# Why doesnt air go into space?

by jamesabc
Tags: space
 P: 22 pressure is high at low altitudes but pressure decreases at higher altitudes. so why doesnt air get sucked into space which is a vacuum? the pressure difference should cause air to flow into space. or am i missing some simple explanation.
 P: 986 Same reason oceans don't get sucked into the air (even though pressure rises as you go down): gravity.
P: 22
 Quote by hamster143 Same reason oceans don't get sucked into the air (even though pressure rises as you go down): gravity.
i understand for water, but for air where gravity is weaker the further you are away. i guess there is no air flow into space because the pressure difference will be getting smaller and smaller as you get higher. but is that to say no air will be expelled into space? like how water evaporates into the air. dono if this is the same thing but gives you the idea.

 PF Gold P: 7,120 Why doesnt air go into space? Think of it this way, if air were to get ejected further out into space, where would it wind back up again soon after?
 PF Gold P: 2,020 Fire something up and gravity pulls it back down,fire it faster and it goes higher and still comes back down but,throw it at a speed equal to or greater than the "escape velocity", then it can keep going and not return.The escape velocity of earth is about eleven thousand km/s and atmospheric temperatures are such that the vast majority of atmospheric molecules do not reach velocities anywhere close to the escape velocity.In short, gravity holds the atmosphere down.
Mentor
P: 17,223
 Quote by jamesabc i understand for water, but for air where gravity is weaker the further you are away.
There is no big difference in the strength of gravity at the top of the atmosphere and at sea level. The atmosphere is a really thin film of gas.
P: 85
 Quote by jamesabc i understand for water, but for air where gravity is weaker the further you are away. i guess there is no air flow into space because the pressure difference will be getting smaller and smaller as you get higher. but is that to say no air will be expelled into space? like how water evaporates into the air. dono if this is the same thing but gives you the idea.

I just wanted to point out that you seem to have the wrong idea about air pressure. Air pressure is caused by the space between the molecules. The difference in air pressure with increasing altitude is due to the molecules being farther apart. For example a certain volume of air at the top of Mount Vernon has fewer molecules than the same volume of air at sea level even though the chemical makeup of both volumes of air is the same. But the fact that air pressure decreases with increasing altitude is not the reason that air does not escape into space. The reason is, as so many have stated, gravity.
P: 15,319
 Quote by Pengwuino Think of it this way, if air were to get ejected further out into space, where would it wind back up again soon after?
Wihle this is not a direct answer to your question, I do think it really warrants some careful thought because it will give a very satisfactory understanding of what happens.

Remember, gravity always pulls downward. If there were air molecules floating around between the Earth and the Moon (yet not at orbital speed), they would quite simply fall Earthward until they encountered enough bouyancy to keep them aloft.
P: 85
 Quote by Pengwuino Think of it this way, if air were to get ejected further out into space, where would it wind back up again soon after?
Well I certainly find this to be an ambiguous statement.
 P: 85 Question for Dave: Why do you feel the need to remind us that gravity pulls downward? And make it clear that you mean towards the center of the earth as downward is ambiguous.
P: 15,319
 Quote by ernestpworrel Well I certainly find this to be an ambiguous statement.
It is not a statement at all; it is an interrogative.

The OP's question was most definitely ambiguous. Not that it was his/her fault at all...

It's just that the list of assumptions is large. Ask a dozen people the question without first stating 'all other factors being the same' and you'll get a dozen assumed scenarios - the most obvious of which is 'a bicycle would not get any traction on moondust, so it will fall over before it even gets moving. This is a percfectly valid assumption - given the ambiguity of the OP's stated scenario, but it probably* doesn't get at what the OP wanted to know.
* a guess. i.e. an assumption.
P: 15,319
 Quote by ernestpworrel Question for Dave: Why do you feel the need to remind us that gravity pulls downward?
What is this "us" thing? I am reminding the OP - the one asking the questions about why air doesn't fly upwards.

 Quote by ernestpworrel And make it clear that you mean towards the center of the earth.
Did you not know that? Or are you just being deliberately obtuse?
 P: 85 Alright, Dave. Why do you think the OP should think carefully about the direction in which gravity always pulls?
P: 85
 Quote by hamster143 Same reason oceans don't get sucked into the air (even though pressure rises as you go down): gravity.
This isn't really a clear answer either.
P: 15,319
 Quote by ernestpworrel Alright, Dave. Why do you think the OP should think carefully about the direction in which gravity always pulls?
Because the OP clearly seems to think that there is some cause for the air to go flying upwards - and contuinue to fly upwards. Once it loses its initial velocity, there is only one force acting on it, and that force is, of course, downward.

The OP knows this, he just hasn't realized that it is the most important factor.

(We are, hopefully, all agreeing to ignore solar winds and such, which will simply complicate the answer without illuminating it.)
P: 775
 Quote by ernestpworrel I just wanted to point out that you seem to have the wrong idea about air pressure. Air pressure is caused by the space between the molecules.
Well, no. The ideal gas equation tells us:

PV = nRT

thus pressure and volume vary with the number of moles of gas components and the temperature of the gas. "Space between the molecules" doesn't cause the pressure, though it does have a relationship with it. Decrease the volume and you increase the pressure of a given number of moles at a given temperature.

But change the temperature and the volume a gas occupies increases, so its density decreases. A gas can have the same pressure at a lower density by getting hotter.

 The difference in air pressure with increasing altitude is due to the molecules being farther apart. For example a certain volume of air at the top of Mount Vernon has fewer molecules than the same volume of air at sea level even though the chemical makeup of both volumes of air is the same. But the fact that air pressure decreases with increasing altitude is not the reason that air does not escape into space. The reason is, as so many have stated, gravity.
Surface pressure is due to the weight of the air on top of a given area. At Earth's sea-level surface , on average, some 10.33 tons of air is weighing down on every square metre. In fact the pressure at every altitude in the atmosphere is principally caused by the weight of the air above it. Air gets compressed by that weight, so it gets denser the closer we get to the ground. So you're right about the molecules being closer together, but the pressure is from the weight of the air itself.

But why doesn't it escape? Any mass of gas at a given temperature has a certain amount of energy per unit mass. Temperature is actually a measure of the average energy of the particles of air. At the very top of the atmosphere, where molecules no longer collide with other molecules if they go straight up, then particles with sufficient energy can escape. There's usually a small probability that a fraction of particles will have enough energy to escape, but it's a very low probability. Only hydrogen atoms typically escape in any number because they're the lightest particles and require the least escape energy. This region of the atmosphere is called the exosphere and it is very, very tenuous. The number of particles at the top, in a given volume, is similar to the solar wind.
 HW Helper P: 7,110 From wiki: Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. http://en.wikipedia.org/wiki/Earth#Upper_atmosphere I don't know how much is gained from capture of solar winds.
PF Gold
P: 2,020
 Quote by qraal Well, no. The ideal gas equation tells us: PV = nRT thus pressure and volume vary with the number of moles of gas components and the temperature of the gas. "Space between the molecules" doesn't cause the pressure, though it does have a relationship with it. Decrease the volume and you increase the pressure of a given number of moles at a given temperature. But change the temperature and the volume a gas occupies increases, so its density decreases. A gas can have the same pressure at a lower density by getting hotter. Surface pressure is due to the weight of the air on top of a given area. At Earth's sea-level surface , on average, some 10.33 tons of air is weighing down on every square metre. In fact the pressure at every altitude in the atmosphere is principally caused by the weight of the air above it. Air gets compressed by that weight, so it gets denser the closer we get to the ground. So you're right about the molecules being closer together, but the pressure is from the weight of the air itself. But why doesn't it escape? Any mass of gas at a given temperature has a certain amount of energy per unit mass. Temperature is actually a measure of the average energy of the particles of air. At the very top of the atmosphere, where molecules no longer collide with other molecules if they go straight up, then particles with sufficient energy can escape. There's usually a small probability that a fraction of particles will have enough energy to escape, but it's a very low probability. Only hydrogen atoms typically escape in any number because they're the lightest particles and require the least escape energy. This region of the atmosphere is called the exosphere and it is very, very tenuous. The number of particles at the top, in a given volume, is similar to the solar wind.
Because of its greater mass the escape of helium is much smaller than that of hydrogen but nevertheless is enough to keep the helium content of the atmosphere in reasonable balance,helium constantly being pumped in by the decay of radon gas.

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