The Potential for Large-Scale Effects Contamination of Earth by putative microorganisms is unlikely to pose a risk of significant ecological impact or other significant harmful effects. The risk is not zero, however. The risk of pathogenesis, or of adverse environmental effects, resulting from inadvertent contamination of Earth with hypothetical martian microbes is lower still, although it is not zero. Accordingly, it is reasonable for NASA to adopt a prudent approach to sample return, erring on the side of caution and safety.
Pathogenesis is even rarer among phototrophs, lithotrophs, and autotrophs.
In contrast, innocuous and mutually beneficial associations between microbes and other terrestrial organisms are so common that strenuous technical efforts are required to prevent the inhabitation of plants and animals by benign microorganisms. In general, microorganisms introduced into natural ecosystems are not known to significantly alter their new environments. While the risk of large-scale effects is low, the consequences are potentially serious. Therefore, unless and until sufficient knowledge of Mars and its environment is available such that assessment of the risk of pathogenesis, environmental disruption, or other harmful effects resulting from the inadvertent contamination of Earth with hypothetical martian microbes can be effectively reduced to zero, due caution and care should be exercised in handling materials returned to Earth from Mars. Samples returned from Mars by spacecraft should be contained 1 and treated as though potentially hazardous until proven otherwise.