New Mars Forums

If you want Mars to retain an atmosphere, all that you need to do is increase the mass of Mars.  Luckily there is a handy source of raw materials next door in the form of the Asteroid Belts.  Also, if you feel the need for more water and organics, you could use cometary cores from the outer solar system.  The side benefit is that you reduce the a number of objects that could attain an Earth-crossing orbit.