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CHAPTER 13 HIGHER ORGANISMS ON MARS CARL SAGAN In the present volume, we are concerned, for the most part, with the possibility that simple living things, chiefly microorganisms, may be found on Mars. Such an approach is certainly justified by terrestrial experience, where microorganisms are ubiquitous over the Earth's surface, and have been throughout geological time. Yet a mission to investigate life on Earth would certainly overlook some items of interest if attention were restricted to microorganisms. The astronomical evidence suggests, but by no means proves, the existence of organisms in the 10~2 cm size range; and much larger plants seem ecologically possible (cf., Chapter 11). Where there are plants, there may be animals; but in the absence of oxygen, their metabolic efficiency might be less than that of terrestrial animals. There is an obvious selective advantage to intelligence, and it would be surprising if Martian animals, if any, lacked at least rudimentary intelligence. Here, we are already on very hypothetical ground. But because it is a topic of great scientific and popular interest, let us proceed even further. Is it possible that there is an indigenous civilization on Mars? Statistically, the likelihood seems very small. The lifetimes of both Earth and Mars are about 5 billion years. If the putative Martians are behind us, it is most likely that they are far behind us, and have not yet achieved a technical civilization. If they are even slightly more advanced than we, 252

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Higher Organisms on Mars 253 their presence should be discernible. The telescope reveals no obvious signs of a reworking of the Martian environment by intelligent beings; although in the inverse situation, the detection of life on Earth with an optical telescope on Mars would be marginal (Chapter 9). The canals of Mars are probably psychophysiological in origin [Antoniadi, 1929; Dollfus, 1961]. A radio observatory of contemporary terrestrial manu- facture on Mars could detect local television broadcasting on Earth. No formal searches have been made from Earth with narrow-bandpass receiv- ers for comparable transmissions on Mars. The radio observations that have been performed are very broad-banded; their function was microwave radiometry, not the detection of intelligent signals. Finally, again extrapo- lating from terrestrial technology, an advanced Martian civilization might be expected, by this time, to have arrived on Earth. There is no evidence for such visits. The arguments from terrestrial analogy have, of course, limited validity, because we do not yet know even one other example of biological evolution. Conceivably, an advanced civilization might develop, but it might not manifest any of the signs of life just mentioned. But if we uncritically apply terrestrial analogy, we find that if the Martians were 5000 years behind us, they would not yet have a civilization; if they were 50 years ahead of us, we should, by now, have deduced their existence. Then the probability of any civilization existing on Mars is 5 X 108/5 X 109 = 10~°; of a technical civilization more advanced than we, 50/(5 X 109) = 10-8. These probabilities are very low. The only serious extant argument supporting intelligent life on Mars was proposed by the Soviet astrophysicist I. S. Shklovsky, and concerns the Martian satellites, Phobos and Deimos [Shklovsky, 1963; Shklovsky and Sagan, 1966]. Observations of the motions of Phobos and Deimos between 1877, the date of their discovery, and 1941 were reduced by B. P. Sharpless in 1945. Sharpless found a secular acceleration for Phobos, the inner satellite, but not for Deimos. If Phobos is an ordinary kind of satellite, its secular acceleration cannot be explained by atmospheric drag; the Martian exosphere is too diffuse by several orders of magnitude. Shklovsky has investigated a large number of other conceivable causes of the observed secular acceleration, including tidal friction, magnetic brak- ing, radiation pressure, and classical celestial mechanical perturbations; he finds that all are inadequate. He then returns to the possibility of atmos- pheric drag, and points out that if Phobos has a mean density of 10~3 gm cm-3 or less, the secular acceleration can be explained by drag on the basis of the probable density of the Martian exosphere. But a solid object of such density is unstable to tidal perturbations, and would long ago have been destroyed. Shklovsky therefore concludes that Phobos must be hol-

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254 SOME EXTRAPOLATIONS AND SPECULATIONS low. Its characteristic diameter would then be 16 km; its characteristic shell thickness, 30 cm; and its characteristic mass, 109 tons. Such an object, Shklovsky says, can only be an artificial satellite of immense propor- tions, launched by a Martian civilization vastly in advance of our own. Since, aside from Phobos and Deimos, there are no present signs of such a civilization, Shklovsky concludes that it is extinct, possibly for several hundreds of millions of years. He goes on to speculate that the function of the satellite may be the communication of the learning accumulated by the Martian civilization, in later epochs when its builders are extinct. The argument for the artificiality of Phobos has not been convincingly criticized on substantive grounds. The only area of controversy concerns the observations themselves. G. A. Wilkins, of the Royal Observatory, Herstmonceux Castle, investigated the problem at the U. S. Naval Observa- tory, several years ago. His conclusions after that work [Wilkins, 1963] are as follows: "So far I have re-reduced practically all of the observations of the satellites of Mars that were made from their discovery up to 1941— i.e., for the period covered by Sharpless' note. . . . The values found for the secular variations of the mean motions were insignificant, but I have not yet fully confirmed this result nor taken into account even those later observations that have been made available to me. "There is no doubt that visual observations of the positions of these satellites with respect to the centre of the disk of the planet are difficult to make, but I do not have sufficient evidence to be able to state that Sharpless' determination was based on inadequate observations or that the method of treatment was not suitable. I consider that the question of the existence of the secular accelerations should be regarded as an open one until a new and more complete analysis of the observations is made; I do not consider that the evidence is sufficiently strong to justify any attempt to look for artificial causes." Subsequently, Wilkins [1965] has reanalyzed the data, and finds two solutions that fit the observations. One solution shows no secular accelera- tion, but the other solution exhibits a large secular acceleration, and both, solutions have approximately equal weight. The observational reality of a secular acceleration, and the validity of Shklovsky's deductions, therefore remain in doubt. A further reduction of observations of Phobos and Deimos, made since 1941, and the acquisition of additional high-precision data on the motion of these satellites, would seem to be of considerable interest.

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Higher Organisms on Mars 255 REFERENCES Antoniadi, E. M. (1929), La Planete Mars, Hermann, Paris. Dollfus, A. (1961), In: Planets and Satellites, G. P. Kuiper and B. M. Middle- hurst, eds., Univ. of Chicago Press, Chicago, Chapter 15. Sharpless, B. P. (1945), Astron. J. 51:185. Shklovsky, I. S. (1963), Vselennaia, Zhizn, Razum, Moscow. Shklovsky, I. S., and C. Sagan (1966), Intelligent Life in the Universe, Holden- Day, San Francisco. Wilkins, G. A. (1963), private communication. Wilkins, G. A. (1965), Proc. Intn'l. Astron. Un. Symp. No. 25, Thessaloniki. In press.

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PART VI APPROACHES TO THE EXPLORATION OF MARS AND REMOTE OBSERVATIONS

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