erry Puke 5 _ UNIVERSE ir n = i ‘ a ae )ht= ooh a ? bd aN Basra Ano ELIZABETH BILSON 4 %Carl Sagan’s Universe Edited by Yervant Terzian Elizabeth Bilson Cornell University Cornell University | CAMBRIDGE i) UNIVERSITY PRESSPUBLISHED BY THE PRESS SYNDICATE OF THE UNIVERSITY OF CAMBRIDGE Tho Pitt Building, Trumpington Stroot, Cambridge CB2 1RP, United Kingdom CAMBRIDGE UNIVERSITY PRESS ‘The Edinburgh Building, Cambridge CB2 2RU, United Kingdom 40 West 20th Street, New York, NY 10011-4211, USA 10 Stamford Road, Cakleigh, Melbourne 3166, Australia © Cambridge University Press 1997 ‘This book is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 1997 Printed in the United States of America ‘Typeset in Melior and Eurostile Library of Congress Cataloging-in-Publication Data Carl Sagan’s universe / edited by Yervant Terzian, Elizaboth Bilson. om. ISBN 0-521-57206-X.— ISBN 0-521-57609-2 (pk) 1. Life on other planets. 2. Science news. 3. Science - Social ‘aspects. 4. Sagan, Carl, 1934-1996. I. Terzian, Yervant, 1939- Il, Bilson, Elizabeth M. QB54.C37 197 500 - dc21 96-40511 cP A catalog record for this book is available from the British Library ISBN 052157286 X hardback ISBN 052157603 2 paperbackContents List of Contributors page ix Preface xi Yervant Terzian and Elizabeth M. Bilson PLANETARY EXPLORATION 1 ‘1 On the Occasion of Car! Sagan's Sixtieth Birthday WESLEY TL HUNTRESS JR @ The Search for the Origins of Life: U.S. Solar System Exploration, 1962-1994 EDWARD C, STONE 8 S&S Highlights of the Russian Planetary Program ROALD SAGDEEV 28 ‘@ _From the Eyepiece to the Footpad: The Search for Life on Mars BRUCE MURRAY 35 LIFE IN THE Cosmos 49 S Environments of Earth and Other Worlds OWEN B. TOON 51 6G The Origin of Life in a Cosmic Context CHRISTOPHER F. CHYBA 64 7 Impacts and Life: Living in a Risky Planetary System DAVID MORRISON 75 8 Extraterrestrial Intelligence: The Significance of the Search FRANK D. DRAKE 87vi 9 Extraterrestrial Intelligence: The Search Programs Contents PauLHOROWTZ 10 Do the Laws of Physics Permit Wormholes for Interstellar Travel and Machines for Time Travel? KIP S. THORNE INTERLUDE 411 The Age of Exploration CARL SAGAN SCIENCE EDUCATION 12 Does Science Need to Be Popularized? ‘ANN DRUYAN 13 Science and Pseudoscience JAMES RAND! 14 Science Education in a Democracy PHILIP MORRISON 415 The Visual Presentation of Science JON LOMBERG 16 Science and the Press WALTER ANDERSON 17 Science and Teaching BILL G, ALDRIDGE SCIENCE, ENVIRONMENT, AND PUBLIC POLICY 16 The Relationship of Science and Power RICHARD L. GARWIN 19 Nuclear-Free World? GEORGI ARBATOV @O Car! Sagan and Nuclear Winter RICHARD P. TURCO 21 Public Understanding of Global Climate Change JAMES HANSEN 22 Science and Religion JOAN B. CAMPBELL 121 135 141 161 163 170 179 190 212 219 221 228 239 247 254Contents vii ———— @3 Speech in Honor of Car! Sagan FRANK PRESS: 261 EPILOGUE 271 24 Car Sagan at Sixty FRANK H. T. RHODES 273List of Contributors BILL G. ALDRIDGE Director, Science Education Solutions Vice President, Airborne Research and Services WALTER ANDERSON Editor, Parade Publications GEORGI ARBATOV Director Emeritus and Chairman of the Governing Board, Institute of U.S. and Canadian Studies Russian Academy of Sciences ELIZABETH M. BILSON Administrative Director, Center for Radiophysics and Space Research, Comell University Editor of this volume JOAN B. CAMPBELL General Secretary, National Council of the Churches of Christ CHRISTOPHER F CHYBA Assistant Professor, Department of Planetary Sciences, The University of Arizona FRANK D. DRAKE Professor of Astronomy, University of California at Santa Cruz ANN DRUYAN Secretary, Federation of American Scientists RICHARD L. GARWIN IBM Fellow Emeritus IBM Research Division JAMES HANSEN Director, NASA Goddard Institute for Space Studies PAUL HOROWITZ Professor of Physics Harvard University WESLEY T. HUNTRESS, JR. Associate Administrator for Space Science, NASA Headquarters JON LOMBERG Senior Advisor The Planetary Society DAVID MORRISON Chief, Space Science Division NASA Ames Research Center PHILIP MORRISON University Professor Emeritus Massachusetts Institute of Technology BRUCE MURRAY Professor of Planetary Sciences Division of Geological & Planetary Sciences, California Institute of Technology FRANK PRESS, Senior Fellow, Carnegie Institution of Washington ieimage not availablePreface When Carl Sagan came to Cornell in 1968 he was young, brilliant, and ambitious; in this respect not so different from other new faculty members. But Sagan had uncommon vision and well-defined purpose. He was fascinated by science and by astronomy in particular, and he believed that key questions concerning the origins of life and the existence of life elsewhere in the universe could be confronted by rational thinking combined with astute research and observation. He was further convinced that what he knew and believed, and what he hoped to discover, had to be effectively communicated to the public policy makers and indeed to the general public at large. He recognized that in a technological society (or in any advanced society, for that matter) science is critical for informed decision making. For nearly three decades we watched Carl Sagan pursue his vision with great dedication and spectacular success. He played a leading role in the American space program since its inception. He briefed the Apollo astronauts before their flights to the Moon and was an exper- imenter on the Mariner, Viking, Voyager, and Galileo expeditions to the planets, He helped solve the mysteries of the high temperature of Venus in terms of a massive greenhouse effect; he explained that the seasonal changes on Mars were caused by windblown dust; and he showed that the reddish haze of Titan was due to organic molecules in its atmosphere. He was a consultant and adviser as well as an im- portant spokesperson for the National Aeronautics and Space Admin- istration (NASA) and the entire scientific community at congressional hearings and in the press. He brought public attention to extremely important environmental and other issues, such as the Nuclear Winter. He was one of the key scientists who organized and inspired programs in the search of extraterrestrial intelligence. Sagan became a best-selling author the world over of books that popularize science and its significance for mankind. In 1978 heimage not availableimage not availableimage not availableOn the Occasion of Carl Sagan’s GOth Birthday WESLEY T. HUNTRESS, JR. NASA Headquarters, Washington, D.C. Carl Sagan is sixty years old, and the Space Age is only thirty-seven years old. Car! is so identified with the Space Age that it is hard to believe that he wasn’t born with it. Over those thirty-seven years of the Space Age, Carl has been the single most-recognized science mission- ary bringing the ideas, excitement, and adventure of space exploration to the general public. One thing that Carl has done so well in his ca- reer is to make both scientist and layman think, and in particular to think about science and space exploration in a much larger societal and historical context. He has caused us to consider what it is about space oxploration that is so fascinating to human beings and why an investment in space is so important for our future. Over his long scientific, literary, and public career, I think that Carl’s greatest accomplishment may be that he has become for many Americans an icon for modern science. For many on this planet, Carl is the personification of space science and exploration. He has reached millions of people with his articles, television appearances, and books. He has explained science and its importance to many an audience. Throughout his work, Carl has carried the message that the commu- nication of science is crucial to its success. Carl realized long ago that scientists have a responsibility to participate in society. Scientists can no longer remain safely in their laboratories and offices, divorced from the rest of the world. Carl recognized early in his career that scientific discoveries have value only if they are shared. Carl understood more than most scientists that science can continue to prosper only if the public can participate in its excitement and thereby support its con- tinuation. There have been those in the science community who have not fully appreciated the value that Carl's approach has brought to the scientific enterprise. The truth is that we need a lot more like him. The chapters of this book clearly reflect the role Car! has played in promoting scionce and the scientific process. They start with a sectionimage not availableimage not availableimage not availableOn the Occasion of Carl Sagan’s 60th Birthday universe, to seek out new planets, and to search for life elsewhere in the galaxy. In fulfilling this mission, we need to use earth-orbiting spacecraft to develop the means to understand life-sustaining pro- cesses on this planet; we need to travel robotically beyond earth in our own solar neighborhood to survey, explore, and sample every ac- cessible body in the Solar System; we need to extend our vision of planetary exploration beyond our own Solar System in order to con- duct an astronomical search for planets around the nearest stars; we need to expand our ability to observe the distant universe in order to survey the universe across the entire electromagnetic spectrum; and, all the while, we need to develop the means for human exploration beyond earth orbit so that we may in the twenty-first century fulfill human destiny to explore beyond earth and to utilize the Solar System for the full benefit of the people of this planet.image not availableimage not availableimage not availableThe Search for the Origins of Life: Solar System Exploration can shed a great deal of light on the conditions under which life can evolve. Where else in the Solar System might liquid water be found? In the 1980s, wo looked farther out in the Solar System, to the giant planets. The first of these is Jupiter. Figure 2.5 shows Jupiter with two of its moons: Io (lef), the closest moon, which is six planetary radii from the center of Jupiter, and Europa, which is about ten planetary radii from the center of the planet. Both satellites, which are about the size of our own Moon, orbit Jupiter in synchronism, with Io orbiting twice for every one orbit of Europa. Because of the gravitational interplay of these two satellites in Jupiter's immense gravitational field, Io's crust is constantly flexed by tidal forces. This flexing generates enough heat to cause extensive volcanic activity, resulting in a constantly renewed surface that is unmarked by impact crators (Figure 2.6). The shades of orange that apparently mark Io's surface are most likely associated with the presence of sulfur, a possibility that Carl studied in some detail. Figure 2.7 shows an interesting Io feature named Loki Patera (the crescent-shaped black spot below center), believed to be a lake of liquid sulfur with a solidified surface crust. The evidence for this is an average surface temperature on Io (which is five times farther from the Sun than Earth is) of 120 kelvins, or 120° above absolute zero, whereas the temperature of Loki Patera is 310 kelvins, warmer than room temperature. Loki Patera and the other black spots seen in the a4 FIGURE 2.3 These channels on the surface of Mars were likely carved by ancient flows of water.image not availableimage not availableimage not availableThe Search for the Origins of Life: U.S. Solar System Exploration 15. FIGURE 2.9 Unlike Io, Europa still has water ~ in the form of an icy crust. FIGURE 2.10 Europe's surface is the smoothest surface observed in the Solar System: its highest features are the white streaks, which are porhaps a fow hundred meters high. (Image processing by U.S. Goological Survoy, Flagstaff, AZ)image not availableimage not availableimage not availableimage not availableimage not availableimage not availableimage not availableimage not availableimage not availableimage not availableimage not availableThe Search for the Origins of Life: Solar System Exploration Solar System is so spread out that many frames were required. In the portrait, the Sun is visible as a bright point, but the planets cannot be easily discerned, except in individual enlargements (with captions) of some of the images. Figure 2.24 shows a frame isolating Earth. In Carl’s words, from this perspective, “Earth is a pale blue dot.” I am confident that in the years ahead, we will continue enlarging our perspective of the origin of life as we undertake new journeys of exploration. 27image not availableimage not availableimage not availableHighlights of the Russian Planetary Program The next episode in the surrealist program of Russia was a project known to many in the United States. We were planning to leunch a huge atmospheric balloon to Venus. The date of launch was choson — 1983. We were doing it together with the French space community, and the idea was that we would celebrate the 200-year anniversary of the Montgolfier brothers’ balloon flight in the atmosphere of the Earth, So, this spirit of celebration, of launching at the important his- toric date, prevailed, but what happened? The project technically was feasible. I'm sure we would still be getting a lot of data from this flight, which was to carry a rather sophisticated scientific payload. Part of it was to be used to study the complicated chemistry of the clouds in the atmosphere of Venus. The reason this project was abandoned almost at tho last minute in 1981 was that we discovered that Bruce Murray was lagging behind schedule in trying to persuade NASA and the admin- istration in this country to launch a spacecraft to encounter Halley’s Comet. Such factors influenced our program so much that we decided to abandon the big balloon celebrating the Montgolfier brothers, but rather to send a spacecraft first to Venus with small balloons and a lander and then after a swing by to encounter Halley's Comet. I think it was probebly the biggest international project, involving about nine nations, in the history of the Soviet program. At that time, not even NASA was involved in such extensive international cooperation, and it brings me to my very first momory of meeting Carl. Theard, of course, a lot about Carl. I read some of the books. One of the books he coauthored with a very close friend of mine and a leading scientist of the Space Research Institute, Iosef Shklovskii. And then in 1976 I was planning to go to the United States and hoping to see Carl during my visit at Cornell, but he was absent from his office at that particular period, so finally we were able to arrange the meeting at the National Airport in Washington. To identify Carl, 1 was given a small photograph. One probably would not be able to find a single living creature on the planet now who would need a photograph to recognize Carl. It was bofore the Cosmos sories. So I kept the photograph and he was very easy to find. What happened then seemed like science fiction. I met the man for the first time, and half an hour later we were talking about how to bring real openness in the Soviet program. We elaborated on a scenario in which one of the next Soviet Veneras landing on the surface of Venus would be coordinated in terms of scientific coverage of the mission, with a parallel American Pioneer Venus mission. Unfortunately, we lived in a different epoch, and only part of this scenario materialized, buta very few years later, in 1986, we fully recovered this scenario when Carl came to Moscow during a real- time encounter with Halley’s Comet. I think it was such an unusual coincidence. We did not try to respect any important Bolshevik dates. After all, we had the excuse that the orbits of comets are controlled by God. But at the very moment when Carl was in real time sitting in theimage not availableimage not availableimage not availableFrom the Eyepiece to the Footpad: The Search for Life on Mars BRUCE MURRAY Division of Geological & Plonetary Sciences California Institute of Technology I would like to amplify Roald Sagdoov's remarks about how the Soviet space program was so driven by milestones and nationalistic symbol- ism. Sagdeev was diplomatic enough not to refer to the U.S. program in the same way. However, the target date for the Viking Lander to go down to the surface of Mars was July 4th, 1976, our 200th anniversary. The landing actually was on the 20th of July, for good technical rea- sons. NASA had the wisdom to back off from the symbolic date to be sure we were technically ready. Thus, nationalistic fervor over space activities was strong also in the United States, but being a more open and pluralistic society, scientists and engineers had a greater influence on policy and technical decisions. U.S. space efforts have been transformed by the end of the Cold War also. It's over, really over. We're all together in a new era of rapidly evolving international cooperation and competition. Now we have to create a new, more international paradigm to motivate ourselves for real space achievement once again. That is what Carl and I and The Planetary Society have tried to focus on. How do we go beyond the bittersweet highs and lows of the Cold War into a new era that will also be distinguished by great scientific accomplishments? How can our children and grandchildren leave their mark on history that will be remembered long into the future? I hope that this conference helps to bring that need and opportunity into focus. Introduction My task is to talk briefly about a big subject - Mars. Fortunately, there is a powerful integrating theme for Mars. It is the search for life. “From the Eyepiece to the Footpad” refers to the allure of life on Mars from the beginning of observing Mars through telescopes in the nineteenth century to the motivation for humans to arrive there in the twenty-firstimage not availableimage not availableimage not availableFrom the Eyepiece to the Footpad 39 FIGURE 4.1 Mariner 4's first glimpse of Mars’ surface. The large crater that dominates this tiny snapshot of Mars’ surface was an astonishing clue to the fact that Mars’ surface was more like tho Moon than the Earth, In this frame, the sunlight is coming from the botiom of the picture. In this primitive digital television picture, there are only 200 picture elements in each direction. on Mars, which meant that there had been no Earth-like erosion and weathering for billions of years and, therefore, no oceans, rainfall, and rivers. We knew right then, from this primitive set of picturos, that Mars was not like the Earth. It didn’t have an Earth-like history. It seemed more like the Moon with a thin atmosphere at this point. Naturally, the expectation of life on Mars plummeted. And there was more to come. My Caltech colleague, Professor Robert Leighton, puzzled about this very thin carbon dioxide atmosphere surround- ing Mars. He made some basic energy calculations and demonstrated clearly that the physical consequence of that carbon dioxide atmo- sphere on Mars is that the frost cap should be carbon dioxide, not water ice! Then, Mariners 6 and 7 flew by Mars in 1969 and reaffirmedimage not availableimage not availableimage not availableFrom the Eyepiece to the Footpad a3 FIGURE 4.5 South polar layered terrains. This preliminary image from Mariner 7 shows the eroding edge of a delicately layered blanket of wind-deposited sediments that characterize both the north and south polar regions of Mars. The sun is coming from the lower left in this image. The regular pattern of black dots is geometric reference points in the focal plate. At the lower portion of the picture, faint craters can be seen from the underlying surface, beneath the blanket. The thin layers are part of an escarpment that is facing equatorward (north). The upper portion of the picture is dominated by a lower left/upper right set of streaks that are believed to be a wind erosional feature called yardangs, carved out in a very smooth top surface of this blanket of wind-deposited material. This image is about 50 kilometers (30 miles) in horizontal dimension. The Viking Search For Life ‘Timed to coincide with the U.S. bicentennial in 1976, the most elab- orate unmanned mission ever deployed arrived at Mars. Figure 4.7 shows a model of the Viking lander. Note the long arm designed to reach out and sample the soil, then drop the samples into a special processing laboratory aboard the sophisticated lander. Included in that laboratory were some of the most sophisticated life-detecting experi- ments ever built. How does one detect life? It’s a tough question. If somebody gives you some stuff, how can you prove something in it is alive? There was only onereally universal way. One has to find evidence that something is roplicating and growing. There were three experiments to look for something growing. Two of them used differing liquid broths, offeringimage not availableimage not availableimage not availableFrom the Eyepiece to the Footpad What's Next? So once again the prospect of life now on Mars’ surface plummeted. Yet Mars may have supported at least simple life forms in the geological past. This is where the convergence of Sagan and Murray came about. Carl’s original optimism about finding a living biota on Mars that we could sample and compare with Earth’s faded. But despite my orig- inal pessimism, there was overwhelming evidence that Mars once had avery active carly history with a lot of water present on the surface in different forms. That time scale seems long enough for life to have formed on Mars analogously to how it must have on the Earth. The implication is that when Mars changed its atmospheric state, probably about three billion years ago, to an extremely hostile one, any surface life that might have developed was not able to survive. So the post-Viking scientific focus has become the search for tell- tale clues to past life. On Earth, the most abundant clues to past life are indirect ones — geological strata high in biogenically deposited cal- cium carbonate, and best, those that contain organic matter. If similar layers of ancient calcium carbonate were discovered on Mars, it would be suggestive of an earlier life-bearing epoch. But, the definitive indi- rect proof would he discovery of deposits of ancient organic matter. Inasmuch as the present surface environment of Mars destroys organic matter as revealed by the Viking lander experiments, we need to know how deep do we have to go to get below this oxidizing layer. A me- ter? Ten meters? A hundred meters? That is a key element. Surely there must be on Mars somewhere relatively uncontaminated, unde- stroyed strata surviving from that early aqueous period. Such locations are what we want to find, especially near enough to the surface that advanced robotic drilling or other systems could search directly for organic material. The full exploitation of such sites could become the scientific objective of future human missions. Building for the Future Following Viking, and the extraordinary exploration of the outer Solar System with the two Voyager spacecraft, the U.S. planetary program collapsed. NASA had become completely committed to a technolog- ical objective, the Space Shuttle, rather than a scientific one. NASA became focused on the means rather than the ends. Space shuttle de- velopment overcame the agency. It was a bad choice to put a human in the loop for everything in space. The United States has abandoned that approach after Challenger, but it cost the U.S. space program nearly fifteen years of progress, The most critical damage was to robotic deep space missions that required the greatest propulsion to get to the planets. a7image not availableimage not availableimage not availableOwen B. Toon recognizing the tenuous nature of the atmosphere of Mars and the pos- sible need to terraform the planet to maintain life as we know it. A giant mountain fictionally mapped near the Martian equator presages the discovery of Mt. Olympus, the largest known volcano in the solar system. After Mariner 9 photographically mapped Mars, Carl Sagan showed that some of the famous canals pictured by the map actually exist. For oxample, the classical canal Agathodaemon, or Coprates (the only canal actually photographed from Earth), corresponds to the canyon named Valles Marineris. However, Valles Marineris is nearly as large as the United States, so it is more a grand-Grand Canyon than an aqueduct. Perhaps the aspect of Edgar Rice Burroughs’ map of Mars that inter- ests Carl the most is not the map itself, but the way it was supposedly obtained. Edgar Rice Burroughs’ hero, John Carter, traveled to Mars by simply lifting his arms. No long and lonely voyage or life support was needed. This idea sounds ridiculous, but truth being as strange as fiction, that is exactly what has been done over the past thirty years or so, An armada of robots has been sent by the nations of Earth to see, feel, and listen to the planets for us. During this spacecraft era we have peeked beneath the veil of mystery surrounding the planets. The planets are no longer mere figments of the imagination; they have become places. A casual glance at a Viking lander image of Mars, like the one shown in Figure 5.1, suggests that Mars might not be such a bad place to visit. People who enjoy a desert lifestyle might even find these scenes to be reminiscent of home and think to themselves, “Here is a great place to take a stroll.” However, such hasty judgments are dangerous. Before we step out on the red planet, we need to know what a visit to Mars actually would be like. We may not be ready to send humans to Mars, but NASA's Ames Research Genter and Arizona State University at Tempe operate a wind tunnel that simulates Martian conditions. Although it is large enough, you cannot stand inside the wind tunnel to see firsthand what Mars is like. However, you can peer through the window to see what the con- sequences of a walk on Mars might be. Some very interesting physical phenomena occur as the atmospheric pressure is reduced from terres- trial to Martian. A jar of water (imagine that this is the blood in your body) left standing inside the wind tunnel looks perfectly normal un- til pressures near those of Mars are reached. Then the water begins to boil furiously, but instead of hot steam it emits a frigid vapor that con- denses to little pellets of ico. Within a few seconds the boiling water freezes solid into a jagged, porous lump of ice. The surface pressure ‘on Mars is equivalent to that beyond an altitude of 100,000 feet above Earth's surface. If you ventured out to explore the Martian desert with- out a pressure suit, your body would literally explode as your blood boiled, killing you before you missed that first breath of oxygen that sustains us on Earth.image not availableimage not availableimage not availableOwen B. Toon our atmosphere. Therefore, the Martian atmospheric nitrogen cannot supply more than about 0.1% of the nitrogen contained in Earth's cur- rent and fossil biomass. Carbon dioxide is often thought of as the key material for making Mars more habitable. Mars’ atmospheric carbon dioxide supply is ca- pable of maintaining a plant world. However, if we wished to reheat Mars using carbon dioxide as a greenhouse gas, we would have to access geologic reservoirs. Unfortunately, the resulting carbon diox- ide atmosphere, which would have to have a total pressure exceeding Earth’s to yield Martian temperatures near freezing, would be toxic to humans even if oxygen were present. In fact, current Mars’ atmo- spheric carbon dioxide amounts are nearly toxic for humans. Green- house gases other than carbon dioxide will be needed if Mars is ever to be a home for unprotected humans. Oxygen, a key requirement for some forms of terrestrial life, is present only in limited quantities in the current Martian atmosphere, but it is plentiful in the soils. The terrestrial oxygen atmosphere was created in the last 25% of Earth’s history as a by-product of life. Oxy- gen availability does not limit our ability to terraform Mars. Terraforming Mars would be a massive project. A synthetic atmo- sphere would have to be produced and constantly maintained. We can envision several ways to possibly achieve such an atmosphere, but lack of knowledge of the mineral reservoirs on Mars prevents us from determining if any of these schemes is practical. Instead of searching for gold, or black gold like terrestrial miners, astronaut prospectors may seek the currency of terraforming-accessible carbon and nitrogen reservoirs in the Martian soil. Of course, planetary-scale terraforming is not the only way to make Mars habitable for humans, Astronauts may terraform small parts of Mars enclosed within protective shells, so that a human colony can be maintained, even if planetary-scale reengineering is just a distant dream. Science fiction writers correctly captured the essence of the Mar- tian environment in the first half of this century. They saw it as a vast desert, with a dwindling atmosphere. However, they envisioned our nearest neighbor, Venus, as a tropical paradise complete with humans and various prehistoric reptiles. But, Venus is more like Hades than the Garden of Eden. If you were a visitor to Venus on an approaching spacecraft you might see the pale yellow clouds as a soft, welcoming haze with var- ious interesting markings in the ultraviolet (Figure 5.3). However, a descent into the cloud tops, which extend nearly 70 kilometers above the surface, would reveal a mist of concentrated sulfuric acid that would badly burn yourskin if you were exposed to it. These clouds are similar to stratospheric volcanic clouds on Earth. A visit to the cloud tops might remind you visually of a smoggy day in Los Angeles. Asimage not availableimage not availableimage not availableOwen B. Toon VENUS EARTH MARS ° 1 2 DISTANCE FROM THE SUN VENUS EARTH ine 2 7 ° ‘SOLAR FLUX EARTH VENUS MARS 2 1 ° SOLAR ENERGY ABSORBED FIGURE 5.6 The relative positions of the planets are illustrated from three perspectives. At the top are the physical positions of the planets. in the middle are the relative amounts of solar energy available at the physical positions of the planets. At the bottom are the amounts of sunlight actually absorbed by the planets. At the bottom it can be seen that Venus, because of its highly rofl clouds, is effectively nearly as far from the sun as is Mars 300,000 times the amount of carbon dioxide in the atmosphere of Earth. Yot there is more to understanding the difference in surface temper- atures between the Earth and Venus than knowing the current levels of atmospheric carbon dioxide. Earth has just as much carbon stored away in the form of limestones in crustal reservoirs as Venus has stored in the form of carbon dioxide in its atmosphere. Figure 5.7 shows that the storage of carbon in different places on Earth is related to liq- uid water. The arrows mark the path taken by terrestrial atmospheric carbon dioxide as it dissolves in fresh and salt water, along with cal- cium ions, which then are used by organisms to form limestone in the oceans. Without liquid water the carbon would build up in our atmo- sphere as carbon dioxide, and Earth would quickly become overheated like Venus. Venus lost its water early in solar system history. Because of the high solar flux at the location of Venus, its original oceans of water vaporized into the atmosphere in a massive runaway greenhouse. At the top of the water-saturated atmosphere, water vapor dissociated into hydrogen and oxygen. The hydrogen escaped to space. So the position of Venus with respect to the sun is an important indirect factor in fixing its current temperature, because its position controlled the composition of the atmosphere.image not availableimage not availableimage not available64 The Origin of Life in a Cosmic Context CHRISTOPHER F. CHYBA: White House Office of Science and Technology Policy* The title of this essay is taken from a paper Carl Sagan published twenty years ago in the journal Origins of Life [Sagan 1974]. “The Ori- gin of Life in a Cosmic Context” was one in a series of publications by Carl on the planetary context for life. Beginning in 1961 with “On the Origin and Planetary Distribution of Life” [Sagan 1961], he continued with the landmark book written with losef Shklovskii, Intelligent Life in the Universe [Shklovskii and Sagan 1966] and the article “Life” in the Encyclopaedia Britannica [Sagan 1970]. The time period of these articles spanned the entire U.S.-Soviet Moon race. But the 1974 paper was special in being the first in that series that could take into account the results of the lunar exploration by the Apollo astronauts. The 1974 paper suggested three reasons why an extraterrestrial context was essential for understanding the origins of terrestrial life: Distinguishing the Contingent from the Necessary. Only the study of extraterrestrial life will allow us to avoid the dangers inherent in try- ing to reach general conclusions about the nature of life based on the single example provided by terrestrial biolog} found ignorance of exobiology, life is a solipsism, is no aspoct of contemporary biology in which we can distinguish the evolutionary accident from the biological sine qua non. We cannot distinguish the contingent from the necessary.” Does life really have to be based on proteins and DNA? Does it even have to be based on car- bon? The danger of parochialism lurks in every conclusion we might reach. “In our present pro- Carl wrote. “There *Current address: Department of Planetary Sciences, University of Arizona, Tucson. AZ85721 USA. "The ideas contained in this paper do not necessarily represent the views of the White House Office of Scionce and Technology Policy or the United States Government.image not availableimage not availableimage not availableChristopher F_Chyba which excavated the 200-kilometer diameter Chicxulub crater {Sharpton et al. 1992], was the impact of an object about the size of Halley’s comet — ten kilometers or so in diameter. It now seems quite likely that the K/T impact had a profound influence on the history of life on Earth. The fact that we are here - rather than, say, six-foot green reptiles ~ may be the result of this impact. By extrapolating the lunar cratering record to the Earth, it is clear that objects the size of the K/T impactor collided with Earth some 10,000 times during its first billion years. At tho time of the origins of life, say about 4 Gyr ago, Earth was sustaining such an impact every few hundred thousand years. ‘The lunar cratering record also reveals that the flux of impact- ing bodies increased in number as something like the 1.6 power of decreasing impactor diameter. Therefore, for example, Earth 4 Gyr ago would also have been experiencing the impact of a 1-kilometer diameter body every 5,000 years or so. Clearly, this was an extremely hostile environment. Darwin, in his famous 1871 letter to Hooker (reproduced, e.g., in [Hartman, Lawless, and Morrison 1985]), imagined that life had originated in some warm little pond. This is a peaceful image, leading one to envision quiet, serene conditions. But the Moon teaches us that this is not the right picture. The right picture is that life on Earth must have originated not in quiet, peaceful circumstances, but rather in extremely violent, impact-ridden ones. The largest craters on the Moon — big, multiringed structures grea- ter than 300 kilometers across — are called basins. We now know of about fifty such lunar basins [Wilhelms 1987], ranging in size up to the South-Pole Aitken basin, about 2.200 kilometers in diameter [Belton et al. 1992]. The largest basin that one can see on the lunar near side is Imbrium, about 1,160 kilometers across. Objects with radii in the range 50-150 kilometers are required to excavate basins of this size. With a terrestrial gravitational cross section nearly 25 times bigger than that of the Moon, and about 50 known lunar basins, it’s clear Earth sustained a large number of giant impacts. The most gigantic of these giant impacts would have had extremely grave consequences for life on Earth. Work by Maher and Stevenson [1988] and Sleep et al. [1989] suggests that the biggest might well have sterilized the surface of the Earth. In this case, the window available for the origin of life was only as wide as the time between the oldest geological evidence for life (either 3.5 or 3.8 Gyr ago) and the final ster- ilizing impact. With the heavy bombardment of the Moon not tailing off until around 3.8 Gyr ago (the Imbrium impact, incidentally, seems to have occurred 3.85 Gyr ago), the timescale for the origin of life on Earth shrinks from nearly a billion years down to around 100 Myr, or even less. The suggestion that the lunar bombardment evident in the Apollo results implied a narrower window for the origin of life thanimage not availableimage not availableimage not availableChristopher F. Chyba organics [McDonald and Bada 1995]. Bigger pieces of Mars, ten meters or more in size, could shield microorganisms from cosmic rays during interplanetary passage, so that it’s possible that microorganisms may have successfully migrated between Mars and Earth early in these planets’ histories. In the future, as we explore Mars, and as we look for signs of an ancient Martian biosphere, it will be precisely those fossils that are easiest to recognize as fossils — because of their similarities to terres- trial microfossils — for which it will be the hardest to tell whether they represent a separate Martian origin of life or merely resulted from a kind of inoculation of Mars by the Earth. (For that matter, they could even represent the now-extinct ancestors of terrestrial life, whose only living descendants are the progeny of those that survived the perilous journey to Earth.) Would the discovery of microscopic fossils that re- sembled terrestrial microorganisms tell us that life on other worlds must find the same solutions to the problems of early evolution? Or would it merely mean that the two worlds had exchanged organisms early in their history? It is not clear that fossil discoveries on Mars will necessarily help in providing the kind of perspective that Carl has asked for. That kind of perspective may be even more difficult to attain than we had hoped.” BIBLIOGRAPHY Alvarez, L. W., Alvarez, W. A., Asaro, F., Michel, H. V. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095-1108. Basaltic Volcanism Study Project 1981. 1981. Basaltic Volcanism on the Terres- trial Planets. New York: Pergamon Press. Belton, M. |. S., Head, J. W., Pieters, C. M., Greeley, R., McEwen, A. S., Neukum, G., Klaasen, K. P., Anger, C. D., Carr, M. H., Chapman, C. R., Davies, N. E., Fanale, F. P., Gierasch, P. J., Greenberg, R., Ingersoll, A. P., Johnson, T., Paczkowski, B., Pilcher, C. B., Veverka, J. 1992. Lunar impact basins and crustal heterogeneity: New western limb and far side data from Galileo. Science 255:570-576. Boston, P. J., Ivanov, M. V., McKay, C. P. 1992. On the possibility of chemosyn- thetic ecosystems in subsurface habitats on Mars. Icarus 95:300-308, Cech, T. R. 1993. The efficiency and versatility of catalytic RNA: Implications for an RNA world. Gene 135:33-36. Chyba, C. F., McDonald, G. D. 1995. The origin of life in the Solar System: Current issues. Annu. Rev. Earth Planet Sci. 24:215-249. *Note added in proof: Since this chapter was written, possible microfossils and other possible signs of life have been reported in the Martian meteorite ALH84001. These claims remain controversial. See McKay, D. S., Gibson, E. K., Thomas-Keprta, K. L., Vali, H., Romanek, C. S., Clemett, S. J., Chillier, X. D. F, Maechling, C. R., Zare, R. N, 1996. Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001. Science 273:924-930.image not availableimage not availableimage not available76 David Morrison the course of biological evolution is profoundly altered. The history of life on this planet, and by implication the origin of humans, is thus closely coupled to the impact history of the Solar System. This paper deals with a new paradigm: that life evolved on Earth in an environment punctuated by impact catastrophes. We have learned that our planet lives in a “bad neighborhood,” with occasional out- bursts of violence that have dramatically influenced our history. We are particularly interested in the contemporary hazard imposed by impacts and with proposals for ways to dea] with these risks. Carl Sagan has played an important role in the revolution we are discussing. As the world’s foremost exponent of planetary exploration, he has done much to stimulate missions to the planets, to plan the critical science that was done on those missions, and to interpret the results for the broadest possible audience. Carl has also played a lead- ing role in recognizing the importance of impacts and the existence of a contemporary threat. And he has been especially sensitive to the geopolitical implications of the impact hazard and the impetus this threat has given to possible defense schemes, many of which are po- tentially more dangerous then the natural hazard they are designed to mitigate. His unique combination of technical expertise and humane wisdom will be important for the public policy debates that will grow around the impact hazard and potential defensive systems. Lessons from the Comet Crash When Comet S-L 9 was discovered, some months after it had broken apart during a very close encounter with Jupiter, it already consisted of more than 20 fragments, each in orbit about Jupiter (Plate VIII). As a result of perturbations introduced by solar gravity, these orbits were distorted to yield a planetary collision the next time around. Over a period of about a week in July 1994, each of the fragments was doomed to impact the planet at 44° south latitude, just over the horizon as seen from the Earth. Although terrestrial telescopes could not image the actual entry and explosion from each impact, they would be well placed to see any large plume of material ejected above the impact site. And within a few minutes of each event, the impact site would be carried into direct view by the rapid rotation of the planet. In the months preceding the impacts, a number of models were developed to predict the consequences as material smashed into the planet at a speed of 70 kilometers/second, more than three times the reentry speed of the Apollo lunar capsules. The stress of deceleration in the Jovian atmosphere would lead to the rapid breakup of each im- pactor. Unfortunately, however, the sizes of the fragments could not be measured, leading to widely divergent predictions for the observ- able phenomena. Models of the tidal breakup of the comet during its 1992 flyby of Jupiter suggested that the largest fragments were no moreimage not availableimage not availableimage not availableimage not availableimage not availableimage not availableimage not availablea4 David Morrison Earth must have formed in the swirling debris of the solar nebula through a process of accretion of smaller bodies. High-velocity impacts generated heat, and eventually the upper layers of the planet melted to form a global ocean of liquid rock. At some point during the period of accretion, we were struck by another coalescing world about the size of Mars today — that is, with a mass about 10% that of the Earth. The smeller, Mars-sized planet was completely destroyed, and even the larger Earth was shattered to its core. Some of that ejected material continued to orbit the Earth as a giant ring, which cooled and collapsed to form our Moon. If the Moon-forming impact had been just a little larger, the Earth itself would have been disrupted. There may have been examples of such planetary collisions during the early days of Solar System history, but if so, the direct evidence is long gone. We do see other examples of planetary peculiarities, however, that are best understood as the product of random collisions. Venus spins in the opposite direction from its orbital motion about the Sun, probably as the result of a late collision that struck it a glancing blow and reversed its direction of rotation, and the small planet Mercury appears to be the metal-rich remnant of a larger parent, stripped of most of its rocky mantle in another giant collision. It is largely a matter of luck that the final prod- uct of this chaos was the four inner planets we have today: Mercury, Venus, Earth, and Mars, plus the Moon. ‘The building blocks of life came to Earth through impacts, as stud- ied by Chris Chyba (at the time at Cornell University) and his col- leagues. In the outer parts of the solar nebula, far from the Sun, tem- peratures were much lower, leading to abundant water ice and other frozen gases such as methane, ammonia, carbon dioxide, carbon mon- oxide, and even ethyl alcohol. The volatiles on Earth must be derived from this distant reservoir in the outer Solar System through cometary bombardmentin the last stages of accretion, which may have extended to several hundred million years after the birth of our planet. Most of the material of the biosphere — and of our own bodies — is comet-stulff, a gift from the outer Solar System. Were it not for this rain of ice and carbon compounds, our planet would be as dry and lifeless as the Moon. Life is a gift from the comets. But the gift did not come without a price to pay. As the rain of cometary materials persisted, the Earth (and presum- ably Mars and Venus as well) built up a thick atmosphere of carbon dioxide and other compounds and developed shallow oceans of liquid water, rich in dissolved organic materials. Such an environment is ex- actly what most scientists think was required for the origin of life. The first self-replicating molecules must have formed in these early seas, perhaps onall three planets. Ifall of the impacting comets were small, this environment might have approximated the “warm little pond” hypothesized by Charles Darwin for the origin of life. However, theimage not availableimage not availableimage not availableFrank D. Drake knowledge that must exist among the intelligent civilizations of our galaxy and other galaxies - information of a scientific and technical nature, as well as philosophical and biological, gathered not only over hundreds of years, as has been our experience, but over literally thou- sands, millions, even (I won't use the “B” word), thousands of millions of years. Surely from this would come a richness of knowledge, even wis- dom, that would help us achieve a higher quality of life, save our resources, give us sophisticated information we would like to have, and within years rather than hundreds of years, or thousands, which it might take us to achieve these same results through our own research. So those, in a nutshell, are the possible significant results of SETI, and of course they are of the most profound consequence and impor- tance to all of us. That is why those of us who have the privilege to work in SETI feel it is a very important project and we give ourselves to it. Now, these ideas are not new. They have been around a long time, as long as people have first understood, as Galileo did, that the other planets were in some way like the Earth. The ideas became even more widespread when we recognized that the stars were objects like our Sun. As the years have gone on, people have speculated or fantasized about life in the universe and particularly intelligent life. An example of early excitement about this subject is the front page from The New York Times, August 17, 1924. (Walter Sullivan is not responsible for this.) It contains drawings, totally wrong, done by Percival Lowell and this very chauvinistic statement that Mars invites mankind to reveal “his” secret. The following Saturday, there was to be a very close approach of Mars to the Earth, and all the world looked. Among all these observations were many attempts to detect signs of intelligent life on Mars. We read, for example, that from the top of the Jungfrau, Swiss astronomers were to flash alight signal to the planet with a giant helioscope. At the same time, there were many attempts to detect radio signals from Mars. Searches for extraterrestrial radio signals are not something solely of our era: they started actually back in the 1890s and reached a peak in 1914. On another front page of The New York Times, from seven days later, there is this announcement, front page news, “Radio Hears Things as Mars Nears Us.” We're supposed to be impressed, because it tells us that it took a twenty-four tube set in England to pick up the strong signals and that Vancouver, for some reason, was ofa special interest to the Martians! We don’t know why that is, and, in fact, the great cryptographer, William Friedman, who had broken the German codes in World War I, was standing by in Washington to decode the messages. Front page news. but as is typical, when it turned out those dots and dashes were from a radio transmitter in Seattle, that was quietly announced on page thirty, where one could read: “MarsExtraterrestrial Intelligence Sails By Us without a Word. Disappointment.” These events produced one of the false alarms that discredited not just searching for life in the universe, but planetary science as a whole for many, many decades. That science only recovered its prestige and escaped its taboo nature with the dawn of the Space Age. It is interesting to point out that at this very same time, something of very great importance to SETI was occurring. Again, ina front page of The New York Times, with the headline: “Talk with Aviator by Radio a Mile over Central Park,” and “Experimenter Catches Words above the Roar of the Airplane.” This may not seem very startling today, but if one looks at the engineering textbooks of that time, one finds that the physics of radio links was very fuzzy. The engineers believed that the design of an electromagnetic communication link required a transmit- ter anda receiver, each of which had a connection to the ground. There was this false concept that somehow radio signals were conducted in a sort of electrical circuit in which one leg of the circuit was in the Earth and the other was through the atmosphere; therefore, without that ground connection one could not communicate. Of course if that were true, it would have terrible consequences for SETI, because there would be no way one star could communicate with another. But we read that the flyer did broadcast directly from the plane. When you read this you find that there was a Lieutenant Connell, on his hands and knees in the open cockpit of this airplane, shouting into a mi- crophone so he could be heard over the roar of the Liberty engine. It was a great moment for SETI. He succeeded with no ground wires! ‘That was a very important breakthrough because it showed that radio didn’t need ground wires and so radio could communicate between the stars. The technological basis for SETI was established. We sense again the interest in the possibility of electromagnetic communication between the stars from a front page of The New York ‘Times, 1933, announcing the discovery of cosmic radio emission. Sur- prisingly, reporters and editors recognized the significance of Karl Jansky’s discovery of cosmic radio emission and put it on the front page. The headline reads, “New Radio Waves Traced to Contor of the Milky Way, Mysterious Static Recorded by K. G. Jansky Held to Differ from Cosmic Ray.” The lowest headline says, “Only Delicate Receiver is Able to Register; No Evidence of Interstellar Signaling.” Even then, they wondered if perhaps radio was the means by which we might contact other civilizations. All of these early ideas were very speculative and they were not properly scientific in that there were no quantitative calculations made whatsoever of, say, what power levels were required to make a de- tectable signal over what distance. These were really hand-waving ideas. It was only in the 1950s and early 1960s when people started to approach the matter in a proper scientific way, in the manner first done in print by Philip Morrison and Guiseppe Cocconi at Cornell.Frank D. Drake Another watershed event at that time was the publication of the book by Carl Sagan and Iosef Shklovskii on SETI, a monumental book that has become a classic. Here Carl took a rather narrowly written book by Shklovskii, written in the context of the limited state of knowledge of the Soviet Union, and added to it a great breadth of scientific and technical knowledge. This created what, to this day, is a handbook for the quantitative analysis of what it might take to detect life elsewhere in the Solar System. So, in the late 1950s, early 1960s, finally, this subject became one that had a proper quantitative scientific basis. We entered what might be called the modern era of SETI. Now, along the way, many discoveries have stimulated interest in this subject and have indicated or provided growing evidence that in- telligent life can be expected to be ubiquitous in the Milky Way and in other galaxies. We've learned that the galaxy is fifteen billion years old. That there are four hundred billion stars, many like the Sun. We have learned how star formation proceeds and that in the clouds of gas and dust that form the stars, we have the chemical precursors of life on Earth. There have been discussions in the previous chapters about these materials being brought to Earth by the comets. We've seen evidence of other solar systems, although not yet another solar system just like our own. A great deal of circumstantial evidence suggests that planetary systems and planets like the Earth are extremely common. We've seen in the planets of the Solar System themselves a great abun- dance of the organic molecules and other materials likely to give rise to life, and even to intelligent life. We know about tantalizing Titan, motivating us, suggesting that the abodes of life are very abundant in the Milky Way and therefore that searches for life are of great value. SETI has significance in areas that are less profound than the ones I first mentioned. It has provided grist for the entertainment industry. ‘The best-selling movies of all time have been based on ideas of SETI. Even now, Carl is working on such a movie, Contact, which will be a source of entertainment, but will also build understanding in the pop- ulation at large about the possibilities of contact with extraterrestrial life. That helps us in carrying out our search. SETI has also turned out to be a wonderful magnet to attract young people to an interest in science. In many places, including our SETI Institute, curriculum materials based on SETI are being developed. These turn out to work extremely well with young people in the real world, because young people are very interested in life in the universe. Once they find out that to understand SETI you have to lean about organic chemistry, or atmospheric structures, and so forth, they're willing to study those things that might otherwise be boring. The existence of SETI activity has stimulated much theoretical work about the propagation of radio waves in the galaxy and more speculative theoretical work, some of it good, some of it not so good. which has to do with the nature and behavior of life in the universe.Extraterrestrial Intelligence SETI is a source of pride in cultures and countries, because we rec- ognize that the pursuit of extraterrestrial intelligent life is one of the most noble, if we can use that word, of human tasks. Both America and the Soviet Union have taken great pride in it, and so have states like West Virginia. A pamphlet for potential tourists to that state points out “buried treasures.” It describes the National Radio Astronomy Obser- vatory as a place where “they listen for signs of life in outer space.” ‘You might think they don’t do anything else there, according to this pamphlet! They just listen for signs of life in outer space! Actually, no major program of this kind has been carried out there in the last twenty years! Other impacts of significance: SETI brings out the very best in people and attracts some of the most talented of our scientists and engineers. Among these is Dr. Jill Tarter, who is the senior project scientist for Project Phoenix, the new name for what was the NASA SETI Search. That search, having lost its federal funding, has become privately based project operated by the SETI Institute. Jill ‘Tarter’s orig- inal name was Jill Comell, and in fact she is a direct descendant of the founder of Cornell University, Ezra Cornell, and got her education in engineering physics at Cornell. Dr. Kent Cullers, Ph.D. in physics, is in charge of the signal detection systems for Project Phoenix, a very daunting task. In Project Phoenix there are some fifty-six million chan- nels of information providing data. Kent Cullers’ task is to find algo- rithms that can search through this in real time for signs of intelligent signals, yet in an affordable way. He has been blind since birth, yet he carries out the most abstruse, difficult mathematics in his head, all the mathematics connected with communication theory and com- puter algorithms. Dale Corson may be surprised to be mentioned in connection with SETI, but he has been important through his role in the construction and the upgrading of the Arecibo telescope reflector. He helped make what is to this day and for the next few decades prob- ably by far the largest radio telescope in the world. It is the preeminent telescope for SETI, the one that gives us our greatest power, the one that we wish to use the most. Project Phoenix has contributed about two million dollars to the further upgrading of Arecibo now under way. Dale Corson and the other people at Cornell have helped in other ways. For example, Jill Cornell wanted to study engineering physics at Cornell. In the will of Ezra Cornell it was specified that all male descendants of Ezra Cornell shall be admitted to Cornell and will at- tend free of all tuition. Jill inquired into this and its lack of political correctness, and of course there was a great deal of discussion in the back room about what to do. In the end, Dale Corson, as president, rec- ognized the unfairness of that stipulation in the will, and quietly took care of it all. As a result, Jill did attend Cornell on a full scholarship and that was an important turning point for SETI, because without Jill, SETI would be a much weaker endeavor. 51FIGURE 8.1 Radio receivers used during the 1960 Ozma Project. Frank D. Drake ‘As has become obvious, the name Cornell pops up repeatedly. Many of the people active in SETI had their roots there: Phil Morri- son, Carl Sagan, jill Tarter, myself, and others. This says, if you think about it, that it's not really a result of chance but the result of the fact that at Cornell University creativity and original ideas have been nurtured and protected. Cornell has protected and nourished the pi- oneers, the people who have interesting but unusual ideas, and from that has come much of the university's greatness, not just in SETI, but in many other areas. What is another significance of SETI? SETI works as a motivator for the development of the very best in frontier technology, particu- larly in radio receiving systems. The first modern SETI system, used in 1960 in Project Ozma, was rather simple. Even so, it cost $2,000 and occupied four banks of equipment, all using vacuum tubes, by the way. See Figure 8.1. This was before the transistor was invented. Itcould monitor one channel at atime. The people who did this exper- iment are shown in Figure 8.2 at a reunion of a few years ago in front of the 85-foot telescope where it took place. In the thirty-four years since Project Ozma, other activities have joined SETI in motivating the development of much better technology such as the Arecibo tele- scope, which has one hundred times the collecting area of that of the Ozma telescope. A picture of its great reflector is shown in Figure 8.3,Extraterrestrial Intelligence ag FIGURE 8.2 ‘The Tatel 85-foot radio telescope in Green Bank, West Virginia, and the team that was involved with the Ozma Project in 1960 (the author of this article is second from the right standing). the reflector reflecting rays to the suspended platform, which is right now undergoing major alterations. With the construction of such telescopes SETI has been given the ability to detect signals from anywhere in the galaxy, signals no stronger than we radiate. Many years ago it was recognized that the real challenge to SETI was not sensitivity so much, but the ability toimage not availableExtraterrestrial Intelligence search through all the possible frequency channels in the microwave spectrum where signals might occur. That has led to the application of some of the most brilliant minds on the planet, and an article is presented by ono in the next chapter, to the development of systems, affordable systems, which can achieve that task. A picture of one of the very first is in Figure 8.4, using digital circuitry in this case to develop a system that monitors about 68,000 channels at once. This system was produced by Paul Horowitz, using mostly very simple off-the-shelf components. Over the years, it has led to much more powerful and complicated systems. These require special-purpose computer chips that have been developed at the cost of about $300,000 for the first one. After that, they cost $50 each, and they are about the size of a postage stamp. They do eighty million flops and 220 million data transfers a second, all to simulate a multichannel radio receiver. One such chip has the same power as a Cray 1 supercomputer programmed to do the same task. We have now incorporated these in our systems, forty- seven of them, so we have in effect forty-seven Cray supercomputers to produce the instruments of Project Phoenix. The equipment again occupies four racks, just like the Ozma receiver did, but in this case the total number of channels is fifty-six million. See Figure 8.5. This system is 100 trillion times more powerful than what we had thirty- four years ago. In fact, the doubling time for improvements has been consistently 250 days; exponential growth has continued. The Project Phoenix equipmentis in atrailer that can be carried ina large transport plane. See Figure 8.6. Right now it is being readied to go to Australia for six months of observations with the Parkes Telescope, the last chance FIGURE 8.4 A digital system that can monitor 68,000 spectral channels at once.936 Frank D. Drake —— FIGURE 8.5 The fity-six- million channel receiver used in Project Phoenix. FIGURE 8.6 The NASA trailer containing the Project Phoenix complex instrumentation ready to be shipped to the Parkes radiotolescope in AustraliaExtraterrestrial Intelligence for many years to observe the southem candidate stars where we might find an extraterrestrial intelligence signal. SETI people are motivated to dream of much bigger systems. We dream of great systems on Earth, as may be built some day as the significance of SETI becomes more widely recognized. Perhaps there will be SETI in space; systoms have boon in the eyes of the designers for many years now. The ultimate dream is that we will achieve those very profound results I mentioned earlier. Lastly, there is one other significant aspect of SETI that Carl has emphasized many times, which comes about when we do SETI and construct messages such as the Pioneer 10 plaque. Carl and I invented that one day during a coffee break in a corridor at a American Astro- nomical Society meeting (I probably shouldn't reveal that; I probably should say there were endless committee meetings and white papers and planning documents, but that’s not so; it all happened over coffee in a hotel). When we not only search but send messages to space, we are actually sending messages to ourselves, reminding us of what hu- man beings can achieve if they work together and use their talents in the very best possible way. SETI sends a message to us that humans can achieve great things. If we work hard enough we can join the com- pany of other creatures in space and receive all of the great bounty that will accrue when, finally, the discovery is made. a7Extraterrestrial Intelligence: The Search Programs PAUL HOROWITZ Harvard University The topic, “Extraterrestrial Intelligence: The Search Programs,” should also include the searchers — you can’t really disembody the personali- ties who motivated these searches and the people who did them from the searches themselves. I'dlike tohighlight some of the giants of SETI, and their predecessors — Hertz, Jansky, Purcell - names you don’t hear so much nowadays. That means many pictures of people, their anten- nas, and their equipment, and only a few equations or graphs. In fact, let’s get the equations over with; Figure 9.1 illustrates SETI Fact Number One: SETI is possible because, as each of us who have become delighted with the subject has discovered rather early on, radio communication is extraordinarily efficient. All of the equations crammed into one figure. It’s an old calculation that those of us in this business have all done, in one form or another: You take a pair of modest-sized radio telescopes, a couple of hundred meters in diameter (that’s smaller than the Arecibo dish, though not by a lot); you space these apart a modest distance, let’s say a thousand light years; you transmit a three-centimeter wave, say; and you ask, for a given amount of energy transmitted, how much is received? Is this crazy scheme going to work? You calculate transmitted energy, areas, gains, and all that sort of thing. And when you're done, you find that a dollar’s worth of energy transmitted results in 4 x 10-1? ergs of energy received out there. Now you should ask yourself, “Well, ten to the minus 12 ergs, not very many ergs, and ergs are pretty darned small; is this even detectable?” Well, you'd be right, it’s nota lot of energy; but the crucial point is that the energy that it has to compete with is even less. The received signal has to compete with cosmic noise, antenna noise, amplifier noise, and so on. How large are these? The delightful fact is that, at these microwave frequencies, you can build receiving systems with large apertures (Arecibo-sized) whose equivalent thermal noise temperatureExtraterrestrial Intelligence ss Galactic Communication via Microwaves? vaaoal je sh ‘$1 = 1OKWh = 4x10’wart-sec = 4x10" erg A (3) = 3x(104F = 3x108om? = BRA, 4x3x3010° * 4x i 7 a 10" (86dB!) & Re Aly, = 3x10"%3%107 = 10!em; 10001y. = 10em 410x410 acdc! 4x3x107 So, $1 transmitted = ugg * 10-er Well? Competes with thermal noise: k — 1.4%10" eng/deg at = 3 om, can achieve T,, = SOK-100K. (sky is cold) 50, KT ~ I FIGURE 9.1 Received energy is =400 kT! Asimple oo calculation If we choose bit rate & integration time so that each bit = 15 kT, demonstrates that na 5 bitsleter then we get ~ 25 “biwtuek: fe. communication by In other words... microwaves is “Interstellar Telegrams Cost $1/Word” extraordinarily efficient. is of the order of a hundred Kelvin or less. And that includes the combined noise contributions of the dish, feeds, amplifier — the works. It is an astronomical fact that the sky itself is extremely cold at these frequencies (which, of course, motivates this choice of wavelength). So, finally, the thermal fluctuation energy that this message has to compete with is something like 10" ergs, four hundred times smaller than the message itself. In other words, a dollar's worth of energy is100 FIGURE 9.2 Even with technology no betier than we have now, galactic ‘communication is a bargain (after E. M. Purcell). Paul Horowitz INTERSTELLAR TELESENS ARE CHEAP Everey RAwATID PR BIT TRAMSHETTED + 0% kWh; cosr PER weRD: FI.00 all it takes. For a dollar you've got four hundred times the fluctuation energy at the receiving end. You pick your bit rate and coding so that each bit is received at fifteen times the noise, say, and that gives you something like twenty-five bits per buck received at the far end. If you group these into letters of five bits each (don’t use ASCII; use Baudot!), and if you speak in words of one syllable, you wind up with the conclusion that interstellar telegrams are cheap (Figure 9.2, an old slide of Ed Purcell’s). This is a remarkable, and essential, fact of SETI - that an interstellar telegram out to a thousand light years costs a mere dollar per word, and that's using only technology that we have on earth now. In fact, we have even larger dishes. The largest on earth is the great 1,000-foot Arecibo dish, with double the area of those I have been discussing. With transmitters and receivers no better than we have right now, two of these things could communicate anywhere across the galaxy (assuming, of course, that they knew to point at each other and to use the same wavelength). This fact is, for most people, coun- terintuitive. Fortunately for SETI, our galaxy is endowed with an ample supply of candidate life sites; it is shown (Figure 9.3), somewhat simplified. It contains something like four hundred thousand million stars, in various colors. It’s a flattened disk a hundred thousand light years in diamoter; we're out in the galactic suburbs, so to speak, at about thirty thousand light years. The Sun is in the plane, as shown, in the middle of a more or less uniform distribution of stars; there are roughly a million stars like the Sun within a thousand light years. That is the range within which we can communicate for a dollar per word.image not availableaa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.106 Paul Horowitz FIGURE 9.7 ‘The first antenna on earth to detect radio waves from neutral hydrogen, in space. informed by radioscience and communication theory — the first search that could have succeeded. Among several legacies of Frank's experi- ment is the apparent mandate that SETI apparatus always occupy four racks, I'll demonstrate this curious effect in subsequent photographs. This opened the floodgates (or perhaps I should say the tricklegates) for subsequent searches, most of which have been done at the hydro- gen hyperfine frequency, or its near relatives. The traditional reason one always hears is that signaling is most efficient in the centimetric region of the spectrum (and it is). To be historically accurate, however, we should note that Frank chose hydrogen for a different reason: He wanted deniability ifaccused of wasting government money for crazy research — this was, after all, a radioastronomy receiver. But this is in fact the efficient wavelength regime. Drake showed that there is no obvious advantage to short or long wavelengths, on the basis of power delivered to a distant observer. What matters, of course, is the competition — the galactic and atmospheric noise backgrounds, which clearly favor contimetric wavelongths if you believe that an efficient strategy is optimum. Let's take now a quick tour through a rogue's gallery of searchers. Palmer and Zuckerman were persistent guys using the NRAO 300- foot radio telescope. They did 500 hours of searching, 674 stars, 21aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.Extraterrestrial Intelligence 114 Galactic Map of Candidate Events as of September 1994 + Events seen on 2 different days (barycentric reference frame) + - Events seen on 2 different days with 2 detections per observation (no reference frame assumptions) 5 ‘ cae FIGURE 9.11 sky visible from Arecibo; in fact, many positions are observed three, Peunat cents four, or five times. The hardware is a four-megachannel Fourier spec- from two yoars of trometer, observing at 430 megahertz. At this frequency one gets lots SERENDIP It's of interference. parasitic operation ‘The idea is to take these seventy trillion signals coming in, and & Arecibo. you look for regularities in frequency and space and time, rejecting everything that smells like a rat: signals that stay on too long while the antenna’s moving, signals that keep coming back at the same fre- quency because they're from some local transmitter, and so on, and in the end you wind up with a residuum of a few hundred interesting events. Figure 9.11 shows the result of two years of running, about two hundred fifty events that were seen on more than one occasion, The statistics happen to be just at the level of chance, and none of these has been seen three times (a single such instance of which would be extremely improbable). This project was done at low cost and with great elegance of design and execution. Meanwhile, at Ohio State University (the antenna that looks like a football field), the longest-running search has just increased its chan- nel count by a factor of 100,000, with the receipt of a four-megachannel system from Berkeley. A photograph of their upgraded receiver is in Figure 9.12, sitting in the leftmost rack of the obligatory four racks of equipment. Perhaps the most tantalizing candidate signal in SETI came from Ohio's search, the celebrated WOW signal, which displayed an uncanny match to the antenna’s beam profile as it drifted through.112 FIGURE 9.12 Four-megachannel Berkeley spectrometer at Ohio State University observatory. Paul Horowitz This event has now occupied hundreds of observing hours in futile attempts at reacquisition. These occasional wonderful signals have been seen by just about every search, invariably operating in a mode that makes immediate reobservation impractical. At the end, I will comment on what should be done about things like that. Hore's another ongoing experiment (Figure 9.13), the infrared ap- paratus of Charlie Townes and Al Betz, looking for carbon dioxide laser beacons. Charlie has always favored infrared and thinks the rest of us are missing out on a good bet. This is elegant physics — they use true coherent detection, in an optical heterodyne system. Finally, our search (META) at Harvard and Buenos Aires, spon- sored primarily by The Planetary Society. My son Jake (Figure 9.14), when he was six years old, is pointing to the Cassegrain radome of the equatorial 84-foot dish at Harvard, Massachusetts. Our apparatus is shown in Figure 9.15, with its eight million channel analyzer, turned on in 1985 with some fanfare (Figure 9.16). What you are seeing, from left to right, is 1) the switch that would turn it on if anybody had a hand on it; 2) the fellow who gave the money to build the stuff that the switch turns on; 3) the fellow who talked this guy into giving the money, and 4) one of the guys who built it A second identical system (META Il) was built by the Argentines, who shipped it to their 30-meter dish, where it is performing a search of the southern sky, and also a set of simultaneous observations of the portion of the sky visible from both observatories. What have we found with these searches? Things like the WOW event, things that go bump in the night, things that never come back.Extraterrestrial Intelligence For instance, in Figure 9.17 is the result of the META search at Harvard, 10" channels examined, during five years of continuous observations. Strong events that survive all tests for fishiness are plotted in this sky plot (reported in Astrophysical Journal, 415, 218 [1993]); the larger points are far too strong to be due to chance, whereas the smaller ones are consistent with the statistical noise tail. What may be rather inter- esting is the clustering of the five strongest signals in an apparently nonrandom arrangement relative to the galactic plane. We've spent many, many days on each one of these, in an attempt at reobservation, and we haven't gotten any to come back. It’s like the cargo cults — we've deployed telescopes, and we'd do anything (even build airstrips!) to make those signals come back. What can one conclude from three decades of negative results in SETI? Of course, it would be much better to have found something — we in the SETI business all wish we had magical results like those that Ed Stone showed this morning from the spectacularly successful program of planetary exploration. But in SETI you either get totally magical results or nothing at all. Carl and I made an effort to extract the most from the “nothing at all” of our search, and it goes something like this: If you believe that there may be supercivilizations, in the Kardashev sense (ones that extract all the energy of sunlight falling on 113 FIGURE 9.13 ‘Ten-micron infrared heterodyne SETI at Mount Wilson.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.Extraterrestrial Intelligence 117 tosen| 0 | O FIGURE 9.17 Surviving events from 5 years of META. Filled circles are 1.4 GHz, open ciscles are 2.8 GHz: the five strongest events are shown as larger circles. The center of the Milky Way is indicated by an X. West (following) East ‘Antenna, (preceding) pattern, ‘Apparent motion of celestial radio source 2 2 FIGURE 9.18 z Dual-beam transit 3 SETI. A third § channel, fed from a low-gain all-sky discone, provides a velo. Time = shown in Table 9.2. With a table like this we can outrun even Frank's optimistic projections. Kent even used Drakian units - ozmas and oz- mas/second (Ozma kept Frank busy for 2 months, back in 1960) — listed as logarithms, because the gains have been so spectacular. You can see that the current and future systems. depending on the kind of signal that you posit, are something between seven and fourteen or- ders of magnitude improved over Ozma in sensitivity. That's not bad for thirty years.118 Paul Horowitz FIGURE 9.19 ‘Two hundred fifty- million-channel spectometer for SETI. BETA operates et 40 billion operations per second, contains 3.4 gigabytes of RAM serving over 200 processors, and produces 250 MByte per second of spectral data. A last comment about these improving searches: we've all learned from these once-a-month or once-in-the-middle-of-the-night events that we've got to include much better schemes for the mitigation of ra- dio frequency interference, occasional errors in the processors, and so on; and we have to include means for quick follow-up, whether there's an operator at the controls or not. All of us pursuing the SETI enter- prise now know this well. Just to give an example, in our new BETA system we've implemented a system with three horns, so that the sig- nal has to display proper sidereal drift through the sky horns while remaining undetected in the terrestrial antenna; BETA also uses re- dundant processors, and it uses extensive parity checking. If a signal passes these tests, the system responds by leapfrogging the antennaaa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a page that is unavailable for viewing or reached your viewing limit for this book.aa You have either reached a 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Peete eMC me Mu Mu ea Meme eRe em UL rte Mea Rte eee a as ad CCS CMe MS Mee em MeL Y earls eM le Oe Me CUM) Lele ee er seer el eo ae Te omer] en eee Ce ae eur meen Ma ste ce Re ra of articles by a distinguished team of authors, and covers the many fee eee et UN meu eee Raia) Beans Deen Re OR eon Te Re ot Om I eal Coe) Ce ae eRe On te Me RCM Co] EU eu Comte cae Mr) Ree ee ROR Mee ie Mis rec eM ato Coe eRe oie Oe RES ie Kd Rec ne ee i Rr ue ht ae one) eee ca ere Ua oats Rae eeu ie Caan Rr Rank an ce PTC uke SCS ae mL aC een mee MCs Rim Imei ae) exploration, the search for extraterrestrial life, or the role of science in Dees ane e Ce oe Me MN Sel: MAC LCL e eee LteLie esc oh LoL Orr eMC sae Mae i ae Me UM cum re] Ce USC TCC amn Nm co RN aM uum ae mar ace RMU aes UST a eee SOI RC eum aRe ek mcs D 5 Cover photo: NASA Hubble Space Telescope photograph P e OR eee] A J a * * CAMBRII UNJVERSITY I ee I) , G3 3 aaah) ISBN 0-521-57603-2 9"780521"576037