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CHAPTER 25 STERILIZATION AND CONTAMINATION: THE NATURE OF THE PROBLEM K. C. ATWOOD The sterilization of objects that may land on extraterrestrial bodies is an accepted feature of any responsibly planned attempt at planetary ex- ploration. It will require vigilant and resourceful persons and substantial facilities and will definitely add to the cost and difficulty of the program. Contamination with terrestrial microbes must be avoided since it may confound future studies of the planetary history, surface characteristics and biota. On the other hand, the requirement that spacecraft withstand sterilization procedures may not always be met without sacrifice of ad- vantages in materials and systems. As to the relative emphasis appropriate to these two considerations, it may be noted that the kinds of difficulties caused by sterilization, e.g., systems failures, launch cancellation or design problems are postponements; they do not preclude ultimate attainment of the scientific goals. In contrast, the conceivable difficulties, real or unreal, that sterilization is intended to prevent may be permanent. They include major uncertainties in regard to the natural state of affairs on the planet, destruction of as yet unforeseen resources and irretrievable forfeit of im- portant objectives of planetary exploration. So far as can be foreseen at present, planetary exploration will be con- fined to the solar system. It is possible that Mars may provide our only chance to study life-forms of entirely separate descent. The importance of this opportunity outranks every prospect; the effect on world thought of the 449

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450 AVOIDING THE CONTAMINATION OF MARS discoveries to be made on Mars may compare with that of the works of Copernicus or Darwin. A careless blunder could destroy the opportunity. The reasons for the sterilization of spacecraft are clear: we do not know enough about Mars to predict with confidence the outcome of microbial contamination of its surface. Regrettable and irreversible results can be imagined. The means of avoiding these results are known; hence we must employ such means. The importance of sterilization of spacecraft, as well as the technical aspects, have been the subject of several prior reports. Phases of the sub- ject that have not been emphasized elsewhere will be given disproportionate attention here. In particular an attempt is made to assess the consequences of contamination of Mars on the basis of the very limited information we now possess and to suggest what information would be most valuable from the standpoint of assessment of these consequences as the exploration of the planet proceeds. PROSPECTS FOR SIGNIFICANT CONTAMINATION OF MARS A lander that carries life detection devices may be the source of con- tamination of its own experiments. More remote consequences of biological contamination would be significant only if the organisms are subsequently recovered, particularly in samples taken at some distance from the point of inoculation, or if their growth measurably alters the chemical constitution of the planetary surface, or if they affect indigenous life forms, e.g., as pathogens. If the organisms do not multiply or if they remain localized, their presence will be irrelevent to subsequent studies of the planet. This statement is based on the dilution factor provided by the Martian surface area, which is about 1014 square meters. Suppose, for example, that a landing capsule deposits 108 viable and environment-resistant organisms (an obvious overestimate) on the Martian surface. If these are distributed uniformly, the probability of detection by a device that exhaustively samples one square meter is only 10~6. If they remain localized, the probability is even less. The detection probability will increase only if the contaminants stay localized and subsequent devices are superimposed. Thus we may con- fidently expect that significant contamination would require both multiplica- tion and dispersion of the introduced organisms. In order to appreciate the bearing of the contamination problem on future scientific investigation and to offer conjectures based on present evidence concerning the likelihood that the introduction of organisms would result in significant contamination, we consider three cases which

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Sterilization and Contamination: Nature of the Problem 451 should, when all their variations are comprised, exhaust the possibilities with regard to the stage of planetary evolution to come under investiga- tion: (1) Primary abiosis, (2) Autochthonous biota present, and (3) Postbiotic stage. We ignore the possibility that the planet is in a biopoietic stage since available evidence points to surface conditions far advanced beyond those conducive to origination of life. It will be assumed from the outset that the surface presents more or less drastic local departure from the average environment. In the present context the important thing about condition (1) is that life may be absent even though the chemogenic prerequisites for biopoiesis were established approximately as they were on Earth. The example pro- vided by the origination of Earth's biota does not tell us how probable such an outcome may be, given a comparable prebiotic opportunity; for reasons discussed elsewhere we cannot dismiss the possibility that the opportunity for prebiotic chemical evolution on Mars during some period in its history was comparable to that on Earth. Thus Mars and Earth may be taken hypothetically to represent independent trials with roughly similar probabilities of success. If no replicating and evolving forms arose on Mars during favorable times, the Martian surface may now contain relict deposits of the presumably rich and varied chemical melange which, with the advent of life on a planet, would soon be recycled and changed beyond recognition. Experiments have shown that substrates utilizable by terrestrial organisms would be abundant in such mixtures; hence, in physically favorable local environments, the stage would be set for explosive multiplication of con- taminants. Theoretically the consequences could be drastic indeed. Given a generation time of one month (a thousandfold submaximal) the popula- tion resulting from a single bacterium after eight years would be sufficient to cover the entire surface of Mars with one gram of bacteria per square centi- meter. Such catastrophic potentialities are not mentioned here to suggest that they are possible on Mars, but rather to illustrate the magnitude of the effect given extensive though suboptimal conditions for microbial growth. If the origination of a system capable of indefinite evolution were com- pleted and refined on Mars, the history of such systems on Earth permits the surmise that condition (2) is more likely than (3); that is, that life has persisted until the present. Earth organisms have evolved to inhabit environments just as remote from those that are believed to have been favorable for biopoiesis as is the average Martian environment. In at least one particular, the oxygen tension, the Earth conditions are more extreme than Martian. It is curious in the light of current concepts of biopoiesis and evolution that the presence of molecular oxygen should persist in many minds as an indispensible condition for advanced life.

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452 AVOIDING THE CONTAMINATION OF MARS In any case, if rapid environment change or catastrophe has not brought an end to previously established life, the organisms must have evolved to keep pace with the progressively more extreme departure of the external environment from that present during the period of biopoiesis. Again, if we follow the example of Earth the modifications to accommodate to such environmental changes would have to be superimposed on certain ubiqui- tous and primitive characteristics such as the chemical identity of the genetic material itself and the specification of functions analogous to our genetic replication, transcription and translation. The internal environment of cells provides the conditions under which the elementary molecular copying processes can occur much as they have from their early beginnings. Thus the internal conditions may be regarded as much closer to those of the biopoietic environment than are the conditions outside the protective enclave of the organism. Further adaptations involve the maintenance of a special environment beyond the boundaries of the organism itself. Despite the strong possibility that the detailed chemical basis will be unique for each system that has had a separate origin, we expect the evolu- tion of elaborate mechanisms for the maintenance of primitive condi- tions within, or in the immediate .neighborhood of, organisms to be a gen- eral feature of life on any advanced planet. On Earth and Mars these primitive conditions may have much in common. The relevance of these considerations to the sterilization question is that the expected physiological and ecological adaptations of organisms to the conditions on an advanced planet are likely to provide local opportunities for the multiplication of adventitious contaminants within or around the indigenous forms. In case autochthonous life has at some time in the past been ex- terminated, the planet could not offer the afore-mentioned biogenically maintained habitats. Suitable microenvironments could, however, be main- tained around special surface features such as fumaroles, warm springs, or buried chemical deposits. The establishment of steady-state populations of contaminants in such localities might temporarily confound investiga- tions more than would the coexistence of autochthonous and adventitious forms. Significant contamination of Mars appears to be a distinct possibility irrespective of the presence or absence of life there. The experimental assessment of the significance of contamination will remain incomplete until the range of different environments is known so that appropriate simulation experiments can be performed. Even then, the interactions with Martian biota will remain a major uncertainty. Since we may assume, however, that large areas of the surface have the average environment, the question of the likelihood of dispersion of contaminants can be discussed in a preliminary way.

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453 THE SPREAD OF CONTAMINANTS Experiments with simulated average environments cannot answer the question whether contaminants would become established on Mars. Such experiments can, however, be helpful in evaluating the possibilities for spread of contaminants; hence—and especially important—the condition that would be found by future explorers of the planet in case contamination from earlier landers had become established. Preliminary studies in environments simulating average Martian condi- tions with a diverse though limited number of microorgansims have revealed none which continue to multiply under those conditions although some have remained viable. We shall make the reasonable but unproven as- sumption that some terrestrial species will be able to survive on Mars but that none can multiply under average Martian conditions. On this assump- tion the establishment of an adventitious population would require the introduction of the landed organisms into a suitable microenvironment. Thus, the multiplication of contaminants would be more or less delayed, perhaps indefinitely, depending on the circumstances of the landing and the accessibility of the special environments. No matter how direct the initial introduction, it will be to some extent selective; we cannot expect the local environment to support all of the species that are potential con- taminants. Subsequent spread from an established growing population might take place by direct extension, by wind, or by transport on (or in) mobile Martian macroorganisms. Spread by direct extension would be limited by topographic barriers unless the relevant microenvironments form an inter- connected unit. Experienced observers represent the dark surface features of Mars as a single network. A similar contiguity of hospitable micro- environments would be especially surprising in view of the scarcity of liquid water; so much so that one would be tempted to consider a cognogenic origin for such a system. In short, it seems impossible on the basis of present knowledge of the average conditions that terrestrial microbes could become widespread on Mars through direct extension of their range unless a stage of cognogeny has been reached, in which case the contamination would not preclude the recognition of Martian organisms, although it might have other undesirable effects. Similarly, the presence of indigenous vectors would imply a Martian biota that could not possibly be confused with any recent terrestrial introduction. Aeolian dispersion remains as a more general basis for planet-wide distribution of contaminants. A primary limitation on this form of trans- port is the rate at which organisms become windborne. On Earth the frequency of organisms in the air varies enormously from place to place,

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454 AVOIDING THE CONTAMINATION OF MARS but is only a very small proportion of the source populations. The occur- rence of dust storms on Mars gives ample evidence of the transport of surface materials from dry areas, but these areas are not the ones in which we should expect contaminants to be concentrated. In the moist micro- environments that would allow multiplication of contaminants, organisms would be much less likely to be lifted by wind, and drying of the region would probably be a prerequisite for rapid or extensive dispersion of its organisms. Windborne organisms and those deposited on the surface would be exposed to the rigors of the Martian environment; thus, dispersion would be selective for species that can withstand desiccation and the freeze-thaw cycles. Solar ultraviolet, which is believed to reach the surface with little atmospheric attenuation, would certainly have a deterrent effect on aeolian spread. While it is true that association with opaque particles will shield the organisms, only a fraction will be completely surrounded by opaque material and hence able to survive indefinitely in fully exposed locations. On the whole, it seems likely that the obstacles to the spread of con- taminants on Mars are formidable. CONTAMINATION AS A SOURCE OF CONFUSION A great deal has been made of the confusion that would result if con- tamination occurred before or near the beginning of a program of biological exploration. A summary of the possible conditions following multiplication and spread of contaminants suggests that the extent to which they would obfuscate scientific investigation has been exaggerated in the context of very thorough studies, but perhaps correctly assessed in the context of preliminary observations. If the planet were barren up to the time of contamination, the biota found later would have three distinctive features. First, few different kinds would be present; second, they would all be classified in Earth taxons and, finally, their distribution on the planet would be superficial. If it were feasible to apply these criteria, they would serve to identify the organisms as adventitious with absolute certainty. It is obvious that the number of species effectively introduced would be small. The sources of contamination during assembly of the vehicle are non-random with respect to species, and further selective attenuation would result from the incomplete sterilization and the Martian environment. Autochthonous biota, on the contrary, would comprise many types. Muta- tion and selection are necessary for any form of life, exobiota included; hence the presence of any evolved organism implies a variety of forms ap- propriate to the variety of ecological opportunities. An absence of auto- trophs would be a very suspicious feature. Terrestrial autotrophs would be

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Sterilization and Contamination: Nature of the Problem 455 unlikely contaminants and the indigenous biota of an advanced planet is expected on thermodynamic grounds to include a larger biomass of auto- trophs than of heterotrophs. The identification of an ostensible exobiont as a member of an Earth taxon would prove not only that it was adventitious, but that the introduc- tion was relatively recent in the time scale of planetary evolution. If a major portion of the evolutionary history of an organism had occurred on Mars it could not fit into any lower taxonomic category now extant on Earth. Evidence of common ancestry would be present, however, if the introduc- tion had been made after ubiquitous Earth biochemistry was established, say in the Precambrian. Statements such as the foregoing are not prejudg- ments of the issues we hope to resolve by exobiological studies; they are substantiated by terrestrial experience. It may be added, parenthetically, that the principles adduced in this hypothetical case of transplanted Earth biota are also generalizable to exotic life because they do not depend on the chemical basis of life, but only on its capacity to evolve by mutation and selection. Autochthonous organisms and their remains would be present in un- disturbed subsurface locations and permafrost in striking contrast to the superficial distribution of recent contaminants. Reliable evidence of the absence of dormant forms in cryptic locations, while they are present super- ficially, would be a strong indication of their recent advent. Having considered the problem of mistaken identity of biota, we turn attention to other changes in the planetary surface. The surface composi- tion of a planet on which life could have arisen, but did not, might be com- pletely obscured by the growth of contaminants if the physical conditions were everywhere favorable. On Mars, however, the average conditions are such that if the contaminants significantly changed the composition of their local environments, examples of the original surface composition would still be preserved by having been dried, frozen, or buried during the history of the planet. The question remains of the feasibility of applying these criteria in case an initial confusion should arise. The taxonomic criterion would be rather easy in an Earth-based laboratory and could be confirmed beyond doubt by means of molecular hybridization. The decision between a recent and a remote introduction might remain equivocal for some time unless return of the organisms could be arranged, with appropriate precautions against back contamination. The other two criteria would require investigations on the planet approximately as elaborate as those now visualized for the automated biological laboratory. By means of still more prolonged in- vestigations, contamination could not only be proved, but dated with a fair degree of precision. We may conclude that contaminants on Mars, if it is

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456 AVOIDING THE CONTAMINATION OF MARS now in a condition of primary abiosis, could under certain circumstances be mistaken for exobionts. The true condition would become evident upon detailed study of the organisms and their surroundings, but the necessary studies would probably be as lengthy and difficult as those for the charac- terization of bona fide exobionts. If Mars is in secondary abiosis, evidences of prior life would probably be found. These evidences might preclude an easy decision on the question of whether the living examples were survivors or adventitious forms, and the extreme improbability of this scenario would only serve to render the erroneous interpretation more plausible. The special difficulty here would be to rule out the possibility that the extinct life, very likely autochthonous, had the same primitive components as the living forms, erroneously sug- gesting that identical systems of macromolecular specification and synthesis had evolved on Mars and Earth. If as seems likely, no decisive differences were apparent from fossil chemistry the alternatives would remain of a terrestrial origin of the living forms versus a common Martian origin for both. Since the latter alternative would indicate unforeseen severe restraints on the variety of solutions to the problem of biopoiesis, an important issue would be confounded. The taxonomic criterion, although powerful, would have to stand alone. The confusion could be resolved, but this would re- quire thorough characterization of the organisms. In this, as in other instances, an adequate series of precontamination tests for terrestrial organisms would be of decisive value. After the dis- tribution and means of dispersion of the contaminating organisms had been studied, it would be possible to interpret prior negative results of these tests more confidently than before, and establish that the organisms found later were indeed contaminants. A positive result at first, on the other hand, would have initiated the study of the bona fide exobionts without delay. If living exobionts are present, they will tend to be better adapted than contaminants to the conditions. Even if terrestrial microbes could ulti- mately compete with Martian counterparts, this would require time for the evolution of special ecotypes. The recognition of contaminants in the presence of exobionts seems feasible and it is difficult to see how their presence would prevent the isolation and study of exobionts. The inter- action of terrestrial microbes with Martian macrobionts, however, is open to any conjecture, including the possibility of various degrees of patho- genicity. It is true that on Earth the host-pathogen relation always involves special adaptations that would have essentially zero probability of pre- existing in mutually exotic forms. We cannot be sure, however, that pathogenicity between mutually exotic forms may not have some basis entirely unanticipated from our experience with monophyletic biota. If

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Sterilization and Contamination: Nature of the Problem 457 this were true, it might or might not confound scientific investigations, but we can imagine many reasons why a pandemic would be regrettable; suffice it to say that the risk should not be taken inadvertently. An early search for macrobionts is essential and the credibility of their presence should not be underestimated. The suggestion of large, complex organisms on Mars is often ridiculed in accordance with the popular belief that inhospitable environments should support only simple or primitive forms. In actuality the opposite is true. Increasing hostility of the environment tends to ex- terminate both primitive and inappropriately specialized forms, and to select increasingly elaborate adaptations to the new conditions. Some of the most successful adaptations that can be imagined, or are known on Earth, can occur only in complex organisms. We may conclude that contamination of Mars would probably not cause permanent confusion in any of the contemplated scientific investigations. The one serious consequence that cannot be evaluated is pathogenicity of terrestrial microbiota for Martian macrobiota. Hence proof of the absence of macrobiota, or proof of non-pathogenicity would be a necessary—but not sufficient—prerequisite to relaxation of sterilization practices. In the foregoing arguments a great deal of confidence has been placed in the uniqueness of each genetic lineage at the level of primary sequence in polypeptides and polynucleotides, and the consequent certainty with which genetic relationships can be established. Clearly, an organism not possess- ing the ubiquitous molecular components of terrestrial biota would be iden- tified as an exobiont. On the other hand, an organism that did possess these components could be subjected to detailed comparisons with known taxa. Amino acid sequences can be determined analytically and nucleotide sequences compared by the techniques of molecular hybridization. The probability of common ancestry approaches unity as the length of common sequence increases. On reasonable assumptions, the coincident sequence required for strong proof of genetic relationship is surprisingly short. Perhaps current opinion on the likelihood of confusion among contaminants and exobionts has not taken the possibilities of molecular taxonomy properly into account. Strong reliance has also been placed on the ecological argument that if life exists at all on Mars, some forms of it will have a practically ubiquitous distribution. The selective advantage of dispersion capability is very strong and is tantamount to selection for prolonged survival in the average, pre- sumably inhospitable areas. This prognostication cannot be proved except by experiment, but it provides a reasonable a priori basis for the belief that the distribution of indigenous and adventitious bionts would differ for a long time following the introduction.

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458 STANDARDS FOR SPACECRAFT STERILIZATION Bacteriological sterility can be expressed as an all-or-none attribute, or as a probability, depending on the criterion employed to ascertain sterility. The all-or-none criteria are tests performed after the sterilization procedure; for example, we inoculate a medium and if nothing grows it is sterile (at least with respect to organisms capable of growth on that medium). Alter- natively, the surviving fraction of organisms can be determined in separate experiments under the sterilizing conditions and a probability obtained that the treated population contains one or more viable organisms. Since in typical cases the surviving fraction declines exponentially with time of treat- ment, the required probability is usually simple to estimate from experi- mental points, and an estimate of the size of the initial population. The testing of spacecraft for sterility is not feasible; therefore the sterility of a spacecraft can be expressed only as a probability. Standardization of the permissible number of viable organisms per spacecraft is a matter of judgment. No particular value can be rigorously defended by appeal to fact or logic, and all attempts to do so turn out upon close scrutiny to be specious. The mechanism by which such values are actually derived can be reduced to two steps. First an all-or-none decision is made by means of the same criteria that would apply if the sterility of the spacecraft could be confirmed experimentally. Second, if by these criteria sterilization is required, then the permissible probability of contamination is fixed at a level which is, in the judgment of the responsible person, indis- tinguishable from zero. Such values, reflecting individual temperament, range from 10~s to 10~6. Sagan and Coleman (Chapter 28) point out that the probability of con- tamination of Mars during a program of scientific investigation is different from the probability of contaminating it in any given mission. They show how the theory of waiting times can be used to compute a standard for spacecraft sterilization such that the risk of contamination prior to comple- tion of the program will not exceed some predesignated value. Required parameters are the number of experiments that must succeed to complete the program, the probability of success per experiment, the number of ex- periments per mission, the probability of failure of a mission as a whole, and the probability of effective contamination per organism deposited. In prac- tice, these values would be extremely uncertain, and changeable during the program. The calculation of the standard for individual missions from pre- designated program characteristics should not obscure the fact that the permissible risk of contamination for the entire program is an arbitrary figure to be assumed a priori for the purpose of computation. This ap-

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Sterilization and Contamination: Nature of the Problem 459 proach does not supply a basis for selection of a standard; its value is in transferring the point of subjective judgment from the individual mission to the anticipated series. Various rationales can be conceived that obscure the arbitrary nature of the standard. For example, the probability is greater than zero that Mars has already been contaminated with organisms from Earth; hence, a risk of contamination no greater than that which has already been sustained might be acceptable. To derive a number on this basis we begin by rejecting the Arrhenius hypothesis having to do with spores, or particles of equivalent size, on the ground that such particles would be insufficiently protected against solar ultraviolet and ionizing radiation to survive the traverse. Macroscopic chunks ejected from the Earth's surface, however, would effectively shield included organisms. It has been estimated that frag- ments with sufficient velocity to reach Mars are ejected by meteoritic im- pacts about once in 107 years; that is, about 103 such instances may have occurred since the Cambrian. If they are ejected in random directions then the probability that a given one impacts on Mars is about 10~9, the ratio of the area of the disk of Mars to the area of the sphere of orbital radius. On these assumptions the expectation that Mars has been contaminated since the Cambrian is 10-°. The mechanism also suffices for Mars to Earth transport, or contamination of both planets by fragments from a common parent body. No such rationale is relevant to the problem, however, and none but an arbitrary standard can be specified. If an arbitrary standard is prescribed, its maintenance in practice may require a more or less reliable estimate of the size and composition of the microbial population in the spacecraft prior to the terminal sterilization. This problem would not arise if the terminal sterilization procedure re- mained harmless to components at arbitrarily low levels of microbial sur- vival. In that case, a fictitious but safe upper limit of the microbial load could be assumed (e.g., the weight of the spacecraft) and the terminal sterilization adjusted accordingly. Such fictitious upper limits are not useful if the damage to components tends to increase with the effectiveness of sterilization. It has been suggested that the bacterial load can be approxi- mated by means of experimental determination of the steady-state popula- tions on representative surfaces, and sampling of components for internal contamination. Estimates obtained in this way would be useful provided that the sources of major fluctuations in number of bacteria per spacecraft can be recognized and controlled. These considerations should make it obvious that a set of procedures will ultimately have to be prescribed, rather than a standard of contamination probability. Such procedures may be designed with an approximate upper limit of contamination probability in mind. Only the procedures themselves, however, can be standardized.

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460 AVOIDING THE CONTAMINATION OF MARS STERILIZATION METHODS The elaboration of useful details pertaining to methods of spacecraft sterilization would require a degree of engineering sophistication beyond the scope and intent of this discussion, or the competence of the author. Methods cannot be discussed intelligently unless the effects of heating and other procedures on different spacecraft components are known. No at- tempt will be made, therefore, to recommend specific procedures. Prin- ciples of redundancy and flexibility are generally applicable, however, no matter how the problems may differ in detail from time to time. As much redundancy should be incorporated into the sterilization program as is feasible from an engineering standpoint. Modules, sub-assemblies and separate components should be sterilized when feasible even though they will later be sterilized again as parts of a larger ensemble, or even though they will be joined to other components that have not been treated in the same way. Reundancy is desirable because it increases the probability that complete sterilization is achieved by some terminal procedure, and because it is a safeguard in case of accidental failures. Even in a complete failure of terminal sterilization the prior measures would not have been in vain, since an innumerable variety of situations can be imagined in which the probability of effective planetary contamination would depend strongly on the number of organisms introduced. The entire process of spacecraft production should be reviewed continuously in an effort to discover oppor- tunities for reduction of bacterial load. Such opportunities should be ex- ploited as they arise, even though a predesignated standard is already being met. The terminal heat soak is a preferred method because it is simple and reliable in principle. No single method, however, is a sine qua non of effective sterilization. Where questions of feasibility arise they can be ap- proached either by rendering existing methods feasible or by finding new methods, and the choice of these approaches should be based on calculable merit rather than on preconceived dogmatism. Given that a sterilization method can be made effective, its figure of merit must also be influenced by its effects on the functioning of spacecraft systems. At present it seems possible that the terminal heat soak can be made feasible even for large and complex spacecraft. ADJUNCTIVE STRATEGEMS Despite all efforts at a prevention the possibility remains that because of an improbable event, or a blunder, Mars will be contaminated at some

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Sterilization and Contamination: Nature of the Problem 461 time during the program of exploration. Accidental impact of an unsteri- lized object may occur. It is also possible that the decision to deliberately allow contamination (e.g., by manned landing) will turn out to be pre- mature, or based on some error of judgment. It would be wise therefore to plan the biological exploration in such a way as to minimize the harm done by such an eventuality. One measure that has been suggested is the early collection of samples in sterile sealed containers. This may be a valuable procedure. Small samples taken at many different locations would be more valuable than one or few large samples. Means would have to be devised to make these samples easy to locate at some indefinite time in the future. The method has the disadvantage that it would not yield informa- tion in time to be useful in planning subsequent missions, since the samples could not be examined until much later. Another stratagem is the early landing of life detection devices designed to detect terrestrial microorganisms and biochemical activities. This would immediately yield evidence concerning the presence or absence of organisms having a superficial resemblance to Earth biota. These would be the important types in case of later confusion. They are also the types expected to inhabit the same microenvironments in which contaminants would thrive. If such exobionts exist it is almost certain that they possess at least as effec- tive dispersion capabilities as those of the putative contaminants; moveover, they have been present since an indefinite time in the past. Therefore some evidence of their presence—dormant forms, microscopic fragments and remnant biochemical activities and materials—would be found everywhere on the planet. If such exobionts are present they would have a very high probability of being detected by means of a few rather imprecisely placed devices, some in light and some in dark areas. Various devices that make use of the criteria of growth, metabolic and biochemical activities would permit deductions concerning the background against which future con- tamination would have to be interpreted. Negative results with devices that detect growth in various media, for example, would strongly suggest that no Martian microbionts can multiply under conditions favorable for con- taminants. It would also suggest, less strongly perhaps, that if Martian microbionts are present (a presence perhaps confirmable by one of the other criteria) favorable microenvironments for contaminants are rare or absent, for otherwise some exobionts would have evolved to utilize this opportunity and hence would be able to grow on some of the test media in common with some terrestrial forms. The devices should perform a sufficient variety of tests to give a fairly distinctive pattern in case of a positive result. They should remain sealed following the experiments, so that examination of their contents may be meaningful if they are recovered. The inclusion of such devices on the

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462 AVOIDING THE CONTAMINATION OF MARS earliest missions is important, since any result, negative or positive, will be valuable for future reference and a much needed guide to action. CRITERIA FOR DISCONTINUANCE OF STERILIZATION The decision that spacecraft landing on Mars need not be sterilized may be based on observational data, experiments, and theory. The necessary observations relate to the range of physical environments, the surface com- position at different sites, and the presence of macrobiota. The purpose of the observations is to permit the accurate simulation of the environments in Earth-based experiments. The experiments concern the fate of terrestrial microbionts in the special Martian conditions known from the foregoing observations. Theoretical estimates of the probabilities of different degrees of evolu- tionary convergence at the molecular level, e.g., Spetner [1964], may permit a decision on whether confusion as to phylogenetic origin could result from contamination. SUMMARY 1. The results of projected scientific investigations on Mars are poten- tially so important that even a remote possibility of invalidating such results should be avoided. Hence a sterilization program is necessary until it becomes more certain that contamination would not have undesirable effects. 2. On the basis of current beliefs concerning average Martian condi- tions, contamination with terrestrial microbes does not seem likely to have significant effects. This fact in no way relieves us of the responsibility to avoid contamination of the planet for the present. REFERENCE Spetner, L. M. (1964), Natural Selection: An Information-transmission Mecha- nism for Evolution. /. Theor. Biol. 7:412-429.