Well, the Lunar atmosphere has a density of 0,000 000 000 000 000 001 times the one of Earth's atmosphere. They defined it as an 'exosphere', with only about 100 molecules pr. cubic centimeter.
In some way, you could call ten molecules an atmosphere if you wanted to. I guess most celestial bodies have a few molecules above the ground, caused by the solar wind or other things.
But interesting.