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SUMMARY AND CONCLUSIONS THE ORIGIN AND NATURE OF LIFE The modern, naturalistic view of life's origin and evolution dates from the foundations of modern biology a century ago. Implicit in the evolu- tionary treatment of life is the proposition that the first appearance of organisms was only a chapter in the natural history of the planet as a whole. Oparin later made this notion explicit in his view that the origin of life was a fully natural, perhaps inevitable, step in the ontogeny of the Earth. Systems capable of self-replication and controlled energy transfer —living organisms—had their origin in the sequence of chemical changes that were part of the planet's early history. The tractability of this great inductive step to further discussion has been enhanced by the progress of terrestrial cellular biology and bio- chemistry over the last few decades. From that progress has emerged a unified picture of life at the subcellular and chemical levels, underlying the unity at higher levels that so largely influenced Darwin. Not only is there a common pattern to the structure of cellular organdles—membranes, mitochondria, nuclear apparatus—but a still more surprising unity is found in its molecular constituents. Everywhere on Earth the essential catalytic functions are discharged by proteins, energy transfer effected by adenosine triphosphate, and the synthesis of proteins today controlled by an elaborate nucleic acid system. The same enzymatic cofactors are found in organism

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4 BIOLOGY AND THE EXPLORATION OF MARS after organism; particular metabolic pathways recur from cell to cell; and everywhere the fundamental functions of information storage and replica- tion are assigned to the nucleic acids. To a significant extent the discussion of life's origin must concern the origin of those molecular types that are crucial in cellular organization: the origin of nucleic acids, of proteins, of carbohydrates, and so on. In the 1950's a series of experiments was initiated in which the synthesis of biologically important compounds was accomplished by application of energy to presumptive primitive environments. The list includes: amino acids and their polymers; carbohydrates and fatty acids; purines and pyrimidines; nucleotides, including adenosine triphosphate, and oligo- nucleotides—every major category of molecular sub-unit of which the cell is built. The credibility of the naturalistic, evolutionary view of life's origin as an exploitation of previous chemical evolution on a sterile Earth is greatly heightened by these results: the great chemical complexity of its molecular constituents does not, in last analysis, require the intervention of the cell itself. The general tenet that life involves no qualitative novelty—no elan vital— goes hand in hand with the more explicit proposition that it is the molec- ular organization, as such, of living things that alone distinguishes them from the non-living. The central issue in discussing origins now concerns not so much the prior evolution of complexity in molecular constituents as the development of their organization into a system that is alive. It is here we lack any sure guides—save one—on the contingency involved; on how improbable it all was. That one lead comes from the great and well-known advances of molecular genetics in the past ten years. The essence of organization in one sense is its improbability, its depen- dence on specification or information. And the most characteristic feature of living organizations—organisms—is their capacity to store and replicate the evolving information on which their existence depends. The high point of our biochemical advance has been identification of the molecular basis of these defining characteristics. It is astonishing how much we have recently learned about the manner in which the information underlying life's organization is encoded in molecular structure; that we understand how that molecular structure is replicated; and further that simple poly- nucleotides have been synthesized in cell-free systems. It remains unclear, of course, what precise sequence of events exploited the opportunities afforded by the purely chemical evolution of the Earth's surface and atmosphere. But at some point in the unknown sequence a community of molecules would have been fully recognizable to us as a living as against a non-living thing: it would have been bounded from its

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Summary and Conclusions 5 environment by a membrane, capable of controlled energy expenditure in fabricating more of itself and endowed with the capacity to store and replicate information. We cannot fully know the precise course of the Earth's early chemical evolution, and the degree of contingency involved in the subsequent transi- tion to a living organization of molecules; and for these reasons we cannot fully assess just how probable or improbable life's origin was at the outset of our own planet's evolution. Nor can we estimate to what extent the emerging picture of a single chemical basis of life on Earth reflects a physical necessity for living organization as against a mixture of physical sufficiency and historical accident. Can the catalysis essential to bio- chemical organization be effected only by proteins containing the 20 amino acids we encounter in cells? Are the nucleic acids the only polymers, for physical reasons, that can carry molecular information on satisfactorily? Or are these and other empirical generalizations about life on Earth, such as optical activity, merely reflections of the historical contingency that gave such molecules first access to living organization, thus preempting the field and precluding realization of other physically sufficient molecular founda- tions for life? To the extent that we cannot answer these questions, we lack a true theoretical biology as against an elaborate natural history of life on this planet. We cannot prejudge the likelihood of life's appearance on Earth; therefore we cannot confidently take the great inductive step when we are told by astronomers that there may be 1020 planetary systems elsewhere in the Universe with histories comparable to our own. One thing is clear: If life is unique to our planet the probability of its origin must be almost unimaginably low. If, on the other hand, the probability is at all appreci- able, life must be abundant in the 1020 planetary systems that fill the sky. At stake in this uncertainty is nothing less than knowledge of our place in nature. It is the major reason why the sudden opportunity to explore a neighboring planet for life is so immensely important. We emphasize that the act of discovery itself would have this great scientific, and for that matter philosophical, impact. But it is also important that discovery would, in another way, be only the beginning. The ex- istence and accessibility of Martian life would mark the beginning of a true general biology, of which the terrestrial is a special case. We would have a unique opportunity to shed new light on the meaning of the astonishing molecular similarity in all terrestrial organisms. Is it there as a physically necessary basis for life? Or is it—physically sufficient but not necessary— a historical accident in the sense that, in another instance of planetary evolution, a different basic chemical complexity could equally well have emerged and preempted the local opportunity for life?

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6 THE POSSIBILITY OF LIFE ON MARS No thoughtful person will disagree with our assertion on the scientific importance of life elsewhere in the solar system. It is, however, another matter to conclude that search for it should proceed at once. The explora- tion will be costly in money and other resources. To undertake it we need some assurance that it is not folly from the outset. Interest immediately focuses on Mars. The nearest and most Earth-like of the planets in the solar system are Mars and Venus, but the surface of Venus has been tentatively excluded as a possible abode of life, because of the probably high surface temperatures. The Martian year is long (687 days), but the length of its day is curiously similar to that of Earth, a fact that to considerable degree ameliorates an otherwise very severe environ- ment. Mars has retained an atmosphere, although it is thin: present estimates of pressure at the surface range from 10 to 80 millibars. The major con- stituents are unidentified, but are thought to be nitrogen and argon. Carbon dioxide has been identified spectroscopically and its proportion estimated to lie between 5 per cent and 30 per cent by volume. Oxygen has been sought but not detected; the sensitivity of measurement implies a propor- tion not greater than 0.1 per cent by volume. Water vapor has also been identified spectroscopically as a minor atmospheric constituent in the amount of 2 X 10"8 g cm"2. (For comparison, approximate terrestrial values of the quantities given above are: surface pressure 1,000 millibars; carbon dioxide 0.03 per cent; oxygen 20 per cent; water vapor 3 g cm"2). The intensity of ultraviolet radiation at the Martian surface may be high by comparison with Earth, but this is not yet certain; some models of the composition of the atmosphere allow for effective shielding. Surface temperatures overlap the range on Earth: at some latitudes and seasons they have a daily high of -f-30°C with a diurnal range of about 100°C. There are two white polar caps whose composition has been the subject of some controversy. The evidence now is clear that they are ice, in the form of hoar frost. They undergo a seasonal waxing and waning, which is probably accompanied by an atmospheric transfer of water vapor from one hemisphere to another. Our knowledge of what lies between the polar caps is limited to the distinction between the so-called "dark" and "bright" areas and their sea- sonal changes. The latter, usually considered "deserts," are an orange-ochre or buff color. The former are much less vividly colored. It is likely that early descriptions of the dark areas as green result from an optical illusion due to contrast with the orange "bright" areas.

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Summary and Conclusions 7 Biological interest nevertheless continues to center on the "dark" areas. In several respects they exhibit the kind of seasonal change one would expect were they due to the presence of organisms absent in the "bright" (desert) areas. In spring, the recession of the ice cap is accompanied by develop- ment of a dark collar at its border, and as the spring advances a wave of darkening proceeds through the dark areas toward the equator and, in fact, overshoots it 20° into the opposite hemisphere. Polarimetric studies suggest that much of the Martian surface may be covered with small sub-mi.'limeter-sized particles. The curve on which this inference is based shows a seasonal displacement in the dark areas, but not in the bright. Infrared absorption features have been attributed to the dark areas, suggesting abundant H—C bonds there, but more recent analysis throws great doubt on this interpretation, leaving us with no definite in- formation, one way or the other, about the existence and distribution of organic matter. Needless to say, none of these inferences about the Martian dark areas demands the presence of organisms for their explanation. Indeed, the question is whether the Martian environment could support life at all; and further, whether its history would have permitted the indigenous origin of life. These are clearly different questions. Our answer to the first question is that we find no compelling evidence that Mars could not support life even of a kind chemically similar to our own. Were oxygen present to the small limiting extent current measurements allow, a fully aerobic respiration would be possible. But even its total absence would not of itself preclude life. One of our more rewarding exercises has been the challenge to construct a Martian ecology assuming the most adverse conditions indicated by present knowledge: it posed no insuperable prob- lem. Some terrestrial organisms have already been shown to survive freeze- thaw cycles of -(-30° to —70°C. Others are known to cope with extremely low humidities and to derive their water supply metabolically. There are many conceivable ways of coping with a strong flux of ultraviolet (and even of exploiting it as an energy source). The history of our own planet provides plenty of evidence that, once attained, living organization is capable of evolving adjustments to very extreme environments. And, finally, we are reminded that the evidence we have on Martian conditions is very coarse-grained, a sort of average that takes account of almost no local variations dependent on topography. Within the range of conditions represented by our present numerical estimates, it is likely that there exist, perhaps abundantly—as on Earth—places where the extremes of tem- perature, aridity, and adverse irradiation are markedly ameliorated. Even the presence of water in the liquid phase is perhaps not unlikely, if only transiently, by season, in the subsoil.

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8 BIOLOGY AND THE EXPLORATION OF MARS A measure of our judgment that niches in the contemporary Martian environment could support life of a sort comparable to that of Earth is provided by our overriding concern with the danger of inadvertently con- taminating Mars with terrestrial organisms. We shall return to this problem later. The other question—whether life in fact is there—depends on our judgment of how probable its origin on Mars has been. The a priori probability of origin we can not assess, even for Earth; it is the principal reason for considering exploration in the first place. Given all the evidence presently available, we believe it entirely reason- able that Mars is inhabited with living organisms and that life inde- pendently originated there. However, it should be clearly recognized that our conclusion that the biological exploration of Mars will be a i rewarding venture does not depend on the hypothesis of Martian life. The scientific questions that ought not to be prejudged are: a. Is terrestrial life unique? The discovery of Martian life, whether extant or extinct, would provide an unequivocal answer. b. What is the geochemical (and geophysical) history of an Earth- like planet undisturbed by living organisms? If we discover that Mars is sterile we may find answers to this alternative and highly significant question. THE SCIENTIFIC AIMS OF MARTIAN EXPLORATION We approach the prospect of Martian exploration as evolutionary biologists. The origin of organisms was a chapter in the natural history of the Earth's surface. The hypothesis to be tested is a generalization from that single case: the origin of living organization is a probable event in the evolution of all planetary crusts that resemble ours. We thus con- ceive the over-all mission as a systematic study of the evolution of the Martian surface and atmosphere: has that evolution included, in some niches of the planet, chemical systems with the kind of organization we would recognize as "living"? Our aims, in summary form, are: (1) determination of the physical and chemical conditions of the Martian surface as a potential environment for life, (2) determination whether life is or has been present on Mars, (3) determination of the characteristics of that life, if present, and (4) investigation of the pattern of chemical evolution without life. This formulation emphasizes that, as biologists, we have as much interest as the planetary astronomers in a thorough study of the meteorology, geo-

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Summary and Conclusions 9 chemistry, geophysics, and topography of Mars. Whatever the outcome of a direct search for life, its full meaning will escape us unless the findings can be related to the prevailing environment. AVOIDING THE CONTAMINATION OF MARS Before proceeding to the more programmatic aspects of the undertaking, we are concerned to single out the task of spacecraft sterilization from the many and diverse problems that Martian exploration will entail. We believe that many of our nonbiologist colleagues have still not fully grasped either the magnitude or the fundamental importance of this issue. Contamination of the Martian surface with terrestrial microbes could irreversibly destroy a truly unique opportunity for mankind to pursue a study of extraterrestrial life. Other future uses of Mars are not evident to us now; whatever they are, they may be clumsily destroyed by premature and uninformed mistakes in our program. We are eager to press Martian exploration as expeditiously as the technology and other factors permit. However, our present sure knowledge of Mars is very slim and so our recommendation to proceed is subject to one rigorous qualification: that no viable terrestrial microorganism reach the Martian surface until we can make a confident assessment of the consequences. In operational context, this means that the probability of a single viable organism reaching the Martian surface be made small enough to meet scientifically acceptable standards. These standards, already established provisionally,* should be continually reexamined in the light of all new information. Moreover, every effort should be made to ensure the continued acceptance by other launching nations of the recommended confidence levels for protection of Mars against contamination. The technical prob- lems precipitated by this demand include the control of trajectories to an accuracy sufficient to prevent the accidental impact of unsterilized payloads, the development of sterilizable spacecraft components for vehicles intended for landing, the development of procedures that will prevent the introduc- tion of microorganisms, and the means for establishing the reliability of the entire program. Since we have not yet succeeded in sterilizing a space vehicle, the problem must be considered unsolved. An energetic program for the development of procedures for sterilizing space vehicles and their components must be implemented immediately if we are to take advantage of the opportunities that will arise between 1969 and 1973. We must guard not only against accidental neglect of necessary safeguards but also against placing ephemeral considerations of prestige * Report of COSPAR Seventh Meeting, Florence, Italy, May 1964, Resolution 26.

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10 BIOLOGY AND THE EXPLORATION OF MARS above enduring scientific significance and utilitarian value in our explora- tion of space. AVENUES OF APPROACH TO THE EXPLORATION OF MARS For convenience, we distinguish four categories of work that can con- tribute to attaining our goals: (a) laboratory work needed to develop techniques for planetary investigations and the knowledge needed to interpret their findings; (b) Earth-bound astronomical studies of Mars; (c) the use of spacecraft for the remote investigation of Mars; and (d) a direct study of the Martian surface by landing missions. LABORATORY WORK The consideration of the evolution of life on Mars raises many problems that can be studied in Earth-based laboratories. Such studies are, in fact, essential to provide the background against which the results of planetary missions must be interpreted. The work includes the chemical analysis of meteorites, especially with respect to their content of organic compounds, and the extension of studies on the spontaneous formation of organic mole- cules and their aggregation into larger units. These investigations may reveal to us not only the mechanism by which the materials essential for living organisms were first formed, but also the origin of reactions and mechanisms that lead to the formation of organized structures and their self-perpetuation. Other possibly interesting lines of effort include study of alternatives to the carbon-water system of biochemistry and simulations of Martian and other planetary environments. While some of these simulated environments may allow terrestrial microorganisms or enzyme systems to function, others may be more conducive to the activity of reaction systems based on alternative biochemistries. It will become clear later that considerable work remains to be done in defining schemes for life detection and in developing the instrumentation to exploit them. EARTH-BOUND ASTRONOMICAL STUDIES OF MARS The observation of Mars from terrestrial observatories enjoys the ad- vantages of economy, absence of weight and size limitations, and high data rate. It is, however, limited by the terrestrial atmosphere in attainable

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Summary and Conclusions 11 resolution and spectral range, and further constrained by daylight and weather. Nevertheless, much valuable work could be conducted at a cost that is low compared to that of space programs if the nation's large instru- ments were made available during prime seeing time for the observation of Mars. The use of 120" and 200" optical telescopes and of the largest radio telescopes and interferometers could rapidly extend our knowledge of Mars. We support the recommendations of another committee of the National Academy of Sciences1 on the need for additional ground-based astronomical facilities. For such facilities to play a significant role in the planning of 1969-73 Mars missions, work on this program must be begun early. USE OF SPACECRAFT FOR REMOTE OBSERVATION OF MARS Some of the observational limitations imposed by the terrestrial environ- ment can be overcome by balloon-borne observatories, but, since they are severely restricted in size and observation time, their usefulness is limited; it is also restricted by absorption in the Earth's atmosphere. The projected Earth-orbiting astronomical observatory overcomes some of these limita- tions, and we believe the observation of Mars, particularly in the ultra- violet, should be included in the plans for its use. It is, however, from Martian fly-by missions and, in particular, from Martian orbiters that the remote observation of that planet is best under- taken. We hope to obtain our first closeup information on the Martian surface from the video scan to be carried out by Mariner IV, and to gain additional knowledge of atmospheric density by observation of the telemetry signals during occultation of the spacecraft. Fly-by missions are, however, severely limited in the time available for observation; they provide at best a fleeting glimpse of the planet. Martian orbiters will be technically possible for the opportunities of 1969 and thereafter. They offer an unparalleled opportunity to scrutinize the planet at comparatively short range. Potential orbiter payloads have been examined by another group, and compositions of such payloads have been suggested for a range of instrument weights up to 200 Ibs (which is within the capability of the Saturn IB-Centaur). For example, a modest payload that any of several vehicles could place in orbit could include instruments for (1) infrared and television mapping; (2) microwave radiometry and bistatic radar; (3) infrared spectrometry; and (4) optical polarimetry. These sensors would yield information on temperatures, surface and atmospheric composition, topography, certain characteristics of surface 1 Ground-Based Astronomy, A Ten-Year Program, National Academy of Sciences Publication No. 1234, 1964.

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12 BIOLOGY AND THE EXPLORATION OF MARS structure, and, most important of all, permit a sustained scrutiny through a full cycle of seasonal change and over a major fraction of the Martian surface. MARTIAN-LANDING MISSIONS: ABL'S SMALL AND LARGE While it is conceivable that the findings of a Martian orbiter could establish the presence of life on the planet, we are in any case convinced that landing missions are essential for adequate Martian exploration. The definition of lander payloads is a complex and demanding task that we have only begun to explore. Their design is to some extent dependent on our knowledge of the struc- ture of the Martian atmosphere. The size of the payload that can be de- posited depends, for instance, on whether the use of a parachute is feasible or whether the density of the atmosphere is so low as to require the use of retrorockets; this is especially critical for small payloads. In this connec- tion, we note the possibility that the density profile of the Martian atmos- phere will be determined by astronomical means, or by Mariner IV, with sufficient precision for the purpose of designing a landing system. A more direct method for studying the Martian atmosphere involves the use of non- survivable atmospheric-entry probes that could transmit information on atmospheric density, structure and composition. Such probes could be launched from either fly-bys or orbiters. Since their design is not de- pendent on atmospheric density, these are useful devices for obtaining advance information, if needed, for the survivable landing of an instrument package. The view has also been presented that a small surviving capsule would have even more value, in that it might determine not only the density profile of the atmosphere, but also its composition at the surface, wind velocity, and other data that would enhance the probability of success of a large lander. However, if we had complete knowledge of these prerequisites for a successful survivable lander, our principal design difficulty would remain: it concerns the problem of life detection. What minimal set of assays will permit us to detect Martian life if it does exist? A debate on this question for the past several years has yielded a variety of competing approaches. Each of these is directed to some manifestation of life according to the cues of terrestrial biology. Needless to say, visual reconnaissance, from micro- scope to telescope, is one of the most attractive of these, for it offers the expectation that many recognizable hints of life would immediately attract our attention. However, we can easily imagine circumstances in which this type of observation would be inconclusive. Many other suggested pro-

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Summary and Conclusions 13 cedures are designed to identify, at the outset, the more fundamental bio- chemical structures and processes that we would, in any case, explore in depth. No one of these analyses, however—whether photosynthesis or respiration, DNA or proteins, growth, enzymes or metabolism, or, in a figurative sense, fleas or elephants—can be sure of finding its target and reliably reporting on it under all circumstances, nor would any single approach satisfy all the particular interests that motivate different in- vestigators in their search. We cannot recount here all our deliberations on the life-detection prob- lem. We have sought the most generalized criteria; among these is net optical activity, which is almost surely the result of steric restrictions imposed by a historical accident in the origin of life. Another is the presence in assays of exponential features that can be ascribed only to growth and reproduction. And we have reconciled ourselves to the fact that early missions should assume an Earth-like carbon-water type of biochemistry as the most likely basis of any Martian life. On that assump- tion, enzymes that should be widespread can be sought and 2rn\vth may be demonstrated by the use of generalized media. The fact remains, and dominates any attempt to define landers for detecting life, that no single criterion is fully satisfactory, especially in the interpretation of some negative results. To achieve the previously stated aims of Martian exploration we must employ as mixed a strategy as possible. Discussion throughout our study has returned repeatedly to the con- clusions that we wouKTnot be convinced by negative answers from single "life detectors"; that, given the hazards of any chemical or metabolic assay, we should ensure some direct visual inspection by television, and that the lander program must ultimately involve an Automated Biological Labora- tory (ABL). The ABL concept is not fully defined: it involves provision for the multiplicity and diversity of chemical analytical techniques and biological assays that our aims call for; it involves, too, the idea of an on- board computer by means of which a variety of programmed assay sequences can be initiated contingently on the results of prior steps; it also involves the idea of a sustained discourse between the computer and in- vestigators on Earth. It is, in short, an ambitious concept. But our pre- liminary scrutiny of the ABL idea suggests that, though ambitious, it is, in principle, realizable with current technology. In the long run, we believe that manned expeditions and the return of Martian samples to the Earth will be part of the exploration of the planet. Neither of these is imminent, but some of our readers will be as surprised as we were to discover that manned Martian missions will probably be feasible in the 1980's. Certainly neither the return of samples nor the

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14 BIOLOGY AND THE EXPLORATION OF MARS sending of men to Mars will be scientifically justifiable until unmanned landings have prepared the way. THE TIMING AND OVER-ALL STRATEGY OF EXPLORATIONS In principle, all of us would prefer a gradualistic approach to the ulti- mate goals of landing a large ABL on Mars and, eventually, of returning samples for study here. It is clear on all grounds—of economy, and scientific prudence—that we should exhaust the possibilities of further progress using Earth-based observations and non-landing missions to Mars. For instance, a strong majority of the working group believes a success- ful orbiter program should precede a landing. The orbiter promises an immense extension of our knowledge of the atmosphere (its density and chemical composition) and surface of Mars. Its capability for sustaining seasonal observation and extensive topographic mapping will permit a thorough re-evaluation of the several Martian features that have been considered suggestive of life. And it will permit a far-better-informed selection of landing site for the ultimate ABL missions. It has the further merit of effecting this substantial step forward with minimum risk of con- taminating the surface. Constraints to proceeding in a completely unhurried, step-by-step fashion arise from several sources, however. They are a combination of celestial mechanics and the operational realities of space research. Any space experiment takes years of preparation and budgetary commitment; the preliminaries to actual flight involve years of experimental design, space- craft development, and the coordination of effort among large numbers of people in a wide range of disciplines. The scientific investigator no longer has the total freedom he usually enjoys to make tentative starts, to explore hunches without full commitment, to stop and follow another course. He is further plagued by the prospect of investing years of work only to en- counter a mission failure or cancellation in which it is all lost—at least until a new opportunity arises, perhaps years hence. He may chafe under these circumstances but he must accept them if he wishes to proceed at all. The kind of Martian lander that we visualize will be a most complex and difficult spacecraft to build and will require the combined efforts of many different scientific specialists. For these reasons, its development will be most costly and time-consuming. A Martian orbiter is also a much larger undertaking than any scientific spacecraft yet flown. The point is that we are confronted with the necessity of near-commitment many years ahead of flight time; and the opportunities for flights to Mars are by no means always at hand. The orbits of Earth and Mars are such that these oppor-

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Summary and Conclusions 15 tunities are now limited to brief windows that recur about every second year but undergo a further, approximately 17-year cycle of favorableness. Our attempt to develop a systematic and gradualistic program is thus con- strained to some extent by the fact that, while favorable opportunities occur in the 1969-1973 period, they will not return before 1984-1985.* We have concluded that the 1969-1973 opportunities can be and should be exploited for a substantial program of planetary missions. By that time, the Saturn booster system will be available, and a four- to five-year lead time is evidently adequate for the development of initial spacecraft. The more detailed planning of planetary missions for 1969-1973 is for the most part outside the scope of this working group's competence and commission: the decisions concerned involve engineering and many other elements with which we did not cope. CONCLUSIONS AND RECOMMENDATIONS THE BIOLOGICAL EXPLORATION OF MARS RECOMMENDED The biological exploration of Mars is a scientific undertaking of the greatest validity and significance. Its realization will be a milestone in the history of human achievement. Its importance and the consequences for biology justify the highest priority among all scientific objectives in space —indeed in the space program as a whole. THE SCIENTIFIC AIMS OF THE EXPLORATION We approach the prospect of Martian exploration not only as biologists but as scientists interested in evolutionary processes over the broadest range. Living systems have emerged as a chapter in the natural history of the Earth's surface. We wish to test the hypothesis that the origin of life is a probable event in the evolution of all planetary environments whose histories resemble ours. We thus conceive the over-all mission as a systematic study of the evolution of the Martian surface and atmosphere: has that evolution in- *For these reasons an alternative strategy has been discussed: it would allow the early use of landing probes, always providing that reliable decontamination systems will have been developed and authenticated. A minority opinion holds that small landers may provide environmental information useful in the design of other space- craft and may succeed more readily than orbiters. According to this view, the way should be left open to their use even though the results obtained may well be less comprehensive.

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16 BIOLOGY AND THE EXPLORATION OF MARS eluded, in some niches of the planet, chemical systems with the degree of complexity, organization, and capacity for evolution we would recognize as "living"? Our specific aims are: (1) determination of the physical and chemical conditions of the Martian surface as a potential environment for life, (2) determination whether life is or has been present on Mars, (3) determination of the characteristics of that life, if present, and (4) investigation of the pattern of chemical evolution without life. AN IMMEDIATE START TO EXPLOIT THE 1969-1973 OPPORTUNITIES A major effort should be initiated immediately to exploit the particularly favorable opportunities of 1969-1973. We are here concurring with the Space Science Board's views that planetary exploration should be the major aim of the nation's space science efforts in the 1970's and 1980's; and, further, that the biological explora- tion of Mars should be the primary focus of the program. AVOIDING THE CONTAMINATION OF MARS: A MAJOR MISSION CONSTRAINT Before proceeding to other aspects of the undertaking, we are concerned to single out the task of prevention of contamination from the many and diverse problems that Martian exploration will entail. Contamination of the Martian surface with terrestrial microbes could irrevocably destroy a truly unique opportunity for mankind to pursue a study of extraterrestrial life. Thus, while we are eager to press Martian exploration as expeditiously as the technology and other factors permit, we insist that our recommendation to proceed is subject to one rigorous qualification: that no viable terrestrial microorganisms reach the Martian surface until we can make a confident assessment of the consequences. PROGRAMMATIC RECOMMENDATIONS Every opportunity for remote observation of Mars by Earth-bound or balloon- and satellite-borne instruments should be exploited. A vigorous program here can yield a very substantial increase in our knowledge of

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Summary and Conclusions 17 Mars before the major program of planetary missions begins in 1969. It has become evident that an adequate program for Martian explora- tion cannot be achieved without using scientific payloads substantially larger than those currently employed in our unmanned space research program. Although predominantly engineering considerations may incline to early use of smaller payloads, we see very substantial advantages in the use, from the outset, of the new generation of large boosters that are expected to become operational toward the end of the present decade. These ad- vantages include: the possibility of avoiding spacecraft obsolescence due to a change in booster; the potential for growth in the versatility of scientific payloads and the relief of pressure on the engineer to design spacecraft to the limit of booster capacity. We deliberately omit an explicit recommendation in favor of any fly-by missions in addition to those already executed or planned for the 1964 (and possibly 1966) opportunities. They yield at best a fleeting glimpse of the planet, and unless they are already so large that they could as well have been orbiters, the array of sensors they carry is small. Given the booster power adequate to deliver it, an orbiter is overwhelmingly preferable. It may well be, however, that strictly engineering considerations will demand some preliminary flights in 1969 and, if these are undertaken, their ex- ploitation as fly-bys could yield worthwhile information. Every effort should be made to achieve a large orbiting mission by 1971 at the latest. This mission should precede the first lander. (A dissenting minority view supports the simultaneous use of small landing probes.) By "large" we mean a scientific payload that would include instrumentation for (a) infrared and television mapping; (b) microwave radiometry and bistatic radar; (c) infrared spectrometry; and (d) optical polarimetry. The success of this mission will depend on the availability of a large booster and a substantial improvement in currently available communications facilities. The first landing mission should be scheduled no later than 1973, and by 1971 if possible. We have not yet outlined what the contents of a large lander should be in terms so specific as those used to describe the orbiter. The central point on which all agree is that the mission ultimately demands a large lander, which we have come to call an ABL (Automated Biological Laboratory). What is unclear at present is how fast such a large lander can be designed and developed from biological and engineering viewpoints. It is clear, however, that the development, both as to conceptual design and as to engineering, will go through several generations. It is hoped that the first generation of an ABL could be used for the 1971 opportunity. The lander we are recommending for 1971 is something short of what is

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18 BIOLOGY AND THE EXPLORATION OF MARS ultimately possible and necessary, but could have a sufficiently diverse array of instrumentation to answer some of the scientific questions we have posed. The task of designing an ABL should be initiated immediately as a con- tinuing project. The contents of landers in 1971 and 1973 will be products of this continuing undertaking. The problems associated with the biological exploration of Mars are diverse, and the task of implementation raises challenges in many respects wholly novel. Orbiter and lander missions alike will involve many different experimenters. The evolution of an optimum scientific payload will require a continuing dialogue among all potential investigators and the engineers responsible for implementing their scientific goals. The undertaking we are recommending cannot proceed without some provision for organizing and sustaining that dialogue on a continuing basis. As the program develops other devices may become more appropriate, but at the outset we believe a standing committee of the Space Science Board will be a useful provision. It should be charged with: (1) a continuing surveillance of progress from a scientific viewpoint; and (2) the responsibility of giving advice to the National Aeronautics and Space Administration.