The Lunar Atmosphere

Although it is commonly understood that the moon has no atmosphere, it does in fact have an extremely tenuous one made up of captured solar wind molocules and out-gassings from radioactive lunar rock. The atmosphere is so thin, that if it were compressed to the same temperature and density as the earth’s, it would fit into a 210 foot cube.

The lunar missions increased the mass of the lunar atmosphere by 30%, which was enough to impact the sensitivity of some of the experiments. After several weeks the atmophere returned to normal having been swept clean by the solar wind that keeps it in relative stasis.

A few other interesting details from aforementioned link…

If the atmosphere were … a factor of just 1000 denser, … the atmosphere would remain stable for tens of thousands of years. … Large scale human activity on the Moon could push the total mass over the limit and create … a stable, if highly tenuous atmosphere which could threaten precisely the most important asset that the Moon can offer us: its sterility and almost atmosphereless nature. There is even some evidence, from the lack of certain sizes of microcraters in surface dust that the Moon did have such an atmosphere in the past, perhaps as much as 10 million times denser than now.

NASA has all the specific data here.