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Why doesn't the outer most gasses in the atmosphere freeze?

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    $\begingroup$ Given that there is a net heat flux outward from the Earth, why would they freeze? Furthermore, there is only a very low pressure in space, even low Earth orbit, and the vapor pressure of the gasses may well still be above the equilibrium vapor pressure. $\endgroup$
    – Jon Custer
    Jan 31 at 22:42
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    $\begingroup$ Related: en.wikipedia.org/wiki/Thermosphere $\endgroup$
    – PM 2Ring
    Jan 31 at 23:45
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    $\begingroup$ To note it, sometimes it does! When parts of the atmosphere condense, falling to the ground, it's generally "precipitation". On Earth, it's mostly water that precipitates, though it can vary on other planets/moons/etc.. $\endgroup$
    – Nat
    Feb 1 at 15:49
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    $\begingroup$ As others have mentioned the low pressure means everything is above its freezing or condensation point and the radiation from the sun has a warming effect, but one thing that does not seem to have been mentioned is that the outer layer is not static. Cold air tends to descend and warm air tends to rise causing a circulation with rising warm air tending to partly warm the upper layers. $\endgroup$
    – KDP
    Feb 1 at 17:00

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Other answers have explained why gas at very low pressure won't freeze. What hasn't been mentioned is that the outermost atmosphere isn't cold.

The coldest layer is around -100°C at 80km but the ionosphere (extending hundreds of kilometres further out) is actually very hot due to absorption of UV and X-rays from the sun: 1500-2000°C or more in terms of molecular kinetic energy, although so thin that its "temperature" is somewhat notional in everyday terms.

https://www.noaa.gov/jetstream/atmosphere/layers-of-atmosphere https://en.wikipedia.org/wiki/Atmosphere_of_Earth

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    $\begingroup$ That's the most important answer: The premise is wrong. $\endgroup$ Feb 1 at 16:00
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    $\begingroup$ And besides, at the points where the atmospheric conditions are right, the atmosphere does freeze, usually starting with its H2O component. $\endgroup$ Feb 1 at 16:02
  • $\begingroup$ Well equilibrium also gets problematic, when you go even further up you get to the interplanetary and interstellar media (and at least some components of those are cold again ...). $\endgroup$ Feb 1 at 16:47
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    $\begingroup$ @Sneftel Lol. Followed (or preceded?) by the iron catastrophe. $\endgroup$ Feb 1 at 17:29
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    $\begingroup$ @Peter-ReinstateMonica Another Empty. Lifeless Planet Found $\endgroup$
    – Sneftel
    Feb 1 at 17:37
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In the outer atmosphere the pressure is very low (and approaches zero when you go further and further away from the earth). So let us look how materials behave at low pressure. Here is a typical phase diagram (pressure vs temperature) from Wikipedia - Phase diagram:

phase diagram
A typical phase diagram. The solid green line shows he behaviour of the melting point for most substances; the dotted green line shows the anomalous behavior of water. The red lines show the sublimation temperature and the blue line the boiling point, showing how they vary with pressure

You see, for very low pressure (i.e. at the bottom edge of the diagram) all materials are gaseous, but not liquid or solid. And that is why the air in the upper atmosphere doesn't freeze.

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    $\begingroup$ Your point isn't wrong, but you did choose a rather atypical phase diagram with respect to water's solid/liquid boundary. The vast majority of phase diagrams have a positive slope for that separation for the region with sensible pressures. (Crazy stuff happens at Mbar, of course.) $\endgroup$ Feb 1 at 11:40
  • $\begingroup$ @JasonPatterson You are right, water is atypical. I updated with a more typical diagram. $\endgroup$ Feb 1 at 12:18
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    $\begingroup$ Might want to update the text as well. It's not obvious that our upper atmosphere maps to an area under the red curve. $\endgroup$
    – chepner
    Feb 1 at 18:31
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    $\begingroup$ Not all materials are gaseous at very low pressure... If that were true, vacuum chambers would be impossible. It is accurate to say that a surprisingly large variety of things are gaseous in equilibrium, which is quite the pain for vacuum system users $\endgroup$
    – Liam Clink
    Feb 2 at 1:28
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    $\begingroup$ @LiamClink Aren't vacuum chambers (the kind with very low pressure) precisely a pain because the walls of the chambers slowly sublimate? If I remember correctly you typically have a normal vacuum pump to empty the chamber of the atmosphere at first, and then a bunch of fans counter spinning at very high rpm to continuously remove the material that sublimates. $\endgroup$
    – Nobody
    Feb 3 at 12:40
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The freezing temperature of the gases depends on the pressure (or rather the resublimation temperature, most gases won't have a liquid phase in between solid and gas at low pressure, helium is the primary exception – it does not have a solid phase at low pressures).

At high altitude the pressure is so low, that even for very low temperatures the equilibrium phase is a gas.

The Wikipedia article on the Clausius-Clapeyron relation tells you more details on the relation between pressure and phase transitions. Especially, have a look at the prototypical phase diagram.

(Also note, that the atmosphere does not really touch space, it slowly fades out – becoming less dense and less dense – there's no sharp edge between the atmosphere and space.)

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    $\begingroup$ To emphasize the last point: trace molecules from Earth's atmosphere can be detected at least as far out as the Moon, and vice-versa, polar lights are essentially the Earth moving through the very far upper levels of the Sun's atmosphere. At 400km, the height of the International Space Station, the atmosphere is still thick enough to slow the ISS down so much that it needs to fire its engines about once a month to speed up again. $\endgroup$ Feb 1 at 7:29
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    $\begingroup$ @JörgWMittag 'speed up'? The drag of the atmosphere actually speeds the ISS up as its orbit is lowered. Raising its energy, raising the orbit, slows it down again. Energy or height is more intuitive than speed in orbit. $\endgroup$
    – Neil_UK
    Feb 1 at 12:41
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    $\begingroup$ @Neil_UK: You are right, of course. I originally had "raise", but then thought this might instill the wrong impression that the ISS would have its thrusters firing down to lift it up. So, I changed it to something talking about firing "prograde", but then thought that term might be confusing as well. But then, obviously, I made it worse by giving understandable, but wrong information. Well, I know why I didn't become a teacher … $\endgroup$ Feb 1 at 13:06
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This is a good question and the answers are truly good. I feel my brain cells deep diving in to these replies. I would like to put forward an answer with more simple logics, if I may. Temperature follows pressure. Currently there is 14.7 pounds of pressure in the earths enviroment. H2O boils 212*F and when I place water in a clear bottle and attach my vacuum pump to such, as the pressure is reduced, so is the boil point of the water. Gas/liquid/solid. All points are lowered. The pressure in space is lower than on earth and these "gasses" never reach the solid stage due to lack of pressure. I am aware this may not be super scientific, there are times I like to just make items simple. Thank You.

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    $\begingroup$ -1. This is exactly replicating the content of the existing (accepted) answers. $\endgroup$
    – Brondahl
    Feb 1 at 14:27

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