Why does our atmosphere stay put?

[Johndw]

If the moon pulls on our ocean bodies resulting in tides, why isnt our atmosphere, which is less dense , sucked into space?

 

[Mark R]

It's still held down by gravity - in the same way that the oceans are.

The atmosphere experiences tidal forces, as does the earth itself (that's why earthquakes are more common around the time of the full moon).

 

[sao123]

Originally posted by: Johndw
If the moon pulls on our ocean bodies resulting in tides, why isnt our atmosphere, which is less dense , sucked into space?

a little know concept called intelligent falling.

 

[CycloWizard]

Think of the free body diagram of a small control volume of gas molecules floating around in our atmosphere. The forces acting on it are:
1. Gravity from Earth - normal to the Earth's surface
2. Shear stress - in the direction of prevailing winds (perhaps tangential to the Earth's surface)
3. Gravity from the Moon - normal to the Moon

There are other factors that would be less important, such as diffusion/Brownian motion, but the first two should suffice as I would guess that they are the dominant forces. So, if you consider such a force balance, you could calculate how much shear stress it would take to throw air out into space. The solution would vary with respect to lattitude. Without actually solving the problem (which could be a bit tricky, though not terribly difficult), I would guess that the highest rate of loss would be at the poles.

 

[Paperdoc]

Whoa! Mark R, that implies that the moon is much heavier and has a geater pulling force to cause earthquakes when it is "full", as opposed to much "smaller". I don't think so.

 

[sao123]

Originally posted by: Paperdoc
Whoa! Mark R, that implies that the moon is much heavier and has a geater pulling force to cause earthquakes when it is "full", as opposed to much "smaller". I don't think so.

the amount of the moon visible because of reflected sunlight does not affect earthquakes, but what I think he meant to say is that when the moon is in spring Tide location earthquakes are more likely, than when the moon is in neap tide position.

http://csep10.phys.utk.edu/astr161/lect/time/spring-neap.jpg


However, Mark is somewhat correct, as when the moon is in spring tide positions... It does coincide with the monthly new/full moon cycle. (assuming a non-eclipse exception) the moon is full when it is on the opposite side of the earth form the sun, and the moon is new when it is on the same side of the earth as the sun.

 

[slatr]

Look up escape velocity

 

[PooBeetle]

Johndw
the answer is that our atmosphere is sucked into space,
and basically, we have already lost it all already.

the tiny bit that remains is statistically irrelevant, less than .01% remains after 4 billion years of the moon, solar wind, photoionization and removal by magnetic fields, etc etc.

 

[DrPizza]

OP, did you notice that you said "less dense" - the amount of pull is directly proportional to the mass of the object being pulled. (That, in addition to everything else people have said above.)

Another thing that helps our atmosphere is the magnetic field of the earth which keeps solar winds from stripping as much of the atmosphere away.

 

[RossGr]

A key factor that has not been mentioned is temperature. Temperature is defined as the aveage kintic energy of the constituant particles.

So the atmosphere tempeature determines the energy of the various gases.

The mass of the atom/molecule is also a factor in the kinetic energy. Since each gas has the same KE, the velocity is determined by the mass. Lighter molecules have a higher velocity. If the velocity of a gas is greater then the earths escape velocity, that gas will escape to space (will not be held by the earths gravitational field).

This is the reason you will not find gases like H and He in our atmosphere.

 

[Jeff7]

Originally posted by: DrPizza
OP, did you notice that you said "less dense" - the amount of pull is directly proportional to the mass of the object being pulled. (That, in addition to everything else people have said above.)

Another thing that helps our atmosphere is the magnetic field of the earth which keeps solar winds from stripping as much of the atmosphere away.

Which I'm not so sure of either - Venus has no magnetic field, yet it's got a much larger atmosphere than we do, and it's closer to the Sun. I've noted this before, but I don't ever remember what the reason is.

 

[Ruptga]

According to Wikipedia "it has been theorized that the much weaker weak magnetic field (only 0.000015 times that of Earth) has caused lighter gases to be knocked out of the atmosphere, leaving only carbon dioxide and small traces of other elements and molecules."