Mars and some other places like Europa are suspected to host or to have hosted life at some point. It's worth applying this policy uniformly because spores are

Mars and some other places like Europa are suspected to host or to have hosted life at some point. It's worth applying this policy uniformly because spores are quite hardy and it can be surprising where fragments of contaminated celestial bodies might end up. Even thoroughly scrambled genetic material could influence the development of extraterrestrial microbes whose biochemistry might or might not be compatible with the genetic language of life on Earth.