they can, though the transportation of the materials as well as the workers are too crucial and expensive..There is also a very high possibility that the people can't go back to earth safely and no one will secure or check the greenhouse unless a robot or something like that surveys the area..Still there is a possibility..about the O2 thingy it will not fly out in space because mars has gravity..and 1 grrenhouse isn't enough for that...hehehe...