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Earth, third planet in distance from the Sun in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans.
Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.
Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and the tiny dwarf planet Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.
Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, indicating that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.
For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas—for example, that the Earth was a sphere and that it traveled around the Sun—were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. And with the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.
In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings were able to study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto, now classified as a dwarf planet. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.
As a result of this recent space exploration, we now know that Earth is one of the most geologically active of all the planets and moons in the solar system. Earth is constantly changing. Over long periods of time land is built up and worn away, oceans are formed and re-formed, and continents move around, break up, and merge.
Life itself contributes to changes on Earth, especially in the way living things can alter Earth’s atmosphere. For example, Earth at one time had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.
A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth’s atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.
Alessandrello, Anna. The Earth: Origins and Evolution. Raintree Steck-Vaughn, 1995. For middle school readers.
Clifford, Nick. Incredible Earth. Firefly, 1996. Mountain-building and other processes; for readers in grades 4 to 6.
Davis, Kenneth C. Don't Know Much About Planet Earth. HarperCollins, 2001. An entertaining, fact-filled trek for readers in grades 5 to 8.
Gallant, Roy A. Dance of the Continents. Marshall Cavendish, 1999. For middle school readers and up.
Gallant, Roy A. Earth's Place in Space. Marshall Cavendish, 1999. For middle school readers and up.
Hooper, Meredith. The Pebble in My Pocket: A History of Our Earth. Viking, 1996. For readers in grades 3 to 6.
Ride, Sally, and Tam O'Shaughnessy. The Third Planet: Exploring the Earth from Space. Crown, 1994. For readers in grades 5 to 7.
Sattler, Helen R. Our Patchwork Planet: The Story of Plate Tectonics. Lothrop, Lee & Shepard, 1995. For middle school readers.
Emiliani, Cesare. Planet Earth: Cosmology, Geology, and the Evolution of Life and Environment. Cambridge University Press, 1992. This compendium of information on planet Earth includes illustrations, a glossary of historical figures, and several appendixes.
Erickson, Jon. Making of the Earth: Geological Forces That Shape Our Planet. Facts on File , 2000. Well-illustrated survey of the evolution of landforms.
Fortey, Richard. Earth: An Intimate History. Knopf, 2005. A lively geological history told through rocks on the face of Earth.
Fortey, Richard. Life: A Natural History of the First Four Billion Years of Life on Earth. Knopf, 1998. Random House, 1999. A paleontologist takes us from the beginnings of life on Earth to the arrival of human beings; for the general reader.
Hartmann, William K., and Ron Miller. The History of Earth: An Illustrated Chronicle of an Evolving Planet. Workman, 1991. Popular account of possible origins of Earth, presented with artist interpretations.
Redfern, Ron. Origins: The Evolution of Continents, Oceans and Life. University of Oklahoma Press, 2001. Well-illustrated account of plate tectonics, geology, and evolution.
Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 5th ed. Wiley, 2003. A recommended textbook.
Morgan, Alan V., B.Sc., M.Sc., Ph.D. Director of the Quaternary Sciences Institute and Professor of Earth Sciences, University of Waterloo.
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.
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