Apparently the surface of Mars is covered with a thin layer of propellants called perchlorates. These are toxic to life – meaning, presumably, life as we’ve found so far on earth – but there may still be “life as we know it” 2 or 3 metres below the surface. That’d still be pretty simple life, tho’: we’re talking bacteria, protozoa, at best tardigrades. The really good news is that it seems as if this toxic layer is not caused by earlier, less scrupulous, human exploration; that would have been a tragedy. Risk of biological or chemical contamination is one of the big problems in space exploration – it’s probably impossible to get even a non-manned probe completely clean, and any humans travelling into space are going to be reservoirs of hostile germs and bugs. Even by studying other planets, we help to destroy them. But we have to do it, it’s in our natures.
WikiP on perchlorates: https://en.wikipedia.org/wiki/Perchlorate#On_Mars
WikiP on the amazing tardigrade: https://en.wikipedia.org/wiki/Tardigrade