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INTRODUCTION The predecessors to this Committee stated in 1974 that "At the present time, Mars is the only real target for exobiological searches in the solar system. All other objects, with the possible exception of Titan, appear to be excluded as possible habitats of life, owing either to the lack of an atmosphere or to tem- perature regimes that are incompatible with complex organic chemistry. This being the case, the return of unambiguous biological data, either positive or negative, from the two Viking '75 spacecraft can be expected to have a major impact on the planetary program. A positive result will initiate a new scien- tific discipline, that of Martian biology. A negative result may terminate the search for extraterrestrial life as a motivation for planetary exploration, al- though interest will remain in the organic chemistry of the solar system." They went on to conclude that "most of the conceivable outcomes of Viking are likely to be viewed as ambiguous or unconvincing," and that "It is diffi- cult to plan responses for this contingency since much depends on the precise nature of the ambiguity."1' P- 172 Much of the Viking data is now in hand, and we are charged with evalu- ating its biological significance and making recommendations for post-Viking biological investigations of Mars in the next decade. The evaluation is dealt with in Part I; the strategic considerations in Part II; and our recommenda- tions in Part III. The discussion will be restricted to Mars. From the point of view of formu- lating strategy for planetary biology in the next decade, we continue to sup- port the opening sentence in the above quotation that, at the present time, Mars is the only real target for exobiological searches in the solar system. While no one can rigorously exclude the possibility of indigenous life forms on the outer planets, the possibilities are too remote and too little is known about the detailed structure and composition of the planets to justify the direct search for existing life at this time. Nevertheless, an understanding of the organic chemistry of the atmospheres of the outer planets is of major biological interest, and gaining that understanding should remain a high priority goal. There are three possibilities for Mars: life exists; life evolved but no longer exists; life never evolved. The discovery of existing life would be tremen- dously exciting. But the other two possibilities would also represent dis- 1

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coveries of profound importance. Venus, Earth, and Mars are roughly similar in size, mass, and distance from the sun. Yet, Venus has a massive atmosphere rich in CO2 and a surface that is an inferno; Mars has a wisp of an atmosphere (also rich in CO2) and a surface that is cold, devoid of liquid water, and ex- posed to highly reactive molecules and intense ultraviolet radiation. Earth has an atmosphere intermediate in density, low in CO2, and rich in oxygen and nitrogen. Its surface temperatures lie predominantly in the range where water is liquid, and a great proportion of its surface is covered with liquid water. And most significant of all, it teems with life. It is customary to think that life exists only on planets that provide the proper conditions for its maintenance. But the realization is growing that life itself may modify a planet's surface and atmosphere to optimize condi- tions for its existence. Even if it were demonstrated that life does not now exist on Mars, the question would remain whether Earth and Mars differed sufficiently in their early histories to permit the origin of life on the former but not the latter. Or, alternatively, did both planets permit the origin of life and then diverge dramatically? If so, did the type and extent of life that evolved play a major role in that divergence? These questions are of fundamental scientific interest, but they may also be questions of fundamental importance to all of us on Earth. We have clearly reached the point where human activities are exerting global effects on the composition of the Earth's atmosphere and perhaps its temperature. At- mospheric pollutants may affect the ozone layer and could modify the Earth's albedo. The burning of fossil fuels has already measurably increased the carbon dioxide content of the atmosphere, and some scenarios predict serious and even devastating consequences if major fractions of our energy requirements continue to be derived from these sources.2 Clearly the sta- bility of equilibria and steady-state processes on the Earth's surface and in its atmosphere to human perturbants, and the role of the Earth's biota in this stability are matters of more than arcane interest. Since the surface of Mars provides a natural global system for comparison with Earth, we submit that studies of biology and of chemical evolution on our neighboring planet will shed important light on these terrestrial questions—questions that could be significant to our ultimate survival.