I have a question about comet/asteroid/etc impacts

[Light Bearer]

Hi there,

So I'm curious to know if it might be possible for an object of sufficient size/mass to, when upon impact, hit with enough force to cause the planets atmosphere to diminish. It's less than a month now until the MSL reaches Mars...I'm pretty pretty excited :|

 

[mfb]

A big object could eject some gas from the atmosphere, but I would not expect any significant effect from this. At the same time, it could release some gas which was bound in Earth or the asteroid before.