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Why doesnt air go into space?

  1. Nov 7, 2009 #1
    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.
  2. jcsd
  3. Nov 7, 2009 #2
    Same reason oceans don't get sucked into the air (even though pressure rises as you go down): gravity.
  4. Nov 7, 2009 #3
    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.
  5. Nov 7, 2009 #4


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    Think of it this way, if air were to get ejected further out into space, where would it wind back up again soon after?
  6. Nov 7, 2009 #5
    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.
    Last edited: Nov 7, 2009
  7. Nov 7, 2009 #6


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    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.
  8. Nov 7, 2009 #7

    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.
  9. Nov 8, 2009 #8


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    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.
  10. Nov 8, 2009 #9
    Well I certainly find this to be an ambiguous statement.
  11. Nov 8, 2009 #10
    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.
  12. Nov 8, 2009 #11


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    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.
  13. Nov 8, 2009 #12


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    What is this "us" thing? I am reminding the OP - the one asking the questions about why air doesn't fly upwards.

    Did you not know that? Or are you just being deliberately obtuse?
  14. Nov 8, 2009 #13
    Alright, Dave. Why do you think the OP should think carefully about the direction in which gravity always pulls?
  15. Nov 8, 2009 #14
    This isn't really a clear answer either.
  16. Nov 8, 2009 #15


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    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.)
  17. Nov 8, 2009 #16
    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.
  18. Nov 8, 2009 #17


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    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.


    I don't know how much is gained from capture of solar winds.
  19. Nov 8, 2009 #18
    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.
  20. Nov 9, 2009 #19
    Radiogenic helium escapes largely via the Polar Wind, which is due to ion pick-up by the Solar Wind around the magnetic poles. Heavier atoms can escape with the Polar Wind that wouldn't otherwise escape, like nitrogen and oxygen. But only a small amount presently does so - mere kilograms per second. As there's thousands of quadrillions of kilograms of atmosphere that means the atmosphere will last for many aeons yet.

    The amount of hydrogen and helium that escape via thermal processes is presently negligible. Not because they can't escape, but because the stratosphere is very dry and so the hydrogen levels available for escape are very low. If the stratosphere became wetter - meaning the troposphere was much hotter - then hydrogen could escape en masse via what's called hydrodynamic outflow. That's how Venus's ocean is believed to have flowed away into space.

    Radon decay is one source of radiogenic helium, though radon itself is produced by uranium decay, of course.
  21. Nov 9, 2009 #20
    Thanks graal.Is it true that lighter atoms such as hydrogen and helium tend to migrate to the upper regions of the atmosphere and therefore become more likely to be carried away by the solar wind?
  22. Nov 9, 2009 #21

    D H

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    In a sense, yes. The lowest three layers of the Earth's atmosphere (troposphere, stratosphere, and mesophere) are of pretty much uniform composition, particular so with long-lived chemical species. (Aside: Short-lived species such as ozone and pollutants tend to be concentrated near their points of origin.) These three lower layers collectively form the homosphere, where turbulent mixing is the dominant atmospheric process. The upper layers of the atmosphere collectively form the heterosphere, where molecular diffusion is the dominant process. The relative concentration of lighter species grows with altitude in the upper atmosphere.
  23. Nov 10, 2009 #22
    The lightest atoms and ions of course are the ones that dominate in the exosphere, from which they can escape, albeit as a rather thin trickle.
  24. Nov 10, 2009 #23
    The simple answer is gravity. The stronger the planet's gravity, the more it will pull anything from space towards it. Rocks, spaceships, mountains...and certainly air. Yeah, if there's too much atmosphere and not enough gravity, then probably the top layers WILL escape into space. Which is probably what happened, and which is why we have the atmosphere we do, and not more.

    Please note this is a very simplified view of this. In reality the reasons why planets have the atmospheres that they do are probably much more complex.
  25. Nov 10, 2009 #24


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    For the record: either about 11 km/s or about 11 thousand m/s :tongue2:
  26. Nov 10, 2009 #25
    Thanks for pointing that out Borek.I can be so dopey at times(most times):wink:
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