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Earth

Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface is made up of the ocean, dwarfing Earth's polar ice, lakes, and rivers. The remaining 29% of Earth's surface is land, consisting of continents and islands. Earth's surface layer is formed of several slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth's liquid outer core generates the magnetic field that shapes the magnetosphere of the Earth, deflecting destructive solar winds.

Earth
A photograph of Earth taken by the crew of Apollo 17 on December 7, 1972. A processed version became widely known as The Blue Marble.[1][2]
Designations
Gaia, Terra, Tellus, the world, the globe
AdjectivesEarthly, terrestrial, terran, tellurian
Orbital characteristics
Epoch J2000[n 1]
Aphelion152100000 km (94500000 mi)[n 2]
Perihelion147095000 km (91401000 mi)[n 2]
149598023 km (92955902 mi)[3]
Eccentricity0.0167086[3]
365.256363004 d[4]
(1.00001742096 aj)
29.78 km/s[5]
(107200 km/h; 66600 mph)
358.617°
Inclination
−11.26064°[5] to J2000 ecliptic
2023-Jan-04[7]
114.20783°[5]
Satellites
Physical characteristics
Mean radius
6371.0 km (3958.8 mi)[9]
Equatorial radius
6378.137 km (3963.191 mi)[10][11]
Polar radius
6356.752 km (3949.903 mi)[12]
Flattening1/298.257222101 (ETRS89)[13]
Circumference
510072000 km2 (196940000 sq mi)[15][n 5]
Volume1.08321×1012 km3 (2.59876×1011 cu mi)[5]
Mass5.97217×1024 kg (1.31668×1025 lb)[16]
(3.0×10−6 M)
Mean density
5.514 g/cm3 (0.1992 lb/cu in)[5]
9.80665 m/s2 (g; 32.1740 ft/s2)[17]
0.3307[18]
11.186 km/s[5] (40270 km/h; 25020 mph)
1.0 d
(24h 00m 00s)
0.99726968 d[19]
(23h 56m 4.100s)
Equatorial rotation velocity
0.4651 km/s[20]
(1674.4 km/h; 1040.4 mph)
23.4392811°[4]
Albedo
Surface temp. min mean max
Celsius −89.2 °C[21] 14 °C (1961–90)[22] 56.7 °C[23]
Fahrenheit −128.5 °F 57 °F (1961–90) 134.0 °F
Surface equivalent dose rate0.274 μSv/h[24]
Atmosphere
Surface pressure
101.325 kPa (at MSL)
Composition by volume
  • 78.08% nitrogen (N2; dry air)[5]
  • 20.95% oxygen (O2)
  • ~ 1% water vapor (climate variable)
  • 0.9340% argon
  • 0.0413% carbon dioxide[25]
  • 0.00182% neon[5]
  • 0.00052% helium
  • 0.00019% methane
  • 0.00011% krypton
  • 0.00006% hydrogen

The atmosphere of Earth consists mostly of nitrogen and oxygen. Greenhouse gases in the atmosphere like carbon dioxide (CO2) trap a part of the energy from the Sun close to the surface. Water vapor is widely present in the atmosphere and forms clouds that cover most of the planet. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. A region's climate is governed not only by latitude but also by elevation and proximity to moderating oceans. In most areas, severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs and greatly impacts life.

Earth is an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. The Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). The Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 380,000 km (1.3 light seconds) and is roughly a quarter as wide as Earth. Through tidal locking, the Moon always faces the Earth with the same side, which causes tides, stabilizes Earth's axis, and gradually slows its rotation.

Earth, like most other bodies in the Solar System, formed 4.5 billion years ago from gas in the early Solar System. During the first billion years of Earth's history, the ocean formed and then life developed within it. Life spread globally and began to affect Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago, and have reached a population of 8 billion today. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted the planet's environment. Today, humanity's impact on Earth's climate, soils, waters, and ecosystems is unsustainable, threatening people's lives and causing widespread extinctions of other life.[26]

Etymology

The Modern English word Earth developed, via Middle English, from an Old English noun most often spelled eorðe.[27] It has cognates in every Germanic language, and their ancestral root has been reconstructed as *erþō. In its earliest attestation, the word eorðe was already being used to translate the many senses of Latin terra and Greek γῆ : the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman Terra/Tellūs and Greek Gaia, Earth may have been a personified goddess in Germanic paganism: late Norse mythology included Jörð ('Earth'), a giantess often given as the mother of Thor.[28]

Historically, earth has been written in lowercase. From early Middle English, its definite sense as "the globe" was expressed as the earth. By the era of Early Modern English, capitalization of nouns began to prevail, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets, though earth and forms with the remain common.[27] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (for example, "Earth's atmosphere") but writes it in lowercase when preceded by the (for example, "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[29]

Occasionally, the name Terra /ˈtɛrə/ is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[30] while in poetry Tellus /ˈtɛləs/ has been used to denote personification of the Earth.[31] Terra is also the name of the planet in some Romance languages (languages that evolved from Latin) like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings (like the Spanish Tierra and the French Terre). The Latinate form Gæa or Gaea (English: /ˈ.ə/) of the Greek poetic name Gaia (Γαῖα; Ancient Greek[ɡâi̯.a] or [ɡâj.ja]) is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis, in which case its pronunciation is /ˈɡ.ə/ rather than the more classical English /ˈɡ.ə/.[32]

There are a number of adjectives for the planet Earth. From Earth itself comes earthly. From the Latin Terra comes terran /ˈtɛrən/,[33] terrestrial /təˈrɛstriəl/,[34] and (via French) terrene /təˈrn/,[35] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/[36] and telluric.[37]

Chronology

Formation

 
Artist's impression of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed

The oldest material found in the Solar System is dated to 4.5682+0.0002
−0.0004
Ga (billion years) ago.[38] By 4.54±0.04 Ga the primordial Earth had formed.[39] The bodies in the Solar System formed and evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and dust (including primordial nuclides). According to nebular theory, planetesimals formed by accretion, with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form.[40]

Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[41] A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object with about 10% of Earth's mass, named Theia, collided with Earth.[42] It hit Earth with a glancing blow and some of its mass merged with Earth.[43][44] Between approximately 4.1 and 3.8 Ga, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.[45]

After formation

 
Pale orange dot artist's impression of the early Earth tinted orange by its methane-rich early atmosphere[46]

Earth's atmosphere and oceans were formed by volcanic activity and outgassing.[47] Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.[48] Sufficient water to fill the oceans may have been on Earth since it formed.[49] In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[50] By 3.5 Ga, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[51]

As the molten outer layer of Earth cooled it formed the first solid crust, which is thought to have been mafic in composition. The first continental crust, which was more felsic in composition, formed by the partial melting of this mafic crust. The presence of grains of the mineral zircon of Hadean age in Eoarchean sedimentary rocks suggests that at least some felsic crust existed as early as 4.4 Ga, only 140 Ma after Earth's formation.[52] There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[53] (1) a relatively steady growth up to the present day,[54] which is supported by the radiometric dating of continental crust globally and (2) an initial rapid growth in the volume of continental crust during the Archean, forming the bulk of the continental crust that now exists,[55][56] which is supported by isotopic evidence from hafnium in zircons and neodymium in sedimentary rocks. The two models and the data that support them can be reconciled by large-scale recycling of the continental crust, particularly during the early stages of Earth's history.[57]

New continental crust forms as a result of plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Over the period of hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to form supercontinents that have subsequently broken apart. At approximately 750 Ma, one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia at 600–540 Ma, then finally Pangaea, which also began to break apart at 180 Ma.[58]

The most recent pattern of ice ages began about 40 Ma,[59] and then intensified during the Pleistocene about 3 Ma.[60] High- and middle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years.[61] The Last Glacial Period, colloquially called the "last ice age", covered large parts of the continents, to the middle latitudes, in ice and ended about 11,700 years ago.[62]

Origin of life and evolution

 
Artist's impression of the Archean, the eon after Earth's formation, featuring round stromatolites which are early oxygen producing forms of life from billions of years ago. After the Late Heavy Bombardment Earth's crust had cooled, its water-rich barren surface is marked by continents and volcanoes, with the Moon still orbiting Earth much closer than today, producing strong tides.[63]

Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose.[64] The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O3) in the upper atmosphere.[65] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[66] 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 Earth's surface.[67] Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia,[68] biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland,[69] and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.[70][71] The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms.[72][73]

During the Neoproterozoic, 1000 to 539 Ma, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity.[74][75] Following the Cambrian explosion, 535 Ma, there have been at least five major mass extinctions and many minor ones.[76][77] Apart from the proposed current Holocene extinction event, the most recent was 66 Ma, when an asteroid impact triggered the extinction of the non-avian dinosaurs and other large reptiles, but largely spared small animals such as insects, mammals, lizards and birds. Mammalian life has diversified over the past 66 Mys, and several million years ago an African ape gained the ability to stand upright.[78] This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the evolution of humans. The development of agriculture, and then civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day.[79]

Future

 
Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7 billion years from now

Earth's expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%.[80] Earth's increasing surface temperature will accelerate the inorganic carbon cycle, reducing CO2 concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 100–900 million years.[81][82] The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.[83] Due to the increased luminosity, Earth's mean temperature may reach 100 °C (212 °F) in 1.5 billion years, and all ocean water will evaporate and be lost to space, which may trigger a runaway greenhouse effect, within an estimated 1.6 to 3 billion years.[84] Even if the Sun were stable, a fraction of the water in the modern oceans will descend to the mantle, due to reduced steam venting from mid-ocean ridges.[84][85]

The Sun will evolve to become a red giant in about 5 billion years. Models predict that the Sun will expand to roughly 1 AU (150 million km; 93 million mi), about 250 times its present radius.[80][86] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU (250 million km; 160 million mi) from the Sun when the star reaches its maximum radius, otherwise, with tidal effects, it may enter the Sun's atmosphere and be vaporized.[80]

Geophysical characteristics

Size and shape

 
Topographic map of Earth; red areas are farthest from Earth's center

The shape of Earth is nearly spherical, with an average diameter of 12,742 kilometers (7,918 mi), making it the fifth largest of the Solar System's planetary sized objects and largest among its terrestrial ones. Due to Earth's rotation its shape is bulged around the Equator and slightly flattened at the poles,[87] resulting in a 43 kilometers (27 mi) larger diameter at the equator than at the poles.[88] Earth's shape therefore is more accurately described as an oblate spheroid.

Earth's shape furthermore has local topographic variations. Though the largest variations, like the Mariana Trench (10,925 meters or 35,843 feet below local sea level),[89] only shortens Earth's average radius by 0.17% and Mount Everest (8,848 meters or 29,029 feet above local sea level) lengthens it by only 0.14%.[n 6][91] Earth's surface is farthest out from Earth's center of mass at its equatorial bulge, making the summit of the Chimborazo volcano in Ecuador (6,384.4 km or 3,967.1 mi) the farthest point.[92][93][94] Parallel to the rigid land topography the Ocean exhibits a more dynamic topography.[95]

The point on Earth's surface closest to the planet's center is Litke Deep in the Arctic Ocean (6,351.7 km (3,946.8 mi) from the center).[96] The deepest point below sea level is, however, Challenger Deep but it is 14.7 km (9.2 mi) farther from Earth's center.[96]

To measure the local variation of Earth's topography, geodesy employs an idealized Earth producing a shape called a geoid. Such a geoid shape is gained if the ocean is idealized, covering Earth completely and without any perturbations such as tides and winds. The result is a smooth but gravitational irregular geoid surface, providing a mean sea level (MSL) as a reference level for topographic measurements.[97]

Surface

 
Earth's surface is with 71% mainly ocean water. While vegitation covers large parts of Earth's land surface,[98] deserts and ice form large parts of Earth's land as well as ocean surface, as distinguishable by colour in this composite image of southern Africa and the ice covered Southern Ocean (grey area) and Antarctica (white area).

The total surface area of Earth is about 510 million km2 (197 million sq mi).[15] Earth's surface can be divided into two hemispheres, such as into the Northern and Southern hemispheres, or the Eastern and Western hemispheres.

Most of the surface is made of water, in liquid form or in smaller amounts as ice. 70.8% (361.13 million km2 (139.43 million sq mi)) of the Earth's surface consists of the interconnected ocean,[99] making it Earth's global ocean or world ocean.[100][101] This makes Earth, along with its vibrant hydrosphere a water world[102][103] or ocean world,[104][105] particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely.[106] The world ocean is commonly divided into the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean and Arctic Ocean, from largest to smallest. Below the ocean's surface are the continental shelf, mountains, volcanoes,[88] oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system.

In contrast, Earth's land makes 29.2%, or 148.94 million km2 (57.51 million sq mi) of Earth's surface area. Earth's land consists of many islands around the globe, but mainly of four continental landmasses, which are from largest to smallest: Afroeurasia,America, Antarctica and Australia.[107][108][109] These landmasses are further broken down and grouped into the continents. The terrain varies greatly and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[110]

The continental crust consists 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.[111] Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[112] 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 Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.[113] Common carbonate minerals include calcite (found in limestone) and dolomite.[114]

Erosion and tectonics, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape Earth's surface over geological time.[115][116] The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. The total arable land is 10.7% of the land surface, with 1.3% being permanent cropland.[117][118] Earth has an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[119]

Tectonic plates

 
Earth's major plates, which are:[120]

Earth's mechanically rigid outer layer, the lithosphere, is divided into tectonic plates. These plates are rigid segments that move relative to each other at one of three boundaries types: at convergent boundaries, two plates come together; at divergent boundaries, two plates are pulled apart; and at transform boundaries, two plates slide past one another laterally. Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur.[121] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates.[122]

As the tectonic plates migrate, oceanic crust is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than 100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 Ma old.[123][124] By comparison, the oldest dated continental crust is 4,030 Ma,[125] although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4,400 Ma, indicating that at least some continental crust existed at that time.[52]

The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include 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 fused with the Indian Plate between 50 and 55 Ma. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/a (3.0 in/year)[126] and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of 10.6 mm/a (0.42 in/year).[127]

Internal structure

Geologic layers of Earth[128]
 
Illustration of Earth's cutaway, not to scale
Depth[129]
(km)
Component
layer name
Density
(g/cm3)
0–60 Lithosphere[n 8]
0–35 Crust[n 9] 2.2–2.9
35–660 Upper mantle 3.4–4.4
660-2890 Lower mantle 3.4–5.6
100–700 Asthenosphere
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1

Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity.[130] The thickness of the crust varies from about 6 kilometers (3.7 mi) under the oceans to 30–50 km (19–31 mi) for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.[131]

Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km (250 and 410 mi) below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[132] Earth's inner core may be rotating at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year, although both somewhat higher and much lower rates have also been proposed.[133] The radius of the inner core is about one-fifth of that of Earth. Density increases with depth, as described in the table on the right.

Among the Solar System's planetary sized objects Earth is the object with the highest density.

Chemical composition

Earth's mass is approximately 5.97×1024 kg (5,970 Yg). 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 aluminum (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[134]

The most common rock constituents of the crust are nearly all oxides: chlorine, sulfur, and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1%. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia, potash, and soda.[135][134]

Heat

 
Global map of heat flow from Earth's interior to the surface

The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232.[136] At the center, the temperature may be up to 6,000 °C (10,830 °F),[137] and the pressure could reach 360 GPa (52 million psi).[138] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately Gyr, twice the present-day heat would have been produced, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.[139][140]

The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42×1013 W.[141] 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.[142] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans because the crust there is much thinner than that of the continents.[143]

Gravitational field

The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth's gravitational field, known as gravity anomalies.[144]

Magnetic field

 
Schematic of Earth's magnetosphere, with the solar wind flows from left to right

The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05×10−5 T, with a magnetic dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average).[145] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[146][147]

The extent of Earth's magnetic field in space defines the magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside magnetosphere into a long tail.[148] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bow shock precedes the dayside magnetosphere within the solar wind.[149] Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[150][151] The ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[152] and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[153][154]

During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the aurora.[155]

Orbit and rotation

Rotation

 
Earth's rotation imaged by Deep Space Climate Observatory, showing axis tilt

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[156] Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day.[157][158]

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[4][n 10] Earth's rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s).[4] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[159]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.[160][161]

Orbit

 
Exaggerated illustration of Earth's elliptical orbit around the Sun, marking that the orbital extreme points (apoapsis and periapsis) are not the same as the four seasonal extreme points (equinox and solstice)

Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the inner Solar System. Earth's average orbital distance is about 150 million km (93 million mi), which is the basis for the Astronomical Unit and is equal to roughly 8.3 light minutes or 380 times Earth's distance to the Moon.

Earth orbits the Sun every 365.2564 mean solar days, or one sidereal year. With an apparent movement of the Sun in Earth's sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12 hours. Due to 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 Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.[5]

The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth-Moon system's common orbit 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 counterclockwise. Viewed from a vantage point above the Sun and Earth's north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth-Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[5][162]

The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[163][n 11] This is the maximum distance at which Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[163] Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm.[164]

Axial tilt and seasons

 
Earth's axial tilt causing different angles of seasonal illumination at different orbital positions around the Sun

The axial tilt of Earth is approximately 23.439281°[4] with the axis of its orbit plane, always pointing towards the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and in the Southern Hemisphere when the Tropic of Capricorn faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere. During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter.[165] Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year, causing a polar night, and this night extends for several months at the poles themselves. These same latitudes also experience a midnight sun, where the sun remains visible all day.[166][167]

By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.[168]

The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[169] The orientation (rather than the angle) of Earth's axis also changes over time, precessing around in a complete circle over each 25,800-year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[170]

In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance causes an increase of about 6.8% in solar energy reaching Earth at perihelion relative to aphelion.[171][n 12] Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere.[172]

Earth–Moon system

Moon

 
Earth–Moon system seen from Mars

The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto.[173][174] The natural satellites of other planets are also referred to as "moons", after Earth's.[175] The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.[43]

The gravitational attraction between Earth and the Moon causes tides on Earth.[176] The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet.[177] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.[178] Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5 in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs/yr—add up to significant changes.[179] During the Ediacaran period, for example, (approximately 620 Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[180]

The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[181] Some theorists think that without this stabilization against the torques applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting large changes over millions of years, as is the case for Mars, though this is disputed.[182][183]

Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[161] This allows total and annular solar eclipses to occur on Earth.[184]

Asteroids and artificial satellites

 
A computer-generated image mapping the prevalence of artificial satellites and space debris around Earth in geosynchronous and low Earth orbit

Earth's co-orbital asteroids population consists of quasi-satellites, objects with a horseshoe orbit and trojans. There are at least five quasi-satellites, including 469219 Kamoʻoalewa.[185][186] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in Earth's orbit around the Sun.[187][188] The tiny near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.[189]

As of September 2021, there are 4,550 operational, human-made satellites orbiting Earth.[8] There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.[n 3] Earth's largest artificial satellite is the International Space Station.[190]

Hydrosphere

 
A view of Earth with its global ocean and cloud cover, which dominate Earth's surface and hydrosphere. At Earth's polar regions Earth's hydrosphere forms larger areas of ice cover.

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 (6,600 ft). The mass of the oceans is approximately 1.35×1018 metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of 361.8 million km2 (139.7 million sq mi) with a mean depth of 3,682 m (12,080 ft), resulting in an estimated volume of 1.332 billion km3 (320 million cu mi).[191] If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km (1.68 to 1.74 mi).[192] About 97.5% of the water is saline; the remaining 2.5% is fresh water.[193][194] Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[195]

In Earth's coldest regions, snow survives over the summer and changes into ice. This accumulated snow and ice eventually forms into glaciers, bodies of ice that flow under the influence of their own gravity. Alpine glaciers form in mountainous areas, whereas vast ice sheets form over land in polar regions. The flow of glaciers erodes the surface changing it dramatically, with the formation of U-shaped valleys and other landforms.[196] Sea ice in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change.[197]

The average salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).[198] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[199] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[200] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[201] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[202]

The abundance of water on Earth's surface is a unique feature that distinguishes it from other planets in the Solar System. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.[203] Despite some moons showing signs of large reservoirs of extraterrestrial liquid water, with possibly even more volume than Earth's ocean, all of them are large bodies of water under a kilometers thick frozen surface layer.[204]

Atmosphere

 
A view of Earth with different layers of its atmosphere visible: the troposphere with its shadows casting clouds and a band of blue sky at the horizon, and above this the lower thermosphere with a line of green airglow around an altitude of 100 km, at the edge of space.

The atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),[205] with a scale height of about 8.5 km (5.3 mi).[5] A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules.[205] Water vapor content varies between 0.01% and 4%[205] but averages about 1%.[5] Clouds cover around two thirds of Earth's surface, more so over oceans than land.[206] The height of the troposphere varies with latitude, ranging between 8 km (5 mi) at the poles to 17 km (11 mi) at the equator, with some variation resulting from weather and seasonal factors.[207]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.[65] This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O2 into O3. The ozone layer blocks ultraviolet solar radiation, permitting life on land.[208] Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[209] This last phenomenon is the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C (0 °F), in contrast to the current +15 °C (59 °F),[210] and life on Earth probably would not exist in its current form.[211]

Weather and climate

 
The ITCZ's band of clouds over the Eastern Pacific and the Americas as seen from space

Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[212] Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface; this lowest layer is called the troposphere.[213] 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 thermal energy.[214]

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°.[215] Ocean heat content and currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[216]

Earth receives 1361 W/m2 of solar irradiance.[217][218] The amount of solar energy that reaches the Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator.[219] Earth's surface can be subdivided 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.[220]

Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology.[221] Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land.[222] Atmospheric circulation also plays an important role: San Francisco and Washington DC are both coastal cities at about the same latitude. San Francisco's climate is significantly more moderate as the prevailing wind direction is from sea to land.[223] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.[224]

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 falls to the surface as precipitation.[214] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or 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, topographic features, and temperature differences determine the average precipitation that falls in each region.[225]

The commonly used Köppen climate classification system has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[215] The Köppen system rates regions based on observed temperature and precipitation.[226] Surface air temperature can rise to around 55 °C (131 °F) in hot deserts, such as Death Valley, and can fall as low as −89 °C (−128 °F) in Antarctica.[227][228]

Upper atmosphere

 
Afterglow of the troposphere (orange), the stratosphere (whitish), the mesosphere (blue) with remains of a spacecraft reentry trail, and above the thermosphere without a visible glow

The upper atmosphere, the atmosphere above the troposphere,[229] is usually divided into the stratosphere, mesosphere, and thermosphere.[209] Each layer 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 geomagnetic fields interact with the solar wind.[230] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km (62 mi) above Earth's surface, is a working definition for the boundary between the atmosphere and outer space.[231]

Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steady loss of the atmosphere into space. Because unfixed hydrogen has a low molecular mass, it can achieve escape velocity more readily, and it leaks into outer space at a greater rate than other gases.[232] The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[233] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[234] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[235]

Life on Earth

 
An animation of the changing density of productive vegitation on land (low in brown; heavy in dark green) and phytoplankton at the ocean surface (low in purple; high in yellow)

Earth is the only known place that is habitable and has hosted life. Earth's life developed in Earth's early bodies of water some hundred million years after Earth formed.

Earth's life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere.[236] Therefore, life has impacted Earth, significantly altering Earth's atmosphere and surface over long periods of time, causing changes like the Great oxidation event.

Earth's life has over time greatly diversified, allowing the biosphere to have different biomes, which are inhabited by comparatively similar plants and animals.[237] The different biomes develope at distinct elevations or water depths, planetary temperature latitudes and on land also with different humidity. Earth's species diversity and biomass reaches a peak in shallow waters and with forests, particularly in equatorial, warm and humid conditions. While freezing polar regions and high altitudes, or extremely arid areas are relatively barren of plant and animal life.[238]

Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain a metabolism.[239] Plants and other organisms take up nutrients from water, soils and the atmosphere. These nutrients are constantly recycled between different species.[240]

Extreme weather, such as tropical cyclones (including hurricanes and typhoons), occurs over most of Earth's surface and has a large impact on life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year.[241] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters.[242] Human impact is felt in many areas due to pollution of the air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.[243] Human activities release greenhouse gases into the atmosphere which cause global warming.[244] This is driving changes such as the melting of glaciers and ice sheets, a global rise in average sea levels, increased risk of drought and wildfires, and migration of species to colder areas.[245]

Human geography

 
A composite image of artificial light emissions at night on a map of Earth

Originating from earlier primates in eastern Africa 300,000 years ago humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly settling Earth's land. In the 20th century Antarctica had been the last continent to see a first and until today limited human presence.

Human population has since the 19th century grown exponentially to seven billion in the early 2010s,[246] and is projected to peak at around ten billion in the second half of the 21st century.[247] Most of the growth is expected to take place in sub-Saharan Africa.[247]

Distribution and density of human population varies greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only the Northern Hemisphere of Earth,[248] partly due to the hemispherical predominance of the world's land mass, with 68% of the world's land mass being in the Northern Hemisphere.[249] Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[250]

Beyond Earth's surface humans have lived on a temporary basis, with only special purpose deep underground and underwater presence, and a few space stations. Human population virtually completely remains on Earth's surface, fully depending on Earth and the environment it sustains. Humans have gone and temporarily stayed beyond Earth with some hundreds of people, since the latter half of the 20th century, and only a fraction of them reaching another celestial body, the Moon.[251][252]

Humans have developed diverse societies and cultures, which have marked Earth significantly. Earth has been the claim of extensive human sedetary, extractive and political activity. Earth's land has been mostly territorially claimed since the 19th century by states, of which today more than 200 exist,[253] with only Antarctica and few areas remaining unclaimed.[254] Most of these states together form the United Nations, the leading worldwide intergovernmental organization,[255] with international governance having provided legal regimes extraterritorially, extanding human governance over the ocean and Antarctica, and therefore all of Earth.

Natural resources and land use

 
Earth's land use for human agriculture

Earth has resources that have been exploited by humans.[256] Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales.[257] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[258] These deposits are used by humans both for energy production and as feedstock for chemical production.[259] Mineral ore bodies have also been formed within the crust through a process of ore genesis, resulting from actions of magmatism, erosion, and plate tectonics.[260] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[261]

Earth's biosphere produces many useful biological products for humans, including food, wood, pharmaceuticals, oxygen, and the recycling of organic waste. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.[262] In 2019, 39 million km2 (15 million sq mi) of Earth's land surface consisted of forest and woodlands, 12 million km2 (4.6 million sq mi) was shrub and grassland, 40 million km2 (15 million sq mi) were used for animal feed production and grazing, and 11 million km2 (4.2 million sq mi) were cultivated as croplands.[263] Of the 12–14% of ice-free land that is used for croplands, 2 percentage points were irrigated in 2015.[264] Humans use building materials to construct shelters.[265]

Humans and the environment

 
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.[266]

Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of greenhouse gases in the atmosphere, altering Earth's energy budget and climate.[244][267] It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.[268] This increase in temperature, known as global warming, has contributed to the melting of glaciers, rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.[245]

The concept of planetary boundaries was introduced to quantify humanity's impact on Earth. Of the nine identified boundaries, five have been crossed: Biosphere integrity, climate change, chemical pollution, destruction of wild habitats and the nitrogen cycle are thought to have passed the safe threshold.[269][270] As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is thought possible to provide all basic physical needs globally within sustainable levels of resource use.[271]

Cultural and historical viewpoint

 
Tracy Caldwell Dyson in the Cupola module of the International Space Station observing the Earth below

Human cultures have developed many views of the planet.[272] The standard astronomical symbols of Earth are a quartered circle,  ,[273] representing the four corners of the world, and a globus cruciger,  . Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity.[274] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[274] The Gaia hypothesis, developed in the mid-20th century, compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.[275][276][277]

Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, called the overview effect, emphasizing its beauty, uniqueness and apparent fragility.[278][279] In particular, this caused a realization of the scope of effects from human activity on Earth's environment. Enabled by science, particularly Earth observation,[280] humans have started to take action on environmental issues globally,[281] acknowledging the impact of humans and the interconnectedness of Earth's environments.

Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. Initial belief in a flat Earth was gradually displaced in Ancient Greece by the idea of a spherical Earth, which was attributed to both the philosophers Pythagoras and Parmenides.[282][283] Earth was generally believed to be the center of the universe until the 16th century, when scientists first concluded that it was a moving object, one of the planets of the Solar System.[284]

It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[285] Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old.[286][287]

See also

Notes

  1. ^ All astronomical quantities vary, both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation, ignoring all periodic variations.
  2. ^ a b aphelion = a × (1 + e); perihelion = a × (1 – e), where a is the semi-major axis and e is the eccentricity. The difference between Earth's perihelion and aphelion is 5 million kilometers.—Wilkinson, John (2009). Probing the New Solar System. CSIRO Publishing. p. 144. ISBN 978-0-643-09949-4.
  3. ^ a b As of 4 January 2018, the United States Strategic Command tracked a total of 18,835 artificial objects, mostly debris. See: Anz-Meador, Phillip; Shoots, Debi, eds. (February 2018). "Satellite Box Score" (PDF). Orbital Debris Quarterly News. 22 (1): 12. Retrieved 18 April 2018.
  4. ^ Earth's circumference is almost exactly 40,000 km because the meter was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
  5. ^ Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover 26 March 2015 at the Wayback Machine datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice sheets of Antarctica and Greenland are counted as land, even though much of the rock that supports them lies below sea level.
  6. ^ If Earth were shrunk to the size of a billiard ball, some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would feel smoother.[90]
  7. ^ Including the Somali Plate, which is being formed out of the African Plate. See: Chorowicz, Jean (October 2005). "The East African rift system". Journal of African Earth Sciences. 43 (1–3): 379–410. Bibcode:2005JAfES..43..379C. doi:10.1016/j.jafrearsci.2005.07.019.
  8. ^ Locally varies between 5 and 200 km.
  9. ^ Locally varies between 5 and 70 km.
  10. ^ The ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—Aoki, S.; Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K. (1982). "The new definition of universal time". Astronomy and Astrophysics. 105 (2): 359–61. Bibcode:1982A&A...105..359A.
  11. ^ For Earth, the Hill radius is  , where m is the mass of Earth, a is an astronomical unit, and M is the mass of the Sun. So the radius in AU is about  .
  12. ^ Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% of the energy at aphelion.

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earth, this, article, about, planet, other, uses, disambiguation, planet, disambiguation, third, planet, redirects, here, other, systems, numbering, planets, planet, history, song, planet, song, third, planet, from, only, astronomical, object, known, harbor, l. This article is about the planet For other uses see Earth disambiguation and Planet Earth disambiguation Third planet redirects here For other systems of numbering planets see Planet History For the song see 3rd Planet song Earth is the third planet from the Sun and the only astronomical object known to harbor life While large volumes of water can be found throughout the Solar System only Earth sustains liquid surface water About 71 of Earth s surface is made up of the ocean dwarfing Earth s polar ice lakes and rivers The remaining 29 of Earth s surface is land consisting of continents and islands Earth s surface layer is formed of several slowly moving tectonic plates which interact to produce mountain ranges volcanoes and earthquakes Earth s liquid outer core generates the magnetic field that shapes the magnetosphere of the Earth deflecting destructive solar winds EarthA photograph of Earth taken by the crew of Apollo 17 on December 7 1972 A processed version became widely known as The Blue Marble 1 2 DesignationsAlternative namesGaia Terra Tellus the world the globeAdjectivesEarthly terrestrial terran tellurianOrbital characteristicsEpoch J2000 n 1 Aphelion152100 000 km 94500 000 mi n 2 Perihelion147095 000 km 91401 000 mi n 2 Semi major axis149598 023 km 92955 902 mi 3 Eccentricity0 0167086 3 Orbital period sidereal 365 256363 004 d 4 1 000017 420 96 aj Average orbital speed29 78 km s 5 107200 km h 66600 mph Mean anomaly358 617 Inclination7 155 to the Sun s equator 1 57869 6 to invariable plane 0 00005 to J2000 eclipticLongitude of ascending node 11 26064 5 to J2000 eclipticTime of perihelion2023 Jan 04 7 Argument of perihelion114 20783 5 Satellites1 natural satellite the Moon5 quasi satellites gt 4 500 operational artificial satellites 8 gt 18 000 tracked space debris n 3 Physical characteristicsMean radius6371 0 km 3958 8 mi 9 Equatorial radius6378 137 km 3963 191 mi 10 11 Polar radius6356 752 km 3949 903 mi 12 Flattening1 298 257222 101 ETRS89 13 Circumference40075 017 km equatorial 24901 461 mi 11 40007 86 km meridional 24859 73 mi 14 n 4 Surface area510072 000 km2 196940 000 sq mi 15 n 5 Volume1 08321 1012 km3 2 59876 1011 cu mi 5 Mass5 97217 1024 kg 1 31668 1025 lb 16 3 0 10 6 M Mean density5 514 g cm3 0 1992 lb cu in 5 Surface gravity9 80665 m s2 1 g 32 1740 ft s2 17 Moment of inertia factor0 3307 18 Escape velocity11 186 km s 5 40270 km h 25020 mph Synodic rotation period1 0 d 24h 00m 00s Sidereal rotation period0 997269 68 d 19 23h 56m 4 100s Equatorial rotation velocity0 4651 km s 20 1674 4 km h 1040 4 mph Axial tilt23 4392811 4 Albedo0 367 geometric 5 0 306 Bond 5 Surface temp min mean maxCelsius 89 2 C 21 14 C 1961 90 22 56 7 C 23 Fahrenheit 128 5 F 57 F 1961 90 134 0 FSurface equivalent dose rate0 274 mSv h 24 AtmosphereSurface pressure101 325 kPa at MSL Composition by volume78 08 nitrogen N2 dry air 5 20 95 oxygen O2 1 water vapor climate variable 0 9340 argon0 0413 carbon dioxide 25 0 00182 neon 5 0 00052 helium0 00019 methane0 00011 krypton0 00006 hydrogenThe atmosphere of Earth consists mostly of nitrogen and oxygen Greenhouse gases in the atmosphere like carbon dioxide CO2 trap a part of the energy from the Sun close to the surface Water vapor is widely present in the atmosphere and forms clouds that cover most of the planet More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation A region s climate is governed not only by latitude but also by elevation and proximity to moderating oceans In most areas severe weather such as tropical cyclones thunderstorms and heatwaves occurs and greatly impacts life Earth is an ellipsoid with a circumference of about 40 000 km It is the densest planet in the Solar System Of the four rocky planets it is the largest and most massive Earth is about eight light minutes away from the Sun and orbits it taking a year about 365 25 days to complete one revolution The Earth rotates around its own axis in slightly less than a day in about 23 hours and 56 minutes The Earth s axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun producing seasons Earth is orbited by one permanent natural satellite the Moon which orbits Earth at 380 000 km 1 3 light seconds and is roughly a quarter as wide as Earth Through tidal locking the Moon always faces the Earth with the same side which causes tides stabilizes Earth s axis and gradually slows its rotation Earth like most other bodies in the Solar System formed 4 5 billion years ago from gas in the early Solar System During the first billion years of Earth s history the ocean formed and then life developed within it Life spread globally and began to affect Earth s atmosphere and surface leading to the Great Oxidation Event two billion years ago Humans emerged 300 000 years ago and have reached a population of 8 billion today Humans depend on Earth s biosphere and natural resources for their survival but have increasingly impacted the planet s environment Today humanity s impact on Earth s climate soils waters and ecosystems is unsustainable threatening people s lives and causing widespread extinctions of other life 26 Contents 1 Etymology 2 Chronology 2 1 Formation 2 2 After formation 2 3 Origin of life and evolution 2 4 Future 3 Geophysical characteristics 3 1 Size and shape 3 2 Surface 3 3 Tectonic plates 3 4 Internal structure 3 5 Chemical composition 3 6 Heat 3 7 Gravitational field 3 8 Magnetic field 4 Orbit and rotation 4 1 Rotation 4 2 Orbit 4 3 Axial tilt and seasons 5 Earth Moon system 5 1 Moon 5 2 Asteroids and artificial satellites 6 Hydrosphere 7 Atmosphere 7 1 Weather and climate 7 2 Upper atmosphere 8 Life on Earth 9 Human geography 9 1 Natural resources and land use 9 2 Humans and the environment 10 Cultural and historical viewpoint 11 See also 12 Notes 13 References 14 External linksEtymologyThe Modern English word Earth developed via Middle English from an Old English noun most often spelled eorde 27 It has cognates in every Germanic language and their ancestral root has been reconstructed as erthō In its earliest attestation the word eorde was already being used to translate the many senses of Latin terra and Greek gῆ ge the ground its soil dry land the human world the surface of the world including the sea and the globe itself As with Roman Terra Tellus and Greek Gaia Earth may have been a personified goddess in Germanic paganism late Norse mythology included Jord Earth a giantess often given as the mother of Thor 28 Historically earth has been written in lowercase From early Middle English its definite sense as the globe was expressed as the earth By the era of Early Modern English capitalization of nouns began to prevail and the earth was also written the Earth particularly when referenced along with other heavenly bodies More recently the name is sometimes simply given as Earth by analogy with the names of the other planets though earth and forms with the remain common 27 House styles now vary Oxford spelling recognizes the lowercase form as the most common with the capitalized form an acceptable variant Another convention capitalizes Earth when appearing as a name for example Earth s atmosphere but writes it in lowercase when preceded by the for example the atmosphere of the earth It almost always appears in lowercase in colloquial expressions such as what on earth are you doing 29 Occasionally the name Terra ˈ t ɛr e is used in scientific writing and especially in science fiction to distinguish humanity s inhabited planet from others 30 while in poetry Tellus ˈ t ɛ l e s has been used to denote personification of the Earth 31 Terra is also the name of the planet in some Romance languages languages that evolved from Latin like Italian and Portuguese while in other Romance languages the word gave rise to names with slightly altered spellings like the Spanish Tierra and the French Terre The Latinate form Gaea or Gaea English ˈ dʒ iː e of the Greek poetic name Gaia Gaῖa Ancient Greek ɡai a or ɡaj ja is rare though the alternative spelling Gaia has become common due to the Gaia hypothesis in which case its pronunciation is ˈ ɡ aɪ e rather than the more classical English ˈ ɡ eɪ e 32 There are a number of adjectives for the planet Earth From Earth itself comes earthly From the Latin Terra comes terran ˈ t ɛr e n 33 terrestrial t e ˈ r ɛ s t r i e l 34 and via French terrene t e ˈ r iː n 35 and from the Latin Tellus comes tellurian t ɛ ˈ l ʊer i e n 36 and telluric 37 ChronologyMain articles History of Earth and Timeline of natural history Formation Further information Early Earth and Hadean Artist s impression of the early Solar System s protoplanetary disk out of which Earth and other Solar System bodies formed The oldest material found in the Solar System is dated to 4 5682 0 0002 0 0004 Ga billion years ago 38 By 4 54 0 04 Ga the primordial Earth had formed 39 The bodies in the Solar System formed and evolved with the Sun In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse which begins to spin and flatten into a circumstellar disk and then the planets grow out of that disk with the Sun A nebula contains gas ice grains and dust including primordial nuclides According to nebular theory planetesimals formed by accretion with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form 40 Estimates of the age of the Moon range from 4 5 Ga to significantly younger 41 A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars sized object with about 10 of Earth s mass named Theia collided with Earth 42 It hit Earth with a glancing blow and some of its mass merged with Earth 43 44 Between approximately 4 1 and 3 8 Ga numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and by inference to that of Earth 45 After formation Main article Geological history of Earth Pale orange dot artist s impression of the early Earth tinted orange by its methane rich early atmosphere 46 Earth s atmosphere and oceans were formed by volcanic activity and outgassing 47 Water vapor from these sources condensed into the oceans augmented by water and ice from asteroids protoplanets and comets 48 Sufficient water to fill the oceans may have been on Earth since it formed 49 In this model atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70 of its current luminosity 50 By 3 5 Ga Earth s magnetic field was established which helped prevent the atmosphere from being stripped away by the solar wind 51 As the molten outer layer of Earth cooled it formed the first solid crust which is thought to have been mafic in composition The first continental crust which was more felsic in composition formed by the partial melting of this mafic crust The presence of grains of the mineral zircon of Hadean age in Eoarchean sedimentary rocks suggests that at least some felsic crust existed as early as 4 4 Ga only 140 Ma after Earth s formation 52 There are two main models of how this initial small volume of continental crust evolved to reach its current abundance 53 1 a relatively steady growth up to the present day 54 which is supported by the radiometric dating of continental crust globally and 2 an initial rapid growth in the volume of continental crust during the Archean forming the bulk of the continental crust that now exists 55 56 which is supported by isotopic evidence from hafnium in zircons and neodymium in sedimentary rocks The two models and the data that support them can be reconciled by large scale recycling of the continental crust particularly during the early stages of Earth s history 57 New continental crust forms as a result of plate tectonics a process ultimately driven by the continuous loss of heat from Earth s interior Over the period of hundreds of millions of years tectonic forces have caused areas of continental crust to group together to form supercontinents that have subsequently broken apart At approximately 750 Ma one of the earliest known supercontinents Rodinia began to break apart The continents later recombined to form Pannotia at 600 540 Ma then finally Pangaea which also began to break apart at 180 Ma 58 The most recent pattern of ice ages began about 40 Ma 59 and then intensified during the Pleistocene about 3 Ma 60 High and middle latitude regions have since undergone repeated cycles of glaciation and thaw repeating about every 21 000 41 000 and 100 000 years 61 The Last Glacial Period colloquially called the last ice age covered large parts of the continents to the middle latitudes in ice and ended about 11 700 years ago 62 Origin of life and evolution Main articles Origin of life Earliest known life forms and Evolutionary history of life Artist s impression of the Archean the eon after Earth s formation featuring round stromatolites which are early oxygen producing forms of life from billions of years ago After the Late Heavy Bombardment Earth s crust had cooled its water rich barren surface is marked by continents and volcanoes with the Moon still orbiting Earth much closer than today producing strong tides 63 Chemical reactions led to the first self replicating molecules about four billion years ago A half billion years later the last common ancestor of all current life arose 64 The evolution of photosynthesis allowed the Sun s energy to be harvested directly by life forms The resultant molecular oxygen O2 accumulated in the atmosphere and due to interaction with ultraviolet solar radiation formed a protective ozone layer O3 in the upper atmosphere 65 The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes 66 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 Earth s surface 67 Among the earliest fossil evidence for life is microbial mat fossils found in 3 48 billion year old sandstone in Western Australia 68 biogenic graphite found in 3 7 billion year old metasedimentary rocks in Western Greenland 69 and remains of biotic material found in 4 1 billion year old rocks in Western Australia 70 71 The earliest direct evidence of life on Earth is contained in 3 45 billion year old Australian rocks showing fossils of microorganisms 72 73 During the Neoproterozoic 1000 to 539 Ma much of Earth might have been covered in ice This hypothesis has been termed Snowball Earth and it is of particular interest because it preceded the Cambrian explosion when multicellular life forms significantly increased in complexity 74 75 Following the Cambrian explosion 535 Ma there have been at least five major mass extinctions and many minor ones 76 77 Apart from the proposed current Holocene extinction event the most recent was 66 Ma when an asteroid impact triggered the extinction of the non avian dinosaurs and other large reptiles but largely spared small animals such as insects mammals lizards and birds Mammalian life has diversified over the past 66 Mys and several million years ago an African ape gained the ability to stand upright 78 This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain which led to the evolution of humans The development of agriculture and then civilization led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day 79 Future Main article Future of Earth See also Global catastrophic risk Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase about 5 7 billion years from nowEarth s expected long term future is tied to that of the Sun Over the next 1 1 billion years solar luminosity will increase by 10 and over the next 3 5 billion years by 40 80 Earth s increasing surface temperature will accelerate the inorganic carbon cycle reducing CO2 concentration to levels lethally low for plants 10 ppm for C4 photosynthesis in approximately 100 900 million years 81 82 The lack of vegetation will result in the loss of oxygen in the atmosphere making animal life impossible 83 Due to the increased luminosity Earth s mean temperature may reach 100 C 212 F in 1 5 billion years and all ocean water will evaporate and be lost to space which may trigger a runaway greenhouse effect within an estimated 1 6 to 3 billion years 84 Even if the Sun were stable a fraction of the water in the modern oceans will descend to the mantle due to reduced steam venting from mid ocean ridges 84 85 The Sun will evolve to become a red giant in about 5 billion years Models predict that the Sun will expand to roughly 1 AU 150 million km 93 million mi about 250 times its present radius 80 86 Earth s fate is less clear As a red giant the Sun will lose roughly 30 of its mass so without tidal effects Earth will move to an orbit 1 7 AU 250 million km 160 million mi from the Sun when the star reaches its maximum radius otherwise with tidal effects it may enter the Sun s atmosphere and be vaporized 80 Geophysical characteristicsFurther information Geophysics Size and shape Main article Figure of the Earth Further information Earth radius Earth s circumference Earth curvature and Geomorphology See also List of highest mountains on Earth Topographic map of Earth red areas are farthest from Earth s center The shape of Earth is nearly spherical with an average diameter of 12 742 kilometers 7 918 mi making it the fifth largest of the Solar System s planetary sized objects and largest among its terrestrial ones Due to Earth s rotation its shape is bulged around the Equator and slightly flattened at the poles 87 resulting in a 43 kilometers 27 mi larger diameter at the equator than at the poles 88 Earth s shape therefore is more accurately described as an oblate spheroid Earth s shape furthermore has local topographic variations Though the largest variations like the Mariana Trench 10 925 meters or 35 843 feet below local sea level 89 only shortens Earth s average radius by 0 17 and Mount Everest 8 848 meters or 29 029 feet above local sea level lengthens it by only 0 14 n 6 91 Earth s surface is farthest out from Earth s center of mass at its equatorial bulge making the summit of the Chimborazo volcano in Ecuador 6 384 4 km or 3 967 1 mi the farthest point 92 93 94 Parallel to the rigid land topography the Ocean exhibits a more dynamic topography 95 The point on Earth s surface closest to the planet s center is Litke Deep in the Arctic Ocean 6 351 7 km 3 946 8 mi from the center 96 The deepest point below sea level is however Challenger Deep but it is 14 7 km 9 2 mi farther from Earth s center 96 To measure the local variation of Earth s topography geodesy employs an idealized Earth producing a shape called a geoid Such a geoid shape is gained if the ocean is idealized covering Earth completely and without any perturbations such as tides and winds The result is a smooth but gravitational irregular geoid surface providing a mean sea level MSL as a reference level for topographic measurements 97 Surface Further information Land Ocean and Sea Earth s surface is with 71 mainly ocean water While vegitation covers large parts of Earth s land surface 98 deserts and ice form large parts of Earth s land as well as ocean surface as distinguishable by colour in this composite image of southern Africa and the ice covered Southern Ocean grey area and Antarctica white area The total surface area of Earth is about 510 million km2 197 million sq mi 15 Earth s surface can be divided into two hemispheres such as into the Northern and Southern hemispheres or the Eastern and Western hemispheres Most of the surface is made of water in liquid form or in smaller amounts as ice 70 8 361 13 million km2 139 43 million sq mi of the Earth s surface consists of the interconnected ocean 99 making it Earth s global ocean or world ocean 100 101 This makes Earth along with its vibrant hydrosphere a water world 102 103 or ocean world 104 105 particularly in Earth s early history when the ocean is thought to have possibly covered Earth completely 106 The world ocean is commonly divided into the Pacific Ocean Atlantic Ocean Indian Ocean Southern Ocean and Arctic Ocean from largest to smallest Below the ocean s surface are the continental shelf mountains volcanoes 88 oceanic trenches submarine canyons oceanic plateaus abyssal plains and a globe spanning mid ocean ridge system In contrast Earth s land makes 29 2 or 148 94 million km2 57 51 million sq mi of Earth s surface area Earth s land consists of many islands around the globe but mainly of four continental landmasses which are from largest to smallest Afroeurasia America Antarctica and Australia 107 108 109 These landmasses are further broken down and grouped into the continents The terrain varies greatly and consists of mountains deserts plains plateaus and other landforms The elevation of the land surface varies from the low point of 418 m 1 371 ft at the Dead Sea to a maximum altitude of 8 848 m 29 029 ft at the top of Mount Everest The mean height of land above sea level is about 797 m 2 615 ft 110 The continental crust consists 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 111 Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together Nearly 75 of the continental surfaces are covered by sedimentary rocks although they form about 5 of the crust 112 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 Earth s surface include quartz feldspars amphibole mica pyroxene and olivine 113 Common carbonate minerals include calcite found in limestone and dolomite 114 Erosion and tectonics volcanic eruptions flooding weathering glaciation the growth of coral reefs and meteorite impacts are among the processes that constantly reshape Earth s surface over geological time 115 116 The pedosphere is the outermost layer of Earth s continental surface and is composed of soil and subject to soil formation processes The total arable land is 10 7 of the land surface with 1 3 being permanent cropland 117 118 Earth has an estimated 16 7 million km2 6 4 million sq mi of cropland and 33 5 million km2 12 9 million sq mi of pastureland 119 Tectonic plates Main articles Plate tectonics and Earth s crust Earth s major plates which are 120 Pacific Plate African Plate n 7 North American Plate Eurasian Plate Antarctic Plate Indo Australian Plate South American Plate Earth s mechanically rigid outer layer the lithosphere is divided into tectonic plates These plates are rigid segments that move relative to each other at one of three boundaries types at convergent boundaries two plates come together at divergent boundaries two plates are pulled apart and at transform boundaries two plates slide past one another laterally Along these plate boundaries earthquakes volcanic activity mountain building and oceanic trench formation can occur 121 The tectonic plates ride on top of the asthenosphere the solid but less viscous part of the upper mantle that can flow and move along with the plates 122 As the tectonic plates migrate oceanic crust is subducted under the leading edges of the plates at convergent boundaries At the same time the upwelling of mantle material at divergent boundaries creates mid ocean ridges The combination of these processes recycles the oceanic crust back into the mantle Due to this recycling most of the ocean floor is less than 100 Ma old The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 Ma old 123 124 By comparison the oldest dated continental crust is 4 030 Ma 125 although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4 400 Ma indicating that at least some continental crust existed at that time 52 The seven major plates are the Pacific North American Eurasian African Antarctic Indo Australian and South American Other notable plates include 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 fused with the Indian Plate between 50 and 55 Ma The fastest moving plates are the oceanic plates with the Cocos Plate advancing at a rate of 75 mm a 3 0 in year 126 and the Pacific Plate moving 52 69 mm a 2 0 2 7 in year At the other extreme the slowest moving plate is the South American Plate progressing at a typical rate of 10 6 mm a 0 42 in year 127 Internal structure Main article Internal structure of Earth Geologic layers of Earth 128 Illustration of Earth s cutaway not to scaleDepth 129 km Component layer name Density g cm3 0 60 Lithosphere n 8 0 35 Crust n 9 2 2 2 935 660 Upper mantle 3 4 4 4660 2890 Lower mantle 3 4 5 6100 700 Asthenosphere 2890 5100 Outer core 9 9 12 25100 6378 Inner core 12 8 13 1Earth s interior like that of the other terrestrial planets is divided into layers by their chemical or physical rheological properties The outer layer is a chemically distinct silicate solid crust which is underlain by a highly viscous solid mantle The crust is separated from the mantle by the Mohorovicic discontinuity 130 The thickness of the crust varies from about 6 kilometers 3 7 mi under the oceans to 30 50 km 19 31 mi for the continents The crust and the cold rigid top of the upper mantle are collectively known as the lithosphere which is divided into independently moving tectonic plates 131 Beneath the lithosphere is the asthenosphere a relatively low viscosity layer on which the lithosphere rides Important changes in crystal structure within the mantle occur at 410 and 660 km 250 and 410 mi below the surface spanning a transition zone that separates the upper and lower mantle Beneath the mantle an extremely low viscosity liquid outer core lies above a solid inner core 132 Earth s inner core may be rotating at a slightly higher angular velocity than the remainder of the planet advancing by 0 1 0 5 per year although both somewhat higher and much lower rates have also been proposed 133 The radius of the inner core is about one fifth of that of Earth Density increases with depth as described in the table on the right Among the Solar System s planetary sized objects Earth is the object with the highest density Chemical composition See also Abundance of elements on Earth Earth s mass is approximately 5 97 1024 kg 5 970 Yg 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 aluminum 1 4 with the remaining 1 2 consisting of trace amounts of other elements Due to mass segregation the core region is estimated to be primarily composed of iron 88 8 with smaller amounts of nickel 5 8 sulfur 4 5 and less than 1 trace elements 134 The most common rock constituents of the crust are nearly all oxides chlorine sulfur and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1 Over 99 of the crust is composed of 11 oxides principally silica alumina iron oxides lime magnesia potash and soda 135 134 Heat Main article Earth s internal heat budget Global map of heat flow from Earth s interior to the surface The major heat producing isotopes within Earth are potassium 40 uranium 238 and thorium 232 136 At the center the temperature may be up to 6 000 C 10 830 F 137 and the pressure could reach 360 GPa 52 million psi 138 Because much of the heat is provided by radioactive decay scientists postulate that early in Earth s history before isotopes with short half lives were depleted Earth s heat production was much higher At approximately 3 Gyr twice the present day heat would have been produced increasing the rates of mantle convection and plate tectonics and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today 139 140 The mean heat loss from Earth is 87 mW m 2 for a global heat loss of 4 42 1013 W 141 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 142 More of the heat in Earth is lost through plate tectonics by mantle upwelling associated with mid ocean ridges The final major mode of heat loss is through conduction through the lithosphere the majority of which occurs under the oceans because the crust there is much thinner than that of the continents 143 Gravitational field Main article Gravity of Earth The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth Near Earth s surface gravitational acceleration is approximately 9 8 m s2 32 ft s2 Local differences in topography geology and deeper tectonic structure cause local and broad regional differences in Earth s gravitational field known as gravity anomalies 144 Magnetic field Main article Earth s magnetic field Schematic of Earth s magnetosphere with the solar wind flows from left to right The main part of Earth s magnetic field is generated in the core the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy The field extends outwards from the core through the mantle and up to Earth s surface where it is approximately a dipole The poles of the dipole are located close to Earth s geographic poles At the equator of the magnetic field the magnetic field strength at the surface is 3 05 10 5 T with a magnetic dipole moment of 7 79 1022 Am2 at epoch 2000 decreasing nearly 6 per century although it still remains stronger than its long time average 145 The convection movements in the core are chaotic the magnetic poles drift and periodically change alignment This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years The most recent reversal occurred approximately 700 000 years ago 146 147 The extent of Earth s magnetic field in space defines the magnetosphere Ions and electrons of the solar wind are deflected by the magnetosphere solar wind pressure compresses the dayside of the magnetosphere to about 10 Earth radii and extends the nightside magnetosphere into a long tail 148 Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind a supersonic bow shock precedes the dayside magnetosphere within the solar wind 149 Charged particles are contained within the magnetosphere the plasmasphere is defined by low energy particles that essentially follow magnetic field lines as Earth rotates 150 151 The ring current is defined by medium energy particles that drift relative to the geomagnetic field but with paths that are still dominated by the magnetic field 152 and the Van Allen radiation belts are formed by high energy particles whose motion is essentially random but contained in the magnetosphere 153 154 During magnetic storms and substorms charged particles can be deflected from the outer magnetosphere and especially the magnetotail directed along field lines into Earth s ionosphere where atmospheric atoms can be excited and ionized causing the aurora 155 Orbit and rotationRotation Main article Earth s rotation Earth s rotation imaged by Deep Space Climate Observatory showing axis tilt Earth s rotation period relative to the Sun its mean solar day is 86 400 seconds of mean solar time 86 400 0025 SI seconds 156 Because Earth s solar day is now slightly longer than it was during the 19th century due to tidal deceleration each day varies between 0 and 2 ms longer than the mean solar day 157 158 Earth s rotation period relative to the fixed stars called its stellar day by the International Earth Rotation and Reference Systems Service IERS is 86 164 0989 seconds of mean solar time UT1 or 23h 56m 4 0989s 4 n 10 Earth s rotation period relative to the precessing or moving mean March equinox when the Sun is at 90 on the equator is 86 164 0905 seconds of mean solar time UT1 23h 56m 4 0905s 4 Thus the sidereal day is shorter than the stellar day by about 8 4 ms 159 Apart from meteors within the atmosphere and low orbiting satellites the main apparent motion of celestial bodies in Earth s sky is to the west at a rate of 15 h 15 min For bodies near the celestial equator this is equivalent to an apparent diameter of the Sun or the Moon every two minutes from Earth s surface the apparent sizes of the Sun and the Moon are approximately the same 160 161 Orbit Main articles Earth s orbit and Earth s location Exaggerated illustration of Earth s elliptical orbit around the Sun marking that the orbital extreme points apoapsis and periapsis are not the same as the four seasonal extreme points equinox and solstice Earth orbits the Sun making Earth the third closest planet to the Sun and part of the inner Solar System Earth s average orbital distance is about 150 million km 93 million mi which is the basis for the Astronomical Unit and is equal to roughly 8 3 light minutes or 380 times Earth s distance to the Moon Earth orbits the Sun every 365 2564 mean solar days or one sidereal year With an apparent movement of the Sun in Earth s sky at a rate of about 1 day eastward which is one apparent Sun or Moon diameter every 12 hours Due to 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 Earth averages about 29 78 km s 107 200 km h 66 600 mph which is fast enough to travel a distance equal to Earth s diameter about 12 742 km 7 918 mi in seven minutes and the distance to the Moon 384 000 km 239 000 mi in about 3 5 hours 5 The Moon and Earth orbit a common barycenter every 27 32 days relative to the background stars When combined with the Earth Moon system s common orbit 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 counterclockwise Viewed from a vantage point above the Sun and Earth s north poles Earth orbits in a counterclockwise direction about the Sun The orbital and axial planes are not precisely aligned Earth s axis is tilted some 23 44 degrees from the perpendicular to the Earth Sun plane the ecliptic and the Earth Moon plane is tilted up to 5 1 degrees against the Earth Sun plane Without this tilt there would be an eclipse every two weeks alternating between lunar eclipses and solar eclipses 5 162 The Hill sphere or the sphere of gravitational influence of Earth is about 1 5 million km 930 000 mi in radius 163 n 11 This is the maximum distance at which Earth s gravitational influence is stronger than the more distant Sun and planets Objects must orbit Earth within this radius or they can become unbound by the gravitational perturbation of the Sun 163 Earth along with the Solar System is situated in the Milky Way and orbits about 28 000 light years from its center It is about 20 light years above the galactic plane in the Orion Arm 164 Axial tilt and seasons Main article Axial tilt Earth Earth s axial tilt causing different angles of seasonal illumination at different orbital positions around the Sun The axial tilt of Earth is approximately 23 439281 4 with the axis of its orbit plane always pointing towards the Celestial Poles Due to Earth s axial tilt the amount of sunlight reaching any given point on the surface varies over the course of the year This causes the seasonal change in climate with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun and in the Southern Hemisphere when the Tropic of Capricorn faces the Sun In each instance winter occurs simultaneously in the opposite hemisphere During the summer the day lasts longer and the Sun climbs higher in the sky In winter the climate becomes cooler and the days shorter 165 Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year causing a polar night and this night extends for several months at the poles themselves These same latitudes also experience a midnight sun where the sun remains visible all day 166 167 By astronomical convention the four seasons can be determined by the solstices the points in the orbit of maximum axial tilt toward or away from the Sun and the equinoxes when Earth s rotational axis is aligned with its orbital axis In the Northern Hemisphere winter solstice currently occurs around 21 December summer solstice is near 21 June spring equinox is around 20 March and autumnal equinox is about 22 or 23 September In the Southern Hemisphere the situation is reversed with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped 168 The angle of Earth s axial tilt is relatively stable over long periods of time Its axial tilt does undergo nutation a slight irregular motion with a main period of 18 6 years 169 The orientation rather than the angle of Earth s axis also changes over time precessing around in a complete circle over each 25 800 year cycle this precession is the reason for the difference between a sidereal year and a tropical year Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth s equatorial bulge The poles also migrate a few meters across Earth s surface This polar motion has multiple cyclical components which collectively are termed quasiperiodic motion In addition to an annual component to this motion there is a 14 month cycle called the Chandler wobble Earth s rotational velocity also varies in a phenomenon known as length of day variation 170 In modern times Earth s perihelion occurs around 3 January and its aphelion around 4 July These dates change over time due to precession and other orbital factors which follow cyclical patterns known as Milankovitch cycles The changing Earth Sun distance causes an increase of about 6 8 in solar energy reaching Earth at perihelion relative to aphelion 171 n 12 Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year This effect is much less significant than the total energy change due to the axial tilt and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere 172 Earth Moon systemFurther information Satellite system astronomy Moon Main articles Moon Lunar theory and Orbit of the Moon Earth Moon system seen from Mars The Moon is a relatively large terrestrial planet like natural satellite with a diameter about one quarter of Earth s It is the largest moon in the Solar System relative to the size of its planet although Charon is larger relative to the dwarf planet Pluto 173 174 The natural satellites of other planets are also referred to as moons after Earth s 175 The most widely accepted theory of the Moon s origin the giant impact hypothesis states that it formed from the collision of a Mars size protoplanet called Theia with the early Earth This hypothesis explains among other things the Moon s relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth s crust 43 The gravitational attraction between Earth and the Moon causes tides on Earth 176 The same effect on the Moon has led to its tidal locking its rotation period is the same as the time it takes to orbit Earth As a result it always presents the same face to the planet 177 As the Moon orbits Earth different parts of its face are illuminated by the Sun leading to the lunar phases 178 Due to their tidal interaction the Moon recedes from Earth at the rate of approximately 38 mm a 1 5 in year Over millions of years these tiny modifications and the lengthening of Earth s day by about 23 µs yr add up to significant changes 179 During the Ediacaran period for example approximately 620 Ma there were 400 7 days in a year with each day lasting 21 9 0 4 hours 180 The Moon may have dramatically affected the development of life by moderating the planet s climate Paleontological evidence and computer simulations show that Earth s axial tilt is stabilized by tidal interactions with the Moon 181 Some theorists think that without this stabilization against the torques applied by the Sun and planets to Earth s equatorial bulge the rotational axis might be chaotically unstable exhibiting large changes over millions of years as is the case for Mars though this is disputed 182 183 Viewed from Earth the Moon is just far enough away to have almost the same apparent sized disk as the Sun The angular size or solid angle of these two bodies match because although the Sun s diameter is about 400 times as large as the Moon s it is also 400 times more distant 161 This allows total and annular solar eclipses to occur on Earth 184 Asteroids and artificial satellites Main articles Near Earth object and Claimed moons of Earth A computer generated image mapping the prevalence of artificial satellites and space debris around Earth in geosynchronous and low Earth orbit Earth s co orbital asteroids population consists of quasi satellites objects with a horseshoe orbit and trojans There are at least five quasi satellites including 469219 Kamoʻoalewa 185 186 A trojan asteroid companion 2010 TK7 is librating around the leading Lagrange triangular point L4 in Earth s orbit around the Sun 187 188 The tiny near Earth asteroid 2006 RH120 makes close approaches to the Earth Moon system roughly every twenty years During these approaches it can orbit Earth for brief periods of time 189 As of September 2021 update there are 4 550 operational human made satellites orbiting Earth 8 There are also inoperative satellites including Vanguard 1 the oldest satellite currently in orbit and over 16 000 pieces of tracked space debris n 3 Earth s largest artificial satellite is the International Space Station 190 HydrosphereMain article Hydrosphere A view of Earth with its global ocean and cloud cover which dominate Earth s surface and hydrosphere At Earth s polar regions Earth s hydrosphere forms larger areas of ice cover 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 6 600 ft The mass of the oceans is approximately 1 35 1018 metric tons or about 1 4400 of Earth s total mass The oceans cover an area of 361 8 million km2 139 7 million sq mi with a mean depth of 3 682 m 12 080 ft resulting in an estimated volume of 1 332 billion km3 320 million cu mi 191 If all of Earth s crustal surface were at the same elevation as a smooth sphere the depth of the resulting world ocean would be 2 7 to 2 8 km 1 68 to 1 74 mi 192 About 97 5 of the water is saline the remaining 2 5 is fresh water 193 194 Most fresh water about 68 7 is present as ice in ice caps and glaciers 195 In Earth s coldest regions snow survives over the summer and changes into ice This accumulated snow and ice eventually forms into glaciers bodies of ice that flow under the influence of their own gravity Alpine glaciers form in mountainous areas whereas vast ice sheets form over land in polar regions The flow of glaciers erodes the surface changing it dramatically with the formation of U shaped valleys and other landforms 196 Sea ice in the Arctic covers an area about as big as the United States although it is quickly retreating as a consequence of climate change 197 The average salinity of Earth s oceans is about 35 grams of salt per kilogram of seawater 3 5 salt 198 Most of this salt was released from volcanic activity or extracted from cool igneous rocks 199 The oceans are also a reservoir of dissolved atmospheric gases which are essential for the survival of many aquatic life forms 200 Sea water has an important influence on the world s climate with the oceans acting as a large heat reservoir 201 Shifts in the oceanic temperature distribution can cause significant weather shifts such as the El Nino Southern Oscillation 202 The abundance of water on Earth s surface is a unique feature that distinguishes it from other planets in the Solar System Solar System planets with considerable atmospheres do partly host atmospheric water vapor but they lack surface conditions for stable surface water 203 Despite some moons showing signs of large reservoirs of extraterrestrial liquid water with possibly even more volume than Earth s ocean all of them are large bodies of water under a kilometers thick frozen surface layer 204 AtmosphereMain article Atmosphere of Earth A view of Earth with different layers of its atmosphere visible the troposphere with its shadows casting clouds and a band of blue sky at the horizon and above this the lower thermosphere with a line of green airglow around an altitude of 100 km at the edge of space The atmospheric pressure at Earth s sea level averages 101 325 kPa 14 696 psi 205 with a scale height of about 8 5 km 5 3 mi 5 A dry atmosphere is composed of 78 084 nitrogen 20 946 oxygen 0 934 argon and trace amounts of carbon dioxide and other gaseous molecules 205 Water vapor content varies between 0 01 and 4 205 but averages about 1 5 Clouds cover around two thirds of Earth s surface more so over oceans than land 206 The height of the troposphere varies with latitude ranging between 8 km 5 mi at the poles to 17 km 11 mi at the equator with some variation resulting from weather and seasonal factors 207 Earth s biosphere has significantly altered its atmosphere Oxygenic photosynthesis evolved 2 7 Gya forming the primarily nitrogen oxygen atmosphere of today 65 This change enabled the proliferation of aerobic organisms and indirectly the formation of the ozone layer due to the subsequent conversion of atmospheric O2 into O3 The ozone layer blocks ultraviolet solar radiation permitting life on land 208 Other atmospheric functions important to life include transporting water vapor providing useful gases causing small meteors to burn up before they strike the surface and moderating temperature 209 This last phenomenon is the greenhouse effect trace molecules within the atmosphere serve to capture thermal energy emitted from the surface thereby raising the average temperature Water vapor carbon dioxide methane nitrous oxide and ozone are the primary greenhouse gases in the atmosphere Without this heat retention effect the average surface temperature would be 18 C 0 F in contrast to the current 15 C 59 F 210 and life on Earth probably would not exist in its current form 211 Weather and climate Main articles Weather and Climate The ITCZ s band of clouds over the Eastern Pacific and the Americas as seen from space Worldwide Koppen climate classifications Earth s atmosphere has no definite boundary gradually becoming thinner and fading into outer space 212 Three quarters of the atmosphere s mass is contained within the first 11 km 6 8 mi of the surface this lowest layer is called the troposphere 213 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 thermal energy 214 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 60 215 Ocean heat content and currents are also important factors in determining climate particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions 216 Earth receives 1361 W m2 of solar irradiance 217 218 The amount of solar energy that reaches the Earth s surface decreases with increasing latitude At higher latitudes the sunlight reaches the surface at lower angles and it must pass through thicker columns of the atmosphere As a result the mean annual air temperature at sea level decreases by about 0 4 C 0 7 F per degree of latitude from the equator 219 Earth s surface can be subdivided 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 220 Further factors that affect a location s climates are its proximity to oceans the oceanic and atmospheric circulation and topology 221 Places close to oceans typically have colder summers and warmer winters due to the fact that oceans can store large amounts of heat The wind transports the cold or the heat of the ocean to the land 222 Atmospheric circulation also plays an important role San Francisco and Washington DC are both coastal cities at about the same latitude San Francisco s climate is significantly more moderate as the prevailing wind direction is from sea to land 223 Finally temperatures decrease with height causing mountainous areas to be colder than low lying areas 224 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 falls to the surface as precipitation 214 Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or 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 topographic features and temperature differences determine the average precipitation that falls in each region 225 The commonly used Koppen climate classification system has five broad groups humid tropics arid humid middle latitudes continental and cold polar which are further divided into more specific subtypes 215 The Koppen system rates regions based on observed temperature and precipitation 226 Surface air temperature can rise to around 55 C 131 F in hot deserts such as Death Valley and can fall as low as 89 C 128 F in Antarctica 227 228 Upper atmosphere Afterglow of the troposphere orange the stratosphere whitish the mesosphere blue with remains of a spacecraft reentry trail and above the thermosphere without a visible glow The upper atmosphere the atmosphere above the troposphere 229 is usually divided into the stratosphere mesosphere and thermosphere 209 Each layer 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 geomagnetic fields interact with the solar wind 230 Within the stratosphere is the ozone layer a component that partially shields the surface from ultraviolet light and thus is important for life on Earth The Karman line defined as 100 km 62 mi above Earth s surface is a working definition for the boundary between the atmosphere and outer space 231 Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth s gravity This causes a slow but steady loss of the atmosphere into space Because unfixed hydrogen has a low molecular mass it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gases 232 The leakage of hydrogen into space contributes to the shifting of Earth s atmosphere and surface from an initially reducing state to its current oxidizing one Photosynthesis provided a source of free oxygen but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere 233 Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth 234 In the current oxygen rich atmosphere most hydrogen is converted into water before it has an opportunity to escape Instead most of the hydrogen loss comes from the destruction of methane in the upper atmosphere 235 Life on EarthMain article Life An animation of the changing density of productive vegitation on land low in brown heavy in dark green and phytoplankton at the ocean surface low in purple high in yellow Earth is the only known place that is habitable and has hosted life Earth s life developed in Earth s early bodies of water some hundred million years after Earth formed Earth s life has been shaping and inhabiting many particular ecosystems on Earth and has eventually expanded globally forming an overarching biosphere 236 Therefore life has impacted Earth significantly altering Earth s atmosphere and surface over long periods of time causing changes like the Great oxidation event Earth s life has over time greatly diversified allowing the biosphere to have different biomes which are inhabited by comparatively similar plants and animals 237 The different biomes develope at distinct elevations or water depths planetary temperature latitudes and on land also with different humidity Earth s species diversity and biomass reaches a peak in shallow waters and with forests particularly in equatorial warm and humid conditions While freezing polar regions and high altitudes or extremely arid areas are relatively barren of plant and animal life 238 Earth provides liquid water an environment where complex organic molecules can assemble and interact and sufficient energy to sustain a metabolism 239 Plants and other organisms take up nutrients from water soils and the atmosphere These nutrients are constantly recycled between different species 240 Extreme weather such as tropical cyclones including hurricanes and typhoons occurs over most of Earth s surface and has a large impact on life in those areas From 1980 to 2000 these events caused an average of 11 800 human deaths per year 241 Many places are subject to earthquakes landslides tsunamis volcanic eruptions tornadoes blizzards floods droughts wildfires and other calamities and disasters 242 Human impact is felt in many areas due to pollution of the air and water acid rain loss of vegetation overgrazing deforestation desertification loss of wildlife species extinction soil degradation soil depletion and erosion 243 Human activities release greenhouse gases into the atmosphere which cause global warming 244 This is driving changes such as the melting of glaciers and ice sheets a global rise in average sea levels increased risk of drought and wildfires and migration of species to colder areas 245 Human geographyMain article Human geography See also World A composite image of artificial light emissions at night on a map of Earth Originating from earlier primates in eastern Africa 300 000 years ago humans have since been migrating and with the advent of agriculture in the 10th millennium BC increasingly settling Earth s land In the 20th century Antarctica had been the last continent to see a first and until today limited human presence Human population has since the 19th century grown exponentially to seven billion in the early 2010s 246 and is projected to peak at around ten billion in the second half of the 21st century 247 Most of the growth is expected to take place in sub Saharan Africa 247 Distribution and density of human population varies greatly around the world with the majority living in south to eastern Asia and 90 inhabiting only the Northern Hemisphere of Earth 248 partly due to the hemispherical predominance of the world s land mass with 68 of the world s land mass being in the Northern Hemisphere 249 Furthermore since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century 250 Beyond Earth s surface humans have lived on a temporary basis with only special purpose deep underground and underwater presence and a few space stations Human population virtually completely remains on Earth s surface fully depending on Earth and the environment it sustains Humans have gone and temporarily stayed beyond Earth with some hundreds of people since the latter half of the 20th century and only a fraction of them reaching another celestial body the Moon 251 252 Humans have developed diverse societies and cultures which have marked Earth significantly Earth has been the claim of extensive human sedetary extractive and political activity Earth s land has been mostly territorially claimed since the 19th century by states of which today more than 200 exist 253 with only Antarctica and few areas remaining unclaimed 254 Most of these states together form the United Nations the leading worldwide intergovernmental organization 255 with international governance having provided legal regimes extraterritorially extanding human governance over the ocean and Antarctica and therefore all of Earth Natural resources and land use Main articles Natural resource and Land use Earth s land use for human agriculture Earth has resources that have been exploited by humans 256 Those termed non renewable resources such as fossil fuels are only replenished over geological timescales 257 Large deposits of fossil fuels are obtained from Earth s crust consisting of coal petroleum and natural gas 258 These deposits are used by humans both for energy production and as feedstock for chemical production 259 Mineral ore bodies have also been formed within the crust through a process of ore genesis resulting from actions of magmatism erosion and plate tectonics 260 These metals and other elements are extracted by mining a process which often brings environmental and health damage 261 Earth s biosphere produces many useful biological products for humans including food wood pharmaceuticals oxygen and the recycling of organic waste The land based ecosystem depends upon topsoil and fresh water and the oceanic ecosystem depends on dissolved nutrients washed down from the land 262 In 2019 39 million km2 15 million sq mi of Earth s land surface consisted of forest and woodlands 12 million km2 4 6 million sq mi was shrub and grassland 40 million km2 15 million sq mi were used for animal feed production and grazing and 11 million km2 4 2 million sq mi were cultivated as croplands 263 Of the 12 14 of ice free land that is used for croplands 2 percentage points were irrigated in 2015 264 Humans use building materials to construct shelters 265 Humans and the environment Main articles Human impact on the environment and Climate change Change in average surface air temperature and drivers for that change Human activity has caused increased temperatures with natural forces adding some variability 266 Human activities have impacted Earth s environments Through activities such as the burning of fossil fuels humans have been increasing the amount of greenhouse gases in the atmosphere altering Earth s energy budget and climate 244 267 It is estimated that global temperatures in the year 2020 were 1 2 C 2 2 F warmer than the preindustrial baseline 268 This increase in temperature known as global warming has contributed to the melting of glaciers rising sea levels increased risk of drought and wildfires and migration of species to colder areas 245 The concept of planetary boundaries was introduced to quantify humanity s impact on Earth Of the nine identified boundaries five have been crossed Biosphere integrity climate change chemical pollution destruction of wild habitats and the nitrogen cycle are thought to have passed the safe threshold 269 270 As of 2018 no country meets the basic needs of its population without transgressing planetary boundaries It is thought possible to provide all basic physical needs globally within sustainable levels of resource use 271 Cultural and historical viewpointMain articles Earth in culture and Earth symbol Tracy Caldwell Dyson in the Cupola module of the International Space Station observing the Earth below Human cultures have developed many views of the planet 272 The standard astronomical symbols of Earth are a quartered circle 273 representing the four corners of the world and a globus cruciger Earth is sometimes personified as a deity In many cultures it is a mother goddess that is also the primary fertility deity 274 Creation myths in many religions involve the creation of Earth by a supernatural deity or deities 274 The Gaia hypothesis developed in the mid 20th century compared Earth s environments and life as a single self regulating organism leading to broad stabilization of the conditions of habitability 275 276 277 Images of Earth taken from space particularly during the Apollo program have been credited with altering the way that people viewed the planet that they lived on called the overview effect emphasizing its beauty uniqueness and apparent fragility 278 279 In particular this caused a realization of the scope of effects from human activity on Earth s environment Enabled by science particularly Earth observation 280 humans have started to take action on environmental issues globally 281 acknowledging the impact of humans and the interconnectedness of Earth s environments Scientific investigation has resulted in several culturally transformative shifts in people s view of the planet Initial belief in a flat Earth was gradually displaced in Ancient Greece by the idea of a spherical Earth which was attributed to both the philosophers Pythagoras and Parmenides 282 283 Earth was generally believed to be the center of the universe until the 16th century when scientists first concluded that it was a moving object one of the planets of the Solar System 284 It was only during the 19th century that geologists realized Earth s age was at least many millions of years 285 Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864 sparking a vigorous debate on the subject it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth s age was established proving the planet to be billions of years old 286 287 See alsoCelestial sphere Earth phase Earth physical characteristics tables Earth science Extremes on Earth List of Solar System extremes Outline of Earth Table of physical properties of planets in the Solar System Timeline of natural history Timeline of the far futureNotes All astronomical quantities vary both secularly and periodically The quantities given are the values at the instant J2000 0 of the secular variation ignoring all periodic variations a b aphelion a 1 e perihelion a 1 e where a is the semi major axis and e is the eccentricity The difference between Earth s perihelion and aphelion is 5 million kilometers Wilkinson John 2009 Probing the New Solar System CSIRO Publishing p 144 ISBN 978 0 643 09949 4 a b As of 4 January 2018 the United States Strategic Command tracked a total of 18 835 artificial objects mostly debris See Anz Meador Phillip Shoots Debi eds February 2018 Satellite Box Score PDF Orbital Debris Quarterly News 22 1 12 Retrieved 18 April 2018 Earth s circumference is almost exactly 40 000 km because the meter was calibrated on this measurement more specifically 1 10 millionth of the distance between the poles and the equator Due to natural fluctuations ambiguities surrounding ice shelves and mapping conventions for vertical datums exact values for land and ocean coverage are not meaningful Based on data from the Vector Map and Global Landcover Archived 26 March 2015 at the Wayback Machine datasets extreme values for coverage of lakes and streams are 0 6 and 1 0 of Earth s surface The ice sheets of Antarctica and Greenland are counted as land even though much of the rock that supports them lies below sea level If Earth were shrunk to the size of a billiard ball some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections whereas much of the planet including the Great Plains and the abyssal plains would feel smoother 90 Including the Somali Plate which is being formed out of the 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