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1 Introduction Understanding the origin and evolution of life has been an important goal of NASA and continues to generate some of the more interesting scientific questions for all mankind: Where did we come from? When did life begin? Does life exist elsewhere? If it exists elsewhere, is life similar to that found on Earth? Although there are many theories regarding these issues, there are as yet no definitive answers. One promising approach to understanding the origin and evolution of life is to search for life elsewhere, primarily on other planets, where physical, hydrological, and geochemical properties might favor (or might have favored in the past) the existence of replicating biotic systems like those found on Earth. If life or evidence of it is found elsewhere, then our views of the evolution of life on Earth may change drastically, and our understanding of life processes and the cosmos will be enhanced dramatically. Although the search for life and/or the chemical precursors of life can be justified in many places in the cosmos, some areas appear more likely than others to yield positive results. As articulated in The Search for Life's Origins, a 1990 report of the Committee on Planetary Biology and Chemical Evolution of the National Research Council,1 "Mars continues to be the extraterrestrial body that holds greatest promise of scientific return on fundamental questions about the origin of life" (p. 71). While the committee agreed that present evidence indicates that extant life on the surface of Mars is not likely, it also stated that "there are reasonable prospects that evidence of chemical evolution and fossil life might be found" (p. 71). Because of these possibilities, the committee recommended that a major objective of future research be "[t]o assess the 13
isotopic, molecular, morphological, and environmental evidence for chemical evolution and the origin of life on Mars" (p. 71). Mars has been the object of intense scientific scrutiny since the invention of the telescope. It has also been the object of intense speculation concerning the possible presence of life on that planet, a possibility that has often enjoyed considerable popular appeal. With the technological advances that accompanied the development of spacecraft, our ability to conduct detailed studies of the planets in the solar system improved dramatically. However, as our knowledge of conditions on the surface of Mars has increased, there has been a concomitant decrease in any expectation that life as we know it could exist on the planet. The Mariner spacecraft, which made both flyby and orbiter measurements, and later the Viking orbiters and landers, provided much new information about the chemical and physical nature of Mars. Viking attempted to look directly for life and for organic molecules commonly associated with life at two landing sites on the surface. No organic matter was found, and most scientists agree that no indications of life were detected. Granting these observations, it is also quite clear, however, that (1) the Viking experiments were performed at only two sites, which may ot have been representative of the whole planet, and (2) the early state of Mars seems to have been very different from its present state and may have been characterized by the presence of abundant liquid water and a more substantial atmosphere.2-4 Given these considerations, the search for life on Mars must include examination of other, more desirable sites (e.g., those where water has been present in the past) where life or a fossil (organismal and/or chemical) record may possibly exist. The possibility that evidence of chemical evolution and/or fossil life might be found on Mars has led many scientists to embrace the conclusion, expressed in The Search for Life's Origins, that continued chemical, physical, environmental, and biological study of Mars is a scientifically sound enterprise. It is thus not surprising that scientists from many different nations are planning to participate in missions to Mars to investigate its properties by using a variety of different approaches, including remote sensing, use of surface landers, sample return, and eventually piloted exploration. Some of these missions are planned to occur within the next few years, including missions that involve landing experimental modules and penetrators on the martian surface. Assuming that Mars will be further investigated, it is imperative that precautions be taken to ensure planetary protection, including protection from both forward and back contamination. The problem of forward contamination includes (1) the invasion of martian ecosystems by organisms from Earth that would be capable of growing and prospering, and (2) contamination by terrestrial biological material that would then be measured by our life-detection experiments. The latter is of great concern 14
as it would compromise a major part of the scientific rationale for the biological study of Mars. The problem of back contamination concerns the possible return of potentially harmful biota to Earth. This issue is driven by many factorsâ societal, political, legal, ethical, and othersâin addition to purely scientific concerns. Back contamination must be given the most serious and careful consideration in missions where samples are to be returned to Earth for analysis, and in piloted missions. To a large extent, the amounts and types of measures needed for protection against back contamination will be established on the basis of data gathered from upcoming missions now in their planning stages. That is, if it is established that life does not exist and has not existed on Mars, then the need for protection of Earth- bound samples will be obviated. On the other hand, if there is a suspicion of extant or past life, then the need for protection will have to be adjusted accordingly. Historically, NASA's interpretation of planetary protection and its implementation of related procedures have focused on specific concerns related to forward and back contamination. First, in the face of possible forward contamination, the concern has been to preserve conditions on the planets for the future conduct of experiments on biological and organic constituents that might lead to insights concerning the origin and evolution of life in the cosmos. Despite other issues such as the dispersal and survival of species, the major focus has been on preserving other planetary environments from contamination by organisms from Earth that might grow there and thus obscure forever any efforts to understand the origin and evolution of life at locations other than Earth. Central to this issue is an assessment of the probability that an earthbound organism could contaminate another planetary body. Contamination in this case includes not only delivery of viable organisms, but also the growth of such organisms on the planet to such an extent that they would compromise future scientific endeavors. The ability to estimate the probability of contamination (Pc) depends on two factors: (1) accurate knowledge of the limits of organisms' ability to survive and grow on Earth and (2) accurate knowledge of the surface conditions of the planet to be visited. Any constraints imposed on a mission to ensure planetary protection from forward contamination will be mission dependent, relying on the best possible information about the conditions that might support growth of any biota from Earth that will survive transit through space. These points are extremely important; as information accumulates about a given extraterrestrial body, assessment of the amount of planetary protection needed to prevent contamination will undoubtedly change accordingly. The process must be iterative and must allow for modifying the techniques to ensure protection as new information is acquired regarding the harshness of the planet and the probability of contamination. 15
It should be obvious that if life is to be detected on Mars (a possibility, given the remarkable sensitivity of modern techniques and approaches), great care must be taken not to compromise such scientific goals by a previous or simultaneous introduction of life forms from Earth. Rigorous precautions were taken during the Viking mission to ensure that no forward contamination occurred. However, substantial amounts of data have accumulated since that time. Thus it is quite possible that the recommendations for planetary protection that guided the Viking mission may not be suitable for missions being flown today or for those flown in the future. An outline of Viking sterilization procedures is shown in Table 1.1, and a more detailed explanation is included in Appendix E of this report. TABLE 1.1 Viking-Level Procedures Recommended for Future Mars Missions Mission Type and Objective Procedure 1. Landers with in situ extant life- Trajectory biasing and orbit detection experiments lifetime Cleaning of componentsa Clean-room assembly Surface cleaning Lander sterilization Protection from recontamination Bioburden assessment 2. Orbiters and landers without in situ Trajectory biasing and orbit extant life-detection experiments lifetime Cleaning of componentsa Clean-room assembly Surface cleaning Bioburden assessment a Levels and types of cleaning will depend on the particular measurements being performed during the mission. For example, the Viking landing craft were cleaned to remove organics to less than 1 ng cm-2 because they were measuring organic molecules. This removal of organic material was accomplished via detergent cleaning, solvent cleaning, and hot helium purges to remove solvents.5 Similar levels of cleaning should be sought in future extant life-detection studies, but these will undoubtedly be modified upwards in landers that have no in situ extant life-detection experiments. The second focus of NASA's planetary protection effort has been, and will continue to be, protection of Earth's biosphere from the possibility of back contamination by any forms of life that may exist on other planets or 16
bodies that might be visited. Both in missions where samples are to be returned to Earth for analysis and in piloted missions returning both samples and crew, a variety of scientific, societal, and legal reasons exist for a planetary protection policy that ensures (1) the integrity and safety of our planet and (2) the rigorous protection of the scientific integrity of the samples. The two goals should be accomplished with a common protocol. REFERENCES 1. Space Studies Board, National Research Council. 1990. The Search for Life's Origins: Progress and Future Directions in Planetary Biology and Chemical Evolution. Committee on Planetary Biology and Chemical Evolution. National Academy Press, Washington, D.C. 2. Carr, M.H. 1987. "Water on Mars." Nature 326:30-35. 3. Baker, V.R. 1982. The Channels of Mars. University of Texas Press, Austin, 198 pp. 4. Gooding, J.R. 1992. "Soil Mineralogy and Chemistry on Mars: Possible Clues from Salts and Clays in SNC Meteorites." Icarus (in press). 5. National Aeronautics and Space Administration (NASA). 1990. Lessons Learned from the Viking Planetary Quarantine and Contamination Control Experience. NASA Contract Document No. NASW-4355. NASA, Washington, D.C. 17