Construction of Planet Earth

The matter produced in the Big Bang was enriched in heavier elements by cycling in and out of stars. Like biological entities, stars form, evolve, and die. In the process of their death, stars ultimately become compact objects such as white dwarfs, neutron stars, or even black holes. On their evolutionary paths to these ends, they eject matter back into space, where it is recycled and further enriched in heavy elements. New stars rise from the ashes of the old. This is why we say that each of the individual atoms in Earth and in all of its creatures—including us—has occupied the interior of at least a few different stars. Just before the sun formed, the atoms that would form Earth and the other planets existed in the form of interstellar dust and gas. Concentration of this interstellar matter formed a nebular cloud, which itself then condensed into the sun, its planets, and their moons.

Let's take a closer look at what happened. The formation process began when a mass of interstellar material became dense and cool enough to grow unstable and gravitationally collapse into itself to form a flattened, rotating cloud—the solar nebula. As the nebula evolved, it quickly assumed the form of a disk-shaped distribution of gas, dust, and rocks orbiting the proto-sun, a short-lived juvenile state of the sun when it was larger, cooler, and less massive and was still gathering mass. The planets formed from this nebula, even though the nebula itself existed for only about 10 million years before the majority of its dust and gas either formed large bodies or was ejected from the solar system.

It would be highly informative to examine similar nebulae around other young stars, but their distance from us is so great, and their size so small, that their details cannot yet be directly imaged with telescopes. Ground-based and space-borne telescopes have, however, revealed several lines of evidence suggesting that disks surround newly forming stars. Among this evidence is a peculiar and spectacular phenomenon that has only recently begun to be understood. Young stars show jets of material radiating away from them. These "bipolar nebulae" are gaseous objects resembling two giant turnips, each with its apex pointing toward the star. The jets appear to be gas ejected perpendicular to disks that apparently exist around the central star. Thus as stars form, they paradoxically also eject matter back into space. The presence of a disk in the equatorial plane of the star forces the ejected material into jets along the polar axes of the spinning system of star and disk.

In the solar nebula, 99% of the mass was gas (mostly hydrogen and helium), and the heavier elements that could exist as solids made up the remaining 1%. Some of the solids were surviving interstellar dust grains; others were formed in the nebula by condensation. This gas played a major role in forming the sun, Jupiter, and Saturn. All of the other planets, the asteroids, and the comets formed primarily from the solids. Solids were only a trace component of the nebula as a whole, but they could undergo a concentration process that gas could not. As the nebula evolved, dust, rocks, and larger solid bodies separated from the gas and became highly concentrated, forming a disk-like sheet in the mid-plane of the solar nebula, in some ways resembling the rings of Saturn.

One of the fundamental processes that led to the production of planets was accretion, the collision of solids and their sticking to one another to form larger and larger bodies. This complex process involved the formation, evolution, destruction, and growth of vast numbers of bodies ranging in size from sand grains to planets. Most of the mass of a planet was accreted from materials in its "feeding zone"—a ring section of the solar nebula disk that extended roughly halfway to the nearest neighboring planets. If viewed from above, the concentric feeding zones could be imagined as a target, with one planet forming in each radial band. The composition of solids varied with distance from the sun, so the nature of each planet was critically influenced by its feeding zone.

The accretion process was responsible for unique and very important aspects of Earth. An enigma of Earth's formation is its composition and particular location in the solar system. As we saw in Chapter 2, Earth formed within the habitable zone of the sun. A grand paradox of terrestrial planets is that if they form close enough to the star to be in its habitable zone, they typically end up with very little water and a dearth of primary life-forming elements such as nitrogen and carbon, compared to bodies that formed in the outer solar system. In other words, the planets that are in the right place, and thus have warm surfaces, contain only minor amounts of the ingredients necessary for life. The accretion process accumulated solids from the nebula, but the composition of solid dust, rocks, and planetesimals in the nebula varied with distance from the sun. At Earth's distance from the center of the solar nebula (see Figure 3.2), the temperature was too high for abundant carbon, nitrogen, or water to be bound in solid materials that could accrete to form

Figure 3.2 The plan of the solar system. The rhythmic geometry of planetary spacings results from planet formation from annular "feeding zones" The asteroid belt and the Kuiper belt of comets are regions where planet growth processes failed and original planetesimals are still preserved. The planet orbits are shown to scale, illustrating how Earth and the other terrestrial planets occupy only the tiny central portion of the solar system. (The sizes of the planets are magnified by a factor of 1000, otherwise, they could not be seen at the scale of planetary orbits.)


planetesimals and planets. Ice and carbon/nitrogen-rich solids were too volatile and had no means of efficiently forming solids in the warm inner regions of the nebula. Thus Earth has only trace amounts of these volatile components, compared to bodies that formed farther from the sun. An excellent example is the case of the carbonaceous meteorites, thought to be samples of typical asteroids formed between Mars and Jupiter. These bodies contain up to 20% water (in hydrous minerals similar to talc) and up to 4% carbon. The bulk of Earth, by comparison, is only 0.1% water and 0.05% carbon.

Had Earth formed from materials similar those in the asteroid belt, farther from the Sun, its ocean could have been hundreds of kilometers deep, and its carbon content would have been higher by many orders of magnitude. Both of these aspects would have resulted in a planet totally covered by water and with vast amounts of CO2 in its atmosphere. The resulting greenhouse heating would have produced Venus-like surface temperatures of hundreds of degrees Celsius, and the surface would have been too hot for the complex organic molecules used by living organisms to survive. Such a planet could have developed more Earth-like conditions only if cataclysmic changes had resulted in the loss to space of most of its oceans and most of its carbon dioxide, and this seems highly unlikely. With even twice as much water, Earth would have ended up as an abyssal planet entirely covered with deep blue water—a true "water world"—and very few nutrients would have been available in the energy-rich surface waters of the ocean.

If natural processes in the nebula had acted in a different way, a radically different Earth might have resulted. For example, the reason why Earth is so carbon-poor is that most of the carbon in the inner parts of the nebula was in the form of carbon monoxide gas. Like hydrogen and helium, gaseous components could not be incorporated. If a way had existed to convert gaseous carbon into solids, then enormous amounts of carbon could have been accreted, and carbon would have been the dominant Earth element. In the cosmic abundance distribution, carbon is half as abundant as oxygen and ten times as abundant as iron, magnesium, and silicon. A genuinely carbon-rich planet would be entirely different from Earth. Imagine a planet with graphite on its surface and diamond and silicon carbide in its interior. Neither of these compounds would allow volcanism or even chemical weathering. Carbon-rich planets are presumably rare, but they probably do occur in exotic planetary systems where oxygen was less abundant than carbon in the planet-forming nebula.

The arrival of the "biogenic elements" on Earth is a matter of considerable speculation, but it is likely that most of them came from the outer regions. In the coldest outer regions of the nebula, water and nitrogen and carbon compounds could condense to form solids. Presolar interstellar solids carrying the light elements were also preserved in this region. Although most of these materials stayed in the outer solar system, some would ultimately have reached Earth by scattering. When they passed near an outer planet, their orbits about the sun could have been significantly altered, sometimes sending them toward the sun, where they might collide with terrestrial planets. Such gravitational effects from encounters with planets can cause aster-oidal and cometary debris, rich in light elements, to assume earth-impacting orbits. This "cross-talk" caused some degree of mixing between different feeding zones and provided a means of bringing the building blocks of life to what might otherwise have been a lifeless planet lacking in many biogenic elements because it formed too close to the sun.

The formation of the giant outer planets is thought to have been particularly effective in scattering volatile-rich planetesimals from the outer regions of the solar system into the inner solar system, the realm of the terrestrial planets. Even today, material from the outer solar system impacts Earth. Most of the mass is in particles a quarter-millimeter in diameter that are derived from comets and asteroids. These materials carry not only carbon, nitrogen, and water but also relatively large amounts of organic material, as was first proved when extraterrestrial amino acids were discovered in the Murchison meteorite that fell in Australia in 1969. Life on Earth formed from organic compounds, and it is possible that prebiotic compounds from the outer solar system stimulated the first steps toward the origin of life on Earth. Thus the outer solar system not only provided the essential elements for life but also may have given the complex organization of the chemical processes of life a critical head start. (In the context of Rare Earth, this "seeding" would not have been unusual for a terrestrial planet. It is reasonable to expect that inner planets in all planetary systems are exposed to organic-rich "manna" from the distant comet cloud systems that invariably surround their central star.)

The scattering process that bestowed on Earth life-giving material from the outer solar system also has a dark side. We have noted that the accretion process never really ended. The rate is many orders of magnitude less than it was 4.5 billion years ago, but, as in any solar system where planets form by accretion of solids, the process still goes on. The annual influx of outer solar system material falling to Earth is 40,000 tons per year. This is mostly in the form of small particles, but larger objects occasionally hit. The small-particle flux is one 10-micron particle per square meter per day and one 100-micron particle per square meter per year. The diameter of a typical human hair is just under 100 microns. Larger objects are increasingly rare, but on the average, an outer solar system object 1 kilometer in diameter randomly impacts Earth every 300,000 years. Collision with a body this size, traveling at a speed of well over 10 kilometers per second results in a very energetic impact event. Every 100 million years, on average, a 10-kilometer object strikes Earth. Such an impact can produce a transient crater tens of kilometers deep and over 200 kilometers in diameter. It can eject enough fine debris into the air to block sunlight from the entire Earth for months. Just such an impact event killed all the dinosaurs on Earth 65 million years ago.

Early in the history of the solar system, the impact rate of very large objects was much higher, and objects struck Earth that were as big as Mars (about half the diameter of Earth). During the first 600 million years of Earth's history, there were impacts of bodies 100 kilometers in size that individually delivered enough energy to heat and sterilize Earth's surface down to depths of several kilometers. The larger impacts would have vaporized the ocean and parts of the crust. The occurrence of impacts that could cause global sterilization raises an intriguing possibility: There may have been occasions when all life on Earth was destroyed by a single impact. The intervals between devastating impacts might have been long enough for life to form and again be annihilated. If life forms easily and quickly when the conditions are right, then life might have formed and been destroyed several times before the era of sterilizing 100-kilometer and larger bodies finally ended. This effect has been called "the impact frustration of the origin of life," because life could not permanently exist on Earth until major impacts had ceased. The giant impacts essentially ended 3.9 billion years ago because most of the large impactors had been swept up by planets, ejected from the solar system, or stored in distant orbits. Over the past 3.9 billion years, impacts have continued, but not by bodies as large as 100 kilometers. Current impactors are comets and asteroids perturbed, by gravitational effects of planets, from their reservoirs in the asteroid and comet belts. The largest of these bodies can have calamitous effects (the impact of a 10-kilometer body probably caused the extinction of the dinosaurs), but they are too small to pose a threat of sterilizing the entire Earth.

The final stages of Earth's assembly process included the impact of several very large objects. In Earth's feeding zone, many celestial bodies were all struggling to grow. During accretion, a given body in a feeding zone would meet one of the following fates:

Growth by assimilation of others

Destruction by high-speed collision

Assimilation into a larger body

Ejection out of the feeding zone

The process resembled a brutal biological competition, and in the end, only a single body survived to become Earth. In the final assembly stages, however, many large bodies orbited within the feeding zone, some as large as the planet Mars. The dramatic collision of these large bodies with the young Earth played a role in determining the initial tilt values of Earth's spin axis, the length of the planet's day, the direction of its spin, and the thermal state of its interior. It is widely believed that the impact of a Mars-sized body was responsible for formation of the Moon, an oddly large satellite relative to the size of its mother planet.

The final composition of Earth had several crucial structural effects. First, enough metal was present in the early Earth to allow formation of an iron- and nickel-rich innermost region, or core, that is partially liquid. This enables Earth to maintain a magnetic field, a valuable property for a planet sustaining life. Second, there were enough radioactive metals such as uranium to make for a long period of radioactive heating of the inner regions of the planet. This endowed Earth with a long-lived inner furnace, which has made possible a long history of mountain building and plate tectonics—also necessary, we believe, to maintaining a suitable habitat for animals. Finally, the early Earth was compositionally able to produce a very thin outer crust of low-density material, a property that allows plate tectonics to operate. The thicknesses and stability of Earth's core, mantle, and crust, could have come about only through the fortuitous assemblage of the correct elemental building blocks.

There is no direct information about Earth's early history because no rocks older than 3.9 billion years have survived. We can say with confidence, however, that the period included episodes of great violence due to the effect of giant impacts. The largest high-velocity collisions would cause heating and actually resurface the planet. Cratering events of the magnitude of those that created the major basins on the Moon (the large circular regions seen without a telescope, including the eye of the "Man in the Moon") may have blown parts of the atmosphere into space. These events may have produced truly horrific environments. Impacts that vaporize large amounts of water and liberate carbon dioxide from surface rocks can lead to phenomenal greenhouse effects. After the direct heating effects due to the kinetic energy of impact have dissipated, the greenhouse gasses linger in the atmosphere and retard the escape of infrared radiation. With its main cooling process blocked, the atmosphere heats up. The greenhouse effect in the dense carbon dioxide atmosphere on modern Venus produces surface temperatures of 450°C; it has been estimated that the vast amount of gas injected into Earth's early atmosphere by giant impacts may have produced surface temperatures hot enough to melt surface rocks!

It is the heritage of terrestrial life that violent events and truly hostile environments preceded it. The violent events of these times may have determined the final abundance of water and carbon dioxide, two compounds that play crucial roles in the ability of Earth to maintain an environment where life can survive. It is interesting to speculate about what would have happened if the final abundance of these had varied. If Earth had had just a little more water, continents would not extend above sea level. Had there been more CO2, Earth would probably have remained too hot to host life, much like Venus.

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