Uranus’s speed is less than that of Jupiter because Uranus is farther from the sun than Jupiter. Predicting: The planet Uranus is about 2,900 million km from the sun. Predict whether its speed is greater or less than Jupiter's speed. Explain your answer. - Observing the Solar System Planet Speed Versus Distance

- Observing the Solar System Previewing Visuals • Preview Figure 2 and Figure 3. Then write two questions you have about Earth’s history in a graphic organizer like the one below. As you read, answer your questions. Models of the Universe Q. What is a geocentric model? A. A model that shows Earth at the center of the revolving planets and stars Q. What is a heliocentric system? A. A model that shows Earth and the other planets revolving around the sun

End of Section:Observing the Solar System

- The Sun The Layers of the Sun • The sun has an interior and an atmosphere, each of which consists of several layers.

- The Sun Outlining The Sun • As you read, make an outline about the sun that you can use for review. Use the red headings for the main topics and the blue headings for the subtopics. • The Sun’s Interior • The Core • The Radiation Zone • The Convection Zone • The Sun’s Atmosphere • The Photosphere • The Chromosphere • The Corona • Features on the Sun • Sunspots • Prominences • Solar Flares • Solar Wind

- The Sun More on the Sun • Click the Planet Diary button for an activity about the sun.

End of Section:The Sun

- The Inner Planets The Inner Planets • The inner planets take up only a small part of the solar system. Note that sizes and distances are not drawn to scale.

- The Inner Planets Earth’s Layers • Earth has three main layers—a crust, a mantle, and a core.

- The Inner Planets Mercury • Mercury is the smallest terrestrial planet and the planet closest to the sun.

- The Inner Planets Venus • Venus’s density and internal structure are similar to Earth’s. But, in other ways, Venus and Earth are very different.

- The Inner Planets Venus • This figure combines images of Venus taken from space with a camera (left) and radar (right). The camera image shows Venus’s thick atmosphere. Radar is able to penetrate Venus’s clouds to reveal the surface. Both images are false color.

- The Inner Planets Mars • Mars is called the “red planet.” When you see it in the sky, it has a slightly reddish tinge. This reddish color is due to the breakdown of iron-rich rocks, which creates a rusty dust that covers much of Mars’s surface.

- The Inner Planets Mars • Mares has ice caps at both poles. Scientists think that a large amount of liquid water flowed on Mars's surface in the distant past.

- The Inner Planets Using Prior Knowledge • Look at the section headings and visuals to see what this section is about. Then write what you know about the inner planets in a graphic organizer like the one below. As you read, write what you learn. What You Know Most of Earth is covered with water. Mercury is closest to the sun. Venus is very hot. Mars is called the “red planet.” What You Learned Earth is unique in our solar system for having liquid water at its surface. Mercury has a greater temperature range than any of the other planets. A day on Venus is longer than its year. The reddish tinge on Mars is caused by the breakdown of iron-rich rocks.

- The Inner Planets Links on the Planets • Click the SciLinks button for links on the planets.

End of Section:The Inner Planets

- The Outer Planets Gas Giants and Pluto • The first four outer planets–Jupiter, Saturn, Uranus, and Neptune–are much larger and more massive than Earth, and they do not have solid surfaces. Pluto is small and rocky.

- The Outer Planets Jupiter’s Structure • Jupiter is composed mainly of the elements hydrogen and helium.

- The Outer Planets Jupiter’s Moons • The astronomer Galileo discovered Jupiter’s four largest moons. They are named Io, Europa, Ganymede, and Callisto.

- The Outer Planets Saturn • Saturn has the most spectacular rings of any planet.

- The Outer Planets Uranus • Although the gas giant Uranus is about four times the diameter of Earth, it is still much smaller than Jupiter and Saturn.

- The Outer Planets Uranus • Uranus’s axis of rotation is tilted at an angle of about 90 degrees from the vertical.

- The Outer Planets Neptune • Neptune is a cold, blue planet. Its atmosphere contains visible clouds.

To calculate the circumference of a circle, use this formula: C = 2πr In the formula, π ≈ 3.14, and r is the circle’s radius, which is the distance from the center of the circle to its edge. The same formula can be used to calculate the circumference of planets, which are nearly spherical. Neptune’s radius at its equator is about 24,800 km. Calculate its circumference. C = 2πr = 2.00 X 3.14 X 24,800 km = 156,000 km - The Outer Planets Circumference

Practice Problem Saturn’s radius is 60,250 km. What is its circumference? 2 X 3.14 X 60,250 km = about 378,800 km - The Outer Planets Circumference

- The Outer Planets Pluto • Pluto has a solid surface and is much smaller and denser than the other outer planets.

- The Outer Planets Identifying Main Ideas • As you read the section “Gas Giants and Pluto,” write the main idea–the biggest or most important idea–in a graphic organizer like the one below. Then write supporting details that further explain the main idea. Main Idea The four gas giants are similar in… Detail Detail Detail Detail Structure– they do not have a solid surface. Atmosphere– thick and made up mainly of hydrogen and helium. Rings– each is surrounded by a set of rings. Size and mass– each is very large and massive.

- The Outer Planets More on the Planets • Click the PHSchool.com button for an activityabout the planets.

End of Section:The Outer Planets

- Comets, Asteroids, and Meteors Structure of a Comet • The main parts of a comet are the nucleus, the coma, and the tail. Most comets have two tails—a bluish gas tail and a white dust tail.

- Comets, Asteroids, and Meteors Comet Orbits • Most comets revolve around the sun in very long, narrow orbits. Gas and dust tails form as the comet approachesthe sun.

- Comets, Asteroids, and Meteors The Asteroid Belt • Most asteroids revolve around the sun between the orbits of Mars and Jupiter. This region is called the asteroid belt.

- Comets, Asteroids, and Meteors Comparing and Contrasting • As you read, compare and contrast comets, asteroids, and meteoroids by completing a table like the one below. Comets, Asteroids, and Meteoroids Feature Comets Asteroids Meteoroids Kuiper belt and Oort cloud Between the orbits of Mars and Jupiter Origin Comets or asteroids Typically less than 1 km; some are more than 300 km in diameter Excluding the tail, about the size of a mountain Smaller than comets or asteroids Size Ice, dust, small rocky particles Composition Rock Rock or dust

- Comets, Asteroids, and Meteors Links on Comets, Asteroids, and Meteors • Click the SciLinks button for links on comets,asteroids, and meteors.

End of Section:Comets, Asteroids, and Meteors

Question Answer - Is There Life Beyond Earth? Asking Questions • Before you read, preview the red headings. In a graphic organizer like the one below, ask a question for each heading. As you read, write answers to your questions. What are the “Goldilocks”conditions? The favorable conditions on Earth that allow life to exist Is there life on Mars? Scientists have not yet found evidence for life on Mars. Why do scientists think Europa might have life? Europa has an ice crust that could have a liquid water ocean underneath.

- Is There Life Beyond Earth? Links on Extraterrestrial Life • Click the SciLinks button for links on extraterrestrial life.

End of Section:Is There Life Beyond Earth?

Graphic Organizer Feature Geocentric System Heliocentric System Object at center Earth Sun Objects that move around center Planets and sun Planets Proposed by Early Greek astronomers Copernicus Supporters Ptolemy Brahe, Kepler, Galileo

End of Section:Graphic Organizer

Table of Contents