Earth, the diversity of putative martian organisms would be correspondingly smaller.

If hypothetical martian organisms are indeed functionally similar to microorganisms on Earth, there would be little threat of widespread ecological disruption resulting from their inadvertent introduction into the biosphere. Such organisms would meet stiff competition for resources in habitable sites, where, if there is water that is at least occasionally liquid, there will be a community of microorganisms that is well adapted to existence at that site and exploits its resources to the limits of their availability.

Extraterrestrial microorganisms would be unlikely to utilize nutrients that Earth organisms do not already consume efficiently. Few compounds containing available potential energy are known that cannot be consumed by terrestrial microorganisms. A great deal of study has been devoted to biodegradation of diverse substances. In most cases, microorganisms have been found that consume energy-yielding compounds. When utilization is limited, it is generally limited by physical constraints such as lack of critical nutrients or physical inaccessibility, rather than by microbial potential. Extraterrestrial organisms would be limited by these same physical constraints.

It is unlikely that putative martian organisms would be capable of out-competing Earth organisms for nutrients. Earth's microorganisms are optimally adapted to their environments as a result of millions of years of intense competition. Laboratory microbes that have been bred and engineered to utilize (and thereby biodegrade) particular substances at an accelerated rate usually fail in the field because they cannot compete with the well-adapted microorganisms that already exist there (Fry and Day, 1992).

The possibility of life on Mars cannot be excluded on the basis of our current understanding of the martian environment (see Chapter 2). Nevertheless, the potential for including a living entity in a sample returned from Mars is judged to be low, especially if the sample is returned from a site that has not been specifically targeted as a possible oasis. The potential for returning an organism that could grow and multiply in the terrestrial environment is lower still. If an organism were returned that could survive on Earth, the potential for large-scale ecological or pathogenic effects still would be low. Any organism that could survive in Earth's environment would meet intense competition from well-adapted terrestrial organisms that occupy their habitats to the limits of available resources. It is especially unlikely that putative martian organisms could be agents of infectious disease. Such a capability requires specific adaptations, for which there would be no selection pressure on Mars, to overcome the elaborate defenses against invasion possessed by terrestrial organisms.

There are large uncertainties associated with these assessments, however, and the risk of potentially harmful effects is not zero.