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CHAPTER 9 REMOTE DETECTION OF TERRESTRIAL LIFE* CARL SAGAN, R. N. COLWELL, S. Q. DUNTLEY, V. R. ESHLEMAN, D. M. GATES, AMRON KATZ, JOSHUA LEDERBERG, HAROLD MASURSKY, D. G. REA, W. G. STROUD, VERNER SUOMI, and RALPH ZIRKIND INTRODUCTION Until very recently, observations of Mars and other planets of possible biological interest could be acquired only from the vicinity of the Earth. To acquire some degree of perspective on the problems of life detection over interplanetary distances, it is useful to consider the inverse problem— that of the detection of life on Earth from the distance of, for example, Mars (cf., Chapter 3). A related problem, somewhat less difficult, which we consider here, is the detection of life on Earth from Earth satellite altitudes. We are interested both in intelligent and in simpler forms of life. In any reconnaissance of the Earth—for example, by photographic means—the reconnaissance expert has access to what is usually called "ground truth," that is, in situ information on the detailed structure of the terrain. In the case of observations of Mars, we lack ground truth. Indeed, Martian ground truth is the goal of reconnaissance of Mars. An open-air theatre, a housing development or an airport are readily identi- fiable in high-resolution photographs of the Earth. The discovery of similar well-ordered geometrical patterns on Mars would certainly be pro- vocative, but by no means could we be sure of their identification. Yet the detection of highly ordered structures on the Martian surface would * Report prepared by Dr. Sagan as chairman of a study group on this subject. 187

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188 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS certainly pinpoint areas deserving closer study. In addition to geometrical patterns that may possibly indicate the presence of intelligent life, the spectral distribution of reflected and emitted radiation may be diagnosti- cally significant for life detection; monochromatic radio signals being one obvious example. In addition, the characteristic infrared absorption fea- tures of organic matter are, at least in principle, detectable, and even with much poorer spectral resolution, the presence of terrestrial plants is indi- cated by their high infrared reflectivity. On the Earth, life is associated with regions of higher temperatures and greater moisture contents, and diagnostic features associated with hot, wet environments should have special weight. We now consider the various possible detection techniques in turn. OPTICAL FREQUENCY RECONNAISSANCE The possibility of optical detection of life on Earth clearly depends on the resolution of the optical system used. With a ground resolution of 10 km, cities, engineering works, cultivated crops—and essentially the entire range of phenomena that we might expect to indicate the presence of life on Earth—should be largely invisible, either because the intrinsic features are below the stated resolution limit or because their contrast with the surroundings is low. At a resolution of 1 km, the situation should improve somewhat. Cities located in high-contrast—for example, grassy— terrain should become barely visible, and many rectilinear features of high contrast with their surroundings should appear marginally. At a resolution of about 0.1 km, the range of detectable objects that indicate intelligent life on Earth should become very large. Roads, bridges and canals, which have high contrast with their surroundings, should be seen fairly easily, even if they are below the theoretical resolving power. Such rectilinear features should be at least several resolution elements long. According to S. Q. Duntley, the most readily detected rectilinear distribution of a given amount of high-contrast material corresponds to a length-to-width ratio of 6:1. There is no increase in detectability when this ratio exceeds 100:1. Reservoirs, some with the outlines of the capital letter "D," should be visible; wakes of ships often extend for many kilometers and would cer- tainly be seen at 0.1 km resolution, and perhaps at the 1 km resolution level. Similarly, atmospheric condensation trails of jet aircraft should be detectable. Estuary and other pollution might be detectable as contrast changes. Shelter belts of trees, firebreaks and transmission lines through forests should begin to become discernible, as should tree plantations, contour farming and, particularly, fallow fields adjacent to growing ones,

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Remote Detection of Terrestrial Life 189 each with regular geometric shapes. Open pits, mine tailings and other industrial artifacts should become visible. It should be possible to follow pollution, condensation trails, and wakes to their sources. When the resolution reaches 10 meters or slightly better, not only are all the foregoing features much more easily seen but also, the detailed contours of major avenues and the entire network of continental auto- mobile highways and railroads should become clearly visible. In some cases, the contours in the terrain carved out for highways or railroads, rather than the structures themselves, will be visible. At this or slightly better resolution, there is a completely new range of detectable phenomena: shadows of living organisms observed at low solar elevation angles. Not only will the characteristic dendritic patterns of trees be visible, but the long shadows of such animals as cows and horses can also be detected when looking at the Earth in late afternoon. Even brief observations should show the articulation of limbs and motion of such animals. These a priori expectations can be tested by an examination of high- altitude photographs of the Earth taken from aircraft and satellites. The Tiros and Nimbus meteorological satellites have yielded about 106 photo- graphs of the Earth at 0.3-3.0 km resolution. These systems are intended primarily for study of terrestrial cloud systems, but the Earth is not per- petually cloudbound and so the photographs can also be examined for evidence of life on Earth. Through late 1964, eight Tiros satellites were launched. The characteristic scientific payload weighs about 300 pounds. The satellite is launched into an approximately circular orbit, with nominal altitudes of 400 statute miles. Tiros is equipped with a 500-line vidicon system and three lens subsystems with fields of view of 12°, 76° and 104°, respectively. Each of the Tiros vehicles has some combination of these three lens systems. At the nominal altitude, the 12° lens gives a resolu- tion of about 0.3 km; the 76° lens, about 3.0 km. The Nimbus meteorological satellite has a payload of about 1000 pounds and is launched into an orbit nominally ranging from 260 to 600 miles. Photographs from 300 miles altitude with a 32° field of view give a ground resolution of about 0.3 km. The wide-angle lens of the Tiros vidicon system observes an area ap- proximately 1000 km by 1000 km. The perpendicular field of view of the narrow-angle lens is approximately 100 km by 100 km. Tiros pictures have an information content of about 1.5 X 106 bits. The Nimbus vidi- con system can accommodate 3.8 X 106 bits. For comparison, hand-held 35 mm cameras from manned orbiting missions yield pictures with about 109 bits information content. The spectral response of both Tiros and Nimbus optical systems lies in the 0.45- to 0.8-micron range. It is generally well-known that both the United States and the Soviet

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190 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS .«.-. • • Figure 1. Nimbus vidicon photograph of the northwestern coast of France. Courtesy of Goddard Space Flight Center, National Aeronautics and Space Administra- tion. Union maintain systems of military reconnaissance satellites for obtaining photographic information either by telemetry or direct recovery. It is easy to compute (cf., Chapter 15) that even with fairly modest apertures, I0- meter ground resolution should be possible from satellite altitudes, and resolutions approaching 1 meter should not be beyond the realm of possi- bility. Indeed, such resolutions would seem to be required if these satellite systems are to possess military utility. Such high-resolution photographs of the Earth are not available for reproduction here, but roughly com- parable photographs can be obtained from airplane altitudes with avia- tion cameras. Typical Nimbus photographs of the Earth in the 1 km resolution range are displayed in Figures 1 and 2. In Figure 1, we see the northwest coast of France and the English Channel. The black, right-angle markings are fiducial standards introduced into the camera system, and the sequenced white-and-black markings are latitude and longitude intervals. Clouds can be seen in the upper portion of the picture. The Cherbourg and Brest peninsulas and the region between the Seine and the Loire are heavily cultivated areas, but there is no apparent sign of such cultivation in this picture. In Figure 2, we see a montage of several photographs of North

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Remote Detection of Terrestrial Life 191 •••• HUDSON BAY LAKE MICHIGAN — YUCATAN «**' HONDURAS ***«rtM««M $ Figure 2. Montage of Nimbus photographs of North America, taken on orbit 20, 29 August 1964, and orbit 35, 30 August 1964. Courtesy of Goddard Space Flight Center, National Aeronautics and Space Administra- tion.

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192 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS Figure 3. Tiros narrow-angle lens photograph of an area near Cochrane, Ontario, Canada. Courtesy of Goddard Space Flight Center, National Aeronautics and Space Administration. America, from Hudson Bay to Nicaragua. These photographs were taken on 29 and 30 August, 1964, when much of the area was cloud-covered. The well-defined cloud pattern in the second frame is associated with the tropical storm Cleo. Note that while the area between Chicago and Mil- waukee, on the shores of Lake Michigan, in the second frame, is almost cloud-free, there is no sign of life, intelligent or otherwise. The vast majority of Nimbus and Tiros pictures are similarly lacking in signs of biological activity. Figure 3 is an exception. It shows an area near Cochrane, Canada, taken with the narrow-angle camera of the Tiros system. The ground resolution is about 0.4 km. The reader's attention is directed to the white orthogonal array in the lower central portion of the photograph. We are seeing forest clearings, logged through this region of Ontario in a rectangular pattern, so that the nearby trees left standing will reforest the logged area. The logged strips are about one mile wide; there are approximately two miles between the strips. After the clearing operation was completed, snow fell, producing the high-contrast effect visible in the photograph.

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Remote Detection of Terrestrial Life 193 But suppose we had observed the same kind of pattern in a photograph of Mars. Would we immediately deduce this entire story about intelligent beings logging in a manner suitable for reforestation, deduce the existence of trees and the necessity for snow? It is much more likely that such details on Mars would be considered enigmatic, and judgment would be withheld until much finer resolution studies, preferably in situ, could be performed. Kilston, Drummond, and Sagan [1965] have made a study of a set of enlargements of the best cloud-free Tiros and Nimbus photographs and have identified several more signs of life on Earth. Many highways in the United States were searched for unsuccessfully. Finally, Interstate High- way 40 was identified in Tennessee, because of its high contrast and because it was clearly distinguishable from the subsidiary rivers in the Mississippi system because it cut them at oblique angles. Actually, it is not the road that is seen, but the swath around the road, cut through the neighboring forests; the width of the swath is only some tens of meters, which is below the ground resolution of the Nimbus photograph in ques- tion. But rectilinear features can be detected, even when they are below the diffraction resolution limit, if they have high contrast, which in this case is provided by the surrounding forests. The road can be followed to Memphis, Tennessee, which is also discernible, although its relative con- trast is low. Condensation trails of jet aircraft and wakes of ships can also be seen in some photographs of the Tiros and Nimbus series. The con- trails are distinguishable because their shadows are visible. From the dis- placement between the contrail and its shadow, the altitude of the aircraft can be determined. The wakes of ships are immediately obvious because of their shape. A further discussion of signs of life in Tiros and Nimbus Earth pho- tography can be found, with relevant photographs and maps, in the refer- ence cited above. The authors conclude that, even under the most opti- mistic assumptions, several thousand high-contrast photographs with reso- lution down to a few tenths of kilometers are necessary before one fairly good indication of life on Earth can be found—and that would be a recti- linear marking indicative of intelligent life. This number provides some measure of the difficulty of a photographic search for extraterrestrial life. For example, the Mariner 4 space vehicle of the United States was de- signed to take approximately 20 photographs of Mars with a ground reso- lution of a few kilometers—that is, about 100 times fewer photographs, each with 10 times poorer resolution, than would be required to detect life on Earth. Searches for seasonal variations in surface albedo, due to the growth and harvesting of high-contrast crops (e.g., cotton) and the annual cycle

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194 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS HpV * Figure 4. Aerial photograph of an area in the Sacramento-San Francisco Bay region of western United States. This photograph was prepared at the Smithsonian Astro- physical Observatory from a photo-mosaic compiled by the Aero Service Corporation, Philadelphia, Penn- sylvania, and provided through the courtesy of Dr. Robert N. Colwell.

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Remote Detection of Terrestrial Life 195 • *&'• J+. & ••*«*+ • Figure 5. Aerial photograph of a California farm taken at a large solar zenith angle. Courtesy of Dr. Robert N. Colwell. of deciduous forests, were unsuccessful [Kilston, Drummond and Sagan, 1965], although this is attributable in part to the transmission filters used in the Tiros and Nimbus systems (see below). When the ground resolution is improved by about a factor 30 over the best Tiros and Nimbus photographs, results such as that of Figure 4 are obtained. This is an aerial photograph prepared from a photo mosaic of the Sacramento-San Francisco Bay area, prepared by Aero Service Corpora- tion, Philadelphia, Pennsylvania, and provided through the courtesy of Robert N. Colwell. Highways, railroads, contour farming, an airport, housing developments, and city streets are all visible in the original, though some of this detail may have been lost in reproduction. Even if we had no previous experience with any of these artifacts of civilization, the regular geometrical arrangements of the structures on this photograph would be readily apparent. It is of interest to inquire how many random photographs of the Earth of this resolution and field of view would be required, before comparable geometrical detail would be obtained. This question has not been put to a rigorous test, but it would appear that a

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196 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS Figure 6. Ranger IX photograph of a rill system on the floor of the crater Alphonsus. The photograph was taken 1 minute, 17 seconds before impact, at an altitude of 115 miles. Courtesy of Jet Propulsion Laboratory and the National Aeronautics and Space Administration. few hundred random photographs, perhaps only a few score, would be adequate. Such a finding underscores the importance of providing photo- graphs of the highest resolution of any extraterrestrial body that we suspect to harbor living organisms. When resolution is improved by another order of magnitude, it is pos- sible to obtain photographs such as that in Figure 5, also provided through the courtesy of Dr. Colwell. This photograph represents the direct detec- tion of a living organism by observation from high altitude. From the

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Remote Detection of Terrestrial Life 197 angle of the afternoon Sun and the length of the shadow, it is possible to deduce the size of the cow. While a very large number of photographs of the Earth with comparable resolution are required before quadrupeds can be detected by their shadows, a systematic program of observations near a planetary terminator can potentially provide unambiguous detection of large organisms and much information about them. Several points emerge from this survey of remote photographic recon- naissance of the Earth. For reasons of efficiency, the constructions of intelligent beings (on planets large enough for Euclidean geometry to be a good approximation) should tend to be rectilinear. Networks and arrays of linear features (cf., Figure 4) should tend to indicate biological activity. However, some features of undoubted nonbiological origin are also linear. Approximately linear rills inside the crater Alphonsus are seen in Figure 6. Faults and other linear features of geological origin should be common features of any planetary or satellite surface. A narrow peninsula in northern Morocco, 25 km long and about 1 km wide, appears as a striking rectilinear feature in a Nimbus photograph of the Earth; however, it is not of biological origin [Kikton, Drummond and Sagan, 1965]. In the deserts of Africa and Asia Minor, there are systems of long, narrow, parallel and almost rectilinear seif sand dunes. These are characteristically several hundred kilometers long, several kilometers wide, and separated, one dune from another, by perhaps 10 km; a photograph of seif dunes in the western Sahara, taken from an unmanned Mercury capsule, and a discussion of the possibility that rectilinear markings reported on Mars are in fact seif chains, can be found in a paper by Gifford [1964]. Before rectilinear features found on other celestial objects can be imputed to biological activity, all reasonable nonbiological alternatives should have been eliminated; and even then, a fairly elaborate array of such features (again, cf., Figure 4) would be required. High-resolution photography of the Moon, both ground-based and from spacecraft, indicates that the dominant processes of deposition and erosion are different from those on Earth. The resulting morphological character of the Moon is distinct and is best displayed in photographs within two degrees of the terminator. Minor differences in elevation are then brought out by the oblique lighting. The spatial patterns due to geological processes on the Moon and the Earth are, in almost all cases, clearly apparent and distinct from familiar patterns of biological origin. However, the interpre- tation of Martian photographs will depend on our ability to characterize the geological processes shaping the surface of that planet, and on the nature of the artifacts—if any—of Martian biological activity. The prospect of detecting terrestrial plants and animals at low solar elevation angles and several meters resolution suggests a possible general-

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202 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS ^>U 1 1 , 1 ' 1 ao '•'-~ V'"'~\ - - e TO B / x («0 ^ < 50 — § I / J40 S ~ I j I "-* ^-^ _l 3O i — ,-*> // SOLID SPECIM EN ?20 i x> POWDERED SP ft' POWDERED RE ECIMEN SIDUE T6A I200*C 10 A. ,,/ / " >> 0 1 1 1 ""I 1 1 1 1 1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 WAVELENGTH.microm 2.0 2.2 2.4 2. Figure 10. Near ultraviolet, visible, and near infrared total re flectances of limonite. The reflectivity of the solid specimen is shown by the unbroken line; the dashed lines show reflectivities of powdered specimens in the 0.1 mm size range; prepared in one case by grinding the solid sample, and in the other case by taking the powdered sample through thermogravimetric analysis to 1200°C. Reproduced from Sagan, Phaneuf, and Ihnat [1965]. The visible and near infrared reflectance and transmittance of some common petals and leaves are shown in Figure 8. The near infrared re- flectivity of vegetation sometimes reaches 70%. In Figure 9 are shown similar curves for lichens. The near infrared reflectivity here is lower, not so much because of the small abundance of chlorophyll as because of the small opportunity for multiple scattering of infrared photons below the epidermis. Note, in the spectra of Figures 8 and 9, that the general re- flectivity in the green (X — 0.55^) of, for example, cottonwood leaves or lichens, while detectable, is not nearly so striking as the increase in reflectivity beyond 0.7/i. Other similar spectra can be found in Gates etal. [1965]. This general behavior, while characteristic of plants, is, unfortunately, not unique to them. In Figure 10, we see the visible and infrared re-

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Remote Detection of Terrestrial Life 203 flectivity of three samples of the mineral limonite (Fe2O3 • nH2O), a solid specimen and two powdered specimens. We see that high reflectivity in the near infrared may also be characteristic of minerals. The reason is not hard to find. Because of the low absorption coefficients in the near infrared expected for fundamental physical reasons, a bulk sample of any powdered mineral whose particle size approximates a few microns should have a very large reflectivity in the near infrared. If it absorbs in the visible, as limonite does because of its iron moiety, its net spectrum will tend to resemble that of many terrestrial plants. Therefore, a finding of low reflectivity in the visible and high reflectivity in the near infrared is not a uniquely diagnostic test for vegetation on another planet. The examples of Figures 9 and 10 are particularly relevant for Mars, which displays a near infrared spectrum rather similar to that of Figure 10. Figure 10 is, in fact, part of the body of evidence suggesting the presence of limonite on Mars [Sagon, Phaneuf, and Ihnat, 1965], although there is other, independent, evidence for its existence [Dollfus, 1957]. Because the reflectivity of Mars in the near infrared is not so striking as that of some green, broad-leaved plants, certain authors (e.g., Kuiper [1952]) have suggested that lichens, with their small near infrared reflectivity, may be better models for Martian organisms. It should be emphasized that the ecology of life on Mars may be extremely different from that of figure 11. Two aerial photographs of the safe region, (left) in visible panchromatic, (right) in the near infrared. Courtesy of Eastman Kodak Company.

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204 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS life on Earth. The organisms may be very sparsely distributed and there- fore make no perceptible contribution to the near infrared reflectivity; or they may utilize light in the near infrared for photosynthesis (as suggested by G. A. Tikhov) and therefore show a low infrared reflectivity. A suitable study of terrestrial vegetation in the photographic infrared from satellite altitudes does not exist in the open literature. Such a study requires a comparison of photographs taken in regions long ward of 0.8/i, where vegetation is highly reflective, with photographs taken in wave- lengths shortward of 0.7/i, where the reflectivity of vegetation is very low. The peak sensitivity of the visible channel of the Tiros radiometer is about 0.7^, but its sensitivity extends to 0.75/t, or even slightly longer wave- lengths. As a result, the passband contains wavelength regjons where vegetation has high reflectivity, as well as wavelength regions where vege- tation has low reflectivity. Thus, in Tiros photographs, vegetation appears neither uniformly brighter nor uniformly darker than the surrounding terrain. Identifications of vegetation from satellite photography—for example, of a conifer forest in France [Kilston, Drummond, and Sagan, 1965]—are not clear-cut. The choice of filters in the Tiros and Nimbus satellites is, of course, not a defect in their design; they were intended for meteorological observations, not for vegetation surveys. Figure 11 shows two photographs, one in the visible and one in the near infrared, of the same vegetated region, containing houses and a waterway. Since liquid water begins to absorb strongly in the near infrared, while vegetation begins to reflect strongly at the same wavelengths, we see that in comparing a photograph taken in the visible with one in the near infrared, the contrast between vegetation and liquid water is re- versed. Similar high-resolution photographs, taken at two wavelength ranges from a Mars orbiter, might be a significant first step in the search for moist, vegetated regions on the planet, but no unique identifications of vegetation by this method are expected. At longer wavelengths, beginning at about 1.2^, the absorption by water and by organic functional groups in the upper layers of the leaf is mani- fested by depressions in the spectrum of light reflected from the leaf. Such absorptions at the methyl and methylene C—H stretching wavelengths near 3.5/i have been observed, for example, by Sinton [1959] and by Rea, Belsky, and Calvin [1963] in the reflection spectra of a variety of ter- restrial plants. At longer wavelengths—for example, in the 4.0-6.5/i region —there are a large number of other organic functional groups of interest. However, especially for Mars, the intensity of reflected sunlight at these wavelengths is likely to be dominated by the thermal emission of the planet itself. Except for very thin covers of organic matter which have an optical depth in the infrared of unity or less, the organic functional groups ap-

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Remote Detection of Terrestrial Life 205 Figure 12. Infrared image of Italy, obtained with the Nimbus I radiometer be- tween 3.4 and 4.2/i at night. Courtesy of Goddard Space Flight Center, National Aeronautics and Space Administration. parently cannot be detected, either in absorption or in emission, over the background infrared thermal emission spectrum (cf., Hovis [1963]). There exists no comprehensive catalogue of the reflectivities of terrains longward of about 0.8/i. The infrared specular reflection spectrum of many plants between 1.5 and 25/i has been reported by Gates and Tantraporn [1952]. Some preliminary measurements obtained from an Aerobee rocket at about 100 km altitude with a 10-channel infrared radiometer, reported by Ralph Zirkind, suggest that terrain observations at a variety of wavelengths in the near infrared may be of considerable interest. For example, with one-line scans from ocean to beach to forest terrain, the filter at 2.3/i with half-width 0.04/i shows approximately a factor of 4 increase in reflectivity over the forest; while at 3.9/i, with a half-width of 0.17ii, approximately a factor of 2 decline in reflectivity is observed. In order to exploit differential wavelength infrared imaging and scanning in applications to other planets, a program of laboratory and field measure- ments on the Earth is needed. Multicolor infrared imagery is feasible, but has never been used for observations of terrain. INFRARED THERMAL MAPPING Some Tiros and Nimbus satellites were equipped for infrared thermal mapping of the Earth. The Tiros infrared radiometers had a ground reso- lution of some 50 km, and with a thermistor bolometer as radiation de- tector, were capable of about 3°K temperature discrimination. Among the channels used were 5.8-6.8/i, 7.5-20.0/i, 8-12/i, and 14.5-15.5/i. Except

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206 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS Figure 13. Two images of a populated region of the Earth, (left) an ordinary photograph obtained in visible light; (right) obtained at night by infrared imaging. Cour- tesy of Republic Aviation Corporation and Dr. Ralph Zirkind.

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Remote Detection of Terrestrial Life 207 for the 8-12/i channel, these wavelength ranges are all in regions of sub- stantial atmospheric absorption. Nimbus has a radiation-cooled lead selenide detector, maintained at temperatures above 200°K. With a channel between 3.4 and 4.2/i, it has a temperature sensitivity of 1 °K and a ground resolution of some 3 km. An area 1000 miles by 1000 miles accommodates approximately 104 bits in the Nimbus radiometer. Espe- cially with Nimbus, good nighttime images of the Earth have been ob- tained. For example, Figure 12 shows an infrared image of Italy, taken at night in emitted light. From detailed studies of single-line scans in such pictures, it is suggested that cities can sometimes be detected as hot spots. For example, Rome appears as an area some 5°K warmer than its environs. Figure 13, reproduced through the courtesy of the Republic Aviation Corporation and Dr. Ralph Zirkind, compares a daytime photo- graph of a populated region of the Earth, taken from aircraft altitudes, with a photograph taken in emitted light at night. The contrast seen in the left-hand photograph is due primarily to differences in thermal conduc- tivity and specific heat capacity of neighboring materials; some retain the heat acquired from sunlight during the daytime much better than others. Infrared thermal mapping is now in increasing use for geological sur- veys. For example, Fischer et al. [1964] describe aerial mapping with infrared imaging radiometers of Hawaiian volcanoes. These volcanoes have since been observed by Nimbus radiometry. Thus, infrared thermal mapping might be of considerable use in studying features of geological interest, as well as, perhaps, in acquiring clues for further biological in- vestigations. We may note that the nighttime hemisphere of Mars has, because of phase angle limitations, never been observed from the Earth. OBSERVATIONS AT MICROWAVE FREQUENCIES Due to the activities of man during the last two or three decades, the brightness temperature of the Earth in the meter wavelength range has increased about a million times [Shklovsky and Sagan, 1966]. The Earth is now the second most powerful radio source in the solar system at these wavelengths, due to television broadcasting intended for local communi- cation on the planet Earth. This fact could have been detected with quite modest equipment from the distance of Mars. It is, of course, by no means clear that an intelligent species developing on some other planet must necessarily undergo the same steps of technological evolution that result in very high brightness temperatures at meter wavelengths. But the ex- ample suggests that narrow-band radio observations may be the easiest method for detecting intelligent extraterrestrial life. While there has been

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208 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS no comprehensive program for observing Mars with narrow-band filters at any radio wavelengths, the few attempts that have been made have proved negative, and nothing approximating intelligible broadcasting has ever been reliably detected. The existence of intelligent life on Earth could most easily be estab- lished from satellite altitudes with small radiotelescopes. Radar reflectivity and depolarization from satellite altitudes can give some information on surface and subsurface composition and structure. Progressive and synoptic observations of the same region as a function of phase angle and of angle to the local surface normal for a variety of wavelengths can give a vast quantity of information on subsurface thermal and electrical properties and the distribution of temperature, granularity, and composition with depth. Like observations on natural terrains at other frequencies, such passive and active microwave observations may be useful for excluding some materials, but they usually cannot provide a unique identification of materials. Occasionally, such techniques can be used to exclude the large- scale presence of organic materials; for an example of the application of such methods to Venus, see Pollack and Sagan [1965]. SUMMARY At kilometer ground resolution, there is generally no sign of life on Earth. Both a priori considerations, based on terrestrial ground truth, and photographs taken by meteorological satellites and high altitude aircraft show that it is very difficult to detect life on Earth by photographic re- connaissance unless the ground resolution is about 0.1 km, or better. At this resolution, rectilinear features of intelligent origin become evident. Photographs of the Earth at 10 m resolution and at low solar elevation angles should permit the detection of the shadows of, e.g., stands of trees or herds of cattle. Perhaps a few thousand randomly distributed photo- graphs of the Earth would be required for a significant detection of life on Earth at 0.1 km resolution; this number can probably be reduced by at least another order of magnitude for observations at 10 m resolution. High reflectivities in the near-infrared are indicative of the presence of vegetation, but not uniquely so. Many inorganic materials show similar behavior. Comparison of photographs of a body of water with vegetation along its banks, obtained in the visible, with similar photographs obtained in the infrared should show a significant reversal in relative contrasts. Infrared reflection spectra of vegetation, obtained from high altitudes, may show characteristic absorption features due to the presence of organic functional groups. Thermal mapping and other infrared techniques may

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Remote Detection of Terrestrial Life 209 be useful in specifying biologically promising locales which are warmer or wetter than their surroundings. Other than high resolution (— 10 m) imaging of the surface, the most reliable technique for the detection of intelligent life on Earth from satellite altitudes appears to be observations of monochromatic emission in the radio-frequency range. REFERENCES Dollfus, A. (1957), Ann. Astrophys., Suppl. 4. Fischer, W. A., R. M. Moxham, F. Polcyn, and G. H. Landis (1964), Science 746:733. Gates, D. M., H. J. Keegan, J. C. Schleter, and V. R. Weidner (1965), Spectral Properties of Plants. To be published. Gates, D. M., and W. Tantraporn (1952), Science 115:613. Gifford, F. A., Jr. (1964), Icarus 3:130. Hovis, W. A. (1964), Science 745:587. Kilston, S. D., R. R. Drummond, and C. Sagan (1965). To be published. Kuiper, G. P. (1952), In: Atmospheres of the Earth and Planets, G. P. Kuiper, ed., University of Chicago Press, Chicago, Chapter 12. Pollack, J. B., and C. Sagan (1965), Icarus 4:62. Rea, D. G., T. Belsky, and M. Calvin (1963), Science 141:923. Sagan, C., J. P. Phaneuf, and M. Ihnat (1965), Icarus 4:43. Shklovsky, I. S., and C. Sagan (1966), Intelligent Life in the Universe, Holden- Day, San Francisco. In press. Sinton, W. M. (1959), Science 130:1234.

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PART V SOME EXTRAPOLATIONS AND SPECULATIONS

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