Interesting you should mention that.
I was actually wondering, after reading about all the water locked deep under the surface of Mars, if a large asteroid impact might release some of that water on the surface. In effect, creating oceans and lakes on the surface. It would have to be a pretty big rock given the 10 mile, deep depth of the water. Deorbiting Diemos or Phobos might do the trick for a big step forward in terraforming.