THE EARTH FORMS
ABOUT 1 SEPTEMBER; ABOUT 668,500 METERS UP THE LINE
Some 13.8 billion years ago, a universe came into existence. Perhaps this was the product of sheer random possibility. Perhaps it was another iteration of a physical reality that has existed and will exist in perpetuity. That universe brought forth all the tendencies, forces, laws, and basic particles by which physical reality as we currently understand it came to be. Those tendencies, forces, laws, and particles first constructed stars, and then regions where stars were created en masse, galaxies. Our galaxy came into being between 11 billion and 12 billion years before the present, and grew to be an immense, slowly rotating spiral with hundreds of billions of stars. Out of that galaxy there emerged, about 4.567 billion years ago, a solar system governed by an above-average sized but still largely unremarkable star, our Sun. The nebular cloud out of which that Sun and the bodies that surrounded it were formed had been enriched by multiple supernova events, which bestowed upon it every naturally occurring element. In the process of forming the solar system containing these elements, a particular region of the nebular cloud brought together collections of “heavy” elements in significant quantities through the process of accretion. The materials that were brought together in this region became a planet heavy with iron, a small world insignificant in size, but one which settled into orbit around the new star in a zone that was drenched in that star’s energy, but not overwhelmed by it. That little world was to become the only place we know with any certainty where life came to be, for we are the inheritors of the first forms it took. We are the inhabitants of that tiny planet, the Earth, which is the stage upon which the human experience has occurred, and the place that gave rise to the consciousness that looks back on its own origins, and attempts to piece together its own story.
Current Ideas About the Formation of the Earth
The formation of planets from protoplanetary disks is an area of intense study. The consensus of scientific opinion is that the Earth and the other planets formed through the accumulation of what are known as planetesimals. Planetesimals are small bodies that gradually come together to form larger bodies, a complex process that is still being elucidated. In our solar nebula, it is now thought by many researchers that minerals containing magnesium and iron formed relatively close to the protosun, in the region we now call the Inner Solar System. It is now thought that when the Sun ignited, and the nuclear fusion process began, the ignition wiped away all the small particles and residual gas that remained in the region that was to be the domain of the inner planets. The only bodies that were unaffected were the planetesimals, which continued to collide and combine until two of them developed significant gravitational fields and dominated the region: Earth and Venus. 1
A reasonably good explanation of planetesimal formation has emerged in recent years. Observations of nascent solar systems have confirmed the essentials of nebular theory, and protoplanetary disks 1 million to 10 million years of age are now studied with regularity. The period of solar system development from 10 million to 100 million years has been facilitated by the study of debris disks, the result of the collisions of larger bodies. These larger bodies may be a system’s first planetesimals.
Astrophysicists have identified three broad phases in the development of planetesimals. At the very smallest size scales, with dust of less than one centimeter (and much of that dust microscopic in size), the processes of chemical bonding and the weak electrical attraction between molecules known as van der Waals forces act to bind them together. A series of intermediate steps (see below) advances the process of formation until objects larger than one kilometer have been formed, at which time gravity takes over as the dominant factor in planetesimal formation.2 It would appear that planetesimals must form within the first few million years of a solar system’s inception, before the great mass of hydrogen and helium in it evaporates/dissipates.3
Within a protoplanetary disk, there are many variations of gas pressure, and solids tend to drift toward the areas with maximum gas pressure.4 If particles of solid matter collide when they are moving slowly enough, if they dissipate enough energy during the collision, and if they are small enough in size when they run into each other, they stick together.5 At different phases in the process of planetesimal formation, we see phenomena that promote the growth of such particles, and other phenomena that promote the concentration of these particles (prior to processes that will once again promote growth). On the very smallest scales, with particles between 10-7 and 10-4 meter in size, the phenomenon of particle sticking, as noted above, promotes growth. When these rudimentary structures reach about 10-4 meters in size, they then become concentrated by settling into thin, vertical layers. Gravitational instability assisted by drag forces (broadly understood as friction) first helps pieces of matter grow to about a meter in size, and then the process of streaming instability, basically the mutual drag forces between solids and gases, helps concentrate these meter-sized objects. Turbulence helps these pieces clump together, which causes the process of gravitational collapse to take over, and objects of half a kilometer in size form. From that point onward, gravitationally driven collisions smash pieces together into larger and larger structures with multiple-kilometer sizes.6
It is thought for this process to gain initial momentum that grains within regions of a protoplanetary disk must exceed a density of greater than 1000 times the ambient gas density to become gravitationally bound. Further, there must be a huge dust abundance spread out widely in the disk. (The high gas pressures that can trap particles can be created by turbulence.) The first generation building blocks of planetesimals may be chondrules, melted pieces of matter found in meteorites. These chondrules were apparently flash-heated at the time of the solar system’s formation, but the manner in which this was done is still unclear. And in the overall formation of planetesimals, the respective roles of sticking and self-gravity are still being elucidated.7
The Place of the Earth in the Solar System
The Earth lies within what astronomers and astrophysicists call the solar system’s snow line. When planets were emerging from our system’s protoplanetary disk, this was the boundary between the region in which it was possible for gases to condense to liquid water and the region in which the temperatures were so low that condensation was not possible. In the region beyond the snow line, any water present took the form of ice. Some researchers have placed the snow line in the range of 1.6 to 1.8 AU, somewhere around 250 million kilometers from the Sun, or just beyond Mars’ orbit. Others postulate that it must have been somewhat farther out, more in the range of 2.7 AU, about 400 million kilometers from our star. A snow line beyond 2.7 AU is considered somewhat unlikely, given our assumptions about the rate at which accretion took place and the amount of mass estimated to have been in the minimum mass solar nebula.8 Scientists believe that the Earth’s position relative to the Sun places it in the region of the solar system most favorable to the development of advanced life forms. It is in a region where its temperatures are somewhat moderate by solar system standards, and the processes by which it formed made it large enough of a body to retain an atmosphere and areas of liquid surface water.
Possible Origin of the Moon
The early solar system was a place of tremendous chaos, characterized by frequent, violent collisions between solid bodies. The best hypothesis we have concerning the origin of the Moon is that while the Earth was still in a formative stage it was struck a glancing blow by a Mars-sized planet. Most of the huge mass of material scoured away by this collision was taken from the Earth’s mantle, naturally, and the pieces of shattered material were confined by the Earth’s gravitational field and eventually coalesced into the Moon. This hypothesis is the most widely accepted one, but there are still unresolved problems with it.9 The Moon is too small to retain any significant atmosphere, although it apparently harbors water (perhaps bestowed upon it by some of the objects that struck it), and it is thought to have been barren throughout its entire 4.5 billion year existence. As planetary satellites go, it is unusually large. The Moon, as we will see, has had and still has a significant influence on the Earth.
The Basic Composition and Structure of the Earth
In the 1970s, research based on the study of meteorites indicated that the Earth is probably around 36% iron, 29% oxygen, 14% silicon, 13% magnesium, and 2% nickel. Of the remaining 6%, calcium, aluminum, and sulphur are most prominent.10 By the late 1990s more refined methods of analysis changed the figures for the overall composition of the Earth somewhat. More current research therefore puts the Earth’s chemical composition at about 32% iron, 30% oxygen, 16% silicon, 15% magnesium, and a little less than 2% nickel.11 However, this distribution of elements is not uniform throughout the interior of the Earth, and the outer crust—the home of our diminutive species—is overwhelmingly composed of oxygen and silicon, with a significant amount of aluminum as well.12
The formation of the Earth into layers, what is known as its differentiation, was of enormous importance.13 In textbooks the stratification of the Earth’s layers sometimes looks as if the Earth’s interior is subdivided into neatly defined, perfectly spherical regions, but such representations are more symbolic than realistic. The layers of the Earth are the crust, the upper mantle, the lower mantle, the core. and the inner core. The outer and inner core together occupy about 15% of the Earth’s volume. The inner core by itself comprises less than 1%. There is, according to recent research, a distinct innermost region of the inner core, a region which makes up a negligible 0.01% of the Earth’s volume.14 In total, about 99% of the Earth’s volume is in the core and the mantle of the planet. The mantle, as a whole, comprises about 84% of the Earth’s volume. Its different regions have different chemical characteristics.15 In fact, the different chemically distinct regions in the mantle follow no neat, perfectly concentric scheme. The mantle appears to have internal boundaries at about 650 kilometers, and there are indications of mantle regions chemically-distinct from others at around 900 kilometers.16 The crust, the layer of the Earth on which we live is, basically, the residual material left over from the processes of melting and cooling that operated during the Earth’s formative period. The crust, in effect, “floats” on the upper mantle, although strictly speaking the upper mantle isn’t liquid. The thickness of the crust varies somewhat from region to region. The great majority of the continental crust in our era is between 30 and 45 kilometers in thickness, with thicknesses of greater than 50 kilometers rare. The crust below the ocean floor is much thinner, and densities of 10 kilometers are common.17
The Earth has a distinct magnetic field, which appears to be generated by its iron core. This magnetic field has shifted many times in the Earth’s history, as evidenced by changing patterns within iron-bearing rocks. Recent evidence indicates the magnetic field has shifted three hundred times just in the last 200 million years.18 It is the Earth’s magnetic field that protects our planet from the potentially destructive effects of the solar “wind”. It also holds the charged particles of the Van Allen Radiation belts in place.
Physical Conditions on the Early Earth
The earliest period of the Earth’s geological history is known as the Hadean Eon, the approximately 700 million year period between 4.5 billion and 3.8 billion years before the present. (Some researchers prefer to define the end of the Hadean as having been around 4 billion ybp.) We are hindered in our study of it by the fact that the very earliest rocks from that era have been destroyed by natural processes, and only the most fragmentary mineral traces remain. Geochemical analysis has been done on samples of crustal material and zircon grains that have survived from the Hadean. Zircons, ZrSiO4, are remarkably durable, and crystals of zircon sometimes have uranium and hafnium in them, which permits their dating by radiometric methods. The very oldest zircons that have been discovered are from the Jack Hills region of Western Australia, and have been reliably dated at 4.4 billion years before the present—only 150 million years after the formation of the solar system itself. These zircons have given geologists intriguing evidence that Earth may have had liquid water and possibly even oceans prior to 4 billion ybp, and that continents might have begun forming in this period as well. Scientists are cautious about these interpretations because such ancient zircons have only been found in one place, and it is not certain whether the conditions they seem to indicate were localized or generalized phenomena.19
In 2010, researchers presented evidence that lavas found on Baffin Island, in Canada’s far north, and in western Greenland are from a deep reservoir of mantle material that has remained hidden since 4.5 billion ybp. This material has so far been analyzed in three different ways, and the three tests taken together strongly suggest that these lavas may be the oldest parts of the ancient Earth ever discovered.20 What they can tell us about the early Hadean awaits further investigation.
It took many millions of years of planetesimal accretion for the Earth to gain its eventual size, although the largest part of the process was probably finished within 10 million years. It is now thought that the violent slamming together of large planetesimals in the latter stages of the Earth’s accretion produced an early planet that was intensely hot, seething with geothermal activity. Add to this the heat coming from radioactive sources within the new planet itself, and a picture of a molten world emerges.21 Indeed, the name Hadean conjures up images from mythology, the early Earth as a “Hades-like” environment. This picture is probably broadly correct, but our understanding of the duration of the hottest and most chaotic period is limited. It appears now that this molten, hellish phase may have lasted only a few million years, and that upon its waning the Earth’s crust began to form.
In all likelihood the Earth underwent very extensive bombardment by asteroids and comets during the Hadean Eon, with asteroids making up the largest number of projectiles. Evidence of the tremendous numbers of such objects in the early solar system can be seen in the scarred surfaces of Mercury, the Moon, and Mars. The Sun during this early period was probably much less luminous, but its output of ultraviolet rays, x-rays, and solar wind was probably much greater.22 Stars gain luminosity as they mature, and the fact that the Sun was less luminous in the days of the early Earth may have had a significant impact on the planet’s development.
Earth scientists are of the opinion that in this earliest phase of the Earth’s history there was nothing resembling tectonic activity of the kind with which we are familiar, the shifting of pieces of the Earth’s outer crust on the mantle. The processes and structures involved in tectonic activity were not yet fully in place, and they would not be for perhaps more than a billion years.23
Possible Origins of the Earth’s Bodies of Water
Although there is seemingly a vast amount of water on this planet, in fact its total mass comprises a surprisingly small percentage of the Earth’s overall mass. But it is the existence of this water that distinguishes the Earth from all other planets in the solar system, and the Earth’s water is thought to have been absolutely crucial in the origin and evolution of life on this planet. Several plausible scenarios have been presented over recent years to account for the presence of the world ocean on the crust of our planet. It has been proposed that the planetesimals that formed the Earth carried trace amounts of water themselves, and that during the process of the cooling of the magma ocean on the early Earth’s surface, as silicon-bearing minerals solidified, this water was extruded. It has been further hypothesized that accompanying this process, and of even more crucial importance in the formation of the ocean, was steam in the early Earth’s atmosphere cooling and condensing into large quantities of liquid water.24 Other researchers contend that the nebular cloud out of which our solar system was born gave the early Earth a hydrogen-rich atmosphere. In this environment, it is thought, oxides present in the magma oxidized the atmospheric hydrogen to produce liquid water.25 Still other scientists argue that while we don’t yet have a definitive explanation for the origin of the Earth’s water, the accretion of water-bearing chondrites in the period after the Earth’s core was formed has the best supporting evidence.26 The role of comets in depositing water on the early Earth has long been debated. Arguments have often centered on the proportion of 2H, deuterium, in water present in comets compared to terrestrial water. Comets have been discounted as a source of the world’s ocean because comets originating in the Kuiper Belt region or the Oort Cloud have a higher proportion of deuterium in them than Earth’s ocean water. But in 2006 scientists from the University of Hawaii presented evidence of comets originating not in the solar system’s more distant regions but in the asteroid belt between Mars and Jupiter. Such objects are known as main-belt comets (MBCs). These comets may have—may have—very well struck the primordial Earth in large numbers and deposited great quantities of water. An analysis of their isotopic composition could revive the case for comets as a major source of the Earth’s water.27
It is entirely possible that the Earth’s water originated in several different ways, perhaps a combination of outgassing (the release of gases from the interior of the Earth), extended, torrential rainfall triggered by the early Earth’s atmospheric chemistry, and water from comets, asteroids, and chondrites together. The question is still open. But however it occurred, it is reasonably certain that there was a significant amount of water on this planet by 4 billion ybp. Liquid water also probably existed in significant quantities on Venus and Mars at one time as well, but it only remains here. The Blue Planet is an anomaly in its home solar system.
Hypotheses About the Early Earth’s Atmosphere
It has been proposed that the Earth has had three different atmospheres in succession, although there is not consensus on this point. Given the extensive volcanism of the Hadean, it is quite possible that the earliest atmosphere was the product of outgassing. At one time it was speculated that the Earth’s early atmosphere may have been highly reduced (that is, one without oxidizing agents, an atmosphere dominated by methane and ammonia) but this is now considered unlikely.28 (There may have been, however, significant quantities of methane, even if not overwhelming ones.) Many scientists are convinced that Earth’s original atmosphere was simply hydrogen gas. It has been thought that this hydrogen atmosphere was quickly lost through dissipation into space, but findings published in 2005 cast doubt on this belief, presenting evidence that heavy concentrations of hydrogen may have existed for perhaps a hundred times longer than previously believed. And why was this significant? It may have helped facilitate the production of chemical compounds conducive to the formation of life:
Although early Earth's atmosphere might have been dominated by CO immediately after the heavy bombardment period, as continents formed on early Earth, the atmospheric CO concentration would decline because of weathering, and the H/C ratio would become suitable for efficient formation of prebiotic organic compounds through electric discharge. Formation of prebiotic organic compounds by electric discharge at this conservative rate in a hydrogen-rich early Earth's atmosphere would have created an ocean with a steady-state amino acid concentration ∼10 mol/liter…which is orders of magnitude greater than the amino acid concentration estimated for a hydrogen-poor early Earth's atmosphere… This amino acid concentration is highly uncertain because neither the production rate nor the destruction rate is well known. In addition, organic films may have formed at the ocean surface, leading to higher concentrations of organic compounds than in the bulk sea water.29
The contention that hydrogen escaped into space much more slowly than believed has its critics, however, and the question is still an open one. But it is an issue of potentially very great significance.
There are researchers who contend that the presence of certain isotopes of krypton and xenon in our current atmosphere may indicate they were deposited by comets or gas clouds that enveloped the early Earth. It is possible, according to these scientists, that many other gases in our atmosphere are of extraterrestrial origin as well.30 Again, given the uncertain state of our knowledge about much of the early Earth, it is possible that a number of different processes were influencing the evolution of the atmosphere and contributing distinct features to it.
The oxygenation of the atmosphere was largely the byproduct of oxygenic photosynthesis, which we will examine in a later chapter. It is not probable that there was any significant atmospheric oxygen in the Hadean environment.
The end result of the process of the Earth’s formation was a planet 12,742 kilometers in diameter (on average; since the Earth is slightly flattened at the poles, its equatorial diameter is greater than its polar diameter) and 40,075 kilometers in circumference, a world of trifling size. The planet has an axial tilt of about 23.5 degrees from perpendicular, a fact of exceeding significance for the characteristics of the Earth’s seasons. The exact cause of the Earth’s tilt, by the way, has not been determined, but it may be related to the violent collisions and bombardments the Earth went through in its early period.31 Over the last 4.5 billion years the length of the day has gradually increased because of the tidal forces exerted by the Moon (and to a lesser extent the Sun and other bodies). In its earliest times the Earth’s day was perhaps no longer than a few hours. Currently the exact length of the sidereal day is 23 hours, 56 minutes, 4.09054 seconds.32 Over historical time (for our purposes, the past 5,000 years) the day may have lengthened by about .0014 of a second per century.33 The sidereal year is 365.25636 days, or 365 days, 6 hours, 9 minutes, and 10 seconds.34 This deviation from an exact 365 day orbital time forced the solar calendar makers in the earliest civilizations to make adjustments. As the length of the year has been ascertained with greater accuracy over the centuries, more adjustments, such as the insertion of leap seconds, have been made. The Sun appears to “rise” in the east because of the counter-clockwise (as seen from the North Pole) rotation of the planet, and the exact direction of the Sun’s rise at a given latitude differs seasonally. And the rotation of the Earth is undergoing a gyroscopic “wobble” known as the precession of the axis, analogous to what happens when the uppermost part of a spinning top begins to gyrate in the shape of a circle. The Earth’s axis goes through this process every 26,000 years.35
So what can we conclude from this brief examination? I believe the most obvious point is that the Earth was—and is—an environment of tremendous dynamism, even volatility. The Earth’s energy-matter reconfigured itself again and again during the Hadean, and at the surface of the Earth, its crust, the interplay between gases erupting from the planet’s interior, the early atmosphere, the earliest bodies of water, the relatively weak but still influential Sun, and objects from outer space crashing into the planet must have produced a drama of unparalleled scope in the geological history of this tiny little terrestrial world. In its first 500 million to 700 million years, the Earth acquired its basic dimensions, its basic internal structure, a relatively stable atmosphere, the first continental landmasses, and the first true world ocean. Most significantly, it was already being differentiated into a series of local environments, areas in which conditions varied, sometimes very greatly, from other regions of the young planet. This phenomenon, while seemingly obvious, is of great importance. It meant that once life became established on this world that it faced a tremendously varied set of conditions and consequently evolved in a very wide variety of ways, as we will see.
The Earth by around 4,000,000,000 years before the present had all the necessary prerequisites for the establishment of life: a position in the solar system that allowed it to absorb great amounts of energy from its local star while not being so close to it that the planet was rendered uninhabitable, sufficient size to maintain an atmosphere, a temperate climate over much of the planet, a variety of elements that formed compounds readily, and widely available water. Most significantly, the interaction of these physical variables produced a suitable chemical environment for the development of metabolic processes and eventually a kind of molecule that could use the energy gathered from such primitive metabolic processes to aid in its own replication. These molecules included carbon atoms, which bonded readily and in a variety of ways with other atoms. It was these replicating molecules that established a sort of chemical template, one that would ensconce itself firmly on the outer regions of our tempestuous planet: if a piece of energy-matter gathered sufficient energy to maintain itself, it could establish a self-reinforcing pattern that would allow it to proliferate. It could, in other words, establish a reproductive synergy, and in replicating it became subject to the processes of natural selection, which we will examine in some detail in the next chapter.
The Earth was ready to bring forth its first life forms, physical entities constructed out of some of the handiest and most common materials the planet had to offer. Life is simply one of the “tricks” energy-matter is capable of performing, and that trick was about to be performed. This event has no doubt happened in many places in the Universe, and in many different ways. But the fact that it happened here, on this planet, meant that the rise of our kind of consciousness became somewhat more probable. This rise was not inevitable, however, not by any means. The life forms that were to evolve on this world would be shaped by its structural, chemical, climatic, and astrophysical features at every conceivable turn. The chemical evolution that swept over the planet’s surface set the stage for the organic evolution that was to produce the stunning array of viral, microbial, fungal, plant, and animal life that has spread to virtually every corner of the Earth’s outer crust. The Earth was a suitable stage for this epic, but it was also an indifferent one. It was not Mother Earth or Mother Nature or any of the other names by which humans have tried to personify it. It was simply a place in the Universe that just happened to have the right circumstances to allow self-reproducing energy-matter to exist. It did not celebrate life’s presence; it will not mourn its absence. And despite the pretensions of the upright ape-like creatures who imagine themselves to be its master, the truth is stark and simple: we live here and survive here only with the Earth’s blind, mindless “permission”—permission that can be withdrawn at any time.