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5 Assessment of the 1978 Report REVIEW The 1978 report of the then Space Science Board's Committee on Planetary Biology and Chemical Evolution established a quarantine policy for exploratory, one-way missions to Mars, Jupiter, Saturn, Uranus, Neptune, and Titan planned for 1974 to 1994.1 The recommendation of the 1978 report was that precautionary measures be taken to minimize forward contamination of these planets by terrestrial microorganisms so as not to jeopardize future life-detection experiments. The criteria used for planetary contamination prior to the 1978 report were those established by international agreement through the Committee on Space Research (COSPAR). They stipulated that the probability of contamination (Pc) should be less than 1 x 10-3 for each planet. The Pc was estimated using a formula that also included the probability of growth (Pg) of a terrestrial microorganism on each of the planets. There was some difficulty in arriving at a sensible and useful Pg, necessitating that the 1978 committee be charged with the task of comprehensively evaluating Pg based on available knowledge of the physical and chemical properties of the surface and atmosphere of each planet and conditions that limit life as we know it. Although the 1978 committee considered the P8 for all the planets being considered for exploration through 1994, the current report is limited to an evaluation of information and past recommendations for Mars. The 1978 report attempted to evaluate the Pg for three separate regions on Mars and included above- and below-surface subpolar areas and the polar caps. Although the committee expressed a reluctance in 43
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recommending a particular value for Pg, they argued that while the Pg for Mars is exceedingly low, the probability is not zero. Furthermore, the Viking mission, although useful in arriving at a Pg for subpolar sites, did not offer any insight on geochemical characteristics and the possibility of liquid water at the polar caps. The committee recommended a Pg of less than 10-10 for the subpolar regions of the planet within 6 centimeters of the surface, less than 10-8 for subsurfaces in subpolar regions, and less than 10-7 for the polar ice caps. These ranges for Pg values reflect Viking data for subpolar regions, including those results that indicated the presence of strong oxidants, observed organic compounds, water, and the possibility that liquid water could exist seasonally and diurnally at the polar caps. The Pg values were arrived at subjectively and have become a matter for debate. It is clear that considerable uncertainty has been engendered by the probabilistic approach to planetary protection. This concern has been restated over the years by virtually every group that has analyzed the problem, and indeed by NASA. Many unknowns must be factored into such elements as the probability of growth of a terrestrial organism on the martian surface, for example, so that estimating the potential for biological contamination of Mars is difficult if not impossible. However, the trend is clear: as we have learned more about Mars, our expectations regarding the likelihood of terrestrial microbial contamination have been reduced, and estimates of the probability of growth have been steadily lowered as a result. Following the 1978 report, whose recommendations were generally accepted, NASA began to look for ways to simplify planetary protection procedures as they applied to particular upcoming planetary missions, and also to minimize the use of mathematical models and quantitative analyses. These studies culminated in a report to COSPAR in 1984 that greatly deemphasized the probabilistic approach and introduced the concept of categories based on target planet and mission type.2 This approach directly reflects the degree of concern for a given planet in the context of a particular type of mission. Five categories of target planet and mission-type combinations and their particular suggested ranges of requirements were proposed in the 1984 report, and these were accepted by COSPAR. The five categories are summarized below; details are contained in the 1984 report (see also Table E.1, Appendix E). Category I missions include any mission to a target planet that is • not of direct interest for understanding the process of chemical evolution. In effect, no protection of such planets (e.g., Mercury, Pluto) is warranted, and no planetary protection requirements are imposed. Category II missions are all types of missions to those target • planets that are of significant interest for understanding the process of chemical evolution, but for which there is only a remote chance that 44
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contamination carried by a spacecraft could jeopardize future exploration. The concern is primarily over unintentional impact, since these missions are not designed to land. Category III missions are certain types of missions (flyby and • orbiter) to a target planet of interest for understanding the chemical evolution and/or the origins of life, or for which scientific opinion suggests a significant chance of contamination that could jeopardize a future biological experiment. Category IV missions are certain types of missions (mostly probe • and lander) to a target planet of interest for understanding chemical evolution and/or the origins of life, or for which scientific opinion suggests a significant chance of contamination that could jeopardize future biological experiments. Category V missions include all Earth-return missions. The • concern is for the protection of the terrestrial system as well as the scientific integrity of the returned sample. These recommendations, made by NASA, were approved by Subcommission F (life sciences) and subsequently by the executive committee of COSPAR, and they have been implemented by NASA. The task group believes that approval and implementation of these recommended categories constitute a significant step forward in the process of simplifying and implementing planetary protection procedures. A goal in this report is to reassess current planetary protection guidelines in light of new knowledge and new technology. The task group was asked to comment only on Mars lander missions that do not involve in situ extant life-detection experiments and has tried to do so, although it was admittedly difficult for task group members to exclude life-detection and sample return missions from their thinking. This group's approach, which is somewhat different from that taken in earlier studies, is intended to contribute to planetary studies as they relate to questions about the origins of life, while keeping secure our profoundly important scientific objectives. RECOMMENDATIONS OF THE TASK GROUP Forward Contamination The task group views the problem of forward contamination as separable into two principal issues. The first centers on the potential for growth, in the martian environment, of whatever fractions of spacecraft populations of microorganisms are able to survive transit from Earth to the surface of Mars. The second involves importation of terrestrial organic contaminants, living or dead, in amounts sufficient to compromise the search for evidence of past or present life on Mars itself. 45
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The guidelines on probability of growth (P8) issued by the Space Science Board in 1978 were recently reassessed in a 1991 NASA report, Planetary Protection Issues for the MESUR Mission: Probability of Growth (Pg).3 Comments and estimates made by the contributors point to Pg values for terrestrial organisms on Mars that are probably lower than the 1978 estimates. Their consensus was that an exceedingly small Pg was necessitated by the low probability of liquid water existing on Mars and the low probability of an appropriate terrestrial organism occupying a particular martian environment and growing there. However, Pg was not judged to be zero because of the possibility that suitable martian microhabitats could conceivably exist. Based on the findings of the MESUR mission workshop on the probability of growth as well as on the arguments presented below, the task group agreed that the Pg value for terrestrial organisms on Mars is so small as to be of no consequence. Therefore, the need for severe reduction of spacecraft bioload solely to prevent the spread of replicating terrestrial organisms on Mars is no longer paramount. However, this is clearly not the case as far as contamination of a possible past or extant martian biosphere is concerned. The reduction of bioload on all lander missions to Mars must continue to be seriously addressed. The sophistication of current molecular analytical techniques is such that single cells are detectable, and so the issue of spacecraft cleanliness is particularly crucial when life-detection experiments are included in the scientific payload. Aside from considerations related to life-detection experiments, spacecraft cleanliness (particularly the biological-organic burden) is extremely important (1) in order to greatly minimize the introduction of foreign material into any site likely to be of biological interest in subsequent missions, and (2) to minimize contamination of experimental devices that are particularly sensitive to biological and chemical contamination (i.e., optic and spectrophotometric devices). The deliberations of the task group on the issue of forward contamination hazards posed by the planned set of U.S. and Soviet lander missions summarized in Chapter 2 were greatly aided by NASA's 1991 report on the MESUR mission and by comprehensive briefings given by experts on matters relevant to this issue (see workshop presentations listed in Appendix C). These deliberations led the task group to unanimous concurrence with the following conclusion: Forward contamination, solely defined as contamination of the martian environment by growth of terrestrial organisms that have potential for growth on Mars, is not a significant hazard. However, forward contamination more broadly defined to include contamination by terrestrial organic matter associated with intact cells or cell components is a significant threat to interpretation of results of in situ experiments specifically designed to search for evidence of extant or fossil martian microorganisms.4 Based on this consensus, the task group makes the following recommendations for control of forward contamination, each tied to specific mission objectives: 46
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1. Landers carrying instrumentation for in situ investigation of extant martian life should be subject to at least Viking-level sterilization procedures. Specific methods for sterilization are to be determined; Viking technology may be adequate, but requirements will undoubtedly be driven by the nature and sensitivity of the particular experiments. The rationale for this requirement is the reduction, to the greatest feasible extent, of contamination by terrestrial organic matter that is deposited at the site by microorganisms or organic residues carried on the spacecraft. This approach, when coupled with molecular analytical methods for assessment of bioload, should allow both elimination of the most troublesome contaminants and an inventory of those few that remain. 2. Spacecraft (including orbiters) without biological experiments should be subject to at least Viking-level presterilization procedures—such as clean-room assembly and cleaning of all components—for reduction of bioload, but such spacecraft need not be sterilized. This recommendation has important implications for the planetary protection program in general, in that it implies that there need be no requirement with regard to orbiter lifetimes if the orbiter is subject to a Viking-level reduction of bioload by clean-room assembly and cleaning. As discussed above, the task group concurs with the conclusion, expressed in NASA's 1991 report,5 that the probability of growth of a terrestrial organism on present-day Mars is essentially zero. However, the task group recommends bioload reduction for anything sent to the martian surface. Major advances in our ability to detect cellular material have occurred over the last decade, and future advances will undoubtedly follow. Reducing contamination of the planet by reducing the bioload on landed vehicles will minimize the chances of jeopardizing future experiments designed to detect material of possible biological origin. These conclusions and recommendations on the issue of forward contamination are based on several considerations discussed earlier in this report. The task group concurs with the MESUR workshop panelists' consensus that Pg is extremely low, and probably significantly below the upper limits estimated by the 1978 committee. Given the likelihood that Pg is extremely low, the task group sees no utility in further attempts to estimate its probable value in various martian environmental regimes. In the absence of crucial data relating to the potential of terrestrial organisms to survive and grow on Mars, such exercises are purely subjective. Although some progress toward quantification of Pg could perhaps be realized in welldesigned laboratory simulation experiments, the task group is not optimistic that the central question of the presence and duration of a liquid water phase in the near-surface martian regolith environment can be unambiguously addressed without more information obtainable possibly only from in situ measurements on Mars itself, or from returned samples—or conceivably from neither. 47
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The task group believes that the recommendations set out above strike an appropriate balance between the obligation for conservatism on the issue of forward contamination insofar as Pg is concerned, and the need to gather the data that will eventually allow that issue to be settled definitively. It is implicit in these recommendations that the approach used in previous attempts to calculate the probability of contamination (Pc) be abandoned. In support of abandoning the method, the task group worked through some sample calculations of Pc to demonstrate the nonutility of the probabilistic approach. Pc is correctly expressed, per unit of microbial burden, as the product of Pg and Pt, where Pt is the probability of an organism's survival during transit from Earth surface to Mars surface. Pt is usually expressed as Pt = P(VT) x P(UV) x P(R) x P(A) x P(SA), with P(VT) and P(UV) representing the probabilities of an organism's surviving exposure to space vacuum and temperature and to ultraviolet radiation, respectively; P(R) the probability of an organism's release from a lander to the martian surface; and P(A) and P(SA) the probabilities of an organism's arriving at the planet and surviving atmospheric entry. Presumption of a successful mission sets P(A) equal to 1 and P(SA) equal to near 1. Data on P(VT) and P(UV) are lacking for most of the recently discovered highly specialized organisms described above, but it is still possible to conservatively estimate their product as 10-1 to 10-2 or less. (The task group notes that appropriate laboratory simulation experiments to evaluate these probabilities for candidate microorganisms are entirely feasible, since both the spacecraft geometry and the characteristics of its space environment can be well determined.) P(R) is interpreted as the probability of release of that fraction of the total bioburden located on surfaces in direct contact with the martian regolith. With special attention to cleaning such surfaces, perhaps combined with prelaunch UV irradiation, it seems feasible to reduce P(R) to 10-2 to 10-3 without total spacecraft sterilization. Then, even with the 1978 SSB value for Pg of less than 10-10, the product of Pg x Pt seems unlikely to exceed about 10-14 per unit of microbial burden. This nominally allows a large bioload approaching 1011 (say, 105 organisms per square centimeter on a spacecraft surface area of 100 square meters) while still retaining the COSPAR value for Pc of 10-3. The task group also notes that this bioload is the total microbial burden. Consideration of only those species with capabilities for surviving in the most extreme environments would reduce Pc for them, probably by a relatively large factor. Another factor to consider is the possibility of such extreme environments existing on Mars, some of which may be hospitable to certain organisms. Clearly, if such niches exist, the Pg may be greater for a population of contaminating organisms if they are widely dispersed, thus raising the probability of their encountering a less hostile environment. It was the intent of the task group to illustrate the uncertainties involved in the probabilistic approach by performing the above calculations. With so many uncertain probabilities multiplied by each 48
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other, the likelihood of achieving a meaningful Pc is very low indeed. When these problems are combined with the fact that the range of environments on Mars is not yet known, the futility of assigning a meaningful Pc is further exemplified. The task group emphasizes that the philosophical intent of the 1978 committee to protect Mars from terrestrial contamination so as not to jeopardize future life-detection experiments on Mars is still profoundly important. Recommendation 1 above deals with the issue of contamination by nonviable but intact cells and biochemical components from terrestrial organisms, independent of whatever low Pg value they may have. Back Contamination A detailed assessment of the complex issue of sample return does not lie within the present charge of this task group. Chapter 6 discusses some of the martian environmental unknowns, and the data required to address them, that will be central to evaluation of possible hazards posed by back contamination. SCIENTIFIC ISSUES—SUMMARY STATEMENT As previously stated, it is the unanimous opinion of the task group that terrestrial organisms have almost no chance of multiplying on the surface of Mars and in fact have little chance of surviving for long periods of time, especially if they are exposed to wind and to UV radiation. However, current techniques to detect life, such as those that use specific biomarkers, are much more sensitive than techniques used at the time of the Viking mission, making contamination a serious threat to experiments designed to look for life on Mars. With regard to this latter point, the recommendation that landers be sterilized if they carry life-detection experiments, but only have reduced bioloads in other instances, has long- range strategic implications. Even if there is no organismal growth, local contamination is to be expected around a nonsterilized spacecraft. Clearly a lander should not return to do life-detection experiments at a site where unsterilized spacecraft have landed previously. For these reasons, the task group believes that it is better to err on the side of caution. Thus the task group recommends that spacecraft be cleaned rigorously to levels that are at least equal if not superior to Viking levels. It does not believe that such constraints are unduly restrictive to subsequent Mars exploration. The task group also recommends that modern methods of bioburden assessment and tabulation be developed for spacecraft destined for Mars missions. 49
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REFERENCES 1. Space Science Board, National Research Council. 1978. Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan. Committee on Planetary Biology and Chemical Evolution. National Academy of Sciences, Washington, D.C. 2. DeVincenzi, D.L., and P.D. Stabekis. 1984. "Revised Planetary Protection Policy for Solar System Exploration." Adv. Space Res. 4:291-295. 3. Klein, H.P. 1991. Planetary Protection Issues for the MESUR Mission: Probability of Growth (Pg). NASA conference publication. NASA Ames Research Center, Moffett Field, Calif. 4. See Klein, H.P., 1991. 5. See Klein, H.P., 1991. 50