What Prevents Air from Flowing Out of Earth's Atmosphere?

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    Atmosphere Space
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Discussion Overview

The discussion revolves around the question of what prevents air from flowing out of Earth's atmosphere into space. It explores concepts related to atmospheric pressure, molecular motion, and the effects of gravity, as well as the balance of atmospheric loss and replenishment processes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why air does not flow into space given the pressure difference between the atmosphere and space.
  • Another participant asserts that gravity prevents air molecules from escaping, emphasizing that if molecular speeds do not exceed escape velocity, they remain bound to Earth.
  • A participant notes that while some air molecules may exceed escape velocity, collisions could prevent them from escaping, raising the question of whether atmospheric density decreases over time.
  • It is mentioned that there is a constant loss of atmosphere, primarily due to solar wind, but various processes contribute to maintaining overall atmospheric pressure.
  • Another point raised is that Earth collects space dust, which may contribute to the atmosphere or settle on the surface.

Areas of Agreement / Disagreement

Participants express differing views on the dynamics of atmospheric loss and the role of gravity, with some acknowledging the complexity of the processes involved. There is no clear consensus on the implications of molecular escape or the overall stability of the atmosphere.

Contextual Notes

Participants reference the Maxwell-Boltzmann distribution and the concept of escape velocity, but the discussion does not resolve the implications of these concepts on atmospheric density over time.

kky
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Just out of curiosity,
We know that the atmosphere is at a pressure of approximately 101 kPa at the sea level and decreases with height.
But in space there is no atmosphere and hence zero pressure; Should that not mean that the air in the atmosphere should flow out into space like in a free expansion?

What is it that prevents the air from flowing out?
 
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Gravity is our friend =]

if the average speed of the molecules does not exceed the escape velocity then the molecules can't escape from the earth. The reason Earth has a thicker atmosphere than Mars is (partly) to do with the fact Mars is smaller and thus was less able to hold onto it.
 
Gravity.
 
Ouch! That hurt.

We know by the maxwell Boltzmann distribution curve that a fraction of the air molecules will be above the escape velocity of the earth. Of course they might collide with other molecules and lose their kinetic energy before they can escape but atleast a few molecules near the edge of the atmosphere will be able to escape.
Then shouldn't there have been a decrease in the density of atmospheric air over time?
 
There is a constant loss of atmosphere, although mostly due to the strpping pressure of the solar wind.
However there are a lot of processes that put new stuff into the atmosphere (ocean, trees, volcanoes) so the overall pressure remains reasonably constant.
 
Also, please be aware that our Earth collects through gravitational attraction many tons of space dust each year. This "dust" may settle to the surface or remain suspended in our atmosphere until it does settle.
 

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