Earth  

Famous "Blue Marble" photograph of Earth, taken from Apollo 17.
Orbital characteristics
Epoch J2000
Aphelion	152,097,701 km 1.0167103335 AU
Perihelion	147,098,074 km 0.9832898912 AU
Semi-major axis	149,597,887.5 km 1.0000001124 AU
Eccentricity	0.016710219
Orbital period	365.256366 days 1.0000175 yr
Average orbital speed	29.783 km/s 107,218 km/h
Inclination	Reference (0) 7.25° to Sun\'s equator
Longitude of ascending node	348.73936°
Argument of perihelion	114.20783°
Satellites	1 (the Moon)
Physical characteristics
Mean radius	6,371.0 km This is the radius that gives a sphere with the same volume as the WGS 84 reference ellipsoid.
Equatorial radius	6,378.1 kmThe WGS 84 reference ellipsoid.
Polar radius	6,356.8 km
Flattening	0.0033528
Circumference	40,075.02 km (equatorial) 40,007.86 km (meridional) 40,041.47 km (mean)
Surface area	510,072,000 km²Pidwirny, Michael (February 2, 2006). "Surface area of our planet covered by oceans and continents.(Table 8o-1)". University of British Columbia, Okanagan. Retrieved on 2007-11-26.  148,940,000 km² land  (29.2 %) 361,132,000 km² water (70.8 %)
Volume	1.0832073×1012 km³
Mass	5.9736×1024 kg
Mean density	5.5153 g/cm³
Equatorial surface gravity	9.780327 m/s² 0.99732 g
Escape velocity	11.186 km/s  40,270 km/h
Sidereal rotation period	0.997258 d 23h 56m 04.09054sYoder, C. F. (1995) p. 12.
Equatorial rotation velocity	465.11 m/s
Axial tilt	23.439281°
Albedo	0.367
Surface temp.    Kelvin    Celsius	min mean max 184 K 287 K 331 K −89 Â°C 14 Â°C 57.7 Â°C
Adjectives	Terrestrial, Terran, Telluric, Tellurian, Earthly
Atmosphere
Surface pressure	101.3 kPa (MSL)
Composition	78.08% Nitrogen (N2) 20.95% Oxygen (O2) 0.93% Argon 0.038% Carbon dioxide Trace water vapor (varies with climate)

Earth (pronounced /ˈ��θ/)Earth (PLANET), entry in the Cambridge Advanced Learner\'s Dictionary, Cambridge University Press, online. Accessed 14-II-2008. is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as the Earth, Planet Earth, and the World, and in some contexts, Gaia and Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:
Blue, Jennifer (July 5, 2007). Descriptor Terms (Feature Types). Gazetteer of Planetary Nomenclature. USGS. Retrieved on 2007-07-05.

Home to millions of species,May, Robert M. (1999). "How many species are there on earth?". Science 241 (4872): 1441-1449. Retrieved on 2007-08-14.  including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago,Dalrymple, G.B. (1991). The Age of the Earth. California: Stanford University Press. ISBN 0-8047-1569-6. Newman, William L. (July 9, 2007). Age of the Earth. Publications Services, USGS. Retrieved on 2007-09-20.Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Geological Society, London, Special Publications 190: 205-221. Retrieved on 2007-09-20. Stassen, Chris (September 10, 2005). The Age of the Earth. The TalkOrigins Archive. Retrieved on 2007-09-20. and life appeared on its surface within a billion years. Since then, Earth\'s biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth\'s magnetic field, blocks harmful radiation, permitting life on land.Harrison, Roy M.; Hester, Ronald E. (2002). Causes and Environmental Implications of Increased UV-B Radiation. Royal Society of Chemistry. ISBN 0854042652. 

Earth\'s outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet\'s surface.Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: Msnbc. "Rover reveals Mars was once wet enough for life", NASA, March 2, 2007. Retrieved on 2007-08-28. Staff. "Simulations Show Liquid Water Could Exist on Mars", University of Arkansas, November 7, 2005. Retrieved on 2007-08-08. As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: G. Tinetti et al. (July, 2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature 448: 169-171.  Earth\'s interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis. The Earth\'s axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,Ahrens, Global Earth Physics: A Handbook of Physical Constants, p. 8. producing seasonal variations on the planet\'s surface with a period of one tropical year (365.24 solar days). Earth\'s only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet\'s rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. (2000). "Source regions and time scales for the delivery of water to Earth". Meteoritics & Planetary Science 35 (6): 1309–1320. Retrieved on 2007-03-06.  Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the Earth\'s orbit, caused by the gravitational influence of other planets, are believed to have given rise to the ice ages that have intermittently covered significant portions of Earth\'s surface in glacial sheets.

History

Main article: History of Earth

Scientists have been able to reconstruct detailed information about the planet\'s past. Earth and the other planets in the Solar System formed 4.54 billion years ago out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth\'s massCanup, R. M.; Asphaug, E. (Fall Meeting 2001). "An impact origin of the Earth-Moon system". Abstract #U51A-02, American Geophysical Union. Retrieved on 2007-03-10.  impacting the Earth in a glancing blow.R. Canup and E. Asphaug (2001). "Origin of the Moon in a giant impact near the end of the Earth\'s formation". Nature 412: 708–712.  Some of this object\'s mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed.Doolittle, W. Ford (February, 2000). "Uprooting the tree of life". Scientific American 282 (6): 90–95. 

The development of photosynthesis allowed the Sun\'s energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.Berkner, L. V.; Marshall, L. C. (1965). "On the Origin and Rise of Oxygen Concentration in the Earth\'s Atmosphere". Journal of Atmospheric Sciences 22 (3): 225–261. Retrieved on 2007-03-05.  True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.Burton, Kathleen (November 29, 2000). Astrobiologists Find Evidence of Early Life on Land. NASA. Retrieved on 2007-03-05.

As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.Murphy, J. B.; Nance, R. D. (1965). "How do supercontinents assemble?". American Scientist 92: 324–33. Retrieved on 2007-03-05. 

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.Kirschvink, J. L. (1992). The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, 51–52. ISBN 0521366151. 

Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.Raup, D. M.; Sepkoski, J. J. (1982). "Mass Extinctions in the Marine Fossil Record". Science 215 (4539): 1501–1503. Retrieved on 2007-03-05.  The last extinction event occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.Gould, Stephan J. (October , 1994). "The Evolution of Life on Earth". Scientific American. Retrieved on 2007-03-05.  This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,Wilkinson, B. H.; McElroy, B. J. (2007). "The impact of humans on continental erosion and sedimentation". Bulletin of the Geological Society of America 119 (1–2): 140–156. Retrieved on 2007-04-22.  affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.Staff. Paleoclimatology - The Study of Ancient Climates. Page Paleontology Science Center. Retrieved on 2007-03-02.

Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.Stern, David P. (November 25, 2001). Planetary Magnetism. NASA. Retrieved on 2007-04-01.

Shape

Main article: Figure of the Earth

Size comparison of inner planets (left to right): Mercury, Venus, Earth, and Mars

The Earth\'s shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters.Milbert, D. G.; Smith, D. A.. Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model. National Geodetic Survey, NOAA. Retrieved on 2007-03-07. The average diameter of the reference spheroid is about 12,742 km. More approximately the distance is 40,000 km/Ï€ because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.Mohr, P.J.; Taylor, B.N. (October, 2000). Unit of length (meter). NIST Reference on Constants, Units, and Uncertainty. NIST Physics Laboratory. Retrieved on 2007-04-23.

The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km larger than the pole to pole diameter.Sandwell, D. T.; Smith, W. H. F. (Jul7 26, 2006). Exploring the Ocean Basins with Satellite Altimeter Data. NOAA/NGDC. Retrieved on 2007-04-21. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls.Staff (November, 2001). WPA Tournament Table & Equipment Specifications. World Pool-Billiards Association. Retrieved on 2007-03-10. Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.Senne, Joseph H. (2000). "Did Edmund Hillary Climb the Wrong Mountain". Professional Surveyor 20 (5). Retrieved on 2007-02-04. 

Chemical composition

See also: Abundance of elements on Earth

The mass of the Earth is approximately 5.98×10²⁴ kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.Morgan, J. W.; Anders, E. (1980). "Chemical composition of Earth, Venus, and Mercury". Proceedings of the National Academy of Science 71 (12): 6973–6977. Retrieved on 2007-02-04. 

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth\'s crust consists of oxygen. The more common rock constituents of the Earth\'s crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Petrology", a publication now in the public domain.

Internal structure

Main article: Structure of the Earth

Earth cutaway from core to exosphere. Not to scale.

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the MohoroviÄ�ić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.Tanimoto, Toshiro (1995). in Thomas J. Ahrens: Crustal Structure of the Earth (PDF), Washington, DC: American Geophysical Union. ISBN 0-87590-851-9. Retrieved on 2007-02-03. 

The geologic component layers of the EarthJordan, T. H. (1979). "Structural Geology of the Earth\'s Interior". Proceedings National Academy of Science 76 (9): 4192–4200. Retrieved on 2007-03-24.  are at the following depths below the surface:Robertson, Eugene C. (July 26, 2001). The Interior of the Earth. USGS. Retrieved on 2007-03-24.

The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.Sanders, Robert. "Radioactive potassium may be major heat source in Earth\'s core", UC Berkeley News, December 10, 2003. Retrieved on 2007-02-28.  At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.Alfè, D.; Gillan, M. J.; Vocadlo, L.; Brodholt, J; Price, G. D. (2002). "The ab initio simulation of the Earth\'s core" (PDF). Philosophical Transaction of the Royal Society of London 360 (1795): 1227–1244. Retrieved on 2007-02-28.  A portion of the core\'s thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.Richards, M. A.; Duncan, R. A.; Courtillot, V. E. (1989). "Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails". Science 246 (4926): 103–107. Retrieved on 2007-04-21. 

Tectonic plates

Main article: Plate tectonics

A map illustrating the Earth\'s major plates.

According to plate tectonics theory, the outermost part of the Earth\'s interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.Seligman, Courtney (2008). The Structure of the Terrestrial Planets. Online Astronomy eText Table of Contents. cseligman.com. Retrieved on 2008-02-28.

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.Kious, W. J.; Tilling, R. I. (May 5, 1999). Understanding plate motions. USGS. Retrieved on 2007-03-02.

The main plates are:Brown, W. K.; Wohletz, K. H. (2005). SFT and the Earth\'s Tectonic Plates. Los Alamos National Laboratory. Retrieved on 2007-03-02.

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yrMeschede, M.; Udo Barckhausen, U. (November 20, 2000). Plate Tectonic Evolution of the Cocos-Nazca Spreading Center. Proceedings of the Ocean Drilling Program. Texas A&M University. Retrieved on 2007-04-02. (3.0 in/yr) and the Pacific Plate moving 52–69 mm/yr (2.1–2.7 in/yr). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr (0.8 in/yr).Staff. GPS Time Series. NASA JPL. Retrieved on 2007-04-02.

Surface

Main articles: Landform and Extreme points of the World

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center\'s TerrainBase Digital Terrain Model.

The Earth\'s terrain varies greatly from place to place. About 70.8%Pidwirny, Michael (2006). Fundamentals of Physical Geography. PhysicalGeography.net. Retrieved on 2007-03-19. of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impactsKring, David A.. Terrestrial Impact Cratering and Its Environmental Effects. Lunar and Planetary Laboratory. Retrieved on 2007-03-22. also act to reshape the landscape.

As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.Duennebier, Fred (August 12, 1999). Pacific Plate Motion. University of Hawaii. Retrieved on 2007-03-14.Mueller, R.D.; Roest, W.R.; Royer, J.-Y.; Gahagan, L.M.; Sclater, J.G. (March 7, 2007). Age of the Ocean Floor Poster. NOAA. Retrieved on 2007-03-14.

The continental plates consist of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.Staff. Layers of the Earth. Volcano World. Retrieved on 2007-03-11. Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.Jessey, David. Weathering and Sedimentary Rocks. Cal Poly Pomona. Retrieved on 2007-03-20. The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth\'s surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.Staff. Minerals. Museum of Natural History, Oregon. Retrieved on 2007-03-20. Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.Cox, Ronadh (2003). Carbonate sediments. Williams College. Retrieved on 2007-04-21.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.Staff (February 8, 2007). The World Factbook. U.S. C.I.A.. Retrieved on 2007-02-25. Close to 40% of the Earth\'s land surface is presently used for cropland and pasture, or an estimated 1.3×10⁷ km² of cropland and 3.4×10⁷ km² of pastureland.FAO Staff (1995). FAO Production Yearbook 1994, Volume 48, Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 9250038445. 

Elevation histogram of the surface of the Earth—approximately 71% of the Earth\'s surface is covered with water.

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 686 m.Mill, Hugh Robert (1893). "The Permanence of Ocean Basins". The Geographical Journal 1 (3): 230–234. Retrieved on 2007-02-25. 

Hydrosphere

Main article: Hydrosphere

The abundance of water on Earth\'s surface is a unique feature that distinguishes the "Blue Planet" from others in the solar system. The Earth\'s hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911 m.Staff. "Deep Ocean Studies". Ocean Studies. RAIN National Public Internet and Community Technology Center. Retrieved on 2006-04-02.Takuyo measurement; see Mariana Trench for details. The average depth of the oceans is 3,794 m, more than five times the average height of the continents.

The mass of the oceans is approximately 1.35×10¹⁸ metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386×10⁹ km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.The total volume of the Earth\'s oceans is: 1.4×10⁹ km³. The total surface area of the Earth is 5.1×10⁸ km². So, to first approximation, the average depth would be the ratio of the two, or 2.7 km. About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.Igor A. Shiklomanov et al (1999). World Water Resources and their use Beginning of the 21st century" Prepared in the Framework of IHP UNESCO. State Hydrological Institute, St. Petersburg. Retrieved on 2006-08-10.

About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.Mullen, Leslie (June 11, 2002). Salt of the Early Earth. NASA Astrobiology Magazine. Retrieved on 2007-03-14. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.Morris, Ron M.. Oceanic Processes. NASA Astrobiology Magazine. Retrieved on 2007-03-14. Sea water has an important influence on the world\'s climate, with the oceans acting as a large heat reservoir.Scott, Michon (April 24, 2006). Earth\'s Big heat Bucket. NASA Earth Observatory. Retrieved on 2007-03-14. Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.Sample, Sharron (June 21, 2005). Sea Surface Temperature. NASA. Retrieved on 2007-04-21.

Atmosphere

Main article: Earth\'s atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 7 km (23,000 ft) at the poles to 17 km (60,000 ft) at the equator, with some variation due to weather factors.^{[citation needed]}

Earth\'s biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere that exists today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth\'s magnetic field, blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth\'s include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.Staff (October 8, 2003). Earth\'s Atmosphere. NASA. Retrieved on 2007-03-21. This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth\'s atmosphere. Without this heat-retention effect, the average surface temperature would be −18 Â°C and life would likely not exist.

Weather and climate

Main articles: Weather and Climate

The Earth\'s atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere\'s mass is contained within the first 11 km (about 7 mi) of the planet\'s surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.Moran, Joseph M. (2005). Weather. World Book Online Reference Center. NASA/World Book, Inc.. Retrieved on 2007-03-17.

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.Berger, Wolfgang H. (2002). The Earth\'s Climate System. University of California, San Diego. Retrieved on 2007-03-24. Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.Rahmstorf, Stefan (2003). The Thermohaline Ocean Circulation. Potsdam Institute for Climate Impact Research. Retrieved on 2007-04-21.

Source regions of global air masses.

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation. Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.Various (July 21, 1997). The Hydrologic Cycle. University of Illinois. Retrieved on 2007-03-24.

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.Staff. Climate Zones. UK Department for Environment, Food and Rural Affairs. Retrieved on 2007-03-24. Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly-used Köppen climate classification system (as modified by Wladimir Köppen\'s student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.

Upper atmosphere

This view from orbit shows the full Moon partially obscured by the Earth\'s atmosphere. NASA image.

See also: Outer space

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere (where the Earth\'s magnetic fields interact with the solar wind).Staff (2004). Stratosphere and Weather; Discovery of the Stratosphere. Science Week. Retrieved on 2007-03-14. An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth\'s surface, is a working definition for the boundary between atmosphere and space.de Córdoba, S. Sanz Fernández (June 21, 2004). 100 km. Altitude Boundary for Astronautics. Fédération Aéronautique Internationale. Retrieved on 2007-04-21.

Due to thermal energy, some of the molecules at the outer edge of the Earth\'s atmosphere have their velocity increased to the point where they can escape from the planet\'s gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate.Liu, S. C.; Donahue, T. M. (1974). "The Aeronomy of Hydrogen in the Atmosphere of the Earth". Journal of Atmospheric Sciences 31 (4): 1118–1136. Retrieved on 2007-03-02.  For this reason, the Earth\'s current environment is oxidizing, rather than reducing, with consequences for the chemical nature of life which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules.Abedon, Stephen T. (March 31, 1997). History of Earth. Ohio State University. Retrieved on 2007-03-19.

The Earth\'s magnetic field, which approximates a dipole.

Magnetic field

Main article: Earth\'s magnetic field

The Earth\'s magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet\

s geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth\'s magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.Fitzpatrick, Richard (February 16, 2006). MHD dynamo theory. NASA WMAP. Retrieved on 2007-02-27.Campbell, Wallace Hall (2003). Introduction to Geomagnetic Fields. New York: Cambridge University Press, p57. ISBN 0521822068. 

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth\'s atmosphere at the magnetic poles, it forms the aurora.Stern, David P. (July 8, 2005). Exploration of the Earth\'s Magnetosphere. NASA. Retrieved on 2007-03-21.

Orbit and rotation

Main article: Earth\'s rotation

An animation showing the rotation of the Earth.

Relative to the background stars, it takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles.Fisher, Rick (January, 30, 1996). Astronomical Times. National Radio Astronomy Observatory. Retrieved on 2007-03-21. From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15\'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. (The apparent sizes of the Sun and the Moon are approximately the same.)

Earth orbits the Sun at an average distance of about 150 million kilometers (93.2 million miles) every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12 hours) eastward. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h or 67,000 mi/h), which is fast enough to cover the planet\'s diameter (about 12,600 km [7,800 mi]) in seven minutes, and the distance to the Moon (384,000 km or 238,000 mi) in four hours.Williams, David R. (September 1, 2004). Earth Fact Sheet. NASA. Retrieved on 2007-03-17.

Earth seen as a tiny dot by the Voyager 1 spacecraft, more than 6 billion kilometers (4 billion miles) from Earth.

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system\'s common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. The orbital and axial planes are not precisely aligned: Earth\'s axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane (which causes the seasons); and the E