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POSTSCRIPT: OCTOBER 1965 During the interval between publication in March 1965 of the Summary and Conclusions of our Study and the appearance of this volume, our knowledge of Mars has been raised to an entirely new level by the success of the Mariner IV mission. On July 15 of this year a spacecraft encoun- tered Mars, passed at 6118 miles, and succeeded in taking twenty-two pictures of its surface that were subsequently transmitted back to Earth. Those pictures represent a major technical and scientific achievement. Our Study began in the summer of 1964. By the time of the 1965 Spring Meeting of the Academy it was possible to present a summary of our general findings to the scientific community. This permitted us to take cognizance of the reactions of our colleagues while work progressed on the various topical chapters of the Study. The time required for the latter has also permitted us to take cognizance of the new information from Mariner IV at a session of the study participants at Stanford on October 24 and 25, 1965.* This meeting formed the basis for this postscript. One of its two * Participants in this session were: K. C. Atwood, R. M. Bock, A. H. Brown, A. L. Burlingame, H. Fernandez-Moran, H. Gaffron, D. A. Glaser, J. Gross, H. K. Hartline, T. Jukes, H. P. Klein, J. Lederberg, E. C. Levinthal, J. McCarthy, A. D. McLaren, S. L. Miller, M. Minsky, J. Or6, G. C. Pimentel, C. S. Pittendrigh, D. C. Rea, A. Rich, C. E. Sagan, E. A. Shneour, L. Stryer, and W. Vishniac. Participants invited specifically to present the results of Mariner IV included R. B. Leighton, V. R. Eshleman, and G. Fjeldbo. 19

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20 BIOLOGY AND THE EXPLORATION OF MARS purposes is to answer the question: Do the results of Mariner IV change our earlier conclusions and recommendations? The other is to re-empha- size some salient features of those conclusions which public reaction to our summary indicates were largely ignored or misunderstood. The essence of our position was, and still is, the immense scientific importance of evaluating the uniqueness of life on Earth; of discovering facts that will permit more valid inference on its abundance in the Universe; and the fact that the new space technology allows us to obtain empirical evidence on the frequency with which living organization and its precursors emerge in the evolutionary history of planets. Given the importance of discovering life there, our judgment was that the exploration of Mars—biologically oriented—was not only fully justified but, given further the opportunity of the space program, it was a clear national goal of the highest scientific merit. To escape that conclusion, we would have had to conclude first that the possibility of Martian life was negligibly low. The decision rests, as it were, on the product of the impor- tance of the discovery and the possibility of attaining it, not on the latter alone. What we concluded on examination of the facts then available was that life, even in essentially terrestrial form, could very well have originated on Mars and have survived in some of its contemporary micro-environ- ments. And the facts revealed by Mariner IV have not changed that judgment. It is, however, important to re-emphasize here a major aspect of our position that critics have unaccountably missed; we sought to emphasize "that our conclusion that the biological exploration of Mars will be a rewarding venture does not depend on the hypothesis of Martian life." Throughout our deliberations we have treated the issues at stake as ques- tions about the general evolutionary processes in nature: in exploring Mars, as biologists, what we are seeking to clarify is the probability of living organization emerging in the sequence of chemical change inherent in a planet's history. It is clear that the positive result of discovering Martian life would be the greatest clarification, empirically demonstrating the probability to be high. But it is not the only result that would be useful. Any knowledge of the history of Martian chemical evolution will contribute to our total understanding of the fundamental issues involved. The origin of terrestrial life was a feature in the evolution of one planet; our attempt to estimate the prevalence of life in nature as a whole hinges precisely on our understanding of planetary evolution in general; and the exploration of Mars is the surest way in the present context to enlarge that under- standing. Our position is then, that Martian exploration, biologically motivated, is fully justified even if life has not emerged there; but we will again be

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Postscript: October 1965 21 misunderstood if that emphasis is taken to mean we believe the chance of discovering fully fledged life to be negligible. The arguments offered by critics reduce to three categories: (1) life can only exist as we know it; (2) the existing Martian environment is too hostile for life, in essentially terrestrial form to be present; and (3) conditions essential to its origin have never existed on Mars. None of these arguments was valid before the Mariner IV results were known and none is compelling now. The biologist is aware of and impressed by the incredible resourcefulness of living systems on Earth. In their evolutionary history they have found ways to exploit the pages of library books, the near boiling water of hot springs, saline pools on the polar ice caps, the bottom of the ocean, and the exposed rocks at over 6,000 m. We cannot conclude that on Mars the temperature range, the low water content, the very low oxygen tension, or even the high ultraviolet flux necessarily create a challenge that defeats the resourcefulness of self-replicating systems. Even before the Mariner pic- tures were available we were wary of taking the average measurements available to us as a guide to the range of microclimates present and the Mariner pictures only reinforced that caution. They reveal differences in elevation of at least 3,000 m; they contain features we cannot exclude as frost on the highlands; they show many features that defy any certain explanation; and, in particular, they raise questions about erosion mecha- nisms we cannot answer as yet; and to that extent they warn strongly against any premature inference on the contemporary environment in gen- eral let alone that of the past. The inference from the Mariner pictures that Mars is currently very dry only reinforces what we already knew. The only novel feature they might imply is that extensive drought has prevailed for a very long time. But even here caution is needed. An initial suggestion that present surface features are about 2 to 5 billion years old has already been challenged by other estimates an order of magnitude smaller. The general and more important point is that analyses of these new and complex data will continue for many years before their full meaning becomes clear. The specific and more urgent point is that they do not prove that Mars is lifeless now and certainly do not prove that its early history was incompatible with life's origin. It is appropriate here to re-emphasize that the central issues at stake are issues of past history; issues on which our inference even for the terrestrial case is uncertain. That very uncertainty is what prompts us to extend our knowledge of life and its chemical precursors to other planets. We cannot conclude that a current absence of hydrogen on Mars means it was always absent and biogenesis therefore impossible. We cannot on the basis of its escape velocity and the absence of a magnetic field conclude that hydrogen escaped in the early history of the planet so rapidly as to preclude at least

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22 BIOLOGY AND THE EXPLORATION OF MARS transiently the existence of sufficiently reducing conditions to permit the origin of life. More generally we cannot without an unwarranted lack of humility pretend we understand life sufficiently to infer from the meager facts available that it is absent on Mars now. Neither the advocate nor the critic of Martian exploration can rest his case on purely empirical and inferential certainty; the judgment for or against the mission rests largely on other grounds. We have deliberately limited ourselves to issues of sci- ence not because we think fiscal policy and allocation of the society's resources are irrelevant but because they are clearly different issues on which we have no special wisdom. It is clear that any final decision on the merits of Martian exploration will devolve strongly on these issues— indeed, they will surely play the major role; but we would insist that in public debate of the venture the distinction between them and the scientific merits of the case be more clearly distinguished than they seem to have been so far.

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PART II LIFE: ITS NATURE AND ORIGIN

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