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

[kky]

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?

 

[SporadicSmile]

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.

 

[mgb_phys]

Gravity.

 

[kky]

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?

 

[mgb_phys]

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.

 

[pallidin]

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.