To Mars, with bugs
Nice article on the science and sense of keeping Mars free of terrestrial pollutants and vice-versa, from treaty obligations to technical implications.
That human explorers will “contaminate” Mars is inevitable — humans contain oceans of bacteria, and a human presence on Mars is sure to leave some behind. Even if all wastes are bagged and returned to Earth (unlikely because of the expense involved), some germs are bound to escape via air leaks, transport on surfaces of Mars suits and other objects that exit the spacecraft, etc.
NASA now takes extensive steps to sterilize unmanned spacecraft so as to keep Earth germs from reaching other planets, something known in the trade as “forward contamination.” Such precautions may be adequate for robotic missions, but it is simply impossible to ensure that missions involving people won’t result in contamination. They will.