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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.

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Life's Matrix

University of California Press

Chapter One

THE FIRST FLOOD

WATER'S ORIGINS

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 sunlight?all, 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 formlessand empty, the Hebrew God was "hovering over the waters."There was no sky, no dry land, until God separated "the water underthe expanse from the water above it" and commanded that "thewater under the sky be gathered to one place." Then the worldemerged?from an infinite primeval ocean.

    This is echoed in similar myths throughout the world. In centraland northern Asia, North America, India, and Russia, a recurringmotif is that of the Earth Diver: an animal or a god whoplunges to the bottom of a primordial ocean to bring up a seed ofearth. The Polynesian cosmogeny reproduces that of the Old Testamentin extraordinary detail: the supreme being, Io says, "Let the waters beseparated, let the heavens be formed, let the earth be!" For the OmahaNative Americans, all creatures once floated disconsolately on a whollysubmerged Earth until a great boulder rose from the deep. In Hindumythology, the sound that embodied Brahma became first water andwind, from which was woven the web of the world. "Darkness was there,all wrapped around by darkness, and all was Water indiscriminate" saysthe beautiful creation hymn of the Rig Veda (3700 B.C.). For the Maya ofCentral America also, the deity Hurakan called forth the land from auniverse of darkness and water.

    Why does this idea of a watery beginning resonate throughout disparatecultures, without heed to the local particulars of geography or religioustradition? Ultimately its origin may be psychological: the land wasknowable for ancient peoples, but the sea was a symbol of the unconscious?somethingmysterious, pristine, unfathomable. I know of nocreation myths where the land came first and the seas followed in a subsequentdeluge.

    Yet land and sea are contemporaneous and complementary in sometraditions. The Judeo-Christian distinction between flesh and blood is adistinction between the earthy and watery aspects of the corpus of theworld. In Norse mythology, the land is the flesh and bones of Ymir, thefirst giant, slain by Odin. His salty blood, gushing from the spear woundin his heart, became the oceans. So too in Chinese myth are land and seacoeval aspects of a primal being, Pan-Ku the sculptor, whose medium washis own body.

    But is there any truth of a more material nature in these myths?wasthe 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 theorigin of the Earth. To look beyond the beginning of our world is to ponderthe eternal: the Chaos of the Greeks, the abyss of fire and ice calledGinnungagap by the Norse, or the supreme deity Akshara-Brahma inHindu tradition. Today, Earth's beginning is merely a local question, amoment of parochial interest in an already mature universe. The real momentof creation goes back at least six billion years beyond that, and it isas fantastic as any myth.

    Origins are seldom uncontentious. Current fashion sometimes has itthat the idea of a cosmic Big Bang is best regarded as our latest culturalmyth, 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 particularkind of confidence that makes science possible. From a scientific perspective,the Big Bang is beyond question still the best model we have forthe birth of the Universe, and rests on some formidable pillars. To addressthe question of why water is what it is, modern cosmology provides aconsistent and explanatory framework in a way that Odin's murder ofYmir 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 Hubblesaw the galaxies of the Universe behaving in the same way, he wasforced 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 theinside?we 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 oncefocused into a much smaller volume, which went Bang! Albert Einsteinhad deduced as much in 1917: when he applied his theory of general relativityto the Universe as a whole, he found the equations predicting that ithad to be either expanding or contracting. That seemed to him then to bea crazy notion, and so he added a "fudge factor" to remove the expansion.But Hubble's discovery persuaded him in 1931 that no fudging wasneeded after all.

    After just one-millionth of a billionth of a second, when according tosome theories the Universe might have been just a few feet across, thetemperature would have been in the region of a billion billion degrees. Insuch extremes, there can be no atoms and molecules, no matter as we currentlyknow 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 twentymillion degrees?about 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 ofthis cosmic fireball. In 1965 Arno Penzias and Robert Wilson at Bell TelephoneLaboratories detected the faint afterglow that pervades the sky: auniform background radiation of microwaves coming from all directions,indicating an average temperature of about five degrees Fahrenheit aboveabsolute zero. This cosmic microwave background is all that is left of theBig 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 livesof stars, rather than divine providence, in our bones and blood. But bonesand blood must come later; for now, I want to follow only the gestation ofthose protons and electrons, and from them the hydrogen, the oxygen?andthe 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 chemicalformula that everyone learns: two atoms of hydrogen welded to oneof oxygen. Their union is a molecule?a duster of atoms. Chop up ablock of ice, and keep chopping?and your finest blade, finer than thekeenest surgical scalpel, will eventually reduce the fragments to these individualthree-atom clusters. If you chop beyond that, you no longer havewater. 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 associationof atoms, divisible into atoms of different natures. Yet water isso fundamental to the world that for millennia it was mistaken, naturallyenough, for an element, something indivisible. Hydrogen and oxygen areelements, because they each contain only one kind of atom. But there isno "water atom"?only a water molecule, made up of two different typesof atom.

    Before making bread, one must make flour; and before water couldcome into the Universe, there had to be hydrogen and oxygen atoms. Butbefore flour comes wheat?and atoms too have more fundamental constituents,Wald's protons and electrons.

    As far as atoms are concerned, protons and electrons are like knivesand forks at the dinner table: no matter how big the table, there are equalnumbers of each. The difference between atoms of different elements?betweenan atom of oxygen and one of carbon, say?is simply that theycontain different numbers of protons. In this regard, the underlying patternof atoms is numerical, as Jacob Bronowski says in The Ascent of Man.An atom with one proton (and one electron) is hydrogen; an atom witheight of each is oxygen. At one level, chemistry is as simple as counting.

    At another level, it is clearly not. For mere proton bookkeeping offersno clue as to why hydrogen atoms join with oxygen atoms in the ratio of2:1, or why sodium (eleven protons) is a soft reactive metal, chlorine (seventeenprotons) a corrosive gas, silicon (fourteen protons) an inert graysolid. To understand any of this, we need to consider how the electronsare deployed: for there are deeper patterns in the arrangement of the electronsthat determine the element's chemical properties.

    Protons and electrons are not, as British physicist J. J. Thomson believedat the turn of the century, lumped together inside the atom as aheterogeneous blob. Rather, they bear to one another something like therelationship of the planets to the Sun, with the electrons orbiting a central,dense nucleus where the protons are. In this "solar system" model ofthe atom, proposed by Thomson's protégé, New Zealander ErnestRutherford, if the nucleus of an atom were scaled up to the size of theSun, then the electrons would be more distant than Neptune's orbit by afactor of about ten. Yet we shouldn't take the model too seriously: electronscan't be pinpointed like planets, and do not follow well-defined ellipticalpaths, but instead occupy regions of space called orbitals. Theseregions, which have the shapes of spheres, lobes, and rings centered onthe nucleus, are best regarded as hazy "electron clouds," rather likeswarms of bees around a hive. From the manner in which an atom's electronsare distributed among the various available orbitals flows the wholeof chemistry.

    Moreover, atomic nuclei grasp their electrons not by the force ofgravity but by electrical attraction: an electron is negatively charged, and aproton has a positive charge of equal magnitude. An atom, with equalnumbers of both particles, is electrically neutral. Electrons, however, canbe stripped away from atoms, rather as a passing star could pull a planetfrom a nearby solar system. The depleted atom then has an excess of protonsover electrons, and so is positively charged. Atoms can also gain anexcess of electrons over protons, and so become negatively charged.These charged atoms are called ions. This is why, even though protons andelectrons are equally represented in a neutral atom, it is the number ofprotons that is the fundamental characteristic of an element. To pull aproton 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 intoa different element entirely.

    Although hydrogen atoms have one proton and oxygen atoms haveeight, oxygen is about sixteen times heavier than hydrogen. There is athird ingredient to the atom?a particle called the neutron, which has virtuallythe same mass as a proton but is electrically neutral. All atoms barhydrogen have neutrons as well as protons in their nuclei, and generallyspeaking the nuclei contain equal numbers of each. The vagueness in thisstatement is, I fear, unavoidable, for two reasons. First, the number ofneutrons tends increasingly to exceed the number of protons for heavieratoms: the proportions are pretty much fifty-fifty for light atoms like carbon,oxygen, and nitrogen, whereas lead atoms have around 40 percentmore neutrons than protons. Second, even atoms of the same elementcan possess different numbers of neutrons. Oxygen atoms can containseven, eight, nine, or ten neutrons to accompany their eight protons,while hydrogen atoms can contain no, one, or two neutrons. These differentforms of atoms of the same element are called isotopes. Most hydrogenatoms have no neutrons; but 0.000015 percent of all of those innature 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 regretforcing it on you so soon. I hope it is of some consolation to learn thatthis is all you will need to know about atoms for the rest of the book. Butthey are the alphabet of chemistry, so we need to be at least on familiarterms with them. Besides, if we are to consider how the Universe cookedup 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 andoxygen. The first ingredient is the easy one: it dropped right out of the BigBang, once things got cool enough. That's to say, protons?the nuclei ofhydrogen atoms?condensed out of the fireball about a millionth of a secondafter time and space were born.

    But at this point the temperature would have been around a trilliondegrees, which is too hot for protons to hold on to electrons. The Universewas then a soup of protons and electrons, seasoned with neutronsand other subatomic particles such as neutrinos, all swimming in aseething broth of X-rays. And for a good few minutes, that's how thingsstayed; the Universe was too hot to be interesting.

    Although protons could not yet combine with electrons, they could atleast team up with each other and with neutrons?for the force that bindsprotons and neutrons together in the nucleus, called the nuclear strongforce, is many, many times stronger than the electrical force of attractionbetween protons and electrons. Just one hundred seconds into the BigBang, with temperatures close to six billion degrees, protons and neutronsbegan to combine to form the nuclei of heavier elements?a processcalled nucleosynthesis. Fusion of these particles led to the formation ofthe nuclei of several light elements: helium-4 (an amalgam of two protonsand two neutrons), lithium (three protons and three or four neutrons),and boron-11 (five protons, six neutrons). About a quarter of themass in the Universe is helium-4, formed by nucleosynthesis in the earlydays of the Big Bang.

    The proportion of the Universe's total mass that comes from all otherelements 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 ismostly helium. Once the temperature had dropped to around 7200°F, nucleibecame able to grasp and retain electrons. Protons teamed up withelectrons, and hydrogen atoms were born.

ATOMCRAFT

    If chemistry had relied solely on the Big Bang, the periodic tablewould be but a short, formless list of half a dozen elements?easier tograsp, perhaps, except that you wouldn't exist to appreciate it. By the timeit had fashioned boron, the Big Bang had exhausted its atom-makingvigor.

    Fortunately for us, gravity came to the rescue. Within the diffuseclouds of matter synthesized in the Big Bang, gravity began the slow butinexorable task of galaxy-building. Where the gas was ever so slightlydenser, the inward tug of gravity was that bit stronger. And so, almostimperceptible variations in density gradually became accentuated, condensinginto ever more compact blobs, like a sheet of rainwater on a windshieldbreaking up into a network of droplets. These amorphous clumpsbecame the precursors of vast galaxy clusters, within which smallerclumps condensed into separate galaxies?a hierarchical fragmentationright down to the scale of the nebulae that would ultimately become stars.

    As the pull of gravity made matter collapse in on itself, the stuffheated up. Stars ignited and began blazing. One by one, the lights cameon again throughout the Universe. The stars are more than mere fireballs?theyare engines of creation, and out of their fiery hearts come theelements 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 sixteenthcentury, unsuspecting all along that the stars possessed the art hehimself sought: the ability to convert one element to another. Stars arethe alchemists of the Universe.

    In the interiors of stars, hydrogen nuclei are fused together to generateheavier elements; this is the process of nuclear fusion, and it is howstars 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 tohelium in this way.

    One often hears that this transmutation of elements is a thoroughlymodern 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 fromParacelsian metaphysics to Enrico Fermi's first atomic pile in Chicago inthe 1940s.

    In 1815 the British chemist William Prout proposed that atoms of theheavier elements were formed by the clustering together of hydrogenatoms, making hydrogen the "first matter," or prote hyle, from which Aristotlehad suggested all matter is composed. Tempting though it is to suggestthat in this way Prout anticipated the twentieth-century discoveries ofnuclear fusion and the structure of the atom, the reason Prout's ideawasn'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 MichaelFaraday remained convinced of the doctrine of elemental transmutationthroughout his life.

    Prout's theory was elaborated on by the French chemist Jean BaptisteDumas in the 1840s. Dumas noted that the atomic weights of some elements,which by then were known with impressive accuracy, were certainlynot whole multiples of the atomic weight of hydrogen, andtherefore these elements could not be made of clusters of hydrogenatoms. Dumas proposed that the fundamental unit of matter might insteadbe some subdivision of the hydrogen atom, perhaps a quarter or ahalf. Unknown to Dumas, the discrepancies are actually a consequence ofthe fact that elements exist in nature as a mixture of isotopes, so that theiraverage mass does not correspond to a whole number of protons. Thelink between these ideas and the chemistry of the extraterrestial Universewas made by Norman Lockyer in the 1870s. During this and the precedingdecade, astronomers detected the fingerprints of many earthly elementsin the light emitted by the Sun and other stars. Lockyer, in parallelwith the Frenchman Pierre Janssen, discovered a new element in 1868purely from its distinctive imprint on the spectrum of sunlight?a seriesof dark bands where the element absorbs light of certain colors. Lockyercalled the element helium (after helios, Greek for the Sun), and it was notfound on Earth until twenty-seven years later.

    Lockyer developed a theory of the "evolution of stars and chemicalelements" which drew explicitly on Dumas's elaboration of Prout's hypothesis.He proposed that heavy elements were made from lighter onesinside stars as the stars cooled from a blue-white brightness to a red dimness?aprogression inferred from the observed colors of different stars.The British chemist William Crookes developed a similar hypothesis inthe 1880s, based on the observation that gases subjected to high voltagescould be decomposed into a plasma, a mixture of ions and electrons.Crookes considered plasmas to be a "fourth state of matter" consisting ofsubatomic particles akin to those postulated by Prout and Dumas. Heconstructed an exotic scheme for the evolution and transmutation of elementsfrom this plasma, which he assumed to be the stuff of stars.

ENTER OXYGEN

    In 1919 the British physicist Francis Aston, working at the CavendishLaboratory of Cambridge University, developed a device that enabled himto measure the relative masses of atomic nuclei with great precision: the"mass spectrograph," which we would now call a mass spectrometer. Hefound that even the nuclear masses of individual isotopes are generallynot exactly whole multiples of hydrogen's; they are somewhat lighter, althoughtypically by a margin of only a fraction of 1 percent. The tiny differencein mass reflects the fact that a huge amount of energy is releasedwhen protons and neutrons combine to form heavier nuclei: the energyaccounts for the "missing mass" and is calculated according to Einstein'sfamous formulation E=mc². For the first time, Aston realized the vast energylurking within the nuclei of atoms. When Ernest Rutherford, the directorof the Cavendish, demonstrated in 1919 that a nuclear transmutationprocess could be induced by artificial means, scientists realized that itmight be possible to extract this energy technologically?for better orworse. French physicist Jean Perrin proposed in the same year that theSun and other stars might derive their energy from the fusion of hydrogento heavier elements. In other words, nuclear fusion might not bejust 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 inthe Sun."

    In the mid-1930s the Russian physicist George Gamow put Perrin'sidea on firmer footing, suggesting that hydrogen was transformed toheavier elements by capturing a succession of protons or neutrons. TheGerman physicist Hans Bethe showed in 1939 that a tiny dose of carbon isneeded to stimulate this process. A newly formed star condensing from agaseous nebula typically contains about 1 percent carbon, primarily in theform of the isotope carbon-12. This can provide the seed for the six-stepsequence of nuclear reactions that converts hydrogen-1 to helium-4. Thecarbon-12 is recycled: consumed at the beginning of the sequence, but regurgitatedat the end. By definition, it acts as a catalyst. This means that atiny 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 helium?and we've seenthat this can happen anyway, without the help of carbon. But in the intermediatesteps of the cycle, other elements are formed: three different isotopesof nitrogen, and one of oxygen (the rare isotope oxygen-15). InBethe's so-called C-N-O cycle, oxygen makes its entrance onto the cosmicstage.

    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 becomesa more important power source than the direct hydrogen-to-helium reactions.Because a star is constantly reiterating this cycle, it maintains asteady amount of carbon, nitrogen, and oxygen in its atmosphere. Clearly,however, this can't be the whole story either. The C-N-O cycle generatesonly 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 hightemperatures, a new set of nuclear reactions becomes possible, in whichoxygen-16, oxygen-17, and fluorine-17 also take part. But this side branchof the C-N-O cycle requires a pinch of oxygen-16. Newly formed stars inthe present-day Universe acquire this oxygen isotope from the interstellarmaterial from which they condense. But where did it come from in thefirst 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 ofwhat we know about the nucleosynthesis of heavy elements in stars. Thereason a star doesn't just keep collapsing once gravity has pulled it togetherfrom gas and dust is that the intense radiation produced by nuclearfusion gives the gas buoyancy, rather as a burner supplies buoyancy to theair in a hot-air balloon. But in the autumn years of a star's life, when it hasburned up most of its hydrogen and its fusion engine grows cooler, thisbuoyancy is lost and the star begins to contract. Gravitational collapsegenerates heat in the star's dense core, which is now mostly helium-4. Atthe same time, the star's outer atmosphere of gas expands and cools to ared glow, and it becomes a red giant. In the hot, dense core, the star startsto burn helium. The nuclei fuse to make new elements whose massesgrow in leaps of four: boron-8, carbon-12, oxygen-16, neon-20, magnesium-24,silicon-28 and beyond. Oxygen-18, meanwhile, is formed fromfusion of helium-4 with nitrogen-14.

    Eventually the helium in the star's core is used up too, and so the starhas to resort to burning whatever it has left, which is mostly carbon andoxygen. This requires still-higher temperatures and pressures, which areconveniently supplied when the diminishing fuel reserves permit furthercontraction, raising the core temperature to around a billion degrees. Atthis point, carbon-12 and oxygen-16 undergo fusion to generate a series ofelements of about twice their mass: sodium-23, silicon-28, phosphorus-31,and sulfur-32. Thereafter, silicon-28 fuses with the helium nuclei producedin other reactions to make elements up to and heavier than iron. Intheir final evolutionary stage, such stars have a concentric-shell structurewith a core of the heaviest elements and successive shells rich in silicon, incarbon/oxygen/nitrogen, helium, and finally hydrogen.

    And there is more. Stars larger than around four times the mass of theSun may end their lives in spectacular fashion: as supernovae, which explodewith a brightness that momentarily outshines the entire galaxy inwhich they reside. When such a star has finally exhausted its supply of nuclearfuel, there is nothing more to prevent the catastrophic collapse ofthe dense core under its own gravity. The inrush of matter in the coregenerates 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 whilethe star's core implodes to unspeakable densities whose inner region is aliquid of neutrons. Here atomic nuclei are unable to retain their separateidentities but instead become crushed to a featureless miasma, and mostof the protons combine with the electrons to produce a preponderance ofneutrons. So the supernova becomes a dark, compact neutron star surroundedby an expanding shell of matter rich in a variety of elements.Such is the energy of the supernova's outburst that new nucleosynthesisreactions are triggered, enriching the debris with very heavy elementssuch as thorium and uranium.

    These elements?the whole periodic table of them?are scatteredthrough space. As a result of supernovae, the void between the stars issprinkled with the raw material from which worlds are made. Walt Whitmananticipated 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." AndTed Hughes, in "Fire Eater," reads the origins of earth and water in thefirmament.

WET SPACE

    Oxygen is the third most abundant element in the Universe?albeit avery poor third to hydrogen and helium, whose primordial generation inthe 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 allbe surprised that water, the combination of the Universe's most popularreactive elements, is so pervasive? This molecule, the matrix of life, is theproduct of the Universe's two most generous acts of creation: the BigBang, which started it all and gave us a cosmos made mostly of hydrogen;and stellar evolution, which reformulates this element, whose very namemeans "water former," into oxygen and all the other elements that makeup the world. Within the imponderable expanses of interstellar space,these two elements unite?and there in the making is the river Nile, theArabian Sea, the clouds and snowflakes, the juice of cells, the ice plains ofNeptune, and who knows what other rivers, oceans, and raindrops onworlds we may never see.

    Every supernova sends a potent brew of atoms and molecules spewingout into the cosmos. But the cosmos is a big place, and even the creativemight of an exploding star is a drop in the ocean. The space betweenthe stars of our galaxy is emptier than the best human-made vacuum; andyet there is enough finely dispersed matter out there to make around tenbillion more stars, one-twentieth of the number in the luminous draperyof the Milky Way. This tenuous stuff is mostly hydrogen, but it has beendelicately seasoned over the aeons with other elements and molecules, adizzy menu of them. You'll find plenty of hydrogen molecules ([H.sub.2], twoatoms hand in hand) out there, but also carbon monoxide, hydrogencyanide, methanol and ethanol, ammonia, formaldehyde, and yes, water.There's solid matter too: tiny grains of silicate minerals, specks of sootand diamond?often with a coating of ice. All you need, in fact, to make aplanet.

    In some parts of the galaxy the gas and dust between the stars isclumpy, forming vast "molecular clouds" which can block out the starlightbeyond to give us fantastic sights like the Horsehead nebula in Orion. Inthese clouds, stars may form as the matter condenses under its own gravity.That water is abundant in these regions was discovered in 1969 by astronomerCharles Townes and his co-workers. The watery signature wasbarely legible: just a single bright peak in a microwave spectrum of coldinterstellar gas. Molecules in interstellar space are usually detected fromthe lines that they strip out of the spectra of light from more distant objects?eachtype of molecule absorbs light at characteristic colors. But thewater that Townes saw was not absorbing the microwave radiation?itwas emitting it. The water was glowing! Improbable as it might seem inthe deep-freeze of space, molecules in interstellar clouds can be pumpedfull of energy. The molecules get "hot" by undergoing collisions in denseregions of the clouds, and they cool again by emitting radiation. The moleculescan synchronize their emission: the radiation emitted by one excitedmolecule can "tickle" a second into emitting too, and before long awhole slew of hot molecules are casting off their excess energy. Much thesame processes are responsible for light emission in some lasers. Becausethe "light" from these collisionally pumped molecular clouds is in the microwaveregion of the spectrum, they are not cosmic lasers but masers(from Microwave-amplified Stimulated Emission of Radiation). WhatTownes and his colleagues saw was the first known astrophysical watermaser. These extensive astrophysical objects are now known to be regionswhere the gas is collapsing to form new stars. Water sends out a signal ofstar formation to the Universe at large.

    Star formation: it's what every world needs. To make a planet, youfirst have to make a sun.

THE GREAT FLOOD

    The ancient Greeks guessed well about our planet's origin, for theybelieved that Mother Earth?Gaia?arose from a primordial Chaos. Chaosis the etymological origin for the word gas, and it was from gas and dustthat the Earth was formed, along with the Sun and our sister planets. Inan inspired guess, Immanuel Kant proposed as much in 1755.

    As a clump of gas collapses within a molecular cloud, it rotates andflattens out into a disk. While most of the matter gathers into the centralcore and is incorporated into the nascent star, some is left farther out inthe disk, where it provides the material for the formation of a planetarysolar system. Several disklike embryonic stars have been seen elsewhere inthe galaxy. Some of the disks are punctuated by ring-shaped voids,thought to be the tracks engraved through the dust by newly formedcirculating planets. This happened in our own stellar disk?the solarnebula?about 4.6 billion years ago, when the Earth was one of those orbitingblobs.

    But planets do not arise fully formed from globules of condensed solarnebula. We know this because there is far less of certain gases?neon,argon, krypton?in today's atmosphere than is thought to have been distributedthrough the solar nebula. Because these gases are chemically un-reactive,we would expect them to remain as abundant as ever they wereif our planet and its atmosphere was just a clump of pristine solar nebulawith its elements rearranged.

    No, planet formation is less stately and more traumatic than this. Theaccretions of gas and dust in the solar nebula formed smaller rocky bodiescalled planetesimals that range in size from boulders to moon-sized asteroids.These swarming planetesimals engaged in fearsome collisions thatsmashed each other to rubble?but the rubble from each collision thencohered into a single, larger object through the tug of its own gravity.Rather like companies, larger planetesimals grew at the expense ofsmaller ones until the disk was swept free of debris and only the planetsremained, the multinational conglomerates of the solar system. The innerplanets?Mercury, Venus, Earth, and Mars?are relatively small, dense,rocky orbs. But out beyond the asteroid belt, where some of the smallerdebris escaped capture, the planets were able to retain vast envelopes ofgases and liquids: here we find the gas giants Jupiter and Saturn, and thefrozen worlds of Uranus, Neptune, and Pluto.

    Earth was not a good vacation destination in those early days. Theheat generated during its formation from colliding planetesimals created aglobal inferno. And around 4.5 billion years ago the Earth seems to havecollided with a planetesimal about the size of Mars. Were this to happentoday, you might as well cancel the papers. Global nuclear war would be apicnic in comparison?an impact this size would almost shatter theplanet, and would certainly extinguish all life. As it was, it sheared offenough material to form the Moon, boiled away any atmosphere that theEarth 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 topole.

    Yet collisions were not wholly destructive. On the contrary, they ultimatelygave the planet an atmosphere, water?and the possibility of harboringlife. In the part of the solar nebula where the Earth condensed,volatile substances like water and carbon dioxide were rare commodities?onlyfarther out, where the temperature was low enough for themto condense and freeze, could they become a major component of planetesimals.These colder bodies could sequester a coating of ice from thegas and dust, just as snowflakes high in our atmosphere sweep up watervapor from the air. Blundering in and out of the nascent inner solar system,such objects most probably added water to the rocky mixture thatwas becoming the Earth.

    To test whether this idea holds water, so to speak, planetary scientiststoday study the composition of meteorites. These cosmic boulders?well,they are more like pebbles on the whole, and some are no bigger thangrains of sand?are mostly the leftovers of planet formation, the bits thatnever quite got incorporated into planets. It's likely, then, that the mixtureof elements and compounds of which they are comprised reflects thecomposition of early planetesimals. They are still raining down on usfrom the skies, albeit in far smaller numbers than when the world wasyoung. Many meteorites do indeed carry a bountiful crust of ice?not justwater ice, but also frozen carbon dioxide, ammonia, and other volatilecompounds. Meteorites called carbonaceous chondrites, which are rich incarbon compounds, can contain up to 20 percent water, either as ice orlocked up in the crystal structures of minerals. The most abundant typeof meteorites, ordinary chondrites, carry much less water?around0.1 percent of their mass. Yet even this would have been more thanenough to fill the oceans if the Earth was formed primarily from planetesimalswith this composition.

    But meteorites are not the only objects still wandering among theplanets. There are itinerants the size of mountains out there, and theycould deliver huge quantities of water to the Earth and its neighbors in aflash. I'm talking about comets, the unruly rabble of the outer solar system.Comets mostly originate in a roughly spherical cloud of objectsstretching way beyond the orbit of the most distant planet, Pluto, perhapsmore than halfway to the nearest neighboring star system. This halo,called the Oort cloud, contains around a million million comets, whoseimmense, looping orbits bring them occasionally sweeping through theinner solar system?as we saw in spectacular fashion with comet Hale-Boppin 1997. They consist mostly of volatile gases condensed into ices, ofwhich by far the most abundant is water. Mixed in with the ice is a scatteringof mineral dust, making comets immense dirty snowballs. Generallythey are several hundred feet to several miles across, and so contain anawesome amount of water. Halley's comet, for instance, is a potato-shapedlump about five by ten miles in size, with a mass of about onehundred billion tons?most of which is ice. A typical comet is still larger,containing around one trillion tons of water. A million comets like thiswould be enough to supply all of Earth's oceans.

    I'm glad to say that comets do not collide with Earth with anythinglike the frequency of small meteorites: the last major collision may havebeen sixty-five million years ago, possibly hastening the dinosaurs' demise.But comets swarmed through the solar system in far greater numberswhen the Earth was forming, and would have crossed paths with theplanet far more regularly, bringing oceans on their backs. It seems thatthe gravitational tug of the outer planets Uranus and Neptune, as well asnearby stars, helped to rearrange the orbits of cometlike planetesimals inthe Oort cloud so that they would pass more often through the inner solarsystem. Meanwhile these and the other giant planets, particularlyJupiter, eventually swept up most of the debris from the solar system andso quieted down the game of cosmic billiards by about a billion years afterthe planets had formed. Had this not happened, huge impacts might havedelayed the appearance of life on Earth for billions of years. So we mayhave our neighbors to thank not only for our oceans but also for the lifethat spawned in them.

    But I'm jumping the gun, for the oceans did not appear until manymillions of years after the planet was formed. Four and a half billion yearsago the Earth was still a molten magma ball, seething from the collisionthat ejected the Moon. As the planet cooled, its constituents separated likecurdled milk. Within about fifty million years, the iron of which much ofthe Earth was comprised had sunk to the core, and the lighter elements?silicon,aluminium, calcium, magnesium, sodium, potassium, and oxygen,along with some remaining iron?formed a rocky crust at the surface, justas slag floats on top of molten iron in a smelter.

    Among all this rocky stuff were the volatile compounds delivered bycollisions as the planet accreted?hydrogen, nitrogen, hydrogen sulfide,carbon oxides, water. While the Earth was molten, these volatile compoundsdissolved in the magma, but as the molten rock cooled and solidified,the vapors were released in a process called degassing. Theatmosphere 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'sultraviolet rays split water in the upper atmosphere into its constituenthydrogen and oxygen atoms, a process called photolysis. The hydrogenthen escapes into space. This water-splitting costs the planet the equivalentof a small lake's worth of water each year. That sounds like a lot?andit certainly would be if it all came from a single lake! But averagedover 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 percentsince the planet was formed.

THE DAY THE RAINS CAME

    Those formative years were steamy times on Earth, for all the waterwas in the sky. And then one day; somewhere between 4.4 and 4.0 billionyears ago, the temperature had fallen far enough for water to condense.Clouds massed in the sky, and the oceans rained down. Sadly, I have toconfess that this would not truly have happened so suddenly, one fine dayin the Hadean era?but I like the image. Yet however you look at it,there's no avoiding the conclusion that a deluge must eventually have ensuedthat leaves the biblical version looking like an April shower. This wasthe original Flood, and had anyone been there to witness it, I don't thinkan ark would have done them much good.

    Far from eradicating life, this deluge set the stage for life's entry. Itturned the face of the world blue and created a planet that exists, in atmosphericscientist James Lovelock's words, as "a strange and beautifulanomaly in our solar system."

Continues...

Excerpted from Life's Matrixby Philip Ball Copyright © 2001 by Philip Ball. Excerpted by permission.
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