["13.45 BYA REIONIZATION OF 13.4 BYA THE FIRST GALAXIES THE UNIVERSE BEGINS START TO FORM structure. Many formed spinning disks, amounts of energy as high-energy (short \u25c0 Merging galaxies with spiral arms; others were egg-shaped wavelength) X-rays, ultraviolet radiation, Astronomers observe elliptical galaxies. But with each merger, and bright visible light. Astronomers first many merging galaxies. the structure was disrupted, only to be detected these energetic galaxies in the Shown here is NGC regained or developed millions or billions 1950s; they made the discoveries with early 4676\u2014also known as of years later. The mergers injected energy radio telescopes, since the short-wavelength the Mice Galaxies\u2014a and mass, too, and the rate of star formation radiation has been stretched to such an pair of colliding galaxies and star death increased. Each star inside extent by the expansion of space that it around 290 million a young galaxy inevitably ended its life in a arrives as long-wavelength infrared and light-years away. powerful supernova explosion that filled the radio waves. Most large galaxies in the galaxy with the elements that would seed the universe today, including our own, still have next generation of stars and even planets. supermassive black holes at their centers. SUPERMASSIVE BLACK HOLES [IN SIMULATIONS] YOU CAN MAKE STARS AND GALAXIES THAT LOOK LIKE THE REAL THING. BUT IT IS THE DARK Although much of the gas and many of the MATTER THAT IS CALLING THE SHOTS. stars stayed in orbit around the center of each galaxy, huge amounts of the matter fell Professor Carlos Frenk, cosmologist, 1951\u2013 toward the center. In large galaxies, the density at the center increased so much that a supermassive black hole (see p.47) formed there. As matter jostled its way in toward the growing black hole, friction heated it to extremely high temperatures, releasing vast 4.7 billion years old Several galaxies have come 13.6 billion years old The galaxy has become stable, together, forming a much larger structure millions merging with others less often. It has a spiral shape, of light-years across. Each small galaxy that merges like that of a hurricane, and a supermassive black hole brings new material, and the increasing density at its core. Fragmented debris of its progenitor leads to a burst of star formation. galaxies lies around it. THE FIRST GALAXIES 49","HARD EVIDENCE HUBBLE EXTREME Relatively nearby DEEP FIELD galaxy looks red as its stars are running low on hydrogen fuel Taken by the Hubble Space Telescope, the eXtreme Deep Field records faint light from thousands of galaxies in a small area of sky. The deepest view of space ever captured, it provides the best evidence we have about the early universe\u2019s stars and galaxies. When we look out into space, we are looking the next eight years, and the addition of an This foreground back in time, because the light from distant infrared camera to the telescope in 2009 star is in our objects left a long time ago. Light that left a meant that objects whose light has been own galaxy galaxy 5 billion years ago will appear redshifted (see p.29) beyond the visible extremely faint, however bright the galaxy spectrum and into the infrared could also be Light from this was at the time. Imaging such a dim object seen. The new observations were combined very faint galaxy, requires a long exposure time\u2014not with the Ultra Deep Field, and the result a fraction of a second, like a typical was published in 2012 as the Hubble called UDFj- photograph, but millions of seconds. eXtreme Deep Field (XDF). Light from the 39546284, took most distant galaxies in the XDF took more In 1995, astronomers pointed NASA\u2019s than 13 billion years to reach us, and they 13.4 billion Hubble Space Telescope at a tiny patch appear one ten-billionth as bright as the light-years to of sky for over 140 hours and combined a dimmest thing visible to the naked eye. total of 342 images into a single, remarkable reach Earth image called the Hubble Deep Field. In Containing evidence of galaxy mergers 2004, NASA scientists produced the even (see p.49), extreme redshifting, and This relatively more remarkable Hubble Ultra Deep gravitational lensing (see p.47), the Hubble nearby object is a Field\u2014an image with an even longer XDF is a significant piece of evidence in spiral galaxy, like exposure, on a different patch of sky. support of the most convincing theories we Observations on that area continued over have about the evolution of the universe. the Milky Way, seen front-on \u25b6 Looking back The largest, brightest objects MORE THAN in the XDF include mature galaxies that appear 9 BILLION YEARS AGO as they were about 5\u20139 billion years ago\u2014when they had grown by merging and were populated 5\u20139 BILLION by second- or third-generation stars. Galaxies YEARS AGO in the background are smaller: young, irregular galaxies seen as they were over 9 billion years ago. The foreground is relatively empty because the XDF team chose an area almost devoid of nearby galaxies and stars in our own galaxy. LESS THAN 5 BILLION YEARS AGO 50 THRESHOLD 2","More recent Distant galaxy Field of view galaxies are appears red due the result of to redshifting Next to the full moon, the Hubble eXtreme mergers of smaller, of its light Deep Field covers a tiny area: less than one older galaxies twenty-millionth of the area of the whole sky. To see the image at its true size, you would need to hold this page about 1,000ft (300m) away. It is remarkable that more than 7,000 galaxies can be seen in such a small field of view\u2014and to think that each tiny dot in the image is a collection of millions or billions of stars frozen in time. XDF\u2019s field of view, with the moon for comparison Early galaxies The XDF gives astronomers a unique view of galaxies as they were during the universe\u2019s first few hundred million years, when they were relatively small, irregularly shaped groups of stars. As they collided and merged, most became spiral shaped because the collisions resulted in rotation. The universe was smaller when the light captured in the XDF left the young galaxies. As space has expanded, the light has been \u201cstretched,\u201d shifting its frequencies toward or even beyond the red end of the spectrum, which is why so many of the XDF galaxies appear reddish. Close-up of heavily redshifted galaxy merger HUBBLE EXTREME DEEP FIELD 51","THRESHOLD","ELEMENTS ARE FORGED We all come from dying stars. All the elements that make up our world originated there. Stars are hungry, and as they use up their fuel, age, and finally die, some of them collapse and go out with a tremendous explosion of energy. But from star death comes new building blocks\u2014the elements\u2014 pushed out into the universe to start something new.","GOLDILOCKS CONDITIONS The formation of the first stars had profound consequences. As well as lighting up the universe, stars act as chemical factories, producing new chemical elements that provide the raw materials for everything else in the universe, including living things. GTIOnhrlsadeivdssietttarysor,tsawnrrghsu,nnichuohycudlpteruooalfrglsefonmrncaet,ther tog Early stars, grouped etheorldinugcalethioyfmudsrsoetgtoeognfeoatnhrmderhheelliiuumm nuclei into galaxies What changed? As stars run out of hydrogen fuel, they begin to collapse, getting hotter and denser. Interstellar space, filled with charged gases","AS ONE ELEMENT First 26 elements RUNS OUT, STAR in periodic table, COLLAPSES AGAIN, up to iron, form LEADING TO SURGE IN TEMPERATURE in this way AND START OF NEW FUSION PROCESS Star collapses for the last time, then explodes in supernova Some new elements mix with other elements from dead stars to form complex molecules ANOTHER TYPE OF Some hydrogen and FUSION REACTION helium will provide raw BEGINS, FORMING materials for next ELEMENTS SUCH generation of stars AS CARBON Process called neutron capture forms even heavier elements, creating all elements up to uranium Universe is now chemically more diverse, with 92 elements Supernova scatters new elements into space","13.6 BYA THE FIRST 13.4 BYA THE FIRST STARS FORM GALAXIES FORM THE LIFE CYCLE OF A STAR Just like humans, stars are born, grow old, and die. The way a star ends its days depends on its mass, with the largest stars exploding as supernovas. These detonations furnished, and continue to furnish, the universe with heavier elements, recycling material ready for it to be turned into new stars. Consequently, the life cycle of stars also A SUPERGIANT STAR CAN balance is maintained, but things change played a crucial role in the emergence when fusion eventually stops. Astronomers refer to a star still fusing hydrogen into of life on Earth. Essential ingredients\u2014 HAVE A VOLUME 8 BILLION helium as a main-sequence star. Once including the calcium in your bones and TIMES THAT OF THE SUN this fusion ceases, the star evolves off the the iron in your blood\u2014were forged inside main sequence. \u25bc Sunlike star stars, only for supernovas to spread them For all but the smallest stars, the core Stars like the sun contracts and the temperature rises to typically live for around far and wide. around 180 million\u00b0F (100 million\u00b0C). 10 billion years. After This is hot enough for helium to fuse into entering a red giant Stars come in a vast array of sizes. will live. The larger the star, the quicker it carbon, which creates enough energy to phase, they form a upset the balance the other way and the planetary nebula\u2014and Astronomers classify them into seven main will consume its nuclear material. O stars star bloats outward. Then, depending usually do not explode on size, it will either turn into a planetary as supernovas. groups from largest to smallest denoted by live fast and die young, often dying out nebula with a white dwarf at the center, or detonate as a supernova, leaving behind the letters O, B, A, F, G, K, and M. Our sun within just a few million years, whereas a neutron star or black hole. is a G star, meaning there are bigger and the smallest stars can eke out their smaller stars out there than our own. The existence for trillions of years. smallest stars, known as dwarfs, are the most common. M stars, for example, LIFE STAGES make up more than 75 percent of all Stars begin their lives as protostars, stars. By contrast, O stars account for formed from clouds of interstellar dust just 0.00003 percent. (see pp.44\u201345). Nuclear processes in a star\u2019s The size of a star also core then shore it up against gravitational governs how long it collapse. For most of a star\u2019s life, this The supply of hydrogen decreases\u2014eventually fusion ceases, and there\u2019s nothing to counteract gravity PROTOSTAR MAIN- MAIN- SEQUENCE SEQUENCE STAR STAR Nuclear fusion Fusion in the core supports The core contracts, and starts, and a the star against the temperature rises to new star, called a protostar, gravitational collapse 100 million degrees is born A cloud of gas and dust The increased temperature allows collapses under gravity helium to fuse into carbon but this to form a protostar creates more energy, causing the star to surge outward 56 THRESHOLD 3","9 BYA THE UNIVERSE CONSISTS OF 6 BYA THE EXPANSION OF THE UNIVERSE 4.6 BYA OUR SOLAR SYSTEM VAST CLUSTERS OF GALAXIES STARTS TO ACCELERATE STARTS TO FORM \u25b6 Low-mass star Stars less than a quarter Hydrogen fusion The star finally These smaller stars are able to of the sun\u2019s mass don\u2019t can continue for runs out of fuel and mix their interiors, meaning that become red giants trillions of years forms a white dwarf the core\u2019s supply of hydrogen gets replenished by the outer PROTOSTAR MAIN-SEQUENCE RED WHITE layers falling toward the STAR DWARF DWARF center\u2014so the core doesn\u2019t contract to start helium fusion. These hot blue stars get Heavier elements The star collapses, and through their nuclear fuse together and the infalling material \u25b6 High-mass star material quickly The evolution of more massive eventually form rebounds outward in a stars is initially similar to that of an iron core violent explosion sunlike stars. But they form red supergiants, instead of red giants, BLACK For the largest stars, and eventually supernovas. The HOLE the iron core becomes star\u2019s ultimate fate depends on a black hole its mass. PROTOSTAR MAIN-SEQUENCE SUPERGIANT TYPE II For smaller stars, a STAR STAR SUPERNOVA neutron star is formed NEUTRON STAR STARS ARE BORN, LIVE\u2014OFTEN FOR BILLIONS OF YEARS\u2014AND DIE\u2026 SOMETIMES IN A SPECTACULAR MANNER. Carl Sagan, American astronomer, 1934\u20131996 This white dwarf is The white dwarf TYPE 1A orbiting a main- can rip material SUPERNOVA sequence star from its partner Half of the star\u2019s mass BINARY ends up in a central, SYSTEM Earth-sized core RED Eventually, the white dwarf GIANT becomes unstable and PLANETARY Initially, the white explodes as a supernova NEBULA dwarf is hot and glows brightly Over time, the white Helium fusion is dwarf fades away less stable, and WHITE DWARF into a black dwarf the star sheds its BLACK DWARF outer layers THE LIFE CYCLE OF A STAR 57","13.6 BYA THE FIRST 13.4 BYA THE FIRST STARS FORM GALAXIES FORM HOW NEW ELEMENTS FORM INSIDE STARS Before the first stars shone, the universe was just a sea of hydrogen, helium, and residual energy from the Big Bang. The chemical diversity in the universe today is due to stars\u2014effectively, vast atom factories\u2014churning primitive materials into more complex elements and then flinging them outward when they die. Inside stars, the temperature is high in uncovering this process and was awarded At this point, the temperature in the core enough to rip electrons away from the the 1967 Nobel Prize in Physics for his has soared to 5.4 billion\u00b0F (3 billion\u00b0C), nuclei of atoms. In the case of hydrogen, work. Crucially, the total mass of the which is enough to force two silicon nuclei this leaves solitary protons (and electrons) products of the pp-chain is less than the together to form iron. In this way, a wealth wandering around the star\u2019s interior. mass of the ingredients entering into it. In of elements builds up in shells within the Matter in this state is known as plasma. the sun, for example, 620 million tons star, resembling the layers of an onion, with Due to their like electric charges, protons repel each other, rather like similar poles of a magnet. NEW ELEMENTS IN STARS FINALLY, I GOT TO CARBON, AND AS YOU ALL KNOW, IN THE CASE OF CARBON THE However, deep in the core of the star, the REACTION WORKS OUT BEAUTIFULLY. temperature and pressure are high enough to squash protons together. Known as Hans Bethe, German\u2013American physicist, 1906\u20132005 nuclear fusion, this process releases energy \u25bc The triple and is the star\u2019s power source. It also exerts of hydrogen (protons) is turned into Hydrogen | 1 alpha process an outward pressure that counters the 616 million tons of helium every second. Helium | 2 In this process, two inward pull of gravity. The missing four million tons of mass Lithium | 3 helium-4 nuclei fuse is converted into energy according to into beryllium-8, which The simplest fusion mechanism is Einstein\u2019s famous equation E = mc\u00b2. Beryllium | 4 becomes carbon-12 called the proton-proton (or pp) chain. Boron | 5 when struck by a third In the first step, one of the fused protons Eventually, the hydrogen in the star\u2019s helium-4 nucleus. turns into a neutron, creating a new core runs out and gravity contracts the core. Carbon | 6 Helium-4 nuclei are proton-neutron pair called a deuteron. The resulting temperature surge allows a Nitrogen | 7 also called alpha This is bombarded by another proton to new fusion mechanism to take over\u2014the particles, and so this create the nucleus of a helium-3 atom. triple alpha process\u2014one which uses Oxygen | 8 mechanism is known as When two of these helium-3 atoms collide, helium-4 nuclei (alpha particles) as its main Fluorine | 9 the triple alpha process. they create a helium-4 nucleus, along with ingredient. This enables two helium nuclei two protons, which can start the whole to fuse into beryllium and then, with the Neon | 10 process again. The German\u2013American addition of a third helium nucleus, into Sodium | 11 physicist Hans Bethe was a key player carbon. In smaller stars, such as the sun, Magnesium | 12 the atom construction process ends here. Aluminum | 13 Gamma ray emitted as two Gamma ray Silicon | 14 helium-4 nuclei fuse to form (high-energy However, larger stars can go on Phosphorus | 15 photons) increasing the diversity of chemical Sulphur | 16 beryllium-8 nucleus elements; once one fusion path runs out, the Chlorine | 17 core contracts and the temperature spikes to Helium-4 Helium-4 kick-start another. Next, carbon fuses with Argon | 18 nucleus, or nucleus helium to form oxygen, which is bombarded Potassium | 19 alpha particle by another helium nucleus to forge neon, which itself is fashioned into magnesium Calcium | 20 Helium-4 by a similar process. The sheer range of Scandium | 21 nucleus possible reactions is vast. Eventually, carbon Titanium | 22 and oxygen fuse together to form silicon. Vanadium | 23 Proton Beryllium-8 Carbon-12 Chromium | 24 Neutron nucleus nucleus Manganese | 25 Helium-4 Helium-4 and beryllium-8 Iron | 26 nucleus nuclei fuse to form carbon-12 nucleus 58 THRESHOLD 3","9 BYA THE UNIVERSE CONSISTS OF 6 BYA THE EXPANSION OF THE UNIVERSE 4.6 BYA OUR SOLAR SYSTEM VAST CLUSTERS OF GALAXIES STARTS TO ACCELERATE STARTS TO FORM iron at its heart. However, because iron common elements in the universe common elements in Earth\u2019s is the most stable of all the elements, it cannot be fused into anything else and ost ost crust fusion ceases. As heavier elements form, M M the process gathers pace\u2014it can take millions of years for a star to exhaust its Helium 23.0% Oxygen 46.0% hydrogen, but the fusion of silicon nuclei to form iron takes just a single day. Oxygen 1.0% Other 0.9% (Titanium 0.66%, NEW ELEMENTS IN SUPERNOVAS Carbon 0.5% Carbon 0.18%) Elements heavier than iron can only be Potassium 1.5% created when a massive star explodes in Other 0.5% Sodium 2.3% a supernova. The next heaviest elements (Neon 0.13%, Magnesium 2.9% are formed by the s-neutron-capture process\u2014\u201cs\u201d stands for slow, as it typically Iron 0.11%, Calcium 5.0% takes hundreds of years. This process Nitrogen 0.10%, actually begins inside stars, but in stars the interactions are extremely slow\u2014they Silicon 0.07%, only speed up once a supernova gets going. Magnesium 0.06%, The earlier transformation of carbon into oxygen, and neon into magnesium, Sulphur 0.05%) created a wealth of additional neutrons. The gradual combination of these excess Hydrogen 75.0% Silicon 27.0% Aluminum 8.1% Iron 6.3% particles with existing nuclei allows elements as heavy as bismuth to form. rapid). The r-process can only happen in form giant molecular clouds that will \u25b2 The distribution However, this process cannot produce any the extreme conditions of a supernova. The eventually collapse to form new stars. of the elements elements heavier than bismuth, because density of neutrons increases greatly during Individual atoms can combine with others The combination of bismuth decays away into polonium before the explosion, and new elements can be in the clouds to form complex molecules, elements found on it can combine with a neutron. A much formed in a fraction of a second. Some of some of which are crucial for life. Earth differs greatly faster neutron capture mechanism is these r-process nuclei later decay away, Astronomers and astrochemists have already from the universe at required\u2014the r-process (\u201cr\u201d stands for creating new elements not fashioned found evidence of these molecules. The large. The lightest directly by either neutron capture process. simplest amino acid\u2014glycine\u2014has been elements, hydrogen and detected in a cloud of gas toward the center helium, were expelled COMPLEX CHEMISTRY of our Milky Way galaxy, as well as in the from Earth\u2019s orbit by the This profusion of material is dispersed nearby Orion Nebula. Amino acids are young sun. Oxygen, the into the wider universe by the force of the regarded as life\u2019s building blocks, so it crust's most abundant supernova. It then mixes with interstellar is possible that the basic ingredients for life element, was created material and debris from other dead stars to were fashioned long before the sun lit up. as life turned carbon dioxide into sugar via photosynthesis. \u25c0 New elements in dying stars As one source of fusion material runs out, gravity contracts the star\u2019s core and triggers further fusion. This successively builds up concentric shells of new elements. The elements become increasingly heavy, as measured by their atomic numbers (the number of protons in the nucleus), which range from 1 to 26. \u25b2 Life\u2019s cosmic origins The building blocks of life have been found in the nearest star-forming region to our solar system, the Orion Nebula. Amino acids combine to create proteins and are a key component of DNA. IRON ASH CORE","13.6 BYA THE FIRST 13.4 BYA THE FIRST STARS FORM GALAXIES FORM WHEN GIANT 5 minutes after STARS EXPLODE core collapse Today we know that supernovas pepper the universe with elements 166 minutes heavier than iron. But our quest to understand these searing explosions dates back to a time long before the advent of our astronomical understanding. We\u2019ve been documenting them for almost 2,000 years. \u25bc Chaco Canyon The earliest recorded evidence of an it was a guest in the night sky for almost These wall markings observed supernova dates back to Chinese two years. The remnant of this colossal in a New Mexico cave astronomers in 185 CE. They documented explosion is the spectacular Crab Nebula show a large star, a the appearance of a sudden bright light in in the constellation Taurus. crescent moon, and the sky that took eight months to fade from a handprint. It has been view. A similar event occurred in 393 CE, ENTER THE TELESCOPE suggested that the local and up to 20 other potential events appear The 1054 event was followed nearly six Anasazi people drew centuries later by the supernovas of 1572 it as a record of the JUST BEFORE A and 1604, the last in the pre-telescope age. 1054 supernova. The latter, known as Tycho\u2019s supernova, SUPERGIANT STAR EXPLODES was the last observed to explode in our AS A SUPERNOVA, ITS Milky Way galaxy. TEMPERATURE REACHES ABOUT However, in more recent times, light reached us in 1987 from an explosion in 180 BILLION\u00b0F (100 BILLION\u00b0C) one of our galaxy\u2019s satellites\u2014the Large Magellanic Cloud. By then, astronomers in Chinese records, although modern were able to observe it with telescopes within astronomers haven\u2019t been able to confirm days of detonation. The Voyager probe, then they were all supernovas. on its way to the farthest planets, was also pointed toward the explosion for a closer One definitive explosion\u2014perhaps the look. Designated SN 1987A, it surprised most famous of the pre-telescope age\u2014was astronomers because the best theories of seen to detonate in 1054. It was observed the day said the star that exploded shouldn\u2019t in Japan and the Middle East, as well as have done so. Consequently, it has become in China. Luminous enough to be seen a valuable source of evidence against which during daylight hours for nearly a month, astronomers can test their theories. Some of their ideas were backed up by SN 1987A, particularly that the radioactive decay of cobalt atoms keeps the supernova remnant bright long after the initial explosion. But some mysteries remain. For example, astronomers have yet to find the neutron star that should have formed at the heart of the dying star. The 1054 supernova and SN 1987A were both Type II supernovas, formed by the core collapse of massive stars. In recent years, astronomers have also been able to pick out some relatively close Type 1a supernovas, which are formed by stars of lower mass. These include SN 2011fe in the Pinwheel Galaxy and SN 2014J in the nearby Cigar Galaxy. 60 THRESHOLD 3","9 BYA THE UNIVERSE CONSISTS OF 6 BYA THE EXPANSION OF THE UNIVERSE 4.6 BYA OUR SOLAR SYSTEM VAST CLUSTERS OF GALAXIES STARTS TO ACCELERATE STARTS TO FORM 27 minutes 50 minutes 120 minutes Simulating a supernova This computer model of SN 1987A was made at the Max Planck Institute for Astrophysics in Germany. Density increases from black through red, orange, and white. A shockwave is expanding through the star\u2019s outer layers of hydrogen. Metals (white) from the core are being expelled rapidly, with turbulence occurring as they collide with gases in the star\u2019s interior. WHEN GIANT STARS EXPLODE 61","13.6 BYA THE FIRST 13.4 BYA THE FIRST STARS FORM GALAXIES FORM \u25bc The Periodic Table Missing elements By arranging the table in terms Atomic number This is the number First presented to the Russian Chemical Society on of the behavior and structure of elements, of protons in the nucleus\u2014just one March 6, 1869 as \u201cthe period system,\u201d this famous Mendeleev was able to spot gaps that suggested in the case of hydrogen depiction of the primary components of matter as-yet-unseen elements, including germanium organizes the elements in an incredibly useful way. Group Vertical columns are called groups. Unstable elements Some elements are not Relative atomic mass This is measured in Group members have similar electron stable and decay over time. Even the most atomic mass units (amu), where 1 amu is equal configurations and so exhibit similar stable form of kurchatovium (now called to 1\/12 of the mass of a carbon atom. This is why chemical properties. Today, 18 groups rutherfordium) will decay to half the original it is called relative\u2014it helps compare the masses are officially recognized amount in just 1 hour 20 minutes of different elements 62 THRESHOLD 3","9 BYA THE UNIVERSE CONSISTS OF 6 BYA THE EXPANSION OF THE UNIVERSE 4.6 BYA OUR SOLAR SYSTEM VAST CLUSTERS OF GALAXIES STARTS TO ACCELERATE STARTS TO FORM MAKING SENSE OF THE ELEMENTS Dmitri Mendeleev The periodic table of the elements is one of the most recognizable icons in Mendeleev is the science. By organizing the elements according to their atomic structure, it name most associated provides a standard way to order and classify them. Of the 118 elements with the Periodic in the table, 92 form inside stars and supernovas. Table. He didn\u2019t win the Nobel Prize, but As the scientific revolution gathered pace, to ensure that sodium is in the same column \u25b2 Organizing he does have an so did the rate at which new elements were as lithium (both are highly reactive). These the elements element named after discovered. Over time, a pattern in their columns, or groups, are the real key to the The elements can be him (Mendelevium), chemical behavior was found. The first table. Mendeleev\u2019s table only had seven grouped according as well as a crater on attempt to organize the elements into groups, but the power of his system was to how they formed. the moon groups came in the late 18th century, when confirmed in the 1890s when the noble Most of the elements French chemist Antoine Lavoisier sorted gases were discovered and fitted in up to uranium formed Period Rows are them into four categories: gases, non- perfectly as an eighth group. as a result of nuclear known as periods. metals, metals, and earths. In 1829, the reactions in stars or Their main function German Johann D\u00f6bereiner noted that WHERE THE ELEMENTS ARE FORGED supernovas. Elements is to make sure that trios of elements had similar chemical The searing heat in the first minutes after heavier than uranium elements with properties. Crucially, he realized that the the Big Bang turned some of the cosmos\u2019s are unstable and similar chemical attributes of one could be predicted from nascent hydrogen into helium via nuclear rarely encountered. properties appear those of the other two. By the 1860s, the fusion (see p.58). After just 20 minutes, in the correct group. British chemist John Newlands had devised fusion stopped and the basic composition KEY There are currently his Law of Octaves, which said that every of the universe was set down as about Formed in Big Bang seven periods eighth element exhibited similar chemical 75 percent hydrogen and 25 percent helium. (hydrogen and helium) behavior. However, on occasion he had It took millions of years for more elements to Formed in stars Tile Each tile to squeeze two elements into the same box, appear. The elements up to and including by fusion (lithium displays a chemical and he did not leave gaps for as-yet- iron form by fusion in stars, whereas many to iron) symbol for the undiscovered elements. This problem beyond iron can only be made in the Formed in stars by element (either one explains why the Russian Dmitri cataclysm of a supernova. neutron capture or two letters), along Mendeleev is often regarded as the father (cobalt to uranium) with information of the periodic table. In 1869, Mendeleev Unstable elements including atomic published a primitive version of the number and relative famous table, leaving gaps based on the atomic mass number \u201cperiodicity\u201d of the known elements. HOW THE TABLE WORKS The elements are organized in order of increasing atomic mass. The horizontal rows are known as periods\u2014a new period begins when the behavior of an element repeats. For example, a new period starts after neon IT IS THE FUNCTION OF SCIENCE TO DISCOVER THE EXISTENCE OF A GENERAL REIGN OF ORDER IN NATURE AND TO FIND THE CAUSES GOVERNING THIS ORDER. Dmitri Mendeleev, Russian chemist, 1834\u20131907 MAKING SENSE OF THE ELEMENTS 63","THRESHOLD","PLANETS FORM As our own star\u2014the sun\u2014ignites, its gravitational pull sweeps up the elements into orbit around it. As they crash together, planets begin to form. While the lighter elements are blown to the outer regions, forming gas giants, close to the sun the heavier elements remain and form rocky planets, including Earth: our home is born.","GOLDILOCKS CONDITIONS When stars were born from the debris of former stars, some chemically rich material was left in orbit. This debris clumped into balls of matter stuck together by gravity and chemical bonds. These structures were planets, and they were far more complex than anything seen before. We now know that this first happened long ago in solar systems GNArenawveiwctyhl,yeamfcocircrmeatleieodlnes,muanendnlitrkseanstard clouadndsoomf cchoellmisiicoanlsly rich matter orbiting the new sta far older than our own. What changed? Matter from dying stars The death of stars builds up an After the formation of a star, material was ever-increasing supply of heavier left orbiting in a disk. The star\u2019s fierce radiation elements, supplementing the blasted light, volatile material, particularly hydrogen hydrogen and helium of the early and helium, far from the star. These gases would go Universe. This results in a more on to form distant gas-giant planets. Nearer to the chemically complex world with star, the heavier, chemically rich materials from the 92 elements that can combine death of previous generations of stars remained solid or liquid and clumped into rocky planets. to form compounds. In our own solar system, one of these planets was Earth. Star nurseries r Clouds of dead star material, rich in heavy elements, such as carbon, oxygen, nitrogen, aluminum, nickel, and iron, gather under weak forces of gravity and electromagnetism. They become sites of new star formation. Shockwave from a supernova A disturbance, such as a shockwave from a neighboring exploding star, may trigger a cloud to begin contracting to form a star. As it slowly collapses, the cloud begins spinning faster and faster, and takes on a disk shape.","Magnetic Movement Solid metal inner field deflects generates Earth\u2019s core and liquid solar wind and metal outer core magnetic field protects atmosphere Semisolid rocky mantle Gaseous While molten, atmosphere planet separates ENERGY OF EARTH\u2019S into layers FORMATION IS SO GREAT, PLANET Solid crust BECOMES A HOT, MOLTEN BALL Liquid water ocean ROCKY PLANETS FORMED One Moon\u2019s gravity giant impact creates Earth\u2019s BY COLLIDING fragments Earth tides, possibly DEBRIS and creates triggering life its moon Conditions for life Moon\u2019s gravity Earth\u2019s stabilizes Earth\u2019s seasons and climate remain axis and spin Heat relatively retained in stable core and drives currents in Hot water and Layers form the mantle minerals reach the geological above the surface at record\u2014a history mid-ocean ridges, where new oceanic written in crust is made Earth\u2019s rocks Rifts create Continental new oceans crust is eroded and new rocks are deposited in layers Volcanoes Deep oceanic Earth\u2019s Continental and mid-ocean trenches created crust splits into landmasses ridges create where plates slip plates, which move against each other as grow new crust underneath they are dragged Continents others by Earth\u2019s moving drift on mantle their moving plates New range Mountains of habitats for built by colonization colliding plates by life Continental crust\u2014a new, light type of crust\u2014is formed","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS OUR SUN IGNITES In an otherwise inconspicuous region of our Milky Way galaxy, a giant cloud of matter began to coalesce. Our sun had a tempestuous birth, heating up and spinning until it exploded into life. An unassuming mass of gas and dust, and pressures counteracted its own measuring only a few gas molecules per gravitational force, blasting ice, rock, and cubic centimeter, floated aimlessly in space. gas away from the center. These materials Eventually, it started to collapse under the flattened in a spinning disk that began weight of its own gravity. to orbit the protosun. It is likely that this collapse was Entering a new phase of intense activity, kick-started by a shockwave from a nearby the protosun began to eject jets of radiation supernova. A rare type of aluminum can from its poles. Fierce winds blasted lighter be found across the solar system, which elements such as hydrogen and helium to may be a potential trace of this supernova. the edge of the protosun\u2019s orbit. Soon, the protosun\u2019s temperature, pressure, and size UNSTOPPABLE FORCE rose even higher, until it had absorbed Whatever the cause, what we do know is 99.9 percent of material from the that over tens of millions of years the cloud original solar nebula. progressively became more dense. In the center, the cloud was at its densest and Despite these events occurring almost hottest\u2014this was the protosun, and it was 5 billion years ago, we can gather clues composed of about 75 percent hydrogen and as to how our sun was born because we 25 percent helium. Extreme temperatures can watch new stars being created elsewhere in the galaxy. THE SUN, WITH ALL OF THOSE PLANETS REVOLVING AROUND IT... CAN STILL RIPEN A BUNCH OF GRAPES AS IF IT HAD NOTHING ELSE IN THE UNIVERSE TO DO. Galileo Galilei, astronomer, 1564\u20131642 Spinning cloud of dust, Dense central protosun Grains of icy dust remain in Sun starts Rocky debris orbits Temperature hydrogen, and helium formed by gravity cold parts of outer disk to shine near to the sun and pressure in sun rise Solar nebula Liquid and gas freeze here, far Rocky dust close Gas and icy particles flattens into a disk away from the protosun\u2019s heat to protosun orbit further out An interstellar cloud of gas and dust begins to Extreme temperatures inside the protosun generate The protosun\u2019s temperature and internal pressure collapse under gravity, spinning and heating up as it energy that counteracts its own gravity. Ice and gas near rise, and it becomes an early sun. Lumps of rock does so. In the hot, dense center, a protosun forms. the protosun burn away, leaving rocky dust particles. and ice orbiting the sun start to collide. 68 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM Bursting into life Intense jets of radiation erupt from the protosun\u2019s poles as it swallows up dust and gas. Fierce winds collide with the surrounding rock and ice that will later form planets. 99 RUNNING HEAD 69","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS Rocky worlds are born As planetesimals within the frost line orbited the young sun, incoming materials approached with ever greater speed. The constant impacts accelerated their growth, pulling even more material toward them. 70 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM THE PLANETS FORM The planets in our solar system started their lives as gas and tiny grains of dust. Formed into a whirling disk by the young sun\u2019s gravitational pull, millions of years of violent collisions would eventually mold the gas and dust into impressive planets, one of which would become our home. Before the modern planets came the DIFFERENT TYPES OF WORLDS In the outer solar system, beyond what planetesimals\u2014the building blocks from astronomers refer to as the frost line, which planets are made. The gathering The distance at which these planetary materials such as water and methane froze together of smaller chunks to form larger seeds formed from the sun determined in the frigid temperatures. With more solid ones is a process known as accretion. whether the eventual planet was made material available, the gravitational pulls primarily of rock or gas. of these larger planetesimals were stronger. ASSEMBLING A PLANET Consequently, lighter elements such as The irregular orbits of the mostly solid In the hot ring of the inner solar hydrogen and helium were more easily materials around the young sun led to system, only materials with very high captured, resulting in the vast gaseous frequent impacts, causing accretion. melting points, such as iron, nickel, and atmospheres typical of Jupiter, Saturn, Initially, grains that were a fraction of an silicon, could survive to be incorporated Uranus, and Neptune. inch grew to foot-sized lumps. It took tens into the rocky planets, Mercury, Venus, to hundreds of millions of years for their our home planet Earth, and Mars. collective gravity to accumulate materials that resulted in planetesimals that THE FORMATION OF THE PLANETS IS LIKE A GIGANTIC stretched miles across. SNOWBALL FIGHT... A PLANET-BALL THAT HAS GATHERED ALL THE SNOWFLAKES IN THE SURROUNDING AREA. The largest planetesimals had enough gravitational power to attract additional Claude All\u00e8gre, scientist and politician, 1937\u2013 material relentlessly. The planetesimals formed by this process of runaway accretion created the embryos of planets. Gravity draws Rock and dust clump Little debris Fully formed Sun Uranus rock and dust into together to form remains in inner rocky inner Earth rings around sun planetesimals planet solar system Sun Mercury Mars Frost line Icy material and gas remains in Kuiper Belt Venus Jupiter outer solar system Sun blasts vapor and other Sun\u2019s radiation boils away most Saturn Neptune Asteroid Belt gases beyond the frost line water and other volatile substances Distant planet continues to pull in ice and gas, becoming a bloated gaseous world Materials and debris left over from the sun\u2019s formation Large planetesimals attracted smaller particles. Stabilization of the solar system took hundreds of orbited the young sun in rings. The inner rings were Their gravitational fields grew stronger as they millions of years (see pp.74\u201375). The gravitational composed of metals and rock; outer rings beyond continued to grow larger. Most of the orbiting interactions of the infant planets settled, eventually the frost line held rock, frozen water, and gases. material was eventually swept up. forming the stable orbits we see today. THE PLANETS FORM 71","HARD EVIDENCE THE IMILAC Metal matrix is made METEORITE of iron and nickel Meteorites\u2014pieces of material that have flown through space and landed on Earth\u2014deliver small time capsules of ancient data. They have drifted since the birth of the solar system, so the information they contain is often older than Earth. Artifacts that were around after the solar pallasite meteorite due to its matrix of metal system formed are still orbiting our sun encapsulating its crystals. Like all pallasites, today as comets and asteroids. They are it originated from the boundary between relics of the early solar system that have the metallic core and the rocky mantle of a remained relatively unchanged due to the planetesimal or asteroid, which broke apart absence of geological activity. When they during the formation of our solar system, land on Earth as meteorites, studying them possibly due to the early sun\u2019s gravitational allows us to journey into the past and test pull. Some small pieces of the mantle fell out our theories of how our solar system, into the molten core during this process. It and our planet, came to be. Tens of then took at least a million years for these thousands of meteorites weighing more chunks to cool into the crystals scattered than 1\u20444oz (10g) land on Earth every throughout the metal you can see here. year, each parachuting down precious information on what the solar system Not only can pallasite meteorites help was like billions of years ago. determine the age of the solar system, they can also provide clues as to its This sample is a slice of a meteorite early chemical composition. Pallasites named \u201cImilac\u201d, which was itself a small such as this one are incredibly rare fragment of almost a ton of material that fell in our Earthly collection\u2014they make into the Atacama Desert, Chile, as part of a up just 0.4 percent of the meteorites single impact event. Imilac is classified as a scientists have gathered up. See-through parts are olivine crystals \u25b2 Orbiting evidence How do we know its age? These ice mountains on Comet 67p, studied by probes in 2014\u201315, are as old as our solar system. Calculating the age of these cosmic fragments The presence of ice in the comet\u2019s interior allows geologists to date the birth of the solar system. demonstrates that water or ice was present This meteorite was once part of an asteroid\u2019s or during the solar system\u2019s formation. planetesimal\u2019s hot interior. When the asteroid cooled sufficiently for its molten rock and metal to freeze, it also sealed in isotopes\u2014unstable, radioactive atoms. Scientists can use a process called radiometric dating (see pp.88\u201389) to put a date on this event. By measuring the present-day densities of the isotopes, geologists can calculate how much radioactive decay has occurred and estimate that the asteroid solidified 4.5 BYA\u2014soon after the birth of the sun. 72 THRESHOLD 4","What happened on impact? During this meteorite\u2019s descent to Earth, it split into fragments as it entered our planet\u2019s atmosphere. Friction heated the surface of this fragment, and a thin crust melted. Outer crystals melted out of the matrix, but crystals in the interior remained cool and intact, because it only took a few seconds to pass through Earth\u2019s atmosphere. Earth\u2019s building block? By comparing the composition of these meteorites to the composition of Earth, geologists can identify the type of planetesimals that came together to form our planet. Like Earth, this meteorite contains iron and nickel\u2014both of which are thought to constitute Earth\u2019s core. Asteroids, dwarf planets, and this pallasite meteorite have remained unchanged since the early solar system and therefore can be key pieces of evidence in determining its history. Planetesimal forming from smaller bodies Crystals from the rocky mantle The crystals are made of olivine and peridot\u2014materials found in tetrataenite, a mineral that can record magnetic fields. Microscopic analysis of these particles demonstrates that when the meteorite was part of an asteroid, the asteroid had a magnetic field\u2014until the core solidified. A thin slice of meteorite under a microscope THE IMILAC METEORITE 73","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS Solar wind KEY Comets and their tails The outer layer of the sun\u2019s Sun\u2019s gravitational field in g (where As a comet nears the sun, heat atmosphere, the chromosphere, 1 g = Earth\u2019s gravity at sea level) vaporizes ice, letting loose dusty emits a stream of highly charged Sunlight intensity in material, forming the dust tail that and extremely hot (1.8 million\u00b0F\/ Watts per sq ft (Watts per sq m) bends as it orbits the sun. A second 1 million\u00b0C) particles outward ion tail that streaks directly behind throughout the solar system. Earth\u2019s the comet\u2019s path is formed through magnetic field (see pp.80\u201381) interactions between the comet protects it from this solar wind and the solar wind 0.000006Asteroid belt In this region, the opposing gravitational 0.00002pulls of Jupiter and the sun cancel each other out, and pull the asteroids in 0.0003 opposite directions. This means they 0.0006 cannot clump together under their 0.001 own gravity and form new planets Gra0vi.t0y04 Venus SATURN Although Venus is the hottest planet, it does not receive the most intense sunlight: JUPITER Mercury is bathed in much greater solar radiation. Venus is hotter because it traps heat from the sun in its dense atmosphere, which is rich in carbon dioxide MERCURY EARTH MARS VENUS Mars Jupiter Rover data suggest Mars was once When our sun ignited (see much warmer and wetter, with a pp.68\u201369), light gases were thicker atmosphere. Mars is blasted into the farthest parts smaller than Earth, so its inner heat of the inner solar system. and activity may have cooled more As Jupiter grew larger, its rapidly, causing its protective gravitational pull captured a magnetic field to switch off. The huge amount of gas to form solar wind would have stripped a giant atmosphere 3,100 most of the atmosphere away miles (5,000km) high 15 (1.4) ASTEROID 51 (4.7)BELT 590 (55) 1,370 (127) 2,610 (243) 9,1S2u0nl(i8g4ht8i)ntensity 74 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM THE SUN TAKES CONTROL \u25c0 Inner solar system Between 4.1 and 3.8 bya, planets shifted their orbits in a cascade of The realm of the eight planets is gravitational disruption. The process left eight major planets in orbits referred to as the inner solar that remain stable to this day. However, the sun controls much more in its neighboring space than just these planets. system. However, that is by no means the end of the sun\u2019s family0.0000002 0.00000007 Scientists have long grappled with the Heavy Bombardment. This occurred when of orbiting objects. There are problem of how the modern solar system a sudden shift in the movements of the gas NEPTUNE came to be. When modeling the evolution of giants and their gravitational fields caused many objects beyond Neptune, the sun\u2019s environment, it was hard to explain a catastrophic torrent of asteroids to fall on including dwarf planets and 1.5 (0.14) its present form if the planets had always the inner solar system, including Earth. comets. Light and gravity been where they are now. Lunar rock samples returned to Earth by the Apollo astronauts point to a clustering spread out from the sun in NICE MODEL of meteor impacts around 3.9 BYA. all directions\u2014each rapidly The present arrangement of the solar According to the Nice Model, the losing intensity with distance system fits with the explanation that the giant planet migration was to blame. four gas giants started out much closer URANUS together: Jupiter moved inward while A MISSING PLANET the other three backed away from the sun. Simulations of the solar system\u2019s infancy Uranus It is even possible that Uranus and Neptune also suggest that our sun once had more The intensity of light may have swapped order. The outward planets. By adding a fifth gas planet to fades over distance: migration of Neptune would have scattered the model, researchers found they could at twice the distance, many of the solar system\u2019s smaller objects get a much better match for the modern sunlight is four times into a region known as the Kuiper Belt. arrangement of planets. We do not have five weaker. Uranus\u2019s orbit is gas planets today, however, so the fifth must 20 times farther from the This simulation is known as the Nice have been ejected from the solar system. sun than Earth\u2019s, so the Model, after the city in France where it was Given that astronomers have recently intensity of sunlight is just devised. If the migration of the gas giants found rogue planets that wander through 1\/400th of that on Earth took place about 600 million years after the empty space with no host star, the idea formation of the solar system, then it might is not as bizarre as it may at first appear. also account for the event known as the Late 3.7 (0.35) Asteroid belt Pluto Jupiter Sedna Earth Eris Sedna Kuiper Belt Oort Cloud \u25b2 Central solar system \u25b2 The Kuiper Belt \u25b2 Outer solar system The sun\u2019s gravity holds four rocky The band of icy objects \u2013 including The Oort Cloud is a large, spherical planets\u2014Mercury, Venus, Earth, and Pluto \u2013 that sits 30\u201350 times further region sparsely populated by comets. Mars\u2014and an asteroid belt. Beyond from the Sun than Earth is known as The sun\u2019s gravity controls their orbits that, the gas giants Jupiter, Saturn, the Kuiper Belt. Objects including Eris up to one light year away, which is the Uranus, and Neptune also orbit the sun. and Sedna orbit even further out. extent of our solar system. THE SUN TAKES CONTROL 75","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS HOW WE FIND SOLAR SYSTEMS For centuries, astronomers have recognized the stars as distant versions Communication hub transmits of our own sun. The stars are so far away that it took until the late data to Earth for eight hours a 20th century to tease out the presence of planets orbiting them day and at speeds of five and to discover new solar systems. megabits per second Stars are often millions of times bigger than shorter orbits mean closer planets. Two dual-speed focuser planets, and their considerable brightness Consequently, astronomers use this distance telescopes with billion-pixel easily overwhelms any light their suites of to estimate the planet\u2019s temperature and cameras housed in spacecraft\u2019s planets happen to reflect. The stars themselves whether it might be habitable. appear only as tiny flecks of light from Earth cylindrical body due to their vast distances\u2014the closest one is GRAVITATIONAL WOBBLE over 25 trillion miles (40 trillion km) away. It The other main way of finding other solar is only in the last few decades that scientists systems is to exploit the two-way nature have developed the technology to spot the of gravity. While stars famously pull on alien worlds orbiting them. planets, planets also pull back on their suns. This slight tugging causes the star to wobble BLOCKING THE LIGHT slightly on the spot. These small changes in While too small and dark to be observed the star\u2019s motion have an effect on the way directly, a planet blocks some of its host star\u2019s we see the light it emits. If wobbling toward light when passing, or \u201ctransiting,\u201d in front us, the star\u2019s light is shifted toward the blue of it. Astronomers can glean a wealth of end of the color spectrum. Conversely, information from this simple event. The if it is moving away from us, the shift is planet\u2019s size, for example, is betrayed by toward the red end (see pp.28\u201329). As the amount of light that is blocked out. more massive planets pull on their A transiting Earth would cause a 0.01 stars with a greater gravitational percent change in the brightness of the sun. force, these color shifts are more pronounced for heavier planets, The time between successive transits allowing astronomers to reveals the duration of the planet\u2019s orbit, estimate the planet\u2019s mass. which in turn discloses its orbital distance: Planet blocks some of its star\u2019s light reaching Earth Star as it orbits Star\u2019s orbit Star Planet\u2019s orbit EarthBrightness Blue light emitted Planet\u2019s gravity as star moves causes its star\u2019s Star brightness toward us orbit to wobble falls as planet passes in front Red light emitted as star moves away Time \u25b2 Finding distant planets Earth Temperature-resistant materials Star brightness (red dots) is sampled cope with conditions between many times. The line shows the average \u25b2 Tracking distant stars as it dips due to the passing planet. As the star wobbles, color shifts -270\u00b0F and 160\u00b0F (-170\u00b0C and 70\u00b0C) in its light tell us its speed of travel toward or away from us. 76 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM Satellite telescope \u25c0 Gaia satellite is 93\/4ft (3m) tall Launched by the European Space Agency (ESA), this spacecraft is able to precisely Silicon carbide pinpoint the location of planets in other structure provides solar systems by measuring their star\u2019s strength and stability brightness and the color of its light. Sun-shield measures 33ft (10m) wide \u25bc Habitable zones Most solar systems have a habitable zone, where liquid water and life could exist. Earth orbits in our solar system\u2019s habitable zone, and other planets in alien solar systems, for example Kepler-452b, orbit in habitable zones of their own. The planet has spent 6 billion years orbiting within its star\u2019s habitable zone\u2014longer than Earth has in its own. Kepler-452b Venus Earth Mercury Mars Sensor within cylinder Kepler- Sun can detect stars 452 Solar system 400,000 times dimmer than human eyes can see Kepler-452 system Habitable zone HOW WE FIND SOLAR SYSTEMS 77","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS EARTH COOLS Early Earth was very different from the warm, blue planet we know today. Its tumultuous first years were dominated by almost constant collisions from elsewhere in the solar system. Initially a giant molten ball of magma, it gradually became a world fit for life. Around 4,560 million years ago, rock floated to the surface. Geologists call Localized and ice orbiting the early sun collided this process \u201cdifferentiation\u201d and it would heating and into a small, rocky planet under the force stabilize Earth\u2019s structure (see pp.80\u201381). melting of rock of gravity. Earth would have looked very different, with no atmosphere and no HELLISH PLANET oceans. The collisions were far from Earth\u2019s earliest period was once believed over\u2014our infant planet was still being to be so hellish that it is named the battered by many objects, some the size Hadean Era\u2014after Hades, the god of of planets. One collision, with an impactor the underworld. It was thought that much about the size of Mars, is thought to have of Earth\u2019s surface remained molten for formed our moon 100 million years later hundreds of millions of years, but recent (see pp.82\u201383). findings are overturning this notion and suggest our planet began to cool more BOMBARDMENT OF EARTH rapidly. It may have had oceans less The energy of these collisions, along with than 200 million years after it formed, that emitted by the radioactive decay of as vapor released by volcanic activity heavier elements, kept early Earth incredibly condensed into water. hot. Much of its material remained molten. This allowed heavier materials, such as iron Spherical shape due and nickel, to sink deep toward the planet\u2019s to larger mass and core. Less dense, rocky materials, such as gravitational field molten magnesium and silicon oxides, Larger clumps of rock and ice formed Rock Craters from impacts A tiny Earth began to form, bearing The gravitational potency of early Earth the scars of continual impacts. Its increased and it attracted impactors, such as bumpy surface was a result of recent asteroids, that were hurtling around the solar Ice additional material. Gravity molded system. Each impactor that joined Earth it into a roughly spherical shape. added to the planet\u2019s mass and gravitational force. This increased the acceleration and Gravity pulled rock energy of the next impactor. and ice together Accretion over many millions of years pulled THE HADEAN ERA, IN WHICH increasingly large clumps of rock and ice (planetesimals) EARTH FORMED, AND IN together. They formed a planetary embryo, which then attracted more material. Lumps of ice that remained WHICH ITS LAYERS STARTED TO intact despite the sun\u2019s heat would later become the STABILIZE, OCCURRED 4.6\u20134 BYA initial source of water on Earth. 78 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM Crust began to form Light liquid rock rose Heavy elements like to the surface iron sank to the center Force of Each impact Earth\u2019s surface became impacts grew now contained a molten mass due to as gravitational immense energy the frequency and force increased power of impacts Early mantle Central had formed metallic core Primordial \u25b2 Layers such as the crust, planet mantle, and core start to form (see pp.80\u201381) as \u25b2 Differentiation\u2014or differentiation continued. movement of Earth\u2019s materials\u2014 The surface cooled and began. Heavier elements sank to solidified into a crust as the the bottom of the magma ocean number of impacts from and lighter material bobbed up space fell. Iron and nickel to the surface. at the Earth\u2019s center formed the planet\u2019s metallic core. \u25b2 Molten surfaces caused by relentless asteroid strikes created a vast ocean of magma. As it was then primarily made of liquid, Earth\u2019s materials had the potential to move. EVEN UNDER THE MOST EXTREME CONDITIONS... EARTH WOULD NOT HAVE BEEN COMPLETELY STERILIZED BY THE BOMBARDMENT. Rock pulled Oleg Abramov, scientist and astronomer, 1978\u2013 toward planet by its gravity \u25c0 Hadean Earth During the Hadean Era, molten lava dominated the surface, and Earth\u2019s atmosphere was devoid of oxygen. The moon, far nearer than it is today, caused huge tides, as a deluge of impactors rained from above. EARTH COOLS 79","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS EARTH SETTLES INTO LAYERS The Earth is formed of distinct layers, and each is made of different materials. The processes responsible for this structure began billions of years ago and continue to shape and influence our planet today. For hundreds of millions of years after the Late Heavy Bombardment about \u25bc Earth\u2019s layers planet formed, Earth was a molten mass. 4.1\u20133.9 BYA (see pp.74\u201375) saw a significant, Layers began to form 4.4\u20133.8 BYA. Our It was still contracting under its own gravity secondary spike in the number of impacts planet is divided here into six layers: and material left over from the solar system\u2019s thumping into Earth. These asteroids and the solid inner core, liquid outer core, formation was still bombarding it. Both comets are thought to have added much semi-solid mantle, solid crust, liquid processes generated heat. Earth\u2019s crust of the water that contributed to the ocean, and gaseous atmosphere. solidified, but the planet continued to primordial oceans. differentiate, settling into its present layers. The lightest materials\u2014gases\u2014escaped FROM CORE TO ATMOSPHERE from the mantle via volcanoes and became Material in the center hardened to form a part of our planet\u2019s carbon dioxide-rich solid inner core, surrounded by a largely atmosphere. Hydrogen and helium were liquid outer core. The fluid in the outer core blasted away by the solar wind, but Earth\u2019s flowed easily, and turbulence within it is gravity was strong enough to hold onto carbon dioxide, nitrogen, water vapor, TEMPERATURES IN EARTH\u2019S and argon. Gaseous oxygen was absent CORE ARE ESTIMATED TO BE from the atmosphere\u2014all of Earth\u2019s oxygen HIGHER THAN 12,000\u00b0F (6,700\u00b0C) was bound into its rocks and water. thought to contribute to Earth\u2019s magnetic EXPLORING INSIDE EARTH field to this day. Above the outer core sits the Our planet\u2019s depths are so hot and under thickest of the layers\u2014the mantle. The next such extreme pressure that we have never layer, formed by molten rock erupting from even penetrated the crust. Instead, scientists the mantle, is the crust, which accounts for have used other methods to deduce what only 0.5 percent of the planet\u2019s thickness. is inside Earth. They knew that there must be significantly heavier material Differentiation continued as water vapor at the center, because the average released by early volcanic activity condensed density of Earth is greater than into water and became the first oceans. The the density at its surface. Studies of the way earthquakes travel and how our magnetic field emerges provide additional clues about the inner structure of Earth. \u25b6 Seismic waves Crust Epicenter of Solid core Vibrations from earthquake made of iron earthquakes are either and nickel sank primary (P) waves or S-waves cannot travel to the center secondary (S) waves. through liquid outer core The speed at which they soon after travel through the planet Earth formed during seismic events can help to determine INNER CORE Earth\u2019s structure. Paths of P-waves refract and wobble Inner core as they travel through each layer Outer core Shadow zone where no waves The flow of liquid iron OUTER CORE can travel due to a change in the and nickel in the outer core Mantle direction of travel at the boundary creates Earth\u2019s magnetic field between mantle and core MANTLE 80 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM Heavier, thinner oceanic crust sits lower on the mantle and forms the deep ocean basins Lighter, thicker continental crust floats higher on the hot mantle and forms dry land, flooded at the edges by shallow seas ATMOSPHERE Gaseous layer around 75 miles (120km) thick contains oxygen, nitrogen, argon, and a small amount of carbon dioxide Layer of water with an average depth of 12,100ft (3.7km) covers two-thirds of Earth\u2019s surface OCEAN Region of charged particles Bow shock held in place by magnetic field; it is sometimes visible as aurorae (Northern Lights) CRUST EARTH SUN Magnetic field lines \u25b2 Natural shield Solar wind show shape and A stream of harmful particles from the deflected strength of field sun\u2014the solar wind\u2014is deflected by by Earth\u2019s Earth\u2019s magnetic field. This field is created magnetic field Semisolid rock in the mantle flows by currents in the liquid-iron core. very slowly in convection currents, which cause plate movements in the crust (see pp.92\u201393) EARTH SETTLES INTO LAYERS 81","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS THE MOON\u2019S ROLE Despite being a relatively small planet, Earth is blessed with a particularly large moon\u2014the fifth largest in the solar system. The moon is our only natural satellite and has had such a significant influence on our planet that it may even have played a role in kick-starting life on Earth. \u25bc Extreme tides If the length of Earth\u2019s existence was Tides were extreme, and biologists have also keeps the tilt of Earth\u2019s axis constant, The Bay of Fundy condensed into a single day, the moon speculated that the intense churning which means our seasons are steady and on Canada\u2019s Atlantic would have formed when the Earth was during these super tides was a key factor repeat predictably. The moon stabilized coast boasts the 10 minutes old. The moon is our planet\u2019s in the mixing of ingredients that led to Earth over time and this has given life widest tidal ranges steadfast partner and it is likely that we life in the first oceans. Over millions of a chance to thrive. on Earth. The water would not be here without it. years, the moon retreated from Earth rises and falls twice due to the moon\u2019s gradually increasing PULLING ON THE PLATES each day by up to It is thought that a giant piece of rock orbital velocity. Today, the moon is the Geologists have speculated that Earth 52 ft (16 m), regularly smashed into our infant planet during its main driver of the roughly daily cycle of is the only planet with plate tectonics submerging the early days. Rock from the impact, while high and low tide, and continues to drift (see pp.92\u201393) because of the early moon\u2019s Hopewell Rocks. in Earth\u2019s orbit, gathered together to form away from Earth at a rate of 1.5in (3.8cm) strong gravitational pull. During Earth\u2019s the moon. As it formed, it was 10 times per year. As it edges further away, tidal hellish Hadean Era, our moon would have closer to Earth than it is currently. strength falls. pulled on the primordial oceans of magma. Theories suggest that the wrench of the THE MOON AND LIFE The tides swirled the oceans, and moon on the cooling liquid rock helped During Earth\u2019s childhood, the moon\u2019s close this helped to spread heat from polar to separate it into the distinct pieces of proximity would have created a considerably equatorial regions, regulating the young crust our planet possesses today. mightier gravitational pull than we feel now. Earth\u2019s temperature. The moon\u2019s gravity 82 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM \u25bc Pull of the moon THE POSSIBILITY DESERVES CONSIDERATION THAT THE The moon\u2019s gravitational force creates tidal bulges FORMATION OF THE MOON... PROVOKED THE ORIGIN on both sides of Earth. On the side facing the moon, OF LIFE ON EARTH. the moon\u2019s gravity pulls the oceans toward it, resulting in high tides. As well as attraction, however, gravity exerts a stretching force on Earth. Counterintuitively, this results in a second high tide facing away from the moon. Moon\u2019s gravity has a stretching Richard Lathe, molecular biologist, c.1950\u2013 effect on Earth along an axis connecting Earth and moon Low tide EARTH High tide High tide Orbit of moon MOON Moon\u2019s gravitational force Stretching effect of moon\u2019s Moon\u2019s gravity pulls gravity acts on both rock and Earth\u2019s oceans ocean, but because the ocean is toward moon liquid, it bulges far more Earth\u2019s direction of rotation; tidal bulges shift as it spins THE MOON\u2019S ROLE 83","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS THE CONTINENTS ARE BORN At some time around 4 BYA, Earth\u2019s crust began moving, forcing some Before continents came cratons\u2014the crust down into the mantle. Magma erupted and cooled into a new, seedlings from which greater swathes of lighter kind of crust\u2014continental crust. It bobbed up higher than the land would grow. Cratons in turn were made surrounding rock, creating the first land masses. The process continues from strings of islands formed from the first today, with 30 percent of our planet\u2019s surface now made of continents. continental crust. The process began in the Archean era (4\u20132.5 BYA). Although Earth First continental crust formed when had cooled since the Hadean era, the planet magma cooled, building a volcanic island was still much hotter than it is today. Earth\u2019s of crystalline rock, typically granite layers had settled, however, and oceans had formed on a solid crust. Primordial crust initially SHALLOW OCEAN VOLCANIC PRIMORDIAL CRUST covered Earth. When SUBDUCTING CRUST ISLAND MANTLE Today, Earth\u2019s crust is made of both two plates of the moving heavy oceanic crust and continental crust, crust met head-on, one Crust was forced down, which is lighter and thicker. The primordial was forced underneath. or subducted, into the crust was uniform, but when currents in In the mantle, its lighter hot mantle and melted Earth\u2019s mantle began dragging on its materials were melted underside (see pp.92\u201393), it began moving, first, and these bubbled splitting into plates. When these plates to the surface. collided, one plate was forced under the other. This triggered a further stage of Melted crust formed magma rich in light elements, such as silicon, oxygen, aluminum, sodium, and potassium Movements of Earth\u2019s Crust continues to be Island collided Some cratons rifted (split) and Erosion of the craton by oceans, crust pushed adjacent forced downward with another island, admitted heavier materials wind, and rain created sediments, islands together and forming a craton from below to fill the gap such as sandstone formed progressively larger masses of light CRATON rock called cratons. But two more processes Subducting crust continued to Heavy magma, rich in magnesium were at work: heavy be pushed into the hot mantle and iron, could push upward material rose to the into rifts in the craton surface where cratons and then melted split, and new. heavy oceanic crust was also Oceanic crust is Craton pushed Volcanic rock, Crust melted Sandstone pushed Basalt from spreading Granite from created where plates destroyed by into other cratons greenstone, formed at base but stayed inland by colliding oceanic ridge could be original islands was separated in oceans. subduction from heavy magma and islands solid on top cratons incorporated into compressed and The first continents continent changed into a eventually formed from banded rock colliding cratons and called gneiss islands. Because they were light, they stayed CONTINENT on the surface, but became composed of a growing variety of rock. Oceanic crust, being heavier, is continuously subducted, and never gets old and complex. It is replaced by new crust at spreading ridges. 84 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM differentiation, in which some primordial rocks dated to 3.6\u20132.7 BYA. In fact we now \u25c0 Nishinoshima crust melted and created lighter material know these land masses have split and In 2013, a new island that bobbed to the surface and solidified, rejoined more than once (see pp.158\u201359), was discovered off forming islands. Over millions of years, and that the cratons that formed the first the coast of Japan. It the movement of Earth\u2019s crust pushed the continents are now scattered across the appeared when lava islands together to form cratons\u2014small modern continents. Even though continents broke through Earth\u2019s protocontinents. Eventually, these cratons change, cratons remain as their stable cores. crust in a burst of collided and coalesced to create successively volcanic activity and larger land masses\u2014the first continents. Continent formation is still occurring. then cooled, following Oceanic crust continues to subduct under the same process that THE FIRST SUPERCONTINENT other oceanic crust, causing magma to push created continents 4\u00a0BYA. By the end of the Archean Era, 2.5 BYA, the to the surface and cool into arcs of volcanic Earth\u2019s surface had 80 percent of the land islands\u2014such as those in the Caribbean. mass it does today, largely gathered together into a supercontinent called Vaalbara. THE OLDEST CONTINENT Vaalbara was formed by colliding cratons WHOSE ROCKS STILL EXIST called Kaapvaal and Pilbara. These survive TODAY IS CALLED \u201cUR\u201d AFTER today, but Kaapvaal is now in South Africa THE ANCIENT SUMERIAN CITY and Pilbara is in Australia, and each has Plates of crust New heavy oceanic crust was created Farther island chains and cratons beneath ocean at a spreading ridge, where the old were built as the creation of crust was diverging; the new rock continental crust continued moved apart formed was heavy, volcanic basalt VOLCANIC VOLCANIC ISLAND ISLAND New oceanic crust Heavy magma rose continued to be created at to fill the gap left by oceanic spreading ridges diverging crust OCEANIC CRUST THE CORES OF CONTINENTS... MAKE UP THE STABLE LITHOSPHERE. THEIR FORMATION... OCCURRED BILLIONS OF YEARS AGO. Nicholas Wigginton, Science editor, c.1970\u2013 THE CONTINENTS ARE BORN 85","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS DATING EARTH The question of Earth\u2019s age has only been resolved in the last few decades. As knowledge increased and scientific techniques were honed, estimates of the age of our planet increased from thousands of years to billions. We now know Earth to be around 4.54 billion years old. It was not always clear if Earth had an in the late 18th century as the tide of \u25b2 Dangerous beliefs origin at all. Ancient Greek philosophers opinion began to turn toward a greater age Bernard Palissy (1509\u20131589) worked as a potter for including Aristotle believed that our planet for the planet. Hutton argued that Hadrian\u2019s most of his life, but he was also a scientist. He put was eternal\u2014it has always been here and Wall, despite being built by Romans in forward his then-radical belief that fossils were always will be. Most civilizations had their England more than 1,000 years previously, prehistoric animals, and not from the biblical flood. own origin stories (see pp.18\u201319), and before had barely eroded. Therefore, other rocks The Catholic Church ultimately imprisoned him. the onset of modern science, religious texts that had been significantly eroded must were the main sources of ideas about Earth\u2019s have been around much longer. Hutton also distance. By the 20th century, the general origins. In 1645, Irish Bishop James Usher noted that layers of rock had not been laid consensus for the age of Earth had leapt famously used the genealogy in the Bible to down continuously, but in separate episodes from thousands of years to tens, if not calculate the date of Earth\u2019s creation as of deposition, leading to \u201cunconforming\u201d hundreds, of millions of years. October 23, 4004 BCE. layers that would have taken millions, not thousands of years, to form. Victorian EARLY SCIENTIFIC IDEAS geologist Charles Lyell agreed with Hutton, Not everyone believed the idea of a young but emphasized the idea of Earth in a state Earth. Back in the 16th century, French of slow, perpetual change. Rates of change thinker Bernard Palissy argued that if the observed in modern times could then be erosion of rocks was caused by the gradual used to estimate rates of change in the past. battering of wind and rain, then Earth must \u25bc Clues in the rocks be much older than a few thousand years. THE DEBATE INTENSIFIES AGE OF RADIOACTIVITY A sketch from 1787 of French natural historian Beno\u00cet de Maillet The discovery of radioactivity by Marie rock layers in Jedburgh, tried to explain why marine fossils were By the middle of the 19th century, attempts Curie in 1903 would enable scientists to find Scotland, shows found at high elevations by wrongly to determine Earth\u2019s age had picked up concrete evidence for Earth\u2019s age. The decay horizontal layers of rock concluding that Earth\u2019s sea level must have steam, and scientists from many different of radioactive atoms in rocks occurs over that sit on top of vertical been much higher in the past. This was long disciplines made estimates. In 1862, millions of years, and the proportion of layers, each from before the discovery of plate tectonics (see physicist William Thompson (later Lord unstable atoms remaining can be measured different periods. This pp.90\u201391). This idea of rates of erosion was Kelvin), imagined our infant planet as a ball to provide an age for the rock (see pp.88\u201389). unconformity served revisited by Scottish geologist James Hutton of molten rock and calculated how long it Over the next 30 years, many scientists used as geologist James would have taken to cool to its present radiometric dating methods to analyze rocks Hutton\u2019s evidence that temperature, concluding 20\u2013400 million from all over the world\u2014arriving at ages Earth was very ancient. years. He did not take into account the between 92 million years and 3 billion years. effect of radioactivity, a phenomenon By the 1960s, the number of ways to use that had yet to be discovered. Lyell radioactivity to date rock samples started to criticized his ideas for being too rise. The precision of these techniques and conservative and inconsistent with the accuracy of the calculated ages steadily what he had learned about the increased. We know now that Earth has deposition of rock layers. Charles been around for close to 4.54 billion years, Darwin joined the debate, stating give or take one percent. Such figures are in On the Origin of Species that Earth supported by the age of meteorites that we must be at least 300 million years think are slightly older than Earth. old in order for chalk deposits in England to have eroded to their FOSSILIZED TREES ON TOP OF A PREHISTORIC SEA BED 6,000FT current state. Charles\u2019s son, (1,800M) HIGH IN THE ANDES astronomer George Darwin, CONVINCED CHARLES DARWIN believed that the moon was THAT EARTH WAS VERY OLD formed from Earth. If so, he reasoned it would have taken at least 56 million years for the moon to reach its current 86 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM \u25bc History in the rocks WITH RESPECT TO HUMAN OBSERVATION, THIS WORLD Rock such as this limestone on a Greek coast, HAS NEITHER A BEGINNING NOR AN END. with its evident long history of deposition, followed by crumpling, followed by erosion, James Hutton, geologist, 1726\u20131797 is the sort of evidence that, in the 18\u201319th centuries, set the minds of pioneering geologists thinking\u2014how much time is needed for all of this geological change?","HARD EVIDENCE ZIRCON CRYSTAL Some ancient crystals have survived 4.4 billion years on Earth. Their persistence provides an excellent opportunity to probe into our planet\u2019s history, and learn more about the origins of life and the first oceans. The Jack Hills of Western Australia are too fierce to support liquid water and home to the oldest material ever found on life, but opinions are beginning to Earth. These tiny zircon crystals are each shift to an Earth that cooled only the size of a dust mite, yet hold within relatively quickly, because them the secrets of our planet\u2019s turbulent the crystals needed those infancy. The oldest crystals date from cool conditions to form. 4.4 BYA\u2014100 million years after a giant impact struck Earth and created the Crystal composition moon\u2014which means that Earth\u2019s solid crust, in which they formed, must be at least Radiometric dating analysis uses a the same age. Zircon is a mineral that device called a mass spectrometer. The contains the element zirconium. It has a rock sample is broken into atoms, then similar hardness to diamond, its more the atoms are ionized (given an electric illustrious cousin\u2014which means zircon charge). As the ions pass through the crystals can survive erosion and other device, magnets sort them according to geological processes, making them an excellent record keeper of Earth\u2019s history. their mass, because the magnets deflect lighter ions more easily. Normally zircon crystals are red, but This allows the sample\u2019s different when scientists bombard them with ions to be identified and their electrons in order to study them, they take precise proportions to be on a blue hue. Analysis of these crystals is measured so the rock\u2019s age subverting previous ideas of the conditions can be determined. on early Earth. It was long thought that our planet\u2019s infancy was a hellscape, one much \u25bc How radiometric dating works Mass spectrometer One-quarter of the One-eighth Uranium atoms are so large and unstable that they original uranium of the uranium decay\u2014they give off radiation and change into Lead atom produced by atoms remain more stable atoms\u2014and they do this at a known the radioactive decay atoms remain rate. Measuring the ratio of uranium in rock to of a uranium atom its final decay product (lead) tells us how much radioactive decay has occurred since the rock formed, and therefore how much time has passed. Uranium atom When the rock formed, the sample 704 million years later, the uranium After 1.406 billion years, more uranium Today, a geologist measures the ratio of contained only uranium as it solidified atoms have decayed, giving off radiation atoms have decayed. The more lead uranium to lead remaining in the rock and from molten rock and crystallized. and changing into lead atoms. found in the rock, the older the sample. dates this rock to 2.112 billion years old. 88 THRESHOLD 4","This particular zircon Evidence of early oceans crystal is 4.4 billion years old By comparing the ratio of oxygen isotopes found within the Jack Hills zircon crystals, scientists have concluded that oceans of liquid water may have been present on Earth as early as 4.4 billion years ago. Isotopes are versions of an atom with differing atomic weight. The ratio of oxygen-18 to oxygen-16 isotopes found in the crystals indicates the presence of liquid water. Earth in the Archean Era, 3.5 BYA Signs of life Earth was previously thought to be inhospitable until 3.8 billion years ago, but isotope analysis of graphite flecks found inside zircon crystals dating back to 4.1 billion years ago suggests that life was present at this earlier time. Graphite is made of carbon, and the ratio of carbon-12 to carbon-13 isotopes in the graphite is characteristic of the ratio produced by living organisms. This zircon crystal is incredibly Protecting the crystals small\u2014measuring just 1\u20448in (0.4mm)\u2014 Around 200,000 zircons have been and is barely visible to the naked eye unearthed in the Jack Hills since the 1980s, and 10 percent of them are more than 3.9 billion years old. The geology of the area is so important that the Australian government has declared the region a geoheritage site, to protect it from future mining activity and preserve its scientific treasures. Jack Hills, Australia ZIRCON CRYSTAL 89","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS CONTINENTS DRIFT The map of our modern world is a familiar image, but this arrangement of continents is a relatively recent development in our planet\u2019s history. Entire continents have split and moved apart over hundreds of millions of years. This idea wasn\u2019t accepted until the late 20th century. The fact that Earth\u2019s land masses have Corner of Africa shifted over time makes sense when looking appears to fit at a map of the world. Some continents snugly with South appear to fit together, like puzzle pieces. America\u2019s coastline However, the notion that these vast land masses could move was long considered \u25b2 First clues expanding. As the planet got bigger, its land outrageous to the scientific community. masses were forced to spread out. Both of Despite scientists\u2019 reservations, the idea has Explorers noticed that the east coast of South these ideas gradually lost support as the been around for centuries, with the Flemish precise physical mechanisms behind them cartographer Abraham Ortelius widely America and the west coast of Africa appeared could not be found. credited as being the first to express such thoughts at the end of the 16th century. to fit together. These maps were drawn by A NEW IDEA In 1912, German scientist Alfred Wegener BRIDGING THE GAP geographer Antonio Snider-Pellegrini in 1858. argued in favor of continental drift. He not In the 19th century, Antonio Snider- only showed matching fossil evidence on Pellegrini created two maps showing the in places now separated by vast oceans. disparate continents, but also concluded ease with which the meandering coastlines This was explained away by the idea that that the types of rock and other geological of the various continents appear to slot into continents were once connected via vast structures were similar too. He decided that place to form one giant supercontinent. land bridges, which have since been eroded this idea could not coexist with the theory of Further evidence that the far-flung away or submerged deep beneath the sea. now-submerged land bridges, so he continents had once been conjoined came suggested that the continents themselves from the fossil record (see pp.158\u201359). Another thorny issue perplexing had moved apart. This offered a potential Scientists were beginning to discover that geologists was the origin of mountain solution to the mountain conundrum. If the fossilized remains of similar animals, ranges, such as the Himalayas. The leading continents were free to roam, then over time and in particular plants, were cropping up idea in the 19th century was that the peaks some could collide. If India had smashed were formed as wrinkles, as Earth cooled into mainland Asia, the Himalayas would \u25b6 Bold ideas and shrank. If that were true, mountain be the result of continental crumpling. German scientist Alfred chains should be spread evenly across the Wegener (1880\u20131930) planet\u2019s surface\u2014and that is not the case. Wegener published his findings the same hoped to collect year, suggesting that Earth\u2019s land masses solid evidence for his Ideas continued to develop at the turn plowed through the sea over time. His work continental drift theory of the 20th century. George Darwin, met with a lukewarm reception from the on his fourth expedition Charles\u2019s son, proposed that the moon scientific community, in part because to Greenland, but he had once formed part of Earth and its died while collecting absence accounted for the vast, landless supplies for his camp. Pacific Ocean. His theory suggested that the continents separated as the moon broke away, explaining their present positions. Another theory was that Earth was 90 THRESHOLD 4","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM \u25c0 Continent scars In 1977, this map, the result of a lifetime\u2019s work by oceanographers and cartographers Marie Tharp and Bruce Heezen, revealed the ocean floor in new detail, providing conclusive evidence for plate tectonics. the idea, arguing that the planet\u2019s crust ruptures at plate boundaries, allowing magma to well up from the mantle. As this material solidifies, it forms a ridge, pushing the existing seafloor apart. So it is not that IT TOOK OVER 300 YEARS FOR THE IDEA OF CONTINENTAL DRIFT FINALLY TO BE ACCEPTED AS FACT he could not provide a plausible reason range and extends through all of its oceans. the continents plow through the ocean crust as to why the continents would drift. He The geologists of the day now had to explain as Wegener had suggested, but rather that incorrectly calculated the rate of their the presence of this ridge, too. It would fall the seafloor itself is growing, carrying away movement and overestimated by a factor to former US Navy officer turned geologist the continents, which are part of moving of 100 compared to today\u2019s accepted tectonic plates (see pp.92\u201393). value, which did not help his cause. Today, these ideas are brought together Wegener\u2019s academic background was as the theory of plate tectonics. It is also a hindrance. Given his training as an supported by observations of Earth from astronomer and meteorologist, many in the space using geodesy, which maps small geological community suggested he did not changes in Earth\u2019s gravity to locate have the expertise required to be taken concentrations of mass. Studies of the seriously. He was not without some support, polarity of Earth\u2019s magnetic field, which is however\u2014British geologist Arthur Holmes known to have flipped frequently over time backed his ideas, arguing as early as 1931 (north becoming south, and vice versa), also that Earth\u2019s mantle contained currents lends weight. The reversals leave stripes of that helped move parts of the crust. I ONCE ASKED ONE OF MY LECTURERS... I WAS TOLD, CLUES FROM THE SEAFLOOR SNEERINGLY, THAT IF I COULD PROVE THERE WAS A It was not until the 1950s that evidence FORCE THAT COULD MOVE CONTINENTS, THEN HE emerged to turn the tide of opinion in MIGHT THINK ABOUT IT. THE IDEA WAS MOONSHINE. Wegener\u2019s favor. In 1953, analysis of rocks in India suggested that it was once in the David Attenborough, natural history broadcaster, 1926\u2013 Southern Hemisphere, bolstering Wegener\u2019s mountain formation argument. Around the Harry Hess to tie all these ideas together. magnetic rock on the ocean floor (see same time, a huge underwater mountain Having used sonar to map the ocean during pp.94\u201395) that allow us to date the bands range\u2014the Mid-Ocean Ridge\u2014was World War II, by the early 1960s, Hess\u2019s and show how fast the seafloor is spreading. discovered. It is Earth\u2019s longest mountain research led him to propose that the continents did indeed drift apart thanks Plate tectonics was not widely accepted to a process called \u201cseafloor spreading.\u201d until the 1970s, when maps of the ocean In 1958, Australian geologist Samuel Carey floor, such as that made by Marie Tharp had suggested that Earth\u2019s surface, its crust, and Bruce Heezen, left no doubt that the was constructed from plates. Hess ran with seafloor was spreading, accounting for continental drift. CONTINENTS DRIFT 91","4.56 BYA SUN 4.54 BYA EARTH 4.53 BYA MOON 4.4 BYA FIRST IGNITES FORMS FORMS OCEANS HOW EARTH\u2019S CRUST MOVES The surface of our planet is sculpted by extremely slow convection currents in the mantle layer below. Earth\u2019s system of plate tectonics sets it apart from the other rocky planets in the solar system, since its surface is constantly changing and is alive with geological activity. \u25bc Volcanic eruption Earth\u2019s surface layer, the crust, is formed TECTONIC PHENOMENA Underwater The Eyjafjallaj\u00f6kull of seven major tectonic plates\u2014African, volcanoes spew volcano in Iceland Antarctic, Eurasian, North American, Where plates meet, a range of tectonic molten lava, which erupts molten magma, South American, Pacific, and Indo- activity may occur, but exactly what along with black clouds Australian\u2014along with several smaller ones. depends on the crust material and the cools into new of ash that fall on the These solid plates float on a semisolid layer direction of movement. There are three oceanic crust ground as added layers called the mantle. Plates move incredibly main types of plate boundary: transform atop Earth\u2019s crust. slowly, typically at about the rate that boundaries, where plates slide or grind past Convection fingernails or human hair grow. Since one another; divergent boundaries, where current causes Earth\u2019s layers stabilized 4 BYA, these they slide apart, allowing magma to cool an upwelling of plates have been constantly moving. into new crust; and convergent boundaries, molten magma where two plates collide head on. Parts of the crust sink and melt at subduction Heat in the core zones, but new crust is made elsewhere causes convection by volcanoes and at mid-ocean ridges, where oceanic crust diverges. currents in the mantle that drive Earthquakes, sudden movements of the movement of Earth\u2019s crust, occur at plate boundaries. At divergent and transform boundaries, tectonic plates they tend to be shallow, whereas collisions at convergent boundaries cause the deepest earthquakes. Where two plates collide, they can push up continental crust to form a mountain range, such as the Himalayas. Those particular mountains were created when the Indian plate slammed into the Eurasian plate around 50 million years ago. EARTH\u2019S SURFACE MOVES \u25b6 Dynamic surface Earth\u2019s crust is constantly changing Convection currents in the mantle are as the plates move, buoyed generated by heat in the core that filters into by currents in the mantle the mantle. Although the mantle is almost below. Depending on solid, it flows slowly, tugging at the base of how the plates interact, the crust and moving the plates. The crust is earthquakes can of two kinds: oceanic crust, which is made of occur and volcanoes dense rock rich in magnesium and iron, and and mountain continental crust, made of rock with lighter chains can form. elements including aluminum. Where the edge of a plate is made of oceanic crust, its Solid crust greater density makes it subduct, or slip underneath, the lighter crust. It then sinks Semisolid deep into the hot mantle, causing an mantle upwelling of molten magma that breaks the surface of the crust as a volcano. Liquid outer core Solid inner core","4.1 BYA FIRST TRACE OF 4 BYA EARTH SETTLES 3.8 BYA EARTH\u2019S CONTINENTS POSSIBLE LIFE INTO LAYERS START TO FORM Divergent plate boundary causes a CONTINENTAL DRIFT... EARTHQUAKES, mid-ocean ridge as magma rises to fill VOLCANICITY... ARE UNDOUBTEDLY the gap left by the separating plates CONNECTED ON A GRAND SCALE. MID-OCEAN RIDGE Alfred Wegener, geologist and meteorologist, 1880\u20131930 OCEAN Volcanic islands are a result of magma from the HOTSPOT mantle that has risen and cooled through the ISLANDS crust\u2014these areas are called hotspots OCEANIC TRENCH Seamounts are hotspots where magma has cooled under water, but has not reached the ocean\u2019s surface Convergent plate boundary causes oceanic crust to be forced underneath (subducted under) lighter continental crust, creating a deep oceanic trench Volcanoes occur at a convergent boundary because of the upwelling of magma created by the subducting, melting crust erupting at the surface Fold mountains are wrinkles in the continental crust, pushed upward due to the pressure of colliding plates Transform plate boundary causes a fault, or crack, in the crust where the plates are sliding past each other. Pressure builds on the plates until they slip suddenly, causing an earthquake MELTING VOLCANO CRUST TECTONIC FAULT","HARD EVIDENCE OCEAN FLOOR In many ways, the ocean floor is a guide to Earth\u2019s history\u2014studying it Caribbean Muertos helps us decipher the mysteries of our planet\u2019s past. Exploring it has even plate is sliding Trough given scientists clues about how life originated. Mapping the ocean floor toward the east reveals a diverse, active landscape full of tectonic phenomena. \u25b2 WEST CARIBBEAN PLATE The depths of the ocean are cold, dark, American plates, where the Puerto Rico and incredibly hostile. At its deepest point, Trench is found, is a particularly active area ANTILLES ARC there are 8.4 tons of water pressing down of the ocean floor. Its unique plate boundary on every square inch (1.2 metric tons per and unusual phenomena provide a rich square centimeter). Such extremes mean resource for scientific research: oceanographers resort to imaging the oceanographers, biologists, seismologists seabed using sonar from the surface. It is (who study earthquakes), and bathymetrists easier for us to get images from Mars than (who study the underwater terrain of lakes map parts of our own seabed. and oceans) all work here, hoping to unlock the secrets of the ocean floor. Despite its inaccessibility, the ocean floor holds clues that are vital in understanding How sonar surveys work the development of Earth\u2019s crust, and also life. Deep ocean exploration is sharpening Multibeam sonar \u25c0 SOUTH our ideas on plate tectonics (see pp.90\u201391). records the time taken The chemically-rich material and heat for sound to bounce back generated by underwater volcanoes found from the seafloor in order on the ocean floor have led biologists to to measure ocean depth. believe that these areas are where the first Oceanographers can use life-forms appeared (see pp.106\u201307). this data to create a colored map of the The deepest places of the ocean floor seafloor, showing its are where two oceanic plates meet and terrain. Side-scan sonar is form an underwater valley\u2014one plate slips more accurate in that the underneath (subducts beneath) the other, intensity of its echoes can creating a V-shaped trench. The deepest reveal whether the ocean ocean trench is the Mariana Trench in floor is rocky (strong) or the Pacific Ocean: its deepest point is at sandy (weak). Marie Tharp 36,070ft (10,994m) below sea level. It could and Bruce Heezen mapped accommodate Mount Everest with about Earth\u2019s ocean floor in the 6,560ft (2,000m) of water to spare. 1950s (see pp.90\u201391). The Puerto Rico Trench in the Atlantic Marie Tharp, oceanographer Ocean has depths greater than 27,560ft (8,400m). The underwater boundary between the Caribbean and North \u25b6 Clues on the ocean floor Magnetic field reversals Cooling material Magma from the mantle breaks through create stripes forces plates apart the crust and forces tectonic plates apart (see pp.92\u201393). As the magma cools to form new crust, minerals in the magma orient themselves in line with Earth\u2019s magnetic field. For reasons unknown, Earth\u2019s north-south polarity reverses from time to time, and over millions of years these reversals are etched into the ocean floor as a series of stripes. Older rock with frozen Molten material The Antilles islands have magnetic alignment from mantle been formed due to both breaks through folding and volcanism at Younger rock with frozen crust this plate boundary magnetic alignment 94 THRESHOLD 4","The island of Puerto Seamount emerging from The most negative gravity \u25bc Exploring the ocean floor KEY Rico rises above the ocean surface becomes anomaly on Earth is found A bathymetry map ocean surface one of the southernmost in the Puerto Rico Trench; generated by sonar reveals Above sea level islands of the Bahamas this indicates an active the underwater terrain of the VIRGIN ISLANDS downward force northeastern corner of the Sea level - 0m 0 ft TROUGH PUERTO RICO Caribbean plate. Differences TRENCH in relief are represented by 2,000m 6,562ft color, highlighting deep sea trenches in purple. 3,000m 9,843ft 4,000m 13,123ft 5,000m 16,404ft 6,000m 19,685ft 7,000m 22,966ft NORTH \u25b6 Tectonic activity NORTH AMERICAN The boundary where the Caribbean and PLATE North American plates meet looks like a convergent plate boundary due to the presence of an ocean trench, which normally indicates subduction, but here, the plates are mainly sliding against one another instead\u2014forming a transform plate boundary. The Caribbean plate is sliding to the east at a rate of 0.8in (2cm) per year. Since it grinds against the North American plate, earthquakes can occur when the plate jerks and slips. Seismologists study the seismic waves produced by the earthquakes. To the east, the Caribbean plate is pushing against the North American plate, resulting in mountain folds that break the surface like the Antilles island arc. ANEGADA North American TROUGH plate is sliding toward the west EAST \u25bc Exploring the deep Compression of the Submersibles are invaluable tools for Caribbean plate\u2019s crust from scientists wanting to explore ocean trenches, the slightly subducting North because they are able to withstand deep-sea American plate creates folds pressure. Robotically-piloted submersibles, or AUVs (autonomous underwater vehicles), are pre-programmed with instructions on where to explore and what to measure. Some submersibles also allow scientists to visit the ocean floor in order to examine and collect samples of both rocks and life-forms for analysis at the surface. Example of an AUV OCEAN FLOOR 95","THRESHOLD","LIFE EMERGES Earth has a privileged position in the solar system\u2014in a band that\u2019s not too cold and not too hot to support liquid water. It is in this vital ingredient that life first emerges. And through a process of natural selection, life evolves from simple bacteria to complex vertebrates, shaping our planet and filling it with astounding diversity.","GOLDILOCKS CONDITIONS On Earth, living organisms emerged from nonliving complex APSlbtaaunbneldet awhnaittbhcitosaomt,lippdloecsxrsuicbshltyeaimnndiclaiqls and minueirdawltshaetedreep ocean, with a source of heat energy chemicals. Life-forms could metabolize, meaning they were able to extract energy from their surroundings. They could also copy themselves and adapt to their environment\u2014 through the process of natural selection. Complex chemicals Rocky planets, such as Earth, are made of a rich variety of elements, including oxygen, silicon, iron, nickel, aluminum, nitrogen, hydrogen, and carbon. The last of these, carbon, can build a large range of complex molecules in combination with other elements. What changed? Chemical reactions produced ever larger and more complex molecules. Molecules with self-copying abilities became more common. Reactions occured that provided both the energy and the means to build more complex molecules. The chemicals of life became packaged inside membranes, forming protocells\u2014the first true living organisms. Heat from Earth\u2019s core The planet\u2019s interior was hot, because of radioactivity and also due to heat left over from its violent formation. The heat energy reached the surface at volcanoes and deep-sea vents. Mineral catalysts The reactions that built the large, complex molecules of life needed to be driven by a chemical booster, or catalyst. Minerals bubbling up from Earth\u2019s mantle at deep-sea vents are thought to be a possible source of those catalysts."]
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