# Asteroids with an atmosphere

What would be the likelihood of an asteroid developing a thin layer of gas? If an asteroid is capable of having an "atmosphere", then how likely would be be for primitive plant life, such as a moss, to grow on the asteroid? The asteroid would stuck in the orbit of a star, in an asteroid belt.

Just for reference, the pressure on the Moon (which is way bigger than an asteroid) is a mere $$10^{-10} \ Pa$$, while on Earth it is $$10^5 \ Pa$$.