the only realistic chance is to find a mega iceteroid than just happens to only need a nudge to get it to crash into mars, preferably going realy real fast and into one of the poles, that then releaes enough water and heat to react most of the nasty stuff and increase the atmospheric pressure. ion engine space tugs, huge ones, and for that matter, crash anything that is possible into the poles. Pro tip, aim for the fourth planet out, #4, red one, you dont want this happening to the planet you are standing on.