Originally Posted by -Apocalypse-
It's a comet so it won't become a meteor, and if this comet enters Mars atmosphere it doesn't have the power to escape it itself, no less cause impact debris that has the necessary escape velocity.
Might want to revise your statement. Meteorites originating from Mars
are not unheard of. To originate from Mars these objects had to have been ejected by an impact. The point here is that the object being ejected need to reach a point in space where the force of gravity exerted by the mass of the planet is zero. An object that can purportedly excavate a 500km diameter crater would have no difficulty ejecting material into space.
It is absolutely possible for an object to enter the atmosphere
of a planet and still escape that planet's gravity well. There is quite a bit of speculation with regards to terraforming regarding atmospheric thickening by skipping
multiple comets off a planet's atmosphere or into the planet itself.
Edit: Typo.Edited by BenC - 3/1/13 at 12:58pm