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What I'm doing is trying to find a way to create an atmospheric cold trap that will maintain a temperature of approximately −200 degrees Celsius (−328 Fahrenheit).

According to Wikipedia

In astronomy, a cold trap is a close to the surface layer of the atmosphere that is substantially colder than both the deeper and higher layers. The temperature of the air drops with increasing height above the surface of the earth reaching a low point (which for the earth resides at about 20 kilometers height). It is called a trap because it keeps ascending gases with high melting points in by freezing them to a solid which then drops back to the planet surface

It's a big part of the reason that all of our water vapor hasn't broken down and had its hydrogen lost to space.

So, how could a cold trap be created that possesses the temperatures I described above while surface temperatures of the planet remain Earth-like. The cold trap doesn't have to be created naturally, nor does it need to be at the same height or anything else as ours. It merely needs to maintain those temperatures in a layer of the atmosphere. For comparison, I want the cold trap to be a little more than 3 times colder than the one here on Earth.

For anyone wondering why I'm asking this, it's a follow up of my terra-forming question here

Thank you for any help or insight!

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  • $\begingroup$ Are you willing to constantly pour energy in it? Or, once created, it should be passive? $\endgroup$ – Mołot Feb 13 '17 at 7:58
  • $\begingroup$ Oxygen and Nitrogen both have boiling points above your required temperature, so the first thing you need is an atmosphere that's still gaseous at your required temperature. $\endgroup$ – Separatrix Feb 13 '17 at 14:11
  • $\begingroup$ What technology can be used? $\endgroup$ – Bellerophon Feb 13 '17 at 21:20
  • $\begingroup$ Any technology can be used, but once it's set up it should be passive. I actually think I may have come up with a solution, and I'll post it tomorrow once I have some more time to think about it. $\endgroup$ – rclev Feb 14 '17 at 3:38
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I think if the planet had an atmosphere that was very thick to keep heat in at ground level, heavy in hydrogen and helium, with a very strong magnetic field to keep solar rays from knocking them away, and an orbit on the outer edge of the Goldilocks zone so things get cold up high.
Having a lot of methane might help as well because it's a very strong greenhouse gas, and is lighter than oxygen. It has a boiling point of -161.5°C, so it would condense out at a lower altitude than the oxygen and nitrogen, keeping the warm air down low, and not at the top of the atmosphere.

Hydrogen has a boiling point of -252°C, and helium's is -268°C, so both gasses would still be in a gas state at -200°C. Oxygen (-183°C) and nitrogen (-195.8°C) (as Separatrix pointed out) would be in a liquid state at that point, so you'd likely get clouds made of liquid oxygen droplets at high altitude, which would condense and fall as oxygen rain, only to boil away before hitting the ground.

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