Life's Matrix: A Biography of Water / Edition 1 available in Paperback
One of the four elements of classical antiquity, water is central to the environment of our planet.
In Life's Matrix, Philip Ball writes of water's origins, history, and unique physical character. As a geological agent, water shapes mountains, canyons, and coastlines, and when unleashed in hurricanes and floods its destructive power is awesome. Ball's provocative exploration of water on other planets highlights the possibilities of life beyond Earth. Life's Matrix also examines the grim realities of depletion of natural resources and its effects on the availability of water in the twenty-first century.
|Publisher:||University of California Press|
|Edition description:||First Edition, With a new preface|
|Product dimensions:||6.00(w) x 9.00(h) x 0.90(d)|
About the Author
Philip Ball studied chemistry at Oxford and received a Ph.D. in physics from the University of Bristol. He has worked for ten years as an editor at Nature magazine. He is the author of Designing the Molecular World: Chemistry for the 21st Century (1994), Made to Measure: New Materials for the 21st Century (1997), and The Self-Made Tapestry: Pattern Formation in Nature.
Read an Excerpt
THE FIRST FLOOD
Surely this is a great part of our dignity ... that we can know, and that through us matter can know itself; that beginning with protons and electrons, out of the womb of time and the vastness of space, we can begin to understand; that organized as in us, the hydrogen, the carbon, the nitrogen, the oxygen, those 16 to 21 elements, the water, the sunlightall, having become us, can begin to understand what they are, and how they came to be.
George Wald, Nobel laureate in medicine
Those stars are the fleshed forebears
Of these dark hills, bowed like labourers,
And of my blood.
Ted Hughes, "Fire Eater"
In the beginning there was water. While the earth was formless and empty, the Hebrew God was "hovering over the waters." There was no sky, no dry land, until God separated "the water under the expanse from the water above it" and commanded that "the water under the sky be gathered to one place." Then the world emergedfrom an infinite primeval ocean.
This is echoed in similar myths throughout the world. In central and northern Asia, North America, India, and Russia, a recurring motif is that of the Earth Diver: an animal or a god who plunges to the bottom of a primordial ocean to bring up a seed of earth. The Polynesian cosmogeny reproduces that of the Old Testament in extraordinary detail: the supreme being, Io says, "Let the waters be separated, let the heavens beformed, let the earth be!" For the Omaha Native Americans, all creatures once floated disconsolately on a wholly submerged Earth until a great boulder rose from the deep. In Hindu mythology, the sound that embodied Brahma became first water and wind, from which was woven the web of the world. "Darkness was there, all wrapped around by darkness, and all was Water indiscriminate" says the beautiful creation hymn of the Rig Veda (3700 B.C.). For the Maya of Central America also, the deity Hurakan called forth the land from a universe of darkness and water.
Why does this idea of a watery beginning resonate throughout disparate cultures, without heed to the local particulars of geography or religious tradition? Ultimately its origin may be psychological: the land was knowable for ancient peoples, but the sea was a symbol of the unconscioussomething mysterious, pristine, unfathomable. I know of no creation myths where the land came first and the seas followed in a subsequent deluge.
Yet land and sea are contemporaneous and complementary in some traditions. The Judeo-Christian distinction between flesh and blood is a distinction between the earthy and watery aspects of the corpus of the world. In Norse mythology, the land is the flesh and bones of Ymir, the first giant, slain by Odin. His salty blood, gushing from the spear wound in his heart, became the oceans. So too in Chinese myth are land and sea coeval aspects of a primal being, Pan-Ku the sculptor, whose medium was his own body.
But is there any truth of a more material nature in these mythswas the world once covered with water? And where has the water come from?
IN THE BEGINNING
In myth, the origin of the Universe is seldom differentiated from the origin of the Earth. To look beyond the beginning of our world is to ponder the eternal: the Chaos of the Greeks, the abyss of fire and ice called Ginnungagap by the Norse, or the supreme deity Akshara-Brahma in Hindu tradition. Today, Earth's beginning is merely a local question, a moment of parochial interest in an already mature universe. The real moment of creation goes back at least six billion years beyond that, and it is as fantastic as any myth.
Origins are seldom uncontentious. Current fashion sometimes has it that the idea of a cosmic Big Bang is best regarded as our latest cultural myth, as much a social construct as the slaying of Ymir. On the one hand, it can only be arrogant to suggest otherwise; on the other, it's this particular kind of confidence that makes science possible. From a scientific perspective, the Big Bang is beyond question still the best model we have for the birth of the Universe, and rests on some formidable pillars. To address the question of why water is what it is, modern cosmology provides a consistent and explanatory framework in a way that Odin's murder of Ymir does not.
Imagine watching a movie of an explosion moments after it has happened. You see many fragments, rushing away from one another within a "bubble" of expanding size. When, in 1929, the astronomer Edwin Hubble saw the galaxies of the Universe behaving in the same way, he was forced to the same conclusion as the one we would reach from the movie: this is the aftermath of an explosion. But Hubble was seeing it from the insidewe are riding on one of those fragments, the Milky Way galaxy. The Universe is getting bigger as all the galaxies rush away from one another. The natural inference is that all the matter in the Universe was once focused into a much smaller volume, which went Bang! Albert Einstein had deduced as much in 1917: when he applied his theory of general relativity to the Universe as a whole, he found the equations predicting that it had to be either expanding or contracting. That seemed to him then to be a crazy notion, and so he added a "fudge factor" to remove the expansion. But Hubble's discovery persuaded him in 1931 that no fudging was needed after all.
After just one-millionth of a billionth of a second, when according to some theories the Universe might have been just a few feet across, the temperature would have been in the region of a billion billion degrees. In such extremes, there can be no atoms and molecules, no matter as we currently know it.
But as it expanded, the temperature of the Universe dropped rapidly. At the end of the first day of creation, it would have been about twenty million degreesabout as hot as the center of a star. The Universe today, with all its stars and supernovae and quasars, is but a dim, cool remnant of this cosmic fireball. In 1965 Arno Penzias and Robert Wilson at Bell Telephone Laboratories detected the faint afterglow that pervades the sky: a uniform background radiation of microwaves coming from all directions, indicating an average temperature of about five degrees Fahrenheit above absolute zero. This cosmic microwave background is all that is left of the Big Bang's fury.
THE FABRIC OF WATER
George Wald's view, quoted at the beginning of this chapter, is mine: understanding what we are composed of, and where that stuff came from, is part of our dignity. It demands, too, a greater humility to read the lives of stars, rather than divine providence, in our bones and blood. But bones and blood must come later; for now, I want to follow only the gestation of those protons and electrons, and from them the hydrogen, the oxygenand the water.
For this is what we're after, these two elements: the H and the O, which unite so readily to create our subject. Water is [H.sub.2]O, the only chemical formula that everyone learns: two atoms of hydrogen welded to one of oxygen. Their union is a moleculea duster of atoms. Chop up a block of ice, and keep choppingand your finest blade, finer than the keenest surgical scalpel, will eventually reduce the fragments to these individual three-atom clusters. If you chop beyond that, you no longer have water. The [H.sub.2]O molecule is the smallest piece of water you can obtain, the basic unit of water.
So here is a central aspect of water's character: it is a compound, an association of atoms, divisible into atoms of different natures. Yet water is so fundamental to the world that for millennia it was mistaken, naturally enough, for an element, something indivisible. Hydrogen and oxygen are elements, because they each contain only one kind of atom. But there is no "water atom"only a water molecule, made up of two different types of atom.
Before making bread, one must make flour; and before water could come into the Universe, there had to be hydrogen and oxygen atoms. But before flour comes wheatand atoms too have more fundamental constituents, Wald's protons and electrons.
As far as atoms are concerned, protons and electrons are like knives and forks at the dinner table: no matter how big the table, there are equal numbers of each. The difference between atoms of different elementsbetween an atom of oxygen and one of carbon, sayis simply that they contain different numbers of protons. In this regard, the underlying pattern of atoms is numerical, as Jacob Bronowski says in The Ascent of Man. An atom with one proton (and one electron) is hydrogen; an atom with eight of each is oxygen. At one level, chemistry is as simple as counting.
At another level, it is clearly not. For mere proton bookkeeping offers no clue as to why hydrogen atoms join with oxygen atoms in the ratio of 2:1, or why sodium (eleven protons) is a soft reactive metal, chlorine (seventeen protons) a corrosive gas, silicon (fourteen protons) an inert gray solid. To understand any of this, we need to consider how the electrons are deployed: for there are deeper patterns in the arrangement of the electrons that determine the element's chemical properties.
Protons and electrons are not, as British physicist J. J. Thomson believed at the turn of the century, lumped together inside the atom as a heterogeneous blob. Rather, they bear to one another something like the relationship of the planets to the Sun, with the electrons orbiting a central, dense nucleus where the protons are. In this "solar system" model of the atom, proposed by Thomson's protégé, New Zealander Ernest Rutherford, if the nucleus of an atom were scaled up to the size of the Sun, then the electrons would be more distant than Neptune's orbit by a factor of about ten. Yet we shouldn't take the model too seriously: electrons can't be pinpointed like planets, and do not follow well-defined elliptical paths, but instead occupy regions of space called orbitals. These regions, which have the shapes of spheres, lobes, and rings centered on the nucleus, are best regarded as hazy "electron clouds," rather like swarms of bees around a hive. From the manner in which an atom's electrons are distributed among the various available orbitals flows the whole of chemistry.
Moreover, atomic nuclei grasp their electrons not by the force of gravity but by electrical attraction: an electron is negatively charged, and a proton has a positive charge of equal magnitude. An atom, with equal numbers of both particles, is electrically neutral. Electrons, however, can be stripped away from atoms, rather as a passing star could pull a planet from a nearby solar system. The depleted atom then has an excess of protons over electrons, and so is positively charged. Atoms can also gain an excess of electrons over protons, and so become negatively charged. These charged atoms are called ions. This is why, even though protons and electrons are equally represented in a neutral atom, it is the number of protons that is the fundamental characteristic of an element. To pull a proton out of an atom, you have to dig it from the dense mass of the nucleus. That takes a huge amount of energy, and it converts the atom into a different element entirely.
Although hydrogen atoms have one proton and oxygen atoms have eight, oxygen is about sixteen times heavier than hydrogen. There is a third ingredient to the atoma particle called the neutron, which has virtually the same mass as a proton but is electrically neutral. All atoms bar hydrogen have neutrons as well as protons in their nuclei, and generally speaking the nuclei contain equal numbers of each. The vagueness in this statement is, I fear, unavoidable, for two reasons. First, the number of neutrons tends increasingly to exceed the number of protons for heavier atoms: the proportions are pretty much fifty-fifty for light atoms like carbon, oxygen, and nitrogen, whereas lead atoms have around 40 percent more neutrons than protons. Second, even atoms of the same element can possess different numbers of neutrons. Oxygen atoms can contain seven, eight, nine, or ten neutrons to accompany their eight protons, while hydrogen atoms can contain no, one, or two neutrons. These different forms of atoms of the same element are called isotopes. Most hydrogen atoms have no neutrons; but 0.000015 percent of all of those in nature have one neutron. This heavier isotope is called heavy hydrogen, hydrogen-2, or deuterium.
This is, I appreciate, the stuff of dry chemistry textbooks, and I regret forcing it on you so soon. I hope it is of some consolation to learn that this is all you will need to know about atoms for the rest of the book. But they are the alphabet of chemistry, so we need to be at least on familiar terms with them. Besides, if we are to consider how the Universe cooked up water, we need to know which ingredients must go into the pot.
THE SOUP GOES COLD
Water is but a simple dish: the recipe tells us to mix hydrogen and oxygen. The first ingredient is the easy one: it dropped right out of the Big Bang, once things got cool enough. That's to say, protonsthe nuclei of hydrogen atomscondensed out of the fireball about a millionth of a second after time and space were born.
But at this point the temperature would have been around a trillion degrees, which is too hot for protons to hold on to electrons. The Universe was then a soup of protons and electrons, seasoned with neutrons and other subatomic particles such as neutrinos, all swimming in a seething broth of X-rays. And for a good few minutes, that's how things stayed; the Universe was too hot to be interesting.
Although protons could not yet combine with electrons, they could at least team up with each other and with neutronsfor the force that binds protons and neutrons together in the nucleus, called the nuclear strong force, is many, many times stronger than the electrical force of attraction between protons and electrons. Just one hundred seconds into the Big Bang, with temperatures close to six billion degrees, protons and neutrons began to combine to form the nuclei of heavier elementsa process called nucleosynthesis. Fusion of these particles led to the formation of the nuclei of several light elements: helium-4 (an amalgam of two protons and two neutrons), lithium (three protons and three or four neutrons), and boron-11 (five protons, six neutrons). About a quarter of the mass in the Universe is helium-4, formed by nucleosynthesis in the early days of the Big Bang.
The proportion of the Universe's total mass that comes from all other elements is tiny, however: about 1 to 2 percent in all. In other words, around three-quarters of the Universe's mass is hydrogen, and the rest is mostly helium. Once the temperature had dropped to around 7200°F, nuclei became able to grasp and retain electrons. Protons teamed up with electrons, and hydrogen atoms were born.
If chemistry had relied solely on the Big Bang, the periodic table would be but a short, formless list of half a dozen elementseasier to grasp, perhaps, except that you wouldn't exist to appreciate it. By the time it had fashioned boron, the Big Bang had exhausted its atom-making vigor.
Fortunately for us, gravity came to the rescue. Within the diffuse clouds of matter synthesized in the Big Bang, gravity began the slow but inexorable task of galaxy-building. Where the gas was ever so slightly denser, the inward tug of gravity was that bit stronger. And so, almost imperceptible variations in density gradually became accentuated, condensing into ever more compact blobs, like a sheet of rainwater on a windshield breaking up into a network of droplets. These amorphous clumps became the precursors of vast galaxy clusters, within which smaller clumps condensed into separate galaxiesa hierarchical fragmentation right down to the scale of the nebulae that would ultimately become stars.
As the pull of gravity made matter collapse in on itself, the stuff heated up. Stars ignited and began blazing. One by one, the lights came on again throughout the Universe. The stars are more than mere fireballsthey are engines of creation, and out of their fiery hearts come the elements needed to make worlds.
TRANSMUTATION MADE REAL
Astronomy is an indispensable art; it should be rightly held in high esteem, and studied earnestly and thoroughly.
So said the itinerant physician and alchemist Paracelsus in the sixteenth century, unsuspecting all along that the stars possessed the art he himself sought: the ability to convert one element to another. Stars are the alchemists of the Universe.
In the interiors of stars, hydrogen nuclei are fused together to generate heavier elements; this is the process of nuclear fusion, and it is how stars conduct nucleosynthesis. Young stars are made mostly of hydrogen, which fuses in three steps to generate helium-4 and a great deal of energy. Over its lifetime, a typical star burns about 12 percent of its hydrogen to helium in this way.
One often hears that this transmutation of elements is a thoroughly modern idea, unrelated in more than a coincidental sense with the alchemists' belief that elements can be interconverted. But on the contrary, it is possible to follow a continuous thread of logic and supposition from Paracelsian metaphysics to Enrico Fermi's first atomic pile in Chicago in the 1940s.
In 1815 the British chemist William Prout proposed that atoms of the heavier elements were formed by the clustering together of hydrogen atoms, making hydrogen the "first matter," or prote hyle, from which Aristotle had suggested all matter is composed. Tempting though it is to suggest that in this way Prout anticipated the twentieth-century discoveries of nuclear fusion and the structure of the atom, the reason Prout's idea wasn't laughed out of court (although it was by no means uncontroversial) was in fact because the legacy of alchemy was still in the air. Indeed, no less a figure than the eminent British chemist and physicist Michael Faraday remained convinced of the doctrine of elemental transmutation throughout his life.
Prout's theory was elaborated on by the French chemist Jean Baptiste Dumas in the 1840s. Dumas noted that the atomic weights of some elements, which by then were known with impressive accuracy, were certainly not whole multiples of the atomic weight of hydrogen, and therefore these elements could not be made of clusters of hydrogen atoms. Dumas proposed that the fundamental unit of matter might instead be some subdivision of the hydrogen atom, perhaps a quarter or a half. Unknown to Dumas, the discrepancies are actually a consequence of the fact that elements exist in nature as a mixture of isotopes, so that their average mass does not correspond to a whole number of protons. The link between these ideas and the chemistry of the extraterrestial Universe was made by Norman Lockyer in the 1870s. During this and the preceding decade, astronomers detected the fingerprints of many earthly elements in the light emitted by the Sun and other stars. Lockyer, in parallel with the Frenchman Pierre Janssen, discovered a new element in 1868 purely from its distinctive imprint on the spectrum of sunlighta series of dark bands where the element absorbs light of certain colors. Lockyer called the element helium (after helios, Greek for the Sun), and it was not found on Earth until twenty-seven years later.
Lockyer developed a theory of the "evolution of stars and chemical elements" which drew explicitly on Dumas's elaboration of Prout's hypothesis. He proposed that heavy elements were made from lighter ones inside stars as the stars cooled from a blue-white brightness to a red dimnessa progression inferred from the observed colors of different stars. The British chemist William Crookes developed a similar hypothesis in the 1880s, based on the observation that gases subjected to high voltages could be decomposed into a plasma, a mixture of ions and electrons. Crookes considered plasmas to be a "fourth state of matter" consisting of subatomic particles akin to those postulated by Prout and Dumas. He constructed an exotic scheme for the evolution and transmutation of elements from this plasma, which he assumed to be the stuff of stars.
In 1919 the British physicist Francis Aston, working at the Cavendish Laboratory of Cambridge University, developed a device that enabled him to measure the relative masses of atomic nuclei with great precision: the "mass spectrograph," which we would now call a mass spectrometer. He found that even the nuclear masses of individual isotopes are generally not exactly whole multiples of hydrogen's; they are somewhat lighter, although typically by a margin of only a fraction of 1 percent. The tiny difference in mass reflects the fact that a huge amount of energy is released when protons and neutrons combine to form heavier nuclei: the energy accounts for the "missing mass" and is calculated according to Einstein's famous formulation E=mc². For the first time, Aston realized the vast energy lurking within the nuclei of atoms. When Ernest Rutherford, the director of the Cavendish, demonstrated in 1919 that a nuclear transmutation process could be induced by artificial means, scientists realized that it might be possible to extract this energy technologicallyfor better or worse. French physicist Jean Perrin proposed in the same year that the Sun and other stars might derive their energy from the fusion of hydrogen to heavier elements. In other words, nuclear fusion might not be just a consequence of the furious solar environment, as Lockyer had supposed, but the cause of it. Arthur Eddington added his approval in 1920: "What is possible in the Cavendish Laboratory may not be too difficult in the Sun."
In the mid-1930s the Russian physicist George Gamow put Perrin's idea on firmer footing, suggesting that hydrogen was transformed to heavier elements by capturing a succession of protons or neutrons. The German physicist Hans Bethe showed in 1939 that a tiny dose of carbon is needed to stimulate this process. A newly formed star condensing from a gaseous nebula typically contains about 1 percent carbon, primarily in the form of the isotope carbon-12. This can provide the seed for the six-step sequence of nuclear reactions that converts hydrogen-1 to helium-4. The carbon-12 is recycled: consumed at the beginning of the sequence, but regurgitated at the end. By definition, it acts as a catalyst. This means that a tiny amount of carbon can facilitate the fusion of a lot of hydrogen.
At first glance, this cycle doesn't seem to get us very much further, since its net result is to transform hydrogen to heliumand we've seen that this can happen anyway, without the help of carbon. But in the intermediate steps of the cycle, other elements are formed: three different isotopes of nitrogen, and one of oxygen (the rare isotope oxygen-15). In Bethe's so-called C-N-O cycle, oxygen makes its entrance onto the cosmic stage.
The C-N-O cycle provides a significant fraction of a star's energy output. In fact, for stars several times more massive than the Sun it becomes a more important power source than the direct hydrogen-to-helium reactions. Because a star is constantly reiterating this cycle, it maintains a steady amount of carbon, nitrogen, and oxygen in its atmosphere. Clearly, however, this can't be the whole story either. The C-N-O cycle generates only oxygen-15, while the isotopes we see in nature are mainly oxygen-16, -17, and -18. And what about all the even heavier elements?
Bethe supplied part of the answer. He showed that at particularly high temperatures, a new set of nuclear reactions becomes possible, in which oxygen-16, oxygen-17, and fluorine-17 also take part. But this side branch of the C-N-O cycle requires a pinch of oxygen-16. Newly formed stars in the present-day Universe acquire this oxygen isotope from the interstellar material from which they condense. But where did it come from in the first place?
The answer was provided in 1957 by Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle, in a paper that still defines today most of what we know about the nucleosynthesis of heavy elements in stars. The reason a star doesn't just keep collapsing once gravity has pulled it together from gas and dust is that the intense radiation produced by nuclear fusion gives the gas buoyancy, rather as a burner supplies buoyancy to the air in a hot-air balloon. But in the autumn years of a star's life, when it has burned up most of its hydrogen and its fusion engine grows cooler, this buoyancy is lost and the star begins to contract. Gravitational collapse generates heat in the star's dense core, which is now mostly helium-4. At the same time, the star's outer atmosphere of gas expands and cools to a red glow, and it becomes a red giant. In the hot, dense core, the star starts to burn helium. The nuclei fuse to make new elements whose masses grow in leaps of four: boron-8, carbon-12, oxygen-16, neon-20, magnesium-24, silicon-28 and beyond. Oxygen-18, meanwhile, is formed from fusion of helium-4 with nitrogen-14.
Eventually the helium in the star's core is used up too, and so the star has to resort to burning whatever it has left, which is mostly carbon and oxygen. This requires still-higher temperatures and pressures, which are conveniently supplied when the diminishing fuel reserves permit further contraction, raising the core temperature to around a billion degrees. At this point, carbon-12 and oxygen-16 undergo fusion to generate a series of elements of about twice their mass: sodium-23, silicon-28, phosphorus-31, and sulfur-32. Thereafter, silicon-28 fuses with the helium nuclei produced in other reactions to make elements up to and heavier than iron. In their final evolutionary stage, such stars have a concentric-shell structure with a core of the heaviest elements and successive shells rich in silicon, in carbon/oxygen/nitrogen, helium, and finally hydrogen.
And there is more. Stars larger than around four times the mass of the Sun may end their lives in spectacular fashion: as supernovae, which explode with a brightness that momentarily outshines the entire galaxy in which they reside. When such a star has finally exhausted its supply of nuclear fuel, there is nothing more to prevent the catastrophic collapse of the dense core under its own gravity. The inrush of matter in the core generates a shock wave, and the star becomes unstable. In an awesome rebound, the outer envelope of the star is cast off and out into space while the star's core implodes to unspeakable densities whose inner region is a liquid of neutrons. Here atomic nuclei are unable to retain their separate identities but instead become crushed to a featureless miasma, and most of the protons combine with the electrons to produce a preponderance of neutrons. So the supernova becomes a dark, compact neutron star surrounded by an expanding shell of matter rich in a variety of elements. Such is the energy of the supernova's outburst that new nucleosynthesis reactions are triggered, enriching the debris with very heavy elements such as thorium and uranium.
These elementsthe whole periodic table of themare scattered through space. As a result of supernovae, the void between the stars is sprinkled with the raw material from which worlds are made. Walt Whitman anticipated this process in 1855 in an inspired poetic leap of imagination: "A leaf of grass is no less than the journey-work of the stars." And Ted Hughes, in "Fire Eater," reads the origins of earth and water in the firmament.
Oxygen is the third most abundant element in the Universealbeit a very poor third to hydrogen and helium, whose primordial generation in the Big Bang ensures that they constitute almost all of the fabric of creation. But helium is unreactive, a cosmic loner. And so should we after all be surprised that water, the combination of the Universe's most popular reactive elements, is so pervasive? This molecule, the matrix of life, is the product of the Universe's two most generous acts of creation: the Big Bang, which started it all and gave us a cosmos made mostly of hydrogen; and stellar evolution, which reformulates this element, whose very name means "water former," into oxygen and all the other elements that make up the world. Within the imponderable expanses of interstellar space, these two elements uniteand there in the making is the river Nile, the Arabian Sea, the clouds and snowflakes, the juice of cells, the ice plains of Neptune, and who knows what other rivers, oceans, and raindrops on worlds we may never see.
Every supernova sends a potent brew of atoms and molecules spewing out into the cosmos. But the cosmos is a big place, and even the creative might of an exploding star is a drop in the ocean. The space between the stars of our galaxy is emptier than the best human-made vacuum; and yet there is enough finely dispersed matter out there to make around ten billion more stars, one-twentieth of the number in the luminous drapery of the Milky Way. This tenuous stuff is mostly hydrogen, but it has been delicately seasoned over the aeons with other elements and molecules, a dizzy menu of them. You'll find plenty of hydrogen molecules ([H.sub.2], two atoms hand in hand) out there, but also carbon monoxide, hydrogen cyanide, methanol and ethanol, ammonia, formaldehyde, and yes, water. There's solid matter too: tiny grains of silicate minerals, specks of soot and diamondoften with a coating of ice. All you need, in fact, to make a planet.
In some parts of the galaxy the gas and dust between the stars is clumpy, forming vast "molecular clouds" which can block out the starlight beyond to give us fantastic sights like the Horsehead nebula in Orion. In these clouds, stars may form as the matter condenses under its own gravity. That water is abundant in these regions was discovered in 1969 by astronomer Charles Townes and his co-workers. The watery signature was barely legible: just a single bright peak in a microwave spectrum of cold interstellar gas. Molecules in interstellar space are usually detected from the lines that they strip out of the spectra of light from more distant objectseach type of molecule absorbs light at characteristic colors. But the water that Townes saw was not absorbing the microwave radiationit was emitting it. The water was glowing! Improbable as it might seem in the deep-freeze of space, molecules in interstellar clouds can be pumped full of energy. The molecules get "hot" by undergoing collisions in dense regions of the clouds, and they cool again by emitting radiation. The molecules can synchronize their emission: the radiation emitted by one excited molecule can "tickle" a second into emitting too, and before long a whole slew of hot molecules are casting off their excess energy. Much the same processes are responsible for light emission in some lasers. Because the "light" from these collisionally pumped molecular clouds is in the microwave region of the spectrum, they are not cosmic lasers but masers (from Microwave-amplified Stimulated Emission of Radiation). What Townes and his colleagues saw was the first known astrophysical water maser. These extensive astrophysical objects are now known to be regions where the gas is collapsing to form new stars. Water sends out a signal of star formation to the Universe at large.
Star formation: it's what every world needs. To make a planet, you first have to make a sun.
THE GREAT FLOOD
The ancient Greeks guessed well about our planet's origin, for they believed that Mother EarthGaiaarose from a primordial Chaos. Chaos is the etymological origin for the word gas, and it was from gas and dust that the Earth was formed, along with the Sun and our sister planets. In an inspired guess, Immanuel Kant proposed as much in 1755.
As a clump of gas collapses within a molecular cloud, it rotates and flattens out into a disk. While most of the matter gathers into the central core and is incorporated into the nascent star, some is left farther out in the disk, where it provides the material for the formation of a planetary solar system. Several disklike embryonic stars have been seen elsewhere in the galaxy. Some of the disks are punctuated by ring-shaped voids, thought to be the tracks engraved through the dust by newly formed circulating planets. This happened in our own stellar diskthe solar nebulaabout 4.6 billion years ago, when the Earth was one of those orbiting blobs.
But planets do not arise fully formed from globules of condensed solar nebula. We know this because there is far less of certain gasesneon, argon, kryptonin today's atmosphere than is thought to have been distributed through the solar nebula. Because these gases are chemically un-reactive, we would expect them to remain as abundant as ever they were if our planet and its atmosphere was just a clump of pristine solar nebula with its elements rearranged.
No, planet formation is less stately and more traumatic than this. The accretions of gas and dust in the solar nebula formed smaller rocky bodies called planetesimals that range in size from boulders to moon-sized asteroids. These swarming planetesimals engaged in fearsome collisions that smashed each other to rubblebut the rubble from each collision then cohered into a single, larger object through the tug of its own gravity. Rather like companies, larger planetesimals grew at the expense of smaller ones until the disk was swept free of debris and only the planets remained, the multinational conglomerates of the solar system. The inner planetsMercury, Venus, Earth, and Marsare relatively small, dense, rocky orbs. But out beyond the asteroid belt, where some of the smaller debris escaped capture, the planets were able to retain vast envelopes of gases and liquids: here we find the gas giants Jupiter and Saturn, and the frozen worlds of Uranus, Neptune, and Pluto.
Earth was not a good vacation destination in those early days. The heat generated during its formation from colliding planetesimals created a global inferno. And around 4.5 billion years ago the Earth seems to have collided with a planetesimal about the size of Mars. Were this to happen today, you might as well cancel the papers. Global nuclear war would be a picnic in comparisonan impact this size would almost shatter the planet, and would certainly extinguish all life. As it was, it sheared off enough material to form the Moon, boiled away any atmosphere that the Earth then possessed, and left the planet a ball of molten rock (magma) for millions of years, its surface awash with a fiery ocean from pole to pole.
Yet collisions were not wholly destructive. On the contrary, they ultimately gave the planet an atmosphere, waterand the possibility of harboring life. In the part of the solar nebula where the Earth condensed, volatile substances like water and carbon dioxide were rare commoditiesonly farther out, where the temperature was low enough for them to condense and freeze, could they become a major component of planetesimals. These colder bodies could sequester a coating of ice from the gas and dust, just as snowflakes high in our atmosphere sweep up water vapor from the air. Blundering in and out of the nascent inner solar system, such objects most probably added water to the rocky mixture that was becoming the Earth.
To test whether this idea holds water, so to speak, planetary scientists today study the composition of meteorites. These cosmic boulderswell, they are more like pebbles on the whole, and some are no bigger than grains of sandare mostly the leftovers of planet formation, the bits that never quite got incorporated into planets. It's likely, then, that the mixture of elements and compounds of which they are comprised reflects the composition of early planetesimals. They are still raining down on us from the skies, albeit in far smaller numbers than when the world was young. Many meteorites do indeed carry a bountiful crust of icenot just water ice, but also frozen carbon dioxide, ammonia, and other volatile compounds. Meteorites called carbonaceous chondrites, which are rich in carbon compounds, can contain up to 20 percent water, either as ice or locked up in the crystal structures of minerals. The most abundant type of meteorites, ordinary chondrites, carry much less wateraround 0.1 percent of their mass. Yet even this would have been more than enough to fill the oceans if the Earth was formed primarily from planetesimals with this composition.
But meteorites are not the only objects still wandering among the planets. There are itinerants the size of mountains out there, and they could deliver huge quantities of water to the Earth and its neighbors in a flash. I'm talking about comets, the unruly rabble of the outer solar system. Comets mostly originate in a roughly spherical cloud of objects stretching way beyond the orbit of the most distant planet, Pluto, perhaps more than halfway to the nearest neighboring star system. This halo, called the Oort cloud, contains around a million million comets, whose immense, looping orbits bring them occasionally sweeping through the inner solar systemas we saw in spectacular fashion with comet Hale-Bopp in 1997. They consist mostly of volatile gases condensed into ices, of which by far the most abundant is water. Mixed in with the ice is a scattering of mineral dust, making comets immense dirty snowballs. Generally they are several hundred feet to several miles across, and so contain an awesome amount of water. Halley's comet, for instance, is a potato-shaped lump about five by ten miles in size, with a mass of about one hundred billion tonsmost of which is ice. A typical comet is still larger, containing around one trillion tons of water. A million comets like this would be enough to supply all of Earth's oceans.
I'm glad to say that comets do not collide with Earth with anything like the frequency of small meteorites: the last major collision may have been sixty-five million years ago, possibly hastening the dinosaurs' demise. But comets swarmed through the solar system in far greater numbers when the Earth was forming, and would have crossed paths with the planet far more regularly, bringing oceans on their backs. It seems that the gravitational tug of the outer planets Uranus and Neptune, as well as nearby stars, helped to rearrange the orbits of cometlike planetesimals in the Oort cloud so that they would pass more often through the inner solar system. Meanwhile these and the other giant planets, particularly Jupiter, eventually swept up most of the debris from the solar system and so quieted down the game of cosmic billiards by about a billion years after the planets had formed. Had this not happened, huge impacts might have delayed the appearance of life on Earth for billions of years. So we may have our neighbors to thank not only for our oceans but also for the life that spawned in them.
But I'm jumping the gun, for the oceans did not appear until many millions of years after the planet was formed. Four and a half billion years ago the Earth was still a molten magma ball, seething from the collision that ejected the Moon. As the planet cooled, its constituents separated like curdled milk. Within about fifty million years, the iron of which much of the Earth was comprised had sunk to the core, and the lighter elementssilicon, aluminium, calcium, magnesium, sodium, potassium, and oxygen, along with some remaining ironformed a rocky crust at the surface, just as slag floats on top of molten iron in a smelter.
Among all this rocky stuff were the volatile compounds delivered by collisions as the planet accretedhydrogen, nitrogen, hydrogen sulfide, carbon oxides, water. While the Earth was molten, these volatile compounds dissolved in the magma, but as the molten rock cooled and solidified, the vapors were released in a process called degassing. The atmosphere that resulted from degassing was very different from today's, consisting mostly of carbon dioxide, nitrogen, and water vapor.
Hydrogen is too light to be retained by the Earth's gravitational field, and was gradually lost from the early atmosphere into space. For this reason, the Earth is steadily losing its water too, albeit very slowly. The Sun's ultraviolet rays split water in the upper atmosphere into its constituent hydrogen and oxygen atoms, a process called photolysis. The hydrogen then escapes into space. This water-splitting costs the planet the equivalent of a small lake's worth of water each year. That sounds like a lotand it certainly would be if it all came from a single lake! But averaged over the amount of water on the planet, the loss is probably quite small: photolysis may have reduced the Earth's water reserves by just 0.2 percent since the planet was formed.
THE DAY THE RAINS CAME
Those formative years were steamy times on Earth, for all the water was in the sky. And then one day; somewhere between 4.4 and 4.0 billion years ago, the temperature had fallen far enough for water to condense. Clouds massed in the sky, and the oceans rained down. Sadly, I have to confess that this would not truly have happened so suddenly, one fine day in the Hadean erabut I like the image. Yet however you look at it, there's no avoiding the conclusion that a deluge must eventually have ensued that leaves the biblical version looking like an April shower. This was the original Flood, and had anyone been there to witness it, I don't think an ark would have done them much good.
Far from eradicating life, this deluge set the stage for life's entry. It turned the face of the world blue and created a planet that exists, in atmospheric scientist James Lovelock's words, as "a strange and beautiful anomaly in our solar system."
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