Wikipedia

Earth

"Planet Earth" redirects here. For other uses, seeEarth (disambiguation)andPlanet Earth (disambiguation).

Earthis the thirdplanetfrom theSunand the onlyastronomical objectknown toharbor life.This is enabled by Earth being anocean world,the only one in theSolar Systemsustaining liquidsurface water.Almost all of Earth's water is contained in its global ocean, covering70.8%ofEarth's crust.The remaining 29.2% of Earth's crust is land, most of which is located in the form ofcontinentallandmasseswithin Earth'sland hemisphere.Most of Earth's land is somewhathumidand covered by vegetation, while largesheets of iceatEarth's polardesertsretain more water than Earth'sgroundwater,lakes, rivers andatmospheric watercombined. Earth's crust consists of slowly movingtectonic plates,which interact to produce mountain ranges,volcanoes,and earthquakes.Earth has a liquid outer corethat generates amagnetospherecapable of deflecting most of the destructivesolar windsandcosmic radiation.

Earth
Photograph of Earth taken by the Apollo 17 mission. The Arabian peninsula, Africa and Madagascar lie in the upper half of the disc, whereas Antarctica is at the bottom.
The Blue Marble,Apollo 17,December 1972
Designations
The world, theglobe,Sol III,Terra, Tellus,Gaia,Mother Earth
AdjectivesEarthly, terrestrial, terran, tellurian
Symbol🜨 and ♁
Orbital characteristics
EpochJ2000[n 1]
Aphelion152097597km (94509065mi)
Perihelion147098450km (91402740mi)[n 2]
149598023km (92955902mi)[1]
Eccentricity0.0167086[1]
365.256363004d[2]
(1.00001742096aj)
29.7827 km/s[3]
(107218km/h;66622mph)
358.617°
Inclination
−11.26064°– J2000 ecliptic[3]
2023-Jan-04[5]
114.20783°[3]
Satellites1, theMoon
Physical characteristics
6371.0km (3958.8mi)[6]
6378.137km (3963.191mi)[7][8]
Polarradius
6356.752km (3949.903mi)[9]
Flattening1/298.257222101(ETRS89)[10]
Circumference
  • 510072000km2
    (196940000sq mi)[12][n 4]
  • Land:148940000km2
    (57510000sq mi)
  • Water:361132000km2
    (139434000sq mi)
Volume1.08321×1012km3(2.59876×1011cu mi)[3]
Mass5.972168×1024kg(1.31668×1025lb)[13]
5.513 g/cm3
(0.1992 lb/cu in)[3]
9.80665m/s2
(32.1740 ft/s2)[14]
0.3307[15]
11.186 km/s (40270km/h;25020mph)[3]
1.0 d
(24h 00 m 00s)
0.99726968d[16]
(23h 56 m 4.100s)
Equatorial rotation velocity
0.4651 km/s[17]
(1674.4km/h;1040.4mph)
23.4392811°[2]
Albedo
Temperature255 K (−18 °C; −1 °F)
(blackbody temperature)[18]
Surfacetemp. min mean max
Celsius[n 5] −89.2 °C 14.76 °C 56.7 °C
Fahrenheit −128.5 °F 58.568 °F 134.0 °F
Surfaceequivalent doserate0.274 μSv/h[22]
−3.99
Atmosphere
Surfacepressure
101.325kPa(at sea level)
Composition by volume
Source:[3]

Earth hasa dynamic atmosphere,which sustains Earth's surface conditions and protects it from mostmeteoroidsandUV-light at entry.It has a composition of primarilynitrogenandoxygen.Water vaporis widely present in the atmosphere,forming cloudsthat cover most of the planet. The water vapor acts as agreenhouse gasand, together with other greenhouse gases in the atmosphere, particularlycarbon dioxide(CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing ofenergy from the Sun's light.This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with theequatorial regionreceiving more sunlight than the polar regions) driveatmosphericandocean currents,producing a globalclimate systemwith differentclimate regions,and a range of weather phenomena such asprecipitation,allowing components such asnitrogentocycle.

Earth isroundedintoan ellipsoidwitha circumferenceof about 40,000 km. It is thedensest planet in the Solar System.Of the fourrocky planets,it is the largest and most massive. Earth is about eightlight-minutesaway from the Sun andorbits it,taking a year (about 365.25 days) to complete one revolution.Earth rotatesaround its own axis in slightly less than a day (in about 23 hours and 56 minutes).Earth's axis of rotationis tilted with respect to the perpendicular to its orbital plane around the Sun, producingseasons.Earth isorbitedby onepermanentnatural satellite,theMoon,whichorbitsEarth at 384,400 km (1.28 light seconds) and is roughly a quarter as wide as Earth. The Moon's gravity helps stabilize Earth's axis, causestidesandgradually slows Earth's rotation.Tidal lockinghas made the Moon always face Earth with the same side.

Earth, like most other bodies in the Solar System,formed 4.5 billion years agofrom gas and dust in theearly Solar System.During the first billion years ofEarth's history,the ocean formed and thenlife developedwithin it. Life spread globally and has been altering Earth's atmosphere and surface, leading to theGreat Oxidation Eventtwo billion years ago.Humansemerged300,000 years agoin Africa and have spread across every continent on Earth. Humans depend on Earth'sbiosphereand natural resources for their survival, but haveincreasingly impacted the planet's environment.Humanity's current impact on Earth's climate and biosphere isunsustainable,threatening the livelihood of humans and many other forms of life, andcausing widespread extinctions.[23]

Etymology

TheModern EnglishwordEarthdeveloped, viaMiddle English,from anOld Englishnoun most often spelledeorðe.[24]It has cognates in everyGermanic language,and theirancestral roothas been reconstructed as*erþō.In its earliest attestation, the wordeorðewas used to translate the many senses ofLatinterraandGreekγῆgē:the ground, itssoil,dry land, the human world, the surface of the world (including the sea), and the globe itself. As with RomanTerra/Tellūs and GreekGaia,Earth may have been apersonified goddessinGermanic paganism:lateNorse mythologyincludedJörð( "Earth" ), a giantess often given as the mother ofThor.[25]

Historically, "Earth" has been written in lowercase. Beginning with the use ofEarly Middle English,itsdefinite senseas "the globe" was expressed as "the earth". By the era ofEarly Modern English,capitalization of nouns began to prevail,andthe earthwas also writtenthe Earth,particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given asEarth,by analogy with the names of theother planets,though "earth" and forms with "the earth" remain common.[24]House stylesnow vary:Oxford spellingrecognizes the lowercase form as the more common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name, such as a description of the "Earth's atmosphere", but employs the lowercase when it is preceded by "the", such as "the atmosphere of the earth". It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[26]

The nameTerra/ˈtɛrə/occasionally is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[27]while in poetryTellus/ˈtɛləs/has been used to denote personification of the Earth.[28]Terrais also the name of the planet in someRomance languages,languages that evolved fromLatin,like Italian andPortuguese,while in other Romance languages the word gave rise to names with slightly altered spellings, like theSpanishTierraand theFrenchTerre.The Latinate formGæaorGaea(English:/ˈdʒiː.ə/) of the Greek poetic nameGaia(Γαῖα;Ancient Greek:[ɡâi̯.a]or[ɡâj.ja]) is rare, though the alternative spellingGaiahas become common due to theGaia hypothesis,in which case its pronunciation is/ˈɡaɪ.ə/rather than the more classical English/ˈɡeɪ.ə/.[29]

There are a number of adjectives for the planet Earth. The word "earthly" is derived from "Earth". From theLatinTerracomesterran/ˈtɛrən/,[30]terrestrial/təˈrɛstriəl/,[31]and (via French)terrene/təˈriːn/,[32]and from the LatinTelluscomestellurian/tɛˈlʊəriən/[33]andtelluric.[34]

Natural history

Main articles:History of EarthandTimeline of natural history

Formation

Further information:Early EarthandHadean
A 2012 artistic impression of the earlySolar System'sprotoplanetary diskfrom which Earth and other Solar System bodies were formed

The oldest material found in theSolar Systemis dated to4.5682+0.0002
−0.0004Ga(billion years) ago.[35]By4.54±0.04 Gathe primordial Earth had formed.[36]The bodies inthe Solar System formed and evolvedwith the Sun. In theory, asolar nebulapartitions a volume out of amolecular cloudby gravitational collapse, which begins to spin and flatten into acircumstellar disk,and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, anddust(includingprimordial nuclides). According tonebular theory,planetesimalsformed byaccretion,with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form.[37]

Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[38]Aleading hypothesisis that it was formed by accretion from material loosed from Earth after aMars-sized object with about 10% of Earth's mass, namedTheia,collided with Earth.[39]It hit Earth with a glancing blow and some of its mass merged with Earth.[40][41]Between approximately 4.1 and3.8 Ga,numerousasteroid impactsduring theLate Heavy Bombardmentcaused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.[42]

After formation

Main article:Geological history of Earth

Earth's atmosphereand oceans were formed byvolcanic activityandoutgassing.[43]Water vapor from these sourcescondensedinto the oceans, augmented by water and ice from asteroids,protoplanets,andcomets.[44]Sufficient water to fill the oceans may have been on Earth since it formed.[45]In this model, atmosphericgreenhouse gaseskept the oceans from freezing when the newly forming Sunhad only 70%of itscurrent luminosity.[46]By3.5 Ga,Earth's magnetic fieldwas established, which helped prevent the atmosphere from being stripped away by thesolar wind.[47]

Pale orange dot,an artist's impression ofEarly Earth,featuring its tinted orangemethane-richearly atmosphere[48]

As the molten outer layer of Earth cooled itformedthe first solidcrust,which is thought to have beenmaficin composition. The firstcontinental crust,which was morefelsicin composition, formed by the partial melting of this mafic crust.[49]The presence of grains of themineral zircon of Hadean ageinEoarcheansedimentary rockssuggests that at least some felsic crust existed as early as4.4 Ga,only140Maafter Earth's formation.[50]There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[51](1) a relatively steady growth up to the present day,[52]which is supported by the radiometric dating of continental crust globally and (2) an initial rapid growth in the volume of continental crust during theArchean,forming the bulk of the continental crust that now exists,[53][54]which is supported by isotopic evidence fromhafniuminzirconsandneodymiumin sedimentary rocks. The two models and the data that support them can be reconciled by large-scalerecycling of the continental crust,particularly during the early stages of Earth's history.[55]

New continental crust forms as a result ofplate tectonics,a process ultimately driven by the continuous loss of heat from Earth's interior. Overthe periodof hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to formsupercontinentsthat have subsequently broken apart. At approximately750 Ma,one of the earliest known supercontinents,Rodinia,began to break apart. The continents later recombined to formPannotiaat600–540 Ma,then finallyPangaea,which also began to break apart at180 Ma.[56]

The most recent pattern ofice agesbegan about40 Ma,[57]and then intensified during thePleistoceneabout3 Ma.[58]High-andmiddle-latituderegions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years.[59]TheLast 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.[60]

Origin of life and evolution

Main articles:Origin of life,Earliest known life forms,andHistory of life

Chemical reactionsled to the first self-replicating molecules about four billion years ago. A half billion years later, thelast common ancestor of all current lifearose.[61]The evolution ofphotosynthesisallowed the Sun's energy to be harvested directly by life forms. The resultantmolecular oxygen(O2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protectiveozone layer(O3) in the upper atmosphere.[62]The incorporation of smaller cells within larger ones resulted in thedevelopment of complex cellscalledeukaryotes.[63]True multicellular organisms formed as cells withincoloniesbecame increasingly specialized. Aided by the absorption of harmfulultraviolet radiationby the ozone layer, life colonized Earth's surface.[64]Among the earliestfossilevidence for life ismicrobial matfossils found in 3.48 billion-year-oldsandstoneinWestern Australia,[65]biogenicgraphitefound in 3.7 billion-year-oldmetasedimentaryrocks inWestern Greenland,[66]and remains ofbiotic materialfound in 4.1 billion-year-old rocks in Western Australia.[67][68]Theearliest direct evidence of lifeon Earth is contained in 3.45 billion-year-oldAustralianrocks showing fossils ofmicroorganisms.[69][70]

An artist's impression of theArchean,theeonafter Earth's formation, featuring roundstromatolites,which are early oxygen-producing forms of life from billions of years ago. After theLate Heavy Bombardment,Earth's crusthad cooled, its water-rich barrensurfaceis marked bycontinentsandvolcanoes,with the Moon still orbiting Earth half as far as it is today, appearing 2.8 times larger and producing strongtides.[71]

During theNeoproterozoic,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 theCambrian explosion,when multicellular life forms significantly increased in complexity.[72][73]Following the Cambrian explosion,535 Ma,there have been at least five majormass extinctionsand many minor ones.[74]Apart from the proposed currentHolocene extinctionevent, themost recentwas66 Ma,whenan asteroid impacttriggered the extinction of 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 past66 Mys,and several million years ago, an Africanapespecies gained the ability to stand upright.[75][76]This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to theevolution of humans.Thedevelopment of agriculture,and thencivilization,led to humans having aninfluence on Earthand the nature and quantity of other life forms that continues to this day.[77]

Future

Main article:Future of Earth
See also:Global catastrophic risk
Conjectured illustration of the scorched Earth after theSunhas entered thered giantphase, about 5–7 billion years from now

Earth's expected long-term future is tied to that of the Sun. Over the next1.1 billion years,solar luminosity will increase by 10%, and over the next3.5 billion yearsby 40%.[78]Earth's increasing surface temperature will accelerate theinorganic carbon cycle,possibly reducingCO2concentration to levels lethally low for current plants (10ppmforC4 photosynthesis) in approximately100–900 million years.[79][80]A lack of vegetation would result in the loss of oxygen in the atmosphere, making current animal life impossible.[81]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 arunaway greenhouse effect,within an estimated 1.6 to 3 billion years.[82]Even if the Sun were stable, a fraction of the water in the modern oceans will descend to themantle,due to reduced steam venting from mid-ocean ridges.[82][83]

The Sun willevolveto become ared giantin about5 billion years.Models predict that the Sun will expand to roughly 1AU(150 million km; 93 million mi), about 250 times its present radius.[78][84]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.[78]

Physical characteristics

Further information:Geophysics

Size and shape

Main article:Figure of the Earth
Further information:Earth radius,Earth's circumference,Earth curvature,andGeomorphology
See also:List of highest mountains on Earth
Earth's western hemisphere showing topography relative to Earth's center instead of tomean sea level,as in common topographic maps

Earth has a rounded shape,throughhydrostatic equilibrium,[85]with an average diameter of 12,742 kilometers (7,918 mi), making it thefifth largestplanetary sizedand largestterrestrial objectof theSolar System.[86]

Due toEarth's rotationit has the shape of anellipsoid,bulging at its Equator;its diameter is 43 kilometers (27 mi) longer there than at itspoles.[87][88] Earth's shape furthermore has localtopographicvariations. Though the largest local variations, like theMariana Trench(10,925 meters or 35,843 feet below local sea level),[89]only shortens Earth's average radius by 0.17% andMount Everest(8,848 meters or 29,029 feet above local sea level) lengthens it by only 0.14%.[n 6][91]Since Earth's surface is farthest out from Earth'scenter of massat its equatorial bulge, the summit of the volcanoChimborazoin Ecuador (6,384.4 km or 3,967.1 mi) is its farthest point out.[92][93]Parallel to the rigid land topographythe Ocean exhibits a more dynamic topography.[94]

To measure the local variation of Earth's topography,geodesyemploys an idealized Earth producing a shape called ageoid.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.[95]

Surface

Further information:Planetary surface,Land cover,Land,Pedosphere,Ocean,Sea,Cryosphere,andPeplosphere
Acompositeimage of Earth, with its different types of surface discernible: Earth's surface dominating Ocean (blue), Africa with lush (green) to dry (brown) land and Earth's polar ice in the form ofAntarctic sea ice(grey) covering theAntarctic or Southern Oceanand theAntarctic ice sheet(white) coveringAntarctica.
ReliefofEarth's crust

Earth's surface is the boundary between the atmosphere, and the solid Earth and oceans. Defined in this way, it has an area of about 510 million km2(197 million sq mi).[12]Earth can be divided into twohemispheres:bylatitudeinto the polarNorthernandSouthernhemispheres; or bylongitudeinto the continentalEasternandWesternhemispheres.

Most of Earth's surface is ocean water: 70.8% or 361 million km2(139 million sq mi).[96]This vast pool of salty water is often called theworld ocean,[97][98]and makes Earth with its dynamichydrospherea water world[99][100]orocean world.[101][102]Indeed, in Earth's early history the ocean may have covered Earth completely.[103]The world ocean is commonly divided into the Pacific Ocean, Atlantic Ocean, Indian Ocean,Antarctic or Southern Ocean,and Arctic Ocean, from largest to smallest. The ocean coversEarth's oceanic crust,with theshelf seascovering theshelvesof thecontinental crustto a lesser extent. The oceanic crust forms largeoceanic basinswith features likeabyssal plains,seamounts,submarine volcanoes,[87]oceanic trenches,submarine canyons,oceanic plateaus,and a globe-spanningmid-ocean ridgesystem.[104]At Earth'spolar regions,theocean surfaceis covered by seasonally variable amounts ofsea icethat often connects with polar land,permafrostandice sheets,formingpolar ice caps.

Earth's land covers 29.2%, or 149 million km2(58 million sq mi) of Earth's surface. The land surface includes many islands around the globe, but most of the land surface is taken by the four continentallandmasses,which are (in descending order):Africa-Eurasia,America (landmass),Antarctica,andAustralia (landmass).[105][106][107]These landmasses are further broken down and grouped into thecontinents.Theterrainof the land surface varies greatly and consists of mountains,deserts,plains,plateaus,and otherlandforms.The elevation of the land surface varies from a low point of −418 m (−1,371 ft) at theDead Sea,to a maximum altitude of 8,848 m (29,029 ft) at the top ofMount Everest.The mean height of land above sea level is about 797 m (2,615 ft).[108]

Land can becoveredbysurface water,snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation,[109]but considerable amounts of land areice sheets(10%,[110]not including the equally large area of land underpermafrost)[111]ordeserts(33%)[112]

Thepedosphereis the outermost layer of Earth's land surface and is composed of soil and subject tosoil formationprocesses. Soil is crucial for land to be arable. Earth's totalarable landis 10.7% of the land surface, with 1.3% being permanent cropland.[113][114]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.[115]

The land surface and theocean floorform the top ofEarth's crust,which together with parts of theupper mantleformEarth's lithosphere.Earth's crust may be divided intooceanicandcontinentalcrust. Beneath the ocean-floor sediments, the oceanic crust is predominantlybasaltic,while the continental crust may include lower density materials such asgranite,sediments and metamorphic rocks.[116]Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust.[117]

Earth's surfacetopographycomprises both thetopography of the ocean surface,and theshapeof Earth's land surface. The submarine terrain of the ocean floor has an averagebathymetricdepth of 4 km, and is as varied as the terrain above sea level. Earth's surface is continually being shaped by internalplate tectonicprocesses includingearthquakesandvolcanism;byweatheringanderosiondriven by ice, water, wind and temperature; and bybiological processesincluding the growth and decomposition ofbiomassintosoil.[118][119]

Tectonic plates

Main article:Plate tectonics
Earth's major plates,which are:[120]

Earth's mechanically rigid outer layer ofEarth's crustandupper mantle,thelithosphere,is divided intotectonic plates.These plates are rigid segments that move relative to each other at one of three boundaries types: atconvergent boundaries,two plates come together; atdivergent boundaries,two plates are pulled apart; and attransform boundaries,two plates slide past one another laterally. Along these plate boundaries, earthquakes,volcanic activity,mountain-building,andoceanic trenchformation can occur.[121]The tectonic plates ride on top of theasthenosphere,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 issubductedunder 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 than100 Maold. The oldest oceanic crust is located in the Western Pacific and is estimated to be200 Maold.[123][124]By comparison, the oldest dated continental crust is4,030 Ma,[125]although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to4,400 Ma,indicating that at least some continental crust existed at that time.[50]

The seven major plates are thePacific,North American,Eurasian,African,Antarctic,Indo-Australian,andSouth American.Other notable plates include theArabian Plate,theCaribbean Plate,theNazca Plateoff the west coast of South America and theScotia Platein the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between50 and 55 Ma.The fastest-moving plates are the oceanic plates, with theCocos Plateadvancing 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 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 theirchemicalor physical (rheological) properties. The outer layer is a chemically distinctsilicatesolid crust, which is underlain by a highlyviscoussolid mantle. The crust is separated from the mantle by theMohorovičić 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 theupper mantleare collectively known as the lithosphere, which is divided into independently moving tectonic plates.[131]

Beneath the lithosphere is theasthenosphere,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 atransition zonethat separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquidouter corelies above a solidinner core.[132]Earth's inner core may be rotating at a slightly higherangular velocitythan 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.The density increases with depth.

Among the Solar System's planetary-sized objects, Earth is theobject with the highest density.

Chemical composition

Further information:Abundance of the chemical elements § Earth
See also:Abundance of elements in Earth's crust

Earth's massis approximately5.97×1024kg(5,970Yg). It is composed mostly of iron (32.1%by mass),oxygen(30.1%),silicon(15.1%),magnesium(13.9%),sulfur(2.9%),nickel(1.8%),calcium(1.5%), andaluminium(1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due togravitational separation,the core is primarily composed of the denser elements: iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[134][49]The most common rock constituents of the crust areoxides.Over 99% of thecrustis composed of various oxides of eleven elements, principally oxides containing silicon (thesilicate minerals), aluminium, iron, calcium, magnesium, potassium, or sodium.[135][134]

Internal heat

Main article:Earth's internal heat budget
A map ofheat flowfrom Earth's interior to the surface of Earth's crust, mostly along theoceanic ridges

The major heat-producingisotopeswithin Earth arepotassium-40,uranium-238,andthorium-232.[136]At the center, the temperature may be up to 6,000 °C (10,830 °F),[137]and the pressure could reach 360GPa(52 millionpsi).[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 approximately3Gyr,twice the present-day heat would have been produced, increasing the rates ofmantle convectionand plate tectonics, and allowing the production of uncommonigneous rockssuch askomatiitesthat are rarely formed today.[139][140]

The mean heat loss from Earth is87 mW m−2,for a global heat loss of4.42×1013W.[141]A portion of the core's thermal energy is transported toward the crust bymantle plumes,a form of convection consisting of upwellings of higher-temperature rock. These plumes can producehotspotsandflood basalts.[142]More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated withmid-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][clarification needed]

Gravitational field

Main article:Gravity of Earth

The gravity of Earth is theaccelerationthat is imparted to objects due to the distribution of mass within Earth. Near Earth's surface,gravitational accelerationis 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 asgravity anomalies.[144]

Magnetic field

Main article:Earth's magnetic field
A schematic view of Earth's magnetosphere withsolar windflowing from left to right

The main part of Earth's magnetic field is generated in the core, the site of adynamoprocess 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, adipole.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 is3.05×10−5T,with amagnetic dipole momentof7.79×1022Am2at 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 causessecular variationof the main field andfield reversalsat 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 themagnetosphere.Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the day-side of the magnetosphere, to about 10 Earth radii, and extends the night-side 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 supersonicbow shockprecedes the day-side magnetosphere within the solar wind.[149]Charged particlesare 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-energyparticlesthat drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[152]and theVan Allen radiation beltsare formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[153][154]

Duringmagnetic stormsandsubstorms,charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth'sionosphere,where atmospheric atoms can be excited and ionized, causing theaurora.[155]

Orbit and rotation

Rotation

Main article:Earth's rotation
Satellitetime lapse imageryof Earth's rotation showing axis tilt

Earth's rotation period relative to the Sun—its mean solar day—is86,400 secondsof mean solar time (86,400.0025SIseconds).[156]Because Earth's solar day is now slightly longer than it was during the 19th century due totidal deceleration,each day varies between0 and 2mslonger than the mean solar day.[157][158]

Earth's rotation period relative to thefixed stars,called itsstellar dayby theInternational Earth Rotation and Reference Systems Service(IERS), is86,164.0989 secondsof mean solar time (UT1), or23h56m4.0989s.[2][n 10]Earth's rotation period relative to theprecessingor moving meanMarch equinox(when the Sun is at 90° on the equator), is86,164.0905 secondsof mean solar time (UT1)(23h56m4.0905s).[2]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 thecelestial 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 orbitandEarth's location
Exaggerated illustration of Earth's elliptical orbit around the Sun, marking that the orbital extreme points (apoapsisandperiapsis) are not the same as the four seasonal extreme points, theequinoxandsolstice

Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of theinner Solar System.Earth's average orbital distance is about 150 million km (93 million mi), which is the basis for theastronomical unit(AU) and is equal to roughly 8.3light minutesor 380 timesEarth's distance to the Moon.Earth orbits the Sun every 365.2564 meansolar days,or onesidereal 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 themeridian.

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 from Earth to the Moon, 384,400 km (238,900 mi), in about 3.5 hours.[3]

The Moon and Earth orbit a commonbarycenterevery 27.32 days relative to the background stars. When combined with the Earth–Moon system's common orbit around the Sun, the period of thesynodic month,from new moon to new moon, is 29.53 days. Viewed from thecelestial north pole,the motion of Earth, the Moon, and their axial rotations are allcounterclockwise.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'saxis is tiltedsome 23.44 degrees from the perpendicular to the Earth–Sun plane (theecliptic), 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 betweenlunar eclipsesandsolar eclipses.[3][162]

TheHill sphere,or thesphere 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 that of 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 theMilky Wayand orbits about 28,000light-yearsfrom its center. It is about 20 light-years above thegalactic planein theOrion 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°[2]with the axis of its orbit plane, always pointing towards theCelestial 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 theNorthern Hemisphereoccurring when theTropic of Canceris facing the Sun, and in theSouthern Hemispherewhen theTropic of Capricornfaces 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 theArctic Circleand below theAntarctic Circlethere is no daylight at all for part of the year, causing apolar night,and this night extends for several months at the poles themselves. These same latitudes also experience amidnight 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 theequinoxes,when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere,winter solsticecurrently occurs around 21 December;summer solsticeis near 21 June, spring equinox is around 20 March andautumnal equinoxis 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 undergonutation;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,precessingaround in a complete circle over each 25,800-year cycle; this precession is the reason for the difference between a sidereal year and atropical 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. Thispolar motionhas multiple, cyclical components, which collectively are termedquasiperiodic motion.In addition to an annual component to this motion, there is a 14-month cycle called theChandler wobble.Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[170]

Earth's annual orbit is elliptical rather than circular, and its closest approach to the Sun is calledperihelion.In modern times, Earth's perihelion occurs around 3 January, and itsaphelionaround 4 July. These dates shift over time due to precession and changes to the orbit, the latter of which follows cyclical patterns known asMilankovitch cycles.The annual change in the 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

Further information:Satellite system (astronomy)

Moon

Main articles:Moon,Lunar theory,andOrbit of the Moon
Earth and the Moon as seen fromMarsby theMars Reconnaissance Orbiter
View of Earth from the Moon by theLunar Reconnaissance Orbiter

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, althoughCharonis larger relative to thedwarf planetPluto.[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, thegiant-impact hypothesis,states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains 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.[40]Computer simulations suggest that two blob-like remnants of this prototype could be inside the Earth.[176][177]

The gravitational attraction between Earth and the Moon causeslunar tideson Earth.[178]The same effect on the Moon has led to itstidal 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.[179]As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to thelunar phases.[180]Due to theirtidal 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 23μs/yr—add up to significant changes.[181]During theEdiacaranperiod, for example, (approximately620 Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[182]

The Moon may have dramatically affected the development of life by moderating the planet's climate.Paleontologicalevidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[183]Some theorists think that without this stabilization against thetorquesapplied 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.[184][185]

Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. Theangular size(orsolid 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.[186]

Asteroids and artificial satellites

Main articles:Near-Earth objectandClaimed moons of Earth
A computer-generated image mapping the prevalence ofartificial satellitesandspace debrisaround Earth ingeosynchronousandlow Earth orbit

Earth'sco-orbital asteroidspopulation consists ofquasi-satellites:objects with ahorseshoe orbitandtrojans.There are at least five quasi-satellites, including469219 Kamoʻoalewa.[187][188]Atrojan asteroidcompanion,2010 TK7,islibratingaround the leadingLagrange triangular point,L4, inEarth's orbitaround the Sun.[189]The tinynear-Earth asteroid2006 RH120makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.[190]

As of September 2021,there are 4,550 operational, human-madesatellitesorbiting Earth.[191]There are also inoperative satellites, includingVanguard 1,the oldest satellite currently in orbit, and over 16,000 pieces of trackedspace debris.[n 13]Earth's largest artificial satellite is theInternational Space Station(ISS).[192]

Hydrosphere

Main article:Hydrosphere
A view of Earth with itsglobal oceanandcloud cover,which dominate Earth's surface andhydrosphere;at Earth'spolarregions, its hydrosphere forms larger areas of ice cover.

Earth's hydrosphere is the sum of Earth's water and its distribution. Most of Earth's hydrosphere consists of Earth's global ocean. Earth's hydrosphere also consists of water in the atmosphere and on land, including clouds, inland seas, lakes, rivers, and underground waters.

The mass of the oceans is approximately 1.35×1018metric tonsor 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).[193]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).[194]About 97.5% of the water issaline;the remaining 2.5% isfresh water.[195][196]Most fresh water, about 68.7%, is present as ice inice capsandglaciers.[197]The remaining 30% isground water,1%surface water(covering only 2.8% of Earth's land)[198]and other small forms of fresh water deposits such aspermafrost,water vaporin the atmosphere, biological binding, etc.[199][200]

In Earth's coldest regions, snow survives over the summer andchanges into ice.This accumulated snow and ice eventually forms intoglaciers,bodies of ice that flow under the influence of their own gravity.Alpine glaciersform in mountainous areas, whereas vastice sheetsform over land in polar regions. The flow of glaciers erodes the surface, changing it dramatically, with the formation ofU-shaped valleysand other landforms.[201]Sea icein the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change.[202]

The averagesalinityof Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).[203]Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[204]The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[205]Sea water has an important influence on the world's climate, with the oceans acting as a largeheat reservoir.[206]Shifts in the oceanic temperature distribution can cause significant weather shifts, such as theEl Niño–Southern Oscillation.[207]

The abundance of water, particularly liquid water, on Earth's surface is a unique feature that distinguishes it from other planets in theSolar System.Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.[208]Despite somemoonsshowing signs of large reservoirs ofextraterrestrial liquid water,with possibly even more volume than Earth's ocean, all of them arelarge bodies of waterunder a kilometers thick frozen surface layer.[209]

Atmosphere

Main article:Atmosphere of Earth
A view of Earth with different layers of its atmosphere visible: thetropospherewith its clouds casting shadows, a band ofstratosphericblue sky at the horizon, and a line of greenairglowof the lowerthermospherearound analtitude of 100 km, at the edge of space

Theatmospheric pressureat Earth's sea level averages 101.325 kPa (14.696 psi),[210]with ascale heightof about 8.5 km (5.3 mi).[3]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.[210]Water vaporcontent varies between 0.01% and 4%[210]but averages about 1%.[3]Clouds coveraround two-thirds of Earth's surface, more so over oceans than land.[211]The height of thetropospherevaries 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.[212]

Earth'sbiospherehas significantly altered itsatmosphere.Oxygenic photosynthesisevolved2.7 Gya,formingthe primarily nitrogen–oxygen atmosphere of today.[62]This change enabled the proliferation ofaerobic organismsand, indirectly, the formation of the ozone layer due to the subsequentconversion of atmosphericO2intoO3.The ozone layer blocksultravioletsolar radiation,permitting life on land.[213]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.[214]This last phenomenon is thegreenhouse effect:trace molecules within the atmosphere serve to capturethermal energyemitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide,methane,nitrous oxide,andozoneare 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),[215]and life on Earth probably would not exist in its current form.[216]

Weather and climate

Main articles:WeatherandClimate
TheITCZ's band of clouds over the Eastern Pacific and the Americas as seen from space
WorldwideKöppen climate classifications

Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.[217]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.[218]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 isatmospheric circulationthat drives the weather and climate through redistribution of thermal energy.[219]

The primary atmospheric circulation bands consist of thetrade windsin the equatorial region below 30° latitude and thewesterliesin the mid-latitudes between 30° and 60°.[220]Ocean heat contentandcurrentsare also important factors in determining climate, particularly thethermohaline circulationthat distributes thermal energy from the equatorial oceans to the polar regions.[221]

Earth receives 1361 W/m2ofsolar irradiance.[222][223]The amount of solar energy that reaches 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.[224]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,temperateandpolarclimates.[225]

Further factors that affect a location's climates are itsproximity to oceans,the oceanic and atmospheric circulation, and topology.[226]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.[227]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.[228]Finally, temperaturesdecrease with heightcausing mountainous areas to be colder than low-lying areas.[229]

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 asprecipitation.[219]Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. Thiswater cycleis 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.[230]

The commonly usedKöppen climate classificationsystem has five broad groups (humid tropics,arid,humid middle latitudes,continentaland coldpolar), which are further divided into more specific subtypes.[220]The Köppen system rates regions based on observed temperature and precipitation.[231]Surfaceair temperature can rise toaround 55 °C (131 °F) inhot deserts,such asDeath Valley,andcan fall as low as−89 °C (−128 °F) inAntarctica.[232][233]

Upper atmosphere

Earth's night-side upper atmosphere appearing from the bottom as bands ofafterglowilluminating thetropospherein orange with silhouettes of clouds, and thestratospherein white and blue. Next themesosphere(pink area) extends to the orange and faintly green line of the lowestairglow,at about one hundred kilometers at theedge of spaceand the lower edge of thethermosphere(invisible). Continuing with green and red bands ofauroraestretching over several hundred kilometers.

The upper atmosphere, the atmosphere above the troposphere,[234]is usually divided into thestratosphere,mesosphere,andthermosphere.[214]Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, theexospherethins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.[235]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. TheKármán line,defined as 100 km (62 mi) above Earth's surface, is a working definition for the boundary between the atmosphere andouter space.[236]

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 steadyloss of the atmosphere into space.Because unfixedhydrogenhas a lowmolecular mass,it can achieveescape velocitymore readily, and it leaks into outer space at a greater rate than other gases.[237]The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initiallyreducingstate 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.[238]Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[239]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.[240]

Life on Earth

Main articles:BiosphereandHistory of life
An animation of the changing density ofproductivevegetation on land (low in brown; heavy in dark green) and phytoplankton at theocean surface(low in purple; high in yellow)

Earth is the only known place that has ever beenhabitablefor 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 particularecosystemson Earth and has eventually expanded globally forming an overarching biosphere.[241]Therefore, life has impacted Earth, significantly altering Earth's atmosphere and surface over long periods of time, causing changes like theGreat Oxidation Event.[242]Earth's life has also over time greatly diversified, allowing the biosphere to have differentbiomes,which are inhabited by comparatively similar plants and animals.[243]The different biomes developed at distinct elevations orwater depths,planetary temperaturelatitudesand on land also with differenthumidity.Earth's species diversityandbiomassreaches a peak in shallow waters and withforests, particularly in equatorial, warm and humid conditions.While freezingpolar regionsandhigh altitudes,orextremely arid areasare relatively barren of plant and animal life.[244]

Earth provides liquid water—an environment where complexorganic moleculescan assemble and interact, and sufficient energy to sustain ametabolism.[245]Plants and other organisms take upnutrientsfrom water, soils and the atmosphere. These nutrients are constantly recycled between different species.[246]

AHigh Desertstorm in theMojave

Extreme weather, such astropical cyclones(includinghurricanesandtyphoons), 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.[247]Many places are subject to earthquakes,landslides,tsunamis,volcanic eruptions,tornadoes,blizzards,floods, droughts,wildfires,and other calamities and disasters.[248]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, speciesextinction,soil degradation,soil depletionanderosion.[249]Human activities release greenhouse gases into the atmosphere which causeglobal warming.[250]This is drivingchangessuch as themelting of glaciers and ice sheets,aglobal rise in average sea levels,increased risk of drought and wildfires, and migration of species to colder areas.[251]

Human geography

Main article:Human geography
See also:World
A composite image ofartificial light emissionsat night on a map of Earth

Originating from earlierprimatesin Eastern Africa 300,000years agohumans have since been migratingand with the advent of agriculture in the 10th millennium BC increasinglysettlingEarth's land.[252]In the 20th centuryAntarcticahad been the last continent to see a first and until today limited human presence.

Human populationhas since the 19th century grown exponentially to seven billion in the early 2010s,[253]and is projected to peak at around ten billion in the second half of the 21st century.[254]Most of the growth is expected to take place insub-Saharan Africa.[254]

Distribution anddensity of human populationvaries greatly around the world with the majority living in south to eastern Asia and 90% inhabiting only theNorthern Hemisphereof Earth,[255]partly due to thehemispherical predominance of the world's land mass,with 68% of the world's land mass being in the Northern Hemisphere.[256]Furthermore, since the 19th century humans have increasingly converged into urban areas with the majority living in urban areas by the 21st century.[257]

Beyond Earth's surface humans have lived on a temporary basis, with only a few special-purpose deepundergroundandunderwaterpresences and a fewspace stations.The human population virtually completely remains on Earth's surface, fully depending on Earth and the environment it sustains. Since the second half of the 20th century, some hundreds of humans have temporarilystayed beyond Earth,a tiny fraction of whom have reached another celestial body, the Moon.[258][259]

Earth has been subject to extensive human settlement, and humans have developed diverse societies and cultures. Most of Earth's land has been territorially claimed since the 19th century bysovereign states(countries) separated bypolitical borders,and205 such statesexist today,[260]with only parts of Antarctica and a few small regionsremaining unclaimed.[261]Most of these states together form theUnited Nations,the leading worldwideintergovernmental organization,[262]which extends human governanceover the oceanandAntarctica,and therefore all of Earth.

Natural resources and land use

Main articles:Natural resourceandLand use
Earth's land use for human agriculture in 2019

Earth has resources that have been exploited by humans.[263]Those termednon-renewable resources,such asfossil fuels,are only replenished over geological timescales.[264]Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[265]These deposits are used by humans both for energy production and as feedstock for chemical production.[266]Mineralorebodies have also been formed within the crust through a process ofore genesis,resulting from actions ofmagmatism,erosion, and plate tectonics.[267]These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[268]

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 upontopsoiland fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.[269]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.[270]Of the 12–14% of ice-free land that is used for croplands, 2percentage pointswere irrigated in 2015.[271]Humans usebuilding materialsto construct shelters.[272]

Humans and the environment

Main articles:Human impact on the environmentandClimate change
Change in average surface air temperature and drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.[273]

Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount ofgreenhouse gasesin the atmosphere, alteringEarth's energy budgetand climate.[250][274]It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.[275]This increase in temperature, known asglobal warming,has contributed to themelting of glaciers,rising sea levels,increased risk of drought and wildfires, and migration of species to colder areas.[251]

The concept ofplanetary boundarieswas 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 thenitrogen cycleare thought to have passed the safe threshold.[276][277]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.[278]

Cultural and historical viewpoint

Main articles:Earth in cultureandEarth in science fiction
Tracy Caldwell Dyson,aNASAastronaut, observing Earth from theCupolamoduleat theInternational Space Stationon 11 September 2010

Human cultureshave developed many views of the planet.[279]The standardastronomical symbolsof Earth are a quartered circle,,[280]representing thefour corners of the world,and aglobus cruciger,.Earth is sometimespersonifiedas adeity.In many cultures it is amother goddessthat is also the primaryfertility deity.[281]Creation mythsin many religions involve the creation of Earth by a supernatural deity or deities.[281]TheGaia 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.[282][283][284]

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 theoverview effect,emphasizing its beauty, uniqueness and apparent fragility.[285][286]In particular, this caused a realization of the scope of effects from human activity on Earth's environment. Enabled by science, particularlyEarth observation,[287]humans have started to takeaction on environmental issuesglobally,[288]acknowledging the impact of humans and theinterconnectedness of Earth's environments.

Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. Initial belief in aflat Earthwas gradually displaced inAncient Greeceby the idea of aspherical Earth,which was attributed to both the philosophersPythagorasandParmenides.[289][290]Earth was generally believed to bethe center of the universeuntil the 16th century, when scientists first concluded that it wasa moving object,one of the planets of the Solar System.[291]

It was only during the 19th century that geologists realizedEarth's agewas at least many millions of years.[292]Lord Kelvinusedthermodynamicsto 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 andradioactive datingwere 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.[293][294]

See also

Notes

  1. ^All astronomical quantities vary, bothsecularlyandperiodically.The quantities given are the values at the instantJ2000.0of the secular variation, ignoring all periodic variations.
  2. ^aphelion =a× (1 +e); perihelion =a× (1 –e), whereais the semi-major axis andeis 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.ISBN978-0-643-09949-4.
  3. ^Earth'scircumferenceis 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.
  4. ^Due to natural fluctuations, ambiguities surroundingice shelves,and mapping conventions forvertical datums,exact values for land and ocean coverage are not meaningful. Based on data from theVector MapandGlobal LandcoverArchived26 March 2015 at theWayback Machinedatasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice sheets ofAntarcticaandGreenlandare counted as land, even though much of the rock that supports them lies below sea level.
  5. ^Source for minimum,[19]mean,[20]and maximum[21]surface temperature
  6. ^If Earth were shrunk to the size of abilliard ball,some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections, whereas much of the planet, including theGreat Plainsand theabyssal plains,would feel smoother.[90]
  7. ^Including theSomali 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 between5 and 200 km.
  9. ^Locally varies between5 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): 359–361.Bibcode:1982A&A...105..359A.
  11. ^For Earth, theHill radiusis,wheremis the mass of Earth,ais an astronomical unit, andMis 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.
  13. ^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.Archived(PDF)from the original on 2 April 2019.Retrieved18 April2018.

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