Considering the Potential Risks from Returned Samples
The potential for adverse environmental or human health effects arising from extraterrestrial materials returned to Earth would depend on a number of factors such as the likelihood of actually finding a living entity on a particular small solar system body targeted for exploration, including such an entity in a returned sample, and returning an entity that could in fact cause significant harmful pathological effects or large-scale ecological impacts if it were inadvertently released on Earth. Concerns about potential risks from returned extraterrestrial materials are not new, having been raised initially more than three decades ago with the return of lunar samples during the Apollo program. In 1997, the National Research Council revisited these issues, focusing on sample return from Mars. Its updated recommendations for handling returned samples and avoiding planetary cross-contamination (NRC, 1997) are summarized in Box 7.1.
Although Mars was the focus, the 1997 report's recommendations were regarded as generally applicable to any mission that could return from an extraterrestrial object to Earth a sample with a similar potential for harboring life. Indeed, based on the systematic approach developed for use in its own study, the Task Group on Sample Return from Small Solar System Bodies found that the 1997 Mars report's recommendations concerning containment and handling of returned samples constituted a suitably strong framework for guiding its deliberations about the need for containment of samples returned from planetary satellites and small solar system bodies.
In recognition of the diverse types and environmental conditions of small solar system bodies considered in this study, the task group adopted a conservative, case-by-case approach (see "Scope and Approach of This Study" in Chapter 1), taking into account information about the different bodies themselves, the natural influx to Earth of different materials, the possible nature of putative extraterrestrial life, and the range of potential adverse effects that might be caused by incoming sample materials. When applied to the solar system bodies addressed in this study—planetary satellites, asteroids and meteorites, comets, and cosmic dust—the six questions formulated by the task group, taken in the general context of the task group's approach, yield only two possible answers, with no intermediate or compromise outcomes identified: either (1) strict containment and handling as outlined in the Mars report (NRC, 1997), or (2) no special containment beyond what is needed for scientific purposes. In certain cases (e.g., P- and D-type asteroids) the limitations of the available data led the task group to be less confident, and therefore more conservative, in its assessment of the need for containment. In other words, if a body is judged, based on available knowledge about its environmental conditions and history, to have a negligible or very low potential to harbor a living entity, it would be appropriate to allow the return of samples from that body without special containment or handling beyond what is needed for scientific purposes. If, on the other hand, a body is judged to have a potential to harbor a living entity or there is reasonable uncertainty in the assessment about a
BOX 7.1 Summary of Recommendations from the 1997 Report Mars Sample Return: Issues and Recommendations
Mars Sample Return: Issues and Recommendations (NRC, 1997) concluded that the possibility of including a living organism, either active or dormant, in a sample returned from Mars cannot be ruled out altogether, although the potential for such an occurrence was judged to be low. Moreover, the report recommended that unless and until sufficient knowledge of Mars and its environment was available, due caution and care should be exercised in handling returned materials that might contain hypothetical martian microorganisms capable of inadvertently contaminating Earth and causing a risk of pathogenesis, environmental disruption, or other harmful effects.
Specifically, the report recommended that samples returned from Mars be contained and treated as though they were potentially hazardous until proven otherwise. Strict containment of all pristine sample material was urged, and special handling procedures were outlined for samples en route to and on Earth. In particular, no uncontained martian materials, including spacecraft surfaces exposed to the martian environment, should be returned to Earth unless sterilized. 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 or not returned to Earth. The integrity of containment should be maintained through reentry of the spacecraft and transfer of the sample to an appropriate receiving facility.
Furthermore, the distribution of unsterilized materials returned from Mars should be controlled and should occur only if rigorous analyses determine that the materials do not contain a biological hazard. Finally, 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.
particular body, then containment and special handling similar to the procedures recommended for samples returned from Mars (NRC, 1997) would be warranted.
LIKELIHOOD OF FINDING AND INCLUDING A LIVING ORGANISM IN SAMPLES FROM DIFFERENT SOLAR SYSTEM BODIES
In the absence of direct evidence of extraterrestrial life forms, consideration needs to be given to the nature of putative life forms based on analysis of terrestrial analogs and biochemical possibilities. Based on current knowledge of the geophysical and geochemical properties of various solar system bodies and contemporary views on the range of conditions under which life can originate, the conditions required for the preservation of metabolically active organisms in terrestrial environments, and the somewhat different conditions needed to preserve living organisms in a dormant form (see discussion in Chapter 1), it is possible to infer the likely metabolic groups of microorganisms that might be found on small bodies. They are most likely to be anaerobic organisms similar to those found in analogous Earth environments, including hydrothermal systems, sea ice, oligotrophic aquatic environments, deep basaltic rock, and soils.
The contemporary views of the nature of different solar system bodies are supported by a considerable body of scientific evidence that has been collected either remotely or from direct analyses of materials delivered to Earth by the natural influx of IDPs, asteroids, and meteorites. Presumably, this natural influx of extraterrestrial materials arriving over time on Earth, albeit with varying transit times in space, has exposed Earth and its biota to risks the same as or similar to those potentially posed by samples deliberately returned from these bodies. Based on the accumulated information and current thinking about the origin, persistence, and preservation of life as we know it, the possibility of including a living organism, either active or dormant, in a sample returned from bodies such as asteroids, comets, planetary satellites, or IDPs is judged by the task group to be very low overall. This judgment is further supported by the complete absence of evidence of the existence of even fossilized extraterrestrial life
forms or chemical fossils in meteorites from planetary satellites or small solar system bodies. However, the task group recognizes that its assessment of the potential for such an occurrence is dependent on a number of environmental and historical factors for each solar system body type, especially the presumed exposure to sterilizing doses of extreme radiation or sterilizing temperature.
Undoubtedly, the knowledge base for the composition and physical-chemical environment of various small solar system bodies will continue to expand as remotely sensed data are received from future exploration missions. Likewise, understanding will increase about the resilience of life in extreme environments, its ability to survive over long periods, and its wide versatility both on Earth and possibly in extraterrestrial environments. In the meantime, because samples are likely to be returned from small solar system bodies before we reach certainty on many important questions, it may not be possible in many cases to predict definitively the presence or absence of living entities on all bodies. Additionally, even if the native composition or current environmental conditions on a specific small body were judged inhospitable to life, the possibility of cross-contamination from impact materials originating from other bodies with more hospitable conditions must be considered. In many cases, concern about cross-contamination is negligible, but in others, it may have real significance.
ANTICIPATING THE PUTATIVE NATURE OF LIFE FROM SMALL SOLAR SYSTEM BODIES
In addition to considering the environment in which a living extraterrestrial entity might be found, it is advisable to consider the nature or type of life that might be found. Based on current understanding of solar system history, two alternative scenarios deserve attention: one involving the possible past exchange of life between Earth and other solar system bodies and the other based on an independent origin of life. In the first scenario, the biochemistry of an extraterrestrial life form might resemble that of a terrestrial organism more or less closely if its ancestor had been transported to the sample site from Earth or if the ancestor of terrestrial life has been transported from the sample site to Earth. If the extraterrestrial organism had evolved independently of terrestrial life, much greater differences would be anticipated, and the extraterrestrial life form might not share a common biochemistry with life as we know it on Earth. In searching for an exotic life form, both possibilities need to be considered, because recent experiments suggest that alternative genetic systems are possible (Egholm et al., 1993; Eschenmoser, 1997). The system studied up to now contain purine and pyrimidine bases, but the possibility of genetic materials that do not contain the standard bases cannot be excluded. In addition to considering the two possibilities discussed above, it will also be important to rule out false positives, that is, organisms or their components that may have been transported from Earth as forward contaminants on launched spacecraft or equipment. Clearly, these considerations have implications for risk assessment and for the design of protocols for systems that will eventually be used for sample handling, life detection studies, and biohazard testing of returned materials.
CONCERNS ABOUT POTENTIAL BIOHAZARDS AND ADVERSE EFFECTS
Samples returned from small solar system bodies could be considered biohazardous or unsafe for two reasons: (1) they could contain living entities that might be pathogenic for Earth organisms or (2) they could be capable of causing ecological disruptions. Although chemical toxicity of returned materials would be of concern for investigators working with the materials, it is not considered further in this context because toxic materials will not replicate and spread, and because appropriate handling and laboratory procedures will be utilized.
As discussed in the Mars sample return report (NRC, 1997), agents of pathogenicity can be divided into two fundamental types: toxic and infectious. In general, biologically induced toxic effects of microorganisms are attributable to cell components or metabolic products that interact incidentally with the nervous or immune systems of other organisms, thereby causing damage. Infectious agents may either have coevolved with their hosts or be opportunistically invasive. They cause adverse effects or damage by multiplying in or on a host: viruses are an example. The risks of pathogenicity from putative life forms on small bodies are considered extremely low, because it is highly unlikely that such extraterrestrial organisms could have evolved pathogenic traits in the absence of host organisms. However, because there are examples of opportunistic pathogens from terrestrial and aquatic environments that have not co-evolved with their hosts, the task group cannot say that the risk is zero.
Ecological disruptions by extraterrestrial microorganisms could, in theory, be caused either by displacement of native life forms or by indirect or direct modification of ecosystems as a result of the activities or presence of organisms from outside the system. Currently, there is very little information on the effects of introduced microorganisms on established microbial communities. In addition, other than documented examples of how environmental perturbations can change species composition, little is known about temporal and spatial variation in natural microbial environments on Earth. Thus, it is the task group's opinion that the probability that an extraterrestrial anaerobic microorganism could contaminate a suitable environment on Earth is very low, but not zero.
As with the case of putative martian organisms, the rationale for the above assessments is based on numerous lines of scientific evidence (e.g., the understanding of basic biogeochemical, ecological, and metabolic processes; microbial metabolic needs; resource availability; environmental conditions; physical constraints; the small amount of material sampled; strict containment and handling protocols; and so on). Clearly, it is not possible to rule out entirely the threat of adverse effects caused by the presence of viable organisms, however rare or hard to detect they may be in returned samples. Therefore, it is prudent to contain and quarantine questionable incoming samples and screen thoroughly for indications of both pathogenicity and ecological disruption, even though it is agreed by the task group that the likelihood of adverse biological effects from returned extraterrestrial samples is very low.
CONTAINMENT AND QUARANTINE FACILITIES
In situations when containment and special handling are warranted for samples returned from particular small solar system bodies, it will be advisable to handle and screen materials in a manner similar to that recommended for Mars samples (NRC, 1997). The requirements for strict containment should apply to all relevant mission activities, starting with collection of materials and separation from the target body, through en route transport of the samples, and ultimately to continued quarantine on Earth at an appropriate receiving facility until comprehensive testing is completed.
The prospect of eventually returning samples from diverse bodies throughout the solar system underscores the need for a specialized sample return facility dedicated to the study and detection of life in extreme environments. Development of relevant methods and technologies for research on returned samples is also needed. In anticipation of the variety of proposed sample return missions (see Table 1.1 in Chapter 1), it will be important to be prepared with a suitably stringent containment and quarantine facility with equipment, protocols, trained personnel, and operating procedures in place well in advance of sample return.
TESTING OF RETURNED SAMPLES
Considering the variety of small bodies and environmental conditions from which samples may be returned in the future, it will be necessary to utilize a comprehensive battery of tests combining life detection studies and biohazard screening for overall evaluation of the returned samples. It is clear that multiple lines of testing will be needed to scan returned samples thoroughly for the presence of biological entities and indications of any potential for harm. The task of developing protocols for screening samples from small solar system bodies will require additional study beyond the scope of this report. The challenges will no doubt be similar to those discussed at a recent workshop at NASA Ames Research Center on developing preliminary protocols for sample return from Mars (DeVincenzi, 1998). An entirely new set of tests and analyses will be needed to reflect major changes in science and technology since the return of lunar samples during the Apollo program. The expectation of working with small quantities of material returned from Mars and other solar system bodies introduces additional requirements that will need attention, such as how to provide capabilities for detection at extremely low levels, how to select representative samples, and how to devise suitable experimental controls for tests done during quarantine.
Although individual analytical methods may have weaknesses, a combination of a variety of methods may suffice to verify with a high level of confidence the presence or absence of self-replicating organisms. To avoid added complications from false positives, it will be critical to ensure that strict controls to prevent forward contamination are used during collection and that stringent laboratory protocols are maintained throughout sample testing.
In general, sample materials should be screened thoroughly for chemical clues or signatures as well as structural and morphological features that could be indicative of the possible existence of living entities (e.g., biogenic compounds, cellular components, and so on). Initial investigations should consider the nature of the organic material present in a sample. Analyses for organic carbon content, amino acids, nucleotides, and the components of terrestrial organisms are very sensitive and could detect very small amounts of compounds associated with microorganisms. More general analytical methods using, for example, mass spectrometry combined with chromatography or pyrolysis would be useful, particularly in establishing the presence of life forms if they are different from those with which we are familiar. An additional set of investigations should address morphology. Examination by optical microscopy and scanning electron microscopy will establish whether or not putative (possibly membrane-enclosed) cellular structures are present. The presence of such structures would be an indication of the possible presence of a living entity. However, it is notoriously difficult to distinguish fossil microorganisms from inorganic artifacts, and so morphological evidence alone may not be conclusive.
Regardless of whether these life-detection tests indicate the possibility of living entities, it will also be important to include a battery of appropriately selected biohazard tests based on culture methods as part of the preliminary sample testing to screen for potential biological activity in the sample. Such tests should focus on indications of pathogenicity and important biogeochemical functions. Considering the advances in science and technology (e.g., advances in molecular biology and biotechnology) in the decades since the original analyses of lunar samples during the Apollo program, it is likely that preliminary life detection and biohazard testing can be done without resorting to the use of whole, multicellular organisms.
Attempts need to be made to culture putative microorganisms. The culture media should be chosen taking into account the nature of the sample and the environment from which it was obtained. Given that many terrestrial microorganisms are not easily cultured, the chances of culturing nonterrestrial organisms may be small. Nonetheless, it will be important to include this analytical approach in whatever preliminary screening of samples is selected. The utility and importance of biohazard tests should not be minimized, despite their shortcomings. In the end, biohazard tests utilizing conventional culture methods may be more helpful for what they do not show; in other words, the absence of growth in test cultures may be sufficient to declare a sample nonhazardous if such results are also accompanied by negative findings in all life detection tests.
Clearly, any indications of biochemicals in combination with morphological evidence of biological structure and/or biological activity in biohazard tests would warrant verification and review by a suitably constituted scientific panel. Moreover, if evidence of viable exogenous biological entities were discovered in samples returned from small bodies, prudence would dictate that they remain segregated from Earth's biosphere in strict quarantine or be made nonviable through sterilization before any distribution outside of quarantine. In the event that fossilized life forms are found and verified, it should be possible to allow distribution under less stringent conditions providing that no other tests indicate signs of living entities in the sample.
For samples returned from planetary satellites and small solar system bodies that warrant containment, the concerns about biohazards or large-scale adverse effects on Earth are similar to those identified earlier for Mars (NRC, 1997). The task group found that the risks of pathogenicity from putative life forms are considered extremely low, because it is highly unlikely that extraterrestrial organisms could have evolved pathogenic traits in the absence of host organisms. However, because there are examples of opportunistic pathogens from terrestrial and aquatic environments that have not co-evolved with their mammalian hosts, the risk cannot be described as zero. The task group likewise found that the risks of ecological disruption from putative life forms are extremely low. The recommendations on containment and handling in the 1997 Mars report represent a strong basic framework for guiding deliberations on the potential risks associated with returned samples warranting containment. Based on current knowledge of the geophysical and geochemical properties of various solar system bodies and of life on Earth, it is unlikely that extraterrestrial organisms could cause harm or thrive in the oxygen-rich environment on Earth as they are likely to be strict anaerobes (see discussion in Chapter 1). Accordingly, the task
group concluded that the probability of an extraterrestrial anaerobic microorganism being able to contaminate a suitable environment on Earth is very low, but not zero.
For overall evaluation of returned samples warranting containment, it will be necessary to apply a comprehensive battery of tests combining both life-detection studies and biohazard screening. The task group concluded that detailed protocols and nondestructive methods still have to be developed to analyze samples, which are anticipated to be small in size but in great demand within the scientific community.
DeVincenzi, D.L. 1998. Workshop report on Mars sample return protocols. NASA Ames Research Center, Mountain View, Calif.
Egholm, M., A.O. Buchardt, L. Christensen, C. Behrens, S.M. Freier, D. Driver, R.H. Berg, S.K. Kim, B. Norden, and P.E. Nielsen. 1993. PNA hybridizes to complementary oligonucleotides obeying the Watson-Crick hydrogen-bonding rules. Nature 365:566-567.
Eschenmoser, A. 1997. Towards a chemical etiology of nucleic acid structure. Origins of Life and Evolution of the Biosphere 27:535-553.
National Research Council (NRC). 1997. Mars Sample Return: Issues and Recommendations. Washington, D.C.: National Academy Press.