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Origin and Evolution of Earth: Research Questions for a Changing Planet
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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Origin and Evolution of Earth: Research Questions for a Changing Planet
How Do Planets Form Around Stars?
We do not know how unique or unusual the Solar System is, but observations of other planetary systems are providing new ideas for how planets form and evolve. Astronomical observations of star-forming regions and young stars, together with hydrodynamic models of star formation, support the conclusion that stars—including the Sun—form by the gravitational collapse of a molecular cloud core composed of materials manufactured and reprocessed in many earlier generations of stars. Because the typical molecular cloud is rotating at the time of collapse, the developing star is surrounded by a rotating disk of gas and dust. Most disks around young stars, as viewed through telescopes, are approximately 99 percent gas and 1 percent dust, but even that
FIGURE 1.1 Hubble Space Telescope images of four protoplanetary disks around young stars in the Orion nebula, located 1,500 light-years from the Sun. The red glow in the center of each disk is a newly formed star approximately 1 million years old. The stars range in mass from 0.3 to 1.5 solar masses. Each image is of a region about 2.6 × 1011 km (400 AU) across and is a composite of three images taken in 1995 with Hubble’s Wide Field and Planetary Camera 2 (WFPC2), through narrow-band filters that admit the light of emission lines of ionized oxygen (represented by blue), hydrogen (green), and nitrogen (red). SOURCE: Mark McCaughrean, Max Planck Institute for Astronomy; C. Robert O’Dell, Rice University; and the National Aeronautics and Space Administration, <http://hubblesite.org/gallery/album/nebula_collection/pr1995045b/>.
small proportion of dust makes the disks opaque at visible wavelengths (Figures 1.1 and 1.2). Gas-giant planets, such as Jupiter and Saturn in our system, are believed to form in such circumstellar disks, but direct astronomical observations of planets forming have not yet been made.
Observations of planets around other nearby stars with masses similar to the Sun indicate that planet formation is a common outcome of star formation, but no star has yet been observed with a system of planets that looks anything like the Solar System. Over 200 extrasolar planets have been discovered by several indirect techniques (e.g., radial velocity of the host star, stellar transit, and microlensing) (Butler et al., 2006; <www.exoplanets.org>). Multiple planets are known to orbit some two dozen stars. The vast majority of these
FIGURE 1.2 Hubble Space Telescope WFPC2 image of Herbig-Haro 30, a prototype of a young (approximately 1-million-year-old) star surrounded by a thin, dark disk and emitting powerful bipolar jets of gas. The disk extends about 6 × 1010 km from left to right in the image, dividing the edge-on nebula in two. The central star is hidden from direct view, but its light reflects off the upper and lower surfaces of the flared disk to produce the pair of reddish nebulae. The gas jets, shown in green, are driven by accretion. SOURCE: Chris Burrows, Space Telescope Science Institute; John Krist, Space Telescope Science Institute; Kare Stapelfeldt, Jet Propulsion Laboratory; and colleagues; the WFPC2 Science Team; and the National Aeronautics and Space Administration, <http://hubblesite.org/gallery/album/entire_collection/pr1999005c/>.
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Origin and Evolution of Earth: Research Questions for a Changing Planet
FIGURE 1.3 Summary of known extrasolar planets sorted by distance from host star and orbital eccentricity. All of the planets in the Solar System have eccentricities of 0.2 or less. SOURCE: Courtesy of Geoffrey Marcy, University of California, Berkeley. Used with permission.
planets are thought to be gas giants on the basis of their masses and densities. Presumably, more gas giants are observed because they are large, and large planets are much easier to detect, leaving open the question of how many terrestrial planets remain hidden from Earth in distant planetary systems. A few “super-Earths,” with masses of several to 10 Earth masses, may be terrestrial planets, but no measurements of the radius or density of these objects has confirmed this. Gas-giant planets appear to be more likely with stars that have proportions of heavier elements (heavier than H, He, and Li) as high as the Sun (Fischer and Valenti, 2005), suggesting that heavy-element concentrations in the circumstellar disk influence the rate or efficiency of planet formation.
Measurements of the masses, orbital distances, and orbital eccentricities (Figure 1.3) of extrasolar planets provide clues about processes that may help determine what the final planetary system looks like. A particularly interesting class of planets, that of gas-giant planets in orbits extremely close to (less than 0.1 AU)1 their host stars—sometimes called “hot Jupiters”—are significant because models have been unable to account for why they form so close to the star (Butler et al., 2006). These hot Jupiters are thought to be telling us that large planets can drift inward toward their star as they form. Models also suggest that planets can under some circumstances drift away from the star, so the ultimate location of the planets may have little to do with where they originally formed. Extrasolar planets more than a few tenths of an AU distant from their host star often have quite eccentric orbits, which contrasts with the Solar System where all of the planets except Mercury have nearly circular orbits.
How Did the Solar System Planets Form?
The Solar System is composed of radically different types of planets. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are distinguished from the inner planets by their large size and low density. The outer planets are the primary products of the planet formation process and comprise almost all of the mass held in the planetary system. They are also the types of planet that are most easily recognized orbiting other stars. The inner planets (Mercury, Venus, Earth, and Mars) are composed mostly of rock and metal, with only minor amounts of gaseous material. There are “standard models” for the formation of both types of planets, but they have serious deficiencies and large uncertainties.
According to the standard model for outer-planet formation, the formation of giant planets starts with condensation and coalescence of rocky and icy material to form objects several times as massive as Earth. These solid bodies then attract and accumulate gas from the circumstellar disk (Pollack et al., 1996). The two largest outer planets, Jupiter and Saturn, seem to fit this model reasonably well, as they consist primarily of hydrogen and helium in roughly solar proportions, but they also include several Earth masses of heavier elements in greater than solar proportions, probably residing in a dense central core. Uranus and Neptune, however, have much lower abundances of hydrogen and helium than Jupiter and Saturn and have densities and atmospheric compositions consistent with a significant component of outer Solar System ices.
An alternative to the standard model is that the rock and ice balls are not needed to induce the formation of gas-giant planets; they can form directly from the gas and dust in the disk, which can collapse under
1
The astronomical unit, or AU, is a unit of length nearly equal to the semimajor axis of Earth’s orbit around the Sun, or about 150 million km.
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its own gravity like miniversions of the Sun (Boss, 2002). In this model the excess abundances of heavy elements in Jupiter and Saturn would have been acquired later by capture of smaller rocky and icy bodies. This model, however, does not account well for the compositions of Uranus and Neptune, which do not have very much gas. Other important questions about the outer planets are when they formed and the extent to which they may have drifted inward or outward from the Sun during and after formation. Where the outer planets were and when is important for understanding how the inner planets formed.
The primary difference between the inner and outer planets (rock versus gas and ice) is thought to reflect the temperature gradient in the solar nebula. Temperatures were relatively high (>1000 K) near the developing Sun, dropping steadily with distance. Near the Sun, mainly silicates and metal would have condensed from the gas (so-called refractory materials), whereas beyond the asteroid belt, temperatures were low enough for ices (i.e., water, methane, ammonia) containing more volatile elements to have condensed, as well as solid silicates. It was once thought that as the nebula cooled, solids formed in a simple unidirectional process of condensation. We now know that solids typically were remelted, reevaporated, and recondensed repeatedly as materials were circulated through different temperature regimes and variously affected by nebular shock waves and collisions between solid objects. Important details of the temperatures of the solar nebula, however, are still uncertain, including such significant issues as peak temperatures, how long they were maintained, and how temperature varied with distance from the Sun and from the midplane of the disk. Defining these conditions is an important part of understanding how the chemical compositions of the planets and meteorites came to be.
The standard model for the formation of the inner planets is somewhat more complicated than the model for outer-planet formation and is based largely on theory and anchored in information from meteorites and observations of disks around other stars (Chambers, 2003). The model strives to explain how a dispersed molecular cloud with a small amount of dust could evolve into solid planets with virtually no intervening gas and how the original mix of chemical elements in the molecular cloud was modified during that evolution. Significant unknowns are how long the process took, how solid materials were able to coagulate into progressively larger bodies, and how and when the residual gas was dissipated. The time for centimeter-sized solid objects to form at Earth’s distance from the Sun, according to the standard model, might have been as short as 10,000 years. These small solid objects were highly mobile, pulled Sun-ward large distances by the Sun’s gravity as a result of drag from the still-present H-He gas. Submeter-sized objects were also strongly affected by turbulence in the gas.
A particular deficiency of the standard model is its inability to describe the formation of kilometer-sized bodies from smaller fragments. The current best guess is that the dust grains aggregated slowly at first, and growth accelerated along with object size as small objects were embedded into larger ones (Weidenschilling, 1997). The aggregation behavior of objects greater than a kilometer in size is better understood: they are less affected by the presence of gas than are smaller pieces, and their subsequent evolution is governed by mutual gravitational attractions. Growth of still larger bodies, or planetesimals, from these kilometer-sized pieces should have been more rapid, especially at first. Gravitational interactions gave the largest planetesimals nearly circular and coplanar orbits—the most favorable conditions for sweeping up smaller objects. This led to runaway growth and formation of Moon- to Mars-sized planetary embryos. Growth would have slowed when the supply of small planetesimals was depleted and the embryos evolved onto inclined, elliptical orbits. Dynamical simulations based on statistical methods and specialized computer codes are finding that a number of closely spaced planetary embryos are likely to have formed about 100,000 years after planetesimals appeared in large numbers (e.g., Chambers, 2003).
The later stages of planet formation took much longer, involved progressively fewer objects, and hence are less predictable (Figure 1.4). The main phase of terrestrial planet formation probably took a few tens of millions of years (Chambers, 2004). The final stages were marked by the occasional collision and merger of planetary embryos, which continued until the orbits of the resulting planets separated sufficiently to be protected from additional major collisions.
Although there are four terrestrial planets, models suggest that the number could easily have been three
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FIGURE 1.4 Results of four representative numerical simulations of the final stage of accretion of the terrestrial planets. The segments in each pie show the fraction of material originating from the four regions of the solar nebula shown by the shades of gray, and the size of the pie is proportional 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 vary. The planets typically receive material from all four zones, with preference for the zones closest to their final orbit location. SOURCE: Chambers (2004). Copyright 2004 by Elsevier Science and Technology Journals. Reprinted with permission.
or five, and they would have been at different distances from the Sun (Figure 1.4). Tidal interactions with nebular gas may have caused early-formed inner planets to migrate inward substantially while they were forming, and several planets may have been lost into the Sun before the gas dispersed (McNeil et al., 2005). The fact that there are no rocky planets beyond Mars is likely a consequence of the presence of the giant planets, particularly Jupiter. The large mass and strong gravitational pull of Jupiter probably prevented the formation of additional rocky planets in the region now occupied by the asteroid belt by disrupting the orbits of bodies in that region before they could form a large planet. Jupiter and Saturn also sent objects from the asteroid belt either out of the Solar System or spiraling into the inner-planet region where they became parts of the planets forming there or fell into the Sun. The asteroids represent the 0.01 percent of material that survived this process.
What Do Meteorites Say About the Origin of Earth?
Earth has undergone so much geological change that we find little evidence in rocks about its origin or even its early development (Question 2). Many meteorites, on the other hand, were not affected by the high-temperature processing that occurs in planetary interiors. They are fragments of, or soil samples from, miniplanets that formed in what is now the asteroid belt just as the Solar System was starting out. Thus, they preserve significant clues about the state of the Solar System when the planets were forming (Figure 1.5). For this reason, studies of meteorites play a major role in helping us understand Earth’s origin.
One gift of meteorites is to reveal the age of the Solar System. Precise radiometric dating of high-temperature inclusions within meteorites shows that the first solid objects in our home system formed 4,567 million years ago (see Box 1.1). We also know that shortly thereafter planetesimals of rock and metal formed and developed iron-rich cores and rocky crusts (see Question 2). Some meteorites are chemically like the Sun (for elements other than H, He, Li, C, N, O, and noble gases), and some of these same meteorites contain tiny mineral grains of dust that survived from earlier generations of stars (see Box 1.2). Other meteorites are parts of small planetary bodies that experienced early volcanism and that were later broken up by collisions. Beyond these clues, meteorites fall short of providing all the information needed to understand Earth, partly because most of them formed far from the Sun (the main asteroid belt is between Mars and Jupiter), and the relationship between meteorites and planets is not fully understood. The systematic collection of well-preserved samples from Antarctica has greatly expanded the number of meteorites available for study and has yielded rarities such as meteorites from Mars and the Moon.
Beyond what they tell us about Earth, meteorites
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FIGURE 1.5 The Allende meteorite, a carbonaceous chondrite, is a mixture of CAIs (calcium-, aluminum-rich inclusions; larger irregularly shaped light-colored objects) and chondrules (round light-colored objects) in a dark-colored matrix of minerals and compounds. The CAIs and chondrules are a high-temperature component that formed and were in some cases reprocessed at temperatures above 1000°C. SOURCE: Hawaii Institute of Geophysics and Planetology. Used with permission.
provide a benchmark for understanding the composition of the Sun and even the Universe as a whole. Most of the visible mass of the Universe, and almost all stars, is composed primarily of hydrogen and helium made during the Big Bang. The rest of the elements—the “heavier” ones with more protons and neutrons in their nuclei—were produced by nucleosynthesis, or thermonuclear reactions within stars. Most nucleosynthesis happens in big stars. These massive stars last only about 10 million to 20 million years before they explode as supernovae. The new elements they make, before and during the explosion, are thrown back into space where they are later recycled into new stars. In the approximately 10 billion years between the origin of the Universe and the origin of the Solar System, hundreds of generations of massive stars have exploded, and over this long period about 1 percent (by weight) of the original H and He has been converted to heavier elements. Meteorites give us the most detailed information about the abundances of these heavier elements.
Meteorites tell us still more about the formation of the Solar System out of the nebular disk. The abundance of heavy elements in the Sun is known moderately well from spectroscopic data. The planets, however, formed from the nebular disk, so it is important to know whether the disk had the same composition 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 studies 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 most elements. The relative amounts of elements and their isotopes can be measured much more precisely on meteoritic materials than by solar spectroscopy, so chondritic meteorites play a special role in helping to understand both Earth and nucleosynthesis in our galaxy. Because chondritic elemental abundances look similar to those of the Sun, the disk likely had about the same composition as the Sun.
What Is the Chemical Composition of Earth?
The most critical question related to the formation of Earth is why the planet has its particular chemical makeup. Although we know quite a lot about this issue, a key unanswered question is the origin of Earth’s water. Earth, like other objects forming near the Sun, is thought to have formed mainly as a relatively high-temperature partial condensate from a gas of solar composition. The uncondensed gas containing water, carbon, and other volatile elements was swept away by the early solar wind or by ultraviolet radiation pressure. Much of the volatile elements that might have been incorporated into the early Earth is thought to have been lost during the intense heating of the Hadean Eon (Question 2).
It has been suggested that the giant planets can pluck materials from the asteroid belt region and throw them in toward the Sun. Objects beyond Mars would have formed in a cooler part of the solar nebula and hence would likely have contained more volatile compounds. Studies of asteroids indicate that meteorites
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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 eventually 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 occurred 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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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 Connecting 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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that have little water are derived from the inner asteroid belt (inward of 2.5 AU), while the volatile element-rich meteorites, some with as much as 20 percent water as well as complex organic compounds, come from farther than 3 AU. These objects from the asteroid belt region may have been the source of Earth’s water and carbon. There is also evidence that much later in the history of the Solar System—500 million to 600 million years after its formation—a large but unknown amount of rocky debris was flung into the inner Solar System, bringing a last barrage of large impacts and finishing off the major construction of the inner planets. However, it is unlikely that this “late heavy bombardment” added enough material to significantly affect Earth’s overall composition.
The aspect of Earth’s composition that is likely best known is the proportion of refractory elements, which form solids at the high temperatures thought to have prevailed in the inner Solar System as the terrestrial planets were forming. Included among the refractory elements are most of Earth’s major components—Si, Mg, Al, and Ca. The relative amounts of refractory elements do not vary much among different classes of the benchmark chondritic meteorites, which is generally 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. Potassium, for example, is relatively volatile, and estimates suggest that Earth has about 10 percent of what was available in the nebula. But exactly how much? The answer is critical because the isotope 40K is radioactive and provides 20 to 40 percent of the heat produced in the early Earth. This heat plays a role in powering the convection in the mantle that drives plate tectonics (Questions 4 and 5).
The chondritic model and the Solar System’s apparent ability to sort chemical elements according to their volatility have proven useful for understanding many aspects of planet formation. But our increasing ability to probe the chemical and isotopic compositions of meteorites and our planet is causing some serious rethinking of long-held models. Unanticipated compositional differences have been discovered between Earth and meteorites and between different types of meteorites. Perhaps the most striking difference is that of the isotopes of oxygen—the most abundant element on Earth (Figure 1.6). Chondritic meteorites have a peculiarly variable proportion of the isotope 16O, and almost every class of meteorites has different proportions of the three oxygen isotopes. Chondritic meteorites, long thought to be the best model for the original Earth, are not like Earth with respect to oxygen isotopes. The one class of meteorites that is like Earth in this respect—enstatite chondrites—would probably be no one’s first choice for Earth’s main building blocks because they do not match Earth for most other ele-
FIGURE 1.6 Representation of the range of values of oxygen isotope ratios on Earth, the Moon, Mars, and different classes of meteorites, including carbonaceous chondrites (CI, CK, CM, CO, CR, CV); ordinary chondrites (H, L, LL); other chondrite groups (R); primitive achondrites (Acapulcoite [Aca], Brachinite [Bra], Lodranite [Lod], Winonaite [Win], Ureilite [Ure]); Howardite, Eucrite, Diogenite [HED] achondrites; aubrite achondrites (Aub); stony-iron meteorites (Pallasites [Pal], Mesosiderite [Mes]); and iron meteorites (IAB-IIICD irons). SOURCE: <http://www4.nau.edu/meteorite/>. Used with permission.
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ments. Moreover, it has recently been reported that the isotopes of neodymium, a lanthanide element that has proven critical for understanding planetary processes (Question 4), are also present in different amounts on Earth and chondritic meteorites (Figure 1.7), as are the isotopes of hafnium and barium.
Although we have long assumed that the isotopic compositions of the elements of the Solar System were mostly homogeneous, and measurements have borne this out in large measure, improved sensitivity is now showing small but significant differences between various planetary bodies. The O isotope differences,
FIGURE 1.7 Reported differences in 142Nd isotopic abundance between Earth, achondritic meteorites (Eucrites), and chondritic meteorites. The ε142Nd value is the difference in the proportion of 142Nd expressed in units of 0.01 percent. 142Nd is the radioactive decay product of the short-lived isotope 146Sm. The differences may reflect deep sequestration of ancient crust formed in the early Earth or differences in refractory element ratios between Earth and chondritic meteorites. SOURCE: Boyet and Carlson (2005). Reprinted with permission of the American Association for the Advancement of Science (AAAS).
for example, suggest that the nebular disk was not entirely homogeneous. While this is a problem in one sense, it is also an opportunity. If we can understand how this heterogeneity arose or was preserved, and what its structure was, we can learn more about how the materials of the nebula were sorted and gathered to produce the planets.
The Nd isotope discrepancy raises a different problem that has not yet been squarely addressed. Studies of asteroids and meteorites show that the process of accretion, whereby small chunks of rock gradually coalesce 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, allowing crystals and liquid to separate. Thus, it was possible 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 example, 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 formation 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 differentiate 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.
Was the Moon Formed by a Giant Impact?
More is known about the Moon than any terrestrial planetary body other than Earth because of the rock samples collected by the U.S. and Soviet lunar missions between 1969 and 1976. The peculiarities of these lunar rocks—their great antiquity, their nearly complete lack
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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) contains about 60 percent of a lunar mass. SOURCE: Canup (2004b). Copyright Elsevier. Reprinted with permission.
of water and other volatile elements and compounds, and the chemical complementarity of the dark lunar basaltic lowlands and the bright highland rocks—led to enormous advances in theories of planet formation. Moon rocks provide one of the most persuasive pieces of evidence that Earth and the Moon have a common origin. The isotopic composition of oxygen varies dramatically within the Solar System (Figure 1.6) but is identical in Earth and the Moon. An important difference is the size of their metallic cores—one-third of the mass of Earth but only about 2 percent of the mass of the Moon. Another difference is that Earth has water, as well as other volatile species and oxidized (ferric) iron; the Moon has virtually no water and all of its iron is in the reduced (ferrous) state.
Studies of lunar rocks have helped persuade many geologists that the Moon was formed when a Mars-sized object collided with the still-forming Earth about 40 million years after the formation of the Solar System. This “giant-impact” hypothesis would explain the relatively large mass of the Moon relative to Earth, the large amount of angular momentum in the Earth-
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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 System. 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: <http://www.nasa.gov/multimedia/imagegallery/image_feature_25.html>.
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 that are only partly understood. It is also thick (30 to 80 km), more silica rich than basalt, and generally old. The average age of continental rocks is about 2,000 million years, but they range from 4,000 million years to effectively 0 million years. Like oceanic crust, continental 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 1.13 Image of Jupiter’s moon, Io, from the National Aeronautics and Space Administration’s Galileo spacecraft. Io is a volcanically active miniplanet, with young crust and no plate tectonics. SOURCE: <http://www2.jpl.nasa.gov/galileo/callisto/PIA00583.html>.
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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 disappear? 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 (Questions 4 and 5). One possibility is that the early Earth had only an oceanic-type crust and no continents. However, virtually all of the rocks preserved from the period 4,000 to 3,600 Ma are continental (3.8 Ga ophiolite in Greenland is an exception; see Furnes et al., 2007), and the only earlier materials are tiny zircon crystals that presumably also come from continental-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 than Hf isotopes; Bennett, 2003). The fact that some of the oldest rocks are water-deposited sediments (3,800-million-year-old rocks from Isua, Greenland) also indicates that there was erosion and transport of sediment, which requires land standing above sea level at that time (Figure 1.14). At the average rate that Earth has been producing continental crust over the past 2 billion years, we would expect one-fifth the mass of the present continental crust to have been produced in the Hadean. However, the total volume of rocks older than 3,600 million years is very small—about 0.0001 percent of the continents. The recent observation that every Earth sample measured is enriched in 142Nd compared to chondritic meteorites suggests very early formation of a crustal component enriched in incompatible elements (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, Earth’s original crust may lie sequestered in the deep Earth today.
The uncertainties about any aspect of Hadean crust are large. Under the conditions of the Hadean Earth, which was hotter, still being hit by meteorites in its waning stages of accretion, and bearing an unknown
FIGURE 1.14 Photograph of exposures of some of Earth’s oldest sedimentary rocks (about 3,700 million years), from the eastern Isua supracrustal belt in West Greenland. Metacherts (light gray) are interlayered with carbonate and calcsilicate metasediments (dark gray). SOURCE: Friend et al. (2007). Reprinted with kind permission of Springer Science and Business Media.
amount and distribution of water, we do not know whether oceanic crust production was similar to that on the modern Earth, whether plate tectonics was operating, and how efficiently continental crust was being formed and recycled. The end of the Hadean, perhaps coincidentally, corresponds to the time of the “late heavy bombardment” of the Moon’s surface, which produced the large lunar impact basins that were subsequently filled with basalt lava flows (Figure 1.12; Box 1.5). Earth probably experienced this bombardment as well, but it is doubtful that such intense bombardment could cause the disappearance of a large preexisting continental crust, given that low-density ancient crust is preserved on the Moon. Rather, vigorous internal convection is more likely responsible for the demise of Earth’s original crust.
Summary
The geological period called the Hadean, which extends from the time of the Moon’s formation to the time when the oldest Earth rocks were formed (~4.5 to 3.9 Ga), is critical to our understanding of planetary evolution. If we are ever to fully appreciate how our
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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 section 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.
Four models of the impact rate of the first billion years of the Moon’s life: a single cataclysm with a late heavy bombardment (LHB), multiple cataclysms 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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planet came to be the home of complex life, we must be able to fill in this enormous gap in the geological record. At present we can construct plausible, but still highly uncertain, models for the Hadean Earth, which are based on our present understanding of planet formation (Question 1), planetary interior processes and material properties (Questions 4 and 6), and climate (Question 7). These models are informed by observations of the Moon and other planets in the Solar System, by measurements made on meteorites and the oldest rocks and minerals on Earth and the Moon, and by our geological understanding of how the modern Earth works. A critical component of understanding Hadean climate is our knowledge of atmospheric processes, but despite the advanced state of models for the modern Earth atmosphere, our understanding of radically different types of planetary atmospheres is still rudimentary.
Recent studies have raised new hope of improving our understanding of the Hadean. New information continues to be gleaned from precise measurement of the isotopic and chemical compositions of ancient zircons and their mineral inclusions. Observations of the Moon, Mars, Venus, and the moons of Jupiter and Saturn have opened new windows for visualizing the early Earth and for documenting what may have been happening in the early Solar System. Comparison of meteorites with Earth rocks has led to better models of Earth’s early internal processes, including the formation of the metallic core, the implantation and loss of gaseous species from Earth’s interior, and the evolution of the crust and mantle.
The future is certain to provide additional breakthroughs. Capabilities for microanalysis of geological materials are improving, and hence the amount of information that can be extracted from even the tiniest samples of old rocks and minerals is increasing rapidly. With concerted effort, it is expected that many more ancient rocks and mineral samples will be found. More precise isotopic measurements are revealing clues to early planetary processes. Planned spacecraft missions to the Moon and Mars will provide critical information about the nature of planets in the Hadean. There is even a chance that pieces of Hadean Earth rocks will be found on the surface of the Moon, sent there by impacts on Earth in the same way that pieces of the Moon and Mars have been sent here.
QUESTION 3:
HOW DID LIFE BEGIN?
The origin of life stands as one of science’s deepest and most challenging questions. It is a historical problem that emerged during a time with little recorded history, so it must be approached mostly through theory and experiment—imaginative efforts to re-create our planet’s early conditions and establish plausible chemical routes to the emergence of life. The goal of understanding life’s beginnings has attracted scientists from geology and from many overlapping disciplines, especially subfields of organic chemistry and molecular biology. In an age of planetary exploration, the origin of life is also an astrobiological issue, currently investigated on Mars, where a sedimentary record of earliest planetary history is preserved, and potentially across the wider stretch of Universe where planets have been detected.
Some of the most fundamental mysteries about the origin of life are geological in nature: From what materials did life originate? When, where, and in what form did life first appear? At its most basic physical level, life is a chemical phenomenon, and because it arose billions of years ago, geologists are intensely interested in creating an accurate picture of the chemical building blocks available to early life.
Top-Down and Bottom-Up Approaches
In The Origin of Species, Charles Darwin (1859) hypothesized that new species arise by the modification of existing ones—that the raw material of life is life. Louis Pasteur, Darwin’s great Parisian contemporary, went a step further. Pasteur decisively refuted the doctrine of spontaneous generation, the long-held view that life can arise de novo from nonliving materials, declaring instead that life springs always from life (Pasteur, 1922-1939). These conclusions, among the most important of 19th-century science, require that forms of life developed in an unbroken pattern of descent through time, with modifications, to produce the biological diversity we see today. And indeed, students of fossils have painstakingly traced such a pattern backward for more than 3 billion years to the time of our planet’s infancy (Knoll, 2003).
Before then, however, somehow and somewhere, the tree of life had to take root from nonliving precursors. Scientists have tried to identify these precursors
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from both the top down and the bottom up (Penny, 2005). Top-down approaches, favored by biologists, look at the complex molecular machinery of living cells for clues about simpler antecedents on the early Earth. Bottom-up approaches, pioneered by chemists, investigate the pathways by which life’s chemical building blocks—the raw materials for top-down research—could have formed from simple inorganic constituents of early environments. These bottom-up approaches require the input of Earth scientists because they specify physical setting, starting materials, energy sources, and chemical catalysts. Did life originate in what Darwin 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 organic 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 composition. 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 early atmosphere and oceans. The Miller-Urey mechanism requires more hydrogen than carbon (Miller and Schlesinger, 1984), and Miller chose his starting mixture to approximate the prebiotic atmosphere as envisioned by his mentor Harold Urey. But since then most atmospheric scientists have adjusted the model to environments that have less hydrogen and therefore are less strongly reducing (Kasting and Catling, 2003). In contrast, Tian et al. (2005) have argued that less hydrogen escaped to space from the early atmosphere than was previously assumed, which implies that while most carbon in the primitive atmosphere was in the form of carbon dioxide, hydrogen gas was also available for organic synthesis, with energy added by lightning. Impacts by iron-rich meteorites might also have transient enrichment in compounds such as carbon monoxide that would have facilitated the synthesis of biologically interesting organic compounds (Kasting, 1990).
FIGURE 1.15 Photograph of the experimental setup of the famous Miller-Urey experiment. An electric spark passes through a chamber containing hydrogen gas, ammonia, methane, and water vapor; as the product of the resulting chemical reaction cools, water condenses, carrying organic molecules to the flask at the bottom of the apparatus, where they can be sampled and analyzed. SOURCE: Bada and Lazcano (2003). Reprinted with permission from AAAS.
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The availability of gases such as hydrogen and carbon monoxide in Earth’s early atmosphere is currently the subject of vigorous debate among Earth scientists, and its outcome will determine how we think about environmental chemistry and the origin of life during Earth’s early development. Whether or not amino acids and other organic molecules were widespread on the early Earth, they existed in some parts of the early Solar System and reached our planet in the form of carbonaceous chondrites. These meteorites contain significant abundances of biologically interesting compounds, as do some interstellar clouds.
How Did Life Arise?
Earth scientists are trying to answer this question by combining field and laboratory studies of the planet’s oldest sedimentary rocks, laboratory simulations, and geochemical theory to define the environmental conditions most likely to have nourished early life. A central question, for example, is what combination of the basic conditions—nitrogen and phosphate availability, electrochemical and acid-base qualities of the environment, and abundances of trace metals and minerals—were the most life enhancing? The challenge is to identify and quantify every one of these conditions to actually estimate the probability of forming life under primitive Earth conditions. Because those conditions are today poorly preserved or absent, geologists must adapt tools of many kinds to infer how life began.
Essential to our understanding of how life emerged from prebiotic chemicals is accurate knowledge of the kinds of catalysts present in the environment. A catalyst is a substance that increases the speed of a chemical reaction, often dramatically. In every cell the complex and coordinated chemical reactions that support life require the action of catalysts, usually enzymatic proteins. Many prebiotic reactions require catalysts as well, not only to support energy-yielding reactions but also to permit the synthesis of the longchain molecules such as nucleic acids and proteins that make up living systems. Some of the most essential catalysts used in experimental approaches to prebiotic chemistry are metal ions, which coordinate chemical reactions in developing metabolism, and mineral surfaces, which provide templates and catalysis in synthesizing biopolymers.
The idea that metal ions, dissolved in early lakes and oceans, might have catalyzed prebiotic chemical reactions follows closely from our knowledge of biochemistry. Biological catalysts commonly depend on the action of a cofactor that contains a metal at its functional heart. For example, a magnesium ion occupies the center of the chlorophyll molecules that trap light energy and drive photosynthesis. Similarly, an iron atom lies at the center of hemoglobin, the molecule that transports oxygen in mammalian respiration. A wide diversity of metals act as important catalysts for biological reactions, especially iron, manganese, magnesium, zinc, copper, cobalt, nickel, and iron-sulfur clusters. Understanding the roles these metals might have played in prebiotic chemistry is a geological question whose answer depends on how the metals were distributed in primitive Earth environments. To find such answers we need integrated data about (1) early crustal differentiation and magma generation (see Question 2), (2) the low-temperature chemistry of weathering, (3) hydrothermal reactions in ancient seafloors, and (4) oxidation-reduction (redox) conditions in early environments. Once we understand these conditions, experiments in prebiotic chemistry can graduate from artificial media doped with single metal ions to complex ionic mixtures informed by Earth science.
The same is true for mineral surfaces, long recognized as potentially important catalysts of prebiotic chemical reactions (Schoonen et al., 2004; Figure 1.16). Clay minerals, for example, have been shown to catalyze the assembly of lipid micelles into vesicles—tiny spheroids that could have governed prebiotic-phase separation on the early Earth (Hanczyc et al., 2003). Clay minerals also catalyze the linkage of nucleotides to form nucleic-acid-like polymers (Orgel, 2004), and pentose sugars (including the biologically important ribose) can be stabilized in the presence of calcium borate minerals (Ricardo et al., 2004). A role in prebiotic chemistry for iron sulfide minerals has been suggested as well, most prominently in Wächtershäuser’s chemically explicit theory of biogenesis around hydrothermal vents (Wächtershäuser, 1988; see Hazen, 2005, for a discussion of recent experimental tests). Continuing advances will require new experiments based on realistic mineral catalysts, as well as constrained theory, experiments, and observations from Earth science (Schoonen et al., 2004). In particular, we need to understand how
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FIGURE 1.16 Diagram showing the role of minerals in prebiotic chemical reactions. SOURCE: Schoonen et al. (2004). Reprinted with permission.
chemical reactions between water and the early crust shaped the chemistry of early environments.
When Did Life Arise?
A second important question flows from the first: When did life arise on our planet? Paleontologists and biogeochemists have long agreed that the origin of life preceded the deposition of minimally metamorphosed sedimentary rock deposited 3,500 to 3,400 Ma. Tiny fossils preserved in sedimentary rocks document microbial diversity in rocks deposited long before animals evolved, and stromatolites—sedimentary structures formed by the interaction of microbial communities and the physical processes of sedimentation—provide independent evidence of widespread microbial life on the early Earth (Knoll, 2003; Figure 1.17). Because bio
FIGURE 1.17 2.76 billion year old stromatolite in Pilbara, Australia. SOURCE: Ohmoto et al. (2005). Reprinted with permission.
logical processes such as photosynthesis tend to enrich 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 microorganisms. Carbon isotopic abundances in 3,500 million year old sedimentary rocks are similar to those found in much younger deposits, suggesting that a biological carbon cycle was established early in our planet’s history. Indeed, highly metamorphosed rocks that are nearly 3,800 million years old contain carbon isotopic abundances suggestive of a still older carbon cycle. It has further been proposed that the high concentrations of organic matter in some of the earliest known shales require primary production by photosynthetic organisms (Sleep and Bird, 2007). In light of these observations, the close molecular similarity of all known 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 mechanism certainly exists—several lines of evidence show that Earth receives a continuing stream of meteorites ejected to space from Mars by meteor impact and that
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some of these meteors could have delivered microbial cargo to Earth. The obvious test is to learn by exploration whether Mars was ever a biological planet. At present we do not know, but exploration of ancient sedimentary rocks on Mars, guided by our geological and paleobiological experiences on Earth, may provide an answer. From orbital observations and the in situ exploration by the Mars rovers Spirit and Opportunity, we know that Mars—unlike Earth—preserves a sedimentary record of surface environments from its first 500 million years (e.g., Squyres et al., 2004). Thus, Martian rocks might preserve a record of prebiotic chemistry, or even nascent life, if such records ever formed. Many scientists have attempted to estimate the odds that life can emerge as a lucky accident, whether on a planet or elsewhere where environmental conditions are favorable. Experiments in prebiotic chemistry will nudge us toward better answers, but what the question really requires is a second example of a living system.
In recent years, however, skeptics, stimulated in part by controversial claims about biological signatures in a Martian meteorite, have challenged the conventional wisdom that terrestrial life arose on Earth prior to 3,500 Ma. Explanations that do not involve biology have been proposed for micron-scale carbon-bearing structures previously interpreted as Earth’s oldest microfossils, for stromatolites, and for carbon isotopic abundances in carbonate minerals and organic matter (e.g., Brasier et al., 2005, 2006). Vigorous defenses of biological interpretations have been mounted (e.g., Schopf et al., 2002; Allwood et al., 2006; Schopf, 2006). At present the weight of evidence favors the hypothesis that life existed 3,500 Ma, and likely existed back at least 3,800 Ma, but much remains to be learned about the nature of early ecosystems. Only careful mapping and stratigraphic analysis will tell us whether our planet preserves an earlier record of its biological (or prebiological) history, and only innovative biogeochemical analyses set in the context of well-corroborated microbial phylogeny will resolve uncertainties about the antiquity and nature of early microorganisms.
What Is Life—and What Is Not Life?
In one way, at least, biologists have it easy: they can evaluate whether a structure is living by testing for evidence of metabolic activity. Does it breathe? Does it eat? Can it move against gravity? Paleontologists have a more difficult task, necessarily judging biogenicity by shape, distribution, and chemistry. No sensible person would doubt that dinosaur skulls excavated from Cretaceous sandstones constitute definitive evidence of ancient life; no known physical processes can produce the complexities of a skull in the absence of biology. Similarly, the preservation of cholestane (the geologically preservable form of cholesterol) in a Jurassic oil tells us that life existed when the oil deposit formed because cholestane does not form abiologically. The problem gets harder when we go backward in time beyond the first appearance of animals ca. 580 Ma. Some microfossils have complicated shapes clearly related to living organisms (Figure 1.18a, b), and an unambiguous record of microfossils goes back some 2,500 million years. Older candidate fossils, however, tend to be poorly preserved and have simple shapes. The tiny spheroid structure in Figure 1.18c is about 3,500 million years old and is made of carbon. It is hard to be sure this is a fossil because such simple structures might well form from physical processes.
The same uncertainties confound investigations of larger scale features of sedimentary rocks that may have been imported by organisms, as well as molecular or isotopic features of ancient organic matter that might reflect biological processes. Stromatolites, for example, are commonly interpreted as the sedimentary products of sediment accretion on ancient lake bottoms and seafloors. Stromatolites formed by trapping, binding, and cementing sediment particles have textures not easily mimicked by purely physical processes, so they provide reliable evidence for life in rocks more than 3,000 million years old (Figure 1.19a). Other stromatolites form by mineral precipitation, however, especially in the oldest sedimentary accumulations, and it is difficult to know what role, if any, life played in their accretion (Figure 1.19b).
The challenge of identifying the geological products of life becomes even more difficult when applied 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 question of paleo- and geobiological exploration of the Solar System is whether a structure (molecular, microscopic texture, or stromatolite) found during planetary exploration can be explained adequately in terms of
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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.
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 Formation, Canada. SOURCE: Courtesy of John Grotzinger, Caltech. Used with permission.
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known physical processes. Some molecular and morphological structures form only by biological processes (cholesterol, dinosaur skulls), while others clearly relate to physical processes (large quartz crystals, for example), and still others exist in a zone of overlap (2-micron spheres, amino acids). We can never eliminate the zone of overlap, but better understanding of the products of both biological and physical processes will better equip us to pursue questions of life’s antiquity on Earth and its distribution through the Solar System.
Is There Life Beyond Earth?
Our understanding of our own origins remains sketchy, but it is expanding at an accelerating pace. Thanks to contributions from many fields and approaches, scientists are better prepared to approach a truly tantalizing question: Are we alone, or has life also evolved elsewhere? If life exists elsewhere, what forms does it take? With continuing planetary exploration, Earth scientists will be able to establish with greater certainty whether life could have originated elsewhere in our Solar System—and even whether organisms could have become established on Earth by meteoritic transfer from another planet. Thanks to discoveries of the National Aeronautics and Space Administration’s rover Opportunity, we now know that around the time life took root on Earth, at least regional environments on Mars’ surface were episodically wet (Knoll et al., 2005). But they were also oxidizing and strongly acidic—serious obstacles to many of the prebiotic chemical pathways thought to have been important on Earth. Was early Mars arid, oxidizing, and acidic globally or just regionally, and when were such environments established? Clay minerals in some of Mars’ oldest terrains may signal that early in its history our neighbor was relatively wet but less acidic (Bibring et al., 2006). Also, carbonate and sulfide minerals precipitated from fluids flowing through crustal fractures document at least transient subterranean environments neither strongly acidic nor oxidizing (McKay et al., 1996). Only further exploration, with Earth and planetary scientists working in partnership, will establish whether life on Earth is unique in our Solar System or merely uniquely successful.
Summary
While synthetic organic chemistry and molecular biology will continue to provide the experimental basis for probing life’s origins, Earth scientists will increasingly specify the conditions under which laboratory experiments are run. Stratigraphers, paleontologists, biogeochemists, and geochronologists can provide sharper constraints on when life arose and the metabolic character of early organisms. Geochemists focused on both crustal differentiation and low-temperature reactions can build an improved sense of redox conditions, weathering reactions, and metal abundances on the early Earth. Modelers can use new data to provide more sophisticated hypotheses about how our planetary surface operated in its infancy, setting the stage for the intercalation of biological processes into the Earth system. And planetary scientists, now exploring Mars and other bodies at a resolution previously limited to Earth, can provide comparable insights about environmental (and, at least potentially, biological) evolution on other planets.
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