why doesnt air go into space?

**DaveC426913** · Nov12-09, 01:15 AM

Originally Posted by qraal

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** · Nov12-09, 02:21 AM

Originally Posted by DaveC426913

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** · Nov12-09, 03:11 AM

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** · Nov12-09, 05:07 AM

Originally Posted by 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.

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.

[quote]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.[quote]

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** · Nov12-09, 06:12 AM

Originally Posted by Dadface

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** · Nov13-09, 10:36 PM

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