Don't assume that people are rational
It would easy to establish one's credentials as a "green" by fighting for the ecology of another world through opposing Mars colonization. At the same time you'd get bonus points by standing up to "mindless progress", colonialism, the use of nuclear energy and the corporate greed of the aerospace industry (military industrial complex). There are too many hot buttons here. The best case scenario would be for Mars to have billion year old fossils but currently be sterile.
Gary