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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 8 Conclusions and Recommendations This chapter summarizes all the findings of the task group and presents its conclusions and recommendations organized to respond specifically to the tasks assigned to it. As pointed out in Chapter 1, the task group considered only two possible containment and handling requirements: 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. ASSESSMENT OF POTENTIAL FOR A LIVING ENTITY TO BE PRESENT IN OR ON SAMPLES RETURNED FROM SMALL SOLAR SYSTEM BODIES Planetary Satellites Satellites are natural consequences of planetary formation processes. They can form around planets through condensation and agglomeration of material from circumplanetary gas and dust disks. Natural satellites can also develop from shorter-lived disks produced by large impacts on a growing planet; such a process may have produced Earth's Moon. Some satellites may be captured objects, that is, objects that formed elsewhere in the solar system but were drawn into orbit around a planet by aerodynamic drag forces generated by passage through an extended early planetary atmosphere. The task group considered the possibility of sample return from the major satellites of the innermost five 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 selection was based on scientific interest and the likelihood of possible sample return missions in the near future. Moon The Moon is a large rocky body with a history dominated by volcanism and by impacts of interplanetary debris. Many samples of lunar rocks and soils were returned to Earth by the U.S. Apollo and Soviet Luna programs. None has been found to contain any evidence of past or present lunar biological activity. Because of this direct evidence, and because there is no indirect evidence for recent or past liquid water on the Moon (although small amounts of polar ice have recently been discovered), the potential for a living entity to be present in returned samples is negligible.
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making Phobos and Deimos The two natural satellites of Mars are small, irregularly shaped rocky objects. With maximum dimensions of 27 km (Phobos) and 15 km (Deimos), they are similar in size and shape to typical asteroids. The origin of Phobos and Deimos is unclear. Both lie in orbits that are low in inclination and nearly circular. Overall, the limited spectral data and poorly determined densities of Phobos and Deimos are broadly consistent with their being similar to C- or P-type asteroids. It is possible that Phobos and Deimos may have experienced liquid water early in their history, but their primitive chemical composition would have led to potentially sterilizing levels of radiation since then. It is unlikely that ice-filled voids, which might have attenuated that radiation, are present today within the upper reaches of Phobos and Deimos that are accessible to sample return missions, because typical subsurface temperatures are far too high for ice to exist in equilibrium. On the other hand, biological materials, if any, could conceivably have been protected in ice pockets at depth for considerable periods of time, transported more recently to near-surface environments by catastrophic collisional disruption, and subsequently reassembled in Mars orbit and by near-surface processes of regolith turnover. Although sampling of any resulting, potentially hazardous material is very unlikely, it cannot be categorically ruled out. Although it is clear that some small fraction of ejecta from impacts on Mars will be transferred to the planet's satellites, such material would be present only in a random (rather than targeted) sampling from the surface of Mars (which is known to be hostile to biological materials) and would be similar to SNC meteorites already striking Earth (which have been found not to be hazardous). Thus, 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. Io Io is the innermost of the galilean satellites, with a radius of more than 1,800 km, making it slightly larger than the Moon. Its composition is dominated by rock and possibly metal. Io is the most volcanically active known body in the solar system. Although abundant biologically useful energy is present, there is no evidence for the present or past existence of solid or liquid water at or beneath the surface. Because essentially all of the material at the surface of Io is volcanic, a good case can be made that all accessible material on the satellite has been heated at some point in time to temperatures higher than the maximum tolerated by any organic material. Io is also exposed to Jupiter's powerful magnetosphere; charged particles trapped in the magnetosphere continually bombard Io's surface at high flux levels and with high energy. This radiation would serve as a powerful inhibitor of biological activity at the surface of Io. Because of the lack of water in any form and the additional sterilizing influence of jovian magnetospheric bombardment on near-surface materials, the potential for a living entity to be present in returned samples is negligible. Europa Europa is one of the solar system bodies that appears to have a potential for past or present life. Europa has a radius of about 1,600 km, slightly less than the Moon's, and it is probably mostly silicate and metal by mass. It has an upper layer, on the order of 100 km deep, composed of liquid and/or solid water. There is evidence of liquid water beneath the icy crust, first surmised from Voyager data and reinforced by Galileo data. Accordingly, the task group found that there is a potential for a living entity to be present in samples returned from Europa. Ganymede Ganymede is the largest satellite in the solar system. With a radius of some 2,600 km, it is larger than the planet Mercury. Ganymede's density is probably roughly 50 percent water by mass. Although there is no evidence of the current presence of liquid water beneath the icy crust, the past presence of liquid water, even if only at a great depth, cannot be ruled out. Hydrothermal activity near the silicate/ice boundary could have occurred through at least some of the satellite's history, and through-going convection in the ice layer could have transported frozen hydrothermal fluids to near-surface regions. It is doubtful, but still possible, that subsurface
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making material was extruded to the surface during the formation of the grooved terrain. Such extrusion, which may have occurred in geologically recent times (e.g., only hundreds of millions of years ago) is certainly a possibility. Accordingly, the task group found that there is a potential for a living entity to be present in samples returned from Ganymede. Callisto Callisto is similar to Ganymede in size (with a radius of approximately 2,400 km) but, unlike its neighboring satellite, exhibits negligible evidence of resurfacing or tectonism. Instead, its history has been dominated by impacts. There is no evidence that liquid water ever existed on Callisto, but the possibility cannot be ruled out. Callisto's moment of inertia is consistent with only modest internal differentiation, a significant finding from a biological perspective because it implies that silicates, rather than being concentrated near the satellite's center as they are in Ganymede, are distributed more nearly uniformly. Without a substantial rocky or rocky-metallic interior, silicate magmatism and hydrothermal activity are unlikely to have taken place within Callisto. Thus, no biologically useful source of energy is likely to have existed, even if liquid water was present at some place or time during the satellite's history. Because Callisto lacks an adequate source of energy to melt ice, and there is no direct evidence for past or present liquid water, the potential for a living entity to be present in returned samples is small. Callisto could be subject to cross-contamination by ejecta from another body in the Jupiter system that has some biological potential (e.g., Europa). However, such material would constitute such a trivial volume of Callisto's surface material that the odds of sampling it would be negligible. Asteroids Asteroids are the remnants of planetesimals—small primordial bodies from which the planets accumulated. Generally, asteroids are relic planetesimals formed in and beyond the asteroid belt (which is located between 2.2 and 3.2 AU from the Sun), as far away from the Sun as the Trojans, which orbit at Jupiter's distance. Those formed in more distant locations are usually considered to be comets. 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. The early environment of C-type asteroids may have been suitable for harboring a dormant living entity in buried ice pockets. There is unequivocal evidence of liquid water having been active within at least some C-type asteroids approximately 4.5 Gyr ago. There is meteoritic evidence that some C-type asteroids have experienced temperatures above 160 °C following aqueous activity, but a substantial fraction have not been so heated. Except for possible localized volumes of water ice in C-types, the interiors of C- and undifferentiated S-type asteroids would have experienced sterilizing doses of radiation from the decay of natural radionuclides during the 4.5 Gyr since cessation of aqueous activity. Meteorites and IDPs have delivered samples of many asteroids to Earth. Whether the C-type material received on Earth is representative of a particular target body remains uncertain, because the sampling of C-type asteroids may be sporadic and nonrepresentative. Meteorites and the precursor fragments from which they are derived may well have been large enough that their interiors have been protected from sterilizing doses of radiation from galactic and solar cosmic rays while in transit to Earth. Furthermore, pockets of volatile compounds within chondritic materials could potentially shield dormant organisms from radiation damage. For these reasons, although the potential for a living entity to be present in returned samples from C-type asteroids is extremely low, the task group could not conclude that it is necessarily zero. To reduce uncertainty when assessing the potential for a living entity to be present on C-type asteroids, it would be helpful to conduct studies to document whether such a body targeted for a sample return mission is similar to nonhazardous meteorites falling naturally on Earth. Very little is known about P- and D-type asteroids. It is plausible that they are like C- type asteroids and dormant comets derived from the Jupiter zone. But caution is warranted as their nature is truly only a matter of
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making speculation. There is a small, perhaps negligible, natural influx to Earth of meteorites from P- and D-type asteroids. For most P- and D-type asteroids that have been observed, there is no evidence that liquid water was present in the past. Because of the general lack of information about P- and D-type asteroids, the potential for a living entity to be present in samples returned from them cannot be determined and, therefore, is considered conservatively by the task group as possible at this time. Undifferentiated metamorphosed asteroids and differentiated asteroids are dry and have been heated to very high temperatures. A minor fraction of S-type asteroids may have experienced a transient episode of aqueous activity, but the great majority of S-types have never been exposed to liquid water. Like C-type asteroids, S- and Q-types would have experienced sterilizing doses of radiation from decay of natural radionuclides during the 4.5 Gyr since cessation of any aqueous activity. Thus, for all S-type asteroids, the potential for a living entity to be present in returned samples is negligible. Liquid water can also be ruled out for all differentiated asteroids, and so the potential for living entities to be present in samples returned from these asteroids is similarly negligible. However, there is clear evidence in meteorites that substantial cross-contamination of material from one asteroid to another has occurred. Therefore, for all undifferentiated metamorphosed asteroids as well as for 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 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. Cometary nuclei are predominantly icy bodies that, most likely, have never melted on large scales. For most of their 4.6-Gyr lives they have been at temperatures below -173 °C (100 K), with those in the Oort Cloud at temperatures on the order of -263 °C (10 K). The surface layers have undergone months-long transient heating to temperatures on the order of 27 °C (300 K) (depending on heliocentric distance) at previous perihelion passages. The generally accepted view is that cometary material is sufficiently porous that the volatile substances escape as vapor rather than remaining trapped as a liquid. Irradiation by galactic cosmic rays of most cometary bodies outside the heliosphere (Oort Cloud comets but not Kuiper Belt comets) would be sufficient to destroy any preexisting life in the outermost tens of meters, and the temperatures are so low that life could not form. The best estimates of the radiation environment of the deep interior indicate that sterilizing doses of radiation may be achieved in time scales of the same order of magnitude as the age of the solar system. Some cometary nuclei have a large quantity of organic material present in the form of volatile and refractory molecules, but the presence of such molecules can be explained by abiotic formation; such molecules have also been detected in the interstellar medium, where they are certainly produced abiotically. It is generally agreed that Earth has received samples of comets. Unsterilized cometary dust has been delivered episodically to the surface of Earth, but it is not clear whether this has occurred within the last 500 years. 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. Cosmic Dust Cosmic dust, or interplanetary dust particles (IDPs), represents a valuable source of material for the evaluation of the composition and characteristics of objects throughout the solar system. Because interplanetary dust particles 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, IDPs themselves cannot be readily assessed by the approach used in this study. 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
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 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 harbor viable organisms or a living entity is negligible. An additional consideration is whether the method of collection for IDPs in returned samples results in spiked heating of the sample to temperatures extreme enough to cause biological sterilization. If so, then no special containment is required, regardless of the source of the IDP. Cosmic dust has been introduced to Earth throughout its history. The accumulation of the various components of IDPs and micrometeorites has had no known adverse effects on Earth's ecosystems. Thus, it is unlikely that a returned sample of this type of extraterrestrial material would pose any kind of threat to Earth's biota or biogeochemical cycles. CONTAINMENT AND HANDLING OF RETURNED SAMPLES The task group's conclusions and recommendations on containment and handling of samples returned from planetary satellites and small solar system bodies are based on its analysis of the potential for a living entity to exist in or on such samples. Table 8.1 summarizes the results of the task group's assessment. 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 TABLE 8.1 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 I No Special Containment and Handling Warranted Beyond What Is Needed for Scientific Purposes II Strict Containment and Handling Warranted Ia High Degree of Confidence Ib Lesser Degree of Confidencea The Moon Phobos Europa Io Deimos Ganymede Dynamically new cometsb Callisto P-type asteroids Interplanetary dust particlesc C-type asteroids D-type asteroids Undifferentiated metamorphosed asteroids Interplanetary dust particlesd Differentiated asteroids 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.
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 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 target bodies. 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, or sampled in a way that would result in exposure to extreme temperatures, e.g., spike heated), 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. 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 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 it is for the Moon and Io and should be reexamined at the time of mission planning on a case-by-case basis. 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. 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 or could cause ecological harm. 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 risk 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 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: 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. 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
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making 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. 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. 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. REFERENCE National Research Council (NRC). 1997. Mars Sample Return: Issues and Recommendations. Washington D.C.: National Academy Press.
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