quantumeffect
 
The term intermolecular interaction is used to describe the interaction between two (or more) distinct molecules.  The strength of the interaction between the particles (here I use the term particle to stand for either an atom or molecule) will govern its physical properties. 

 

 
Mike, the main reason why the Earth's atmosphere doesn't float off into space is the same reason we don't float off into space - gravity.
 
Incidentally, I deliberately italicized 'float' because there are a number of other factors that could explain why a planet might lose it's atmosphere, but these factors would have to exert a force on the atmosphere's gaseous atoms and molecules greater than the pull of gravity holding them there.
 
Again, my memory is a little rusty on this, but I'm pretty certain outside influences such as the solar wind (a constant stream of charged particles emitted by the sun) can react with atmospheric particles and give them enough kinetic energy to reach escape velocity. The Earth's magnetic field (which extends out into space around the planet) actively repels the solar wind (electrical charge and magnetic force are essentially the same thing) preventing most of the harmful particles from reaching our atmosphere.
 
Most of the solar wind is diverted around the Earth, but some of the charged particles are trapped and deposited in radiation belts, the most well-known of which are the Van Allen belts. When these trapped and highly energetic charged particles collide with atoms of the upper atmosphere, some of the energy released is dispersed as light (of many different wavelengths, depending on the energy released) and we see this as a display of aurora. Aurora are normally associated with the Earth's polar regions as these regions are where the magnetic field is strongest. 
 
To create a planet's magnetic field, the major factor is that its iron core has to be molten - the movement of the charged particles in this physical state generates the field.
 
It's generally thought that the solar wind was the major factor in Mars losing its atmosphere. Mars once had a molten magnetic core, and its atmosphere was protected from the solar wind in much the same way as ours is. However, as Mars's mass is only around 10% of that of the Earth, its core was under significantly less pressure (and consequently temperature) and so its core solidified around four billion years ago. Without the protection afforded by a magnetic field, Mars's atmosphere was exposed to the solar wind and has been eroded away ever since.

I just want cool gloves made of water like that.

mike_mccue
BTW Steve,
 I just went off and read about Gravitational force. It gets over my head real quickly.

Are you upside down?

Wouldn't that be fun.

What I want to know is:
 
Where is Howard Wolowitz from the Big Bang Theory?
 
Once he sees this, he'll be able to wash the marks put on his face by the others.

I was really just making some random comments about intermolecular interactions in a general sense (addressing the question about a common force) not really addressing your question about the effect of gravity on the atmosphere.  Wrt to gases, sometimes we need to take intermolecular interactions into account (explaining why something is in the gas phase) and sometimes we ignore them (PV=nRT equations). 
 
Steve did a good job of covering the roll of gravity.
 
I will add that when I am looking at a problem and trying to figure if something is going to happen or not and there are different forces at work (some of them competing) … I will look at it in terms of energy balances.
 
If the force of gravity is holding the atmosphere to the Earth but the pull of gravity decreases as distance increases (or in this case very high altitudes) it seems perfectly reasonable to wonder why gas particles in the upper atmosphere don’t ultimately diffuse (not sure if that is the correct term but you get the point) into space.
 
I did some calculations and the thing is … the gravitational force between the Earth and an oxygen molecule at sea level is 5.22x10^-25 N and the force between the Earth and an oxygen molecule at 100,000 meters above the Earth is 5.06x10^-25 N.  So, even at extremely high altitudes the force of gravity is still about 97% of what it is at sea level (assuming I did my calculations correctly ... I can post them if you would like).
 
But let’s assume for the sake of argument a very slight decrease in the gravitational force may allow some molecules to escape.
 
At higher altitudes, the temperature is lower than it is at the surface suggesting the average kinetic energy of the gas particles (or in other words, the motion of the gas particles) is lower than that of the gas particles near the surface (I am guessing this to be true).  The kinetic energy is a competing force; I would suspect that if the kinetic energy of the gas particles is great enough, the particles will escape the Earth’s gravitational field.
 
So, I don’t know enough about it to tell you quantitatively where you need to be in terms of (1) the gravitational force between the Earth and the gas particles in the atmosphere and (2) the kinetic energy of the gas particles in the atmosphere … but, it seems to me that if we assume the gas particles possess less kinetic energy at high altitudes than they do at sea level, and that the gravitation force doesn’t really change that much between sea level and very high altitudes then we can venture a guess that gravity can hold onto the upper atmosphere.        

Hi Steve, Thanks very much for taking the time to put all that into perspective and make it seem understandable.
 
Hi Dave,
 What can I say? Your explanation has exceeded my optimism. Thanks for taking the time to lay out the framework for a comparison/contrast of attractive forces and especially for going to the trouble of quantifying the effects of gravity.
 
 When you first mentioned the 97% my *intuition* reacted with a short burst of "how can that be?" but then I realized that I was confusing myself by not accounting for the effects of barometric pressure and by simply thinking about gravity.
 
 I can't make a sensible excuse, because it doesn't make sense. I was distracted by the idea that air pressure lowers rapidly with altitude and now recognize that the weight (I hope I use the correct word there) of any single molecule doesn't change with the same schedule.
 
 Your explanation about the temperatures and kinetic energy is fascinating.
 
 Thanks for entertaining my interest in this subject... I've always found it very interesting and try to understand as much as can as a lay person.
 
 You have made it very easy to have learned a greater appreciation of the subtleties.
 
 I don't think I can thank you enough. Thanks!
 
 
 best regards,
mike

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Hi,
 
Such a wonderful physics lesson ... it's awesome!
 
Kids could use seeing that and learning a thing or two!