Thats
one of the many many theories of comet creation that is, near misses from asteroids around atmospheric planets, somehow in their wake they collect a bit of atmosphere, it freezes and then does the whole gas emission thingy when it warms up a bit.
(
Steerable_forehead Our Lady of Blessed Acceleration,don't fail me now,
Wed 24 Mar 2010, 20:13,
archived)