If the earth's gravity exerts a net downward gravitational force on all air molecules, how come the molecules don't eventually lose their momentum and all settle down? How is the atmosphere is still miles thick after billions of years?
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The key ingredient is temperature.
If it were zero then all the air would indeed just fall down to the ground (actually, this is a simplification I'll address later). As you increase the temperature the atoms of the ground will start to wiggle more and they'll start to kick the air molecules giving them non-zero average height. So the atmosphere would move a little off the ground. The bigger the temperature is the higher the atmosphere will reach.
Note: there are number of assumptions above that simplify the picture. They are not that important but I want to provide a complete picture:
- Even at the zero temperature the molecules would wiggle a little because of quantum mechanics
- the atmosphere would freeze at some point (like 50K) so under that temperature it would just lie on the ground
- I assumed that the ground and the atmosphere have the same temperature because they are in the thermal equilibrium; in reality their temperatures can differ a little because of additional slow heat-transfer processes
To leave you with something a little bit more concrete (although quite simplified), statistical mechanics tells us that probability $p$ that some portion of the system will occupy a given energy $E$ level is $p \propto \exp({-E \over kT})$ where $k$ is a Boltzmann constant (note that you have to normalize this probability to 1 by summing over all energy levels; that's why it's only proportional, $\propto$).
For the atmosphere of ideal gas where molecules have only potential energy (kinetic energy is not important here because the molecules don't interact with each other in the ideal gas) we have $E = mgh$. This gives us $p \propto \exp({-mgh \over kT})$. You can see that atmosphere "wants" to get lower ($p$ is bigger for $h$ lower) but the temperature "forces" it to go up. Play with the equation for different values of $T$ and $g$ (gravity strength) to see what happens.
Marek
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First of all, gravity does continually accelerate the air molecules. I don't see how this could make them lose their momentum.
What is the net effect of gravity on the atmosphere? Simple, gravity prevents the atmosphere from flying off in space, and instead it keeps it comfortably wrapped around our planet!
The reason why the atmosphere is still thick after billions of years is because you have two net effects on the air molecules, gravity, which keeps it as close as possible to the ground, and inertia, who has the opposite net effect. So as long as the molecules do not slow down they "orbit" our planet.
The reason is the same as why is the moon orbiting the Earth after billions of years. There's a balance between the kinetic energy of the moon and the gravitational potential energy - or a balance between gravity and inertia.
The other answers give you a summary explanation of what determines the temperature of the atmosphere and hence its molecules' average velocity. The reality is way more complicated as the temperature of the atmosphere is not constant with height and you have to take into account many more factors like varying pressure, convection and so on. Modelling the Earth's atmosphere accurately is very complicated.
In conclusion the basic mechanisms are outlined above. I hope they answer your question.
Sklivvz
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1I don't understand the talk about inertia and comparison to moon and Earth. This problem has nothing to do with inertia. There is only energy (molecules want to get as close to Earth as possible) vs. entropy (molecules want to occupy as big phase space as possible and so they want to go higher) battle. Of course, in reality the picture is complicated by many processes, but this is the main point. – Marek Dec 18 '10 at 16:17
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1@Marek, why would entropy decrease if the molecules fell down the gravity well? Gravity increases entropy (see http://www.mathpages.com/home/kmath573/kmath573.htm), or maybe I am misunderstanding your comment? – Sklivvz Dec 18 '10 at 16:35
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Huh? I never said anything like that. Systems in equilibrium want to minimize their energy and maximize their entropy. But this can't be done at the same time so there is a battle between the two phenomena (which can be made precise by Boltzmann statistics). But your answer doesn't talk about temperature/entropy part of the problem at all and therefore it is wrong. – Marek Dec 18 '10 at 16:37
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@Marek, the talk about inertia is the following: gravitational attraction keeps the atmosphere wrapped around us. Without gravitation the atmosphere would simply float away because molecules going outwards would keep their constant motion due to inertia. – Sklivvz Dec 18 '10 at 16:38
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@Marek: "you don't agree with me, and are therefore wrong" is a poor argument. The phenomenon can be explained without using entropy, in my opinion (although you can use it if you like). – Sklivvz Dec 18 '10 at 16:39
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@Sklivvz: without gravitation there would be no pressure at all (because there is no box to contain the gas). So this misses the most important point about gases. – Marek Dec 18 '10 at 16:41
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@Sklivvz: yeah that would indeed be a poor argument so it's good that I didn't make such an argument ;-) I made an argument that any answer that avoids entropy is wrong because entropy is absolutely necessary to explain any phenomenon that has to do with thermodynamics and temperature. This is one of them. I am sorry that you are not able to see this basic point. – Marek Dec 18 '10 at 16:42
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@Sklivvz: and I just realized that I am being horrible again. So I will take some time to cool down before I'll write anything more. Sorry if I offended you. It's just that I get really agitated when talking about statistical physics :-) – Marek Dec 18 '10 at 16:51
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@Marek: I was answering the question: "How is the atmosphere is still miles thick after billions of years?". The answer is, because it doesn't fly away, and that's the effect of gravity - the OP seems to think that the effect of gravity is making stuff settle down, which is wrong. Gravity doesn't make anything settle down. – Sklivvz Dec 18 '10 at 16:54
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1@Sklivvz: actually, gravity makes things settle down! Let a ball fall down from your hand. It will bounce a few times and then settle down. If there were no Sun, all of the atmosphere would do the same after some time. The only reason it doesn't is that ground is hot and wiggles around a lot and this is what gives the air molecules their dissipated energy back. This point is completely missing in your answer and so it is just wrong (at the very least, it's incomplete), I am sorry :-) – Marek Dec 18 '10 at 17:38
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2@Marek, I would say that is inelasticity and friction that make the ball settle down. Without them the ball would simply bounce forever. – Sklivvz Dec 18 '10 at 17:50
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1@Sklivvz: yeah. But Earth is certainly not a perfectly elastic rigid body, so you have to take both of these effects (inelasticity of the ground and non-zero temperature of the ground) into account somehow. – Marek Dec 18 '10 at 17:58
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@Marek: I did, "The other answers give you a summary explanation of what determines the temperature of the atmosphere and hence its molecules' average velocity." I did not claim the others were wrong. I merely completed their answers, since nobody ever spoke about gravity (and the question specifically mentions it). – Sklivvz Dec 18 '10 at 19:20
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@Sklivvz: nobody spoke about gravity? Really? And what do you think my $E = mgh$ is talking about? Or gigacyan's $h={v_0^2 \over 2g}$? And kalle's answer? I suggest you read the answers again if you missed the talk about gravity ;-) – Marek Dec 19 '10 at 02:06
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@Marek: I don't think that ideal gas equations apply or are relevant in this case, though. Also the assumption that the Sun heats the atmosphere is a bit shaky. The Sun heats the planet, mostly, and the planet heats the atmosphere. Also space cools down the atmosphere correspondingly - nobody mentioned that - which is what keeps the average temperature constant. There is no equilibrium in the system. The atmosphere is basically kept in orbit, but some of it is actually lost as we move through space, etc. There's a lot that has not been said. – Sklivvz Dec 23 '10 at 09:56
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@Sklivvz: seems you haven't read my answer at all :-) The interaction of the atmosphere with the ground is the fundamental part of my answer. True, I don't mention all the effects that happen but they are not that important. To the first approximation, ideal gas atmosphere with constant temperature is a good picture to make. You can make the gas non-ideal, introduce more gas types, make temperature height dependent, add thermal radiation of both atmosphere and ground, add heat and molecular fluxes, etc. But these just complicate picture and distract from the core of the matter. – Marek Dec 23 '10 at 10:06
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@Marek: gigacyan's answer makes the wrong predictions, kalle's makes the wrong assumptions. I am not sure about yours, but it would be great if you could plug in some numbers, calculate the thickness and see if it agrees with http://www.funtrivia.com/askft/Question12158.html – Sklivvz Dec 23 '10 at 10:11
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@Marek: I had read your answer, but you were asking me about 3 different answers (I was referring to the other two). I think your answer is pretty good, as I've explained above. – Sklivvz Dec 23 '10 at 10:13
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@Sklivvz: aah, fair enough. I lost the context because it has been few days since I asked :-) And yes, perhaps I should update my answer to include additional phenomena. Thermal radiation is definitely important because it's the reason why we need Sun (and therefore "hot" ground) in the first place, as you pointed out. – Marek Dec 23 '10 at 10:16
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The other answers are correct but to understand them you have to get an idea of how much thermal energy does an average molecule have.
According to Maxwell-Boltzmann distribution, the most probable speed of an air (say, nitrogen) molecule at room temperature is $v_p = \sqrt { \frac{2kT}{m} } = 422 m/s$. Without collisions with other molecules it can travel upwards $h=\frac{v_{0}^{2}}{2g}$ = 9 kilometers before the gravity stops it and pulls back to Earth. Basically, potential energy of molecules in gravitational field is too small compared to their kinetic energy to keep them low.
Update: Still, gravity is the reason why we have atmosphere after billions of years.
gigacyan
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1incorrect, the atmosphere is hundreds of kilometers thick. http://www.funtrivia.com/askft/Question12158.html – Sklivvz Dec 23 '10 at 10:07
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2At this point I'd like to express big thanks to @Sklivvz because he made me look at what the real atmosphere looks like. I have to say I was stunned by e.g. the fact that menopause is about $170K$ because of $CO_2$ cooling while slightly above it thermosphere is about $2000K$ hot; that exosphere reaches as far as half the way to the moon (where solar wind finally overcomes gravity) and that there exists turbopause after which gases no longer mix with each other. I never imagined atmosphere to be such a lively place :-) – Marek Dec 23 '10 at 13:03
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Of course, the above is just a little excerpt of all the processes that go on in the atmosphere that I found interesting. By no means is it an exhaustive list. – Marek Dec 23 '10 at 13:05
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@Sklivvz: I know that and I didn't say that all molecules stop at 9 km. I just tried to give feeling of how strong thermal motion is compared to Earth's gravity. – gigacyan Dec 23 '10 at 17:28
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The current answers all dive into the nitty-gritty, so I thought I'd provide a more conceptual answer. First of all, if you ignore all interaction, radiative, etc. effects and just consider air to be an ideal gas under the influence of gravity, then it wouldn't settle down because of conservation of energy - there's nowhere for the air molecules' initial kinetic energy to go.
But in practice, air does have a small but nonzero viscosity, so over the course of billions of years, at zero temperature internal friction would slow down the air molecules (radiating the lost kinetic energy away as heat) and they would indeed settle down to the ground.
But the constant influx of energy from the Sun keeps the atmosphere at high temperature. The key insight is that the state with all the air stationary down at the ground has extremely low entropy - much less than the state where the molecules are whizzing around all over the place. Since high temperature favors states with high entropy, the atmospheric temperature keeps the air molecules up in the air because that state has such high entropy.
If the Sun were to suddenly go dark, then the Earth's atmosphere would eventually cool down to the point where it would indeed settle down to the ground (at least until it reaches a density high enough that interactions between the molecules becomes important). In practice, this won't happen, because when the Sun dies, it will become a red giant that engulfs the Earth completely.
tparker
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This is because the sun is supplying them with kinetic energy all the time, which you know is temperature. Then the ideal gas law states p*V=nRT, so the volume keeps pretty much constant.
The pressure is causes by the electrostatic repulsion between the molecules pushing them apart, and the gravity pulling them towarsd earth.
Another important reason they dont 'settle down' is that temperature is the lowest form of energy, this means that what you actually ask for cannot happen in any closed system, the temperature energy can only go towards heating something else, the core of the atmosphere is in this sense not a perfect analogy, because some of its energy will be "lost" to heating the earth and the oceans, and most will be radiated into the universe.
TROLLHUNTER
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But what pressure is keeping them up? It seems like there's plenty of free space in air. – wrongusername Dec 18 '10 at 04:43
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@wrongusername: imagine a gas in a box. The only pressure there is, is exerted on the walls of the box. The molecules bounce off the wall and go back. But our atmosphere doesn't really have walls. There is only wall on the bottom which we call ground. So molecules are in fact falling down and then jumping up all the time. But this wouldn't work if the temperature of the ground were zero because bouncing off the ground dissipates energy and after a little while atmosphere would fall down (same way the ball does if you let it fall). You need "hot" ground to give some energy back to the air. – Marek Dec 18 '10 at 16:27
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@wrongusername: of course, I was talking just about ideal gas. In that case you don't really have any pressure in the atmosphere because molecules don't interact with each other. But if you'd put some object there (like balloon) it would get hit by the molecules all the time and it would experience the pressure. So it's important to distinguish between the pressure the air is able to exert and the actual pressure that happens by bouncing off the walls. By the way, the pressure would obey the same exponential law I stated above for probability. So that it would be pretty low up high (cont.) – Marek Dec 18 '10 at 16:30
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@wrongusername: in accordance with gigacyan's answer. When the molecules fly 9 km up they have to overcome gravitational attraction and lose almost all they kinetic energy. So on average the molecule 9km up is very slow and therefore exerts almost no pressure (on a balloon that would happen to be there, say). – Marek Dec 18 '10 at 16:32
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The sun keeps the earth warm. Without it, the atmosphere would freeze solid.
grizzly adam
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If the atmosphere were made out of ideal gas, it wouldn't freeze (and there are actually some non-ideal real gases that don't freeze). So this answer doesn't address the main point of why doesn't gas fall down. – Marek Dec 18 '10 at 16:35
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@Sklivvz: no. Only individual particles. But average height (which is the only thing that is important here) doesn't change in first order. – Marek Dec 23 '10 at 10:13
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@Marek: it was a point about terminology. I think we both understand the physics. :-) – Sklivvz Dec 23 '10 at 10:15
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@Sklivvz: well, if you imagine the gas as little non-interacting balls (as is the case in ideal gas), then the particles would just fell down to the ground and lie there forever (at least Boltzmann says so :-)). – Marek Dec 23 '10 at 10:20
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Gravitational force is the weakest of all the natural fundamental force. After ascending to certain height due to thermal energy of expansion caused by the Sun's heating it and its own KE of collisions among other gas molecules, equilibrium is established between the said two forces . Moons Gravity also contributes to the ascent of the gases molecules against Earth's G.
Brandon Enright
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It is because the force of attractions between the particles become strong as they get cold by law of thermodynamic so the move towards earth to get heat. And when they got heat there force 9f attractions become loose and act as single molecule so they move upward
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"Ideal gas"
If we consider an ideal gas, the molecules of which do not collide, then each of them behaves as a tennis ball, studied it Physics 101: it falls to the ground and bounces back, performs a parabolic trajectory, bounces again, etc. In this sense the molecules do fall to the ground - they can't settle, because their energy is conserved.
Real world
What the above naïve picture misses is the collisions between molecules, which are implied even in the ideal gas - as they are responsible for the establishment of the thermodynamic equilibrium (if molecules are truly non-interacting, then their energy is conserved and the energy distribution remains the same as it was in the beginning, as can be seen from the first paragraph.) Still, in a sufficiently rarefied gas, molecules do hit the ground.
This is not the case in air, where the mean free path is about a hundred of nanometers. Thus, a molecule that is falling down would be scattered and reverse its velocity. This shortness of the mean free path that justifies the hydrodynamic approximation, in which volumes of hot air rise, as if confined in a balloon (rather than passing through the cold air).
Water droplets do fall
Let us however think instead of molecules of water droplets, suspended in the air. When the droplets are small and dense, they behave pretty much like an ideal gas, and remain suspended in the air. However, as the droplets collide, they merge and grow, becoming rarefied enough to actually fall... and then it rains!
Roger V.
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Hydrostatic balance...the vacuum of space wants to suck all the molecules of air out but it's perfectly counter balanced with the gravity of Earth...all the while we keep spinning and rotating thousands of miles per hour in space around a giant nuclear reactor just waiting to destroy us all :)
Ali K.
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Why doesn't the earth's gravity reduce temperature by pulling down the molecules?. The answer would be conservation of energy, lack of friction, and the elastic nature of colisions between molecules. – Malabarba Dec 18 '10 at 20:10