The Restless Sea: Exploring the World Beneath the Wavesby Robert Kunzig
“Meticulously researched and elegantly written, this book answers questions about the ocean I didn’t even know I had. For the first time, I feel I truly understand the other two-thirds of our planet. Anyone who loves the sea should read this book.” Sebastian Junger, author of The Perfect Storm
A vivid, up-to-date tour of the Earth's last
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“Meticulously researched and elegantly written, this book answers questions about the ocean I didn’t even know I had. For the first time, I feel I truly understand the other two-thirds of our planet. Anyone who loves the sea should read this book.” Sebastian Junger, author of The Perfect Storm
A vivid, up-to-date tour of the Earth's last frontier, a remote and mysterious realm that nonetheless lies close to the heart of even the most land-locked reader. The sea covers seven-tenths of the Earth, but we have mapped only a small percentage of it. The sea contains millions of species of animals and plants, but we have identified only a few thousand of them. The sea controls our planet's climate, but we do not really understand how. The sea is still the frontier, and yet it seems so familiar that we sometimes forget how little we know about it. Just as we are poised on the verge of exploiting the sea on an unprecedented scale-mining it, fertilizing it, fishing it out-this book reminds us of how much we have yet to learn. More than that, it chronicles the knowledge explosion that has transformed our view of the sea in just the past few decades, and made it a far more interesting and accessible place. From the Big Bang to that far-off future time, two billion years from now, when our planet will be a waterless rock; from the lush crowds of life at seafloor hot springs to the invisible, jewel-like plants that float at the sea surface; from the restless shifting of the tectonic plates to the majestic sweep of the ocean currents, Kunzig's clear and lyrical prose transports us to the ends of the Earth.
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Three hundred thousand years after the Big Bang, when the primordial fireball had cooled to a mere 5,000 degrees or so, electrons fell into orbit around protons. The change was sudden, as if a spell had been cast. One instant the electrons were running amok, like children in a schoolyard; the next instant, the universe had expanded and cooled and the negatively charged electrons had slowed just enough to succumb to the positive pull of the protons. The particles lined up two by two. Astronomers speak of this as the moment when the fog cleared, and light could at last course through the universe unimpeded. But to someone interested in Earth's ocean and its origin, the moment has a different significance. A single electron orbiting a single proton is a hydrogen atom; and although individual particles would part and reunite countless times in the eons that followed, it was in that early epoch that the universe acquired its basic supply of hydrogen.
When light stopped being intercepted all the time by matter, matter stopped being pushed around by light. The primeval radiation had been like a wind from all directions, and like the wind it had combed things flat. Freed at last from this smoothing pressure, matter could pursue its own natural tendency to clump. Hydrogen clumped into ever larger clouds, and soon those clouds collapsed, under the force of their own gravitation, to form embryonic galaxies. Within the large clouds, fragments collapsed to form stars. In the cores of stars, history was reversed. Temperatures rose again to what they had been afew minutes after the Big Bang; pressures rose even higher. But now these extraordinary conditions were maintained, not for a transient moment, but for millions of years. There was time now for hydrogen to fuse into two-protoned helium, and for three helium nuclei to fuse into carbon. There was time as well for a carbon nucleus to collect another helium and become oxygen.
As stars evolved, they assembled the elements one after another, on up the periodic table to iron. When a star's core is solid iron, however, fusion stops. Without an internal heat source, the star can no longer fight gravity. It collapses in on itself, compresses itself, rebounds off a 12-mile-wide core of incompressible denseness, and thenall this in millisecondsblasts itself apart, leaving only that dense cinder behind. Layers of hot star-gas millions of miles thick and laced with millions of years worth of fusion products shoot into space at millions of miles per hour. There they ram into clouds of primeval hydrogen left over from the Big Bang. Thus do interstellar clouds become seeded with heavier elements.
And thus, many billions of years ago, somewhere in our galaxy and probably in a billion other galaxies too, did hydrogen first encounter oxygen.
* * *
The recipe for making water was discovered in 1781 by an odd English aristocrat, a man so afraid of women he would fire his own maid if he encountered her on the stairs, so afraid of his peers he might cry out and leave the room if one addressed him, and so afraid of everyone he would take his constitutional at dusk to avoid being seen. According to a contemporary account, Henry Cavendish "was somewhat above the middle size, his body rather thick, and his neck rather short. He stuttered a little...." Another report suggests he smelled bad, but that may be an embellishment. He certainly always wore the same outmoded gray suit. It was men such as Henry Cavendish who established the presumption of wacky eggheadness that scientists to this day must labor under until they prove themselves innocent. Cavendish was the genuine article. He might well have ended up in a public asylum had he not been rich enough to afford his own private one: a large house at Clapham, near London, with a back stair for the maids. There he became instead a remarkable experimental chemist.
By the late eighteenth century, when Cavendish did his great work, three of the four elements into which the ancient Greeks had divided the physical worldearth, air, fire, and waterhad lost their elemental status. There were several kinds of earth, several different airs, and fire was not an element but a process. But water was still water, pure and simple. Even Cavendish, who did the experiment that was to topple this intellectual vestige, could never bring himself to see water in any other way.
In 1766, he had isolated what we now know as hydrogen. But Cavendish thought it was phlogiston: a mythical fire essence invented by German chemists, the "oily earth" that was supposedly expelled from a substance when it burned, leaving behind a "stony earth," or ash. Hydrogen burned clean, or so it seemed, and so it seemed a good candidate for pure phlogiston. Eight years later, when Cavendish's compatriot Joseph Priestley isolated a gas that caused a candle to burn furiously and kept a mouse alive for weeks in a bell jar, both he and Cavendish interpreted the discovery in terms of the phlogiston theory. They decided that this exceptionally pure air must be "dephlogisticated air," great for burning things in because it had plenty of room for phlogiston. In fact it was oxygengreat for burning things in because things that burn do not lose phlogiston; they gain oxygen.
Priestley, a carefree empiricist who did experiments almost at random to amuse himself and his friends, soon had the idea of burning Cavendish's phlogiston (hydrogen) in his own dephlogisticated air (oxygen). Holding a lighted candle to the mixture, he succeeded in generating a nice bang. In 1937, the same sort of bang destroyed the Hindenburg, the German zeppelin. But Priestley made little of his result. In 1781, Cavendish repeated the experiment more carefully. He noticed that hydrogen did in fact leave a residue when it burned, in the form of a fine mist on the walls of the experimental vessel. He also noticed that the mist was water.
Eventually Cavendish established that if you mixed two parts phlogisticated with one part dephlogisticated air and ignited the lot, then all the air disappeared, and the weight of the water you produced was equal to the original weight of the gases. Cavendish had discovered the right proportions, but he still thought that elemental water was somehow springing whole from each of the two gases.
Two centuries later, when phlogiston and the Greek concept of elements are dead history rather than vital organizing (and eye-blinkering) principles, it is hard to understand his point of view. Antoine-Laurent Lavoisier, cool rationalist of the French enlightenment, was the first to abandon the Greek prejudice. For years he had been quietly undermining the phlogiston theory; for years he had believed that burning is oxidation. When he heard of Cavendish's results, in June 1783, he immediately repeated Cavendish's experiment; tried, in fact, to steal credit for it, for his genius was neither humble nor generous. Yet it was Lavoisier who grasped that the water in Cavendish's vessel had not condensed out of the two gases but had been synthesized from themfrom oxygen and the substance that Lavoisier named hydrogen, or "source of water." When Lavoisier triumphantly declared the German phlogiston theory dead, students in Berlin burned him in effigy. Lavoisier responded by staging a ritual oxidation of the book written by Georg Ernst Stahl, chief proponent of phlogiston.
After Lavoisier, water was no longer an element. It was oxidized hydrogen, or burnt hydrogen. Cavendish and Priestley were too old to accept the fact, but the next generation of chemists grew up with it. By the mid-nineteenth century, water was known to be a molecule consisting of two hydrogen atoms and one oxygen: [H.sub.2]O. With the discovery that water was a compound, not an element, the Greeks had finally been booted out of modern chemistry. Yet a legacy of their thought remained. Although the Greeks did not know the structure of water, they, like the Babylonians and Egyptians before them, appreciated its primordial importance. Homer, in a rare cosmogonical moment in the Iliad, made water the source of deity ("I am off to the ends of the fruitful, teeming earth," Hera says to Aphrodite in Book 14, "to visit Ocean, fountainhead of the gods....") Later, in the sixth century B.C., when Thales of Miletus conceived the first nonreligious cosmogonythe first rational attempt to make sense of the worldhe made water its foundation. The world rested on an ocean of water, he said; and all life sprang from water. Thales had the right idea.
* * *
As the structure of the atom has been unraveled in the twentieth century, so has the structure of the water molecule. It accounts for water's remarkable properties, which is to say that it accountsin the sense of being a necessary conditionfor the most distinctive properties of our planet, including the fact that it is inhabited.
Water exists because it solves a problem that oxygen has. An oxygen atom has six negatively charged electrons in its outer shell, orbiting the positively charged nucleus, but it has room for and yearns to be filled by two more. Two hydrogen atoms donate the missing electrons and are thereby bound tightly to the oxygen. The eight electrons form four pairs (magnetism keeps each pair together) that separate as far as they can (electrostatic repulsion drives the pairs apart). The result is a four-cornered molecule, a tetrahedron. Projecting from two corners are hydrogen nuclei, that is, individual protons. The other two corners are naked electron pairs. The protons give one side of the water molecule a positive electric charge, while the electrons make the other side negative.
This electric polarization has profound consequences. The naked electron pairs attract their opposite numbers-hydrogen nuclei in other water molecules. The resulting bond is called a hydrogen bond. As long as the forces of chaosthat is, heatare not extreme, hydrogen bonds link water molecules in a loose association. Each molecule can join hands with a neighbor at the four corners of the pyramid, grabbing a hydrogen at two corners and a pair of oxygen electrons at the other two. The liason is fleeting, though; it is like the passing touch exchanged by dancers in a quadrille, as they constantly change partners. Water molecules change partners billions of times a second.
Their dance, not surprisingly, is not as orderly as a quadrille, and two centuries after Cavendish and Lavoisier, its rules are only beginning to be understood. But its effects are clear enough. It allows water to exist as a liquid on Earth. Without hydrogen bonds, each molecule would go its own way, and water would exist only as a gas. Hydrogen sulfide, the closest molecular analogue to water, does not form hydrogen bondssulfur atoms are apparently too big to submit to such constraintsand as a result it does not form a liquid until the temperature drops below -109 degrees Fahrenheit. (That is fortunate, because otherwise our landscape might be dotted with pure pools of rotten-egg smell.) Hydrogen bonds keep water liquid below 212 degrees. They make it possible for Earth to have an ocean.
They also make the ocean Earth's primary distributor of heat. To increase the temperature of a liquid, you have to increase the velocity of its molecules, which in the case of water means you have to overcome the hydrogen-bond restraints. As a result water requires a greater input of heat to raise its temperature by a given amount than does any other liquid. That allows it to store heat and transport it great distances. On Earth, the ocean soaks up heat from the sun in the tropics, where sunlight is plentiful, and transports it to higher latitudes, which can use the warmth. Without the Gulf Stream, for instance, the English would live in a gray and icy land. They have hydrogen bonds to thank (among other things) for their relative prosperity.
Sunlight warms water by causing the molecules to vibrate, straining at their hydrogen bonds, pushing and pulling their neighbors. Water molecules in a liquid can only vibrate in certain modes, however, corresponding to distinct energies, and those energies, in turn, match the energy contained in red light. Thus seawater quickly absorbs red sunlight, whereas it scatters most of the blue. That is why the ocean looks blue to uswhen it does not have green or red algae in it, that is, or brown or yellow sediment.
But as sunlight penetrates into the upper layers of the ocean, more and more of all its wavelengths are either absorbed or reflected at each successive layer. By a depth of 500 feet only around 1 percent of it is left. No sunlight at all penetrates below a depth of 3,000 feet or so. Below that depth everything is utterly dark and utterly invisible from the ocean surface.
Because of water, then, we live in a divided world. Thirty percent of it we can see; 70 percent of it we cannot, because it is covered by water, and so we tend to ignore it. It is worth dwelling for a moment on this fact that we take for granted. If we lived on Mars or Venus, our world would contain no insurmountable frontiers. We could get in our sports utility vehicle and drive around the planet on a continuous route, swerving only to avoid mountains and chasms. We could map all parts of it in equal detailas indeed we have mapped Venusinstead of covering most of it with great swatches of featureless blue. Our world would not end arbitrarily at coastlines. We have the structure of the water molecule to thank, and its propensity to form hydrogen bonds, for our existence on Earthwe could not, in fact, live on Mars or Venusbut also for our monumental ignorance of most of the planet.
The ignorance begins with what sounds like a child's question, but ought to be everyone's first question when standing on the beach: All this water, a billion trillion tons of it, this stuff that makes our planet what it is and makes us who we arewhere did it come from? And why did it come here?
* * *
A few days before Christmas in 1968, Albert Cheung saw water in space. There was more of it than he or anyone else had ever dreamed he might find. Cheung has since returned to Hong Kong to manage the family business, but at the time he was a graduate student in astronomy at the University of California at Berkeley. His adviser was a physicist named Charles Townes, who had only just switched over to astronomy. Some years before, Townes had conceived the wild notion that molecules, including water, might be plentiful in space.
The idea was wild to most astronomers, anyway. Space is vast, they reasoned, and atoms are small. In a typical cubic foot of our galaxy, there are only a couple of hundred hydrogen atoms, and each atom is just a few billionths of an inch across. Two sparrows released on opposite sides of the Earth and flying randomly have a greater chance of colliding in midair than two atoms have of meeting to form a molecule in most parts of interstellar space. What is more, space is bathed in ultraviolet starlightthe radiation our atmospheric ozone layer still protects us fromwhich carries enough energy to knock molecules apart. Infrequent births and rapid deaths should result in a relatively small population of molecules between the stars, or so most astronomers figured. "The problem was that astronomers thought they understood interstellar space better than they actually did," Townes recalls, not without satisfaction. "That's why they felt stable molecules couldn't exist and there was no point in looking for them. So they never looked."
Townes, on the other hand, was not overly constrained by knowledge of interstellar space when he first hatched the idea of looking for water and other molecules there. But he had a deep knowledge of how a molecule goes about emitting electromagnetic radiation. As it steps from a higher energy level down to a lower oneeach level corresponding to a rotation or a vibration that only that particular molecule is capable of executinga molecule surrenders energy in the form of radiation. In 1953, Townes had found a way of getting a large population of ammonia molecules to rotate at the same rapid frequency, poised on the brink of a downward energy step. By stimulating them with microwave radiation of precisely that energy, he got them all to take the step at the same time. Thus orchestrated, the molecules delivered themselves of a coherent blast of microwave radiation. Townes had invented the maserthe forerunner and microwave equivalent of the laser. He got the Nobel Prize in 1964.
By then he was bored with masers. The field he had startedthe whole idea, he has said, came to him while sitting on a park bench in Washington, D.C., waiting for a restaurant to open for breakfasthad gotten too crowded for him. In 1967, when he moved to Berkeley, he encountered some of the few astronomers who had taken up his earlier suggestion and were looking for molecules in space. Townes decided to get into the act himself, and to start by looking for his old friend, ammonia.
With Cheung and a postdoc named David Rank, Townes built a receiver for the ammonia frequency. An astronomer named Jack Welch helped them install it on the University of California's new radio dish, at Hat Creek in northern California. Pointing the dish toward the center of our galaxy, they pulled in ammonia's 23,870 megahertz signal, the same signal that had coursed through Townes's first maser all those years ago, right away. The ammonia molecules in space were not masingthey were not radiating in lockstepbut they were there. Next Cheung and Townes tuned their receiver to 22,235 megahertz. They pulled in a signal at that frequency, toothe signal that a water molecule makes, floating in space and rotating in its lopsided, rabbit-eared way, with its electrons sliding back and forth like the electrons in a tiny radio antenna.
One evening in December of that same year, 1968, Cheung was back up at Hat Creek. He had the radio dish pointed at the Orion Nebulathe gorgeous glowing cloud, 1,600 light years away, that those of us without telescopes see as the middle "star" in the hunter Orion's sword. Townes was at his home in Berkeley, entertaining the rest of his students and staff at a Christmas party. Sometime during the evening the phone rang: it was Cheung, and he was excited. "It must be raining in Orion," he reported. "There's water everywhere!"
The signal from Orion was far more powerful than what Cheung and Townes had observed coming from the galactic center. The water molecules in Orion were giving off as much energy at a single frequency as the sun emits at all frequencies. It did not take Townes long to figure out what was going on. Fifteen years earlier he had built the first maser in his lab; he and Cheung were using a maser in their receiver to amplify the weak signals they were expecting from space, but in this case the signal coming was amplified already. It was so strong it could have been discovered decades earlier, before Townes's invention, had anyone bothered to point an antenna at Orion and tune it to the right frequency. Cheung and Townes had discovered a water maser in space.
By now it was becoming clear what the astronomers' mistake had been, in assuming all those years that molecules were too unlikely to form in interstellar space and too fragile to withstand the ultraviolet onslaught. Those assumptions were true enough for most parts of our galaxy. But they were not true in scattered dark patches, patches where astronomers had been unable to find any signals at all at the frequencies favored by lone atoms. The dark patches had remained mysterious. There was a patch like that in Orion: It loomed around the visible nebula and behind it, like a larger, hidden truth.
Now it too turned out to be a cloud: a giant cloud of molecular gas, in which matter is packed a million times more densely than it is in ordinary interstellar space. That is still only a hundred-trillionth as dense as the air in our atmosphere. But it is dense enough to allow molecules to form readily; and it is dense enough, given that the cloud is a hundred or more light-years across, to prevent ultraviolet light or any other light from penetrating to its interior. That is why the cloud appears dark. It is also cold, around -400 degrees Fahrenheit. And as astronomers have come to realize in the past three decades, the molecular cloud in Orion and others like itthere are many in our galaxyare more than just oddities. They are the places where stars are born. Their density and coldness allows gravity to triumph over heat, which is the first requirement if a gas cloud is to collapse in on itself to form a star.
What is more, molecular clouds are lousy with water. In their protected depths, hydrogen and oxygenhydrogen made in the Big Bang, oxygen made in the interior of starsare always uniting to form [H.sub.2]O. The process is not simple. The two flighty hydrogens must first meet on the surface of a frozen dust grain, which holds onto them long enough for them to join hands; and there are several more steps after that. But the process is clearly effective. The microwave signal of rotating water molecules has been detected in just about every molecular cloud. (Unlike ultraviolet or visible light, it is not absorbed by the cloud layers and travels easily all the way to Earth.) The mass of the water in all the clouds in our galaxy is about the mass of a million suns. In most cases, as in Orion, the water is acting as a maser.
It is not clear exactly how interstellar masers work. But it must have something to do with the fact that new stars are forming in the cloud. In the early stages of its life, a star, particularly a massive one, seems to cast off a lot of its mass as a dense wind. Traveling at speeds as high as 250,000 miles per hour, the wind slams into the surrounding gas cloud and compresses it. Water masers are often found at this shock front. The compression of the cloud may bring enough water molecules together in a small enough space to be able to radiate in concert; and radiation from the hot young star may provide the pumping energy needed to excite the molecules and get the maser going.
However they work, water masers are heraldspowerful radio stations, among the most powerful in our galaxy, beaming out the message that a star is born. Four and a half billion years ago, when our own sun emerged from the black, a water maser may have broadcast news of the glad event to the rest of the galaxy. Certainly there would have been no shortage of water in the spinning cloud of gas and dust that gave rise to the sun and its retinue of planets. In such a cloud, water is everywhere. The interesting question is how so much of it ended up in liquid form on just one of the nine planets, the third one from the sunEarth.
* * *
"About ten years ago, while I was on a ride in what used to be called an amusement park," writes George Wetherill, who knows as much as anyone on this Earth about the processes that created it, "a disembodied head appeared above me out of the darkness. Its lips quivered as it intoned: 'So you want to know how the world began? Very well then, I will show you. But remember, you fool, no one asked you to take this journey!'" Wetherill is a modest man. "There really are no bona fide experts on the formation of the Earth," he says. The same is true of the question of how Earth ended up with an ocean.
It was not always thus. Time was when every Great Thinker seemed to be taking a pop at explaining where Earth and the rest of the solar system came from. The first and greatest of these men, at least in the modern scientific era, were Immanuel Kant and Pierre-Simon Marquis de Laplace. (The latter, incidentally, was in Lavoisier's laboratory the day he repeated Cavendish's historic experiment on the composition of water.) Independently of each other, Kant and Laplace conceived essentially the same theory, which came to be called the nebular theory. It attributed the birth of the solar system to a spinning nebula, or cloud, of gas. As the nebula cooled, it shrank, and the sun turned on in its center. As the nebula shrank, it had to spin faster; the rapid spinning produced a centrifugal force that fought gravity and caused the shrinking nebula to leave behind rings of gas, one after another. From those rings the planets condensed.
The nebular theory was neat and pretty. It also explained one of the most conspicuous mysteries of the solar system, which is that all the planets revolve about the sun in the same direction. George W. Wetherill, however, is not overly impressed, at least in this context, by Immanuel Kant and Pierre-Simon Marquis de Laplace. Although at its core the nebular theory expresses a truth that everyone now acceptsthat a spinning cloud was the progenitor of the solar systemas far as Wetherill is concerned that is not saying very much. How, exactly, do you get a planet like Earth out of a spinning cloud? "It's amazing how much of this field never got around to making planets," says Wetherill.
There are only two ways of doing it. The first is to have the planet condense from something biggera giant gaseous protoplanet. This was an approach many researchers followed until well into the 1960s. But they never spelled out in any detail how the process might have worked, and researchers today generally assume it could not have worked, at least in the case of Earth and the three other rocky planets of the inner solar system. Instead Wetherill and others, following the lead of a Russian planetologist named V. S. Safronov, have adopted the second method of building a planet: assembling it brick by brick, from ever smaller pieces.
The smallest bricks are the tiny dust grains that crystallize out of the gaseous molecules and atoms in all molecular clouds. Just what such grains might look like is an open question, but they include two basic categories of material: rock and ice, that is, solid crystals made of heavy elements and solid crystals made of light ones. In the molecular cloud that hatched our solar system, there must have been rocky grains of pure silicon, grains of iron and other metals, and icy grains of carbon compounds such as methane. Many grains may have had rocky cores with icy coatings. Certainly the cloud contained an abundance of water ice, either as compact crystals or as fluffy snowflakes.
The birth of our solar system began when a dusty cloud, a small fragment of a giant molecular cloud like the one in Orion, somehow became denser than its surroundings and somehow was pushed into collapse, perhaps by the explosion of a nearby star. As the central bulk of the cloud collapsed into a sun, in a hundred thousand years or so, the rest of it began to rotate rapidly, preserving in its motion all the rotational momentum of the original cloud. The centrifugal force of the rotation saved this dust-spiked gas from being swallowed by the newborn star. Instead it drifted down and out, snowing gently into the plane of the sun's equator, where it formed a broad, flat disk.
This diska solar nebula, as it is called, with a nod at Kant and Laplacewas denser in the center, near the sun, than it was in its outer reaches. It was also hotter in the centerperhaps as hot as 2,200 degrees Fahrenheit, in the region where Earth now finds itself, whereas out beyond Uranus the temperature would have been more like -400 degrees. And because the central part of the disk was hotter, it was also rockier. Only rocky grains could survive the intense heat; ice, including water ice, would have been vaporized. As a result, the planets that later formed in the inner part of the solar nebulaMercury, Venus, Earth, and Marsare mostly rock. The rapid swirling rush of dust in this region may have produced such static electricity that the dust was sometimes rent by thunderclaps and lightningsound and fury signifying, in this case, the impending birth of a planet.
As the dust grains snowed down into the solar disk, and even more once they were there, they began to collide and stick together. Clumps formed, then bigger clumps, until soon the space that is now occupied by the inner planets was littered with ten billion rocks the size of asteroids, about 5 or 10 miles across. These rocks were large enough to attract one another gravitationally. As they orbited the sun in the same direction, they whipped one another to higher velocities, but whenever they collided they slowed one another down. It was Safronov who first calculated that the net effect of these competing influences would be to keep the relative velocities of the rocks low, and thus the collisions between them gentlegentle enough that two colliding rocks would often stick together rather than ricochet apart. Within a million years after the sun first coalesced, most of the dust in the inner solar system had settled onto 30 or so planet embryos, the size of Mercury.
Wetherill has calculated what happened next. Now all gentleness faded; now it was survival of the fittest and luckiest embryo. The inner solar system, home today to four planets, was not big enough for 30 Mercuries orbiting on concentric circles. But the embryos did not stay on safe, concentric paths for long. The gravitational attraction among them soon forced the issue. The embryos' orbits became more elliptical, and their paths began to crossand so they began to collide, not gently, but at speeds of tens of thousands of miles per hour. Only the embryos that started out somewhat larger than the rest could survive such impacts, and they proceeded to grow even larger by absorbing their victims. Within ten to a hundred million years, the planetary barroom brawl had produced two winners, Earth and Venus. Mars and Mercury, which are much smaller, survived only by cowering in corners, well out of the way of the bullies.
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Robert Kunzig is European editor of Discover magazine, based in Dijon, France. His writing about the ocean has won the AAAS-Westinghouse Science Journalism Award and the Walter Sullivan Award for Excellence in Science Journalism.
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