The Potential for Large-Scale Effects
Contamination of Earth by putative martian microorganisms is unlikely to pose a risk of significant ecological impact or other significant harmful effects. 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 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.
Contrary to popular perception, the capacity for inducing disease (pathogenesis) is rare among Earth's microbes. Despite the stunning diversity of Earth's microbial communities and their wide-ranging physiological and metabolic properties, only a tiny fraction of chemoorganoheterotrophic (see Box 1 for explanation of terms) microbes produce adverse effects in host organisms. 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.
Recommendation. Samples returned from Mars by spacecraft should be contained1 and treated as though potentially hazardous until proven otherwise. No uncontained martian materials, including spacecraft surfaces that have been exposed to the martian environment, should be returned to Earth unless sterilized.
Recommendation. If sample containment cannot be verified en route to Earth, the sample, and any spacecraft components that may have been exposed to the sample, should either be sterilized in space or not returned to Earth.
Recommendation. Integrity of containment should be maintained through reentry of the spacecraft and transfer of the sample to an appropriate receiving facility.
Recommendation. Controlled distribution of unsterilized materials returned from Mars should occur only if rigorous analyses determine that the materials do not contain a biological hazard. If any portion of the sample is removed from containment prior to completion of these analyses, it should first be sterilized.
Samples returned from the martian surface, unless returned from sites specifically targeted as possible oases, are unlikely to harbor extant life as we know it, and there may be some pressure to reduce planetary protection requirements on subsequent sample-return missions if prior samples are found to be sterile. Presumably, however, subsequent missions will be directed toward locations on Mars where extant life is more plausible based on data acquired from an integrated exploration program, including prior sample-return missions. Thus, planetary protection measures may become more rather than less critical as the exploration program evolves. At some point it may be reasonable to relax the requirements, but this should only be done after careful scientific review by an independent body.
Recommendation. The planetary protection measures adopted for the first Mars sample-return missions should not be relaxed for subsequent missions without thorough scientific review and concurrence by an appropriate independent body.
THE POTENTIAL FOR PATHOGENIC EFFECTS
Pathogenesis can be divided into two fundamental types: toxic and infectious. Generally, toxic effects of microorganisms are attributable to cell components or metabolic products that incidentally damage other organisms. Certain bacteria, algae, and fungi, as well as some animals and many plants, produce substances that interact with the nervous or immune systems of animals. This
Box 1 Classification Nomenclature
A nomenclature based on the classification of microorganisms by nutritional requirements is designed to reflect three properties: the principal process for generating metabolic energy, the source of electrons for energy-converting reactions, and the form of environmental carbon assimilated for growth. The processes for generating metabolic energy can be divided into two classes: chemical oxidation and light absorption, designated by the prefixes chemo- and photo-, respectively. Likewise, the sources of electrons for biological energy-converting reactions can be divided into two broad classes: inorganic and organic, designated by the prefixes litho- and organo-, respectively. Finally, the sources of carbon assimilated for growth can be classified as either inorganic or organic, designated by the prefixes auto- and hetero-, respectively. A photoorganoheterotroph, for example, uses light energy to excite electrons extracted from organic compounds to support the assimilation and transformation of carbon from organic compounds. If humans were microbes, they would be classified as chemoorganoheterotrophs.
interaction is usually irrelevant to the existence of the producing organism but may be damaging or even fatal to the infected organism.
Infectious agents, which may be actively or opportunistically invasive, must multiply in or on the host in order to cause damage. The capacity of a microbe to infect a host requires an intimate interaction between the pathogen and the host and often depends on highly specific interactions between cell surfaces of the host and pathogen. Above all, infectious agents must overcome the defenses that have evolved in most potential hosts as a consequence of persistent, unremitting challenges by potential pathogens on Earth. Living organisms defend themselves mechanically and chemically in diverse ways against agents that are themselves constantly changing.
The chances that invasive properties would have evolved in putative martian microbes in the absence of evolutionary selection pressure for such properties is vanishingly small. Subcellular disease agents, such as viruses and prions, are biologically part of their host organisms, and an extraterrestrial source of such agents is extremely unlikely.
THE POTENTIAL FOR ECOLOGICAL EFFECTS
Although hypothetical extraterrestrial biota could have properties different from those of Earth biota, it must be assumed that the chemical reactions governing their metabolism would be largely the same. Putative martian microorganisms would likely be functionally similar to some of Earth's soil bacteria, but because the range of habitats available on Mars is much narrower than that on
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.