As advances in the biological and planetary sciences enable a shift from mere observation to active exploration of the solar system, space missions are increasingly likely to collect samples from planetary satellites and small solar system bodies and return them to Earth for study. This is an exciting development that offers the opportunity to search for extraterrestrial life forms and improve understanding of the origin and composition of the solar system. But sample return also involves potential risks that need to be understood and managed properly.
Accordingly, the National Aeronautics and Space Administration (NASA) asked the Space Studies Board of the National Research Council (NRC) to assess the potential for a living entity to be contained in or on samples returned from planetary satellites and other small solar system bodies such as asteroids and comets. In response to NASA's request, the Space Studies Board established the Task Group on Sample Return from Small Solar System Bodies to address the following specific tasks:
Assess the potential for a living entity to be contained in or on samples returned from planetary satellites or primitive solar system bodies, such as asteroids, comets, and meteoroids;
Identify detectable differences among small solar system bodies that would affect the above assessment;
Identify scientific investigations that need to be conducted to reduce the uncertainty in the above assessment; and
Assess the potential risk posed by samples returned directly to Earth from spaceflight missions, as compared to the natural influx of material that enters Earth's atmosphere as interplanetary dust particles, meteorites, and other small impactors.
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 for samples returned from Mars and updated previous recommendations (NRC, 1992) for handling returned samples and avoiding planetary cross-contamination (NRC, 1997). This report of the Task Group on Sample Return from Small Solar System Bodies builds on and extends that earlier work.
Because there is no direct evidence that a living entity evolved or exists on any small solar system body, the task group examined indirect evidence based on data from Earth, meteorites, and the Moon and on astronomical
observations of distant objects in an effort to assess whether NASA needs to treat samples returned from small solar system bodies differently from samples returned from Mars. To identify the requirements for the origin and survival of living organisms, the task group examined 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. Based on this analysis, the task group identified six parameters (liquid water, energy sources, organic compounds, temperature, radiation intensity, and natural influx to Earth) as relevant to its assessment and formulated the following six questions to help determine how returned samples should be handled.
Does the preponderance of scientific evidence indicate that there was never liquid water in or on the target body?
Does the preponderance of scientific evidence indicate that metabolically useful energy sources were never present?
Does the preponderance of scientific evidence indicate that there was never sufficient organic matter (or CO2 or carbonates and an appropriate source of reducing equivalents)1 in or on the target body to support life?
Does the preponderance of scientific evidence indicate that subsequent to the disappearance of liquid water, the target body has been subjected to extreme temperatures (i.e., >160 °C)?
Does the preponderance of scientific evidence indicate that there is or was sufficient radiation for biological sterilization of terrestrial life forms?
Does the preponderance of scientific evidence indicate that there has been a natural influx to Earth, e.g., via meteorites, of material equivalent to a sample returned from the target body?
For the purposes of this report, the term ''preponderance of scientific evidence" is not used in a legal sense but rather is intended to connote a nonquantitative level of evidence compelling enough to research scientists in the field to support an informed judgment. In applying the questions, the task group drew on existing data on the origin, composition, and environmental conditions (past and present) of each small body or planetary satellite examined and then determined whether the quality and weight of the evidence were convincing enough to allow making judgments and deriving findings. The answers to the questions, taken together, were used by the task group to reach a considered conclusion that the potential for a living entity to be in on a returned sample was either "negligible" or "not negligible." Because of the incomplete current state of knowledge about small solar system bodies, there are no definitive answers to the questions, and so all judgments regarding biological potential are qualitative (not quantitative).
The questions allow for a conservative, case-by-case approach to assessing whether or not special physical and biological isolation and handling of returned samples (containment) would be warranted, taking into account information about the different small bodies, natural influx to Earth of material from small bodies, and the possible nature of putative extraterrestrial life. An answer of "yes" to any question argues against the need for special containment beyond what is needed for scientific purposes. (Sample-handling requirements to support scientific investigations are currently under study by NASA.) For containment procedures to be necessary, an answer of "no" needs to be returned to all six questions. For such samples, strict containment and handling as outlined in Chapter 7 are required.
The task group chose to consider only two possible alternatives for containment and handling of samples returned from small solar system bodies: either (1) strict containment and handling of returned samples as outlined in the Mars report (NRC, 1997) or (2) no special containment beyond what is needed for scientific purposes. The task group ruled out intermediate or compromise procedures involving partial containment. In certain cases (e.g., P- and D-type asteroids) the limitations of the available data led the task group to be less certain, and therefore more conservative, in its assessment of the need for containment.
The following section summarizes the task group's findings with regard to the potential for a living entity to be present in samples returned from select planetary satellites and small solar system bodies. The selection was
based on scientific interest and the likelihood of possible sample return missions to those destinations in the near future. These findings provided the basis for the task group's conclusions and recommendations, which are presented directly afterward.
Satellites are natural consequences of planetary formation processes. The task group considered the possibility of sample return from the major satellites of the innermost planets. These include the satellite of Earth (the Moon), satellites of Mars (Phobos and Deimos), and selected satellites of Jupiter (Io, Europa, Ganymede, and Callisto). The potential for a living entity to be present in samples returned from the Moon and Io is negligible. The potential for a living entity to be present in samples returned from Phobos, Deimos, and Callisto is extremely low, but the task group could not conclude that it is necessarily zero. Importantly, the task group found that there is a significant potential for a living entity to be present in samples returned from Europa and Ganymede.
Asteroids are the remnants of planetesimals—small primordial bodies from which the planets accumulated. Common asteroid types include undifferentiated, primitive types (C-, B-, and G-types); undifferentiated metamorphosed types (Q- and S-types [ordinary chondrites]); and differentiated types (M-, V-, J-, A-, S- [stony irons], and E-types). Other types of asteroids have been defined, including the common P- and D-types in the outer parts of the asteroid belt, but little is known about their composition and origin. Others are subdivisions of the types listed above, whereas still others are rare, new types, generally seen only among the population of very small asteroids. For undifferentiated, primitive (C-type) asteroids, the potential for a living entity to be contained in returned samples is extremely low, but the task group could not conclude that it is necessarily zero. Because of a fundamental lack of information about P- and D-type asteroids, the potential for a living entity to be present in returned samples cannot be determined and, therefore, was considered conservatively by the task group as possible at this time. For all C-type asteroids, undifferentiated metamorphosed asteroids, and differentiated asteroids, the potential for a living entity to be present in returned samples is extremely low, but the task group could not conclude that it is necessarily zero.
Comets are believed to have formed in the protoplanetary disk, at distances from the Sun ranging from the distance of proto-Jupiter to far beyond the distance of proto-Neptune. It is unlikely that a living entity could exist on comets, but the possibility cannot be completely ruled out except in a few cases, such as in the outer layers of Oort Cloud comets entering the solar system for the first time. Thus, the potential for a living entity to be present in returned samples from all comets was considered by the task group to be extremely low, but the task group could not conclude that it is necessarily zero.
Because interplanetary dust particles (IDPs) are derived from a variety of sources, including interstellar grains and debris from comets, asteroids, and possibly planetary satellites, IDPs cannot be viewed as a distinct target body. As a result, the assessment approach used in this study does not lend itself readily to IDPs. Instead, the task group considered the potential source(s) of any IDPs that might be returned in samples. For the purposes of this study, IDPs are viewed as originating from either a single identifiable parent body or multiple sources. Particles collected near a particular solar system body are viewed as originating from that body, possibly including grains recently released from that body. Thus, the potential for a living entity to be present in returned samples, and the
associated containment requirements, will be the same as those for the parent body. On the other hand, IDPs collected in the interplanetary medium may represent a mixture of dust originating from many parent bodies. Because IDPs in the interstellar medium are exposed to sterilizing doses of radiation, the potential for IDPs to contain viable organisms or a living entity is negligible.
CONCLUSIONS AND RECOMMENDATIONS
Table ES.1 summarizes the task group's assessment of the level of containment and handling warranted for samples returned from the planetary satellites and small solar system bodies examined in this study. Box ES.1 summarizes the requirements that apply to samples for which strict containment and handling are advisable. It is important to note that the task group's recommended approach is provided only as a guide and not as an inflexible protocol for determining whether containment is required. The final decision must be based on the best judgment of the decision makers at the time and, when possible, on experience with samples returned previously from the target bodies.
Containment of Returned Samples
On the basis of available information about the Moon, Io, dynamically new comets (specifically the outer 10 meters), and interplanetary dust particles (sampled from the interplanetary medium, sampled near the Moon or Io,
TABLE ES.1 Summary of Currently Recommended Approach to Handling Samples Returned from Planetary Satellites and Small Solar System Bodies Assessed by the Task Group on Sample Return from Small Solar System Bodies
No Special Containment and Handling Warranted Beyond
What Is Needed for Scientific Purposes
Strict Containment and Handling Warranted
High Degree of Confidence
Lesser Degree of Confidencea
Dynamically new cometsb
Interplanetary dust particlesc
Undifferentiated metamorphosed asteroids
Interplanetary dust particlesd
All other comets
Interplanetary dust particlese
a Subcolumn Ib lists those bodies for which confidence in the recommended approach is still high but for which there is insufficient information at present to express it absolutely. This lesser degree of confidence does not mean that containment is warranted for those bodies; rather, it means that continued scrutiny of the issue is warranted for the listed bodies as new data become available. The validity of the task group's conclusion that containment is not warranted for the bodies listed in Ib should be evaluated, on a case-by-case basis, by an appropriately constituted advisory committee in light of the data available at the time that a sample return mission to the body is planned.
b Samples from the outer 10 meters of dynamically new comets.
c Interplanetary dust particles sampled from the interplanetary medium and from the parent bodies listed in subcolumn Ia.
d Interplanetary dust sampled from the parent bodies in column II and collected in a way that would not result in exposure to extreme temperatures.
e Interplanetary dust sampled from the parent bodies listed in subcolumn Ib.
BOX ES.1 Summary of Requirements for Samples That Need Strict Containment and Handling
All samples returned from planetary satellites and small solar system bodies that must be contained should be treated as potentially hazardous until proven otherwise. As in the 1997 Mars report (NRC, 1997), strict containment is recommended for all pristine sample material, and special handling procedures are needed for samples en route to and on Earth. 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. Integrity of containment should be maintained through reentry of the spacecraft and transfer of the sample to an appropriate receiving facility. Furthermore, distribution of unsterilized materials returned from small bodies should be controlled and should occur only if rigorous analysis shows that the materials do not present a biological hazard. Finally, the planetary protection measures adopted for the first sample return mission to a small solar system body should not be relaxed for subsequent missions without a thorough scientific review and concurrence by an appropriate independent body.
or sampled in a way that would result in exposure to extreme temperatures), the task group concluded with a high degree of confidence that no special containment is warranted for samples returned from those bodies beyond what is needed for scientific purposes.
Recommendation: Samples returned from the Moon, Io, the outer 10 meters of dynamically new comets, and interplanetary dust particles (from the interplanetary medium, near the Moon, Io, or dynamically new comets), or sampled in a way that would result in exposure to extreme temperatures (e.g., spike heated), should not be contained or handled in a special way beyond what is needed for scientific purposes.
For samples returned from Phobos and Deimos, Callisto, C-type asteroids, undifferentiated metamorphosed asteroids, differentiated asteroids, and comets other than dynamically new comets, the potential for a living entity in or on a returned sample is extremely low, but the task group could not conclude that it is zero. Based on the best available data at the time of this study, the task group concluded that containment is not warranted for samples returned from these bodies or from interplanetary dust particles collected near these bodies. However, this conclusion is less firm than the conclusion for the Moon and Io and should be reexamined at the time of mission planning on a case-by-case basis.
Recommendation: For samples returned from Phobos and Deimos, Callisto, C-type asteroids, undifferentiated metamorphosed asteroids, differentiated asteroids, comets other than dynamically new ones, and interplanetary dust particles sampled near these bodies, a conservative, case-by-case approach should be used to assess the containment and handling requirements. NASA should consult with or establish an advisory committee with expertise in the planetary and biological sciences relevant to such an assessment. The goal of such an assessment should be to use any new, relevant data to evaluate whether containment is still not warranted. This assessment should take into account all available information about the target body, the natural influx to Earth of relevant materials, and the likely nature of any putative living entities. Such an advisory committee should include both NASA and non-NASA experts and should be established as early in the mission planning process as possible.
For samples returned from Europa and Ganymede, the task group concluded that strict containment and handling requirements are warranted. Because the knowledge base for P- and D-type asteroids is highly speculative, the task group concluded conservatively that strict containment and handling requirements are warranted at
this time. Strict containment and handling requirements are also warranted for interplanetary dust particles collected near these bodies unless they are sampled in a way that would result in exposure to extreme temperatures, e.g., spike heated.
Recommendation: Based on currently available information, samples returned from Europa, Ganymede, P- and D-type asteroids, and interplanetary dust particles sampled near these bodies should be contained and handled similarly to samples returned from Mars (NRC, 1997). Interplanetary dust particles sampled in a way that would result in exposure to extreme temperatures, e.g., spike heated, should not be contained or handled in a special way beyond what is needed for scientific purposes.
Handling of Returned Samples
For samples that are returned from planetary satellites and small solar system bodies and 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 concluded that the risks of pathogenicity from putative life forms are 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 hosts, the risk cannot be described as zero. The recommendations on containment and handling in the Mars report (NRC, 1997) represent a strong basic framework for addressing potential risks associated with returned samples warranting containment.
The microbial species composition of most anaerobic environments on Earth is not known, and consequently it is also not known how the species composition of these anaerobic microbial communities might change over time, what environmental factors might influence these changes, or what the incidence of and successful colonization by new species of microorganisms in these habitats might be. Accordingly, the task group concluded that although there is a low likelihood of a viable anaerobic microorganism surviving transport through space and finding a suitable anaerobic habitat on Earth, growth in a suitable habitat if found might be possible. This conclusion is necessary because of the current lack of information about anaerobic environments on Earth that may be analogous to environments on other solar system bodies, and the likelihood that the metabolic properties of such an extraterrestrial anaerobe would resemble an Earth anaerobe from a similar environment.
For overall evaluation of returned samples that warrant containment, it will be necessary to apply a comprehensive battery of tests combining both life-detection studies and biohazard screening.
Recommendation: Returned samples judged to warrant containment should be quarantined and screened thoroughly for indications of a potential for pathogenicity and ecological disruption, even though the likelihood of adverse biological effects from returned extraterrestrial samples is very low.
Recommendation: NASA should consult with or establish an advisory committee of experts from the scientific community when developing protocols and methods to examine returned samples for indicators of past or present extraterrestrial life forms.
Recommendation: The planetary protection measures adopted for the first sample return mission to a small body whose samples warrant special handling and containment should not be relaxed for subsequent missions without a thorough scientific review and concurrence by an appropriate independent body.
Scientific Investigations to Reduce Uncertainty
Identified by the task group in Chapters 2 through 6 is scientific research that could help to reduce the uncertainty in its assessment of the potential for a living entity to be contained in or on samples returned from planetary satellites and small solar system bodies. Because most of the suggested research topics are general in scope, they are not repeated here. However, one topic is of sufficient importance that it requires emphasis.
Because organisms subjected to sterilizing conditions for a sufficient time period pose no threat to terrestrial ecosystems, it is important to assemble a database on the survival capacity of a wide range of terrestrial organisms under extreme conditions. Despite the existence of a rich literature on the survival of microorganisms exposed to radiation and high temperatures, the studied taxa represent only a small sampling of the microbial diversity known to exist in the biosphere and, in general, have not been taken from extreme environments. Little is known about the radiation and temperature resistance of microorganisms from environments on Earth that have the chemical and physical characteristics likely to be encountered in or on small solar system bodies.
Recommendation: NASA should sponsor research that will lead to a better understanding of the radiation and temperature resistance of microorganisms from environments on Earth that have the chemical and physical characteristics likely to be encountered in or on small solar system bodies. Information on the survival of organisms subjected to long- or short-term ionizing radiation needs to be collected for both metabolically active and dormant stages of diverse groups of microorganisms, including hyperthermophiles, oligotrophic chemoorganotrophs, and chemolithoautotrophs. Likewise, it is important to establish short- and long-term temperature survival curves for similarly broad groups of metabolically active and dormant organisms. In particular, data are required on survival of diverse microorganisms under flash heating (1- to 10-second exposures) to temperatures between 160 °C and 400 °C.
National Research Council (NRC). 1992. Biological Contamination of Mars: Issues and Recommendations. Washington D.C.: National Academy Press.
National Research Council (NRC). 1997. Mars Sample Return: Issues and Recommendations. Washington D.C.: National Academy Press.