Where were you when I laid the foundation of the earth? Tell me, if you have understanding. Who determined its measurements—surely you know! Or who stretched the line upon it? On what were its bases sunk, or who laid its cornerstone when the morning stars sang together and all the heavenly beings shouted for joy? Or who shut in the sea with doors when it burst out from the womb?—when I made the clouds its garment, and thick darkness its swaddling band, and prescribed bounds for it, and set bars and doors, and said, “Thus far shall you come, and no farther, and here shall your proud waves be stopped”?
—Job 38:4–11 (NRSV)
Black-and-white rendering of a color mosaic of the Orion nebula, in the constellation of Orion, from images taken with NASA’s Hubble Space Telescope. Orion shows prominently in the early winter evenings at northern latitudes. The Orion nebula is visible to the naked eye and is located to the south (or north, in the southern hemisphere) of the three bright stars that form Orion’s belt. Embedded in this image are at least 153 glowing embryotic solar systems called protoplanetary disks. Each of these will eventually form a system of planets orbiting a central star. Object names: Orion Nebula, M42, NGC 1976. Image Type: Astronomical. Source: NASA (http://hubblesite.org/andtheHubbleHeritageTeam), C. R. O’Dell and S. K. Wong (Rice University). Material credited to Space Telescope Science Institute on this site was created, authored, and/or prepared for NASA under Contract NAS5–26555.
In the so-called priestly account of the origin of the Earth found in Genesis 1:1–10, a universe of water exists, and the formation of light initiates Earth’s creation. “In the beginning when God created the heavens and the earth, the earth was a formless void and darkness covered the face of the deep, while a wind from God swept over the face of the waters. Then God said, ‘Let there be light’; and there was light.” The water is partitioned into two parts on the second day. “And God said, ‘Let there be a dome in the midst of the waters, and let it separate the waters from the waters.’” One part of this partitioned universe of waters then is separated into sky and sea. “So God made the dome and separated the waters that were under the dome from the waters that were above the dome. And it was so. God called the dome Sky.” On the third day, land and sea are separated from the other beneath the dome of the sky. “God said, ‘Let the waters under the sky be gathered together into one place, and let the dry land appear. And it was so. God called the dry land Earth, and the waters that were gathered together he called Seas.’” This sky-domed earth and sea sat under a sky-lined bubble in the watery universe.
The creation accounts of other cultures in the Middle Eastern region have similarities with the biblical priestly account. They postulate a creation that involves a primordial ocean often represented as a goddess. In these creation stories, this ocean deity is often somehow divided into components—the land, seas, and sky of Earth. For example, in the Babylonian creation mythology Enŭma Elish, Marduk, the storm god, slays Tiamat, a primordial sea goddess and mother of the first generation of Babylonian deities. Marduk then divides her corpse to create the sea and the land.1 Nearby in time and space on the Mediterranean coast of what is now Syria, Yamm, whose name in the Ugaritic Baal legend means “the Sea,” must be defeated before cosmic order and subsequent creation can be established. In Sumerian mythology, Nammu, also a primordial sea goddess, gives birth to the heaven and earth. She later creates humankind.2 In the priestly creation in Genesis and as in the Baal and Enŭma Elish legends, a water-filled universe precedes divine creation.3
The creation account in Job differs from the watery priestly account and of the Ugaritic, Babylonian, and Sumerian “wet creation” accounts. In the Joban creation, an initial divine construction project with foundations and cornerstones produces land. Then the sea bursts forth from the womb. The infant sea is wrapped in clouds and swaddled in darkness. The oceanic part of creation is not a dividing event about the separation of primordial waters or the dissection of sea goddesses. It is a birth with God as a divine midwife to a powerful infant whose seething waves pulsate with tides and storms.4 God instructs this newborn where its boundaries are and where it should not go. Joban creation involves elements of birth and childrearing, elements that match the other procreation-centered aspects in the whirlwind questions about animals that follow in the text.
Behind all the creation accounts, biblical, scientific, or otherwise, lies an essential and difficult question: “Where did all of this come from?” The current scientific explanation is that the Earth coalesced from an eddy in a vast swirling flat disk of space dust whirling ever faster around a forming star, a star we call the Sun. This creation seems remarkable when expressed as a single statement. Indeed, it is no less fantastic than creation stories from other cultures—that the Earth was made by a water beetle bringing up the mud that then spread to form the land,5 that it was coughed up from the stomach of a solitary god named Bumba,6 or that it was constructed from the corpse of Ymir, a frost giant.7 One significant aspect of the whirling-disk-of-space-dust creation account is that this process can actually be observed taking place across the universe in different stages of completion by using modern instruments such as the Spitzer and the Hubble space telescopes. A second significant aspect is that it explains some of the regular features of our solar system. What are these patterns, and what do they tell us about the origins of the major Earth systems, the land, the seas, and the atmosphere?
PATTERNS IN THE SOLAR SYSTEM
The formation of our solar system began about 4.6 billion years ago with a collapse in a cloud of celestial dust like the ones currently forming protoplanetary disks in the Orion Nebula and shown in the illustration at the front of this chapter. There are regularities in our solar system that are consistent with such a whirling disk coalescing to form the Sun and its planets. Before discussing the processes that formed our solar system, it is useful to point out some of the planetary patterns generated by the process.8
Regularities in the Planets
The planets in the solar system are sorted according to their composition. The planets in orbits near the Sun, called the terrestrial planets (Mercury, Venus, Earth, and Mars), are relatively small, rocky, and dense. Their atmospheres represent a minor fraction of their mass. They rotate slowly and have no or few moons. The planets that are further from the Sun, the Jovian planets (Jupiter, Saturn, Uranus, and Neptune), are large and are almost all atmosphere—they are primarily made of gases. They spin rapidly and have many moons; they also all have rings.
Regularities in Orbits
Planets do not clump together; each planet is relatively isolated in space. They are also spread with regularity. Each planet from Mercury out to Saturn (skipping the asteroid belt) is roughly twice as far from the Sun as its next inward neighbor. The orbits of the planets are ellipses, but they are close to being circles, except for Mercury’s strongly eccentric orbit, which is likely caused by this innermost planet’s interaction with the nearby Sun. The orbits of the planets all lie in nearly the same plane. The orbital plane for the Earth is called the ecliptic. The plane of each planet’s orbit aligns with the others to within a few degrees (except for Mercury, at 7°); thus the planetary orbits in the solar system all lie on a flat disk.
Regularities in Rotations
The planets orbit the Sun in the same direction as the Sun’s rotation on its axis—counterclockwise, if viewed from the direction of north on Earth. Further, the direction of the rotation of most planets is the same as the Sun’s spin (counterclockwise). The exceptions are Venus, which spins in the opposite direction (retrograde), and Uranus, which has its axis tipped to lie in line with the plane of the Uranian orbit around the Sun.
Regularities in Moons
Most of the moons revolve about their planet in the same direction as their planet rotates on its axis. Some moons, such as those of Jupiter, resemble the arrangement seen in the solar system and revolve about their planet on roughly the same plane as the planet’s equator.
These regular patterns are seen in the eight planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, listed in their order from the Sun. Their regularities remind one of a set of Chinese carved ivory balls with an intricate nest of carved globes, one inside the other, connoting pattern within pattern created at the hand of a remarkable craftsman.
When Galileo Galilei observed the rotation of the moons around Jupiter, he realized that the observable patterns for the solar system suggested that the other planets did not rotate about the Earth, a position strongly held by the Catholic Church at the time. His initial glimpse of the remarkable order of the solar system eventually led Galileo to a sentence of heresy and a life of house arrest under the Inquisition in Rome on July 1633.9
Now demoted from its status as a planet to that of a dwarf planet, Pluto does not conform to the aforementioned planetary regularities. Its orbit is well off the plane of the eight planets (over 17° off) and is highly eccentric (noncircular). Unlike the other “gas giant” planets that lie beyond Mars’s orbit, Pluto is rocky. Named in 1930, Pluto was demoted from planetary status at the twenty-fifth General Assembly of the International Astronomical Union meeting in Prague in August 2006. The assembly defined a planet as a celestial body that orbits the Sun, that is not a moon, that is massive enough that its gravity has compressed it to a nearly spherical shape, and that has cleared the neighborhood around its orbit. Pluto fails in this last regard, because its gravity has not captured all of the material in its vicinity. Thus, Pluto is considered a dwarf planet.
So is Ceres, a spherical asteroid and dwarf planet found in the asteroid belt. Its gravity has not cleared the ice and rock asteroids in its orbit. The asteroid belt is a ring of primordial material from the nebula that produced our solar system. Found between Mars and Jupiter, its components seem to be composed of three broad categories of material.10 About 75 percent of the asteroids are carbon rich, 17 percent are rocklike and rich in silicates,11 and the remaining metallic asteroids appear to be rich in iron and nickel.12 Ceres was discovered by Giuseppe Piazzi of the Academy of Palermo, Sicily, on January 1, 1801.13 It is made of rock (at its core) and ice. It is about 950 kilometers in diameter. More will be learned about this dwarf planet when the NASA Dawn Discovery mission reaches it in 2015.14
There are other dwarf planets, and likely more will be discovered, particularly given our increasing observational ability. Officially, the International Astronomical Union lists three dwarf planets: Ceres (the only known round asteroid), Pluto (the now-demoted planet), and Eris (a dwarf planet far from the Sun that seems a bit larger than Pluto).15 Eris and Pluto, along with forty-two objects with a size large enough to be round (having over a 400-kilometer diameter), have been found in a region called the Kuiper belt, which is located beyond the orbit of Neptune at a distance equal to thirty to fifty times the distance from Earth to the Sun. One dwarf planet–sized object, Sedna, has been identified in the region beyond the Kuiper Belt. Mike Brown, the discoverer of Eris, reckons that the number of dwarf planets could be around two hundred for the region in the Kuiper belt but that even more, perhaps around two thousand, will be found when the region beyond the Kuiper belt is explored.16
THE EARLY FORMATION OF THE SOLAR SYSTEM AND EARTH
So what processes explain the patterns we see in the planets, the chemical composition of the primordial material in the asteroid belt, the Kuiper belt, and other regions, along with the collage of planets and dwarf planets and the cosmic menagerie of comets, larger comets called centaurs, meteorites, and asteroids? The starting point for the current explanation is a large cloud of stellar dust like the Orion nebula.
Stars have life cycles. They form, they grow, they ignite with internal thermonuclear reactions, they burn up their nuclear fuel, they collapse, and they explode. The patterns of these cycles are observable in the sense that we can piece together the typical star life-cycle sequence by looking at different stars at different stages of the process. The stellar explosions at the end of this cycle form vast clouds of space dust called nebulas, which are the starting point of the next cycle. The patterns are regular, but the details of the specific path and duration of a star’s life cycle vary with a star’s size.
The nebula that produced our Sun was a cloud of dust produced from the dying explosions of a previous generation of stars. With the stardust were gases, mostly hydrogen and helium, left from the initial creation of the universe. The solar nebula that spawned our Sun was a large rotating interstellar cloud in orbit around the center of the Milky Way galaxy. From the isotopes of elements found in meteorites, the dust cloud appears to have been relatively well mixed.17 About 4.54 billion years ago, something caused the solar nebula to collapse from its gravity. This cause appears to have left at least some “fingerprints”—notably an unusual radioactive isotope of iron (60Fe), which is produced in the fierce conditions of a supernova, a large stellar explosion.18 Material with this isotopic signature seems to have been blown from a nearby supernova into the dust cloud that produced our solar system. The shock from that stellar explosion initiated a collapse of the dust cloud and the formation of the protostar that would become the Sun.19
The supernova-triggered collapse was intensified by positive feedback—the increased mass of the developing Sun increased its gravitational pull, and the increased gravity allowed the embryonic Sun to capture more material. Eventually, the Sun captured over 99 percent of the matter in our solar system. When any spinning mass is pulled toward a center, conservation of angular momentum increases the number of rotations per unit time. An example is the increased rate of rotation of a spinning figure skater as she pulls her extended arms into her body. Conservation of angular momentum caused the material spinning into the embryonic Sun to rotate at greater and greater speeds as it was drawn into the Sun. The rapidly rotating dust cloud whirled into a flattened disk spinning about the Sun’s equator.
Eventually, the Sun gained enough mass that the internal pressure and heat from the gravitational compression of the material forming it ignited a thermonuclear reaction that generated heat from the fusion of hydrogen atoms into helium. This lit the Sun, and it became a star emitting sunlight and the stream of high-velocity particles, mostly electrons and protons, called the solar wind. The geomagnetic storms that can interfere with radios and communication satellites and even knock out power grids on Earth are consequences of solar wind. Solar wind also causes the tails of comets always to point away from the Sun. The dramatic aurora borealis and aurora australis (the northern and southern lights, respectively) originate through interactions between the Earth’s magnetic field and the solar wind.
In the solar protoplanetary disk, like the disks seen today in the Orion Nebula, specks of dust bump into one another and form larger bits of dust. The bits of stardust over time collide to form grit, then grains, then pebbles, then cobbles, boulders, hills, mountains…After tens or hundreds of thousands of years into this process, many ten-kilometer objects spin and collide in the disk forming around the developing Sun.20 Eventually, the growth of these ever-larger pieces of matter form embryonic planets with enough gravity to pull in materials at higher rates than could be obtained from random collisions alone. As these merge to form planets, their gravity increases, which “sweeps” other material in their orbits. The increasing gravity in an accreting planet whirls up a smaller version of the protoplanetary disk that surrounds the Sun (and of which the accreting planet is a part). Eddies in these smaller whirling disks then become moons, which orbit the planet. The moons’ orbits have the same spin as the planet, which has the same spin as the direction of its own orbit around the Sun. Our planet Earth formed by this process in the protoplanetary disk of our Sun.
As the Earth’s gravity pulled matter from its surroundings, its growing gravity compressed it into a sphere. The heat from this compression, along with the heat generated by the decay of radioactive isotopes and from the heat from the bombardment of meteors upon the Earth, turned the planet into a glowing molten ball. Needless to say, molten rock is hot, and many elements (for example, zinc, lead, and sodium) were vaporized by the heat. The gases in this first Earth atmosphere were whisked away by the solar wind simply because the small Earth did not have enough gravity to hold them. A similar process can be seen today, 4.5 billion years after the solar system’s formation, in the ongoing loss of Venus’s atmosphere.21 Venus has a tail observable by space probes. Venus’s tail, pushed by the solar wind, almost reaches out as far as Earth’s orbit.
When matter is heated to high temperatures, the patterns in the wavelengths of radiation that are emitted are called a spectrum. Different elements emit different spectra. Thus, the light radiation emitted from the Sun spectrographically reveals the proportions of the elements in this gigantic repository of the primordial dust that originally formed our solar system. The chemical composition of the protoplanetary disk that formed the solar system also can be estimated from chemical analysis of meteorites and by analyzing the spectra from comets. This allows an estimate of the composition of the material that originally whirled up to become the Sun and solar system.
As it consolidated into a spherical shape, the molten Earth also began to separate into different components. The heavier molten iron sank to the center of the forming planet, and the lighter melted rock floated on top of this molten iron core. This process changed the chemical makeup to what would become the crust of the Earth. Molten iron, like more familiar liquids such as water, alcohol, or gasoline, dissolves some substances but not others. The elements that are soluble in molten iron are called siderophilic (iron-loving) elements. These, including gold, cobalt, and nickel, among many others,22 dissolved into the iron of the molten Earth and ended up concentrated in the Earth’s metallic core. Other elements, such as magnesium, calcium, aluminum, and the rare earth elements, are lithophilic (rock loving)23 and remained in the rocky crust of the Earth.24
Elements determined from the spectrography of the Sun’s emitted radiation can be compared to the nonvolatile, rock-loving elements found in the oldest rocks on the Earth’s surface. Similar determinations can be made by analyzing meteorites that have fallen to Earth and from the spectra emitted from light reflected off of comets. The proportions of these lithophilic elements and the ratios of their isotopes in these indicate that Earth, the Sun, meteorites, and comets all derive from the same whirling stellar dust cloud. The missing elements that boiled away to be whisked off by the solar wind, as well as the other siderophilic elements dissolved in molten iron to be carried to the Earth’s core, do not match—just as one would expect.
The proportions of volatile, lithophilic, and siderophilic elements (and their isotopes) in the ancient rocks of the Earth’s crust provide a unique signature—a fingerprint reflecting the details of Earth’s formation processes. Some rocky meteorites, thought to have been blasted into space by a Martian collision with asteroids, have a different elemental signature for these elements than do ancient Earth rocks. In addition, the ratios of the isotopes of other elements indicate that they are made from the same stardust as Earth. An ancient rock from Mars would have a different elemental signature from ancient Earth rock, because the details of a Martian rock’s formation would reflect Mars’s planetary formation, which would yield subtle chemical differences from an Earth rock.
Earth is the first planet from the Sun to have a moon, and Earth’s moon is an unusually large one. Moon rocks brought to Earth by the U.S. Apollo astronauts, by the Soviet robotic Luna mission, and from lunar meteorites knocked from the Moon have an Earth fingerprint.25 The Moon somehow came from the Earth’s surface. How did this happen?
THE BIRTH OF THE MOON
The formation of Earth was a violent process. The “clearing its orbit of large objects” part of the definition of a planet sounds much, much gentler than the actual process, which features a violent bombardment of the forming planet by objects kilometers across hurtling in from space. As material on the protoplanetary disk coalesced, the collisions between these objects became less frequent, but the sizes of the objects became larger. About 40 million years into the formation of the solar system, the forming Earth collided with a Mars-sized planet (about half the diameter of Earth) named Theia.26 In Greek mythology, Theia was the titan who was the mother of Selene, the Moon. This is an apt name: the collision between Theia and Earth is thought to have produced the Moon.
Theia is postulated to have overtaken the Earth’s orbit from a more-or-less shared orbit. Theia hit the Earth with a glancing blow, which knocked it off kilter and gave its axis a 23.5° tilt. The collision was relatively slow in astrophysical terms but fast by normal human reckoning, around 14,400 kilometers per hour. The core of Theia would have eventually merged with the Earth’s core.27 The impact would have pulverized the surface of both planets to form a dust cloud around the Earth. Eventually this cloud would have spun up into a disk around the Earth. The Moon coalesced from this whirling dust cloud. Thus, the Moon was made of rock from the Earth’s surface. This source material was depleted of iron-loving elements, which already would have been collected into the molten iron cores of Earth and Theia before their impact.
Questions remain to be answered in this theory of the Moon’s origin. The surfaces of the Moon and of the Earth actually are too similar—computer simulations of the collision between Earth and Theia predict that the Moon mostly would be composed of material from the Theian surface material.28 At the time of writing this book, two gravity-sensing GRAIL satellites are in a tandem orbit around the Moon mapping its internal structure.29 They will map the fine-scale variations in the Moon’s gravitational field to determine its internal structure. Hopefully, this mission will reveal more about the Moon’s core and help us better understand the details of the Moon’s origin.
A BRIEF ACCOUNT OF THE ORIGINS OF MAJOR PHYSICAL SYSTEMS OF THE EARTH
The first 700 million years of the Earth’s history is called the Hadean eon, after the Greek god Hades, god of the underworld. Hades is an appropriate eponym for this period in the history of our then hellish planet. The Hadean is the interval between the agglomeration of the Earth into a great molten ball up until the end of the time of the heavy meteorite bombardments.30 If one looks at the full moon, the signature of the so-called Late Heavy Meteorite Bombardment is easily seen on the Moon’s scarred, cratered face. The geological record of this period is sparse because the crust of the planet was undergoing active change both from geological dynamics and meteorite impacts.
The Oceans and the Lands
Zircons, prism-shaped mineral crystals of zirconium silicate (ZrSiO4), are among the scant surviving relics of the Hadean. Zircons have some uranium embedded in their crystalline matrix. The radioactive decay of the isotopes of this uranium produces isotopes of lead and provides the dates of the formation of zircons far back into the past. This uranium-lead clock provides what turns out to be an estimate of the minimum age of the oceans.31 Zircons found in marine sediments from Greenland date back to 3.85 billion years ago. Even older are a few samples of zircons found in Western Australia, which appear to have been deposited in liquid water even earlier in the Hadean.32 These imply an origin of the Earth’s ocean as early as 4.4 billion years ago.33 They also indicate a continental crust existing at the same time.34 This ancient crust demonstrated plate tectonics, the moving of continents, whose collisions are associated with earthquakes, volcanoes, and the building of mountains and ocean trenches on modern Earth.35
Whence came the oceans? There has been and remains considerable scientific discussion on the source of the Earth’s water. The first issue in these debates contrasts two fundamental hypotheses regarding the accumulation of water on Earth. Was Earth’s water delivered at the initial accretion of the planet? Or has water continued to be delivered to the Earth right up to the present?36 The hypothesis of continued water delivery derives from satellite measurements, which indicates a considerable influx of water from small comets.37 This implies an ever-growing Earth ocean. If the input of water observed in the falls of “microcomets” now observed also occurred across all of Earth’s history, this accumulation adds up to about three times the mass of the water found in the current oceans.
However, that there was early formation of deep ocean water is indicated by the existence of minerals (over three billion years old) that could have only formed in water over two miles deep.38 The most accepted hypothesis is that the planetary inventory of water, which amounts to about 0.02 percent of the Earth’s mass, was available soon after the Earth formed.39 Like today’s ocean, this early ocean was salty.40
But one still needs to explain where the early ocean water might have come from. Currently there are several theories on the origin of Earth’s water. These indicate several possible sources and potential combinations of these sources. Prominent among these sources:
1. The water could have originated in the very material from which the Earth originally formed. The heat generated by the gravitational compression of primordial Earth produced a very hot, molten planet. As the heat radiated into space, the Earth cooled and an atmosphere formed from volatile chemicals cooked out of material that made up the coalescing planet. This atmosphere apparently had enough mass to provide a surface pressure large enough to allow liquid water to exist on the Earth’s surface. Recall that water boils at lower temperatures in the lower air pressures of high altitudes (and vice versa), so one needs a relatively dense atmosphere to hold water in liquid form. On the one hand, some calculations of how much water could have come from the Earth’s crust indicate that not enough water would have been produced to fill the oceans.41 On the other hand, some studies suggest that volcanoes do produce enough water and gases, indicating that volcanism could have produced both the ocean and the atmosphere. The problem is that the ratios of stable hydrogen ions in the oceans do not match those for the material thought to have whirled up originally to form the Earth, based on the ratios of hydrogen isotopes in the Sun42 and the atmospheres of the larger planets.43 It seems that the part of our solar system with isotope signatures that match the oceans should occur in planetesimals formed in the region of Jupiter and Saturn and in the region of the outer part of the asteroid belt.44 This suggests the next possible source for ocean water.
2. Comets; other objects from beyond the orbit of Neptune, now the outermost planet after the demotion of Pluto; or water-rich meteorites originating in the outer reaches of the main asteroid belt could have brought water to supply the world’s oceans. Measurements of the ratio of the two hydrogen isotopes45 found in the carbon-rich chondritic asteroids resemble the ratios found in the oceans, but this is not the case for hydrogen ratios measured in comets and trans-Neptunian objects. This indicates these asteroids as a source of the ocean’s water. We only really have measurements for a small number of comets (Hykutake, Hale-Bopp, and Halley).46 If these are indicative of all the comets past and present, then comets did not contribute much water to the seas, perhaps 10 percent of the total.47 However, if the isotope ratios of the early comets were different, then the contribution from comets could be different as well.48
The current dominant view is that the waters of the oceans originated from small chondritic asteroids from the outer reaches of the asteroid belt and were violently delivered near the end of the accretion of the Earth.49
The Atmosphere and Life
What of our atmosphere? The Earth lost its initial atmosphere to the solar winds, which stole it from the growing Earth’s weak gravity. It may have lost another atmosphere after its collision with Theia. Very significantly, Earth’s current atmosphere bears the signature of life. It is full of oxygen—an element that, without its constant replenishment by the photosynthesis of plants, would combine with other elements comprising the Earth’s surface and be lost from the atmosphere.
With hot land surfaces and boiling seas, the hellish conditions of the Hadean eon are hostile to life as we think of it. Even when the surface cooled to an average surface temperature below the boiling point of water, the occasional large asteroid impact could generate enough heat to once again boil the oceans.50 A logical question regarding the origin of life on Earth is to ascertain when the planet began to have the conditions that would allow life to survive. Life is fragile, but it is also tougher than one might first think. The current “record” for a lifeform surviving at high temperatures is above the boiling point of water at 122°C for a strain of microorganisms brought up from a hydrothermal vent found in the abyssal depths of the Indian Ocean and grown in a laboratory.51 The microorganism, Methanopyrus kandleri strain 116, is an archaeon. Archaeons, members of domain Archaea, once were considered a form of bacteria named the archaebacteria. After further research, they seem to have a long and separate evolutionary history.52 They also have a quite different biochemical makeup from bacteria.53
Archaeons are ancient microorganisms. These one-celled organisms are very simple—they do not have a cell nucleus or any other membrane-covered structures inside their cells. They are ubiquitous organisms found in a wide range of conditions54 and may represent as much as 20 percent of the Earth’s living mass.55 Significantly for the current discussion, they can live in extremely hostile environments. Some of the first archaeons described were isolated from hot springs, salt lakes, geysers, hydrothermal vents in the deep ocean, and oil wells. A new species of the genus Picrophilus has been found in sulfuric geothermal springs with a pH between 0 and 2,56 Carnobacterium pleistocenium were found to be alive after being frozen for 32,000 years in permafrost in Alaska,57 and Thermococcus alkaliphilus can grow in strong alkali conditions with a pH of 10.5.58 Archaeons could live on present-day Mars, and they also could have lived early in the Hades of the Hadean.
Archaeons may not have been the “last common ancestor” for all life on Earth. Indeed, life may not have even developed and evolved on Earth, although this is hard to prove and generally thought unlikely.59 When did the Earth cool into the habitable range for life? If one expands the range of life-supporting environments to include the wide range of conditions that can be tolerated by various species of archaea, it could have been quite early in the Earth’s history but also quite briefly.60 Another complexity involves the degree to which the young Earth cooled. Based on astronomical observations of younger stars otherwise like the Sun, the early Sun should have been about 25 to 30 percent less bright than today. The eventual cooling of the Earth to balance the heat input from this dimmer young Sun should be at a temperature below freezing. The early Earth may have been warmed by the greenhouse effects of water vapor, carbon dioxide, and methane in its atmosphere or by a change in the amount of solar radiation reflected by the Earth into space. Or it may have frozen in the dim sunlight. Debate remains.
Early fossil evidence of mats of microorganisms referred to as stromolites, as well as of microscopic fossils of living organisms, date from 3.496 billion years ago.61 Life could have developed and evolved in a hyperthermophile Eden with high-heat-loving organisms developing during a brief period, perhaps a million years in hot ocean water following an asteroid impact, or in a hydrothermal zone of the ocean that was hot but deep enough to be shielded from the effects of asteroid sea-boiling and the associated planetary steambath.62 Or perhaps initial life was not heat loving at all. Perhaps life diversified into all sorts of environments but then was wiped out by an asteroid impact—except for the forms that had evolved to tolerate hot conditions.63 This “Noah hypothesis” could produce multiple locations where life survived—perhaps bacteria evolving in one location and archaeons in another to found two lineages of descendants.64 The sea-boiling collisions with asteroids could have selected life for tolerance of high heat (the Noah hypothesis) as could the hot-but-cooling Earth associated with the formation of liquid water (the Eden hypothesis). A third alternative is that life evolved on Mars, given its (at the time) slightly different and more beneficent conditions than those on Earth.65 Bits of material knocked off from Mars by asteroid impacts could then seed the Earth with life. Of course, life has not been found on Mars, but should it be found its nature could reveal much about the development of life.
The U.S. National Aeronautics and Space Administration (NASA) planned missions to land unmanned probes on Mars starting in 1966 under the name Voyager Mars Program. The program was cancelled in 1968 but rose from its ashes as the highly successful Viking missions, which orbited Mars with satellite sensors and landed robotic rovers on the surface. The Viking mission had the objectives of mapping the Martian surface in high resolution, analyzing Mars’s atmosphere, and searching for signs of past or present life.66 From speculations as early as 1958, details of a mission to Mars and the question of how one might detect “life” in the broadest sense captured the imagination of researchers from a variety of fields. How would you detect life on another planet? James Lovelock, an inventor, sensor designer, and systems thinker, directed: “Search for the presence of compounds in the planet’s atmosphere, which are incompatible on a long-term basis. For example, oxygen and hydrocarbons co-exist in Earth’s atmosphere.”67 This insight led Lovelock to a global view that the biogeochemical cycles of materials at the planetary scale produce a holistic feedback system analogous to the physiological feedbacks in an organism: the Gaia hypothesis.68
Indeed, our air carries the signature of life.69 It is 78 percent nitrogen, with a concentration influenced by bacteria,70 archaeons, and more and more by human activities. It is 21 percent oxygen, the product of plant photosynthesis. Carbon dioxide, used by green plants in photosynthesis, is rare, composing only around 0.039 percent of the atmosphere. Our air has a trace amount of methane, generated by microbes. Mars and Venus, our nearest neighbors, have atmospheres that are 96.5 percent carbon dioxide in the dense atmosphere of Venus and 95 percent carbon dioxide in the thin atmosphere of Mars. Nitrogen is rare in the atmospheres of both planets (3.5 percent of the Venusian atmosphere, 2.7 percent of Mars’s). Mars has 0.13 percent oxygen and a whiff of methane that does not demonstrate life’s presence there but is tantalizing, nonetheless.
The signature of life has probably been writ upon Earth’s atmosphere for a long time.71 The Hadean eon ended about 3.8 billion years ago, when the Archean eon (sometimes called the Archeozooic) began. Understanding the proportions of gases in the early atmosphere requires a complicated observational, experimental, and theoretical synthesis. Significant in these determinations is reconciling the apparent warmth of the Earth despite the dim Sun.72 The early Earth’s atmosphere until well toward the end of the Archean was high in carbon dioxide but also methane, the signature of the metabolic wastes of anaerobic archaeons and bacteria.73 The oxygen-producing photosynthesis process removes carbon dioxide from the atmosphere and replaces it with oxygen. The atmosphere also loses some of its methane from its reaction with the increased atmospheric oxygen. This removes greenhouse gases from the atmosphere and possibly induces glaciation, even a frozen tropics, on a “snowball Earth.” The increase in atmospheric oxygen also promotes the formation of ozone in the upper atmosphere, which shields living cells on the Earth’s surface from ultraviolet sunlight—an additional feedback system. There may have been a considerable length of time during the Archean when Earth flipped between being a snowball and a warm planet, the latter stage perhaps triggered by volcanoes and other geological events.74
At the end of the Archean, around 2.45 billion years ago, the forces for oxygen scored a major victory, captured the control of the makeup of the atmosphere for photosynthetic organisms, and likely threw the planet into an ice age.75 This Earth has retained its oxygenated atmosphere despite even the occasional asteroid impact, the movement of the continents, and volcanic eruptions. If Earth has moved into a relatively stable condition with respect to oxygen in its atmosphere, even given the regular increase in the heat influx from a gradually warming Sun, the climate continues to vary in ways that strongly affect life when there are changes in the land, atmosphere, and oceans.
The interaction of these large systems will be taken up in chapters 5 and 7, which discuss the interactions of land, air, and ocean. It is important to stress here that these examples are but a glimmer of the complexity of interactions that occur on different time scales from decades, to centuries, to millennia, to millions and multiple millions of years—a marvelously complex system that comprises the clockwork of our planet’s functioning.
The creation account in the whirlwind speech is one of seven biblical accounts of creation.76 Each of these reflects a different aspect of the creation. Prior to the Babylonian captivity, YHWH, the God of Israel, was an example of monolatry, the worship of a single god but without the claim that it is the only god.77 One sees this from Deuteronomy 32:7–9 (NRSV): “Remember the days of old, consider the years long past; ask your father, and he will inform you; your elders, and they will tell you. When the Most High apportioned the nations, when he divided humankind, he fixed the boundaries of the peoples according to the number of the gods; The Lord’s own portion was his people, Jacob his allotted share.” By the end of the Babylonian captivity, YHWH took on the images, authority, and attributes of other deities from the region. In absorbing the qualities of these other gods, Israel’s God also displaced them. The former belief system of monolatry became a monotheistic system.78 One of the significant aspects of this transition is the role of God as creator.79 The theological importance of God as a creator is clearly evident from the first questions of the whirlwind speech, “Where were you when I laid the foundation of the earth? Tell me, if you have understanding” (Job 38:4, NRSV). At the same time, understanding creation and the Earth processes that follow is central to scientific inquiry. This is all the more so today, when the actions of humans appear as agents of planetary change. Both science and religion need to understand the process of cosmic creation. Hopefully, each can learn from the other.
In chapter 1, a point was made that belief is a weaker basis for argument in science. Our solar system’s cosmic origins, when separated from the direct evidence for its plausibility, would seem as hard to imagine at face value as other alternative accounts from a diversity of other religious traditions. The scientific narrative of the origins of our solar system is a ripping yarn: supernova detonations, colossal cosmic vortices, thermonuclear star ignition under the crush of gravity, and interplanetary collision. It is a tale with a rich filigree of details: rare isotopes of elements that can only be cooked in an exploding star’s death, symmetries in planetary orbits, atmospheres forming and boiling away, celestial organization arising from seeming chaos. It is still a tale not completely told, and there is much more to be learned. If humans are the focus of this particular story, they show up very late in the last paragraph of the last chapter of a very long epic, an epic that beggars belief.
Certainly, we live in an exceedingly exciting time for astronomers, cosmologists, and astrophysicists. New technologies (satellites, space missions, and the capability to measure the isotopes of elements) meld with innovative theorists and clever interpreters of observational data. This synthesis produces a heady mixture of creativity, argument, and discovery. There are riddles to be solved and questions to be asked. And this is essentially what science is about.
