What is Asreroids composition atmosphere?

Answer 1

I think you may have meant to ask "what is the composition of the atmosphere of an asteroid?"

If so, the answer is "asteroids don't have atmospheres." Most planets have atmospheres because they are immensely massive. Mercury, which is the smallest planet, is a thousand times more massive than Ceres, the biggest asteroid, yet it is not massive enough to keep an atmosphere in the solar wind to which it is subject.

Ceres, is about 1000km across, and contains between a third and half the mass of all the asteroids put together, so most asteroids are just boulders. And a boulder isn't big enough to hold on to an atmosphere.