It doesn’t, it’s a one way trip. It’s much harder to take off from Mars than from the Moon. There are planetary protection issues also for returning a spacecraft from Mars because there’s still some chance of present day life on the surface of Mars. We’d need to know it is safe to return or else sterilize the samples or make sure that anything returned is contained in such away that it can’t introduce Mars microbes to Earth. The risks from microbes from another planet released in the wild by mistake (if they exist) are similar to the risks from creating synthetic microbes in the laboratory not based on DNA and releasing them in the wild.