This is old news in astronomy circles but I just started thinking about it recently – we may have been polluting Mars for decades. I don’t mean landing probes, rovers, and the like on its surface since the first Viking mission in 1976. Microbes may have been hitching a ride on those probes and spreading to a degree in the areas surrounding those landing sites. There are two big ways this could be bad thing. It is possibly the pollution (even if accidental) of another world and it can interfere with scientific investigations to detect native Martian life.
NASA has long since been cleaning, baking, and otherwise disinfecting its probes before they ever get close to a launch pad. They are nowgiving a special eye to making sure the probes are absolutely sterile. There are some environments on Mars in which Earth-native microbes could survive and grow.
There is nothing intrinsically bad about Earth life growing on Mars. In a way, it’s kind of uplifting and encouraging. It would mean that life is possible on Mars, it could be common in the universe, and it might be possible for us to grow our own food there, a necessary first step toward colonization. The greatest drawback would be that it could contaminate Martian soil samples and make it difficult to determine if life already exists there.
Mars, for the most part, is untouched by human hands. It is a perfectly natural world. As missions to Mars become more common, we will have to decide how important it is to preserve Mars. Maybe we should declare certain areas of the planet to be off-limits to human exploration in order to preserve them, in much the same way we create national parks and reserves to preserve nature here on Earth.
This is really something that bears consideration as we plan for more missions to Mars. We have already made a mark upon our own world; we should be careful not to do the same to others.