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Earth, Planet

Earth, planet

Earth is the third of nine planets in our solar system . Its surface is mostly water (about 70%), and it has a moderately dense nitrogen and oxygen atmosphere that supports life. Rich in iron and nickel, Earth is a dense, molten oblate sphere with a solid core and a thin outer crust . Earth rotates about its polar axis as it revolves around the Sun . Earth has one natural satellite , the Moon . A complete revolution of Earth around the Sun takes about one year, while a rotation on its axis takes one day. The surface of Earth is constantly changing, as tectonic plates slowly move about on the turbulent foundation of partially molten rock beneath them. Collisions between landmasses build mountains; erosion wears them down. Slow changes in the climate cause equally slow changes in vegetation and animals.

Earth orbits the Sun at a distance of about 93,000,000 mi (150,000,000 km), taking 365.25 days to complete one revolution. Earth is small by planetary standards, only one-tenth the size of Jupiter. The equatorial radius of the earth is about 6,378 km. The polar radius of the earth is about 6,357 km. The difference is due to centrifugal flattening. Earth's mass is estimated at approximately 6.0×1024 kg. The difference is due to centrifugal flattening. Unlike the outer planets, which are composed mainly of light gases, Earth is made of heavy elements such as iron and nickel, and is, therefore, much more

dense. These characteristicssmall and denseare typical of the inner four terrestrial planets.

About 4.5 to 5.1 billion years ago, the solar system formed from a contracting cloud of interstellar gas. The cloud underwent compression and heating as it shrank, until its central part blazed forth as the mature, stable star. As the Sun formed, the surrounding gas cloud flattened into a disk. In this disk, the first solid particles formed and then grew as they accreted additional matter from the surrounding gas. Soon sub-planetary bodies, called planetesimals, built up, and then they collided and merged, forming the planets. The high temperatures in the inner solar system ensured that only the heavy elements, those that form rock and metal, could survive in solid form.

Thus were formed the small, dense terrestrial planets. Hot at first due to the collisions that formed it, Earth began to cool. Its components began to differentiate, or separate themselves according to their density. To its core went the heavy abundant elements, iron and nickel. Outside the core were numerous elements compressed into a dense but pliable substance called the mantle. Finally, a thin shell of cool, oxygen- and silicon-rich rock formed at Earth's surface: the crust, or lithosphere . Formation of the crust from the initial molten mass took half a billion years.

Earth's atmosphere formed as a result of outgassing of carbon dioxide from its interior, and accretion of gases from space , including elements brought to Earth by comets . The lightest elements, such as helium and most of the hydrogen, escaped to space, leaving behind an early atmosphere consisting of hydrogen compounds such as methane and ammonia as well as water vapor and nitrogen- and sulfur-bearing compounds released by volcanoes. Carbon dioxide was also plentiful, but was soon dissolved in ocean waters and deposited in carbonate rocks. As the gases cooled, they condensed, and rains inundated the planet. The lithosphere was uneven, containing highlands made of buoyant rock such as granite , and basins of heavy, denser basalt . Into these giant basins the rains flowed, forming the oceans . Eventually life forms appeared, and over the course of a billion years, plants enriched the atmosphere with oxygen, finally producing the nitrogen-oxygen atmosphere we have today.

Earth's crust is in a constant, though slow, state of change. Landmasses move, collide, and break apart according to a process called plate tectonics . The lithosphere (the outer crust and a portion of the upper mantle) is not one huge shell of rock; it is composed of several large pieces called plates. These pieces are constantly in motion, because Earth's interior is dynamic, with its core still molten and with large-scale convective currents in the upper mantle. The thermal forces move the plates a few centimeters a year, but this is enough to have profound consequences over the long expanse of geologic time .

For instance, the center of the North American continent is the wide open expanse of the Great Plains and the Canadian Prairies. On the eastern edge, the rolling folds of the Appalachian Mountains grace western North Carolina, Virginia, and Pennsylvania. In the west, the jagged, crumpled Rockies thrust skyward, tall, stark, and snow-capped.

These two great ranges represent one of the two basic land-altering processes: mountain building. Two hundred million years ago, North America was moving east, driven by plate tectonics. In a shattering, slow-motion collision, it rammed into what is now Europe and North Africa . The land crumpled, and the ancient Appalachians rose. At that time, they were the mightiest mountains on Earth. A hundred million years later, North America was driven back west. Now the western edge of the continent rumbled along over the Pacific plate, and about 80 million years ago, a massive spate of mountain building formed the Rockies.

During the time since the Appalachians rose, the other land-altering process, erosion, has reshaped the landscape. Battered by wind and water, their once sheer mountain flanks have been worn into the low, rolling hills of today.

Mountain building can be seen today in the Himalayas, which are still rising as India moves northward into Asia , crumpling parts of Nepal and Tibet. Erosion rules in Arizona's Grand Canyon , which gradually is deepening and widening as the Colorado River slices now into ancient granite two billion years old.

This unending cycle of mountain building (caused by movement of the crustal plates) and erosion (by wind and water) has formed every part of Earth's surface today. Where there are mountains, as in the long ranks of the Andes or the Urals, there is subterranean conflict. Where a crustal plate rides over another one, burying and melting it in the hot regions below the lithosphere, volcanoes rise, dramatically illustrated by Mt. St. Helens in Washington and the other volcanoes that line the Pacific rim. Where lands lie wide and arid, they are sculpted into long, scalloped cliffs, as one sees in the deserts of New Mexico, Arizona, and Utah.

Earth is mostly covered with water. The Pacific Ocean covers nearly half of Earth; from the proper vantage point in space above the middle of the Pacific Ocean, one would see nothing but water, dotted here and there with tiny islands, with only Australia and the coasts of Asia and the Americas rimming the edge of the globe.

The lithosphere rides on a pliable layer of rock in the upper mantle called the asthenosphere . Parts of the lithosphere are made of relatively light rocks (continental crust), while others (oceanic crust) are made of heavier, denser rocks. Just as corks float mostly above water while ice cubes float nearly submerged, the less dense parts of the lithosphere ride higher on the asthenosphere than the more dense ones. Earth therefore has huge basins, and early in the planet's history, these basins filled with water condensing and raining out of the primordial atmosphere. Additional water was brought to Earth by the impacts of comets, whose nuclei are made of water ice.

The atmosphere has large circulation patterns, and so do the oceans. Massive streams of warm and cold water flow through them. One of the most familiar is the Gulf Stream , which brings warm water up the eastern coast of the United States.

Circulation patterns in the oceans and in the atmosphere are driven by temperature differences between adjacent areas and by the rotation of Earth, which helps create circular, or rotary, flows. Oceans play a critical role in the overall energy balance and weather patterns. Storms are ultimately generated by moisture in the atmosphere, and evaporation from the oceans is the prime source of such moisture. Oceans respond less dramatically to changes in energy input than land does, so the temperature over a given patch of ocean is far more stable than one on land.

Earth's atmosphere is the gaseous region above its outer crust, composed of nitrogen (78% by number), oxygen (21%), and other gases (1%). It is only about 50 mi (80 km) from the ground to space: on a typical, 12-in (30 cm) globe the atmosphere would be less than 2 mm thick. The atmosphere has several layers. The most dense and significant of these is the troposphere ; all weather occurs in this layer, and commercial jets cruise near its upper boundary, 6 mi (10 km) above Earth's surface. The stratosphere lies between 6 and 31 mi (10 and 50 km) above, and it is here that the ozone layer lies. In the mesosphere and the thermosphere one finds auroras occurring after eruptions on the Sun; radio communications "bounce" off the ionosphere back to Earth.

The atmosphere is an insulator of almost miraculous stability. Only 50 mi (80 km) away is the cold of outer space, but the surface remains temperate. Heat is stored by the land and the atmosphere during the day, but the resulting heat radiation (infrared) from the surface is prevented from radiating away by gases in the atmosphere that trap infrared radiation. This is the well-known greenhouse effect , and it plays an important role in the atmospheric energy budget. According to some models, a global temperature decrease of two degrees could trigger the next advance of an ice age, while an increase of three degrees could melt the polar ice caps, submerging nearly every coastal city in the world.

Despite this overall stability, the troposphere is nevertheless a turbulent place. It is in a state of constant circulation, driven by Earth's rotation as well as the constant heating and cooling that occurs during each 24-hour period.

The largest circulation patterns in the troposphere are the Hadley cells. There are three of them in each hemisphere, with the middle, or Ferrel cell, lying over the latitudes spanned by the continental United States. Northward-flowing surface air in the Ferrel cell is deflected toward the east by the Coriolis force, with the result that windsand weather systemsmove from west to east in the middle latitudes of the northern hemisphere.

Near the top of the troposphere are the jet streams, fast-flowing currents of air that circle Earth in sinuous paths.

Circulation on a smaller but violent scale appears in the cyclones and anticyclones, commonly called low and high pressure cells. Lows typically bring unsettled or stormy weather, while highs mean sunny skies. Weather in most areas follows a basic pattern of alternating calm weather and storms, as the endless progression of highs and lows, generated by Earth's rotation and temperature variation, passes by. This is a great simplification, however, and weather in any given place may be affected, or even dominated, by local features. The climate in Los Angeles, California is entirely different from that in Las Vegas, Nevada though the two cities are not very far apart. Here, local featuresspecifically, the mountains between themare as important as the larger circulation patterns.

Earth has a magnetic field that extends tens of thousands of kilometers into space and shields Earth from most of the solar wind, a stream of particles emitted by the Sun. Sudden enhancements in the solar wind, such as a surge of particles ejected by an eruption in the Sun's atmosphere, may disrupt the magnetic field, temporarily interrupting long-range radio communications and creating brilliant displays of auroras near the poles, where the magnetic field lines bring the charged particles close to the earth's surface.

Farther out, at a mean distance of about 248,400 mi (400,000 km), is Earth's only natural satellite, the Moon. Some astronomers assert that the Earth and the Moon should properly be considered a "double planet," since the Moon is larger relative to our planet than the satellites of most other planets.

The presence of life on Earth is, as far as we know, unique. Men have walked on the Moon, and there is no life on our barren, airless satellite. Unmanned spacecraft have landed on Venus and Mars and have flown close to every other planet in the solar system except Pluto. The most promising possibility, Mars, yielded nothing to the automated experiments performed by the Viking and Mars Surveyor spacecraft that searched for signs of extraterrestrial life.

See also Atmospheric composition and structure; Biogeochemical cycles; Continental drift theory; Cosmology; Earth science; Earth, interior structure; Evolution, evidence of; Evolutionary mechanisms; Gaia hypothesis; Geochemistry; Geologic time; Global warming; Greenhouse gases and greenhouse effect; History of exploration I (Ancient and classical); History of exploration II (Age of exploration); History of exploration III (Modern era); Hydrologic cycle; Landforms; Landscape evolution; Latitude and longitude; Marine transgression and marine regression; Miller-Urey experiment; Oceanography; Origin of life; Orogeny; Revolution and rotation; Space and planetary geology; Supercontinents; Terra satellite and Earth Observing Systems (EOS); Uniformitarianism; Weather and climate

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planet Earth

planet Earth

Mean distance from the Sun

149.6 million km (93 million mi)

Average speed around the Sun

108,000 km/h (66,600 mph)


c.4600 million years


5.9 3 1021 tonnes

Density (water = 1)



1,083,230 3 106 cu km (260,000 3 106 cu mi)


510 million sq km (197 million sq mi)

Land surface

149 million sq km (58 million sq mi) = 29.2% of total area

Water surface

361 million sq km (139 million sq mi) = 70.7% of total area

Equatorial circumference

40,077 km (24,904 mi)

Polar circumference

40,009 km (24,862 mi)

Equatorial diameter

12,757 km (7927 mi)

Polar diameter

12,714 km (7900 mi)

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