QUESTION:
What would happen if you smashed an ice comet or a small moon made of mostly
ice into Mars?

ANSWER from Bruce Jakosky on October 9, 1997:
The impact of a small icy  comet or asteroid onto the Martian  surface
would generate enough heat from the force of the impact that it  would
vaporize all of the  ice.  Once the  ice was in the  atmosphere in the
form of water vapor, several things  would happen.  Some of the  water
would rain  out in  the surrounding  areas; however,  this water would
quickly (days or weeks) re-evaporate into the atmosphere.  Some of the
water would form thick clouds; again, these eventually would evaporate
or sublime  back into  vapor.   Eventually, all  or almost  all of the
water would make it to the  polar caps.  Those are the  coldest places
on the planet  averaged over a  year, and the  water would freeze  out
there.  In fact, we see large deposits of water ice at the poles,  and
some of this no doubt has been supplied from comets.