So, a neat thing, depending on how you look at it, about greenhouse gases is that while it's causing a raise in temperature near the surface it actually makes the upper parts of the atmosphere decrease in temperature. Venus is a good example of this.

Now on another note, atmospheric escape depends significantly on the planet/moon's escape velocity and the atmospheric temperature. The lower the temperature then the lower escape velocity a planet can have and still maintain their atmospheres for a significant amount of time. Titan is a good example of this.

What I'm curious about is if a smaller planet, like perhaps our moon, had a larger amount of greenhouse gases than here on Earth which in turn would cause a significantly lower temperature in the upper atmosphere which then would that be enough to slow down gases that are attempting to escape to space, so that the atmosphere would last at least for millions of years. (As of now, the best calculations I have seen so far puts an Earthlike atmosphere on the moon lasting for roughly 700,000 years).

It is my thoughts that the upper atmosphere would work similar to the cold trap we have on Earth, only instead of freezing the gases it instead slows them down enough for the Moon to pull them back in.

Any opinions on this or calculations for this?

P.s. A cold trap is an area in our atmosphere that gets so cold that water freezes and falls back down. It's the primary reason our water hasn't floated off into space.

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    $\begingroup$ Please consider asking that question on physics staxechange. You will probably find more people capable for such calculations there. $\endgroup$
    – running.t
    Oct 27, 2017 at 10:37
  • $\begingroup$ For now I'll leave it here, but if I don't get an answer this week I'll make sure to try other exchanges. Thanks! $\endgroup$
    – rclev
    Oct 27, 2017 at 13:14
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    $\begingroup$ I think you should link these "calculations" you keep referring to because I am unconvinced that green house gases are a determinate factor in atmospheric stability. To my knowledge planetary mass, distance from a star, and presence of a magnetosphere are bigger more critical factors. $\endgroup$
    – anon
    Oct 27, 2017 at 14:29
  • $\begingroup$ @anon I apologize, but I think you misread my question or perhaps I can phrase it better. All those things you listed are much more important, but what I'm wondering is if the cooling effect of greenhouse gases in the upper atmosphere could have an offsetting effect on Jean's Escape and extend the length of time an atmosphere can survive on the moon. $\endgroup$
    – rclev
    Oct 27, 2017 at 16:01
  • $\begingroup$ How does this differ from worldbuilding.stackexchange.com/questions/70898/… $\endgroup$ Oct 27, 2017 at 17:17

1 Answer 1


Great question!

I don't believe that a "cold trap" would form in the topmost layers and inhibit escape via Jeans escape or pickup by a stellar wind. In order for a gas atom or molecule to escape the planet, three things must happen:

  1. The particle must be moving faster than the escape speed of the planet.
  2. The particle must be moving in the upward direction.
  3. The particle must not collide with another gas atom (or anything else) on its outward trip. (It must have a large "mean free path.")

Upper atmospheres are highly rarefied, so the odds of a collision between an outwardly-bound atom and another atom in that region are very small. A collision, if it happened, would transfer some of the kinetic energy from the escaping particle to the other, and may indeed lose enough energy (or change direction enough) such that it no longer escapes. However, this collision is extremely unlikely!

  • $\begingroup$ Just nitpicking here... If you have reached escape velocity of a "parent" body, and points 1 and 3 of your question are followed, then any direction that does not include a collision path with the parent body will result in an escape. Not just upwards. Normal, antinormal, prograde (supposing it is not the same as radial out) etc. would all work. $\endgroup$ Oct 27, 2017 at 21:34

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