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CHAPTER 14 SOME TERRESTRIAL PROGRAMS S. L. MILLER, G. C. PIMENTEL, and CARL SAGAN INTRODUCTION The exploration of Mars by means of spacecraft launched from the Earth is, at best, a very difficult undertaking. Opportunities are relatively infre- quent and the experimental difficulties are formidable. When, as in the present case, plans for the investigations must take into account the possi- bility of encountering biological phenomena, these difficulties are further accentuated, for the recognition of life cannot yet be reduced to a simple and economical experimental procedure. In these circumstances, it is only prudent to seek assistance from investi- gations in the laboratory and from observations from the Earth and its vicinity. Preparations of this sort include laboratory work on the chemistry of biopoiesis, collection and analysis of meteorites, studies of biological tolerance of, and adaptation to, the simulated planetary environment, and the development of experimental methods for the characterization of life and of its environment. While astronomical studies of Mars are often limited by the Earth's atmosphere, many further contributions are to be expected from the appli- cation of modern techniques and instruments. All of these studies merit encouragement, for not only will they facilitate the definition and planning of the planetary missions to come but they will also help in the interpretation of the results. 259

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260 APPROACHES AND REMOTE OBSERVATIONS The sections that follow contain some additional suggestions for Earth- based work. They are not intended to be exhaustive, but rather to indicate the range of topics of interest. Discussions of the very important problems of developing methods and apparatus for later use on the surface of Mars are to be found in Part VII. RESEARCH IN PREBIOLOGICAL CHEMISTRY S. L. MILLER Several discussions in this volume indicate the importance of understand- ing the processes leading up to the beginning of life on the Earth. This is important from the standpoint of the search for life on Mars because it will give significant boundary conditions for the design of life-detection experiments. At the same time one of the principle reasons for searching for life on Mars is to provide guidance and clues in studying how the process took place on the Earth. Thus each of these two areas of science— the search for life on Mars and the origin of life—benefits from progress in the other. The full meaning of a discovery of life on Mars and an investigation of its properties will not be elucidated until we have a greater understanding than we presently have of the origin of life. It follows that the study of chemical evolution is an integral part of the program of Martian exploration. However, this does not mean that the chemical evolution program should be a large one. This is not an area of science where a "crash program" or massive infusion of money will yield commensurate progress. Progress in the past has been almost entirely the result of a good idea combined with a simple experiment. Only modest amounts of money have been used by modern scientific standards. Care should be taken to ensure that the experiments are reasonable models of prebiological conditions. Considerable latitude can be envisioned in these conditions, and there will be legitimate differences of opinion, but it is clear that there are limits. For example, 100 per cent H2SO4 can by no stretch of the imagination be considered a suitable solvent for a prebiological experiment. At the present stage of development of this field, it seems clear that theoretical discussions are not likely to result in progress comparable to that to be expected from a good experiment. Chapter 2 of this volume discusses those specific problems in which no progress has been made as well as the areas where limited progress has been achieved but on which further experiments are needed. These areas

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Some Terrestrial Programs 261 and problems will not be repeated here, since the synthesis of all com- pounds of biological interest, from amino acids to polynucleotides, needs further investigation. In most areas it is not sufficient to report that a compound can be synthesized under one set of conditions. A study of the yield as a function of the concentration of reactants, temperature, pH, light intensity, etc., should be included. The mechanism of synthesis should also be studied. It would also be helpful to have much greater knowledge of the stabilities of organic compounds, both in aqueous solution and in the dry state. Some work in this area has been done by Abelspn and Vallentyne with amino acids, but more extensive data are needed for amino acids, and data should be obtained for the other organic compounds of biological interest. It might be desirable to support a search for evidence of life in the early Precambrian. This does not mean that Precambrian geology in general should be supported. The earliest evidence of life is 2.7 billion years ago, and there is considerable evidence of life of 1.5 billion years. The need is to fill in the 1.5 to 2.7 billion year gap, and most important is to find evidence of life more than 2.7 billion years old. POSSIBLE RESEARCH EXPLORATION OF EXOTIC BIOCHEMISTRY GEORGE C. PIMENTEL The optimum content of a laboratory program in support of a search for exotic biochemistries is decidedly more difficult to determine than if the search were for terrestrial biochemistries. An approach that readily comes to mind is the study of the effects of simulated planetary environments on terrestrial organisms. Yet this approach is, at best, peripheral to the problem. To be sure, the adaptability (or lack thereof) of terrestrial microorganisms to a given non-Earth-like environment might give sugges- tive leads to the possible line of evolution for life that might originate on that planet. More likely, the experiment would define only adaptability limits and "built-in" environmental preferences of an Earth-evolved or- ganism; thus a terrestrial cell might adapt to a hydrocarbon sea by sur- rounding itself with a suitable membrane to protect its aqueous cellular chemistry. An organism that evolved in the hydrocarbon sea would be encouraged by its environment to utilize a non-aqueous medium in its cells. We seem to be left with limitless possibilities, in the building of exotic

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262 APPROACHES AND REMOTE OBSERVATIONS chemical systems that might support life but without knowledge of the appropriate chemical constraints. The situation is not hopeless, however. What is needed are studies that strive to transfer into other chemical sys- tems the properties and processes that characterize living systems here on Earth. Such transfer will serve to generalize our concept of a living system and, perhaps it will aid us in distinguishing possible exotic biochemistries from improbable ones. To be specific, but not comprehensive, there would be value in the examination of chemical systems that retain similarity to our biochemistry while pointedly abandoning certain essential characteristics. The following specific proposals may furnish illustrative and suggestive examples that might serve as guides in evaluating and encouraging this type of research. a) Investigation of processes analogous to photosynthesis but based upon ultraviolet light. b) Formation of protein-like structures from o-amino acids that have high solubility in hydrocarbon solvents. c) Synthesis of a high-temperature analogue of the protein structure in which the strong skeletal bonds (the amide linkages) are replaced by still stronger bonds (e.g., — Si — O — Si — bonds) and the weak bonds that fix molecular conformation (the hydrogen bonds) are replaced by C-C bonds. d) Synthesis of a low-temperature protein analogue in which more labile bonds (e.g., — N — N — N) replace the skeletal bonds and the weak bonds are replaced by still weaker interactions (e.g., charge-transfer com- plexes). e) Study of stereospecific polymerization. f) Study of chemical information storage in non-proteinoid polymers. On a more fundamental level, there would be benefit in any study of the chemistry of a planetary atmosphere and its surface in advance of direct exploration. Any study that forecasts the chemical environment to be found will suggest the chemical constraints that do exist for life on that planet. Again, illustrative examples are probably useful but to serve as comparisons, not as boundaries. a) Prediction of gravitational fractionation of planetary atmospheres. b) Laboratory investigation of chemical processes initiated in model planetary atmospheres by vacuum ultraviolet light. c) Investigation of the planetary atmospheric composition by spectro- scopic studies from afar (e.g., from earth-orbiting platforms). d) Investigation of the planetary surface temperature and its variability from afar (e.g., again from earth-orbiting platforms).

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263 ASTRONOMICAL STUDIES: USE OF TERRESTRIAL, BALLOON, ROCKET-BORNE, AND ORBITING OBSERVATORIES CARL SAGAN The utility of observations of Mars from the vicinity of the Earth, com- pared with observations from the vicinity of Mars is discussed in Chapter 15. The specific programs recommended there include further ground- based observations in the middle infrared (cf., Chapter 3); ground-based searches in the photographic infrared for molecular oxygen; ground-based microwave interferometric observations of Martian surface roughness and possible seasonal variations in Martian surface roughness; balloon-borne photographic and infrared spectrometric reconnaissance of Mars; and high-resolution ultraviolet spectroscopy of Mars from orbiting astronomical observatories. These programs are particularly well suited to their respec- tive platforms, and would permit the allocation of scientific payload in space vehicles bound for Mars to experiments that could not be performed so well in any other way. In addition to the foregoing, there are programs of infrared spectroscopy devoted to a better determination of Martian surface pressures; infrared bolometric surveys in the 8-13^ and 20/i win- dows; and long-term synoptic visual and photographic observations of Mars that are being pursued at the present time, and which are potentially very rich sources of information about Mars.