Top 50 words repeated twice, next 100 once, plus phrases. The risk is not zero, however. As discussed in Chapter 2, the possibility that a living organism, either active or dormant, could be included in a sample returned from Mars cannot be ruled out altogether, although the potential for such an occurrence is judged to be low. The risk of pathogenesis, or of adverse environmental effects, resulting from inad- vertent 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, innocu- ous 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. Samples returned from Mars by spacecraft should be con- tainedi and treated as though potentially hazardous until proven otherwise. No uncontained martian materials, including spacecraft surfaces that have been ex- posed to the martian environment, should be returned to Earth unless sterilized.