Further Reading

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Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New York: W. H. Freeman, 2008.

Encyclopedia of Astronomy and Astrophysics. CRC Press, Taylor and Francis Group. Available online. URL: http://eaa.crcpress.com/. Accessed October 24, 2008.

"ScienceDaily: Astrophysics News." Science Daily LLC. Available online. URL: http://www.sciencedaily.com/ news/space_time/astrophysics/. Accessed October 24, 2008.

Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West, 1991.

Earth Earth is the third planet from the center of our solar system, located between Venus and Mars at a distance of 93 million miles (150 x 106 km) from the Sun. It has a mean radius of 3,960 miles (6,371 km), a surface area of 2.04 x 108 square miles (5.101 x 108 km2), and an average density of 5.5 grams per cubic centimeter. As one of the terrestrial planets (Mercury, Venus, Earth, and Mars), Earth is composed of solid rock, with silicate minerals being the most abundant in the outer layers and a dense iron-nickel alloy forming the core material.

Earth and other planets condensed from a solar nebula about 5 billion years ago. In this process a swirling cloud of hot dust, gas, and protoplanets collided with one another, eventually forming the main planets. The accretion of Earth was a high-temperature process that allowed melting of the early Earth and segregation of the heavier metallic elements such as iron (Fe) and nickel (Ni) to sink to the core, and for the lighter rocky elements to float upward. This process led to the differentiation of Earth into several concentric shells of contrasting density and composition, and was the main control on the large-scale structure of Earth today.

The main shells of Earth include the crust, a light outer shell 3-43 miles (5-70 km) thick. This is followed inward by the mantle, a solid rocky layer extending to 1,802 miles (2,900 km). The outer core is a molten metallic layer extending to 3,170 miles (5,100 km) depth and the inner core is a solid metallic layer extending to 3,958 miles (6,370 km). With the acceptance of plate tectonics in the 1960s, geologists recognized that the outer parts of Earth were also divided into several zones that had very different mechanical properties. The outer shell of Earth was divided into many different rigid plates all moving with respect to each other, with some of them carrying continents in continental drift. This outer rigid layer became known as the lithosphere; it ranges from 45 to 95 miles (75-150 km) thick. The lithosphere is essentially floating on a denser, but partially molten layer of rock in the upper mantle known as the asthenosphere (or weak sphere). The weakness of this layer allows the plates on the surface of Earth to move about.

The most basic division of Earth's surface shows that it is divided into continents and ocean basins, with oceans occupying about 60 percent of the surface and continents 40 percent. Mountains are

Earth seen by Apollo 17 crew: Africa and Madagascar at center of field of view (NASA)

elevated portions of the continents. shorelines are where the land meets the sea, whereas continental shelves are broad to narrow areas underlain by continental crust, covered by shallow water. The shelves drop off to continental slopes, consisting of steep drop-offs from shelf edges to the deep ocean basin, and at the continental rise the slope flattens to merge with the deep-ocean abyssal plains. ocean ridge systems are subaquatic mountain ranges where seafloor spreading creates new ocean crust. Mountain belts on Earth are of two basic types. orogenic belts are linear chains of mountains, largely on the continents, that contain highly deformed, contorted rocks that represent places where lithospheric plates have collided or slid past one another. The midocean ridge system is a 40,000-mile (65,000-km) long mountain ridge that represents vast outpourings of young lava on the ocean floor and places where new oceanic crust is being generated by plate tectonics. After it is formed, it moves away from the ridge crests, and new magmatic plates fill the space created by the plates drifting apart. The oceanic basins also contain long, linear, deep-ocean trenches that are up to several kilometers deeper than the surrounding ocean floor and locally reach depths of seven miles (14 km) below the sea surface. These are places where the oceanic crust is sinking back into the mantle of Earth, completing the plate tectonic cycle for oceanic crust.

External layers of Earth include the hydrosphere, consisting of the ocean, lakes, streams, and the atmosphere. The air/water interface is very active, for here erosion breaks rocks down into loose debris—the regolith.

The hydrosphere is a dynamic mass of liquid, continuously on the move, including all the water in oceans, lakes, streams, glaciers, and groundwater, although most water is in the oceans. The hydrologic cycle describes changes, both long- and short-term, in Earth's hydrosphere. It is powered by heat from the sun, which causes evaporation and transpiration. This water then moves in the atmosphere and precipitates as rain or snow, which then drains off in streams, evaporates, or moves as groundwater, eventually to begin the cycle over and over again.

The atmosphere is the sphere around Earth consisting of the mixture of gases called air. It is hundreds of kilometers thick and is always moving, because more of the sun's heat is received per unit area at the equator than at the poles. The heated air expands and rises to where it spreads out, cools and sinks, and gradually returns to the equator. The effects of Earth's rotation modify this simple picture of the atmosphere's circulation. The Coriolis effect causes any freely moving body in the Northern Hemisphere to veer to the right and toward the left in the southern hemisphere.

The biosphere is the totality of Earth's living matter and partially decomposed dead plants and animals. It is made up largely of the elements carbon, hydrogen, and oxygen. When these organic elements decay, they may become part of the regolith and are returned through geological processes to the lithosphere, atmosphere, or hydrosphere.

See also atmosphere; biosphere; energy in the Earth system; lithosphere; magnetic field, Magnetosphere; mantle.

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