The terraforming of Mars was well underway, but not complete, at the onset of the Nano Plague. Now, several hundred years of genetically modified flora, a few comets, and a plethora of atmospheric processing stations later, Mars is not too horribly worse than many places in Tibet. Which is to say, the temperatures are cold, the air pressure is somewhat low, the oxygen isn't actually that bad, but a facemask and thermal suit is still considered normal outerwear, even if not strictly necessary at all altitudes and times of year. The population of Mars is much smaller than Earth's but boasts higher percentages of modified human gene stock and Mechanists. This is both because these types are often more comfortable in Martian climes than unmodified humans, and because the Purist plurality on Mars is both much smaller and less extreme than the Purist majority on Earth. It is the capital of the Confederation of Inhabited Worlds.