Maintain atmosphere on moon using global warming

Question

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.

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!