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Chapter 7 EXOBIOLOGY The biological examination of a planet other than the Earth was considered at length in Biology and the Exploration of Mars.* The scientific justifications, the proposed general strategies for various kinds of explorations, and the detailed review of many subordinate aspects of this general topic presented in this volume constitute a useful background for current planning of planetary exploration. What is Life? Life is not a thing in itself; rather it refers to a state of chemical complex- ity. Its physical basis occurs in discrete units called living organisms. A living organism is a coherent, heterogeneous, and thermodynamically improbable assemblage of molecules that is far out of equilibrium with its immediate surroundings and exhibits self-coordinated internal functions. It interacts with its environment as an open system through which a continual flow of energy and materials must occur, channelled by a highly specific catalysis that enables this living system to be self-maintaining and self-replicating. Essential to the catalyzed energy flow and to the replicative process are large molecules of especially high information content, the structural requirements for which can be met, we believe, only by a chemistry based on carbon. The chemical and energetic changes that constitute the metabolism of the organism can occur only in association with a polar solvent; of the solvents, liquid water appears to be the only reasonable possibility. The requirements that the chemistry be based on carbon and that the solvent be water establish a limited temperature range, depen- dent on solute content and atmospheric pressure, within which the essentially aqueous living system can continue to function. We thus assume a carbon-water biochemistry. The relative cosmic abundance of the pertinent elements, the spontaneous formation of organic compounds, the ability of carbon to combine with many other elements and especially with itself to form large compounds stable enough for continuity and labile enough for metabolism, and the anomalous behavior of water (with temperature) speak for this assumption. Exotic biochemistries based, say, on silicon are quite unlikely. How to Search The search for life should be preceded by an identification of environments where conditions are compatible with the existence of living systems. Thus, compounds of carbon, some water (not necessarily oceans), and a temperature range that permits water to exist in liquid form at least part of the time are absolute requirements. Additionally, the ultimate energy source needed to maintain a highly evolved system of living organisms is radiant energy, and the form in which carbon enters the open system is most likely to be carbon dioxide. Therefore a search for life could be rewarding where light can penetrate and where there is diffusional access to (X^. Exploring a planetary surface with respect to these environmental variables will tell us whether life is possible there and in what specific regions it is most likely to be found. Such an examination of the environment is not, however, a search for life itself. To demonstrate in fact that extraterrestrial life exists, we must either *Biology and the Exploration of Mars: Report of a study held under the auspices of the Space Science Board, 1964-1965, C. S. Pittendrigh, W. Vishniac, and J. P. T. Pearman, eds., NAS-NRC Pub. 1296, Washington, D C., 1966. -46-
-47- (a) identify organisms by imaging methods (by recognizing an unmistakably biologically derived morphology), (b) identify environmental factors that point incontestably to the influence of life processes (certain isotope fractionations, major departures from chemical equilibrium, consumption or production of CO^ or other specific substances) or (c) observe an increase of biomass or a catalytic activity for which a nonbiological explanation would be considered most unlikely. Any or all of these things can be accomplished upon the planetary surface or on a returned sample here on the Earth. Consequences of Discovering Extraterrestrial Life or of Negative Results The discovery that life exists on another planet would yield unique information on the origin of life anywhere. If the extraterrestrial life were fundamentally different in its chemical organization from life on the Earth, we should have to con- clude that it originated independently of terrestrial life. This would imply strongly that life is a common phenomenon in the universe. If the extraterrestrial biota bore a close chemical resemblance to terrestrial life, we would entertain the possibility that life originated on one planet and was transferred to the other. This would suggest that the origin of life is an unlikely event, and that transferral of living matter through interplanetary space is more probable than is now generally thought to be the case. The discovery of life on another planet would have consequences far beyond its immediate scientific implications. Such a discovery would be one of the momentous events of human history. Its effects on man's view of himself, of nature, and of the universe would be profound and far-reaching. Failure to find life on a planet whose environment was compatible with life would imply that the origin of life is an unusual event, not a predictable outcome of geo- chemical processes. This information, too, would be of scientific value for it would demand a reappraisal of the widely held assumption that life inevitably originates in any hospitable environment. Strategy and Tactics Our choice of planetary search targets is limited. Besides Earth, only Mars appears to be suitable for life as we know it. Mars provides an environment of light, water (albeit in severely limiting amounts), and CC^. The temperature range is com- patible with terrestrial life. The Venus surface is too hot, and the idea of a floating biota at great altitude, while not to be rejected out of hand, requires special and complicated assumptions. Too little is known about Jupiter, and the outer planets are too cold. In common with his colleagues in other disciplines, the biologist searching for life is interested in atmospheric composition, soil structure and composition, water economy, meteorological data, temperatures, and other physical parameters such as radi- ation flux. The biologist is primarily interested in data pertaining to the vicinity of the surface, i.e., the microclimate immediately above, at, or just below, the surface. In atmospheric analyses the gases of interest to him include, besides the major compon- ent, C02, the small amounts, if any, of H^S, NO, NH3, HCN, CH,, CO, N-, 0 , and volatile organic compounds. He is also interested in the noble gases, He, Ne, A, which can tell something of the atmospheric history of the planet. . Soil analysis to him means in par- ticular a search for organic compounds and determination of water content. He searches for departures from predictable equilibria: both in chemical compositions, which are thermodynamically unlikely unless some continuous chemical activity (possibly biologic- al) regenerates them, or in isotopic distributions which suggest a continuous fraction- ation. One exclusively biological experiment is the attempt to observe "active biochemistry,"
-48- be it growth or a catalytic activity that is unlikely to be of nonbiological origin. The measurement of temperature is bound to be of some use, but its biological sig- nificance (with respect to determining the presence or absence of life) is low. Hence this is a low risk -low yield measurement. The attempt to determine active biochem- istry is risky, but an affirmative observation would have a very high yield indeed. The strategy of a biological mission should therefore be to carry out a variety of observations among which risk and yield are properly balanced. The determination of atmospheric composition, for example, carries with it little risk because we are bound to obtain useful information and are independent of the mechanically difficult problem of obtaining a solid sample. At the same time, it provides us with significant yield since the information is of biological relevance (more so if carried out over several diurnal cycles), and can even be indicative of life if it shows the atmosphere to be far from an equilibrium mixture. Conclusive Negative Results It is easy to describe the kinds of evidence that would demonstrate that Mars is inhabited, but is it possible to prove the negative? What observations would convince biologists that Mars is a dead planet? The following, taken together, would, we be- lieve, constitute such proof for most biologists: (a) Demonstration that the observed seasonal changes (wave of darkening) on the planet result from nonbiological causes (b) Finding that Mars has negligible amounts of water (c) Finding that the Martian atmosphere is essentially in chemical equilibrium (or, more correctly, does not depart significantly from the steady state expected from interaction of the atmosphere with solar radiation) (d) Demonstration that Martian soil at a number of different sites contains organic matter no different than that expected from meteoritic infall (e) Failure to find evidences of the existence of liquid water on the planet in the past. Such evidence could be obtained by photoimaging from an orbiter and by chemical analysis of the soil (specifically, with reference to the presence or absence of clay minerals or other hydrated material) (f) Negative results in life-seeking experiments Timing An early biological mission to Mars is desirable because of: (a) Scientific and philosophical significance. The discovery of life on another planet would, as noted above, be one of the momentous events of human history, with pro- found and far-reaching implications. It is within our capacity, for a relatively small expenditure of money and effort, to reap a tremendous harvest. (b) The problem of possible contanimation. Many biologists consider Mars to be a suitable environment for the multiplication of certain types of terrestrial microorg- anisms. This opinion is not unanimous, but so long as the issue remains unsettled, it will be prudent to carry out biological studies on Mars at the earliest opportunity. Thus the planet can be examined before any rocket-borne organisms can have altered the Martian ecology, and the question of whether terrestrial microorganisms can infect Mars will be resolved. If the answer is negative, spacecraft sterilization thereafter will be unnecessary. A Program for 1969-1973 The 1969 Mariner-Mars fly-by spacecraft are, at this writing, well on their way to completion. These spacecraft, if successful, will greatly enlarge our knowledge of Mars. They will acquire new photographs of the Martian surface at higher resolutions than any yet obtained, and of the entire planet at lower resolutions. They will make radiometric
-49- measurements of the surface temperature and spectrometric analyses of the atmosphere from the neighborhood of the planet. Our attitudes toward Mars as a possible habitat for life will be strongly influenced by the results of this mission. It is equally true, however, that the Mariner '69 fly-bys will not settle the question of life on Mars and that further study of the planet will be needed. The Mars 1971 orbiter program recommended elsewhere in this report will observe the important wave-of-darkening phenomenon, whose existence first led astronomers to suggest that Mars is an inhabited planet. It will map most of the planet photographic- ally, and it will seek locales especially favorable for life by examining the surface and atmosphere for evidences of higher-than-average water content. These results will strongly influence the choice of a landing site for the Mars 1973 lander. The 1973 Titan/Centaur-launched Mars orbiter/lander which is recommended in this report will be the first U. S. planetary entry-and-lander mission. A suggested landed payload for biological investigation of the planet is described in Chapter 2. The entry-lander capsule will be accompanied by an orbiter whose primary function will be to support the capsule as a relay link. There is a strong possibility that a soft landing by means of retro-descent would contaminate the atmosphere and surface, and thereby invalidate the scientific experiments. A "hard" or "rough" landing is there- fore far the preferable for this mission unless it can be demonstrated that the retro- descent will not, in fact, disturb the area to be sampled nor interfere with the at- mospheric analyses to be obtained during entry. Second-Generation Lander After the biologically significant missions recommended by this study (1969 Mars fly-by, 1971 Mars orbiter, and 1973 Mars orbiter/lander) have been completed, what will remain to be done? If we assume that after we have digested the results of these missions Mars will command enhanced biological interest, then NASA should be pre- pared to optimize its exobiological effort and doubtless will be justified in raising priorities for experimental work on the Martian surface. The next generation lander/ orbiter, which we recommend should be planned for 1975, will build on earlier mission results and therefore will be able to carry out more sophisticated experiments. Surface exploration by means of a roving vehicle carrying a complement of scientific instruments - a mobile laboratory for biochemical and related studies -- would be very desirable, allow- ing us to take advantage of a potentially great opportunity to study an exotic biota in detail. Instruments on the vehicle might include "wet chemical" apparatus not recommend- ed for the 1973 lander payload. NASA should carry out studies to determine whether a Titan/Centaur vehicle would provide the payload capacity to accomplish a suitably am- bitious mission at the 1975 opportunity. Alternatively, if Saturn-class vehicles are made available to the planetary program for a 1975 mission, it would provide an oppor- tunity to introduce a mission which will effectively utilize this capacity into the planning for Martian exploration. In conclusion, we emphatically support one of NASA's major goals: to increase our understanding of the origin and nature of life. We recognize that this is achievable mainly through survivable Martian lander missions when properly supported by fly-by, orbiter, and entry science. In terms of the particular missions recommended in this Study, we note that the 1969 fly-bys followed by the 1971 orbiter and the subsequent 1973 orbiter/lander are rational steps that could place us in position, for the first time, to make close-up observations of the kind that could establish whether living org- anisms are present or absent on Mars.
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