Why Doesn't Air Get Sucked into Space?

[russ_watters]

Is it a question of sufficient gravity to hold it or is it a question of insufficient external forces to take it away...

If the primary mode is molecules reaching escape velocity, there need not be any external forces to take it away.

 

[qraal]

Into space...because maybe there's not enough gravity to hold the atmosphere? The sun heats up the gas, probably to beyond escape velocity in some instance, and the gas escapes...

I'm not sure about the exact mechanism, but I see either very thin or no atmosphere around small planets, and I never heard of an asteroid with an atmosphere. This leads me to believe a planet can only hold so much gas around it. Up to a certain point I'm sure, where the atmosphere itself will make its own gravity.

And as per my disclaimer, I'm open to correction, and I'm not certain if Earth itself is large enough to hold as big an atmosphere as we throw at it. I imagine there's a limit...

There several limits, though they require some interesting circumstances. One limit is when the atmosphere is no longer of negligible mass and it causes a planet to capture more gases - that's about 10 Earth masses. The cores of the gas giants hit that size, then began "sucking up" the surrounding nebula gas with their gravity. Jupiter was most successful at this, but the nebula gas was eventually blown away by the young Sun's solar-wind.

Another limit is when the atmosphere of a body overflows the gravitational boundaries between it and a larger mass that it's orbitting. A small Moon-like object compresses an atmosphere only very slightly and if the atmosphere is too heavy it extends out to several times the size of the body, eventually coming under the control of the larger planet that the moon is orbitting. But such a small object is unlikely to retain such an atmosphere for very long. Instead back when the ice moons of the gas giants were newly formed they would have had large, dense and hot atmospheres that would have extended out to their gravitational boundaries. What atmosphere cooled before it escaped was retained by the moon - mostly water vapour that condensed as very temporary oceans before they froze over.

But not all the moons formed quick enough to ever get so hot. Callisto formed so gradually that the ice/rock mix it is composed of never separated out. Instead Callisto's interior is frozen water-logged mud. Ganymede formed hotter and separated out into a core, mantle and crust. Likewise Europa and Io, though both are much drier than their outer cousins.

 

[DaveC426913]

Another limit is when the atmosphere of a body overflows the gravitational boundaries between it and a larger mass that it's orbitting. A small Moon-like object compresses an atmosphere only very slightly and if the atmosphere is too heavy it extends out to several times the size of the body, eventually coming under the control of the larger planet that the moon is orbitting.

And the other way around, too.

Apparently, during Pluto's summer, its atmo expands enough that some of its extremely tenuous atmo is swapped with its moon Charon.

 

[qraal]

And the other way around, too.

Apparently, during Pluto's summer, its atmo expands enough that some of its extremely tenuous atmo is swapped with its moon Charon.

Possible, though no gas stays for long on Charon. It's spectrum is consistent with age darkened water ice and seems free of methane or nitrogen, both of which feature prominently on Pluto. So there's a good example of what mass is needed for retention of a minimal atmosphere (N2/CH4 in equilibrium with their solid phase) after 4.5 Gyr. Charon apparently has lost all its open-to-the-sky volatile ices, probably via them merely subliming away and being blown off by the solar-wind. Other large TNOs like Eris show signs of freshly resublimated methane ice, then below a certain radius there's only cosmic-ray darkened methane ice - in otherwords no atmosphere sticks around long enough to resolidify as fresh ice.

 

[Kmenex]

So the force of gravity is large enough that it keeps the gas molecules from filling the void...

I have heard that space is an almost perfect vacuum, with some very small pico torr pressures. I know that down here on earth, gas filling a vacuum can be a violent affair.

If i have a dual chamber down here on Earth where one chamber is a vacuum and the other is filled with a gas, if i open a gateway between the two chambers the gas will very quickly move to fill the vacuum. However, can i affect the "speed" at which it fills to vacuum by cooling the gas, logically it seems so. This happens even if i arrange the apparatus so that the vacuum chamber is above the gas and it's all inline with the direction of gravity.

So, why does the gas quickly fill the vacuum down here at earth? why does the gas not stay in the bottom chamber due to gravity? Does it have anything to do with the volume of the evacuated chamber?

If i increase the volume of the evacuated chamber by a very large amount will the gas reach a "volume of equilibrium" whereby the volume of the gas does not change much?

 

[qraal]

So the force of gravity is large enough that it keeps the gas molecules from filling the void...

I have heard that space is an almost perfect vacuum, with some very small pico torr pressures. I know that down here on earth, gas filling a vacuum can be a violent affair.

Because the gas is under pressure at sea level conditions. Think about it... 10 tons of force on every square metre of area. 100 kiloNewtons per square metre. Violent.

If i have a dual chamber down here on Earth where one chamber is a vacuum and the other is filled with a gas, if i open a gateway between the two chambers the gas will very quickly move to fill the vacuum. However, can i affect the "speed" at which it fills to vacuum by cooling the gas, logically it seems so.

How cool do you want to go? The energy of individual gas molecules decreases linearly with temperature, but their speed with the square root of the energy. Low mass molecules - even nitrogen and oxygen are in that class - have high speeds at STP. Dropping from 298 K to just 29.8 K only decreases the average molecular speed to "merely" 163 m/s in the case of N₂... or it does if it doesn't freeze. Hydrogen is still moving at 600 m/s and helium is doing 430 m/s.

Problem with going really cold is that intermolecular forces begin to dominate and the molecules become "sticky" with each other, forming liquids or ices.

This happens even if i arrange the apparatus so that the vacuum chamber is above the gas and it's all inline with the direction of gravity.

So, why does the gas quickly fill the vacuum down here at earth? why does the gas not stay in the bottom chamber due to gravity? Does it have anything to do with the volume of the evacuated chamber?

How high do you want the chamber? Consider N₂ again. At 29.8 K its molecules are doing 163 m/s. How many metres of fall at 1 gee (9.80665 m/s²) is that equal to? Easy computation g.h = 1/2.v² ...which means the required h is 1,355 metres. This is known as the scale height, and because the temperature is a measure of average energy, that means a gas thins out 1/e for each scale height the gas rises against gravity, at constant temperature.

However there is a complication. If a gas suddenly goes from one small volume to a larger one, then it undergoes what's called adiabatic cooling
because it expends internal energy by expanding. This cooling can be quite significant and can even cause a gas to change phase.

If i increase the volume of the evacuated chamber by a very large amount will the gas reach a "volume of equilibrium" whereby the volume of the gas does not change much?

If you expand the volume quickly and the amount of gas remains constant, then it will get colder and colder. But it will still move to try to fill the volume - so long as it doesn't freeze.

 

[hasnain721]

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.

You mean eleven km/s ?

[edit] ah been pointed out already!

 

[Rhine720]

Earth's Gravity Is pulling even past the moon. The air would have to be "sucked" pretty far.

 

[Kmenex]

Because the gas is under pressure at sea level conditions. Think about it... 10 tons of force on every square metre of area. 100 kiloNewtons per square metre. Violent.

If i have a dual chamber down here on Earth where one chamber is a vacuum and the other is filled with a gas, if i open a gateway between the two chambers the gas will very quickly move to fill the vacuum. However, can i affect the "speed" at which it fills to vacuum by cooling the gas, logically it seems so.

How cool do you want to go? The energy of individual gas molecules decreases linearly with temperature, but their speed with the square root of the energy. Low mass molecules - even nitrogen and oxygen are in that class - have high speeds at STP. Dropping from 298 K to just 29.8 K only decreases the average molecular speed to "merely" 163 m/s in the case of N₂... or it does if it doesn't freeze. Hydrogen is still moving at 600 m/s and helium is doing 430 m/s.

Problem with going really cold is that intermolecular forces begin to dominate and the molecules become "sticky" with each other, forming liquids or ices.



How high do you want the chamber? Consider N₂ again. At 29.8 K its molecules are doing 163 m/s. How many metres of fall at 1 gee (9.80665 m/s²) is that equal to? Easy computation g.h = 1/2.v² ...which means the required h is 1,355 metres. This is known as the scale height, and because the temperature is a measure of average energy, that means a gas thins out 1/e for each scale height the gas rises against gravity, at constant temperature.

However there is a complication. If a gas suddenly goes from one small volume to a larger one, then it undergoes what's called adiabatic cooling
because it expends internal energy by expanding. This cooling can be quite significant and can even cause a gas to change phase.



If you expand the volume quickly and the amount of gas remains constant, then it will get colder and colder. But it will still move to try to fill the volume - so long as it doesn't freeze.

Qraal,

I just read your response and wanted to thank you for the extremely interesting response and for sharing your knowledge. After reading i was pondering the possibility of "micro phase patterns" that existed in the upper atmosphere. I mean how much gas mass is floating around the earth? and how big is the space (aka container) which the gas is expanding into? Seems like it would def be "big enough" for the phase change you were talking about... Discounting the violent display of cosmic energies.

Perhaps this could play a role in how a planet traps it's gases? I wonder about the "gravitational" attraction of a small "micro domain" of "phase" is vrs the same attraction to the same mass of element within a different phase...

 

[qraal]

Qraal,

I just read your response and wanted to thank you for the extremely interesting response and for sharing your knowledge. After reading i was pondering the possibility of "micro phase patterns" that existed in the upper atmosphere. I mean how much gas mass is floating around the earth? and how big is the space (aka container) which the gas is expanding into? Seems like it would def be "big enough" for the phase change you were talking about... Discounting the violent display of cosmic energies.

Perhaps this could play a role in how a planet traps it's gases? I wonder about the "gravitational" attraction of a small "micro domain" of "phase" is vrs the same attraction to the same mass of element within a different phase...

Hi Kmenex
What kind of phase changes did you think I was referring to exactly?

As for gas mass, there's quite a lot very close to the ground, but not so much further up. In the real atmosphere there's of course the ionosphere and its interaction with Earth's magnetic fields, the incoming solar wind and a whole bunch of other energy flows and mass sources which my idealisation doesn't capture. But such extremely rarefied gases and plasmas are a tiny fraction of the atmosphere and don't really play a role in 'capturing' the gas. Gravity is sufficient. On a smaller world then the magnetic environment plays a much more prominent role, as a potentially erosive and preservative force.

 

[Kmenex]

I thought you were referring to a change from gas to liquid or solid during the adiabatic cooling.

So we have a gravitation force that holds gas close to the earth. I was wondering if this minuscule amount of gas that makes it "high enough" undergoes a phase change, perhaps forming micro liquid droplets or ice-lets.

Also i was wondering if these "micro phase domains" might have different gravitational attraction then say, an equivalent mass of gas in the gas phase.

 

[qraal]

I thought you were referring to a change from gas to liquid or solid during the adiabatic cooling.

So we have a gravitation force that holds gas close to the earth. I was wondering if this minuscule amount of gas that makes it "high enough" undergoes a phase change, perhaps forming micro liquid droplets or ice-lets.

Some rather unexpected chemicals undergo phase changes in unexpected parts of the Earth's atmosphere - sulfuric acid being the most surprising to my mind, which forms droplets just like in the atmosphere of Venus, but not as prominently. Such condensate gets water clouds started, so it's an important phase change. Water vapour freezing out before it hits the stratosphere is the most important example because it stops hydrogen loss. But the chief atmospheric gases never get cold enough.

Also i was wondering if these "micro phase domains" might have different gravitational attraction then say, an equivalent mass of gas in the gas phase.

No. Same mass has the same attraction. Of course droplets fall when they get big enough, but that's caused by a difference in viscosity, not gravity. However non-gravitational forces can be important - ice crystals can pick up a charge and conceivably could be electromagnetically levitated in the right conditions.