Salem Press

Solar System
Earth's Origin

Categories: Earth; Planets and Planetology

The Earth's early formation, its subsequent internal differentiation, its active plate tectonics, and its external weathering have left little substantive evidence of its origin intact for direct study. Much about the materials and formative processes involved in the planet's origin can be deduced, however, from seismology, geomagnetics, and by studying meteorites and comets.

Overview
In order to understand the origins of the Earth, it is necessary to be aware of the sources of the materials from which it is made. The matter from which the Earth and the entire universe is made was created in the big bang, about 13 to 14 billion years ago. The processes that occurred in the first few minutes after the big bang produced all the hydrogen and most of the helium in the universe today; trace amounts of lithium and beryllium also were formed.

Later, after stars formed, they produced all the other chemical elements through nuclear fusion reactions, also called nucleosynthesis. For most of their lives, stars generate the energy to shine by fusing lighter atomic nuclei together to make heavier atomic nuclei. This requires high temperatures and densities, the conditions that exist in the interiors of stars. The first step is the fusion of four hydrogen nuclei into one helium nucleus. The next step is the fusion of three helium nuclei into one carbon nucleus, maybe adding a fourth helium to produce oxygen. This is the end of energy generation and nucleosynthesis for a Sun-like star before it puffs off its outer layers as a planetary nebula, and the exposed core cools and fades to end its life as a white dwarf and ultimately as a black dwarf. (Note that planetary nebulae have nothing to do with planets or the formation of planets. The name dates back to the 1700's, when, viewed through telescopes of that time, they looked fuzzy, like nebulae, and round, like planets).

More massive stars have a more spectacular a demise. After carbon and oxygen have formed, further fusion reactions continue to form heavier elements up to iron. The production of elements even heavier than iron does not generate energy but requires the input of energy. The iron core collapses, and the outer layers collapse on top of it and then rebound, tearing the star apart in a supernova explosion. The tremendous energy released in a supernova permits the formation of the rest of the chemical elements. The chemical elements produced during the star's life and death are dispersed by the supernova explosion into interstellar space, there to enrich clouds of gas called nebulae (containing mostly hydrogen and some helium) in the heavier chemical elements.

The Sun, planets, and other bodies of the solar system formed as the result of the gravitational contraction of part of such a nebula about 4.5 to 4.6 billion years ago. The portion that would become the solar system, called the solar nebula, initially was perhaps about a light-year (about 9.5 trillion kilometers) across and was composed of about 74 percent hydrogen, about 24 percent helium, and about 2 percent all the other chemical elements. The exact mechanism responsible for the initiation of the solar nebula's contraction are still speculative. It may have involved the compression of the nebula as it passed through a spiral arm density wave as the nebula orbited the center of our galaxy, the Milky Way. It may have been triggered by a shock wave propagating through the nebula when a nearby massive star went supernova. It is generally agreed that once started, gravitational effects within the solar nebula kept the process going.

Any initial slow rotation of the solar nebula increased its speed with the contraction of the nebula to conserve angular momentum. (The same effect is seen on spinning figure skaters, whose rotation speed increases as they bring their arms close to their bodies.) The increase in rotation speed caused the nebula first to become oblate and eventually to form a flattened equatorial disk. Most of the solar nebula's mass concentrated at the center of the disk, forming the proto-Sun, which grew hotter by gravitational contraction.

As the proto-Sun formed at the center, fractional condensation began—a process in which gaseous matter solidifies into small, sand-sized grains only in regions where the ambient temperature is below the material's melting point. Only metallic grains of iron and nickel condensed close to the proto-Sun. Farther out, where the temperature was lower, they were joined by grains of silicate minerals. Still farther out, various ices of water, carbon dioxide, methane, and ammonia could condense. These solid grains collided with one another and stuck together in a process called accretion, forming planetesimals that grew in size. Within a time span of a few tens of millions to perhaps one hundred million years, the largest planetesimals grew into protoplanets, while the smaller ones became the many satellites and other minor members of the solar system.

As the protoplanets continued to grow, their gravitational influence grew as well. They could attract greater amounts of disk material, thus accelerating their growth while at the same time sweeping the surrounding interplanetary space clean. Solar radiation could then penetrate the space between the Sun and the planets, bringing light and heat to their still-evolving surfaces. It was during this time that the planets started to evolve in different ways. The third planet from the Sun, Earth, and its inner solar system neighbors (Mercury, Venus, and Mars) had relatively weak gravitational fields, which, coupled with their now high surface temperatures and exposure to the solar wind, caused them to lose significant amounts of the lighter gases. This first atmosphere, probably consisting of hydrogen, helium, methane, ammonia, carbon dioxide, and water vapor (gases common in the solar nebula), escaped from the inner planets and was blown away into the outer solar system. The outer planets, Jupiter and beyond—because of their colder temperatures, their greater masses, and consequently the lessened influence of the solar wind—were not so affected. As a result, they became the low-density gas/liquid/ice "giant" planets with small, rocky/metallic cores.

As the accretion process drew to a close, the Earth (along with the other inner planets) was subjected to a final intense bombardment of impacting planetesimals. As each colliding object struck the Earth's surface, its energy of motion was converted into heat energy. Furthermore, radioactivity levels were much higher in the very early Earth, since many of the radioactive elements with shorter half-lives had not yet decayed. As the Earth grew larger in size, it tended to insulate itself, making it more difficult for the energy released by radioactive decay in its interior to reach its surface and escape. All of these effects served to increase the early Earth's temperature to the point that it at least partially melted, allowing chemical differentiation and the development of the Earth's layered internal structure. Molten blobs of heavy metals like iron and nickel sank to the center, forming the iron-rich core. Less dense silicate and oxide minerals remained behind, forming the mantle. The surface also melted, forming a magma ocean perhaps a few hundred kilometers deep. Eventually the surface cooled and hardened into a thin, primitive basaltic crust, probably similar to present-day ocean-floor crust.

At the same time, outgassing released gases trapped in the interior at a prodigious pace, producing the Earth's second atmosphere, probably consisting mostly of water vapor, carbon dioxide, and sulfur dioxide, with smaller amounts of nitrogen, hydrogen sulfide, and other gases, but no free oxygen. (Outgassing continues today through volcanoes, fissures, and fumaroles, but at a much reduced rate.)

As the Earth cooled, water vapor in the atmosphere condensed, formed clouds, and fell as rain, forming the first streams and oceans. Due to the abundant carbon dioxide along with sulfur dioxide and hydrogen sulfide in the atmosphere, this early rain was highly acidic, resulting in rapid chemical weathering of surface rocks. The weathering products were carried by streams into the oceans, rapidly increasing their salinity. By about 4 billion years ago, the oceans had reached nearly their present volume and degree of saltiness. Large amounts of carbon dioxide from the atmosphere dissolved in the oceans, combined with other dissolved materials, and precipitated out as sediment (mostly as the mineral calcite, as calcium carbonate). As other gases were removed from the atmosphere, nitrogen remained, eventually becoming the major atmospheric constituent.

Probably by about 3.8 billion years ago, the first life appeared. Organic molecules, including amino acids, may have formed in the Earth's early atmosphere and oceans with solar ultraviolet light, lightning, or deep-sea hydrothermal vents providing the needed energy input, or they may have been delivered to the Earth's surface by impacts of comets, asteroids, and meteoroids. The earliest living organisms were anaerobic (able to survive without oxygen), since there was no free oxygen in the atmosphere and oceans. Then cyanobacteria and possibly other early organisms developed photosynthesis, using sunlight to turn water and carbon dioxide into sugars for food. This reaction released free oxygen into the oceans and atmosphere, and life evolved to utilize it to extract energy from food.

Thus the Earth was transformed from its formative stages to what it is today, a place where life thrives, where rocks are formed and weathered on the surface, and where a dynamic interior drives tectonic processes.

Methods of Study
Much of what is known about the origins of the Earth is derived by studying meteorites and comets as well as from seismology and geomagnetics. Meteorites and comets are unaltered or little-altered examples of early solar-system materials, providing information on the composition of and processes that occurred in the early solar nebula and the various types of bodies that formed from it. Seismology and geomagnetics give researchers clues about the internal structure of the planet.

Meteorites are extraterrestrial pieces of rock or metal that survived their fall through the Earth's atmosphere. The combined total composition of all meteorites is probably representative of the rocky and metallic material from which the inner planets--Mercury, Venus, Earth, and Mars--formed. Comets provide evidence of the more volatile components of the early solar system. They are composed of various "dirty" ices, indicative of materials blown out from the inner solar system by the solar wind but not before a portion was incorporated into the accreting planetesimals.

Data obtained from seismology (the study of the transmission of earthquake shock waves through the Earth) led to the discovery that the Earth's interior is divided into several distinct layers or zones. Observations of a change in speed of seismic waves near the Earth's surface led to the discovery of the Mohorovicic (Moho) discontinuity, the boundary between the crust and mantle. A low seismic velocity zone is now recognized in the upper mantle below the Moho and is used to define the lower boundary of the Earth's rigid lithosphere and the top of the "plastic," deformable asthenosphere. Another seismic discontinuity 2,900 kilometers below the surface delineates where the solid mantle is separated from the molten outer core. Later seismic work revealed the existence of a solid inner core.

Studies of the magnetic field of the Earth also give support to the zonal nature of the planet's interior. Hypotheses concerning the generation of the magnetic field within the Earth assume an iron-nickel-rich core (not unlike the iron-nickel meteorites) with a solid interior surrounded by a molten outer part. This combination could produce an electric dynamo that could sustain a magnetic field.

All these disciplines provide evidence of the Earth's formation in the early solar nebula. As more data are obtained, the picture of the Earth's origin becomes clearer and more refined.

Context
An understanding of the Earth's origin has many practical benefits. For example, the genesis of ore bodies is invaluable to prospecting for new resources. By knowing the products of various processes in the past, one is better able to predict human impact on present environments. Planetary engineering can use such information for the modification or preservation of conditions on the Earth, and maybe someday on other planets such as Mars. Meteorite size and shape studies were employed by space engineers in designing reentry vehicles and in studying their aerodynamic properties. Theories about material behavior in zero-gravity conditions similar to those in the solar nebula have led to experiments on manufacturing techniques in Earth orbit that are impossible to conduct on the Earth's surface.

Bruce D. Dod

Further Reading
Beatty, J. Kelly, and Brian O'Leary. The New Solar System. 2d ed. New York: Cambridge University Press, 1981. A general overview of the solar system and its components, this book has been organized around comparative planetology. A discussion of the various aspects of the genesis of the planets is scattered throughout. Draws heavily on the results of space exploration and on the interdisciplinary use of science to illustrate many concepts. Contains abundant illustrations, such as full-color photographs, artwork, graphs, and charts.

Brush, Stephen G. Nebulous Earth: The Origin of the Solar System and the Core of the Earth from Laplace to Jeffreys. New York: Cambridge University Press, 1996. An in-depth reference work detailing twentieth century theories on solar system formation. Also discusses lunar origin theories.

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. New York: Addison-Wesley, 2008. A well-written college-level textbook for introductory astronomy courses. Offers an entire chapter on the formation of planets.

Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages Through the Universe, 3d ed. New York: Brooks/Cole, 2006. A well-written, thorough college textbook for introductory astronomy courses. Several sections deal with the origin of the solar system.

Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed. New York: W. H. Freeman, 2007. College-level introductory astronomy textbook, thorough and well written. Part of one chapter deals with the origin of the solar system, and part of another specifically addresses the development of the Earth.

Hartmann, William K. Moons and Planets, 5th ed. New York: Brooks/Cole, 2005. A college textbook beyond the introductory level, with an approach based on comparative planetology. Offers much information on the origin of the solar system in general and the Earth in particular.

Horton, E., and John H. Jones, eds. Origin of the Earth. New York: Oxford University Press, 1990. A collection of articles written by experts in the field. Focuses on the study of the Earth and origins of the Earth-Moon system.

Hutchison, Robert. The Search for Our Beginning. New York: Oxford University Press, 1983. The author addresses the problem of determining the processes involved in the formation of the Earth and other solar system bodies through the analysis of meteorites. Links astrophysics, geology, cosmochemistry, organic chemistry, and astronomy using meteoritics as the common ground. Provides historical perspectives along with space exploration results. Contains some fine illustrations, both in color and in black and white.

Morrison, David, and Tobias Owen. The Planetary System. Reading, Mass.: Addison-Wesley, 1988. Designed as a text for a college course in planetology, this book contains many references to the origins of the solar system and its individual components. Comparative planetology based on space exploration results, meteoritics, and other sources are utilized throughout the text to illustrate some of the evolutionary phases in the development of the planets and other solar system objects. Extensively illustrated.

Ozima, Minoru. The Earth: Its Birth and Growth. Translated by J. F. Wakabayashi. New York: Cambridge University Press, 1981. Traces the genesis of the Earth and its growth while highlighting problems addressed by isotope geochemistry. The past 4.5 billion years are sketched in terms that are easy to comprehend. Alternative hypotheses and explanations are considered.

Schneider, Stephen E., and Thomas T. Arny. Pathways to Astronomy. New York: McGraw-Hill, 2007. A thorough college textbook for introductory astronomy courses, divided into many short sections on specific topics. An entire unit is devoted to the origin of the solar system.

Smart, William M. The Origin of the Earth. 2d ed. New York: Cambridge University Press, 1953. An older work useful for those interested in earlier hypotheses and theories of the Earth's origin.

Tarbuck, Edward J., and Frederick K. Lutgens. Earth: An Introduction to Physical Geology, 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2008. This college-level textbook for introductory geology courses is well written and fully illustrated. Contains a chapter on the origin and historical development of the Earth.

Wasson, John T. Meteorites: Their Record of Early Solar-System History. New York: W. H. Freeman, 1985. Written as a text for a course on solar-system genesis, this book includes topics on meteorite classification, properties, formation, and compositional evidence linking meteorite groups with individual planets. Describes how researchers use meteorites to determine conditions in the formative periods of Earth and other planets. Includes many graphs, charts, and illustrations.

See Also
Big Bang; Universe: Evolution; Universe: Expansion; Earth-Moon Relations; Earth-Sun Relations; Solar System: Element Distribution; Solar System: Origins.