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If gases spread out to fill their container, why doesn't the atmosphere escape from earth? It's not gravity;heavier-than-air gases in a beaker don't stay at the bottom, so how does this work?
Question
#72337. Asked by cactus95. (Nov 15 06 7:04 PM)
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zbeckabee
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The EXOSPHERE begins at an altitude of about 500 to 700 Km above Earth and extends out to INTERPLANETARY SPACE. At these altitudes, ATOMS and IONS are Very Far Apart. Some gases in the EXOSPHERE actually escape into SPACE. Particles of the SOLAR WINDS are concentrated into Radiation Layers at about 3000 and 16 000 Km above the Earth's Surface. These Layers of the EXOSPHERE are the VAN ALLEN BELTS and are held in place by Earth's MAGNETIC FIELD.
 http://www.sirinet.net/~jgjohnso/physci.html
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Brainyblonde
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The Earth's atmosphere is kept in place because of the gravitational pull the Earth exerts on particles in the atmosphere. But part of the Earth's atmosphere does float out into space. The part that is lost out into space is the extremely light particles like Hydrogen which can escape the Earth's gravity. But the Earth's atmosphere is also replenished by releases from planetary volcanism.
http://www.windows.ucar.edu/tour/link=/kids_space/qearth_atm.html
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Arpeggionist
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It's by the same principal that keeps our planets and comet belts orbiting the sun. Occasionally a comet from the Oort cloud might be driven away into interstellar space, but mostly they keep orbiting the sun.
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Gnomon
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It is gravity. Gases can be thought of as billions of tiny bouncing balls (known as molecules). They bounce off the ground and go very high, but gravity pulls them back down. They also bounce off each other, so that the ball that leaves the ground may not get very high before its momentum is transferred to another ball, pushing it upwards.
The reason the heavy gases don't form a layer at the bottom of the atmosphere with the lighter gases on top is that a gas consists mainly of empty space. The gaps between the balls are much bigger than the balls. So A dense gas does not provide a suitable base for a light gas to sit on - the molecules of the light gas keep going until they hit the ground and bounce.
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