system
Given the following post, respond as if you're commenting on Reddit:
human
Ignoring the difficulty of capturing a comet or adjusting its orbit: If we could arrange for a comet to strike Mars, what would the effect be? Could we terraform Mars by hitting it with a cubic mile of water?

As a child, I loved "The Martian Way" - a short story by Asimov where 2nd generation Martian colonists establish their independence by "stealing" a piece of Saturn's rings and bringing it back to Mars as a water supply. These days we know that his story was based on bad information - for one thing, Saturn's rings aren't made from convenient cubic-mile chunks and, for another, we now know that Mars has no atmosphere to speak of, making it hard for a colony to get established in the first place.

But I still wonder if there was a nugget of gold in that old story: What if we grabbed a chunk of ice & methane from the outer solar system and just dropped it on the Red Planet? Could we restore Mars' atmosphere that way? If we did, how long would it last before the solar wind stripped it off again?
gpt
The smart thing to do would be not to smash a comet into Mars directly, but to aerobrake one in Mars' atmosphere. The temperature of atmospheric entry would be enough to not only break up & melt the comet, but also split the water molecules into their constituent oxygen & hydrogen. 

If you planned the comet's entry into the atmosphere *just so*, you'd be able to control how much of the O & H got liberated, or re-combined back into water after the fact. So you'd get to choose the proportions of oxygen, hydrogen & water you introduced. 

Oxygen is obviously very handy! The hydrogen would be so much lighter than the rest of the atmospheric gases that it would rise to the atmosphere's upper layers, and get preferentially stripped away, protecting the more valuable oxygen & water vapour.

However, it would a good hundred comets to so much as double the mass of Mars' atmosphere. So you're better off grabbing a Kuiper Belt Object.

Even then, the real issue with terraforming Mars is a lack of nitrogen. You ideally want some sort of inert gas to make up the bulk of the atmosphere. Too much oxygen, and everything is flammable. too much carbon dioxide and people can't survive. Nitrogen is also vital to plant (and therefore animal) life on earth, in the form of the nitrogen cycle.