Moving resource extraction off-planet (metals from asteroids, nuclear and solar energy creation) and moving manufacturing off-planet will truly save the environment.
A pretty good, if very old, popular intro to this kind of thinking was Heppenheimer's Colonies in Space (free on the web now).
Mars seems unpromising because it's at the bottom of a gravity well, has an inconvenient atmosphere, should be preserved for scientific research while there's still a chance of traces of life, etc. Asteroids seem way more useful, and the Moon is nearer.
How are you polluting another world if there is no life there in the first place?
What we ought to be doing with Mars is terraforming it. I'd start by collecting extremophile bacteria from Earth and letting it loose on Mars and see if any of it takes hold.