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CHAPTER 3 THE SOUR SYSTEM AS AN ABODE OF LIFE CARL SAGAN INTRODUCTION By most astronomical criteria, there seems nothing extraordinary about our corner of the Universe. We live on one of nine planets of a common G dwarf star situated on the outskirts of a typical spiral galaxy. There are some 1011 other stars within our Galaxy. A visual impression of the number of stars in a galaxy may be garnered from Figure 1, a photograph of a very small region of our Galaxy, in the constellation Sagittarius. There are at least 1010 other galaxies within the presently accessible Universe. The total number may approach 1011 or 1012 as astronomers probe even further into the recesses of space, especially if—as present radioastronomical evidence suggests—the very distant galaxies are packed relatively closely together. The total number of galaxies may be literally infinite, for we do not know whether our Universe is bounded. Evidence from the rates of stellar rota- tion, from periodic perturbations of nearby stars, from theories of the origin of our solar system, and the example of our Copernican heritage all suggest that the formation of planets is the rule, rather than the exception, and that the origin of planetary systems is a general occurrence in the course of stellar evolution. According to present ideas of stellar and planetary cosmogony, the solar system was formed by the gravitational contraction and condensation of a 73

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74 THE COSMIC SETTING Figure 1. Photograph of a star cloud in the constellation Sagit- tarius. Photographed in red light with the 48-inch Schmidt camera at Mt. Palomar Observatory. (Courtesy of Mt. Wilson and Palomar Observatories.)

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The Solar System as an Abode of Life 75 cloud of interstellar gas and dust (see, e.g., Jastrow and Cameron [1963]). While the exact details of this condensation are still under discussion, it is quite certain that objects ranging from interstellar grains (.—- 1O5 cm in diameter) to supergiant stars (— 1013 cm in diameter) are being formed today. The prevalence of binary and multiple star systems suggests that many of the larger objects condense collectively. If the condensation has a mass much larger than that of the planet Jupiter, in time the interior tem- peratures, steadily increasing during the accretion process, become so high that thermonuclear reactions are initiated, and the star reaches the main sequence in the Hertzsprung-Russell diagram. Most stars then radiate quite stably for many billions of years. In the case of an object less massive than Jupiter, the interior temperatures reached are not adequate to permit thermo- nuclear reactions, and the object may become a non-self-luminous planet, or a black dwarf star. Many of the nearest stars exhibit periodic perturbations in their proper motions that can best be explained by the presence of a dark companion of mass intermediate between that of the sun and that of Jupiter. For example, Barnard's star, the second nearest star system to us (counting a Centauri A, B, and C as one system), has at least one dark companion of mass 50 per cent greater than that of Jupiter [van de Kamp, 1963]. Lalande 21185, the fifth nearest system by the same reckoning, has a companion with a mass about ten times that of Jupiter. Thi farther these systems are from the Earth, the more difficult it is to detect dark companions by perturbations in the stellar proper motion. A planet with the mass of the Earth could not be so detected, even in the nearest systems. But the statistics are already good enough to imply strongly that the formation of planets is a common, if not invariable, accompaniment to the formation of stars. If this is so, there must be at least 1021 to 1023 other planets in the Universe. Thus, if the Earth is the only abode of life, the probability of the origin of life must be as small as 10 21 to 10~23. Especially in the con- text of contemporary experiments on the origin of life (cf., Chapter 2), it seems much more likely that the development of life is a routine accompani- ment of the development of planets. This contention is not proved; at the moment, it is at best a likely story. However, just as the finding of dark planetary companions to the nearest stars strengthens the view that most stars have planetary systems, so would the finding of life on a nearby planet strengthen the contention that the origin and evolution of life is a common cosmic occurrence. In addition to the philosophical excitement that the discovery of even one example of extraterrestrial life would provide, the characterization of any extraterrestrial biological system would provide an ingredient currently lacking in biology: perspective. Since all organisms that the biologist can

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76 THE COSMIC SETTING now study are almost certainly relatives, common descendants from a single instance of the origin of life, it is difficult to determine which biological char- acteristics are evolutionary accidents and which are necessary for living systems in general. We expect that because of different adaptations to dif- ferent environmental circumstances, extraterrestrial life forms will differ fundamentally from terrestrial organisms. It is also possible, however, that strong biochemical or even morphological and physiological similarities will be found. Only by investigating extraterrestrial life can we acquire the perspective to separate the accidental from the inevitable in biology. The quest for life beyond the Earth can be pursued in three ways. First, we may study contemporary terrestrial biology and organic chemistry to approach the problem of the origin of life on Earth. If it appears relatively likely that life emerged in the primitive terrestrial environment, it may follow that the origin of life is a fairly general planetary phenomenon. Second, we can investigate the physical environments of the planets, and determine whether conditions there are so severe that living processes are entirely excluded, even allowing for the adaptability of life. Third, the planets may be studied for direct evidence of indigenous life forms. All three approaches are beset with uncertainties and it is important to state at the outset that no completely convincing evidence exists for extra- terrestrial life. The problem often reduces to probability considerations and estimates of observational reliability. At convenient places in the marshalling of evidence, I shall try to pause and give brief expression to alternative interpretations. In almost all cases, an optimistic view can be found which holds that the evidence is strongly suggestive of, or at worst, not inconsistent with, the existence of extraterrestrial life; and a pessimistic view can be found, which holds that the evidence adduced in favor of extraterrestrial life is unconvincing, irrelevant, or has an alternative, non- biological explanation. I leave it to the reader to pick his own way among the factions. THE ORIGIN OF LIFE The question of the nature of life is relevant to any consideration of its origins. Perhaps remarkably, there is no definition of life that is acceptable to the large majority of biologists. The energetics, homeostasis and relative environmental isolation of many living things have convinced one group of workers that the origin of metabolism is the critical event in the origin of life. They have regarded as significant primitive Earth simulation experi- ments in which an enclave of the experimental medium is separated from the remainder by a membranous barrier. Such enclaves, including the

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The Solar System as an Abode of Life 77 coacervates of Oparin [1957] and the microspheres of Fox [Fox and Yuyama, 1963], also provide a local concentration of organic materials. For such workers, the synthesis of something resembling a cell would be required for the laboratory synthesis of life. In another view, evolution by natural selection is the sine qua non of liv- ing systems. Without evolution, it is argued, nothing recognizable as living would ever have developed. For natural selection to proceed, a self- replicating, mutating system is required. It then follows that the critical event in the origin of life is the origin of a self-replicating, but not invariably meticulous, molecular system. The genetic view and the enclave view are not mutually exclusive. It is really not of much operational significance whether some forms of primi- tive metabolism preceded replication, or whether some form of primitive replication preceded metabolism. It is generally recognized that replication, mutation, metabolism and isolation of the organism from its environment are characteristic features of contemporary organisms, and must have been important in the early development of life. Self-replication in contemporary terrestrial organisms occurs through the mediation of the nucleic acids. In the now confirmed Watson-Crick model of deoxyribonucleic acid (DNA), two polynucleotide strands are helically wound about each other. Each strand is a nucleotide polymer; each nucleo- tide is a nucleoside phosphate; and each nucleoside is composed of the five- carbon sugar deoxyribose and one of the four bases adenine, guanine, cyto- sine and thymine. Replication occurs when the two strands (held together by weak hydrogen bonds) become uncoupled, and each synthesizes its complementary strand from precursors in the cell medium. A good degree of precision in replication is insured by the fact that, for stereochemical reasons, adenine will bond only with thymine; and guanine, only with cytosine. Cellular metabolism is also controlled by DNA, the information for the entire functioning of the cell being contained in the sequencing of the four kinds of nucleotides along the DNA strand. The information contained in this four-letter code is transcribed on the proteins of the cytoplasm, con- structed of approximately 20 different sub-units, the amino acids. This translation of the four-letter DNA code to the 20-letter protein code is mediated by ribonucleic acid (RNA), whose structure is only slightly dif- ferent from that of DNA. The proteins synthesized according to the nuclear instructions include enzymes that control the rate of chemical reaction in the cytoplasm, and thereby, the metabolism of the cell. The transcription ratio from DNA to RNA is one-to-one, one base of DNA transcribing to one base of RNA. The RNA-protein transcription ratio is three-to-one, three RNA bases carrying the information for the construction of one amino acid,

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78 THE COSMIC SETTING and 3n bases carrying the information for the construction of the n-amino acid-containing protein. These transcriptions require a stereochemically and enzymatically sophisticated apparatus in contemporary cells, and it is immediately clear that, at the time of the origin of life, control by the nucleic acids of their immediate environment could not have been nearly so elaborate nor so complete as it is today. There is no evidence that molecules of such complexity are being pro- duced abiologically on the Earth today. Could the conditions in the early history of the Earth have been more favorable for abiological synthesis of organic molecules? If planetary systems condense, as we think, from the interstellar medium, they should originally have a so-called "cosmic" distribution of the elements —that is, a distribution characteristic of the interstellar material and of most stars. This composition is outlined in Table 1, where we see that by far the most abundant element in the Universe is hydrogen. The next most abundant chemically reactive elements are oxygen, nitrogen and carbon. Thus we expect that any cold, non-dilute, cosmic environment should have large amounts of the fully saturated hydrides of these elements, i.e., H2, H2O, NH3, and CH4. Comets and interstellar grains are believed to consist primarily of these compounds, the relative proportions depending on the ambient temperatures and the vapor pressures. The Earth and the other planets, early in their history, should also have been composed of these substances. We have direct evidence that the early atmosphere of the Earth was lost to space, because the noble gases, which generally remain chemically inactive, have been preferentially fractionated on the Earth (compared to other cosmic material that we can investigate spectroscopically). Such a fractionation may have occurred by thermal escape during the early history of the Earth, when the acceleration due to gravity may have been small, or the exosphere temperature high [Kuiper, 1952; Sagan, 1966]. Alternatively, the fractionation may be due to ambi- polar diffusion in the magnetic field of the condensing solar nebula [Jokipii, 1964]. In either case, before fractionation occurred, much precipitation and compound-formation must have occurred in the Earth's primitive reduc- ing atmosphere. This material, sequestered in the forming Earth, must later have been outgassed as the Earth became heated by radioactive decay of now-extinct radionuclides, by gravitational accretion, and by tidal forces. Thus, as the Earth approached its present form, its atmosphere must have retained its reducing character. The transition from a reducing to an oxidizing atmosphere is attributed to two causes: (1) plant photosynthesis; and (2) ultraviolet photodissocia- tion of water in the upper atmosphere and the preferential gravitational escape of hydrogen. It is not known whether photosynthesis is the main

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The Solar System as an Abode of Life 79 TABLE 1. Distribution of the Elements Atom Relative Atomic Weight Relative Cosmic Abundance (by number) Hydrogen 1.0 10,000,000. Helium 4.0 1,400,000. Lithium 6.9 0.003 Carbon 12.0 3,000. Nitrogen 14.0 910. Oxygen 16.0 6,800. Neon 20.2 2,800. Sodium 23.0 17. Magnesium 24.3 290. Aluminum 27.0 19. Phosphorus 31.0 3. Potassium 39.1 0.8 Argon 40.0 42. Calcium 40.1 17. Iron 55.8 80. cause of the transition [Sagan, 1966]. The present atmospheres of the Jovian planets, Jupiter, Saturn, Uranus and Neptune, retain their reducing character; are probably composed primarily of hydrogen, helium, methane, ammonia and water; and therefore are close in composition to the primi- tive atmosphere of the Earth. When energy is supplied to a mixture of methane, ammonia, water and hydrogen, in the presence of liquid water—that is, in a laboratory simulation of the primitive environment of the Earth—complex organic compounds of distinctly contemporary biological significance are produced. A survey of these laboratory experiments is presented in Chapter 2, and in The Origin of Prebiological Systems, S. W. Fox, ed. [1965]. Amino acids, pentose and hexose sugars, nucleosides, nucleoside phosphates and polypeptides have all been produced under conditions that resemble, more or less, the primitive terrestrial environment. It has been pointed out independently by Beadles [1960] and by Sagan [1961a, 1964] that spon- taneous copolymerization of nucleoside triphosphates may have been a key event in the origin of the first self-replicating system. The picture that emerges of the origin of life is, then, as follows: The application of energy to the primitive reducing atmosphere causes the efficient production of sugars and bases. Phosphates are presumably present in the primitive oceans. The interaction of sugars, bases and phos- phates—especially after ultraviolet irradiation—leads directly to the syn-

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80 THE COSMIC SETTING thesis of nucleoside triphosphates. In periods of time long compared with the lifetimes of contemporary organisms, but short compared with geological time, reactions among the nucleoside triphosphates lead to the formation of nucleic acids. The nucleic acids then prime the synthesis of identical molecules from the organically rich surrounding waters. Random errors in the replication process lead to occasional deviations in the nucleic acid structure. These deviations precisely reproduce themselves, so that, in the course of time, many varieties of nucleic acids can be found in the primitive oceans. However, this cannot be the whole story. In our hypothetical recon- struction of the origin of life, natural selection has not yet come into play. For evolution to have proceeded, the nucleic acids must, in some way, have controlled the environment in primitive times. Natural selection operates only on the phenotype and not on the genotype. Something analogous to the nuclear DNA-messenger-RNA-adapter-RNA-enzyme transcription sequence, but much simpler, must be postulated for the primitive oceans; the contemporary triplet code is far too complex [Rich, 1962; Stent, 1962]. A singlet coding directly between DNA and polypeptides would satisfy this requirement, but it is not known whether such a configuration is sterically feasible. If DNA itself had some weak catalytic properties, or protein some weak self-replicating properties, the difficulties would be alleviated. Alterna- tively, it is possible that the information content of the first living systems was contained in RNA, and that DNA played no fundamental primitive role. The necessity for a transcription intermediary would thereby be obviated. As time progressed, there may have been a selective advantage to storing the genetic inheritance in a molecule less labile than RNA. DNA admirably serves this function. Such a possibility of a secondary origin of DNA is consistent with Horowitz' [1945] hypothesis on the origin of biochemical reaction chains, in which successive members of contemporary enzyme- catalyzed reaction chains arose in a sequence opposite to the one in which they are utilized today. At the present time, this remains one of several major uncertainties faced by the genetic approach to the origin of life. Another uncertainty concerns the assumption that nucleoside triphos- phates will spontaneously polymerize in times short compared with 5 X 109 years. An investigation of these reaction rates would be very useful. Other agents—for example, ultraviolet light—may have been active in acceler- ating the rate of nucleoside triphosphate polymerization in the primitive synthesis of polynucleotides. In all primitive Earth simulation experiments, only a few reactants, all of high purity, have been used. However, the primitive terrestrial oceans, or any other hypothetical media for the origin of life, were certainly not composed of these pure substances alone. In the primitive Earth, there

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The Solar System as an Abode of Life 81 must have been major contamination by large numbers of other sub- stances. Some of them would accelerate the production of compounds of biological interest, others would impede their synthesis or destroy them once they had been formed [Abelson, 1961; Horowitz and Miller, 1962]. Until the immensely difficult task of allowing for all probable contaminants has been made, the successful laboratory synthesis of substances relevant to the origin of life can have only a vague resemblance to the chemical events of primitive times. The striking success of such simulation experiments should not obscure the existence of this complication. Nevertheless, this success itself—the preferential production of just those compounds that are of bio- logical interest—suggests that the contaminants did not entirely alter the course of abiological organic synthesis. Finally, the question arises whether such complex molecules as the nucleic acids really were implicated in the origin of life. Might they not be a rela- tively recent evolutionary sophistication? And might the first self-replicating system have been composed of other molecules altogether? Answers to such questions can only be provided experimentally. The success in producing nucleic acid precursors in simulated primitive environments at least sug- gests that polynucleotides may indeed have been the first self-replicating systems. It is a logical consequence of the genetic approach to the origin of life that the subsequent development of living systems was an evolutionary at- tempt at local retention of the primitive environment, so that nucleic acid survival and replication could be maintained. The organism can then be viewed as a temporary repository for the genetic material, a situation aptly summarized in Samuel Butler's aphorism, "The hen is the egg's way of making another egg." At the turn of the century, when it was much more difficult to imagine how life came into being from inanimate matter than it is today, Arrhenius [1903] proposed the panspermia hypothesis. This was a stopgap measure that attributed the origin of life on Earth to the germination of a spore wafted by radiation pressure to the Earth from some other planet. The problem of the origin of life was thereby deferred, but not answered. It is now possible to perform some computations on the panspermia hypothesis. The preliminary results are as follows: Some bacterial and fungal spores are of the proper size range to be driven by solar radiation pressure out of our solar system entirely, if somehow they could first escape from the Earth's gravitational field. However, long before such a spore could reach even the orbit of Mars, it would receive a lethal dose of solar ultraviolet radiation. If we surround the spore with ultraviolet-absorbing protective material, then it does not escape from the solar system at all; because of the increased mass, it falls into the Sun. Particles that successfully enter our

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82 THE COSMIC SETTING solar system must have a different range of sizes from the particles that are driven away from the Sun by radiation pressure. The hotter the star, the more massive the spore that can be driven out by radiation pressure; but also the greater the administered ultraviolet radiation dose. The spore must also survive solar x rays and protons and galactic cosmic rays. The last will be significant for very long journeys. Even if an occasional spore does escape from a planetary system, it is geometrically very unlikely that it will encounter another suitable planet and there initiate life—space is very empty. These limitations of geometry and radiation damage seem quite general, and make it appear very unlikely that life in our solar system was initiated, through the Arrhenius mechanism, by a spore from some other planetary system. If the origin of life is a necessary consequence in a relatively short time period of the physics and chemistry of primitive planetary environments, and if, on the average, each star has a planetary system, it follows that life must be pervasive throughout the Universe. Organisms with our level of intelligence and technical civilization have emerged on our planet about midway in the residence time of our Sun on the main sequence. Large numbers of stars within our Galaxy are older than the Sun, and many of them may contain planetary systems on which life is flourishing. We do not know whether the emergence of intelligence and technical civilization, in a variety of local forms, is a necessary eventuality in the evolution of life, or whether it involves a concatenation of many very unlikely circumstances. But the large number of presumptive planets and the belief that there is nothing extraordinary about our sector of the Universe, both argue for the contention that there may be other intelligences and other technical civiliza- tions in our Galaxy. Some of them may be far in advance of ourselves. Project Ozma, an attempt to listen for radio signals from c Eridani and T Ceti, two nearby stars of approximately solar spectral type, was con- ducted unsuccessfully at the National Radio Astronomy Observatory in 1961 [Drake, 1965]. But the search was restricted to a short period of time, a specific frequency and two stars. The potentialities of interstellar communication with other intelligent species are enormous, and further in- vestigations along these lines could be immensely rewarding. ENVIRONMENTS FOR EXTRATERRESTRIAL LIFE We wish to inquire whether other bodies in our solar system are capable of supporting indigenous life. Accordingly, we must first attempt to specify the range of planetary parameters consistent with the existence of living systems.

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The Solar System as an Abode of Life 83 Temperature For a given molecular system, the temperatures may be too high, in which case thermal degradation will occur; or the temperatures may be too low, in which case the rate of chemical reaction may be biologically insignificant. However, as pointed out in Chapter 12, we must not be too provincial in assessing such temperature ranges. Chemistries are known, or can be anticipated, that utilize conceivably abundant but terrestrially unfamiliar compounds. Such compounds may be stable at several hundred °C or enter into reactions that proceed at substantial rates at — 100°C. It is also important to bear in mind that a variety of temperatures may be found on any given planet. As we shall see, the temperatures of the planets in our solar system are such that probably not one can be excluded as a pos- sible abode of life on grounds of temperature alone. Solvent System In terrestrial life forms, water serves as the primary solvent system. It functions as an interaction medium and as a thermal reservoir. Water possesses these and other properties of biological significance partly be- cause of hydrogen bonding between adjacent molecules in the liquid state. Water also has the advantage of being a liquid over a fairly wide range of expected planetary temperatures. In eutectic salt solutions, open pools of water can be stable at temperatures several tens of degrees below 0°C, and pure liquid water can be maintained at even lower temperatures in sub- surface capillaries. Other solvents also have large dielectric constants and large thermal inertias. Chief among these are ammonia, hydrogen cyanide and hydrogen fluoride. The possibility that hydrogen fluoride is a significant solvent sys- tem on other planets can be excluded on the basis of cosmic abundance. The expected abundance of hydrogen cyanide is greater, but is also prob- ably too low to be of major exobiological significance. Liquid ammonia, however, should be abundant in highly reducing planetary atmospheres. The temperature range over which it maintains liquid properties is, however, small. As described in Chapter 12, many of the properties of water that we consider essential for living systems are probably not essential. Being com- posed largely of water ourselves, we may be unjustifiably unsympathetic to other possibilities. For example, mixtures of hydrocarbons have many use- ful solvent properties, and they also may have high abundances on planets with reducing atmospheres.

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The Solar System as an Abode of Life 103 itself remarkable, a fact that can perhaps best be appreciated by con- sidering the circumstances reversed. Imagine that we are situated on Mars and provided with the same kinds of astronomical instruments that exist on Earth today. Is there life on Earth? The largest engineering works would be invisible. In a preliminary survey of about 104 Tiros photographs of Earth, with much better re- solving power than could be obtained with a 200-inch aperture from Mars, only one image showed any clear sign of the works of man [Stroud, 1963]. A more detailed discussion of problems in planetary biological recon- naissance is presented in Chapter 9. The diffuse emission from large cities with high surface brightness, such as Los Angeles, would be marginally detectable from Mars, but interpretation would not be easy. Emission lines of neon, argon, krypton and sodium might be discovered. The observa- tions themselves would be difficult, because the night hemisphere of Earth could be observed at night on Mars only when the Earth lies low in the evening Martian sky. Seasonal color changes of crops and deciduous forests would be observed, for example, in the American midwest, or in the Ukraine, but vexing questions would arise on the reliability of Martian color vision, ocular physiology and the chromatic aberration of telescope objectives. Even if the color changes were accepted as real, a variety of interpretations could be proposed; some might be inorganic and still fea- sible. A search might be made for H-C absorption features in the 3.5/t region in sunlight reflected from the Earth. It is unlikely that these at- tempts would be successful, if laboratory spectra of terrestrial vegetation performed on our planet are any guide. Perhaps this failure would be attributed to the fact that the intensity distribution curves for reflected sunlight and for thermal emission from the Earth cross over in the 3.5/t region. Until fairly recently, bright flashes of light would be discernible at times. They would last only several seconds and there would be some evidence of their recurrence only in a few restricted locales, such as Eniwetok and Novaya Zemlya. It is doubtful whether these would be considered evidence for life on Earth, much less intelligent life. If the hypothetical Martians had radio reception equipment and chose to scan Earth in narrow wave bands, they would certainly be rewarded, if that is the word, by television transmission from Earth. There would be an intensity maximum when the North American continent faced Mars and it would doubtless be pos- sible to determine that this radio frequency emission was not entirely random noise. Of course, the hypothetical Martians would have to have developed radio technology; they also would have to think of selecting a narrow radio frequency bandpass in observing the Earth. The capability alone is not the only prerequisite. For example, there has been no program

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104 THE COSMIC SETTING of narrow bandpass radio frequency observations of Mars made from this planet. Of course, we feel that the prospects for intelligent life on Mars are very small. The preceding intellectual exercise suggests that there might be life forms on Mars—in fact, fairly complex organisms—that would be entirely undetected from Earth. But the advent of interplanetary spaceflight pro- vides a reasonable expectation that such possibilities will at last be ex- perimentally tested. EVIDENCE FOR LIFE ON MARS The Martian surface is, in general, divided into three kinds of regions: bright areas, which are probably deserts; dark areas; and the polar caps, which we know from polarimetric and spectrometric evidence to be made of ordinary ice hoarfrost. Two high quality photographs of Mars, taken with the superb seeing conditions of the Pic du Midi Observatory in France, are displayed in Figure 11. Figure 12 is the International Astronomical Union Mars cartography. The relative configurations of bright and dark areas displayed are those common to photographs of the last several decades. Visual observations show much greater detail, the dark areas breaking up into mottled fine structure. However, because of difficulties in the reproducibility of these details when observations of different ob- servers are compared, the map has indicated only those features which have been, beyond doubt, present on Mars in recent years. The general coloration of the Martian dark areas is a neutral gray; the bright areas, a kind of orange ochre, or buff. Primarily due to con- trast with the orange deserts and to chromatic aberration in early refracting telescopes, it was formerly believed that the dark areas were predomi- nantly green. This now appears to be a psychophysiological and optical distortion. A second observation that once was thought to be evidence for life on Mars was of rectilinear features extending from the dark areas and crossing over enormous distances through the bright areas. These so- called "canals" were interpreted as massive engineering works of a race of intelligent Martians. While there is little doubt that rectilinear features can be seen on the planet, observations under the best seeing conditions show a resolution of the canals into disconnected fine detail, quite analo- gous to observations of the dark areas under the best seeing conditions [Antoniadi, 1929; Dollfus, 1961]. Nevertheless, there must be some ex- planation for the disconnected detail which the eye organizes into recti- linear features. It has recently been suggested [Gifford, 1964] that they

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The Solar System as an Abode of Life 105 Figure 11. Typical high-quality photographs of Mars, taken at Pic du Midi Observatory, in the French Pyrenees. On the left is a photograph taken in 1941, near the maxi- mum of the wave of darkening. Syrtis Major appears foreshortened at the bottom right of the photograph. Nepenthes-Thoth can be seen with a break in it, ex- tending left from Syrtis Major. The appearance of Nepenthes-Thoth in this photograph may be com- pared with that of Figure 12, which is based upon later photographs. On the right is a photograph taken at the same observatory in 1954. The southern polar cap has a greater extent, and the contrast between bright and dark areas is generally less than in 1941. Sinus Meridian! is at left center, Soils Lacus at right center. (Courtesy of Meudon Documentation Center, International Astronomical Union.) are sand dunes of considerable length, which may be expected under the Martian atmospheric pressure regimes and expected sizes of the particles in the Martian deserts. The present evidence for life on Mars is based on three kinds of ob- servations: visual, photographic and photometric; polarimetric; and in- frared spectrometric. It has long been known that as the polar ice cap in a given hemisphere undergoes its seasonal regression, a wave of dark- ening courses across the dark regions of Mars, crossing the equator and traveling some twenty degrees of latitude into the opposite hemisphere. As the wave front progresses, the dark areas decrease in albedo, their boundaries with the adjoining deserts become more sharply denned, and occasionally, fairly delicate color changes can also be observed, individual regions of Mars undergoing their own characteristic seasonal color cycle.

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106 THE COSMIC SETTING 8 ass;;s---.9gsss 3 ' 1' Mh gs J -S .2 3 1!o . 3 b .2 e „ l-2J||rfT8l|41l 8l^||!i||lj|, tfalll' "o !? "2. if f *ilj?. ii I i^b

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The Solar System as an Abode of Life 107 At the same time, a dark collar, variously reported as black, brown or blue, is observed to follow the retreating edge of the polar cap on its journey toward the pole. There is no doubt about either darkening phe- nomenon; and the wave of darkening has been adequately registered photo- metrically [Focas, 1959]. Some dispute exists about the reality of the color changes. It is fairly certain that the color changes in the last few decades are not nearly so pronounced as the color changes recorded by earlier observers, who reported vivid blues, greens and a variety of other hues. Changes in contrast of dark neutrally colored surfaces adjacent to highly colored surfaces may appear to the eye as true color changes, when in fact there has been no change in the spectral distribution of the absorp- tion by the dark area [Schmidt, 1959; Hurvich and Hurvich, 1964]. Thus, the seasonal color changes of the Martian dark areas may be en- tirely a psychophysiological phenomenon. The biological interpretation of the wave of darkening is as follows: The Martian dark areas are covered with organisms—possibly, but not necessarily, plants—whose metabolism is very sensitive to the availability of water. During most of the year, they are in a dormant state. As the wave front of water from the vaporizing polar cap arrives, the local water abundance surpasses the critical value for metabolism, and a rapid growth and proliferation of the organisms occur. The changes in albedo and color of the dark areas are attributed to these metabolic activities. As the water vapor wave front passes, the local water abundance declines, and the organisms once again fall into a state of minimal metabolism. In Figure 13 the progress of the Martian wave of darkening is plotted as a function of latitude and season of the year. The high-contrast, medium- contrast and low-contrast thirds (all compared with the invariant bright- ness of the deserts) are illustrated by the different degrees of irregular shading. The southern wave of darkening is the more pronounced, in that the contrast with the deserts reaches higher values than are observed in the northern wave. We see that the southern hemisphere wave of dark- ening begins at the end of southern hemisphere spring; and the northern hemisphere wave of darkening begins in the middle of northern hemisphere spring. Where there is no shading, the available data are sparse. The wave of darkening here plotted is taken from the photometric data of Focas [1959]. Superposed on this diagram are isotherms computed from the inclina- tion of the axis of rotation of Mars to the ecliptic plane, and from the eccentricity of the Martian orbit. The temperatures have been calculated for an almost transparent atmosphere and for a perfectly insulating surface with a bolometric albedo of 10 per cent. Therefore, they refer approxi- mately to the dark nuclei that are observed under highest resolution within

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108 THE COSMIC SETTING HELIOCENTRIC LONGITUDE, Ti 270- 300- 330- 0- 30- 60- 90- 120' ISO- ISO- 210' 240- 6OO 687 90 180 270 36O 450 340 EARTH DAYS |* AUTUMN | WINTER | SPRING | SUMMER NORTHERN SEASONS A = O.IO,T=0.0 Figure 13. Thermal regimes of the Martian wave of darken- ing. See text for details. the dark areas. The temperatures indicated are mean temperatures from sunrise to sunset within the dark nuclei at the appropriate latitude and season. We observe that the wave of darkening generally follows the loci of maximum temperature. Similar conclusions follow for other choices of the bolometric aIbedo and the transparency of the Martian atmosphere. Thus, the wave of darkening proceeds from the receding edge of the polar cap through those areas of Mars which are at moderate temperatures. In the dark nuclei of the dark areas, these temperatures may be equable, even by terrestrial standards. Computations based upon this method, using the aIbedo of the dark or bright areas as a wJiole, give results consistent with those obtained observationally by infrared bolometry. This correlation between the loci of the wave of darkening and the loci of highest temperature on the planet is consistent with the view that the wave of darkening is due to biological activity of organisms whose optimum temperatures are similar to the optimum temperatures of familiar ter- restrial organisms. But other, nonbiological alternatives have been sug- gested. Arrhenius [1918] postulated that the Martian dark areas are covered with deliquescent or hygroscopic salts, whose color and aIbedo vary with the relative humidity. It has not been possible to specify which salts change color and aIbedo at Martian humidity and temperature

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The Solar System as an Abode of Life 109 regimes, in a manner that might explain the visual observations; but neither has anyone suggested which Martian organisms might account for the observations. There is strong polarimetric evidence [Dollfus, 1963] that the dark areas are not covered with semi-transparent crystals, as the pres- ence of large amounts of deliquescent salts would imply; but it is not known whether an amount of such salts too small to be detected polarimetrically could still be detected photometrically and colorimetrically. Occasionally, a dark area may appear rather suddenly in a region that was formerly pri- marily desert. A spectacular instance of this sort occurred several decades ago, in the Thoth-Nepenthes area (latitude +20°, longitude 260° on Figure 12.) Advocates of the biological explanation hold that such in- cursions represent ecological successions, possibly stimulated by some change in the physical environment—for example, fumarole eruptions. Observations of the fractional polarization of sunlight reflected from the Martian dark areas yield a characteristic polarization curve when plotted as a function of phase angle (the angle Sun-Mars-Earth). This curve can then be compared with laboratory samples [Dollfus, 1957]. Such comparisons suggest that the Martian dark areas are covered with small, very opaque granules in the size range from 0.1 mm diameter down. The polarization curve for the bright areas shows similar features and must have a similar interpretation. Attempts to match the polarization curve for the bright areas have led to an apparently unique identification with a metal- lically absorbing iron oxide polyhydrate, the mineral limonite. The polari- zation curve for the dark areas is even more extreme, compared with the several hundreds of terrestrial samples that Dollfus has investigated. The nature of this "super-limonite" is a mystery, but apparently it must be attributed to an unusually dark substance, consistent with the observed very low albedo of the dark areas. The polarimetric identification of limonite provides, with no additional assumptions, a ready explanation of the albedo and reflectivity of the Martian bright areas, matching the observed re- flection spectrum from the visible through the near infrared. The polarization curve for the dark areas shows a seasonal displacement which the polarization curve for the bright areas does not [Dollfus, 1957]. The only successful laboratory reconstructions of this displacement require either a change in the size distribution of the opaque particles, or a change in their opacity. A biological interpretation of this phenomenon holds that the dark areas of Mars are covered by organisms in the 0.1 mm size range, which proliferate as the wave of darkening arrives. In a fairly brief period of time, the smaller, recently formed organisms must grow to "adult" size, so that the winter form of the polarization curve can be reestablished. The inorganic interpretation of the polarization phenomenon holds that a reproducible physical redistribution of sizes of inorganic grains occurs

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110 THE COSMIC SETTING at the time of the wave of darkening. Perhaps the winds that may accompany the water vapor front redistribute the surface dust, which in the absence of winds has settled, with the larger particles deepest. It must also be asked why the bright areas of Mars, which are presumably nonliving, are covered by the same kind of granules that cover the dark areas. A uniformitarian explanation for the existence of small, opaque particles in both the bright and the dark areas is desirable. There is, at the present time, no acceptable inorganic model to explain the wave of darkening and the polarization observations. However, such models should be pursued. A successful model should explain, in terms of allowable meteorological postulates and geochemically plausible mate- rials, not only the obvious photometric and polarimetric data, but also a wide range of other phenomena. These include the reappearance of the dark areas after a dust storm, the absence of dark clouds obscuring the bright areas, the presence of finely pulverized material in the dark areas as well as the bright areas, and the differences in radar reflectivities between the bright and the dark areas. It may be that these phenomena are explicable in terms of systematic elevation differences between bright and dark areas, of seasonal variations in the prevailing wind patterns on Mars, and of the size distribution of finely pulverized materials; but no such detailed model has yet been formulated. For this reason it seems desirable also to pursue the hypothesis that the wave of darkening and the polarization phenomena are of biological origin. In recent years, observations of the infrared spectra of the dark regions of Mars have been reported to show absorption features at about 3.45/i, 3.58^ and 3.69/i [Sinton, 1957; 1959; 1961; Moroz, 1964]. Several at- tempts have been made to account for these absorptions in terms of the putative presence on Mars of organic or inorganic compounds known to show similar spectral features (e.g., compounds containing methyl or methylene groups, aldehydes, inorganic carbonates, deuterated water). Nevertheless, while suggestive, none of these hypotheses has been entirely satisfactory [Lederberg, 1959; Sinton, 1961; Colthup, 1961; 1965; Rea, 1962; Rea, Belsky and Calvin, 1963; Shirk, Hazeltine and Pimentel, 1965]. More recently, reexamination of the experimental evidence suggests that the two long-wavelength Sinton bands are due to deuterated water (HDO) in the atmosphere of the Earth [Rea, O'Leary and Sinton, 1965]. The remaining band's Martian origin remains unquestioned; it has been at- tributed both to organic molecules and to carbonates. Problems of this sort emphasize the difficulties inherent in detailed astro- nomical studies of planetary surfaces by conventional means, no matter how painstaking. However, improvements in technique and the prospect of new

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The Solar System as an Abode of Life 111 methods of observation hold the promise of more exact formulation—and exclusion—of hypotheses in such cases. The foregoing observations and their possible interpretations, both in- organic and biological, were discussed at some length in a Conference on Remote Investigations of Martian Biology, Cambridge, Mass., 1964 (Proceedings to be published). With the still limited information currently available, it was nevertheless possible to formulate several hypothetical Martian ecologies. A description and elaboration of one of them is given in Chapter 11 of the present volume. In this model organic matter on Mars is imagined to be metabolized respiratively, with limonite serving as electron acceptor. The pool of organic matter is maintained photosyn- thetically, by processes that do not involve the the release of molecular oxygen. The result of such an exercise is, of course, not to demonstrate the exist- ence of life on Mars, but merely to show that its existence does violence to none of the facts of familiar biochemistry. There are, in addition, possibili- ties of more exotic biochemistries (cf., Chapter 12). Of all the possible extraterrestrial habitats within our solar system and beyond, Mars is clearly the most promising. But at the present time, we have no rigorous answers to these enigmatic questions. Rigor can be pro- vided only through a comprehensive program of observation from the vicin- ity of the Earth, from the vicinity of Mars, and eventually from unmanned and manned vehicles on the surface of Mars. With such a program, we may be fortunate enough to witness in our lifetimes the solution, one way or another, of the tantalizing problem of life on Mars, with all the scientific and philosophical implications that such a solution would suggest. REFERENCES Abelson, P. H. (1961), paper presented at the Denver meeting of the American Association for the Advancement of Science. Anders, E. (1963), Ann. N. Y. Acad. Sci. 108:514. Antoniadi, E. M. (1929), La Planete Mars 1659-1929, Hermann, Paris. Arrhenius, S. (1908), Worlds in the Making, Harper, New York. Arrhenius, S. (1918), The Destinies of the Stars, Putnam, New York. Beadle, G. W. (1960), Accad. naz. Lincei, Roma 47:301.. Binder, A. B., and D. P. Cruikshank (1964), Icarus. 3:299. Bottema, M., W. Plummer, and J. Strong (1964), Astrophys. J. 759:1021. Bottema, M., W. Plummer, J. Strong, and R. Zander (1964), Astrophys. J. 740:1640. Chamberlain, J. W. (1965), Astrophys. J. 747:1184. Claus, G., and B. Nagy (1961), Nature 792:594. Claus, G., B. Nagy, and D. L. Europa (1963), Ann. N. Y. Acad. Sci. 108:580.

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112 THE COSMIC SETTING Colthup, N. B. (1961), Science 132:529. Colthup, N. B. (1965), private communication. Dollfus, A. (1957), Ann. d'Astrophys. Suppl. 4. Dollfus, A. (1961), Chap. 15 of Planets and Satellites, G. P. Kuiper and B. M. Middlehurst, eds., Univ. of Chicago Press, Chicago. Dollfus, A. (1963), private communication. Drake, F. D. (1965), The Radio Search for Intelligent Extraterrestrial Life, Pergamon Press. To be published. Fitch, F. W., and E. Anders (1963a), Ann. N. Y. Acad. Sci. 108:495. Fitch, F. W., and E. Anders (19636), Science 140:1097. Focas, J. H. (1959), Comptes Rendus Acad. Sci., Paris 248:626. Fox, S. W., ed. (1965), The Origins of Prebiological Systems, Academic Press, New York. Fox, S. W., and S. Yuyama (1963), Ann. N. Y. Acad. Sci. 108:487. Gallet, R. (1963), private communication. Gates, D. M., and C. C. Shaw (1960), J. Opt. Soc. Amer. 50:876. Gifford, F. A., Jr. (1964), Icarus 5:130. Gold, T. (1963), private communication. Hayatsu, R. (1964), Kept. EFINS 64-67, Univ. of Chicago. Horowitz, N. H. (1945), Proc. Natl. Acad. Sci. U. S. 37:153. Horowitz, N. H., and S. L. Miller (1962), Fortschr. Chem. Org. Naturstoffe 20:423. Hurvich, L. M., and D. J. Hurvich (1964), Proc. Internatl. Conf. on Remote Investigations of Martian Biology, C. Sagan, ed. To be published. Jastrow, R., and A. G. W. Cameron, eds. (1963), Origin of the Solar System, Academic Press, New York. Jokipii, J. R. (1964), Icarus 3:248. Kamp, P. van de (1963), Astron. J. 68:515. Kellerman, K. I. (1964), paper presented at the General Assembly, Interna- tional Astronomical Union, Hamburg. Kiess, C. C., S. Karrer, and H. K. Kiess (1960), Publ. Astron. Soc. Pacific 72:256. Kozyrev, N. A. (1962), In: Physics and Astronomy of the Moon, Z. Kopal, ed., Academic Press, New York, p. 361. Krotikov, V. D., and V. S. Troitskii (1964), In: Life Sciences and Space Re- search II, M. Florkin and A. Dollfus, eds., North-Holland, Amsterdam; p. 145. Kuiper, G. P. (1944), Astrophys. J. 700:378. Kuiper, G. P. (\952),Atomospheres of the Earth and Planets, University of Chi- cago Press, Chicago. Lederberg, J. (1959), private communication. Lederberg, J., and C. Sagan (1962), Proc. Natl. Acad. Sci., U. S. 48:1473. Meinschein, W. G., B. Nagy, and D. J. Hennessy (1963), Ann. N. Y. Acad. Sci. 108:553. Moroz, V. I. (1963), Astronomicheskii Tsirkular, No. 270. Moroz, V. I. (1964), Soviet Astron. J. 8:273. Moroz, V. I. (1965), Soviet Astron. J. 8:566. Nagy, B., M. T. J. Murphy, V. E. Modzeleski, G. Rouser, G. Claus, D. J. Hennessy, U. Colombo, and F. Gazzarrini (1964), Nature 202:228.

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The Solar System as an Abode of Life 113 Oparin, A. I. (1957), The Origin of Life on the Earth, Academic Press, New York. Packer, E., S. Scher, and C. Sagan (1963), Icarus 2:293. Pollack, J. B., and C. Sagan (1965), Icarus 4:62. Rea, D. G. (1962), Space Science Rev. 7:159. Rea, D. G., T. Belsky, and M. Calvin (1963), Science 141:923. Rea, D. G., B. T. O'Leary, and W. M. Sinton (1965), Science 747:1286. Rich, A. (1962), In: Horizons in Biochemistry, M. Kasha and B. Pullman, eds., p. 103, Academic Press, New York. Sagan, C. (1961a), Radiation Research 75:174. Sagan, C. (19616), Organic Matter and the Moon, Natl. Acad. Sci.-Natl. Res. Council Publ. 757. Sagan, C. (1964), In: Life Sciences and Space Research II, M. Florkin and A. Dollfus, eds., North-Holland, Amsterdam; p. 35. Sagan, C. (1966), Origins of the Atmospheres of the Earth and Planets, Internat. Dictionary of Geophysics, S. K. Runcorn, ed.; Section I, H. C. Urey, Section Ed. Pergamon Press, London. Sagan, C., P. L. Hanst, and A. T. Young (1965), Planet. Space Sci. 13:73. Sagan, C., and S. L. Miller (1960), Astronom. J. 65:499. Sagan, C., and J. B. Pollack (1966). To be published. Schmidt, 1. (1959), Proc. Lunar and Planetary Explor. Colloq. 1, No. 6, p. 19. Shirk, J. S., W. A. Haseltine, and G. C. Pimentel (1965), Science 147:48. Sinton, W. M. (1957), Astrophys. J. 726:231. Sinton, W. M. (1959), Science 750:1234. Sinton, W. M. (1961), Science 132:529. Sinton, W. M. (1965), private communication. Sisler, F. D. (1961), Proc. Lunar and Planetary Explor. Colloq. 2, No. 4, p. 67. Spinrad, H. (1962), Publ. Astron. Soc. Pacific 74:156. Stent, G. (1962), private communication. Urey, H. C. (1961), Astrophys. J. 134:268. Urey, H. C. (1962), Nature 793:1119. Walker, R. G., and C. Sagan (1965), Icarus, in press. Young, R. S., P. H. Deal, J. Bell, and J. L. Allen (1964), In: Life Sciences and Space Research II, M. Florkin and A. Dollfus, eds., North-Holland, Amster- dam, p. 105.