Now that we're looking at colonizing other planets, our neighbor Mars stands as the best candidate for our initial attempt since it's nearest to us.

And though it has water locked-in as ice in one of it's [polar] caps, Mars doesn't have the other pre-requisite to life - an atmosphere.

Hence, today's proposal regarding "terra-forming" Mars (making it Earth-like) involves bombarding it with TONS of our nukes in order to create a dust of particulates & CO2. These will serve as the primordial atmosphere there.

*This is a rather "elementary" discussion of "terra-forming."