Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: benm on 29/03/2019 14:29:26
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Ralph has a question:
Why, do not the atmospheric gases separate and stay apart in accordance with their individual specific gravities?
Does anyone have an answer (without gas-bagging)?
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There is a very small amount of separation, but this is largely due to the fact that most of the gases that make up the atmosphere have very similar molecular masses (N2 28, O2 32, Ar 40).
The difference in gravitational potential energy that would occur by stratifying would be less than the energy equivalent of the decrease in entropy (due to unmixing) at the temperature that the atmosphere is. Same reason that vodka doesn't spontaneously separate into water and ethanol (which is about 20% less dense than water).
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The simple answer is that diffusion mixes them.
Strictly speaking, they would segregate , but only to a tiny extent. (and so does the vodka)
Thermal currents will mix them anyway.
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At the microscopic scale, molecules in the air are charging around at speeds around the speed of sound. This causes a lot of mixing at microscopic scales.
At medium scales (such as shown on weather maps), high and low pressure regions cause winds and storms that mix the air on scales of hundreds of kilometers.
At larger scales, heating from the sun, and the rotation of the Earth produce large-scale mixing of the atmosphere, on scales of thousands of kilometers.
Our Earth is a pretty mixed-up place!
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They do separate, despite thermal and turbulent mixing. The upper atmosphere does contain slighty less oxygen and a lot less carbon dioxide than the lower atmosphere.