1
Origins

The modern study of Earth is ultimately rooted in humankind’s desire to understand its origins. Although it was once assumed that intelligent life was unique to Earth, we have now gained an appreciation that even though it may not be unique, the existence of advanced life on planets may well be uncommon. None of the other planets of the Solar System are presently suitable for the complex life forms that exist on Earth, and we have yet to identify other stars that have planets much like Earth. Although the odds are good that there is other life in our galaxy, this inference has not been confirmed.

Considering the apparent rarity of Earth-like life, it is natural to want to understand what went into making Earth suitable for life and how life arose. Pursuing these questions leads us to fundamental issues about how stars and planets form and evolve and to questions about how the modern Earth works, from the innermost core to the atmosphere, oceans, and land surface. This chapter presents three questions related specifically to origins—one regarding the origin of Earth and other planets and one regarding the origin of life. These two questions are separated by a third that deals with Earth’s earliest history: the 500 million to 700 million years between the time of the origin of the Solar System and the oldest significant rock record preserved on Earth. During this early, still poorly understood, stage of Earth’s development, tremendous changes must have taken place, accompanied by myriad catastrophic events, all leading ultimately to a setting in which life could develop and eventually thrive.

QUESTION 1:
HOW DID EARTH AND OTHER PLANETS FORM?

One of the most challenging and relevant questions about Earth’s formation is why our planet is the only one in the Solar System with abundant liquid water at its surface and abundant carbon in forms that can be used to make organic matter. This question is part of a broader set: why the inner planets are rocky and the outer planets are gaseous; how the growth and orbital evolution of the outer planets influenced the inner Solar System; why all of the largest planets are so different from one another; and how typical our Solar System is within the Milky Way galaxy. Although these questions are longstanding, the answers are only now emerging from new insights provided by astronomy, isotopic chemistry, Solar System exploration, and advanced computing. And although we know in general how to make a planet like Earth—starting with some stardust and allowing gravity, radiation, and thermodynamics to do their parts—our answers often serve only to refine our questions. For example, the details of Earth’s chemical composition—such as how much of the heat-producing elements uranium, thorium, and potassium it contains; how much oxygen and carbon it contains; and how it came to have its particular allotment of noble gases and other minor constituents—turn out to be critical to models of Earth’s geological processes and, ultimately, to understanding why Earth has remained suitable for life over most of its history.



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1 Origins T he modern study of Earth is ultimately rooted QUESTION 1: HOW DID EARTH AND in humankind’s desire to understand its origins. OTHER PLANETS FORM? Although it was once assumed that intelligent One of the most challenging and relevant questions life was unique to Earth, we have now gained an ap- about Earth’s formation is why our planet is the only preciation that even though it may not be unique, one in the Solar System with abundant liquid water at the existence of advanced life on planets may well be its surface and abundant carbon in forms that can be uncommon. None of the other planets of the Solar used to make organic matter. This question is part of System are presently suitable for the complex life forms a broader set: why the inner planets are rocky and the that exist on Earth, and we have yet to identify other outer planets are gaseous; how the growth and orbital stars that have planets much like Earth. Although the evolution of the outer planets influenced the inner Solar odds are good that there is other life in our galaxy, this System; why all of the largest planets are so different inference has not been confirmed. from one another; and how typical our Solar System is Considering the apparent rarity of Earth-like life, within the Milky Way galaxy. Although these questions it is natural to want to understand what went into mak- are longstanding, the answers are only now emerging ing Earth suitable for life and how life arose. Pursuing from new insights provided by astronomy, isotopic these questions leads us to fundamental issues about chemistry, Solar System exploration, and advanced how stars and planets form and evolve and to questions computing. And although we know in general how to about how the modern Earth works, from the inner- make a planet like Earth—starting with some stardust most core to the atmosphere, oceans, and land surface. and allowing gravity, radiation, and thermodynamics This chapter presents three questions related specifi- to do their parts—our answers often serve only to re- cally to origins—one regarding the origin of Earth and fine our questions. For example, the details of Earth’s other planets and one regarding the origin of life. These chemical composition—such as how much of the heat- two questions are separated by a third that deals with producing elements uranium, thorium, and potassium Earth’s earliest history: the 500 million to 700 million it contains; how much oxygen and carbon it contains; years between the time of the origin of the Solar Sys- and how it came to have its particular allotment of tem and the oldest significant rock record preserved on noble gases and other minor constituents—turn out to Earth. During this early, still poorly understood, stage be critical to models of Earth’s geological processes and, of Earth’s development, tremendous changes must ultimately, to understanding why Earth has remained have taken place, accompanied by myriad catastrophic suitable for life over most of its history. events, all leading ultimately to a setting in which life could develop and eventually thrive. 

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 ORIGIN AND EVOLUTION OF EARTH how do Planets Form around stars? small proportion of dust makes the disks opaque at visible wavelengths (Figures 1.1 and 1.2). Gas-giant We do not know how unique or unusual the Solar Sys- planets, such as Jupiter and Saturn in our system, are tem is, but observations of other planetary systems are believed to form in such circumstellar disks, but direct providing new ideas for how planets form and evolve. astronomical observations of planets forming have not Astronomical observations of star-forming regions and yet been made. young stars, together with hydrodynamic models of star Observations of planets around other nearby stars formation, support the conclusion that stars—includ- with masses similar to the Sun indicate that planet ing the Sun—form by the gravitational collapse of a formation is a common outcome of star formation, but molecular cloud core composed of materials manufac- no star has yet been observed with a system of planets tured and reprocessed in many earlier generations of that looks anything like the Solar System. Over 200 stars. Because the typical molecular cloud is rotating at extrasolar planets have been discovered by several in- the time of collapse, the developing star is surrounded direct techniques (e.g., radial velocity of the host star, by a rotating disk of gas and dust. Most disks around stellar transit, and microlensing) (Butler et al., 2006; young stars, as viewed through telescopes, are approxi- ). Multiple planets are known to mately 99 percent gas and 1 percent dust, but even that orbit some two dozen stars. The vast majority of these FIGURE 1.1 Hubble Space Telescope images of four proto- FIGURE 1.2 Hubble Space Telescope WFPC2 image of planetary disks around young stars in the Orion nebula, located Herbig-Haro 30, a prototype of a young (approximately 1- 1,500 light-years from the Sun. The red glow in the center of million-year-old) star surrounded by a thin, dark disk and each disk is a newly formed star approximately 1 million years emitting powerful bipolar jets of gas. The disk extends about old. The stars range in mass from 0.3 to 1.5 solar masses. Each 6 × 1010 km from left to right in the image, dividing the edge-on image is of a region about 2.6 × 1011 km (400 AU) across and nebula in two. The central star is hidden from direct view, but is a composite of three images taken in 1995 with Hubble’s its light reflects off the upper and lower surfaces of the flared Wide Field and Planetary Camera 2 (WFPC2), through nar- disk to produce the pair of reddish nebulae. The gas jets, shown row-band filters that admit the light of emission lines of ionized in green, are driven by accretion. SOURCE: Chris Burrows, oxygen (represented by blue), hydrogen (green), and nitrogen Space Telescope Science Institute; John Krist, Space Telescope (red). SOURCE: Mark McCaughrean, Max Planck Institute for Science Institute; Kare Stapelfeldt, Jet Propulsion Laboratory; Astronomy; C. Robert O’Dell, Rice University; and the National and colleagues; the WFPC2 Science Team; and the National Aeronautics and Space Administration, . org/gallery/album/entire_collection/pr1999005c/>.

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 ORIGINS for why they form so close to the star (Butler et al., 215 Planets Msin i < 15 M JUP 2006). These hot Jupiters are thought to be telling us 0.8 that large planets can drift inward toward their star as Orbital Eccentricity they form. Models also suggest that planets can under 0.6 some circumstances drift away from the star, so the ultimate location of the planets may have little to do 0.4 with where they originally formed. Extrasolar planets more than a few tenths of an AU distant from their host 0.2 star often have quite eccentric orbits, which contrasts with the Solar System where all of the planets except 0.0 Mercury have nearly circular orbits. Earth 0.1 1.0 Semimajor Axis (AU) how did the solar system Planets Form? Figure 1.3.eps FIGURE 1.3 Summary of known extrasolar planets sorted by The Solar System is composed of radically different distance from host star and orbital eccentricity. All of the planets types of planets. The outer planets ( Jupiter, Saturn, in the Solar System have eccentricities of 0.2 or less. SOURCE: Uranus, and Neptune) are distinguished from the inner Courtesy of Geoffrey Marcy, University of California, Berkeley. Used with permission. planets by their large size and low density. The outer planets are the primary products of the planet forma- tion process and comprise almost all of the mass held in planets are thought to be gas giants on the basis of their the planetary system. They are also the types of planet masses and densities. Presumably, more gas giants are that are most easily recognized orbiting other stars. The observed because they are large, and large planets are inner planets (Mercury, Venus, Earth, and Mars) are much easier to detect, leaving open the question of how composed mostly of rock and metal, with only minor many terrestrial planets remain hidden from Earth in amounts of gaseous material. There are “standard mod- distant planetary systems. A few “super-Earths,” with els” for the formation of both types of planets, but they masses of several to 10 Earth masses, may be terrestrial have serious deficiencies and large uncertainties. planets, but no measurements of the radius or density According to the standard model for outer-planet of these objects has confirmed this. Gas-giant planets formation, the formation of giant planets starts with appear to be more likely with stars that have propor- condensation and coalescence of rocky and icy material tions of heavier elements (heavier than H, He, and to form objects several times as massive as Earth. These Li) as high as the Sun (Fischer and Valenti, 2005), solid bodies then attract and accumulate gas from the suggesting that heavy-element concentrations in the circumstellar disk (Pollack et al., 1996). The two largest circumstellar disk influence the rate or efficiency of outer planets, Jupiter and Saturn, seem to fit this model planet formation. reasonably well, as they consist primarily of hydrogen Measurements of the masses, orbital distances, and and helium in roughly solar proportions, but they also orbital eccentricities (Figure 1.3) of extrasolar planets include several Earth masses of heavier elements in provide clues about processes that may help determine greater than solar proportions, probably residing in a what the final planetary system looks like. A par- dense central core. Uranus and Neptune, however, have ticularly interesting class of planets, that of gas-giant much lower abundances of hydrogen and helium than planets in orbits extremely close to (less than 0.1 AU)1 Jupiter and Saturn and have densities and atmospheric their host stars—sometimes called “hot Jupiters”—are compositions consistent with a significant component significant because models have been unable to account of outer Solar System ices. An alternative to the standard model is that the rock and ice balls are not needed to induce the forma- 1The astronomical unit, or AU, is a unit of length nearly equal tion of gas-giant planets; they can form directly from to the semimajor axis of Earth’s orbit around the Sun, or about the gas and dust in the disk, which can collapse under 150 million km.

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0 ORIGIN AND EVOLUTION OF EARTH its own gravity like miniversions of the Sun (Boss, that evolution. Significant unknowns are how long the 2002). In this model the excess abundances of heavy process took, how solid materials were able to coagulate elements in Jupiter and Saturn would have been ac- into progressively larger bodies, and how and when the quired later by capture of smaller rocky and icy bodies. residual gas was dissipated. The time for centimeter- This model, however, does not account well for the sized solid objects to form at Earth’s distance from the compositions of Uranus and Neptune, which do not Sun, according to the standard model, might have been have very much gas. Other important questions about as short as 10,000 years. These small solid objects were the outer planets are when they formed and the extent highly mobile, pulled Sun-ward large distances by the to which they may have drifted inward or outward from Sun’s gravity as a result of drag from the still-present the Sun during and after formation. Where the outer H-He gas. Submeter-sized objects were also strongly planets were and when is important for understanding affected by turbulence in the gas. how the inner planets formed. A particular deficiency of the standard model is its The primary difference between the inner and inability to describe the formation of kilometer-sized outer planets (rock versus gas and ice) is thought to bodies from smaller fragments. The current best guess reflect the temperature gradient in the solar nebula. is that the dust grains aggregated slowly at first, and Temperatures were relatively high (>1000 K) near growth accelerated along with object size as small ob- the developing Sun, dropping steadily with distance. jects were embedded into larger ones (Weidenschilling, Near the Sun, mainly silicates and metal would have 1997). The aggregation behavior of objects greater than condensed from the gas (so-called refractory materials), a kilometer in size is better understood: they are less whereas beyond the asteroid belt, temperatures were affected by the presence of gas than are smaller pieces, low enough for ices (i.e., water, methane, ammonia) and their subsequent evolution is governed by mutual containing more volatile elements to have condensed, gravitational attractions. Growth of still larger bodies, as well as solid silicates. It was once thought that or planetesimals, from these kilometer-sized pieces as the nebula cooled, solids formed in a simple uni- should have been more rapid, especially at first. Gravi- directional process of condensation. We now know tational interactions gave the largest planetesimals that solids typically were remelted, reevaporated, and nearly circular and coplanar orbits—the most favorable recondensed repeatedly as materials were circulated conditions for sweeping up smaller objects. This led through different temperature regimes and variously to runaway growth and formation of Moon- to Mars- affected by nebular shock waves and collisions between sized planetary embryos. Growth would have slowed solid objects. Important details of the temperatures of when the supply of small planetesimals was depleted the solar nebula, however, are still uncertain, including and the embryos evolved onto inclined, elliptical orbits. such significant issues as peak temperatures, how long Dynamical simulations based on statistical methods they were maintained, and how temperature varied with and specialized computer codes are finding that a distance from the Sun and from the midplane of the number of closely spaced planetary embryos are likely disk. Defining these conditions is an important part of to have formed about 100,000 years after planetesimals understanding how the chemical compositions of the appeared in large numbers (e.g., Chambers, 2003). planets and meteorites came to be. The later stages of planet formation took much The standard model for the formation of the in- longer, involved progressively fewer objects, and hence ner planets is somewhat more complicated than the are less predictable (Figure 1.4). The main phase of model for outer-planet formation and is based largely terrestrial planet formation probably took a few tens on theory and anchored in information from mete- of millions of years (Chambers, 2004). The final stages orites and observations of disks around other stars were marked by the occasional collision and merger of (Chambers, 2003). The model strives to explain how planetary embryos, which continued until the orbits of a dispersed molecular cloud with a small amount of the resulting planets separated sufficiently to be pro- dust could evolve into solid planets with virtually no tected from additional major collisions. intervening gas and how the original mix of chemical Although there are four terrestrial planets, models elements in the molecular cloud was modified during suggest that the number could easily have been three

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 ORIGINS FIGURE 1.4 Results of four representa- Simulation tive numerical simulations of the final stage of accretion of the terrestrial planets. The 1 segments in each pie show the fraction of material originating from the four regions of the solar nebula shown by the shades 2 of gray, and the size of the pie is propor- tional to the volume of each planet. In each simulation the largest planet has a size similar to Earth’s, but there can be either two or three other planets, and the sizes 3 vary. The planets typically receive material from all four zones, with preference for the zones closest to their final orbit location. 4 SOURCE: Chambers (2004). Copyright 2004 by Elsevier Science and Technology Journals. Reprinted with permission. or five, and they would have been at different dis- belt just as the Solar System was starting out. Thus, Figure 1.4.eps tances from the Sun (Figure 1.4). Tidal interactions they preserve significant clues about the state of the with nebular gas may have caused early-formed inner Solar System when the planets were forming (Figure planets to migrate inward substantially while they were 1.5). For this reason, studies of meteorites play a major forming, and several planets may have been lost into role in helping us understand Earth’s origin. the Sun before the gas dispersed (McNeil et al., 2005). One gift of meteorites is to reveal the age of the The fact that there are no rocky planets beyond Mars Solar System. Precise radiometric dating of high- is likely a consequence of the presence of the giant temperature inclusions within meteorites shows that planets, particularly Jupiter. The large mass and strong the first solid objects in our home system formed gravitational pull of Jupiter probably prevented the 4,567 million years ago (see Box 1.1). We also know formation of additional rocky planets in the region now that shortly thereafter planetesimals of rock and metal occupied by the asteroid belt by disrupting the orbits formed and developed iron-rich cores and rocky crusts of bodies in that region before they could form a large (see Question 2). Some meteorites are chemically like planet. Jupiter and Saturn also sent objects from the the Sun (for elements other than H, He, Li, C, N, O, asteroid belt either out of the Solar System or spiraling and noble gases), and some of these same meteorites into the inner-planet region where they became parts contain tiny mineral grains of dust that survived from of the planets forming there or fell into the Sun. The earlier generations of stars (see Box 1.2). Other mete- asteroids represent the 0.01 percent of material that orites are parts of small planetary bodies that experi- survived this process. enced early volcanism and that were later broken up by collisions. Beyond these clues, meteorites fall short of providing all the information needed to understand What do meteorites say about the origin of Earth, partly because most of them formed far from earth? the Sun (the main asteroid belt is between Mars and Earth has undergone so much geological change that Jupiter), and the relationship between meteorites and we find little evidence in rocks about its origin or planets is not fully understood. The systematic collec- even its early development (Question 2). Many me- tion of well-preserved samples from Antarctica has teorites, on the other hand, were not affected by the greatly expanded the number of meteorites available for high-temperature processing that occurs in planetary study and has yielded rarities such as meteorites from interiors. They are fragments of, or soil samples from, Mars and the Moon. miniplanets that formed in what is now the asteroid Beyond what they tell us about Earth, meteorites

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 ORIGIN AND EVOLUTION OF EARTH moderately well from spectroscopic data. The planets, however, formed from the nebular disk, so it is impor- tant to know whether the disk had the same composi- tion as the Sun, and whether it was homogeneous or varied significantly in composition, perhaps with radial distance from the proto-Sun. The standard model for the composition of the solar nebula is based on stud- ies of a class of meteorites called chondrites (Figure 1.5). Chondrites, the commonest type of meteorites, are stony bodies formed from the accretion of dust and small grains that were present in the early Solar System. They are often used as reference points for chemicals present in the original solar nebula. The most primitive of these objects—those least altered by heat and pressure—are carbonaceous chondrites, whose chemical compositions match that of the Sun for FIGURE 1.5 The Allende meteorite, a carbonaceous chondrite, is a mixture of CAIs (calcium-, aluminum-rich inclusions; larger most elements. The relative amounts of elements and irregularly shaped light-colored objects) and chondrules (round their isotopes can be measured much more precisely light-colored objects) in a dark-colored matrix of minerals and on meteoritic materials than by solar spectroscopy, compounds. The CAIs and chondrules are a high-temperature so chondritic meteorites play a special role in helping component that formed and were in some cases reprocessed at temperatures above 1000°C. SOURCE: Hawaii Institute of to understand both Earth and nucleosynthesis in our Geophysics and Planetology. Used with permission. galaxy. Because chondritic elemental abundances look similar to those of the Sun, the disk likely had about the same composition as the Sun. provide a benchmark for understanding the composi- tion of the Sun and even the Universe as a whole. Most What is the chemical composition of earth? of the visible mass of the Universe, and almost all stars, is composed primarily of hydrogen and helium made The most critical question related to the formation during the Big Bang. The rest of the elements—the of Earth is why the planet has its particular chemical “heavier” ones with more protons and neutrons in makeup. Although we know quite a lot about this is- their nuclei—were produced by nucleosynthesis, or sue, a key unanswered question is the origin of Earth’s thermonuclear reactions within stars. Most nucleo- water. Earth, like other objects forming near the Sun, synthesis happens in big stars. These massive stars last is thought to have formed mainly as a relatively high- only about 10 million to 20 million years before they temperature partial condensate from a gas of solar explode as supernovae. The new elements they make, composition. The uncondensed gas containing water, before and during the explosion, are thrown back into carbon, and other volatile elements was swept away by space where they are later recycled into new stars. In the the early solar wind or by ultraviolet radiation pressure. approximately 10 billion years between the origin of the Much of the volatile elements that might have been Universe and the origin of the Solar System, hundreds incorporated into the early Earth is thought to have of generations of massive stars have exploded, and over been lost during the intense heating of the Hadean this long period about 1 percent (by weight) of the orig- Eon (Question 2). inal H and He has been converted to heavier elements. It has been suggested that the giant planets can Meteorites give us the most detailed information about pluck materials from the asteroid belt region and throw the abundances of these heavier elements. them in toward the Sun. Objects beyond Mars would Meteorites tell us still more about the forma- have formed in a cooler part of the solar nebula and tion of the Solar System out of the nebular disk. The hence would likely have contained more volatile com- abundance of heavy elements in the Sun is known pounds. Studies of asteroids indicate that meteorites

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 ORIGINS BOX 1.1 Time, the Early Solar System, and the Age of Earth The initial events in the formation of the Sun, meteorites, and Earth and other planets took place in only a few million years, about 4,567 million years ago (Ma). Documenting early Solar System timescales is therefore a substantial challenge. Advances in geochronological techniques are beginning to enable the sequence of events to be discerned. The most primitive chondritic meteorites contain inclusions made up of minerals that condense at high temperature from a gas of solar nebula composition. These objects, called calcium-aluminum-rich inclusions (CAI), have recently been precisely dated using the decay of uranium to lead, where time is measured by the accumulation of the lead decay products formed at 4,567 (±1) Ma. This age is now generally accepted as “time zero” for the Solar System. The U-Pb method gives the most precise and accurate ages for these ancient objects partly because the radioactive decay constants for 238U and 235U are precisely known. Once the absolute time is established using the long-lived radioactive isotopes of uranium, the sequence of events within the first few million years of the Solar System can be studied using isotopes with much shorter half lives (extinct radionuclides). These isotopes were present in the early Solar System because they had been produced in stars just prior to the beginning of the Solar System and were part of the molecular cloud that collapsed to form the Sun. Subsequently, virtually every atom of these short-lived radioactive isotopes that existed at time zero has now decayed to the daughter isotope. The isotopes used for this purpose are 26Al, 53Mn, 244Pu, 182Hf, 60Fe, and 129I and their corresponding decay products 26Mg, 53Cr, 136Xe, 182W, 60Ni, and 129Xe. The resulting sequence of events is summarized in the figure. How old is Earth? Although the start of the Solar System is well dated at 4,567 Ma, at that time and shortly after only the pieces that would eventu- ally come together to make Earth were present. About half or more of the planet was probably assembled by 4,550 Ma, and the Moon-forming impact, now generally thought to culminate the main phase of Earth’s formation, happened at about 4,530 Ma. Earth probably continued to accumulate small amounts of material, some of them perhaps quite significant chemically, until as late as 4,450 Ma. A short episode of renewed accretion may have oc- curred much later, at 4,000 to 3,900 Ma. Summary of recent geochronological data and models for the sequence and timing of events in the early Solar System. SOURCE: Adapted from Halliday (2006).

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 ORIGIN AND EVOLUTION OF EARTH BOX 1.2 Presolar Grains On the basis of characteristically anomalous isotope ratios (Lewis et al., 1987), we now recognize and can study “presolar grains”—bits of stardust manufactured by individual stars before the birth of our Solar System that are preserved in primitive meteorites. Each of these grains contains chemical elements that were made or reprocessed by an individual star. How stars produce the heavier elements (from iron to uranium) was highlighted in Con- necting Quarks with the Cosmos as one of the 11 major science questions for cosmology in the new century (NRC, 2003a). Geochemists will play a key role in addressing this question because the relative abundances of elements and isotopes in the different types of presolar grains provide the most specific and detailed data for checking our understanding of how chemical elements are produced in different types of stars (Zinner, 2003). Electron microscope images of presolar grains representing materials that were manufactured by individual stars and condensed in the outflow of material marking the end of that star’s life cycle. Typical sizes are given in microns (µm), and typical abundances are given in parts per million (ppm) and parts per billion (ppb) by weight. SOURCE: Nanodiamond image courtesy of Tyrone Daulton, Washington University, Meteorite Magazine; graphite image courtesy of Sachiko Amari, Washington University; oxide image courtesy of Larry Nittler, Carnegie Institution of Washington. Used with permission. SiC image from Bernatowicz et al. (2003), copyright 2003 by Elsevier Science and Technology Journals. Reproduced with permission.

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 ORIGINS that have little water are derived from the inner asteroid The chondritic model and the Solar System’s ap- belt (inward of 2.5 AU), while the volatile element-rich parent ability to sort chemical elements according to meteorites, some with as much as 20 percent water as their volatility have proven useful for understanding well as complex organic compounds, come from farther many aspects of planet formation. But our increasing than 3 AU. These objects from the asteroid belt region ability to probe the chemical and isotopic compositions may have been the source of Earth’s water and carbon. of meteorites and our planet is causing some serious There is also evidence that much later in the history rethinking of long-held models. Unanticipated com- of the Solar System—500 million to 600 million years positional differences have been discovered between after its formation—a large but unknown amount of Earth and meteorites and between different types of rocky debris was flung into the inner Solar System, meteorites. Perhaps the most striking difference is bringing a last barrage of large impacts and finishing off that of the isotopes of oxygen—the most abundant the major construction of the inner planets. However, element on Earth (Figure 1.6). Chondritic meteorites it is unlikely that this “late heavy bombardment” added have a peculiarly variable proportion of the isotope 16O, and almost every class of meteorites has different enough material to significantly affect Earth’s overall composition. proportions of the three oxygen isotopes. Chondritic The aspect of Earth’s composition that is likely best meteorites, long thought to be the best model for the known is the proportion of refractory elements, which original Earth, are not like Earth with respect to oxygen form solids at the high temperatures thought to have isotopes. The one class of meteorites that is like Earth prevailed in the inner Solar System as the terrestrial in this respect—enstatite chondrites—would probably planets were forming. Included among the refractory be no one’s first choice for Earth’s main building blocks elements are most of Earth’s major components—Si, because they do not match Earth for most other ele- Mg, Al, and Ca. The relative amounts of refractory elements do not vary much among different classes of the benchmark chondritic meteorites, which is gener- ally taken as a strong argument that Earth is not much different from the meteorites. For the more volatile elements, which evaporate more easily, there are wide and puzzling variations throughout the Solar System. Oxygen is one example. Si, Mg, and Fe readily combine with oxygen to form SiO2, MgO, and FeO. On Earth almost all of the Si and Mg occur as oxides, but only about 20 percent of the Fe is combined with O; the rest is metallic Fe that resides in Earth’s core. The size of the core is therefore a rough measure of the amount of oxygen that Earth has. Most meteorites have different Fe/FeO ratios, and at least two of the other terrestrial planets have a different ratio of metallic core to silicate mantle. Elements of intermediate volatility also raise important questions of chemical evolution. Potas- FIGURE 1.6 Representation of the range of values of oxygen sium, for example, is relatively volatile, and estimates isotope ratios on Earth, the Moon, Mars, and different classes of suggest that Earth has about 10 percent of what was meteorites, including carbonaceous chondrites (CI, CK, CM, CO, available in the nebula. But exactly how much? The CR, CV); ordinary chondrites (H, L, LL); other chondrite groups answer is critical because the isotope 40K is radioactive (R); primitive achondrites (Acapulcoite [Aca], Brachinite [Bra], Lodranite [Lod], Winonaite [Win], Ureilite [Ure]); Howardite, and provides 20 to 40 percent of the heat produced in Eucrite, Diogenite [HED] achondrites; aubrite achondrites (Aub); the early Earth. This heat plays a role in powering the stony-iron meteorites (Pallasites [Pal], Mesosiderite [Mes]); and convection in the mantle that drives plate tectonics iron meteorites (IAB-IIICD irons). SOURCE: . Used with permission.

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6 ORIGIN AND EVOLUTION OF EARTH ments. Moreover, it has recently been reported that the for example, suggest that the nebular disk was not isotopes of neodymium, a lanthanide element that has entirely homogeneous. While this is a problem in one proven critical for understanding planetary processes sense, it is also an opportunity. If we can understand (Question 4), are also present in different amounts on how this heterogeneity arose or was preserved, and Earth and chondritic meteorites (Figure 1.7), as are the what its structure was, we can learn more about how isotopes of hafnium and barium. the materials of the nebula were sorted and gathered Although we have long assumed that the isotopic to produce the planets. compositions of the elements of the Solar System The Nd isotope discrepancy raises a different prob- were mostly homogeneous, and measurements have lem that has not yet been squarely addressed. Studies borne this out in large measure, improved sensitivity is of asteroids and meteorites show that the process of now showing small but significant differences between accretion, whereby small chunks of rock gradually co- various planetary bodies. The O isotope differences, alesce to form larger and larger bodies and eventually planets, is not one directional. When objects collide, they are almost as likely to blow each other apart as they are to coalesce. In addition, there is evidence that small accreting bodies become hot enough to melt, al- lowing crystals and liquid to separate. Thus, it was pos- sible to differentiate (make heterogeneous by internal processes) smaller bodies and then blast material off them that is chemically different from the bulk object. This process would create differentiated objects that could eventually become part of the planets (or be lost into the Sun or ejected from the Solar System). In this view we cannot expect even the refractory elements to be present in exactly the same proportions everywhere, and this would have enormous implications. For ex- ample, if we relax the requirement that Earth be exactly chondritic for the elements Nd and Sm, we reach a different interpretation of the subsequent evolution of Earth’s mantle and crust (Question 4). If the Hf/W ratio of Earth is not chondritic, the timing of forma- tion of Earth’s metallic core, as estimated by W isotope data, changes (see Question 2). We now know that even small bodies were able to partially melt and differenti- ate into core and mantle and that the mantle could potentially be removed from the core by an impact. So the timing and mechanism of formation of planetary metallic cores and the abundances of trace metals in planetary mantles have to be viewed in this context. FIGURE 1.7 Reported differences in 142Nd isotopic abun- dance between Earth, achondritic meteorites (Eucrites), and Was the moon Formed by a Giant impact? chondritic meteorites. The ε142Nd value is the difference in the proportion of 142Nd expressed in units of 0.01 percent. 142Nd is More is known about the Moon than any terrestrial the radioactive decay product of the short-lived isotope 146Sm. planetary body other than Earth because of the rock The differences may reflect deep sequestration of ancient crust samples collected by the U.S. and Soviet lunar missions formed in the early Earth or differences in refractory element ratios between Earth and chondritic meteorites. SOURCE: Boyet between 1969 and 1976. The peculiarities of these lunar and Carlson (2005). Reprinted with permission of the American rocks—their great antiquity, their nearly complete lack Association for the Advancement of Science (AAAS).

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 ORIGINS FIGURE 1.8 Snapshots in a numerical simulation of the Moon-forming giant impact. Times are shown in hours and color scales with particle temperature in K; frames (a) through (e) are views onto the plane of the impact; particles with T > 6440 K are shown in red. Distances are shown in units of 1,000 km. Frame (f) is the final state viewed edge on; here the temperature scale has been shifted so that red corresponds to T > 9110 K. The large orbiting clump in (d) and (e) con- tains about 60 percent of a lunar mass. SOURCE: Canup (2004b). Copyright Elsevier. Reprinted with permission. the Moon. Another difference is that Earth has water, of water and other volatile elements and compounds, as well as other volatile species and oxidized (ferric) and the chemical complementarity of the dark lunar iron; the Moon has virtually no water and all of its iron basaltic lowlands and the bright highland rocks—led is in the reduced (ferrous) state. to enormous advances in theories of planet formation. Studies of lunar rocks have helped persuade many Moon rocks provide one of the most persuasive pieces geologists that the Moon was formed when a Mars- of evidence that Earth and the Moon have a common sized object collided with the still-forming Earth origin. The isotopic composition of oxygen varies dra- about 40 million years after the formation of the Solar matically within the Solar System (Figure 1.6) but is System. This “giant-impact” hypothesis would explain identical in Earth and the Moon. An important differ- the relatively large mass of the Moon relative to Earth, ence is the size of their metallic cores—one-third of the the large amount of angular momentum in the Earth- mass of Earth but only about 2 percent of the mass of

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 ORIGIN AND EVOLUTION OF EARTH to effectively 0 million years. Like oceanic crust, conti- nental crust appears to be “recycled” to the mantle, but at an unknown rate. The surface of average continental crust stands about 5 km higher than the surface of the average oceanic crust, so Earth’s water is collected in the basins underlain by oceanic crust, and there is abundant dry land rather than a globe-encircling ocean. Crusts are widely variable throughout the Solar System and offer no clear insight about what Earth’s earliest crust was like. Samples returned by astronauts showed that the Moon’s light-colored highland crust is very old (ca. 4,400 million years) and probably formed from feldspar crystals that floated to the surface after the Moon-forming impact when it was largely molten (Figure 1.12). The crust of Mars appears to be variable in age, but most is extremely old (Frey, 2006). The crust of Venus is much less well known, but a large fraction is thought to be young (Hansen, 2005; Basilevsky and Head, 2006). The crusts of the larger moons of Jupiter and Saturn seem to resemble our conceptions of the early Earth in interesting ways. Jupiter’s moon Io, for FIGURE 1.12 The heavily cratered light-colored areas of the lunar surface, the lunar “highlands,” reflect the intense rain of meteorites that occurred in the earliest history of the Solar Sys- tem. The highlands are composed of rock made mostly of a single mineral, plagioclase feldspar, which floated to the surface as the magma ocean crystallized, at about 4,500 Ma. The large, dark lunar “seas,” or maria, are huge impact basins that formed mostly between 4,000 and 3,900 Ma and are evidence of a late heavy meteorite bombardment that would also have affected Earth (see Box 1.5). The lunar maria are filled with dark lava flows of basalt that formed 3,900 to 3,300 Ma. The lower crater density in the maria indicates that the meteorite flux dropped off considerably by the time the lava flows formed. SOURCE: . by moving downward in subduction zones. Oceanic crust is thin (about 6 to 8 km), submerged under the oceans, and relatively young; its average age is about 60 million years, which is only 1.4 percent of Earth’s age. The continents, which are mostly above sea level, are underlain by a different kind of crust. Continental crust is a quilt of rocks of vastly different compositions, textures, and ages and forms by multistage processes FIGURE 1.13 Image of Jupiter’s moon, Io, from the National that are only partly understood. It is also thick (30 to Aeronautics and Space Administration’s Galileo spacecraft. 80 km), more silica rich than basalt, and generally old. Io is a volcanically active miniplanet, with young crust and no plate tectonics. SOURCE: . million years, but they range from 4,000 million years

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 ORIGINS example, which is rocky and about the size of Earth’s Moon, is thought to have a young crust (Figure 1.13), which it resurfaces rapidly by continuing volcanism. However, none of the rocky planets or moons have Earth’s crustal resurfacing mechanism of plate tectonics (see Question 5). One of the most obvious qualities of Earth’s early crust is that it no longer exists. Why did it all disap- pear? The type of crust most likely to be preserved is continental crust, since virtually no oceanic crust endures for more than about 200 million years before descending into the mantle at subduction zones (Ques- tions 4 and 5). One possibility is that the early Earth had only an oceanic-type crust and no continents. FIGURE 1.14 Photograph of exposures of some of Earth’s However, virtually all of the rocks preserved from the oldest sedimentary rocks (about 3,700 million years), from the period 4,000 to 3,600 Ma are continental (3.8 Ga eastern Isua supracrustal belt in West Greenland. Metacherts ophiolite in Greenland is an exception; see Furnes et (light gray) are interlayered with carbonate and calcsilicate al., 2007), and the only earlier materials are tiny zircon metasediments (dark gray). SOURCE: Friend et al. (2007). Reprinted with kind permission of Springer Science and Busi- crystals that presumably also come from continental- ness Media. type rocks such as granite. Isotopic evidence suggests the presence of pre-3.8 Ga continental crust, although the relative proportion varies with isotopic system (Nd isotopes suggest a greater proportion of ancient crust amount and distribution of water, we do not know than Hf isotopes; Bennett, 2003). The fact that some of whether oceanic crust production was similar to that the oldest rocks are water-deposited sediments (3,800- on the modern Earth, whether plate tectonics was op- million-year-old rocks from Isua, Greenland) also erating, and how efficiently continental crust was being indicates that there was erosion and transport of sedi- formed and recycled. The end of the Hadean, perhaps ment, which requires land standing above sea level at coincidentally, corresponds to the time of the “late that time (Figure 1.14). At the average rate that Earth heavy bombardment” of the Moon’s surface, which has been producing continental crust over the past 2 produced the large lunar impact basins that were sub- billion years, we would expect one-fifth the mass of the sequently filled with basalt lava flows (Figure 1.12; Box present continental crust to have been produced in the 1.5). Earth probably experienced this bombardment as Hadean. However, the total volume of rocks older than well, but it is doubtful that such intense bombardment 3,600 million years is very small—about 0.0001 percent could cause the disappearance of a large preexisting of the continents. The recent observation that every continental crust, given that low-density ancient crust Earth sample measured is enriched in 142Nd compared is preserved on the Moon. Rather, vigorous internal to chondritic meteorites suggests very early formation convection is more likely responsible for the demise of of a crustal component enriched in incompatible ele- Earth’s original crust. ments (such as the light rare-earth elements) and its removal from the accessible portions of Earth (Boyet and Carlson, 2005). If this interpretation is correct, summary Earth’s original crust may lie sequestered in the deep The geological period called the Hadean, which ex- Earth today. tends from the time of the Moon’s formation to the The uncertainties about any aspect of Hadean crust time when the oldest Earth rocks were formed (~4.5 are large. Under the conditions of the Hadean Earth, to 3.9 Ga), is critical to our understanding of planetary which was hotter, still being hit by meteorites in its evolution. If we are ever to fully appreciate how our waning stages of accretion, and bearing an unknown

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6 ORIGIN AND EVOLUTION OF EARTH BOX 1.5 Late Heavy Bombardment A major scientific discovery that came out of the Apollo program is that at about 3.9 Ga the Moon was pummeled by several 100-km asteroids (or comets) and by hundreds of 10-km asteroids (Wilhelms, 1987). The craters they made carved the face of the Moon. Because Earth’s effective cross sec- tion is 20 times bigger than the Moon’s, Earth must have been hit 20 times as often. But not only was Earth hit by a hundred 100-km asteroids, statistics imply that it was also hit by a dozen bodies bigger than any that hit the Moon. The biggest would have been comparable to Vesta or Pallas, the largest asteroids now in the asteroid belt. Whether these impacts marked the tail end of a sustained bombardment dating back to the accretion of the planets or whether they record a catastrophic event, such as a sudden influx of planetesimals to the inner Solar System due to rapid migration of the giant planets (e.g., Gomes et al., 2005), is contentious but of great importance to the Hadean environment. Examples of both possibilities are shown in the figure. 104 50 Myr half-life Impact Rate (Relative to Today) 1000 100 Myr half-life multiple 100 cataclysms 10 LHB Single Cataclysm 1 4.4 4.2 4 3.8 3.6 Time (Gyrs) Four models of the impact rate of the first billion years of the Moon’s life: a single figure.eps late heavy bombardment (LHB), multiple cataclysms Box 1.5 cataclysm with a throughout the Hadean, and sustained bombardments (denoted 50-Myr [million year] half life and 100-Myr half life). The single- and multiple-cataclysm curves are schematic representations, and the sustained bombardment curves are standard impact rates based on lunar crater counts and surface ages of the Apollo landing sites and impact basins. The 100-Myr half-life curve incorporates the age of the Imbrium impact basin and is more consistent with terrestrial and Vestan impact records than the 50-Myr half life curve, which incorporates the age of the Nectaris impact basin. SOURCE: Courtesy of Kevin Zahnle, National Aeronautics and Space Administration. Available data offer some support for a late cataclysm, but not for the enormous hidden impacts implicit in monotonic decline. The most telling argument against a huge unseen Hadean impact flux is that it does not explain anything else in the Solar System that needs explaining. By contrast, a cataclysm (or cataclysms) fits in well with current concepts of how a solar system might evolve. All that is required is a rearrangement of the architecture of the Solar System; such rearrangements are a natural consequence of the dynamical evolution of a swarm of planets (the Moon-forming impact provides a cogent example) and are expected to occur on every timescale (Gomes et al., 2005). Before the cataclysm, impact rates would have been higher than they are today, because there were more stray bodies in the Solar System. The impacts of the late heavy bombardment would have posed a recurrent hazard to life on Earth. Impacts by asteroids as big as Pallas or Vesta would have been big enough to boil away the oceans and leave Earth enveloped in 1500 K steam. The lunar impact record suggests that one or two of these struck Earth ca. 4.0 Ga. Conditions a few hundred meters underground would be little changed and life could have gone on (Sleep et al., 1989; Zahnle and Sleep, 1997). Later, smaller impacts may have boiled half the ocean and left the rest a scalding brine. It is this scale of event that suggests that life on Earth may have descended from organisms that either lived in hydrothermal systems or were extremely tolerant of heat and salt. It has been widely postulated that all life appears to descend from thermophilic organisms (Wiegel and Adams, 1998). Whether this means that life originated in such environments or that life survived only in such environments is debated. If the latter, the thermophilic root implies that life on Earth arose in the Hadean during the age of impacts.

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 ORIGINS planet came to be the home of complex life, we must QUESTION 3: HOW DID LIFE BEGIN? be able to fill in this enormous gap in the geological The origin of life stands as one of science’s deepest and record. At present we can construct plausible, but still most challenging questions. It is a historical problem highly uncertain, models for the Hadean Earth, which that emerged during a time with little recorded history, are based on our present understanding of planet for- so it must be approached mostly through theory and ex- mation (Question 1), planetary interior processes and periment—imaginative efforts to re-create our planet’s material properties (Questions 4 and 6), and climate early conditions and establish plausible chemical routes (Question 7). These models are informed by obser- to the emergence of life. The goal of understanding vations of the Moon and other planets in the Solar life’s beginnings has attracted scientists from geology System, by measurements made on meteorites and the and from many overlapping disciplines, especially sub- oldest rocks and minerals on Earth and the Moon, and fields of organic chemistry and molecular biology. In by our geological understanding of how the modern an age of planetary exploration, the origin of life is also Earth works. A critical component of understanding an astrobiological issue, currently investigated on Mars, Hadean climate is our knowledge of atmospheric where a sedimentary record of earliest planetary history processes, but despite the advanced state of models is preserved, and potentially across the wider stretch of for the modern Earth atmosphere, our understanding Universe where planets have been detected. of radically different types of planetary atmospheres Some of the most fundamental mysteries about the is still rudimentary. origin of life are geological in nature: From what mate- Recent studies have raised new hope of improving rials did life originate? When, where, and in what form our understanding of the Hadean. New information did life first appear? At its most basic physical level, life continues to be gleaned from precise measurement is a chemical phenomenon, and because it arose billions of the isotopic and chemical compositions of ancient of years ago, geologists are intensely interested in creat- zircons and their mineral inclusions. Observations of ing an accurate picture of the chemical building blocks the Moon, Mars, Venus, and the moons of Jupiter and available to early life. Saturn have opened new windows for visualizing the early Earth and for documenting what may have been Top-down and Bottom-Up approaches happening in the early Solar System. Comparison of meteorites with Earth rocks has led to better models In The Origin of Species, Charles Darwin (1859) hy- of Earth’s early internal processes, including the forma- pothesized that new species arise by the modification of tion of the metallic core, the implantation and loss of existing ones—that the raw material of life is life. Louis gaseous species from Earth’s interior, and the evolution Pasteur, Darwin’s great Parisian contemporary, went a of the crust and mantle. step further. Pasteur decisively refuted the doctrine of The future is certain to provide additional break- spontaneous generation, the long-held view that life throughs. Capabilities for microanalysis of geological can arise de novo from nonliving materials, declaring materials are improving, and hence the amount of in- instead that life springs always from life (Pasteur, 1922- formation that can be extracted from even the tiniest 1939). These conclusions, among the most important samples of old rocks and minerals is increasing rapidly. of 19th-century science, require that forms of life With concerted effort, it is expected that many more developed in an unbroken pattern of descent through ancient rocks and mineral samples will be found. More time, with modifications, to produce the biological precise isotopic measurements are revealing clues to diversity we see today. And indeed, students of fossils early planetary processes. Planned spacecraft missions have painstakingly traced such a pattern backward for to the Moon and Mars will provide critical informa- more than 3 billion years to the time of our planet’s tion about the nature of planets in the Hadean. There infancy (Knoll, 2003). is even a chance that pieces of Hadean Earth rocks Before then, however, somehow and somewhere, will be found on the surface of the Moon, sent there the tree of life had to take root from nonliving precur- by impacts on Earth in the same way that pieces of the sors. Scientists have tried to identify these precursors Moon and Mars have been sent here.

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 ORIGIN AND EVOLUTION OF EARTH from both the top down and the bottom up (Penny, environments that have less hydrogen and therefore are 2005). Top-down approaches, favored by biologists, less strongly reducing (Kasting and Catling, 2003). In look at the complex molecular machinery of living cells contrast, Tian et al. (2005) have argued that less hydro- for clues about simpler antecedents on the early Earth. gen escaped to space from the early atmosphere than Bottom-up approaches, pioneered by chemists, inves- was previously assumed, which implies that while most tigate the pathways by which life’s chemical building carbon in the primitive atmosphere was in the form blocks—the raw materials for top-down research— of carbon dioxide, hydrogen gas was also available for could have formed from simple inorganic constituents organic synthesis, with energy added by lightning. Im- of early environments. These bottom-up approaches re- pacts by iron-rich meteorites might also have transient quire the input of Earth scientists because they specify enrichment in compounds such as carbon monoxide physical setting, starting materials, energy sources, and that would have facilitated the synthesis of biologically chemical catalysts. Did life originate in what Darwin interesting organic compounds (Kasting, 1990). envisaged as a “warm little pond,” perhaps a tidal pool repeatedly dried and refreshed? Or might life be rooted among hydrothermal vents? Could life’s origins even lie beyond Earth? Experimental approaches to prebiotic chemistry must be framed in terms of environments likely to have formed life’s incubators, and only Earth scientists can inform us about the physical and chemical characteristics of these settings. a search for clues in the laboratory We have understood for more than half a century that modern laboratory experiments can shed light on our search for life’s beginning. In the classic Miller-Urey experiment, Stanley Miller (1953) ran an electric spark through a mixture of water vapor, ammonia, methane, and hydrogen gas, generating a complex array of or- ganic molecules, including amino acids, the building blocks of proteins (Figure 1.15). Intermediate products in amino acid synthesis included formaldehyde (from which sugars can be synthesized) and hydrogen cyanide (the starting material for abiotic synthesis of the bases that specify information in nucleic acids). In this experiment the spark serves as a proxy for lightning in the early atmosphere. The gas mixture approximates one hypothesis for atmospheric com- position. As it turns out, the success of Miller-Urey and other experiments in prebiotic chemistry depends critically on the relative amounts of gases present in the FIGURE 1.15 Photograph of the experimental setup of the early atmosphere and oceans. The Miller-Urey mecha- famous Miller-Urey experiment. An electric spark passes through nism requires more hydrogen than carbon (Miller a chamber containing hydrogen gas, ammonia, methane, and and Schlesinger, 1984), and Miller chose his starting water vapor; as the product of the resulting chemical reaction cools, water condenses, carrying organic molecules to the flask mixture to approximate the prebiotic atmosphere as at the bottom of the apparatus, where they can be sampled and envisioned by his mentor Harold Urey. But since then analyzed. SOURCE: Bada and Lazcano (2003). Reprinted with most atmospheric scientists have adjusted the model to permission from AAAS.

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 ORIGINS The availability of gases such as hydrogen and car- The idea that metal ions, dissolved in early lakes bon monoxide in Earth’s early atmosphere is currently and oceans, might have catalyzed prebiotic chemi- the subject of vigorous debate among Earth scientists, cal reactions follows closely from our knowledge of and its outcome will determine how we think about biochemistry. Biological catalysts commonly depend environmental chemistry and the origin of life during on the action of a cofactor that contains a metal at its Earth’s early development. Whether or not amino acids functional heart. For example, a magnesium ion occu- and other organic molecules were widespread on the pies the center of the chlorophyll molecules that trap early Earth, they existed in some parts of the early Solar light energy and drive photosynthesis. Similarly, an iron System and reached our planet in the form of carbona- atom lies at the center of hemoglobin, the molecule ceous chondrites. These meteorites contain significant that transports oxygen in mammalian respiration. A abundances of biologically interesting compounds, as wide diversity of metals act as important catalysts for do some interstellar clouds. biological reactions, especially iron, manganese, mag- nesium, zinc, copper, cobalt, nickel, and iron-sulfur clusters. Understanding the roles these metals might how did life arise? have played in prebiotic chemistry is a geological ques- Earth scientists are trying to answer this question by tion whose answer depends on how the metals were dis- combining field and laboratory studies of the planet’s tributed in primitive Earth environments. To find such oldest sedimentary rocks, laboratory simulations, and answers we need integrated data about (1) early crustal geochemical theory to define the environmental condi- differentiation and magma generation (see Question tions most likely to have nourished early life. A central 2), (2) the low-temperature chemistry of weathering, question, for example, is what combination of the basic (3) hydrothermal reactions in ancient seafloors, and conditions—nitrogen and phosphate availability, elec- (4) oxidation-reduction (redox) conditions in early trochemical and acid-base qualities of the environment, environments. Once we understand these conditions, and abundances of trace metals and minerals—were experiments in prebiotic chemistry can graduate from the most life enhancing? The challenge is to identify artificial media doped with single metal ions to complex and quantify every one of these conditions to actually ionic mixtures informed by Earth science. estimate the probability of forming life under primitive The same is true for mineral surfaces, long rec- Earth conditions. Because those conditions are today ognized as potentially important catalysts of prebiotic poorly preserved or absent, geologists must adapt tools chemical reactions (Schoonen et al., 2004; Figure 1.16). of many kinds to infer how life began. Clay minerals, for example, have been shown to cata- Essential to our understanding of how life emerged lyze the assembly of lipid micelles into vesicles—tiny from prebiotic chemicals is accurate knowledge of spheroids that could have governed prebiotic-phase the kinds of catalysts present in the environment. A separation on the early Earth (Hanczyc et al., 2003). catalyst is a substance that increases the speed of a Clay minerals also catalyze the linkage of nucleotides chemical reaction, often dramatically. In every cell to form nucleic-acid-like polymers (Orgel, 2004), and the complex and coordinated chemical reactions that pentose sugars (including the biologically important support life require the action of catalysts, usually ribose) can be stabilized in the presence of calcium bo- enzymatic proteins. Many prebiotic reactions require rate minerals (Ricardo et al., 2004). A role in prebiotic catalysts as well, not only to support energy-yielding chemistry for iron sulfide minerals has been suggested reactions but also to permit the synthesis of the long- as well, most prominently in Wächtershäuser’s chemi- chain molecules such as nucleic acids and proteins that cally explicit theory of biogenesis around hydrothermal make up living systems. Some of the most essential vents (Wächtershäuser, 1988; see Hazen, 2005, for a catalysts used in experimental approaches to prebiotic discussion of recent experimental tests). Continuing chemistry are metal ions, which coordinate chemi- advances will require new experiments based on realistic cal reactions in developing metabolism, and mineral mineral catalysts, as well as constrained theory, experi- surfaces, which provide templates and catalysis in ments, and observations from Earth science (Schoonen synthesizing biopolymers. et al., 2004). In particular, we need to understand how

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0 ORIGIN AND EVOLUTION OF EARTH FIGURE 1.16 Diagram show- ing the role of minerals in prebi- otic chemical reactions. SOURCE: Schoonen et al. (2004). Reprinted with permission. chemical reactions between water and the early crust logical processes such as photosynthesis tend to enrich shaped the chemistry of early environments. organic molecules in the lighter stable form of carbon (12C) relative to its heavier forms, we can estimate when carbon began to be trapped by photosynthetic microor- When did life arise? ganisms. Carbon isotopic abundances in 3,500 million A second important question flows from the first: year old sedimentary rocks are similar to those found When did life arise on our planet? Paleontologists and in much younger deposits, suggesting that a biologi- biogeochemists have long agreed that the origin of life cal carbon cycle was established early in our planet’s preceded the deposition of minimally metamorphosed history. Indeed, highly metamorphosed rocks that are sedimentary rock deposited 3,500 to 3,400 Ma. Tiny nearly 3,800 million years old contain carbon isotopic fossils preserved in sedimentary rocks document mi- abundances suggestive of a still older carbon cycle. It crobial diversity in rocks deposited long before animals has further been proposed that the high concentra- evolved, and stromatolites—sedimentary structures tions of organic matter in some of the earliest known formed by the interaction of microbial communities shales require primary production by photosynthetic and the physical processes of sedimentation—provide organisms (Sleep and Bird, 2007). In light of these ob- independent evidence of widespread microbial life on servations, the close molecular similarity of all known the early Earth (Knoll, 2003; Figure 1.17). Because bio- species strongly suggests that all living organisms are descended from a common ancestor that lived nearly 4 billion years ago. did life originate more Than once? We cannot tell how many times life arose. Life may have originated many times on the young Earth, with the ancestor of present life persisting by good luck (chance survival of primordial mass extinctions) or good genes (outcompeting other early life forms). But experiments can help us understand whether there is more than one route to life. There is no reason these routes must all be terrestrial, and some scientists have speculated that terrestrial life was seeded from afar, most likely from Mars (Weiss et al., 2000). A mecha- nism certainly exists—several lines of evidence show FIGURE 1.17 2.76 billion year old stromatolite in Pilbara, that Earth receives a continuing stream of meteorites Australia. SOURCE: Ohmoto et al. (2005). Reprinted with ejected to space from Mars by meteor impact and that permission.

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 ORIGINS some of these meteors could have delivered microbial eat? Can it move against gravity? Paleontologists have cargo to Earth. The obvious test is to learn by explo- a more difficult task, necessarily judging biogenicity ration whether Mars was ever a biological planet. At by shape, distribution, and chemistry. No sensible per- present we do not know, but exploration of ancient son would doubt that dinosaur skulls excavated from sedimentary rocks on Mars, guided by our geological Cretaceous sandstones constitute definitive evidence of and paleobiological experiences on Earth, may provide ancient life; no known physical processes can produce an answer. From orbital observations and the in situ ex- the complexities of a skull in the absence of biology. ploration by the Mars rovers Spirit and Opportunity, we Similarly, the preservation of cholestane (the geologi- know that Mars—unlike Earth—preserves a sedimen- cally preservable form of cholesterol) in a Jurassic oil tary record of surface environments from its first 500 tells us that life existed when the oil deposit formed million years (e.g., Squyres et al., 2004). Thus, Martian because cholestane does not form abiologically. The rocks might preserve a record of prebiotic chemistry, problem gets harder when we go backward in time or even nascent life, if such records ever formed. Many beyond the first appearance of animals ca. 580 Ma. scientists have attempted to estimate the odds that life Some microfossils have complicated shapes clearly can emerge as a lucky accident, whether on a planet or related to living organisms (Figure 1.18a, b), and an elsewhere where environmental conditions are favor- unambiguous record of microfossils goes back some able. Experiments in prebiotic chemistry will nudge 2,500 million years. Older candidate fossils, however, us toward better answers, but what the question really tend to be poorly preserved and have simple shapes. requires is a second example of a living system. The tiny spheroid structure in Figure 1.18c is about In recent years, however, skeptics, stimulated in 3,500 million years old and is made of carbon. It is hard part by controversial claims about biological signatures to be sure this is a fossil because such simple structures in a Martian meteorite, have challenged the conven- might well form from physical processes. tional wisdom that terrestrial life arose on Earth prior The same uncertainties confound investigations of to 3,500 Ma. Explanations that do not involve biology larger scale features of sedimentary rocks that may have have been proposed for micron-scale carbon-bearing been imported by organisms, as well as molecular or structures previously interpreted as Earth’s oldest isotopic features of ancient organic matter that might microfossils, for stromatolites, and for carbon isotopic reflect biological processes. Stromatolites, for example, abundances in carbonate minerals and organic matter are commonly interpreted as the sedimentary products (e.g., Brasier et al., 2005, 2006). Vigorous defenses of of sediment accretion on ancient lake bottoms and sea- biological interpretations have been mounted (e.g., floors. Stromatolites formed by trapping, binding, and Schopf et al., 2002; Allwood et al., 2006; Schopf, 2006). cementing sediment particles have textures not easily At present the weight of evidence favors the hypothesis mimicked by purely physical processes, so they pro- that life existed 3,500 Ma, and likely existed back at vide reliable evidence for life in rocks more than 3,000 least 3,800 Ma, but much remains to be learned about million years old (Figure 1.19a). Other stromatolites the nature of early ecosystems. Only careful mapping form by mineral precipitation, however, especially in and stratigraphic analysis will tell us whether our planet the oldest sedimentary accumulations, and it is difficult preserves an earlier record of its biological (or prebio- to know what role, if any, life played in their accretion logical) history, and only innovative biogeochemical (Figure 1.19b). analyses set in the context of well-corroborated mi- The challenge of identifying the geological prod- crobial phylogeny will resolve uncertainties about the ucts of life becomes even more difficult when applied antiquity and nature of early microorganisms. to ancient rocks of Mars or other planets. We have no confidence that the diversity of life on Earth exhausts all possibilities for living systems. Thus, the guiding What is life—and What is Not life? question of paleo- and geobiological exploration of the In one way, at least, biologists have it easy: they can Solar System is whether a structure (molecular, micro- evaluate whether a structure is living by testing for scopic texture, or stromatolite) found during planetary evidence of metabolic activity. Does it breathe? Does it exploration can be explained adequately in terms of

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 ORIGIN AND EVOLUTION OF EARTH a b c FIGURE 1.18 (a) Fossil of a eukaryotic microorganism preserved in ca. 580 Ma phosphorite from the Doushantuo Formation, China. The fossil is 250 microns across. (b) Branching cyanobacterium preserved in ca. 800 Ma chert from the Upper Eleonore Bay Group, Greenland. The fossil is 500 microns from left to right. (c) Paired 4-micron-wide carbonaceous spheroids in ca. 3,500 Ma cherts from the Onverwacht Group, South Africa. Are these fossils? SOURCE: Courtesy of Andrew Knoll, Harvard University. a b FIGURE 1.19 (a) Stromatolite built by the trapping and building of sediment particles by microbial communities—1,500 Ma Bil’yakh Group, Siberia. SOURCE: Courtesy of Andrew Knoll, Harvard University. (b) Stromatolites built of seafloor precipitate structures that are composed of calcium carbonate crystals without any obvious templating influence of microorganisms—1,900 Ma Rocknest Forma- tion, Canada. SOURCE: Courtesy of John Grotzinger, Caltech. Used with permission.

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 ORIGINS known physical processes. Some molecular and mor- Clay minerals in some of Mars’ oldest terrains may phological structures form only by biological processes signal that early in its history our neighbor was rela- (cholesterol, dinosaur skulls), while others clearly relate tively wet but less acidic (Bibring et al., 2006). Also, to physical processes (large quartz crystals, for exam- carbonate and sulfide minerals precipitated from fluids ple), and still others exist in a zone of overlap (2-micron flowing through crustal fractures document at least spheres, amino acids). We can never eliminate the zone transient subterranean environments neither strongly of overlap, but better understanding of the products of acidic nor oxidizing (McKay et al., 1996). Only fur- both biological and physical processes will better equip ther exploration, with Earth and planetary scientists us to pursue questions of life’s antiquity on Earth and working in partnership, will establish whether life on its distribution through the Solar System. Earth is unique in our Solar System or merely uniquely successful. is There life Beyond earth? summary Our understanding of our own origins remains sketchy, but it is expanding at an accelerating pace. Thanks to While synthetic organic chemistry and molecular biol- contributions from many fields and approaches, scien- ogy will continue to provide the experimental basis for tists are better prepared to approach a truly tantalizing probing life’s origins, Earth scientists will increasingly question: Are we alone, or has life also evolved else- specify the conditions under which laboratory experi- where? If life exists elsewhere, what forms does it take? ments are run. Stratigraphers, paleontologists, biogeo- With continuing planetary exploration, Earth scientists chemists, and geochronologists can provide sharper will be able to establish with greater certainty whether constraints on when life arose and the metabolic life could have originated elsewhere in our Solar Sys- character of early organisms. Geochemists focused tem—and even whether organisms could have become on both crustal differentiation and low-temperature established on Earth by meteoritic transfer from an- reactions can build an improved sense of redox condi- other planet. Thanks to discoveries of the National tions, weathering reactions, and metal abundances on Aeronautics and Space Administration’s rover Op- the early Earth. Modelers can use new data to provide portunity, we now know that around the time life took more sophisticated hypotheses about how our planetary root on Earth, at least regional environments on Mars’ surface operated in its infancy, setting the stage for surface were episodically wet (Knoll et al., 2005). But the intercalation of biological processes into the Earth they were also oxidizing and strongly acidic—serious system. And planetary scientists, now exploring Mars obstacles to many of the prebiotic chemical pathways and other bodies at a resolution previously limited to thought to have been important on Earth. Was early Earth, can provide comparable insights about environ- Mars arid, oxidizing, and acidic globally or just region- mental (and, at least potentially, biological) evolution ally, and when were such environments established? on other planets.

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