Re: Arthur C Clarke and 'The Sentinel'
Planetary atmospheres are formed from outgassing from the surface. On the earth subsequent biological processes have modified surface outgassing to modify the atmosphere too.
The key to atmosphere is to capture the gasses from the surface, for that you need a global magnetic field strong enough to divert the solar wind, which causes the atmosphere to form and also forms a double stage radiation shield which encourages life.
So if the moon ever had a decent global magnetic field - which it almost certainly did at one point early on - it would have had an atmosphere, the same with Mars. Venus has a decent magnetic field and therefore has an atmosphere, but the heat means that it's quite unlike earth's: it's too near the sun for it to be benign which is why heavy gasses like CO2 dominate.