The delta V requirements to go from the outer solar system to Mars are far less than those required to go from Earth to Mars. Plus, an ammonia iceteroid can be

The delta V requirements to go from the outer solar system to Mars are far less than those required to go from Earth to Mars. Plus, an ammonia iceteroid can be moved around virtually for free (with significant mass lost upon arrival) since ammonia can serve as rocket fuel, and thus the iceteroid can act as a giant fuel tank.