lecture16spring2009 - Astronomy100Dr.Rhodes Lecture Chapter...

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Astronomy 100 - Dr. Rhodes Lecture # 16 - 2/23/2009
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Chapter 7: A Planetary Overview Planetary Atmospheres We have also learned that the escape velocity is the minimum velocity an object must have in order to escape the gravitational attraction of an object such as a planet. Earth’s escape velocity is 11 km/s. The Moon’s is only 2.5 km/s. Phobos (a moon of Mars) is so small that its escape velocity is about 50 km/hr.
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Chapter 7: A Planetary Overview Planetary Atmospheres Gases and Escape Velocity We have already learned the following properties of a gas: 1. As gas molecules interact, different molecules have different speeds. 2. The average speed of the molecules depends upon the temperature of the gas. 3. At a given temperature, the less massive molecules have greater speeds than do the heavier molecules.
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Chapter 7: A Planetary Overview Planetary Atmospheres The temperature of a substance is defined by the average energy of its molecules. There is little free hydrogen in Earth’s atmosphere because low-mass hydrogen molecules can achieve escape velocity at the temperatures of the upper atmosphere. On the sunlit side of the Moon even molecules of oxygen and nitrogen—so prevalent in Earth’s atmosphere—can achieve escape velocity in the low lunar gravity.
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Chapter 7: A Planetary Overview Planetary Atmospheres The Atmospheres of the Planets If a planet’s escape speed is at least five to six times the average speed of molecules at that planet’s atmospheric temperature, then that planet will retain an atmosphere for more than 5 billion years. Because of their size (and mass) the Jovian planets have retained almost all of their gases.
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This Graph of the Escape speeds And surface Temperatures of The Planets is an Example of a Nomograph
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Chapter 7: A Planetary Overview Terrestrial—versus—Jovian Planets Near the Sun Small Mostly solid Low mass Slow rotation No rings High density Thin atmosphere Few moons Far from the Sun Large Mostly liquid & gas Great mass Fast rotation Rings Low density Dense atmosphere Many moons
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Chapter 7: A Planetary Overview The Formation of the Solar System Two categories of competing theories: evolutionary and catastrophic . Evidentiary Clues from the Data A successful theory must be able to explain the following data: 1. All the planets revolve around the Sun in the same direction, and all planetary orbits are nearly circular (except for Mercury and Pluto).
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Chapter 7: A Planetary Overview The Formation of the Solar System 2. All of the planets lie in nearly the same plane of revolution.
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