"'Contamination by even one Earth bacterium may be a serious issue of environmental ethics,' McKay contends." But I'd wager this guy believes that Darwinism is "proven".
If Darwinism is proven, then it's "survival of the fittest" and whatever we do to Mars is Mars' tough luck. Too bad, indigenous Martian lifeforms: you've been out-competed. So sorry, but you're obsolete, and evolution has no qualms about disposing of obsolete species.
I find it interesting that environmentalists--most of whom contend that evolution is proven--nonetheless subscribe to the notion of "human exceptionalism" that comes to us from Christianity, which holds that humans are a special case.
If you believe that Man evolved from lower life forms, he's part of nature, and anything he does is part of nature. Like a beaver building a dam, we build...well, all kinds of stuff, which alters our environment to make it more to our liking. This means that if we go to Mars and bring all our bugs with us, and whatever Martian life that may exist is out-competed and goes extinct, well--it's all part of nature. It's evolution in action: only the strong survive.
On the other hand, if you believe that Man is exceptional in creation, and that anything he does to nature is intereference, then you must also accept the fact that human exceptionalism is a strike against evolution. If Man is not a part of nature, he has to have come from somewhere, and creationism is the only viable alternative to evolution.
You guys can't have it both ways. If we evolved, then anything we do to Mars is okay, because it's evolution in action. If Man is exceptional, then evolution isn't "proven".