Atmospheres - The properties of each planet's atmosphere...

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Atmospheres A planet's atmosphere helps shield a planet's surface from harsh radiation from the Sun and it moderates the amount of energy lost to space from the planet's interior. An atmosphere also makes it possible for liquid to exist on a planet's surface by supplying the pressure needed to keep the liquid from boiling away to space---life on the surface of a planet or moon requires an atmosphere. All of the planets started out with atmospheres of hydrogen and helium. The inner four planets (Mercury, Venus, Earth, and Mars) lost their original atmospheres. The atmospheres they have now are from gases released from their interiors, but Mercury and Mars have even lost most of their secondary atmospheres. The outer four planets (Jupiter, Saturn, Uranus, and Neptune) were able to keep their original atmospheres. They have very thick atmospheres with proportionally small solid cores while the the inner four planets have thin atmospheres with proportionally large solid parts.
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Unformatted text preview: The properties of each planet's atmosphere are summarized in the Planet Atmospheres table (will appear in a new window). Two key determinants in how thick a planet's atmosphere will be are the planet's escape velocity and the temperature of the atmosphere. Escape of an Atmosphere The thickness of a planet's atmosphere depends on the planet's gravity and the temperature of the atmosphere. A planet with weaker gravity does not have as strong a hold on the molecules that make up its atmosphere as a planet with stronger gravity. The gas molecules will be more likely to escape the planet's gravity. If the atmosphere is cool enough, then the gas molecules will not be moving fast enough to escape the planet's gravity. But how strong is ``strong enough'' and how cool is ``cool enough'' to hold onto an atmosphere? To answer that you need to consider a planet's escape velocity and how the molecule speeds depend on the temperature ....
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