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Preventing the Forward Contamination of Mars 8 A Path Forward for Planetary Protection in the 21st Century Increased scientific understanding of the martian environment (see Chapter 4) and the ability of microorganisms to survive in severe conditions (see Chapter 5) have important implications for the planetary protection of Mars. Ongoing missions such as Mars Global Surveyor, Mars Odyssey, Mars Express, and Spirit and Opportunity, as well as continuing ground-based observations, are producing a rolling wave of scientific discoveries about Mars (see Chapter 3). Anticipated Mars missions will likely travel to locations with greater potential for the survival and possibly the growth of Earth microbes. The science and engineering community needs to ensure, on an ongoing basis, that planetary protection policy and practices reflect current scientific and technical understanding and capabilities. As a result of its study, the Committee on Preventing the Forward Contamination of Mars found that (1) many of the existing policies and practices for preventing the forward contamination of Mars are outdated in light of new scientific evidence about Mars and current research on the ability of microorganisms to survive in severe conditions on Earth; (2) a host of research and development efforts are needed to update planetary protection requirements so as to reduce the uncertainties in preventing the forward contamination of Mars; (3) updating planetary protection practices will require additional budgetary, management, and infrastructure support; and (4) updating planetary protection practices will require a roadmap, including a transition plan with interim requirements, and a schedule. In addition, the committee found that scientific data from ongoing Mars missions may point toward the possibility that Mars could have locales that would permit the growth of microbes brought from Earth, or that could even harbor extant life (although this remains unknown),1 and that these intriguing scientific results raise potentially important questions about protecting the planet Mars itself, in addition to protecting the scientific investigations that might be performed there. Drawing on information presented in Chapters 1 through 7 of this report, the committee presents below its findings and recommendations for preventing the forward contamination of Mars. The recommendations are organized according to five themes: (1) protection of mission science and protection of the planet, (2) programmatic support, (3) research and reconnaissance, (4) transition to a new approach, and (5) interim requirements. The committee urges that its recommendations be considered as a package; the recommendations build one on the other to establish a new planetary protection framework. Chapter 9 presents a suggested transition plan and process for implementing the committee’s recommendations. 1 See Chapters 4 and 5 and references therein.
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Preventing the Forward Contamination of Mars EXPANDING THE PURPOSE OF PLANETARY PROTECTION: SAFEGUARDING OF INDIGENOUS LIFE AS WELL AS PROTECTION OF MISSION SCIENCE? Historically, planetary protection policy has addressed the concern that the forward contamination of planetary environments by terrestrial organisms could compromise spacecraft investigations sent to identify indigenous life.2 As a result, current practice imposes the strictest standards of cleanliness on those spacecraft that will conduct life-detection experiments. Other spacecraft that will not search for life are required to meet less stringent standards. Although this policy may succeed in protecting the integrity of Mars mission science during the near-term period of biological exploration, recent discoveries suggest that there may be numerous (and potentially difficult to detect) environments on Mars where the probability of growth for terrestrial organisms is substantially higher than previously thought. If so, there is the potential that the lower standard of cleanliness afforded for spacecraft that do not include life-detection experiments may allow the introduction of terrestrial organisms into sensitive environments where they may reproduce. The ethical and policy implications of questions about protection of the planet Mars are not currently addressed by either the Outer Space Treaty or COSPAR policies. Although they fall outside the mandate of the current committee, the committee believes that their consideration should have high priority. The need for urgency in deliberations on protection of the planet Mars as well as protection of science is underscored by the present uncertainty about the distribution of such sensitive martian environments, the failure rate and cleanliness levels of Mars landers, and the projected rapid pace of future spacecraft investigations (see Box 8.1). For these reasons, the committee believes that it is important that NASA and its international partners address questions about the protection of the planet Mars as expeditiously as possible. Recommendation 1. In light of new knowledge about Mars and the diversity and survivability of terrestrial microorganisms in extreme environments, NASA should work with COSPAR and other appropriate organizations to convene, at the earliest opportunity, an international workshop to consider whether planetary protection policies for Mars should be extended beyond protecting the science to include protecting the planet. This workshop should focus explicitly on (1) ethical implications and the responsibility to explore Mars in a manner that minimizes the harmful impacts of those activities on potential indigenous biospheres (whether suspected or known to be extant), (2) whether revisions to current planetary protection policies are necessary to address this concern, and (3) how to involve the public in such a dialogue about the ethical aspects of planetary protection. PROGRAMMATIC SUPPORT Many existing planetary protection practices stem from the R&D on planetary protection that was conducted during the 1970s in preparation for sending the Viking life-detection missions to Mars. Since that time, knowledge about Earth organisms and their ability to survive under severe conditions has advanced considerably, and the potential presence of such organisms on spacecraft may warrant alternative approaches to reducing bioburden. Over the last 30 years as well, new technologies for assessing microbial diversity and reducing bioburden on spacecraft have been developed. If applied properly, they should allow researchers, engineers, and planetary protection officials to improve both microbial detection and bioburden reduction methods compared to those currently being used (see Chapter 6). Transitioning NASA’s planetary protection practices so that they reflect current scientific understanding of Mars and microbiology and also benefit from the use of advanced technologies will require investing in a series of R&D efforts on and assessments of new technologies that can be applied to the implementation of planetary protection policies. It will also require a structure for managing such research efforts in coordination with the engineering, spacecraft and instrument development, and science communities at NASA 2 This position originated in the decade before the promulgation of the Outer Space Treaty of 1967 (see Chapter 1).
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Preventing the Forward Contamination of Mars BOX 8.1 Perspectives on the Potential for Contamination of Mars Posed by Past and Future Missions Many committee members agreed with issues regarding the potential for contamination of the planet Mars posed by past and future missions. Indeed, COSPAR’s current planetary protection policy focuses implicitly on protecting the planets for scientific exploration. Over the years, debates about standards of cleanliness for spacecraft have centered on whether the standards are strict enough to ensure the integrity of life-detection efforts during the period of biological exploration (see Chapters 1 and 2). More broadly, it is also uncertain what the long-term fate or effects of terrestrial microbes might be on the potential indigenous biosphere, locally or planetwide. Since the start of robotic exploration of Mars, 12 spacecraft have landed or crashed on the planet’s surface (Table 1.1). Each was cleaned to the bioburden levels deemed appropriate for its individual mission at the time of its launch.1 Even so, given current recognition of the potential nature, diversity, and distribution of habitable environments on Mars, the adequacy of these cleanliness levels is now questioned. Thus, the committee’s recommendations 12 through 14 advise more stringent requirements for bioburden reduction for Mars missions. Much of the uncertainty about the risks of contaminating the martian biosphere involves the survival and growth rates of microbial contaminants that may be present on spacecraft when they arrive. The problem is compounded when one considers the very long term. Current climate models suggest that, at times of high obliquity (i > 45°), summertime surface temperatures at polar and near-polar latitudes may exceed the melting point of water for continuous periods of many months. Such conditions may be repeated annually for perhaps thousands of years (i.e., for as long as the high-obliquity phase of the 105-year obliquity cycle continues; see Chapter 4 and Appendix F). The effect is to make these ice-rich, high-latitude environments among the most potentially habitable surface environments on Mars for the survival and growth of terrestrial microbes. In addition to the uncertainty about whether these environments may become habitable in response to future environmental or climatic change, and whether terrestrial microbes introduced to Mars today could survive for 104 to 105 years until the climate again swings, there is concern about the potential for spacecraft to contribute to possibly irreversible contamination of these environments, despite compliance with planetary protection controls. Not only are these questions scientifically relevant for life-detection and planetary exploration efforts, but they also have potentially major implications for the martian biosphere itself. Ramifications extend into the ethical and philosophical realms, beyond the purview of this committee and current international policies. Committee perspectives differed on but included the view that upcoming missions demonstrate the urgency with which such ethical concerns should be considered, even if those missions comply with existing planetary protection policies. Some committee members believed that missions that are in compliance with existing policies should not be subjected to further scrutiny. However, others thought it important to discuss missions that could potentially have irreparable effects on a martian biosphere. Two missions were cited as illustrative of this concern. The Mars Polar Lander (MPL) experienced a failure during its final descent in December 1999 and crashed into the ice-rich south polar layered deposits. Because the spacecraft did not include a life-detection experiment, it was cleaned to the Category IVa (Viking pre-sterilization) standard. As a result, surviving pieces of the spacecraft, bearing relatively high bioburdens of terrestrial organisms, may have become embedded in the polar ice. The Phoenix Scout mission (2007) is being sent to investigate a similar high-latitude, ice-rich environment and is being cleaned to approximately the same continues 1 Few data on planetary protection measures taken for missions of the former Soviet Union are available. However, NASA memoranda from Lawrence B. Hall, NASA’s planetary protection officer in 1972, suggest that the Soviets applied bioburden reduction measures to the Mars 2 and 3 spacecraft, including the use of methyl bromide-ethylene oxide gas. The memoranda include summary translations of Soviet documents describing their practices. Soviet planetary protection measures were judged by Hall to “approximate compliance with COSPAR constraints,” assuming that the Soviet program “did, or will, carry out the measures described.”
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Preventing the Forward Contamination of Mars standard. However, those portions of the spacecraft that are expected to come into direct contact with the surface (i.e., its footpads and robotic arm-mounted shovel) are being cleaned to the more stringent Viking post-sterilization (IVb) standard. A significant concern is whether, by accident or nominal operations, such spacecraft could contribute to the irreversible contamination of sensitive environments, despite compliance with current planetary protection controls. In addition to its relevance to life-detection and planetary exploration efforts, this situation has potentially major implications for the long-term health and survival of a martian biosphere, should one exist. Within the committee, reaction to this concern was considerably varied: at one extreme, there was a call for the immediate review of planetary protection requirements for all missions in development, while at the other extreme, it was held that before the acquisition of additional data, no immediate action should be taken. The majority of the committee members found themselves somewhere between these two views. headquarters, NASA centers, and in universities, research laboratories, and industry, as well as with the international community. The committee recognizes that such research efforts have cost implications; however, in the committee’s view, the Mars Exploration Program’s focus on the search for past and present life (Chapters 1 and 3) requires that additional resources be committed for updating planetary protection practices, and that this be done in a sustainable way that ensures a new generation of scientists and engineers with expertise in this area. Such investment would also introduce innovation that could potentially lead to techniques for planetary protection that are faster, more accurate, and more effective at reducing bioburden on spacecraft bound for Mars, all of which could be cost-effective in the long term. Recommendation 2. NASA should establish and budget adequately for, on an ongoing basis, a coordinated research initiative, management capability, and infrastructure to research, develop, and implement improved planetary protection procedures. The research initiative should include a training component to encourage the growth of national expertise relevant to planetary protection. Currently, planetary protection often is not emphasized until the spacecraft production process. The committee supports the Jet Propulsion Laboratory’s plans to assess spacecraft design and development processes with a view toward considering planetary protection at the earliest stages of a mission project. Bioburden reduction will be most effective and most efficient if it is built into mission planning and design from the earliest stages. Recommendation 3. Future missions to Mars should plan for the effective implementation of planetary protection requirements at the earliest stages of mission and instrument design, and engineers should be provided with a selection of effective, certified tools for bioburden reduction. NASA’s Mars Exploration Program is planning a series of missions of increasing scientific and technological capability that will, inter alia, explore potential martian habitats for life (see Chapter 3). The resulting wave of data may significantly change the scientific understanding of Mars and its environment. Scientific understanding of microorganisms on Earth is also increasing rapidly, and molecular methodology has revealed that more than 99 percent of suspected terrestrial microbial species still remain largely uncharacterized (see Chapter 5). The analysis of these forthcoming data may suggest the need to either relax or increase the stringency of planetary protection requirements for preventing forward contamination. The committee believes that the current pace of exploration and discovery on Mars places unprecedented pressure on the adequacy of planetary protection requirements and protocols, a situation that will require dialogue involving a broad range of scientific viewpoints, ongoing dynamic adaptation, and continuing oversight by all governing and implementing organizations.
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Preventing the Forward Contamination of Mars Recommendation 4. NASA should establish an independent review panel that meets every 3 years to (1) consider the latest scientific information about Mars, as well as about Earth microorganisms, and recommend to NASA appropriate modifications to NASA’s planetary protection implementation requirements as needed in light of new knowledge; and (2) identify and define the highest-priority measurements needed at Mars to inform future assessments and possible modifications of planetary protection requirements. The first meeting of the review panel should be held in 2008. Meetings should occur every 3 years thereafter, unless major changes in understanding of Mars or other factors related to planetary protection require meetings on an urgent basis. NEEDED RESEARCH AND RECONNAISSANCE Planetary protection policy and practice should be based on the most up-to-date understanding of the microbial bioburden on and embedded in spacecraft sent to Mars and the kinds of environments relative to life that may be encountered there. Modern means to reduce bioload, in addition to dry-heat sterilization, should be widely available for use in spacecraft and component manufacture. This section summarizes key recommendations intended to ensure that the research necessary to meet these objectives is conducted. Although recorded data on bioburden levels during assembly, test, and launch operations (ATLO) are sufficient to certify compliance with current requirements for total bioload (densities and numbers), there is little information to indicate the taxonomic diversity and densities of microbes on spacecraft hardware or in clean-room areas. The committee commends NASA’s support of the preliminary work in this area (Dickinson et al., 2004a,b; Venkateswaran et al., 2001, 2003).3 Information on the diversity and density of microbes on spacecraft hardware could be useful in reconsidering the probability of growth of transported terrestrial microbes, given recent discoveries about martian environmental conditions. In addition, a more complete understanding of actual transported bioburdens would be useful in designing controls to rule out false positives during future life-detection investigations. Moreover, because molecular techniques are constantly advancing, and because the future may reveal the need for more information about what organisms may have been delivered from Earth to particular sites on Mars, it is important that routine and long-term archiving of environmental samples, as well as phylogenetic data, be adequately supported. Recommendation 5. NASA should require the routine collection of phylogenetic data to a statistically appropriate level to ensure that the diversity of microbes in assembly, test, and launch operations (ATLO) environments, and in and on all NASA spacecraft to be sent to Mars, is reliably assessed.4 NASA should also require the systematic archiving of environmental samples from ATLO environments and from all spacecraft to be sent to Mars. On the basis of current knowledge about Mars (see Chapter 4), the committee believes that psychrophilic or psychrotrophic organisms are those most likely to grow in a martian near-surface environment (Chapter 5), although this evaluation may evolve as knowledge of Mars improves.5 For example, detection of active near-surface hydrothermal vents would expand the class of microorganisms that could grow in the martian near-surface. Recommendation 4 of this report addresses the need to update planetary protection requirements, if necessary, in 3 As noted in Chapter 6, NASA is currently investigating advanced microbial detection and bioburden reduction methods, but additional research on these technologies is needed. 4 Analytical models such as rarefaction or Bayesian inference should be used to ensure the completeness of a survey, and a sufficient number of nucleotides per gene sequence should be used to differentiate among unique sequences (e.g., Altekruse et al., 2003). 5 The committee recognizes that liquid water could also exist in the warm, deep subsurface of Mars and create the conditions for a possible biosphere. However, the deep subsurface is largely inaccessible at present, although such environments may need further consideration as technology advances and knowledge increases over time.
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Preventing the Forward Contamination of Mars light of new science and technology. It is thus important to improve knowledge of organisms’ ability to survive in various environmental conditions, as well as to improve bioburden reduction measures. A deeper understanding of psychrophiles and psychrotrophs may help in determining what is required to sterilize naturally occurring populations of these organisms and the extent of their ability to withstand mild or strong heat treatment. It would be especially valuable to know whether psychrophiles and psychrotrophs are sensitive to mild heat treatment that would not harm spacecraft components. Recommendation 6. NASA should sponsor research on those classes of microorganisms most likely to grow in potential martian environments. Given current knowledge of the Mars environment, it is most urgent to conduct research on psychrophiles and psychrotrophs, including their nutritional and growth characteristics, their susceptibility to freeze-thaw cycles, and their ability to replicate as a function of temperature, salt concentration, and other environmental factors relevant to potential spaceflight and to martian conditions. This recommended research should be expanded to include other classes of organisms if new scientific results suggest the existence of hydrothermal vents or other types of near-surface environments where microorganisms could grow, or as the ability to access deeper subsurface environments improves. Over the past several decades, little research has been done to update the assigned embedded microbe density values used in NASA’s planetary protection implementation requirements (Chapter 6). In the absence of new data, flight projects are required to assign values (number of microbes per cubic centimeter) for the bioburden implied by electronic piece parts or other nonmetallic materials used in spacecraft. Assigning these values amounts to reading an upper limit from a data table constructed on the basis of measurements made three decades ago. It is possible that modern materials, owing both to temperatures that some achieve during construction and to cleaner assembly environments, may have lower bioburdens than projects must currently assume. However, these details are not known. Research is needed on methods for determining actual levels of bioburden in encapsulated components for modern spacecraft materials, and models should be developed for extrapolating these levels to components that cannot be directly assayed. Recommendation 7. NASA should ensure that research is conducted and appropriate models developed to determine the embedded bioburden (the bioburden buried inside nonmetallic spacecraft material) in contemporary and future spacecraft materials. Requirements for assigned values of embedded bioburden should be updated as the results of such research become available. At present, dry-heat sterilization is the only method available for reduction of the microbial bioburden (as assessed through spore counts) of spacecraft hardware to Viking post-sterilization levels. Some newer materials and electronics, however, are incompatible with heat sterilization. Although NASA’s implementation documents allow the use of alternative methods for reducing bioburden, NASA’s requirements also stipulate that the user must provide conclusive data on biological effectiveness and on reproducibility, as well as demonstrate no reduction in hardware reliability (see Chapter 6). The potential for some techniques to leave organic residues is also important to examine when assessing alternative bioburden reduction techniques. Recommendation 8. NASA should sponsor studies of bioburden reduction techniques that are alternatives to dry-heat sterilization. These studies should assess the compatibility of these methods with modern spacecraft materials and the potential that such techniques could leave organic residue on the spacecraft. Studies of bioburden reduction methods should use naturally occurring microorganisms associated with spacecraft and spacecraft assembly areas in tests of the methods. Nonliving organic compounds, which may or may not have been originally derived from living organisms, could confound scientific investigations of Mars, especially those that involve the search for life. Consideration of nonliving organic contaminants should thus be an integral part of the requirements for spacecraft cleanliness. However, too little research has been conducted to understand the risks that nonliving contaminants, including possibly spacecraft propellants, might pose for life-detection or other scientific experiments.
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Preventing the Forward Contamination of Mars Recommendation 9. NASA should sponsor research on nonliving contaminants of spacecraft, including the possible role of propellants for future Mars missions (and the potential for contamination by propellant that could result from a spacecraft crash), and their potential to confound scientific investigations or the interpretation of scientific measurements, especially those that involve the search for life. These research efforts should also consider how propulsion systems for future missions could be designed to minimize such contamination. As discussed in Chapter 4, the potential for water distributed in the near surface and subsurface of Mars has significant implications with respect to preventing the forward contamination of Mars. Although recent missions are producing a wealth of data, scientific investigations have not yet yielded results detailed enough to distinguish among special and nonspecial regions on Mars. The committee believes that data from present and planned future missions also lack the fidelity to allow definitive conclusions about the distribution of water in the near subsurface; therefore, it finds that additional measurements are needed to understand with confidence the near-surface distribution of water on Mars and “a region within which terrestrial organisms are likely to propagate, or a region which is interpreted to have a high potential for the existence of extant Martian life forms” (COSPAR, 2003, p. 71)—that is, special regions. Examples of the kind of measurements needed, and the required spatial resolution and ground-truth that could make such a determination possible, are given in Chapter 4. Recommendation 10. NASA’s Mars Exploration Office should assign high priority to defining and obtaining measurements needed to distinguish among special and nonspecial regions on Mars. TRANSITION TO A NEW APPROACH Important to NASA’s current planetary protection implementation requirements are detailed procedures for assessing and verifying the cleanliness of ATLO environments and spacecraft surfaces, as well as for documenting avoidance of recontamination before launch (see Chapter 2 and Appendix C). However, swab culturing is no longer the optimal way of determining ATLO environment or spacecraft bioburden. It can take up to 3 days to yield results—a period that adds to the time pressures of meeting spacecraft assembly deadlines and that makes enforcing planetary protection requirements more difficult. Several methods are available that can more directly estimate the total viable cells on spacecraft and provide near-real-time results as compared with the spore-count assay methods currently used to assess bioburden on spacecraft (see Chapter 6). Nevertheless, NASA’s progress in introducing alternative methods of bioburden assessment has been slow. The committee found no defined standard certification method or process in place for screening and approving promising new methods for assaying or reducing bioburden, nor any clear indication of how and when to discontinue the use of current methods based on culture growth. In addition, the absence of extensive comparative archival data about microbial diversity, as well as serious time and cost constraints, have limited the introduction of any innovative approaches to implementation of planetary protection policies. The committee notes that the NRC’s previous report on planetary protection for Mars urged that “efforts should be made to adopt current molecular analytical methods for use in bioburden assessment and inventory procedures for spacecraft assembly and launch for future missions, and also to develop new methods for the same purpose” (NRC, 1992, p. 18). Although some progress has been made along these lines (see Chapter 6), the committee believes that NASA should set a specific date for completion of a transition to use of advanced methods and should apply the necessary resources to meet that goal. The committee believes that, with a dedicated research program and appropriate budget, the transition to the new methods described here can be fully implemented in time for missions that will launch in 2016. Recommendation 11. NASA should take the following steps to transition toward a new approach to assessing the bioburden on spacecraft: Transition from the use of spore counts to the use of molecular assay methods that provide rapid estimates of total bioburden (e.g., via limulus amebocyte lysate (LAL) analysis) and estimates of viable
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Preventing the Forward Contamination of Mars bioburden (e.g., via adenosine triphosphate (ATP) analysis). These determinations should be combined with the use of phylogenetic techniques to obtain estimates of the number of microbes present with physiologies that might permit them to grow in martian environments. Develop a standard certification process to transition the new bioassay and bioburden assessment and reduction techniques to standard methods. Complete the transition and fully employ molecular assay methods for missions to be launched in 2016 and beyond. The committee agrees with the 1992 NRC report that the probability of growth (Pg) of terrestrial microorganisms on Mars cannot currently be reliably estimated. However, the committee is concerned that, given current knowledge of Mars, Pg might in fact prove to be greater than suggested by earlier reports (NRC, 1978, 1992). The committee considered returning to a Pg-based approach to planetary protection but concluded that the unknowns remain too great to do so now (see Chapter 5). However, implementation of the DNA phylogenetic methods recommended in this report, coupled with increasing knowledge of the martian environment, might make it possible to return to a Pg-based approach with much greater confidence. Once spacecraft bioburden is understood on a species-by-species basis, it may prove possible to radically alter current planetary protection requirements and protocols. For example, it may be proved that broad classes of organisms have essentially no chance of growth at a particular martian landing site, whereas, say, psychrophiles may have nonvanishing probabilities of growth. In this case, Equation 5.1 could be employed to demonstrate that no particular bioburden reduction need be employed for most organisms on the spacecraft. However, certain species, were they present, would have to be rigorously eliminated. In some cases, elimination of particular species on spacecraft components might prove easier and less demanding than with Viking-style baking. Greater knowledge of the diversity and number of species on outbound Mars spacecraft is critical. Such data are important in reducing uncertainty in assessments of the bioburden reduction needed to prevent the forward contamination of Mars. INTERIM REQUIREMENTS The recommendations in this report are intended to provide a path toward a transition by NASA to planetary protection practices and policies that will reflect current science and technology. A time line for the implementation of these recommendations is offered in Chapter 9. The committee anticipates that it will take until 2016 before R&D efforts can be conducted and their results used to develop updated planetary protection methods and techniques that are fully implemented on missions to Mars. However, because a number of Mars missions are planned for launch prior to 2016, the committee recognizes the need for an interim plan that updates existing planetary protection requirements to reflect new scientific knowledge about Mars and terrestrial microorganisms. The recommendations presented below, based on existing planetary protection protocols, concern planetary protection requirements that should be implemented during the interim period from now until the transition to new practices is completed. These recommended requirements are intended to reflect the best current scientific understanding of terrestrial microorganisms and the martian environment, incorporating the new knowledge acquired since publication of the 1992 NRC report. Because knowledge of Mars is changing so rapidly, the committee has tried to build appropriate flexibility into these requirements. Recommended Changes for Category IV Missions Chapter 2 of this report describes the current COSPAR planetary protection categories, based on the destination body in the solar system and type of mission to be flown (see Table 2.2). Category III applies to flyby and orbiter missions that should not make direct contact with a planet or its atmosphere. Category IV applies to direct-contact missions, including landers, penetrators, and atmospheric probes. Determining the level of bioburden reduction needed to avoid confounding a mission’s own measurements (for life detection or otherwise) is scientifically crucial, and it is the responsibility of a given mission’s planners.
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Preventing the Forward Contamination of Mars Failure to reduce bioburden or certain nonliving contaminants appropriately, and thus to prevent payload investigations from being compromised, would be a mistake that could cause partial mission failure. Different levels of cleanliness, both with respect to viable microorganisms (Chapter 6) and with respect to nonliving contaminants (Chapter 7), might be required for missions flying different kinds of instruments, and so there is no single or uniform requirement that planetary protection should impose for this purpose. In particular, some types of biodetection experiments may require extraordinary levels of cleanliness, whereas others may have much less stringent requirements. Thus the committee believes that the IVa and IVb mission categories are no longer a useful way to determine planetary protection requirements for Mars landers. Rather, Mars mission categories should depend on spacecraft destination, regardless of mission instrument payload. To avoid confusion with Categories IVa through IVc, the committee defines new categories—IVs and IVn. Category IVs applies to missions that are landing or crashing in, or traversing, excavating, or drilling into, special regions. In contrast, Category IVn missions are those that are not going to a special region. Recommendation 12. For the interim period until updated planetary protection methods and techniques can be fully implemented, NASA should replace categories IVa through IVc for Mars exploration with two categories, IVn and IVs. Category IVs applies to missions that are landing or crashing in, or traversing, excavating, or drilling into, special regions; Category IVn applies to all other category IV missions. Each mission project should (in addition to meeting the requirements imposed by Categories IVn and IVs) ensure that its cleanliness with respect to bioburden and nonliving contaminants of concern is sufficient to avoid compromising its experiments, in consultation with NASA’s planetary protection officer. As discussed in Chapter 4, scientific results from recent Mars orbiters and landers have confirmed the existence of past water on Mars and suggest that liquid water may currently exist, at least transiently, on the planet. Recent results make it substantially more likely that transient liquid water may exist near the surface at many locations on Mars, and it is difficult on the basis of current knowledge to declare with confidence that any particular Mars regions are free of this possibility. Researchers do not currently have the data necessary to distinguish special regions on Mars from regions that are not special. Current and planned investigations of water on Mars, although very important, involve issues of spatial resolution and interpretive ambiguities that render them unlikely to fully resolve this ambiguity. Along with these ambiguities are uncertainties regarding the diversity of the viable bioburden on spacecraft that could make contact with the martian surface, as well as the potential for such organisms to grow in martian environments. The committee thus found that information is currently insufficient to make use of Category IVn. Until measurements are made that permit distinguishing confidently between regions that are special on Mars and those that are not, NASA should treat all direct-contact missions (i.e., all Category IV missions) as being in Category IVs. Recommendation 13. Until measurements are made that permit distinguishing confidently between regions that are special on Mars and those that are not, NASA should treat all direct-contact missions (i.e., all Category IV missions) as Category IVs missions. The independent panel referred to in Recommendation 4 could be the body that recommends, as more knowledge becomes available, whether areas may be appropriately designated as Category IVn rather than Category IVs. Note added in proof—The following text change was approved and made after release of the prepublication copy of this report: The phrase “in consultation with NASA’s planetary protection officer” was added to Recommendation 12.
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Preventing the Forward Contamination of Mars Bioburden Reduction Requirements for Category IV Missions The committee considered at length a range of levels of bioburden reduction appropriate to Category III and Category IVn and Category IVs missions. It evaluated a number of factors: Definition of “special region.” A “special region” (see Chapter 2) is defined as “a region within which terrestrial organisms are likely to propagate, or a region which is interpreted to have a high potential for the existence of extant martian life forms. Given current understanding, this is to apply to regions where liquid water is present or may occur” (COSPAR, 2003, p. 71). The COSPAR IVc classification (see Chapter 2) was needed in part to prevent a “dirty” spacecraft (one sterilized to only IVa, or Viking pre-sterilization, levels; see Table 2.1) from being sent to a special region, even if that particular spacecraft did not have life-detection experiments on board. Therefore, the committee could not see permitting a Category IVs mission to fly with only Viking presterilization bioburden protection. Moreover, microorganisms could be introduced into a potential liquid water environment not only by sampling arms or rover wheels that might make direct contact with Mars, but also from any exposed surface on the spacecraft from which a microorganism might be saltated6 or lofted into contact with that environment. Kinetics of growth and the period of biological exploration. At the same time, the committee concluded that, on the basis of current understanding, Mars surface or near-surface environments are likely to be cold ones where liquid water, if ever present, was present only diurnally (and perhaps even then only seasonally) (see Chapter 4). It might be possible for certain Earth microorganisms to survive under such conditions, but extrapolations from Earth experience suggests that most terrestrial organisms would not survive, and that those that would survive (most likely, psychrophilic or psychrotrophic organisms) would likely have slow growth kinetics, that is, long generation times—long enough so that during the period of biological exploration, the organisms would not produce sufficient copies of themselves to pose a likely threat to life-detection measurements made during that period (see Chapter 5). Possibility of long-lived water. There could be some surface or near-surface locations on Mars, as well as deeper subsurface locations, where long-lived liquid water could be present (see Chapter 4). None has yet been unambiguously observed, although such environments, at least in the subsurface, are widely anticipated. A dramatic example would be the discovery of hydrothermal vents at the martian surface. Sites of long-lived liquid water could be particularly amenable to the sustained growth of certain Earth microorganisms. Probability of a crash. Table 1.1 displays the outcome of all missions of all nationalities sent from Earth to Mars, and in particular, tracks which of these missions failed, and of those that failed, which crashed onto the martian surface. If the two penetrators associated with the Mars Polar Lander mission are counted as having failed after landing, the compilation shows that 12.5 percent of U.S. Mars landers have crashed. Historically, one in eight U.S. Category IV missions has crashed. Crashes may take many different forms and have varied consequences, however. For example, supersonic impacts would likely expose all surviving surfaces to the martian environment, and even some embedded bioburden. However, many, but not all, spacecraft components would in such a case experience great bioburden reduction through extreme surface heating. The most likely type of crash impact, terminal impact, would expose only some of the nominally nonexposed surfaces. Some missions, such as those using martian airplanes, might involve low-velocity crashes as part of a nominal mission plan. Penetrator missions are designed for high-speed impact, but they can also break up if their angle of attack is too great at impact. Under certain crash scenarios, higher levels of planetary protection would be required. However, making this determination requires detailed knowledge of each particular mission, including the landing technique and its history of success or failure, the nature of the lander, the presence or absence of radioisotope thermal generators (discussed below), the nature of the landing site, and many other factors. Therefore, a mission-by-mission analysis will be necessary.7 6 “Saltation” is the process by which a small particle is lifted off a surface without enough velocity to place it into suspension, but with enough velocity to move it downwind in a series of little jumps. 7 The committee was briefed on the details of one such mission analysis by Brian K. Muirhead, chief engineer, JPL, “Mars Science Laboratory Planetary Protection Categorization Strategy,” briefing to NRC Space Studies Board, May 5, 2004, Diversa Corporation, San Diego, California.
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Preventing the Forward Contamination of Mars TABLE 8.1 Recommended Levels of Bioburden Reduction for the Interim Period Level Requirement Representative Scenario 1 Viking lander pre-sterilization total bioburden (fewer than 3 × 105 total surface spores) and 300 spores per square meter.a Category IVn 2 Viking pre-sterilization levels required for the bulk spacecraft plus Viking post-sterilization on all exposed surfaces.a,b The latter is to be understood as an areal (surface density) measurement. Explicitly, Viking post-sterilization levels correspond to a reduction of 1 × 10–4 times the Viking pre-sterilization upper limit of 300 spores per square meter. All Category IVs 3 Viking pre-sterilization levels required for the bulk spacecraft b plus Viking post-sterilization on all surfaces, including those not exposed under nominal (e.g., no-crash) conditions.a Explicitly, Viking post-sterilization levels correspond to a reduction of 1 × 10–4 times the Viking pre-sterilization upper limit of 300 spores per square meter. Category III missions that do not meet existing requirements for probable orbital lifetime 4 Viking post-sterilization bioburden reduction for the whole spacecraft.a Currently, this would likely mean baking the spacecraft in a manner similar to that employed in the Viking mission, although the committee encourages NASA to investigate other technologies to this same end. Category IVs missions accessing locations determined to have long-lived liquid water 5 The committee cannot currently specify the technology that could become available to attain zero microorganisms on Mars-bound spacecraft. Bioburden reduction techniques more effective than those applied today may be or may soon be available for use on spacecraft (see Chapter 6). A level 5 bioburden reduction level would represent the implementation of these techniques, to achieve bioburden reduction significantly more rigorous than that obtained for the Viking landers. Category IVs missions accessing locations determined to have long-lived liquid water aSee Table 2.2 for descriptions of Viking pre- and post-sterilization requirements. b“Bulk spacecraft” refers to the entire nonmetallic volume of the spacecraft, including entry, descent, and landing systems. For missions that include both orbiters and direct contact components, the appropriate level must be determined separately for each type of component. An exposed surface is a surface that freely communicates with the martian atmosphere or surface. All external surfaces on a lander would count as exposed surfaces, but interior surfaces might do so as well if they were not fully enclosed or shielded from the atmosphere by submicron filters. Potential for radioisotope thermal generators to create liquid water. Some future landers may include radioisotope thermal generators (RTGs) or possibly even nuclear reactors. Under some crash conditions, RTGs could be released from the spacecraft yet remain intact or largely intact. In such a scenario, it might be possible, given certain crash site characteristics, for an RTG to produce sufficient heat to create its own long-lived liquid water environment. That result is far from guaranteed, owing both to the many different crash outcomes that are possible and to the thermal output of the particular RTG to be flown (e.g., if heat is conducted away sufficiently quickly, melting may never take place). The committee considered all these factors,8 with due regard for the substantial uncertainties that they present, with respect to Mars, Earth microbiology, and particular characteristics of future missions and corresponding crash scenarios. Consequently, the committee recommends a new set of levels for bioburden reduction for Category IV missions. The interim classification scheme presented below is based on current protocols (i.e., Viking-era protocols and the use of spore counts for determining bioburden) that should be put in place until the committee’s recommendations for the transition to modern methods are implemented. 8 This included consideration of input from Brian K. Muirhead, chief engineer, JPL, “Mars Science Laboratory Planetary Protection Categorization Strategy,” briefing to NRC Space Studies Board, May 5, 2004, Diversa Corporation, San Diego, California.
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Preventing the Forward Contamination of Mars The committee concluded that five levels of bioburden reduction would be sufficient, during the interim period envisioned, to cover appropriate bioload requirements for Category III and IV missions to Mars (see Table 8.1). The committee considered the level of bioburden reduction that should apply to different categories of missions, taking particular account of the findings described in Chapters 4 and 5. The committee found that Category IVn missions should satisfy level 1 bioburden requirements. Until NASA obtains further measurements to distinguish special regions on Mars from nonspecial regions, Category IVn will be a null set. Therefore, all Category IV missions for the time being should be treated as Category IVs missions. Recommendation 14. NASA should ensure that all Category IVs missions to Mars satisfy at least level 2 bioburden reduction requirements.9 For each Category IVs mission, NASA’s planetary protection officer should appoint an independent, external committee with appropriate engineering, martian geological, and biological expertise to recommend to NASA’s planetary protection officer whether a higher level of bioburden reduction is required. This analysis should be completed by the end of Phase A (performance of the concept study) for each mission. Key points that the appointed committee should consider in reaching this determination include previous spacecraft experience, crash scenarios, and modeling for the mission, including the likely extent of release to the martian environment of organisms on nominally nonexposed surfaces or embedded in spacecraft components, as well as the presence and likely fates of radioisotope thermal generators or other sources of significant heat present on the spacecraft. Recommendation 15. NASA should sponsor research on how to implement level 3, 4, and 5 bioburden reduction requirements in practical ways. This research should include techniques to reduce the surface bioburden to post-Viking sterilization levels, and maintain those levels, for both exposed and nonexposed spacecraft surfaces, through new techniques (such as the use of hydrogen-peroxide vapor), component bagging, heating, or other means. Recommendation 16. Any mission to Mars that will access regions or sites that have been determined to have or are strongly suspected to have long-lived liquid water should satisfy at least level 4 bioburden reduction requirements. Category III Requirements Category III missions, typically orbiters, are missions that are not expected to be direct-contact missions (see Chapter 2) but that have the potential for crashing on Mars (see Table 1.1 for the crash history of such missions). Category III mission crashes will be supersonic crashes, following extreme heating of many, but not all, spacecraft components. Note added in proof—The following text changes were approved and made after release of the prepublication copy of this report: In Recommendation 14, a footnote was added to the first sentence; in the second sentence, the word “determine” was replaced by the phrase “recommend to NASA’s planetary protection officer.” 9 In this chapter, the committee defines level 2 as corresponding to the Viking-level pre-sterilization required for the bulk spacecraft plus Viking post-sterilization for all exposed surfaces; the latter is to be understood as an areal (surface density) measurement. Explicitly, Viking post-sterilization levels correspond to a reduction of 1 × 10–4 times the Viking pre-sterilization upper limit of 300 spores per square meter. Level 2 requirements (see Table 8.1) are not identical to those previously applied to Category IVs missions (Table 2.2), as is readily seen by comparing Tables 8.1 and 2.1. The committee also draws a distinction between mission categorization (based on mission destination) and bioburden reduction levels; e.g., Category IVs missions will typically be level 2 missions, but under some circumstances a decision could be made to require level 3 or higher for a particular Category IVs mission.
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Preventing the Forward Contamination of Mars Recommendation 17. NASA should take the following approach to preventing the forward contamination of Mars from Category III missions: Category III missions should be required to have orbital lifetimes of 20 years and 50 years, and the probability of impact over those time periods should be below 1 percent and 5 percent, respectively. Category III missions unable to meet these requirements should satisfy at least level 3 bioburden reduction requirements. For each Category III mission that cannot meet the orbital lifetime requirements, NASA’s planetary protection officer should appoint an independent, external committee with appropriate engineering, martian geological, and biological expertise to recommend to NASA’s planetary protection officer whether a higher level of bioburden reduction is required. This analysis should be completed by the end of Phase A for each mission. In reaching this determination, the appointed committee should consider previous experience, crash scenarios, and modeling for the mission, including the likely extent of release to the martian environment of organisms on nominally nonexposed surfaces or embedded in spacecraft components, as well as the presence and likely fates of radioisotope thermal generators or other sources of significant heat present on the spacecraft. Chapter 9 presents a roadmap showing how the various recommendations fit together, as well as a time line for meeting the milestones that are required. REFERENCES Altekruse, S.F., F. Elvinger, Y. Wang, and K. Ye. 2003. A model to estimate the optimal sample size for microbiological surveys. Appl. Environ. Microbiol . 69: 6174-6178. Baker, A., and J.D. Rummel. 2005. Planetary Protection Issues in the Human Exploration of Mars. Final Report and Proceedings, February 10-12, 2004, Cocoa Beach, Fla. NASA/CP-2005-213461. NASA Ames Research Center, Mountain View, Calif. COSPAR. 2003. Report on the 34th COSPAR Assembly, COSPAR Information Bulletin, No. 156, April. Elsevier Science Ltd., Oxford, United Kingdom, pp. 24 and 67-74. Dickinson, D.N., M.T. La Duc, W.E. Haskins, I. Gornushkin, J.D. Winefordner, D.H. Powell, and K. Venkateswaran. 2004a. Species differentiation of a diverse suite of Bacillus spores using mass spectrometry based protein profiling. Appl. Environ. Microbiol. 70: 475-482. Dickinson, D.N., M.T. La Duc, M. Satomi, J.D. Winefordner, D.H. Powell, and K. Venkateswaran. 2004b. MALDI-TOFMS compared with other polyphasic taxonomy approaches for the identification and classification of Bacillus pumilis spores. J. Microbiol. Methods 58(1): 1-12. National Research Council (NRC). 1978. Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan. National Academy of Sciences, Washington, D.C. NRC. 1992. Biological Contamination of Mars: Issues and Recommendations. National Academy Press, Washington, D.C. Venkateswaran, K., M. Satomi, S. Chung, R. Kern, R. Koukol, C. Basic, and D. White. 2001. Molecular microbial diversity of spacecraft assembly facility. Syst. Appl. Microbiol. 24: 311-320. Venkateswaran, K., N. Hattori, M.T. La Duc., and R. Kern. 2003. ATP as a biomarker of viable microorganisms in clean-room facilities. J. Microbiol. Methods 52: 367-377.
Representative terms from entire chapter: