"Fair enough.... and so what stops a molecule in the atmosphere from simply leaving? A force is working on it to stop it leaving... kinetic energy is replaced by gravitational potential energy??"
Pretty much, yes. Just like everything else here on Earth, a given gas molecule would need to reach escape velocity (11.2 km/s, about Mach 33) in order to leave permanently into space. Any slower than that, and gravity reels it back in every time. But in that round trip, the net work done by gravity is still zero. The gas molecule just converts kinetic energy to gravitational potential, then converts gravitational potential back to kinetic energy. I
