Suppose a large, 'dirty snowball' comet, 25-50km in diameter or more, were to hit Mars at some speed, say 75,000 km/h. I am quite certain it would vapourize on impact. And the impact will create super-heated high-velocity shock waves which will travel around Mars at least once, probably several times. What I want to know, is what happens then? Especially, how long will the steam condense, and rain fall?