Physical Science 8th grade (1).pdf

D a year is the time it takes a planet to revolve

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d. A year is the time it takes a planet to revolve once around the sun. Which planet has the longest year? The shortest year? e. Which planet is the most dense? The least dense? f. Which planet is approximately 10 AU from the sun? 9. Why are we able to see a certain comet one year but not again until many years later? 10. What is the difference between a meteor and a meteorite? 11. What is the asteroid belt and where is it located? 12. Why are the orbits of the planets slightly elliptical instead of being perfect circles? 13. Compared with Earth’s diameter, Saturn’s diameter is roughly: a) the same b) 5 times larger c) 10 times larger d) 50 times larger
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321 15.2 T HE P LANETS C HAPTER 15: T HE S OLAR S YSTEM 15.2 The Planets The nine planets of our solar system together contain 250 times the surface area of Earth. This vast territory includes environments baked by heat and radiation (Mercury) and frozen far colder than ice (Pluto). Venus, the most Earth-like planet in size, has a surface atmosphere of hot dense sulfuric acid that would be instantly fatal to any form of life on Earth. Our own crystal blue world is unique in having the right balance of temperature and environment to sustain life — or is it? Might there be unusual kinds of life unknown to us on the other planets? Scientists have recently discovered living organisms that feed off hot sulfur emissions from volcanoes on the ocean floor. These organisms might be able to survive on Venus. With a combined surface area 1,700 times the size of North America, the planets are an unexplored frontier full of discoveries waiting to be made. Mercury Mercury Mercury, the closest planet to the sun, is the second smallest (after Pluto) in both size and mass. Mercury appears to move quickly across the night sky because its period of revolution is the shortest of all of the planets. Only 40 percent larger than Earth’s moon, Mercury is a rocky, cratered world, more like the moon than like Earth. Like the moon, Mercury has almost no atmosphere (except for traces of sodium). Mercury has no moons. Surface environment Of all the planets, Mercury has the most extreme variations in temperature. The side of Mercury that faces the sun is very hot, about 400°C, while the other side is very cold, about -170°C. This is partly because Mercury’s rotation is locked in a 3:2 ratio with its orbit. The planet completes three “Mercury days” every two “Mercury years.” This also translates into one day on Mercury being about 59 Earth days long, and a year on Mercury being not much longer, about 88 Earth days. Figure 15.9: Mercury was named for the messenger of the Roman gods because of its quick motion in the sky (image from radar maps, NASA) Type: Rocky Moons: none Distance from sun: 0.39 AU Diameter: 0.38 of Earth Surface gravity: 38% of Earth Surface temp.: -170 to 400°C Atmosphere: none Length of day: 59 Earth days Length of year: 88 Earth days Shortest flight to Earth: 2.3 AU Travel time from Earth: 3 months
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322 U NIT 6 A STRONOMY .
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