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Ballantine Books Edition, September 1997 ISBN: 0-345-37659-5 Scanned: December, 2000 V.1.0 Formatted for viewing in Word 97 CARL SAGAN PALE BLUE DOT A V I S I O N O F T H E H U M A N F U T UR E I N S PA C E F O R S A M Another wanderer, May your generation see Wonders undreamt. 2 SPACECRAFT EXPLORATION OF THE SOLAR SYSTEM NOTABLE EARLY ACHIEVEMENTS UNITED STATES 1958 First scientific discovery in space-Van Allen radiation belt (Explorer 1) 1959 First television images of the Earth from space (Explorer 6) 1962 First scientific discovery in interplanetary space -direct observation of the solar wind (Mariner 2) 1962 First scientifically successful planetary mission (Mariner 2 to Venus) 1962 First astronomical observatory in space (OSO-1) 1968 First manned orbit of another world (Apollo 8 to the Moon) 1969 First landing of humans on another world (Apollo 11 to the Moon) 1969 First samples returned to Earth from another world (Apollo 11 to the Moon) 1971 First manned roving vehicle on another world (Apollo 15 to the Moon) 1971 First spacecraft to orbit another planet (Mariner 9 to Mars) 1973 First flyby of Jupiter (Pioneer 10) 1974 First dual-planet mission (Mariner 10 to Venus and Mercury) 1974 First flyby of Mercury (Mariner 10) 1976 First successful Mars landing; first spacecraft to search for life on another planet (Viking 1) 1977 First flybys of Saturn (Pioneer 11) 1981 First manned reusable spacecraft (STS-1) 1980- First satellite to be retrieved, repaired, 1984 and redeployed in space (Solar Maximum Mission) 1985 First distant cometary encounter (International Cometary Explorer to Comet Giacobini-Zimmer) 1986 First flyby of Uranus (Voyager 2) 1989 First flyby of Neptune (Voyager 2) 1992 First detection of the heliopause (Voyager) 1992 First encounter with a main-belt asteroid (Galileo to Gaspra) 5 WANDERERS: AN INTRODUCTION But tell me, who are they, these wanderers . . .? —RAINER MARIA RILKE, "THE FIFTH ELEGY" (1923) We were wanderers from the beginning. We knew every stand of tree for a hundred miles. When the fruits or nuts were ripe, we were there. We followed the herds in their annual migrations. We rejoiced in fresh meat. through stealth, feint, ambush, and main-force assault, a few of us cooperating accomplished what many of us, each hunting alone, could not. We depended on one another. Making it on our own was as ludicrous to imagine as was settling down. Working together, we protected our children from the lions and the hyenas. We taught them the skills they would need. And the tools. Then, as now, technology was the key to our survival. When the drought was prolonged, or when an unsettling chill lingered in the summer air, our group moved on—sometimes to unknown lands. We sought a better place. And when we couldn't get on with the others in our little nomadic band, we left to find a more friendly bunch somewhere else. We could always begin again. For 99.9 percent of the time since our species came to be, we were hunters and foragers, wanderers on the savannahs and the steppes. There were no border guards then, no customs officials. The frontier was everywhere. We were bounded only by the Earth and the ocean and the sky—plus occasional grumpy neighbors. When the climate was congenial, though, when the food was plentiful, we were willing to stay put. Unadventurous. Overweight. Careless. In the last ten thousand years—an instant in our long history— we've abandoned the nomadic fife. We've domesticated the plants and animals. Why chase the food when you can make it come to you? For all its material advantages, the sedentary life has left us edgy, unfulfilled. Even after 400 generations in villages and cities, we haven't forgotten. The open road still softly calls, like a nearly forgotten song of childhood. We invest far-off places with a certain romance. This appeal, I suspect, has been meticulously crafted by natural selection as an essential element in our survival. Long summers, mild winters, rich harvests, plentiful game—none of them lasts forever. It is beyond our powers to predict the future. Catastrophic events have a way of sneaking up on us, of catching us unaware. Your own life, or your band's, or even your species' might be owed to a restless few—drawn, by a craving they can hardly articulate or understand, to undiscovered lands and new worlds. Herman Melville, in Moby Dick, spoke for wanderers in all epochs and meridians: "I am tormented with an everlasting itch for things remote. I love to sail forbidden seas . . ." To the ancient Greeks and Romans, the known world comprised Europe and an attenuated Asia and Africa, all surrounded by an impassable World Ocean. Travelers might encounter 6 inferior beings called barbarians or superior beings called gods. Every tree had its dryad, every district its legendary hero. But there were not very many gods, at least at first, perhaps only a few dozen. They lived on mountains, under the Earth, in the sea, or up there in the sky. They sent messages to people, intervened in human affairs, and interbred with us. As time passed, as the human exploratory capacity hit its stride, there were surprises: Barbarians could be fully as clever as Greeks and Romans. Africa and Asia were larger than anyone had guessed. The World Ocean was not impassable. There were Antipodes.* Three new continents existed, had been settled by Asians in ages past, and the news had never reached Europe. Also the gods were disappointingly hard to find. The first large-scale human migration from the Old World to the New happened during the last ice age, around 11,500 years ago, when the growing polar ice caps shallowed the oceans and made it possible to walk on dry land from Siberia to Alaska. A thousand years later, we were in Tierra del Fuego, the southern tip of South America. Long before Columbus, Indonesian argonauts in outrigger canoes explored the western Pacific; people from Borneo settled Madagascar; Egyptians and Libyans circumnavigated Africa; and a great fleet of ocean going junks from Ming Dynasty China crisscrossed the Indian Ocean, established a base in Zanzibar, rounded the Cape of Good Hope, and entered the Atlantic Ocean. In the fifteenth through seventeenth centuries, European sailing ships discovered new continents (new, at any rate, to Europeans) and circumnavigated the planet. In the eighteenth and nineteenth centuries, American and Russian explorers, traders, and settlers raced west and east across two vast continents to the Pacific. This zest to explore and exploit, however thoughtless its agents may have been, has clear survival value. It is not restricted to any one nation or ethnic group. It is an endowment that all members of the human species hold in common. Since we first emerged, a few million years ago in East Africa, we have meandered our way around the planet. There are now people on every continent and the remotest islands, from pole to pole, from Mount Everest to the Dead Sea, on the ocean bottoms and even, occasionally, in residence 200 miles up—humans, like the gods of old, living in the sky. These days there seems to be nowhere left to explore, at least on the land area of the Earth. Victims of their very success the explorers now pretty much stay home. Vast migrations of people—some voluntary, most not— have shaped the human condition. More of us flee from war, oppression, and famine today than at any other time in human history. As the Earth's climate changes in the coming decade. there are likely to be far greater numbers of environmental refugees. Better places will always call to us. Tides of people will continue to ebb and flow across the planet. But the lands we run to now have already been settled. Other people, often unsympathetic to our plight, are there before us. * * * * "As to the fable that there are Antipodes," wrote St. Augustine in the fifth century, "that is to say, men on the opposite side of the earth, where the sun rises when it sets to us, men who walk with their feet opposite ours, that is on I'll ground credible." Even if some unknown landmass is there, and not just ocean, "there was only one pair of original ancestors, and it is inconceivable that such distant regions should have been peopled by Adam's descendants.'' 7 LATE IN THE NINETEENTH CENTURY, Leib Gruber was growing up 111 Central Europe, in an obscure town in the immense, polyglot, ancient Austro-Hungarian Empire. His father sold fish when he could. But times were often hard. As a young man, the only honest employment Leib could find was carrying people across the nearby river Bug. The customer, male or female, would mount Leib's back; in his prized boots, the tools of his trade, he would wade out in a shallow stretch of the river and deliver his passenger to the opposite bank. Sometimes the water reached his waist. There were no bridges here, no ferryboats. Horses might have served the purpose, but they had other uses. That left Leib and a few other young men like him. They had no other uses. No other work was available. They would lounge about the riverbank, calling out their prices, boasting to potential customers about the superiority of their drayage. They hired themselves out like four-footed animals. My grandfather was a beast of burden I don't think that in all his young manhood Leib had ventured more than a hundred kilometers from his little hometown of Sassow. But then, in 1904, he suddenly ran away to the New World to avoid a murder rap, according to one family legend. He left his young wife behind. How different from his tiny back-water hamlet the great German port cities must have seemed, how vast the ocean, how strange the lofty skyscrapers and endless hub-bub of his new land. We know nothing of his crossing, but have found the ship's manifest for the journey undertaken later by his wife, Chaiya joining Leib after he had saved enough to bring her over. She traveled in the cheapest class on the Batavia, a vessel of Hamburg registry. There's something heartbreakingly terse about the document: Can she read or write? No. Can she speak English? No. How much money does she have? I can imagine her vulnerability and her shame as she replies, "One dollar." She disembarked in New York, was reunited with Leib, lived just long enough to give birth to my mother and her sister, and then died from "complications" of childbirth. In those few years in America, her name had sometimes been anglicized to Clara. A quarter century later, my mother named her own firstborn, a son, after the mother she never knew. OUR DISTANT ANCESTORS, watching the stars, noted five that did more than rise and set in stolid procession, as the so-called "fixed" stars did. These five had a curious and complex motion. Over the months they seemed to wander slowly among the stars. Sometimes they did loops. Today we call them planets, the Greek word for wanderers. It was, I imagine, a peculiarity our ancestors could relate to. We know now that the planets are not stars, but other worlds, gravitationally lashed to the Sun. Just as the exploration of the Earth was being completed, we began to recognize it as one world among an uncounted multitude of others, circling the Sun or orbiting the other stars that make up the Milky Way galaxy. Our planet and our solar system are surrounded by a new world ocean the depths of space. It is no more impassable than the last. Maybe it's a little early. Maybe the time is not quite yet. But those other worlds— promising untold opportunities—beckon. In the last few decades, the United States and the former Soviet Union have accomplished something stunning and historic—the close-up examination of all those points of light, from Mercury to Saturn, that moved our ancestors to wonder and to science. Since the advent of successful interplanetary flight in 1962, our machines have flown by, orbited, or landed 10 C H A P T E R 1 YOU ARE HERE The entire Earth is but a point, and the place of our own habitation but a minute corner of it. —MARCUS AURELIUS, ROMAN EMPEROR, MEDITATIONS, BOOK 4 (CA. 170) As the astronomers unanimously teach, the circuit of the whole earth, which to us seems endless, compared with the greatness of the universe has the likeness of a mere tiny point. —AMMIANUS MARCELLINUS ACA. 330-395, THE LAST MAJOR ROMAN HISTORIAN, IN THE CHRONICLE OF EVENTS The spacecraft was a long way from home, beyond the orbit of the outermost planet and high above the ecliptic plane—which is an imaginary flat surface that we can think of as something like a racetrack in which the orbits of the planets are mainly confined. The ship was speeding away from the Sun at 40,000 miles per hour. But in early February of 1990, it was overtaken by an urgent message from Earth. Obediently, it turned its cameras back toward the now-distant planets. Slewing its scan platform from one spot in the sky to another, it snapped 60 pictures and stored them in digital form on its tape recorder. Then, slowly, in March, April, and May, it radioed the data back to Earth. Each image was composed of 640,000 individual picture elements ("pixels"), like the dots in a newspaper wire-photo or a pointillist painting. The spacecraft was 3.7 billion miles away from Earth, so far away that it took etch pixel 5½ hours, traveling at the speed of light, to reach us. The pictures might have been returned earlier, but the big radio telescopes in California, Spain, and Australia that receive these whispers from the edge of the Solar System had responsibilities to other ships that ply the sea of space among them, Magellan, bound for Venus, and Galileo on its tortuous passage to Jupiter. Voyager 1 was so high above the ecliptic plane because, in 1981, it had made a close pass by Titan, the giant moon of Saturn. Its sister ship, Voyager 2, was dispatched on a different trajectory, within the ecliptic plane, and so she was able to perform her celebrated explorations of Uranus and Neptune The two Voyager robots have explored four planets and nearly sixty moons. They are triumphs of human engineering an. one of the glories of the American space program. They will he in the history books when much else about our time forgotten. The Voyagers were guaranteed to work only until the Saturn encounter. I thought it might be a good idea, just after Saturn, to have them take one last glance homeward. From Saturn, I knew the Earth would appear too small for Voyager to make out any detail. Our planet would be just a point of light, a lonely pixel, hardly distinguishable from the many other points of light Voyager could see, nearby planets and far-off suns. But precise because of the obscurity of our world thus revealed, such picture might be worth having. 11 Mariners had painstakingly mapped the coastlines of the continents. Geographers had translated these findings into charts and globes. Photographs of tiny patches of the Earth had been obtained first by balloons and aircraft, then by rockets in brief ballistic flight, and at last by orbiting spacecraft—giving a perspective like the one you achieve by positioning your eyeball about an inch above a large globe. While almost everyone is taught that the Earth is a sphere with all of us somehow glued to it by gravity, the reality of our circumstance did not really begin to sink in until the famous frame-filling Apollo photograph of the whole Earth—the one taken by the Apollo 17 astronauts on the last journey of humans to the Moon. It has become a kind of icon of our age. There's Antarctica at what Americans and Europeans so readily regard as the bottom, and then all of Africa stretching up above it: You can see Ethiopia, Tanzania, and Kenya, where the earliest humans lived. At top right are Saudi Arabia and what Europeans call the Near East. Just barely peeking out at the top is the Mediterranean Sea, around which so much of our global civilization emerged. You can make out the blue of the ocean, the yellow-red of the Sahara and the Arabian desert, the brown-green of forest and grassland. And yet there is no sign of humans in this picture, not our reworking of the Earth's surface, not our machines, not ourselves: We are too small and our statecraft is too feeble to be seen by a spacecraft between the Earth and the Moon. From this vantage point, our obsession with nationalism is nowhere in evidence. The Apollo pictures of the whole Earth conveyed to multitudes something well known to astronomers: On the scale of worlds—to say nothing of stars or galaxies—humans are inconsequential, a thin film of life on an obscure and solitary lump of rock and metal. It seemed to me that another picture of the Earth, this one taken from a hundred thousand times farther away, might help in the continuing process of revealing to ourselves our true circumstance and condition. It had been well understood by the scientists and philosophers of classical antiquity that the Earth was a mere point in a vast encompassing Cosmos, but no one had ever seen it as such. Here was our first chance (and perhaps also our last for decades to come). Many in NASA's Voyager Project were supportive. But from the outer Solar System the Earth lies very near the Sun, like a moth enthralled around a flame. Did we want to aim the camera so close to the Sun as to risk burning out the spacecraft's vidicon system? Wouldn't it be better to delay until all the scientific images from Uranus and Neptune, if the spacecraft lasted that long, were taken? And so we waited— and a good thing too—from 1981 at Saturn, to 1986 at Uranus, to 1989, when both spacecraft had passed the orbits of Neptune and Pluto. At last the time came But there were a few instrumental calibrations that needed to be done first, and we waited a little longer. Although the spacecraft were in the right spots, the instruments were still working beautifully, and there were no other pictures to take, a few project personnel opposed it. It wasn't science, they said. Then we discovered that the technicians who devise and transmit the radio commands to Voyager were, in a cash-strapped NASA, to be laid off immediately or transferred to other jobs. If the picture were to be taken, it had to be done right then. At the last minute actually, in the midst of the Voyager 2 encounter with Neptune, the then NASA Administrator, Rear Admiral Richard Truly, stepped in and made sure that these images were obtained. The space scientists Candy Hansen of NASA's Jet Propulsion Laboratory (JPL) and Carolyn Porco of 12 University of Arizona designed the command sequence and calculated the camera exposure times. So here they are—a mosaic of squares laid down on top of the planets and a background smattering of more distant stars. We were able to photograph not only the Earth, but also five other of the Sun's nine known planets. Mercury, the innermost, was lost in the glare of the Sun, and Mars and Pluto were too small, too dimly lit, and/or too far away. Uranus and Neptune are so dim that to record their presence required long exposures; accordingly, their images were smeared because of spacecraft motion. This is how the planets would look to an alien spaceship approaching the Solar System after a long interstellar voyage. From this distance the planets seem only points of light, smeared or unsmeared—even through the high-resolution telescope aboard Voyager. They are like the planets seen with the naked eye from the surface of the Earth—luminous dots, brighter than most of the stars. Over a period of months the Earth, like the other planets, would seem to move among the stars. You cannot tell merely by looking at one of these dots what it's like, what's on it, what its past has been, and whether, n this particular epoch, anyone lives there. Because of the reflection of sunlight off the spacecraft, the Earth seems to be sitting in a beam of light, as if there were some special significance to this small world. But it's just an accident of geometry and optics. The Sun emits its radiation equitably in all directions. Had the picture been taken a little earlier or a little later, there would have been no sunbeam highlighting the Earth. And why that cerulean color? The blue comes partly from the sea, partly from the sky. While water in a glass is transparent, It absorbs slightly more red light than blue. If you have tens of meters of the stuff or more, the red light is absorbed out and what gets reflected back to space is mainly blue. In the same way, a short line of sight through air seems perfectly transparent. Nevertheless—something Leonardo da Vinci excelled at portraying—the more distant the object, the bluer it seems. Why? Because the air scatters blue light around much better than it does red. So the bluish cast of this dot comes from its thick but transparent atmosphere and its deep oceans of liquid water. And the white? The Earth on an average day is about half covered with white water clouds. We can explain the wan blueness of this little world because we know it well. Whether an alien scientist newly arrived at the outskirts of our solar system could reliably deduce oceans and clouds and a thickish atmosphere is less certain. Neptune, for instance, is blue, but chiefly for different reasons. From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, ever king and peasant, every young couple in love, every moth and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar,” every "supreme leader," every saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam. 15 beings evolve who would one day capture a little of that galactic light, and try to puzzle out what had sent it on its way. And after the Earth dies, some 5 billion years from now, after it is burned to a crisp or even swallowed by the Sun, there will be other worlds and stars and galaxies coming into being— and they will know nothing of a place once called Earth. IT ALMOST NEVER FEELS like prejudice. Instead, it seems fitting and just—the idea that, because of an accident of birth, our group (whichever one it is) should have a central position in the social universe. Among Pharaonic princelings and Plantagenet pretenders, children of robber barons and Central Committee bureaucrats, street gangs and conquerors of nations, members of confident majorities, obscure sects, and reviled minorities, this self-serving attitude seems as natural as breathing. It draws sustenance from the same psychic wellsprings as sexism, racism, nationalism, and the other deadly chauvinisms that plague our species. Uncommon strength of character is needed to resist the blandishments of those who assure us that we have an obvious, even God- given, superiority over our fellows. The more precarious our self-esteem, the greater our vulnerability to such appeals. Since scientists are people, it is not surprising that comparable pretensions have insinuated themselves into the scientific worldview. Indeed, many of the central debates in the history of science seem to be, in part at least, contests over whether humans are special. Almost always, the going-in assumption is that we are special. After the premise is closely examined, though, it turns out—in dishearteningly many cases—that we are not. Our ancestors lived out of doors. They were as familiar with the night sky as most of us are with our favorite television programs. The Sun, the Moon, the stars, and the planets all rose in the east and set in the west, traversing the sky overhead in the interim. The motion of the heavenly bodies was not merely a diversion, eliciting a reverential nod and grunt; it was the only way to tell the time of day and the seasons. For hunters and gatherers, as well as for agricultural peoples, knowing about the sky was a matter of life and death. How lucky for us that the Sun, the Moon, the planets, and the stars are part of some elegantly configured cosmic clockwork! It seemed to be no accident. They were put here for a purpose, for our benefit. Who else makes use of them? What else are they good for? And if the lights in the sky rise and set around us, isn't it evident that we're at the center of the Universe? These celestial bodies—so clearly suffused with unearthly powers, especially the Sun on which we depend for light and heat—circle us like courtiers fawning on a king. Even if we had not already guessed, the most elementary examination of the heavens reveals that we are special. The Universe seems designed for human beings. It's difficult to contemplate these circumstances without experiencing stirrings of pride and reassurance. The entire Universe, made for us! We must really be something. This satisfying demonstration of our importance, buttressed by daily observations of the heavens, made the geocentrist conceit a transcultural truth—taught in the schools, built into the language, part and parcel of great literature and sacred scripture. Dissenters were discouraged, sometimes with torture and death. It is no wonder that for the vast bulk of human history, no one questioned it. 16 It was doubtless the view of our foraging and hunting ancestors. The great astronomer of antiquity, Claudius Ptolemaeus (Ptolemy), in the second century knew that the Earth was a sphere, knew that its size was "a point" compared to the distance of the stars, and taught that it lay "right in the middle of the heavens." Aristotle, Plato, St. Augustine, St. Thomas Aquinas, and almost all the great philosophers and scientists of all cultures over the 3,000 years ending in the seventeenth century bought into this delusion. Some busied themselves figuring out how the Sun, the Moon, the stars, and the planets could be cunningly attached to perfectly transparent, crystalline spheres—the big spheres, of course, centered on the Earth—that would explain the complex motions of the celestial bodies so meticulously chronicled by generations of astronomers. And they succeeded: With later modifications, the geocentric hypothesis adequately accounted for the facts of planetary motion as known in the second century, and in the sixteenth. From there it was only a slight extrapolation to an even more grandiose claim—that the "perfection" of the world would be incomplete without humans, as Plato asserted in the Timaeus. "Man . . . is all," the poet and cleric John Donne wrote in 1625. "He is not a piece of the world, but the world itself; and next to the glory of God, the reason why there is a world." And yet—never mind how many kings, popes, philosophers, scientists, and poets insisted on the contrary—the Earth through those millennia stubbornly persisted in orbiting the Sun. You might imagine an uncharitable extraterrestrial observer looking down on our species over all that time—with us excitedly chattering, "The Universe is created for us! We're at the center! Everything pays homage to us!"—and concluding that our pretensions are amusing, our aspirations pathetic, that this must be the planet of the idiots. But such a judgment is too harsh. We did the best we could. There was an unlucky coincidence between everyday appearances and our secret hopes. We tend not to be especially critical when presented with evidence that seems to confirm our prejudices. And there was little countervailing evidence. In muted counterpoint, a few dissenting voices, counseling humility and perspective, could be heard down through the centuries. At the dawn of science, the atomist philosophers of ancient Greece and Rome— those who first suggested that matter is made of atoms—Democritus, Epicurus, and their followers (and Lucretius, the first popularizer of science) scandalously proposed many worlds and many alien life forms, all made of the same kinds of atoms as we. They offered for our consideration infinities in space and time. But in the prevailing canons of the West, secular and sacerdotal, pagan and Christian, atomist ideas were reviled. Instead, the heavens were not at all like our world. They were unalterable and "perfect." The Earth was mutable and "corrupt." The Roman statesman and philosopher Cicero summarized the common view: "In the heavens . . . there is nothing of chance or hazard, no error, no frustration, but absolute order, accuracy, calculation and regularity." Philosophy and religion cautioned that the gods (or God) were far more powerful than we, jealous of their prerogatives and quick to mete out justice for insufferable arrogance. At the same time, these disciplines had not a clue that their own teaching of how the Universe is ordered was a conceit and a delusion. Philosophy and religion presented mere opinion—opinion that might be overturned by observation and experiment—as certainty. This worried them not at all. That some of their deeply held beliefs might turn out to be mistakes was a possibility hardly considered. Doctrinal humility 17 was to be practiced by others. Their own teachings were inerrant and Infallible. In truth, they had better reason to be humble than they knew. BEGINNING WITH COPERNICUS in the middle sixteenth century, the issue was formally joined. The picture of the Sun rather than the Earth at the center of the Universe was understood to be dangerous. Obligingly, many scholars were quick to assure the religious hierarchy that this newfangled hypothesis represented no serious challenge to conventional wisdom. In a kind of split-brain compromise, the Sun-centered system was treated as a mere computational convenience, not an astronomical reality that is, the Earth was really at the center of the Universe, as everybody knew; but if you wished to predict where Jupiter would be on the second Tuesday of November the year after next, you were permitted to pretend that the Sun was at the center. Then you could calculate away and not affront the Authorities.* "This has no danger in it," wrote Robert Cardinal Bellarmine, the foremost Vatican theologian in the early seventeenth century, and suffices for the mathematicians. But, to affirm that the Sun is really fixed in the center of the heavens and that the Earth revolves very swiftly around the Sun is a dangerous thing, not only irritating the theologians and philosophers, but injuring our holy faith and making the sacred scripture false." "Freedom of belief is pernicious," Bellarmine wrote on another occasion. "It is nothing but the freedom to be wrong." Besides, if the Earth was going around the Sun, nearby stars should seem to move against the background of more distant stars as, every six months, we shift our perspective from one side of the Earth's orbit to the other. No such "annual parallax" had been found. The Copernicans argued that this was because the stars were extremely far away—maybe a million times more distant than the Earth is from the Sun. Perhaps better telescopes, in future times, would find an annual parallax. The geocentrists considered this a desperate attempt to save a flawed hypothesis, and ludicrous on the face of it. When Galileo turned the first astronomical telescope to the sky, the tide began to turn. He discovered that Jupiter had a little retinue of moons circling it, the inner ones orbiting faster than the outer ones, just as Copernicus had deduced for the motion of the planets about the Sun. He found that Mercury and Venus went through phases like the Moon (showing they orbited the Sun). Moreover, the cratered Moon and the spotted Sun challenged the perfection of the heavens. This may in part constitute the sort of trouble Tertullian was worried about thirteen hundred years earlier, when he pleaded, "If you have any sense or modesty, have done with prying into the regions of the sky, into the destiny and secrets of the universe." * Copernicus' famous book was first published with an introduction by the theologian Andrew Osiander, inserted without the knowledge of the dying astronomer. Osiander's well-meaning attempt to reconcile religion and Copernican astronomy ended with these words: "[L]et no one expect anything in the way of certainty of astronomy, since astronomy can offer us nothing certain, lest, if anyone take as true that which has been constructed for .mother use, he go away from this discipline a bigger fool than w hen he cane to it." Certainty could be found only in religion. 20 C H A P T E R 3 THE GREAT DEMOTIONS [One philosopher] asserted that he knew the whole secret . . . [H]e surveyed the two celestial strangers from top to toe, and maintained to their faces that their persons, their worlds, their suns, and their stars, were created solely for the use of man. At this assertion our two travelers let themselves fall against each other, seized with a fit of . . . inextinguishable laughter. --VOLTAIRE, MICROMEGAS. A PHILOSOPHICAL HISTORY (1752) In the seventeenth century there was still some hope that, even if the Earth was not the center of the Universe, it might be the only "world." But Galileo's telescope revealed that "the Moon certainly does not possess a smooth and polished surface" and that other worlds might look "just like the face of the Earth itself." The Moon and the planets showed unmistakably that they had as much claim to being worlds as the Earth does—with mountains, craters, atmospheres, polar ice caps, clouds, and, in the case of Saturn, a dazzling, unheard-of set of circumferential rings. After millennia of philosophical debate, the issue was settled decisively in favor of "the plurality of worlds." They might be profoundly different from our planet. None of them might be as congenial for life. But the Earth was hardly the only one. This was the next in the series of Great Demotions, downlifting experiences, demonstrations of our apparent insignificance, wounds that science has, in its search for Galileo's facts, delivered to human pride. WELL, SOME HOPED, even if the Earth isn't at the center of the Universe, the Sun is. The Sun is our Sun. So the Earth is approximately at the center of the Universe. Perhaps some of our pride could in this way be salvaged. But by the nineteenth century, observational astronomy had made it clear that the Sun is but one lonely star in a great self-gravitating assemblage of suns called the Milky Way Galaxy. Far from being at the center of the Galaxy, our Sun with its entourage of dim and tiny planets lies in an undistinguished sector of an obscure spiral arm. We are thirty thousand light years from the Center. Well, our Milky Way is the only galaxy. The Milky Way Galaxy is one of billions, perhaps hundreds of billions of galaxies notable neither in mass nor in brightness nor in how its stars are configured and arrayed. Some modern deep sky photographs show more galaxies beyond the Milky Way than stars within the Milky Way. Every one of them is an island universe containing perhaps a hundred billion suns. Such an image is a profound sermon on humility. 21 Well, then, at least our Galaxy is at the center of the Universe. No, this is wrong too. When the expansion of the Universe was first discovered, many people naturally gravitated to the notion that the Milky Way was at the center of the expansion, and all the other galaxies running away from us. We now recognize that astronomers on any galaxy would see all the others running away . from them; unless they were very careful, they would all conclude that they were at the center of the Universe. There is, in fact, no center to the expansion, no point of origin of the Big Bang, at least not in ordinary three-dimensional space. Well, even if there are hundreds of billions of galaxies, each with hundreds of billions of stars, no other star has planets. If there are n other planets beyond our Solar System, perhaps there's no other life in the Universe. Our uniqueness might then be saved. Sing planets are small and feebly shine by reflected sunlight, they're hard to find. Although applicable technology is improving wit breathtaking speed, even a giant world like Jupiter, orbiting the nearest star, Alpha Centauri, would still be difficult to detect. Iii our ignorance, the geocentrists find hope. There was once a scientific hypothesis—not just well received but prevailing—that supposed our solar system to have formed through the near collision of the ancient Sun with another star; the gravitational tidal interaction pulled out tendrils of sunstuff that quickly condensed into planets. Since space is mainly empty and near stellar collisions most rare, it was concluded that few other planetary systems exist—perhaps only one, around that other star that long ago co-parented the worlds of our solar system. Early in my studies, I was amazed and disappointed that such a view had ever been taken seriously, that for planets of other stars, absence of evidence had been considered evidence of absence. Today we have firm evidence for at least three planets orbiting an extremely dense star, the pulsar designated B1257+12, about which I'll say more later. And we've found, for more than half the stars with masses like the Sun's, that early in their careers they're surrounded by great disks of gas aid dust out of which planets seem to form. Other planetary systems now look to be a cosmic commonplace, maybe even worlds something like the Earth. We should be able, in the next few decades, to inventory at least the larger planets, if they exist, of hundreds of nearby stars. Well, if our position in space doesn't reveal our special role, our position in time does: We've been in the Universe since The Beginning (give or take a few days). We've been given special responsibilities by the Creator. It once seemed very reasonable to think of the Universe as beginning just a little before our collective memory is obscured by the passage of time and the illiteracy of our ancestors. Generally speaking, that's hundreds or thousands of years ago. Religions that purport to describe the origin of the Universe often specify—implicitly or explicitly—a date of origin of roughly such vintage, a birthday for the world. If you add up all the "begats" in Genesis, for example, you get an age for the Earth: 6,000 years old, plus or minus a little. The universe is said to be exactly as old as the Earth. This is still the standard of Jewish, Christian, and Moslem fundamentalists and is clearly reflected in the Jewish calendar. But so young a Universe raises an awkward question: How is it that there are astronomical objects more than 6,000 light-years away? It takes light a year to travel a light-year, 10,000 years to travel 10,000 light-years, and so on. When we look at the center of the Milky Way Galaxy, the light we see left its source 30,000 years ago. The nearest spiral galaxy like our 22 own, M31 in the constellation Andromeda, is 2 million light-years away, so we are seeing it as it was when the light from it set out on its long journey to Earth—2 million years ago. And when we observe distant quasars 5 billion light-years away, we are seeing them as they were 5 billion years ago, before the Earth was formed. (They are, almost certainly, very different today.) If, despite this, we were to accept the literal truth of such religious books, how could we reconcile the data? The only plausible conclusion, I think, is that God recently made all the photons of light arriving on the Earth in such a coherent format as to mislead generations of astronomers into the misapprehension that there are such things as galaxies and quasars, and intentionally driving them to the spurious conclusion that the Universe is vast and old. This is such a malevolent theology I still have difficulty believing that anyone, no matter how devoted to the divine inspiration of any religious book, could seriously entertain it. Beyond this, the radioactive dating of rocks, the abundance of impact craters on many worlds, the evolution of the stars, and the expansion of the Universe each provides compelling and independent evidence that our Universe is many billions of years old—despite the confident assertions of revered theologians that a world so old directly contradicts the word of God, and that at any rate information on the antiquity of the world is inaccessible except to faith.* These lines of evidence, as well, would have to be manufactured by a deceptive and malicious deity— unless the world is much older than the literalists in the Judeo-Christian-Islamic religion suppose. Of course, no such problem arises for those many religious people who treat the Bible and the Qur'an as historical and moral guides and great literature, but who recognize that the perspective of these scriptures on the natural world reflects the rudimentary science of the time in which they were written. Ages rolled by before the Earth began. More ages will run their course before it is destroyed. A distinction needs to be drawn between how old the Earth is (around 4.5 billion years) and how old the Universe is (about 15 billion years since the Big Bang). The immense interval of time between the origin of the Universe and our epoch was two-thirds over before the Earth came to be. Some stars and planetary systems are billions of years younger, others billions of years older. But in Genesis, chapter 1, verse 1, the Universe and the Earth are created on the same day. The Hindu-Buddhist-Jain religion tends not to confound the two events. As for humans, we're latecomers. We appear in the last instant of cosmic time. The history of the Universe till now was 99.998 percent over before our species arrived on the scene. In that vast sweep of aeons, we could not have assumed any special responsibilities for our planet, or life, or anything else. We were not here. Well, if we can't find anything special about our position or our epoch, maybe there's something special about our motion. Newton and all the other great classical physicists held that the velocity of the Earth in space constituted a "privileged frame of reference." That's actually what it was called. Albert Einstein, a keen critic of prejudice and privilege all his life, considered * St. Augustine, in The City of God, says, "As it is not yet six thousand years since the first man . . . are not those to be ridiculed rather than refuted who try to persuade us of anything regarding a space of time so different from, and contrary to, the ascertained truth? . . . We, being sustained by divine authority in the history of our religion, have no doubt that whatever is opposed to it is most false." He excoriates the ancient Egyptian tradition that the world is at much as a hundred thousand years old as "abominable lies." St. Thomas Aquinas, in the Summa Theologica, flatly states that "the newness of the world cannot be demonstrated from the world itself." They were so sure. 25 What about the related matter of whether we are capable of creating intelligences smarter than ourselves? Computers routinely do mathematics that no unaided human can manage, outperform world champions in checkers and grand masters in chess, speak and understand English and other languages, write presentable short stories and musical compositions, learn from their mistakes, and competently pilot ships, airplanes, and spacecraft. Their abilities steadily improve. They're getting smaller, faster, and cheaper. Each year, the tide of scientific advance laps a little further ashore on the island of human intellectual uniqueness with its embattled castaways. If, at so early a stage in our technological evolution, we have been able to go so far in creating intelligence out of silicon and metal, what will be possible in the following decades and centuries? What happens when smart machines are able to manufacture smarter machines? PERHAPS THE CLEAREST INDICATION that the search for an unmerited privileged position for humans will never be wholly abandoned is what in physics and astronomy is called the Anthropic Principle. It would be better named the Anthropocentric Principle. It comes in various forms. The "Weak" Anthropic Principle merely notes that if the laws of Nature and the physical constants— such as the speed of light, the electrical charge of the electron, the Newtonian gravitational constant, or Planck's quantum mechanical constant had been different, the course of events leading to the origin of humans would never have transpired. Under other laws and constants, atoms would not hold together, stars would evolve too quickly to leave sufficient time for life to evolve on nearby planets, the chemical elements of which life is made would never have been generated, and so on. Different laws, no humans. There is no controversy about the Weak Anthropic Principle: Change the laws and constants of Nature, if you could, and a very different universe may emerge—in many cases, a universe incompatible with life.* The mere fact that we exist implies (but does not impose) constraints on the laws of Nature. In contrast, the various "Strong" Anthropic Principles go much farther; some of their advocates come close to deducing that the laws of Nature and the values of the physical constants were established (don't ask how or by Whom) so that humans would eventually come to be. Almost all of the other possible universes, they say, are inhospitable. In this way, the ancient conceit that the Universe was made for us is resuscitated. To me it echoes Dr. Pangloss in Voltaire's Candide, convinced that this world, with all its imperfections, is the best possible. It sounds like playing my first hand of bridge, winning, knowing that there are 54 billion billion billion (5.4 X 1028) possible other hands that I was equally likely to have been dealt . . . and then foolishly concluding that a god of bridge exists and favors me, a god who arranged the cards and the shuffle with my victory foreordained from The Beginning. We do not know how many other winning hands there are in the cosmic deck, how many other kinds of universes, laws of Nature, and physical constants: that could also lead to life and Intelligence and perhaps even delusions of self-importance. Since we know next to nothing * Our universe is almost incompatible with life—or at least what we understand as necessary for life: Even if every star in a hundred billion galaxies had an Earthlike planet, without heroic technological measures life could prosper in only about 10-37 the volume of the Universe. For clarity, let's write it out: only 0.000 000 000 000 000 000 000 000 000 000 000 000 1 of our universe is hospitable to life. Thirty-six zeroes before the one. The rest is cold, radiation-riddled black vacuum. 26 about how the Universe was made—or even if it was made—it's difficult to pursue these notions productively. Voltaire asked "Why is there anything?" Einstein's formulation was to ask whether God had any choice in creating the Universe. But if the Universe is infinitely old—if the Big Bang some 15 billion years ago is only the most recent cusp in an infinite series of cosmic contractions and expansions—then it was never created and the question of why it is as it is is rendered meaningless. If, on the other hand, the Universe has a finite age, why is it the way it is? Why wasn't it given a very different character? Which laws of Nature go with which others? Are there meta- laws specifying the connections? Can we possibly discover them? Of all conceivable laws of gravity, say, which ones can exist simultaneously with which conceivable laws of quantum physics that determine the very existence of macroscopic matter? Are all laws we can think of possible, or is there only a restricted number that can somehow be brought into existence? Clearly we have not a glimmering of how to determine which laws of Nature are "possible" and which are not. Nor do we have more than the most rudimentary notion of what correlations of natural laws are "permitted." For example, Newton's universal law of gravitation specifies that the mutual gravitational force attracting two bodies towards each other is inversely proportional to the square of how far they are apart. You move twice as far from the center of the Earth and you weigh a quarter as much; ten times farther and you weigh only a hundredth of your ordinary weight; etc. It is this inverse square law that permits the exquisite circular and elliptical orbits of planets around the Sun, and moons around the planets—as well as the precision trajectories of our interplanetary spacecraft. If r is the distance between the centers of two masses, we say that the gravitational force varies as 1/r2. But if this exponent were different—if the gravitational law were 1/r4, say, rather than 1/r2 —then the orbits would not close; over billions of revolutions, the planets would spiral in and be consumed in the fiery depths of the Sun, or spiral out and be lost to interstellar space. If the Universe were constructed with an inverse fourth power law rather than an inverse square law, soon there would be no planets for living beings to inhabit. So of all the possible gravitational force laws, why are we so lucky as to live in a universe sporting a law consistent with life? First of course, we're so "lucky," because if we weren't, we wouldn't be here to ask the question. It is no mystery that inquisitive beings who evolve on planets can be found only in universes that admit planets. Second, the inverse square law is not is the only one consistent with stability over billions of years. Any power law less steep than 1/r3 (1/r2.99 or 1/r, for example) will keep a planet in the vicinity of a circular orbit even if it's given a shove. We have a tendency to overlook the possibility that other conceivable laws of Nature might also be consistent with life. But there's a further point: It's not arbitrary that we have an inverse square law of gravitation. When Newton's theory is understood in terms of the more encompassing general theory of relativity, we recognize that the exponent of the gravity law is 2 because the number of physical dimensions we live in is 3. All gravity laws aren't available, free for a Creator's choosing. Even given an infinite number of three-dimensional universes for some great god to tinker with, the 27 gravity law would always lave to be the law of the inverse square. Newtonian gravity, we might say, is not a contingent facet of our universe, but a necessary one. In general relativity, gravity is due to the dimensionality and curvature of space. When we talk about gravity we are talking about local dimples in space-time. This is by no means obvious and even affronts commonsense notions. But when examined deeply, the ideas of gravity and mass are not separate matters, but ramifications of the underlying geometry of space-time. I wonder if something like this doesn't apply generally to all anthropic hypotheses. The laws or physical constants on which our lives depend turn out to be members of a class, perhaps even a vast class, of other laws and other physical constants—but some of these are also compatible with a kind of life. Often we do not (or cannot) work through what those other universes allow. Beyond that, not every arbitrary choice of a law of Nature or a physical constant may be available, even to a maker of universes. Our understanding of which laws of Nature and which physical constants are up for grabs is fragmentary at best. Moreover, we have no access to any of those putative alternative universes. We have no experimental method by which anthropic hypotheses may be tested. Even if the existence of such universes were to follow firmly from well-established theories—of quantum mechanics or gravitation, say—we could not be sure that there weren't better theories that predict no alternative universes. Until that time comes, if it ever does, it seems to me premature to put faith in the Anthropic Principle as an argument for human centrality or uniqueness. Finally, even if the Universe were intentionally created to allow for the emergence of life or intelligence, other beings may exist on countless worlds. If so, it would be cold comfort to anthropocentrists that we inhabit one of the few universes that allow life and intelligence. There is something stunningly narrow about how the Anthropic Principle is phrased. Yes, only certain laws and constants of nature are consistent with our kind of life. But essentially the same laws and constants are required to make a rock. So why not talk about a Universe designed so rocks could one day come to be, and strong and weak Lithic Principles? If stones could philosophize, I imagine Lithic Principles would be at the intellectual frontiers. There are cosmological models being formulated today in which even the entire Universe is nothing special. Andrei Linde, formerly of the Lebedev Physical Institute in Moscow and now at Stanford University, has incorporated current understanding of the strong and weak nuclear forces and quantum physics into a new cosmological model. Linde envisions a vast Cosmos, much larger than our Universe—perhaps extending to infinity both in space and time—not the paltry 15 billion light-years or so in radius and 15 billion years in age which are the usual understanding. In this Cosmos there is, as here, a kind of quantum fluff in which tiny structures— much smaller than an electron—are everywhere forming, reshaping, and dissipating; in which, as here, fluctuations in absolutely empty space create pairs of elementary particles—an electron and a positron, for example. In the froth of quantum bubbles, the vast majority remain submicroscopic. But a tiny fraction inflate, grow, and achieve respectable universehood. They are so far away from us, though—much farther than the 15 billion light-years that is the conventional scale of our universe—that, if they exist, they appear to be wholly inaccessible and undetectable. Most of these other universes reach a maximum size and then collapse, contract to a point, and disappear forever. Others may oscillate. Still others may expand without limit. In 30 intellect done such violence to their own senses as to prefer what reason told them over what sensible experience plainly showed them . . . The Church declared, in its indictment of Galileo, The doctrine that the earth is neither the center of the universe nor immovable, but moves even with a daily rotation, is absurd, and both psychologically and theologically false, and at the least an error of faith. Galileo replied, The doctrine of the movements of the earth and the fixity of the sun is condemned on the ground that the Scriptures speak in many places of the sun moving and the earth standing still . . . It is piously spoken that the Scriptures cannot lie. But none will deny that they are frequently abstruse and their true meaning difficult to discover, and more than the bare words signify. I think that in the discussion of natural problems we ought to begin not with the Scriptures, but with experiments and demonstrations. But in his recantation (June 22, 1633) Galileo was made to say, Having been admonished by the Holy Office entirely to abandon the false opinion that the Sun was the center of the universe and immovable, and that the Earth was not the center of the same and that it moved . . . I have been . . . suspected of heresy, that is, of having held and believed that the Sun is the center of the universe and immovable, and that the Earth is not the center of the same, and that it does move . . . 1 abjure with a sincere heart and unfeigned faith, I curse and detest the same errors and heresies, and generally all and every error and sect contrary to the Holy Catholic Church. It took the Church until 1832 to remove Galileo's work from its list of books which Catholics were forbidden to read at the risk of dire punishment of their immortal souls. Pontifical disquiet with modern science has ebbed and flowed since the time of Galileo. The high-water mark in recent history is the 1864 Syllabus of Errors of Pius IX, the pope who also convened the Vatican Council at which the doctrine of papal infallibility was, at his insistence, first proclaimed. Here are a few excerpts: Divine revelation is perfect and, therefore, it is not subject to continual and indefinite progress in order to correspond with the progress of human reason . . . No man is free to embrace and profess that religion which he believes to be true, guided by the light of reason . . . The Church has power to define dogmatically the religion of the Catholic Church to be the only true religion . . . It is necessary even in the present day that the Catholic religion shall be held as the only religion of the state, to the exclusion of all other forms of worship . . . The civil liberty of every mode of worship, and full power given to all of openly and publicly manifesting their opinions and their ideas conduce more easily to corrupt the morals and minds of the people . . . The Roman Pontiff cannot and ought not to reconcile himself or agree with, progress, liberalism and modern civilization. 31 To its credit, although belatedly and reluctantly, the Church in 1992 repudiated its denunciation of Galileo. It still cannot quite bring itself, though, to see the significance of its opposition. In a 1992 speech Pope John Paul II argued, From the beginning of the Age of Enlightenment down to our own day, the Galileo case has been a sort of "myth" in which the image fabricated out of the events is quite far removed from reality. In this perspective, the Galileo case was a symbol of the Catholic Church's supposed rejection of scientific progress, or of "dogmatic" obscurantism opposed to the free search for truth. But surely the Holy Inquisition ushering the elderly and infirm Galileo in to inspect the instruments of torture in the dungeons of the Church not only admits but requires just such an interpretation. This was not mere scientific caution and restraint, a reluctance to shift a paradigm until compelling evidence, such as the annual parallax, was available. This was fear of discussion and debate. Censoring alternative views and threatening to torture their proponents betray a lack of faith in the very doctrine and parishioners that are ostensibly being protected. Why were threats and Galileo's house arrest needed? Cannot truth defend itself in its confrontation with error? The Pope does, though, go on to add: The error of the theologians of the time, when they maintained the centrality of the earth, was to think that our understanding of the physical world's structure was in soiree way imposed by the literal sense of Sacred Scriptures. Here indeed considerable progress has been made—although proponents of fundamentalist faiths will be distressed to hear from the Pontiff that Sacred Scripture is not always literally true. But if the Bible is not everywhere literally true, which parts are divinely inspired and which are merely fallible and human? As soon as we admit that there are scriptural mistakes (or concessions to the ignorance of the times), then how can the Bible be an inerrant guide to ethics and morals? Might sects and individuals now accept as authentic the parts of the Bible they like, and reject those that are inconvenient or burdensome? Prohibitions against murder, say, are essential for a society to function, but if divine retribution for murder is considered implausible, won't more people think they can get away with it? Many felt that Copernicus and Galileo were up to no good and erosive of the social order. Indeed any challenge from any source, to the literal truth of the Bible might have such consequences. We can readily see how science began to make people nervous. Instead of criticizing those who perpetuated the myths, public rancor was directed at those who discredited them. OUR ANCESTORS UNDERSTOOD origins by extrapolating from their own experience. How else could they have done it? So the Universe was hatched from a cosmic egg, or conceived in the 32 sexual congress of a mother god and a father god, or was a kind of product of the Creator's workshop—perhaps the latest of many flawed attempts. And the Universe was not much bigger than we see, and not much older than our written or oral records, and nowhere very different from places that we know. We've tended in our cosmologies to make things familiar. Despite all our best efforts, we've not been very inventive. In the West, Heaven is placid and fluffy, and Hell is like the inside of a volcano. In many stories, both realms are governed by dominance hierarchies headed by gods or devils. Monotheists talked about the king of kings. In every culture we imagined something like our own political system running the Universe. Few found the similarity suspicious. Then science came along and taught us that we are not the measure of all things, that there are wonders unimagined, that the Universe is not obliged to conform to what we consider comfortable or plausible. We have learned something about the idiosyncratic nature of our common sense. Science has carried human self-consciousness to a higher level. This is surely a rite of passage, a step towards maturity. It contrasts starkly with the childishness and narcissism of our pre-Copernican notions. But why should we want to think that the Universe was made for us? Why is the idea so appealing? Why do we nurture it? Is our self-esteem so precarious that nothing short of a universe custom-made for us will do? Of course it appeals to our vanity. "What a man desires, he also imagines to be true," said Demosthenes. "The light of faith makes us see what we believe," cheerfully admitted St. Thomas Aquinas. But I think there may be something else. There's a kind of ethnocentrism among primates. To whichever little group we happen to be born, we owe passionate love and loyalty. Members of other groups are beneath contempt, deserving of rejection and hostility. That both groups are. of the same species, that to an outside observer they are virtually indistinguishable, makes no difference. This is certainly the pattern among the chimpanzees, our closest relatives in the animal kingdom. Ann Druyan and I have described how this way of viewing the world may have made enormous evolutionary sense a few million years ago, however dangerous it has become today. Even members of hunter-gatherer groups—as far from the technological feats of our present global civilization as it is possible for humans to be—solemnly describe their little band, whichever it is, as "the people." Everyone else is something different, something less than human. If this is our natural way of viewing the world, then it should occasion no surprise that every time we make a naive judgment about our place in the Universe—one untempered by careful and skeptical scientific examination—we almost always opt for the centrality of our group and circumstance. We want to believe, moreover, that these are objective facts, and not our prejudices finding a sanctioned vent. So it's not much fun to have a gaggle of scientists incessantly haranguing us with "You're ordinary, you're unimportant your privileges are undeserved, there's nothing special about you." Even unexcitable people might, after a while, grow annoyed at this incantation and those who insist on chanting it. It almost seems that the scientists are getting some weird satisfaction out of putting humans down. Why can't they find a way in which we're superior? Lift our spirits! Exalt us! In such debates science, with its mantra of discouragement, feels cold and remote, dispassionate, detached, unresponsive to human needs. 35 think straight. "All that we pass on to our children" in the scientific age, Appleyard complains, "is the conviction that nothing is true, final or enduring, including the culture from which they sprang." How right he is about the inadequacy of our legacy. But would it be enriched by adding baseless certainties? He scorns "the pious hope that science and religion are independent realms which can easily be separated." Instead, "science, as it is now, is absolutely not compatible with religion." But isn't Appleyard really saying that some religions now find it difficult to make unchallenged pronouncements on the nature of the world that are straight-out false? We recognize that even revered religious leaders, the products of their time as we are of ours, may have made mistakes. Religions contradict one another on small matters, such as whether we should put on a hat or take one off on entering a house of worship, or whether we should eat beef and eschew pork or the other way around, all the way to the most central issues, such as whether there are no gods, one God, or many gods. Science has brought many of us to that state in which Nathaniel Hawthorne found Herman Melville: "He can neither believe, nor be comfortable in his unbelief." Or Jean-Jacques Rousseau: "They had not persuaded me, but they had troubled me. Their arguments had shaken me without ever convincing me . . . It is hard to prevent oneself from believing what one so keenly desires." As the belief systems taught by the secular and religious authorities are undennined, respect for authority in general probably does erode. The lesson is clear: Even politics] leaders must be wary of embracing false doctrine. This is not a failing of science, but one of its graces. Of course, worldview consensus is comforting, while clashes of opinion may be unsettling, and demand more of us. But unless we insist, against all evidence, that our ancestors were perfect, the advance of knowledge requires us to unravel and then restitch the consensus they established. In some respects, science has far surpassed religion in delivering awe. How is it that hardly any major religion has looked at science and concluded, "This is better than we thought! The Universe is much bigger than our prophets said, grander, more subtle, more elegant. God must be even greater than we dreamed"? Instead they say, "No, no, no! My god is a little god, and I want him to stay that way." A religion, old or new, that stressed the magnificence of the Universe as revealed by modern science might be able to draw forth reserves of reverence and awe hardly tapped by the conventional faiths. Sooner or later, such a religion will emerge. IF YOU LIVED two or three millennia ago, there was no shame in holding that the Universe was made for us. It was an appealing thesis consistent with everything we knew; it was what the most learned among us taught without qualification. But we have found out much since then. Defending such a position today amounts to willful disregard of the evidence, and a flight from self-knowledge. Still, for many of us, these deprovincializations rankle. Even if they do not fully cant' the day, they erode confidence—unlike the happy anthropocentric certitudes, rippling with social utility, of an earlier age. We long to be here for a purpose, even though, despite much self- 36 deception, none is evident. "The meaningless absurdity of life," wrote Leo Tolstoy, "is the only incontestable knowledge accessible to man." Our time is burdened under the cumulative weight of successive debunkings of our conceits: We're Johnny-come-latelies. We live in the cosmic boondocks. We emerged from microbes and muck. Apes are our cousins. Our thoughts and feelings are not fully under our own control. There may be much smarter and very different beings elsewhere. And on top of all this, we're making a mess of our planet and becoming a danger to ourselves. The trapdoor beneath our feet swings open. We find ourselves in bottomless free fall. We are lost in a great darkness, and there's no one to send out a search party. Given so harsh a reality, of course we're tempted to shut our eyes and pretend that we're safe and snug at home, that the fall is only a bad dream. We lack consensus about our place in the Universe. There is no generally agreed upon long- term vision of the goal of our species—other than, perhaps, simple survival. Especially when times are hard, we become desperate for encouragement, unreceptive to the litany of great demotions and dashed hopes, and much more willing to hear that we're special, never mind if the evidence is paper-thin. If it takes a little myth and ritual to get us through a night that seems endless, who among us cannot sympathize and understand? But if our objective is deep knowledge rather than shallow reassurance, the gains from this new perspective far outweigh the losses. Once we overcome our fear of being tiny, we find ourselves on the threshold of a vast and awesome Universe that utterly dwarfs—in time, in space, and in potential—the tidy anthropocentric proscenium of our ancestors. We gaze across billions of light-years of space to view the Universe shortly after the Big Bang, and plumb the fine structure of matter. We peer down into the core of our planet, and the blazing interior of our star. We read the genetic language in which is written the diverse skills and propensities of every being on Earth. We uncover hidden chapters in the record of our own origins, and with some anguish better understand our nature and prospects. We invent and refine agriculture, without which almost all of us would starve to death. We create medicines and vaccines that save the lives of billions. We communicate at the speed of light, and whip around the Earth in an hour and a half. We have sent dozens of ships to more than seventy worlds, and four spacecraft to the stars. We are right to rejoice in our accomplishments, to be proud that our species has been able to see so far, and to judge our merit in part by the very science that has so deflated our pretensions. To our ancestors there was much in Nature to be afraid of—lightning, storms, earthquakes, volcanos, plagues, drought, long winters. Religions arose in part as attempts to propitiate and control, if not much to understand, the disorderly aspect of Nature. The scientific revolution permitted us to glimpse an underlying ordered Universe in which there was a literal harmony of the worlds (Johannes Kepler's phrase). If we understand Nature, there is a prospect of controlling it or at least mitigating the harm it may bring. In this sense, science brought hope. Most of the great deprovincializing debates were entered into with no thought for their practical implications. Passionate and curious humans wished to understand their actual circumstances, how unique or pedestrian they and their world are, their ultimate origins and destinies, how the Universe works. Surprisingly, some of these debates have yielded the most profound practical benefits. The very method of mathematical reasoning that Isaac Newton 37 introduced to explain the motion of the planets around the Sun has led to most of the technology Of our modern world. The Industrial Revolution, for all its shortcomings, is still the global model of how an agricultural nation can emerge from poverty. These debates have bread-and-butter consequences. It might have been otherwise. It might have been that the balance lay elsewhere, that humans by and large did not want to yaw about a disquieting Universe, that we were unwilling to hermit challenges to the prevailing wisdom. Despite determined resistance in every age, it is very much to our credit that we have allowed ourselves to follow the evidence, to draw conclusions that at first seem daunting: a Universe so much larger and older that our personal and historical experience is dwarfed and humbled, a Universe in which, every day, suns are born and worlds obliterated, a Universe in which humanity, newly arrived, clings to an obscure clod of matter. How much more satisfying had we been placed in a garden custom-made for us, its other occupants put there for us to use as we saw fit. There is a celebrated story in the Western tradition like this, except that not quite everything was there for us. There was one particular tree of which we were not to partake, a tree of knowledge. Knowledge and understanding and wisdom were forbidden to us in this story. We were to be kept ignorant. But we couldn't help ourselves. We were starving for knowledge—created hungry, you might say. This was the origin of all our troubles. In particular, it is why we no longer live in a garden: We found out too much. So long as we were incurious and obedient, I imagine, we could console ourselves with our importance and centrality, and tell ourselves that we were the reason the Universe was made. As we began to indulge our curiosity, though, to explore, to learn how the Universe really is, we expelled ourselves from Eden. Angels with a flaming sword were set as sentries at the gates of Paradise to bar our return. The gardeners became exiles and wanderers. Occasionally we mourn that lost world, but that, it seems to me, is maudlin and sentimental. We could not happily have remained ignorant forever. There is in this Universe much of what seems to he design. Every time we come upon it, we breathe a sigh of relief. We are forever hoping to find, or at least safely deduce, a Designer. But instead, we repeatedly discover that natural processes—collisional selection of worlds, say, or natural selection of gene pools, or even the convection pattern in a pot of boiling water—can extract order out of chaos, and deceive us into deducing purpose where there is none. In everyday life, we often sense—in the bedrooms of teenagers, or in national politics—that chaos is natural, and order imposed from above. While there are deeper regularities in the Universe than the simple circumstances we generally describe as orderly, all that order, simple and complex, seems to derive from laws of Nature established at the Big Bang (or earlier), rather than as a consequence of belated intervention by an imperfect deity. "God is to be found in the details" is the famous dictum of the German scholar Abu Warburg. But, amid much elegance and precision, the details of life and the Universe also exhibit haphazard, jury-rigged arrangements and much poor planning. What shall we make of this: an edifice abandoned early in construction by the architect? The evidence, so far at least and laws of Nature aside, does not require a Designer. Maybe there is one hiding, maddeningly unwilling to be revealed. Sometimes it seems a very slender hope. The significance of our lives and our fragile planet is then determined only by our own 40 a very abundant material in the Universe; polar caps made of solid water would be a reasonable guess, as well as clouds of solid and liquid water. You might also he tempted by the idea that the blue stuff is enormous quantities— kilometers deep—of liquid water. The suggestion is bizarre, though, at least as far as this solar system is concerned, because surface oceans of liquid water exist nowhere else. When you look in the visible and near-infrared spectrum for telltale signatures of chemical composition, sure enough you discover water ice in the polar caps, and enough water vapor in the air to account for the clouds; this is also just the right amount that must exist because of evaporation if the oceans are in fact made of liquid water. The bizarre hypothesis is confirmed. The spectrometers further reveal that the air on this world is one fifth oxygen, O2. No other planet in the Solar System has anything close to so much oxygen. Where does it all come from? The intense ultraviolet light from the Sun breaks water, H20, down into oxygen and hydrogen, and hydrogen, the lightest gas, quickly escapes to space. This is a source of O2, certainly, but it doesn't easily account for so much oxygen. Another possibility is that ordinary visible light, which the Sun pours out in vast amounts, is used on Earth to break water apart—except that there's no known way to do this without life. There would have to be plants, life-forms colored by a pigment that strongly absorbs visible light, that knows how to split a water molecule by saving up the energy of two photons of light, that retains the H and excretes the O, and that uses the hydrogen thus liberated to synthesize organic molecules. The plants would have to be spread over much of the planet. All this is asking a lot. If you're a good skeptical scientist, so much Oz would not be proof of life. But it certainly might be cause for suspicion. With all that oxygen you're not surprised to discover ozone (O3) in the atmosphere, because ultraviolet light makes ozone out of molecular oxygen (O2). The ozone then absorbs dangerous ultraviolet radiation. So if the oxygen is due to life, there's a curious sense in which the life is protecting itself. But this life "night be mere photosynthetic plants. A high level of intelligence is not implied. When you examine the continents more closely, you find there are, crudely speaking, two kinds of regions. One shows the spectrum of ordinary rocks and minerals as found on many worlds. The other reveals something unusual: a material, covering vast areas, that strongly absorbs red light. (The Sun, of course, shines in light of all colors, with a peak in the yellow.) This pigment might be just the agent needed if ordinary visible light is being used to break water apart and account for the oxygen in the air. It's another hint, this time a little stronger, of life, not a bug here and there, but a planetary surface overflowing with life. The pigment is in fact chlorophyll: It absorbs blue light as well as red, and is responsible for the fact that plants are green. What you're seeing is a densely vegetated planet. So the Earth is revealed to possess three properties unique at least in this solar system— oceans, oxygen, life. It's hard not to think they're related, the oceans being the sites of origin, and the oxygen the product, of abundant life. When you look carefully at the infrared spectrum of the Earth, you discover the minor constituents of the air. In addition to water vapor, there's carbon dioxide (CO2), methane (CH4), and other gases that absorb the heat that the Earth tries to radiate away to space at night. These 41 gases warm the planet. Without them, the Earth would everywhere be below the freezing point of water. You've discovered this world's greenhouse effect. Methane and oxygen together in the same atmosphere is peculiar. The laws of chemistry are very clear: In an excess of O2, CH4 should be entirely converted into H2O and CO2, The process is so efficient that not a single molecule in all the Earth's atmosphere should be methane. Instead, you find that one out of every million molecules is methane, ail immense discrepancy. What could it mean? The only possible explanation is that methane is being injected into the Earth's atmosphere so quickly that its chemical reaction with Oz can't keep pace. Where does all this methane come from? Maybe it seeps out of the deep interior of the Earth—but quantitatively this doesn't seem to work, and Mars and Venus don't have anything like this much methane. The only alternatives are biological, a conclusion that makes no assumptions about the chemistry of life, or what it looks like, but follows merely from how unstable methane is in an oxygen atmosphere. In fact, the methane arises from such sources as bacteria in bogs, the cultivation of rice, the burning of vegetation, natural gas from oil wells, and bovine flatulence. In an oxygen atmosphere, methane is a sign of life. That the intimate intestinal activities of cows should be detectable from interplanetary space is a little disconcerting, especially when so much of what we hold dear is not. But an alien scientist flying by the Earth would, at this point, be unable to deduce bogs, rice, fire, oil, or cows. Just life. All the signs of life that we've discussed so far are due to comparatively simple forms (the methane in the rumens of cows is generated by bacteria that homestead there). Had your spacecraft flown by the Earth a hundred million years ago, in the age of the dinosaurs when there were no humans and no technology, you would still have seen oxygen and ozone, they chlorophyll pigment, and far too much methane. At present, though, your instruments are finding signs not just of life, but of high technology—something that couldn't possibly have been detected even a hundred years ago: You are detecting a particular kind of radio wave emanating from Earth. Radio waves don't necessarily signify life and intelligence Many natural processes generate them. You've already found radio emissions from other, apparently uninhabited worlds—generated by electrons trapped in the strong magnetic fields of planets, by chaotic motions at the shock front that separates these magnetic fields from the interplanetary magnetic field, and by lightning. (Radio "whistlers" usually sweep from high notes to low, and then begin again.) Some of these radio emissions are continuous; some come in repetitive bursts; some last a few minutes and then disappear. But this is different: A portion of the radio transmission from Earth is at just the frequencies where radio waves begin to leak out of the planet's ionosphere, the electrically charged region above the stratosphere that reflects and absorbs radio waves. There is a constant central frequency for each transmission, added to which is a modulated signal (a complex sequence of ons and offs). No electrons in magnetic fields, no shock waves, no lightning discharges can generate something like this. Intelligent life seems to be the only possible explanation. Your conclusion that the radio transmission is due to technology on Earth holds no matter what the ons and offs mean: You don't have to decode the message to be sure it is a 42 message. (This signal is really, let us suppose, communications from the U.S. Navy to its distant nuclear-armed submarines.) So, as an alien explorer, you would know that at least one species on Earth has achieved radio technology. Which one is it? The beings that make methane? Those that generate oxygen? The ones whose pigment colors the landscape green? Or somebody else, somebody more subtle, someone not otherwise detectable to a spacecraft plummeting by? To search for this technological species, you might want to examine the Earth at finer and finer resolution— seeking, if not the beings themselves, at least their artifacts. You look first with a modest telescope, so the finest detail you can resolve is one or two kilometers across. You can make out no monumental architecture, no strange formations, no unnatural reworking of the landscape, no signs of life. You see a dense atmosphere in motion. The abundant water must evaporate and then rain back down. Ancient impact craters, apparent on the Earth's nearby Moon, are almost wholly absent. There must, then, be a set of processes whereby new land is created and then eroded away in much less time than the age of this world. Running water is implicated. As you look with finer and finer definition you find mountain ranges, river valleys, and many other indications that the planet is geologically active. There are also odd places surrounded by vegetation, but which are themselves denuded of plants. They look like discolored smudges on the landscape. When you examine the Earth at about 100-meter resolution, everything changes. The planet is revealed to be covered with straight lines, squares, rectangles, circles—sometimes huddling along river banks or nestling on the lower slopes of mountains, sometimes stretching over plains, but rarely in deserts or high mountains, and absolutely never in the oceans. Their regularity, complexity, and distribution would be hard to explain except by life and intelligence, although a deeper understanding of function and purpose might be elusive. Perhaps you would conclude only that the dominant life-forms have a simultaneous passion for territoriality and Euclidean geometry. At this resolution you could not see them, much less know them. Many of the devegetated smudges are revealed to have an underlying checkerboard geometry. These are the planets cities. Over much of the landscape, and not just in the cities, there is a profusion of straight lines, squares, rectangles, circles. The dark smudges of the cities are revealed to be highly geometrized, with only a few patches of vegetation—themselves with highly regular boundaries—left intact. There are occasional triangles, and in one city there is even a pentagon. When you take pictures at a meter resolution or better, you find that the crisscrossing straight lines within the cities and the long straight lines that join them with other cities are filled with streamlined, multicolored beings a few meters in length, politely running one behind the other, in long, slow orderly procession. They are very patient. One stream of beings stops so another stream can continue at right angles. Periodically, the favor is returned. At night, they turn on two bright lights in front so they can see where they're going. Some, a privileged few, go into little houses when their workday is done and retire for the night. Most are homeless and sleep in the streets. At last! You've detected the source of all the technology. the dominant life-forms on the planet. The streets of the cities and the roadways of the countryside are evidently built for their 45 oceans, polar ice, life, and intelligence. The use of instruments and protocols developed to explore the planets to monitor the environmental health of our own—something NASA is now doing in earnest—was described by the astronaut Sally Ride as "Mission to Planet Earth." Other members of the NASA scientific team who worked with me on Galileo's detection of life on Earth were Drs. W. Reid Thompson, Cornell University; Robert Carlson, JPL; Donald Gurnett, University of Iowa; and Charles Hord, University of Colorado. Our success in detecting life on Earth with Galileo, without making any assumptions beforehand about what kind of life it must be, increases our confidence that when we fail to find life on other planets, that negative result is meaningful. Is this judgment anthropocentric, geocentric, provincial? I don't think so. We're not looking only for our kind of biology. Any widespread photosynthetic pigment, any gas grossly out of equilibrium with the rest of the atmosphere, any rendering of the surface into highly geometrized patterns, any steady constellation of lights on the night hemisphere, any non-astrophysical sources of radio emission would betoken the presence of life. On Earth we have found of course only our type, but many other types would have been detectable elsewhere. We have not found them. This examination of the third planet strengthens our tentative conclusion that, of all the worlds in the Solar System, only ours is graced by life. We have just begun to search. Maybe life is hiding on Mars or Jupiter, Europa or Titan. Maybe the Galaxy is filled with worlds as rich in life as ours. Maybe we are on the verge of making such discoveries. But in terms of actual knowledge, at this moment the Earth is unique. No other world is yet known to harbor even a microbe, much less a technical civilization. 46 C H A P T E R 6 THE TRIUMPH OF VOYAGER They that go down to the sea in ships, that do business in great waters; these see the works of the Lord, and his wonders in the deep. —PSALMS, 107 (CA. 150 B.C) The visions we offer our children shape the future. It matters what those visions are. Often they become self-fulfilling prophecies. Dreams are maps. I do not think it irresponsible to portray even the direst futures; if we are to avoid them, we must understand that they are possible. But where are the alternatives? Where are the dreams that motivate and inspire? We long for realistic maps of a world we can be proud to give to our children. Where are the cartographers of human purpose? Where are the visions of hopeful futures, of technology as a tool for human betterment and not a gun on hair trigger pointed at our heads? NASA, in its ordinary course of doing business, offers such a vision. But in the late 1980s and early '90s, many people saw the U.S. space program as, instead, a succession of catastrophes—seven brave Americans killed on a mission whose main function was to put up a communications satellite that could have been launched at less cost without risking anybody; a billion-dollar telescope sent up with a bad case of myopia; a spacecraft to Jupiter whose main antenna—essential for returning data to Earth—did not unfurl; a probe lost just as it was about to orbit Mars. Some people cringe every time NASA describes as exploration sending a few astronauts 200 miles up in a small capsule that endlessly circles the Earth and goes nowhere. Compared to the brilliant achievements of robotic missions, it is striking how rarely fundamental scientific findings emerge from manned missions. Except for repairing ineptly manufactured or malfunctioning satellites, or launching a satellite that could just as well have been sent up in an unmanned booster, the manned program has, since the 1970s, seemed unable to generate accomplishments commensurate with the cost. Others looked at NASA as a stalking horse for grandiose schemes to put weapons into space, despite the fact that an orbiting weapon is in many circumstances a sitting duck. And NASA showed many symptoms of an aging, arteriosclerotic, overcautious, unadventurous bureaucracy. The trend is perhaps beginning to be reversed. But these criticisms—many of them surely valid—should not blind us to NASA triumphs in the same period: the first exploration of the Uranus and Neptune systems, the in-orbit repair of the Hubble space telescope, the proof that the existence of galaxies is compatible with the Big Bang, the first close-up observations of asteroids, mapping Venus pole to pole, monitoring ozone depletion, demonstrating the existence of a black hole with the mass of a billion suns at the 47 center of a nearby galaxy, and a historic commitment to joint space endeavors by the U.S. and Russia. There are far-reaching, visionary, and even revolutionary implications to the space program. Communications satellites link up the planet, are central to the global economy, and, through television, routinely convey the essential fact that We live in a global community. Meteorological satellites predict the weather, save lives in hurricanes and tornados, and avoid many billions of dollars in crop losses every year. Military-reconnaissance and treaty- verification satellites make nations and the global civilization more secure; in a world with tens of thousands of nuclear weapons, they calm the hotheads and paranoids on all sides; they are essential tools for survival on a troubled and unpredictable planet. Earth-observing satellites, especially a new generation soon to be deployed, monitor the health of the global environment: greenhouse warming, topsoil erosion, ozone layer depletion, ocean currents, acid rain, the effects of floods and droughts, and new dangers we haven't yet discovered. This is straightforward planetary hygiene. Global positioning systems are now in place so that your locale is radio-triangulated by several satellites. Holding a small instrument the size of a modern shortwave radio, you can read out to high precision your latitude and longitude. No crashed airplane, no ship in fog and shoals, no driver in an unfamiliar city need ever be lost again. Astronomical satellites peering outward from Earth's orbit observe with unsurpassed clarity—studying questions ranging from the possible existence of planets around nearby stars to the origin and fate of the Universe. Planetary probes from close range explore the gorgeous array of other worlds in our solar system comparing their fates with ours. All of these activities are forward-looking, hopeful, stirring and cost-effective. None of them requires "manned"* spaceflight. A key issue facing the future of NASA and addressed in this book is whether the purported justifications for human spaceflight are coherent and sustainable. Is it worth the cost? But first, let's consider the visions of a hopeful future vouchsafed by robot spacecraft out among the planets. VOYAGER 1 AND VOYAGER 2 are the ships that opened the Solar System for the human species, trailblazing a path for future generations. Before their launch, in August and September 1977, we were almost wholly ignorant about most of the planetary part of the Solar System. In the next dozen years, they provided our first detailed, close-up information on many new worlds— some of them previously known only as fuzzy disks in the eyepieces of ground-based telescopes, some merely as points of light, and some whose very existence was unsuspected. They are still returning reams of data. These spacecraft have taught us about the wonders of other worlds, about the uniqueness and fragility of our own, about beginnings and ends. They have given us access to most of the * Since women astronauts and cosmonauts of several nations have flown in space, "manned" is just flat-out incorrect. I've attempted to find an alternative to this widely used term, coined in a more unselfconsciously sexist age. I tried "crewed" for a while, but in spoken language it lends itself to misunderstanding. "Piloted" doesn't work, because even commercial airplanes have robot pilots. "Manned and womanned" is just, but unwieldy. Perhaps the best compromise is "human," which permits us to distinguish crisply between human and robotic missions. But every now and then, 1 find "human" not quite working, and to my dismay "manned" slips back in. 50 The flow of electricity through the innards of the spacecraft would generate enough magnetism to overwhelm the sensitive instrument that measures interplanetary magnetic fields. So the magnetometer is placed at the end of along boom, far from the offending electrical currents. With other projections, it gives Voyager alter a slightly porcupine appearance. Cameras, infrared and ultraviolet spectrometers, and an instrument called a photopolarimeter are on a scan platform that swivels on command so these device can be aimed at a target world. The spacecraft must know where Earth is if the antenna is to be pointed properly and the data rereceived back home. It also needs to know where the Sun is and at least one bright star, so it can orient itself in three dimensions and point properly toward any passing world. If you can't point the cameras, it does no good to be able to return pictures over billions of miles. Each spacecraft cost about as much as a single modern strategic bomber. But unlike bombers, Voyager cannot, once launched, be returned to the hangar for repairs. The ship's computers and electronics are therefore designed redundantly. Much key machinery, including the essential radio receiver, had at least one backup—waiting to be called upon should the hour of need ever arrive. When either Voyager finds itself in trouble, the computers use branched contingency tree logic to work out the appropriate course of action. If that doesn't work, the ship radios home for help. As the spacecraft journeys increasingly far from Earth, the roundtrip radio travel time also increases, approaching eleven hours by the time Voyager is at the distance of Neptune. Thus, in case of emergency, the spacecraft needs to know how to put itself into a safe standby mode while awaiting instructions from Earth. As it ages, more and more failures are expected, both in its mechanical parts and in its computer system, although there is no sign, even now, of a serious memory deterioration, some robotic Alzheimer's disease. This is not to say that Voyager is perfect. Serious mission-threatening, white-knuckle mishaps did occur. Each time, special teams of engineers—some of whom had been with the Voyager program since its inception—were assigned to "work" the problem. They would study the underlying science and draw upon their previous experience with the failed subsystems. They would experiment with identical Voyager spacecraft equipment that had never been launched, or even manufacture a large number of components of the sort that failed in order to gain some statistical understanding of the failure mode. In April 1978, almost eight months after launch, and while the ship was approaching the asteroid belt, an omitted ground command—a human error—caused Voyager 2's on-board computer to switch from the prime radio receiver to its backup, During the next ground transmission to the spacecraft, the backup receiver refused to lock onto the signal from Earth. A component called a tracking loop capacitor had failed. After seven days in which Voyager 2 was entirely out of contact, its fault protection software suddenly commanded the backup receiver to be switched off and the prime receiver to be switched back on. Mysteriously—to this day, no one knows why—the prime receiver failed moments later. It was never heard from again. To top it off, the on-board computer now foolishly insisted on using the failed primary receiver. Through an unlucky concatenation of human and robotic error, the spacecraft was now in real jeopardy. No one could think of a way to get Voyager 2 to revert to the backup receiver. Even if it did, the backup receiver couldn't receive the commands from Earth, because of that failed capacitor. There were many project personnel who feared that all was lost. 51 But after a week of obdurate unresponsiveness to all commands, instructions to switch automatically between receivers were accepted and programmed into the skittish onboard computer. During that same week the JPL engineers designed an innovative command frequency control procedure to make sure that essential orders would be understood by the damaged backup receiver. The engineers were now able to recommunicate, at least in a rudimentary way, with the spacecraft. Unfortunately the backup receiver now turned giddy, becoming extremely sensitive to stray heat dumped when various components of the spacecraft powered up or down. In the following months the JPL engineers devised and conducted tests that let them thoroughly understand the thermal implications of most spacecraft operational modes: What would prevent and what would permit receipt of commands from Earth? With this information, the backup receiver problem was entirely circumvented. It subsequently acquired all the commands sent from Earth on how to gather data in the Jupiter, Saturn, Uranus, and Neptune systems. The engineers had saved the mission. (To be on the safe side, during most of Voyager 2's subsequent flight a nominal data-taking sequence for the next planet to be encountered was always sitting in the on-board computers—should the spacecraft again become deaf to entreaties from home.) Another heart-wrenching failure occurred just after Voyager 2 emerged from behind Saturn (as seen from the Earth) in August 1981. The scan platform had been moving feverishly— pointing here and there among the rings, moons, and the planet itself during the all-too-brief moments of close approach. Suddenly, the platform jammed. A stuck scan platform is a maddening predicament: knowing that the spacecraft is flying past wonders that have never been witnessed, that we will not see again for years or decades, and the incurious spacecraft staring fixedly off into space, ignoring everything. The scan platform is driven by actuators containing gear trains. So first the JCL engineers ran an identical copy of a flight actuator in a simulated mission. This actuator failed after 348 turns; the actuator on the spacecraft had failed after 352 turns. The problem turned out to be a lubrication failure. Good to know, but what to do about it? Plainly, it would be impossible to overtake Voyager with ail oilcan. The engineers wondered whether they could restart the tailed actuator by alternate heating and cooling; maybe the resulting thermal stresses would induce the components of the actuator to expand and contract at different rates and unjam the system. They tested this notion with specially manufactured actuators in the laboratory, and then jubilantly found that in this way they could start the scan platform up again in space. Project personnel also devised ways to diagnose any additional trend toward actuator failure early enough to work around the problem. Thereafter, Voyager 2's scan platform worked perfectly. All the pictures taken in the Uranus and Neptune systems owe their existence to this work. The engineers had saved the day again. Voyagers 1 and 2 were designed to explore the Jupiter and Saturn systems only. It is true that their trajectories would carry them on past Uranus and Neptune, but officially these planets were never contemplated as targets for Voyager exploration: The spacecraft were not supposed to last that long. Because of our wish to fly close to the mystery world Titan, Voyager 9 was flung by Saturn on a path that could never encounter any other known world; it is Voyager 2 that flew on to Uranus and Neptune with brilliant success. At these immense distances, sunlight is 52 getting progressively dimmer, and the radio signals transmitted to Earth are getting progressively fainter. These were predictable but still very serious problems that the JPL engineers and scientists also had to solve. Because of the low light levels at Uranus and Neptune, the Voyager television cameras were obliged to take long time exposures. But the spacecraft was hurtling so fast through, say, the Uranus system (at about 35,000 miles per hour) that the image would have been smeared or blurred. To compensate, the entire spacecraft had to be moved during the time exposures to cancel out the motion, like panning in the direction opposite yours while taking a photograph of a street scene from a moving car. This may sound easy, but it's not: You have to neutralize the most innocent of motions. At zero gravity, the mere start and stop of the on-board tape recorder can jiggle the spacecraft enough to smear the picture. This problem was solved by sending up commands to the spacecraft's little rocket engines (called thrusters), machines of exquisite sensitivity. With a little puff of gas at the start and stop of each data-taking sequence, the thrusters compensated for the tape-recorder jiggle by turning the entire spacecraft just a little. To deal with the low radio power received at Earth, the engineers devised a new and more efficient way to record and transmit the data, and the radio telescopes on Earth were electronically linked together with others to increase their sensitivity. Overall, the imaging system worked, by many criteria, better at Uranus and Neptune than it did at Saturn or even at Jupiter. Voyager may not yet be done exploring. There is, of course, a chance that some vital subsystem will fail tomorrow, but as far as the radioactive decay of the plutonium power source is concerned, the two Voyager spacecraft should be able to return data to Earth roughly through the year 2015. Voyager is an intelligent being—part robot, part human. It extends the human senses to far- off worlds. For simple tasks and short-term problems, it relies on its own intelligence; but for more complex tasks and longer-term problems, it turns to the collective intelligence and experience of the JPL engineers. This trend is sure to grow. The Voyagers embody the technology of the early 1970s; if spacecraft were designed for such a mission today, they would incorporate stunning advances in artificial intelligence, in miniaturization, in data-processing speed, in the ability to self-diagnose and repair, and in the propensity to learn from experience They would also be much cheaper. In the many environments too dangerous for people, on Earth as well as in space, the future belongs to robot-human partnerships that will recognize the two Voyagers as antecedents and pioneers. For nuclear accidents, mine disasters, undersea exploration and archaeology, manufacturing, prowling the interiors of volcanos, and household help, to name only a few potential applications, it could make an enormous difference to have a ready corps of smart, mobile, compact, commandable robots that can diagnose and repair their own malfunctions. There are likely to be many more of this tribe in the near future. It is conventional wisdom now that anything built by the government will be a disaster. But the two Voyager spacecraft were built by the government (in partnership with that other bugaboo, academia). They came in at cost, on time, and vastly exceeded their design specifications—as well as the fondest dreams of their makers. Seeking not to control, threaten, wound, or destroy, these elegant machines represent the exploratory part of our nature set free to 55 nucleotide bases, the building blocks of the nucleic acids. Mist before the origin of life, where did these molecules come from? There are only two possibilities: from the outside or from the inside. We know that vastly more comets and asteroids were hitting the Earth than do so today, that these small worlds are rich storehouses of complex organic molecules, and that some of these molecules escaped being fried on impact. Here I'm describing homemade, not imported, goods: the organic molecules generated in the air and waters of the primitive Earth. Unfortunately, we don't know very much about the composition of the early air, and organic molecules are far easier to make in some atmospheres than in others. There couldn't have been much oxygen, because oxygen is generated by green plants and there weren't any green plants yet. There was probably more hydrogen, because hydrogen is very abundant in the Universe and escapes from the upper atmosphere of the Earth into space better than any other atom (because it's so light). If we can imagine various possible early atmospheres, we can duplicate them in the laboratory, supply some energy, and see which organic molecules are made and in what amounts. Such experiments have over the years proved provocative and promising. But our ignorance of initial conditions limits their relevance. What we need is a real world whose atmosphere still retains some of those hydrogen-rich gases, a world in other respects something like the Earth, a world in which the organic building blocks of life are being massively generated in our own time, a world we can go to to seek our own beginnings. There is only one such world in the Solar System.* That world is Titan, the big moon of Saturn. It's about 5,150 kilometers (3,200 miles) in diameter, a little less than half the size of the Earth. It takes 16 of our days to complete one orbit of Saturn. No world is a perfect replica of any other, and in at least one important respect Titan is very different from the primitive Earth: Being so far from the Sun, its surface is extremely cold, far below the freezing point of water, around 180° below zero Celsius. So while the Earth at the time of the origin of life was, as now, mainly ocean-covered, plainly there can be no oceans of liquid water on Titan. (Oceans made of other stuff are a different story, as we shall see.) The low temperatures provide an advantage, though, because once molecules are synthesized on Titan, they tend to stick around: The higher the temperature, the faster molecules fall to pieces. On Titan the molecules that have been raining down like manna from heaven for the last 4 billion years might still be there, largely unaltered, deep-frozen, awaiting the chemists from Earth. THE INVENTION OF THE TELESCOPE In the seventeenth century led to the discovery of many new worlds. In 1610 Galileo first spied the four large satellites of Jupiter. It looked like a miniature solar system, the little moons racing around Jupiter as the planets were thought by Copernicus to orbit the Sun. It was another blow to the geocentrists. Forty-five years later, the celebrated Dutch physicist Christianus Huygens discovered a moon moving about the planet Saturn and named it Titan.† It was a dot of light a billion miles away, gleaming in reflected sunlight. From the time of * There could have been none. We're very lucky that there is such a world study. The others ill have too much hydrogen, or not enough, or no atmosphere at all. † Not because he thought it remarkably large. but because in Greek mythology members of the generation preceding the Olympian gods—Saturn, his siblings, and his cousins—were called Titans. 56 its discovery, when European men wore long curly wigs, to world War II, when American men cut their hair down to stubble, almost nothing more was discovered about Titan except the fact it had a curious, tawny color. Ground-based telescopes could, even in principle, barely make out some enigmatic detail. The Spanish astronomer J. Comas Sola reported at the turn of the twentieth century some faint and indirect evidence of an atmosphere. In a way, I grew up with Titan. I did my doctoral dissertation at the University of Chicago under the guidance of Gerard P. Kuiper, the astronomer who made the definitive discovery that Titan has an atmosphere. Kuiper was Dutch and in a direct line of intellectual descent from Christianus Huygens. In 1914, while making a spectroscopic examination of Titan, Kuiper was astonished to find the characteristic spectral features of the gas methane. When he pointed the telescope at Titan, there was the signature of methane.* When he pointed it away, not a hint of methane. But moons were not supposed to hold onto sizable atmospheres, and the Earth's Moon certainly doesn't. Titan could retain an atmosphere, Kuiper realized, even though its gravity was less than Earth's, because its upper atmosphere is very cold. The molecules simply aren't moving fast enough for significant numbers to achieve escape velocity and trickle away to space. Daniel Harris, a student of Kuiper's, showed definitively that Titan is red. Maybe we were looking at a rusty surface, like that of Mars. If you wanted to learn more about Titan, you could also measure the polarization of sunlight reflected off it. Ordinary sunlight is unpolarized. Joseph Veverka, now a fellow faculty member at Cornell University, was my graduate student at Harvard University, and therefore, so to speak, a grandstudent of Kuiper's. In his doctoral work, around 1970, he measured the polarization of Titan and found that it changed as the relative positions of Titan, the Sun, and the Earth changed. But the change was very different from that exhibited by, say, the Moon. Veverka concluded that the character of this variation was consistent with extensive clouds or haze on Titan. When we looked at it through the telescope, we weren't seeing its surface. We knew nothing about what the surface was like. We had no idea how fat below the clouds the surface was. So, by the early 1970s, as a kind of legacy from Huygens and his line of intellectual descent, we knew at least that Titan has a dense methane-rich atmosphere, and that it's probable enveloped by a reddish cloud veil or aerosol haze. But what kind of cloud is red? By the early 1970s my colleague Bishun Khare and I had been doing experiments at Cornell in which we irradiated various methane-rich atmospheres with Ultraviolet light or electrons and were generating reddish or brownish solids; the stuff would coat the interiors of our reaction vessels. It seemed to me that, if methane-rich Titan had red-brown clouds, those clouds might very well be similar to what we were making in the laboratory. We called this material tholin, after a Greek word for "muddy." At the beginning we had yen little idea what it was made of. It was some organic stew made by breaking apart our starting molecules, and allowing the atoms—carbon, hydrogen, nitrogen—and molecular fragments to recombine. The word "organic" carries no imputation of biological origin; following long-standing chemical usage dating back mots than a century, it merely describes molecules built out of car * Titan's atmosphere has no detectable oxygen, so methane is not wildly out of chemical equilibrium—as it is on Earth—and its presence is in no way a sign of life 57 bon atoms (excluding a few very simple ones such as carbon monoxide, CO, and carbon dioxide, CO2). Since life on Earth is based oil organic molecules, and since there was a time before there was life on Earth, some process must have made organic molecules on our planet before the time of the first organism. Something sitar, I proposed, might be happening on Titan today. The epochal event in our understanding of Titan was the arrival in 1980 and 1981 of the Voyager 1 and 2 spacecraft in the Saturn system. The ultraviolet, infrared, and radio instruments revealed the pressure and temperature through the atmosphere—from the hidden surface to the edge of space. We learned how high the cloud tops are. We found that the air on Titan is composed mainly of nitrogen, N2, as on the Earth today. The other principal constituent is, as Kuiper found, methane. CH4 the starting material from which carbon-based organic molecules are generated there. A variety of simple organic molecules was found, present as gases, mainly hydrocarbons and nitriles. The most complex of them have four "heavy" (carbon and/or nitrogen) atoms. Hydrocarbons are molecules composed of carbon and hydrogen atoms only, and are familiar to us as natural gas, petroleum, and waxes. (They're quite different from carbohydrates, such as sugars and starch, which also have oxygen atoms.) Nitriles are molecules with a carbon and nitrogen atom attached in a particular way. The best known nitrile is HCN, hydrogen cyanide, a deadly gas for humans. But hydrogen cyanide is implicated in the steps that on Earth led to the origin of life. Finding these simple organic molecules in Titan's upper atmosphere—even if present only in a part per million or a part per billion—is tantalizing. Could the atmosphere of the primeval Earth have been similar? There's about ten times more air oil Titan than there is on Earth today, but the early Earth may well have had a denser atmosphere. Moreover, Voyager discovered an extensive region of energetic electrons and protons surrounding Saturn, trapped by the planet's magnetic field. During the course of its orbital motion around Saturn, Titan bobs in and out of this magnetosphere. Beams of electrons (plus solar ultraviolet light) fall on the upper air of Titan, just as charged particles (plus solar ultraviolet light) were intercepted by the atmosphere of the primitive Earth. So it's a natural thought to irradiate the appropriate mixture of nitrogen and methane with ultraviolet light or electrons at very low pressures, and find out what more complex molecules can be made. Can we simulate what's going on in Titan's high atmosphere? In our laboratory at Cornell—with my colleague W. Reid Thompson playing a key role—we've replicated some of Titan's manufacture of organic gases. The simplest hydrocarbons on Titan are manufactured by ultraviolet light from the Sun. But for all the other gas products, those made most readily by electrons in the laboratory correspond to those discovered by Voyager on Titan, and in the same proportions. The correspondence is one to one. The next most abundant gases that we've found in the laboratory will be looked for in future studies of Titan. The most complex organic gases we make have six or seven carbon and/or nitrogen atoms. These product molecules are on their way to forming tholins. WE HAD HOPED FOR A BREAK In the weather as Voyager 1 approached Titan. A long distance away, it appeared as a tiny disk; at closest approach, our camera's field of view was filled by a 60 What I've just told you is a kind of scientific progress report. Tomorrow there might be a new finding that clears up these mysteries and contradictions. Maybe there's something wrong with Muhleman's radar results, although it's hard to see what it might be: His system tells him he's seeing Titan when it's nearest, when he ought to be seeing Titan. Maybe there's something wrong with Dermott's and my calculation about the tidal evolution of the orbit of Titan, but no one has been able to find any errors so far. And it's Bard to see how ethane can avoid condensing out at the surface of Titan. Maybe, despite the low temperatures, over billions of years there's been a change in the chemistry; maybe some combination of comets impacting from the sky and volcanoes and other tectonic events, helped along by cosmic rays, can congeal liquid hydrocarbons, turning them into some complex organic solid that reflects radio waves back to space. Or maybe something reflective to radio waves is floating on the ocean surface. But liquid hydrocarbons are very underdense: Every known organic solid, unless extremely frothy, would sink like a stone in the seas of Titan. Dermott and I now wonder whether, when we imagined continents and oceans on Titan, we were too transfixed by our experience on our own world, too Earth-chauvinist in our thinking. Battered, cratered terrain and abundant impact basins cover other moons in the Saturn system. If we pictured liquid hydrocarbons slowly accumulating on one of those worlds, we would wind up not with global oceans, but with isolated large craters filled, although not to the brim, with liquid hydrocarbons. Many circular seas of petroleum, some over a hundred miles across, would be splattered across the surface—but no perceptible waves would be stimulated by distant Saturn and, it is conventional to think, no ships, no swimmers, no surfers, and no fishing. Tidal friction should, we calculate, be negligible in such a case, and Titan's stretched-out, elliptical orbit would not have become so circular. We can't know for sure until we start getting radar or near-infrared images of the surface. But perhaps this is the resolution of our dilemma: Titan as a world of large circular hydrocarbon lakes, more of them in some longitudes than in others. Should we expect an icy surface covered with deep tholin sediments, a hydrocarbon ocean with at most a few organic encrusted islands poking up here and there, a world of crater lakes, or something more subtle that we haven't yet figured out? This isn't just an academic question, because there's a real spacecraft being designed to go to Titan. In a joint NASA/ESA program, a spacecraft called Cassini will be launched in October 1997—if all goes well. With two flybys of Venus, one of Earth, and one of Jupiter for gravitational assists, the ship will, after a seven-year voyage, be injected into orbit around Saturn. Each time the spacecraft comes close to Titan, the moon will be examined by an array of instruments, including radar. Because Cassini will be so much closer to Titan, it will be able to resolve many details on Titan's surface indetectable to Muhleman's pioneering Earth-based system. It's also likely that the surface can be viewed in the near infrared. Maps of the hidden surface of Titan may be in our hands sometime in the summer of 2004. Cassini is also carrying an entry probe, fittingly called Huygens, which will detach itself from the main spacecraft and plummet into Titan's atmosphere. A great parachute will be deployed. The instrument package will slowly settle through the organic haze down into the lower atmosphere, through the methane clouds. It will examine organic chemistry as it descends, and—if it survives the landing—on the surface of this world as well. 61 Nothing is guaranteed. But the mission is technically feasible, hardware is being built, an impressive coterie of specialists, including many young European scientists, are hard at work on it, and all the nations responsible seem committed to the project. Perhaps it will actually come about. Perhaps winging across the billion miles of intervening interplanetary space will be, in the not too distant future. news about how far along the path to life Titan has come. 62 C H A P T E R 8 THE FIRST NEW PLANET 1 implore you, you do not hope to be able to give the reasons for the number of planets, do you? This worry has been resolved . . . — JOHANNES KEPLER, EPITOME OF COPERNICAN ASTRONOMY, BOOK 4 / 1621 Before we invented civilization, our ancestors lived mainly in the open, out under the sky. Before we devised artificial lights and atmospheric pollution and modern forms of nocturnal entertainment, we watched the stars. There were practical calendrical reasons, of course, but there was more to it than that. Even today, the most jaded city dweller can be unexpectedly moved upon encountering a clear night sky studded with thousands of twinkling stars. When it happens to me after all these years, it still takes my breath away. In every culture, the sky and the religious impulse are intertwined. I lie back in an open field and the sky surround me. I'm overpowered by its scale. It's so vast and so far away that my own insignificance becomes palpable. But I don't feel rejected by the sky. I'm a part of it, tiny, to be sure, but everything is tiny compared to that overwhelming immensity, And when I concentrate on the stars, the planets, and their motions, I have an irresistible sense of machinery, clockwork, elegant precision working on a scale that, however lofty our aspirations, dwarfs and humbles us. Most of the great inventions in human history—from stone tools and the domestication of fire to written language—were made by unknown benefactors. Our institutional memory of long- gone events is feeble. We do not know the name of that ancestor who first noted that planets were different from stars. She or he must have lived tens, perhaps even hundreds of thousands of years ago. But eventually people all over the world understood that five, no more, of the bright points of light that grace the night sky break lockstep with the others over a period of months, moving strangely-almost as if they had minds of their own. Sharing the odd apparent motion of these planets were the Sun and Moon, making seven wandering bodies in all. These seven were important to the ancients, and they named them after gods not any old gods, but the main gods, the chief gods, the ones who tell other gods (and mortals) what to do. One of the planets, bright and slow-moving, was named by the Babylonians after Marduk, by the Norse after Odin, by the Greeks after Zeus, and by the Romans after 65 WHEN CLAIMS OF NEW WORLDS WERE MADE In the late eighteenth century, the force of such numerological arguments had much dissipated. Still, it was with a real sense of surprise that people heard in 1781 about a new planet, discovered through the telescope. New moons were comparatively unimpressive, especially after the first six or eight. But that there were new planets to be found and that humans had devised the means to do so were both considered astonishing, and properly so. If there is one previously unknown planet, there may be many more—in this solar system and in others. Who can tell what might be found if a multitude of new worlds are hiding in the dark? The discovery was made not even by a professional astronomer but by William Herschel, a musician whose relatives had come to Britain with the family of another anglified German, the reigning monarch and future oppressor of the American colonists, George III. It became Herschel's wish to call the planet George ("George's Star," actually), after his patron. but, providentially, the name didn't stick. (Astronomers seem to have been very busy buttering up kings.) Instead, the planet that Herschel found is called Uranus (an inexhaustible source of hilarity renewed in each generation of English-speaking nine-year-olds). It is named after the ancient sky god who, according to Greek myth, was Saturn's father and thus the grandfather of the Olympian gods. We no longer consider the Sun and Moon to be planets, and ignoring the comparatively insignificant asteroids and comets, count Uranus as the seventh planet in order from the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto). It is the first planet unknown to the ancients. The four outer, Jovian, planets turn out to be very different from the four inner, terrestrial, planets. Pluto is a separate case. As the years passed and the quality of astronomical instruments unproved, we began to learn more about distant Uranus. What reflects the dim sunlight back to us is no solid surface, but atmosphere and clouds just as for Titan, Venus, Jupiter Saturn, and Neptune. The air on Uranus is made of hydrogen and helium, the two simplest gases. Methane and other hydrocarbons are also present. Just below the clouds visible to Earthbound observers is a massive atmosphere with enormous quantities of ammonia, hydrogen sulfide, and, especially, water. At depth on Jupiter and Saturn, the pressures are so great that atoms sweat electrons, and the air becomes a metal. That does not seem to happen on less massive Uranus, because the pressures at depth are less. Still deeper, discovered only by its subtle tugs on Uranus' moons, wholly inaccessible to view, under the crushing weight of the overlying atmosphere, is a rocky surface. A big Earthlike planet is hiding down there, swathed in an immense blanket of air. The Earth's surface temperature is due to the sunlight it intercepts. Turn off the Sun and the planet soon chills—not to trifling Antarctic cold, not just so cold that the oceans freeze, but to a cold so intense that the very air precipitates out, forming a ten-meter-thick layer of oxygen and nitrogen snows covering the whole planet. The little bit of energy that trickles up from the Earth's hot interior would be insufficient to melt these snows. For Jupiter, Saturn, and Neptune it's different. There's about as much heat pouring out from their interiors as they acquire from the warmth of the distant Sun. Turn off the Sun, and they would be only a little affected. But Uranus is another story. Uranus is an anomaly among the Jovian planets. Uranus is like the Earth: There's very little intrinsic heat pouring out. We have no good understanding of why this should be, why Uranus—which in many respects is so similar to Neptune—should lack a 66 potent source of internal heat. For this reason, among others, we cannot say we understand what is going on in the deep interiors of these mighty worlds. Uranus is lying on its side as it goes around the Sun. In the 1990s. the south pole is heated by the Sun, and it is this pole that Earthbound observers at the end of the twentieth century see when they look at Uranus. It takes Uranus 84 Earth years to make one circuit of the Sun. So in the 2030s, the north pole will be sunward (and Earthward). In the 2070s the south pole will be pointing to the Sun once again. In between, Earthbound astronomers will be looking mainly at equatorial latitudes. All the other planets spin much more upright in their orbits. No one is sure of the reason for Uranus' anomalous spin; the most promising suggestion is that sometime in its early history, billions of years ago, it was struck by a rogue planet, about the size of the Earth, in a highly eccentric orbit. Such a collision, if it ever happened, must have worked much tumult in the Uranus system; for all we know, there may be other vestiges of ancient havoc still left for us to find. But Uranus' remoteness tends to guard its mysteries. In 1977 a team of scientists led by James Elliot, then of Cornell University, accidentally discovered that, like Saturn, Uranus has rings. The scientists were flying over the Indian Ocean in a special NASA airplane—the Kuiper Airborne Observatory—to witness the passage of Uranus in front of a star. (Such passages, or occultations as they're called, happen from time to time, precisely because Uranus slowly moves with respect to the distant stars.) The observers were surprised to find that the star winked on and off several times just before it passed behind Uranus and its atmosphere, then several times more just after it emerged. Since the patterns of winking on and off were the same before and after occultation, this finding (and much subsequent work) has led to the discovery of nine very thin, very dark circumplanetary rings, giving Uranus the appearance of a bull's-eye in the sky. Surrounding the rings, Earthbound observers understood were the concentric orbits of the five moons then known: Miranda, Ariel, Umbriel, Titania, and Oberon. They're named after characters in Shakespeare's A Midsummer Night's Dream and The Tempest, and in Alexander Pope's The Rape of the Lock. Two of them were found by Herschel himself. The innermost of the five, Miranda, was discovered as recently as 1948, by my teacher G. P. Kuiper.* I remember how great an achievement the discovery of a new moon of Uranus was considered back then. The near-infrared light reflected by all five moons subsequently revealed the spectral signature of ordinary water ice on their Surfaces. And no wonder—Uranus is so far from the Sun that it is no brighter there at noontime than it is after sunset on Earth. The temperatures are frigid. Any water must be frozen. A REVOLUTION IN OUR UNDERSTANDING of the Uranus system—the planet, its rings, and its moons—began on January 24, 1986. On that day, after a journey of 8½ years, the Voyager 2 spacecraft sailed very near to Miranda, and hit the bull's-eye in the sky. Uranus' gravity then * He so named it because of the words spoken by Miranda, the heroine of The Tempest: "O brave new world, That has such people in't." (To Which Prospero replies, "'Tis new to thee." Just so. Like all the other worlds in the Solar System, Miranda is about 4.5 billion years old.) 67 flung it on to Neptune. The spacecraft returned 4,300 close-up pictures of the Uranus system and a wealth of other data. Uranus was found to be surrounded by an intense radiation belt, electrons and protons trapped by the planet's magnetic field. Voyager flew through this radiation belt, measuring the magnetic field and the trapped charged particles as it went. It also detected—in changing timbres, harmonies, and nuance, but mainly in fortissimo—a cacophony of radio waves generated by the speeding, trapped particles. Something similar was discovered on Jupiter and Saturn and would be later found at Neptune—but always with a theme and counterpoint characteristic of each world. On Earth the magnetic and geographical poles are quite close together. On Uranus the magnetic axis and the axis of rotation are tilted away from each other by some 60 degrees. No one yet understands why: Some have suggested that we are catching Uranus in a reversal of its north and south magnetic poles, as periodically happens on Earth. Others propose that this too is the consequence of that mighty, ancient collision that knocked the planet over. But we do not know. Uranus is emitting much more ultraviolet light than it's receiving from the Sun, probably generated by charged particles leaking out of the magnetosphere and striking its upper atmosphere. From a vantage point in the Uranus system, the spacecraft examined a bright star winking on and off as the rings of Uranus passed by. New faint dust bands were found. From the perspective of Earth, the spacecraft passed behind Uranus; so the radio signals it was transmitting back home passed tangentially through the Uranian atmosphere, probing it—to below its methane clouds. A vast and deep ocean, perhaps 8,000 kilometers thick, of super- heated liquid water floating in the air is inferred by some. Among the principal glories of the Uranus encounter were the pictures. With Voyager's two television cameras, we discovered ten new moons, determined the length of the day in the clouds of Uranus (about 17 hours), and studied about a dozen rings. The most spectacular pictures were those returned from the five larger, previously known moons of Uranus, especially the smallest of them, Kuiper's Miranda. Its surface is a tumult of fault valleys, parallel ridges, sheer cliffs, low mountains, impact craters, and frozen floods of once-molten surface material. This turmoiled landscape is unexpected for a small, cold, icy world so distant from the Sun. Perhaps the surface was melted and reworked in some long-gone epoch when a gravitational resonance between Uranus, Miranda, and Ariel pumped energy from the nearby planet into Miranda's interior. Or perhaps we are seeing the results of the primordial collision that is thought to have knocked Uranus over. Or, just conceivably, maybe Miranda was once utterly destroyed, dismembered, blasted into smithereens by a wild careening world, with many collision fragments still left in Miranda's orbit. The shards and remnants, slowly colliding, gravitationally attracting one another, may have re-aggregated into just such a jumbled, patchy, unfinished world as Miranda is today. For me, there's something eerie about the pictures of dusky Miranda, because I can remember so well when it `vas only a faint point of light almost lost in the glare of Uranus, discovered through great difficulty by dint of the astronomer's skills and patience. In only half a lifetime it has gone from an undiscovered world to a destination whose ancient and idiosyncratic secrets have been at least partially revealed. 70 Uranus and Neptune are fundamentally rock and ice worlds swaddled in dense atmospheres that hide them from view. Neptune is four times bigger than the Earth. When we look down on its cool, austere blueness, again we are seeing only atmosphere and clouds—no solid surface. Again, the atmosphere is made mainly of hydrogen and helium, with a little methane and traces of other hydrocarbons. There may also be some nitrogen. The bright clouds, which seem to be methane crystals, float above thick, deeper clouds of unknown composition. From the motion of the clouds we discovered fierce winds, approaching the local speed of sound. A Great Dark Spot was found, curiously at almost the same latitude as the Great Red Spot on Jupiter. The azure color seems appropriate for a planet named after the god of the sea. Surrounding this dimly lit, chilly, stormy, remote world is—here also—a system of rings, each composed of innumerable orbiting objects ranging in size from the fine particles in cigarette smoke to small trucks. Like the rings of the other Jovian planets, those of Neptune seem to be evanescent—it is calculated that gravity and solar radiation will disrupt them in much less than the age of the Solar System. If they are destroyed quickly, we must see them only because they were made recently. But how can rings be made? The biggest moon in the Neptune system is called Triton.* Nearly six of our days are required for it to orbit Neptune, which—alone among big moons in the Solar System—it does in the opposite direction to which its planet spins (clockwise if we say Neptune rotates counterclockwise). Triton has a nitrogen-rich atmosphere, somewhat similar to Titan's; but, because the air and haze are much thinner, we can see its surface. The landscapes are varied and splendid. This is a world of ices—nitrogen ice, methane ice, probably underlain by more familiar water ice and rock. There are impact basins, which seem to have been flooded with liquid before refreezing (so there once were lakes on Triton); impact craters; long crisscrossing valleys; vast plains covered by freshly fallen nitrogen snow; puckered terrain that resembles the skin of a cantaloupe; and more or less parallel, long, dark streaks that seem to have been blown by the wind and then deposited on the icy surface despite how sparse Triton's atmosphere is (about 1/10,000 the thickness of the Earth's). All the craters on Triton are pristine—as if stamped out by some vast milling device. There are no slumped walls or muted relief. Even with the periodic falling and evaporation of snow, it seems that nothing has eroded the surface of Triton in billions of years. So the craters that were gouged out during the formation of Triton must have all been filled in and covered over by some early global resurfacing event. Triton orbits Neptune in the opposite direction to Neptune's rotation—unlike the situation with the Earth and its moon, and with most of the large moons in the Solar System. If Triton had formed out of the same spinning disk that made Neptune, it ought to be going around Neptune in the same direction that Neptune rotates. So Triton was not made from the original local nebula around Neptune, but arose somewhere else—perhaps far beyond Pluto—and was by chance gravitationally captured when it passed too close to Neptune. This * Robert Goddard, the inventor of the modern liquid-fueled rocket, envisioned a time when expeditions to the stars would be outfitted on and launched from Triton. This was in a 1927 afterthought to a 1918 handwritten manuscript called "The Last Migration." Considered much too daring for publication, it was deposited in a friend's safe. The cover page bears a warning: "The[se] notes should be read thoroughly only by an optimist." 71 event should have raised enormous solid-body tides in Triton, melting the surface and sweeping away all the past topography. In some places the surface is as bright and white as freshly fallen Antarctic snows (and may offer a skiing experience unrivaled in all the Solar System). Elsewhere there's a tint, ranging from pink to brown. One possible explanation: Freshly fallen snows of nitrogen, methane, and other hydrocarbons are irradiated by solar ultraviolet light and by electrons trapped in the magnetic field of Neptune, through which Triton plows. We know that such irradiation will convert the snows (like the corresponding gases) to complex, dark, reddish organic sediments, ice tholins—nothing alive, but here too composed of some of the molecules implicated in the origin of life on Earth four billion years ago. In local winter, layers of ice and snow build up on the surface. (Our winters, mercifully, are only 4 percent as long.) Through the spring, they are slowly transformed, more and more reddish organic molecules accumulating. By summertime, the ice and snow have evaporated; the gases so released migrate halfway across the planet to the winter hemisphere and there cover the surface with ice and snow again. But the reddish organic molecules do not vaporize and are not transported—a lag deposit, they are next winter covered over by new snows, which are in turn irradiated, and by the following summer the accumulation is thicker. As time goes on, substantial amounts of organic matter are built up on the surface of Triton, which may account for its delicate color markings. The streaks begin in small, dark source regions, perhaps when the warmth of spring and summer heats subsurface volatile snows. As they vaporize, gas comes gushing out as in a geyser, blowing off less-volatile surface snows and dark organics. Prevailing low-speed winds carry away the dark organics, which slowly sediment out of the thin air, are deposited on the ground, and generate the appearance of the streaks. This, at least, is one reconstruction of recent Tritonian history. Triton may have large, seasonal polar caps of smooth nitrogen ice underlying layers of dark organic materials. Nitrogen snows seem recently to have fallen at the equator. Snowfalls, geysers, windblown organic dust, and high-altitude hazes were entirely unexpected on a world with so thin an atmosphere. Why is the air so thin? Because Triton is so far from the Sun. Were you somehow to pick this world up and move it into orbit around Saturn, the nitrogen and methane ices would quickly evaporate, a much denser atmosphere of gaseous nitrogen and methane would form, and radiation would generate an opaque tholin haze. It would become a world very like Titan. Conversely, if you moved Titan into orbit about Neptune, almost all its atmosphere would freeze out as snows and ices, the tholin would fall out and not be replaced, the air would clear, and the surface would become visible in ordinary light. It would become a world very like Triton. These two worlds are not identical. The interior of Titan seems to contain much more ice than that of Triton, and much less rock. Titan's diameter is almost twice that of Triton. Still, if placed at the same distance from the Sun they would look like sisters. Alan Stern of the Southwest Research Institute suggests that they are two members of a vast collection of small worlds rich in nitrogen and methane that formed in the early Solar System. Pluto, yet to be visited by a spacecraft, appears to be another member of this group. Many more may await 72 discovery beyond Pluto. The thin atmospheres and icy surfaces of all these worlds are being irradiated—by cosmic rays, if nothing else and nitrogen—rich organic compounds are being formed. It looks as if the stuff of life is sitting not just on Titan, but throughout the cold, dimly lit outer reaches of our planetary system. Another class of small objects has recently been discovered, whose orbits take them—at least part of the time—beyond Neptune and Pluto. Sometimes called minor planets or asteroids, they are more likely to be inactive comets (with no tails, of course; so far from the Sun, their ices cannot readily vaporize). But they are much bigger than the run-of-the-mill comets we know. They may be the vanguard of a vast array of small worlds that extends from the orbit of Pluto halfway to the nearest star. The innermost province of the Oort Comet Cloud, of which these new objects may be members, is called the Kuiper Belt, after my mentor Gerard Kuiper, who first suggested that it should exist. Short-period comets—like Halley's—arise in the Kuiper Belt, respond to gravitational tugs, sweep into the inner part of the Solar System, grow their tails, and grace our skies. Back in the late nineteenth century, these building blocks of worlds—then mere hypotheses—were called "planetesimals." The flavor of the word is, I suppose, something like that of "infinitesimals": You need an infinite number of them to make anything. It's not quite that extreme with planetesimals, although a very large number of them would be required to make a planet. For example, trillions of bodies each a kilometer in size would be needed to coalesce to make a planet with the mass of the Earth. Once there were much larger numbers of worldlets in the planetary part of the Solar System. Most of them are now gone—ejected into interstellar space, fallen into the Sun, or sacrificed in the great enterprise of building moons and planets. But out beyond Neptune and Pluto the discards, the leftovers that were never aggregated into worlds, may be waiting—a few largish ones in the 100-kilometer range, and stupefying numbers of kilometer-sized and smaller bodies peppering the outer Solar System all the way out to the Oort Cloud. In this sense there are planets beyond Neptune and Pluto—but they are not nearly as big as the Jovian planets, or even Pluto. Larger worlds may, for all we know, also be hiding in the dark beyond Pluto, worlds that can properly be called planets. The farther away they are, the less likely it is that we would have detected them. They cannot lie just beyond Neptune, though; their gravitational tugs would have perceptibly altered the orbits of Neptune and Pluto, and the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft. The newly discovered cometary bodies (with names like 1992QB and 1993FW) are not planets in this sense. If our detection threshold has just encompassed them, many more of them probably remain to be discovered in the outer Solar System—so far away that they're hard to see from Earth, so distant that it's a long journey to get to them. But small, quick ships to Pluto and beyond are within our ability. It would make good sense to dispatch one by Pluto and its moon Charon, and then, if we can, to make a close pass by one of the denizens of the Kuiper Comet Belt. The rocky Earthlike cores of Uranus and Neptune seem to have accreted first, and then gravitationally attracted massive amounts of hydrogen and helium gas from the ancient nebula out of which the planets formed. Originally, they lived in a hailstorm. Their gravities were just sufficient to eject icy worldlets, when they came too close, far out beyond the realm of the 75 but as far as we could see, no life. There was no oxygen in their atmospheres, and no gases profoundly out of chemical equilibrium, as methane is in the Earth's oxygen. Many of the worlds were painted with subtle colors, but none with such distinctive, sharp absorption features as chlorophyll provides over much of the Earth's surface. On very few worlds was Voyager able to resolve details as small as a kilometer across. By this standard, it would not have detected even our own technical civilization had it been transplanted to the outer Solar System. But for what it's worth, we found no regular patterning, no geometrization, no passion for small circles, triangles, squares, or rectangles. There were no constellations of steady points of light on the night hemispheres. There were no signs of a technical civilization reworking the surface of any of these worlds. The Jovian planets are prolific broadcasters of radio waves—generated in part by the abundant trapped and beamed charged particles in their magnetic fields, in part by lightning, and in part by their hot interiors. But none of this emission has the character of intelligent life—or so it seems to the experts in the field. Of course our thinking may be too narrow. We may be missing something. For example, there is a little carbon dioxide in the atmosphere of Titan, which puts its nitrogen/methane atmosphere out of chemical equilibrium. I think the CO2 is provided by the steady pitter-patter of comets falling into Titan's atmosphere—but maybe not. Maybe there's something on the surface unaccountably generating CO2 in the face of all that methane. The surfaces of Miranda and Triton are unlike anything else we know. There are vast chevron-shaped landforms and crisscrossing straight lines that even sober planetary geologists once mischievously described as "highways." We think we (barely) understand these landforms in terms of faults and collisions, but of course we might be wrong. The surface stains of organic matter—sometimes, as on Triton, delicately hued—are attributed to charged particles producing chemical reactions in simple hydrocarbon ices, generating more complex organic materials, and all this having nothing to do with the intermediation of life. But of course we might be wrong. The complex pattern of radio static, bursts, and whistles that we receive from all four Jovian planets seems, in a general way, explicable by plasma physics and thermal emission. (Much of the detail is not yet well understood.) But of course we might be wrong. We have found nothing on dozens of worlds so clear and striking as the signs of life found by the Galileo spacecraft in its passages by the Earth. Life is a hypothesis of last resort. You invoke it only when there's no other way to explain what you see. If I had to judge, I would say that there's no life on any of the worlds we've studied, except of course our own. But I might be wrong, and, right or wrong, my judgment is necessarily confined to this Solar System. Perhaps on some new mission we'll find something different, something striking, something wholly inexplicable with the ordinary tools of planetary science—and tremulously, cautiously, we will inch toward a biological explanation. However, for now nothing requires that we go down such a path. So far, the only life in the Solar System is that which comes from Earth. In the Uranus and Neptune systems, the only sign of life has been Voyager itself. As we identify the planets of other stars, as we find other worlds of roughly the size and mass of the Earth, we will scrutinize them for life. A dense oxygen atmosphere may be detectable even on a world we've never imaged. As for the Earth, that may by itself be a sign of 76 life. An oxygen atmosphere with appreciable quantities of methane would almost certainly be a sign of life, as would modulated radio emission. Someday, from observations of our planetary system or another, the news of life elsewhere may be announced over the morning coffee. THE VOYAGER SPACECRAFT are bound for the stars. They are on escape trajectories from the Solar System, barreling along at almost a million miles a day. The gravitational fields of Jupiter, Saturn, Uranus, and Neptune have flung them at such high speeds that they have broken the bonds that once tied them to the Sun. Have they left the Solar System yet? The answer depends very much on how you define the boundary of the Sun's realm. If it's the orbit of the outermost good-sized planet, then the Voyager spacecraft are already long gone; there are probably no undiscovered Neptunes. If you mean the outermost planet, it may be that there are other—perhaps Triton-like—planets far beyond Neptune and Pluto; if so, Voyager 1 and Voyager 2 are still within the Solar System. If you define the outer limits of the Solar System as the heliopause—where the interplanetary particles and magnetic fields are replaced by their interstellar counterparts—then neither Voyager has yet left the Solar System, although they may do so in the next few decades.* But if your definition of the edge of the Solar System is the distance at which our star can no longer hold worlds in orbit about it, then the Voyagers will not leave the Solar System for hundreds of centuries. Weakly grasped by the Sun's gravity, in every direction in the sky, is that immense horde of a trillion comets or more, the port Cloud. The two spacecraft will finish their passage through the Oort cloud in another 20,000 years or so. Then, at last, completing their long good-bye to the Solar System, broken free of the gravitational shackles that once bound them to the Sun, the Voyagers will make for the open sea of interstellar space. only then will Phase Two of their mission begin. Their radio transmitters long dead, the spacecraft will wander for ages in the calm, cold interstellar blackness—where there is almost nothing to erode them. Once out of the Solar System, they will remain intact for a billion years or more, as they circumnavigate the center of the Milky Way galaxy. We do not know whether there are other space-faring civilizations in the Milky Way. If they do exist, we do not know how abundant they are, much less where they are. But there is at least a chance that sometime in the remote future one of the Voyagers will be intercepted and examined by an alien craft. Accordingly, as each Voyager left Earth for the planets and the stars, it carried with it a golden phonograph record encased in a golden, mirrored jacket containing, among other things; greetings in 59 human languages and one whale language; a 12-minute sound essay including a * Radio signals that both Voyagers detected in 1992 are thought to arise from the collision of powerful gusts of solar wind with the thin gas that lies between the stars. From the immense .power of the signal (over 10 trillion watts), the distance to the heliopause can be estimated: about 100 times the Earth's distance from the Sun. At the speed it's leaving the Solar System, Voyager 1 might pierce the heliopause and enter interstellar space around the year 2010. If its radioactive power source is still working, news of the crossing will be radioed back to the stay-at-homes on Earth. The energy released by the collision of this shock wave with the heliopause makes it the most powerful source of radio emission in the Solar System. It makes you wonder whether even stronger shocks in other planetary systems might he detectable by our radio telescope. 77 kiss, a baby's cry, and an EEG record of the meditations of a young woman in love; 116 encoded pictures, on our science, our civilization, and ourselves; and 90 minutes of the Earth's greatest hits—Eastern and Western, classical and folk, including a Navajo night chant, a Japanese shakuhachi piece, a Pygmy girl's initiation song, a Peruvian wedding song, a 3,000-year-old composition for the ch'in called "Flowing Streams," Bach, Beethoven, Mozart, Stravinsky, Louis Armstrong, Blind Willie Johnson, and Chuck Berry's "Johnny B. Goode." Space is nearly empty. There is virtually no chance that one of the Voyagers will ever enter another solar system—and this is true even if every star in the sky is accompanied by planets. The instructions on the record jackets, written in what we believe to be readily comprehensible scientific hieroglyphics, can be read, and the contents of the records understood, only if alien beings, somewhere in the remote future, find Voyager in the depths of interstellar space. Since both Voyagers will circle the center of the Milky Way Galaxy essentially forever, there is plenty of time for the records to be found—if there's anyone out there to do the finding. We cannot know how much of the records they would understand. Surely the greetings will be incomprehensible, but their intent may not be. (We thought it would be impolite not to say hello.) The hypothetical aliens are bound to be very different from us—independently evolved on another world. Are we really sure they could understand anything at all of our message? Every time I feel these concerns stirring, though, I reassure myself. Whatever the incomprehensibilities of the Voyager record, any alien ship that finds it will have another standard by which to judge us. Each Voyager is itself a message. In their exploratory intent, in the lofty ambition of their objectives, in their utter lack of intent to do harm, and in the brilliance of their design and performance, these robots speak eloquently for us. But being much more advanced scientists and engineers than we—otherwise they would never be able to find and retrieve the small, silent spacecraft in interstellar space—perhaps the aliens would have no difficulty understanding what is encoded on these golden records. Perhaps they would recognize the tentativeness of our society, the mismatch between our technology and our wisdom. Have we destroyed ourselves since launching Voyager, they might wonder, or have we gone on to greater things? Or perhaps the records will never be intercepted. Perhaps no one in five billion years will ever come upon them. Five billion years is a long time. In five billion years, all humans will have become extinct or evolved into other beings, none of our artifacts will have survived on Earth, the continents will have become unrecognizably altered or destroyed, and the evolution of the Sun will have burned the Earth to a crisp or reduced it to a whirl of atoms. Far from home, untouched by these remote events, the Voyagers, bearing the memories of a world that is no more, will fly on. 80 light, are more efficiently scattered than the longer wavelengths, those that we sense as orange and red light. When we look up on a cloudless day and admire the blue sky, we are witnessing the preferential scattering of the short waves in sunlight. This is called Rayleigh scattering, after the English physicist who offered the first coherent explanation for it. Cigarette smoke is blue for just the same reason: The particles that make it up are about as small as the wavelength of blue light. So why is the sunset red? The red of the sunset is what's left of sunlight after the air scatters the blue away. Since the atmosphere is a thin shell of gravitationally bound gas surrounding the solid Earth, sunlight must pass through a longer slant path of air at sunset (or sunrise) than at noon. Since the violet and blue waves are scattered even more during their now-longer path through the air than when the Sun is overhead, what we see when we look toward the Sun is the residue—the waves of sunlight that are hardly scattered away at all, especially the oranges and reds. A blue sky makes a red sunset. (The noontime Sun seems yellowish partly because it emits slightly more yellow light than other colors, and partly because, even with the Sun overhead, some blue light is scattered out of the sunbeams by the Earth's atmosphere.) It is sometimes said that scientists are unromantic, that their passion to figure out robs the world of beauty and mystery. But is it not stirring to understand how the world actually works— that white light is made of colors, that color is the way we perceive the wavelengths of light, that transparent air reflects light, that in so doing it discriminates among the waves, and that the sky is blue for the same reason that the sunset is red? It does no harm to the romance of the sunset to know a little bit about it. Since most simple molecules are about the same size (roughly a hundred millionth of a centimeter), the blue of the Earth's sky doesn't much depend on what the air is made of—as long as the air doesn't absorb the light. Oxygen and nitrogen molecules don't absorb visible light; they only bounce it away in some other direction. Other molecules, though, can gobble up the light. Oxides of nitrogen—produced in automotive engines and in the fires of industry—are a source of the murky brown coloration of smog. Oxides of nitrogen (made from oxygen and nitrogen) do absorb light. Absorption, as well as scattering, can color a sky. OTHER WORLDS, OTHER SKIES: Mercury, the Earth's Moon, and most satellites of the other planets are small worlds; because of their feeble gravities, they are unable to retain their atmospheres—which instead trickle of into space. The near-vacuum of space then reaches the ground. Sunlight strikes their surfaces unimpeded, neither scattered nor absorbed along the way. The skies of these worlds are black, even at noon. This has been witnessed firsthand so far by only 12 humans, the lunar landing crews of Apollos 11, 12, and 14-17. A full list of the satellites in the Solar System, known as of this writing, is given in the accompanying table. (Nearly half of them were discovered by Voyager.) All have black skies— except Titan of Saturn and perhaps Triton of Neptune, which are big enough to have atmospheres. And all asteroids as well. Venus has about 90 times more air than Earth. It isn't mainly oxygen and nitrogen as here— it's carbon dioxide. But carbon dioxide doesn't absorb visible light either. What would the sky look like from the surface of Venus if Venus had no clouds. With so much atmosphere in the way, not only are violet and blue waves scattered, but all the other colors as well-green yellow, 81 orange, red. The air is so thick, though, that hardly any blue light makes it to the ground; it's scattered back to space by successive bounces higher up. Thus, the light that does reach the ground should be strongly reddened-like an Earth sunset all over the sky. Further, sulfur in the high clouds will stain the sky yellow. Pictures taken by the Soviet Venera landers confirm that the skies of Venus are a kind of yellow-orange. SIXTY-TWO WORLDS FOR THE THIRD MILLENNIUM: KNOWN MOONS OF THE PLANETS (AND ONE ASTEROID)— LISTED IN ORDER OF DISTANCE FROM THEIR PLANET EARTH, 1 MARS, 2 IDA, 1 JUPITER, 16 SATURN, 18 URANUS, 15 NEPTUNE, 8 PLUT0, 1 Moon Phobos Dactyl Metis Pan Cordelia Naiad Charon Deimos Adrastea Atlas Ophelia Thalassa Amalthea Prometheus Bianca Despina Thebe Pandora Cressida Galatea to Epimetheus Desdemona Larissa Europa Janus Juliet Proteus Ganymede Mimas Portia Triton Callisto Enceladus Rosalind Nereid Leda Tethys Belinda Himalia Telesto Puck Lysithea Calypso Miranda Elara Diane Ariel Ananke Helene Umbriel Carme Rhea Titania Pasiphae Titan Oberon Sinope Hyperion Iapetus Phoebe Mars is a different story. It is a smaller world than Earth, with a much thinner atmosphere. The pressure at the surface of Mars is, in fact, about the same as the altitude in the Earth's stratosphere to which Simons rose. So we might expect the Martian sky to be black or purple- black. The first color picture from the surface of Mars was obtained in July 1976 by the American Viking 1 lander—the first spacecraft to touch down successfully on the surface of the Red Planet. The digital data were dutifully radioed from Mars back to Earth, and the color picture assembled by computer. To the surprise of all the scientists and nobody else, that first image, released to the press, showed the Martian sky to be a comfortable, homey blue— impossible for a planet with so insubstantial an atmosphere. Something had gone wrong. The picture on your color television set is a mixture of three monochrome images, each in a different color of light—red, green, and blue. You can see this method of color compositing in video projection systems, which project separate beams of red, green, and blue light to generate a full-color picture (including yellows). To get the right color, your set needs to mix or balance these three monochrome images correctly. If you turn up the intensity of, say, blue, the picture will appear too blue. Any picture returned from space requires a similar color balance. Considerable discretion is sometimes left to the computer analysts in deciding this balance. The 82 hiking analysts were not planetary astronomers, and with this first color picture from Mars they simply mixed the colors until it looked "right." We are so conditioned by our experience on Earth that "right," of course, means a blue sky. The color of the picture was soon corrected— using color calibration standards placed for this very purpose on board the spacecraft—and the resulting composite showed no blue sky at all; rather it was something between ochre and pink. Not blue, but hardly purple-black either. This is the right color of the Martian sky. Much of the surface of Mars is desert—and red because the sands are rusk. There are occasional violent sandstorms that lift fine particles from the surface high into the atmosphere. It takes a long time for them to fall out, and before the sky has fully cleaned itself, there's always another sandstorm. Global or near-global sandstorms occur almost every Martian year. Since rusty particles are always suspended in this sky, future generations of humans, born and living out their lives on Mars, will consider that salmon color to be as natural and familiar as we consider our homey blue. From a single glance at the daytime sky, they'll probably be able to tell how long it's been since the last big sandstorm. The planets in the outer Solar System—Jupiter, Saturn, Uranus, and Neptune— are of a different sort. These are huge worlds with giant atmospheres made mainly of hydrogen and helium. Their solid surfaces are so deep inside that no sunlight penetrates there at all. Down there, the sky is black, with no prospect of a sunrise—not ever. The perpetual starless night is perhaps illuminated on occasion by a bolt of lightning. But higher in the atmosphere, where the sunlight reaches, a much more beautiful vista awaits. On Jupiter, above a high-altitude haze layer composed of ammonia (rather than water) ice particles, the sky is almost black. Farther down, in the blue sky region, are multicolored clouds—in various shades of yellow-brown, and of unknown composition. (The candidate materials include sulfur, phosphorus, and complex organic molecules.) Even farther down, the sky will appear red-brown, except that the clouds there are of varying thicknesses, and where they are thin, you might see a patch of blue. Still deeper, we gradually return to perpetual night. Something similar is true on Saturn, but the colors there are more muted. Uranus and especially Neptune have an uncanny, austere blue color through which clouds— some of them a little whiter—are carried by high-speed winds. Sunlight reaches a comparatively clean atmosphere composed mainly of hydrogen and helium but also rich in methane. Long paths of methane absorb yellow and especially red light and let the green and blue filter through. A thin hydrocarbon haze removes a little blue. There may be a depth where the sky is greenish. Conventional wisdom holds that the absorption by methane and the Rayleigh scattering of sunlight by the deep atmosphere together account for the blue colors on Uranus and Neptune. But analysis of Voyager data by Kevin Baines of JPL seems to show that these causes are insufficient. Apparently very deep—maybe in the vicinity of hypothesized clouds of hydrogen sulfide—there is an abundant blue substance. So far no one has been able to figure out what it might be. Blue materials are very rare in Nature. As always happens in science, the old mysteries are dispelled only to be replaced by new ones. Sooner or later we'll find out the answer to this one, too. ALL WORLDS WITH NONBLACK SKIES have atmospheres. If you're standing on the surface and 85 landings—the last making a bull's-eye in the crater Alphonsus. But time was short for the Venus mission, and cameras were heavy. There were those who maintained that cameras weren't really scientific instruments, but rather catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to answer a single straightforward, well-posed scientific question. I thought myself that whether there are breaks in the clouds was one such question. I argued that cameras could also answer questions that we were too dumb even to pose. I argued that pictures were the only way to show the public—who were, after all, footing the bill—the excitement of robotic missions. At any rate, no camera was flown, and subsequent missions have, for this particular world, at least partly vindicated that judgment: Even at high resolution from close flybys, in visible light it turns out there are no breaks in the clouds of Venus, any more than in the clouds of Titan.* These worlds are permanently overcast. In the ultraviolet there is detail, but due to transient patches of high-altitude overcast, far above the main cloud deck. The high clouds race around the planet much faster than the planet itself turns: super-rotation. We have an even smaller chance of seeing the surface in the ultraviolet. When it became clear that the atmosphere of Venus was much thicker than the air on Earth—as we now know, the pressure at the surface is ninety times what it is here—it immediately followed that in ordinary visible light we could not possibly see the surface, even if there were breaks in the clouds. What little sunlight is able to make its tortuous way through the dense atmosphere to the surface would be reflected back, all right; but the photons would be so jumbled by repeated scattering off molecules in the lower air that no image of surface features could be retained. It would be like a "whiteout" in polar snowstorm. However, this effect, intense Rayleigh scattering, declines rapidly with increasing wavelength; in the near-infrared, it was easy to calculate, you could see the surface if there were breaks in the clouds or if the clouds were transparent there. So in 1970 Jim Pollack, Dave Morrison, and I went to the McDonald Observatory of the University of Texas to try to observe Venus in the near-infrared. We "hypersensitized" our emulsions; the good old-fashioned† glass photographic plates were treated with ammonia, and sometimes heated or briefly illuminated, before being exposed at the telescope to light from Venus. For a time the cellars of McDonald Observatory reeked of ammonia. We took many pictures. None showed any detail. We concluded that either we hadn't gone far enough into the infrared or the clouds of Venus were opaque and unbroken in the near infrared. More than 20 years later, the Galileo spacecraft, making a close flyby of Venus, examined it with higher resolution and sensitivity, and at wavelengths a little further into the infrared than we were able to reach with our crude glass emulsions. Galileo photographed great mountain ranges. * For Titan, imaging revealed a succession of detached hazes above the main layer of aerosols. So Venus works out to be the only world in the Solar System for which spacecraft cameras working in ordinary visible light haven't discovered something important. Happily, we've now returned pictures from almost every world we've visited. (NASA's International Cometary Explorer, which raced through the tail of Comet Giacobini-Zimmer in 1985, flew blind, be devoted to charged particles and magnetic fields.) † Today many telescopic images are obtained with such electronic contrivances as charge-coupled devices and diode arrays, and processed by computer—all technologies unavailable to astronomers in 1970. 86 We already knew of their existence, though; a much more powerful technique had earlier been employed: radar. Radio waves effortlessly penetrate the clouds and thick atmosphere of Venus, bounce off the surface, and return to Earth, where they are gathered in and used to make a picture. The first work had been done, chiefly, by. American ground-based radar at JPL's Goldstone tracking station in the Mojave Desert and at the Arecibo Observatory in Puerto Rico, operated by Cornell University. Then the U.S. Pioneer 12, the Soviet Venera 15 and ?6 and the U.S. Magellan missions inserted radar telescopes into orbit around Venus and mapped the place pole to pole. Each spacecraft would transmit a radar signal to the surface and then catch it as it bounced back. From how reflective each patch of surface was and how long it took the signal to return (shorter from mountains, longer from valleys), a detailed map of the entire surface was slowly and painstakingly constructed. The world so revealed turns out to be uniquely sculpted by lava flows (and, to a much lesser degree, by wind), as described in the next chapter. The clouds and atmosphere of Venus have now become transparent to us, and another world has been visited by the doughty robot explorers from Earth. Our experience with Venus is now being applied elsewhere—especially to Titan, where once again impenetrable clouds hide an enigmatic surface, and radar is beginning to give us hints of what might lie below. VENUS HAD LONG BEEN THOUGHT of as our sister world. It is the nearest planet to the Earth. It has almost the same mass, size, density, and gravitational pull as the Earth does. It's a little closer to the Sun than the Earth, but its bright clouds reflect more sunlight back to space than our clouds do. As a first guess you might very well imagine that, under those unbroken clouds, Venus was rather like Earth. Early scientific speculation included fetid swamps crawling with monster amphibians, like the Earth in the Carboniferous Period; a world desert; a global petroleum sea; and a seltzer ocean dotted here and there with limestone-encrusted islands. While based on some scientific data, these “models" of Venus—the first dating from the beginnings of the century, the second from the 1930s, and the last two from the raid-1950s—were little more than scientific romances, hardly constrained by the sparse data available. Then, in 1956, a report was published in The Astrophysical Journal by Cornell H. Mayer and his colleagues. They had pointed a newly completed radio telescope, built in part for classified research, on the roof of the Naval Research Laboratory in Washington, D.C., at Venus and measured the flux of radio waves arriving at Earth. This was not radar: No radio waves were bounced off Venus. This was listening to radio waves that Venus on its own emits to space. Venus turned out to be much brighter than the background of distant stars and galaxies. This in itself was not very surprising. Every object warmer than absolute zero (-273°C) gives off radiation throughout the electromagnetic spectrum, including the radio region. You, for example, emit radio waves at an effective or "brightness" temperature of about 35°C, and if you were in surroundings colder than you are, a sensitive radio telescope could detect the faint radio waves you are transmitting in all directions. Each of us is a source of cold static. What was surprising about Mayer's discovery was that the brightness temperature of Venus is more than 300°C, far higher than the surface temperature of the Earth or the measured infrared 87 temperature of the clouds of Venus. Some places on Venus seemed at least 200° hotter than the normal boiling point of water. What could this mean? Soon there was a deluge of explanations. I argued that the high radio brightness temperature was a direct indication of a hot surface, and that the high temperatures were due to a massive carbon dioxide/water vapor greenhouse effect—in which some sunlight is transmitted through the clouds and heats the Surface, but the surface experiences enormous difficulty in radiating back to space because of the high infrared opacity of carbon dioxide and water vapor. Carbon dioxide absorbs at a range of wavelengths through the infrared, but there seemed to be "windows" between the CO2 absorption bands through which the surface could readily cool off to space. Water vapor, though, absorbs at infrared frequencies that correspond in part to the windows in the carbon dioxide opacity. The two gases together, it seemed to me, could pretty well absorb almost all the infrared emission, even if there was very little water vapor— something like two picket fences, the slats of one being fortuitously positioned to cover the gaps of the other. There was another very different category of explanation, in which the high brightness temperature of Venus had nothing to do with the ground. The surface could still be temperate, clement, congenial. It was proposed that some region in the atmosphere of Venus or in its surrounding magnetosphere emitted these radio waves to space. Electrical discharges between water droplets in the Venus clouds were suggested. A glow discharge in which ions and electrons recombined at twilight and dawn in the upper atmosphere was offered. A very dense ionosphere had its advocates, in which the mutual acceleration of unbound electrons ("free-free emission") gave off radio waves. (One proponent of this idea even suggested that the high ionization required was due to an average of 10,000 times greater radioactivity on Venus than on Earth—perhaps from a recent nuclear war there.) And, in the light of the discovery of radiation from Jupiter's magnetosphere, it was natural to suggest that the radio emission came from an immense cloud of charged particles trapped by some hypothetical very intense Venusian magnetic field. In a series of papers I published in the middle 1960s, many in collaboration with Jim Pollack,* these conflicting models of a high hot emitting region and a cold surface were subjected to a critical analysis. By then we had two important new clues: the radio spectrum of Venus, and the Mariner 2 evidence that the radio emission was more intense at the center of the disk of Venus than toward its edge. By 1967 we were able to exclude the alternative models with some confidence, and conclude that the surface of Venus was at a scorching and un-Earthlike temperature, in excess of 400°C. But the argument was inferential, and there were many intermediate steps. We longed for a more direct measurement. In October 1967—commemorating the tenth anniversary of Sputnik 1—the Soviet Venera 4 spacecraft dropped an entry capsule into the clouds of Venus. It returned data from the hot lover atmosphere, but did not survive to the surface. One day later, the United States spacecraft * James B. Pollack made important contributions to every area of planetary science. He was my first graduate student and a colleague ever since. He converted NASA's Ames Research Center into a world leader in planetary research and the post-doctoral training of planetary scientists. His gentleness was as extraordinary as his scientific abilities. He died in 1994 at the height of his powers. 90 C H A P T E R 12 THE GROUND MELTS Midway between Thera and Therasia, fires broke forth from the sea and continued for four days, so that the whole sea boiled and blazed, and the fires cast up an island which was gradually elevated as though by levers . . . After the cessation of the eruption, the Rhodians, at the time of their maritime supremacy, were first to venture upon the scene and to erect on the island a temple. —STRABO, GEOGRAPHY (CA. 7 B.C) All over the Earth, you can find a kind of mountain with one striking and unusual feature. Any child can recognize it: The top seems sheared or squared off: If you climb to the summit or fly over it, you discover that the mountain has a hole or crater at its peak. In some mountains of this sort, the craters are small; in others, they are almost as big as the mountain itself. Occasionally, the craters are filled with water. Sometimes they're filled with a more amazing liquid: You tiptoe 10 the edge, and see vast, glowing lakes of yellow-red liquid and fountains of fire. These holes in the tops of mountains are called calderas, after the word "caldron," and the mountains on which they sit are known, of course, as volcanos—after Vulcan, the Roman god of fire. There are perhaps 600 active volcanos discovered on Earth. Some, beneath the oceans, are yet to be found. A typical volcanic mountain looks safe enough. Natural vegetation runs up its sides. Terraced fields decorate its flanks. Hamlets and shrines nestle at its base. And yet, without warning, after centuries of lassitude, the mountain may explode. Barrages of boulders, torrents of ash drop out of the sky. Rivers of molten rock come pouring down its sides. All over the Earth people imagined that an active volcano was an imprisoned giant or demon struggling to get out. The eruptions of Mt. St. Helens and Mt. Pinatubo are recent reminders, but examples can be found throughout history. In 1902 a hot, glowing volcanic cloud swept down the slopes of Mt. Pelee and killed 35,000 people in the city of St. Pierre on the Caribbean island of Martinique. Massive mudflows from the eruption of the Nevado del Ruiz volcano in 1985 killed more than 25,000 Colombians. The eruption of Mt. Vesuvius in the first century buried in ash the hapless inhabitants of Pompeii and Herculaneum and killed the intrepid naturalist Pliny the Elder as he made his way up the side of the volcano, intent on arriving at a better understanding of its 91 workings. (Pliny was hardly the last: Fifteen volcanologists have been killed in sundry volcanic eruptions between 1979 and 1993.) The Mediterranean island of Santorin (also called Thera) is in reality the only part above water of the rim of a volcano now inundated by the sea.* The explosion of the Santorin volcano in 1623 B.C. may, some historians think, have helped destroy the great Minoan civilization on the nearby island of Crete and changed the balance of power in early classical civilization. This disaster may be the origin of the Atlantis legend as related by Plato, in which a civilization was destroyed "in a single day and night of misfortune." It must have been easy back then to think that a god was angry. Volcanos have naturally been regarded with fear and awe. When medieval Christians viewed the eruption of Mt. Hekla in Iceland and saw churning fragments of soft lava suspended over the summit, they imagined they were seeing the souls of the damned awaiting entrance to Hell. "Fearful howlings, weeping and gnashing of teeth," "melancholy cries and loud wailings" were dutifully reported. The glowing red lakes and sulfurous gases within the Hekla caldera were thought to be a real glimpse into the underworld and a confirmation of folk beliefs in Hell sand, by symmetry, in its partner, Heaven). A volcano is, in fact, an aperture to an underground realm much vaster than the thin surface layer that humans inhabit, and far more hostile. The lava that erupts from a volcano is liquid rock—rock raised to its melting point, generally around 1000°C. The lava emerges from a hole in the Earth; as it cools and solidifies, it generates and later remakes the flanks of a volcanic mountain. The most volcanically active locales on Earth tend to be along ridges on the ocean floor and island arcs—at the junction of two great plates of oceanic crust—either separating from each other, or one slipping under the other. On the seafloor there are long zones of volcanic eruptions—accompanied by swarms of earthquakes and plumes of abyssal smoke and hot water—that we a are just beginning to observe with robot and manned submersible vehicles. Eruptions of lava must mean that the Earth's interior is extremely hot. Indeed, seismic evidence shows that, only a few hundred kilometers beneath the surface, nearly the entire body of the Earth is at least slightly molten. The interior of the Earth is hot, in part, because radioactive elements there, such as uranium, give off heat as they decay; and in part because the Earth retains some of the original heat released in its formation, when many small worlds fell together by their mutual gravity to make the Earth, and when iron drifted down to form our planet's core. The molten rock, or magma, rises through fissures in the surrounding heavier solid rocks. We can imagine vast subterranean caverns filled with glowing, red, bubbling, viscous liquids that shoot up toward the surface if a suitable channel is by chance provided. The magma, called lava as it pours out of the summit caldera, does indeed arise from the underworld. The souls of the damned have so far eluded detection. Once the volcano is fully built from successive outpourings, and the lava is no longer spewing up into the caldera, then it becomes just like any other mountain—slowly eroding because of rainfall and windblown debris and, eventually, the movement of continental plates * The eruption of a nearby submarine volcano and the rapid construction Of' new island in 197 B.C. are described by Strabo in the epigraph to this chapter. 92 across the Earth's surface. "How many years can a mountain exist before it is washed to the sea?" asked Bob Dylan in the ballad "Blowing in the Wind." The answer depends on which planet we're talking about. For the Earth, it's typically about ten million years. So mountains, volcanic and otherwise, must be built on the same timescale; otherwise the Earth would be everywhere smooth as Kansas.* Volcanic explosions can punch vast quantities of matter—mainly fine droplets of sulfuric acid—into the stratosphere. There, for a year or two, they reflect sunlight back to space and cool the Earth. This happened recently with the Philippine volcano, Mt. Pinatubo, and disastrously in 1815-16 after the eruption of the Indonesian volcano Mt. Tambora, which resulted in the famine-ridden "year without a summer." A volcanic eruption in Taupo, New Zealand, in the year 177 cooled the climate of the Mediterranean, half a world away, and dropped fine particles onto the Greenland ice cap. The explosion of Mt. Mazama in Oregon (which left the caldera now called Crater Lake) in 4803 B.C. had climatic consequences throughout the northern hemisphere. Studies of volcanic effects on the climate were on the investigative path that eventually led to the discovery of nuclear winter. They provide important tests of our use of computer models to predict future climate change. Volcanic particles injected into the upper air are also an additional cause of thinning of the ozone layer. So a large volcanic explosion in some unfrequented and obscure part of the world can alter the environment on a global scale. Both in their origins and in their effects, volcanos remind us of how vulnerable we are to minor burps and sneezes in the Earth's internal metabolism, and how important it is for us to understand how this subterranean heat engine works. IN THE FINAL STAGES of formation of the Earth—as well as the Moon, Mars, and Venus— impacts by small worlds are thought to have generated global magma oceans. Molten rock flooded the pre-existing topography. Great floods, tidal waves kilometers high, of flowing, red- hot liquid magma welled up from the interior and poured over the surface of the planet, burying everything in their path: mountains, channels, craters, perhaps even the last evidence of much earlier, more clement times. The geological odometer was reset. All accessible records of surface geology begin with the last global magma flood. Before the, cool and solidify, oceans of lava may be hundreds or even thousands of kilometers thick. In our time, billions of years later, the surface of such a world may be quiet, inactive, with no hint of current vulcanism. Or there may be—as on Earth—a few small-scale but active reminders of an epoch when the entire surface was flooded with liquid rock. In the early days of planetary geology, ground-based telescopic observations were all the data we had. A fervent debate had been running for half a century on whether the craters of the Moon were due to impacts or volcanos. A few low mounds with summit calderas were found— almost certainly lunar volcanos. But the big craters—bowl or pan-shaped and sitting on the flat ground and not the tops of mountains—were a different story. Some geologists saw in them similarities with certain highly eroded volcanos on Earth. Others did not. The best counter- argument was that we know there are asteroids and comets that fly past the Moon; they must hit * Even with its mountains and submarine trenches, our planet is astonishingly smooth. If the Earth were the size of a billiard ball, the largest protuberances would be less than a tenth of a millimeter in size—on the threshold of being too small to see or feel. 95 million years* must have been eradicated—on a planet almost certainly 4.5 billion years old. There is only one plausible erosive agent adequate for what we see: vulcanism. All over the planet craters, mountains, and other geological features have been inundated by seas of lava that once welled up from the inside, flowed far, and froze. After examining so young a surface covered with congealed magma, you might wonder if there are any active volcanos left. None has been found for certain, but there are a few—for example, one called Maat Mons—that appear to be surrounded by fresh lava and which may indeed still be churning and belching. There is some evidence that the abundance of sulfur compounds in the high atmosphere varies with time, as if volcanos at the surface were episodically injecting these materials into the atmosphere. When the volcanos are quiescent, the sulfur compounds simply fall out of the air. There's also disputed evidence of lightning playing around the mountaintops of Venus, as sometimes happens on active volcanos on Earth. But we do not know for certain whether there is ongoing vulcanism on Venus. That's a matter for future missions. Some scientists believe that until about 500 million years ago the Venus surface was almost entirely devoid of landforms. Streams and oceans of molten rock were relentlessly pouring out of the interior, filling in and covering over any relief that had managed to form. Had you plummeted down through the clouds in that long-ago time, the surface would have been nearly uniform and featureless. At night the landscape would have been hellishly glowing from the red heat of molten lava. In this view, the great internal heat engine of Venus, which supplied copious amounts of magma to the surface until about 500 million years ago, has now turned off. The planetary heat engine has finally run down. In another provocative theoretical model, this one by the geophysicist Donald Turcotte, Venus has plate tectonics like the Earth's but it turns off and on. Right now, he proposes, the plate tectonics are off; "continents" do not move along the surface, do not crash into one another, do not thereby raise mountain ranges, and are not later subducted into the deep interior. After hundreds of millions of years of quiescence, though, plate tectonics always breaks out and surface features are flooded by lava, destroyed by mountain building, subducted, and otherwise obliterated. The last such breakout ended about 500 million years ago, Turcotte suggests, and everything has been quiet since. However, the presence of coronae may signify—on timescales that are geologically in the near future—that massive changes on the surface of Venus are about to break out again. EVEN MORE UNEXPECTED than the great Martian volcanos or the magma-flooded surface of Venus is what awaited us when the Voyager 1 spacecraft encountered lo, the innermost of the four * The age of the Venus surface, as determined by Magellan radar imagery, puts an additional nail in the coffin of the thesis of Immanuel Velikovsky—who around 1950 proposed, to surprising media acclaim, that 3,500 years ago Jupiter spat out a giant "comet" which made several grazing collisions with the Earth, causing various events chronicled in the ancient books of many peoples (such as the Sun standing still on Joshua's command), and then transformed itself into the planet Venus. There ire still people N% ho take these notions seriously. 96 large Galilean moons of Jupiter, in March 1979. There we found a strange, small, multihued world positively awash in volcanos. As we watched in astonishment, eight active plumes poured gas and fine particles up into the sky. The largest, now called Pele—after the Hawaiian volcano goddess—projected a fountain of material 250 kilometers into space, higher above the surface of Io than some astronauts have ventured above the Earth. By the time Voyager 2 arrived at Io, four months later, Pele had turned itself off, although six of the other plumes were still active, at least one new plume had been discovered, and another caldera, named Surt, had changed its color dramatically. The colors of Io, even though exaggerated in NASA's color-enhanced images, are like none elsewhere in the Solar System. The currently favored explanation is that the Ionian volcanos are driven not by upwelling molten rock, as on the Earth, the Moon, Venus, and Mars, but by upwelling sulfur dioxide and molten sulfur. The surface is covered with volcanic mountains, volcanic calderas, vents, and lakes of molten sulfur. Various forms and compounds of sulfur have been detected on the surface of Io and in nearby space—the volcanos blow some of the sulfur off Io altogether.* These findings have suggested to some an underground sea of liquid sulfur that issues to the surface at points of weakness, generates a shallow volcanic mound, trickles downhill, and freezes, its final color determined by its temperature on eruption. On the Moon or Mars, you can find many places that have changed little in a billion years. On Io, in a century, much of the surface should be reflooded, filled in or washed away by new volcanic flows. Maps of Io will then quickly become obsolete, and cartography of Io will have become a growth industry. All this seems to follow readily enough from the Voyager observations. The rate at which the surface is covered over by current volcanic flows implies massive changes in 50 or 100 years, a prediction that luckily can be tested. The Voyager images of to can be compared with much poorer images taken by ground-based telescopes 50 years earlier, and by the Hubble Space Telescope 13 years later. The surprising conclusion seems to be that the big surface markings on Io have hardly changed at all. Clearly, we're missing something. A VOLCANO in one sense represents the insides of a planet gushing out, a wound that eventually heals itself by cooling, only to be replaced by new stigmata. Different worlds have different insides. The discovery of liquid-sulfur vulcanism on Io was a little like finding that an old acquaintance, when cut, bleeds green. You had no idea such differences were possible. He seemed so ordinary. We are naturally eager to find additional signs of vulcanism on other worlds. On Europa, the second of the Galilean moons of Jupiter and Io's neighbor, there are no volcanic mountains at all; but molten ice—liquid water—seems to have gushed to the surface through an enormous number of crisscrossing dark markings before freezing. And further out, among the moons of Saturn, there are signs that liquid water has gushed up from the interior and wiped away impact craters. Still, we have never seen anything that might plausibly be an ice volcano in either the Jupiter or * Io's volcanos are also the copious source of electrically charged atoms such as oxygen and sulfur that populate a ghostly, doughnut-shaped tube of matter that surrounds Jupiter. 97 Saturn systems. On Triton, we may have observed nitrogen or methane vulcanism. The volcanos of other worlds provide a stirring spectacle. They enhance our sense of wonder, our joy in the beauty and diversity of the Cosmos. But these exotic volcanos perform another service as well: They help us to know the volcanos of our own world—and perhaps will help one day even to predict their eruptions. If we cannot understand what's happening in other circumstances, where the physical parameters are different, how deep can our understanding be of the circumstance of most concern to us? A general theory of vulcanism must cover all cases. When we stumble upon vast volcanic eminences on a geologically quiet Mars; when we discover the surface of Venus wiped clean only yesterday by floods of magma; when we find a world melted not by the heat of radioactive decay, as on Earth, but by gravitational tides exerted by nearby worlds; when we observe sulfur rather than silicate vulcanism; and when we begin to wonder, in the moons of the outer planets, whether we might be viewing water, ammonia, nitrogen, or methane vulcanism—then we are learning what else is possible.