Hmm. A bunch of people commenting here don't seem to realize that the reason that Mars has such a miserably thin atmosphere is because it has a terribly weak ma

Hmm. A bunch of people commenting here don't seem to realize that the reason that Mars has such a miserably thin atmosphere is because it has a terribly weak magnetosphere which is insufficient to prevent solar radiation from stripping away the atmosphere. So, you can generate as much atmosphere on Mars as you care to imagine, but gradually it will all dissipate away into space, unless you construct networks of structures to contain it. The most common theory I've read on this is that a massive impact on the Martian planet a few billion years ago not only ejected a lot of its material but also changed the magnetizing dynamo effect of its core (or halted it altogether, not sure). The sun then gradually blew the atmosphere off of the planet. I'm a huge fan of efforts to colonize Mars; this is just one of the challenges of doing it that I know of. Terraforming in the sense that most people imagine is unfortunately not possible.