial planets, because they are all rocky, like Earth. In Latin, terra means “earth.” The terrestrial planets are made up mostly of solid rock, and they have metallic cores. Their surfaces have bowl-shaped depressions called impact craters. These craters formed when other objects in space collided with the planets. What Are the Characteristics of Mercury? Mercury is the planet closest to the sun. It circles the sun every 88 Earth days, and it rotates on its axis once every 59 Earth days. Its surface has many craters, and it has cliffs that are hundreds of kilometers long. These cliffs may be wrinkles that developed in the crust when the molten core cooled and shrank. Mercury has almost no atmosphere. The lack of an atmosphere and a slow rotation cause temperatures on Mercury to vary widely. During the day, the temperature may reach as high as 427 °C. At night, the temperature may drop to –173 °C. READING TOOLBOX Organize As you read this section, use a four-corner fold to compare the inner planets. Label the outer flaps with the planet names. Describe each planet underneath the correct flap. READING CHECK 1. Explain What are two characteristics the terrestrial planets have in common? Mercury Crust Mantle Characteristics of Mercury Core Diameter 4,880 km, or 38% of Earth’s diameter Density 5.4 g/cm3, or 98% of Earth’s density Surface gravity 38% of Earth’s surface gravity Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 417 Planets of the Solar System Name SECTION 3 Class Date The Inner Planets continued Critical Thinking 2. Identify How long does it take for Venus to revolve around the sun? What Are the Characteristics of Venus? Venus is the second planet from the sun. It has an orbital period of 225 Earth days. In some ways, Venus is much like Earth. The two planets have similar size, mass, and density. However, Venus rotates very slowly, only once every 243 Earth days. Venus and Earth also differ in other ways. Venus Crust Mantle Core LOOKING CLOSER 3. Compare Which planet is smallest—Mercury, Venus, or Earth? Characteristics of Venus Diameter 12,100 km, or 95% of Earth’s diameter Density 5.2 g/cm3, or 95% of Earth’s density Surface gravity 91% of Earth’s surface gravity VENUS’S ATMOSPHERE READING CHECK 4. Identify Which two features are responsible for high temperatures on Venus? Venus and Earth have very different atmospheres. The atmospheric pressure of Venus is 90 times the pressure on Earth. Venus’s atmosphere is composed of about 96% carbon dioxide. High carbon dioxide levels and the planet’s nearness to the sun make temperatures on Venus very high. The large amount of carbon dioxide prevents most of the radiation that enters Venus’s atmosphere from escaping. The heat remains trapped at Venus’s surface, raising temperatures. This process is called a greenhouse effect. On Venus, a runaway greenhouse effect raises the average surface temperature to 464 °C. Venus also has sulfur dioxide droplets in its upper atmosphere. These droplets form a cloud layer that reflects sunlight. The reflection is so strong that Venus is one of the brightest objects in the night sky. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 418 Planets of the Solar System Name SECTION 3 Class Date The Inner Planets continued SURFACE FEATURES OF VENUS Scientists have used probes and satellites to study Venus. Data show that Venus has a rocky landscape. The surface is made of basalt and granite. Scientists also discovered landforms such as mountains, volcanoes, lava plains, and sand dunes. The surface of Venus has some craters. All the craters are about the same age, and they are relatively young. This evidence makes scientists think that volcanic activity changes Venus’s surface. Energy inside the planet heats the interior over time. Volcanoes erupt and cover the surface of the planet with lava. The lava buries older craters. Scientists think that this kind of event probably won’t happen again for another 100 million years. What Are the Characteristics of Earth? Earth is the third planet from the sun. Its orbital period is 365.25 days. Earth completes one rotation on its axis about every 24 hours. Earth has had a very active geologic history. Over the last 250 million years, Earth’s continents separated from a single landmass and moved to their present positions. Weathering and erosion have changed (and continue to change) the surface of Earth. Surface temperatures on Earth range from −90 °C to 60 °C. Earth’s average temperature is 15 °C. Earth’s unique atmosphere and distance from the sun allow water to exist in a liquid state here. Mercury and Venus are so close to the sun that any liquid water would boil. The other planets are so far from the sun that any water would freeze. Earth is the only planet scientists know of that has oceans of liquid water. READING CHECK 5. Describe What evidence do scientists have that volcanic activity completely changes the surface of Venus? Earth Lithosphere Asthenosphere Mesosphere Characteristics of Earth Outer core Inner core Diameter 12,756 km Density 5.515 g/cm3 Surface gravity 9.8 m/s2 Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 419 Planets of the Solar System Name SECTION 3 Class Date The Inner Planets continued Critical Thinking 6. Apply Concepts What would have happened if Earth’s oceans did not absorb carbon dioxide in the atmosphere? LIFE ON EARTH Earth is the only known planet that has the proper combination of water, temperature, and oxygen to support life. Scientists think that as oceans formed on Earth, carbon dioxide from the atmosphere dissolved in the oceans. This process kept carbon dioxide from building up in the atmosphere. With less carbon dioxide in the atmosphere, less heat was trapped at Earth’s surface. As a result, Earth maintained the moderate temperatures living things need to survive. Plants and cyanobacteria added oxygen to the atmosphere. As oxygen levels increased, living things that breathe oxygen began to develop. What Are the Characteristics of Mars? READING CHECK 7. Explain Why do Mars and Earth have similar seasons? Mars is the fourth planet from the sun. It has an average distance of about 228 million kilometers from the sun. Its orbital period is 687 Earth days, and it rotates on its axis every 24 h 37 min. Mars’s axis tilts at almost the same angle that Earth’s axis does. Thus, Mars’s seasons are similar to those on Earth. Massive volcanoes on the surface of Mars are evidence that the planet was once geologically active. A system of deep canyons also covers part of the surface. Scientists think that the canyons are cracks that formed in the crust as the planet cooled. Water may have then further eroded the cracks. Mars Crust Critical Thinking 8. Apply Prior Knowledge On which planet would you weigh more—Earth or Mars? Explain your answer. Mantle Core Characteristics of Mars Diameter 6,800 km, or 53% of Earth’s diameter Density 3.9 g/cm3, or 71% of Earth’s density Surface gravity 38% of Earth’s surface gravity Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 420 Planets of the Solar System Name SECTION 3 Class Date The Inner Planets continued MARTIAN VOLCANOES Tharsis Montes is one of several volcanic regions on Mars. Volcanoes in this region are 100 times as large as Earth’s largest volcano. The biggest volcano on Mars is Olympus Mons. It is about three times as tall as Mount Everest. Its base is about the size of Nebraska. Scientists think that Olympus Mons has grown so large because Mars has no moving tectonic plates. As a result, the mountain may have built up around a steady magma source for millions of years. Scientists are not yet sure whether Martian volcanoes are still active. However, they have observed certain events that produced seismic waves. These events are called marsquakes. WATER ON MARS The pressure and temperature on Mars are too low for water to exist as a liquid on the planet’s surface. However, several NASA missions have found evidence that liquid water did exist on the surface in the past. Many surface features look like water eroded them. Other features might be evidence of flood plains produced by massive volumes of water. The surface temperatures on Mars can range from 20 °C to –130 °C, depending on location and season. Although most of the water on Mars is trapped in polar icecaps, frozen water also exists below the surface. If liquid water exists below Mars’s surface, the odds of finding life on the planet would increase. However, scientists have not yet found evidence for life on Mars. READING CHECK 9. Explain Why do scientists look at Mars for signs of life? Scientists think liquid water possibly formed these gullies on Mars. Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 421 Planets of the Solar System Name Class Date Section 3 Review SECTION VOCABULARY terrestrial planet one of the highly dense planets nearest to the sun; Mercury, Venus, Mars, and Earth 1. Identify Relationships What is the relationship between the terms inner planet and terrestrial planet? 2. Summarize Use the graphic organizer below to describe the characteristics of the inner planets. Mercury Orbital Period Rotation Period Temperature Range 88 Earth days 59 Earth days –173 °C to 427 °C Venus Size Ranking (1 is largest, 4 is smallest) up to 464 °C Earth 1 Mars 3. Identify What characteristics of Earth make it possible for life to exist here? 4. Describe Why does Mercury have such a large daily temperature range? 5. Explain Why does Venus have such a high surface temperature? 6. Explain Why do scientists think that the volcanoes on Mars may still be active? Copyright © Holt McDougal. All rights reserved. Holt McDougal Earth Science 422 Planets of the Solar System