Mars and the Asteroid Belt
"Humans have reached the asteroid belt and have returned..."
Colonization of Mars is a relatively recent initiative; the technology to terraform planets is less than 50 years old, and still in its infancy. Only a concerted effort by both human and Zentradi forces after the Zentradi War rendered it readily habitable to humans.
Martian colonization began shortly after the Zentradi War, using a combination of Earth-made nanotechnology and Zentradi know-how and manpower. Mars has an atmosphere which is livable for short durations outside thanks to environmental remediation nanomachines, but the majority of actual habitation and day-to-day life takes place in domed colonies.
Over the course of 0095, most Martian colonies were destroyed by the Jovian Lizards, setting habitation back easily two decades. There are a handful of cities that remain, mostly due to excessive fortifications designed to give war-inclined Zentradi something to do. The primary habitation that remains, Alba City, was one of Mars's largest cities even before most of the others were destroyed.
Zeon remnant forces also have a fairly strong presence on Mars; to an even greater extent than on Earth, these pockets are left alone.
The Asteroid Belt
The belt of asteroids past Mars used to be a place for those fleeing the Earth Sphere to hide; now, it's just another stop on the resource highway. Most of what's out at the asteroid belt is mass drivers and small colonies that support -- and in turn get their support from -- the Jupiter Energy Fleet that moves through there. The food production caps are much more relaxed here than in near-Earth colonies, and enforcement would be difficult anyway; the atmosphere in the asteroid belt is lax. Everyone lives and lets live; it's also notoriously easy to start a new life here, as belt colonies tend to be more than happy to help new residents create new legal identities.