Forward Contamination

Forward contamination is the passing of earth organisms to other space objects (of particular relevance, the Moon and Mars). The issue is discussed in Time Magazine.

Even though Phoenix was assembled in a special clean room to minimize bacterial contamination, and its arm, which would have direct contact with Martian ice, was heat-sterilized before launch, it's likely that dozens or more species of microbes hitched a ride on Phoenix's 10-month trip to Mars. Once on Mars, it's possible that bacteria shielded by the structure of the spacecraft from the harsh Martian UV radiation could stay alive, in dormancy, for hundreds of thousands of years. And if native microbes do exist on Mars — nothing has been found yet, but scientists hold out hope that the ice present on parts of the planet harbors life — there's a risk that foreign bacteria could contaminate or somehow change the development of their Martian counterparts.